publicationDate,title,abstract,id 2014-01-05,Hybrid paramagnetic-ferromagnetic quantum computer design based on electron spin arrays and a ferromagnetic nanostripe,"General design for the practical implementation of hybrid paramagnetic-ferromagnetic quantum computer.",1401.0878v1 2002-12-02,Theoretical description of the ferromagnetic $π$-junctions near the critical temperature,"The theory of ferromagnetic Pi-junction near the critical temperature is presented. It is demonstrated that in the dirty limit the modified Usadel equation adequately describes the proximity effect in ferromagnets. To provide the description of an experimentally relevant situation, oscillations of the Josephson critical current are calculated as a function of ferromagnetic layer thickness for different transparencies of the superconductor-ferromagnet interfaces.",0212031v1 2004-10-14,Spin dependent transport in ferromagnetic/superconductor/ferromagnetic single electron transistor,"Ferromagnetic single electron transistors with Al islands and orthogonal ferromagnetic leads (Co) are fabricated using ebeam lithography followed by shadow evaporation techniques. I-V characteristics exhibit typical single electron tunneling effects. Transport measurements performed in external magnetic field show that, when the two ferromagnetic leads are in antiparallel configuration, spin imbalance leads to a suppression of superconductivity.",0410351v1 1997-12-18,From Nagaoka's ferromagnetism to flat-band ferromagnetism and beyond: An introduction to ferromagnetism in the Hubbard model,"This is a self-contained review about ferromagnetism in the Hubbard model, which should be accessible to readers with various backgrounds who are new to the field. We describe Nagaoka's ferromagnetism and flat-band ferromagnetism in detail, giving all necessary backgrounds as well as complete (but elementary) mathematical proofs. By studying an intermediate model called long-range hopping model, we also demonstrate that there is indeed a deep relation between these two seemingly different approaches to ferromagnetism. We further discuss some attempts to go beyond these approaches. We briefly discuss recent rigorous example of ferromagnetism in the Hubbard model which has neither infinitely large parameters nor completely flat bands. We give preliminary discussions about possible experimental realizations of the (nearly-)flat-band ferromagnetism. Finally we focus on some theoretical attempts to understand metallic ferromagnetism. We discuss three artificial one-dimensional models in which the existence of metallic ferromagnetism can be easily proved.",9712219v3 2010-12-01,The second order dense ferromagnetic-ferromagnetic phase transition,"The fcc spin-1 Ising (BEG) model has a dense ferromagnetic ($df$) ground state instead of the ferromagnetic ground state at low temperature region and exhibits the dense ferromagnetic ($df$) - ferromagnetic ($F$) phase transition for $d=D/J=2.9$, $k=K/J=-0.5$, $\ell =L/J=0$ and $h=H/J=0$. The critical behavior of the dense ferromagnetic ($df$) - ferromagnetic ($F$) phase transition has been investigated using the cellular automaton cooling and heating algorithms. The universality class and the type of the dense ferromagnetic ($df$) - ferromagnetic ($F$) phase transition have been researched within the framework of the finite - size scaling, the power law relations and the probability distribution. The results show that the dense ferromagnetic- ferromagnetic phase transition is of the second order and the model shows universal second order Ising critical behavior at $d=2.9$ parameter value through $k=-0.5$ line.",1012.0195v1 2006-11-09,Conductance spectra of ferromagnetic superconductors: Quantum transport in a ferromagnetic metal/non-unitary ferromagnetic superconductor junction,"Recent findings of superconductors that simultaneously exhibit multiple spontaneously broken symmetries, such as ferromagnetic order or lack of an inversion center and even combinations of such broken symmetries, have led to much theoretical and experimental research. We consider quantum transport in a junction consisting of a ferromagnetic metal and a non-unitary ferromagnetic superconductor. It is shown that the conductance spectra provides detailed information about the superconducting gaps, and is thus helpful in determining the pairing symmetry of the Cooper pairs in ferromagnetic superconductor.",0611242v1 2018-03-09,Non-local coupling between antiferromagnets and ferromagnets in cavities,"Microwaves couple to magnetic moments in both ferromagnets and antiferromagnets. Although the magnons in ferromagnets and antiferromagnets radically differ, they can become entangled via strong coupling to the same microwave mode in a cavity. The equilibrium configuration of the magnetic moments crucially governs the coupling between the different magnons because the antiferromagnetic and ferromagnetic magnons have the opposite spins when their dispersion relations cross. We derive analytical expressions for the coupling strengths and find that the coupling between antiferromagnets and ferromagnets is comparable to the coupling between two ferromagnets. Our findings reveal a robust link between cavity spintronics with ferromagnets and antiferromagnets.",1803.03486v1 2021-11-24,Platform for controllable Majorana zero modes using superconductor/ferromagnet heterostructures,"We propose a novel platform for the creation and manipulation of Majorana zero modes consisting of a ferromagnetic metallic wire placed between conventional superconductors which are in proximity to ferromagnetic insulators. Our device relies on the interplay of applied supercurrents and exchange fields emerging from the ferromagnetic insulators. We assert that the proposed superconductor/ferromagnet heterostructures exhibit enhanced controllability, since topological superconductivity can be tuned apart from gate voltages applied on the ferromagnetic wire, also by manipulating the applied supercurrents and/or the magnetisation of the ferromagnetic insulators.",2111.12344v2 2001-07-02,Microscopic theory of non local pair correlations in metallic F/S/F trilayers,"We consider a microscopic theory of F/S/F trilayers with metallic or insulating ferromagnets. The trilayer with metallic ferromagnets is controlled by the formation of non local pair correlations among the two ferromagnets which do not exist with insulating ferromagnets. The difference between the insulating and ferromagnetic models can be understood from lowest order diagrams. Metallic ferromagnets are controlled by non local pair correlations and the superconducting gap is larger if the ferromagnetic electrodes have a parallel spin orientation. Insulating ferromagnets are controlled by pair breaking and the superconducting gap is smaller if the ferromagnetic electrodes have a parallel spin orientation. The same behavior is found in the presence of disorder in the microscopic phase variables and also in the presence of a partial spin polarization of the ferromagnets. The different behaviors of the metallic and insulating trilayers may be probed in experiments.",0107038v3 2008-01-22,Quantum Oscillations of Tunnel Magnetoresistance Induced by Spin-Wave Excitations in Ferromagnet-Ferromagnet-Ferromagnet Double Barrier Tunnel Junctions,"The possibility of quantum oscillations of the tunnel conductance and magnetoresistance induced by spin-wave excitations in a ferromagnet-ferromagnet-ferromagnet double barrier tunnel junction, when the magnetizations of the two side ferromagnets are aligned antiparallel to that of the middle ferromagnet, is investigated in a self-consistent manner by means of Keldysh nonequilibrium Green function method. It has been found that owing to the s-d exchange interactions between conduction electrons and the spin density induced by spin accumulation in the middle ferromagnet, the differential conductance and the TMR indeed oscillate with the increase of bias voltage, being consistent with the phenomenon that is observed recently in experiments. The effects of magnon modes, the energy levels of electrons as well as the molecular field in the central ferromagnet on the oscillatory transport property of the system are also discussed.",0801.3350v1 2009-10-20,Enhancement of ferromagnetism by p-wave Cooper pairing in superconducting ferromagnets,"In superconducting ferromagnets for which the Curie temperature $T_{m}$ exceeds the superconducting transition temperature $T_{c}$, it was suggested that ferromagnetic spin fluctuations could lead to superconductivity with p-wave spin triplet Cooper pairing. Using the Stoner model of itinerant ferromagnetism, we study the feedback effect of the p-wave superconductivity on the ferromagnetism. Below $T_{c}$, the ferromagnetism is enhanced by the p-wave superconductivity. At zero temperature, the critical Stoner value for itinerant ferromagnetism is reduced by the strength of the p-wave pairing potential, and the magnetization increases correspondingly. More important, our results suggest that once Stoner ferromagnetism is established, $T_m$ is unlikely to ever be below $T_c$. For strong and weak ferromagnetism, three and two peaks in the temperature dependence of the specific heat are respectively predicted, the upper peak in the latter case corresponding to a first-order transition.",0910.3873v1 2012-05-24,Ferromagnetic features on zero-bias conductance peaks in ferromagnet/insulator/superconductor junction,"We present a formula for tunneling conductance in ballistic ferromagnet/ferromagnetic insulator/superconductor junctions where the superconducting state has opposite spin pairing symmetry. The formula can involve correctly a ferromagnetism has been induced by effective mass difference between up- and down-spin electrons. Then, this effective mass mismatch ferromagnet and standard Stoner ferromagnet have been employed in this paper. As an application of the formulation, we have studied the tunneling effect for junctions including spin-triplet p-wave superconductor. The conductace spectra show a clear difference between two ferromagnets depending upon the way of normalization of the conductance. Especially, a essential difference is seen in zero-bias conductance peaks reflecting characteristics of each ferromagnets. From obtained results, it will be suggested that the measurements of the tunneling conductance in the junction provide us a useful information about the mechanism of itinerant ferromagnetism in metal.",1205.5394v2 2003-11-19,Two-dimensional array of magnetic particles: The role of an interaction cutoff,"Based on theoretical results and simulations, in two-dimensional arrangements of a dense dipolar particle system, there are two relevant local dipole arrangements: (1) a ferromagnetic state with dipoles organized in a triangular lattice, and (2) an anti-ferromagnetic state with dipoles organized in a square lattice. In order to accelerate simulation algorithms we search for the possibility of cutting off the interaction potential. Simulations on a dipolar two-line system lead to the observation that the ferromagnetic state is much more sensitive to the interaction cutoff $R$ than the corresponding anti-ferromagnetic state. For $R \gtrsim 8$ (measured in particle diameters) there is no substantial change in the energetical balance of the ferromagnetic and anti-ferromagnetic state and the ferromagnetic state slightly dominates over the anti-ferromagnetic state, while the situation is changed rapidly for lower interaction cutoff values, leading to the disappearance of the ferromagnetic ground state. We studied the effect of bending ferromagnetic and anti-ferromagnetic two-line systems and we observed that the cutoff has a major impact on the energetical balance of the ferromagnetic and anti-ferromagnetic state for $R \lesssim 4$. Based on our results we argue that $R \approx 5$ is a reasonable choice for dipole-dipole interaction cutoff in two-dimensional dipolar hard sphere systems, if one is interested in local ordering.",0311462v1 1997-06-06,Hole doping and weak quenched disorder effects on the 1D Kondo lattice for ferromagnetic Kondo couplings,"We investigate the one-dimensional Kondo lattice model (1D KLM) for ferromagnetic Kondo couplings, by using the bosonization method. The ferromagnet ic 2-leg spin ladder and the S=1 antiferromagnet occur as new one-dimensional Kondo insulators, for a half-filled band. First,a very small hole-doping makes the charge sector massless and it can produce, either an incommensurate RKKY interaction or a S=1 ferromagnet according to the strength of the ferromagnetic Kondo coupling. Second, we investigate, the effects of a weak and quenched disorder on these two Kondo insulators, by applying renormalization group methods: the Anderson localization is suppressed only in the context of a strong ferromagnetic Kondo coupling.",9706068v1 1999-07-13,Interface resistance in ferromagnet/superconductor junctions,"Results of theoretical study of spin-polarized tunneling in ferromagnet/superconductor junctions are presented. Spin and charge currents are calculated as a function of applied voltage and spin polarization in a ferromagnet. The model takes into account the splitting of different spin subbands in a ferromagnet and impurity scattering in the contact. The excess resistance of an FS contact due to the charge-imbalance in a superconductor is calculated for the first time. The results have implications for spin-coupled magnetoresistance in ferromagnet/superconductor contacts and for measuring spin polarization in ferromagnets.",9907194v1 2000-07-27,Mesoscopic Ferromagnet/Superconductor Junctions and the Proximity Effect,"We have measured the electrical transport of submicron ferromagnets (Ni) in contact with a mesoscopic superconductor (Al) for a range of interface resistances. In the geometry measured, the interface and the ferromagnet are measured separately. The ferromagnet itself shows no appreciable superconducting proximity effect, but the ferromagnet/superconductor interface exhibits strong temperature, field and current bias dependences. These effects are dependent on the local magnetic field distribution near the interface arising from the ferromagnet. We find that the temperature dependences may be fit to a modified version of the Blonder-Tinkham-Klapwijk theory for normal-superconductor transport.",0007433v1 2000-10-26,Magnon-assisted Andreev reflection in a ferromagnet-superconductor junction,"We study subgap transport in a ferromagnet-superconductor junction at low temperature due to Andreev reflection. The mismatch of spin polarized current in the ferromagnet and spinless current in the superconductor results in an additional contact resistance which can be reduced by magnon emission in the ferromagnet. Using the s-f model and focusing on half-metallic ferromagnets, we calculate the corresponding nonlinear contribution to the current which is asymmetric with respect to the sign of the bias voltage and is related to the density of states of magnons in the ferromagnet.",0010421v1 2001-08-27,Gate-induced band ferromagnetism in an organic polymer,"We propose that a chain of five-membered rings (polyaminotriazole) should be ferromagnetic with an appropriate doping that is envisaged to be feasible with an FET structure. The ferromagnetism is confirmed by a spin density functional calculation, which also shows that ferromagnetism survives the Peierls instability. We explain the magnetism in terms of Mielke and Tasaki's flat-band ferromagnetism with the Hubbard model. This opens a new possibility of band ferromagnetism in purely organic polymers.",0108413v1 2002-01-18,Spin separation in digital ferromagnetic heterostructures,"In a study of the ferromagnetic phase of a multilayer digital ferromagnetic semiconductor in the mean-field and effective-mass approximations, we find the exchange interaction to have the dominant energy scale of the problem, effectively controlling the spatial distribution of the carrier spins in the digital ferromagnetic heterostructures. In the ferromagnetic phase, the majority and minority carriers tend to be in different regions of the space (spin separation). Hence, the charge distribution of carriers also changes noticeably from the ferromagnetic to the paramagnetic phase. An example of a design to exploit these phenomena is given.",0201326v1 2002-09-19,Thermodynamic properties of ferromagnetic/superconductor/ferromagnetic nanostructures,"The theoretical description of the thermodynamic properties of ferromagnetic/superconductor/ferromagnetic (F/S/F) systems of nanoscopic scale is proposed. Their superconducting characteristics strongly depend on the mutual orientation of the ferromagnetic layers. In addition, depending on the transparency of S/F interfaces, the superconducting critical temperature can exhibit four different types of dependences on the thickness of the F-layer. The obtained results permit to give some practical recommendations for the spin-valve effect experimental observation. In this spin-valve sandwich, we also expect a spontaneous transition from parallel to anti-parallel ferromagnetic moment orientation, due to the gain in the superconducting condensation energy.",0209466v1 2002-12-16,Odd triplet superconductivity in superconductor/ferromagnet multilayered structures,"We demonstrate that in multilayered superconductor-ferromagnet structures a non-collinear alignment of the magnetizations of different ferromagnetic layers generates a triplet superconducting condensate which is odd in frequency. This triplet condensate coexists in the superconductors with the conventional singlet one but decays very slowly in the ferromagnet, which should lead to a large Josephson effect between the superconductors separated by the ferromagnet. Depending on the mutual direction of the ferromagnetic moments the Josephson coupling can be both of 0 and $\pi $ type.",0212348v3 2003-07-15,Electronic structures and ferromagnetism in transition metals codoped ZnO,"We have investigated electronic structures and magnetic properties of potential ZnO based diluted magnetic semiconductors: (Fe, Co) and (Fe, Cu) codoped ZnO. The origins of ferromagnetism are shown to be different between two. (Fe, Co) codoped ZnO does not have a tendency of Fe-O-Co ferromagnetic cluster formation, and so the double exchange mechanism will not be effective. In contrast, (Fe, Cu) codoped ZnO has a tendency of the Fe-O-Cu ferromagnetic cluster formation with the charge transfer between Fe and Cu, which would lead to the ferromagnetism through the double-exchange mechanism. The ferromagnetic and nearly half-metallic ground state is obtained for (Fe, Cu) codoped ZnO.",0307359v1 2003-11-14,Transport through ferromagnet/superconductor interfaces,"The differential resistance of submicron-size ferromagnet/superconductor interface structures shows asymmetries as a function of the current through the ferromagnet/superconductor interface. These asymmetries are a consequence of spin-polarized electron transport from the ferromagnet to the superconductor, coupled with the Zeeman-splitting of the superconducting quasiparticle density of states. They are sensitive to the orientation of the magnetization of the ferromagnet, as the magnetic field required to spin-split the quasiparticle density of states can be provided by the ferromagnetic element itself.",0311334v2 2003-12-01,Microscopic theory of equilibrium properties of F/S/F trilayers with weak ferromagnets,"The aim of this paper is to explain the non monotonic temperature dependence of the self-consistent superconducting gap of ferromagnet/superconductor/ferromagnet (F/S/F) trilayers with weak ferromagnets in the parallel alignment (equivalent to F/S bilayers). We show that this is due to Andreev bound states that compete with the formation of a minigap. Using a recursive algorithm we discuss in detail the roles of various parameters (thicknesses of the superconductor and ferromagnets, relative spin orientation of the ferromagnets, exchange field, temperature, disorder, interface transparencies).",0312029v2 2004-08-19,Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn$_2$,"We report a study of the ferromagnetism of ZrZn$_{2}$, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures $T$ as function of pressure $p$. We find that the ordered ferromagnetic moment disappears discontinuously at $p_c$=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetisation isotherms up to 12 T in the $p-T$ plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn$_{2}$, thus strongly suggesting that QPT in itinerant ferromagnets are always first order.",0408424v1 2005-04-21,Magnetic semiconductor artificial atom with many particles: Thomas-Fermi model and ferromagnetic phases,"Many-particle electron states in semiconductor quantum dots with carrier-mediated ferromagnetism are studied theoretically within the self-consistent Boltzmann equation formalism. Depending on the conditions, a quantum dot may contain there phases: partially spin-polarized ferromagnetic, fully spin-polarized ferromagnetic, and paramagnetic phases. The physical properties of many-body ferromagnetic confined systems come from the competing carrier-mediated ferromagnetic and Coulomb interactions. The magnetic phases in gated quantum dots with holes can be controlled by the voltage or via optical methods.",0504559v2 2006-09-05,Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet,"We report direct electrical detection of spin pumping, using a lateral normal metal/ferromagnet/normal metal device, where a single ferromagnet in ferromagnetic resonance pumps spin polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a d.c. voltage due to the spin dependent conductivities of the ferromagnet. By comparing different contact materials (Al and /or Pt), we find, in agreement with theory, that the spin related properties of the normal metal dictate the magnitude of the d.c. voltage.",0609089v1 2008-01-10,Controllable spin transport in ferromagnetic graphene junctions,"We study spin transport in normal/ferromagnetic/normal graphene junctions where a gate electrode is attached to the ferromagnetic graphene. We find that due to the exchange field of the ferromagnetic graphene, spin current through the junctions has an oscillatory behavior with respect to the chemical potential in the ferromagnetic graphene, which can be tuned by the gate voltage. Especially, we obtain a controllable spin current reversal by the gate voltage. Our prediction of high controllability of spin transport in ferromagnetic graphene junction may contribute to the development of the spintronics.",0801.1552v2 2008-03-31,Optical Orientation in Ferromagnet/Semiconductor Hybrids,"The physics of optical pumping of semiconductor electrons in the ferromagnet/semiconductor hybrids is discussed. Optically oriented semiconductor electrons detect the magnetic state of the ferromagnetic film. In turn, the ferromagnetism of the hybrid can be controlled optically with the help of the semiconductor. Spin-spin interactions near the interface ferromagnet/semiconductor play crucial role in the optical readout and the manipulation of ferromagnetism.",0803.4401v1 2009-06-01,Current-voltage characteristics of tunable ferromagnet-silicon-ferromagnet channels in the spin blockade regime,"The steady current-voltage characteristics of ferromagnet-silicon-ferromagnet channels with tunable emitter and collector polarizations are investigated in the presence of spin blockade generalizing the model developed by Pershin Yu V and Di Ventra M (2008 {\it Phys. Rev.} B {\bf 77} 073301). The dependence of the critical current on both collector and emitter polarizations is obtained analytically. It is found that the current amplitude in the channel can be effectively tuned by varying the difference between the collector and emitter ferromagnet polarizations which allows to perform the magnetic manipulation of the electrical current in wide class of both n- and p-doped, low- and high-Ohmic semiconductor channels coupled to ferromagnetic leads.",0906.0246v1 2009-09-27,Correlated versus Ferromagnetic State in Repulsively Interacting Two-Component Fermi Gases,"Whether a spin-1/2 Fermi gas will become ferromagnetic as the strength of repulsive interaction increases is a long-standing controversial issue. Recently this problem is studied experimentally by Jo et al, Science, 325, 1521 (2009) in which the authors claim a ferromagnetic transition is observed. This work is to point out the results of this experiment can not distinguish whether the system is in a ferromagnetic state or in a non-magnetic but strongly short-range correlated state. A conclusive experimental demonstration of ferromagnetism relies on the observation of ferromagnetic domains.",0909.4917v2 2013-02-13,Theory of ferromagnetism in vanadium-oxide based perovskites,"The conditions under which ferromagnetism may occur in transition metal oxides with partially filled $t_{2g}$ shells such as vanadium-based perovskites are studied using a combination of density functional and single-site dynamical mean field methods. For reasonable values of the correlation strength, rotations of the VO$_6$ octahedra play an important role in enabling ferromagnetism, with ferromagnetism typically occurring for rotations larger than a nonzero critical value. Ferromagnetism is suppressed near the Mott insulating phase but the phase boundary is otherwise only weakly dependent on carrier concentration. Design rules are suggested for new oxide systems exhibiting ferromagnetism.",1302.3062v2 2014-07-18,Local characterization of ferromagnetic properties in ferromagnet/superconductor bilayer by Point Contact Andreev Reflection Spectroscopy,"We realized point contact spectroscopy experiment on ferromagnet/superconductor bilayers. Differential conductance curves show several features that we explained within Bogoliubov-de Gennes formalism considering the presence of two interfaces in the normal-metal-tip/ferromagnet/superconductor device. We demonstrate that such configuration is suitable as local probe of the spin polarization and thickness of ferromagnetic layer, directly on bilayer areas. This is due to the high sensitivity of the Andreev surface states to the physical properties of the ferromagnetic interlayer.",1407.4906v1 2011-11-08,Unusual ferromagnetism in nanoparticles of doped oxides and manganites,"The observation of unusually large ferromagnetism in the nanoparticles of doped oxides and enhanced ferromagnetic tendencies in manganite nanoparticles have been in focus recently. For the transition metal-doped oxide nanoparticles a phenomenological `charge transfer ferromagnetism' model is recently proposed by Coey et al. From a microscopic calculation with charge transfer between the defect band and mixed valent dopants, acting as reservoir, we show how the unusually high ferromagnetic response develops. The puzzle of nanosize-induced ferromagnetic tendencies in manganites is also addressed within the same framework where lattice imperfections and uncompensated charges at the surface of the nanoparticle are shown to reorganize the surface electronic structures with enhanced double exchange.",1111.1778v1 2018-09-23,Ferromagnetic resonance in thin ferromagnetic film with surface anisotropy,"The ferromagnetic resonance frequencies are obtained for a thin ferromagnetic film with surface anisotropy for the cases when the external magnetic field is applied perpendicularly or parallel to the film surface, and for various combinations of boundary conditions on the film surface. It is shown that in the presence of surface anisotropy the ferromagnetic resonance frequency essentially depends both on the film thickness and on the value of the surface anisotropy constant. The results obtained provide a basis for the correct interpretation of experimental data obtained by means of broadband ferromagnetic resonance in thin film structures.",1809.08644v1 2019-12-20,Magnetic field-controlled 0-$π$ transitions and their experimental signatures in superconductor-ferromagnet-superconductor junctions,"Superconductor-ferromagnet-superconductor Josephson junctions are known to exist in the $0$ and $\pi$ states with the transitions between them controlled by the temperature and ferromagnetic interlayer thickness. We demonstrate that these transitions can be controlled also by the external magnetic field directed perpendicular to the layers. By varying the ratio of diffusion coefficients in superconducting and ferromagnetic layers, these field-controlled transitions can be made detectable for arbitrary large value of the exchange energy in the ferromagnet. We also show that the $0$-$\pi$ transitions in the perpendicular field can be observed as the specific features of the flux-flow conductivity dependencies on the ferromagnetic thickness in accordance with recent experimental results.",1912.10144v1 2020-09-04,Detection of Ferromagnetic Resonance from 1 nm-thick Co,"To explore the further possibilities of nanometer-thick ferromagnetic films (ultrathin ferromagnetic films), we investigated the ferromagnetic resonance (FMR) of 1 nm-thick Co film. Whilst an FMR signal was not observed for the Co film grown on a SiO2 substrate, the insertion of a 3 nm-thick amorphous Ta buffer layer beneath the Co enabled the detection of a salient FMR signal, which was attributed to the smooth surface of the amorphous Ta. This result implies the excitation of FMR in an ultrathin ferromagnetic film, which can pave the way to controlling magnons in ultrathin ferromagnetic films.",2009.01977v1 2021-04-16,Ferromagnetism in 2D Vanadium Diselenide,"Two-dimensional (2D) Van der Waals ferromagnets carry the promise of ultimately miniature spintronics and information storage devices. Among the newly discovered 2D ferromagnets all inherit the magnetic ordering from their bulk ancestors. Here we report a new 2D ferromagnetic semiconductor at room temperature, 2H phase vanadium diselenide (VSe2) which show ferromagnetic at 2D form only. This unique 2D ferromagnetic semiconductor manifests an enhanced magnetic ordering owing to structural anisotropy at 2D limit.",2104.07937v1 2021-11-30,Majorana Zero Modes in Ferromagnetic Wires without Spin-Orbit Coupling,"We present a novel controllable platform for engineering Majorana zero modes. The platform consists of a ferromagnetic metallic wire placed among conventional superconductors, which are in proximity to ferromagnetic insulators. We demonstrate that Majorana zero modes emerge localised at the edges of the ferromagnetic wire, due to the interplay of the applied supercurrents and the induced by proximity exchange fields with conventional superconductivity. Our mechanism does not rely on the pairing of helical fermions by combining conventional superconductivity with spin-orbit coupling, but rather exploits the misalignment between the magnetization of the ferromagnetic insulators and that of the ferromagnetic wire.",2111.15260v1 2011-11-14,Fermi liquid theory and ferromagnetism,"There is demonstrated that an isotropic ferromagnetic Fermi liquid reveals instability of the ferromagnetic state in respect to the transversal inhomogeneous deviations of magnetization from equilibrium. The result was obtained by derivation of the spin waves spectrum by means of kinetic equation.",1111.3208v2 2019-03-15,Novel critical behavior of magnetization in URhSi:Similarities to uranium ferromagnetic superconductors UGe$_2$ and URhGe,"We study the critical behavior of dc magnetization in the uranium ferromagnet URhSi around the paramagnetic to ferromagnetic phase transition at T_C~ 10 K with a modified Arrott plot, a Kouvel-Fisher plot, the critical isotherm analysis and the scaling analysis. URhSi is isostructural to uranium ferromagnetic superconductors URhGe and UCoGe. The critical exponent beta for the temperature dependence of the spontaneous magnetization below TC, gamma for the magnetic susceptibility, and delta for the magnetic isotherm at T_C in URhSi have been determined as beta = 0.300 +- 0.002, gamma = 1.00 +- 0.02, and delta = 4.38 +- 0.04 by the scaling analysis and the critical isotherm analysis. These critical exponents fulfill the Widom scaling law delta = 1+ gamma/beta. Magnetization has strong uniaxial magnetic anisotropy in the ferromagnetic state of URhSi. However, the universality class of the ferromagnetic transition does not belong to the 3D Ising system with short-range exchange interactions between magnetic moments (beta = 0.325, gamma = 1.241, and delta = 4.82). The obtained exponents in URhSi are similar to those in the uranium ferromagnetic superconductors UGe_2 and URhGe, and uranium ferromagnets UIr and U(Co_0.98Os_0.02)Al. We have previously reported the unconventional critical behavior of magnetization in the uranium ferromagnetic superconductors [N. Tateiwa et al. Phys. Rev. B 89, 064420 (2014)]. The universality class of the ferromagnetic transition in URhSi may belong to the same one in the uranium ferromagnetic superconductors and the uranium ferromagnets. The unconventional critical behavior of the magnetization in the uranium compounds cannot be understood with previous theoretical interpretations of critical phenomena. The absence of the superconductivity in URhSi is discussed from several viewpoints.",1903.06841v1 2024-01-08,Ferromagnetic Materials for Josephson π Junctions,"The past two decades have seen an explosion of work on Josephson junctions containing ferromagnetic materials. Such junctions are under consideration for applications in digital superconducting logic and memory. In the presence of the exchange field, spin-singlet Cooper pairs from conventional superconductors undergo rapid oscillations in phase as they propagate through a ferromagnetic material. As a result, the ground-state phase difference across a ferromagnetic Josephson junction oscillates between 0 and $\pi$ as a function of the thickness of the ferromagnetic material. $\pi$-junctions have been proposed as circuit elements in superconducting digital logic and in certain qubit designs for quantum computing. If a junction contains two or more ferromagnetic layers whose relative magnetization directions can be controlled by a small applied magnetic field, then the junction can serve as the foundation for a memory cell. Success in all of those applications requires careful choices of ferromagnetic materials. Often, materials that optimize magnetic properties do not optimize supercurrent propagation, and vice versa. In this review we discuss the significant progress that has been made in identifying and testing a wide range of ferromagnetic materials in Josephson junctions over the past two decades. The review concentrates on ferromagnetic metals, partly because eventual industrial applications of ferromagnetic Josephson junctions will most likely start with metallic ferromagnets (either in all metal junctions or junctions containing also an insulating layer). We will briefly mention work on non-metallic barriers, including ferromagnetic insulators, and some of the exciting work on spin-triplet supercurrent in junctions containing noncollinear magnetic inhomogeneity.",2401.04219v2 1997-07-03,Hund's-Rule Coupling Effect in Itinerant Ferromagnetism,"We present a general model which includes the ferromagnetic Kondo lattice and the Hubbard model as special cases. The stability of the ferromagnetic state is investigated variationally. We discuss the mechanism of ferromagnetism in metallic nickel, emphasizing the importance of orbital degeneracy and the effect of the Hund's-rule coupling.",9707032v1 1999-09-06,Mesoscopic mechanism of exchange interaction in magnetic multilayers,"We discuss a mesoscopic mechanism of exchange interaction in ferromagnet-normal metal-ferromagnet multilayers. We show that in the case when the metal's thickness is larger than the electron mean free path, the relative orientation of magnetizations in the ferromagnets is perpendicular. The exchange energy between ferromagnets decays with the metal thickness as a power law.",9909064v1 2000-11-21,Spin and Orbital Order in Itinerant Ferromagnets,"The long-standing problem of the effect of correlations on the ferromagnetism of is apparently nearing solution. The ferromagnetism of transition metals compounds, for instance doped manganites, poses a new question: is there some kind of orbital order coexisting with itinerant ferromagnetism? The ideas and techniques introduced by Gutzwiller should be of use again.",0011354v1 2002-09-04,Theory of exchange coupling in disordered magnetic multilayers,"We consider mechanism of exchange coupling based on interaction between electrons in nonmagnetic layer. Depending on ratio of inverse time of diffusion of electrons between ferromagnetic layers and ferromagnetic splitting of conducting electrons this mechanism describes transition from ferromagnetic to noncollinear ordering of magnetizations of ferromagnetic layers.",0209084v1 2003-05-26,Homogeneous Phase of Coexistence of Spin-Triplet Superconductivity and Ferromagnetism,"The coexistence of a homogeneous (Meissner-like) phase of spin-triplet superconductivity and ferromagnetism is investigated within the framework of a phenomenological model of spin-triplet ferromagnetic superconductors. The results are discussed in view of application to metallic ferromagnets as UGe$_2$, ZrZn$_2$, URhGe, and Fe.",0305602v1 2003-07-18,Induced Ferromagnetism due to Superconductivity in Superconductor-Ferromagnet structures,"We consider a superconductor-ferromagnet (S/F) structure and assume that above the superconducting transition temperature $T_{c}$ the magnetic moment exists only in F. {In a simple model of the ferromagnet (the exchange field is of the ferromagnetic type for all energies)}we show by an explicit calculation that below $T_{c}$ the magnetic moment may penetrate the superconductor. {In this model} its direction in S is opposite {to the magnetization of free electrons} in the ferromagnet. The magnetization spreads over a large distance which is of the order of the superconducting coherence length $\xi_{S}$ and can much exceed the ferromagnet film thickness. At the same time the magnetic moment in the ferromagnet is reduced. This inverse proximity effect may explain the reduction in magnetization observed in recent experiments and may lead to a strong interaction between the ferromagnetic layers in F/S/F structures.",0307468v3 2004-11-18,Density of states in spin-valve structure with superconducting electrodes,"Energy variation of the density of states (DOS) has been calculated in the superconductor/ferromagnet/ferromagnet/superconductor structure (SFFS) in the frame of Gorkov equations taking into account the s-d electron scattering in the ferromagnetic layers. DOS behavior is presented for the antiparallel and parallel magnetic moments alignment of two adjacent F layers. The cases of small and large values of exchange ferromagnetic field are discussed.",0411470v1 2005-01-10,Domain wall effects in ferromagnet-superconductor structures,"We investigate how domain structure of the ferromagnet in superconductor-ferromagnet heterostructures may change their transport properties. We calculate the distribution of current in the superconductor induced by magnetic field of Bloch domain walls, find the ``lower critical'' magnetization of the ferromagnet that provides vortices in the superconductor.",0501210v1 2006-07-05,Theory of the spin-torque-driven ferromagnetic resonance in a ferromagnet/normal-metal/ferromagnet structure,"We present a theoretical analysis of current driven ferromagnetic resonance in a ferromagnet/normal-metal/ferromagnet tri-layer. This method of driving ferromagnetic resonance was recently realized experimentally by Tulapurkar et al. [Nature 438, 339 (2005)] and Sankey et al. [Phys. Rev. Lett. 96, 227601 (2006)]. The precessing magnetization rectifies the alternating current applied to drive the ferromagnetic resonance and leads to the generation of a dc voltage. Our analysis shows that a second mechanism to generate a dc voltage, rectification of spin currents emitted by the precessing magnetization, has a contribution to the dc voltage that is of approximately equal size for the thin ferromagnetic films used in the experiment.",0607145v2 2007-08-16,Spin-transfer torques in anti-ferromagnetic metals from first principles,"In spite of the absence of a macroscopic magnetic moment, an anti-ferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting anti-ferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in anti-ferromagnetic non-collinear spin valves and domain walls. We report a first principles study on the electronic transport properties of anti-ferromagnetic systems. The current-induced spin torques acting on the magnetic moments are comparable with those in conventional ferromagnetic materials, leading to measurable angular resistances and current-induced magnetization dynamics. In contrast to ferromagnets, spin torques in anti-ferromagnets are very nonlocal. The torques acting far away from the center of an anti-ferromagnetic domain wall should facilitate current-induced domain wall motion.",0708.2143v2 2009-03-12,Magnetic screening properties of superconductor-ferromagnet bilayers,"We study theoretically the magnetic screening properties of thin, diffusive superconductor/ferromagnet bilayers subject to a perpendicular magnetic field. We find that the effective penetration depth characterizing the magnetic response oscillates with the thickness of the ferromagnetic layer on the scale of the ferromagnetic coherence length.",0903.2245v2 2010-02-12,Quantum Spin Tomography in Ferromagnet-Normal Conductors,"We present a theory for a complete reconstruction of non-local spin correlations in ferromagnet-normal conductors. This quantum spin tomography is based on cross correlation measurements of electric currents into ferromagnetic terminals with controllable magnetization directions. For normal injectors, non-local spin correlations are universal and strong. The correlations are suppressed by spin-flip scattering and, for ferromagnetic injectors, by increasing injector polarization.",1002.2499v1 2010-06-29,Ferromagnetic phase diagram of neutron matter,"The magnetic properties of matter under extreme conditions are of particular importance to understanding the neutron star interior. One contributing factor to the magnetic field of a neutron star could be the ferromagnetic phase of nuclear matter. In this letter we present a self-consistent, relativistic description of ferromagnetism in dense matter, within which the ferromagnetic phase diagram for neutron matter is calculated.",1006.5564v1 2011-12-28,Current conservation and ratio rules in magnetic metals with Coulomb repulsion,"From general considerations of spin-symmetry breaking associated with (anti-)ferromagnetism in metallic systems with Coulomb repulsion, we obtain interesting and simple all-order rules involving the ratios of the densities of states. These are exact for ferromagnetism under reasonable conditions, and nearly exact for anti-ferromagnetism. In the case of ferromagnetism, the comparison with the available experimental and theoretical numbers yields favourable results.",1112.6036v1 2019-06-26,"Room-temperature insulating ferromagnetic (Ni,Co)1+2xTi1-xO3 thin films","Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. We show that the uniaxial $A^{1+2x}$Ti$^{4+}$$_{1-x}$O$_3$ (ATO), $A=$Ni$^{2+}$,Co$^{2+}$ and $0.6\bar I_{c}$. In a weak magnetic field, ferromagnetism makes immense contribution to the magnetization density. While at a high magnetic field, the diamagnetism inclines to saturate. Evidence for condensation can be seen in the magnetization density at weak magnetic field.",1208.1580v1 2014-02-05,Re-entrant superspin glass phase in La$_{0.82}$Ca$_{0.18}$MnO$_3$ ferromagnetic insulator,"We report results of magnetization and ac susceptibility measurements down to very low fields on a single crystal of the perovskite manganite, La$_{0.82}$Ca$_{0.18}$MnO$_3$. This composition falls in the intriguing ferromagnetic insulator region of the manganite phase diagram. In contrast to earlier beliefs, our investigations reveal that the system is magnetically (and in every other sense) single-phase with a ferromagnetic ordering temperature of $\sim$ 170 K. However, this ferromagnetic state is magnetically frustrated, and the system exhibits pronounced glassy dynamics below 90 K. Based on measured dynamical properties, we propose that this quasi-long-ranged ferromagnetic phase, and associated superspin glass behavior, is the true magnetic state of the system, rather than being a macroscopic mixture of ferromagnetic and antiferromagnetic phases as often suggested. Our results provide an understanding of the quantum phase transition from an antiferromagnetic insulator to a ferromagnetic metal via this ferromagnetic insulating state as a function of $x$ in La$_{1-x}$Ca$_x$MnO$_3$, in terms of the possible formation of magnetic polarons.",1402.1148v1 2016-02-01,Hubbard models with nearly flat bands: Ground-state ferromagnetism driven by kinetic energy,"We consider the standard repulsive Hubbard model with a flat lowest-energy band for two one-dimensional lattices (diamond chain and ladder) as well as for a two-dimensional lattice (bilayer) at half filling of the flat band. The considered models do not fall in the class of Mielke-Tasaki flat-band ferromagnets, since they do not obey the connectivity conditions. However, the ground-state ferromagnetism can emerge, if the flat band becomes dispersive. To study this kinetic-energy-driven ferromagnetism we use perturbation theory and exact diagonalization of finite lattices. We find as a typical scenario that small and moderate dispersion may lead to a ferromagnetic ground state for sufficiently large on-site Hubbard repulsion $U>U_c$, where $U_c$ increases monotonically with the acquired bandwidth. However, we also observe for some specific parameter cases, that (i) ferromagnetism appears at already very small $U_c$, (ii) ferromagnetism does not show up at all, (iii) the critical on-site repulsion $U_c$ is a nonmonotonic function of the bandwidth, or that (iv) a critical bandwidth is needed to open the window for ground-state ferromagnetism.",1602.00439v1 2016-11-25,Phase diagram of UCoGe,"The temperature-pressure phase diagram of ferromagnetic superconductor UCoGe includes four phase transitions. They are between the paramagnetic and the ferromagnetic states with the subsequent transition in the superconducting ferromagnetic state and between the normal and the superconducting states after which has to occur the transition to the superconducting ferromagnetic state. Here we have developed the Landau theory description of the phase diagram and established the specific ordering arising at each type of transition. The phase transitions to the ferromagnetic superconducting state are inevitably accompanied by the emergency of screening currents. The corresponding magnetostatics considerations allow to establish the significant difference between the transition from the ferromagnetic to the ferromagnetic superconducting state and the transition from the superconducting to the ferromagnetic superconducting state.",1611.08440v4 2016-08-03,Evolution of magnetic fluctuations through the Fe-induced paramagnetic to ferromagnetic transition in Cr$_2$B,"In itinerant ferromagnets, the quenched disorder is predicted to dramatically affect the ferromagnetic to paramagnetic quantum phase transition driven by external control parameters at zero temperature. Here we report a study on Fe-doped Cr$_2$B, which, starting from the paramagnetic parent, orders ferromagnetically for Fe-doping concentrations $x$ larger than $x_{\rm c}=2.5$\%. In parent Cr$_2$B, $^{11}$B nuclear magnetic resonance data reveal the presence of both ferromagnetic and antiferromagnetic fluctuations. The latter are suppressed with Fe-doping, before the ferromagnetic ones finally prevail for $x>x_{\rm c}$. Indications for non-Fermi liquid behavior, usually associated with the proximity of a quantum critical point, were found for all samples, including undoped Cr$_2$B. The sharpness of the ferromagnetic-like transition changes on moving away from $x_{\rm c}$, indicating significant changes in the nature of the magnetic transitions in the vicinity of the quantum critical point. Our data provide constraints for understanding quantum phase transitions in itinerant ferromagnets in the limit of weak quenched disorder.",1608.01136v1 2019-06-17,Ferromagnetism and superconductivity in twisted double bilayer graphene,"We present a theory of competing ferromagnetic and superconducting orders in twisted double bilayer graphene (TDBG). In our theory, ferromagnetism is induced by Coulomb repulsion, while superconductivity with intervalley equal-spin pairing can be mediated by electron-acoustic phonon interactions. We calculate the transition temperatures for ferromagnetism and superconductivity as a function of moir\'e band filling factor, and find that superconducting domes can appear on both the electron and hole sides of the ferromagnetic insulator at half filling. We show that the ferromagnetic insulating gap has a dome shape dependence on the layer potential difference, which provides an explanation to the experimental observation that the ferromagnetic insulator only develops over a finite range of external displacement field. We also verify the stability of the half-filled ferromagnetic insulator against two types of collective excitations, i.e., spin magnons and valley magnons.",1906.07302v2 2019-10-04,Intrinsic 2D Ferromagnetism in V5Se8 Epitaxial Thin Films,"The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy (MBE) exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is 'originally antiferromagnetic (AFM)'. Moreover, thickness-dependence measurements reveal that this newly-developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.",1910.01959v1 2018-03-08,Hole-doping-induced half-metallic ferromagnetism in highly-air-stable PdSe2 monolayer under uniaxial stress,"Two-dimensional (2D) high-temperature ferromagnetic materials are important for spintronic application. Fortunately, a highly-air-stable PdSe$_2$ monolayer semiconductor has been made through exfoliation from the layered bulk material. It is very highly desirable to realize robust ferromagnetism, even half-metallic ferromagnetism (100\% spin polarization), in such excellent nonmagnetic monolayer semiconductors. Here, the first-principles investigation shows that the PdSe$_2$ monolayer can be made to attain Stoner ferromagnetism with the maximal Curie temperature reaching to 800K, and the hole concentration threshold for ferromagnetism decreases with applied uniaxial stress. Furthermore, half-metallicity can be achieved in some hole concentration regions. For the strain of 10\% (uniaxial tensile stress of 4.4 N/m), the monolayer can attain half-metallic ferromagnetism up to 150 K. The magnetic anisotropic energy is suitable to not only stabilizing the 2D ferromagnetism but also realizing fast magnetization reversal. The magnetization can be also controlled by applying a transverse uniaxial stress. The highly-air-stable PdSe$_2$ monolayer, with these advantages, should be promising for spintronic applications.",1803.03079v1 2018-08-12,Resonant Driving induced Ferromagnetism in the Fermi Hubbard Model,"In this letter we consider quantum phases and the phase diagram of a Fermi Hubbard model under periodic driving that has been realized in recent cold atom experiments, in particular, when the driving frequency is resonant with the interaction energy. Due to the resonant driving, the effective Hamiltonian contains a correlated hopping term where the density occupation strongly modifies the hopping strength. Focusing on half filling, in addition to the charge and spin density wave phases, large regions of ferromagnetic phase and phase separation are discovered in the weakly interacting regime. The mechanism of this ferromagnetism is attributed to the correlated hopping because the hopping strength within a ferromagnetic domain is normalized to a larger value than the hopping strength across the domain. Thus, the kinetic energy favors a large ferromagnetic domain and consequently drives the system into a ferromagnetic phase. We note that this is a different mechanism in contrast to the well-known Stoner mechanism for ferromagnetism where the ferromagnetism is driven by interaction energy.",1808.03966v2 2019-12-03,Nanoscale Tantalum Layer Controlling the Magnetic Coupling between Two Ferromagnetic Electrodes via Insulator of a Magnetic Tunnel Junction,"Ability to tailor the nature of the magnetic coupling between two ferromagnetic electrodes can enable the realization of new spintronics device systems. This paper discusses our finding that deposition of an ultrathin tantalum (Ta) on the NiFe top electrode reversed the nature of inter-ferromagnetic electrode coupling. We observed that the deposition of ~ 5 nm Ta on the top of a magnetic tunnel junction with Ta( 2 nm)/Co(5 nm)/NiFe (5 nm)/AlOx( 2 nm)/NiFe (10-15 nm) configuration changed the magnetic coupling between two ferromagnetic electrodes from antiferromagnetic to ferromagnetic. We investigated Ta effect using multiple magnetic characterizations like ferromagnetic resonance, magnetometry, and polarized neutron reflectometry. Ferromagnetic resonance characterization was very sensitive for detecting the changes in magnetic coupling via the insulating spacer. This simple approach of adding Ta film to alter the magnetic coupling can impact the other burgeoning areas like molecular spintronics. We found that preexisting magnetic coupling between two ferromagnetic electrodes impacted the resultant magnetic properties of magnetic tunnel junctions based molecular spintronics devices.",1912.01331v1 2021-12-23,Monte Carlo investigation of phase changes and the order of transition of Ising modeled single-walled Nanotube,"The Monte Carlo analysis for the magnetic response of a single-walled nanotube using the Metropolis and Wang Landau algorithms is reported in the present paper. The nanotube architecture used in the present study utilizes the spin half Ising model with nearest neighbors interaction and obtained various magnetic orderings namely, ferromagnetic, G-type anti-ferromagnetic, A-type anti-ferromagnetic, and C-type anti-ferromagnetic. It is also found that the phase changes from ferromagnetic/anti-ferromagnetic to paramagnetic with the modification of system's control parameters. The transition temperatures is determined for various interaction strength in the absence of magnetic field and for fixed interaction strength with the inclusion of external magnetic field. The present study confirms the transition from ferromagnetic /anti-ferromagnetic to paramagnetic is a second order transition.",2112.12393v2 2022-12-23,Surface ferromagnetism in a chiral topological semimetal CoSi,"Despite the chiral topological semimetal CoSi is known as bulk diamagnetic, it shows unusual surface ferromagnetism of debatable origin. The ferromagnetic ordering has been attributed to the distorted bonds, the superlattice of ordered vacancies, or even to topological surface textures due to the spin polarization in the neighboring Fermi arcs. We experimentally compare magnetization reversal curves for initially oxidized CoSi single crystals and cleaved samples with a fresh, oxide-free surface. While the oxidized CoSi samples do not show sizable ferromagnetism, the fresh CoSi surface gives a strong ferromagnetic response, which is accompanied by the pronounced modulation of the angle dependence of magnetization, as it can be expected for easy and hard axes in a ferromagnet. In addition to the first order reversal curves analysis, this observation allows us to distinguish between different mechanisms of the ferromagnetic ordering in CoSi single crystals. We conclude that the surface states-induced RKKY interaction between distorted bonds near the sample surface is responsible for the strong ferromagnetic multi-domain behavior for freshly cleaved samples.",2212.12383v2 2023-03-28,Flat-band ferromagnetism in the SU($N$) Hubbard and Kondo lattice models,"We develop a general theory of flat-band ferromagnetism in the SU($N$) Fermi-Hubbard model, which describes the behavior of $N$-component fermions with SU($N$) symmetric interactions. We focus on the case where the single-particle spectrum has a flat band and establish a necessary and sufficient condition for the SU($N$) Hubbard model to exhibit ferromagnetism when the number of particles is the same as the degeneracy. We show that the occurrence of ferromagnetism is equivalent to the irreducibility of the projection matrix onto the space of single-particle ground states. We also demonstrate that this result can be exploited to establish a rigorous result for the ferromagnetic SU($N$) Kondo lattice model with a flat band. Specifically, we prove that when the SU($N$) Hubbard model is ferromagnetic, the ferromagnetic SU($N$) Kondo lattice model with the same hopping matrix also exhibits SU($N$) ferromagnetism.",2303.15820v2 2023-05-08,Enhanced Itinerant Ferromagnetism in Hole-doped Transition Metal Oxides: Beyond the Canonical Double Exchange Mechanism,"Here we demonstrate the occurrence of robust itinerant ferromagnetism in Mott-Hubbard systems at both low and high doping concentrations. Specifically, we study the effect of hole doping on the experimentally synthesized LaCrAsO via first-principles calculations and observe that the parent G-type antiferromagnetism vanishes quickly at low doping concentration ($x$ $\sim$ 0.20) and the system becomes ferromagnetic metal due to the canonical double exchange (CDE) mechanism. As $x$ continues to increase, the onsite energy difference between Cr 3$d$ and As 4$p$ orbitals decreases and the system transitions to a ferromagnetic negative charge-transfer energy metal. Therefore, the itinerant ferromagnetism doesn't terminate at intermediate $x$ as CDE mechanism usually predicts. Furthermore, our calculations reveal that both nearest and next-nearest ferromagnetic exchange coupling strengths keep growing with $x$, showing that ferromagnetism caused by negative charge-transfer energy state is ""stronger"" than that of CDE picture. Our work not only unveils an alternative mechanism of itinerant ferromagnetism, but also has the potential to attract immediate interest among experimentalists.",2305.04459v1 2023-11-15,Polaronic mechanism of Nagaoka ferromagnetism in Hubbard models,"The search for elusive Nagaoka-type ferromagnetism in the Hubbard model has recently enjoyed renewed attention with the advent of a variety of experimental platforms enabling its realization, including moir\'e materials, quantum dots, and ultracold atoms in optical lattices. Here, we demonstrate a universal mechanism for Nagaoka ferromagnetism (that applies to both bipartite and nonbipartite lattices) based on the formation of ferromagnetic polarons consisting of a dopant dressed with polarized spins. Using large-scale density-matrix renormalization group calculations, we present a comprehensive study of the ferromagnetic polaron in an electron-doped Hubbard model, establishing various polaronic properties such as its size and energetics. Moreover, we systematically probe the internal structure of the magnetic state$\unicode{x2014}$through the use of pinning fields and three-point spin-charge-spin correlation functions$\unicode{x2014}$for both the single-polaron limit and the high-density regime of interacting polarons. Our results highlight the crucial role of mobile polarons in the birth of global ferromagnetic order from local ferromagnetism and provide a unified framework to understand the development and demise of the Nagaoka-type ferromagnetic state across dopings.",2311.09279v1 1993-12-21,Magnetic Properties of the Spin-1/2 Ferromagnetic-Ferromagnetic-Antiferromagnetic Trimerized Heisenberg Chain:,"The magnetic properties of the ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin-1/2 Heisenberg chain are studied theoretically. The high temperature susceptibilty and the ground state saturation magnetic field are calculated and the exchange energies of the trimer compound 3CuCl${}_2\cdot$2dx are determined. The magnetization curve is obtained by numerical diagonalization of finite size systems. The result explains the low temperature magnetization data for 3CuCl${}_2\cdot$2dx with the exchange energies obtained as above. It is predicted that the magnetization curve has a plateau at 1/3 of the saturation magnetization if the ferromagnetic exchange energy is comparable to or smaller than the antiferromagnetic exchange energy.",9312084v1 1997-11-10,Magneto-Coulomb Oscillation in Ferromagnetic Single Electron Transistors,"The mechanism of the magneto-Coulomb oscillation in ferromagnetic single electron transistors (SET's) is theoretically considered. Variations in the chemical potentials of the conduction electrons in the ferromagnetic island electrode and the ferromagnetic lead electrodes in magnetic fields cause changes in the free energy of the island electrode of the SET. Experimental results of the magneto-Coulomb oscillation in a Ni/Co/Ni ferromagnetic SET are presented and discussed. Possible applications of this phenomenon are also discussed.",9711075v2 1997-12-31,The influence of long-range hopping on ferromagnetism in the Hubbard model,"The phase diagram of the Hubbard model in an external magnetic field is examined by extrapolation of small-cluster exact-diagonalization calculations. Using a general expression for the hopping matrix elements ($t_{ij}\sim q^{|i-j|}$) the influence of long-range hopping (band asymmetry) on ferromagnetism in this model is studied. It is found that the long-range hopping (nonzero $q$) stabilizes ferromagnetism in an external magnetic field for $n > 1$. In the opposite limit $n \leq 1$ the fully polarized ferromagnetic state is generally suppressed with increasing $q$. The critical value of magnetic field $h$ below which the ferromagnetic state becomes unstable is calculated numerically.",9712324v1 1998-09-03,Spin-Dependent Coulomb Blockade in Ferromagnet/Normal-Metal/Ferromagnet Double Tunnel Junctions,"We study theoretically the spin-dependent transport in ferromagnet/normal-metal/ferromagnet double tunnel junctions by special attention to cotunneling in the Coulomb blockade region. The spin accumulation caused by cotunneling squeezes the Coulomb blockade region when the magnetizations in the ferromagnetic electrodes are antiparallel. Outside the squeezed Coulomb blockade region, we propose a new anomalous region, where the sequential tunneling in one of the spin bands is suppressed by the Coulomb blockade and that in the other is not. In this region, the tunnel magnetoresistance oscillates as a function of bias voltage. The temperature dependences of the tunnel magnetoresistance and the magnitude of the spin accumulation are calculated.",9809058v1 1998-11-24,Local Density of States in Superconductor-Ferromagnetic Hybrid Systems,"In this Letter we discuss various properties of the local density of states (DOS) for a superconductor-ferromagnet hybrid system. The DOS is modified at small energies on both sides of the interface. Due to the interplay of superconductivity and ferromagnetism, the local DOS depends on the spin direction. The spin polarization effects extend over a long distance from the interface both in the superconductor and in the ferromagnet. If the ferromagnet is of finite lenght, the DOS shows a (spin dependent) gap.",9811334v1 1999-01-27,Spin-Imbalance and Magnetoresistance in Ferromagnet/Superconductor/Ferromagnet Double Tunnel Junctions,"We theoretically study the spin-dependent transport in a ferromagnet/super- conductor/ferromagnet double tunnel junction. The tunneling current in the antiferromagnetic alignment of the magnetizations gives rise to a spin imbalance in the superconductor. The resulting nonequilibrium spin density strongly suppresses the superconductivity with increase of bias voltage and destroys it at a critical voltage Vc. The results provide a new method not only for measuring the spin polarization of ferromagnets but also for controlling superconductivity and tunnel magnetoresistance (TMR) by applying the bias voltage.",9901300v2 1999-04-19,Ferromagnetism in single-band Hubbard models with a partially flat band,"A Hubbard model with a single, partially flat band has ferromagnetic ground states. It is shown that local stability of ferromagnetism implies its global stability in such a model: The model has only ferromagnetic ground states if there are no single spin-flip ground states. Since a single-band Hubbard model away from half filling describes a metal, this result may open a route to metallic ferromagnetism in single band Hubbard models.",9904258v1 1999-10-25,Stability of Ferromagnetism in Hubbard models with degenerate single-particle ground states,"A Hubbard model with a N_d-fold degenerate single-particle ground state has ferromagnetic ground states if the number of electrons is less or equal to N_d. It is shown rigorously that the local stability of ferromagnetism in such a model implies global stability: The model has only ferromagnetic ground states, if there are no single spin-flip ground states. If the number of electrons is equal to N_d, it is well known that the ferromagnetic ground state is unique if and only if the single-particle density matrix is irreducible. We present a simplified proof for this result.",9910385v1 1999-11-30,Coexistence of superconductivity and ferromagnetism in ferromagnetic metals,"In this paper we address the question of coexistence of superconductivity and ferromagnetism in the high temperature superconductor RuSr$_2$GdCu$_2$O$_{8-\delta}$. Using a field theoretical approach we study a one-fermion effective model of a ferromagnetic superconductor in which the quasiparticles responsible for the ferromagnetism form the Cooper pairs as well. We discuss the physical features which are different in this model and the standard BCS model and consider their experimental consequences.",9911489v1 2000-02-10,Cryptoferromagnetic state in superconductor-ferromagnet multilayers,"We study a possibility of a non-homogeneous magnetic order (cryptoferromagnetic state) in heterostructures consisting of a bulk superconductor and a ferromagnetic thin layer that can be due to the influence of the superconductor. The exchange field in the ferromagnet may be strong and exceed the inverse mean free time. A new approach based on solving the Eilenberger equations in the ferromagnet and the Usadel equations in the superconductor is developed. We derive a phase diagram between the cryptoferromagnetic and ferromagnetic states and discuss the possibility of an experimental observation of the CF state in different materials.",0002151v1 2001-03-14,Multispin Ising spin glasses with ferromagnetic interactions,"We consider the thermodynamics of an infinite-range Ising p-spin glass model with an additional r-spin ferromagnetic interaction. For r=2 there is a continuous transition to a ferromagnetic phase, while for r>2 the transition is first order. We find both glassy and non-glassy ferromagnetic phases, with replica symmetry breaking of both the one step and full varieties. We obtain new results for the case where r=p>2, demonstrating the existence of a non-glassy ferromagnetic phase, of significance to error-correcting codes.",0103292v2 2001-07-30,Andreev Reflection in Narrow Ferromagnet/Superconductor Point Contacts,"The Andreev reflection of narrow ferromagnet/superconductor point contacts is theoretically studied. We show that the conductance quantization depends on whether the contact region is superconducting or ferromagnetic as well as on the strength of the exchange field in the ferromagnet. The Andreev reflection of the ferromagnetic contact is more suppressed than that of the superconducting contact. We also show that the conductance-voltage curve has a bump at zero bias-voltage if there is no interfacial-scattering. On the contrary, the conductance-voltage curve shows a dip if the system has an interfacial-scattering.",0107588v1 2001-10-22,Anomalous Hall effect in ferromagnetic semiconductors,"We present a theory of the anomalous Hall effect in ferromagnetic (Mn,III)V semiconductors. Our theory relates the anomalous Hall conductance of a homogeneous ferromagnet to the Berry phase acquired by a quasiparticle wavefunction upon traversing closed paths on the spin-split Fermi surface of a ferromagnetic state. It can be applied equally well to any itinerant electron ferromagnet. The quantitative agreement between our theory and experimental data in both (In,Mn)As and (Ga,Mn)As systems suggests that this disorder independent contribution to the anomalous Hall conductivity dominates in diluted magnetic semiconductors.",0110484v1 2001-11-15,A Model for Superconductivity in Ferromagnetic ZrZn2,"This article proposes that superconductivity in the ferromagnetic state of ZrZn$_2$ is stabilized by an exchange-type interaction between the magnetic moments of triplet-state Cooper pairs and the ferromagnetic magnetization density. This explains why superconductivity occurs in the ferromagnetic state only, and why it persists deep into the ferromagnetic state. The model of this article also yields a particular order parameter symmetry, which is a prediction that can be checked experimentally.",0111292v2 2002-03-07,Ferromagnetism in Oriented Graphite Samples,"We have studied the magnetization of various, well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite to investigate the recently reported ferromagnetic-like signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers - to minimize the diamagnetic background - show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.",0203153v1 2002-04-12,Superconducting states in ferromagnetic metals,"The symmetry of the superconducting states arising directly from ferromagnetic states in the crystals with cubic and orthorombic symmetries is described. The symmetry nodes in the quasiparticle spectra of such the states are pointed out if they exist. The superconducting phase transition in the ferromagnet is accompanied by the formation of superconducting domain structure consisting of complex conjugate states imposed on the ferromagnet domain structure with the opposite direction of the magnetization in the adjacent domains. The interplay between stimulation of a nonunitary superconducting state by the ferromagnetic moment and supression of the superconductivity by the diamagnetic orbital currents is established.",0204263v3 2002-08-15,High Ferromagnetic Transition Temperature (172K) in Mn delta-doped GaAs with p-type Selective Doping,"We have found high ferromagnetic transition temperature in Mn delta-doped GaAs-based heterostructures grown on GaAs(001) substrates by molecular beam epitaxy. A 0.3 ML Mn d-doped GaAs samples showed high resistivity at low temperature and did not show a ferromagnetic behavior. However, in a selectively doped heterostructure (Mn delta-doped GaAs / Be-doped AlGaAs), where holes were supplied from the Be-doped AlGaAs layer, clear ferromagnetic order was observed. The ferromagnetic transition temperature of the selectively doped heterostructure was as high as 172K with suitable low-temperature (LT) annealing treatment.",0208299v1 2002-08-28,Absence of an equilibrium ferromagnetic spin glass phase in three dimensions,"Using ground state computations, we study the transition from a spin glass to a ferromagnet in 3-d spin glasses when changing the mean value of the spin-spin interaction. We find good evidence for replica symmetry breaking up till the critical value where ferromagnetic ordering sets in, and no ferromagnetic spin glass phase. This phase diagram is in conflict with the droplet/scaling and mean field theories of spin glasses. We also find that the exponents of the second order ferromagnetic transition do not depend on the microscopic Hamiltonian, suggesting universality of this transition.",0208566v2 2002-12-30,Quantum Critical Behavior in Disordered Itinerant Ferromagnets: Instability of the Ferromagnetic Phase,"An effective field theory is derived that describes the quantum critical behavior of itinerant ferromagnets as the transition is approached from the ferromagnetic phase. This complements a recent study of the critical behavior on the paramagnetic side of the phase transition, and investigates the role of the ferromagnetic Goldstone modes near criticality. We find that the Goldstone modes have no direct impact on the critical behavior, and that the critical exponents are the same as determined by combining results from the paramagnetic phase with scaling arguments.",0212636v2 2003-04-04,Dynamic exchange coupling and Gilbert damping in magnetic multilayers,"We theoretically study dynamic properties of thin ferromagnetic films in contact with normal metals. Moving magnetizations cause a flow of spins into adjacent conductors, which relax by spin flip, scatter back into the ferromagnet, or are absorbed by another ferromagnet. Relaxation of spins outside the moving magnetization enhances the overall damping of the magnetization dynamics in accordance with the Gilbert phenomenology. Transfer of spins between different ferromagnets by these nonequilibrium spin currents leads to a long-ranged dynamic exchange interaction and novel collective excitation modes. Our predictions agree well with recent ferromagnetic-resonance experiments on ultrathin magnetic films.",0304116v1 2003-06-12,Magnetoelectronic Spin Echo,"We predict a spin echo in electron transport through layered ferromagnetic-normal-ferromagnetic metal structures: whereas a spin current polarized perpendicular to the magnetization direction decays when traversing a single homogeneous ferromagnet on the scale of the ferromagnetic spin-coherence length, it reappears by adding a second identical but antiparallel ferromagnet. This re-entrant transverse spin current resembles the spin-echo effect in the magnetization of nuclei under pulsed excitations. We propose an experimental setup to measure the magnetoelectronic spin echo.",0306344v2 2003-07-13,Crossed Andreev Reflection in Structures Consisting of a Superconductor with Ferromagnetic Leads,"A theory of crossed Andreev reflection in structures consisting of a superconductor with two ferromagnetic leads is presented. The electric current due to the crossed Andreev reflection strongly depends on the relative orientation of the magnetization of two ferromagnetic leads. It is shown that the dependence of the electric current and the magnetoresistance on the distance between two ferromagnetic leads is understood by considering the interference between the wave functions in ferromagnets. The current and the magnetoresistance are calculated as functions of the exchange field and the height of the interfacial barriers.",0307307v2 2003-07-31,Carrier induced ferromagnetism in room temperature ferromagnetic semiconductor rutile TiO2 doped with Co,"A ferromagnetic semiconductor, rutile (Ti,Co)O2, exhibits anomalous Hall effect at room temperature. Strong dependence of the anomalous Hall effect, as well as the magneto-optic response, on the carrier concentration suggests carrier induced ferromagnetism in this compound. Both ferromagnetic responses are caused by the charge carriers at the band edge of host semiconductor, indicating possibilities of spintronics devices operable at room temperature.",0307760v1 2004-04-21,Reversible 300K Ferromagnetic Ordering in a Diluted Magnetic Semiconductor,"The discovery of reversible 300 K ferromagnetic ordering in a diluted magnetic semiconductor is reported. Switching of room-temperature ferromagnetism between ""on"" and ""off"" states is achieved in Co2+:ZnO by lattice incorporation and removal of the native n-type defect, interstitial Zn. Spectroscopic and magnetic data implicate a double-exchange mechanism for ferromagnetism. These results demonstrate for the first time reversible room-temperature ferromagnetic ordering in a diluted magnetic semiconductor, and present new opportunities for integrating magnetism and conductivity in semiconductor sensor or spin-based electronics devices.",0404518v1 2004-05-20,Ferromagnetic and random spin ordering in diluted magnetic semiconductors,"In a diluted magnetic semiconductor system, the exchange interaction between magnetic impurities has two independent components: a direct antiferromagnetic interaction and a ferromagnetic interaction mediated by charge carriers. Depending on the system parameters, the ground state of the system may be ordered either ferromagnetically or randomly. In this paper we use percolation theory to find the ferromagnetic transition temperature and the location of the quantum critical point separating the ferromagnetic phase and a valence bond glass phase.",0405467v2 2004-08-12,Spin Screening and Antiscreening in a Ferromagnet/Superconductor Heterojunction,"We present a theoretical study of spin screening effects in a ferromagnet/superconductor (F/S) heterojunction. It is shown that the magnetic moment of the ferromagnet is screened or antiscreened, depending on the polarization of the electrons at the Fermi level. If the polarization is determined by the electrons of the majority (minority) spin band then the magnetic moment of the ferromagnet is screened (antiscreened) by the electrons in the superconductor. We propose experiments that may confirm our theory: for ferromagnetic alloys with certain concentration of Fe or Ni ions there will be screening or antiscreening respectively. Different configurations for the density of states are also discussed.",0408273v1 2004-10-27,Suppression of the superconducting critical current of Nb in bilayers of Nb/SrRuO3,"In bilayers consisting of ferromagnetic and superconducting films, the ferromagnetic film in its domain state induces inhomogeneous distribution of magnetic fields in the superconducting film. When the ferromagnetic film has bubble magnetic domains in a labyrinth structure, it has been found that the pinning of the vortices increases; hence, the critical current of the superconducting film becomes larger. Here we study the effect of parallel ferromagnetic domain structure in Nb/SrRuO3 on the critical current of Nb with current flowing perpendicularly to the domains and find that in this case the ferromagnetic domain structure decreases the critical current.",0410715v2 2005-04-18,Itinerant Ferromagnetism in an Ultracold Atom Fermi Gas,"We address the possible occurrence of ultracold atom ferromagnetism by evaluating the free energy of a spin polarized Fermi gas to second order in its interaction parameter. We find that Hartree-Fock theory underestimates the tendency toward ferromagnetism, predict that the ferromagnetic transition is first order at low temperatures, and point out that the spin coherence time of gases prepared in a ferromagnetic state is strongly enhanced as the transition is approached. We relate our results to recent experiments.",0504459v1 2005-11-03,Superconducting phase coherent electron transport in proximity conical ferromagnets,"We report superconducting phase-periodic conductance oscillations in ferromagnetic wires with interfaces to conventional superconductors. The ferromagnetic wires were made of Ho, a conical ferromagnet. The distance between the interfaces was much larger than the singlet superconducting penetration depth. We explain the observed oscillations as due to the long-range penetration of an unusual ""helical"" triplet component of the order parameter that is generated at the superconductor/ferromagnet interfaces and maintained by the intrinsic rotating magnetization of Ho.",0511077v1 2006-03-03,Theoretical Study on Coexistence of Ferromagnetism and Superconductivity,"On the basis of a two-dimensional t-t' Hubbard model in ferromagnetic and paramagnetic states, the triplet superconducting mechanism is investigated by the third-order perturbation theory with respect to the on-site Coulomb interaction U. In general, the superconducting state is more stable in the paramagnetic state than in the ferromagnetic state. As a special case, the dominant ferromagnetic superconductivity is obtained by the electron-electron correlation between the electronlike majority and holelike minority bands. Furthermore, it is pointed out that in some cases the two bands play an essential role for the coexistence of superconductivity and ferromagnetism.",0603066v1 2007-01-06,Spin Ladder with Anisotropic Ferromagnetic Legs in a Transverse Magnetic Field,"We study the ground state phase diagram of a two-leg spin ladder with anisotropic ferromagnetic leg couplings under the influence of a symmetry breaking transverse magnetic field by the exact diagonalization technique. In the case of antiferromagnetic coupling between legs we identified two phase transitions in the plane of magnetic field vs interchain coupling strength. The first one corresponds to the transition from the gapped rung-singlet phase to the gapped stripe-ferromagnetic phase. The second one represents the transition from the gapped stripe-ferromagnetic phase into the fully polarized ferromagnetic phase.",0701127v1 2007-06-28,Penetration of Andreev bound states into the ferromagnet in a SrRuO$_{3}$/(110)YBa$_2$Cu$_3$O$_{7-δ}$ bilayer: a scanning tunneling spectroscopy study,"Scanning tunneling spectroscopy of thin epitaxial $SrRuO_{3}/(110)YBa_2Cu_3O_{7-\delta}$ ferromagnet/superconductor bilayers, reveal a clear penetration of the Andreev bound states into the ferromagnetic layer. The penetration is manifested in the density of states of the ferromagnet as a split zero bias conductance peak with an imbalance between peak heights. Our data indicate that the splitting occurs at the superconductor side as a consequence of induced magnetization, confirming recent theoretical predictions. The imbalance is attributed to the spin polarization in the ferromagnet.",0706.4208v1 2007-10-30,Devitrification of a glass-like arrested ferromagnetic phase in La0.5Ca0.5MnO3,"Magnetization measurements in La0.5Ca0.5MnO3 manganite show that the high-temperature long-range ferromagnetic-metallic phase transforms to antiferromagnetic-insulating phase, although a fraction of ferromagnetic-metallic phase undergoes glass-like kinetic arrest and coexists at low temperature with the equilibrium antiferromagnetic-insulating phase. We show here through resistivity measurements that the residual arrested ferromagnetic-metallic fraction can be converted to the equilibrium antiferromagnetic-insulating phase by successive annealing at higher temperatures, possibly through heterogeneous nucleation of equilibrium phase. Significantly, larger fractions of this glassy ferromagnetic-metallic phase can be obtained by cooling in higher fields and larger conversion to equilibrium antiferromagnetic-insulating phase results.",0710.5585v1 2008-03-03,Ferromagnetic Pairing States on Two-Coupled Chains,"We propose a concrete model which exhibits ferromagnetism and electron-pair condensation simultaneously. The model is defined on two chains and consists of the electron hopping term, the on-site Coulomb repulsion, and a ferromagnetic interaction which describes ferromagnetic coupling between two electrons, one on a bond in a chain and the other on a site in the other chain. It is rigorously shown that the model has fullypolarized ferromagnetic pairing ground states. The higher dimensional version of the model is also presented.",0803.0174v2 2009-02-02,Evolution of the local superconducting density of states in ErRh$_4$B$_{4}$ close to the ferromagnetic transition,"We present local tunneling spectroscopy experiments in the superconducting and ferromagnetic phases of the reentrant superconductor ErRh$_4$B$_{4}$. The tunneling conductance curves jump from showing normal to superconducting features within a few mK close to the ferromagnetic transition temperature, with a clear hysteretic behavior. Within the ferromagnetic phase, we do not detect any superconducting correlations. Within the superconducting phase we find a peculiar V-shaped density of states at low energies, which is produced by the magnetically modulated phase that coexists with superconductivity just before ferromagnetism sets in.",0902.0308v1 2009-04-02,Probing interaction-induced ferromagnetism in optical superlattices,"We propose a controllable method for observing interaction induced ferromagnetism in ultracold fermionic atoms loaded in optical superlattices. We first discuss how to probe and control Nagaoka ferromagnetism in an array of isolated plaquettes (four lattice sites arranged in a square). Next, we show that introducing a weak interplaquette coupling destroys the ferromagnetic correlations. To overcome this instability we propose to mediate long-range ferromagnetic correlations among the plaquettes via double-exchange processes. Conditions for experimental realization and techniques to detect such states are discussed.",0904.0412v2 2009-05-12,Trends in Ferromagnetism in Mn doped dilute III-V alloys from a density functional perspective,"Mn doping in dilute III-V alloys has been examined as a route to enhance ferromagnetic stability. Strong valence band bowing is expected at the dilute limit, implying a strong modification of the ferromagnetic stability upon alloying, with even an increase in some cases. Using first principle electronic structure calculations we show that while codoping with a group V anion enhances the ferromagnetic stability in some cases when the effects of relaxation of the lattice are not considered, strong impurity scattering in the relaxed structure result in a reduction of the ferromagnetic stability.",0905.1762v1 2010-08-13,Neutral skyrmion configurations in the low-energy effective theory of spinor condensate ferromagnets,"We study the low-energy effective theory of spinor condensate ferromagnets for the superfluid velocity and magnetization degrees of freedom. This effective theory describes the competition between spin stiffness and a long-ranged interaction between skyrmions, topological objects familiar from the theory of ordinary ferromagnets. We find exact solutions to the non-linear equations of motion describing neutral configurations of skyrmions and anti-skyrmions. These analytical solutions provide a simple physical picture for the origin of crystalline magnetic order in spinor condensate ferromagnets with dipolar interactions. We also point out the connections to effective theories for quantum Hall ferromagnets.",1008.2239v1 2010-09-03,Josephson current and $π-$state in ferromagnet with embedded superconducting nanoparticles,"On the basis of Usadel equations we investigate superconductor/ferromagnet/superconductor (S/F/S) hybrid systems which consist of superconducting nanostructures (spheres, rods) embedded in ferromagnetic metal. The oscillations of the critical current of the S/F/S Josephson junctions with the thickness of ferromagnetic spacer between superconducting electrodes are studied. We demonstrate that the $\pi $ state can be realized in such structures despite of a dispersion of the distances between different parts of the electrodes. The transitions between 0 and $\pi $ states at some thickness of ferromagnetic spacer can be triggered by temperature variation.",1009.0643v1 2010-09-19,Itinerant ferromagnetism in a two-dimensional atomic gas,"Motivated by the first experimental evidence of ferromagnetic behavior in a three-dimensional ultracold atomic gas, we explore the possibility of itinerant ferromagnetism in a trapped two-dimensional atomic gas. Firstly, we develop a formalism that demonstrates how quantum fluctuations drive the ferromagnetic reconstruction first order, and consider the consequences of an imposed population imbalance. Secondly, we adapt this formalism to elucidate the key experimental signatures of ferromagnetism in a realistic trapped geometry.",1009.3614v1 2011-03-31,Pressure-induced s-band ferromagnetism in alkali metals,"First-principles density-functional-theory calculations show that compression of alkali metals stabilizes open structures with localized interstitial electrons which may exhibit a Stoner-type instability towards ferromagnetism. We find ferromagnetic phases of the lithium-IV-type, simple cubic, and simple hexagonal structures in the heavier alkali metals, which may be described as s-band ferromagnets. We predict that the most stable phases of potassium at low temperatures and pressures around 20 GPa are ferromagnets.",1103.6141v1 2011-06-20,Ferromagnetic state and phase transitions,"Evidence is summarized attesting that the standard exchange field theory of ferromagnetism by Heisenberg has not been successful. It is replaced by the crystal field and a simple assumption that spin orientation is inexorably associated with the orientation of its carrier. It follows at once that both ferromagnetic phase transitions and magnetization must involve a structural rearrangement. The mechanism of structural rearrangements in solids is nucleation and interface propagation. The new approach accounts coherently for ferromagnetic state and its manifestations.",1106.3795v1 2011-08-05,Thermoelectric detection of ferromagnetic resonance of a nanoscale ferromagnet,"We present thermoelectric measurements of the heat dissipated due to ferromagnetic resonance of a Permalloy strip. A microwave magnetic field, produced by an on-chip coplanar strip waveguide, is used to drive the magnetization precession. The generated heat is detected via Seebeck measurements on a thermocouple connected to the ferromagnet. The observed resonance peak shape is in agreement with the Landau-Lifshitz-Gilbert equation and is compared with thermoelectric finite element modeling. Unlike other methods, this technique is not restricted to electrically conductive media and is therefore also applicable to for instance ferromagnetic insulators.",1108.1286v1 2011-08-24,Ferromagnetism and Superconductivity in Uranium Compounds,"Recent advances on ferromagnetic superconductors, UGe2, URhGe and UCoGe are presented. The superconductivity (SC) peacefully coexists with the ferromagnetism (FM), forming the spin-triplet state of Cooper pairs. The striking new phenomena, such as SC reinforced by the magnetic field, are associated with Ising-type ferromagnetic fluctuations. A variety of ferromagnetic ordered moments between UGe2, URhGe and UCoGe affords to understand the relation between FM, tricriticality and SC.",1108.4807v1 2012-07-06,Double Exchange Ferromagnetism in the Peierls Insulator State,"We study the effects of opening of the band gap on the double exchange ferromagnetism. Applying the density-matrix renormalization group method and an analytical expansion from the dimer limit to the one-dimensional double exchange model, we demonstrate for a relevant region of the exchange coupling that, in the weak dimerization regime, the Peierls gap opens in the fully spin-polarized conduction band without affecting its ferromagnetism, whereas in the strong dimerization regime, the ferromagnetism is destroyed and the Mott gap opens instead, leading the system to the antiferromagnetic quasi-long-range order. An insulator version of the double exchange ferromagnetism is thus established.",1207.1525v1 2012-11-12,Spin polarization and magnetoresistance through a ferromagnetic barrier in bilayer graphene,"We study spin dependent transport through a magnetic bilayer graphene nanojunction configured as two dimensional normal/ferromagnetic/normal structure where the gate-voltage is applied on the layers of ferromagnetic graphene. Based on the fourband Hamiltonian, conductance is calculated by using Landauer Butikker formula at zero temperature. For parallel configuration of the ferromagnetic layers of bilayer graphene, the energy band structure is metallic and spin polarization reaches to its maximum value close to the resonant states, while for antiparallel configuration, the nanojunction behaves as a semiconductor and there is no spin filtering. As a result, a huge magnetoresistance is achievable by altering the configurations of ferromagnetic graphene especially around the band gap.",1211.2701v1 2013-01-03,Integrable Heisenberg Ferromagnet Equations with self-consistent potentials,"In this paper, we consider some integrable Heisenberg Ferromagnet Equations with self-consistent potentials. We study their Lax representations. In particular we give their equivalent counterparts which are nonlinear Schr\""odinger type equations. We present the integrable reductions of the Heisenberg Ferromagnet Equations with self-consistent potentials. These integrable Heisenberg Ferromagnet Equations with self-consistent potentials describe nonlinear waves in ferromagnets with some additional physical fields.",1301.1649v2 2013-03-16,Designing ferromagnetism in vanadium-oxide based superlattices,"Motivated by recent reports (Phys. Rev. B\textbf{80}, 241102) of room-temperature ferromagnetism in vanadium-oxide based superlattices, a single-site dynamical mean field study of the dependence of the paramagnetic-ferromagnetic phase boundary on superlattice geometry was performed. An examination of variants of the experimentally determined crystal structure indicate that ferromagnetism is found only in a small and probably inaccessible region of the phase diagram. Design criteria for increasing the range over which ferromagnetism might exist are proposed.",1303.4016v2 2015-11-09,Ferromagnetic instability of interlayer floating electrons in quasi-two-dimensional electride Y$_2$C,"Ab initio electronic structure calculations show that the recently identified quasi-two-dimensional electride Y$_2$C is a weak itinerant ferromagnet or at least close to a ferromagnetic instability. The ferromagnetism is induced by the electride electrons, which are loosely bound around interstitial sites and overlap with each other to form two-dimensional interlayer conduction bands. The semimetallicity and two-dimensionality of the band structure are the key to understanding this ferromagnetic instability.",1511.02561v1 2013-08-08,Ferromagnetism of the Repulsive Atomic Fermi Gas: three-body recombination and domain formation,"The simplest model for itinerant ferromagnetism, the Stoner model, has so far eluded experimental observation in repulsive ultracold fermions due to rapid three-body recombination at large scattering lengths. Here we show that a ferromagnetic phase can be stabilised by imposing a moderate optical lattice. The reduced kinetic energy drop upon formation of a polarized phase in an optical lattice extends the ferromagnetic phase to smaller scattering lengths where three-body recombination is small enough to permit experimental detection of the phase. We also show, using time dependent density functional theory, that in such a setup ferromagnetic domains emerge rapidly from a paramagnetic initial state.",1308.1961v1 2016-11-09,Theoretical study of the stripline ferromagnetic resonance response of metallic ferromagnetic films based on an analytical model,"We develop an advanced analytical model for calculating the broadband stripline ferromagnetic resonance (FMR) response for metallic ferromagnetic films, taking into account the exchange interaction as well as the exchange boundary conditions at the film surface. This approach leads to simple analytical expressions in the Fourier space. As a result, a numerical code which implements inverse Fourier transform of these equations is very quick. This allows us to explore a wide space of parameters as numerical examples of application of this theory. In particular, we investigate the joint effect of microwave eddy current shielding and magnetisation pinning at the ferromagnetic film surfaces on the shape of the stripline FMR response of the film.",1611.02834v1 2018-01-05,Magnetoresistance originated from charge-spin conversion in ferromagnet,"Transverse magnetoresistance in a ferromagnetic/nonmagnetic/ferromagnetic trilayer originated from charge-spin conversion by anomalous Hall effect is investigated theoretically. Solving the spin diffusion equation in bulk and using the spin-dependent Landauer formula at the ferromagnetic/nonmagnetic interface, an analytical formula of the transverse resistivity is obtained. The charge-spin conversion by the anomalous Hall effect contributes to the magnetoresistance in a manner proportional to the square of the spin anomalous Hall angle. The angular dependence of the magnetoresistance is basically identical to that of planar Hall effect, but has an additional term which depends on the relative angle of the magnetizations in two ferromagnets.",1801.01623v1 2018-07-26,Phase separation and proximity effects in itinerant ferromagnet-superconductor heterostructures,"Heterostructures made of itinerant ferromagnets and superconductors are studied. In contrast to most previous models, ferromagnetism is not enforced as an external Zeeman field but induced in a correlated single-band model (CSBM) that displays itinerant ferromagnetism as a mean-field ground state. This allows us to investigate the influence of an adjacent superconducting layer on the properties of the ferromagnet in a self-consistent Bogoliubov-de Gennes approach. The CSBM displays a variety features not present in the Zeeman exchange model that influence the behavior of order parameters close to the interface, as e.g. phase separation and the competition between magnetism and superconducting orders.",1807.10387v1 2020-08-15,The inverse proximity effect in strong ferromagnet-superconductor structures,"The magnetization in a superconductor induced due to the inverse proximity effect is investigated in hybrid bilayers containing a superconductor and a ferromagnetic insulator or a strongly spin-polarized ferromagnetic metal. The study is performed within a quasiclassical Green function framework, wherein Usadel equations are solved with boundary conditions appropriate for strongly spin-polarized ferromagnetic materials. A comparison with recent experimental data is presented. The singlet to triplet conversion of the superconducting correlations as a result of the proximity effect with a ferromagnet is studied.",2008.06757v1 2022-10-17,Flat band induced room temperature ferromagnetism in two dimensional systems,"The remarkable and fascinating properties of two-dimensional materials have raised them to the rank of most promising candidates for technological applications. In particular, the possibility of long-range ferromagnetic order in 2D materials is generating a growing excitement. Here, we demonstrate that flat bands (FBs) may pave the way to room temperature ferromagnetism in 2D compounds. Indeed, the magnetic exchanges between localized spins are largely dominated by the FB-FB contribution. This contribution is ferromagnetic and scales linearly with the local coupling, thus leading to cross-over temperatures (ferromagnetic phase/ paramagnetic phase) higher by an order of magnitude than those currently reported in experiments. Cross-over temperatures as high as $600~ K$ could be reached in micrometer-sized FB systems.",2210.08896v1 2023-03-21,Unique Itinerant Ferromagnetism in 4d-electron System Ca2RuO4,"We have studied the magnetic properties of pressure-induced ferromagnet Ca2RuO4 to reveal the uniqueness of the 4d-electron ferromagnetism in the quasi-two-dimensional conductor. The magnetic parameters have been estimated from the paramagnetic susceptibility and the magnetisation process under pressure up to 2 GPa. The parameters can well be interpreted on the basis of the self-consistent renormalization theory of spin fluctuation for 3D-itinerant ferromagnet. Nevertheless, the metallic Ca2RuO4 shows quite strong anisotropy not only in the conductivity but also in the magnetisation process. Such the strong anisotropy is rare for an itinerant ferromagnet and is a unique characteristic of the 4d electron system Ca2RuO4.",2303.11604v1 2023-04-23,Spin-triplet superconductivity from quantum-geometry-induced ferromagnetic fluctuation,"We show that quantum geometry induces ferromagnetic fluctuation resulting in spin-triplet superconductivity. The criterion for ferromagnetic fluctuation is clarified by analyzing contributions from the effective mass and quantum geometry. When the non-Kramers band degeneracy is present near the Fermi surface, the Fubini-Study quantum metric strongly favors ferromagnetic fluctuation. Solving the linearized gap equation with the effective interaction obtained by the random phase approximation, we show that the spin-triplet superconductivity is mediated by quantum-geometry-induced ferromagnetic fluctuation.",2304.11536v2 2001-04-12,Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density,"We studied ferromagnetism in the one-dimensional Hubbard model with doubly degenerate atomic orbitals by means of the density-matrix renormalization-group method and obtained the ground-state phase diagrams. It was found that ferromagnetism is stable from low to high (0< n < 1.75) electron density when the interactions are sufficiently strong. Quasi-long-range order of triplet superconductivity coexists with the ferromagnetic order for a strong Hund coupling region, where the inter-orbital interaction U'-J is attractive. At quarter-filling (n=1), the insulating ferromagnetic state appears accompanying orbital quasi-long-range order. For low densities (n<1), ferromagnetism occurs owing to the ferromagnetic exchange interaction caused by virtual hoppings of electrons, the same as in the quarter-filled system. This comes from separation of the charge and spin-orbital degrees of freedom in the strong coupling limit. This ferromagnetism is fragile against variation of band structure. For high densities (n>1), the phase diagram of the ferromagnetic phase is similar to that obtained in infinite dimensions. In this case, the double exchange mechanism is operative to stabilize the ferromagnetic order and this long-range order is robust against variation of the band-dispersion. A partially polarized state appears in the density region 1.680$ in the large limit of band multiplicity. Ferromagnetic instability occurs, when the sum of the two exchange interactions is ferromagnetic and it overcomes the quenching of magnetic moments by the Kondo effect or local quantum spin fluctuations and the suppression of magnetic instability by the mode-mode coupling among intersite spin fluctuations.",0108269v1 2002-09-05,Ground state properties of ferromagnetic metal/conjugated polymer interfaces,"We theoretically investigate the ground state properties of ferromagnetic metal/conjugated polymer interfaces. The work is partially motivated by recent experiments in which injection of spin polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger (SSH) Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a half-metallic ferromagnetic colossal magnetoresistance (CMR) oxide which has explicit structural degrees of freedom. The Fermi energy of the magnetic metallic contact is adjusted to control the degree of electron transfer into the polymer. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. Bipolarons are the lowest energy charge state in the bulk polymer for the nondegenerate SSH model Hamiltonian. As a result electrons (or holes) transferred into the bulk of the polymer form spinless bipolarons. However, there can be spin density in the polymer localized near the interface.",0209145v1 2003-03-12,Spin-Flip Scattering Effect on the Current-Induced Spin Torque in Ferromagnet-Insulator-Ferromagnet Tunnel Junctions,"We have investigated the current-induced spin transfer torque of a ferromagnet-insulator-ferromagnet tunnel junction by taking the spin-flip scatterings into account. It is found that the spin-flip scattering can induce an additional spin torque, enhancing the maximum of the spin torque and giving rise to an angular shift compared to the case when the spin-flip scatterings are neglected. The effects of the molecular fields of the left and right ferromagnets on the spin torque are also studied. It is found that $\tau ^{Rx}/I_{e}$ ($\tau ^{Rx}$ is the spin-transfer torque acting on the right ferromagnet and $I_{e}$ is the tunneling elcetrical current) does vary with the molecular fields. At two certain angles, $\tau ^{Rx}/I_{e}$ is independent of the molecular field of the right ferromagnet, resulting in two crossing points in the curve of $\tau ^{Rx}/I_{e}$ versus the relevant orientation for different molecular fields.",0303215v1 2003-03-20,"Towards a Fully Ab-Initio Description of the Diluted Magnetic Semiconductor $Ga_{1-x}Mn_{x}As$. Ferromagnetism, Electronic Structure, and Optical Response","There are two competing views of itinerant ferromagnetism, the first viewing ferromagnetism as resulting from the indirect coupling between local moments via the itinerant carrier dynamics, the so-called RKKY mechanism, while in the alternative picture, ferromagnetism results from the spin polarization of itinerant carriers by the strong atomic Hund interaction - the so-called double exchange (DE) scenario. Which view describes the ferromagnetism in diluted magnetic semiconductors, materials with promise for spintronic applications, is still unclear. Here, we describe the detailed physical response of the prototype material $Ga_{1-x}Mn_{x}As$ using a combination of first-principles bandstructure with methods based on dynamical mean field theory to incorporate strong, dynamical correlations {\it and} intrinsic as well as extrinsic disorder in one single theoretical picture. We show how ferromagnetism is driven by DE, in agreement with very recent observations, along with a good quantitative description of the details of the electronic structure, as probed by scanning tunnelling microscopy (STM) and optical conductivity. Our results show how ferromagnetism can be driven by DE even in diluted magnetic semiconductors with small carrier concentration.",0303427v1 2004-06-21,"Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N","The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is extensively studied over a vast range of Mn concentrations. We calculate the electronic structures of these materials using density functional theory in both the local spin density approximation and the LDA+U scheme, that we have now implemented in the code SIESTA. For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the ferromagnetism, with an exchange constant $N\beta$ =~ -2.8 eV. This is smaller than that obtained with LSDA, which overestimates the exchange coupling between Mn ions and the As $p$ holes. In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the double-exchange mechanism, since a hole of strong $d$ character is found at the Fermi level in both the LSDA and the LDA+U approaches. In this case the coupling between the Mn ions decays rapidly with the Mn-Mn separation. This suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a robust ferromagnetic phase at large Mn concentration coexisting with a diluted weak ferromagnetic phase.",0406464v1 2005-04-08,Effect of an inhomogeneous exchange field on the proximity effect in disordered superconductor-ferromagnet hybrid structures,"We investigate the effect of an inhomogeneous exchange field on the proximity effect in superconductor-ferromagnet hybrid structures within the quasi-classical theory of superconductivity. As an example we study the proximity effect in a superconductor-ferromagnet bilayer with an in-plane spiral magnetic order in the ferromagnet. This model simulates a multiple magnetic domain structure where the domain walls of the N\'{e}el type are of equal size as the domains. Triplet correlations are induced in the bilayer by the superconducting proximity effect, which are sensitive to the local quantization axis of the exchange field in the ferromagnet. The coexistence of singlet and triplet pair correlations in the bilayer results into a sensitivity of the superconducting transition temperature on the spatial variation of the exchange field in the ferromagnetic layer. We show that the inhomogeneity tends to suppress the oscillating behavior of the pair amplitudes in the ferromagnet. As a result, the superconducting critical temperature is found to be strongly dependent on the spiral wavevector. We study the spin-dependent local density of states and the effect of an induction of a spin magnetization in the superconductor.",0504198v2 2005-12-29,Ferromagnetic phase transition in spinor Bose gases,"The achievement in cooling alkali atomic gases, such as $^{87}$Rb, $^{23}$Na and $^{7}$Li, to quantum degeneracy opens up a way to study magnetism in spinor bosons, because these constituent atoms usually have a hyperfine spin degree of freedom. This article reviews several basic problems related to the ferromagnetic phase transition in spinor atomic Bose gases from a theoretical perspective. After a brief discussion on various possible origins of the ferromagnetic interaction, the phase diagram of the ferromagnetically coupled spinor bosons is investigated. It is found that the ferromagnetic transition occurs always above Bose-Einstein condensation and the Bose condensate is fully polarized. The low-lying collective excitations of the spinor condensate, including spin and density modes, are discussed. The spectrum of the density mode is of the Bogliubov form and the spin wave spectrum has a $k^2$-formed dispersion relation at long wavelengths. The spin-wave stiffness coefficient contains contribut",0512693v1 2006-07-11,Significant enhancement of ferromagnetism in Zn$_{1-x}$Cr$_{x}$Te doped with iodine as an n-type dopant,"The effect of additional doping of charge impurities was investigated in a ferromagnetic semiconductor Zn$_{1-x}$Cr$_{x}$Te. It was found that the doping of iodine, which is expected to act as an n-type dopant in ZnTe, brought about a drastic enhancement of the ferromagnetism in Zn$_{1-x}$Cr$_{x}$Te while the grown films remained electrically insulating. In particular, at a fixed Cr composition of x = 0.05, the ferromagnetic transition temperature Tc increased up to 300K at maximum due to the iodine doping from Tc = 30K of the undoped counterpart, while the ferromagnetism disappeared due to the doping of nitrogen as a p-type dopant. The observed systematic correlation of ferromagnetism with the doping of charge impurities of both p- and n-type, suggesting a key role of the position of Fermi level within the impurity d-state, is discussed on the basis of the double exchange interaction as a mechanism of ferromagnetism in this material.",0607273v1 2007-03-15,Signatures of retroreflection and induced triplet electron-hole correlations in ferromagnet/s-wave superconductor structures,"We present a theoretical study of a ferromagnet/s-wave superconductor junction to investigate the signatures of induced triplet correlations in the system. We apply the extended BTK-formalism and allow for an arbitrary magnetization strength/direction of the ferromagnet, a spin-active barrier, Fermi-vector mismatch, and different effective masses in the two systems. It is found that the phase associated with the $xy$-components of the magnetization in the ferromagnet couples with the superconducting phase and induces spin-triplet pairing correlations in the superconductor, if the tunneling barrier acts as a spin-filter. This feature leads to an induced spin-triplet pairing correlation in the ferromagnet, along with a spin-triplet electron-hole coherence due to an interplay between the ferromagnetic and superconducting phase. As our main result, we investigate the experimental signatures of retrorelection, manifested in the tunneling conductance of a ferromagnet/s-wave superconductor junction with a spin-active interface.",0703398v1 2007-05-21,Evidence for ferromagnetic spin-pairing superconductivity in UGe$_2$: A $^{73}$Ge-NQR study under pressure,"We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe$_2$ via $^{73}$Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure ($P$). Measurements of the nuclear spin-lattice relaxation rate $(1/T_1)$ have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.",0705.2944v1 2008-03-21,Induced Triplet Pairing in clean s-wave Superconductor/Ferromagnet layered structures,"We study induced triplet pairing correlations in clean ferromagnet/superconductor/ferromagnet heterostructures. The pairing state in the superconductor is the conventional singlet s-wave, and the angle $\alpha$ between the magnetizations of the two ferromagnetic layers is arbitrary. We use a numerical fully self-consistent solution of the microscopic equations and obtain the time-dependent triplet correlations via the Heisenberg equations of motion. We find that in addition to the usual singlet correlations, triplet correlations, odd in time as required by the Pauli principle, are induced in both the ferromagnets and the superconductor. These time-dependent correlations are largest at times of order of the inverse of the Debye cutoff frequency, $\omega_D$, and we find that within that time scale they are often spatially very long ranged. We discuss the behavior of the characteristic penetration lengths that describe these triplet correlations. We also find that the ferromagnets can locally magnetize the superconductor near the interface, and that the local magnetization then undergoes strongly damped oscillations. The local density of states exhibits a variety of energy signatures, which we discuss, as a function of ferromagnetic strength and $\alpha$.",0803.3174v1 2008-12-11,Coexistence of triplet superconductivity and itinerant ferromagnetism,"The triplet superconductivity in $UGe_2$ and $URhGe$ coexists with itinerant ferromagnetism such that in the pressure-temperature phase diagram the whole region occupied by the superconducting state is situated inside a more vast ferromagnetic region. In the same family metal UCoGe the pressure dependent critical lines T_{Curie}(P) and T_{sc}(P) of the ferromagnet and the superconducting phase transitions intersect each other. The two-band multidomain superconducting ferromagnet state arises at temperatures below both of these lines. Here I describe the symmetry and the order parameters of the paramagnet as well of the multidomain ferromagnet superconducting states. The Josephson coupling between two adjacent ferromagnet superconducting domains is discussed.",0812.2171v2 2010-08-02,A Search for Defect Related Ferromagnetism in SrTiO$_3$,"Room temperature ferromagnetic hysteresis is observed in commercial SrTiO$_3$ substrates purchased from a variety of suppliers. It is shown that the ferromagnetic signal comes from the unpolished surfaces. Surface impurity phases cannot be detected using either x-ray diffraction or energy dispersive x-ray spectra on the unpolished surfaces. However, a possible correlation between surface disorder (xray diffraction peak linewidth) and ferromagnetism is observed. Ar ion bombardment (10keV-90 keV) can be used to produce surface layer disorder but is not found to induce ferromagnetism. Annealing of the substrates at temperatures ranging from 600 to 1100 $^\circ$C is found to alter the hysteresis curves differently depending on whether the annealing is performed in air or in vacuum. Identical annealing behaviour is observed if the substrates are artificially spiked with iron. This suggests that the ferromagnetic hysteresis of as purchased SrTiO$_3$ could be due to Fe contamination of the unpolished surfaces. In addition, it is shown that no ferromagnetism is observed in samples that contain $10^{19}-10^{21}$ cm$^{-3}$ of oxygen vacancies if all the faces are polished.",1008.0299v1 2011-04-12,Properties of Ferromagnetic Superconductors,"Thanks to the discovery in the last decade of three uranium ferromagnetic superconductors, UGe2, URhGe and UCoGe, the fascinating aspects of the interplay between the triplet state of Cooper pairing and ferromagnetism have emerged. Furthermore, as the ferromagnetic properties in the normal state are quite different with respect to the proximity of the ferromagnetic-paramagnetic instabilities, the feedback with the coexistence of superconductivity gives rise to quite different boundaries in pressure and magnetic field. Special attention is given on the location of the materials with respect to the tricriticality and on the reinforcement of superconductivity in a transverse field response with respect to the direction of the ferromagnetic sublattice magnetization. The other facts of the interplay between ferromagnetism and superconductivity is briefly mentioned.",1104.2279v1 2012-03-16,Universal low-temperature tricritical point in metallic ferromagnets and ferrimagnets,"An earlier theory of the quantum phase transition in metallic ferromagnets is revisited and generalized in three ways. It is shown that the mechanism that leads to a fluctuation-induced first-order transition in metallic ferromagnets with a low Curie temperature is valid, (1) irrespective of whether the magnetic moments are supplied by the conduction electrons or by electrons in another band, (2) for ferromagnets in the XY and Ising universality classes as well as for Heisenberg ferromagnets, and (3) for ferrimagnets as well as for ferromagnets. This vastly expands the class of materials for which a first-order transition at low temperatures is expected, and it explains why strongly anisotropic ferromagnets, such as UGe2, display a first-order transition as well as Heisenberg magnets.",1203.3826v1 2013-02-05,Reflection and refraction process of spinwave in a ferromagnet/frustrated ferromagnet junction system,"Frustration introduces a nontrivial dispersion relation of spinwave even in a ferromagnetic phase in a spin system. We study the reflection and refraction process of spinwaves in the ferromagnet/frustrated ferromagnet junction system by using the Holstein-Primakoff spinwave expansion and taking the large-S limit. We discuss the relation between the incident angle and refraction angle of spinwave, namely, the Snell's low of spinwave. As concrete examples of frustrated ferromagnets, we study the fully polarized ferromagnet phases in the J1-J2 chains and the J1-J2 model on the square lattice. The interesting refraction processes, such as the splitting of the incident spinwave and the negative refraction, are discussed. We also study the transmittance and reflectance in these concrete models.",1302.1003v2 2013-05-14,Interface induced room-temperature ferromagnetism in hydrogenated epitaxial graphene,"Due to the predominantly surface character of graphene, it is highly suitable for functionalization with external atoms and/or molecules leading to a plethora of new and interesting phenomena. Here we show ferromagnetic properties of hydrogen-functionalized epitaxial graphene on SiC. Ferromagnetism in such a material is not directly evident as it is inherently composed of only non-magnetic constituents. Our results nevertheless show strong ferromagnetism, which cannot be explained by simple magnetic impurities. The ferromagnetism is unique to hydrogenated epitaxial graphene on SiC, where interactions with the interfacial buffer layer play a crucial role. We argue that the origin of the observed ferromagnetism is governed by electron correlation effects of the narrow Si-dangling-bond (Si-DB) states in the buffer layer exchange-coupled to localized states in the hydrogenated graphene layer. This forms a quasi-three-dimensional ferromagnet with a Curie temperature higher than 300 K.",1305.3158v1 2014-04-22,Superconductivity-induced Magnetic Modulation in a Ferromagnet Through an Insulator in LaCaMnO3\SrTiO3\YBa2Cu3O7-δ Hybrid Heterostructures,"Coexistence of ferromagnetic and superconducting orders and their interplay in ferromagnet-superconductor heterostructures is a topic of intense research. While it is well known that proximity of a ferromagnet suppresses superconducting order in the superconductor, there exist few studies indicating the proximity of a superconductor suppressing ferromagnetic order in a ferromagnet. Here we demonstrate a rare observation of the suppression of ferromagnetic order in a LaCaMnO3 layer separated from a YBa2Cu3O7-{\delta} layer by a thin insulator (SrTiO3). Polarized neutron reflectivity measurements on LaCaMnO3\SrTiO3\YBa2Cu3O7-{\delta} trilayer deposited on [001] SrTiO3 single crystal substrates shows the emergence of a thin magnetic dead layer in LaCaMnO3 adjacent to the insulating layer below its superconducting transition temperature of YBa2Cu3O7-{\delta}. Further, the magnetic dead layer grows in thickness when the insulating layer is made thinner. This indicates a possible tunneling of the superconducting order-parameter through the insulating SrTiO3 inducing modulation of magnetization in LaCaMnO3.",1404.5456v1 2014-10-17,Coexistence of ferromagnetism and superconductivity in iron based pnictides: a time resolved magnetooptical study,"Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains.",1410.4651v1 2014-10-19,Coexistence and competition of ferromagnetism and p-wave superconductivity in holographic model,"By combining a holographic p-wave superconductor model and a holographic ferromagnetism model, we study the coexistence and competition of ferromagnetism and p-wave superconductivity. It is found that the results depend on the self-interaction of magnetic moment of the complex vector field and which phase appears first. In the case that the ferromagnetic phase appears first, if the interaction is attractive, the system shows the ferromagnetism and superconductivity can coexist in low temperatures. If the interaction is repulsive, the system will only be in a pure ferromagnetic state. In the case that the superconducting phase appears first, the attractive interaction will leads to a magnetic p-wave superconducting phase in low temperatures. If the interaction is repulsive, the system will be in a pure p-wave superconducting phase or ferromagnetic phase when the temperature is lowered.",1410.5080v2 2014-11-19,Dynamical spin injection at a quasi-one-dimensional ferromagnet-graphene interface,"We present a study of dynamical spin injection from a three-dimensional ferromagnet into two-dimensional single-layer graphene. Comparative ferromagnetic resonance (FMR) studies of ferromagnet/graphene strips buried underneath the central line of a coplanar waveguide show that the FMR linewidth broadening is the largest when the graphene layer protrudes laterally away from the ferromagnetic strip, indicating that the spin current is injected into the graphene areas away from the area directly underneath the ferromagnet being excited. Our results confirm that the observed damping is indeed a signature of dynamical spin injection, wherein a pure spin current is pumped into the single-layer graphene from the precessing magnetization of the ferromagnet. The observed spin pumping efficiency is difficult to reconcile with the expected backflow of spins according to the standard spin pumping theory and the characteristics of graphene, and constitutes an enigma for spin pumping in two-dimensional structures.",1411.5339v2 2014-12-14,Magnetic properties of spin-1/2 Fermi gases with ferromagnetic interaction,"We investigate the magnetic properties of spin-$1/2$ charged Fermi gases with ferromagnetic coupling via mean-field theory, and find the interplay among the paramagnetism, diamagnetism and ferromagnetism. Paramagnetism and diamagnetism compete with each other. When increasing the ferromagnetic coupling the spontaneous magnetization occurs in a weak magnetic field. The critical ferromagnetic coupling constant of the paramagnetic phase to ferromagnetic phase transition increases linearly with the temperature. Both the paramagnetism and diamagnetism increase when the magnetic field increases. It reveals the magnetization density $\bar M$ increases firstly as the temperature increases, and then reaches a maximum. Finally the magnetization density $\bar M$ decreases smoothly in the high temperature region. The domed shape of the magnetization density $\bar M$ variation is different from the behavior of Bose gas with ferromagnetic coupling. We also find the curve of susceptibility follows the Curie-Weiss law, and for a given temperature the susceptibility is directly proportional to the Land\'{e} factor.",1412.4297v2 2014-12-22,Possibility of ferromagnetic neutron matter,"We study ferromagnetism at high density of neutrons in the QCD hadron phase, by using the simplest chiral effective model incorporating magnetic fields and the chiral anomaly. Under the assumption of spatial homogeneity, we calculate the energy density as a function of neutron density, with a magnetization and a neutral pion condensation a la Dautry and Neyman. We find that at a high density the energy of the ferromagnetic order is lower than that of the ordinary neutron matter, and the reduction effect is enhanced by the anomaly. Compared to the inhomogeneous phase with the alternating layer structure, our ferromagnetic phase turns out to be unfavored. However, once an axial vector meson condensation is taken into account in our simplest model, the ferromagnetic energy density is lowered significantly, which still leaves some room for a possible realization of a QCD ferromagnetic phase and ferromagnetic magnetars.",1412.6960v2 2015-01-31,Room temperature local ferromagnetism and nanoscale domain growth in the ferromagnetic semiconductor GeFe,"We investigate the local electronic structure and magnetic properties of the group IV based ferromagnetic semiconductor, GeFe, using soft X ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have an unusually large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin = 0.3. We find that local ferromagnetic domains, which are formed through ferromagnetic exchange interactions in the high Fe content regions of the GeFe films, exist at room temperature, well above the Curie temperature. We demonstrate the first observation of the intriguing nanoscale domain growth process in which ferromagnetic domains expand as the temperature decreases, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.",1502.00118v1 2015-05-06,"Magnetization Process of the n-type Ferromagnetic Semiconductor (In,Fe)As:Be Studied by X-ray Magnetic Circular Dichroism","In order to investigate the mechanism of ferromagnetic ordering in the new n-type magnetic semiconductor (In,Fe)As co-doped with Be, we have performed X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) studies of ferromagnetic and paramagnetic samples. The spectral line shapes suggest that the ferromagnetism is intrinsic originating from Fe atoms incorporated into the Zinc-blende-type InAs lattice. The magnetization curves of Fe measured by XMCD were well reproduced by the superposition of a Langevin function representing superparamagnetic (SPM) behavior of nano-scale ferromagnetic domains and a T-linear function representing Curie-Weiss paramagnetism even much above the Curie temperatures. The data at 20 K showed a deviation from the Langevin behavior, suggesting a gradual establishment of macroscopic ferromagnetism on lowering temperature. The existence of nano-scale ferromagnetic domains indicated by the SPM behavior suggests spatial fluctuations of Fe concentration on the nano-scale.",1505.01402v1 2017-01-11,Emergent incommensurate correlations in the frustrated ferromagnetic spin-1 chains,"We study the frustrated ferromagnetic spin-1 chains, where the ferromagnetic nearest-neighbor coupling competes with the antiferromagnetic next-nearest-neighbor coupling. We use the density matrix renormalization group to obtain the ground states. Through the analysis of spin-spin correlations we identify the double Haldane phase as well as the ferromagnetic phase. It is shown that the ferromagnetic coupling leads to incommensurate correlations in the double Haldane phase. Such short-range correlations transform continuously into the ferromagnetic instability at the transition to the ferromagnetic phase. We also compare the results with the spin-1/2 and classical spin systems, and discuss the string orders in the system.",1701.02832v1 2014-05-08,Peculiar long-range supercurrent in SFS junction containing a noncollinear magnetic domain in the ferromagnetic region,"We study the supercurrent in a superconductor-ferromagnet-superconductor heterostructure containing a noncollinear magnetic domain in the ferromagnetic region. It is demonstrated that the magnetic domain can lead to a spin-flip process, which can reverse the spin orientations of the singlet Cooper pair propagating through the magnetic domain region. If the ferromagnetic layers on both sides of magnetic domain have the same features, the long-range proximity effect will take place. That is because the singlet Cooper pair will create an exact phase-cancellation effect and gets an additional $\pi$ phase shift as it passes through the entire ferromagnetic region. Then the equal spin triplet pair only exists in the magnetic domain region and can not diffuse into the other two ferromagnetic layers. So the supercurrent mostly arises from the singlet Cooper pairs and the equal spin triplet pairs are not involved. This behavior is quite distinct from the common knowledge that long-range supercurrent induced by inhomogeneous ferromagnetism stems from the equal spin triplet pairs. The result we presented here provides a new way for generating the long-range supercurrent.",1405.1951v2 2016-05-24,Superconductivity in Uranium Ferromagnets,"The theoretical description and the survey of physical properties of superconducting states in the uranium ferromagnetic materials are presented. On the basis of microscopic theory is shown that the coupling between the electrons in these ferromagnetic metals by means of magnetization fluctuations gives rise the triplet pairing superconducting state and the general form of the order parameter dictated by the symmetry is established. The theory allows to explain some specific observations including peculiar phenomenon of reentrant superconductivity in URhGe in magnetic field perpendicular to the direction of spontaneous magnetization. In addition we describe several particular topics relating to uranium superconducting ferromagnets: (i) critical magnetic relaxation in dual localized-itinerant ferromagnets, (ii) phase transition to ferromagnetic state in Fermi liquid and UGe$_2$, (iii) superconducting ordering in ferromagnetic metals without inversion symmetry.",1605.07319v3 2016-08-11,Magnetoresistance generated from charge-spin conversion by anomalous Hall effect in metallic ferromagnetic/nonmagnetic bilayers,"A theoretical formulation of magnetoresistance effect in a metallic ferromagnetic/nonmagnetic bilayer originated from the charge-spin conversion by the anomalous Hall effect is presented. Analytical expressions of the longitudinal and transverse resistivities in both nonmagnet and ferromagnet are obtained by solving the spin diffusion equation. The magnetoresistance generated from charge-spin conversion purely caused by the anomalous Hall effect in the ferromagnet is found to be proportional to the square of the spin polarizations in the ferromagnet and has fixed sign. We also find additional magnetoresistances in both nonmagnet and ferromagnet arising from the mixing of the spin Hall and anomalous Hall effects. The sign of this mixing resistance depends on those of the spin Hall angle in the nonmagnet and the spin polarizations of the ferromagnet.",1608.03359v2 2016-12-19,Spontaneous Distortion and Ferromagnetism Induced by Quantum-well States in Pd(100) Ultrathin Films,"We study the crystal structure of Pd(100) ultrathin films, which show ferromagnetism induced by the quantum confinement effect, using in-situ X-ray crystal truncation rod measurement and density functional calculation. The energy gain due to the appearance of ferromagnetism in Pd results in flatter and uniform film growth of ferromagnetic Pd films compared with paramagnetic Pd. In addition, ferromagnetic Pd films expand the lattice constant in order to suppress the increase in kinetic energy of electrons accompanied by the occurrence of exchange splitting. Although the traditional theory of magnetism in metals indicates that the increase in density of states that induces ferromagnetism (Stoner criterion), our present finding reveals a mechanism of modulation in the density of states via the appearance of ferromagnetism, i.e., the inverse mechanism of Stoner's theory.",1612.06216v1 2017-06-24,Random-field-induced disordering mechanism in a disordered ferromagnet: Between the Imry-Ma and the standard disordering mechanism,"Random fields disorder Ising ferromagnets by aligning single spins in the direction of the random field in three space dimensions, or by flipping large ferromagnetic domains at dimensions two and below. While the former requires random fields of typical magnitude similar to the interaction strength, the latter Imry-Ma mechanism only requires infinitesimal random fields. Recently, it has been shown that for dilute anisotropic dipolar systems a third mechanism exists, where the ferromagnetic phase is disordered by finite-size glassy domains at a random field of finite magnitude that is considerably smaller than the typical interaction strength. Using large-scale Monte Carlo simulations and zero-temperature numerical approaches, we show that this mechanism applies to disordered ferromagnets with competing short-range ferromagnetic and antiferromagnetic interactions, suggesting its generality in ferromagnetic systems with competing interactions and an underlying spin-glass phase. A finite-size-scaling analysis of the magnetization distribution suggests that the transition might be first order.",1706.07904v3 2017-10-30,RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$: From a ferromagnetic superconductor to a superconducting ferromagnet,"The intrinsically hole-doped RbEuFe$_4$As$_4$ exhibits bulk superconductivity at $T_{\mathrm{sc}}=36.5$ K and ferromagnetic ordering in the Eu sublattice at $T_\mathrm{m}=15$ K. Here we present a hole-compensation study by introducing extra itinerant electrons via a Ni substitution in the ferromagnetic superconductor RbEuFe$_4$As$_4$ with $T_{\mathrm{sc}}>T_{\mathrm{m}}$. With the Ni doping, $T_{\mathrm{sc}}$ decreases rapidly, and the Eu-spin ferromagnetism and its $T_{\mathrm{m}}$ remain unchanged. Consequently, the system RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ transforms into a superconducting ferromagnet with $T_{\mathrm{m}}>T_{\mathrm{sc}}$ for $0.07\leq x\leq0.08$. The occurrence of superconducting ferromagnets is attributed to the decoupling between Eu$^{2+}$ spins and superconducting Cooper pairs. The superconducting and magnetic phase diagram is established, which additionally includes a recovered yet suppressed spin-density-wave state.",1710.10895v2 2018-01-26,Enhancement of exchange bias in ferromagnetic/antiferromagnetic core-shell nanoparticles through ferromagnetic domain wall formation,"The spin configuration in the ferromagnetic part during the magnetization reversal plays a crucial role in the exchange bias effect. Through Monte Carlo simulation, the exchange bias effect in ferromagnetic-antiferromagnetic core-shell nanoparticles is investigated. Magnetization reversals in the ferromagnetic core were controlled between the coherent rotation and the domain wall motion by modulating ferromagnetic domain wall width with parameters of uniaxial anisotropy constant and exchange coupling strength. An anomalous monotonic dependence of exchange bias on the uniaxial anisotropy constant is found in systems with small exchange coupling, showing an obvious violation of classic Meiklejohn-Bean model, while domain walls are found to form close to the interface and propagate in the ferromagnetic core with larger uniaxial anisotropy in both branches of the hysteresis. The asymmetric magnetization reversal with the formation of a spherical domain wall dramatically reduces the coercive field in the ascending branch, leading to the enhancement of the exchange bias. The results provide another degree of freedom to optimize the magnetic properties of magnetic nanoparticles for applications.",1801.08743v1 2017-09-27,Effects of geometrical frustration on ferromagnetism in the Hubbard model on the Shastry-Sutherland lattice,"The small-cluster exact-diagonalization calculations and the projector quantum Monte Carlo method are used to examine the competing effects of geometrical frustration and interaction on ferromagnetism in the Hubbard model on the Shastry-Sutherland lattice. It is shown that the geometrical frustration stabilizes the ferromagnetic state at high electron concentrations ($n \gtrsim 7/4$), where strong correlations between ferromagnetism and the shape of the noninteracting density of states are observed. In particular, it is found that ferromagnetism is stabilized only for these values of frustration parameters, which lead to the single peaked noninterating density of states at the band edge. Once, two or more peaks appear in the noninteracting density of states at the band egde the ferromagnetic state is suppressed. This opens a new route towards the understanding of ferromagnetism in strongly correlated systems.",1709.09461v1 2019-03-16,$S$ =1/2 ferromagnetic Heisenberg chain in a verdazyl-based complex,"We present a model compound for the $S$=1/2 ferromagnetic Heisenberg chain composed of the verdazyl-based complex $[$Zn(hfac)$_2]$$[$4-Cl-$o$-Py-V-(4-F)$_2]$. $Ab$ $initio$ MO calculations indicate a predominant ferromagnetic interaction forming an $S$=1/2 ferromagnetic chain. The magnetic susceptibility and specific heat indicate a phase transition to an AF order owing to the finite interchain couplings. We explain the magnetic susceptibility and magnetization curve above the phase transition temperature based on the $S$=1/2 ferromagnetic Heisenberg chain. The magnetization curve in the ordered phase is described by a conventional AF two-sublattice model. Furthermore, the obtained magnetic specific heat reproduces the almost temperature-independent behavior of the $S$=1/2 ferromagnetic Heisenberg chain. In the low-temperature region, the magnetic specific heat exhibits $\sqrt{T}$ dependence, which is attributed to the energy dispersion in the ferromagnetic chain.",1903.06896v1 2019-04-02,Quantum storage in quantum ferromagnets,"We must protect inherently fragile quantum data to unlock the potential of quantum technologies. A pertinent concern in schemes for quantum storage is their potential for near-term implementation. Since Heisenberg ferromagnets are readily available, we investigate their potential for robust quantum storage. We propose to use permutation-invariant quantum codes to store quantum data in Heisenberg ferromagnets, because the ground space of any Heisenberg ferromagnet must be symmetric under any permutation of the underlying qubits. By exploiting an area law on the expected energy of Pauli errors, we show that increasing the effective dimension of Heisenberg ferromagnets can improve the storage lifetime. When the effective dimension of Heisenberg ferromagnets is maximal, we also obtain an upper bound for the storage error. This result relies on perturbation theory, where we use Davis' divided difference representation for Fr{\'e}chet derivatives along with the recursive structure of these divided differences. Our numerical bounds allow us to better understand how quantum memory lifetimes can be enhanced in Heisenberg ferromagnets.",1904.01458v3 2020-03-30,Impact of Next-Nearest-Neighbor hopping on Ferromagnetism in Diluted Magnetic Semiconductors,"Being a wide band gap system GaMnN attracted considerable interest after the discovery of highest reported ferromagnetic transition temperature $T_C$ $\sim$ 940 K among all diluted magnetic semiconductors. Later it become a debate due to the observation of either a ferromagnetic state with very low $T_C$ $\sim$ 8 K or sometimes no ferromagnetic state at all. We address these issues by calculating the ferromagnetic window, $T_C$ Vs $p$, within a $t-t'$ Kondo lattice model using a spin-fermion Monte-Carlo method on a simple cubic lattice. We exploit the next-nearest-neighbor hopping $t'$ to tune the degree of delocalization of the free carriers and show that carrier localization (delocalization) significantly widen (shrunken) the ferromagnetic window with a reduction (enhancement) of the optimum $T_C$. We connect our results with the experimental findings and try to understand the ambiguities in ferromagnetism in GaMnN.",2003.13327v2 2020-11-27,d0 Ferromagnetism in Ag-doped Monoclinic ZrO2 Compounds,"Recently d0 or intrinsic ferromagnetism was believed to provide an alternative pathway to transition metal induced ferromagnetism in oxide. In pursuit of augmenting the area of d0 ferromagnetism; we have undertaken to study the crystal structure and magnetic properties of Ag-doped ZrO2 compounds. Polycrystalline samples of Zr1-xAgxO2 (with x=0, 0.02, 0.04, 0.06 and 0.08) were prepared by solid-state reaction route. All the prepared compounds are found to crystallize in monoclinic symmetry of ZrO2. In our study, pure ZrO2 compound exhibits paramagnetic behavior. However, the Ag-doped ZrO2 compounds exhibit ferromagnetic to paramagnetic transition. The Curie temperature was found to increase from 28.7 K for x=0.02 to 173.2 K for x= 0.08 doped ZrO2. Thus, the introduction of Ag in ZrO2 induces ferromagnetism with a large ThetaC. The measurements of hysteresis curves indicate that Ag doped ZrO2 compounds exhibit hysteresis loops with a coercivity of around 1350 Oe. Moreover, increase in Ag concentration resulted increase in the value of saturation magnetization (MS); the maximum value of MS was recorded as 0.01 {\mu}B/Ag ion for x= 0.06 sample. The sintering of sample at high temperature (13500C) diminishes the ferromagnetism and it leads to paramagnetic behaviour.",2011.13727v1 2021-12-29,A unified theory of ferromagnetic quantum phase transitions in heavy fermion metals,"Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh$_6$Ge$_4$ and its distinction from other U-based heavy fermion metals such as UGe$_2$, we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large-$N$ Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three different ferromagnetic phases including a half-metallic spin selective Kondo insulator with a constant magnetization. The Fermi surface topologies are found to change abruptly between different phases, consistent with that observed in UGe$_2$. At finite temperatures, we predict the development of Kondo hybridization well above the ferromagnetic long-range order and its relocalization near the phase transition, in good agreement with band measurements in CeRh$_6$Ge$_4$. Our results highlight the importance of magnetic anisotropy and provide a unified theory for understanding the ferromagnetic quantum phase transitions in heavy fermion metals.",2112.14515v1 2022-01-26,Local ferromagnetic resonance measurements of mesoscopically patterned ferromagnets using deterministically placed nanodiamonds,"Nitrogen-vacancy centers in diamond have recently been established as effective sensors of the magnetization dynamics in vicinal ferromagnetic materials. We demonstrate sub-100 nm placement accuracy of nitrogen-vacancy-containing nanodiamonds and use these as local sensors that probe optically detected ferromagnetic resonance in mesoscopically patterned Permalloy islands. These measurements reveal variations in the ferromagnetic resonance signal at different sites on these structures with distinct behavior in the edge and the bulk of patterned features. These test measurements establish an easily implemented approach for spatially targeted measurements of spin dynamics in mesoscale ferromagnets. In principle, the methodology can also be extended to local studies of nanoscale ferromagnets such as single magnetic nanowires and nanoparticles.",2201.10722v1 2022-02-03,On the Divergence of the Ferromagnetic Susceptibility in the SU(N) Nagaoka-Thouless Ferromagnet,"Using finite temperature strong coupling expansions for the SU(N) Hubbard Model, we calculate the thermodynamic properties of the model in the infinite-$U$ limit for arbitrary density $0\leq \rho \leq 1$ and all $N$. We express the ferromagnetic susceptibility of the model as a Curie term plus a $\Delta \chi$, an excess susceptibility above the Curie-behavior. We show that, on a bipartite lattice, graph by graph the contributions to $\Delta \chi$ are non-negative in the limit that the hole density $\delta=1-\rho$ goes to zero. By summing the contributions from all graphs consisting of closed loops we find that the low hole-density ferromagnetic susceptibility diverges exponentially as $\exp{\Delta /T}$ as $T \to 0$ in two and higher dimensions. This demonstrates that Nagaoka-Thouless ferromagnetic state exists as a thermodynamic state of matter at low enough density of holes and sufficiently low temperatures. The constant $\Delta$ scales with the SU(N) parameter $N$ as $1/N$ implying that ferromagnetism is gradually weakened with increasing $N$ as the characteristic temperature scale for ferromagnetic order goes down.",2202.01611v1 2022-04-04,Hole-doping induced ferromagnetism in 2D materials,"Two-dimensional (2D) ferromagnetic materials are considered as promising candidates for the future generations of spintronic devices. Yet, 2D materials with intrinsic ferromagnetism are scarce. High-throughput first-principles simulations are performed in order to screen 2D materials that present a non-magnetic to a ferromagnetic transition upon hole doping. A global evolutionary search is subsequently performed, in order to identify alternative possible atomic structures of the eligible candidates, and 122 materials exhibiting a hole-doping induced ferromagnetism are identified. Their energetic and dynamic stability, as well as their magnetic properties under hole doping are investigated systematically. Half of these 2D materials are metal halides, followed by chalcogenides, oxides and nitrides, some of them having predicted Curie temperatures above 300 K. The exchange interactions responsible for the ferromagnetic order in these 2D materials are also discussed. This work not only provides theoretical insights into hole-doped 2D ferromagnetic materials, but also enriches the family of 2D magnetic materials for possible spintronic applications.",2204.01551v2 2022-06-12,Interference phenomena in Josephson junctions with ferromagnetic bilayers: Spin-triplet correlations and resonances,"We study the Josephson effect in planar $SF_1F_2S$ junctions that consist of conventional $s$-wave superconductors ($S$) connected by two metallic monodomain ferromagnets ($F_1$ and $F_2$) with arbitrary transparency of interfaces. We solve the scattering problem in the clean limit based on the Bogoliubov-de Gennes equation for both spin-singlet and odd in frequency spin-triplet pairing correlations. We calculate numerically the Josephson current-phase relation $I(\phi)$. While the first harmonic of $I(\phi)$ is completely generated by spin-singlet and short-range spin-triplet superconducting correlations, for noncollinear magnetizations of ferromagnetic layers the second harmonic has an additional long-range spin-triplet component. Therefore, for strong ferromagnetic influence, the long-range spin-triplet contribution to the second harmonic dominates. We find an exception due to the geometric resonance for equal ferromagnetic layers when the first harmonic is strongly enhanced. Both first and second harmonic amplitudes oscillate with ferromagnetic layer thicknesses due to $0-\pi$ transitions. We study the influence of interface transparencies and find additional resonances for finite transparency of interface between ferromagnetic layers.",2206.05770v3 2023-08-04,Anisotropy of the spin Hall effect in a Dirac ferromagnet,"We study the intrinsic spin Hall effect of a Dirac Hamiltonian system with ferromagnetic exchange coupling, a minimal model combining relativistic spin-orbit interaction and ferromagnetism. The energy bands of the Dirac Hamiltonian are split after introducing a Stoner-type ferromagnetic ordering which breaks the spherical symmetry of pristine Dirac model. The totally antisymmetric spin Hall conductivity (SHC) tensor becomes axially anisotropic along the direction of external electric field. Interestingly, the anisotropy does not vanish in the asymptotic limit of zero magnetization. We show that the ferromagnetic ordering breaks the spin degeneracy of the eigenfunctions and modifies the selection rules of the interband transitions for the intrinsic spin Hall effect. The difference in the selection rule between the pristine and the ferromagnetic Dirac phases causes the anisotropy of the SHC, resulting in a discontinuity of the SHC as the magnetization, directed orthogonal to the electric field, is reduced to zero in the ferromagnetic Dirac phase and enters the pristine Dirac phase.",2308.02336v1 2023-08-29,Theory of Fractionally-magnetized Quantum Ferromagnet,"We present a theory to realize entangled quantum spin states with fractional magnetization. The origin of magnetization reduction is partly emergent antiferromagnetism, that is, spin-liquefaction of ferromagnetism. We study a ferromagnetic bilinear coupling region of the spin-$S$ $({\geqq} 1)$ bilinear-biquadratic spin chain based on (i) a rigorous eigenstate correspondence between the spin-$S$ model and spin-$\frac12$ model and (ii) a numerical exact-diagonalization calculation up to $S=3$. As a result, we obtain a fractional magnetized $M=1-1/(2S)$ phase, where ground states have quantum entanglement-reflecting corresponding spin-$\frac12$ antiferromagnetic ground states in a ferromagnetic background. This spin-liquefaction theory of ferromagnets can be generalized to any-dimensional lattices even under a magnetic field. This fractional ferromagnetism opens the new research field of quantum ferromagnets.",2308.15372v1 2023-10-03,Stray magnetic fields from elliptical-shaped and stadium-shaped ferromagnets,"An artificial spin ice consisting of numerous ferromagnets has attracted attention because of its applicability to practical devices. The ferromagnets interact through their stray magnetic field and show various functionality. The ferromagnetic element in the spin ice was recently made in elliptical-shape or stadium-shape. The former has a narrow edge, expecting to generate a large stray magnetic field. The latter has a large volume and is also expected to generate a large stray magnetic field. Here, we estimate the stray magnetic field by numerically integrating the solution of the Poisson equation. When magnetization is parallel to an easy axis, the elliptical-shaped ferromagnet generates a larger stray magnetic field than the stadium-shaped ferromagnet. The stray magnetic fields from both ferromagnets for arbitrary magnetization directions are also investigated.",2310.01721v2 2023-10-09,The influence of metallic overlayers on ferromagnetism in LaMnO$_3$,"LaMnO$_3$ (LMO) thin films epitaxially grown on SrTiO$_3$ (STO) usually exhibit ferromagnetism above a critical layer thickness. We report the use of scanning SQUID microscopy (SSM) to study the suppression of the ferromagnetism in STO/LMO/metal structures. By partially covering the LMO surface with a metallic layer, both covered and uncovered LMO regions can be studied simultaneously. While Au does not significantly influence the ferromagnetic order of the underlying LMO film, a thin Ti layer induces a strong suppression of the ferromagnetism, over tens of nanometers, and a large change in the out-of-plane lattice parameter. We relate the suppression of the ferromagnetism to the scavenging of oxygen and diffusion of Ti approximately 5 nanometers deep into the film, which takes place at timescales of days. Furthermore, we demonstrate that by patterning Ti/Au overlayers, we can define ferromagnetic structures down to sub-micrometer scales.",2310.05514v1 2023-12-23,Room temperature ferromagnetic semiconductors through metal-semiconductor transition in monolayer MnSe2,"To realize room temperature ferromagnetic semiconductors is still a challenge in spintronics. Recent experiments have obtained two-dimensional (2D) room temperature ferromagnetic metals, such as monolayer MnSe2. In this paper, we proposed a way to obtain room temperature ferromagnetic semiconductors through metal-semiconductor transition. By the density functional theory calculations, a room temperature ferromagnetic semiconductor is obtained in monolayer MnSe2 with a few percent tensile strains, where a metal-semiconductor transition occurs with 2.2% tensile stain. The tensile stains raise the energy of d orbitals of Mn atoms and p orbitals of Se atoms near the Fermi level, making the Fermi level sets in the energy gap of bonding and antibonding states of these p and d orbitals, and opening a small band gap. The room temperature ferromagnetic semiconductors are also obtained in the heterostructures MnSe2/X (X = Al2Se3, GaSe, SiH, and GaP), where metal-semiconductor transition happens due to the tensile strains by interface of heterostructures. In addition, a large magneto-optical Kerr effect (MOKE) is obtained in monolayer MnSe2 with tensile strain and MnSe2-based heterostructures. Our theoretical results pave a way to obtain room temperature magnetic semiconductors from experimentally obtained 2D room temperature ferromagnetic metals through metal-semiconductor transitions.",2312.15259v1 2024-02-11,Quantum geometric bound for saturated ferromagnetism,"Despite its abundance in nature, predicting the occurrence of ferromagnetism in the ground state is possible only under very limited conditions such as in a flat band system with repulsive interaction or in a band with a single hole under infinitely large Coulomb repulsion, etc. Here, we propose a general condition to achieve saturated ferromagnetism based on the quantum geometry of electronic wave functions in itinerant electron systems. By analyzing multi-band repulsive Hubbard models with an integer band filling, relevant to either ferromagnetic insulators or semimetals, we propose a rigorous quantum geometric upper bound on the spin stiffness. By employing this geometric bound, we establish that saturated ferromagnetism is prohibited in the absence of interband coupling, even when the local Hubbard repulsion is infinitely large. As a corollary, this shows that saturated ferromagnetism is forbidden in any half-filled Hubbard model. We also derive the condition that the upper bound of the spin stiffness can be completely characterized by the Abelian quantum metric. We believe that our findings reveal a profound connection between quantum geometry and ferromagnetism, which can be extended to various symmetry-broken ground states in itinerant electronic systems.",2402.07171v1 2007-01-05,Colossal electroresistance in ferromagnetic insulating state of single crystal Nd$_0.7$Pb$_0.3$MnO$_3$,"Colossal electroresistance (CER) has been observed in the ferromagnetic insulating (FMI) state of a manganite. Notably, the CER in the FMI state occurs in the absence of magnetoresistance (MR). Measurements of electroresistance (ER) and current induced resistivity switching have been performed in the ferromagnetic insulating state of a single crystal manganite of composition Nd$_0.7$Pb$_0.3$MnO$_3$ (NPMO30). The sample has a paramagnetic to ferromagnetic (Curie) transition temperature, Tc = 150 K and the ferromagnetic insulating state is realized for temperatures, T <~ 130 K. The colossal electroresistance, arising from a strongly nonlinear dependence of resistivity ($\rho$) on current density (j), attains a large value ($\approx 100%$) in the ferromagnetic insulating state. The severity of this nonlinear behavior of resistivity at high current densities is progressively enhanced with decreasing temperature, resulting ultimately, in a regime of negative differential resistivity (NDR, d$\rho$/dj < 0) for temperatures <~ 25 K. Concomitant with the build-up of the ER however, is a collapse of the MR to a small value (< 20%) even in magnetic field, H = 7 T. This demonstrates that the mechanisms that give rise to ER and MR are effectively decoupled in the ferromagnetic insulating phase of manganites. We establish that, the behavior of ferromagnetic insulating phase is distinct from the ferromagnetic metallic (FMM) phase as well as the charge ordered insulating (COI) phase, which are the two commonly realized ground state phases of manganites.",0701090v1 2008-03-03,Phase separation and the effect of quenched disorder in $Pr_{0.5}Sr_{0.5}MnO_3$,"The nature of phase separation in $Pr_{0.5}Sr_{0.5}MnO_3$ has been probed by linear as well as nonlinear magnetic susceptibilities and resistivity measurements across the 2nd order paramagnetic to ferromagnetic transition ($T_C$) and 1st order ferromagnetic to antiferromagnetic transition ($T_N$). We found that the ferromagnetic (metallic) clusters, which form with the onset of long-range order in the system at $T_C$, continuously decrease their size with the decrease in temperature and coexist with non-ferromagnetic (insulating) clusters. These non-ferromagnetic clusters are identified to be antiferromagnetic. Significantly, it is shown that they do not arise because of the superheating effect of the lower temperature 1st order transition. Thus reveals unique phase coexistence in a manganite around half-doping encompassing two long-range order transitions. Both the ferromagnetic and antiferromagnetic clusters form at $T_C$ and persist much below $T_N$. Substitution of quenched disorder (Ga) at Mn-site promotes antiferromagnetism at the cost of ferromagnetism without adding any magnetic interaction or introducing any significant lattice distortion. Moreover, increase in disorder decreases the ferromagnetic cluster size and with 7.5% Ga substitution clusters size reduces to the single domain limit. Yet, all the samples show significant short-range ferromagnetic interaction much above $T_C$. Resistivity measurements also reveal the novel phase coexistence identified from the magnetic measurements. It is significant that, increase in disorder up to 7.5% increases the resistivity of the low temperature antiferromagnetic phase by about four orders.",0803.0085v1 2011-01-25,Charge and spin transport through a ferromagnet/insulator/unconventional superconductor junction,"We analyze the charge and spin transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. For the ferromagnetic side we assume that ferromagnetism may be driven by an unequal mass renormalization of oppositely polarized carriers, i.e. a spin bandwidth asymmetry, and/or by a rigid splitting of up-and down-spin electron bands, as in a standard Stoner ferromagnet, whereas the superconducting side is assumed to exhibit a d-wave symmetry of the order parameter, which can be pure or accompanied by a minority component breaking time-reversal symmetry. Several remarkable features in the charge conductance arise in this kind of junction, providing useful information about the mechanism of ferromagnetism in the ferromagnetic electrode, as well as of the order parameter symmetry in the superconducting one. In particular, we show that when a time-reversal symmetry breaking superconductor is considered, the use of the two kinds of ferromagnet mentioned above represents a valuable tool to discriminate between the different superconducting mixed states. We also explain how this junction may mimic a switch able to turn on and off a spin current, leaving the charge conductance unchanged, and we show that for a wide range of insulating barrier strengths, a spin bandwidth asymmetry ferromagnet may support a spin current larger than a standard Stoner one.",1101.4782v1 2014-04-05,Theory of ferromagnetic superconductors with spin-triplet electron pairing,"A general quasi-phenomenological theory that describes phases and phase transitions of ferromagnetic superconductors with spin-triplet electron Cooper pairing is presented. The theory is based on extended Ginzburg-Landau expansion in powers of superconducting and ferromagnetic order parameters. A simple form for the dependence of theory parameters on the pressure ensures a correct theoretical outline of the temperature-pressure phase diagram where a stable phase of coexistence of p-wave superconductivity and itinerant ferromagnetism appears. This new theory is in an excellent agreement with the experimental data for intermetallic compounds, for example, UGe2, URhGe, UCoGe, and UIr that are experimentally proven to be itinerant ferromagnets exhibiting spin-triplet superconductivity. The mechanism of appearance of superconductivity due to itinerant ferromagnetism ($M$-trigger effect) is established and demonstrated. On the basis of the same theory, basic features of quantum phase transitions in this type of ferromagnetic superconductors are explained in agreement with the experimental data. The theory allows for a classification of the spin-triplet ferromagnetic superconductors in two different types: type I and type II. The classification is based on quantitative criteria, i.e., on simple relations between theory parameters. Both theory and experiment indicate that the two types of p-wave ferromagnetic superconductors are well distinguished by essential differences in their physical properties.",1404.1497v1 2014-12-08,Voltage Controlled Magnetic Anisotropy Based Low Energy Switching of a Ferromagnet on a Topological Insulator,"We present a novel memory device that consists of a thin ferromagnetic layer of Fe deposited on topological insulator thin film, Bi2Se3. The ferromagnetic layer has perpendicular anisotropy, due to MgO deposited on the top surface of Fe. When current is passed on the surface of Bi2Se3, the surface of the Bi2Se3 becomes spin polarized and strong exchange interaction occurs between the d electrons in the ferromagnet and the electrons conducting the current on the surface of the Bi2Se3. Part of the current is shunted through the ferromagnet which generates spin transfer torque in the ferromagnet. The combination of the spin transfer torque and exchange interaction torque along with voltage-controlled magnetic anisotropy (VCMA) allows ultralow-energy switching of the ferromagnet. We perform micromagnetic simulations and predict switching time of the order of 2.5 ns and switching energy of the order of 0.45fJ for a ferromagnetic bit with thermal stability of 43kBT. Such ultralow-energy and high-speed VCMA-induced switching of a perpendicular anisotropy ferromagnet on a topological insulator could be utilized for energy-efficient memory design.",1412.2434v3 2017-09-18,Magnetic field driven enhanced ferroelectric switching in self-grown ferroelectric-ferromagnetic composite in the BiFeO3-BaTiO3 multiferroic alloy system,"Over the years attempts have been made to compensate for the inherent weaknesses in the bulk state of the multiferroic BiFeO3, such as high leakage current and the absence of ferromagnetic correlation, and exploit its magnetoelectric potential by forming solid solutions with other perovskites. Studies in the recent few years have shown that alloying of BiFeO3 with BaTiO3, both with and without additives, can induce both ferroelectric and ferromagnetic switching. While the coexistence of both the ferroic orders is encouraging from the view point of technological applications, the origin of ferromagnetism in this system remains elusive. Here, we synthesized such compositions and carried out a detailed structural analysis employing magnetic separation of the powder particles. We found that the origin of ferromagnetism lies in the spontaneous precipitation of a minor ( ~ 1 wt %) barium hexaferrite (BaFe9O19) phase, often undetected in routine x-ray diffraction studies of powders sampled from the entire specimen. We also demonstrate that inspite of the insignificant fraction the ferrimagnetic phase, this self-grown composite exhibit noticeably enhanced ferroelectric switching in the presence of external magnetic field. We obtained a dc magnetoelectric coupling of ~ 9 x 10-8 s/m, a value which is comparable to what has been reported for layered ferroelectric/ferromagnetic laminates and bilayer thin film ferroelectric-ferromagnetic hetrostructures. Our study suggests that reasonably large magnetoelectric coupling is realizable in simple 0-3 ferroelectric-ferromagnetic bulk composites provided synthesis strategies are developed which induces spontaneous precipitation of the ferromagnetic phase in small volume fraction to ensure good insulating behaviour of the composite thus developed.",1709.05898v1 2020-05-30,Magnetization dynamics in proximity-coupled superconductor/ferromagnet/superconductor multilayers,"In this work, magnetization dynamics is studied in superconductor/ferromagnet/superconductor three-layered films in a wide frequency, field, and temperature ranges using the broad-band ferromagnetic resonance measurement technique. It is shown that in presence of both superconducting layers and of superconducting proximity at both superconductor/ferromagnet interfaces a massive shift of the ferromagnetic resonance to higher frequencies emerges. The phenomenon is robust and essentially long-range: it has been observed for a set of samples with the thickness of ferromagnetic layer in the range from tens up to hundreds of nanometers. The resonance frequency shift is characterized by proximity-induced magnetic anisotropies: by the positive in-plane uniaxial anisotropy and by the drop of magnetization. The shift and the corresponding uniaxial anisotropy grow with the thickness of the ferromagnetic layer. For instance, the anisotropy reaches 0.27~T in experiment for a sample with 350~nm thick ferromagnetic layer, and about 0.4~T in predictions, which makes it a ferromagnetic film structure with the highest anisotropy and the highest natural resonance frequency ever reported. Various scenarios for the superconductivity-induced magnetic anisotropy are discussed. As a result, the origin of the phenomenon remains unclear. Application of the proximity-induced anisotropies in superconducting magnonics is proposed as a way for manipulations with a spin-wave spectrum.",2006.00348v1 2022-06-07,Spin and charge transport in ferromagnet-superconductor-ferromagnet heterostructures: Stoner versus spin mass mismatch mechanism,"We study transport phenomena through a ballistic ferromagnet-superconductor-ferromagnet (F/S/F) junction, comparing the case in which the ferromagnetic order in the two F layers is of the standard Stoner type with the case where it is driven by a spin mass mismatch (SMM). It is shown that the two mechanisms lead to a different behavior in the charge and the spin conductances, especially when compared to the corresponding non-superconducting ferromagnet-normal-ferromagnet (F/N/F) junctions. In particular, when the injected current is perpendicular to the barrier, for high barrier transparency and large magnetization of the F layers, the large mass mismatch gives rise to an enhancement of both low-bias charge and spin conductances of the F/S/F junction, which is not observed in the equal-mass case. When all the allowed injection directions are considered, the low bias enhancement of the charge conductance for SMM leads still holds for high barrier transparency and large magnetization of the F layers. However, in the case of non-transparent interfaces, spin transport with SMM ferromagnets exhibits an opposite sign response with respect to the Stoner case at high biases for all magnetization values, also manifesting a significant amplification induced by superconductivity at the gap edge. The above mentioned differences can be exploited to probe the nature of the electronic mechanism underlying the establishment of the ferromagnetic order in a given material.",2206.03335v1 2023-06-11,Ferromagnetic Superconductivity in Two-dimensional Niobium Diselenide,"The co-existence of ferromagnetism and superconductivity becomes possible through unconventional pairing in the superconducting state. Such materials are exceedingly rare in solid-state systems but are promising platforms to explore topological phases, such as Majorana bound states. Theoretical investigations date back to the late 1950s, but only a few systems have so far been experimentally identified as potential hosts. Here, we show that atomically-thin niobium diselenide (NbSe$_2$) intercalated with dilute cobalt atoms spontaneously displays ferromagnetism below the superconducting transition temperature ($T_c$). We elucidate the origin of this phase by constructing a magnetic tunnel junction that consists of cobalt and cobalt-doped niobium diselenide (Co-NbSe$_2$) as the two ferromagnetic electrodes, with an ultra-thin boron nitride as the tunnelling barrier. At a temperature well below $T_c$, the tunnelling magnetoresistance shows a bistable state, suggesting a ferromagnetic order in Co-NbSe$_2$. We propose a RKKY exchange coupling mechanism based on the spin-triplet superconducting order parameter to mediate such ferromagnetism. We further perform non-local lateral spin valve measurements to confirm the origin of the ferromagnetism. The observation of Hanle precession signals show spin diffusion length up to micrometres below Tc, demonstrating an intrinsic spin-triplet nature in superconducting NbSe$_2$. Our discovery of superconductivity-mediated ferromagnetism opens the door to an alternative design of ferromagnetic superconductors",2306.06659v1 2023-09-21,Ferromagnetic quantum critical point in a locally noncentrosymmetric and nonsymmorphic Kondo metal,"Quantum critical points (QCPs), zero-temperature phase transitions, are windows to fundamental quantum-mechanical phenomena associated with universal behaviour and can provide parallels to the physics of black holes. Magnetic QCPs have been extensively investigated in the vicinity of antiferromagnetic order. However, QCPs are rare in metallic ferromagnets due to the coupling of the order parameter to electronic soft modes [1,2]. Recently, antisymmetric spin-orbit coupling in noncentrosymmetric systems was suggested to protect ferromagnetic QCPs [3]. Nonetheless, multiple centrosymmetric materials host FM QCPs, suggesting a more general mechanism behind their protection. In this context, CeSi$_{2-\delta}$, a dense Kondo lattice crystallising in a centrosymmetric structure, exhibits ferromagnetic order when Si is replaced with Ag. We report that the Ag-substitution controls the strength of the Kondo coupling, leading to a transition between paramagnetic and ferromagnetic Kondo phases. Remarkably, a ferromagnetic QCP accompanied by concurrent strange-metal behaviour emerges. Herein, we suggest that, despite the centrosymmetric structure, spin-orbit coupling arising from the local noncentrosymmetric structure, in combination with nonsymmorphic symmetry, can protect ferromagnetic QCPs. Our findings present a unique example of Kondo coupling-driven ferromagnetic QCP through chemical doping and offer a general guideline for discovering new ferromagnetic QCPs.",2309.12497v1 1993-12-03,Dynamic Domains Above the Ferromagnetic Resonance Instability,"A simplified model is introduced and analysed to show, that for the Landau-Lifshitz equation stable, steady state solutions of domain type exist in ferromagnetic systems, strongly driven by external transverse fields. These dynamic domain states are able to describe the drastic reduction of the power absorption found experimentally above the instability of the homogeneous magnetisation. The excitations of the domain states are presented and the relevance of the model for real ferromagnets is discussed.",9312015v1 1995-12-06,Ferromagnetism in the Hubbard Model on the Infinite Dimensional fcc Lattice,"The Hubbard model on the fcc lattice is studied in the limit of infinite spatial dimensions. At sufficiently strong interaction finite temperature Quantum Monte Carlo calculations yield a second order phase transition to a highly polarized ferromagnetic state. Ferromagnetism is stable over a wide hole doping regime but not for electron doping. A possible non-Fermi liquid behavior is discussed.",9512044v1 1997-07-29,Spin Wave Instability of Itinerant Ferromagnet,"We show variationally that instability of the ferromagnetic state in the Hubbard model is largely controlled by softening of a long-wavelength spin-wave excitation, except in the over-doped strong-coupling region where the individual-particle excitation becomes unstable first. A similar conclusion is drawn also for the double exchange ferromagnet. Generally the spin-wave instability may be regarded as a precursor of the metal-insulator transition.",9707300v1 1999-01-14,Spin dynamics in the generalized ferromagnetic Kondo model for manganites,"Dynamical spin susceptibility is calculated for the generalized ferromagnetic Kondo model which describes itinerant $e_{g}$ electrons interacting with localized $t_{2g}$ electrons with antiferromagnetic coupling. The calculations done in the mean field approximation show that the spin-wave spectrum of the system in ferromagnetic state has two branches, acoustic and optic ones. Self-energy corrections to the spectrum are considered and the acoustic spin-wave damping is evaluated.",9901141v1 2000-03-10,Quantum spin hydrodynamics and a new spin-current mode in ferromagnetic metals,"We derive the quantum spin hydrodynamic equations in a ferromagnetic metal. From these equations we show the existence of a new massive spin-current mode. This mode can be observed in neutron scattering experiments and we discuss the difficulties in seeing it. At the end we discuss the existence of this mode in localized ferromagnets.",0003187v2 2000-07-31,Ferromagnetic ordering in a generalized Hubbard model: weak intra-atomic interaction limit,"In the present work ferromagnetic ordering in the Hubbard model generalized by taking into account the inter-atomic exchange interaction and correlated hopping in partially filled narrow band is considered. Expressions for the magnetization and Curie temperature as functions of model parameters and band filling are obtained in the case of weak intra-atomic Coulomb interaction. Condition of ferromagnetic state realization is found. The obtained results indicate the important role of correlated hopping.",0007501v1 2000-12-22,The re-examined phenomenological phase transitions theory for ferromagnets,"The existence of the linear on the order parameter term of the thermodynamic functions expansion near the critical point is justified. The criticism of the arguments, used for the rejection of the odd-power expansion terms of the ferromagnets thermodynamic functions is presented. It is shown, that taking into account the linear term in expansion one achieves the consentency with experimental data on the magnetization behavior near the transition temperature in ferromagnets.",0012431v1 2001-06-06,Ferromagnetism in a lattice of Bose condensates,"We show that an ensemble of spinor Bose-Einstein condensates confined in a one dimensional optical lattice can undergo a ferromagnetic phase transition and spontaneous magnetization arises due to the magnetic dipole-dipole interaction. This phenomenon is analogous to ferromagnetism in solid state physics, but occurs with bosons instead of fermions.",0106118v1 2001-07-13,Skyrmion Physics in Bose-Einstein Ferromagnets,"We show that a ferromagnetic Bose-Einstein condensate has not only line-like vortex excitations, but in general also allows for point-like topological excitations, i.e., skyrmions. We discuss the thermodynamic stability and the dynamic properties of these skyrmions for both spin-1/2 and ferromagnetic spin-1 Bose gases.",0107302v1 2001-11-29,Origin of room-temperature ferromagnetism in Mn doped semiconducting CdGeP2,"CdGeP2 chalcopyrites doped with Mn have been recently found to exhibit room temperature ferromagnetism. Isovalent substitution of the Cd site is expected, however, to create antiferromagnetism, in analogy with the well-known CdTe:Mn (d^5) case. However, chalcopyrite semiconductors exhibit low-energy intrinsic defects. We show theoretically how ferromagnetism results from the interaction of Mn with hole-producing intrinsic defects.",0111570v1 2002-11-21,"Phase Separation, Magnetism and Superconductivity in Rutheno-Cuprates","The properties of the superconducting ferromagnets RuSr2(Gd/Eu)Cu2O8 and RuSr2(Ln/Ce)2Cu2O10 are reviewed. The problem of coexistence and competition between magnetic and superconducting states is discussed. A model of phase separation into ferromagnetic and anti-ferromagnetic structures is proposed.",0211492v1 2003-10-29,"Comment on ``Coexistence of superconductivity and ferromagnetism in ferromagnetic metals""","We argue that a single-band itinerant electron model with short-range interactions, proposed by Karchev et al. (cond-mat/9911489) and investigated further by Jackiewicz et al. (cond-mat/0302449), cannot describe the coexistence of superconducting and ferromagnetic order.",0310704v1 2003-12-22,Spin dynamics of the ferromagnetic superconductor UGe$_{2}$,"Inelastic neutron scattering was used to study the low energy magnetic excitations of the ferromagnetic superconductor UGe$_{2}$. The ferromagnetic fluctuations are of Ising nature with a non-conserved magnetization and have an intermediate behavior between localized and itinerant magnetism.",0312584v1 2004-03-20,Nonequilibrium Dynamic Phase transitions in ferromagnetic systems: Some new phenomena,"The nonequilibrium dynamic phase transition in ferromagnetic systems is reviewed. Very recent results of dynamic transition in kinetic Ising model and that in Heisenberg ferromagnet is discussed.",0403530v2 2004-04-01,Mean Field in Long-Range Ferromagnets and Periodic Boundary Conditions,"Periodic boundary conditions are applied to a ferromagnetic spin lattice. A symmetrical lattice and its contributions all over space are being used. Results, for the Ising model with ferromagnetic interaction that decays as a $1/r^{D+\nu}$ law, are discussed in the mean field approximation",0404032v1 2004-06-16,Superconductivity in ferromagnetic metals and in compounds without inversion centre,"The symmetry properties and the general overview of the superconductivity theory in the itinerant ferromagnets and in materials without space parity are presented. The basic notions of unconventional superconductivity are introduced in broad context of multiband superconductivity which is inherent property of ferromagnetic metals or metals without centre of inversion.",0406371v1 2005-02-03,Carrier-induced ferromagnetism in a diluted Hubbard model,"Carrier-induced ferromagnetism is investigated in a diluted Hubbard model for ordered impurity arrangements. The delicate competition between particle-hole processes contributing to the spin couplings results in a rich variety of behaviour. The ferromagnetic transition temperature obtained within the spin-fluctuation theory is in good agreement with reported values for $\rm Ga_{1-x} Mn_x N$.",0502085v1 1999-09-06,Riemannian 2-D conical geometry of Heisenberg ferromagnets,"An exact 2-dimensional conical Riemannian defect solution of 3-dimensional Euclidean Einstein equations of stresses and defects representing a shear-free Heisenberg ferromagnet is given.The system is equivalent to the Einstein equations in vacuum.Geodesics of magnetic monopoles around the ferromagnet are also investigated.",9909019v2 2005-10-06,Ferromagnetism in quark matter and origin of the magnetic field in compact stars,"Two magnetic aspects of quark matter, ferromagnetism and spin density wave, are discussed in the temperature-density plane. Some implications of ferromagnetism are suggested on relativistic heavy-ion collisions and compact stars.",0510068v1 2000-02-09,Existence of Algebraic Decay in Nonabelian Ferromagnets,"The low temperature regime of nonabelian two-dimensional ferromagnets is investigated. The method involves mapping such models into certain site-bond peroclation processes and using ergodicity in a novel fashion. It is concluded that all ferromagnets possessing a continuous symmetry (abelian or not) exihibit algebraic decay of correlations at sufficiently low temperatures.",0002028v1 2007-04-24,Theory of the tunneling spectroscopy of ferromagnetic superconductors,"We study tunneling conductance in normal metal / insulator / ferromagnetic superconductor junctions. The tunneling spectra show a clear difference between spin-singlet s-wave pairing, spin-triplet opposite spin pairing and spin-triplet equal spin pairing: These pairings exhibit, respectively, gap struture, double peak structure and zero bias peak in the spectra. The obtained result may serve as a tool for determining the pairing symmetry of ferromagnetic superconductors.",0704.3125v1 2007-12-19,Dynamic Phase Transitions for Ferromagnetic Systems,"This article presents a phenomenological dynamic phase transition theory for ferromagnetism, leading to a precise description of the dynamic transitions, and to a physical predication on the spontaneous magnetization. The analysis also suggests asymmetry of fluctuations in both the ferromagnetism and the PVT systems.",0712.3082v1 2008-04-22,Pressure-temperature phase diagram of ferromagnetic superconductors,"The symmetry approach to the description of the (P,T) phase diagram of ferromagnet superconductors with triplet pairing is developed. Taking into account the recent experimental observations made on UCoGe it is considered the case of a crystal with orthorhombic structure and strong spin-orbital coupling. It is shown that formation of ferromagnet superconducting state from a superconducting state is inevitably accompanied by the first order type transition.",0804.3546v1 2008-05-26,Magnetic anisotropy in ferromagnetic Josephson junctions,"Magnetotransport measurements were done on $\Nb/\Al_2\O_3/\Cu/\Ni/\Nb$ superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Depending on ferromagnetic $\Ni$ interlayer thickness and geometry the standard (1d) magnetic field dependence of critical current deviates from the text-book model for Josephson junctions. The results are qualitatively explained by a short Josephson junction model based on anisotropy and 2d remanent magnetization.",0805.3925v2 2008-09-21,Quantized Conductance of a Single Magnetic Atom,"A single Co atom adsorbed on Cu(111) or on ferromagnetic Co islands is contacted with non-magnetic W or ferromagnetic Ni tips in a scanning tunneling microscope. When the Co atom bridges two non-magnetic electrodes conductances of 2e^2/h are found. With two ferromagnetic electrodes a conductance of e^2/h is observed which may indicate fully spin-polarized transport.",0809.3567v1 2008-12-09,Spin-Wave Relaxation in a Quantum Hall Ferromagnet,"We study spin wave relaxation in quantum Hall ferromagnet regimes. Spin-orbit coupling is considered as a factor determining spin nonconservation, and external random potential as a cause of energy dissipation making spin-flip processes irreversible. We compare this relaxation mechanism with other relaxation channels existing in a quantum Hall ferromagnet.",0812.1703v1 2010-02-18,Paramagnetic limiting in ferromagnetic superconductors with triplet pairing,"The spin susceptibility in the uranium ferromagnet superconductors is calculated. There is shown that the absence of superconductivity paramagnetic limitation for the field directions perpendicular to the direction of the spontaneous magnetization is explained by the itinerant ferromagnet band splitting rather than by a rotation of magnetization toward the external field direction. The qualitative description of the upper critical field temperature dependence is given.",1002.3510v1 2010-07-26,Thermodynamics of classical frustrated spin chain at the ferromagnet-helimagnet transition point,"Low-temperature thermodynamics of the classical frustrated ferromagnetic spin chain is studied. Using transfer-matrix method we found the behavior of the correlation function and zero-field susceptibility at the ferromagnetic-helical transition point. It is shown that the critical exponent for the susceptibility is changed from 2 to 4/3 at the transition point.",1007.4536v1 2011-05-10,Triplet superconductivity in a ferromagnetic vortex,"We argue that odd-frequency triplet superconductivity can be conveniently realized in hybrid superconductor-ferromagnet (SF) structures with a ferromagnetic vortex. We demonstrate that due to proximity-induced long-range triplet pairing such SFS junctions can sustain appreciable supercurrent which can be directly measured in experiments.",1105.1916v1 2013-10-12,Spin diffusion and magnetoresistance in ferromagnet/topological-insulator junctions,"We study spin and charge diffusion in metallic-ferromagnet/topological-insulator junctions. The coupled diffusion equations are derived perturbatively with respect to the strength of the interlayer tunneling. We calculate spin accumulation in the ferromagnet and junction magnetoresistance associated with a current bias along the interface.",1310.3354v2 2020-03-27,Spin-polarized Current-driven Ferromagnetic Domain Wall Motion with a Skyrmion Building Block,"The purpose of the research is the construction of the analytical model for description of spin-polarized current-driven ferromagnetic domain wall motion with a skyrmion building block. The dependence of velocity of ferromagnetic domain wall motion with a skyrmion building block is found as a function of driving torques and an external magnetic field strength.",2003.12270v1 2022-08-26,Mechanics of a ferromagnetic domain wall,"This paper gives a pedagogical introduction to the mechanics of ferromagnetic solitons. We start with the dynamics of a single spin and develop all the tools required for the description of the dynamics of solitons in a ferromagnet.",2208.12799v3 2023-04-21,Soft Magnons in Anisotropic Ferromagnets,"We discuss spin-wave transport in anisotropic ferromagnets with an emphasis on the zeroes of the band edges as a function of a magnetic field. An associated divergence of the magnon spin should be observable by enhanced magnon conductivities in non-local experiments, especially in two-dimensional ferromagnets.",2304.10709v1 1999-07-28,Emergence of magnetic field due to spin-polarized baryon matter in neutron stars,"A model of the ferromagnetic origin of magnetic fields of neutron stars is considered. In this model, the magnetic phase transition occurs inside the core of neutron stars soon after formation. However, owing to the high electrical conductivity the core magnetic field is initially fully screened. We study how this magnetic field emerges for an outside observer. After some time, the induced field that screens the ferromagnetic field decays enough to uncover a detectable fraction of the ferromagnetic field. We conjecture that weak fields of millisecond pulsars of 10^8-10^9 G could be identified with ferromagnetic fields of unshielded fraction f=10^-4 resulting from the decay of screening fields by a factor 1-f in 10^8 yr since their birth.",9907393v1 2004-10-09,Phase transitions to spin-triplet ferromagnetic superconductivity in neutron stars,"Effects of the anisotropy of Cooper pairs in spin-triplet ferromagnetic superconductors are investigated on the basis of the Ginzburg-Landau theory. A special attention is paid to the triggering of the superconducting state by the ferromagnetic order. The ground states of these superconductors are outlined and discussed. The idea about a possible coexistence of ferromagnetism and spin-triplet superconductivity in neutron stars is introduced.",0410246v1 1993-07-08,Macroscopic Quantum Tunneling of a Domain Wall in a Ferromagnetic Metal,"The macroscopic quantum tunneling of a planar domain wall in a ferromagnetic metal is studied based on the Hubbard model. It is found that the ohmic dissipation is present even at zero temperature due to the gapless Stoner excitation, which is the crucial difference from the case of the insulating magnet. The dissipative effect is calculated as a function of width of the wall and is shown to be effective in a thin wall and in a weak ferromagnet. The results are discussed in the light of recent experiments on ferromagnets with strong anisotropy. PACS numbers:75.60.Ch, 03.65.Sq, 75.10.Lp",9307012v1 1994-02-08,Method of effective potential for quantum Heisenberg ferromagnet theory,"Self - consistent temperature dependence of the average magnetization in quantum Heisenberg ferromagnet is obtained as a first approximation of perturbation theory on an inverse radius by application of the functional method for quantum ferromagnet. The experimental data are compared with the theoretical results for {\bf EuO} ferromagnet. A quantitative agreement is observed between theoretical and experimental results outside interval near Curie temperature. The theory is of critical behavior in the vicinity of Curie temperature $T_c$. Theoretical $T_c$ value itself is agreed with the experimental one with an accuracy of several percents.",9402037v1 1994-04-08,Excitation Spectrum of the Spin-1/2 Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chain:,"The natural explanation of the excitation spectrum of the spin-1 antiferromagnetic Heisenberg chain is given from the viewpoint of the spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain. The energy spectrum of the latter is calculated with fixed momentum $k$ by numerical diagonalization of finite size systems. It consists of a branch of propagating triplet pair (triplet wave) and the continuum of multiple triplet waves for weak ferromagnetic coupling. As the ferromagnetic coupling increases, the triplet wave branch is absorbed in the continuum for small $k$, reproducing the characteristics of the spin-1 antiferromagnetic Heisenberg chain.",9404017v1 1994-07-22,Numerical Study of a Two-Dimensional Quantum Antiferromagnet with Random Ferromagnetic Bonds,"A Monte Carlo method for finite-temperature studies of the two-dimensional quantum Heisenberg antiferromagnet with random ferromagnetic bonds is presented. The scheme is based on an approximation which allows for an analytic summation over the realizations of the randomness, thereby significantly alleviating the ``sign problem'' for this frustrated spin system. The approximation is shown to be very accurate for ferromagnetic bond concentrations of up to ten percent. The effects of a low concentration of ferromagnetic bonds on the antiferromagnetism are discussed.",9407088v1 1995-10-04,Thermodynamics of Quantum Hall Ferromagnets,"The two-dimensional interacting electron gas at Landau level filling factor $\nu =1$ and temperature $T=0$ is a strong ferromagnet; all spins are completely aligned by arbitrarily weak Zeeman coupling. We report on a theoretical study of its thermodynamic properties using a many-body perturbation theory approach and concentrating on the recently measured temperature dependence of the spin magnetization. We discuss the interplay of collective and single-particle aspects of the physics and the opportunities for progress in our understanding of itinerant electron ferromagnetism presented by quantum Hall ferromagnets.",9510021v1 1996-02-11,Theory on the Itinerant Ferromagnetism in the $3d$-transition metal systems,"Keeping nickel, cobalt and iron in mind, we investigate the origin of the itinerant ferromagnetism. In so doing, we generalize the Gutzwiller approximation. In that,we take account of the effect of the band degeneracy and the Hund's-rule coupling in addition to the on-site repulsion. After the discussion on nickel, the condition for the incomplete ferromagnetism, observed in cobalt and iron, is argued. Phase diagrams, which show the interplay between the band shape peculiarity and the Hund's-rule coupling, are given. It is found that for the $3d$-transition metal systems, both of the Hund's-rule coupling and the special feature of the density of states are necessary to explain the itinerant ferromagnetism.",9602058v1 1996-12-10,Band ferromagnetism versus collective Kondo state in lattice fermion models,"It is becoming widely recognized that, contrary to earlier expectations, the usual one-band Hubbard model does not give an explanation for itinerant ferromagnetism. After reviewing the status of magnetic ordering in the one-band model, we discuss the possibility of ferromagnetism in some recently introduced two-band Hubbard models, and in generalized Anderson lattices. It is argued that these two classes of models are closely related and that it is their common feature that the ferromagnetic phase has to compete with a collective Kondo state.",9612090v1 1997-08-20,Ferromagnetism in the large-U Hubbard model,"We study the Hubbard model on a hypercubic lattice with regard to the possibility of itinerant ferromagnetism. The Dynamical Mean Field theory is used to map the lattice model on an effective local problem, which is treated with help of the Non Crossing Approximation. By investigating spin dependent one-particle Green's functions and the magnetic susceptibility, a region with nonvanishing ferromagnetic polarization is found in the limit $U\to\infty$. The $\delta$-T-phase diagram as well as thermodynamic quantities are discussed. The dependence of the Curie temperature on the Coulomb interaction and the competition between ferromagnetism and antiferromagnetism are studied in the large $U$ limit of the Hubbard model.",9708156v1 1997-09-12,S-wave Superconductivity in Weak Ferromagnetic Metals,"We investigate the behavior of weak ferromagnetic metals close to the ferromagnetic critical point. We show that in the limit of small magnetic moment the low temperature metallic phase is rigorously described by a local ferromagnetic Fermi liquid that has a momentum-independent self-energy. Whereas, non-Fermi liquid features develop at higher temperatures. Furthermore, we find that an instability towards s-wave superconductivity is possible when the exchange splitting is comparable to the superconducting gap.",9709143v1 1997-09-26,Dynamics of a ferromagnetic domain wall and the Barkhausen effect,"We derive an equation of motion for the the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium and we study the associated depinning transition. The long-range dipolar interactions set the upper critical dimension to be $d_c=3$, so we suggest that mean-field exponents describe the Barkhausen effect for three-dimensional soft ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a function of the field driving rate and the intensity of the demagnetizing field, and find results in quantitative agreement with experiments on crystalline and amorphous soft ferromagnetic alloys.",9709300v1 1998-01-30,Disorder and the quantum Hall ferromagnet,"The distinguishing feature of the quantum Hall ferromagnet is the identity between the electrical and topological charge densities of a spin distortion. In addition to the wealth of physics associated with Skyrmionic excitations of the quantum Hall ferromagnet, this identification permits a rather curious coupling of spinwaves to the disorder potential. A wavepacket of spinwaves has an associated, oscillating dipole charge distribution, due to the non-linear form of the topological density. We investigate the way in which this coupling modifies the conductivity and temperature dependence of magnetization of the quantum Hall ferromagnet.",9801324v1 1998-03-18,Spinor Bose Condensates in Optical Traps,"In an optical trap, the ground state of spin-1 Bosons such as $^{23}$Na, $^{39}$K, and $^{87}$Rb can be either a ferromagnetic or a ""polar"" state, depending on the scattering lengths in different angular momentum channel. The collective modes of these states have very different spin character and spatial distributions. While ordinary vortices are stable in the polar state, only those with unit circulation are stable in the ferromagnetic state. The ferromagnetic state also has coreless (or Skyrmion) vortices like those of superfluid $^{3}$He-A. Current estimates of scattering lengths suggest that the ground states of $^{23}$Na and $^{87}$Rb condensate are a polar state and a ferromagnetic state respectively.",9803231v1 1998-10-09,Low-Energy Structure of Heisenberg Ferrimagnetic Spin Chains,"Static and dynamic structure factors of Heisenberg ferrimagnetic spin chains are numerically investigated. There exist two distinct branches of elementary excitations, which exhibit ferromagnetic and antiferromagnetic aspects. The ferromagnetic feature is smeared out with the increase of temperature, whereas the antiferromagnetic one persists up to higher temperatures. The scattering intensity is remarkably large at lower boundaries of the ferromagnetic and antiferromagnetic spectra. All these observations are consistent with the ferromagnetic-to-antiferromagnetic crossover in the thermal behavior which has recently been reported.",9810113v1 1998-11-26,Competition between glassiness and order in a multi-spin glass,"A mean-field multi-spin interaction spin glass model is analyzed in the presence of a ferromagnetic coupling. The static and dynamical phase diagrams contain four phases (paramagnet, spin glass, ordinary ferromagnet and glassy ferromagnet) and exhibit reentrant behavior. The glassy ferromagnet phase has anomalous dynamical properties. The results are consistent with a nonequilibrium thermodynamics that has been proposed for glasses.",9811373v2 1998-12-10,Effect of Ferromagnetic Spin Correlations on Superconductivity in Ferromagnetic Metals,"We study the renormalization of the quasiparticle properties in weak ferromagnetic metals, due to spin fluctuations, away from the quantum critical point for small magnetic moment. We explain the origin of the s-wave superconducting instability in the ferromagnetic phase and find that the vertex corrections are small and Migdal's theorem is satisfied away from the quantum critical point.",9812171v1 1999-01-21,Interplay between Mesoscopic and Microscopic Fluctuations in Ferromagnets,"A model of a ferromagnet is considered, in which there arise mesoscopic fluctuations of paramagnetic phase. The presence of these fluctuations diminishes the magnetization of the ferromagnet, softens the spin-wave spectrum, increases the spin-wave attenuation, shortens the magnon free path, lowers the critical point, and can change the order of phase transition. A special attention is paid to the interplay between these mesoscopic paramagnetic fluctuations and microscopic fluctuations due to magnons. One of the main results of this interplay is an essential extension of the region of parameters where the ferromagnet-paramagnet phase transition is of first order.",9901213v1 1999-01-26,Resonant transmission of normal electrons through Andreev states in ferromagnets,"Giant oscillations of the conductance of a superconductor - ferromagnet - superconductor Andreev interferometer are predicted. The effect is due to the resonant transmission of normal electrons through Andreev levels when the voltage $V$ applied to the ferromagnet is close to $2h_0/e$ ($h_0$ is the spin-dependant part of the electron energy). The effect of bias voltage and phase difference between the superconductors on the current and the differential conductance is presented. These efects allow a direct spectroscopy of Andreev levels in the ferromagnet.",9901278v1 1999-07-06,A generalization of the spherical model: Reentrant phase transition in the spin one ferromagnet,"By mapping the hamiltonian of the spin one ferromagnet onto that of the classical spherical model we investigate the possible phase transitions and the phase diagram of the spin one ferromagnet. Similarly to what happens in the spherical model we find no phase transition in one and two dimensions. Nonetheless, for three dimensions we obtain a phase diagram in which the most important and unexpected feature is the existence of a ferromagnetic-paramagnetic transition at low temperatures.",9907073v1 1999-10-25,Spin injection and magnetoresistance in ferromagnet/superconductor/ferromagnet tunnel junctions,"We theoretically study the spin-dependent transport in a ferromagnet/superconductor/ferro-magnet double barrier tunnel junction. The spin-polarized tunneling currents give rise to spin imbalance in the superconductor. The resulting nonequilibrium spin density suppresses the superconductivity with increase of the tunneling currents. We focus on the effect of asymmetry in the double tunnel junction, where the barrier height of the tunnel junction and the spin-polarization of the ferromagnets are different, on spin injection, and discuss how the superconductivity is suppressed in the asymmetric junction. Our results explain recent experimental results on the critical current suppression in high-$T_c$ SCs by spin injection.",9910388v1 1999-11-22,Quasiparticle energy spectrum in ferromagnetic Josephson weak links,"The quasiparticles energy spectrum in clean ferromagnetic weak links between conventional superconductors is calculated. Large peaks in DOS, due to a special case of Andreev reflection at the ferromagnetic barrier, correspond to spin-splitt bound states. Their energies are obtained as a function of the barrier thickness, exchange field strength, and of the macroscopic phase difference $\phi$ at the link, related to the Josephson current. In the ground state, $\phi$ can be 0 or $\pi$, depending on the ferromagnetic barrier influence. Conditions for the appearence of the zero-energy bound states (ZES) and for the spin polarized ground state (SPGS) are obtained analytically. It is shown that ZES appear only outside the weak link ground state.",9911339v1 1999-11-23,New Spin-Wave Mode in Weak Ferromagnetic Fermi Liquids,"We study a phenomenological model for weak ferromagnetic Fermi liquids and investigate the properties of the spin waves in the model. The Landau kinetic equation is used to derive, in addition to the known Goldstone mode, a new spin-wave mode -- the first Silin-like ferromagnetic mode. We discuss the role of the interaction parameter F^a_1 on the behavior of the Goldstone mode and the first Silin-like ferromagnetic mode.",9911363v1 1999-12-11,p>2 spin glasses with first order ferromagnetic transitions,"We consider an infinite-range spherical p-spin glass model with an additional r-spin ferromagnetic interaction, both statically using a replica analysis and dynamically via a generating functional method. For r>2 we find that there are first order transitions to ferromagnetic phases. For r

=p only the replica symmetric phase exists.",9912201v2 1999-12-13,Spin Anisotropy and Quantum Hall Effect in the Kagome Lattice - Chiral Spin State based on a Ferromagnet-,"A ferromagnet with spin anisotropies on the 2D Kagome lattice is theoretically studied. This is a typical example of the flat-band ferromagnet. The Berry phase induced by the tilting of the spins opens the band gap and quantized Hall conductance \sigma_{xy}=\pm e^2/h is realized without external magnetic field. This is the most realistic chiral spin state based on the ferromagnetism. We also discuss the implication of our results to anomalous Hall effect observed in the metallic pyrochlore ferromagnets R_2Mo_2O_7(R=Nd, Sm, Gd).",9912206v1 1999-12-22,Probing the spin polarization in ferromagnets,"The emission of correlated electrons from an itinerant ferromagnet following the impact of a polarized electron beam is analyzed in terms of irreducible tensorial parameters that can be measured. Under favorable conditions, specified in this work, these parameters are related to the spin polarization in the ferromagnet. The formal results are illustrated by numerical studies of the polarized electron pair emission from a Fe(110) surface and a novel technique for the investigation of magnetic properties of ferromagnets is suggested.",9912409v1 1999-12-29,Theory of Disordered Itinerant Ferromagnets II: Metal-Insulator Transition,"The theory for disordered itinerant ferromagnets developed in a previous paper is used to construct a simple effective field theory that is capable of describing the quantum phase transition from a ferromagnetic metal to a ferromagnetic insulator. It is shown that this transition is in the same universality class as the one from a paramagnetic metal to a paramagnetic insulator in the presence of an external magnetic field, and that strong corrections to scaling exist in this universality class. The experimental consequences of these results are discussed.",9912478v2 2000-03-03,Ordered phase in the two-dimensional randomly coupled ferromagnet,"True ground states are evaluated for a 2d Ising model with random near neighbor interactions and ferromagnetic second neighbor interactions (the Randomly Coupled Ferromagnet). The spin glass stiffness exponent is positive when the absolute value of the random interaction is weaker than the ferromagnetic interaction. This result demonstrates that in this parameter domain the spin glass like ordering temperature is non-zero for these systems, in strong contrast to the 2d Edwards-Anderson spin glass.",0003042v2 2000-04-25,Weak ferromagnetism of quasi-one-dimensional S=1/2 antiferromagnet BaCu$_2$Ge$_2$O$_7$,"Weak ferromagnetism of quasi-one-dimensional S = 1/2 antiferromagnet BaCu$_2$Ge$_2$O$_7$ is studied by the magnetization measurement. The spontaneous magnetization appears along the b axis. The local symmetry between the in-chain nearest neighbor spins allows the presence of Dzyaloshinskii-Moriya interaction, and the only possible spatial configuration of the weak ferromagnetic moment per spin determines the sign of the inter-chain interaction. A weak $a$-axis magnetic field can change the direction of the magnetization to the $a$-axis direction, which shows that the spin chain forms a weakly coupled weak-ferromagnetic chain system.",0004403v2 2000-05-09,Spin-Waves in itinerant ferromagnets,"We introduce a novel approach for the investigation of spin-wave excitations in itinerant ferromagnets. Our theory is based on a variational treatment of general multi-band Hubbard models which describe elements and compounds of transition metals. The magnon dispersion is derived approximately as the energy of a variational spin-wave state in the limit of large spatial dimensions. A numerical evaluation of our results is feasible for general multi-band models. As a first application we consider a model with two degenerate orbitals per lattice site. From our results we can conclude that spin-wave excitations in strong itinerant ferromagnets are very similar to those in ferromagnetic spin systems.",0005154v1 2000-07-05,"Competition of charge, orbital, and ferromagnetic correlations in layered manganites","The competition of charge, orbital, and ferromagnetic interactions in layered manganites is investigated by magneto-Raman scattering spectroscopy. We find that the colossal magnetoresistance effect in the layered compounds results from the interplay of the orbital and ferromagnetic double-exchange correlations. Inelastic scattering by charge-order fluctuations dominates the quasiparticle dynamics in the ferromagnetic-metal state. The scattering is suppressed at low frequencies, consistent with the opening of a charge-density wave pseudogap.",0007082v1 2000-07-06,Detecting Gapless Excitations above Ferromagnetic Domain Walls,"In a two or three dimensional ferromagnetic XXZ model, a low energy excitation mode above a magnetic domain wall is gapless, whereas all of the usual spin wave excitations moving around the whole crystal are gapful. Although this surprising fact was already proved in a mathematically rigorous manner, the gapless excitations have not yet been detected experimentally. For this issue, we show theoretically that the gapless excitations appear as the dynamical fluctuations of the experimental observable, magnetoresistance, in a ferromagnetic wire. We also discuss other methods (e.g., ferromagnetic resonance and neutron scattering) to detect the gapless excitations experimentally.",0007099v1 2000-07-17,Long Range Proximity Effect in Hybrid Ferromagnetic/Superconducting Nanostructures,"We find that the dependence on temperature and magnetic field of the electrical resistance of diffusive ferromagnetic nano-wires measured with superconducting electrodes changes drastically with the distance, $L$, between the ferromagnet/superconductor contacts, however is remarkably similar for the wires with the same $L$ ranging from 300 nm to 1000 nm, prepared under identical conditions. The result gives an evidence for the long-range superconductor-induced changes in transport properties of ferromagnetic nano-wires.",0007278v1 2000-08-09,Bulk experimental evidence of half-metallic ferromagnetism in doped manganites,"We report precise measurements and quantitative data analysis on the low-temperature resistivity of several ferromagnetic manganite films. We clearly show that there exists a T^{4.5} term in low-temperature resistivity, and that this term is in quantitative agreement with the quantum theory of two-magnon scattering for half metallic ferromagnets. Our present results provide the first bulk experimental evidence of half-metallic ferromagnetism in doped manganites.",0008151v1 2000-09-22,"spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models","The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site complete graph extended with on-site repulsion is obtained from the underlying spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic ground state is realized for 1 and N+1 electrons only. We identify the large density of states to be responsible for the suppression of the ferromagnetic state and argue that a similar situation is encountered in the Kagome, pyrochlore, and other lattices with flat bands in their one-particle density of states.",0009358v1 2000-12-06,Magnons in the ferromagnetic Kondo-lattice model,"The magnetic properties of the ferromagnetic Kondo-lattice model (FKLM) are investigated. Starting from an analysis of the magnon spectrum in the spin-wave regime, we examine the ferromagnetic stability as a function of the occupation of the conduction band $n$ and the strength $J$ of the coupling between the localised moments and the conduction electrons. From the properties of the spin-wave stiffness $D$ the ferromagnetic phase at zero temperature is derived. Using an approximate formula the critical temperature $T_c$ is calculated as a function of $J$ and $n$.",0012086v1 2000-12-22,Quantum Spin Fluctuations as a Source of Long-Range Proximity Effects in Diffusive Ferromagnet-Superconductor Structures,"We show that quantum spin fluctuations in inhomogeneous ferromagnets drastically affect the Andreev reflection of electrons and holes at a ferromagnet-superconductor interface. As a result a strong long-range proximity effect appears, associated with electron-hole spin triplet correlations and persisting on a lenght scale typical for non-magnetic materials, but anomalously large for ferromagnets.",0012437v1 2001-01-17,Design of a semiconductor ferromagnet in a quantum dot artificial crystal,"We present the theoretical design of quantum dot (QD) artificial ferromagnetic crystals. The electronic structure calculations based on local spin density approximation (LSDA) show that our designed QD artificial crystal from a structure comprising the crossing 0.104-micrometer-wide InAs quantum wires (an effective Kagome lattice) has flat band characteristics. Our examined QD artificial crystal has the ferromagnetic ground state when the flat band is half-filled, even though it contains no magnetic elements. The ferromagnetic and the paramagnetic state can be freely switched by changing the electron filling via a gate voltage.",0101252v1 2001-01-25,Ferromagnetism in magnetically doped III-V semiconductors,"The origin of ferromagnetism in semimagnetic III-V materials is discussed. The indirect exchange interaction caused by virtual electron excitations from magnetic impurity level in the bandgap to the valence band can explain ferromagnetism in GaAs(Mn) no matter samples are degenerated or not. Formation of ferromagnetic clusters and percolation picture of phase transition describes well all available experimental data and allows to predict the Mn-composition dependence of transition temperature in wurtzite (Ga,In,Al)N epitaxial layers.",0101389v1 2001-02-21,Inverse proximity effect in superconductors near ferromagnetic material,"We study the electronic density of states in a mesoscopic superconductor near a transparent interface with a ferromagnetic metal. In our tunnel spectroscopy experiment, a substantial density of states is observed at sub-gap energies close to a ferromagnet. We compare our data with detailed calculations based on the Usadel equation, where the effect of the ferromagnet is treated as an effective boundary condition. We achieve an excellent agreement with theory when non-ideal quality of the interface is taken into account.",0102367v2 2001-03-01,A ferromagnet with a glass transition,"We introduce a finite-connectivity ferromagnetic model with a three-spin interaction which has a crystalline (ferromagnetic) phase as well as a glass phase. The model is not frustrated, it has a ferromagnetic equilibrium phase at low temperature which is not reached dynamically in a quench from the high-temperature phase. Instead it shows a glass transition which can be studied in detail by a one step replica-symmetry broken calculation. This spin model exhibits the main properties of the structural glass transition at a solvable mean-field level.",0103026v2 2001-04-26,On metallic ferromagnetism of a generalized Hubbard model with correlated hopping,"In the paper a possibility of metallic ferromagnetic state realization in a generalized Hubbard model with more complete accounting of electron-electron interactions, in particular, the correlated hopping and exchange interaction integrals is investigated. Recently obtained by means of mean-field approximation single electron energy spectrum is used for the description of finite temperature properties of the system. In the paper the expression for the critical temperature of ferromagnet-paramagnet transition is found, the behaviour of temperature dependencies of magnetization and paramagnetic susceptibility is analyzed. Taking into account the correlated hopping allows to explain some peculiarity of ferromagnetic behaviour of transition metals, their alloys and compounds.",0104515v1 2001-05-08,Dissimilar Ferromagnetic Layer Dependence of Hot Electron Magnetotransport,"The material dependence of hot electron magnetotransport in a spin-valve transistor has been theoretically explored. We calculate the parallel and anti-parallel collector current changing the types and relative spin orientation of the ferromagnetic layers. The magnetocurrent has been presented as well. In this calculations, spin dependent self energy effect of hot electron in ferromagnetic materials has been taken into account. The results show that the magnetotransport property strongly depends on the combination of different ferromagnetic metal layers since the hot electron has different inelastic scattering strength in each material, and the hot electron spin polarization enters importantly into the magnetocurrent at finite temperatures. This calculations may suggest the guide for searching the best structural combination of hot electron magnetoelectronic device such as a spin-valve transistor.",0105168v1 2001-07-12,Magnetic exchange interaction induced by a Josephson current,"We show that a Josephson current flowing through a ferromagnet-normal-metal-ferromagnet trilayer connected to two superconducting electrodes induces an equilibrium exchange interaction between the magnetic moments of the ferromagnetic layers. The sign and magnitude of the interaction can be controlled by the phase difference between the order parameters of the two superconductors. We present a general framework to calculate the Josephson current induced magnetic exchange interaction in terms of the scattering matrices of the different layers. The effect should be observable as the periodic switching of the relative orientation of the magnetic moments of the ferromagnetic layers in the ac Josephson effect.",0107258v1 2001-08-24,Influence of disorder on the ferromagnetism in diluted magnetic semiconductors,"Influence of disorder on the ferromagnetic phase transition in diluted (III,Mn)V semiconductors is investigated analytically. The regime of small disorder is addressed, and the enhancement of the critical temperature by disorder is found both in the mean field approximation and from the analysis of the zero temperature spin stiffness. Due to disorder, the spin wave fluctuations around the ferromagnetically ordered state acquire a finite mass. At large charge carrier band width, the spin wave mass squared becomes negative, signaling the breakdown of the ferromagnetic ground state and the onset of a noncollinear magnetic order.",0108396v2 2002-01-08,"Feromagnetic ""$π$""-junctions","We investigate Josephson coupling through a ferromagnetic thin film using Superconductor-Insulator-Ferromagnet-Superconductor planar junctions. Damped oscillations of the critical current are observed as a function of the ferromagnetic layer thickness. We show that they result from the exchange energy gained or lost by a quasiparticle Andreev-reflected at the ferromagnet-superconductor interface. The critical current cancels out at the transition from positive (""0"") to negative (""$\pi$"") coupling, in agreement with theoretical calculations.",0201104v2 2002-03-15,Modified spin-wave study of random antiferromagnetic-ferromagnetic spin chains,"We study the thermodynamics of one-dimensional quantum spin-1/2 Heisenberg ferromagnetic system with random antiferromagnetic impurity bonds. In the dilute impurity limit, we generalize the modified spin-wave theory for random spin chains, where local chemical potentials for spin-waves in ferromagnetic spin segments are introduced to ensure zero magnetization at finite temperature. This approach successfully describes the crossover from behavior of pure one-dimensional ferromagnet at high temperatures to a distinct Curie behavior due to randomness at low temperatures. We discuss the effects of impurity bond strength and concentration on the crossover and low temperature behavior.",0203326v1 2002-04-08,Pressure Effect on the Superconducting and Magnetic Transitions of the Superconducting Ferromagnet RuSr2GdCu2O8,"The superconducting ferromagnet RuSr2GdCu2O8 was investigated at high pressure. The intra-grain superconducting transition temperature, Tc, is resolved in ac-susceptibility as well as resistivity measurements. It is shown that the pressure shift of Tc is much smaller than that of other high-Tc compounds in a similar doping state. In contrast, the ferromagnetic transition temperature, Tm, increases with pressure at a relative rate that is about twice as large as that of Tc. The high-pressure data indicate a possible competition of the ferromagnetic and superconducting states in RuSr2GdCu2O8.",0204185v1 2002-04-20,Coexistence of ferromagnetism and superconductivity,"A comprehensive theory is developed that describes the coexistence of p-wave, spin-triplet superconductivity and itinerant ferromagnetism. It is shown how to use field-theoretic techniques to derive both conventional strong-coupling theory, and analogous gap equations for superconductivity induced by magnetic fluctuations. It is then shown and discussed in detail that the magnetic fluctuations are generically stronger on the ferromagnetic side of the magnetic phase boundary, which substantially enhances the superconducting critical temperature in the ferromagnetic phase over that in the paramagnetic one. The resulting phase diagram is compared with the experimental observations in UGe_2 and ZrZn_2.",0204440v2 2002-05-23,Ferromagnetism in the Hubbard model with a generalized type of hopping,"The extrapolation of small-cluster exact-diagonalization calculations is used to examine ferromagnetism in the one-dimensional Hubbard model with a generalized type of hopping. It is found that the long-range hopping with power decaying hopping amplitudes ($t_{ij}\sim q^{|i-j|}$) stabilizes the ferromagnetic state for a wide range of electron interactions $U$ and electron concentrations $n$. The critical value of the interaction strength $U_c(q)$ above which the ferromagnetic state becomes stable is calculated numerically and the ground-state phase diagram of the model (in the $U$-$q$ plane) is presented for physically the most interesting cases.",0205481v1 2002-05-27,Parity Effect and Tunnel Magnetoresistance of Ferromagnet / Superconductor / Ferromagnet Single-Electron Tunneling Transistors,"We theoretically study the tunnel magnetoresistance(TMR) of ferromagnet / superconductor / ferromagnet single-electron tunneling transistors with a special attention to the parity effect. It is shown that in the plateau region, there is no spin accumulation in the island even at finite bias voltage. However, the information of the injected spin is carried by the excess electron and thus the TMR exists. The spin relaxation rate of the excess electron can be estimated from the TMR. We also show that the TMR increases with decreasing the size of the superconducting island.",0205541v1 2002-08-01,Magneto-optical study of ZnO based diluted magnetic semiconductors,"Magneto-optical properties of ZnO:Co and ZnO:Ni films were measured. Magnetization measurements show that some of the films are paramagnetic and others are ferromagnetic. Magnetic circular dichroism clarified that Zn1-xCoxO and Zn1-xNixO included in samples are paramagnetic diluted magnetic semiconductors. Ferromagnetic precipitations seem to be responsible for the observed ferromagnetic behaviors. Criteria to judge the ferromagnetic DMS are also discussed.",0208010v1 2002-09-30,Phase diagrams of spin ladders with ferromagnetic legs,"The low-temperature properties of the spin S=1/2 ladder with anisotropic ferromagnetic legs are studied using the continuum limit bosonization approach. The weak-coupling ground state phase diagram of the model is obtained for a wide range of coupling constants and several unconventional gapless ''spin-liquid'' phases are shown to exist for ferromagnetic coupling. The behavior of the ladder system in the vicinity of the ferromagnetic instability point is discussed in detail.",0209677v1 2002-11-28,Proximity effect in ferromagnet-superconductor hybrid structures: role of the pairing symmetry,"The spatial variations of the pair amplitude, and the local density of states in d-wave or s-wave superconductor-ferromagnet hybrid structures are calculated self consistently using the Bogoliubov-deGennes formalism within the two dimensional extended Hubbard model. We describe the proximity effect in superconductor / ferromagnet (SF) bilayers, FSF trilayers, and interfaces between a superconductor and a ferromagnetic domain wall. We investigate in detail the role played by the pairing symmetry, the exchange field, interfacial scattering and crossed Andreev reflection.",0211663v1 2002-12-05,Dynamic stiffness of spin valves,"The dynamics of the magnetic order parameters of ferromagnet/normal-metal/ferromagnet spin valves and isolated ferromagnets may be very different. We investigate the role of the nonequilibrium spin-current exchange between the ferromagnets in the magnetization precession and switching. We find a (low-temperature) critical current bias for a coherent current-induced magnetization excitation in spin valves, which unifies and generalizes previous ideas of Slonczewski and Berger. In the absence of an applied bias, the effect of the spin transfer can be expressed as magnetic--configuration-dependent Gilbert damping.",0212130v2 2003-02-17,Charge-ordered ferromagnetic phase in manganites,"A mechanism for charge-ordered ferromagnetic phase in manganites is proposed. The mechanism is based on the double exchange in the presence of diagonal disorder. It is modeled by a combination of the Ising double-exchange and the Falicov-Kimball model. Within the dynamical mean-field theory the charge and spin correlation function are explicitely calculated. It is shown that the system exhibits two successive phase transitions. The first one is the ferromagnetic phase transition, and the second one is a charge ordering. As a result a charge-ordered ferromagnetic phase is stabilized at low temperature.",0302328v1 2003-04-15,Unconventional superconducting states induced in a ferromagnet by a d-wave superconductor,"We develop a quasi-classical theory for the superconducting proximity effect in a ballistic ferromagnetic layer in contact with a d-wave superconductor. In agreement with recent experiments we find that the density of states oscillate around the normal state value with varying the thickness of the ferromagnetic layer. We show that the phase, the amplitude, and the period of these oscillations depend on the orientation of the superconductor. This effect reveals spatial oscillations and anisotropy of the induced superconducting correlations in the ferromagnet.",0304336v1 2003-04-28,Ferromagnetism and Superconductivity in the multi-orbital Hubbard Model: Hund's Rule Coupling versus Crystal-Field Splitting,"The multi-orbital Hubbard model in one dimension is studied using the numerical diagonalization method. Due to the effect of the crystal-field splitting $\Delta$, the fully polarized ferromagnetism which is observed in the strong coupling regime becomes unstable against the partially polarized ferromagnetism when the Hund's rule coupling $J$ is smaller than a certain critical value of order of $\Delta$. In the vicinity of the partially polarized ferromagnetism, the orbital fluctuation develops due to the competition between the Hund's rule coupling and the crystal-field splitting. The superconducting phase with the Luttinger liquid parameter $K_{\rho}>1$ is observed for the singlet ground state in this region.",0304620v1 2003-05-14,Atomic thickness hybrid F/S/F structures,"We propose an exactly solvable model to describe the properties of atomic thickness hybrid ferromagnet-superconductor-ferromagnet (F/S/F) structures. We show that the superconducting critical temperature is always higher for antiparallel orientation of the ferromagnetic moments. However at low temperature the superconducting gap occurs to be larger for parallel orientation of the ferromagnetic moments. This leads to a peculiar temperature dependence of the proximity effect in (F/S/F) structures.",0305320v2 2003-05-26,Phase diagram of a class of spin-triplet ferromagnetic superconductors,"We investigate thermodynamic phases, including the phase of coexistence of superconductivity and ferromagnetism, the possible phase transitions of first and second order, and the shape of the phase diagram in mean-field approximation for a phenomenological model of spin-triplet ferromagnetic superconductors. The results are discussed in view of application to metallic ferromagnets as UGe$_2$, ZrZn$_2$, URhGe, and Fe.",0305585v1 2003-05-27,Double Magnetic Transition in Pr0.5Sr0.5CoO3,"We report studies of polycrystalline samples of the metallic ferromagnet Pr0.5Sr0.5CoO3 through measurements of the magnetization, a.c. magnetic susceptibility, resistivity, and specific heat. We find an unusual anomaly around TA = 120 K, much below the ferromagnetic transition (TC = 226 K). The anomaly is manifested in field cooled magnetization as a downward step in low fields (H less than or equal to 0.01 T) but is transformed into an upward step for H greater than or equal to 0.05 T. The anomaly cannot be easily attributed to antiferromagnetic ordering, but may correspond to a second ferromagnetic transition or an alteration of the ferromagnetic state associated with orbital ordering.",0305621v1 2003-07-11,Orange Peel coupling in granular ferromagnetic films,"We present magnetoresistance (MR) measurements performed on magnetic tunnel junctions in which one of the electrodes is a granular ferromagnetic film. These junctions exhibit a zero field resistance dip. The dip magnitude depends on the size of the grains. We interpret these results as a consequence of the orange peel effect between the continuous ferromagnetic film and the magnetic grains. The coupling is found to be much stronger than that between continuous ferromagnetic layers.",0307274v1 2003-08-26,Field dependence of the electronic phase separation in Pr0.67Ca0.33MnO3 by small angle magnetic neutron scattering,"We have studied by small angle neutron scattering the evolution induced by the application of magnetic field of the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in a crystal of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$. The results are compared to magnetic measurements which provide the evolution of the ferromagnetic fraction. These results show that the growth of the ferromagnetic phase corresponds to an increase of the thickness of the ferromagnetic ''cabbage'' sheets.",0308534v1 2003-10-01,Thermodynamic properties of the phase transitions in a class of spin-triplet ferromagnetic superconductors,"Magnetic susceptibility, entropy and specific heat are calculated at the equilibrium points of phase transition to a phase of coexistence of ferromagnetic order and superconductivity in a new class of spin-triplet ferromagnetic superconductors. The results are discussed in view of application to metallic ferromagnets as UGe$_2$, ZrZn$_2$, URhGe.",0310016v2 2003-10-06,"Ferromagnetism in (In,Mn)As alloy thin films grown by metalorganic vapor phase epitaxy","Ferromagnetic properties of In1-xMnxAs thin films were investigated. Room temperature ferromagnetic order was observed in nominally single-phase films with x = 0.01-0.10. Magnetization measurements indicated that these In1-xMnxAs samples had a Curie temperature of 333 K. The Curie temperature was independent of Mn concentration. The temperature dependent magnetization along with the magnitude of the saturation magnetization and microstructural data indicate that the source of the high-temperature ferromagnetism in single-phase films is not attributable to MnAs nanoprecipitates. The high transition temperature is attributed to the presence of near neighbor Mn pairs.",0310104v1 2003-12-12,Role of Inter-site Hybrid Interactions in Itinerant Ferromagnetism,"We study role of inter-site hybrid interactions in deciding ferromagnetic state in the itinerant electron (narrow band) systems like some of the transition metals. We have considered Hubbard like tight binding model alongwith exchange and hybrid interactions. All interactions have been treated within mean-field approximation. It is found here that hybrid interactions play significant role at zero as well as infinite temperatures in deciding on-set of ferromagnetic state. We have studied variation of effective mass of up and down spin electrons in ferromagnetic state as a function of temperature. Also studied are magnetic susceptibility, optical conductivity and magnetoresistance as a function of temperature.",0312309v1 2003-12-18,Coherently photo-induced ferromagnetism in diluted magnetic semiconductors,"Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-bandgap laser radiation . The mechanism for photo-induced ferromagnetism is coherence between conduction and valence bands induced by the light which leads to an optical exchange interaction. The ferromagnetic critical temperature T_C depends both on the properties of the material and on the frequency and intensity of the laser and could be above 1 K.",0312445v1 2003-12-18,Controlled normal and inverse magnetoresistance and current-driven magnetization switching in magnetic nanopillars,"Combining pairs of ferromagnetic metals with different signs of scattering anisotropies, let us independently invert the magnetoresistance and the direction of current-driven switching in ferromagnetic/non-magnetic/ferromagnetic metal nanopillars. We show all four combinations of normal and inverse behaviors, at both room temperature and 4.2K. In all cases studied, the direction of switching is set by the net scattering anisotropy of the fixed (polarizing) ferromagnet. We provide simple arguments for what we see.",0312475v1 2003-12-18,Ferromagnetism in the Hubbard model with long-range and correlated hopping,"The extrapolation of small-cluster exact-diagonalization calculations is used to examine ferromagnetism in the one-dimensional Hubbard model with long-range and correlated hopping. It is found that the correlated hopping term stabilizes the ferromagnetic state for a wide range of electron interactions $U$ and electron concentrations $n$. The critical value of the interaction strength $U_c$ above which the ferromagnetic state becomes stable is calculated numerically and the ground-state phase diagram of the model is presented for physically the most interesting cases.",0312486v1 2004-01-16,Ferromagnetism in Fe-doped SnO2 thin films,"Thin films grown by pulsed-laser deposition from targets of Sn0.95Fe0.05O2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 Am2kg-1, respectively. The 57Fe Mossbauer spectra show the iron is all high-spin Fe3+ but the films are magnetically inhomogeneous on an atomic scale, with only 23 % of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 Bohr magnetons, is greater than for any simple iron oxide. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is proposed to explain the high Curie temperature",0401293v1 2004-02-28,Superconductor-Ferromagnet Bi-Layers: a Comparison of s-Wave and d-Wave Order Parameters,"We study superconductor-ferromagnet bi-layers, not only for s-wave but also for d-wave superconductors. We observe oscillations of the critical temperature when varying the thickness of the ferromagnetic layer for both s-wave and d-wave superconductors. However, for a rotated d-wave order parameter the critical temperature differs considerably from that for the unrotated case. In addition we calculate the density of states for different thicknesses of the ferromagnetic layer; the results reflect the oscillatory behaviour of the superconducting correlations.",0403009v1 2004-04-07,Interaction Between Superconducting and Ferromagnetic Order Parameters in Graphite-Sulfur Composites,"The superconductivity of graphite-sulfur composites is highly anisotropic and associated with the graphite planes. The superconducting state coexists with the ferromagnetism of pure graphite, and a continuous crossover from superconducting to ferromagnetic-like behavior could be achieved by increasing the magnetic field or the temperature. The angular dependence of the magnetic moment m(alpha) provides evidence for an interaction between the ferromagnetic and the superconducting order parameters.",0404169v1 2004-04-08,Incomplete Andreev reflection in a clean Superconductor/Ferromagnet/Superconductor junction,"We study the Josephson effect in a clean Superconductor-Ferromagnet-Superconductor junction for arbitrarily large spin polarizations. The Andreev reflection at a clean Ferromagnet-Superconductor interface is incomplete, and Andreev channels with a large incidence angle are progressively suppressed with increasing exchange energy. As a result, the critical current exhibits oscillations as a function of the exchange energy and of the length of the ferromagnet and has a temperature dependence which deviates from the one predicted by the quasiclassical theory.",0404215v3 2004-04-12,Phases and phase transitions in spin-triplet ferromagnetic superconductors,"Recent results for the coexistence of ferromagnetism and unconventional superconductivity with spin-triplet Cooper pairing are reviewed on the basis of the quasi-phenomenological Ginzburg-Landau theory. New results are reported. The results are discussed in view of applications to metallic compounds as UGe2, URhGe, ZrZn2.",0404261v1 2004-04-24,Spin and Charge Pumping by Ferromagnetic-Superconductor Order Parameters,"We study transport in ferromagnetic-superconductor/normal-metal systems. It is shown that charge and spin currents are pumped from ferromagnetic superconductors into adjacent normal metals by adiabatic changes in the order parameters induced by external electromagnetic fields. Spin and charge pumping identify the symmetry of the superconducting order parameter, e.g., singlet pairing or triplet pairing with opposite or equal spin pairing. Consequences for ferromagnetic-resonance experiments are discussed.",0404591v2 2004-06-25,Correlation Effects in Band Ferromagnetism,"We analyze the influence of different on-site and inter-site interactions on the Curie temperature of transition metal magnetic elements. The numerical calculations show the well known result that the on-site Coulomb repulsion helps ferromagnetism mainly at half-filling of the band, where the 3d elements are antiferromagnetic (Cr, Mn). The inter-site interactions, which in the new approximation decrease the width of the band, favor ferromagnetism at both ends of the band. At the same time they lower the ferromagnetic Curie temperature towards experimental values, removing the paradox which persisted for a long time.",0406634v1 2004-07-08,Singlet-Triplet Mixing in Superconductor-Ferromagnet Hybrid Devices,"We develop a theory which describes hybrid structures consisting out of superconducting and ferromagnetic parts. We give two examples for applications. First, we consider a hybrid structure containing a strong ferromagnet in the ballistic limit. Second, we study for a weak ferromagnet the influence of a domain wall on the superconducting proximity effect. In both cases we account quantitatively for the mixing between singlet and triplet correlations.",0407194v1 2004-10-21,"Search For Hole Mediated Ferromagnetism In Cubic (Ga,Mn)N","Results of magnetisation measurements on p-type zincblende-(Ga,Mn)N are reported. In addition to a small high temperature ferromagnetic signal, we detect ferromagnetic correlation among the remaining Mn ions, which we assign to the onset of hole-mediated ferromagnetism in (Ga,Mn)N.",0410551v1 2005-01-18,Ferromagnetism of Weakly-Interacting Electrons in Disordered Systems,"It was realized two decades ago that the two-dimensional diffusive Fermi liquid phase is unstable against arbitrarily weak electron-electron interactions. Recently, using the nonlinear sigma model developed by Finkelstein, several authors have shown that the instability leads to a ferromagnetic state. In this paper, we consider diffusing electrons interacting through a ferromagnetic exchange interaction. Using the Hartree-Fock approximation to directly calculate the electron self energy, we find that the total energy is minimized by a finite ferromagnetic moment for arbitrarily weak interactions in two dimensions and for interaction strengths exceeding a critical proportional to the conductivity in three dimensions. We discuss the relation between our results and previous ones.",0501431v1 2005-02-14,"Interlayer Exchange Coupling in (Ga,Mn)As-based Superlattices","The interlayer coupling between (Ga,Mn)As ferromagnetic layers in all-semiconductor superlattices is studied theoretically within a tight-binding model, which takes into account the crystal, band and magnetic structure of the constituent superlattice components. It is shown that the mechanism originally introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe superlattices, explains the experimental results observed in ferromagnetic semiconductor structures, i.e., both the antiferromagnetic coupling between ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer structures. The model allows also to predict (Ga,Mn)As-based structures, in which an antiferromagnetic interlayer coupling could be expected.",0502321v1 2005-02-16,Ferromagnetism in a hard-core boson model,"The problem of ferromagnetism -- associated with a ground state with maximal total spin -- is discussed in the framework of a hard-core model, which forbids the occupancy at each site with more than one particle. It is shown that the emergence of ferromagnetism on finite square lattices crucially depends on the statistics of the particles. Fermions (electrons) lead to the well-known instabilities for finite hole densities, whereas for bosons (with spin) ferromagnetism appears to be stable for all hole densities.",0502384v1 2005-02-18,Resonances in Ferromagnetic Gratings Detected by Microwave Photoconductivity,"We investigate the impact of microwave excited spin excitations on the DC charge transport in a ferromagnetic (FM) grating. We observe both resonant and nonresonant microwave photoresistance. Resonant features are identified as the ferromagnetic resonance (FMR) and ferromagnetic antiresonance (FMAR). A macroscopic model based on Maxwell and Landau-Lifschitz equations reveals the macroscopic nature of the FMAR. The experimental approach and results provide new insight in the interplay between photonic, spintronic, and charge effects in FM microstructures.",0502442v1 2005-03-11,Ferromagnetic fluctuation and superconductivity in Na_0.35CoO_2*1.3H_2O: FLEX study of multi-orbital Hubbard model,"Spin and charge fluctuations and superconductivity in Na_0.35CoO_2*1.3H_2O are studied based on a multi-orbital Hubbard model. By applying the fluctuation exchange (FLEX) approximation, we show that the Hund's-rule coupling between the Co t2g orbitals causes ferromagnetic spin fluctuation. Triplet pairing is favored by this ferromagnetic fluctuation on the hole-pocket band. We propose that, in Na_0.35CoO_2*1.3H_2O, Co t2g orbitals and inter-orbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor mediated by inter-orbital-interaction-induced ferromagnetic fluctuation.",0503262v1 2005-04-14,Ferromagnetism and Curie temperature of Vanadium-doped Nitrides,"Electronic structures, exchange interaction mechanism between magnetic ions and Curie temperature of Vanadium - doped Nitrides (AlN, GaN, and InN) are studied within KKR-LSDA-CPA. It is found that the ferromagnetic super-exchange interaction mechanism is dominant at low concentrations of Vanadium, but the anti-ferromagnetic super-exchange interaction appears and reduces the stabilization of ferromagnetism at sufficiently high concentrations (x > 0.10), especially for Vanadium-doped AlN and Vanadium- doped GaN. The estimation of the Curie temperature within the mean field approximation shows the Curie temperature of Vanadium-doped Nitrides exceeding the room temperature with a few constituents of Vanadium.",0504355v1 2005-05-30,"Comment on ""Domain Structure in a Superconducting Ferromagnet""","According to Faure and Buzdin [Phys. Rev. Lett. 94, 187202 (2005)], in a superconducting ferromagnet a domain structure with a period small compared with the London penetration depth can arise. They claim that this contradicts to the conclusion of Sonin [Phys. Rev. B, 66, 100504 (2002)] that ferromagnetic domain structure in the Meissner state of a superconducting ferromagnet is absent in equilibrium. This contradiction is imaginary, based on misinterpretation of the results of these two papers.",0505712v2 2005-06-21,Enhancement of spin stiffness with dilution in a ferromagnetic Kondo lattice model,"Carrier-induced ferromagnetism is investigated in a diluted ferromagnetic Kondo lattice model for several ordered impurity arrangements on square and cubic lattices, allowing for quantitative comparison with different theoretical pictures. The spin stiffness is found to be optimized with respect to hole doping concentration, exchange interaction strength, as well as spin dilution due to a competition involving magnitude of carrier spin polarization and its oscillation length scale. The ferromagnetic transition temperature determined within the spin-fluctuation theory is in good agreement with experimental values for $\rm Ga_{1-x}Mn_xAs$.",0506523v2 2005-07-05,300 K Ferromagnetic Magnetic Circular Dichroism in Co2+-doped ZnO Activated by Shallow Donors,"Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300 K ferromagnetic MCD signal was observed between 1.4 and 4.0 eV after introducing shallow donor states by exposure of paramagnetic Co2+:ZnO films to zinc vapor. The new sub-bandgap ferromagnetic MCD intensity is attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic Co2+:ZnO.",0507121v1 2005-07-25,Surface magnetic anisotropy at a compensated interface of ferromagnetic antiferromagnetic bilayer,"The periodic deviations of ferromagnetic and antiferromagnetic spins from corresponding uniform configurations are shown to be energetically favorable close to a compensated interface of ferromagnetic antiferromagnetic bilayer. The amplitude of the deviations decreases exponentially into the film volumes as a function of a coordinate perpendicular to the interface. The interaction energy is found to be proportional to a square of a scalar product of unit ferromagnetic and antiferromagnetic vectors.",0507578v1 2005-09-05,CeMnNi4: A soft ferromagnet with a high degree of transport spin polarization,"In this letter we introduce a new soft ferromagnetic compound, i.e. CeMnNi4, which exhibits a large moment (~4.95mu_B/Mn) and high degree of spin polarization. The system has a ferromagnetic transition temperature of 148K. Isothermal magnetization measurements at 5K reveal that the material is a soft ferromagnet with a magnetization saturating at about 500Oe and a coercive field of < 5 Oe. We determine the transport spin polarization of this material from Point Contact Andreev Reflection measurements to be 66% thereby making this material potentially important for spintronic applications.",0509101v2 2005-10-24,Charge Transfer Excited State Contributions to Polarity Dependent Ferromagnetism in ZnO Diluted Magnetic Semiconductors,"A close link between the charge transfer electronic structures and polarity dependent high-TC ferromagnetism of TM2+:ZnO DMSs (TM2+ = 3d ions) is demonstrated. Trends in ferromagnetism across the 3d series of TM2+:ZnO DMSs predicted from their charge transfer energies reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.",0510644v1 2005-12-13,Doping change and distortion effect on double-exchange ferromagnetism,"Doping change and distortion effect on the double-exchange ferromagnetism are studied within a simplified double-exchange model. The presence of distortion is modelled by introducing the Falicov-Kimball interaction between itinerant electrons and classical variables. By employing the dynamical mean-field theory the charge and spin susceptibility are exactly calculated. It is found that there is a competition between the double-exchange induced ferromagnetism and disorder-order transition. At low temperature various long-range order phases such as charge ordered and segregated phases coexist with ferromagnetism depending on doping and distortion. A rich phase diagram is obtained.",0512265v1 2005-12-24,Low-temperature thermodynamics of one-dimensional alternating-spin Heisenberg ferromagnets,"Motivated by a novel bimetallic chain compound in which alternating magnetic centers are ferromagnetically coupled, we investigate thermodynamic properties of one-dimensional spin-$(S,s)$ Heisenberg ferromagnets both numerically and analytically. On the one hand, quantum Monte Carlo calculations illuminate the overall thermal behavior. The specific heat may exhibit a double-peaked structure at intermediate temperatures for $S\agt 3s$ in general. On the other hand, a modified spin-wave theory precisely describes the low-temperature properties. Expanding the specific heat and the magnetic susceptibility, we reveal an analogy and a contrast between mixed-spin ferromagnets and ferrimagnets.",0512630v1 2006-04-28,Evidence for Carrier-Induced High-Tc Ferromagnetism in Mn-doped GaN film,"A GaN film doped with 8.2 % Mn was grown by the molecular-beam-epitaxy technique. Magnetization measurements show that this highly Mn-doped GaN film exhibits ferromagnetism above room temperature. It is also revealed that the high-temperature ferromagnetic state is significantly suppressed below 10 K, accompanied by an increase of the electrical resistivity with decreasing temperature. This observation clearly demonstrates a close relation between the ferromagnetism with extremely high-Tc and the carrier transport in the Mn-doped GaN film.",0604647v1 2006-05-23,Evidence for Crossed Andreev Reflections in bilayers of (100)YBCO and the itinerant ferromagnet SrRuO3,"Scanning tunneling spectroscopy measurements on thin epitaxial SrRuO3/(100)YBCO ferromagnet/superconductor bilayers, reveal localized regions in which the superconductor order parameter penetrates the ferromagnet to more than 26 nm, an order of magnitude larger than the coherence length in the ferromagnetic layer. These regions consist of narrow (< 10 nm) and long strips, separated by at least 200 nm, consistent with the known magnetic domain wall structure in SrRuO3. We attributed this behavior to Crossed Andreev Reflections, taking place in the vicinity of the magnetic domain walls.",0605573v1 2006-08-01,Voltage generation by ferromagnetic resonance,"A ferromagnet can resonantly absorbs rf radiation to sustain a steady precession of the magnetization around an internal or applied magnetic field. We show that under these ferromagnetic resonance (FMR) conditions, a dc voltage is generated at a normal-metal electric contact to a ferromagnet with spin-flip scattering. This mechanism allows an easy electric detection of magnetization dyamics.",0608022v1 2006-09-06,Stability of Ferromagnetism in Hubbard models on two-dimensional line graphs,"It is well known that the Hubbard model on a line graph has a flat band and ferromagnetic ground states in a certain density range. We show that for a Hubbard model on a line graph of a planar bipartite graph the ferromagnetic ground state is stable if one adds a special contribution to the kinetic energy which lifts the degeneracy of the lowest single particle state. Stability holds for sufficiently strong repulsion U. The model has extended single particle eigenstates, no degeneracy, and no band gap. It is therefore a good candidate for metallic ferromagnetism.",0609142v2 2006-09-17,Universal conductance fluctuations in epitaxial GaMnAs ferromagnets: structural and spin disorder,"Mesoscopic transport measurements reveal a large effective phase coherence length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal ferromagnets. Universal conductance fluctuations of single nanowires are compared for epilayers with a tailored anisotropy. At large magnetic fields, quantum interferences are due to structural disorder only, and an unusual behavior related to hole-induced ferromagnetism is evidenced, for both quantum interferences and decoherence. At small fields, phase coherence is shown to persist down to zero field, even in presence of magnons, and an additional spin disorder contribution to quantum interferences is observed under domain walls nucleation.",0609410v1 2006-10-25,Domain-Wall Waves (2D Magnons) in Superconducting Ferromagnets,"Propagation of the magnetization waves along domain walls (2D magnons) in a superconducting ferromagnet has been studied theoretically. The magnetostatic fields (long-range dipole-dipole interaction) have a crucial effect on the spectrum of 2D magnons. But this effect is essentially affected by the superconducting Meissner currents, which screen the magnetostatic fields and modify the long-wavelength spectrum from square-root to linear. The excitation of the domain wall waves by an electromagnetic wave incident on a superconducting-ferromagnet sample has been considered. This suggests using measurements of the surface impedance for studying the domain wall waves, and eventually for effective probing of superconductivity-ferromagnetism coexistence.",0610699v1 2006-12-10,Nonequilibrium Transport in Superconductor/Ferromagnet/Superconductor Diffusive Junctions: Interplay between Proximity Effect and Ferromagnetism,"The theory of the I-V characteristics in diffusive superconductor/weak ferromagnet/superconductor (SFS) junction is developed. We show that the exchange field $h$ of the ferromagnet manifests itself as an additional conductance peak at $eV \sim \Delta+h$ in the phase-coherent regime, when the Thouless energy is of the order of superconducting order parameter. The excess current exhibits non-monotonous dependence on the exchange field and non-trivial temperature behavior, which is strongly influenced by the temperature dependence of the exchange field.",0612239v1 2007-01-27,Spin blockade at semiconductor/ferromagnet junctions,"We study theoretically extraction of spin-polarized electrons at nonmagnetic semiconductor/ferromagnet junctions. The outflow of majority spin electrons from the semiconductor into the ferromagnet leaves a cloud of minority spin electrons in the semiconductor region near the junction, forming a local spin-dipole configuration at the semiconductor/ferromagnet interface. This minority spin cloud can limit the majority spin current through the junction creating a pronounced spin-blockade at a critical current. We calculate the critical spin-blockade current in both planar and cylindrical geometries and discuss possible experimental tests of our predictions.",0701678v2 2007-02-12,Crystalline phases in chiral ferromagnets: Destabilization of helical order,"In chiral ferromagnets, weak spin-orbit interactions twist the ferromagnetic order into spirals, leading to helical order. We investigate an extended Ginzburg-Landau theory of such systems where the helical order is destabilized in favor of crystalline phases. These crystalline phases are based on periodic arrangements of double-twist cylinders and are strongly reminiscent of blue phases in liquid crystals. We discuss the relevance of such blue phases for the phase diagram of the chiral ferromagnet MnSi.",0702287v2 2007-02-17,Magnetic Anisotropy of Co2+ as Signature of Intrinsic Ferromagnetism in ZnO:Co,"We report on the magnetic properties of thoroughly characterized Zn1-xCoxO epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an ""easy plane"" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.",0702410v1 2007-02-21,Enhancement of ferromagnetism upon thermal annealing in pure ZnO,"We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.",0702486v3 2003-09-08,Color Ferromagnetism of Quark Matter and Quantum Hall States of Gluons in SU(3) Gauge Theory,"We show a possibility that a color ferromagnetic state exists in SU(3) gauge theory of quark matter with two flavors. Although the state involves three types of unstable modes of gluons, all of these modes are stabilized by forming a quantum Hall state of one of the modes. We also show that at large chemical potential, a color superconducting state (2SC) appears even in the ferromagnetic state. This is because Meissner effect by condensed anti-triplet quark pairs does not work on the magnetic field in the ferromagnetic state.",0309066v1 2004-08-11,A Ferromagnetic Lieb-Mattis Theorem,"We prove ferromagnetic ordering of energy levels for XXX Heisenberg chains of any spin and XXZ spin chains with all spins equal to 1/2. Ferromagnetic ordering means that the minimum energies in the invariant subspaces of fixed total spin are monotone decreasing as a function of the total spin. This result provides a ferromagnetic analogue of the well-known theorem by Lieb and Mattis about ordering of energy levels in antiferromagnetic and ferrimagnetic systems on bipartite graphs.",0408020v3 2003-01-31,Competition of ferromagnetic and antiferromagnetic spin ordering in nuclear matter,"In the framework of a Fermi liquid theory it is considered the possibility of ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear matter with Skyrme effective interaction. The zero temperature dependence of ferromagnetic and antiferromagnetic spin polarization parameters as functions of density is found for SkM$^*$, SGII effective forces. It is shown that in the density domain, where both type of solutions of self--consistent equations exist, ferromagnetic spin state is more preferable than antiferromagnetic one.",0301101v1 2004-11-17,Universality in the entanglement structure of ferromagnets,"Systems of exchange-coupled spins are commonly used to model ferromagnets. The quantum correlations in such magnets are studied using tools from quantum information theory. Isotropic ferromagnets are shown to possess a universal low-temperature density matrix which precludes entanglement between spins, and the mechanism of entanglement cancellation is investigated, revealing a core of states resistant to pairwise entanglement cancellation. Numerical studies of one-, two-, and three-dimensional lattices as well as irregular geometries showed no entanglement in ferromagnets at any temperature or magnetic field strength.",0411125v2 1996-03-27,Ferromagnetic ground states of the Hubbard model on a complete graph,"We use group theory to derive the exact analytical expression of the ferromagnetic ground states of the Hubbard model on a complete graph for arbitrary lattice sites f and for arbitrary fillings $N$. We find that for $t>0$ and for $N=f+1$ the ground state is maximally ferromagnetic with total spin $S=(f-1)/2$. For $N > f+1$ the ground state is still ferromagnetic but becomes degenerate with respect to $S$.",9603008v1 2007-04-30,Spectroscopy and critical temperature of diffusive superconducting/ferromagnetic hybrid structures with spin-active interfaces,"The description of the proximity effect in superconducting/ferromagnetic heterostructures requires to use spin-dependent boundary conditions. Such boundary conditions must take into account the spin dependence of the phase shifts acquired by electrons upon scattering on the boundaries of ferromagnets. The present article shows that this property can strongly affect the critical temperature and the energy dependence of the density of states of diffusive heterostructures. These effects should allow a better caracterisation of diffusive superconductor/ferromagnet interfaces.",0704.3975v3 2007-05-23,Role of long range ferromagnetic order in the electronic structure of Sr$_{1-x}$Ca$_x$RuO$_3$,"We investigate the role of long range ferromagnetic order in the electronic structure of Sr$_{1-x}$Ca$_x$RuO$_3$ using high resolution photoemission spectroscopy. SrRuO$_3$ is a ferromagnetic metal but isostructural, isoelectronic CaRuO$_3$ is an enhanced paramagnet. Surface spectra of CaRuO$_3$ exhibit temperature induced modifications. This is not significant in other compositions. This may be attributed to the structural changes observed in previous studies. Interestingly, the bulk spectra reveal unusual spectral changes exhibiting large decrease in the coherent feature intensity corresponding to only ferromagnetic samples, although the Ru moment is very similar in all the compositions.",0705.3299v1 2007-06-05,Room-temperature ferromagnetism in nanoparticles of superconducting materials,"Nanoparticles of superconducting YBa2Cu3O7-delta (YBCO) (Tc = 91 K) exhibit ferromagnetism at room temperature while the bulk YBCO, obtained by heating the nanoparticles at high temperature (940 degree C), shows a linear magnetization curve. Across the superconducting transition temperature, the magnetization curve changes from that of a soft ferromagnet to a superconductor. Furthermore, our experiments reveal that not only nanoparticles of metal oxides but also metal nitrides such as NbN (Tc = 6 - 12 K) and delta-MoN (Tc ~ 6 K) exhibit room-temperature ferromagnetism.",0706.0634v1 2007-06-30,Zooming into the coexisting regime of ferromagnetism and superconductivity in ErRh4B4 single crystals,"High resolution measurements of the dynamic magnetic susceptibility are reported for ferromagnetic re-entrant superconductor, ErRh$_{4}$B$_{4}$. Detailed investigation of the coexisting regime reveals unusual temperature-asymmetric and magnetically anisotropic behavior. The superconducting phase appears via a series of discontinuous steps upon warming from the ferromagnetic normal phase, whereas the ferromagnetic phase develops via a gradual transition. A model based on local field inhomogeneity is proposed to explain the observations.",0707.0029v1 2007-07-22,Magnetic State Modification Induced by Superconducting Response in Ferromagnet/Superconductor Hybrids,"Magnetization measurements in superconductor/ferromagnet Nb/Co superlattices show a complex behavior as a function of temperature, applied field and sample history. In base to a simple model it is shown that this behavior is due to an interplay between the superconductor magnetization temperature dependence, the ferromagnet magnetization time dependence, and the stray fields of both materials. It is also shown that the magnetic state of the Co layers is modified by the Nb superconducting response, implying that the problem of a superconductor/ferromagnetic heterogeneous sample has to be solved in a self-consistent manner.",0707.3265v1 2007-08-13,Spin valve effect by ballistic transport in ferromagnetic metal (MnAs) / semiconductor (GaAs) hybrid heterostructures,"We demonstrate the spin valve effect by ballistic transport in fully epitaxial MnAs ferromagnetic metal / GaAs semiconductor / GaAs:MnAs granular hybrid heterostructures. The GaAs:MnAs material contains ferromagnetic MnAs nanoparticles in a GaAs matrix, and acts as a spin injector and a spin detector. Although the barrier height of the GaAs/MnAs interface was found to be very small, relatively large magnetoresistance was observed. This result shows that by using ballistic transport, we can realize a large spin valve effect without inserting a high tunnel barrier at the ferromagnetic metal / semiconductor interface.",0708.1681v1 2007-08-23,Current-induced torques due to compensated antiferromagnets,"We analyse the influence of current induced torques on the magnetization configuration of a ferromagnet in a circuit containing a compensated antiferromagnet. We argue that these torques are generically non-zero and support this conclusion with a microscopic NEGF calculation for a circuit containing antiferromagnetic NiMn and ferromagnetic Co layers. Because of symmetry dictated differences in the form of the current-induced torque, the phase diagram which expresses the dependence of ferromagnet configuration on current and external magnetic field differs qualitatively from its ferromagnet-only counterpart.",0708.3231v1 2007-08-24,Enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems,"We analyzed the enhancement of the Gilbert damping constant due to spin pumping in non-collinear ferromagnet / non-magnet / ferromagnet trilayer systems. We show that the Gilbert damping constant depends both on the precession angle of the magnetization of the free layer and on the direction of the magntization of the fixed layer. We find the condition to be satisfied to realize strong enhancement of the Gilbert damping constant.",0708.3323v1 2007-09-12,Two Ferromagnetic Phases in Spin-Fermion Systems,"We consider spin-fermion systems which get their magnetic properties from a system of localized magnetic moments being coupled to conducting electrons. The dynamical degrees of freedom are spin-$s$ operators of localized spins and spin-1/2 fermi operators of itinerant electrons. We develop modified spin-wave theory and obtain that system has two ferromagnetic phases. At the characteristic temperature T* the magnetization of itinerant electrons becomes zero, and high temperature ferromagnetic phase (T*T_\mathrm{C}$, which reflects characteristics of itinerant ferromagnets including a spin-orbit interaction.",2110.06478v1 2022-01-16,Ferromagnetic resonance modulation in $d$-wave superconductor/ferromagnetic insulator bilayer systems,"We investigate ferromagnetic resonance (FMR) modulation in $d$-wave superconductor (SC)/ferromagnetic insulator (FI) bilayer systems theoretically. The modulation of the Gilbert damping in these systems reflects the existence of nodes in the $d$-wave SC and shows power-law decay characteristics within the low-temperature and low-frequency limit. Our results indicate the effectiveness of use of spin pumping as a probe technique to determine the symmetry of unconventional SCs with high sensitivity for nanoscale thin films.",2201.06060v2 2022-02-03,Circular photogalvanic effects in topological insulator/ferromagnet hybrid structures,"We study laser driven spin-current effects at ferromagnet/topological-insulator interfaces by two complementary experimental approaches. The DC photocurrent is studied in ferromagnet/topological-insulator bilayers with high spatial resolution. Dynamic interface currents are explored via the emission of terahertz radiation emitted by these currents with high temporal resolution. From our experiments, we reveal a lateral and dynamic interaction of the ferromagnet and the topological insulator interface.",2202.01522v1 2022-12-22,Planar Magnetic Paul Traps for Ferromagnetic Particles,"We present a study on the trapping of hard ferromagnetic particles using alternating magnetic fields, with a focus on planar trap geometries. First, we realize and characterize a magnetic Paul trap design for millimeter-size magnets based on a rotating magnetic potential. Employing a physically rotating platform with two pairs of permanent magnets with opposite poles, we show stable trapping of hard ferromagnets a centimeter above the trap and demonstrate that the particle shape plays a critical role in the trapping. Finally, we propose a chip trap design that will open a path to studies of gyromagnetic effects with ferromagnetic micro-particles.",2212.11622v1 2023-01-26,Anisotropic spin-current spectroscopy of ferromagnetic superconducting gap symmetries,"We develop a microscopic theory of tunneling spin transport at the magnetic interface between a ferromagnetic insulator and a ferromagnetic superconductor (FSC) driven by ferromagnetic resonance. We show that the spin susceptibilities of the FSC can be extracted from the spin currents by tuning the easy axis of the FSC, and thus the spin currents can be a probe for the symmetries of the spin-triplet Cooper pairing. Our results will offer a route to exploiting the synergy of magnetism and superconductivities for spin devices.",2301.11027v3 2023-12-28,Detecting bulk carbon ferromagnetism in graphene multi-edge structure,"The emergence of bulk carbon ferromagnetism is long-expected over years. At nanoscale, carbon ferromagnetism was detected by analyzing the magnetic edge states via scanning tunneling microscopy(STM), and its origin can be explained by local redistribution of electron wave function. In larger scale, carbon ferromagnetism can be created by deliberately producing defects in graphite, and detected by macroscopic technical magnetization. Meanwhile, it becomes crucial to determine that the detected magnetization is originated from carbon rather than from magnetic impurities. One solution is X-ray magnetic circular dichroism (XMCD). Nonetheless, a reproducible, full section of XMCD spectrum across C-1s absorption energy has not appeared yet, which should be decisive for assuring the indisputable existence of bulk carbon ferromagnetism. Besides, the lack of direct observation on the atomic structure of the ferromagnetic carbon leaves the structural origin of its ferromagnetism still in mist. In this work, for detecting bulk carbon ferromagnetism, we managed to grow all-carbon film consisting of vertically aligned graphene multi-edge (VGME), which wove into a three-dimensional hyperfine-porous network. Magnetization (M-H) curves and XMCD spectra co-confirmed bulk carbon ferromagnetism of VGME at room temperature, with the average unit magnetic momentum of ~0.0006 miuB/atom. The influence of magnetic impurities on magnetization was excluded by both absorption spectra and inductively coupled plasma mass spectrometry measurements. The spin transfer behavior also verified the long-range and robust feature of the bulk carbon ferromagnetism. Our work provides direct evidence of elementary resolved bulk carbon ferromagnetism at room temperature and clarifies its origin from pi-electrons at graphene edges.",2312.16925v1 2000-04-05,Ferromagnetism of quark liquid and magnetars,"Spontaneous magnetization of quark liquid is examined on the analogy with that in electron gas. It is pointed out that quark liquid has potential to be ferromagnetic at rather low densities, around nuclear saturation density. Somme comments are given as for implications on magnetars.",0004062v1 1996-05-24,Ferromagnetism in the Hubbard model with an infinite-range hopping,"We prove, as recently conjectured, that the ground state of the Hubbard Hamiltonian with an infinite-range hopping, when the number of electrons $N_e=N+1$ ($N$ being the number of sites), is ferromagnetic fully polarized.",9605156v1 1996-06-06,Spherical P-spin glass at $P\to\infty$ and the information storing by continuous spins,"The $P\to\infty$ limit was considered in the spherical P-spin glass. It is possible to store information in the vacuum configuration of ferromagnetic phase. Maximal allowed level of noise was calculated in ferromagnetic phase.",9606034v1 1996-12-05,DMRG study of ferromagnetism in a one-dimensional Hubbard model,"The one dimensional Hubbard model with nearest and (negative) next-nearest neighbour hopping has been studied with the density-matrix renormalization group (DMRG) method. A large region of ferromagnetism has been found for finite density and finite on-site interaction.",9612056v1 1997-03-28,Optical Spectra in the Ferromagnetic States near the Charge Ordering,"The optical conductivity is studied numerically for the ferromagnetic metallic state close to the charge ordering observed in perovskite manganites.",9703245v1 1998-05-15,The gauge equivalence of the Zakharov equations and (2+1)-dimensional continuous Heisenberg ferromagnetic models,"The gauge equivalence between the (2+1)-dimensional Zakharov equations and (2+1)-dimensional integrable continuous Heisenberg ferromagnetic model is established. Also their integrable reductions are shown explicitly.",9805187v1 2003-04-18,Elementary Excitations of Ferromagnetic Metal Nanoparticles,"We present a theory of the elementary spin excitations in transition metal ferromagnet nanoparticles which achieves a unified and consistent quantum description of both collective and quasiparticle physics. The theory starts by recognizing the essential role played by spin-orbit interactions in determining the energies of ferromagnetic resonances in the collective excitation spectrum and the strength of their coupling to low-energy particle-hole excitations. We argue that a crossover between Landau-damped ferromagnetic resonance and pure-state collective magnetic excitations occurs as the number of atoms in typical transition metal ferromagnet nanoparticles drops below approximately $10^4$, approximately where the single-particle level spacing, $\delta$, becomes larger than, $\sqrt{\alpha} E_{\rm res}$, where $E_{\rm res}$ is the ferromagnetic resonance frequency and $\alpha$ is the Gilbert damping parameter. We illustrate our ideas by studying the properties of semi-realistic model Hamiltonians, which we solve numerically for nanoparticles containing several hundred atoms. For small nanoparticles, we find one isolated ferromagnetic resonance collective mode below the lowest particle-hole excitation energy, at $E_{\rm res} \approx 0.1$ meV. The spectral weight of this pure excitation nearly exhausts the transverse dynamical susceptibility spectral weight. As $\delta$ approaches $\sqrt{\alpha} E_{\rm res}$, the ferromagnetic collective excitation is more likely to couple strongly with discrete particle-hole excitations. In this regime the distinction between the two types of excitations blurs. We discuss the significance of this picture for the interpretation of recent single-electron tunneling experiments.",0304427v1 2004-07-28,"Comment on ""Exchange interaction parameters and adiabatic spin-wave spectra of ferromagnets: A 'renormalized magnetic force theorem'""","This is a comment on publication in Phys. Rev. Lett. 90, 087205 (2003): ``Exchange interaction parameters and adiabatic spin-wave spectra of ferromagnets: A `renormalized magnetic force theorem'''",0407739v1 2005-11-21,"Reply to ""Comment on ""Domain Structure in a Superconducting Ferromagnet""""","This short article is a reply to the comment from E.B. Sonin on our paper on the domain structure in a superconducting ferromagnet published in Phys. Rev. Lett..",0511517v1 2003-10-09,A Criterion for Ferromagnetism in the Isotropic Quantum Heisenberg Model,"A criterion for ferromagnetism is presented suggesting a line of proof to rigorously establish the phase transition. Spectral information will be required in certain invariant subspaces of the Hamiltonian, hopefully the relatively crude estimates needed will be not too difficult to establish.",0310013v1 2008-04-10,Nonergodic thermodynamics of disordered ferromagnets and ferroelectrics,"Phenomenological thermodynamic theory describing the properties of metastable states in disordered ferromagnets and ferroelectrics with frustrative random interactions is developed and its ability to describe various nonergodic phenomena in real crystals is demonstrated.",0804.1713v1 2009-02-10,Nucleation of superconductivity and vortex matter in superconductor - ferromagnet hybrids,"The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially-separated conventional low-temperature superconductor (S) and ferromagnet (F), is reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field H_{ext}, different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affect the nucleation of superconductivity leading to an exotic dependence of the critical temperature T_{c} on H_{ext}. Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this ""soft"" S/F hybrids requires rearrangements of both, superconducting and ferromagnetic characteristics, as compared with ""hard"" S/F structures.",0902.1630v1 2010-04-16,How to recognize a nearly-flat-band ferromagnet by means of thermodynamic measurements?,"We make an attempt at unveiling the thermodynamic ""signature"" of a specific class of electronic systems, the so called nearly-flat-band paramagnets and ferromagnets that can theoretically be described by appropriate versions of the Hubbard model.",1004.2786v1 2010-07-15,Variational cluster approach to ferromagnetism in infinite dimensions and in one-dimensional chains,"The variational cluster approach (VCA) is applied to study spontaneous ferromagnetism in the Hubbard model at zero temperature. We discuss several technical improvements of the numerical implementation of the VCA which become necessary for studies of a ferromagnetically ordered phase, e.g. more accurate techniques to evaluate the variational ground-state energy, improved local as well as global algorithms to find stationary points, and different methods to locate the magnetic phase transition. Using the single-site VCA, i.e. the dynamical impurity approximation (DIA), the ferromagnetic phase diagram of the model in infinite dimensions is worked out. The results are compared with previous dynamical mean-field studies for benchmarking purposes. The DIA results provide a unified picture of ferromagnetism in the infinite-dimensional model by interlinking different parameter regimes that are governed by different mechanisms for ferromagnetic order. Using the DIA and the VCA, we then study ferromagnetism in one-dimensional Hubbard chains with nearest and next-nearest-neighbor hopping t2. In comparison with previous results from the density-matrix renormalization group, the phase diagram is mapped out as a function of the Hubbard-U, the electron filling and t2. The stability of the ferromagnetic ground state against local and short-range non-local quantum fluctuations is discussed.",1007.2517v1 2011-07-15,Ferromagnetic-glassy transitions in three-dimensional Ising spin glasses,"We investigate the ferromagnetic-glassy transitions which separate the low-temperature ferromagnetic and spin-glass phases in the temperature-disorder phase diagram of three-dimensional Ising spin-glass models. For this purpose, we consider the cubic-lattice +-J (Edwards-Anderson) Ising model with bond distribution $P(J) = p \delta(J - 1) + (1-p) \delta(J + 1)$, and present a numerical Monte Carlo study of the critical behavior along the line that marks the onset of ferromagnetism. The finite-size scaling analysis of the Monte Carlo data shows that the ferromagnetic-glassy transition line is slightly reentrant. As a consequence, for an interval of the disorder parameter p, around p=0.77, the system presents a low-temperature glassy phase, an intermediate ferromagnetic phase, and a high-temperature paramagnetic phase. Along the ferromagnetic-glassy transition line magnetic correlations show a universal critical behavior with critical exponents nu=0.96(2) and eta=-0.39(2). The hyperscaling relation beta/nu = (1 + eta)/2 is satisfied at the transitions, so that beta/nu = 0.305(10). This magnetic critical behavior represents a new universality class for ferromagnetic transitions in Ising-like disordered systems. Overlap correlations are apparently not critical and show a smooth behavior across the transition.",1107.3005v1 2012-06-15,"Comment on ""Anisotropic Critical Magnetic Fluctuations in the Ferromagnetic Superconductor UCoGe""","We have presented the potential explanation of nonvanishing at q=0 Landau damping measured experimentally in ferromagnetic compounds UGe2 and UCoGe based on possible intersection of the Fermi sheets corresponding different bands.",1206.3468v1 2013-02-08,"Reply to the Comment on ""Unified Formulism of Andreev Reflection at a Ferromagnetic/Superconductor Interface"" by Eschrig et al","Reply to the Comment on ""Unified Formulism of Andreev Reflection at a Ferromagnetic/Superconductor Interface"" by Eschrig et al",1302.2166v1 2018-11-29,Ferromagnetic Neutron Stars in Scalar-Tensor Theories of Gravity,"Ferromagnetic spin ordering can take place in neutron stars. This phase transition alters the neutron star equation of state. Here, applying the scalar-tensor theories of gravity, we investigate the structure of neutron stars which are in the ferromagnetic phase. Considering the equation of state of ferromagnetic neutron matter with Skyrme-type interactions at zero temperature and using the scalar-tensor theories of gravity with sufficiently negative coupling constant, we explore the spontaneous scalarization in ferromagnetic neutron stars. In this regard, the mass versus the central density, the profiles of scalar field and mass and density, the central scalar field for different central densities, and the mass-radius relation of ferromagnetic neutron stars are presented. Moreover, we investigate the influences of the coupling constant on the scalarization of ferromagnetic neutron stars. The effects of the coupling constant on the critical densities of scalarization and the scalar charge of ferromagnetic neutron stars are also calculated. In addition, we study the maximum value of the coupling constant at which the spontaneous scalarization takes place presenting the influence of the neutron matter equation of state on this critical value.",1811.12090v2 2014-03-25,Unconventional critical scaling of magnetization in uranium ferromagnetic superconductors UGe$_2$ and URhGe,"We report a dc magnetization study of the critical phenomenon around the ferromagnetic transition temperature T_C in high-quality single crystals of uranium ferromagnetic superconductors UGe2 and URhGe. The critical exponents, beta for the temperature dependence of the magnetization below T_C, gamma for the magnetic susceptibility, and delta for the magnetic isothermal at T_C have been determined with a modified Arrott plot, a Kouvel-Fisher plot, and the scaling analysis. Magnetization in the ferromagnetic state has strong uniaxial magnetic anisotropy in the two compounds. However, the universality class of the critical phenomena do not belong to the three dimensional (3D) Ising system. Although the values of beta in UGe2 and URhGe are close to those in the 3D magnets, the values of gamma are close to unity, that expected from the mean field theory. Similar critical exponents have been reported previously for the 3D Ising ferromagnet UIr where superconductivity appears under high pressure. The critical behavior may be limited to a very narrow Ginzburg critical region of 1 mK because of the strong itinerant character of the 5f electrons in the ferromagnetic superconductor UCoGe where the mean field behavior of the magnetization has been reported. The unconventional critical scaling of magnetization in UGe2, URhGe and UIr cannot be explained via previous approaches to critical phenomena. The ferromagnetic correlation between the 5f electrons differs from that in the 3D Ising system and this difference may be a key point for the understanding of the ferromagnetism where superconductivity emerges.",1403.6197v1 2008-07-22,Anisotropic tunneling magnetoresistance and tunneling anisotropic magnetoresistance: spin-orbit coupling in magnetic tunnel junctions,"The effects of the spin-orbit interaction on the tunneling magnetoresistance of ferromagnet/semiconductor/normal metal tunnel junctions are investigated. Analytical expressions for the tunneling anisotropic magnetoresistance (TAMR) are derived within an approximation in which the dependence of the magnetoresistance on the magnetization orientation in the ferromagnet originates from the interference between Bychkov-Rashba and Dresselhaus spin-orbit couplings that appear at junction interfaces and in the tunneling region. We also investigate the transport properties of ferromagnet/semiconductor/ferromagnet tunnel junctions and show that in such structures the spin-orbit interaction leads not only to the TAMR effect but also to the anisotropy of the conventional tunneling magnetoresistance (TMR). The resulting anisotropic tunneling magnetoresistance (ATMR) depends on the absolute magnetization directions in the ferromagnets. Within the proposed model, depending on the magnetization directions in the ferromagnets, the interplay of Bychkov-Rashba and Dresselhaus spin-orbit couplings produces differences between the rates of transmitted and reflected spins at the ferromagnet/seminconductor interfaces, which results in an anisotropic local density of states at the Fermi surface and in the TAMR and ATMR effects. Model calculations for Fe/GaAs/Fe tunnel junctions are presented. Furthermore, based on rather general symmetry considerations, we deduce the form of the magnetoresistance dependence on the absolute orientations of the magnetizations in the ferromagnets.",0807.3564v1 2017-12-19,"Ba(Zn,Co)2As2: a II-II-V Diluted Ferromagnetic Semiconductor with N-type Carriers","Diluted ferromagnetic semiconductors (DMSs) that combine the properties of semiconductors with ferromagnetism have potential application in spin-sensitive electronics (spintronics) devices. The search for DMS materials exploded after the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a series of DMS compounds isostructural to iron-based superconductors have been reported. Among them, the highest Curie temperature $T_C$ of 230 K has been achieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As, are p-type, i.e., the carriers that mediate ferromagnetism are holes. For practical applications, DMS with n-type carriers are also advantageous. Here we report the successful synthesis of a II-II-V diluted ferromagnetic semiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization measurements show that the ferromagnetic transition occurs up to $T_{C} \sim$ 45 K. Hall effect and Seebeck effect measurements jointly confirm that the dominant carriers are electrons. Through muon spin relaxation ($\mu$SR), a volume sensitive magnetic probe, we have also confirmed that the ferromagnetism in Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static.",1712.06764v1 2018-09-14,Non-collinearity and spin frustration in the itinerant kagome ferromagnet Fe3Sn2,"Frustrated itinerant ferromagnets, with non-collinear static spin structures, are an exciting class of material as their spin chirality can introduce a Berry phase in the electronic scattering and lead to exotic electronic phenomena such as the anomalous Hall effect (AHE). This study presents a reexamination of the magnetic properties of Fe$_3$Sn$_2$, a metallic ferromagnet, based on the 2-dimensional kagome bilayer structure. Previously thought of as a conventional ferromagnet, we show using a combination of SQUID measurements, symmetry analysis and powder neutron diffraction, that Fe$_3$Sn$_2$ is a frustrated ferromagnet with a temperature-dependent non-collinear spin structure. The complexity of the magnetic interactions is further evidenced by a re-entrant spin glass transition ($T_f\simeq80$\,K) at temperatures far below the main ferromagnetic transition ($T\mathrm{_C}$ = 640\,K). Fe$_3$Sn$_2$ therefore provides a rare example of a frustrated itinerant ferromagnet. Further, as well as being of great fundamental interest our studies highlight the potential of Fe$_3$Sn$_2$ for practical application in spintronics technology, as the AHE arising from the ferromagnetism in this material is expected to be enhanced by the coupling between the conduction electrons and the non-trivial magnetic structure over an exceptionally wide temperature range.",1809.05466v1 2018-10-30,Effect of Coulomb interaction on the two-dimensional electronic structure of the van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$,"In order to investigate the electronic properties of the semiconducting van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$ (CGT), where ferromagnetic layers are bonded through van der Waals forces, we have performed angle-resolved photoemission spectroscopy (ARPES) measurements and density-functional-theory (DFT+U) calculations. The valence-band maximum at the {\Gamma} point is located $\sim$ 0.2 eV below the Fermi level, consistent with the semiconducting property of CGT. Comparison of the experimental density of states with the DFT calculation has suggested that Coulomb interaction between the Cr 3d electrons U$_{\rm eff}$ $\sim$ 1.1 eV. The DFT+U calculation indicates that magnetic coupling between Cr atoms within the layer is ferromagnetic if Coulomb U $_{\rm eff}$ is smaller than 3.0 eV and that the inter-layer coupling is ferromagnetic below U$_{\rm eff}$ $\sim$ 1.0 eV. We therefore conclude that, for U$_{\rm eff}$ deduced by the experiment, the intra-layer Cr-Cr coupling is ferromagnetic and the inter-layer coupling is near the boundary between ferromagnetic and antiferromagnetic, which means experimentally deduced U$_{\rm eff}$ is consistent with theoretical ferromagnetic condition.",1810.12564v4 2020-12-18,Tunable ferromagnetism at non-integer filling of a moiré superlattice,"The flat bands resulting from moir\'e superlattices in magic-angle twisted bilayer graphene (MATBG) and ABC-trilayer graphene aligned with hexagonal boron nitride (ABC-TLG/hBN) have been shown to give rise to fascinating correlated electron phenomena such as correlated insulators and superconductivity. More recently, orbital magnetism associated with correlated Chern insulators was found in this class of layered structures centered at integer multiples of n0, the density corresponding to one electron per moir\'e superlattice unit cell. Here we report the experimental observation of ferromagnetism at fractional filling of a flat Chern band in an ABC-TLG/hBN moir\'esuperlattice. The ferromagnetic state exhibits prominent ferromagnetic hysteresis behavior with large anomalous Hall resistivity in a broad region of densities, centered in the valence miniband at n = -2.3 n0. This ferromagnetism depends very sensitively on the control parameters in the moir\'e system: not only the magnitude of the anomalous Hall signal, but also the sign of the hysteretic ferromagnetic response can be modulated by tuning the carrier density and displacement field. Our discovery of electrically tunable ferromagnetism in a moir\'e Chern band at non-integer filling highlights the opportunities for exploring new correlated ferromagnetic states in moir\'e heterostructures.",2012.10075v1 2019-03-28,High spin-polarization in the low Curie temperature complex itinerant ferromagnet EuTi$_{1-x}$Nb$_x$O$_3$,"The physical systems with ferromagnetism and ""bad"" metallicity hosting unusual transport properties are playgrounds of novel quantum phenomena. Recently EuTi$_{1-x}$Nb$_x$O$_3$ emerged as a ferromagnetic system where non-trivial temperature dependent transport properties are observed due to coexistence and competition of various magnetic and non-magnetic scattering processes. In the ferromagnetic state, the resistivity shows a $T^2$ temperature dependence possibly due to electron-magnon scattering and above the Curie temperature $T_c$, the dependence changes to $T^{3/2}$ behaviour indicating a correlation between transport and magnetic properties. In this paper, we show that the transport spin-polarization in EuTi$_{1-x}$Nb$_x$O$_3$, a low Curie temperature ferromagnet, is as high ($\sim 40\%$) as that in some of the metallic ferromagnets with high Curie temperatures. In addition, owing to the low Curie temperature of EuTi$_{1-x}$Nb$_x$O$_3$, the temperature ($T$) dependence of $P_t$ could be measured systematically up to $T_c$ which revealed a proportionate relationship with magnetization $M_s$ vs. $T$. This indicates that such proportionality is far more universally valid than the ferromagnets with ideal parabolic bands. Furthermore, our band structure calculations not only helped understand the origin of such high spin polarization in EuTi$_{1-x}$Nb$_x$O$_3$ but also provided a route to estimate the Hubbard $U$ parameter in complex metallic ferromagnets in general using experimental inputs.",1903.12022v1 2019-11-26,Modulating Super-Exchange Strength to Achieve Robust Ferromagnetic Couplings in Two-Dimensional Semiconductors,"Low-dimensional semiconducting ferromagnets have attracted considerable attention due to their promising applications as nano-size spintronics. However, realizing robust ferromagnetic couplings that can survive at high temperature is restrained by two decisive factors: super-exchange couplings and anisotropy. Despite widely explored low-dimensional anisotropy, strengthening super-exchange couplings has rarely been investigated. Here, we found that ligands with lower electronegativity can strengthen ferromagnetic super-exchange couplings and further proposed the ligand modulation strategy to enhance the Curie temperature of low-dimensional ferromagnets. Based on the metallic CrX2 (X = S, Se, Te) family, substituting ligand atoms by halides can form stable semiconducting phase as CrSeCl, CrSeBr and CrTeBr. It is interesting to discover that, the nearest ferromagnetic super-exchange couplings can be strengthened when substituting ligands from S to Se and Te. Such evolution originates from the enhanced electron hopping integral and reduced energy intervals between d and p orbits. While the second nearest anti-ferromagnetic couplings are also benefitted due to delocalized p-p interactions. Finally, ligand modulation strategy is applied in other ferromagnetic monolayers, further verifying our theory and providing a fundamental understanding on controlling super-exchange couplings in low-dimension.",1911.11333v1 2020-05-28,Spin-Pumping-Induced Non-Linear Electric Current on the Surface of a Ferromagnetic Topological Insulator,"We investigate the spin-pumping-induced electric current on the surface of a three-dimensional topological insulator hybridized with a ferromagnet, namely, ferromagnetic topological insulator. In order to do this, we establish the microscopic formalism and construct the perturbation theory using a Keldysh Green's function approach. We analyze how this electric current is generated by an exchange interaction and an external ac magnetic field, which is the driving force of ferromagnetic resonance as well as the spin pumping. The mechanism is as follows. First, the ferromagnetic resonance is driven and a zero-momentum magnon emerges. It is the fluctuation from the saturation magnetization pointing parallel to the precession axis of the ferromagnetic resonance. After then, the spin pumping is generated with the zero-momentum magnon being the carrier of spin. The zero-momentum magnon and the topological insulator surface state couples through the exchange interaction and the spin carried by the magnon is transferred to it. Owing to the spin-momentum locking, the transferred spin is converted into the momentum of topological insulator surface state leading to the generation of electric current flowing perpendicular to the precession axis of the ferromagnetic resonance. It is quadratic in the amplitude of external ac magnetic field whereas it is linear to the strength of the exchange interaction. The associated electric voltage is described by the spectrum of zero-momentum magnon. The non-linearity of spin-pumping-induced electric current in the ac magnetic field as well as the linearity in the exchange-interaction strength reflects that the surface of ferromagnetic topological insulator has a high-performing functionality of generating the electric charge current by magnetic controlling.",2005.13850v1 2020-11-24,Ground state properties and exact thermodynamics of a 2-leg anisotropic spin ladder system,"We study a frustrated two-leg spin ladder with alternate isotropic Heisenberg and Ising rung exchange interactions, whereas, interactions along legs and diagonals are Ising-type. All the interactions in the ladder are anti-ferromagnetic in nature and induce frustration in the system. This model shows four interesting quantum phases: (i) stripe rung ferromagnetic (SRFM), (ii) stripe rung ferromagnetic with edge singlet (SRFM-E), (iii) anisotropic antiferromagnetic (AAFM), and (iv) stripe leg ferromagnetic (SLFM) phase. We construct a quantum phase diagram for this model and show that in stripe rung ferromagnet (SRFM), the same type of sublattice spins (either $S$ or $\sigma$-type spins) are aligned in the same direction. Whereas, in anisotropic antiferromagnetic phase, both $S$ and $\sigma$-type of spins are anti-ferromagnetically aligned with each other, two nearest $S$ spins along the rung form an anisotropic singlet bond whereas two nearest $\sigma$ spins form an Ising bond. In large Heisenberg rung exchange interaction limit, spins on each leg are ferromagnetically aligned, but spins on different legs are anti-ferromagnetically aligned. The thermodynamic quantities like $Cv(T)$, $\chi(T)$ and $S(T)$ are also calculated using the transfer matrix method for different phase. The magnetic gap in the SRFM and the SLFM can be notice from $\chi(T)$ and $Cv(T)$ curves.",2011.11901v1 2021-03-17,Tunable electronic structure and magnetic anisotropy in bilayer ferromagnetic semiconductor Cr2Ge2Te6,"The emergence of ferromagnetism in two-dimensional van der Waals materials has aroused broad interest. However, the ferromagnetic instability has been a problem remained. In this work, by using the first-principles calculations, we identified the critical ranges of strain and doping for the bilayer Cr2Ge2Te6 within which the ferromagnetic stability can be enhanced. Beyond the critical range, the tensile strain can induce the phase transition from the ferromagnetic to the antiferromagnetic, and the direction of magnetic easy axis can be converted from out-of-plane to in-plane due to the increase of compressive strain, or electrostatic doping. We also predicted an electron doping range, within which the ferromagnetism can be enhanced, while the ferromagnetic stability was maintained. Moreover, we found that the compressive strain can reverse the spin polarization of electrons at the conduction band minimum, so that two categories of half-metal can be induced by controlling electrostatic doping in the bilayer Cr2Ge2Te6. These results should shed a light on achieving ferromagnetic stability for low-dimensional materials.",2103.09456v1 2021-05-11,Pressure-enhanced ferromagnetism in layered CrSiTe3 flakes,"The research on van der Waals (vdW) layered ferromagnets have promoted the development of nanoscale spintronics and applications. However, low-temperature ferromagnetic properties of these materials greatly hinder their applications. Here, we report pressure-enhanced ferromagnetic behaviours in layered CrSiTe3 flakes revealed by high-pressure magnetic circular dichroism (MCD) measurement. At ambient pressure, CrSiTe3 undergoes a paramagnetic-to-ferromagnetic phase transition at 32.8 K, with a negligible hysteresis loop, indicating a soft ferromagnetic behaviour. Under 4.6 GPa pressure, the soft ferromagnet changes into hard one, signalled by a rectangular hysteretic loop with remnant magnetization at zero field. Interestingly, with further increasing pressure, the coercive field (H_c) dramatically increases from 0.02 T at 4.6 GPa to 0.17 T at 7.8 GPa, and the Curie temperature (T_c^h: the temperature for closing the hysteresis loop) also increases from ~36 K at 4.6 GPa to ~138 K at 7.8 GPa. The influences of pressure on exchange interactions are further investigated by density functional theory calculations, which reveal that the in-plane nearest-neighbor exchange interaction and magneto-crystalline anisotropy increase simultaneously as pressure increases, leading to increased H_c and T_c^h in experiments. The effective interaction between magnetic couplings and external pressure offers new opportunities for both searching room-temperature layered ferromagnets and designing pressure-sensitive magnetic functional devices.",2105.04740v1 2022-02-09,"Ferromagnetism induced by hybridization of Fe 3d orbitals with ligand InSb bands in n-type ferromagnetic semiconductor (In,Fe)Sb","Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC > 300 K). To clarify the mechanism of the high-TC ferromagnetism, we have investigated the electronic structure and magnetic properties of an (In,Fe)Sb thin film by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe L2,3 edges. The magnetic-field dependence of the XMCD spectra reveals that there are ferromagnetic-like Fe and paramagnetic-like Fe components in the (In,Fe)Sb thin film. The XAS and XMCD spectra of the ferromagnetic-like and paramagnetic-like Fe components resemble those of other Fe-doped FMSs and extrinsic oxides, respectively. The finite value of the ratio between the orbital and spin magnetic moments estimated by applying the XMCD sum rules indicates that the valence state of the Fe ions substituting for the In sites in (In,Fe)Sb is not purely ionic Fe3+, but intermediate between Fe3+ and Fe2+. The qualitative correspondence between the magnetic-field dependence of the visible-light magnetic circular dichroism intensity and that of the XMCD intensity demonstrates that the Zeeman splitting of the InSb band is proportional to the net magnetization of the doped Fe. These results suggest that the ferromagnetism of (In,Fe)Sb originates from the Fe 3d orbitals hybridized with the host InSb bands.",2202.04286v1 2022-04-26,Bulk domain Meissner state in the ferromagnetic superconductor EuFe$_{2}$(As$_{0.8}$P$_{0.2}$)$_{2}$: Consequence of compromise between ferromagnetism and superconductivity,"Small-angle neutron scattering (SANS) measurements are performed on the ferromagnetic superconductor EuFe$_{2}$(As$_{0.8}$P$_{0.2}$)$_{2}$ ($T\rm_{sc}=22.5\,$K) to probe the delicate interplay between ferromagnetism and superconductivity. A clear signature of large ferromagnetic domains is found below the ferromagnetic ordering temperature $T\rm_{C}$ = 18.5 K. In a small temperature interval of $\sim$ 1.5 K below $T\rm_{C}$, additional SANS signal is observed, of which the indirect Fourier transform reveals characteristic length scales in between $\sim$ 80 nm to $\sim$ 160 nm. These nanometer-scaled domain structures are identified to result from an intermediate inhomogeneous Meissner effect denoted domain Meissner state, which was recently observed on the surface of EuFe$_{2}$(As$_{0.79}$P$_{0.21}$)$_{2}$ crystals by means of magnetic force microscopy [V. S. Stolyarov $\mathit{et}$ $\mathit{al.}$, Sci. Adv. 4, 1061 (2018)], ascribing to the competition between ferromagnetism and superconductivity. Our measurements clearly render the domain Meissner state as a bulk phenomenon and provide a key solution to the mystery regarding the intriguing coexistence of strong ferromagnetism and bulk superconductivity in these compounds.",2204.12094v1 2022-06-16,Tunable Ferromagnetism in LaCoO3 Epitaxial Thin Films,"Ferromagnetic insulators play a crucial role in the development of low-dissipation quantum magnetic devices for spintronics. Epitaxial LaCoO3 thin film is a prominent ferromagnetic insulator, in which the robust ferromagnetic ordering emerges owing to epitaxial strain. Whereas it is evident that strong spin-lattice coupling induces ferromagnetism, the reported ferromagnetic properties of epitaxially strained LaCoO3 thin films were highly consistent. For example, even under largely modulated degree of strain, the reported Curie temperatures of epitaxially strained LaCoO3 thin films lie within 80-85 K, without much deviation. In this study, substantial enhancement (~18%) in the Curie temperature of epitaxial LaCoO3 thin films is demonstrated via crystallographic orientation dependence. By changing the crystallographic orientation of the films from (111) to (110), the crystal-field energy was reduced and the charge transfer between the Co and O orbitals was enhanced. These modifications led to a considerable enhancement of the ferromagnetic properties (including the Curie temperature and magnetization), despite the identical nominal degree of epitaxial strain. The findings of this study provide insights into facile tunability of ferromagnetic properties via structural symmetry control in LaCoO3.",2206.08196v1 2022-07-28,Evolution of short-range magnetic correlations in ferromagnetic Ni-V alloys,"We experimentally study how the magnetic correlations develop in a binary alloy close to the ferromagnetic quantum critical point with small-angle neutron scattering (SANS). Upon alloying the itinerant ferromagnet nickel with vanadium, the ferromagnetic order is continuously suppressed. The critical temperature Tc vanishes when vanadium concentrations reach the critical value of xc=0.116 indicating a quantum critical point separating the ferromagnetic and paramagnetic phases. Earlier magnetization and $\mu$SR data have indicated the presence of magnetic inhomogeneities in Ni(1-x)V(x) and, in particular, recognize the magnetic clusters close to xc, on the paramagnetic and on the ferromagnetic sides with nontrivial dynamical properties [R. Wang et al., Phys. Rev. Lett. 118, 267202 (2017)]. We present the results of SANS study with full polarization analysis of polycrystalline Ni(1-x)V(x) samples with x=0.10 and x=0.11 with low critical temperatures Tc below 50 K. For both Ni-V samples close to xc we find isotropic magnetic short-range correlations in the nanometer-scale persisting at low temperatures. They are suppressed gradually in higher magnetic fields. In addition, signatures of long-range ordered magnetic domains are present below Tc. The fraction of these magnetic clusters embedded in the ferromagnetic ordered phase grows towards xc and agrees well with the cluster fraction estimate from the magnetization and $\mu$SR data. Our SANS studies provide new insights into the nature of the inhomogeneities in a ferromagnetic alloy close to a quantum critical point.",2207.14196v2 2023-03-14,The unusual distribution of spin-triplet supercurrents in disk-shaped Josephson junctions,"The phenomenon of s-wave spin triplet Cooper pairs induced in ferromagnetic metals has been researched now for more than a decade, and its main aspects are well understood. Crucial in converting s-wave singlet pairs in the superconductor to s-wave triplets in the ferromagnet is the engineering of well-defined magnetic inhomogeneity (the 'generator') at the interface with the superconductor. Vertical layer stacks are typically used as such, where two separate thin ferromagnetic layers with homogeneous but non-collinear magnetizations, provide the inhomogeneity. Alternatively, magnetic textures, like ferromagnetic domain walls and vortices, are possible triplet generators, although they are far less studied. In this paper we review our experiments on lateral disk-shaped Josephson junctions where a ferromagnetic bottom layer provides a weak link with a vortex magnetization imposed by the shape of the disk. We present three different junction configurations, exhibiting their own generator mechanism. In the first, we utilize the non-collinearity with a second ferromagnetic layer to produce the triplet correlations. The second configuration consists of only the bottom ferromagnet and the superconducting contacts; it relies on the vortex magnetization itself to generate the spin-polarized supercurrents. In the third case we exploit an intrinsic generator by combining a conventional superconductor (NbTi) and a half-metallic ferromagnetic oxide (La$_{0.7}$Sr$_{0.3}$MnO$_3$). We find strong supercurrents in all cases. A particularly interesting finding is that the supercurrents are strongly confined at the rims of the device, independent of the generating mechanism, but directly related to their triplet nature. What causes these rim currents remains an open question.",2303.07993v1 2024-01-31,Ferromagnetic Semiconductors and Spintronic Devices,"Ferromagnetic semiconductors play a crucial role in spintronic devices, enabling effective control of electron spin over charge. This study explores their unique properties, ongoing advancements in spin control, and potential integration into next-generation semiconductor technologies.",2401.17554v1 1995-02-24,Ferromagnetism in Hubbard Models: Low Density Route,"Thirty years ago the Hubbard model was introduced by Gutzwiller, Hubbard and Kanamori with the main purpose of mimicking the ferromagnetism of transition metals. Soon after, Nagaoka and Thouless pointed out a basic mechanism for ferromagnetism in strongly correlated electron systems by studying the motion of a single hole in a half--filled Hubbard model. This important work was hoped to shed light onto metallic ferromagnetism from the low doping regime. Unfortunately, this low doping route towards ferromagnetism has not been successful as far as rigorous results for finite doping concentrations are concerned. In the work presented here, we start from the opposite limit of low particle concentrations. In this limit we provide the first proof of a fully polarized metallic ground state for a Hubbard model. The proof proceeds by mapping Hubbard ``zigzag'' chains onto a continuum model with an additional degree of freedom and local first Hund's rule coupling. For this model the maximum total spin multiplet is shown to be the unique ground state for infinite Hubbard coupling. Our proof may open a low density route towards the understanding of the ferromagnetism of Hubbard models.",9502104v1 1999-08-02,Aging phenomena in spin glass and ferromagnetic phases: domain growth and wall dynamics,"We compare aging in a disordered ferromagnet and in a spin glass, by studying the different phases of a reentrant system. We have measured the relaxation of the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the temperature is lowered (`chaos-like' effect) is observed in both phases. The memory of previous aging at a higher temperature can be retrieved upon re-heating, but in the ferromagnetic phase it can rapidly be erased by the growth of ferromagnetic domains. We interpret the behaviour observed in the ferromagnetic phase in terms of a combination of domain growth and pinned wall reconformations, and suggest that aging in spin glasses is dominated by such wall reconformation processes.",9908030v2 1999-11-11,"Field-induced segregation of ferromagnetic nano-domains in Pr$_{0.5}$Sr$_{0.5}$MnO$_3$, detected by $^{55}$Mn NMR","The antiferromagnetic manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ was investigated at low temperature by means of magnetometry and $^{55}$Mn NMR. A field-induced transition to a ferromagnetic state is detected by magnetization measurements at a threshold field of a few tesla. NMR shows that the ferromagnetic phase develops from zero field by the nucleation of microscopic ferromagnetic domains, consisting of an inhomogeneous mixture of tilted and fully aligned parts. At the threshold the NMR spectrum changes discontinuously into that of a homogeneous, fully aligned, ferromagnetic state, suggesting a percolative origin for the ferromagnetic transition.",9911164v1 2000-01-26,Double-exchange is not the cause of ferromagnetism in doped manganites,"The coexistence of ferromagnetism and metallic conduction in doped manganites has long been explained by a double-exchange model in which the ferromagnetic exchange arises from the carrier hopping. We evaluate the zero-temperature spin stiffness D(0) and the Curie temperature T_{C} on the basis of the double-exchange model using the measured values of the bare bandwidth W and the Hund's rule coupling J_{H}. The calculated D(0) and T_{C} values are too small compared with the observed ones even in the absence of interactions. A realistic onsite interorbital Coulomb repulsion can reduce D(0) substantially in the case of a 2-orbital model. Furthermore, experiment shows that D(0) is simply proportional to x in La_{1-x}Sr_{x}MnO_{3} system, independent of whether the ground state is a ferromagnetic insulator or metal. These results strongly suggest that the ferromagnetism in manganites does not originate from the double-exchange interaction. On the other hand, an alternative model based on the d-p exchange can semi-quantitatively explain the ferromagnetism of doped manganites at low temperatures.",0001390v4 2000-10-31,Superconducting crossed correlations in ferromagnets: implications for thermodynamics and quantum transport,"It is demonstrated that non local Cooper pairs can propagate in ferromagnetic electrodes having an opposite spin orientation. In the presence of such crossed correlations, the superconducting gap is found to depend explicitly on the relative orientation of the ferromagnetic electrodes. Non local Cooper pairs can in principle be probed with dc-transport. With two ferromagnetic electrodes, we propose a ``quantum switch'' that can be used to detect correlated pairs of electrons. With three or more ferromagnetic electrodes, the Cooper pair-like state is a linear superposition of Cooper pairs which could be detected in dc-transport. The effect also induces an enhancement of the ferromagnetic proximity effect on the basis of crossed superconducting correlations propagating along domain walls.",0010490v3 2000-11-22,"Do As antisites destroy the ferromagnetism of (Ga,Mn)As?","The effect of the inclusion of As antisites in the diluted magnetic semiconductor (Ga,Mn)As is studied within density functional theory in the local spin density approximation. In the case of homogeneous distribution of Mn ions we find that the ferromagnetism is largely weakened by the presence of the antisites. This is due to compensation of the free holes which mediate the long range ferromagnetic order. In contrast, when two Mn ions are coupled through only one As ion, ferromagnetic and antiferromagnetic order compete. In this case the magnetic ground state depends on: i) the position of the As antisites relative to the Mn, and ii) the As antisite concentration. We explain our results using a model of competing antiferromagnetic super-exchange and ferromagnetic double-exchange via localized Zener carriers. The strong dependence of the ferromagnetic order on the microscopic configuration accounts for the large variation in experimental data",0011372v1 2000-12-12,Metallic Ferromagnetism - an Electronic Correlation Phenomenon,"New insights into the microscopic origin of itinerant ferromagnetism were recently gained from investigations of electronic lattice models within dynamical mean-field theory (DMFT). In particular, it is now established that even in the one-band Hubbard model metallic ferromagnetism is stable at intermediate values of the interaction U and density n on regular, frustrated lattices. Furthermore, band degeneracy along with Hund's rule couplings is very effective in stabilizing metallic ferromagnetism in a broad range of electron fillings. DMFT also permits one to investigate more complicated correlation models, e.g., the ferromagnetic Kondo lattice model with Coulomb interaction, describing electrons in manganites with perovskite structure. Here we review recent results obtained with DMFT which help to clarify the origin of band-ferromagnetism as a correlation phenomenon.",0012203v1 2001-02-22,Low-Temperature Properties of Quasi-One-Dimensional Molecule-Based Ferromagnets,"Quantum and thermal behaviors of low-dimensional mixed-spin systems are investigated with particular emphasis on the design of molecule-based ferromagnets. One can obtain a molecular ferromagnet by assembling molecular bricks so as to construct a low-dimensional system with a magnetic ground state and then coupling the chains or the layers again in a ferromagnetic fashion. Two of thus-constructed quasi-one-dimensional bimetallic compounds are qualitatively viewed within the spin-wave treatment, one of which successfully grows into a bulk magnet, while the other of which ends in a singlet ground state. Then, concentrating on the ferrimagnetic arrangement on a two-leg ladder which is well indicative of general coupled-chain ferrimagnets, we develop the spin-wave theory and fully reveal its low-energy structure. We inquire further into the ferromagnetic aspect of the ferrimagnetic ladder numerically calculating the sublattice magnetization and the magnetic susceptibility. There exists a moderate coupling strength between the chains in order to obtain the most ferromagnetic ferrimagnet.",0102417v2 2001-05-15,Shot noise in ferromagnet--normal metal systems,"A semiclassical theory of the low frequency shot noise in ferromagnet - normal metal systems is formulated. Non-collinear magnetization directions of the ferromagnetic leads, arbitrary junctions and the elastic and inelastic scattering regimes are considered. The shot noise is governed by a set of mesoscopic parameters that are expressed in terms of the microscopic details of the junctions in the circuit. Explicit results in the case of ballistic, tunnel, and diffusive junctions are evaluated. The shot noise, the current and the Fano factor are calculated for a double barrier ferromagnet - normal metal - ferromagnet system. It is demonstrated that the shot noise can have a non-monotonic behavior as a function of the relative angle between the magnetizations of the ferromagnetic reservoirs.",0105305v2 2001-06-08,Subgap transport in ferromagnet-superconductor junctions due to magnon-assisted Andreev reflection,"We propose a new process of magnon-assisted Andreev reflection at a ferromagnetic metal - superconductor interface, which consists of the simultaneous injection of a Cooper pair from the superconductor and the emission of a magnon inside the ferromagnet. At low temperature this process represents an additional channel for sub-gap transport across an FS interface, which lifts restrictions on the current resulting from the necessity to match spin-polarized current in the ferromagnet with spin-less current in the superconductor. For a junction between a superconductor and a ferromagnet with an arbitrary degree of polarization, the inelastic magnon-assisted Andreev reflection process would manifest itself as a nonlinear addition to the $I(V)$ characteristics which is asymmetric with respect to the sign of the bias voltage and is related to the density of states of magnons in the ferromagnet. Expressions for the subgap $I(V)$ characteristics are given for arbitrary interfacial quality whilst the limiting cases of uniformly transparent and disordered interfaces are discussed in detail.",0106161v1 2001-09-26,Ferromagnetic Domain Walls in finite systems: mean-field critical exponents and applications,"The distribution of magnetic moments in finite ferromagnetic bodies was first investigated by Landau and Lifshitz in a famous paper [\textit{Phys. Z. Soviet Union}, \textbf{8}, 153 (1935)], where they obtained the domain structure of a ferromagnetic crystal at low temperatures, in the regime of saturated magnetization. In this article, we investigate the general properties of ferromagnetic domain walls of uniaxial crystals from the view point of the Landau free energy. We present the basic ideas at an introductory level, for non-experts. Extending the formalism to the vicinity of the Curie temperature, where a general qualitative description by the Landau theory of phase transitions can be applied, we find that domain walls tend to suppress the layers, leading to a continuous vanishing of the domain structure with anomalous critical exponents. In the saturated regime, we discuss the role of domain walls in mesoscopic systems and ferromagnetic nanojunctions, relating the observed magnetoresistance with promising applications in the recent area of spintronics.",0109499v2 2002-05-30,Collective ferromagnetism in two-component Fermi-degenerate gas trapped in finite potential,"Spin asymmetry of the ground states is studied for the trapped spin-degenerate (two-component) gases of the fermionic atoms with the repulsive interaction between different components, and, for large particle number, the asymmetric (collective ferromagnetic) states are shown to be stable because it can be energetically favorable to increase the fermi energy of one component rather than the increase of the interaction energy between up-down components. We formulate the Thomas-Fermi equations and show the algebraic methods to solve them. From the Thomas-Fermi solutions, we find three kinds of ground states in finite system: 1) paramagnetic (spin-symmetric), 2) ferromagnetic (equilibrium) and 3) ferromagnetic (nonequilibrium) states. We show the density profiles and the critical atom numbers for these states obtained analytically, and, in ferromagnetic states, the spin-asymmetries are shown to occur in the central regions of the trapped gas, and grows up with increasing particle number. Based on the obtained results, we discuss the experimental conditions and current difficulties to realize the ferromagnetic states of the trapped atom gas, which should be overcome.",0205638v2 2002-10-06,Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems,"We present a theory of Andreev reflection in a ferromagnet/superconductor/ferromagnet double junction system. The spin polarized quasiparticles penetrate to the superconductor in the range of penetration depth from the interface by the Andreev reflection. When the thickness of the superconductor is comparable to or smaller than the penetration depth, the spin polarized quasiparticles pass through the superconductor and therefore the electric current depends on the relative orientation of magnetizations of the ferromagnets. The dependences of the magnetoresistance on the thickness of the superconductor, temperature, the exchange field of the ferromagnets and the height of the interfacial barriers are analyzed. Our theory explains recent experimental results well.",0210127v1 2002-10-18,Effect of Impurities and Effective Masses on Spin-Dependent Electrical Transport in Ferromagnet-Normal Metal-Ferromagnet Hybrid Junctions,"The effect of nonmagnetic impurities and the effective masses on the spin-dependent transport in a ferromagnet-normal metal-ferromagnet junction is investigated on the basis of a two-band model. Our results show that impurities and the effective masses of electrons in two ferromagnetic electrodes have remarkable effects on the behaviors of the conductance, namely, both affect the oscillating amplitudes, periods, as well as the positions of the resonant peaks of the conductance considerably. The impurity tends to suppress the amplitudes of the conductance, and makes the spin-valve effect less obvious, but under certain conditions the phenomenon of the so-called impurity-induced resonant tunneling is clearly observed. The impurity and the effective mass both can lead to nonmonotonous oscillation of the junction magnetoresistance (JMR) with the incident energy and the thickness of the normal metal. It is also observed that a smaller difference of the effective masses of electrons in two ferromagnetic electrodes would give rise to a larger amplitude of the JMR.",0210392v1 2003-01-20,Magnetic Polarons in the 1D FM Kondo Model,"The ferromagnetic Kondo model with classical corespins is studied via unbiased Monte-Carlo simulations. We show that with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation in one-dimensional chains but rather stabilizes individual ferromagnetic polarons. Within the ferromagnetic polaron picture, the pseudogap in the one-particle spectral function A_k(\omega) can easily be explained. Ferromagnetic polarons also clear up a seeming failure of the double-exchange mechanism in explaining the comparable bandwidths in the ferromagnetic and paramagnetic phase. For our analysis, we extend a simplified model, the finite temperature uniform hopping approach (UHA), to include polarons. It can easily be evaluated numerically and provides a simple quantitative understanding of the physical features of the ferromagnetic Kondo model.",0301350v2 2003-03-01,Relaxor ferromagnetic behavior below the antiferromagnetic transition in La0.5Ca0.5MnO3,"A detailed reinvestigation of magnetization in La0.5Ca0.5MnO3 reveals that although a field of H = 7 T applied at T = 5 K after zero field cooling is insufficient to convert the low temperature charge ordered antiferromagnetic phase into ferromagnetic, annealing in a magnetic field as small as H = 1 T induces ferromagnetic clusters in the charge ordered matrix. The volume phase fraction of the ferromagnetic clusters increases nearly linearly with the annealing field until Han = 4 T and then changes dramatically from about 11 % at 5 T to 60 % at Han = 7 T. This is analogous to the field-induced micro to macro polar domains in relaxor ferroelectrics. It is suggested that charge ordered clusters coexist with ferromagnetic phase above the Neel temperature (TN) and these clusters which transform into ferromagnetic by external magnetic fields are supercooled below TN. Our results will be relevant to the observation of many orders of decrease in the resistivity found for H << HC, where HC is the critical field for metamagnetic transition.",0303007v1 2003-04-24,Ferromagnetism in a dilute magnetic semiconductor -- Generalized RKKY interaction and spin-wave excitations,"Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been studied using i) a single-impurity based generalized RKKY approach which goes beyond linear response theory, and ii) a mean-field-plus-spin-fluctuation (MF+SF) approach within a (purely fermionic) Hubbard-model representation of the magnetic impurities, which incorporates dynamical effects associated with finite frequency spin correlations in the ordered state. Due to a competition between the magnitude of the carrier spin polarization and its oscillation length scale, the ferromagnetic spin coupling is found to be optimized with respect to both hole doping concentration and impurity-carrier spin coupling energy $J$ (or equivalently $U$). The ferromagnetic transition temperature $T_c$, deteremined within the spin-fluctuation theory, corresponds closely with the observed $T_c$ values. Positional disorder of magnetic impurities causes significant stiffening of the high-energy magnon modes. We also explicitly study the stability/instability of the mean-field ferromagnetic state, which highlights the role of competing AF interactions causing spin twisting and noncollinear ferromagnetic ordering.",0304557v2 2003-06-05,Andreev drag effect in ferromagnetic-normal-superconducting systems,"We investigate conductances and current correlations in a system consisting of a normal multichannel conductor connected to one superconducting and two ferromagnetic electrodes. For antiparallel orientation of the ferromagnet polarizations, current injection from one ferromagnet can, due to Andreev reflection, lead to a net drag of current from the second ferromagnet toward the superconductor. We present the conditions for the Andreev drag in terms of the degree of lead polarizations, contact conductances and spin-flip scattering. Remarkably, both equilibrium and nonequilibrium zero-frequency current correlations between the ferromagnets become positive even in the presence of spin relaxation.",0306132v2 2003-06-25,Static magnetic response of clusters in Co_{0.2}Zn_{0.8}Fe_{1.95}Ho_{0.05}O_{4} spinel oxide,"Earlier investigation of Co_{0.2}Zn_{0.8}Fe_{1.95}Ho_{0.05}O_{4} spinel has shown the existence of ""super-ferromagnetic "" clusters containing Fe^{3+} and Ho^{3+} ions along with small size clusters of Fe^{3+} ions (Bhowmik et al, J. Magn. Magn. Mater. {247}, 83 (2002)). Here, We report the static magnetic response of these clusters. The experimental data suggests some interesting magnetic features, such as, enhancement of magnetization; re-entrant magnetic transitions with paramagnetic to ferromagnetic state below 225 K and ferromagnetic to spin glass state below 120 K; appearance of field induced ferromagnetism. We also observe an unusual maximum in the thermoremanent magnetization (TRM) vs temperature data. Our measurements suggest that this unusuality in TRM is related to the blocking of ""super-ferromagnetic"" clusters ,out of the ferromagnetic state, along their local anisotropy axis.",0306653v1 2004-01-15,Driven Random Field Ising Model: some exactly solved examples in threshold activated kinetics,"The random field Ising model driven by a slowly varying uniform external field at zero temperature provides a caricature of several threshold activated systems. In this model, the non-equilibrium response of the system can be obtained analytically on a Bethe lattice if the initial state of the system has all spins aligned parallel to each other. We consider ferromagnetic as well as anti-ferromagnetic interactions. The ferromagnetic model exhibits avalanches and non-equilibrium critical behavior. The anti-ferromagnetic model is marked by the absence of these features. The ferromagnetic model is Abelian, and the anti-ferromagnetic model is non-Abelian. Theoretical approaches based on the probabilistic method are discussed in the two cases, and illustrated by deriving some basic results.",0401252v1 2004-04-06,"Topological Defects in Ferromagnetic, Antiferromagnetic and Cyclic Spinor Condensates -- A Homotopy Theory","We apply the homotopy group theory in classifying the topological defects in atomic spin-1 and spin-2 Bose-Einstein condensates. The nature of the defects depends crucially on the spin-spin interaction between the atoms. We find the topologically stable defects both for spin-1 ferromagnetic and anti-ferromagnetic states, and for spin-2 ferromagnetic and cyclic states. With this rigorous approach we clarify the previously controversial identification of symmetry groups and order parameter spaces for the spin-1 anti-ferromagnetic state, and show that the spin-2 cyclic case provides a rare example of a physical system with non-Abelian line defects, like those observed in biaxial nematics. We also show the possibility to produce vortices with fractional winding numbers of 1/2, 1/3 and their multiples in spinor condensates.",0404138v1 2004-05-17,Itinerant Ferromagnetism and Superconductivity,"Superconductivity has again become a challenge following the discovery of unconventional superconductivity. Resistance-free currents have been observed in heavy-fermion materials, organic conductors and copper oxides. The discovery of superconductivity in a single crystal of $UGe_2$, $ZrZn_2$ and $URhGe$ revived the interest in the coexistence of superconductivity and ferromagnetism. The experiments indicate that: i)The superconductivity is confined to the ferromagnetic phase. ii)The ferromagnetic order is stable within the superconducting phase (neutron scattering experiments). iii) The specific heat anomaly associated with the superconductivity in these materials appears to be absent. The specific heat depends on the temperature linearly at low temperature. I present a review of the recent experimental results and the basic theoretical ideas concerning ferromagnetic superconductivity (FM-superconductivity) induced by the ferromagnetic spin fluctuations. A particular attention is paid to the magnon exchange mechanism of FM-superconductivity.",0405371v2 2004-06-18,Spin pumping and magnetization dynamics in ferromagnet-Luttinger liquid junctions,"We study spin transport between a ferromagnet with time-dependent magnetization and a conducting carbon nanotube or quantum wire, modeled as a Luttinger liquid. The precession of the magnetization vector of the ferromagnet due for instance to an outside applied magnetic field causes spin pumping into an adjacent conductor. Conversely, the spin injection causes increased magnetization damping in the ferromagnet. We find that, if the conductor adjacent to the ferromagnet is a Luttinger liquid, spin pumping/damping is suppressed by interactions, and the suppression has clear Luttinger liquid power law temperature dependence. We apply our result to a few particular setups. First we study the effective Landau-Lifshitz-Gilbert (LLG) coupled equations for the magnetization vectors of the two ferromagnets in a FM-LL-FM junction. Also, we compute the Gilbert damping for a FM-LL and a FM-LL-metal junction.",0406437v1 2004-09-21,Ferromagnet-superconductor proximity effect: The clean limit,"We study theoretically the influence of ferromagnetic metals on a superconducting film in the clean limit. Using a self-consistent solution of the Bogoliubov--de Gennes equation for a ferromagnet-superconductor-ferromagnet double junction we calculate the pair potential and conductance spectra as a function of the superconducting layer thickness $d$ for different strengths of ferromagnets and interface transparencies. We find that the pair potential and the critical temperature are weakly perturbed by the exchange interaction and do not drop to zero for any finite $d$. On the other hand, for thin superconducting films charge transport is spin polarized and exhibits a significant dependence on the ferromagnetic strength and magnetization alignment.",0409544v3 2004-10-12,"Ferromagnetic Polarons, Phase Separation, Stripes and Polaron Lattice: The two- and three-dimensional ferromagnetic Kondo Model","We investigate the two- and three-dimensional ferromagnetic Kondo lattice model by unbiased Monte Carlo simulations. A phase diagram for the two-dimensional model is presented, in which the stability of magnetic order and ferromagnetic polarons is examined with respect to the antiferromagnetic superexchange J' and temperature. The Monte Carlo simulations reveal that J' > 0.02 strengthens individual polarons while small J' < 0.02 favors larger clusters and phase separation except for small doping. Lowering the temperature stabilizes ferromagnetic polarons for realistic J' > 0.01, while phase separation is only favored for very small J' < 0.01. Our Monte Carlo simulations show that low temperatures can lead to diagonal or vertical stripes depending on J'. Simulations for three-dimensional systems yield ferromagnetic polarons, which form a `polaron lattice' at higher doping levels 0.2 < x < 0.23, when independent polarons do no longer fit into the system. No tendency to phase separation is observed in three dimensions.",0410274v1 2004-12-13,Evidence for Uniform Coexistence of Ferromagnetism and Unconventional Superconductivity in UGe_2: A ^73Ge-NQR Study under Pressure,"We report on the itinerant ferromagnetic superconductor UGe_2 through ^73Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum signals a first-order transition from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical pressure of P_x ~ 1.2 GPa. The superconductivity exhibiting a maximum value of T_sc=0.7 K at P_x ~ 1.2 GPa, was found to take place in connection with the P-induced first-order transition. The nuclear spin-lattice relaxation rate 1/T_1 has probed the ferromagnetic transition, exhibiting a peak at the Curie temperature as well as a decrease without the coherence peak below T_sc. These results reveal the uniformly coexistent phase of ferromagnetism and unconventional superconductivity with a line-node gap. We remark on an intimate interplay between the onset of superconductivity and the underlying electronic state for the ferromagnetic phases.",0412311v1 2005-02-09,HfO$_2$: a new direction for intrinsic defect driven ferromagnetism,"In view of the recent experimental reports of unexpected ferromagnetism in HfO$_{2}$ thin films, we carried out first principles investigations looking for magnetic order possibly brought about by the presence of small concentrations of intrinsic point defects. {\it Ab initio} electronic structure calculations using density functional theory (DFT) show that isolated cation vacancy sites in HfO$_{2}$ lead to the formation of high spin defect states. Furthermore these appear to be ferromagnetically coupled with a rather short range magnetic interaction, resulting in a ferromagnetic ground state for the whole system. More interestingly, the occurrence of these high spin states and ferromagnetism is in the low symmetry monoclinic phase of HfO$_{2}$. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.",0502246v1 2005-05-24,Proximity effects in superconductor-ferromagnet heterostructures,"The very special characteristic of the proximity effect in superconductor-ferromagnet systems is the damped oscillatory behavior of the Cooper pair wave function in a ferromagnet. In some sense, this is analogous to the inhomogeneous superconductivity, predicted long time ago by Larkin and Ovchinnikov (1964), and Fulde and Ferrell (1964), and constantly searched since that. After the qualitative analysis of the peculiarities of the proximity effect in the presence of the exchange field, the author provides a unified description of the properties of the superconductor-ferromagnet heterostructures. Special attention is paid to the striking non-monotonous dependance of the critical temperature of the multilayers and bilayers on the ferromagnetic layer thickness and conditions of the realization of the ""Pi""- Josephson junctions. The recent progress in the preparation of the high quality hybrid systems permitted to observe on experiments many interesting effects, which are also discussed in the article. Finally, the author analyzes the phenomenon of the domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers.",0505583v1 2005-10-14,Phase diagram of a dilute ferromagnet model with antiferromagnetic next-nearest-neighbor interactions,"We have studied the spin ordering of a dilute classical Heisenberg model with spin concentration $x$, and with ferromagnetic nearest-neighbor interaction $J_1$ and antiferromagnetic next-nearest-neighbor interaction $J_2$. Magnetic phases at absolute zero temperature $T = 0$ are determined examining the stiffness of the ground state, and those at finite temperatures $T \neq 0$ are determined calculating the Binder parameter $g_L$ and the spin correlation length $\xi_L$. Three ordered phases appear in the $x-T$ phase diagram: (i) the ferromagnetic (FM) phase; (ii) the spin glass (SG) phase; and (iii) the mixed (M) phase of the FM and the SG. Near below the ferromagnetic threshold $x_{\rm F}$, a reentrant SG transition occurs. That is, as the temperature is decreased from a high temperature, the FM phase, the M phase and the SG phase appear successively. The magnetization which grows in the FM phase disappears in the SG phase. The SG phase is suggested to be characterized by ferromagnetic clusters. We conclude, hence, that this model could reproduce experimental phase diagrams of dilute ferromagnets Fe$_x$Au$_{1-x}$ and Eu$_x$Sr$_{1-x}$S.",0510372v1 2005-11-15,Precipitation of amorphous ferromagnetic semiconductor phase in epitaxially grown Mn-doped Ge thin films,"We investigated the origin of ferromagnetism in epitaxially grown Mn-doped Ge thin films. Using low-temperature molecular beam epitaxy, Mn-doped Ge films were successfully grown without precipitation of ferromagnetic Ge-Mn intermetallic compounds, such as Mn5Ge3. Magnetic circular dichroism measurements revealed that the epitaxially grown Mn-doped Ge films exhibited clear ferromagnetic behavior, but the Zeeman splitting observed at the critical points was not induced by the s,p-d exchange interactions. High-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy analyses show phase separation of amorphous Ge1-xMnx clusters with high Mn content from a Mn-free monocrystalline Ge matrix. Since amorphous Ge1-xMnx was characterized as a homogeneous ferromagnetic semiconductor, the precipitation of the amorphous Ge1-xMnx clusters is the origin of the ferromagnetic semiconductor behavior of the epitaxially grown Mn-doped Ge films.",0511361v1 2005-11-17,Intrinsic mechanism for high temperature ferromagnetism in GaMnN,"Considerable efforts have been devoted recently to synthesize diluted magnetic semiconductors having ferromagnetic properties at room temperature because of their technological impacts for spintronic devices. In 2001 successful growth of GaMnN films showing room temperature ferromagnetism and p-type conductivity has been reported. The estimated Curie temperature was 940K at 5.7% of Mn, which is highest among diluted magnetic semiconductors ever been reported. However, the electronic mechanism behind the ferromagnetic behaviour has still been controversial. Here we show experimental evidence using the ferromagnetic samples that Mn atoms are substitutionally dissolved into the GaN lattice and they exhibit mixed valences of +2 (majority) and +3 (minority). The p-type carrier density decreases significantly at very low temperatures. At the same time, magnetization dramatically decreases. The results imply that the ferromagnetic coupling between Mn atoms is mediated by holes in the mid-gap Mn band.",0511435v1 2006-01-02,Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets,"We present a variational calculation of the spin wave excitation spectrum of double--exchange ferromagnets in different dimensions. Our theory recovers the Random Phase approximation and 1/S expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three--body correlations between a Fermi sea electron--hole pair and a magnon excitation and show that they strongly affect the spin dynamics in the parameter range relevant to experiments in the manganites. The manifestations of these correlations are many-fold. We demonstrate that they significantly change the ferromagnetic phase boundary. In addition to a decrease in the magnon stiffness, we obtain an instability of the ferromagnetic state against spin wave excitations close to the Brillouin zone boundary.Within a range of intermediate concentrations, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness, which changes into hardening for other concentrations. We discuss the relevance of these results to experiments in colossal magnetoresistance ferromagnets.",0601025v2 2006-08-09,Temperature dependence of transport spin polarization in NdNi5 measured using Point Contact Andreev reflection,"We report a study in which Point contact Andreev reflection (PCAR) spectroscopy using superconducting Nb tip has been carried out on NdNi5, a ferromagnet with a Curie temperature of TC~7.7K. The measurements were carried out over a temperature range of 2-9K which spans across the ferromagnetic transition temperature. From an analysis of the spectra, we show that (i) the temperature dependence of the extracted value of transport spin polarization closely follows the temperature dependence of the spontaneous magnetization; (ii) the superconducting quasiparticle lifetime shows a large decrease close to the Curie temperature of the ferromagnet. We attribute the latter to the presence of strong ferromagnetic spin fluctuations in the ferromagnet close to the ferromagnetic transition temperature.",0608214v3 2006-11-14,Re-entrant ferromagnetism in a generic class of diluted magnetic semiconductors,"Considering a general situation where a semiconductor is doped by magnetic impurities leading to a carrier-induced ferromagnetic exchange coupling between the impurity moments, we show theoretically the possible generic existence of three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3) between two ferromagnetic phases, arises from a subtle competition between indirect exchange induced by thermally activated carriers in an otherwise empty conduction band versus the exchange coupling existing in the impurity band due to the bound carriers themselves. We comment on the possibility of observing such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic oxides.",0611384v1 2007-02-22,Ferromagnetism in Fe-doped ZnO Nanocrystals: Experimental and Theoretical investigations,"Fe-doped ZnO nanocrystals are successfully synthesized and structurally characterized by using x-ray diffraction and transmission electron microscopy. Magnetization measurements on the same system reveal a ferromagnetic to paramagnetic transition temperature > 450 K with a low-temperature transition from ferromagnetic to spin-glass state due to canting of the disordered surface spins in the nanoparticle system. Local magnetic probes like EPR and Mossbauer indicate the presence of Fe in both valence states Fe2+ and Fe3+. We argue that the presence of Fe3+ is due to the possible hole doping in the system by cation (Zn) vacancies. In a successive ab-initio electronic structure calculation, the effects of defects (e.g. O- and Zn-vacancy) on the nature and origin of ferromagnetism are investigated for Fe-doped ZnO system. Electronic structure calculations suggest hole doping (Zn-vacancy) to be more effective to stabilize ferromagnetism in Fe doped ZnO and our results are consistent with the experimental signature of hole doping in the ferromagnetic Fe doped ZnO samples.",0702525v1 2007-07-10,Superexchange induced canted ferromagnetism in dilute magnets,"We argue, in contrast to recent studies, that the antiferromagnetic superexchange coupling between nearest neighbour spins does not fully destroy the ferromagnetism in dilute magnets with long-ranged ferromagnetic couplings. Above a critical coupling, we find a \textit{canted} ferromagnetic phase with unsaturated moment. We have calculated the transition temperature using a simplified local Random Phase Approximation procedure which accounts for the canting. For the dilute magnetic semiconductors, such as GaMnAs, using \textit{ab-initio} couplings allows us to predict the existence of a canted phase and provide an explanation to the apparent contradictions observed in experimental measurements. Finally, we have compared with previous studies that used RKKY couplings and reported non-ferromagnetic state when the superexchange is too strong. Even in this case the ferromagnetism should remain essentially stable in the form of a canted phase.",0707.1522v1 2007-08-30,A dynamical mean-field theory study of Nagaoka ferromagnetism,"We revisit Nagaoka ferromagnetism in the U=oo Hubbard model within the dynamical mean-field theory (DMFT) using the recently developed continuous time quantum Monte Carlo method as the impurity solver. The stability of Nagaoka ferromagnetism is studied as a function of the temperature, the doping level, and the next-nearest-neighbor lattice hopping t'. We found that the nature of the phase transition as well as the stability of the ferromagnetic state is very sensitive to the t' hopping. Negative t'=-0.1t stabilizes ferromagnetism up to higher doping levels. The paramagnetic state is reached through a first order phase transition. Alternatively, a second order phase transition is observed at t'=0. Very near half-filling, the coherence temperature T_{coh} of the paramagnetic metal becomes very low and ferromagnetism evolves out of an incoherent metal rather than conventional Fermi liquid. We use the DMFT results to benchmark slave-boson method which might be useful in more complicated geometries.",0708.4240v2 2007-10-11,Analytical investigation of magnetic field distributions around superconducting strips on ferromagnetic substrates,"The complex-field approach is developed to derive analytical expressions of the magnetic field distributions around superconducting strips on ferromagnetic substrates (SC/FM strips). We consider the ferromagnetic substrates as ideal soft magnets with an infinite magnetic permeability, neglecting the ferromagnetic hysteresis. On the basis of the critical state model for a superconducting strip, the ac susceptibility $\chi_1'+i\chi_1''$ of a SC/FM strip exposed to a perpendicular ac magnetic field is theoretically investigated, and the results are compared with those for superconducting strips on nonmagnetic substrates (SC/NM strips). The real part $\chi_1'$ for $H_0/j_cd_s\to 0$ (where $H_0$ is the amplitude of the ac magnetic field, $j_c$ is the critical current density, and $d_s$ is the thickness of the superconducting strip) of a SC/FM strip is 3/4 of that of a SC/NM strip. The imaginary part $\chi_1''$ (or ac loss $Q$) for $H_0/j_cd_s<0.14$ of a SC/FM strip is larger than that of a SC/NM strip, even when the ferromagnetic hysteresis is neglected, and this enhancement of $\chi_1''$ (or $Q$) is due to the edge effect of the ferromagnetic substrate.",0710.2151v1 2008-01-07,Magnetization reversal driven by spin-injection : a mesoscopic spin-transfer effect,"A mesoscopic description of spin-transfer effect is proposed, based on the spin-injection mechanism occurring at the junction with a ferromagnet. The effect of spin-injection is to modify locally, in the ferromagnetic configuration space, the density of magnetic moments. The corresponding gradient leads to a current-dependent diffusion process of the magnetization. In order to describe this effect, the dynamics of the magnetization of a ferromagnetic single domain is reconsidered in the framework of the thermokinetic theory of mesoscopic systems. Assuming an Onsager cross-coefficient that couples the currents, it is shown that spin-dependent electric transport leads to a correction of the Landau-Lifshitz-Gilbert equation of the ferromagnetic order parameter with supplementary diffusion terms. The consequence of spin-injection in terms of activation process of the ferromagnet is deduced, and the expressions of the effective energy barrier and of the critical current are derived. Magnetic fluctuations are calculated: the correction to the fluctuations is similar to that predicted for the activation. These predictions are consistent with the measurements of spin-transfer obtained in the activation regime and for ferromagnetic resonance under spin-injection.",0801.1019v1 2008-04-03,Pressure-temperature Phase Diagram of Polycrystalline UCoGe Studied by Resistivity Measurement,"Recently, coexistence of ferromagnetism (T_Curie = 2.8K) and superconductivity (T_sc = 0.8K) has been reported in UCoGe, a compound close to a ferromagnetic instability at ambient pressure P. Here we present resistivity measurements under pressure on a UCoGe polycrystal. The phase diagram obtained from resistivity measurements on a polycrystalline sample is found to be qualitatively different to those of all other ferromagnetic superconductors. By applying high pressure, ferromagnetism is suppressed at a rate of 1.4 K/GPa. No indication of ferromagnetic order has been observed above P ~ 1GPa. The resistive superconducting transition is, however, quite stable in temperature and persists up to the highest measured pressure of about 2.4GPa. Superconductivity would therefore appear also in the paramagnetic phase. However, the appearance of superconductivity seems to change at a characteristic pressure P* ~ 0.8GPa. Close to a ferromagnetic instability, the homogeneity of the sample can influence strongly the electronic and magnetic properties and therefore bulk phase transitions may differ from the determination by resistivity measurements.",0804.0500v1 2008-06-01,Spin-glass instability of short-range spherical ferromagnet,"In structurally disordered ferromagnets the weak random dipole-dipole exchange may transform the polydomain state into a spin-glass one. To some extent the properties of such phase in disordered isotropic ferromagnet can be qualitatively described by the spherical model with the short-range ferromagnetic interaction and weak frustrated infinite-range random-bond exchange. This model is shown to predict that spin-glass phase substitute the ferromagnetic one at the arbitrary small disorder strength and that its thermodynamics has some similarity to that of polydomain state along with some significant distinctions. In particular, the longitudinal susceptibility at small fields becomes frozen below transition point at a constant value depending on the disorder strength, while the third order nonlinear magnetic susceptibilitiy exhibits the temperature oscillations in small field near the transition point. The relation of these predictions to the experimental data for some disordered isotropic ferromagnets is discussed.",0806.0125v1 2008-09-12,Proximity effect and its enhancement by ferromagnetism in high-temperature superconductor-ferromagnet structures,"We consider a bi-layer consisting of a $d-$wave layerd superconductor and diffusive ferromagnet with a domain wall (DW). The $c-$axis in the superconductor and DW in the ferromagnet are assumed to be perpendicular to the interface. We demonstrate that in such a heterostructure the inhomogeneous exchange field enhances the proximity effect. It is shown that, whereas in the absence of the exchange field the $d-$wave condensate decays in the normal metal on the mean free path $l$, the superconductivity penetrates the ferromagnet along the DW over much larger distances. This happens because the presence of DW results in a generation of an odd frequency triplet s-wave component of the condensate. The phenomenon discovered here may help to explain a recent experiment on high temperature superconductor-ferromagnet bi-lyers.",0809.2199v2 2008-09-23,Electrical detection of spin pumping: dc voltage generated by ferromagnetic resonance at ferromagnet/nonmagnet contact,"We describe electrical detection of spin pumping in metallic nanostructures. In the spin pumping effect, a precessing ferromagnet attached to a normal-metal acts as a pump of spin-polarized current, giving rise to a spin accumulation. The resulting spin accumulation induces a backflow of spin current into the ferromagnet and generates a dc voltage due to the spin dependent conductivities of the ferromagnet. The magnitude of such voltage is proportional to the spin-relaxation properties of the normal-metal. By using platinum as a contact material we observe, in agreement with theory, that the voltage is significantly reduced as compared to the case when aluminum was used. Furtheremore, the effects of rectification between the circulating rf currents and the magnetization precession of the ferromagnet are examined. Most significantly, we show that using an improved layout device geometry these effects can be minimized.",0809.3859v1 2008-10-01,Lattice disorder and Ferromagnetism in La0.67Ca0.33MnO3 nanoparticle,"We study the ferromagnetism of La0.67Ca0.33MnO3 in bulk polycrystalline, nanocrystalline and amorphous phase. The structural change from crystalline phase to amorphous phase exhibited a systematic decrease of TC(paramagnetic to ferromagnetic transition temperature) and spontaneous magnetization (MS). The experimental results suggested few more features, e.g., appearance of large magnetic irreversibility in the temperature dependence of magnetization, lack of magnetic saturation at high magnetic field, blocking of magnetization below TB, and enhancement of coercivity. In addition, the magnetic phase transition near to TC has changed from first order character in bulk sample to second order character in nanocrystalline and amorphous samples. We understand the observed magnetic features as the effects of decreasing particle size and increasing magnetic (spin- lattice) disorder. We noted that magnetic dynamics of amorphous samples is distinctly different from the nanocrystalline samples. The ferromagnetism of amorphous samples are comparable with the properties of reported amorphous ferromagnetic nanoparticles. We also demonstrate the effect of disorder shell in controlling the dynamics of ferromagnetic cores.",0810.0090v1 2008-10-12,The Quantum Noise of Ferromagnetic $π$-Bloch Domain Walls,"We quantify the probability per unit Euclidean-time of reversing the magnetization of a $\pi$-Bloch vector, which describes the Ferromagnetic Domain Walls of a Ferromagnetic Nanowire at finite-temperatures, by evaluating the saddlepoint solution of the grand canonical partition function for the Ferromagnetic Nanowire consisting of $N$ such soliton and anti-soliton states. Our approach, based on Langer's Theory, treats the double Sine-Gordon model that defines the $\pi$-Bloch vectors via a procedure of nonperturbative renormalization, and uses importance sampling methods to minimise the free energy of the system, and identify the saddlepoint solution corresponding to the reversal probability. We identify that whilst the general solution for the free energy minima cannot be expressed in closed form, we can obtain a closed expression for the saddlepoint by maximizing the entanglement entropy of the system. We use this approach to quantify the geometric and non-geometric contributions to the entanglement entropy of the Ferromagnetic Nanowire, defined between entangled Ferromagnetic Domain Walls, and evaluate the Euclidean-time dependence of the domain wall width and angular momentum transfer at the domain walls, which has been recently proposed as a mechanism for Quantum Memory Storage.",0810.2087v2 2008-11-09,Thermodynamics of ferromagnetic superconductors with spin-triplet electron pairing,"We present a general thermodynamic theory that describes phases and phase transitions of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory is based on extended Ginzburg-Landau expansion in powers of superconducting and ferromagnetic order parameters. We propose a simple form for the dependence of theory parameters on the pressure that allows correct theoretical outline of the temperature-pressure phase diagram for which at low temperatures a stable phase of coexistence of p-wave superconductivity and itinerant ferromagnetism appears. We demonstrate that the theory is in an agreement with the experimental data for some intermetallic compounds that are experimentally proven to be itinerant ferromagnetic exhibiting spin-triplet superconductivity. Some basic features of quantum phase transitions in such systems are explained and clarified. We propose to group the spin-triplet ferromagnetic superconductors in two different types of thermodynamic behavior, on the basis of quantitative criterion deduced from the present theory and the analysis of experimental data.",0811.1344v3 2008-12-04,Metallic Ferromagnetism in the Kondo Lattice,"Metallic magnetism is both ancient and modern, occurring in such familiar settings as the lodestone in compass needles and the hard drive in computers. Surprisingly, a rigorous theoretical basis for metallic ferromagnetism is still largely missing. The Stoner approach perturbatively treates Coulomb interactions when the latter need to be large, while the Nagaoka approach incorporates thermodynamically negligible electrons into a half-filled band. Here, we show that the ferromagnetic order of the Kondo lattice is amenable to an asymptotically exact analysis over a range of interaction parameters. In this ferromagnetic phase, the conduction electrons and local moments are strongly coupled but the Fermi surface does not enclose the latter (i.e. it is small). Moreover, non-Fermi liquid behavior appears over a range of frequencies and temperatures. Our results provide the basis to understand some long-standing puzzles in the ferromagnetic heavy fermion metals, and raises the prospect for a new class of ferromagnetic quantum phase transitions.",0812.0819v3 2009-02-24,Evidence for Induced Magnetization in Superconductor-Ferromagnet Hetero-structures: a Scanning Tunnelling Spectroscopy Study,"We performed scanning tunneling spectroscopy of c-axis oriented YBCO films on top of which ferromagnetic SRO islands were grown epitaxially in-situ. When measured on the ferromagnetic islands, the density of states exhibits small gap-like features consistent with the expected short range penetration of the order parameter into the ferromagnet. However, anomalous split-gap structures are measured on the superconductor in the vicinity of ferromagnetic islands. This observation may provide evidence for the recently predicted induced magnetization in the superconductor side of a superconductor/ ferromagnet junction. The length scale of the effect inside the superconductor was found to be an order of magnitude larger than the superconducting coherence length. This is inconsistent with the theoretical prediction of a penetration depth of only a few superconducting coherence lengths. We discuss a possible origin for this discrepancy.",0902.4140v1 2009-03-10,Reproducibility of room temperature ferromagnetism in Zn0.95Mn0.05O and its understanding,"The present work reproduces the room temperature ferromagnetism by doping magnetic Mn atoms in diamagnetic ZnO. The ferromagnetic ordering is extended up to 640 K in mechanical milled Zn0.95Mn0.05O samples. The bulk and nanocrystalline samples are stabilized in hexagonal crystal structure with space group p63mc. The grain size and lattice strain of the samples, estimated from XRD spectrum using Williamson-Hall plot, showed a significant variation with milling time. Surface structure (morphology, distribution of grains and elements) is observed to be reasonably good from SEM picture and EDX spectrum at room temperature. The ferromagnetic ordering in bulk, milled and alloyed samples is primarily due to the diffusion of Mn(2+) ions into the lattice sites of ZnO. The enhancement of magnetic moment and ferromagnetic ordering temperature with reducing the grain size is discussed in terms of the existing theoretical predictions and experimental works. The role of mechanical milling induced disorder for the enhancement of room temperature ferromagnetism in Zn0.95Mn0.05O (dilute magnetic semiconductor) is also highlighted.",0903.1691v1 2009-10-18,"Response to comment by Tin-Lun Ho on ""Itinerant Ferromagnetism in a Strongly Interacting Fermi Gas of Ultracold Atoms"", Science 325, 1521 (2009)","Ho claims in his comment that our experiment is direct evidence that itinerant ferromagnetism does not exist in ultracold Fermi gases. This claim is incorrect and based on an invalid estimate of relaxation times and an erroneous interpretation of the detectability of ferromagnetic domains. We point out that the experimental evidence is consistent with the existence of ferromagnetism, but further experiments are needed to distinguish a ferromagnetic ground state from a non-magnetic ground state with ferromagnetic correlations.",0910.3419v2 2009-11-10,Co-doped Ceria: Tendency towards ferromagnetism driven by oxygen vacancies,"We perform an electronic structure study for cerium oxide homogeneously-doped with cobalt impurities, focusing on the role played by oxygen vacancies and structural relaxation. By means of full-potential ab-initio methods, we explore the possibility of ferromagnetism as observed in recent experiments. Our results indicate that oxygen vacancies seem to be crucial for the appearance of a ferromagnetic alignment among Co impurities, obtaining an increasing tendency towards ferromagnetism with growing vacancy concentration. The estimated couplings cannot explain though, the experimentally observed room-temperature ferromagnetism. In this systematic study, we draw relevant conclusions regarding the location of the oxygen vacancies and the magnetic couplings involved. In particular, we find that oxygen vacancies tend to nucleate in the neighborhood of Co impurities and we get a remarkably strong ferromagnetic coupling between Co atoms and the Ce^{3+} neighboring ions. The calculated magnetic moments per cell depend on the degree of reduction which could explain the wide spread in the magnetization values observed in the experiments.",0911.1959v1 2010-06-15,Field-induced multiple metamagnetization in phase transition from paramagnetic austenite to ferromagnetic martensite in MnNi1-xFexGe,"Tailoring the phase stability and magnetic structure by combining the antiferromagnetic MnNiGe martensite with ferromagnetic MnFeGe austenite, a magnetic-field-induced martensitic transformation has been achieved in MnNi1-xFexGe system. The doped Fe enables the isolated structural and magnetic transitions of MnNiGe to coincide at room temperature and also establishes ferromagnetic couplings in six-Mn-centering-one-Fe-atom clusters. The ferromagnetic clusters transit the antiferromagnetic state in martensite to a ferromagnetic state and the alloys are further induced to a stronger ferromagnetic state by external field, resulting in a large magnetization difference up to 50 emu/g across the transformation. The field-induced martensitic transformation thus experiences a multiple metamagnetization process.",1006.2907v1 2010-07-08,Absence of Meissner State and Robust Ferromagnetism in the Superconducting State of UCoGe: Possible Evidence of Spontaneous Vortex State,"We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of $T_{{\rm SC}} \sim 0.5$~K and $T_{{\rm Curie}} \sim 2.5$~K, respectively). In the normal, ferromagnetic state ($T_{{\rm SC}} < T < T_{{\rm Curie}}$), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below $T_{{\rm SC}}$, the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism is not completely shielded, and there is no Meissner region, a magnetic field region of $H < H_{\rm c1}$ (a lower critical field). From these results, we suggest that UCoGe is the first material in which ferromagnetism robustly survives in the superconducting state and a spontaneous vortex state without the Meissner state is realized.",1007.1344v1 2011-01-25,Does a ferromagnet with spin-dependent masses produce a spin-filtering effect in a ferromagnetic/insulator/superconductor junction?,"We analyze charge transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. We take into account the possibility that ferromagnetism in the first electrode may be driven by a mass renormalization of oppositely polarized carriers, i.e. by a spin bandwidth asymmetry, rather than by a rigid splitting of up-and down-spin electron bands as in a standard Stoner ferromagnet. By evaluating the averaged charge conductance for both an s- and a $d_{x^2-y^2}$-wave order parameter for the S side, we show that the mass mismatch in the ferromagnetic electrode may mimic a spin active barrier. Indeed, in the $s$-wave case we show that under suitable conditions the spin dependent conductance of minority carriers below the energy gap $\Delta_0$ can be larger than for majority carriers, and lower above $\Delta_0$. On the other hand, for a d_{x^2-y^2}-wave superconductor similar spin-dependent effects give rise to an asymmetric peak splitting in the conductance. These results suggest that the junction may work as a spin-filtering device.",1101.4780v1 2011-04-07,Itinerant ferromagnetism in an interacting Fermi gas with mass imbalance,"We study the emergence of itinerant ferromagnetism in an ultra-cold atomic gas with a variable mass ratio between the up and down spin species. Mass imbalance breaks the SU(2) spin symmetry leading to a modified Stoner criterion. We first elucidate the phase behavior in both the grand canonical and canonical ensembles. Secondly, we apply the formalism to a harmonic trap to demonstrate how a mass imbalance delivers unique experimental signatures of ferromagnetism. These could help future experiments to better identify the putative ferromagnetic state. Furthermore, we highlight how a mass imbalance suppresses the three-body loss processes that handicap the formation of a ferromagnetic state. Finally, we study the time dependent formation of the ferromagnetic phase following a quench in the interaction strength.",1104.1439v2 2013-01-09,Weak ferromagnetism with the Kondo screening effect in the Kondo lattice systems,"We carefully consider the interplay between ferromagnetism and the Kondo screening effect in the conventional Kondo lattice systems at finite temperatures. Within an effective mean-field theory for small conduction electron densities, a complete phase diagram has been determined. In the ferromagnetic ordered phase, there is a characteristic temperature scale to indicate the presence of the Kondo screening effect. We further find two distinct ferromagnetic long-range ordered phases coexisting with the Kondo screening effect: spin fully polarized and partially polarized states. A continuous phase transition exists to separate the partially polarized ferromagnetic ordered phase from the paramagnetic heavy Fermi liquid phase. These results may be used to explain the weak ferromagnetism observed recently in the Kondo lattice materials.",1301.1771v3 2013-05-13,Ferromagnetism and Fermi-surface transition in the periodic Anderson model: Second-order phase transition without symmetry breaking,"We study ferromagnetism in the periodic Anderson model with and without a magnetic field by the Gutzwiller theory. We find three ferromagnetic phases: a weak ferromagnetic phase (FM0), a half-metallic phase without Fermi surface for the majority spin (FM1), and a ferromagnetic phase with almost completely polarized f-electrons (FM2). The Fermi surface changes from the large Fermi-surface in the paramagnetic state to the small Fermi-surface in FM2. We also find that the transitions between the ferromagnetic phases can be second-order phase transitions in spite of the absence of symmetry breaking. While we cannot define an order parameter for such transitions in an ordinary way, the topology of the Fermi surface characterizes the transitions, i.e., they are Lifshitz transitions.",1305.2653v1 2013-05-20,The 'Higgs' amplitude mode in weak ferromagnetic metals,"Using Ferromagnetic Fermi liquid theory, Bedell and Blagoev derived the collective low-energy excitations of a weak ferromagnet. They obtained the well-known magnon (Nambu-Goldstone) mode and found a new gapped mode that was never studied in weak ferromagnetic metals. In this article we have identified this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identified as the Higgs since it can be show that it corresponds to a fluctuation of the amplitude of the order parameter. We use this model to describe the itinerant-electron ferromagnetic material MnSi. By fitting the model with the existing experimental results, we calculate the dynamical structure function and see well-defined peaks contributed from the magnon and the Higgs. Our estimates of the relative intensity of the Higgs amplitude mode suggest that it can be seen in neutron scattering experiments on MnSi.",1305.4674v2 2013-07-30,Interfacial Ferromagnetism in LaNiO3/CaMnO3 Superlattices,"We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.",1307.8050v1 2014-02-23,Superconductivity and Ferromagnetic Quantum Criticality in Uranium Compounds,"We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, together with the ferromagnetic quantum criticality and paramagnetic singularity on the Ising 5f-itinerant system UCoAl. Thanks to the variety of ordered moment in ferromagnetic superconductors from 1.5 muB to 0.05 muB, interesting systematic changes or similarities are clarified. All ferromagnetic superconductors show large upper critical field Hc2, and the field-reentrant (-reinforced) phenomena are observed in the field-temperature phase diagram, when the pressure or field direction is tuned for particular conditions. These phenomena are well explained by the ferromagnetic longitudinal fluctuations, which are induced by the magnetic field in transverse configurations. The large Hc2 might be also associated with possible additional effects of Fermi surface instabilities, such as Lifshitz-type singularities.",1402.5628v1 2014-04-08,Ferromagnetism and impurity band in a new magnetic semiconductor: InMnP,"We have synthesized ferromagnetic InMnP, a member of III-Mn-V ferromagnetic semiconductor family, by Mn ion implantation and pulsed laser annealing. Clear ferromagnetic hysteresis loops and a perpendicular magnetic anisotropy are observed up to a Curie temperature of 42 K. Large values of negative magnetoresistance and magnetic circular dichroism as well as anomalous Hall effect are further evidences of a ferromagnetic order in InMnP. An effort is made to understand the transport mechanism in InMnP using the theoretical models. We find that the valence band of InP does not merge with the impurity band of the heavily doped ferromagnetic InMnP. Our results suggest that impurity band conduction is a characteristic of Mn-doped III-V semiconductors which have deep Mn-acceptor levels.",1404.2023v1 2014-08-05,Tunable asymmetric magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films,"We investigate the magnetization dynamics through the magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films. We observe that the magnetoimpedance response is dependent on the thickness of the non-magnetic Cu spacer material, a fact associated to the kind of the magnetic interaction between the ferromagnetic layers. Thus, we present an experimental study on asymmetric magnetoimpedance in ferromagnetic films with biphase magnetic behavior and explore the possibility of tuning the linear region of the magnetoimpedance curves around zero magnetic field by varying the thickness of the non-magnetic spacer material, and probe current frequency. We discuss the experimental magnetoimpedance results in terms of the different mechanisms governing the magnetization dynamics at distinct frequency ranges, quasi-static magnetic properties, thickness of the non-magnetic spacer material, and the kind of the magnetic interaction between the ferromagnetic layers. The results place ferromagnetic films with biphase magnetic behavior exhibiting asymmetric magnetoimpedance effect as a very attractive candidate for application as probe element in the development of auto-biased linear magnetic field sensors.",1408.0992v1 2014-11-05,Homogenization of Composite Ferromagnetic Materials,"Nowadays, nonhomogeneous and periodic ferromagnetic materials are the subject of a growing interest. Actually such periodic configurations often combine the attributes of the constituent materials, while sometimes, their properties can be strikingly different from the properties of the different constituents. These periodic configurations can be therefore used to achieve physical and chemical properties difficult to achieve with homogeneous materials. To predict the magnetic behavior of such composite materials is of prime importance for applications. The main objective of this paper is to perform, by means of Gamma-convergence and two-scale convergence, a rigorous derivation of the homogenized Gibbs-Landau free energy functional associated to a composite periodic ferromagnetic material, i.e. a ferromagnetic material in which the heterogeneities are periodically distributed inside the ferromagnetic media. We thus describe the Gamma-limit of the Gibbs-Landau free energy functional, as the period over which the heterogeneities are distributed inside the ferromagnetic body shrinks to zero.",1411.1231v1 2014-11-14,Pairing gaps near ferromagnetic quantum critical points,"We address the quantum-critical behavior of a two-dimensional itinerant ferromagnetic systems described by a spin-fermion model in which fermions interact with close to critical bosonic modes. We consider Heisenberg ferromagnets, Ising ferromagnets, and the Ising nematic transition. Mean-field theory close to the quantum critical point predicts a superconducting gap with spin-triplet symmetry for the ferromagnetic systems and a singlet gap for the nematic scenario. Studying fluctuations in this ordered phase using a nonlinear sigma model, we find that these fluctuations are not suppressed by any small parameter. As a result, we find that a superconducting quasi-long-range order is still possible in the Ising-like models but long-range order is destroyed in Heisenberg ferromagnets.",1411.4055v2 2014-12-03,Heat-driven spin transport in a ferromagnetic metal,"As a non-magnetic heavy metal is attached to a ferromagnet, a vertically flowing heat-driven spin current is converted to a transverse electric voltage, which is known as the longitudinal spin Seebeck effect (SSE). If the ferromagnet is a metal, this voltage is also accompanied by voltages from two other sources, i.e. the anomalous Nernst effect in both the ferromagnet and the proximity-induced ferromagnetic boundary layer. By properly identifying and carefully separating those different effects, we find that in this pure spin current circuit the additional spin current drawn by the heavy metal generates another significant voltage by the ferromagnetic metal itself which should be present in all relevant experiments.",1412.1139v1 2014-12-04,Thermal and electrical quantum Hall effects in ferromagnet-topological insulator-ferromagnet junction,"We present the theoretical description for a class of experimental setups that measure quantum Hall coefficients in ferromagnet-topological insulator-ferromagnet (FM-TI-FM) junctions. We predict that varying the magnetization direction in ferromagnets, one can change the induced Hall voltage and transverse temperature gradient from the maximal values, corresponding to the quantized Hall coefficients, down to their complete suppression to zero. We provide detailed analysis of thermal and electrical Hall resistances as functions of the magnetization directions in ferromagnets, the spin-scattering time in TI, and geometrical positions of FM leads and measurement contacts.",1412.1697v3 2014-12-22,Control of Edge Currents at a Ferromagnet - Triplet Superconductor Interface by Multiple Helical Majorana Modes,"We study the spin and charge currents flowing at the interface of an itinerant ferromagnet with a topological spin-triplet superconductor having different number of time-reversal-invariant Majorana helical modes. Depending on the number of helical modes, the capacity of carrying spin and charge currents is shown to be directly related to the amplitude and orientation of the ferromagnetic magnetization with respect to the superconducting $\vec{d}$-vector. Differently from the one-helical mode spin-triplet superconductor, we find that the presence of a finite amount of electronic hybridization with the two pairs of Majorana helical modes leads to nonvanishing charge current independently of the ferromagnetic exchange. The competition between the two pairs of Majorana helical modes remarkably yields a spin-current response that is almost constant in the range of weak to intermediate ferromagnetism. The behavior of the spin current is tightly linked to the direction of the spin-polarization in the ferromagnet and tends to be flatten for a magnetization that is coplanar to the spin-triplet $\vec{d}$-vector independently of the number of helical modes.",1412.7053v1 2015-01-13,Doping control of realization of an extended Nagaoka ferromagnetic state from the Mott state,"Inspired by the Nagaoka ferromagnetism, we propose an itinerant model to study the transition between the Mott singlet state and a ferromagnetic state by emulating a doping process in finite lattices. In the Nagaoka ferromagnetism, the total spin of the system takes the maximum value when an electron is removed from the half-filled system. To incorporate a procedure of the electron removal, our model contains extra sites as a reservoir of electrons, and the chemical potential of the reservoir controls the distribution of electrons. As a function of the chemical potential, the system exhibits ground-state phase transitions among various values of the total spin, including a saturated ferromagnetic state due to the Nagaoka mechanism at finite hole density. We discuss the nature of the ferromagnetism by measuring various physical quantities, such as the distribution of electrons, the spin correlation functions, the magnetization process in the magnetic field, and also the entanglement entropy.",1501.02899v1 2015-02-10,The effect of the heating rate on the order to order phase transition,"The simple cubic spin-1 Ising (BEG) model exhibits the ferromagnetic (F) - ferromagnetic (F) phase transition at low temperature region for the interval 1.40= 2 in any dimension and for any lattice in which anomalous ground states are realized in a spin-1/2 Heisenberg model.",2310.10287v1 2023-10-16,Trompe L'oeil Ferromagnetism: magnetic point group analysis,"Ferromagnetism can be characterized by various unique phenomena such as non-zero magnetization (inducing magnetic attraction/repulsion), diagonal piezomagnetism, nonreciprocal circular dichroism (such as Faraday effect), odd-order (including linear) anomalous Hall effect, and magneto-optical Kerr effect. We identify all broken symmetries requiring each of the above phenomena, and also the relevant magnetic point groups (MPGs) with those broken symmetries. All of ferromagnetic point groups, relevant for ferromagnets, ferri-magnets and weak ferromagnets, can certainly exhibit all of these phenomena, including non-zero magnetization. Some of true antiferromagnets, which are defined as magnets with MPGs that do not belong to ferromagnetic point groups, can display these phenomena through magnetization induced by external perturbations such as applied current, electric fields, light illumination, and strain. Such MPGs are identified for each external perturbation. Since high-density and ultrafast spintronic technologies can be enabled by antiferromagnets, our findings will be an essential guidance for the future magnetism-related science as well as technology.",2310.10562v1 2024-01-23,Altermagnetism with Non-collinear Spins,"Altermagnetism is introduced as a category of magnetic states with vanishing net magnetic moment, and consists of collinear alternating (i.e., antiferromagnetic-like) spins and alternating variations of local structures around spins in such a way that the symmetry allows ferromagnetic behaviors such as anomalous Hall effect and magneto-optical Kerr effects. Altermagnets exhibiting ferromagnetic behaviors without any external perturbations (called type-I) turn out to belong to the ferromagnetic point group, and represent a form of weak ferromagnetic states. Other altermagnets (called type-II and type-III) can have ferromagnetic behaviors only with external perturbations, which conserve parity-time reversal (PT) symmetry. All types of altermagnets themselves have broken PT symmetry. The concept of altermagnetism can be extended to accommodate multiple spin and local-structure variations (thus, include non-collinear spins), and this extended form of altermagnetism offers an intriguing opportunity to leverage complementary advantages of both ferromagnetism and antiferromagnetism, and thus holds great promise for, for example, various spintronic applications.",2401.13069v1 2024-03-12,Probing quantum criticality in ferromagnetic CeRh6Ge4,"CeRh$_6$Ge$_4$ is unusual in that its ferromagnetic transition can be suppressed continuously to zero temperature, i.e., to a ferromagnetic quantum-critical point (QCP), through the application of modest hydrostatic pressure. This discovery has raised the possibility that the ferromagnetic QCP may be of the Kondo-breakdown type characterized by a jump in Fermi volume, to which thermopower S measurements should be sensitive. Though $S/T$ changes both sign and magnitude around the critical pressure P$_{c}\approx{}0.8$ GPa, these changes are not abrupt but extend over a pressure interval from within the ferromagnetic state up to P$_c$. Together with temperature and pressure variations in electrical resistivity and previously reported heat capacity, thermopower results point to the near coincidence of two sequential effects near P$_c$, delocalization of 4f degrees-of-freedom through orbital-selective hybridization followed by quantum criticality of itinerant ferromagnetism.",2403.07812v1 2024-03-14,Electronic structure of above-room-temperature van der Waals ferromagnet Fe$_3$GaTe$_2$,"Fe$_3$GaTe$_2$, a recently discovered van der Waals ferromagnet, demonstrates intrinsic ferromagnetism above room temperature, necessitating a comprehensive investigation of the microscopic origins of its high Curie temperature ($\textit{T}$$_C$). In this study, we reveal the electronic structure of Fe$_3$GaTe$_2$ in its ferromagnetic ground state using angle-resolved photoemission spectroscopy and density functional theory calculations. Our results establish a consistent correspondence between the measured band structure and theoretical calculations, underscoring the significant contributions of the Heisenberg exchange interaction ($\textit{J}$$_{ex}$) and magnetic anisotropy energy to the development of the high-$\textit{T}$$_C$ ferromagnetic ordering in Fe$_3$GaTe$_2$. Intriguingly, we observe substantial modifications to these crucial driving factors through doping, which we attribute to alterations in multiple spin-splitting bands near the Fermi level. These findings provide valuable insights into the underlying electronic structure and its correlation with the emergence of high-$\textit{T}$$_C$ ferromagnetic ordering in Fe$_3$GaTe$_2$.",2403.09846v1 2024-03-29,Piezomagnetism in the Ising ferromagnet URhGe,"Piezomagnetism, linear response between strain and magnetic field, is relatively unexplored cross-correlation but has promising potential as a novel probe of time-reversal-symmetry breaking in various classes of materials. Interestingly, there has been no report of piezomagnetism in ferromagnets, most archetypal time-reversal-symmetry-broken materials. This half-century absence of piezomagnetic ferromagnets is attributable to complications originating from multiple-domain states, as well as from changes in the magnetic point group by rotation of magnetic moment. Here, we report characteristic V-shaped magnetostriction in the Ising itinerant ferromagnet URhGe, observed by simultaneous multi-axis strain measurement technique utilizing optical fiber Bragg grating sensors. This novel magnetostriction occurs only under fields along the c axis and does not scale with the square of magnetization. Such unconventional feature indicates piezomagnetism as its origin. Our observation, marking the first report of piezomagnetism in ferromagnets, is owing to the mono-domain switching and the Ising magnetization. The obtained piezomagnetic coefficients are fairly large, implying that Ising ferromagnets are promising frontiers when seeking for materials with large piezomagnetic responses.",2403.19998v1 2010-03-29,Ferromagnetism in defect-ridden oxides and related materials,"The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO2 (rutile) containing 1 - 5% of iron cations and find that the room-temperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir in the present case a mixture Fe2+ and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons Ntot, the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d-d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as nonmagnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an an anhysteretic magnetization curve which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1-2 % of the volume of the films is magnetically ordered.",1003.5558v1 2016-01-08,Coexistence of interacting-ferromagnetic and small-antiferromagnetic clusters in La$_{0.5}$Ba$_{0.5}$CoO$_3$,"We report detailed dc magnetization, linear and non-linear ac susceptibility measurements on the hole doped disordered cobaltite La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters possibly start a magnetic ordering at the Curie temperature of 201.5(5)~K. The non-ferromagnetic phases mainly consist of antiferromagnetic-clusters with size smaller than the ferromagnetic-clusters. Below Curie temperature the system exhibits an irreversibility in the field cooled and zero field cooled magnetization and frequency dependence in the peak of ac susceptibility. These dynamical features indicate towards the possible coexistence of spin-glass phase along with ferromagnetic-clusters similar to La$_{1-x}$Sr$_{x}$CoO$_3$ (x$\geq$0.18), but the absence of field divergence in third harmonic of ac susceptibility and zero field cooled memory clearly rule out any such possibility. We argue that the spin-glass phase in La$_{1-x}$Sr$_{x}$CoO$_3$ (x$\geq$0.18) is associated with the presence of incommensurate antiferromagnetic ordering in non-ferromagnetic phases which is absent in La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our analysis show that the observed dynamical features in La$_{0.5}$Ba$_{0.5}$CoO$_3$ are possibly due to progressive thermal blocking of ferromagnetic-clusters which is further confirmed by the Wohlfarth's model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with $\tau_0\approx10^{-9}$s. This together with the existence of an AT line in H-T space indicates the presence of significant inter-cluster interaction among these ferromagnetic-clusters.",1601.01995v1 1999-08-10,Existence of Saturated Ferromagnetic and Spiral States in 1D Lieb-Ferrimagnetic Models away from Half-Filling,"In order to study conditions for the appearance of ferromagnetism in a wide filling region, we investigate numerically three types of one-dimensional Lieb-ferrimagnetic Hubbard models: a periodic diamond (PD) chain, a periodic alternately-attached leg (PAAL) chain and an open diamond (OD) chain. All of these models have a flat band (or equivalently, degenerate single-electron eigenvalues). The PD and OD chains commonly have a local-loop structure. Nagaoka's theorem holds only in the PD chain. At half-filling, it have been rigorously proven that all of these models are ferrimagnet. Away from half-filling, however, quite different magnetic properties are found. In the fillings 1/3< rho_e <1/2, the ground state of the PD chain for a infinitely-large U is the extended ferromagnetic state, that is, the saturated ferromagnetic state or the spiral state for odd or even number of electrons, respectively. In the PAAL chain, on the other hand, there is no magnetic order. Thus, the flat band is found to be not a sufficient condition of the extended ferromagnetic state. We find, moreover, that the saturated ferromagnetism appears in the OD chain, although the Nagaoka theorem does not hold on this chain. This indicates that the local-loop structure plays an important role on the appearance of the extended ferromagnetic state.",9908144v1 1999-12-29,Theory of Disordered Itinerant Ferromagnets I: Metallic Phase,"A comprehensive theory for electronic transport in itinerant ferromagnets is developed. We first show that the Q-field theory used previously to describe a disordered Fermi liquid also has a saddle-point solution that describes a ferromagnet in a disordered Stoner approximation. We calculate transport coefficients and thermodynamic susceptibilities by expanding about the saddle point to Gaussian order. At this level, the theory generalizes previous RPA-type theories by including quenched disorder. We then study soft-mode effects in the ferromagnetic state in a one-loop approximation. In three-dimensions, we find that the spin waves induce a square-root frequency dependence of the conductivity, but not of the density of states, that is qualitatively the same as the usual weak-localization effect induced by the diffusive soft modes. In contrast to the weak-localization anomaly, this effect persists also at nonzero temperatures. In two-dimensions, however, the spin waves do not lead to a logarithmic frequency dependence. This explains experimental observations in thin ferromagnetic films, and it provides a basis for the construction of a simple effective field theory for the transition from a ferromagnetic metal to a ferromagnetic insulator.",9912477v2 2000-03-23,Conductance modulation by spin precession in non-collinear ferromagnet-normal metal-ferromagnet systems,"We study diffusive transport through ferromagnet - normal metal - ferromagnet (F-N-F) systems, with arbitrary but fixed magnetization directions of the ferromagnetic reservoirs and orientations of a magnetic field applied to the normal metal. For non-collinear configurations, the complex mixing conductance describes the transport of spins non-collinear to the magnetizations of the ferromagnetic reservoirs. When the imaginary part of the mixing conductance is different from zero, the total conductance of the system in the presence of a magnetic field can be asymmetric with respect to time reversal. The total conductance changes non-monotonically with the magnetic field strength for different magnetic configurations. This modulation of the conductance is due to the precession of the spin accumulation in the normal metal. The difference between the conductance of the parallel and antiparallel configurations can be either positive or negative as a function of the applied magnetic field. This effect should be best observable on Al single crystals attached to ferromagnetic electrodes by means of tunnel junctions or metallic contacts.",0003382v4 2002-11-14,Ferromagnetism of a graphite nodule from the Canyon Diablo meteorite,"There have recently been various reports of weak ferromagnetism in graphite (1,2) and synthetic carbon materials (3) such as rhombohedral C60 (4), as well as a theoretical prediction of a ferromagnetic instability in graphene sheets (5). With very small ferromagnetic signals, it is difficult to be certain that the origin is intrinsic, rather than due to minute concentrations of iron-rich impurities. Here we take a different experimental approach to study ferromagnetism in graphitic materials, by making use of meteoritic graphite, which is strongly ferromagnetic at room temperature. We examined ten samples of extraterrestrial graphite from a nodule in the Canyon Diablo meteorite. Graphite is the major phase in every sample but there are minor amounts of magnetite, kamacite, akaganeite, and other phases. By analysing the phase composition of a series of samples, we find that these iron-rich minerals can only account for about two-thirds of the observed magnetization. The remainder is somehow associated with graphite, corresponding to an average magnetization of 23 Am2kg-1, or 0.05 Bohr magnetons per carbon atom. The magnetic ordering temperature is near 570 K. We suggest that the ferromagnetism is a magnetic proximity effect induced at the interface with magnetite or kamacite inclusions.",0211275v1 2002-12-20,Magnon Exchange Mechanism of Ferromagnetic Superconductivity,"The magnon exchange mechanism of ferromagnetic superconductivity (FM-superconductivity) was developed to explain in a natural way the fact that the superconductivity in $UGe_2$, $ZrZn_2$ and $URhGe$ is confined to the ferromagnetic phase.The order parameter is a spin anti-parallel component of a spin-1 triplet with zero spin projection. The transverse spin fluctuations are pair forming and the longitudinal ones are pair breaking. In the present paper, a superconducting solution, based on the magnon exchange mechanism, is obtained which closely matches the experiments with $ZrZn_2$ and $URhGe$. The onset of superconductivity leads to the appearance of complicated Fermi surfaces in the spin up and spin down momentum distribution functions. Each of them consist of two pieces, but they are simple-connected and can be made very small by varying the microscopic parameters. As a result, it is obtained that the specific heat depends on the temperature linearly, at low temperature, and the coefficient $\gamma=\frac {C}{T}$ is smaller in the superconducting phase than in the ferromagnetic one. The absence of a quantum transition from ferromagnetism to ferromagnetic superconductivity in a weak ferromagnets $ZrZn_2$ and $URhGe$ is explained accounting for the contribution of magnon self-interaction to the spin fluctuations' parameters. It is shown that in the presence of an external magnetic field the system undergoes a first order quantum phase transition.",0212520v1 2005-09-04,Itinerant Ferromagnetism and Metamagnetism in Cr Doped Perovskite Ruthenates,"We report results of structural, magnetic and transport properties of single crystal CaRu1-xCrxO3 (0≤x≤0.36) and SrRu1-xCrxO3 (0≤x≤0.30). Cr substitution as low as x=0.08 drives CaRu1-xCrxO3 from the paramagnetic state to an itinerant ferromagnetic state with field-driven first-order metamagnetic transitions leading to a sizeable saturation moment (~0.4B/f.u.within the ab plane). The ferromagnetism occurs abruptly and reaches as high as TC=123 K for x=0.22. The Cr-driven ferromagnetism is highly anisotropic, suggesting an important role for spin-orbit coupling. Lattice constant and magnetic measurements strongly support the valence of the Cr as tetravalent (Cr4+, 3d2 configuration). Cr substitution for Ru in SrRuO3 (TC=165 K) enhances the itinerant ferromagnetism, with TC reaching 290 K for x=0.30, consistent with Cr-induced ferromagnetism in paramagnetic CaRuO3. Preliminary pressure-dependent magnetization of CaRu0.85Cr0.15O3 shows strong enhancement of the saturation magnetization (25% for P~0.7 GPa). All results indicate a coupling of Ru 4d and Cr 3d electrons that is unexpectedly favorable for itinerant ferromagnetism which often exists delicately in the ruthenates.",0509084v1 2007-11-07,Spin-flip scattering and non-ideal interfaces in dirty ferromagnet/superconductor junctions,"We study the proximity-induced superconducting correlations as well as the local density of states of a ferromagnet, in a ferromagnet/s-wave superconductor heterostructure. We include the effects of spin-flip scattering, non-ideal interfaces, and the presence of impurities in the sample. We employ the quasiclassical theory of superconductivity, solving the Usadel equation with emphasis on obtaining transparent analytical results. As our main result, we report that in a certain parameter regime the spatial oscillations of the anomalous (superconducting) part of the Green's function induced in the ferromagnet by the proximity effect from the s-wave superconductor, are damped out due to the presence of spin-flip processes. As a consequence, spin-flip scattering may under certain conditions actually enhance the local density of states due to the oscillatory behaviour of the latter in ferromagnet/superconductor structures. We also conjecture that the damping could be manifested in the behaviour of the critical temperature ($T_c$) of the s-wave superconductor in contact with the ferromagnet. More specifically, we argue that the non-monotonic decrease of $T_c$ in ferromagnet/s-wave superconductor junctions without magnetic impurities is altered to a monotonic, non-oscillatory decrease when the condition $1>16\tau_\text{sf}^2h^2$ is fulfilled, where $\tau_\text{sf}$ is the spin-flip relaxation time and $h$ is the exchange field.",0711.1062v1 2007-12-10,Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies,"Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were performed in the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transitions were reported to occur at $T_{\rm Curie} \sim 3$ K and $T_S \sim 0.8$ K (N. T. Huy {\it et al.}, Phys. Rev. Lett. {\bf 99} (2007) 067006), in order to investigate the coexistence of ferromagnetism and superconductivity as well as the normal-state and SC properties from a microscopic point of view. From the nuclear spin-lattice relaxation rate $1/T_1$ and Knight-shift measurements, we confirmed that ferromagnetic fluctuations which possess a quantum critical character are present above $T_{\rm Curie}$ and the occurrence of ferromagnetic transition at 2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal state show that UCoGe is an itinerant ferromagnet similar to ZrZn$_2$ and YCo$_2$. The onset SC transition was identified at $T_S \sim 0.7$ K, below which $1/T_1$ of 30 % of the volume fraction starts to decrease due to the opening of the SC gap. This component of $1/T_1$, which follows a $T^3$ dependence in the temperature range of $0.3 - 0.1$ K, coexists with the magnetic components of $1/T_1$ showing a $\sqrt{T}$ dependence below $T_S$. From the NQR measurements in the SC state, we suggest that the self-induced vortex state is realized in UCoGe.",0712.1403v2 2009-07-21,Search for Ferromagnetism in doped semiconductors in the absence of transition metal ions,"In contrast to semiconductors doped with transition metal magnetic elements, which become ferromagnetic at temperatures below ~ 100K, semiconductors doped with non-magnetic ions (e.g. silicon doped with phosphorous) have not shown evidence of ferromagnetism down to millikelvin temperatures. This is despite the fact that for low densities the system is expected to be well modeled by the Hubbard model, which is predicted to have a ferromagnetic ground state at T=0 on 2- or 3-dimensional bipartite lattices in the limit of strong correlation near half-filling. We examine the impurity band formed by hydrogenic centers in semiconductors at low densities, and show that it is described by a generalized Hubbard model which has, in addition to strong electron-electron interaction and disorder, an intrinsic electron-hole asymmetry. With the help of mean field methods as well as exact diagonalization of clusters around half filling, we can establish the existence of a ferromagnetic ground state, at least on the nanoscale, which is more robust than that found in the standard Hubbard model. This ferromagnetism is most clearly seen in a regime inaccessible to bulk systems, but attainable in quantum dots and 2D heterostructures. We present extensive numerical results for small systems that demonstrate the occurrence of high-spin ground states in both periodic and positionally disordered 2D systems. We consider how properties of real doped semiconductors, such as positional disorder and electron-hole asymmetry, affect the ground state spin of small 2D systems. We also discuss the relationship between this work and diluted magnetic semiconductors, such as Ga_(1-x)Mn_(x)As, which though disordered, show ferromagnetism at relatively high temperatures.",0907.3671v1 2010-02-11,The frustrated spin-1/2 J1-J2 Heisenberg ferromagnet on the square lattice: Exact diagonalization and Coupled-Cluster study,"We investigate the ground-state magnetic order of the spin-1/2 J1-J2 Heisenberg model on the square lattice with ferromagnetic nearest-neighbor exchange J1<0 and frustrating antiferromagnetic next-nearest neighbor exchange J2>0. We use the coupled-cluster method to high orders of approximation and Lanczos exact diagonalization of finite lattices of up to N=40 sites in order to calculate the ground-state energy, the spin-spin correlation functions, and the magnetic order parameter. We find that the transition point at which the ferromagnetic ground state disappears is given by J2^{c1}=0.393|J1| (exact diagonalization) and J2^{c1}=0.394|J1| (coupled-cluster method). We compare our results for ferromagnetic J1 with established results for the spin-1/2 J1-J2 Heisenberg model with antiferromagnetic J1. We find that both models (i.e., ferro- and antiferromagnetic J1) behave similarly for large J2, although significant differences between them are observed for J2/|J1| \lesssim 0.6. Although the semiclassical collinear magnetic long-range order breaks down at J2^{c2} \approx 0.6J1 for antiferromagnetic J1, we do not find a similar breakdown of this kind of long-range order until J2 \sim 0.4|J1| for the model with ferromagnetic J1. Unlike the case for antiferromagnetic J1, if an intermediate disordered phase does occur between the phases exhibiting semiclassical collinear stripe order and ferromagnetic order for ferromagnetic J1 then it is likely to be over a very small range below J2 \sim 0.4|J1|.",1002.2299v1 2010-02-12,Magnetic materials and the problem of thermal Casimir force,"We investigate the thermal Casimir interaction between two magnetodielectric plates made of real materials. On the basis of the Lifshitz theory, it is shown that for diamagnets and for paramagnets in the broad sense (with exception of ferromagnets) the magnetic properties do not influence the magnitude of the Casimir force. For ferromagnets, taking into account the realistic dependence of magnetic permeability on frequency, we conclude that the impact of magnetic properties on the Casimir interaction arises entirely from the contribution of the zero-frequency term in the Lifshitz formula. The computations of the Casimir free energy and pressure are performed for the configurations of two plates made of ferromagnetic metals (Co and Fe), for one plate made of ferromagnetic metal and the other of nonmagnetic metal (Au), for two ferromagnetic dielectric plates (on the basis of polystyrene), and for a ferromagnetic dielectric plate near a nonmagnetic metal plate. The dielectric permittivity of metals is described using both the Drude and the plasma model approaches. It is shown that the Casimir repulsion through the vacuum gap can be realized in the configuration of a ferromagnetic dielectric plate near a nonmagnetic metal plate described by the plasma model. In all cases considered, the respective analytical results in the asymptotic limit of large separations between the plates are obtained. The impact of the magnetic phase transition through the Curie temperature on the Casimir interaction is considered. In conclusion, we propose several experiments allowing to determine whether the magnetic properties really influence the Casimir interaction and to independently verify the Drude and plasma model approaches to the thermal Casimir force.",1002.2484v1 2010-09-14,Spin-triplet Supercurrent through Inhomogeneous Ferromagnetic Trilayers,"Motivated by a recent experiment [J. W. A. Robinson, J. D. S. Witt and M. G. Blamire, Science, \textbf{329}, 5987 (2010)], we here study the possibility of establishing a long-range spin-triplet supercurrent through an inhomogeneous ferromagnetic region consisting of a Ho$\mid$Co$\mid$Ho trilayer sandwiched between two conventional s-wave superconductors. We utilize a full numerical solution in the diffusive regime of transport and study the behavior of the supercurrent for various experimentally relevant configurations of the ferromagnetic trilayer. We obtain qualitatively very good agreement with experimental data regarding the behavior of the supercurrent as a function of the width of the Co-layer, $L_\text{Co}$. Moreover, we find a synthesis of 0-$\pi$ oscillations with superimposed rapid oscillations when varying the width of the Ho-layer which pertain specifically to the spiral magnetization texture in Ho. We are not able to reproduce the anomalous peaks in the supercurrent observed experimentally in this regime, but note that the results obtained are quite sensitive to the exact magnetization profile in the Ho-layers, which could be the reason for the discrepancy between our model and the experimental reported data for this particular aspect. We also investigate the supercurrent in a system where the intrinsically inhomogeneous Ho ferromagnets are replaced with domain-wall ferromagnets, and find similar behavior as in the Ho$\mid$Co$\mid$Ho case. Furthermore, we propose a novel type of magnetic Josephson junction including only a domain-wall ferromagnet and a homogeneous ferromagnetic layer, which in addition to simplicity regarding the magnetization profile also offers a tunable long-range spin-triplet supercurrent. Finally, we discuss some experimental aspects of our findings.",1009.2754v1 2012-10-19,"Ferromagnetic structures in Mn2CoGa and Mn2CoAl doped by Co, Cu, V, and Ti","The structure and magnetic properties in doped Heusler alloys of Mn2CoGa and Mn2CoAl have been investigated by experiments and calculations. The main group elements of Ga and Al are substituted by the magnetic or non-magnetic transition metals, Co, Cu, V, and Ti in the alloy systems. Three kinds of local ferromagnetic structures, Co-Mn-Co, Mn-Co-Mn and Mn-Co-V, have been found. They embed in the native ferrimagnetic matrix and increase the magnetization with different increments. The Co-Mn-Co ferromagnetic structure shows the largest increment of 6.18{\mu}B /atom. In addition, interesting results for non-magnetic Cu increasing the magnetization and the V atom having a large ferromagnetic moment of about 1.0{\mu}B have been obtained. The exchange interaction energy can be increased by the newly added Co and depleted by supporting a ferromagnetic coupling in other substitution cases, and showing the variation of the TC. Our calculation of electronic structure verifies the strong d-d hybridization when the three ferromagnetic structures are achieved. It has also been found that the covalent effect from the Ga and Al determines the generation of the local ferromagnetic structure and the tolerance for dopant content.",1210.5357v1 2013-07-22,Self-consistent Gutzwiller study of bcc Fe: interplay of ferromagnetic order and kinetic energy,"The Gutzwiller technique has long been known as a method to include correlations in electronic structure calculations. Here we implement an ab-initio Gutzwiller+LDA calculation, and apply it to a classic problem, the ferromagnetism of bulk bcc Fe, whose nature has attracted recent interest. In the conventional Stoner-Wohlfarth model, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion at the extra cost of some increase of electron kinetic energy. Density functional theory within the spin polarized local density approximation (LDA) has long supported that picture, showing that ferromagnetic alignment causes band narrowing and a corresponding wavefunction localization, whence a kinetic energy increase. However, because of its inadequate treatment of strong intra-site correlations for localized d orbitals, LDA cannot be relied upon, particularly when it comes to separately describing fine potential and kinetic energy imbalances. With ab-initio Gutzwiller+LDA, we indeed find that the effect of correlations is to dramatically reverse the balance, the ferromagnetic ordering of Fe in fact causing a decrease of kinetic energy, at the cost of some increase of potential energy. The underlying physical mechanism, foreshadowed long ago by Goodenough and others, and more recently supported by LDA+DMFT calculations, is that correlations cause eg and t2g 3d orbitals to behave very differently. Weakly dispersive eg states are spin-polarized and almost localized, while, more than half filled, the t2g are broad band, fully delocalized states. Owing to intra-atomic Hund's rule exchange which aligns eg and t2g spins, the propagation of itinerant t2g holes is only allowed when different atomic spins are ferromagnetically aligned. We thus conclude that double exchange is at work already in the most popular ferromagnetic metal.",1307.5738v1 2013-11-29,Magnon radiation by moving Abrikosov vortices in ferromagnetic superconductors and superconductor-ferromagnet multilayers,"In systems combining type-II superconductivity and magnetism the non-stationary magnetic field of moving Abrikosov vortices may excite spin waves, or magnons. This effect leads to the appearance of an additional damping force acting on the vortices. By solving the London and Landau-Lifshitz-Gilbert equations we calculate the magnetic moment induced force acting on vortices in ferromagnetic superconductors and superconductor/ferromagnet superlattices. If the vortices are driven by a dc force, magnon generation due to the Cherenkov resonance starts as the vortex velocity exceeds some threshold value. For an ideal vortex lattice this leads to an anisotropic contribution to the resistivity and to the appearance of resonance peaks on the current voltage characteristics. For a disordered vortex array the current will exhibit a step-like increase at some critical voltage. If the vortices are driven by an ac force with a frequency \omega, the interaction with magnetic moments will lead to a frequency-dependent magnetic contribution \eta_M to the vortex viscosity. If \omega is below the ferromagnetic resonance frequency \omega_F, vortices acquire additional inertia. For \omega > \omega_F dissipation is enhanced due to magnon generation. The viscosity \eta_M can be extracted from the surface impedance of the ferromagnetic superconductor. Estimates of the magnetic force acting on vortices for the U-based ferromagnetic superconductors and cuprate/manganite superlattices are given.",1311.7620v1 2014-07-22,Fermi Surface Instabilities in Ferromagnetic Superconductor URhGe,"The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well explained by two contributions, namely by the normal Hall effect and by the large anomalous Hall effect due to skew scattering. The large change in the Hall coefficient is observed at low fields between the paramagnetic and ferromagnetic states for H // c-axis (easy-magnetization axis) in the orthorhombic structure, indicating that the Fermi surface is reconstructed in the ferromagnetic state below the Curie temperature (T_Curie=9.5K). At low temperatures (T << T_Curie), when the field is applied along the b-axis, the reentrant superconductivity was observed in both the Hall resistivity and the magnetoresistance below 0.4K. Above 0.4K, a large jump with the first-order nature was detected in the Hall resistivity at a spin-reorientation field H_R ~ 12.5T, demonstrating that the marked change of the Fermi surface occurs between the ferromagnetic state and the polarized state above H_R. The results can be understood by the Lifshitz-type transition, induced by the magnetic field or by the change of the effective magnetic field.",1407.5799v1 2014-10-18,Colossal proximity effect in a superconducting triplet spin valve based on halfmetallic ferromagnetic CrO2,"Ferromagnets can sustain supercurrents through the formation of equal spin triplet Cooper pairs and the mechanism of odd-frequency pairing. Since such pairs are not broken by the exchange energy of the ferromagnet, superconducting triplet correlations are long-ranged and spin-polarized, with promises for superconducting spintronics devices. The main challenge is to understand how triplets are generated at the superconductor (S)/ ferromagnet (F) interface. Here we use the concept of a so-called triplet spin valve (TSV) to investigate the conversion of singlets in a conventional superconductor to triplets in the halfmetallic ferromagnet CrO_2. TSV's are composed of two ferromagnetic layers (separated by a thin normal metal (N) layer) and a superconductor (F_1/N/F_2/S). The package F_1/N/F_2 generates triplets in F_1 when the magnetization directions of the F_{1,2}-layers are not collinear. This drains singlet pairs from the S-layer, and triplet generation is therefore signalled by a decrease of the critical temperature $T_c$. Recently, experiments with TSV's were reported with Co draining layers, using in-plane fields, and finding T_c-shifts up to 100~mK. Using CrO_2 instead of Co and rotating a magnetic field from in-plane to out-of-plane, we find strong T_c variations of almost a Kelvin up to fields of the order of a Tesla. Such strong drainage is consistent with the large lengths over which supercurrents can flow in CrO_2, which are significantly larger than in conventional ferromagnets. Our results point to the special interest of halfmetals for superconducting spintronics.",1410.4973v1 2015-02-10,Disentangling relativistic spin torques in a ferromagnet/semiconductor bilayer,"Recently discovered relativistic spin torques induced by a lateral current at a ferromagnet/paramagnet interface are a candidate spintronic technology for a new generation of electrically-controlled magnetic memory devices. Phenomenologically, the torques have field-like and antidamping-like components with distinct symmetries. Microscopically, they are considered to have two possible origins. In one picture, a spin-current generated in the paramagnet via the relativistic spin Hall effect (SHE) is absorbed in the ferromagnet and induces the spin transfer torque (STT). In the other picture, a non-equilibrium spin-density is generated via the relativistic inverse spin galvanic effect (ISGE) and induces the spin-orbit torque (SOT) in the ferromagnet. From the early observations in paramagnetic semiconductors, SHE and ISGE are known as companion phenomena that can both allow for electrically aligning spins in the same structure. It is essential for our basic physical understanding of the spin torques at the ferromagnet/paramagnet interface to experimentally disentangle the SHE and ISGE contributions. To achieve this we prepared an epitaxial transition-metal-ferromagnet/semiconductor-paramagnet single-crystal structure and performed a room-temperature vector analysis of the relativistic spin torques by means of the all-electrical ferromagnetic resonance (FMR) technique. By design, the field-like torque is governed by the ISGE-based mechanism in our structure while the antidamping-like torque is due to the SHE-based mechanism",1502.02870v1 2015-05-14,Superconductivity in the ferromagnetic semiconductor SmN,"The discovery of materials that simultaneously host different phases of matter has often initially confounded, but ultimately enhanced, our basic understanding of the coexisting types of order. The associated intellectual challenges, together with the promise of greater versatility for potential applications, have made such systems a focus of modern materials science. In particular, great efforts have recently been devoted to making semiconductors ferromagnetic and metallic ferromagnets superconducting. Here we report the unprecedented observation of a heavily donor-doped ferromagnetic semiconductor, SmN, becoming superconducting with ferromagnetism remaining intact. The extremely large exchange splitting of the conduction and valence bands in this material necessitates that the superconducting order hosted by SmN is of an unconventional triplet type, most likely exhibiting p-wave symmetry. Short range spin fluctuations, which are thought to be the cause of pairing interactions in currently known triplet superconductors, are quenched in SmN, suggesting its superconductivity to be the result of phonon- or Coulomb-mediated pairing mechanisms. This scenario is further supported by the inferred heavy mass of superconducting charge carriers. The unique near-zero magnetisation associated with the ferromagnetic state in SmN further aids its coexistence with superconductivity. Presenting this novel material system where semiconducting, ferromagnetic and superconducting properties are combined provides a versatile new laboratory for studying quantum phases of matter. Moreover it is a major step towards identifying materials that merge superconductivity and spintronics, urgently needed to enable the design of electronic devices with superior functionality.",1505.03621v2 2015-07-15,"Conductance, Valley and Spin polarization and Tunnelling magneto-resistance in ferromagnetic-normal-ferromagnetic junctions of silicene","We investigate charge conductance and spin and valley polarization along with the tunnelling magneto-resistance (TMR) in silicene junctions composed of normal silicene and ferromagnetic silicene. We show distinct features of the conductances for parallel and anti-parallel spin configurations and the TMR, as the ferromagnetic-normal-ferromagnetic (FNF) junction is tuned by an external electric field. We analyse the behavior of the charge conductance and valley and spin polarizations in terms of the independent conductances of the different spins at the two valleys and the band structure of ferromagnetic silicene and show how the conductances are affected by the vanishing of the propagating states at one or the other valley. In particular, unlike in graphene, the band structure at the two valleys are independently affected by the spin in the ferromagnetic regions and lead to non-zero, and in certain parameter regimes, pure valley and spin polarizations, which can be tuned by the external electric field. We also investigate the oscillatory behavior of the TMR with respect to the strength of the barrier potential (both spin-independent and spin-dependent barriers) in the normal silicene region and note that in some parameter regimes, the TMR can even go from positive to negative values, as a function of the external electric field.",1507.04225v2 2015-11-24,"Coexistence of superconductivity and itinerant ferromagnetism in Sr0.5Ce0.5FBiS2-xSex (x = 0.5 and 1.0), the first non-U material with Tc < TFM","We have carried out detailed magnetic and transport studies of the new Sr0.5Ce0.5FBiS2-xSex (x = 0.5, 1) superconductors derived by doping Se in Sr0.5Ce0.5FBiS2. Se-doping produces several effects: it suppresses semiconducting like behavior observed in the undoped Sr0.5Ce0.5FBiS2, ferromagnetic ordering temperature, TFM, decreases considerably from 7.5 K (in Sr0.5Ce0.5FBiS2) to 3.5 K and superconducting transition temperature, Tc, gets enhanced slightly to 2.9 - 3.3 K. Thus in these Se-doped materials, TFM is just marginally higher than Tc. Magnetization studies provide an evidence of bulk superconductivity in Sr0.5Ce0.5FBiS2-xSex. Quite remarkably, as compared with the effective paramagnetic Ce-moment (~ 2.2 muB), the ferromagnetically ordered Ce-moment in the superconducting state is rather small (~ 0.1 muB). To the best of our knowledge, the title compounds are the first Ce-based superconducting itinerant ferromagnetic materials (Tc < TFM). We stress that Ce-4f electrons are responsible for both superconductivity and ferromagnetism just as U-5f electrons are in UCoGe. Furthermore, a novel feature of these materials is a dual hysteresis loop corresponding to both the ferromagnetism and the coexisting superconductivity. Such features of Sr0.5Ce0.5FBiS2-xSex put these materials apart from the well known U-containing superconducting ferromagnets reported so far.",1511.07692v2 2016-06-15,Topological Magnon Bands in Ferromagnetic Star Lattice,"The experimental observation of topological magnon bands and thermal Hall effect in a kagom\'e lattice ferromagnet Cu(1-3, bdc) has inspired the search for topological magnon effects in various insulating ferromagnets that lack an inversion center allowing a Dzyaloshinskii-Moriya (DM) spin-orbit interaction. The star lattice (also known as the decorated honeycomb lattice) ferromagnets is an ideal candidate for this purpose because it is a variant of the kagom\'e lattice with additional links that connect the up-pointing and down-pointing triangles. This gives rise to twice the unit cell of the kagom\'e lattice, hence a more interesting topological magnon effects. In particular, the triangular bridges on the star lattice can be coupled either ferromagnetically or antiferromagnetically which is not possible on the kagome lattice ferromagnets. Here, we study DM-induced topological magnon bands, chiral edge modes, and thermal magnon Hall effect on the star lattice ferromagnet in different parameter regimes. The star lattice can also be visualized as the parent material from which topological magnon bands can be realized for the kagom\'e and honeycomb lattices in some limiting cases.",1606.04904v8 2018-04-12,Spin Transport in Half-Metallic Ferromagnet-Superconductor Junctions,"We investigate the charge and spin transport in half-metallic ferromagnet ($F$) and superconductor ($S$) nanojunctions. We utilize a self-consistent microscopic method that can accommodate the broad range of energy scales present, and ensures proximity effects that account for the interactions at the interfaces are accurately determined. Two experimentally relevant half-metallic junction types are considered: The first is a $F_1 F_2 S$ structure, where a half-metallic ferromagnet $F_1$ adjoins a weaker conventional ferromagnet $F_2$. The current is injected through the $F_1$ layer by means of an applied bias voltage. The second configuration involves a $S F_1 F_2 F_3 S$ Josephson junction whereby a phase difference $\Delta\varphi$ between the two superconducting electrodes generates the supercurrent flow. In this case, the central half-metallic $F_2$ layer is surrounded by two weak ferromagnets $F_1$ and $F_3$. By placing a ferromagnet with a weak exchange field adjacent to an $S$ layer, we are able to optimize the conversion process in which opposite-spin triplet pairs are converted into equal-spin triplet pairs that propagate deep into the half-metallic regions in both junction types. For the tunnel junctions, we study the bias-induced local magnetization, spin currents, and spin transfer torques for various orientations of the relative magnetization angle $\theta$ in the $F$ layers. We find that the bias-induced equal-spin triplet pairs are maximized in the half-metal for $\theta\approx90^\circ$ and as part of the conversion process, are anticorrelated with the opposite-spin pairs. We show that the charge current density is maximized, corresponding to the occurrence of a large amplitude of equal-spin triplet pairs, when the exchange interaction of the weak ferromagnet is about $0.1E_F.$",1804.04275v1 2020-07-28,"Critical behavior of ferromagnets CrI3, CrBr3, CrGeTe3, and anti-ferromagnet FeCl2: a detailed first-principles study","We calculate the Curie temperature of layered ferromagnets, chromium tri-iodide (CrI3), chromium tri-bromide (CrBr3), chromium germanium tri-telluride (CrGeTe3), and the Neel temperature of a layered anti-ferromagnet iron di-chloride (FeCl2), using first-principles density functional theory calculations and Monte-Carlo simulations. We develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature. We provide a unified method to obtain the magnetic exchange parameters (J) for an effective Heisenberg Hamiltonian from first-principles, taking into account both the magnetic ansiotropy as well as the out-of-plane interactions. We obtain the magnetic phase change behavior, in particular the critical temperature, from the susceptibility and the specific-heat, calculated using the three-dimensional Monte-Carlo (Metropolis) algorithm. The calculated Curie temperatures for ferromagnetic materials (CrI3, CrBr3 and CrGeTe3), match very well with experimental values. We show that the interlayer interaction in bulk CrI3 with R3 stacking is significantly stronger than the C2/m stacking, in line with experimental observations. We show that the strong interlayer interaction in R3 CrI results in a competition between the in-plane and the out-of-plane magnetic easy axis. Finally, we calculate the Neel temperature of FeCl2 to be 47 +- 8 K, and show that the magnetic phase transition in FeCl2 occurs in two steps with a high-temperature intralayer ferromagnetic phase transition, and a low-temperature interlayer anti-ferromagnetic phase transition.",2007.14379v3 2017-01-28,Dominant Majorana bound energy and critical current enhancement in ferromagnetic-superconducting topological insulator,"Among the potential applications of topological insulators, we theoretically study the coexistence of proximity-induced ferromagnetic and superconducting orders in the surface states of a 3-dimensional topological insulator. The superconducting electron-hole excitations can be significantly affected by the magnetic order induced by a ferromagnet. In one hand, the surface state of the topological insulator, protected by the time-reversal symmetry, creates a spin-triplet and, on the other hand, magnetic order causes to renormalize the effective superconducting gap. We find Majorana mode energy along the ferromagnet/superconductor interface to sensitively depend on the magnitude of magnetization $m_{zfs}$ from superconductor region, and its slope around perpendicular incidence is steep with very low dependency on $m_{zfs}$. The superconducting effective gap is renormalized by a factor $\eta(m_{zfs})$, and Andreev bound state in ferromagnet-superconductor/ferromagnet/ferromagnet-superconductor (FS/F/FS) Josephson junction is more sensitive to the magnitude of magnetizations of FS and F regions. In particular, we show that the presence of $m_{zfs}$ has a noticeable impact on the gap opening in Andreev bound state, which occurs in finite angle of incidence. This directly results in zero-energy Andreev state being dominant. By introducing the proper form of corresponding Dirac spinors for FS electron-hole states, we find that via the inclusion of $m_{zfs}$, the Josephson supercurrent is enhanced and exhibits almost abrupt crossover curve, featuring the dominant zero-energy Majorana bound states.",1701.08277v1 2016-05-25,Exploring the ferromagnetic behaviour of a repulsive Fermi gas via spin dynamics,"Ferromagnetism is a manifestation of strong repulsive interactions between itinerant fermions in condensed matter. Whether short-ranged repulsion alone is sufficient to stabilize ferromagnetic correlations in the absence of other effects, like peculiar band dispersions or orbital couplings, is however unclear. Here, we investigate ferromagnetism in the minimal framework of an ultracold Fermi gas with short-range repulsive interactions tuned via a Feshbach resonance. While fermion pairing characterises the ground state, our experiments provide signatures suggestive of a metastable Stoner-like ferromagnetic phase supported by strong repulsion in excited scattering states. We probe the collective spin response of a two-spin mixture engineered in a magnetic domain-wall-like configuration, and reveal a substantial increase of spin susceptibility while approaching a critical repulsion strength. Beyond this value, we observe the emergence of a time-window of domain immiscibility, indicating the metastability of the initial ferromagnetic state. Our findings establish an important connection between dynamical and equilibrium properties of strongly-correlated Fermi gases, pointing to the existence of a ferromagnetic instability.",1605.07850v2 2017-07-15,Itinerant ferromagnetism in actinide 5f electrons system: Phenomenological analysis with spin fluctuation theory,"We have carried out an analysis of magnetic data in 69 uranium, 7 neptunium and 4 plutonium ferromagnets with the spin fluctuation theory developed by Takahashi (Y. Takahashi, J. Phys. Soc. Jpn. 55, 3553 (1986)). The basic and spin fluctuation parameters of the actinide ferromagnets are determined and the applicability of the spin fluctuation theory to actinide 5f system has been discussed. Itinerant ferromagnets of the 3d transition metals and their intermetallics follow a generalized Rhodes-Wohlfarth relation between p_eff/p_s and T_C/T_0, viz., p_eff/p_s ~ (T_C/T_0)^(-3/2). Here, p_s, p_eff, T_C, and T_0 are the spontaneous and effective magnetic moments, the Curie temperature and the width of spin fluctuation spectrum in energy space, respectively. The same relation is satisfied for T_C/T_0 < 1.0 in the actinide ferromagnets. However, the relation is not satisfied in a few ferromagnets with T_C/T_0 ~1.0 that corresponds to local moment system in the spin fluctuation theory. The deviation from the theoretical relation may be due to several other effects not included in the spin fluctuation theory such as the crystalline electric field effect on the 5f electrons from ligand atoms. The value of the spontaneous magnetic moment p_s increases linearly as a function of T_C/T_0 in the uranium and neptunium ferromagnets below (T_C/T_0)_kink = 0.32 +- 0.02 where a kink structure appears in relation between the two quantities. p_s increases more weakly above (T_C/T_0)_kink. A possible interpretation with the T_C/T_0-dependence of p_s is given.",1707.04772v1 2017-10-04,Possible evidence for spin-transfer torque induced by spin-triplet supercurrent,"Cooper pairs in superconductors are normally spin singlet. Nevertheless, recent studies suggest that spin-triplet Cooper pairs can be created at carefully engineered superconductor-ferromagnet interfaces. If Cooper pairs are spin-polarized they would transport not only charge but also a net spin component, but without dissipation, and therefore minimize the heating effects associated with spintronic devices. Although it is now established that triplet supercurrents exist, their most interesting property - spin - is only inferred indirectly from transport measurements. In conventional spintronics, it is well known that spin currents generate spin-transfer torques that alter magnetization dynamics and switch magnetic moments. The observation of similar effects due to spin-triplet supercurrents would not only confirm the net spin of triplet pairs but also pave the way for applications of superconducting spintronics. Here, we present a possible evidence for spin-transfer torques induced by triplet supercurrents in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions. Below the superconducting transition temperature T_c, the ferromagnetic resonance (FMR) field at X-band (~ 9.0 GHz) shifts rapidly to a lower field with decreasing temperature due to the spin-transfer torques induced by triplet supercurrents. In contrast, this phenomenon is absent in ferromagnet/superconductor (F/S) bilayers and superconductor/insulator/ferromagnet/superconductor (S/I/F/S) multilayers where no supercurrents pass through the ferromagnetic layer. These experimental observations are discussed with theoretical predictions for ferromagnetic Josephson junctions with precessing magnetization.",1710.01534v1 2018-09-13,Spin-current driven spontaneous coupling of ferromagnets,"A theoretical framework is proposed for the spin-current driven synchronized self-oscillations in ferromagnets in the spin Hall geometry. The spin current generated by the spin Hall effect in a bottom nonmagnetic heavy metal excites a self-oscillation of the magnetization in an attached ferromagnet through spin-transfer effect. The spin current simultaneously creates spin accumulation inside the ferromagnet. Therefore, when the top surfaces of two ferromagnets are connected by a nonmagnetic material having a long spin diffusion length, another spin current flows according to the gradient of the spin accumulations between the ferromagnets. This additional spin current excites an additional spin torque leading to a coupled motion of the magnetizations. This coupling mechanism comes purely from spin degree of freedom in the system without using electric and/or magnetic interactions. The additional spin torque acts as a repulsive force between the magnetizations, and prefers an antiphase synchronization between the oscillators. The phase difference in a synchronized state is determined by the competition between this additional spin torque and spin pumping. Eventually, either an in-phase or antiphase synchronization is spontaneously excited in the individual ferromagnets, depending on the current magnitude. These conclusions are obtained by deriving the theoretical formula of the additional spin torque from the diffusive spin transport theory and solving the equation of motion of the magnetizations both numerically and analytically.",1809.05195v1 2019-06-10,Direct Imaging Revealing Halved Ferromagnetism in Tensile-Strained LaCoO3 Thin Films,"The enigma of the emergent ferromagnetic state in tensile-strained LaCoO3 thin films remains to be explored because of the lack of a well agreed explanation. The direct magnetic imaging technique using a low-temperature magnetic force microscope (MFM) is critical to reveal new aspects of the ferromagnetism by investigating the lateral magnetic phase distribution. Here we show the experimental demonstration of the rare halved occupation of the ferromagnetic state in tensile-strained LaCoO3 thin films on SrTiO3 substrates using the MFM. The films have uniformly strained lattice structure and minimal oxygen vacancies (less than 2%) beyond the measurement limit. It is found that percolated ferromagnetic regions with typical sizes between 100 nm and 200 nm occupy about 50% of the entire film, even down to the lowest achievable temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Preformed ferromagnetic droplets were still observed when the temperature is 20 K above the Curie temperature indicating the existence of possible Griffiths phase. Our study demonstrated a sub-micron level phase separation in high quality LaCoO3 thin films, which has substantial implications in revealing the intrinsic nature of the emergent ferromagnetism.",1906.03825v1 2019-06-24,"Weak ferromagnetism in hexagonal Mn3Z (Z=Sn, Ge, Ga) alloys","We present combined spin model and first principles electronic structure calculations to study the weak ferromagnetism in bulk Mn$_3$Z (Z=Sn, Ge, Ga) compounds. The spin model parameters were determined from a spin-cluster expansion technique based on the relativistic disordered local moment formalism implemented in the screened Korringa--Kohn--Rostoker method. We describe the magnetic ground state of the system within a three-sublattice model and investigate the formation of the weak ferromagnetic states in terms of the relevant model parameters. First, we give a group-theoretical argument how the point-group symmetry of the lattice leads to the formation of weak ferromagnetic states. Then we study the ground states of the classical spin model and derive analytical expressions for the weak ferromagnetic distortions by recovering the main results of the group-theoretical analysis. As a third approach we obtain the weak ferromagnetic ground states from self-consistent density functional calculations and compare our results with previous first principles calculations and with available experimental data. In particular, we demonstrate that the orbital moments follow a decomposition predicted by group theory. For a deeper understanding of the formation of weak ferromagnetism we selectively trace the effect of the spin-orbit coupling at the Mn and Z sites. In addition, for the case of Mn$_3$Ga, we gain information on the role of the induced moment of Ga from constrained local density functional calculations.",1906.10062v1 2019-09-24,Controlling the strength of ferromagnetic order in YBa$_2$Cu$_3$O$_7$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ multilayers,"With dc magnetisation and polarized neutron reflectometry we studied the ferromagnetic response of YBa$_2$Cu$_3$O$_7$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A-type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural details of the YBCO layer on which it is deposited, for others (B-type) the ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of similar structural quality, but electron energy-loss spectroscopy (EELS) studies with a scanning transmission electron microscope reveal an enhanced average Mn oxidation state of +3.5 for the A-type as opposed to the B-type samples for which it is close to the nominal value of +3.33. The related, additional hole doping of the A-type LCMO layers, which likely originates from La and/or Mn vacancies, can explain their fragile ferromagnetic order since it places them close to the boundary of the ferromagnetic order at which even weak perturbations can induce an antiferromagnetic or glassy state. On the other hand, we show that the B-type samples allow one to obtain YBCO/LCMO heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic LCMO layers.",1909.10997v1 2020-12-03,Ferromagnetic and spin-glass like transition in the $q$-neighbor Ising model on random graphs,"The $q$-neighbor Ising model is investigated on homogeneous random graphs with a fraction of edges associated randomly with antiferromagnetic exchange integrals and the remaining edges with ferromagnetic ones. It is a nonequilibrium model for the opinion formation in which the agents, represented by two-state spins, change their opinions according to a Metropolis-like algorithm taking into account interactions with only a randomly chosen subset of their $q$ neighbors. Depending on the model parameters in Monte Carlo simulations phase diagrams are observed with first-order ferromagmetic transition, both first- and second-order ferromagnetic transitions and second-order ferromagnetic and spin-glass-like transitions as the temperature and fraction of antiferromagnetic exchange integrals are varied; in the latter case the obtained phase diagrams qualitatively resemble those for the dilute spin-glass model. Homogeneous mean-field and pair approximations are extented to take into account the effect of the antiferromagnetic exchange interactions on the ferromagnetic phase transition in the model. For a broad range of parameters critical temperatures for the first- or second-order ferromagnetic transition predicted by the homogeneous pair approximation show quantitative agreement with those obtained from Monte Carlo simulations; significant differences occur mainly in the vicinity of the tricritical point in which the critical lines for the second-order ferromagnetic and spin-glass-like transitions meet.",2012.01948v1 2017-09-12,Macroscopic phase separation of superconductivity and ferromagnetism in Sr0.5Ce0.5FBiS2-xSex revealed by muSR,"The compound Sr$_{0.5}$Ce$_{0.5}$FBiS$_{2}$ belongs to the intensively studied family of layered BiS$_2$ superconductors. It attracts special attention because superconductivity at $T_{sc} = 2.8$ K was found to coexist with local-moment ferromagnetic order with a Curie temperature $T_C = 7.5$ K. Recently it was reported that upon replacing S by Se $T_C$ drops and ferromagnetism becomes of an itinerant nature (Thakur et al., Sci. Reports 6, 37527 (2016)). At the same time $T_{sc}$ increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study ($\mu$SR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr$_{0.5}$Ce$_{0.5}$FBiS$_{2-x}$Se$_x$ with $x=0.5$ and $1.0$. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For $x=0.5$ and $x=1.0$ we find a ferromagnetic volume fraction of $\sim \, 8 \%$ and $\sim \, 30 \%$ at $T=0.25$ K, well below $T_{C} = 3.4$ K and $T_C = 3.3$ K, respectively. For $x=1.0$ ($T_{sc} = 2.9$ K) the superconducting phase occupies the remaining sample volume ($\sim \, 70 \%$), as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated.",1709.03632v1 2018-03-07,Two-Dimensional Itinerant Ising Ferromagnetism in Atomically thin Fe3GeTe2,"Recent discoveries of intrinsic two-dimensional (2D) ferromagnetism in insulating/semiconducting van der Waals (vdW) crystals open up new possibilities for studying fundamental 2D magnetism and devices employing localized spins. However, a vdW material that exhibits 2D itinerant magnetism remains elusive. In fact, the synthesis of such single-crystal ferromagnetic metals with strong perpendicular anisotropy at the atomically thin limit has been a long-standing challenge. Here, we demonstrate that monolayer Fe3GeTe2 is a robust 2D itinerant ferromagnet with strong out-of-plane anisotropy. Layer-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanying a fast drop of the Curie temperature from 207 K down to 130 K in the monolayer. For Fe3GeTe2 flakes thicker than ~15 nm, a peculiar magnetic behavior emerges within an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces a novel atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant Ising ferromagnetism and for engineering spintronic vdW heterostructures.",1803.02559v1 2019-02-11,"Magnetization process of the insulating ferromagnetic semiconductor (Al,Fe)Sb","We have studied the magnetization process of the new insulating ferromagnetic semiconductor (Al,Fe)Sb by means of x-ray magnetic circular dichroism. For an optimally doped sample with 10% Fe, a magnetization was found to rapidly increase at low magnetic fields and to saturate at high magnetic fields at room temperature, well above the Curie temperature of 40 K. We attribute this behavior to the existence of nanoscale Fe-rich ferromagnetic domains acting as superparamagnets. By fitting the magnetization curves using the Langevin function representing superparamagnetism plus the paramagnetic linear function, we estimated the average magnetic moment of the nanoscale ferromagnetic domain to be 300-400 $\mu_{B}$, and the fraction of Fe atoms participating in the nano-scale ferromagnetism to be $\sim$50%. Such behavior was also reported for (In,Fe)As:Be and Ge:Fe, and seems to be a universal characteristic of the Fe-doped ferromagnetic semiconductors. Further Fe doping up to 14% led to the weakening of the ferromagnetism probably because antiferromagnetic superexchange interaction between nearest-neighbor Fe-Fe pairs becomes dominant.",1902.03742v1 2019-02-19,Micromagnetometry of two-dimensional ferromagnets,"The study of atomically thin ferromagnetic crystals has led to the discovery of unusual magnetic behaviour and provided insight into the magnetic properties of bulk materials. However, the experimental techniques that have been used to explore ferromagnetism in such materials cannot probe the magnetic field directly. Here, we show that ballistic Hall micromagnetometry can be used to measure the magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from encapsulated graphene. We use the micromagnetometry technique to study atomically thin chromium tribromide (CrBr3). We find that the material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. We also find that the magnetic response of CrBr3 varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism.",1902.06988v2 2019-11-17,A Monte Carlo Study on the Variation of Residual Magnetisation with the Ratio of Coupling Strengths and Non-magnetic Impurities in an Ising Trilayer,"We have studied a spin-1/2, ABA, Ising trilayer system with two different types of interactions, in-plane ferromagnetic and out-of-plane anti-ferromagnetic, among the lattice sites, on different layers. In the pure case, devoid of any impurity, we employed Monte-Carlo method with single spin-flip Metropolis algorithm to find out the anti-ferromagnetic critical temperature (N\'eel temperature) and another lower temperature, called the compensation temperature, both with total magnetisation zero, in accordance with already established results for square lattice. Then non-magnetic impurities, (spin value= 0) were implanted on each layer at randomly picked sites and their concentration was increased in steps from 5% to 20%. The ratio of inter-planar anti-ferromagnetic to mid-layer ferromagnetic coupling strength as well as the ratio of top and bottom layer ferromagnetic to mid layer ferromagnetic coupling strength were also varied and N\'eel and compensation temperatures, both were observed shifting towards lower temperature values with increase in concentration of impurities in the lattice for any fixed ratio of different coupling strengths, in absence of any external magnetic field. In addition, the magnitude of the residual magnetisation i.e. the ratio of the peak value of the magnetisation in between N\'eel and compensation points and the saturated value of magnetisation, was also observed to vary with different values of controlling parameters.",1911.07261v2 2020-03-09,Spin-Triplet Superconductivity in UTe2 and Ferromagnetic Superconductors,"The spin-triplet state is most likely realized in uranium ferromagnetic superconductors, UGe2, URhGe, UCoGe. The microscopic coexistence of ferromagnetism and superconductivity means that the Cooper pair should be realized under the strong internal field due the ferromagnetism, leading to the spin-triplet state with equal spin pairing. The field-reinforced superconductivity, which is observed in all three materials when the ferromagnetic fluctuations are enhanced, is one of the strong evidences for the spin-triplet superconductivity. We present here the results of a newly discovered spin-triplet superconductor, UTe2, and compare those with the results of ferromagnetic superconductors. Although no magnetic order is found in UTe2, there are similarities between UTe2 and ferromagnetic superconductors. For example, the huge upper critical field exceeding the Pauli limit and the field-reentrant superconductivity for H || b-axis are observed in UTe2, URhGe and UCoGe. We also show the specific heat results on UTe2 in different quality samples, focusing on the residual density of states in the superconducting phase.",2003.04055v1 2020-03-31,"Observation of a possible diluted ferromagnetism above room temperature in cobalt-substituted LaTa(O,N)3-d","Since 2000, the intensive effort in materials research to develop a diluted magnetic semiconductor exhibiting high-temperature (HT) ferromagnetism above room temperature was not successful. Here, the possible first bulk diluted HT-ferromagnetic non-metallic materials, based on the perovskite-type oxynitrides LaTa1-xCox(O,N)3-d (x = 0.01, 0.03, 0.05) are realized. The Curie temperature of the synthesized powders exceeds 600 K and the sample magnetizations are large enough to be directly attracted by permanent magnets. Cobalt clusters as a possible source for the observed HT-ferromagnetism can be excluded, since all applied characterization methods verify phase purity. Applied conventional and element-specific magnetometry imply ferromagnetic intermediate spin (IS) Co3+ which is included in a ferromagnetic host matrix. This indicates a complex magnetic interplay between the existing crystal structure, the observed anionic vacancies, and the introduced cobalt ions. These results lay the foundation for the experimental investigation and design of further diluted HT-ferromagnetic semiconductors.",2003.14259v4 2020-06-24,Ferromagnetic induced Kondo effect in graphene with a magnetic impurity,"We investigate the many-body effects of a magnetic adatom in ferromagnetic graphene by using the numerical renormalization group method. The nontrivial band dispersion of ferromagnetic graphene gives rise to interesting Kondo physics different from that in conventional ferromagnetic materials. For a half-filled impurity in undoped graphene, the presence of ferromagnetism can bring forth Kondo correlations, yielding two kink structures in the local spectral function near the Fermi energy. When the spin splitting of local occupations is compensated by an external magnetic field, the two Kondo kinks merge into a full Kondo resonance characterizing the fully screened ground state. Strikingly, we find the resulting Kondo temperature monotonically increases with the spin polarization of Dirac electrons, which violates the common sense that ferromagnetic bands are usually detrimental to Kondo correlations. Doped ferromagnetic graphene can behave as half metals, where its density of states at the Fermi energy linearly vanishes for one spin direction but keeps finite for the opposite direction. In this regime, we demonstrate an abnormal Kondo resonance that occurs in the first spin direction, while completely absent in the other one.",2006.13582v1 2021-11-10,Layer-controlled Ferromagnetism in Atomically Thin CrSiTe$_3$ Flakes,"The research on two-dimensional (2D) van der Waals (vdW) ferromagnets has promoted the development of ultrahigh-density and nanoscale data storage. However, intrinsic ferromagnetism in layered magnets is always subject to many factors, such as stacking orders, interlayer couplings, and the number of layers. Here, we report a magnetic transition from soft to hard ferromagnetic behaviors as the thickness of CrSiTe$_3$ flakes decreases down to several nanometers. Phenomenally, in contrast to the negligible hysteresis loop in the bulk counterparts, atomically thin CrSiTe$_3$ shows a rectangular loop with finite magnetization and coercivity as thickness decreases down to ~8 nm, indicative of a single-domain and out-of-plane ferromagnetic order. We find that the stray field is weakened with decreasing thickness, which suppresses the formation of the domain wall. In addition, thickness-dependent ferromagnetic properties also reveal a crossover from 3 dimensional to 2 dimensional Ising ferromagnets at a ~7 nm thickness of CrSiTe$_3$, accompanied by a drop of the Curie temperature from 33 K for bulk to ~17 K for 4 nm sample.",2111.05495v1 2022-04-05,Cr doping-induced ferromagnetism in the spin-glass Cd1-xMnxTe studied by x-ray magnetic circular dichroism,"The prototypical diluted magnetic semiconductor Cd1-xMnxTe is a spin glass (x<0.6) or an antiferromagnet (x>0.6), but becomes ferromagnetic upon doping with a small amount of Cr atoms substituting for Mn. In order to investigate the origin of the ferromagnetism in Cd1-x-yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Cr and Mn L2,3 edges. Thin films were grown by molecular beam epitaxy with a fixed Mn content of x = 0.2 and varying Cr content in the range of y = 0 - 0.04. Measured XAS and XMCD spectra indicate that both Cr and Mn atoms are divalent and that the ferromagnetic or superparamagnetic components of Cr and Mn are aligned in the same directions. The magnetization of Mn increases with increasing Cr content. These results can be explained if ferromagnetic interaction exists between neighboring Mn and Cr ions although interaction between Mn atoms is largely antiferromagnetic. We conclude that each ferromagnetic or superparamagnetic cluster consists of ferromagnetically coupled several Cr and a much larger number of Mn ions.",2204.02042v1 2022-08-11,Ferromagnetism and Skyrmions in the Hofstadter-Fermi-Hubbard Model,"Strongly interacting fermionic systems host a variety of interesting quantum many-body states with exotic excitations. For instance, the interplay of strong interactions and the Pauli exclusion principle can lead to Stoner ferromagnetism, but the fate of this state remains unclear when kinetic terms are added. While in many lattice models the fermions' dispersion results in delocalization and destabilization of the ferromagnet, flat bands can restore strong interaction effects and ferromagnetic correlations. To reveal this interplay, here we propose to study the Hofstadter-Fermi-Hubbard model using ultracold atoms. We demonstrate, by performing large-scale DMRG simulations, that this model exhibits a lattice analog of the quantum Hall ferromagnet at magnetic filling factor $\nu=1$. We reveal the nature of the low energy spin-singlet states around $\nu\approx1$ and find that they host quasi-particles and quasi-holes exhibiting spin-spin correlations reminiscent of skyrmions. Finally, we predict the breakdown of flat-band ferromagnetism at large fields. Our work paves the way towards experimental studies of lattice quantum Hall ferromagnetism, including prospects to study many-body states of interacting skyrmions and explore the relation to high-$T_{\rm c}$ superconductivity.",2208.05960v2 2023-08-06,Anatomy of spin Hall effect in ferromagnetic metals,"The spin Hall effect in nonmagnetic materials has been intensively studied and became one of the most crucial spin-charge conversion mechanism in spintronics. However, the spin Hall effect in ferromagnetic metals has been less investigated and remains unclear. In this work, we investigate the spin Hall effect in representative ferromagnetic alloy by using first-principles calculations. We first clarify the spin Hall effect into three different types including conventional (CSHE), spin anomalous (SAHE) and magnetic spin Hall effect (MSHE) and then calculate the corresponding spin Hall conductivity and spin Hall angle for (Fe, Co, Ni)Pt, NiFe and CoFe alloy. We find the above three spin Hall mechanisms do coexist in ferromagnetic metals. Particularly, for Pt-based ferromagnetic alloy, a sizable conventional and magnetic spin Hall angles comparable to that of Pt have been predicted. The remarkable unconventional spin Hall effect in ferromagnetic metal may enrich the spin-charge conversion phenomena. For instance, the spin current generated by remarkable MSHE with out-of-plane spin-polarization should be helpful for field-free switching of perpendicular magnetization through spin-orbit torque effect. This work may stimulate future studies on the spin Hall effect in ferromagnetic metals and pave their promising applications for spin-charge conversion devices in spintronics.",2308.03111v1 2023-08-08,Activity-induced ferromagnetism in one-dimensional quantum many-body systems,"We study a non-Hermitian quantum many-body model in one dimension analogous to the Vicsek model or active spin models, and investigate its quantum phase transitions. The model consists of two-component hard-core bosons with ferromagnetic interactions and activity, i.e., spin-dependent asymmetric hopping. Numerical results show the emergence of a ferromagnetic order induced by the activity, a quantum counterpart of flocking, that even survives in the absence of ferromagnetic interaction. We confirm this phenomenon by proving that activity generally increases the ground state energies of the paramagnetic states, whereas the ground state energy of the ferromagnetic state does not change. By solving the two-particle case, we find that the effective alignment is caused by avoiding the bound state formation due to the non-Hermitian skin effect in the paramagnetic state. We employ a two-site mean-field theory based on the two-particle result and qualitatively reproduce the phase diagram. We further numerically study a variant of our model with the hard-core condition relaxed, and confirm the robustness of ferromagnetic order emerging due to activity.",2308.04382v4 2023-08-24,Nearly-room-temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2,"Itinerant ferromagnetism at room temperature is a key ingredient for spin transport and manipulation. Here, we report the realization of nearly-room-temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (323 K - 337 K) is almost unchanged when thickness is down to 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical calculations further indicate that the ferromagnetism persists in monolayer Fe4CoGeTe2. In addition to the robust ferromagnetism down to the ultrathin limit, Fe4CoGeTe2 exhibits an unusual temperature- and thickness-dependent intrinsic anomalous Hall effect. We propose that it could be ascribed to the dependence of band structure on thickness that changes the Berry curvature near the Fermi energy level subtly. The nearly-room-temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2 provide opportunities to understand the exotic transport properties of two-dimensional van der Waals magnetic materials and explore their potential applications in spintronics.",2308.12765v1 2023-08-28,Anisotropic magnetism and band evolution induced by ferromagnetic phase transition in titanium-based kagome ferromagnet SmTi3Bi4,"Kagome magnets with diverse topological quantum responses are crucial for next-generation topological engineering. The anisotropic magnetism and band evolution induced by ferromagnetic phase transition (FMPT) is reported in a newly discovered titanium-based kagome ferromagnet S mTi3 Bi4, which features a distorted Ti kagome lattice and S m atomic zig-zag chains. Temperature-dependent resistivity, heat capacity, and magnetic susceptibility reveal a ferromagnetic ordering temperature Tc of 23.2 K. A large magnetic anisotropy, observed by applying the magnetic field along three crystallographic axes, identifies the b axis as the easy axis. Angle-resolved photoemission spectroscopy with first-principles calculations unveils the characteristic kagome motif, including the Dirac point at the Fermi level and multiple van Hove singularities. Notably, a band splitting and gap closing attributed to FMPT is observed, originating from the exchange coupling between S m 4 f local moments and itinerant electrons of the kagome Ti atoms, as well as the time-reversal symmetry breaking induced by the long-range ferromagnetic order. Considering the large in-plane magnetization and the evolution of electronic structure under the influence of ferromagnetic ordering, such materials promise to be a new platform for exploring the intricate electronic properties and magnetic phases based on the kagome lattice.",2308.14349v2 2023-09-12,Proximity-induced interfacial room-temperature ferromagnetism in semiconducting Fe3GeTe2,"The discoveries of two-dimensional ferromagnetism and magnetic semiconductors highly enrich the magnetic material family for constructing spin-based electronic devices but with an acknowledged challenge that the Curie temperature (Tc) is usually far below room temperature. Many efforts such as voltage control and magnetic ion doping are currently underway to enhance the functional temperature, in which the involvement of additional electrodes or extra magnetic ions limits their plenty of applications in practical devices. Here we demonstrate that the magnetic proximity, a robust effect but with elusive mechanisms, can induce room-temperature ferromagnetism at the interface between sputtered Pt and semiconducting Fe3GeTe2, both of which do not show ferromagnetism at 300 K. The independent electrical and magnetization measurements, structure analysis, and control samples with Ta highlighting the role of Pt confirm that the ferromagnetism with the Tc of above 400 K arises from the Fe3GeTe2/Pt interfaces, rather than Fe aggregation or other artificial effects. Moreover, contrary to conventional ferromagnet/Pt structures, the spin current generated by the Pt layer is enhanced more than two times at the Fe3GeTe2/Pt interfaces, indicating the potential applications of the unique proximity effect in building high-efficient spintronic devices. These results may pave a new avenue to create room-temperature functional spin devices based on low-Tc materials and provide clear evidences of magnetic proximity effects by using non-ferromagnetic materials.",2309.06204v1 2023-11-13,Robust magnetic proximity induced anomalous Hall effect in a room temperature van der Waals ferromagnetic semiconductor based 2D heterostructure,"Developing novel high-temperature van der Waals ferromagnetic semiconductor materials and investigating their interface coupling effects with two-dimensional topological semimetals are pivotal for advancing next-generation spintronic and quantum devices. However, most van der Waals ferromagnetic semiconductors exhibit ferromagnetism only at low temperatures, limiting the proximity research on their interfaces with topological semimetals. Here, we report an intrinsic, van der Waals layered room-temperature ferromagnetic semiconductor crystal, FeCr0.5Ga1.5Se4 (FCGS), with a Curie temperature as high as 370 K, setting a new record for van der Waals ferromagnetic semiconductors. The saturation magnetization at low temperature (2 K) and room temperature (300 K) reaches 8.2 emu/g and 2.7 emu/g, respectively. Furthermore, FCGS possesses a bandgap of approximately 1.2 eV, which is comparable to the widely used commercial silicon. The FCGS/graphene heterostructure exhibits an impeccably smooth and gapless interface, thereby inducing a robust magnetic proximity coupling effect between FCGS and graphene. After the proximity coupling, graphene undergoes a charge carrier transition from electrons to holes, accompanied by a transition from non-magnetic to ferromagnetic transport behavior with robust anomalous Hall effect. Notably, the anomalous Hall effect remains robust even temperatures up to 400 K.",2311.07183v1 2023-12-07,Direct and indirect spin current generation and spin-orbit torques in ferromagnet/nonmagnet/ferromagnet trilayers,"Spin-orbit torques in ferromagnet/nonmagnet/ferromagnet trilayers are studied using a combination of symmetry analysis, circuit theory, semiclassical simulations, and first-principles calculations using the non-equilibrium Green's function method with supercell disorder averaging. We focus on unconventional processes involving the interplay between the two ferromagnetic layers, which are classified into direct and indirect mechanisms. The direct mechanism involves spin current generation by one ferromagnetic layer and its subsequent absorption by the other. In the indirect mechanism, the in-plane spin-polarized current from one ferromagnetic layer ``leaks'' into the other layer, where it is converted into an out-of-plane spin current and reabsorbed by the original layer. The direct mechanism results in a predominantly dampinglike torque, which damps the magnetization towards a certain direction $\mathbf{s}_d$. The indirect mechanism results in a predominantly fieldlike torque with respect to a generally different direction $\mathbf{s}_f$. Similar to the current-in-plane giant magnetoresistance, the indirect mechanism is only active if the thickness of the nonmagnetic spacer is smaller than or comparable to the mean-free path. Numerical calculations for a semiclassical model based on the Boltzmann equation confirm the presence of both direct and indirect mechanisms of spin current generation. First-principles calculations reveal sizeable unconventional spin-orbit torques in Co/Cu/Co, Py/Cu/Py, and Co/Pt/Co trilayers and provide strong evidence of indirect spin current generation.",2312.04538v1 2023-12-12,Dzyaloshinskii-Moriya interaction inducing weak ferromagnetism in centrosymmetric altermagnets and weak ferrimagnetism in noncentrosymmetric altermagnets,"The Dzyaloshinskii-Moriya interaction (DMI) has explained successfully the weak ferromagnetism in some centrosymmetric antiferromagnets. However, in the last years, it was generally claimed that the DMI is not effective in centrosymmetric systems. We reconciled these views by separating the conventional antiferromagnets and altermagnets. Altermagnets represent collinear antiferromagnetic compounds with spin-up and spin-down sublattices connected only by mirror and roto-translational symmetries. Consequently, the system shows even-parity wave spin order in the k-space lifting the Kramer's degeneracy in the non-relativistic band structure. We emphasize that the DMI can create weak ferromagnetism in centrosymmetric altermagnets while it is not effective in centrosymmetric conventional antiferromagnets. Additionally, DMI can create weak ferromagnetism or weak ferrimagnetism in noncentrosymmetric altermagnets. Once the spin-orbit coupling is included in an altermagnetic system without time-reversal symmetry, the components of spin moments of the two sublattices along the Neel vector are antiparallel but the other two spin components orthogonal to the N\'eel vector can be either parallel or antiparallel for centrosymetric systems. For noncentrosymmetric systems, we can have different bands showing parallel or antiparallel spin components resulting in weak ferrimagnetism. We can divide the altermagnetic compounds into classes based on the weak ferromagnetism or weak ferrimagnetism properties. The weak ferromagnetism and weak ferrimagnetism induced by DMI is a property exclusively of the altermagnets, not present in either ferromagnets or conventional antiferromagnets so we propose that altermagnets should be classified based on this property.",2312.07678v2 2012-05-21,Random Transverse Field Ising model on the Cayley Tree : analysis via Boundary Strong Disorder Renormalization,"Strong Disorder Renormalization for the Random Transverse Field Ising model leads to a complicated topology of surviving clusters as soon as $d>1$. Even if one starts from a Cayley tree, the network of surviving renormalized clusters will contain loops, so that no analytical solution can been obtained. Here we introduce a modified procedure called 'Boundary Strong Disorder Renormalization' that preserves the tree structure, so that one can write simple recursions with respect to the number of generations. We first show that this modified procedure allows to recover exactly most of the critical exponents for the one-dimensional chain. After this important check, we study the RG equations for the quantum Ising model on a Cayley tree with a uniform ferromagnetic coupling $J$ and random transverse fields with support $[h_{min},h_{max}]$. We find the following picture (i) for $J>h_{max}$, only bonds are decimated, so that the whole tree is a quantum ferromagnetic cluster (ii) for $J0$) ground states with maximum spin are stable already at finite Hubbard interaction $U>U_c$. For non-bipartite lattices this requires a hopping amplitude $t\leq0$. For vanishing $F$ one obtains $U_c\to\infty$ as in Nagaoka's theorem. This shows that the exchange interaction $F$ is important for stabilizing ferromagnetism at finite $U$. Only in the special case $X=t$ the ferromagnetic state is stable even for $F=0$, provided the lattice allows the hole to move around loops.",9511060v1 1998-04-08,Ferromagnetism in the Hubbard model with orbital degeneracy in infinite dimensions,"We study the ferromagnetism due to orbital degeneracy in the Hubbard model in infinite dimensions. The model contains the intraorbital repulsion $U$, the interorbital repulsion $U^\prime$, the exchange $J$ (Hund coupling) and the pair hopping $J^\prime$, where all of them originate from the on-site Coulomb interaction. The ground state of the effective one-site problem was obtained by exact diagonalizations. At the 1/4-filling, we found two insulating phases; one is a ferromagnetic phase with alternating orbital order and the other is antiferromagnetic one with uniform orbital order. If electrons are doped into the 1/4-filling, the ferromagnetic phase still survives and becomes metallic, while the antiferromagnetic phase disappears. This result indicates that the double-exchange mechanism is relevant to stabilize metallic ferromagnetism in infinite dimensions.",9804096v2 1998-04-09,Metallic ferromagnetism: Progress in our understanding of an old strong-coupling problem,"Metallic ferromagnetism is in general an intermediate to strong coupling phenomenon. Since there do not exist systematic analytic methods to investigate such types of problems, the microscopic origin of metallic ferromagnetism is still not sufficiently understood. However, during the last two or three years remarkable progress was made in this field: It is now certain that even in the one-band Hubbard model metallic ferromagnetism is stable in dimensions $d=1,$ 2, and $\infty$ on regular lattices and at intermediate values of the interaction $U$ and density $n$. In this paper the basic questions and recent insights regarding the microscopic conditions favoring metallic ferromagnetism in this model are reviewed. These findings are contrasted with the results for the orbitally degenerate case.",9804112v2 1998-10-21,Ferromagnetic ground state of an orbital degenerate electronic model for transition-metal oxides: exact solution and physical mechanism,"We present an exact ground state solution of a one-dimensional electronic model for transition-metal oxides in the strong coupling limit. The model contains doubly degenerated orbit for itinerant electrons and the Hund coupling between the itinerant electrons and localized spins. The ground state is proven to be a full ferromagnet for any density of electrons. Our model provides a rigorous example for metallic ferromagnetism in narrow band systems. The physical mechanism for ferromagnetism and its relevance to high-dimensional systems, like R$_{1-x}$X$_x$MnO$_3$, are discussed. Due to the orbital degeneracy of itinerant electrons, the superexchange coupling can be ferromagnetic rather than antiferromagnetic in the one-band case.",9810268v1 1999-08-02,Breakdown of the magnetization plateau of the S=1/2 ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin chain with anisotropy,"We study the breakdown of the magnetization plateau at the magnetization M=M_{S}/3 (M_{S} is the saturation magnetization) of the S=1/2 anisotropic spin chain with ferromagnetic-ferromagnetic-antiferromagnetic interactions. We consider the model with the isotropic ferromagnetic (trimer) coupling J_{F}, and anisotropic antiferromagnetic coupling (J_{x}=J_{y}=J_{AF} and J_{z}=\Delta J_{AF}). For the limit of large \gamma\equiv J_{F}/J_{AF}, the model is equivalent to the S=3/2 XXZ chain with the exchange anisotropy \Delta. There is a phase transition between the plateau (small-\gamma) and the no-plateau (large-\gamma) regions. This phase transition is of the Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from the numerical diagonalization data. For \Delta=1, in particular, the phase transition between the plateau and the no-plateau regions occurs at the point \gamma_{c}=15.4.",9908027v2 2000-02-10,The ground state of Sr3Ru2O7 revisited; Fermi liquid close to a ferromagnetic instability,"We show that single-crystalline Sr3Ru2O7 grown by a floating-zone technique is an isotropic paramagnet and a quasi-two dimensional metal as spin-triplet superconducting Sr2RuO4 is. The ground state is Fermi liquid with very low residual resistivity (3 micro ohm cm for in-plane currents) and a nearly ferromagnetic metal with the largest Wilson ratio Rw>10 among paramagnets so far. This contrasts with the ferromagnetic order at Tc=104 K reported on single crystals grown by a flux method [Cao et al., Phys. Rev. B 55, R672 (1997)]. We have also found a dramatic changeover from paramagnetism to ferromagnetism under applied pressure. This suggests the existence of a substantial ferromagnetic instability on the verge of a quantum phase transition in the Fermi liquid state.",0002147v2 2000-03-27,Pseudospin Anisotropy Classification of Quantum Hall Ferromagnets,"Broken symmetry ground states with uniform electron density are common in quantum Hall systems when two Landau levels simultaneously approach the chemical potential at integer filling factor $\nu$. The close analogy between these two-dimensional electron system states and conventional itinerant electron ferromagnets can be emphasized by using a pseudospin label to distinguish the two Landau levels. As in conventional ferromagnets, the evolution of the system's state as external field parameters are varied is expected to be sensitive to the dependence of ground state energy on pseudospin orientation. We discuss the predictions of Hartree-Fock theory for the dependence of the sign and magnitude of the pseudospin anisotropy energy on the nature of the crossing Landau levels. We build up a classification scheme for quantum Hall ferromagnets that applies for single layer and bilayer systems with two aligned Landau levels distinguished by any combination of real-spin, orbit-radius, or growth direction degree-of-freedom quantum numbers. The possibility of in-situ tuning between easy-axis and easy-plane quantum Hall ferromagnets is discussed for biased bilayer systems with total filling factors $\nu=3$ or $\nu=4$. Detailed predictions are made for the bias dependence of pseudospin reversal properties in $\nu=3$ bilayer systems.",0003430v1 2000-04-05,Linear response conductance and magneto-resistance of ferromagnetic single-electron transistors,"The current through ferromagnetic single-electron transistors (SET's) is considered. Using path integrals the linear response conductance is formulated as a function of the tunnel conductance vs. quantum conductance and the temperature vs. Coulomb charging energy. The magneto-resistance of ferromagnet-normal metal-ferromagnet (F-N-F) SET's is almost independent of the Coulomb charging energy and is only reduced when the transport dwell time is longer than the spin-flip relaxation time. In all-ferromagnetic (F-F-F) SET's with negligible spin-flip relaxation time the magneto-resistance is calculated analytically at high temperatures and numerically at low temperatures. The F-F-F magneto-resistance is enhanced by higher order tunneling processes at low temperatures in the 'off' state when the induced charges vanishes. In contrast, in the 'on' state near resonance the magneto-resistance ratio is a non-monotonic function of the inverse temperature.",0004082v3 2000-05-11,Spin-accumulation and Andreev-reflection in a mesoscopic ferromagnetic wire,"The electron transport though ferromagnetic metal-superconducting hybrid devices is considered in the non-equilibrium Green's function formalism in the quasiclassical approximation. Attention if focused on the limit in which the exchange splitting in the ferromagnet is much larger than the superconducting energy gap. Transport properties are then governed by an interplay between spin-accumulation close to the interface and Andreev reflection at the interface. We find that the resistance can either be enhanced or lowered in comparison to the normal case and can have a non-monotonic temperature and voltage dependence. In the non-linear voltage regime electron heating effects may govern the transport properties, leading to qualitative different behaviour than in the absence of heating effects. Recent experimental results on the effect of the superconductor on the conductance of the ferromagnet can be understood by our results for the energy-dependent interface resistance together with effects of spin- accumulation without invoking long range pairing correlations in the ferromagnet.",0005188v1 2000-07-27,Superconductivity and Ferromagnetism from Effective Mass Reduction,"Within a simple model Hamiltonian, both superconductivity and metallic ferromagnetism can be understood as arising from lowering of kinetic energy as the ordered state develops, due to a reduction in the carriers effective mass, or equivalently, a bandwidth expansion. Experimental manifestation of this physics has been detected in both high Tc superconductors and large magnetoresistance ferromagnets, as an anomalous transfer of spectral weight in optical absorption from high to low frequencies as the ordered state develops. It is proposed that this general principle is common to the essential physics of superconductivity and ferromagnetism in nature, and hence that these effects in optical properties, although often smaller in magnitude, should exist in all superconductors and metallic ferromagnets.",0007453v1 2000-08-30,"Ferromagnetism in the Periodic Anderson Model: A Comparison of Spectral Density Approximation (SDA), Modified Alloy Analogy (MAA) and Modified Perturbation Theory (MPT)","We compare different approximation schemes for investigating ferromagnetism in the periodic Anderson model. The use of several approximations allows for a detailed analysis of the implications of the respective methods, and also of the mechanisms driving the ferromagnetic transition. For the Kondo limit, our results confirm a previously proposed mechanism leading to ferromagnetic order, namely an RKKY exchange mediated via the formation of Kondo screening clouds in the conduction band. The contrary case is found in the intermediate-valence regime. Here, the bandshift correction ensuring a correct high-energy expansion of the self-energy is essential. Inclusion of damping effects reduces stability of the ferromagnetic phase.",0008445v1 2000-09-08,Ferromagnetism in the Two-Dimensional Periodic Anderson Model,"Using the constrained-path Monte Carlo method, we studied the magnetic properties of the two-dimensional periodic Anderson model for electron fillings between 1/4 and 1/2. We also derived two effective low energy theories to assist in interpreting the numerical results. For 1/4 filling we found that the system can be a Mott or a charge transfer insulator, depending on the relative values of the Coulomb interaction and the charge transfer gap between the two non-interacting bands. The insulator may be a paramagnet or antiferromagnet. We concentrated on the effect of electron doping on these insulating phases. Upon doping we obtained a partially saturated ferromagnetic phase for low concentrations of conduction electrons. If the system were a charge transfer insulator, we would find that the ferromagnetism is induced by the well-known RKKY interaction. However, we found a novel correlated hopping mechanism inducing the ferromagnetism in the region where the non-doped system is a Mott insulator. Our regions of ferromagnetism spanned a much smaller doping range than suggested by recent slave boson and dynamical mean field theory calculations, but they were consistent with that obtained by density matrix renormalization group calculations of the one-dimensional periodic Anderson model.",0009128v1 2001-01-17,Weak localization in ferromagnets with spin-orbit interaction,"Weak localization corrections to conductivity of ferromagnetic systems are studied theoretically in the case when spin-orbit interaction plays a significant role. Two cases are analyzed in detail: (i) the case when the spin-orbit interaction is due to scattering from impurities, and (ii) the case when the spin-orbit interaction results from reduced dimensionality of the system and is of the Bychkov-Rashba type. Results of the analysis show that the localization corrections to conductivity of ferromagnetic metals lead to a negative magnetoresistance -- also in the presence of the spin-orbit scattering. Positive magnetoresistance due to weak antilocalization, typical of nonmagnetic systems, does not occur in ferromagnetic systems. In the case of two-dimensional ferromagnets, the quantum corrections depend on the magnetization orientation with respect to the plane of the system.",0101260v2 2001-06-29,Stabilization of d-Band Ferromagnetism by Hybridization with Uncorrelated Bands,"We investigate the influence of s-d or p-d hybridization to d-band ferromagnetism to estimate the importance of hybridization for the magnetic properties of transition metals. To focus our attention to the interplay between hybridization and correlation we investigate a simple model system consisting of two non-degenerated hybridized bands, one strongly correlated, the other one quasi-free. To solve this extended Hubbard model, we apply simple approximations, namely SDA and MAA, that, concerning ferromagnetism in the single-band model, are known to give qualitatively satisfactory results. This approach allows us to discuss the underlying mechanism, by which d-band ferromagnetism is influenced by the hybridization on the basis of analytical expressions. The latter clearly display the order and the functional dependencies of the important effects. It is found, that spin-dependent inter-band particle fluctuations cause a spin-dependent band shift and a spin-dependent band broadening of the Hubbard bands. The shift stabilizes, the broadening tends to destabilize ferromagnetism. Stabilization requires relatively high band distances and small hybridization matrix elements. Super-exchange and RKKY coupling are of minor importance.",0106638v1 2001-07-12,P-wave pairing and ferromagnetism in the metal-insulator transition in two dimensions,"Based on recent experimental evidence for a spin polarized ground state in the insulating phase of the two-dimensional electron system, we propose that ferromagnetic spin fluctuations lead to an attractive interaction in the triplet channel and cause p-wave pairing in the conducting phase. We use the Landau Fermi liquid phenomenology to explain how the enhanced spin susceptibility near the critical density yields an attractive potential, in a similar mechanism to superfluidity in $^3$He. As the density is decreased, the p-wave order parameter undergoes a transition from a unitary to a nonunitary state, in which it coexists with ferromagnetism for a range of densities. As the density is further reduced, the pairing amplitude vanishes and the system is described by a ferromagnetic insulator. Thus, we find two quantum critical points as a function of density associated with the polarization of the paired state and ferromagnetism. We explain the magnetotransport measurements in parallel and perpendicular magnetic fields and propose a shot noise experiment to measure the pair charge.",0107271v1 2001-08-29,Current and Spin-Torque in Double Tunnel Barrier Ferromagnet - Superconductor - Ferromagnet Systems,"We calculate the current and the spin-torque in small symmetric double tunnel barrier ferromagnet - superconductor - ferromagnet (F-S-F) systems. Spin-accumulation on the superconductor governs the transport properties when the spin-flip relaxation time is longer than the transport dwell time. In the elastic transport regime, it is demonstrated that the relative change in the current (spin-torque) for F-S-F systems equals the relative change in the current (spin-torque) for F-N-F systems upon changing the relative magnetization direction of the two ferromagnets. This differs from the results in the inelastic transport regime where spin-accumulation suppresses the superconducting gap and dramatically changes the magnetoresistance [S. Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. 82, 3911 (1999)]. The experimental relevance of the elastic and inelastic transport regimes, respectively, as well as the reasons for the change in the transport properties are discussed.",0108490v2 2001-11-14,Extrinsic Magnetotransport Phenomena in Ferromagnetic Oxides,"This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surfacial and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin-polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.",0111263v1 2002-01-02,Kinetic exchange vs. room temperature ferromagnetism in diluted magnetic semiconductors,"Guided by the internal-reference rule and the known band offsets in III-V and II-VI diluted magnetic semiconductors, we discuss the feasibility of obtaining p-type conductivity, required for the carrier-induced ferromagnetism, as well as the cases for which the doping by shallow impurities may lead to the ferromagnetism driven by the double exchange. We consider the dependence of kinetic exchange on the p-d hybridization, on the electronic configurations of the magnetic ions, and on the energies of the charge transfer between the valence band of host materials and the magnetic ions. In the case of Mn-based II-VI compounds, the doping by acceptors is necessary for the hole-induced ferromagnetism. The latter is, however, possible without any doping for some of Mn-, Fe- or Co-based III-V magnetic semiconductors. In nitrides with Fe or Co carrier-induced ferromagnetism with T_C>300K is expected in the presence of acceptor doping.",0201012v1 2002-01-10,"Curie Temperature Trends in (III,Mn)V Ferromagnetic Semiconductors","We present a theoretical survey of ferromagnetic transition temperatures in cubic (III,Mn)V semiconductors based on a model with $S=5/2$ local moments exchange-coupled to itinerant holes in the host semiconductor valence band. Starting from the simplest mean-field theory of this model, we estimate the $T_c$ enhancement due to exchange and correlation in the itinerant-hole system, and the $T_c$ suppression due to collective fluctuations of the ordered moments. We show that high critical temperatures in these ferromagnetic semiconductors require both the large magnetic susceptibility contribution from the valence band's heavy holes and the large spin stiffness that results from strong spin-orbit coupling in the valence band. Our calculations demonstrate that this model for the ferromagnetism of these systems is fully consistent with the room-temperature ferromagnetism reported for Mn doped nitrides.",0201157v2 2002-01-29,Inhomogeneous magnetism induced in a superconductor at superconductor-ferromagnet interface,"We study a magnetic proximity effect at superconductor (S) - ferromagnet (F) interface. It is shown that due to an exchange of electrons between the F and S metals ferromagnetic correlations extend into the superconductor, being dependent on interface parameters. We show that ferromagnetic exchange field pair breaking effect leads to a formation of subgap bands in the S layer local density of states, that accommodate only one spin-polarized quasiparticles. Equilibrium magnetization leakage into the S layer as function of SF interface quality and a value of ferromagnetic interaction have also been calculated. We show that a damped-oscillatory behavior versus distance from SF interface is a distinguished feature of the exchange-induced magnetization of the S layer.",0201534v2 2002-03-08,Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures,"We study a simple microscopic model for thin, ferromagnetic, metallic layers on semi-infinite bulk superconductor. We find that for certain values of the exchange spliting, on the ferromagnetic side, the ground states of such structures feature spontaneously induced spin polarized currents. Using a mean-field theory, which is selfconsistent with respect to the pairing amplitude $\chi$, spin polarization $\vec{m}$ and the spontaneous current $\vec{j}_s$, we show that not only there are Andreev bound states in the ferromagnet but when their energies $E_n$ are near zero they support spontaneous currents parallel to the ferromagnetic-superconducting interface. Moreover, we demonstrate that the spin-polarization of these currents depends sensitively on the band filling.",0203184v1 2002-03-19,Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator: Pr0.8Ca0.2MnO3,"The Pr(1-x}CaxMnO3 system exhibits a ferromagnetic insulating state for the composition range x<0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr0.8Ca0.2MnO3 single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr{3+} ions.",0203365v1 2002-04-12,Disorder and interactions in quantum Hall ferromagnets: effects of disorder in Skyrmion physics,"We present a Hartree-Fock study of the competition between disorder and interactions in quantum Hall ferromagnets near $\nu=1$. We find that the ground state at $\nu=1$ evolves with increasing interaction strength from a quasi-metallic paramagnet, to a partially spin-polarized ferromagnetic Anderson insulator, and to a fully spin-polarized ferromagnet with a charge gap. Away from $\nu=1$, the ground state evolves from a conventional Anderson insulator, to a conventional quasiparticle glass, and finally to a ferromagnetic Skyrmion quasiparticle glass. These different regimes can be measured in low-temperature transport and NMR experiments. We present calculations for the NMR spectra in different disorder regimes.",0204288v1 2002-06-02,Ground state of a double-exchange system containing impurities: bounds of ferromagnetism,"We study the boundary between ferromagnetic and non-ferromagnetic ground state of a double-exchange system with quenched disorder for arbitrary relation between Hund exchange coupling and electron band width. The boundary is found both from the solution of the Dynamical Mean Field Approximation equations and from the comparison of the energies of the saturated ferromagnetic and paramagnetic states. Both methods give very similar results. To explain the disappearance of ferromagnetism in part of the parameter space we derive from the double-exchange Hamiltonian with classical localized spins in the limit of large but finite Hund exchange coupling the $t-J$ model (with classical localized spins).",0206018v2 2002-06-27,Magnon-Mediated Superconductivity in Itinerant Ferromagnets,"The present paper discusses magnon-mediated superconductivity in ferromagnetic metals. The mechanism explains in a natural way the fact that the superconductivity in UGe_2, ZrZn_2 and URhGe is apparently confined to the ferromagnetic phase.The order parameter is a spin anti-parallel component ofa spin-1 triplet with zero spin projection. The transverse spinfluctuations are pair forming and the longitudinal ones are pair breaking.The competition between magnons and paramagnons explains the existence of two successive quantum phase transitions in UGe_2, from ferromagnetism to ferromagnetic superconductivity, and at higher pressure to paramagnetism. The maximum T_{SC} results from the suppression of the paramagnon contribution. To form a Cooper pair an electron transfers from one Fermi surface to the other. As a result, the onset of superconductivity leads to the appearance of two Fermi surfaces in each of the spin up and spin down momentum distribution functions. This fact explains the linear temperaturedependence at low temperature of the specific heat, and the experimental results for UGe_2.",0206534v2 2002-07-02,"Magnon Exchange Mechanism of Superconductivity: ZrZn_2, URhGe","The magnon exchange mechanism of superconductivity was developed to explain in a natural way the fact that the superconductivity in $UGe_2$, $ZrZn_2$ and $URhGe$ is confined to the ferromagnetic phase.The order parameter is a spin anti-parallel component of a spin-1 triplet with zero spin projection. The transverse spin fluctuations are pair forming and the longitudinal ones are pair breaking. In the present paper, a superconducting solution, based on the magnon exchange mechanism, is obtained which closely matches the experiments with $ZrZn_2$ and $URhGe$. The onset of superconductivity leads to the appearance of complicated Fermi surfaces in the spin up and spin down momentum distribution functions. Each of them consist of two pieces, but they are simple-connected and can be made very small by varying the microscopic parameters. As a result, it is obtained that the specific heat depends on the temperature linearly, at low temperature, and the coefficient $\gamma=\frac {C}{T}$ is smaller in the superconducting phase than in the ferromagnetic one. The absence of a quantum transition from ferromagnetism to ferromagnetic superconductivity in a weak ferromagnets $ZrZn_2$ and $URhGe$ is explained accounting for the contribution of magnon self-interaction to the spin fluctuations' parameters. It is shown that in the presence of an external magnetic field the system undergoes a first order quantum phase transition.",0207078v1 2002-09-18,Charge ordered ferromagnetic phase in La0.5Ca0.5MnO3,"Mixed valent manganites are noted for their unusual magnetic,electronic and structural phase transitions. The La1-xCaxMnO3 phase diagram shows that below transition temperatures in the range 100-260 K, compounds with 0.2 < x < 0.5 are ferromagnetic and metallic whereas those with 0.5 < x < 0.9 are antiferromagnetic and charge ordered. In a narrow region around x = 0.5, these totally dissimilar states are thought to coexist. Uehara et al. have shown that charge order and charge disorder can coexist in the related compound La0.25Pr0.375Ca0.375MnO3. Here, we present electron microscopy data for La0.5Ca0.5MnO3 that sheds light on the distribution of coexisting phases and uncovers a novel and unexpected phase. Using electron holography and Fresnel imaging, we find micron sized ferromagnetic regions spanning several grains coexisting with similar sized regions with no local magnetisation. Holography shows that the ferromagnetic regions have a local magnetisation of 3.4 +- 0.2 mB/Mn (the spin aligned value is 3.5 mB/Mn). We use electron diffraction and dark field imaging to show that charge order exists in regions with no net magnetisation and, surprisingly, can also occur in ferromagnetic regions.",0209436v1 2002-12-02,Dilatometry study of the ferromagnetic order in single-crystalline URhGe,"Thermal expansion measurements have been carried out on single-crystalline URhGe in the temperature range from 2 to 200 K. At the ferromagnetic transition (Curie temperature T_C = 9.7 K), the coefficients of linear thermal expansion along the three principal orthorhombic axes all exhibit pronounced positive peaks. This implies that the uniaxial pressure dependencies of the Curie temperature, determined by the Ehrenfest relation, are all positive. Consequently, the calculated hydrostatic pressure dependence dT_C/dp is positive and amounts to 0.12 K/kbar. In addition, the effective Gruneisen parameter was determined. The low-temperature electronic Gruneisen parameter \Gamma_{sf} = 14 indicates an enhanced volume dependence of the ferromagnetic spin fluctuations at low temperatures. Moreover, the volume dependencies of the energy scales for ferromagnetic order and ferromagnetic spin fluctuations were found to be identical.",0212038v1 2003-01-07,Ferromagnetism in the Hubbard model: A constructive approach,"It is believed that strong ferromagnetic orders in some solids are generated by subtle interplay between quantum many-body effects and spin-independent Coulomb interactions between electrons. Here we describe our rigorous and constructive approach to ferromagnetism in the Hubbard model, which is a standard idealized model for strongly interacting electrons in a solid. We introduce a class of Hubbard models in any dimensions which are nonsingular in the sense that both the Coulomb interaction and the density of states (at the Fermi level) are finite. We then prove that the ground states of the models exhibit saturated ferromagnetism, i.e., have maximum total spins. Combined with our earlier results, the present work provides nonsingular models of itinerant electrons with only spin-independent interactions where low energy behaviors are proved to be that of a ``healthy'' ferromagnetic insulator.",0301071v4 2003-03-15,Tuning of the superconducting and ferromagnetic transitions by Cu doping for Ru in GdSr2RuCu2O8,"In order to explore the possibility of tuning superconducting and ferromagnetic transitions by Cu doping (for Ru) in GdSr2RuCu2O8, we have carried out synthesis and characterization of GdSr2Ru{1-x}Cu{2+x)O8 (x = 0, 0.05, 0.1, 0.2) and studied their physical properties. Coexistence of superconductivity and ferromagnetism is observed in all the Cu doped samples studied here. The zero field susceptibility data suggests formation of a spontaneous vortex phase. Cu doping decreases the ferromagnetic Curie temperature, whereas the superconducting transition temperature increases until an optimal concentration x~0.1. This reflects an increase in hole transfer to the CuO2 planes and reduction of ferromagnetic order within the ruthenate layers.",0303287v1 2003-05-15,Ferromagnetism and Metamagnetism in Copper-doped Germanium Clathrate,"Cu-doped type-I germanium clathrate can exhibit dilute magnetism, including ferromagnetism, antiferromagnetism, and metamagnetic transitions up to 90 K. 63Cu NMR measurements confirm that these transitions are due to a dilute composition of magnetic defects coupled by conduction electrons, behavior similar to that of magnetic semiconductors. Magnetic measurements indicate localized magnetic moments, attributed to clusters of magnetic ions, with competition between ferromagnetic and antiferromagnetic exchange, and also indications of glassy behavior in the ferromagnetic phase. NMR Knight shifts and relaxation times show that the conduction band is metallic with a large Korringa ratio. Comparison to a mean-field theory for the ordering behavior gives a good accounting for the ferromagnetic transition.",0305369v1 2003-05-27,"Ferromagnetism and metallic state in digital (Ga,Mn)As heterostructures","We present an extensive density functional theory study of the electronic, magnetic and transport properties of GaAs and AlAs digital ferromagnetic heterostructures. These can be obtained by $\delta$-doping with Mn the GaAs layers of a GaAs/AlAs superlattice. Our analysis spans a range of Mn concentrations and considers the presence of compensating defects such as As antisites. In the defect-free case all the heterostructures studied present an half-metallic electronic structure. In contrast when As antisites are present the half-metallic state is destroyed and the heterostructures behave as dirty planar metals. In this case they show a large $p$-type metallic conductance in the Mn plane mainly due to majority spin electrons, and an $n$-type hopping-like conductance in the GaAs planes mainly due to minority spin electrons. This suggests that if the As antisites can be kept far from the Mn planes, spatial separation of the different spin currents can be achieved. Finally we show that in the case of AlAs/(Ga,Mn)As digital ferromagnetic heterostructures the AlAs/GaAs valence band offset produces an additional confining potential for the holes responsible for the ferromagnetism. Therefore the ferromagnetic coupling between the Mn ions becomes larger and more robust to the presence of As antisites.",0305615v1 2003-06-11,Diameter dependence of ferromagnetic spin moment in Au nanocrystals,"Au nanoparticles exhibit ferromagnetic spin polarization and show diameter dependence in magnetization. The magnetic moment per Au atom in the particle attains its maximum value at a diameter of about 3 nanometer (nm) in the Magnetization-Diameter curve. Because Au metal is a typical diamagnetic material, its ferromagnetic polarization mechanism is thought to be quite different from the ferromagnetism observed in transition metals. The size effect strongly suggests the existence of some spin correlation effect at the nanoscale. The so-called ``Fermi hole effect'' is the most probable one given in the free electron gas system. Ferromagnetism in Au nanoparticles is discussed using this model.",0306261v2 2003-06-25,Ferromagnetic insulating phase in Pr{1-x}Ca{x}MnO3,"A ferromagnetic insulating (FM-I) state in Pr0.75Ca0.25MnO3 has been studied by neutron scattering experiment and theoretical calculation. The insulating behavior is robust against an external magnetic field, and is ascribed to neither the phase separation between a ferromagnetic metallic (FM-M) phase and a non-ferromagnetic insulating one, nor the charge ordering. We found that the Jahn-Teller type lattice distortion is much weaker than PrMnO3 and the magnetic interaction is almost isotropic. These features resembles the ferromagnetic metallic state of manganites, but the spin exchange interaction J is much reduced compared to the FM-M state. The theoretical calculation based on the staggered type orbital order well reproduces several features of the spin and orbital state in the FM-I phase.",0306631v1 2003-08-25,Non-Fermi liquid behavior near the ferromagnetic quantum critical point in the series Ca(x)Sr(1-x)RuO(3),"A series of epitaxial films were grown across the solid solution Ca(x)Sr(1-x)RuO(3) in order to pinpoint the ferromagnetic to paramagnetic quantum phase transition in this system and to study the evolution of transport and magnetic properties in its vicinity. The ferromagnetic Tc of SrRuO(3) was found to decrease linearly with Ca doping levels up to 70%. Further doping resulted in the abrupt elimination of ferromagnetic order, and the onset of low temperature (< 10K) non-Fermi liquid (NFL) resistivity of the form rho ~ rho(0 + AT^(1.5) for samples with x <= 0.75 <= 1.0. The resistivity exponent of 1.5 matches that previously observed for MnSi, indicating possible universality of the NFL behavior of itinerant ferromagnets. Field-dependent specific heat measurements on bulk samples at compositions near the quantum phase transition provide additional evidence for NFL behavior (C/T ~ log T) and show the conditions nder which spin fluctuations contribute to the specific heat.",0308514v2 2003-09-11,Resistance control of a magnetoresistive manganite by spin-injection,"We report a new spin-injection effect found for a manganite using a specially fabricated sample. A wire of La0.7Sr0.3MnO3 was patterned by means of focused ion beam etching, and the central part was subsequently irradiated with Ga+ ions lightly. The ferromagnetic Curie temperature was reduced locally by the irradiation from Tc to Tc', and thus a sequential ferromagnetic/paramagnetic/ferromagnetic structure was realized along the wire between Tc and Tc'. The injection of spin-polarized current from the ferromagnetic manganite into the paramagnetic part rendered the latter ferromagnetic and more conductive. This can be explained by assuming the suppression of spin fluctuation in the paramagnet by the injected spins.",0309274v1 2003-10-07,Weak ferromagnetism in magnetoelectrics LiCoPO4 and LiNiPO4,"The magnetic properties of the antiferromagnetic magnetoelectric single-crystals LiCoPO4 and LiNiPO4 were investigated using the SQUID-magnetometer. The magnetization was measured in a longitudinal experimental geometry (M // H) along the crystallographic axes a, b and c. The carried out investigations have shown that these compounds exhibit weak ferromagnetism. The weak ferromagnetic (WFM) moment is directed along the antiferromagnetic axis in both crystals. But the temperature behaviours of the weak ferromagnetic moments in LiCoPO4 and LiNiPO4 are different. The weak ferromagnetic moment in LiCoPO4 changes non-monotonically. The essential horizontal negative bias of the magnetic hysteresis loop is observed in LiCoPO4",0310156v1 2003-10-10,The Silicon Inversion Layer With A Ferromagnetic Gate: A Novel Spin Source,"Novel spin transport behavior is theoretically shown to result from replacing the usual metal (or poly-silicon) gate in a silicon field-effect transistor with a ferromagnet, separated from the semiconductor by an ultra-thin oxide. The spin-dependent interplay between the drift current (due to a source-drain bias) and the diffusion current (due to carrier leakage into the ferromagnetic gate) results in a rich variety of spin dependence in the current that flows through such a device. We examine two cases of particular interest: (1) creating a 100% spin-polarized electrical current and (2) creating a pure spin current without a net electrical current. A spin-valve consisting of two sequential ferromagnetic gates is shown to exhibit magnetoresistance dependent upon the relative orientations of the magnetization of the two ferromagnets. The magnetoresistance ratio grows to arbitrarily large values in the regime of low source-drain bias, and is limited only by the spin-flip time in the channel.",0310259v1 2003-11-19,Shape memory ferromagnets,"In ferromagnetic alloys with shape memory large reversible strains can be obtained by rearranging the martensitic domain structure by a magnetic field. Magnetization through displacement of domain walls is possible in the presence of high magnetocrystalline anisotropy, when martensitic structure rearrangement is energetically favorable compared to the reorientation of magnetic moments. In ferromagnetic Heusler alloys Ni$_{2+x}$Mn$_{1-x}$Ga the Curie temperature exceeds the martensitic transformation temperature. The fact that these two temperatures are close to room temperature offers the possibility of magnetically controlling the shape and size of ferromagnets in the martensitic state. In Ni$_{2+x}$Mn$_{1-x}$Ga single crystals, a reversible strain of $\sim 6$% is obtained in fields of $\sim 1$ T.",0311433v1 2004-01-27,Non-linear sigma-model for odd triplet superconductivity in superconductor/ferromagnet structures,"We consider some properties of odd frequency triplet superconducting condensates. In order to describe fluctuations we construct a supermatrix sigma-model for the superconductor/ferromagnet or superconductor/normal-metal structures. We show that an odd frequency triplet superconductor, when in isolation or coupled to a normal metal, generally displays behaviour comparable to a superconductor with the usual singlet pairings. However, for spin dependent systems such as the superconductor/ferromagnet the two types of superconductor have quite different behaviour. We discuss this difference by considering transformations under which the sigma-model is invariant. Finally, we calculate the low energy density of states in a ferromagnet coupled to a singlet superconductor. If odd frequency triplet components are induced in the ferromagnet the density of states will have a micro-gap similar to the micro-gap found in normal metals coupled to a superconductor.",0401530v1 2004-03-08,Mean-field magnetization relaxation in conducting ferromagnets,"Collective ferromagnetic motion in a conducting medium is damped by the transfer of the magnetic moment and energy to the itinerant carriers. We present a calculation of the corresponding magnetization relaxation as a linear-response problem for the carrier dynamics in the effective exchange field of the ferromagnet. In electron systems with little intrinsic spin-orbit interaction, a uniform magnetization motion can be formally eliminated by going into the rotating frame of reference for the spin dynamics. The ferromagnetic damping in this case grows linearly with the spin-flip rate when the latter is smaller than the exchange field and is inversely proportional to the spin-flip rate in the opposite limit. These two regimes are analogous to the ""spin-pumping"" and the ""breathing Fermi-surface"" damping mechanisms, respectively. In diluted ferromagnetic semiconductors, the hole-mediated magnetization can be efficiently relaxed to the itinerant-carrier degrees of freedom due to the strong spin-orbit interaction in the valence bands.",0403224v2 2004-05-05,Spin waves in a Bose Ferromagnet,"It is shown that the ferromagnetic transition takes place always above Bose-Einstein condensation in ferromagnetically coupled spinor Bose gases. We describe the Bose ferromagnet within Ginzburg-Landau theory by a ""two-fluid"" model below Bose-Einstein condensation. Both the Bose condensate and the normal phase are spontaneously magnetized. As a main result we show that spin waves in the two fluids are coupled together so as to produce only one mixed spin-wave mode in the coexisting state. The long wavelength spectrum is quadratic in the wave vector ${\bf k}$, consistent with usual ferromagnetism theory, and the spin-wave stiffness coefficient $c_s$ includes contributions from both the two phases, implying the ""two-fluid"" feature of the system. $c_s$ can show a sharp bend at the Bose-Einstein condensation temperature.",0405094v1 2004-06-11,F/S interfaces: point contact versus atomic thickness geometries,"We contrast perturbative expansions of ferromagnet / superconductor interfaces in two geometries: (i) a point contact geometry where a single weak link connects a 3D ferromagnet to a 3D superconductor and (ii) an atomic thickness geometry with an infinite planar interface connecting a quasi-2D ferromagnet to a quasi-2D superconductor. Perturbation theories are rather different in the two approaches but they both break down at order $t^4$ ($t$ is the tunnel amplitude). The regimes of strong ferromagnets are in a qualitative agreement in both geometries. The regime of weak ferromagnets exists only for the atomic thickness geometry and is related to Andreev bound states due to lateral confinement in the superconductor.",0406280v1 2004-06-29,Ferromagnetism in substituted zinc oxide,"Room-temperature ferromagnetism is observed in (110) oriented ZnO films containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There are large moments, 1.9 and 0.5 muB/atom for Co- and Ti-substituted oxides, respectively. Sc-substituted ZnO shows also a moment of 0.3 muB/Sc. Magnetization is very anisotropic, with variations of up to a factor three depending on the orientation of the applied field relative to the R-cut sapphire substrates. Results are interpreted in terms of a spin-split donor impurity band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold. The variation of the ferromagnetism with oxygen pressure used during film growth is evidence of a link between ferromagnetism and defect concentration.",0406719v1 2004-07-05,Absence of ferromagnetism in Mn- and Co-doped ZnO,"Following the theoretical predictions of ferromagnetism in Mn- and Co-doped ZnO, several workers reported ferromagnetism in thin films as well as in bulk samples of these materials. While some observe room-temperature ferromagnetism, others find magnetization at low temperatures. Some of the reports, however, cast considerable doubt on the magnetism of Mn- and Co-doped ZnO. In order to conclusively establish the properties of Mn- and Co-doped ZnO, samples with 6 percent and 2 percent dopant concentrations, have been prepared by the low-temperature decomposition of acetate solid solutions. The samples have been characterized by x-ray diffraction, EDAX and spectroscopic methods to ensure that the dopants are substitutional. All the Mn- and Co-doped ZnO samples (prepared at 400 deg C and 500 deg C) fail to show ferromagnetism. Instead, their magnetic properties are best described by a Curie-Weiss type behavior. It appears unlikely that these materials would be useful for spintronics, unless additional carriers are introduced by some means.",0407108v2 2004-07-16,Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 made from Colloidal Nanocrystals,"Colloidal cobalt-doped TiO2 (anatase) nanocrystals were synthesized and studied by electronic absorption, magnetic circular dichroism, transmission electron microscopy, magnetic susceptibility, cobalt K-shell X-ray absorption spectroscopy, and extended X-ray absorption fine structure measurements. The nanocrystals were paramagnetic when isolated by surface-passivating ligands, weakly ferromagnetic (Ms = 1.5 x 10-3 mB/Co2+ at 300 K) when aggregated, and strongly ferromagnetic (up to Ms = 1.9 mB/Co2+ at 300 K) when spin-coated into nanocrystalline films. X-ray absorption data reveal that cobalt is in the Co2+ oxidation state in all samples. In addition to providing strong experimental support for the existence of intrinsic ferromagnetism in cobalt-doped TiO2, these results demonstrate the possibility of using colloidal TiO2 diluted magnetic semiconductor nanocrystals as building blocks for assembly of ferromagnetic semiconductor nanostructures with potential spintronics applications.",0407451v1 2004-09-06,Ferromagnet-Superconductor Hybrids,"A new class of phenomena discussed in this review is based on interaction between spatially separated, but closely located ferromagnets and superconductors. They are called Ferromagnet-Superconductor Hybrids (FSH). These systems include coupled smooth and textured Ferromagnetic and Superconducting films, magnetic dots, wires etc.The interaction may be provided by the magnetic flux from magnetic textures and supercurrents. The magnetic flux from magnetic textures or topological defects can pin vortices or create them, changing drastically the properties of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem leading to formation of coupled magnetic-superconducting topological defects. We discuss possible experimental realization of the FSH. The presence of ferromagnetic layer can change dramatically the properties of the superconducting film due to proximity effect. We discuss experimental and theoretical studies of the proximity effect in the FSH including transition temperature and order parameter oscillations and triplet superconductivity.",0409137v1 2004-09-08,Tailoring of ferromagnetic Pr0.85Ca0.15MnO3/ferroelectric Ba0.6Sr0.4TiO3 superlattices for multiferroic properties,"Superlattices composed of ferromagnetic Pr0.85Ca0.15MnO3 and ferroelectric Ba0.6Sr0.4TiO3 layers were fabricated on (100) SrTiO3 substrates by a pulsed-laser deposition method. The capacitance and resistive parts of the samples were analyzed from the complex impedance measurements, performed on the samples using a special experimental set-up. The superlattice with larger ferroelectric thickness shows unique characteristics which are not present in the parent ferromagnetic thin film. The superlattice show both ferromagnetic and ferroelectric transitions which is an evidence for the coexistence of both the properties. The high magnetoresistance (40 % at 80K) shown by the superlattice can be attributed to the coupling between ferromagnetic and ferroelectric layers, i.e, to the magnetoelectric effect.",0409182v1 2004-12-01,Magnetoresistive Effects in Ferromagnet-Superconductor Multilayers,"We consider a nanoscale system consisting of Manganite-ferromagnet and Cuprate-superconductor multilayers in a spin valve configuration. The magnetization of the bottom Manganite-ferromagnet is pinned by a Manganite-antiferromagnet. The magnetization of the top Manganite-ferromagnet is coupled to the bottom one via indirect exchange through the superconducting layers. We study the behavior of the critical temperature and the magnetoresistance as a function of an externally applied parallel magnetic field, when the number of Cuprate-superconductor layers are changed. There are two typical behaviors in the case of a few monolayers of the Cuprates: a) For small magnetic fields, the critical temperature and the magnetoresistance change abruptly when the flipping field of the top Manganite-ferromagnet is reached. b) For large magnetic fields, the multilayered system re-enters the zero-resistance (superconducting) state after having become resistive (normal).",0412005v1 2005-01-25,Spin-polarized transport through a single-level quantum dot in the Kondo regime,"Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter $U$. Exchange interaction between the dot and ferromagnetic electrodes is taken into account {\it via} an effective molecular field. The following situations are analyzed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization of electron states at the Fermi level, and (iii) one of the two electrodes is nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak in the density of states (DOS) becomes spin-split when the total exchange field acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads to splitting and suppression of the corresponding zero bias anomaly in the differential conductance.",0501605v1 2005-01-28,One-dimensional itinerant ferromagnets with Heisenberg symmetry and the ferromagnetic quantum critical point,"We study one-dimensional itinerant ferromagnets with Heisenberg symmetry near a ferromagnetic quantum critical point. It is shown that the Berry phase term arises in the effective action of itinerant ferromagnets when the full SU(2) symmetry is present. We explicitly demonstrate that dynamical critical exponent of the theory with the Berry term is $z=2 +{\rm O}(\epsilon^2)$ in the sense of $\epsilon$ expansion, as previously discovered in the Ising limit. It appears, however, that the universality class at the interacting fixed point is not the same. We point out that even though the critical theory in the Ising limit can be obtained by the standard Hertz-Millis approach, the Heisenberg limit is expected to be different. We also calculate the exact electron Green functions $G(x,t=0)$ and $G(x=0,t)$ near the transition in a range of temperature, which can be used for experimental signatures of the associated critical points.",0501689v3 2005-03-04,Can the interface between a non-ideal ferromagnet and a semiconductor quantum wire acts as an ideal spin filter?,"The problem of spin injection across the interface between a non-ideal ferromagnet (less than 100% spin polarization) and a semiconductor (paramagnetic) quantum wire is examined in the presence of Rashba spin orbit coupling and an axial magnetic field along the wire axis. The field is caused by the ferromagnet magnetized along the wire axis. At low temperatures and for certain injection energies, the interface can act as an ideal spin filter allowing injection only from the majority spin band of the ferromagnet. Thus, 100% spin filtering can take place even if the ferromagnet itself is less than 100% spin polarized. Below a critical value of the magnetic field, there are two injection energies for which ideal (100%) spin filtering is possible; above this critical field, there is only one such injection energy.",0503112v1 2005-03-08,"Ferromagnetic Semiconductors: Moving Beyond (Ga,Mn)As","The recent development of MBE techniques for growth of III-V ferromagnetic semiconductors has created materials with exceptional promise in spintronics, i.e. electronics that exploit carrier spin polarization. Among the most carefully studied of these materials is (Ga,Mn)As, in which meticulous optimization of growth techniques has led to reproducible materials properties and ferromagnetic transition temperatures well above 150 K. We review progress in the understanding of this particular material and efforts to address ferromagnetic semiconductors as a class. We then discuss proposals for how these materials might find applications in spintronics. Finally, we propose criteria that can be used to judge the potential utility of newly discovered ferromagnetic semiconductors, and we suggest guidelines that may be helpful in shaping the search for the ideal material.",0503185v1 2005-03-28,Giant oscillations of the density of states and the conductance in a ferromagnetic conductor coupled to two superconductors,"Giant oscillations of the density of electronic states and the differential conductance of a superconductor-ferromagnet-superconductor structure are predicted for the case when the exchange energy, $I_0$, due to the interaction between the electron spin and the spontaneous moment of the ferromagnet is smaller than the superconductor energy gap, $\Delta$. The effect is due to the extremely large degeneration of the energy level $\epsilon = I_0$ when the superconductor phase difference $\phi$ is close to odd multiple of $\pi$ ($\epsilon$ is the electron energy measured from the Fermi-energy). This quantum interference effect persists even in long ferromagnetic bridges whose length much exceeds the ''magnetic length'' ($\hbar v_{F}/I_{0}$ in the ballistic regime and $\sqrt{\hbar D/I_0}$ in the diffusive regime; $D$ is the electron diffusion constant). The predicted effect allows a direct spectroscopy of Andreev levels in the ferromagnet as well as a direct measurement of the exchange energy,$I_0$.",0503643v1 2005-05-04,Meissner phases in spin-triplet ferromagnetic superconductors,"We present new results for the properties of phases and phase transitions in spin-triplet ferromagnetic superconductors. The superconductivity of the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the presence of spontaneous magnetization. The mixed phase is stable but the other superconducting phases that usually exist in unconventional superconductors are either unstable or for particular values of the parameters of the theory some of them are metastable at relatively low temperatures in a quite narrow domain of the phase diagram. Phase transitions from the normal phase to the phase of coexistence is of first order while the phase transition from the ferromagnetic phase to the coexistence phase can be either of first or second order depending on the concrete substance. Cooper pair and crystal anisotropies determine a more precise outline of the phase diagram shape and reduce the degeneration of ground states of the system but they do not change drastically phase stability domains and thermodynamic properties of the respective phases. The results are discussed in view of application to metallic ferromagnets as UGe2, ZrZn2, URhGe.",0505087v1 2005-05-11,Stoner gap in the superconducting ferromagnet UGe2,"We report the temperature ($T$) dependence of ferromagnetic Bragg peak intensities and dc magnetization of the superconducting ferromagnet UGe2 under pressure ($P$). We have found that the low-$T$ behavior of the uniform magnetization can be explained by a conventional Stoner model. A functional analysis of the data produces the following results: The ferromagnetic state below a critical pressure can be understood as the perfectly polarized state, in which heavy quasiparticles occupy only majority spin bands. A Stoner gap $\Delta(P)$ decreases monotonically with increasing pressure and increases linearly with magnetic field. We show that the present analysis based on the Stoner model is justified by a consistency check, i.e., comparison of density of states at the Fermi energy deduced from the analysis with observed electronic specific heat coeffieients. We also argue the influence of the ferromagnetism on the superconductivity.",0505266v2 2005-06-16,Meissner superconductivity in itinerant ferromagnets,"Recent results about the coexistence of ferromagnetism and unconventional superconductivity with spin-triplet Cooper pairing are reviewed on the basis of the quasi-phenomenological Ginzburg-Landau theory. The superconductivity in the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the spontaneous magnetization. The mixed phase is stable whereas the other superconducting phases that usually exist in unconventional superconductors are either unstable or metastable at relatively low temperatures in a quite narrow domain of the phase diagram and the stability properties are determined by the particular values of Landau parameters. The phase transitions from the normal phase to the phase of coexistence is of first order while the phase transition from the ferromagnetic phase to the coexistence phase can be either of first or second order depending on the concrete substance. The Cooper pair and crystal anisotropy are relevant to a more precise outline of the phase diagram shape and reduce the degeneration of the ground states of the system. The results are discussed in view of application to itinerant ferromagnetic compounds as UGe2, ZrZn2, URhGe.",0506396v1 2005-07-01,Critical fields and spontaneous vortex state in the weak-ferromagnetic superconductor RuSr2GdCu2O8,"A spontaneous vortex state (SVS) between 30 K and 56 K was observed for the weak-ferromagnetic superconductor RuSr2GdCu2O8 with ferromagnetic Curie temperature TC = 131 K and superconducting transition temperature Tc = 56 K. The low field (20 G) Superconducting hysteresis loop indicates a narrow Meissner state region within average lower critical field Bc1(T)= Bc1(0)[1 - (T/T0)2], with average Bave c1 (0) = 12 G and T0 = 30 K. Full Meissner shielding signal in very low applied field indicates an ab-plane Bab c1(0) ~ 4 G with an estimated anisotropic parameter g ~ 7 for this layered system. The existence of a spontaneous vortex state between 30 K and 56 K is the result of weak-ferromagnetic order with a net spontaneous magnetic moment of ~ 0.1 muB/Ru, which generates a weak magnetic dipole field around 10 G in the CuO2 bi-layers. The upper critical field Bc2 varies linearly as (1 - T/Tc) up to 7-T field. The vortex melting line Bm varies as (1 - T/Tm)3.5 with melting transition temperature Tm = 39 K and a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order.",0507014v1 2005-07-03,Coulomb Interactions and Ferromagnetism in Pure and Doped Graphene,"We study the presence of ferromagnetism in the phase diagram of the two-dimensional honeycomb lattice close to half-filling (graphene) as a function of the strength of the Coulomb interaction and doping. We show that exchange interactions between Dirac fermions can stabilize a ferromagnetic phase at low doping when the coupling is sufficiently large. In clean systems, the zero temperature phase diagram shows both first order and second order transition lines and two distinct ferromagnetic phases: one phase with only one type of carriers (either electrons or holes) and another with two types of carriers (electrons and holes). Using the coherent phase approximation (CPA) we argue that disorder further stabilizes the ferromagnetic phase.",0507061v2 2005-10-13,Direct Measurements of Spin-Dependent and Coherent Effects in Conductance of a Ferromagnet/Superconductor System,"In the systems ferromagnet/superconductor ([Fe, Ni]/In), the temperature dependent transport has been investigated within the temperature range including superconducting transition temperature for Indium. It has been found that when Indium becomes superconducting, the system resistance acquires two positive additions. The first, numerically equal to the resistance of a ferromagnet part, of order of a spin--polarized region in length, corresponds to the manifestation of ""spin accumulation"" effect. The second agrees in magnitude with an interference reduction in the ferromagnet conductance over the coherence length for singlet ""Andreev pairs"" which is established by the exchange field in the ferromagnet. From the experimental data, the degree of current spin polarization, the coherence length in the exchange field, and the lower limit of spin--relaxation length in Fe and Ni have been estimated.",0510352v1 2005-12-15,Nematic order in square lattice frustrated ferromagnets,"We present a new scenario for the breakdown of ferromagnetic order in a two-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. In this, dynamical effects lead to the formation of two-magnon bound states, which undergo Bose-Einstein condensation, giving rise to bond-centered nematic order. This scenario is explored in some detail for an extended Heisenberg model on a square lattice. In particular, we present numerical evidence confirming the existence of a state with d-wave nematic correlations but no long range magnetic order, lying between the saturated ferromagnetic and collinear antiferromagnetic phases of the ferromagnetic J1-J2 model. We argue by continuity of spectra that this phase is also present in a model with 4-spin cyclic exchange.",0512349v1 2006-02-15,A spin triplet supercurrent through the half-metallic ferromagnet CrO2,"In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur when spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental setup is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.",0602359v1 2006-02-28,Magnetic field dependence of the magnetic phase separation in Pr1-xCaxMnO3 manganites studied by small-angle neutron scattering,"Transport properties of manganese oxides suggest that their colossal magnetoresistance (CMR) is due to percolation between ferromagnetic metallic (FM) clusters in an antiferromagnetic insulating (AFI) matrix. We have studied small-angle neutron scattering under applied magnetic field in CMR Pr1-xCaxMnO3 crystals for x around 0.33. Quantitative analysis of the small-angle magnetic neutron scattering shows that the magnetic heterogeneities take place at different scales. At the mesoscopic scale (200nm), the inhomogeneities correspond to the percolation of the conducting ferromagnetic phase into the insulating phases. It is at the origin of the colossal magnetoresistance of the compound. The other inhomogeneities are nanoscopic: inside the antiferromagnetic phase (AFI), there exist small ferromagnetic clusters. Inside the ferromagnetic phase which exists in absence of magnetic field in some compounds and is in fact insulating (FI), there also exist small non ferromagnetic objects. No evolution of this nanostructure is observed when the magnetic field is applied. The existence of such nanoscale objects is discussed in relation to the cationic disorder of these compounds.",0602668v1 2006-04-20,Influence of the microstructure on the magnetism of Co-doped ZnO thin films,"The prediction of ferromagnetism at room temperature in Co-ZnO thin films has generated a large interest in the community due to the possible applications. However, the results are controversial, going from ferromagnetism to non-ferromagnetism, leading to a large debate about its origin (secondary phase, Co clusters or not). By carefully studying the micro-structure of various Co-ZnO films, we show that the Co2+ partly substitutes the ZnO wurtzite matrix without forming Co clusters. Surprisingly, the ferromagnetism nature of the films disappears as the Co content increases. In addition, our results suggest that the observed ferromagnetism is likely associated to a large amount of defects- close to the interface and strongly depending on the growth temperature- which may explained the spreading of the results.",0604468v1 2006-05-09,"Superexchange in Dilute Magnetic Dielectrics: Application to (Ti,Co)O_2","We extend the model of ferromagnetic superexchange in dilute magnetic semiconductors to the ferromagnetically ordered highly insulating compounds (dilute magnetic dielectrics). The intrinsic ferromagnetism without free carriers is observed in oxygen-deficient films of anatase TiO_2 doped with transition metal impurities in cation sublattice. We suppose that ferromagnetic order arises due to superexchange between complexes [oxygen vacancies + magnetic impurities], which are stabilized by charge transfer from vacancies to impurities. The Hund rule controls the superexchange via empty vacancy related levels so that it becomes possible only for the parallel orientation of impurity magnetic moments. The percolation threshold for magnetic ordering is determined by the radius of vacancy levels, but the exchange mechanism does not require free carriers. The crucial role of the non-stoichiometry in formation of the ferromagnetism makes the Curie temperatures extremely sensitive to the methods of sample preparation.",0605242v1 2006-05-14,Ballistic transport in ferromagnet-superconductor-ferromagnet trilayers with arbitrary orientation of magnetizations,"Transport phenomena in clean ferromagnet-superconductor-ferromagnet (FSF) trilayers are studied theoretically for a general case of arbitrary orientation of in-plane magnetizations and interface transparencies. Generalized expressions for scattering probabilities are derived and the differential conductance is computed using solutions of the Bogoliubov-de Gennes equation. We focus on size and coherence effects that characterize ballistic transport, in particular on the subgap transmission and geometrical oscillations of the conductance. We find a monotonic dependence of conductance spectra and magnetoresistance on the angle of misorientation of magnetizations as their alignment is changed from parallel to antiparallel. Spin-triplet pair correlations in FSF heterostructures induced by non-collinearity of magnetizations are investigated by solving the Gor'kov equations in the clean limit. Unlike diffusive FSF junctions, where the triplet correlations have a long-range monotonic decay, we show that in clean ferromagnet-superconductor hybrids both singlet and triplet pair correlations induced in the F layers are oscillating and power-law decaying with the distance from the S-F interfaces.",0605355v1 2006-05-23,Josephson coupling through ferromagnetic heterojunctions with noncollinear magnetizations,"We study the Josephson effect in clean heterojunctions that consist of superconductors connected through two metallic ferromagnets with insulating interfaces. We solve the scattering problem based on the Bogoliubov--de Gennes equation for any relative orientation of in-plane magnetizations, arbitrary transparency of interfaces, and mismatch of Fermi wave vectors. Both spin singlet and triplet superconducting correlations are taken into account, and the Josephson current is calculated as a function of the ferromagnetic layers thicknesses and of the angle $\alpha$ between their magnetizations. We find that the critical Josephson current $I_c$ is a monotonic function of $\alpha$ when the junction is far enough from $0-\pi$ transitions. This holds when ferromagnets are relatively weak. For stronger ferromagnets, variation of $\alpha$ induces switching between 0 and $\pi$ states and $I_c(\alpha)$ is non-monotonic function, displaying characteristic dips at the transitions. However, the non-monotonicity is the effect of a weaker influence of the exchange potential in the case of non-parallel magnetizations. No substantial impact of spin-triplet superconducting correlations on the Josephson current has been found in the clean limit. Experimental control of the critical current and $0-\pi$ transitions by varying the angle between magnetizations is suggested.",0605562v2 2006-06-06,Variational Mote Carlo Study of Flat Band Ferromagnetism -- Application to CeRh_3 B_2,"A new mechanism for ferromagnetism in CeRh_3B_2 is proposed on the basis of variational Monte Carlo results. In a one-dimensional Anderson lattice where each 4f electron hybridizes with a ligand orbital between neighboring Ce sites, ferromagnetism is stabilized due to a nearly flat band which is a mixture of conduction and 4f electron states. Because of the strong spin-orbit interaction in 4f electron states, and of considerable amount of hybridization in the nearly flat band, the magnetic moments from 4f and conduction electrons tend to cancel each other. The resultant ferromagnetic moment becomes smaller as compared with the local 4f moment, and the Fermi surface in the ferromagnetic ground state is hardly affected by the presence of 4f electrons. These theoretical results are consistent with experimental observations in CeRh_3B_2 by neutron scattering and dHvA effects.",0606143v1 2006-06-13,Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$,"We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. We discuss field- and temperature-induced trends within the framework of the Tomioka-Tokura global electronic phase diagram picture and suggest that controlled disorder near a phase boundary can be used to tune the magnetodielectric response. Remnants of the spin-glass insulator to ferromagnetic metallic transition can also drive 300 K color changes in (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$.",0606339v1 2006-06-19,Magnetic phase diagram of the Kondo lattice model with quantum localized spins,"The magnetic phase diagram of the ferromagnetic Kondo lattice model is determined at T=0 in 1D, 2D, and 3D for various magnitudes of the quantum mechanical localized spins ranging from S=1/2 to classical spins. We consider the ferromagnetic phase, the paramagnetic phase, and the ferromagnetic/antiferromagnetic phase separated regime. There is no significant influence of the spin quantum number on the phase boundaries except for the case S=1/2, where the model exhibits an instability of the ferromagnetic phase with respect to spin disorder. Our results give support, at least as far as the low temperature magnetic properties are concerned, to the classical treatment of the S=3/2-spins in the intensively investigated manganites, for which the ferromagnetic Kondo-lattice model is generally employed to account for magnetism.",0606485v1 2006-08-04,Compositional tuning of ferromagnetism in Ga1-xMnxP,"We report the magnetic and transport properties of Ga1-xMnxP synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga1-xMnxAs, Ga1-xMnxP displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga1-xMnxAs, Ga1-xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.",0608133v1 2006-09-07,"Mn-doped Ga(As,P) and (Al,Ga)As ferromagnetic semiconductors","A remarkable progress towards functional ferromagnetic semiconductor materials for spintronics has been achieved in p-type (Ga,Mn)As. Robust hole-mediated ferromagnetism has, however, been observed also in other III-V hosts such as antimonides, GaP or (Al,Ga)As which opens a wide area of possibilities for optimizing the host composition towards higher ferromagnetic Curie temperatures. Here we explore theoretically ferromagnetism and Mn incorporation in Ga(As,P) and (Al,Ga)As ternary hosts. While alloying (Ga,Mn)As with Al has only a small effect on the Curie temperature we predict a sizable enhancement of Curie temperatures in the smaller lattice constant Ga(As,P) hosts. Mn-doped Ga(As,P) is also favorable, as compared to (Al,Ga)As, with respect to the formation of carrier and moment compensating interstitial Mn impurities. In (Ga,Mn)(As,P) we find a marked decrease of the partial concentration of these detrimental impurities with increasing P content.",0609158v1 2006-12-01,Theory of digital magneto resistance in ferromagnetic resonant tunneling diodes,"We propose a ferromagnetic spintronic system, which consists of two serial connected resonant tunneling diodes. One diode is nonmagnetic whereas the other comprises a ferromagnetic emitter and quantum well. Using a selfconsistent coherent transport model we show that the current-voltage characteristic of the ferromagnetic diode can be strongly modulated by changing the relative orientation of the magnetizations in the emitter and quantum well, respectively. By a continuous change of the relative magnetization angle the total resistance exhibits a discrete jump realizing digital magneto resistance. The interplay between the emitter's Fermi energy level and the relative magnetization orientations allows to tailor the current voltage characteristics of the ferromagnetic diode from ohmic to negative differential resistance regime at low voltages.",0612025v1 2006-12-23,Magnetic phase diagram of Ce2Fe17,"Rare-earth-based permanent-magnet materials rich in iron have relatively low ferromagnetic ordering temperatures. This is believed to be due to the presence of antiferromagnetic exchange interactions, besides the ferromagnetic interactions responsible for the magnetic order. The magnetic properties of Ce2Fe17 are anomalous. Instead of ferromagnetic, it is antiferromagnetic, and instead of one ordering temperature, it shows two, at the Neel temperature TN ~ 208 K and at TT ~ 124 K. Ce2Fe17, doped by 0.5% Ta, also shows two ordering temperatures, one to an antiferromagnetic phase, at TN ~ 214 K, and one to a ferromagnetic phase, at T0 ~ 75 K. In order to clarify this behavior, single-crystalline samples were prepared by solution growth, and characterized by electron microscopy, single crystal x-ray diffraction, temperature-dependent specific heat, and magnetic field and temperature-dependent electrical resistivity and magnetization. From these measurements, magnetic H-T phase diagrams were determined for both Ta-doped Ce2Fe17 and undoped Ce2Fe17. These phase diagrams can be very well described in terms of a theory that gives magnetic phase diagrams of systems with competing antiferro- and ferromagnetism.",0612603v1 2007-01-14,Sr1.5Ba0.5Zn2Fe12O22 Hexaferrites Ferromagnetic Resonance and Nonlinear Excitation for Magneto-Electric Devices,"A magneto electric (ME) effect was reported [1] for the insulator material of helimagnetic hexaferrite of Sr1.5Ba0.5Zn2Fe12O22. In this study we are interested in investigating this materials ferromagnetic resonance to define its internal anisotropy fields. By using a vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) analysis we realized that this helical hexaferrite has about -0.5 kOe exchange anisotropy field of HE. Also, by using an alternative free energy model we derived this materials Polder tensor and its ferromagnetic resonance condition. Meanwhile, we were able to show a potential magneto electric coupling by nonlinear excitation of planar helical hexaferrite, in which a magnetic nonlinear excitation is integrated into dielectric permittivity. Index Terms, Ferrites, ferromagnetic resonances, and magnetic nonlinear excitations.",0701309v2 2007-01-19,"Reversible Ferromagnetic Switching in Zno:(Co,Mn) Powders","We report here on the magnetic properties of ZnO:Mn and ZnO:Co doped nanoparticles. We have found that the ferromagnetism of ZnO:Mn can be switched on and off by consecutive low-temperature annealings in O2 and N2 respectively, while the opposite phenomenology was observed for ZnO:Co. These results suggest that different defects (presumably n-type for ZnO:Co and p-type for ZnO:Mn) are required to induce a ferromagnetic coupling in each case. We will argue that ferromagnetism is likely to be restricted to a very thin, nanometric layer, at the grain surface. These findings reveal and give insight into the dramatic relevance of surface effects for the occurrence of ferromagnetism in ZnO doped oxides.",0701473v1 2007-01-25,Role of magnetic scattering in 0-pi transitions in a superconductor/ferromagnetic metal/superconductor junction,"We study the influence of magnetic scattering on the Josephson critical current, Ic, in a superconductor/ferromagnetic metal/superconductor (SFS) junction by a tunneling Hamiltonian approach. An analytical formula of Ic is given in the fourth order perturbation theory as regards the tunneling matrix element. The Ic exhibits the damped oscillatory dependence on the thickness of the ferromagnetic metal, d, and shows the transition between 0- and pi-states with d. When the superconducting transition temperature is comparable to the ferromagnetic Curie temperature, the period of oscillation is obviously changed by increasing temperature, T, due to the magnetic scattering, which induces the 0-pi transition with T. The magnetic scattering provides rich variety of Josephson effect in the SFS junction. Our results present an appropriate condition of a superconductor and a ferromagnetic metal to control the 0- and the pi-states.",0701610v2 2007-02-20,Spin dynamics in a superconductor / ferromagnet proximity system,"The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is measured. By varying the temperature and thickness of the Nb the role of the superconductivity on the whole ferromagnetic layer in these heterostructures is explored. The change in the spin transport properties below the superconducting transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a sharpening in the resonance of the ferromagnet, or a decrease in the effective Gilbert damping co-efficient. This dynamic proximity effect is in contrast to low frequency studies in these systems, where the effect of the superconductor is confined to a small region in the ferromagnet. We interpret this in terms of the spin pumping model.",0702461v1 2007-04-03,Fabrication of half metallicity in a ferromagnetic metal,"We investigate the growth of half metallic phase in a ferromagnetic material using state-of-the-art full potential linearized augmented plane wave method. To address the issue, we have substituted Ti at the Ru-sites in SrRuO3, where SrRuO3 is a ferromagnetic material. Calculated results establish Ti4+ valence states (similar to SrTiO3), which was predicted experimentally. Thus, Ti substitution dilutes the Ru-O-Ru connectivity, which is manifested in the calculated results in the form of significant band narrowing leading to finite gap between t2g and eg bands. At 75% substitution, a large gap (> 2 eV) appears at the Fermi level, e_F in the up spin density of states, while the down spin states contributes at e_F characterizing the system a half-metallic ferromagnet. The t2g - eg gap can be tailored judiciously by tuning Ti concentrations to minimize thermal effects, which is often the major bottleneck to achieve high spin polarization at elevated temperatures in other materials. This study, thus, provides a novel but simple way to fabricate half-metallicity in ferromagnetic materials, which are potential candidates for spin-based technology.",0704.0321v1 2007-08-10,Superconductivity on the border of weak itinerant ferromagnetism in UCoGe,"We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T_{C}= 3 K and a small ordered moment $m_{0}$= 0.03 $\mu_B$. Superconductivity is observed with a resistive transition temperature T_{s} = 0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T_{s}, and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.",0708.1388v1 2007-08-21,Magnetic and transport properties of the one-dimensional ferromagnetic Kondo lattice model with an impurity,"We have studied the ferromagnetic Kondo lattice model (FKLM) with an Anderson impurity on finite chains with numerical techniques. We are particularly interested in the metallic ferromagnetic phase of the FKLM. This model could describe either a quantum dot coupled to one-dimensional ferromagnetic leads made with manganites or a substitutional transition metal impurity in a MnO chain. We determined the region in parameter space where the impurity is empty, half-filled or doubly-occupied and hence where it is magnetic or nonmagnetic. The most important result is that we found, for a wide range of impurity parameters and electron densities where the impurity is magnetic, a singlet phase located between two saturated ferromagnetic phases which correspond approximately to the empty and double-occupied impurity states. Transport properties behave in general as expected as a function of the impurity occupancy and they provide a test for a recently developed numerical approach to compute the conductance. The results obtained could be in principle reproduced experimentally in already existent related nanoscopic devices or in impurity doped MnO nanotubes.",0708.2826v2 2007-08-27,Determination of Penetration Depth of Transverse Spin Current in Ferromagnetic Metals by Spin Pumping,"Spin pumping in nonmagnetic/ferromagnetic metal multilayers is studied both theoretically and experimentally. We show that the line widths of the ferromagnetic resonance (FMR) spectrum depend on the thickness of the ferromagnetic metal layers, which must not be in resonance with the oscillating magnetic field. We also show that the penetration depths of the transverse spin current in ferromagnetic metals can be determined by analyzing the line widths of their FMR spectra. The obtained penetration depths in NiFe, CoFe and CoFeB were 3.7 [nm], 2.5 [nm] and 12.0 [nm], respectively.",0708.3528v3 2007-09-10,Exotic Ground State Phases of $S=1/2$ Heisenberg $Δ$-Chain with Ferromagnetic Main Chain,"The ground state phase diagram of the spin-1/2 Heisenberg frustrated $\Delta$-chain with a ferromagnetic main chain is investigated. In addition to the ferromagnetic phase, various nonmagnetic ground states are found. If the ferromagnetic coupling between apical spins and the main chain is strong, this model is approximated by a spin-1 bilinear-biquadratic chain and the spin quadrupolar phase with spin-2 gapless excitation is realized in addition to the Haldane and ferromagnetic phases. In the regime where the coupling between the apical spins and the main chain is weak, the numerical results which suggest the possibility of a series of phase transitions among different nonmagnetic phases are obtained. Physical pictures of these phases are discussed based on the numerical results.",0709.1423v3 2007-10-31,Charge Density Wave Driven Ferromagnetism in the Periodic Anderson Model,"We demonstrate the existence of ferromagnetism in the Periodic Anderson Model (PAM) at conduction-band filling near a quarter. We show that this ferromagnetism is not supported by Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions but is instead driven by the precursors of charge density wave (CDW) formation in the conduction electron band. To study the effect of spatial correlations, we compare Dynamical Mean field Approximation (DMFA) and Dynamical Cluster Approximation (DCA) results. We find that both RKKY and CDW driven ferromagnetism persist as short-range correlations are incorporated into the theory. Both DMFA and DCA show the precursors of CDW formation through the strong enhancement of the d-electron CDW susceptibility as the temperature decreases, up to the ferromagnetic transition temperature. In addition, the DCA captures the signal of a band gap opening due to Peierls instability.",0710.5937v1 2007-11-02,Origin of ferromagnetic response in diluted magnetic semiconductors and oxides,"This paper reviews the present understanding of the origin of ferromagnetic response of diluted magnetic semiconductors and diluted magnetic oxides as well as in some nominally magnetically undoped materials. It is argued that these systems can be grouped into four classes. To the first belong composite materials in which precipitations of a known ferromagnetic, ferrimagnetic or antiferromagnetic compound account for magnetic characteristics at high temperatures. The second class forms alloys showing chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent. To the third class belong (Ga,Mn)As, heavily doped p-(Zn,Mn)Te, and related semiconductors. In these solid solutions the theory built on p-d Zener's model of hole-mediated ferromagnetism and on either the Kohn-Luttinger kp theory or the multi-orbital tight-binding approach describes qualitatively, and often quantitatively many relevant properties. Finally, in a number of carrier-doped DMS and DMO a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation.",0711.0340v1 2007-11-29,The underscreened Kondo lattice model applied to heavy fermion uranium compounds,"We present theoretical results for the underscreened Kondo lattice model with localized S=1 spins coupled to a conduction band through a Kondo coupling, $J_K$, and interacting among them ferromagnetically. We use a fermionic representation for the spin operators and expand the Hamiltonian in terms of bosonic fields. For large values of $J_K$, we obtain a ferromagnetically ordered solution and a Kondo regime with a Kondo temperature, $T_K$, larger than the Curie temperature, $T_C$. This finding suggests a scenario for a coexistence of Kondo effect and ferromagnetic order. In some uranium compounds, like $UTe$ or $UCu_{0.9}Sb_{2}$, this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order $T_C \sim 100K$ and exhibit a Kondo behavior for $T > T_C$. The proposed underscreened Kondo lattice model accounts well for the coexistence between magnetic order and Kondo behavior and yields to a new ``ferromagnetic Doniach diagram''.",0711.4765v1 2007-12-18,Defect-induced ferromagnetism in fullerenes,"Based on the ab initio electronic structure calculations the picture of ferromagnetism in polimerized C60 is proposed which seems to explain the whole set of controversial experimental data. We have demonstrated that, in contrast with cubic fullerene, in rhombohedral C60 the segregation of iron atoms is energetically unprofitable which is a strong argument in favor of intrinsic character of carbon ferromagnetism which can be caused by vacancies with unpaired magnetic electrons. It is shown that: (i) energy formation of the vacancies in the rhombohedral phase of C60 is essentially smaller than in the cubic phase, (ii) there is a strong ferromagnetic exchange interactions between carbon cages containing the vacancies, and (iii) the fusion of the magnetic vacancies into nonmagnetic bivacancy is energetically profitable. The latter can explain a fragility of the ferromagnetism.",0712.2928v2 2008-01-03,Fabrication and Characterization of Short Josephson Junctions with Stepped Ferromagnetic Barrier,"We present novel low-T_c superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions with planar and stepped ferromagnetic interlayer. We optimized the fabrication process to set a step in the ferromagnetic layer thickness. Depending on the thickness of the ferromagnetic layer the ground state of the SIFS junction has a phase drop of either 0 or pi. So-called 0-pi Josephson junctions, in which 0 and pi ground states compete with each other, were obtained. These stepped junctions may have a double degenerate ground state, corresponding to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum \Phi_0. Here, we limit the presentation to static properties of short junctions.",0801.0515v2 2008-01-11,Quasiparticle Trapping In Three Terminal Ferromagnetic Tunneling Devices,"Hybrid Superconductor/Ferromagnet structures have been investigated recently to address the interplay between ferromagnetism and superconductivity. They also open up new routes for the investigation of out of equilibrium superconductivity. Here, we show how it is possible for out of equilibrium excitations produced in a superconducting thin film (S) to be localized in a ferromagnetic trap (F). Specifically, a ferromagnetic nano-volume in good contact with S represents a potential well for the quasiparticles (QPs) at the gap edge. As the superconducting proximity effect is highly suppressed in F, QPs get efficiently trapped and they share their energy with the free electrons in the trap. The electronic temperature Te in the trap can be increased by up to 60% from the bath temperature at 320 mK as measured by tunneling spectroscopy using a second junction.",0801.1799v3 2008-01-31,Enhancement of ferromagnetism by Co and Ni substitution in the perovskite LaBiMn2O6+d,"The substitution of cobalt and nickel for manganese in the perovskite manganate LaBiMn2O6+d; has been realized, leading to the perovskites LaBiMn2-x(M)xO6+d, with M = Co, Ni and x =0-2/3. In contrast to the literature those phases are found to be orthorhombic with Pnma symmetry. More importantly, it is shown that ferromagnetism is enhanced, TC being increased from 80 K for the parent compound (x = 0) to 97 K for Ni-phase, and to 130 K for the Co-phase. Moreover, a strong competition between ferromagnetism and a glassy-ferromagnetic state in the case of nickel or a spin-glass behaviour in the case of cobalt is observed. These phenomena are interpreted in the frame of a phase separation scenario, where the ferromagnetic Mn4+/Ni2+ and Mn4+/Co2+ interactions reinforce the Mn3+/Mn4+ interactions. These compounds are found to be insulating with a relatively large positive value of thermoelectric power.",0801.4916v1 2008-02-25,Current-induced spin torques in III-V ferromagnetic semiconductors,"We formulate a theory of current-induced spin torques in inhomogeneous III-V ferromagnetic semiconductors. The carrier spin-3/2 and large spin-orbit interaction, leading to spin non-conservation, introduce significant conceptual differences from spin torques in ferromagnetic metals. We determine the spin density in an electric field in the weak momentum scattering regime, demonstrating that the torque on the magnetization is intimately related to spin precession under the action of both the spin-orbit interaction and the exchange field characteristic of ferromagnetism. The spin polarization excited by the electric field is smaller than in ferromagnetic metals and, due to lack of angular momentum conservation, cannot be expressed in a simple closed vectorial form. Remarkably, scalar and spin-dependent scattering do not affect the result. We use our results to estimate the velocity of current-driven domain walls.",0802.3717v2 2008-04-11,Simultaneous suppression of ferromagnetism and superconductivity in UCoGe by Si substitution,"We investigate the effect of substituting Si for Ge in the ferromagnetic superconductor UCoGe. Dc-magnetization, ac-susceptibility and electrical resistivity measurements on polycrystalline UCoGe$_{1-x}$Si$_x$ samples show that ferromagnetic order and superconductivity are progressively depressed with increasing Si content and simultaneously vanish at a critical concentration $x_{cr} \simeq 0.12$. The non-Fermi liquid temperature variation in the electrical resistivity near $x_{cr}$ and the smooth depression of the ordered moment point to a continuous ferromagnetic quantum phase transition. Superconductivity is confined to the ferromagnetic phase, which provides further evidence for magnetically mediated superconductivity.",0804.1891v1 2008-05-17,Ferromagnetic tendency at the surface of CE charge-ordered manganites,"Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally the opposite effect has been recently observed: ferromagnetism appears in some nano-sized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half-doping n=0.5, using open boundary conditions to simulate the surface of either bulk or nano-sized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.",0805.2702v2 2008-08-17,Inhomogeneous Phases in a Double-Exchange Magnet with Long Range Coulomb Interactions,"We consider a model with competing double-exchange (ferromagnetic) and super-exchange (anti-ferromagnetic) interactions in the regime where phase separation takes place. The presence of a long range Coulomb interaction frustrates a macroscopic phase separation, and favors microscopically inhomogeneous configurations. We use the variational Hartree-Fock approach, in conjunction with Monte-Carlo simulations to study the geometry of such configurations in a two-dimensional system. We find that an array of diamond shaped ferromagnetic droplets is the preferred configuration at low electronic densities, while alternating ferromagnetic and anti-ferromagnetic diagonal stripes emerge at higher densities. These findings are expected to be relevant for thin films of colossal magneto-resistive manganates.",0808.2290v2 2008-09-17,The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions,"We study the effect of Coulomb interactions in transition metal oxides junctions. In this paper we analyze charge transfer at the interface of a three layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose a charge model considering a few atomic planes within each layer and obtain results for the magnetic coupling between the ferromagnetic layers. For large number of planes in the paramagnetic spacer we find that the coupling oscillates with the same period as in RKKY but the amplitude is sensitive to the Coulomb energy. At small spacer thickness however, large differences may appear as function of : the number of electrons per atom in the ferromagnetics and paramagnetics materials, the dielectric constant at each component, and the charge defects at the interface plane emphasizing the effects of charge transfer.",0809.2984v1 2008-09-25,Magnetic Behavior of Single Crystalline Pr_{5}Ge_{3} and Tb_{5}Ge_{3}Compounds,"The results of the magnetization studies on Pr_{5}Ge_{3}and Tb_{5}Ge_{3} single crystals are reported. Single Crystals of Pr_{5}Ge_{3} and Tb_{5}Ge_{3} compounds were successfully grown by Czochralski method. These compounds crystallize in a Mn_{5}Si_{3} type hexagonal structure with space group P6_{3}/mcm. Ferromagnetic correlations set in at around 36 K in Pr_{5}Ge_{3} in the ab plane followed by an antiferromagnetic transition at 13 K. Along the c-axis the magnetization shows a ferromagnetic transition around 13 K with an overall ferrimagnetic behavior. At 2K, the magnetic isotherm of the compound along [0001] direction is typical for a ferromagnet, while a field induced ferromagnetic type response is observed along the [10\overline{\mathit{1}}0] direction. Hexagonal ab plane or [10\overline{\mathit{1}}0] direction was found to be the easy axis of magnetization. Tb_{5}Ge_{3} orders antiferromagneticaly at 85 K with the hexagonal ab plane as easy axis of magnetization. The compound shows a field induced ferromagnetic behavior in its magnetic isotherm at 2 K.",0809.4467v2 2008-10-07,Nontrivial interplay between superconductivity and spin-orbit coupling in non-centrosymmetric ferromagnets,"Motivated by the recent discoveries of ferromagnetic and non-centrosymmetric superconductors, we present a mean-field theory for a superconductor that \textit{both} lacks inversion symmetry and displays ferromagnetism, a scenario which is believed to be realized in UIr. We study the interplay between the order parameters to clarify how superconductivity is affected by the presence of ferromagnetism and spin-orbit coupling. One of our key findings is that the spin-orbit coupling seems to enhance both ferromagnetism and superconductivity in all spin channels. We discuss our results in the context of the heavy fermion superconductor UIr and analyze possible symmetries of the order parameter by the group theory method.",0810.1281v1 2008-10-08,Transverse spin diffusion in ferromagnets,"We discuss the dissipative diffusion-type term of the form $\mathbf{m}\times\nabla^2\partial_t\mathbf{m}$ in the phenomenological Landau-Lifshitz equation of ferromagnetic precession, which describes enhanced Gilbert damping of finite-momentum spin waves. This term arises physically from itinerant-electron spin flows through a perturbed ferromagnetic configuration and can be understood to originate in the ferromagnetic spin pumping in the continuum limit. We develop a general phenomenology as well as provide microscopic theory for the Stoner and s-d models of ferromagnetism, taking into account disorder and electron-electron scattering. The latter is manifested in our problem through the Coulomb drag between the spin bands. The spin diffusion is identified in terms of the transverse spin conductivity, in analogy with the Einstein relation in the kinetic theory.",0810.1340v2 2008-10-09,Polaron formation in the optimally doped ferromagnetic manganites La0.7Ba0.3MnO3 and La0.7Ba0.3MnO3,"The nature of the polarons in the optimally doped colossal magnetoresistive (CMR) materials La0.7Ba0.3MnO3 (LBMO) and La0.7Sr0.3MnO3 (LSMO) is studied by elastic and inelastic neutron scattering. In both materials, dynamic nanoscale polaron correlations develop abruptly in the ferromagnetic state. However, the polarons are not able to lock-in to the lattice and order, in contrast to the behavior of La0.7Ca0.3MnO3. Therefore ferromagnetic order in LBMO and LSMO survives their formation, explaining the conventional second order nature of the ferromagnetic--paramagnetic transition. Nevertheless, the results demonstrate that the fundamental mechanism of polaron formation is a universal feature of these ferromagnetic perovskite manganites.",0810.1772v2 2008-11-03,Coexistence of Ferromagnetism with Spin Triplet Superconductivity,"The experimental results for ZrZn$_{2}$, URhGe, and in some pressure ranges also for UGe$_{2}$, have shown that the ferromagnetic superconductors are weak itinerant ferromagnets. Guided by these results we describe the phenomenon of coexistence between equal spin triplet pairing superconductivity (SC) and ferromagnetism (F) using the extended Stoner model, which includes in Hamiltonian the on-site Coulomb interaction, $U$, and occupation dependent hopping integral. We use Hartree-Fock (H-F) approximation and the Green functions technique. In the H-F approximation the on-site Coulomb interaction plays the role of the on-site exchange (Hund's) field. All inter-site interactions will have included the inter-site kinetic correlation, $$, within the H-F approximation. We introduce the pressure-dependence to the hopping integral. Numerical results are compared with experimental data for ZrZn$_{2}$. The kinetic correlation creates the superconductivity without help of negative values of Coulomb interactions. The model can explain stimulation of triplet SC by the weak itinerant ferromagnetism. This effect was observed experimentally in ZrZn$_{2}$. Numerical analysis also confirms the experimental effect of decrease in critical temperatures (Curie and superconducting) with increasing external pressure.",0811.0278v1 2008-11-21,Room temperature ferromagnetism in carbon-implanted ZnO,"Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e. ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.",0811.3487v1 2008-12-02,Hybridization of Spin and Plasma Waves in Josephson Tunnel Junctions Containing a Ferromagnetic Layer,"We study dynamics of tunnel Josephson junctions with a thin ferromagnetic layer F [superconductor-insulator-ferromagnet-superconductor (SIFS) junctions]. On the basis of derived equations relating the superconducting phase and magnetic moment to each other we analyze collective excitations in the system and find a new mode which is a hybrid of plasmalike and spin waves. The latter are coupled together in a broad range of parameters characterizing the system. Using the solution describing the collective modes we demonstrate that besides the Fiske steps new peaks appear on the I-V characteristics due to oscillations of the magnetic moment M in the ferromagnetic layer. Thus, by measuring the I-V curve of the SIFS junctions, one can extract information about the spectrum of spin excitations in the ferromagnet F.",0812.0512v3 2008-12-10,Possible d0 ferromagnetism in MgO doped with nitrogen,"We study the possibility of d0 ferromagnetism in the compound MgO doped with nitrogen (N). The Haldane-Anderson impurity model is formulated within the tight-binding approximation for determining the host band-structure and the impurity-host hybridization. Using the quantum Monte Carlo technique, we observe a finite local moment for an N impurity, and long-range ferromagnetic correlations between two N impurities. The ferromagnetic correlations are strongly influenced by the impurity bound state. When the ferromagnetic correlation between a pair of impurities is mapped onto the isotropic Heisenberg model for two spin-1/2 particles, the effective exchange constant J12 is found to increase with increasing temperature. Similar temperature dependence of J12 is also obtained in other diluted magnetic semiconductors, such as zincblende ZnO doped with Mn. The temperature dependence of J12 suggests that the mapping of the full Hamiltonian onto the spin Hamiltonian cannot fully describe the magnetic correlations for the diluted magnetic semiconductors at least in the limit of low impurity spin.",0812.1836v1 2008-12-15,Polaron Exchange Model for Ferromagnetic Ordering in Manganite Films,"In doped manganites, the strong electron-phonon coupling due to the Jahn-Teller effect localizes the conduction-band electrons as polarons. This results in polarons are carriers responsible for transport and ferromagnetic ordering rather than the bare eg electrons, and sequentially polaron exchange model is emerged for describing ferromagnetic ordering. In Pr0.7(Sr1-xCax)0.3MnO3(x=0.3-0.6) epitaxial thin films, for higher-temperature paramagnetic state and lower-temperature ferromagnetic state, both the temperature dependent transports present behaviors of small polaron; for paramagnetic-ferromagnetic transition, the experimental data of Curie temperature are well described by an energy balance expression induced by polaron exchange model. These results demonstrate that the polaron models are proper ways to describe the strongly correlated electrons in the doped manganites.",0812.2722v1 2009-01-28,Variational Monte Carlo study of ferromagnetism in the two-orbital Hubbard model on a square lattice,"To understand effects of orbital degeneracy on magnetism, in particular effects of Hund's rule coupling, we study the two-orbital Hubbard model on a square lattice by a variational Monte Carlo method. As a variational wave function, we consider a Gutzwiller projected wave function for a staggered spin and/or orbital ordered state. We find a ferromagnetic phase with staggered orbital order around quarter-filling, i.e., electron number n=1 per site, and an antiferromagnetic phase without orbital order around half-filling n=2. In addition, we find that another ferromagnetic phase without orbital order realizes in a wide filling region for large Hund's rule coupling. These two ferromagnetic states are metallic except for quarter filling. We show that orbital degeneracy and strong correlation effects stabilize the ferromagnetic states.",0901.4415v1 2009-04-03,Muon spin rotation and relaxation in the superconducting ferromagnet UCoGe,"We report zero-field muon spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon spin precession frequency below the Curie temperature $T_C = 3$ K. The $\mu^+$ precession frequency persists below the bulk superconducting transition temperature $T_{sc} = 0.5$ K, where it measures a local magnetic field $B_{loc} = 0.015$ T. The amplitude of the $\mu$SR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.",0904.0532v1 2009-05-29,Ultracold Gases of Ytterbium: Ferromagnetism and Mott States in an SU(6) Fermi System,"It is argued that ultracold quantum degenerate gas of ytterbium $^{173}$Yb atoms having nuclear spin $I = 5/2$ exhibits an enlarged SU$(6)$ symmetry. Within the Landau Fermi liquid theory, stability criteria against Fermi liquid (Pomeranchuk) instabilities in the spin channel are considered. Focusing on the SU$(n > 2)$ generalizations of ferromagnetism, it is shown within mean-field theory that the transition from the paramagnet to the itinerant ferromagnet is generically first order. On symmetry grounds, general SU$(n)$ itinerant ferromagnetic ground states and their topological excitations are also discussed. These SU$(n > 2)$ ferromagnets can become stable by increasing the scattering length using optical methods or in an optical lattice. However, in an optical lattice at current experimental temperatures, Mott states with different filling are expected to coexist in the same trap, as obtained from a calculation based on the SU$(6)$ Hubbard model.",0905.4948v3 2009-06-09,Stable half-metallic ferromagnetism in nonstoichiometric cubic binary chromium chalcogenides,"We find that three nonstoichiometric cubic binary chromium chalcogenides, namely Cr3S4, Cr3Se4, and Cr3Te4, are stable half-metallic ferromagnets with wide half-metallic gaps on the basis of systematic state-of-the-arts first-principles calculations. We optimize their structures, and then calculate their magnetic moments, electronic structures, formation heats, and elastic moduli and investigate their structural stability and robustness of ferromagnetism against antiferromagnetic fluctuations. Our calculated results show that the three sulvanite phases are structurally stable and ferromagnetically robust, and hence could be realized as epitaxial thin films. We attribute the structural and ferromagnetic stability and the better half-metallicity to their special effective Cr valence 2.667+. These findings will open doors for much more high-performance spintronic materials compatible with current semiconductor technology.",0906.1794v2 2009-07-16,Itinerant Ferromagnetism in a Fermi Gas of Ultracold Atoms,"Can a gas of spin-up and spin-down fermions become ferromagnetic due to repulsive interactions? This question which has not yet found a definitive theoretical answer was addressed in an experiment with an ultracold two-component Fermi gas. The observation of non-monotonic behavior of lifetime, kinetic energy, and size for increasing repulsive interactions provides strong evidence for a phase transition to a ferromagnetic state. It implies that itinerant ferromagnetism of delocalized fermions is possible without lattice and band structure and validates the most basic model for ferromagnetism introduced by Stoner.",0907.2888v1 2009-08-17,Enhancement of superconductivity near the ferromagnetic quantum critical point in UCoGe,"We report a high-pressure single crystal study of the superconducting ferromagnet UCoGe. Ac-susceptibility and resistivity measurements under pressures up to 2.2 GPa show ferromagnetism is smoothly depressed and vanishes at a critical pressure $p_c = 1.4$ GPa. Near the ferromagnetic critical point superconductivity is enhanced. Upper-critical field measurements under pressure show $B_{c2}(0)$ attains remarkably large values, which provides solid evidence for spin-triplet superconductivity over the whole pressure range. The obtained $p-T$ phase diagram reveals superconductivity is closely connected to a ferromagnetic quantum critical point hidden under the superconducting `dome'.",0908.2325v1 2009-08-20,Pressure-phase diagram of UCoGe by ac-susceptibility and resistivity measurements,"UCoGe is one of the few compounds showing the coexistence of ferromagnetism and superconductivity at ambient pressure. With T_Curie = 3 K and T_SC = 0.6 K it is near a quantum phase transition; the pressure needed to suppress the magnetism is slightly higher than 1 GPa. We report simultaneous resistivity and ac-susceptibility measurements under pressure on a polycrystal with very large single-crystalline domains and a resistivity ratio of about 6. Both methods confirm the phase diagram established before by resistivity measurements on a polycrystal. The ferromagnetic phase is suppressed for P approximately 1.2 GPa. Astonishingly, the superconductivity persists at pressures up to at least 2.4 GPa. In other superconducting and ferromagnetic heavy fermion compounds like UGe2 and URhGe, the superconducting state is situated only inside the larger ferromagnetic region. Therefore, UCoGe seems to be the first example where superconductivity extends from the ferromagnetic to the paramagnetic region.",0908.2945v1 2009-09-30,Two-Dimensional Spin Dynamics in the Itinerant Ferromagnet LaCoPO Revealed by Magnetization and $^{31}$P-NMR Measurements,"We have performed magnetization and $^{31}$P-NMR measurements on the itinerant ferromagnet LaCoPO (Curie temperature $T_{\rm Curie}\sim 44$ K) with a layered structure in order to investigate spin dynamics in the paramagnetic state. The linear scaling between the Knight shift $K$ at the P site and the bulk susceptibility $\chi$ above $T_{\rm Curie}$ indicates that the P nucleus is suitable for investigating magnetic properties. The temperature and magnetic field dependences of the nuclear spin-lattice relaxation rate divided by the temperature $1/T_1T$ at the P site show characteristic features of itinerant ferromagnets, such as ZrZn$_2$ and Y(Co$_{1-x}$Al$_x$)$_2$. In addition, the relationship between $1/T_1T$ and $\chi$ above $T_{\rm Curie}$ suggests that ferromagnetic fluctuations possess a two-dimensional (2D) characteristic. The present data show that LaCoPO is a unique ferromagnet, where the 2D fluctuations anticipated from the crystal structure are predominant down to almost $T_{\rm Curie}$.",0909.5641v1 2009-10-12,Room-temperature ferromagnetism in graphite driven by 2D networks of point defects,"Ferromagnetism in carbon-based materials is appealing for both applications and fundamental science purposes because carbon is a light and bio-compatible material that contains only s and p electrons in contrast to traditional ferromagnets based on 3d or 4f electrons. Here we demonstrate direct evidence for ferromagnetic order locally at defect structures in highly oriented pyrolytic graphite (HOPG) with magnetic force microscopy and in bulk magnetization measurements at room temperature. Magnetic impurities have been excluded as the origin of the magnetic signal after careful analysis supporting an intrinsic magnetic behavior of carbon. The observed ferromagnetism has been attributed to originate from unpaired electron spins localized at grain boundaries of HOPG. Grain boundaries form two-dimensional arrays of point defects, where their spacing depends on the mutual orientation of two grains. Depending on the distance between these point defects, scanning tunneling spectroscopy of grain boundaries showed two intense split localized states for small distances between defects (< 4 nm) and one localized state at the Fermi level for large distances between defects (> 4 nm).",0910.2130v1 2009-11-02,Inverse Spin-Galvanic Effect in a Topological-Insulator/Ferromagnet Interface,"When a ferromagnet is deposited on the surface of a topological insulator the topologically protected surface state develops a gap and becomes a 2-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a large effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.",0911.0106v1 2009-11-09,High frequency susceptibility of a weak ferromagnet with magnetostrictive magnetoelectric coupling: using heterostructures to tailor electromagnon frequencies,"In the first part of this work we calculate the high frequency magnetoelectric susceptibility of a simultaneously ferroelectric and canted antiferromagnetic (also know as weak ferromagnetic) thin film with magnetostrictive magnetoelectric coupling. We show that a dynamic coupling exists between the ferroelectric and optic antiferromagnetic excitations. In the second part of the paper, we calculate using an effective medium method the susceptibility of a heterostructure comprising alternating thin films of such a material together with a ferromagnet. Dipolar magnetic fields serve to couple the ferromagnetic and optic antiferromagnetic modes, which in turn couples the ferromagnetic and ferroelectric excitations. This provides a mechanism for creating ""electromagnon"" modes in the microwave regime which may be useful for applications.",0911.1577v1 2009-11-15,Effect of three-body loss on itinerant ferromagnetism in an atomic Fermi gas,"A recent experiment has provided the first evidence for itinerant ferromagnetism in an ultracold atomic gas of fermions with repulsive interactions. However, the gas in this regime is also subject to significant three-body loss. We adopt an extended Hertz-Millis theory to account for the effect of loss on the transition and on the ferromagnetic state. We find that the losses damp quantum fluctuations and thereby significantly increase the critical interaction strength needed to induce ferromagnetism. This effect may resolve a discrepancy between the experiment and previous theoretical predictions of the critical interaction strength. We further illuminate the impact of loss by studying the collective spin excitations in the ferromagnet. Even in the fully polarized state, where loss is completely suppressed, spin waves acquire a decay rate proportional to the three-body loss coefficient.",0911.2839v1 2010-01-27,The development of ferromagnetism in the doped topological insulator Bi2-xMnxTe3,"The development of ferromagnetism in Mn-doped Bi2Te3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi2-xMnxTe3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi1.96Mn0.04Te3 and Bi1.91Mn0.09Te3 a second order ferromagnetic transition is observed, with Tc ~ 9-12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi2Te3 crystals are p-type. Angle resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi2Te3 are also present in ferromagnetic Mn-doped Bi2-xMnxTe3, and that the dispersion relations of the surface states are changed in a subtle fashion.",1001.4834v1 2010-02-07,\textit{Additional} carrier-mediated ferromagnetism in GdN,"The mechanism behind ferromagnetic exchange interaction in GdN is not well understood. It has been argued that it can be due to fourth order cross process of \textit{d-f} mixing and \textit{d-f} exchange. An alternative explanation suggests an anti- ferromagnetic interaction between Gd \textit{d} and N \textit{p} induced moments on the rock salt structure which aligns the nearest neighbor Gd \textit{f} moments ferromagnetically through the \textit{d-f} exchange. In this paper we present results of Curie temperature in GdN as a function of carrier density calculated within our multiband modified RKKY- like exchange interaction. It includes realistic bandstructure of the 5\textit{d} conduction band as an input for single particle energies. We analyze the possibility of carrier- mediated ferromagnetism in GdN and also demonstrate a simple phenomenological model which justifies the role of charge carriers.",1002.1426v1 2010-02-11,Spin drag in an ultracold Fermi gas on the verge of a ferromagnetic instability,"Recent experiments [Jo et al., Science 325, 1521 (2009)] have presented evidence of ferromagnetic correlations in a two-component ultracold Fermi gas with strong repulsive interactions. Motivated by these experiments we consider spin drag, i.e., frictional drag due to scattering of particles with opposite spin, in such systems. We show that when the ferromagnetic state is approached from the normal side, the spin drag relaxation rate is strongly enhanced near the critical point. We also determine the temperature dependence of the spin diffusion constant. In a trapped gas the spin drag relaxation rate determines the damping of the spin dipole mode, which therefore provides a precursor signal of the ferromagnetic phase transition that may be used to experimentally determine the proximity to the ferromagnetic phase.",1002.2371v2 2010-02-18,Ferromagnetic transition in the double-exchange model on the pyrochlore lattice,"The double-exchange model, which has been extensively studied in the context of colossal magneto-resistance in perovskite manganese oxides, is known to exhibit a ferromagnetic metallic state at low temperatures because of the interplay between localized moments and itinerant electrons through the Hund's-rule coupling. Here we investigate numerically the ferromagnetic transition in the double-exchange model defined on the frustrated pyrochlore lattice as a simple model for ferromagnetic pyrochlore oxides. We demonstrate that the finite-size corrections are largely reduced by implementing averages over the twisted boundary conditions in the Monte Carlo simulation, which enables to estimate the ferromagnetic transition temperature in relatively small size clusters. The estimate is compared with that for the non-frustrated cubic lattice system.",1002.3642v1 2010-02-25,In-Plane Magnetoresistance on the Surface of Topological Insulator,"We study the tunneling magneto-transport properties of the Ferromagnetic Insulator-Normal Insulator-Ferromagnetic Insulator(F$\mid$N$\mid$F) and Ferromagnetic Insulator-Barrier Insulator-Ferromagnetic Insulator (F$\mid$B$\mid$F) junctions on the surface of topological insulator in which in-plane magnetization directions of both ferromagnetic sides can be parallel and antiparallel. We derive analytical expressions for electronic conductances of the two mentioned junctions with both parallel and antiparallel directions of magnetization and using them calculate magnetoresistance of the two junctions. We use thin barrier approximation for investigating the F$\mid$B$\mid$F junction. We find that although magnetoresistance of the F$\mid$N$\mid$F and F$\mid$B$\mid$F junctions are tunable by changing the strength of magnetization texture, they show different behaviors with variation of magnetization. In contrast to the magnetoresistance of F$\mid$N$\mid$F, magnetoresistance of F$\mid$B$\mid$F junctions shows very smooth enhance by increasing the strength of magnetization. We suggest an experimental set up to detect our predicted effects.",1002.4882v2 2010-03-18,Magnetoresistance and transistor-like behavior of double quantum dots connected to ferromagnetic and superconductor leads,"The electric current and the magnetoresistance effect are studied in a double quantum-dot system, where one of the dots QDa is coupled to two ferromagnetic electrodes (F1,F2), while the second QDb is connected to a superconductor S. For energy scales within the superconductor gap, electric conduction is allowed by Andreev reflection processes. Due to the presence of two ferromagnetic leads, non-local crossed Andreev reflections are possible. We found that the magnetoresistance sign can be changed by tuning the external potential applied to the ferromagnets. In addition, it is possible to control the current of the first ferromagnet (F1) through the potential applied to the second one (F2). We have also included intradot interaction and gate voltages at each quantum dot and analyzed their influence through a mean field approximation. The interaction reduces the current amplitudes with respect to the non-interacting case, but the switching effect still remains as a manifestation of quantum coherence, in scales of the order of the superconductor coherence length.",1003.3688v2 2010-03-24,Strain-induced half-metallic ferromagnetism in zinc blende CrP/MnP superlattice: First-principles study,"Using first-principles calculations within generalized gradient approximation, the electronic and magnetic properties of zinc blende (zb) CrP/MnP superlattice are investigated. The equilibrium lattice constant is calculated to be $5.33\,$\AA. The stability of ferromagnetic zb CrP/MnP superlattice against antiferromagnetism is considered and it is found that the ferromagnetic CrP/MnP superlattice is more stable than the antiferromagnetic one. It is shown that at the equilibrium lattice constant the CrP/MnP superlattice does not show any half metallicity mainly due to the minority $t_{2g}$ states of Cr and Mn. However, if strain is imposed on the CrP/MnP superlattice then the minority $t_{2g}$ electrons shift to higher energies and the proposed superlattice becomes a half-metal ferromagnet. The effect of tetragonal and orthorhombic distortions on the half metallicity of zb CrP/MnP superlattice is also discussed. It is also shown that InP-CrP/MnP/InP is a true half-metal ferromagnet. The half metallicity and magnetization of these superlattices are robust against tetragonal/ orthorhombic deformation.",1003.5142v2 2010-08-18,Phase-separated Ferromagnetism in Spin-imbalanced Fermi Atoms Loaded on an Optical Ladder: a DMRG study,"We consider repulsively-interacting cold fermionic atoms loaded on an optical ladder lattice in a trapping potential. The density-matrix renormalization-group method is used to numerically calculate the ground state for systematically varied values of interaction U and spin imbalance p in the Hubbard model on the ladder. The system exhibits rich structures, where a fully spin polarized phase, spatially separated from other domains in the trapping potential, appears for large enough U and p. The phase-separated ferromagnetism can be captured as a real-space image of the energy gap between the ferromagnetic and other states arising from a combined effect of Nagaoka's ferromagnetism extended to the ladder and the density dependence of the energy separation between competing states. We also predict how to maximize the ferromagnetic region.",1008.3055v2 2010-09-07,Three dimensional instability of flexible ferromagnetic filament loop,"Dynamics of flexible ferromagnetic filaments in an external magnetic field is considered. We report the existence of a buckling instability of the ferromagnetic filament at the magnetic field reversion, which leads to the formation of a metastable loop. Its relaxation through three dimensional transformation of the configurations is observed experimentally and confirmed by numerical simulations. Bending modulus of the flexible ferromagnetic filaments synthesized by linking micron size core-shell ferromagnetic particles with DNA fragments is estimated by comparison of the parameters of the loops observed in the experiment with theoretical calculations. Formation of the loop and its relaxation are characterized by the numerically calculated writhe number. The relaxation time of the loop allows us to estimate the hydrodynamic drag of the filament.",1009.1276v1 2010-09-09,Antiferromagnetic Order and Phase Coexistence in Antisite Disordered Double Perovskites,"In addition to the well known ferromagnetism, double perovskites are also expected to exhibit antiferromagnetic (AF) order driven by electron delocalisation. This has been seen in model Hamiltonian studies and confirmed via ab initio calculations. The AF phases should occur, for example, on sufficient electron doping of materials like Sr_2FeMoO_6 (SFMO) via La substitution for Sr. Clear experimental indication of such AF order is limited, possibly because of increase in antisite disorder with La doping on SFMO, although intriguing signatures of non ferromagnetic behaviour are seen. We study the survival of electronically driven antiferromagnetism in the presence of spatially correlated antisite disorder and extract the signals in magnetism and transport. We discover that A and G type AF order, that is predicted in the clean limit, is actually suppressed less strongly than ferromagnetism by antisite disorder. The AF phases are metallic, and, remarkably, more conducting that the ferromagnet for similar antisite disorder. We also highlight the phase coexistence window that connects the ferromagnetic regime to the A type antiferromagnetic phase.",1009.1709v1 2010-10-08,Angular Dependence of the Superconducting Transition Temperature in Ferromagnet-Superconductor-Ferromagnet Trilayers,"The superconducting transition temperature, $T_c$, of a ferromagnet (F) - superconductor (S) - ferromagnet trilayer depends on the mutual orientation of the magnetic moments of the F layers. This effect has been previously observed in F/S/F systems as a $T_c$ difference between parallel and antiparallel configurations of the F layers. Here we report measurements of $T_c$ in CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments of the CuNi ferromagnets. The observed angular dependence of $T_c$ is in qualitative agreement with a F/S proximity theory that accounts for the odd triplet component of the condensate predicted to arise for non-collinear orientation of the magnetic moments of the F layers.",1010.1752v1 2010-11-02,Tricritical point and wing structure in the itinerant ferromagnet UGe2,"Precise resistivity measurements on the ferromagnetic superconductor UGe2 under pressure p and magnetic field H reveal a previously unobserved change of the anomaly at the Curie temperature. Therefore, the tricritical point (TCP) where the paramagnetic-to-ferromagnetic transition changes from a second order to a first order transition is located in the p-T phase diagram. Moreover, the evolution of the TCP can be followed under the magnetic field in the same way. It is the first report of the boundary of the first order plane which appears in the p-T-H phase diagram of weak itinerant ferromagnets. This line of critical points starts from the TCP and will terminate at a quantum critical point. These measurements provide the first estimation of the location of the quantum critical point in the p-H plane and will inspire similar studies of the other weak itinerant ferromagnets.",1011.0754v2 2010-12-06,Diamagnetic critical singularity in unconventional ferromagnetic superconductors,"The scaling properties of the free energy, the diamagnetic moment, and the diamagnetic susceptibility above the phase transition from the ferromagnetic phase to the phase of coexistence of ferromagnetic order and superconductivity in unconventional ferromagnetic superconductors with spin-triplet (p-wave) electron paring are considered. The crossover from weak to strong magnetic induction is described for both quasi-2D (thin films) and 3D (bulk) superconductors. The singularities of diamagnetic moment and diamagnetic susceptibility are dumped for large variations of the pressure and, hence, such singularities could hardly be observed in experiments. The results are obtained within Gaussian approximation on the basis of general theory of ferromagnetic superconductors with p-wave electron pairing.",1012.1205v3 2011-01-03,Effect of size reduction on the ferromagnetism of the manganite La1-xCaxMnO3 (x = 0.33),"In this paper we report an investigation on the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of $\mathrm{La_{0.67}Ca_{0.33}MnO_3}$ using magnetic measurements and neutron diffraction. The investigation was made on nanoparticles with crystal size down to $15$ nm. The neutron data show that even down to a size of $15$ nm the nanoparticles show finite spontaneous magnetization ($M_S$) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic to paramagnetic transition temperature $T_C$ with size $d$ and found that $T_C$ initially enhances on size reduction, but for $d < 50$ nm it decreases again. The initial enhancement in $T_C$ was related to an increase in the bandwidth that occured due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined form the neutron data. The size reduction also changes the nature of the ferromagnetic to paramagnetic transition from first order to second order with critical exponents approaching mean field values. This was explained as arising from a truncation of the coherence length by the finite sample size.",1101.0494v1 2011-01-21,Asymmetric nano graphene model applied to graphite-like room-temperature ferromagnetism,"Room temperature ferromagnetic materials composed only by light elements like carbon, hydrogen and/or nitrogen, so called carbon magnet, are very attractive for creating new material categories both in science and industry. Recently several experiments suggest ferromagnetic features at a room temperature, especially in graphite base materials. This paper reveals a mechanism of such ferromagnetic features by modeling nanometer size asymmetric graphene molecule by using both a semi-empirical molecular orbital method and a first principle density function theory. Asymmetrically dihydrogenated zigzag edge graphene molecule shows that high spin state is more stable in total molecular energy than low spin state. Proton ion irradiation play an important role to create such asymmetric features. Also, nitrogen contained graphite ferromagnetism is explained by a similar asymmetric molecule model.",1101.4079v1 2011-01-31,"Electronic structure, magnetism and spin fluctuations in the superconducting weak ferromagnet Y4Co3","Results of the first principles study on the electronic structure and magnetism of the superconducting weak ferromagnet Y$_4$Co$_3$, are presented. Using the full potential Korringa-Kohn-Rostoker (FP-KKR) method, densities of states, dispersion curves and magnetic moments were calculated for quasi-ordered structural model of the compound in the framework of the local density approximation. Spin-polarized KKR calculations confirm that weak ferromagnetic properties of Y$_4$Co$_3$ can be attributed to only one cobalt atom located on (2b) site in the unit cell, while other twenty Co and Y atoms acts as a diamagnetic environment. Moreover, the magnetic Co atoms form a quasi-one-dimensional chains along $z$ direction. The magnitude of Co(2b) magnetic moment ($0.55~\mu_B$) markedly overestimates the experimental value (0.23~$\mu_B$), which suggests the importance of spin fluctuations in this system. Calculated distribution of spin magnetization in the unit cell provides a background for discussion of the coexistence of ferromagnetism and superconductivity in Y$_4$Co$_3$. Finally, the effect of pressure on magnetism is discussed and compared with experimental data, also supporting weak ferromagnetic behaviors in the system.",1101.5932v1 2011-02-17,Ferromagnetic instability and finite-temperature properties of two-dimensional electron systems with van Hove singularities,"We study a ferromagnetic tendency in the two-dimensional Hubbard model near van Hove filling by using a functional renormalization-group method. We compute temperature dependences of magnetic susceptibilities including incommensurate magnetism. The ferromagnetic tendency is found to occur in a dome-shaped region around van Hove filling with an asymmetric property: incommensurate magnetism is favored near the edge of the dome above van Hove filling whereas a first-order-like transition to the ferromagnetic ground state is expected below van Hove filling. The dome-shaped phase diagram is well captured in the Stoner theory by invoking a smaller Coulomb interaction. Triplet p-wave superconductivity tends to develop at low temperatures inside the dome and extends more than the ferromagnetic region above van Hove filling.",1102.3606v1 2011-04-18,The Kondo effect in magnetic impurities and ferromagnetic contacts,"Planar macroscopic magnetic tunnel junctions exhibit well defined zero bias anomalies when a thin layer of ferromagnetic CoFe(B) nanodots is inserted within a MgO based tunnel barrier. The conductance curves exhibit a single and a double peak, respectively, for anti-parallel and parallel alignment of the magnetizations of the electrodes which sandwich the tunnel barrier. This leads to a suppression of the tunneling magnetoresistance near zero bias. We show that the double peak structure indicates that the zero-bias anomaly is spin-split due to a magnetic exchange interaction between the magnetic nanodots and the ferromagnetic electrodes. Using a model based on an Anderson quantum dot coupled to ferromagnetic leads, we show that these results imply the coexistence of a Kondo effect and ferromagnetism.",1104.3403v1 2011-06-15,Dissipative hydrodynamic equation of a ferromagnetic Bose-Einstein condensate: Analogy to magnetization dynamics in conducting ferromagnets,"The hydrodynamic equation of a spinor Bose-Einstein condensate (BEC) gives a simple description of spin dynamics in the condensate. We introduce the hydrodynamic equation of a ferromagnetic BEC with dissipation originating from the energy dissipation of the condensate. The dissipative hydrodynamic equation has the same form as an extended Landau-Lifshitz-Gilbert (LLG) equation, which describes the magnetization dynamics of ferromagnets interacting with spin-polarized currents. Employing the dissipative hydrodynamic equation, we demonstrate the magnetic domain pattern dynamics of a ferromagnetic BEC in the presence and absence of a current of particles, and discuss the effects of the current on domain pattern formation. We also discuss the characteristic lengths of domain patterns that have domain walls with and without finite magnetization.",1106.2876v4 2011-06-29,"Ferromagnetism, spiral magnetic structures and phase separation in the two-dimensional Hubbard model","The quasistatic approximation and equation-of-motion decoupling for the electron Green's functions are applied to trace the effect of electronic dispersion and electron correlations on the ferromagnetism of two-dimensional itinerant-electron systems. It is found that next-nearest-neighbor hopping t' is of crucial importance for ferromagnetism formation yielding the magnetic phase diagram which is strongly asymmetric with respect to half-filling. At small t' in the vicinity of half-filling the ferromagnetic phase region is restricted by the spin-density wave instability, and far from half-filling by one-particle (spin-polaron) instability. At t' close to t/2 ferromagnetism is stabilized at moderate Hubbard U due to substantial curvature of the Fermi surface which passes in the vicinity of the van Hove singularity points. The results obtained are of possible importance for high-T_c compounds and layered ruthenates.",1106.5842v2 2011-09-29,Orbital Ferromagnetism and Quantum Collapse in Stellar Plasmas,"The possibility of quantum collapse and characteristics of nonlinear localized excitations is examined in dense stars with Landau orbital ferromagnetism in the framework of conventional quantum magnetohydrodynamics (QMHD) model including Bohm force and spin-orbit polarization effects. Employing the concepts of effective potential and Sagdeev pseudopotential, it is confirmed that the quantum collapse and Landau orbital ferromagnetism concepts are consistent with the magnetic field and mass-density range present in some white dwarf stars. Furthermore, the value of ferromagnetic-field found in this work is about the same order of magnitude as the values calculated earlier. It is revealed that the magnetosonic nonlinear propagations can behave much differently in the two distinct non-relativistic and relativistic degeneracy regimes in a ferromagnetic dense astrophysical object. Current findings should help to understand the origin of the most important mechanisms such as gravitational collapse and the high magnetic field present in many compact stars.",1109.6674v1 2011-10-05,Molecule Induced Strong Exchange Coupling between Ferromagnetic Electrodes of a Magnetic Tunnel Junction,"Multilayer edge molecular spintronics device (MEMSD) approach can produce novel logic and memory units for the computers. MEMSD are produced by bridging the molecular channels across the insulator, in the exposed edge region(s) of a magnetic tunnel junction (MTJ). The bridged molecular channels start serving as the dominant exchange coupling medium between the two ferromagnetic electrodes of a MTJ. Present study focus on the effect of molecule enhanced exchange coupling on the magnetic properties of the MTJ. This paper shows that organometallic molecular clusters (OMCs) strongly increased the magnetic coupling between the two ferromagnetic electrodes. SQUID magnetometer showed that OMCs transformed the typical hysteresis magnetization curve of a Co/NiFe/AlOx/NiFe MTJ into linear one. Ferromagnetic resonance studies showed that OMC bridges affected the two fundamental resonance peaks of the Co/NiFe/AlOx/NiFe MTJ. According to magnetic force microscopy, OMCs caused the disappearance of magnetic contrast from the Co/NiFe/AlOx/NiFe tunnel junction area. These three independent and complimentary experiments, suggested the development of extremely strong interlayer exchange coupling. This work delineated a practical route to control the exchange coupling between ferromagnetic electrodes. Ability to tailor magnetic coupling can lead to the development of molecule based quantum computation device architecture.",1110.0885v1 2012-02-23,Spin current and polarization reversal through a single-molecule magnet with ferromagnetic electrodes,"We theoretically study the spin-polarized transport through a single-molecule magnet, which is weakly coupled to ferromagnetic leads, by means of the rate-equation approach. We consider both the ferromagnetic and antiferromagnetic exchange-couplings between the molecular magnet and transported electron-spin in the nonlinear tunneling regime. For the ferromagnetic exchangecoupling, spin current exhibits step- and basin-like behaviors in the parallel and antiparallel configurations respectively. An interesting observation is that the polarization reversal of spin-current can be realized and manipulated by the variation of bias voltage in the case of antiferromagnetic exchange-coupling with antiparallel lead-configuration, which may be useful in the development of spintronic devices, while the bias voltage can only affect the magnitude of spin-polarization in the ferromagnetic coupling.",1202.5173v1 2012-02-24,Quantum Griffiths singularities in ferromagnetic metals,"We present a theory of the quantum Griffiths phases associated with the ferromagnetic quantum phase transition in disordered metals. For Ising spin symmetry, we study the dynamics of a single rare region within the variational instanton approach. For Heisenberg symmetry, the dynamics of the rare region is studied using a renormalization group approach. In both cases, the rare region dynamics is even slower than in the usual quantum Griffiths case because the order parameter conservation of an itinerant ferromagnet hampers the relaxation of large magnetic clusters. The resulting quantum Griffiths singularities in ferromagnetic metals are stronger than power laws. For example, the low-energy density of states $\rho(\epsilon)$ takes the asymptotic form $\exp[\{-\tilde{\lambda}\log (\epsilon_0/\epsilon)\}^{3/5}]/\epsilon$ with $\tilde{\lambda}$ being non-universal. We contrast these results with the antiferromagnetic case in which the systems show power-law quantum Griffiths singularities in the vicinity of the quantum critical point. We also compare our result with existing experimental data of ferromagnetic alloy ${\rm{Ni}}_{x}{\rm{V}}_{1-x}$.",1202.5562v3 2012-06-01,d0 Ferromagnetic Interface Between Non-magnetic Perovskites,"We use computational and experimental methods to study d0 ferromagnetism at a charge- imbalanced interface between two perovskites. In SrTiO3/KTaO3 superlattice calculations, the charge imbalance introduces holes in the SrTiO3 layer, inducing a d0 ferromagnetic half-metallic 2D electron gas at the interface oxygen 2p orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a gen- eral property of hole-type d0 perovskite interfaces. Atomically sharp epitaxial d0 SrTiO3/KTaO3, SrTiO3 /KNbO3 and SrTiO3 /NaNbO3 interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest the behavior is due to high density of states and exchange coupling at the oxygen t1g band in comparison with the more studied d band t2g symmetry electron gas.",1206.0140v2 2012-06-10,High bias anomaly in YBa2Cu3O7/LaMnO_{3+delta}/YBa2Cu3O7 Superconductor/Ferromagnetic Insulator/Superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator,"We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+\delta}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $T keeps the same, but the bond-length variance varies, we are able to demonstrate a parabolic curve of Tc versus mean bond length . A much higher Tc ~ 177 K than that found in orthorhombic SrRuO3 can be obtained from the curve at a bond length which makes the geometric factor t = /(sqrt(2)) = 1. This new result reveals not only that the ferromagnetism in the ruthenates is extremely sensitive to the lattice strain, but also that it has an important implication for exploring the structure-property relationship in a broad range of oxides with perovskite or perovskite-related structure.",1309.3359v1 2013-09-20,"Control of ferromagnetism by manipulating the carrier wavefunction in ferromagnetic semiconductor (In,Fe)As quantum wells","We demonstrated the control of ferromagnetism in a surface quantum well containing a 5-nm-thick n-type ferromagnetic semiconductor (In,Fe)As layer sandwiched between two InAs layers, by manipulating the carrier wavefunction. The Curie temperature (Tc) of the (In,Fe)As layer was effectively changed by up to 12 K ({\Delta}Tc/Tc = 55%). Our calculation using the mean-field Zener theory reveals an unexpectedly large s-d exchange interaction in (In,Fe)As. Our results establish an effective way to control the ferromagnetism in quantum heterostructures of n-type FMSs, as well as require reconsideration on the current understanding of the s-d exchange interaction in narrow gap FMSs.",1309.5283v1 2013-09-26,Proximity effect of spin orbit coupling in Pt/Co2FeAl and Pt/Permalloy bilayers_810571,"Proximity effect of spin orbit coupling is investigated through anomalous Hall effect in Pt/Co2FeAl and Pt/Permalloy bilayers. A series of nontrivial magnetotransport behaviors, resulting from a strong impact of phonons on skew scattering, is observed in these films with ultrathin ferromagnetic layers. The parameters representing skew scattering, side jump and intrinsic contributions are dramatically enhanced when the ferromagnetic layer is very thin, and they have clear linear dependences on the reciprocal of ferromagnetic layer thickness, indicating a powerful influence of Pt/Ferromagnet interface. Further study on Cu/Co2FeAl and Ta/Co2FeAl bilayers reveals that a simple interface scattering without intense spin orbit coupling is not sufficient to trigger such a phenomenon. The proximity effect of spin orbit coupling is thus suggested to occur at the Pt/Ferromagnet interface, and as a result quite large anomalous Hall angle (0.036) and Nernst angle (0.23) are confirmed in the Pt/CFA films at room temperature.",1309.7023v1 2014-04-30,Spin Controlled Coexistence of 0 and π States in SFSFS Josephson Junctions,"Using the Keldysh-Usadel formalism, we theoretically study the $0$-$\pi$ transition profiles and current-phase relations of magnetic $SFSFS$ and $SFSFFS$ Josephson nanojunctions in the diffusive regime. By allowing the magnetizations of the ferromagnetic layers to take arbitrary orientations, the strength and direction of the charge supercurrent flowing through the ferromagnetic regions can be controlled via the magnetization rotation in one of the ferromagnetic layers. Depending on the junction parameters, we find opposite current flow in the ferromagnetic layers, revealing that remarkably such configurations possess well-controlled $0$- and $\pi$-states simultaneously, creating a three-terminal $0$-$\pi$ spin switch. We demonstrate that the spin-controlled $0$-$\pi$ profiles trace back to the proximity induced odd-frequency superconducting correlations generated by the ferromagnetic layers. It is also shown that the spin-switching effect can be more pronounced in $SFSFFS$ structures. The current-phase relations reveal the important role of the middle $S$ electrode, where the spin controlled supercurrent depends crucially on its thickness and phase differences with the outer $S$ terminals.",1405.0012v1 2015-12-27,Topological properties of ferromagnetic superconductors,"A variety of heavy fermion superconductors, such as UCoGe, UGe$_2$, and URhGe exhibit a striking coexistence of bulk ferromagnetism and superconductivity. In these systems, the magnetic moment decreases with pressure, and vanishes at a ferromagnetic quantum critical point (qcp). Remarkably, the superconductivity in UCoGe varies smoothly with pressure across the qcp and exists in both the ferromagnetic and paramagnetic regimes. We argue that in UCoGe, spin-orbit interactions stabilize a time-reversal invariant odd-parity superconductor in the high pressure paramagnetic regime. Based on a simple phenomenological model, we predict that the transition from the paramagnetic normal state to the phase where superconductivity and ferromagnetism coexist, is a first-order transition.",1512.08286v2 2015-12-30,Topological insulator associated with quantum anomalous Hall phase in ferromagnetic perovskite superlattices,"We do a search for topological insulators which are associated with ferromagnetic ordering and show anomalous quantum Hall effect, among transition metal oxide superlattices taking the parent compounds as LaAlO3 and SrTiO3. Among the various superlattices which are studied here, (LaAlO3)10/(LaOsO3)2 exhibits a ferromagnetic ground state with a topologically non-trivial energy gap when a spin-orbit interaction is turned on. The study of transverse conductivity shows that the system has quantized Hall conductivity inside the topological energy gap without applying any external magnetic field. The ferromagnetic order parameters and the ordering temperature (Tc) have been estimated by taking a simple Heisenberg model of ferromagnetism.",1512.08843v1 2016-08-06,Spin Transfer Torque in the Semiconductor/Ferromagnetic Structure in the Presence of Rashba Effect,"Spin transfer torque in magnetic structures occurs when the transverse component of the spin current that flows from the nonmagnetic medium to ferromagnetic medium are absorbed by the interface. In this paper, considering the Rashba effect on semiconductor region, we have discussed the spin transfer torque in semiconductor/ferromagnetic structure and obtained the components of spin-current density for two models: (I)-single electron and (II)- the distribution of electrons. We have shown that whatever the difference between Fermi surfaces in semiconductor and Fermi spheres for the up and down spins in ferromagnetic increase, the transmission probability decreases. The obtained results for the values used in this article illustrate that Rashba effect increases the difference between a Fermi sphere in semiconductors and Fermi sphere for the up and down spins in ferromagnetic. The results also show that the Rashba effect, brings an additional contribution to the components of spin transfer torque, which is not exist in the absence of the Rashba interaction. Moreover, the Rashba term has also different effects on the transverse components of the spin torque transfer.",1608.02129v1 2016-08-09,"Local atomic and magnetic structure of dilute magnetic semiconductor (Ba,K)(Zn,Mn)$_2$As$_2$","We have studied the atomic and magnetic structure of the dilute ferromagnetic semiconductor system (Ba,K)(Zn,Mn)$_2$As$_2$ through atomic and magnetic pair distribution function analysis of temperature-dependent x-ray and neutron total scattering data. We detected a change in curvature of the temperature-dependent unit cell volume of the average tetragonal crystallographic structure at a temperature coinciding with the onset of ferromagnetic order. We also observed the existence of a well-defined local orthorhombic structure on a short length scale of $\lesssim 5$ \AA, resulting in a rather asymmetrical local environment of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic alignment of Mn spins along the crystallographic $c$-axis, with robust nearest-neighbor ferromagnetic correlations that exist even above the ferromagnetic ordering temperature. We discuss these results in the context of other experiments and theoretical studies on this system.",1608.02684v1 2016-12-27,Isotope shift of the ferromagnetic transition temperature in itinerant ferromagnets,"We present a theory of the isotope effect of the Curie temperature $T_{\rm c}$ in itinerant ferromagnets. The isotope effect in ferromagnets occurs via the electron-phonon vertex correction and the effective attractive interaction mediated by the electron-phonon interaction. The decrease of the Debye frequency increases the relative strength of the Coulomb interaction, which results in a positive isotope shift of $T_{\rm c}$ when the mass $M$ of an atom increases. Following this picture, we evaluate the isotope effect of $T_{\rm c}$ by using the Stoner theory and a spin-fluctuation theory. When $T_{\rm c}$ is large enough as large as or more than 100K, the isotope effect on $T_{\rm c}$ can be measurable. Recently, precise measurements on the oxygen isotope effect on $T_{\rm c}$ have been performed for itinerant ferromagnet SrRuO$_3$ with $T_{\rm c}\sim 160$K. A clear isotope effect has been observed with the positive shift of $T_{\rm c}\sim 1$K by isotope substitution ($^{16}O\rightarrow ^{18}O$). This experimental result is consistent with our theory.",1612.08511v1 2018-01-08,Stability of the Nagaoka-type Ferromagnetic State in a $t_{2g}$ Orbital System on a Cubic Lattice,"We generalize the previous exact results of the Nagaoka-type itinerant ferromagnetic states in a three dimensional $t_{2g}$-orbital system to allow for multiple holes. The system is a simple cubic lattice with each site possessing $d_{xy}$, $d_{yz}$, and $d_{xz}$ orbitals, which allow two-dimensional hopping within each orbital plane. In the strong coupling limit of $U\to \infty$, the orbital-generalized Nagaoka ferromagnetic states are proved degenerate with the ground state in the thermodynamic limit when the hole number per orbital layer scales slower than $L^{\frac{1}{2}}$. This result is valid for arbitrary values of the ferromagnetic Hund's coupling $J>0$ and inter-orbital repulsion $V\ge 0$. The stability of the Nagaoka-type state at finite electron densities with respect to a single spin-flip is investigated. These results provide helpful guidance for studying the mechanism of itinerant ferromagnetism for the $t_{2g}$-orbital materials.",1801.02583v2 2018-10-20,Dynamics of a Ferromagnetic Particle Levitated Over a Superconductor,"Under conditions where the angular momentum of a ferromagnetic particle is dominated by intrinsic spin, applied torque is predicted to cause gyroscopic precession of the particle. If the particle is sufficiently isolated from the environment, a measurement of spin precession can potentially yield sensitivity to torque beyond the standard quantum limit. Levitation of a micron-scale ferromagnetic particle above a superconductor is a possible method of near frictionless suspension enabling observation of ferromagnetic particle precession and ultrasensitive torque measurements. We experimentally investigate the dynamics of a micron-scale ferromagnetic particle levitated above a superconducting niobium surface. We find that the levitating particles are trapped in potential minima associated with residual magnetic flux pinned by the superconductor and, using an optical technique, characterize the quasiperiodic motion of the particles in these traps.",1810.08748v5 2019-06-18,Discovery of room temperature ferromagnetism in metal-free organic semiconductors,"Creating magnetic semiconductors that work at warm circumstance is still a great challenge in the physical sciences. Here, we report the discovery of ferromagnetism in the metal-free perylene diimide semiconductor, whose Curie temperature is higher than 400 Kelvin. A solvothermal approach is used to reduce and dissolve the rigid-backbone perylene diimide crystallites, and radical anion aggregates were fabricated by the subsequent self-assembly and oxidation process. Magnetic measurements exhibit the ferromagnetic ordering with the saturated magnetization of 0.48 $\mu_{\rm B}$ per molecule and the appreciable magnetic anisotropy. X-ray magnetic circular dichroism spectra suggest the ferromagnetism stems from $\pi$ orbitals of radicals. Our findings unambitiously demonstrate the long-range ferromagnetic ordering can survive at room temperature in organic semiconductors, although which are intuitively regarded to be nonmagnetic.",1906.07531v2 2019-07-10,Dynamical mean-field study of Vanadium diselenide monolayer ferromagnetism,"To understand the magnetism of VSe$_2$, whose monolayer form has recently been reported to be a room temperature ferromagnet, Here, the combined method of conventional density functional theory with dynamical mean-field theory has been adopted. This higher-level computation method enables us to resolve many of existing controversies and contradictions in between theory and experiment. First of all, this new approach is shown to give the correct magnetic properties of both bulk and two-dimensional limit of VSe$_2$ which demonstrates its superiority to the conventional methods. The results demonstrate that monolayer VSe$_2$ without charge density waves is a ferromagnet with ordering temperature of 250K. From the direct simulation of temperature-dependent magnetic susceptibility and ordered moment, it is shown that its ferromagnetism is clearly two-dimensional in nature. Further, it is shown that this ferromagnetic order is vulnerable to extra charge dopings which provides the important insight to elucidate recent experimental controversies.",1907.04790v3 2019-10-09,Nature vs. Nurture: Dynamical Evolution in Disordered Ising Ferromagnets,"We study the predictability of zero-temperature Glauber dynamics in various models of disordered ferromagnets. This is analyzed using two independent dynamical realizations with the same random initialization (called twins). We derive, theoretically and numerically, trajectories for the evolution of the normalized magnetization and twin overlap as the system size tends to infinity. The systems we treat include mean-field ferromagnets with light-tailed and heavy-tailed coupling distributions, as well as highly-disordered models with a variety of other geometries. In the mean-field setting with light-tailed couplings, the disorder averages out and the limiting trajectories of the magnetization and twin overlap match those of the homogenous Curie--Weiss model. On the other hand, when the coupling distribution has heavy tails, or the geometry changes, the effect of the disorder persists in the thermodynamic limit. Nonetheless, qualitatively all such random ferromagnets share a similar time evolution for their twin overlap, wherein the two twins initially decorrelate, before either partially or fully converging back together due to the ferromagnetic drift.",1910.04144v1 2019-10-15,CrAs monolayer: Low buckled two-dimensional half-metal ferromagnet,"\textit{Ab-initio} calculations based on density functional theory (DFT) are performed to study the structural, electronic, and magnetic properties of two-dimensional (2D) free-standing honeycomb CrAs. We show that CrAs has low buckled stable structure. Magnetic CrAs has larger buckling than non-magnetic CrAs. 2D-CrAs is a ferromagnetic semiconductor for lattice constant $a \leq 3.71$\AA, and above this lattice constant CrAs is a half-metal ferromagnet. 2D-CrAs is shown to be half-metal ferromagnetic with magnetic moment of 3.0$\mu_{\rm{B}}$ per unit cell, at equilibrium structure. The $d_{z}^{2}$ orbital of $e_{g}$ band is completely empty in the spin-down state whereas it is almost occupied in the spin-up state, and the magnetic moment in the $e_{g}$ band is mainly dominated by the $d_{z}^{2}$ orbital of Cr. The $d_{zx}/d_{zy}$ and $d_{xy}$ orbitals of $t_{2g}$ band are partially occupied in the spin-up state and behaves as metal whereas they are insulator in the spin-down state. Phonon calculations confirm the thermodynamic stability of 2D-CrAs. The ferromagnetic (FM) and antiferromagnetic (AFM) interaction between the Cr atoms reveal that the FM state is more stable than the AFM state of 2D-CrAs.",1910.06935v1 2019-10-24,Stabilization of coupled Dzyaloshinskii domain walls in fully compensated synthetic anti-ferromagnets,"We examine the combined effects of interlayer exchange coupling (IEC) and the interfacial Dzyaloshinskii-Moriya Interaction (DMI) on the structure of magnetic domain walls in fully compensated synthetic anti-ferromagnets (SAFs). Ir-based SAFs with ferromagnetic (FM) layers based on [Pt/(Co/Ni)M]N were characterized by Lorentz transmission electron microscopy (LTEM). The multi-layer design of the individual ferromagnetic layers enables control of the interfacial Dzyaloshinskii-Moriya interaction (via 'M') and, in turn, the structure and chirality of domain walls (DWs). We compare the Fresnel-mode LTEM images in SAF designs with only a change in the purported strength of the DMI. The existence of anti-ferromagnetically coupled Dzyaloshinskii domain walls (DWs) in a high DMI SAF is confirmed through application of in-situ perpendicular magnetic field and sample tilt. This conclusion is based on a unique set of conditions required to observe contrast in Fresnel-mode LTEM, which we outline in this document.",1910.11421v1 2020-02-07,Anomalous current-voltage characteristics of SFIFS Josephson junctions with weak ferromagnetic interlayers,"We present a quantitative study of the current-voltage characteristics (CVC) of SFIFS Josephson junctions (S denotes bulk superconductor, F - metallic ferromagnet, I - insulating barrier) with weak ferromagnetic interlayers in the diffusive limit. The problem is solved in the framework of the nonlinear Usadel equations. We consider the case of a strong tunnel barrier such that the left SF and the right FS bilayers are decoupled. We calculate the density of states (DOS) in SF bilayers using a self-consistent numerical method. Then we obtain the CVC of corresponding SFIFS junctions, and discuss their properties for different set of parameters including the thicknesses of ferromagnetic layers, the exchange field, and the magnetic scattering time. We observe the anomalous nonmonotonic CVC behavior in case of weak ferromagnetic interlayers, which we ascribe by DOS energy dependencies in case of small exchange fields in F layers.",2002.02725v1 2020-11-30,Topological superconductivity in tripartite superconductor-ferromagnet-semiconductor nanowires,"Motivated by recent experiments searching for Majorana zero modes in tripartite semiconductor nanowires with epitaxial superconductor and ferromagnetic-insulator layers, we explore the emergence of topological superconductivity in such devices for paradigmatic arrangements of the three constituents. Accounting for the competition between magnetism and superconductivity, we treat superconductivity self consistently and describe the electronic properties, including the superconducting and ferromagnetic proximity effects, within a direct wave-function approach. We conclude that the most viable mechanism for topological superconductivity relies on a superconductor-semiconductor-ferromagnet arrangement of the constituents, in which spin splitting and superconductivity are independently induced in the semiconductor by proximity and superconductivity is only weakly affected by the ferromagnetic insulator.",2012.00055v1 2021-01-08,Appearance of ferromagnetism in Pt(100) ultrathin films originated from quantum-well states,"Ferromagnetism was observed in a Pt(100) ultrathin film deposited on a SrTiO3(100) substrate. The ferromagnetism, which appears in films with thicknesses of 2.2-4.4 nm, periodically changes with a period of approximately 1 nm (5-6 monolayers) depending on the film thickness. This is consistent with the period derived from the quantum-well states formed in the thin film. X-ray magnetic circular dichroism measurements show the evidence of appearance of intrinsic ferromagnetism in Pt(100) ultra-thin film. In addition, present results suggest a possibility that the orbital magnetic moment of pure Pt is much smaller than that of the Pt/ferromagnetic multilayer system. These results will provide a potential new mechanism for origin of the large magnetic anisotropy in Pt components.",2101.02818v4 2017-09-25,Exfoliation and van der Waals heterostructure assembly of intercalated ferromagnet Cr1/3TaS2,"Ferromagnetic van der Waals (vdW) materials are in demand for spintronic devices with all-two-dimensional-materials heterostructures. Here, we demonstrate mechanical exfoliation of magnetic-atom-intercalated transition metal dichalcogenide Cr1/3TaS2 from its bulk crystal; previously such intercalated materials were thought difficult to exfoliate. Magnetotransport in exfoliated tens-of-nanometres-thick flakes revealed ferromagnetic ordering below its Curie temperature TC ~ 110 K as well as strong in-plane magnetic anisotropy; these are identical to its bulk properties. Further, van der Waals heterostructure assembly of Cr1/3TaS2 with another intercalated ferromagnet Fe1/4TaS2 is demonstrated using a dry-transfer method. The fabricated heterojunction composed of Cr1/3TaS2 and Fe1/4TaS2 with a native Ta2O5 oxide tunnel barrier in between exhibits tunnel magnetoresistance (TMR), revealing possible spin injection and detection with these exfoliatable ferromagnetic materials through the vdW junction.",1709.08313v1 2017-11-07,Quantum tricritical point in the temperature-pressure-magnetic field phase diagram of CeTiGe$_3$,"We report the temperature-pressure-magnetic field phase diagram of the ferromagnetic Kondo-lattice CeTiGe$_3$ determined by means of electrical resistivity measurements. Measurements up to $\sim$ 5.8 GPa reveal a rich phase diagram with multiple phase transitions. At ambient pressure, CeTiGe$_3$ orders ferromagnetically at $T_\text{C}$ = 14 K. Application of pressure suppresses $T_\text{C}$, but a pressure induced ferromagnetic quantum criticality is avoided by the appearance of two new successive transitions for $p$ $>$ 4.1 GPa that are probably antiferromagnetic in nature. These two transitions are suppressed under pressure, with the lower temperature phase being fully suppressed above 5.3 GPa. The critical pressures for the presumed quantum phase transitions are $p_1$ $\cong$ 4.1 GPa and $p_2$ $\cong$ 5.3 GPa. Above 4.1 GPa, application of magnetic field shows a tricritical point evolving into a wing structure phase with a quantum tricritical point at 2.8 T at 5.4 GPa, where the first order antiferromagnetic-ferromagnetic transition changes into the second order antiferromagnetic-ferromagnetic transition.",1711.02605v2 2018-03-11,Electromagnetic proximity effect in planar superconductor-ferromagnet structures,"The spread of the Cooper pairs into the ferromagnet in proximity coupled superconductor - ferromagnet (SF) structures is shown to cause a strong inverse electromagnetic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, the characteristic length of the above inverse electrodynamic effect is of the order of the London penetration depth, which usually much larger than the superconducting coherence length. The corresponding spontaneous currents appear even in the absence of the stray field of the ferromagnet and are generated by the vector-potential of magnetization near the S/F interface and they should be taken into account at the design of the nanoscale S/F devices. Similarly to the well-known Aharonov-Bohm effect, the discussed phenomenon can be viewed as a manifestation of the role of vector potential in quantum physics.",1803.03926v2 2018-03-27,Spin flip scattering engendered quantum spin torque in a Josephson junction,"We examine a Josephson junction with two Ferromagnets and a spin flipper sandwiched between two superconductors. In such Ferromagnetic Josephson junctions, equilibrium spin torque exists only when Ferromagnets are misaligned. This is explained via the ""conventional"" mechanism of spin transfer torque, which owes its origin to the misalignment of two Ferromagnets. However, we see surprisingly when the magnetic moments of the Ferromagnets are aligned parallel or antiparallel, there is a finite equilibrium spin torque due to the quantum mechanism of spin-flip scattering. We explore the properties of this unique spin-flip scattering induced equilibrium quantum spin torque, especially its tunability via exchange coupling and phase difference across the superconductors.",1803.10105v2 2018-06-07,Ferromagnet/Semiconductor/Ferromagnet Hybrid Trilayers grown using solid phase epitaxy,"The direct growth of semiconductors over metals by molecular beam epitaxy is a difficult task due to the large differences in crystallization energy between these types of materials. This aspect is problematic in the context of spintronics, where coherent spin-injection must proceed via ballistic transport through sharp interfacial Schottky barriers. We report the realization of single-crystalline ferromagnet/semiconductor/ferromagnet hybrid trilayers using solid-phase epitaxy, with combinations of Fe3Si, Co2FeSi, and Ge. The slow annealing of amorphous Ge over Fe3Si results in a crystalline filmlm identified as FeGe2. When the annealing is performed over Co2FeSi, reflected high-energy electron diffraction and X-ray diffraction indicate the creation of a different crystalline Ge(Co,Fe,Si) compound, which also preserves growth orientation. It was possible to observe independent magnetization switching of the ferromagnetic layers in a Fe3Si/FeGe2/Co2FeSi sample, thanks to the different coercive fields of the two metals and to the quality of the interfaces. This result is a step towards the implementation of vertical spin-selective transistor-like devices.",1806.02733v1 2018-06-11,Competing magnetic correlations across the ferromagnetic quantum critical point in the Kondo system CeTi$_{1-x}$V$_x$Ge$_3$: $^{51}$V NMR as a local probe,"$^{51}$V nuclear magnetic resonance (NMR) and magnetization studies on CeTi$_{1-x}$V$_x$Ge$_3$ have been performed to explore the evolution from the ferromagnetic ($x = 0.113$) to the antiferromagnetic Kondo lattice state ($x = 1$), with focus on the emergence of a possible ferromagnetic quantum critical point (FMQCP) at $x_c \approx 0.4$. From the temperature dependence of the nuclear spin-lattice relaxation rate, $1/T_1T$, and the Knight shift, \textit{K}, for $x=0.113$ and $x=1$ a considerable competition between ferro- and antiferromagnetic correlations is found. Around the critical concentration ($x = 0.35, 0.405$) quantum-critical spin fluctuations entail weak antiferromagnetic spin fluctuations admixed with ferromagnetic spin fluctuations. The FMQCP in CeTi$_{1-x}$V$_x$Ge$_3$ therefore is not purely ferromagnetic in nature.",1806.04111v3 2018-07-31,New room-temperature ferromagnet: B-added Pd0.75Mn0.25 alloy,"Mn-based room-temperature ferromagnets attract considerable attention due to their high ordered Mn moment. We have found that a Pd0.75Mn0.25 alloy with added B (Pd0.75Mn0.25Bx) shows room-temperature soft ferromagnetism, whereas the parent Pd0.75Mn0.25 alloy is a spin-glass system. The saturated Mn moment at room temperature systematically increases to 2.68\muB/Mn as x increases to 0.125. The maximum Curie temperature of 390 K is also realized at an x of 0.125. The experimental results suggest a tunable soft ferromagnetism, which is governed only by the boron concentration. Our results will pave the way in chemical control of room-temperature bulk ferromagnetism in Mn compounds based on the addition of an atom with a small atomic radius.",1807.11640v1 2018-07-31,Spin absorption at ferromagnetic-metal/platinum-oxide interface,"We investigate the absorption of a spin current at a ferromagnetic-metal/Pt-oxide interface by measuring current-induced ferromagnetic resonance. The spin absorption was characterized by the magnetic damping of the heterostructure. We show that the magnetic damping of a Ni$_{81}$Fe$_{19}$ film is clearly enhanced by attaching Pt-oxide on the Ni$_{81}$Fe$_{19}$ film. The damping enhancement is disappeared by inserting an ultrathin Cu layer between the Ni$_{81}$Fe$_{19}$ and Pt-oxide layers. These results demonstrate an essential role of the direct contact between the Ni$_{81}$Fe$_{19}$ and Pt-oxide to induce sizable interface spin-orbit coupling. Furthermore, the spin-absorption parameter of the Ni$_{81}$Fe$_{19}$/Pt-oxide interface is comparable to that of intensively studied heterostructures with strong spin-orbit coupling, such as an oxide interface, topological insulators, metallic junctions with Rashba spin-orbit coupling. This result illustrates strong spin-orbit coupling at the ferromagnetic-metal/Pt-oxide interface, providing an important piece of information for quantitative understanding the spin absorption and spin-charge conversion at the ferromagnetic-metal/metallic-oxide interface.",1807.11806v1 2018-08-03,Definition of the interlayer interaction type in magnetic multilayers analyzing the shape of the ferromagnetic resonance peaks,"We present theoretical study of ferromagnetic resonance in a system of two coupled magnetic layers. We show that an interaction between the layers leads to the occurrence of the so-called Fano resonance. The Fano resonance changes the shape of the ferromagnetic resonance peak. It introduces a peak asymmetry. The asymmetry type is defined by the sign of the interaction between the magnetic layers. Therefore, studying the shape of the ferromagnetic resonance peaks one can define the type of the interlayer coupling (ferromagnetic or antiferromagnetic). We show that using numerical simulations one can estimate a magnitude of the interaction by fitting the asymmetric resonance peaks.",1808.01296v1 2018-08-25,Resonance spin transfer torque in ferromagnetic/normal/ferromagnetic spin-valve structure of topological insulators,"We theoretically study the spin current and spin-transfer torque generation in a conventional spin- valve hybrid structure of type ferromagnetic/normal metal/ferromagnetic (FM/NM/FM) made of the topological insulator (TI), in which a gate voltage is attached to the normal layer. We demonstrate the penetration of the spin-transfer torque into the right ferromagnetic layer and show that, unlike graphene spin-valve junction, the spin-transfer torque in TI is very sensitive to the chemical potential of the NM region. As an important result, by changing the chemical potential of the NM spacer and magnetization directions, one can control all components of the STT. Interestingly, both the resonance spin current and the resonance spin-transfer torque appear for energies determined from a resonance equation. By increasing the chemical potential of the NM spacer, the amplitude of the STTs decreases while at large chemical potentials of $\mu_N$ there are intervals of chemical potential in which both the spin current and the spin-transfer torque become zero. These findings could open new perspectives for applications in spin-transfer torque magnetic random access memory (STT-MRAM) devices based on TI.",1808.08379v1 2018-12-03,The Magnetic Properties of 1111-type Diluted Magnetic Semiconductor (La$_{1-x}$Ba$_{x}$)(Zn$_{1-x}$Mn$_{x}$)AsO in the Low Doping Regime,"We investigated the magnetic properties of (La$_{1-x}$Ba$_{x}$)(Zn$_{1-x}$Mn$_{x}$)AsO with $x$ varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of $x$ $\leq$ 0.01, the system remains paramagnetic down to the lowest measurable temperature of 2 K. Only when the doping level increases to $x$ = 0.02 does the ferromagnetic ordering appear. Our analysis indicates that antiferromagnetic exchange interactions dominate for $x$ $\leq$ 0.01, as shown by the negative Weiss temperature fitted from the magnetization data. The Weiss temperature becomes positive, i.e., ferromagnetic coupling starts to dominate, for $x$ $\geq$ 0.02. The Mn-Mn spin interaction parameter $\mid$$2J/k_B$$\mid$ is estimated to be in the order of 10 K for both $x$ $\leq$ 0.01 (antiferromagnetic ordered state) and $x$ $\geq$ 0.02 (ferromagnetic ordered state). Our results unequivocally demonstrate the competition between ferromagnetic and antiferromagnetic exchange interactions in carrier-mediated ferromagnetic systems.",1812.00824v1 2018-12-05,Strong correlation between ferromagnetic superconductivity and pressure-enhanced ferromagnetic fluctuations in UGe$_2$,"We have measured magnetization at high pressure in the uranium ferromagnetic superconductor UGe$_2$ and analyzed the magnetic data using Takahashi's spin fluctuation theory. There is a peak in the pressure dependence of the width of the spin fluctuation spectrum in the energy space $T_0$ at $P_x$, the phase boundary of FM1 and FM2 where the superconducting transition temperature $T_{sc}$ is highest. This suggests a clear correlation between the superconductivity and pressure-enhanced magnetic fluctuations developed at $P_x$. The pressure effect on $T_{Curie}/T_0$, where $T_{Curie}$ is the Curie temperature, suggests that the less itinerant ferromagnetic state FM2 is changed to a more itinerant one FM1 across $P_x$. Peculiar features in relations between $T_0$ and $T_{sc}$ in uranium ferromagnetic superconductors UGe$_2$, URhGe and UCoGe are discussed in comparison with those in high-$T_c$ cuprate and heavy fermion superconductors.",1812.01771v2 2018-12-14,VI3 - a new layered ferromagnetic semiconductor,"Two-dimensional (2D) materials are promising candidates for next-generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here we report the discovery of ferromagnetism in a layered van der Waals semiconductor, VI3, which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI3-type structure (R-3, No.148) at room temperature, and our experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI3 becomes ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical band gap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.",1812.05982v1 2019-01-21,Non-collinear coupling across RuCo and RuFe alloys,"Spintronic applications, which rely on spin torques for operation, would greatly benefit from a non-collinear alignment between magnetizations of adjacent ferromagnetic layers for maximum performance and reliability. We demonstrate that such an alignment can be created and controlled by coupling two ferromagnetic layers across magnetic coupling layers. These coupling layers consist of a non-magnetic material, Ru, alloyed with ferromagnetic elements of Co or Fe. Changing the composition and thickness of the coupling layer enables control of the relative angle between the magnetizations of ferromagnetic layers between 0 and 180 degrees. The onset of the non-collinear alignment between ferromagnetic layers coincide with the advent of magnetic order in the coupling layer. This study will map the range of concentrations and thicknesses of RuCo and RuFe coupling layers that give rise to non-collinearity between Co layers.",1901.07055v1 2019-02-01,Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport,"The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-pi transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3 K, and the first evidence of an incomplete 0-pi transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations.",1902.00406v1 2019-02-22,A highly sensitive magnetic sensor using a 2D van der Waals ferromagnetic material,"Two-dimensional (2D) van der Waals ferromagnetic materials are emerging as promising candidates for applications in ultra-compact spintronic nanodevices, nanosensors, and information storage. Our recent discovery of the strong room temperature ferromagnetism in single layers of VSe2 grown on graphite or MoS2 substrate has opened new opportunities to explore these ultrathin magnets for such applications. In this paper, we present a new type of magnetic sensor that utilizes the single layer VSe2 film as a highly sensitive magnetic core. The sensor relies in changes in resonance frequency of the LC circuit composed of a soft ferromagnetic microwire coil that contains the ferromagnetic VSe2 film subject to applied DC magnetic fields. The sensitivity of the sensor reaches an extremely high value of 16x10^6 Hz/Oe, making it an excellent candidate for a wide range of magnetic sensing applications.",1902.08365v1 2019-02-22,Strongly Enhanced Gilbert Damping in 3d Transition Metal Ferromagnet Monolayers in Contact with Topological Insulator Bi2Se3,"Engineering Gilbert damping of ferromagnetic metal films is of great importance to exploit and design spintronic devices that are operated with an ultrahigh speed. Based on scattering theory of Gilbert damping, we extend the torque method originally used in studies of magnetocrystalline anisotropy to theoretically determine Gilbert dampings of ferromagnetic metals. This method is utilized to investigate Gilbert dampings of 3d transition metal ferromagnet iron, cobalt and nickel monolayers that are contacted by the prototypical topological insulator Bi2Se3. Amazingly, we find that their Gilbert dampings are strongly enhanced by about one order in magnitude, compared with dampings of their bulks and free-standing monolayers, owing to the strong spin-orbit coupling of Bi2Se3. Our work provides an attractive route to tailoring Gilbert damping of ferromagnetic metallic films by putting them in contact with topological insulators.",1902.08700v1 2019-04-03,Ferromagnetic Instability for single-band Hubbard model in the strong-coupling regime,"We study a ferromagnetic instability in a doped single-band Hubbard model by means of dynamical mean-field theory with the continuous-time quantum Monte Carlo simulations. Examining the effect of the strong correlations in the system on the hypercubic and Bethe lattice, we find that the ferromagnetically ordered state appears in the former, while it does not in the latter. We also reveal that the ferromagnetic order is more stable in the case that the noninteracting DOS exhibits a slower decay in the high-energy region. The present results suggest that, in the strong-coupling regime, the high-energy part of DOS plays an essential role for the emergence of the ferromagnetically ordered state, in contrast to the Stoner criterion justified in the weak interaction limit.",1904.01724v1 2019-04-10,Intrinsic Ferromagnetism in Electrenes,"We report intrinsic ferromagnetism in monolayer electrides or electrenes, in which excess electrons act as anions. Our first-principles calculations demonstrate that magnetism in such electron-rich two-dimensional (2D) materials originates from the anionic electrons rather than partially filled d orbitals, which is fundamentally different from ferromagnetism found in other 2D intrinsic magnetic materials. Taking the honeycomb LaBr$_2$ (La$^{3+}$Br$^{-}_{2}\cdot e^{-}$) as an example, our calculations reveal that the excess electron is localized at the center of the hexagon, which leads to strong Stoner-instability of the associated states at the Fermi energy, resulting in spontaneous magnetization and formation of a local moment. The overlap of extended tails of the wave functions of these electrons mediates a long-range ferromagnetic interaction, contributing to a Curie temperature ($T_\textrm{c}$) of 235 K and a coercive field ($H_\textrm{c}$) of 0.53 T, which can be further enhanced by hole doping. The dual nature, localization and extension, of the electronic states suggests a unique mechanism in such magnetic-element-free electrenes as intrinsic 2D ferromagnets.",1904.04952v1 2019-04-24,Ultra-low-power orbital-controlled magnetization switching using a ferromagnetic oxide interface,"A major challenge in spin-based electronics is reducing power consumption for magnetization switching of ferromagnets, which is being implemented by injecting a large spin-polarized current. The alternative approach is to control the magnetic anisotropy (MA) of the ferromagnet by an electric field. However, the voltage-induced MA is too weak to deterministically switch the magnetization without an assisting magnetic field, and the strategy towards this goal remains elusive. Here, we demonstrate a new scheme of orbital-controlled magnetization switching (OCMS): A sharp change in the MA is induced when the Fermi level is moved between energy bands with different orbital symmetries. Using a ferromagnetic oxide interface, we show that OCMS can be used to achieve a deterministic and magnetic-field-free 90 degree-magnetization switching solely by applying an extremely small electric field of 0.05 V/nm with a negligibly small current density of 10^-2 A/cm^2. Our results highlight the huge potential of band engineering in ferromagnetic materials for efficient magnetization control.",1904.10599v1 2019-08-07,"Structural, Electronic and Magnetic Properties of Bulk and Epitaxial LaCoO$_3$ through Diffusion Monte Carlo","Magnetism in lanthanum cobaltite (LCO, LaCoO$_3$) appears to be strongly dependent on strain, defects, and nanostructuring. LCO on strontium titanate (STO, SrTiO$_3$) is a ferromagnet with an interesting strain relaxation mechanism that yields a lattice modulation. However, the driving force of the ferromagnetism is still controversial. Experiments debate between a vacancy-driven or strain-driven mechanism for the ferromagnetism of epitaxial LCO. We found that a weak lateral modulation of the superstructure is sufficient to promote ferromagnetism. We find that ferromagnetism appears under uniaxial compression and expansion. Although earlier experiments suggest that bulk LCO is nonmagnetic, we find an antiferromagnetic ground state for bulk LCO. We discuss the recent experiments which indicate a more complicated picture for bulk magnetism and a closer agreement with our calculations. Role of defects are also discussed through excited state calculations.",1908.02811v3 2019-08-16,Spin Seebeck effect in the layered ferromagnetic insulators CrSiTe$_3$ and CrGeTe$_3$,"We have studied the longitudinal spin Seebeck effect (LSSE) in the layered ferromagnetic insulators CrSiTe$_3$ and CrGeTe$_3$ covered by Pt films in the measurement configuration where spin current traverses the ferromagnetic Cr layers. The LSSE response is clearly observed in the ferromagnetic phase and, in contrast to a standard LSSE magnet Y$_3$Fe$_5$O$_{12}$, persists above the critical temperatures in both CrSiTe$_3$/Pt and CrGeTe$_3$/Pt samples. With the help of a numerical calculation, we attribute the LSSE signals observed in the paramagnetic regime to exchange-dominated interlayer transport of in-plane paramagnetic moments reinforced by short-range ferromagnetic correlations and strong Zeeman effects.",1908.05807v1 2019-08-25,Anomalous Hall effect mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2,"The recent emergence of two-dimensional (2D) van der Waals ferromagnets has provided a new platform for exploring magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices. Despite intensive studies, the anomalous Hall effect (AHE) mechanisms in 2D van der Waals ferromagnets have not been investigated yet. In this paper, we report the AHE mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2 via systematically measuring Fe0.29TaS2 devices with thickness from 14 nm to bulk single crystal. The AHE mechanisms are investigated via the scaling relationship between the anomalous Hall and channel conductivities. As the Fe0.29TaS2 thickness decreases, the major AHE mechanism changes from extrinsic scattering to intrinsic contribution. The crossover of the AHE mechanisms is found to be highly associated with the channel conductivities as the Fe0.29TaS2 thickness varies.",1908.09280v1 2019-08-26,Phase-controlled spin and charge currents in superconductor-ferromagnet hybrids,"We investigate spin-dependent quasiparticle and Cooper-pair transport through a central node interfaced with two superconductors and two ferromagnets. We demonstrate that voltage biasing of the ferromagnetic contacts induces superconducting triplet correlations on the node and reverses the supercurrent flowing between the two superconducting contacts. We further find that such triplet correlations can mediate a tunable spin current flow into the ferromagnetic contacts. Our key finding is that unequal spin-mixing conductances for the two interfaces with the ferromagnets result in equal-spin triplet correlations on the node, detectable via a net charge current between the two magnets. Our proposed device thus enables the generation, control, and detection of the typically elusive equal-spin triplet Cooper pairs.",1908.09610v1 2019-12-03,Large Resistance Change on Magnetic Tunnel Junction based Molecular Spintronics Devices,"Molecular bridges covalently bonded to two ferromagnetic electrodes can transform ferromagnetic materials and produce intriguing spin transport characteristics. This paper discusses the impact of molecule induced strong coupling on spin transport. To study the molecular coupling effect organometallic molecular complex (OMC) was bridged between two ferromagnetic electrodes of a magnetic tunnel junction (Ta/Co/NiFe/AlOx/NiFe/Ta) along the exposed side edges. OMCs induced strong iter-ferromagnetic electrode coupling to yield drastic changes in transport properties of the magnetic tunnel junction testbed at the room temperature. These OMCs also transformed the magnetic properties of magnetic tunnel junctions. SQUID and ferromagnetic resonance studies provided insightful data to explain transport studies on the magnetic tunnel junction based molecular spintronics devices.",1912.01305v1 2020-04-30,Controlled Two-Dimensional Ferromagnetism in 1T-CrTe$_2$. The role of charge density wave and strain,"Transition metal dichalcogenides are promising candidates to show long-range ferromagnetic order in the single-layer limit. Based on ab initio calculations, we report the emergence of a charge density wave (CDW) phase in monolayer 1T-CrTe$_2$. We demonstrate that this phase is the ground state in the single-layer limit at any strain value. We obtain an optical phonon mode of $1.96$ THz that connects CDW phase with the undistorted 1T phase. Localization of the $a_{1g}$ orbital of CrTe$_2$ produces an out-of-plane orientation of the magnetic moments, circumventing the restrictions of the Mermin-Wagner theorem and producing ferromagnetic long-range order in the two-dimensional limit. This orbital-localization is enhanced by the CDW phase. Tensile strain also increases the localization of this orbital driving the system to become ordered. CrTe$_2$ becomes an example of a material where the CDW phase produces the stabilization of the long-range ferromagnetic order. Our results show that both strain and phase switching are mechanisms to control the 2D ferromagnetic order of CrTe$_2$.",2005.00097v2 2020-05-12,Thermoelectricity carried by proximity-induced odd-frequency pairing in ferromagnet/superconductor junctions,"We explore the role of proximity-induced odd-frequency pairing in the thermoelectricity of a ferromagnet when coupled to a conventional $s$-wave spin-singlet superconductor through a spin-active interface. By varying both the polarization and its direction in the ferromagnet and the interfacial spin-orbit interaction strength, we analyze the behavior of all proximity-induced pair amplitudes in the ferromagnet and their contributions to the thermoelectric coefficients. Based on our results for the Seebeck coefficient, we predict that odd-frequency spin-triplet Cooper pairs are much efficient than the conventional spin-singlet even-frequency pairs in enhancing thermoelectricity of the junction, especially mixed-spin triplet pairing is favorable. Our results on the thermoelectric figure of merit show that ferromagnet/superconductor junctions are very good thermoelectric systems when superconductivity is dominated by odd-frequency pairing.",2005.05950v2 2020-06-03,Superconductivity-induced change in magnetic anisotropy in epitaxial ferromagnet-superconductor hybrids with spin-orbit interaction,"The interaction between superconductivity and ferromagnetism in thin film superconductor/ferromagnet heterostructures is usually reflected by a change in superconductivity of the S layer set by the magnetic state of the F layers. Here we report the converse effect: transformation of the magnetocrystalline anisotropy of a single Fe(001) layer, and thus its preferred magnetization orientation, driven by the superconductivity of an underlying V layer through a spin-orbit coupled MgO interface. We attribute this to an additional contribution to the free energy of the ferromagnet arising from the controlled generation of triplet Cooper pairs, which depends on the relative angle between the exchange field of the ferromagnet and the spin-orbit field. This is fundamentally different from the commonly observed magnetic domain modification by Meissner screening or domain wall-vortex interaction and offers the ability to fundamentally tune magnetic anisotropies using superconductivity - a key step in designing future cryogenic magnetic memories.",2006.02118v2 2020-06-09,The spin-polarized ferromagnetic state of a cold Fermi gas,"The spin-polarized ferromagnetic state of a cold Fermi gas is investigated for interacting and non-interacting charge-neutral and $\beta$-equilibrated gases. The standard minimal couplings between the magnetic field and the fermions' charges and magnetic dipole moments define the fermions' interaction with the magnetic field. Assuming a variable coupling strength between the magnetic field and the fermion (baryon) dipole moments, it is shown that a ferromagnetized state can be achieved that corresponds to a lower energy spin-polarized state with a magnetic field entirely due to the gas's magnetic response. We find that, depending on the density, a very large increase in the baryon dipole moments is needed to achieve this ferromagnetized state. While the required increase seems unlikely, the induced magnetic field is of the order $\sim10^{17}$ gauss. Furthermore, while externally magnetized Fermi gases have an anisotropic pressure, the pressure of the ferromagnetized gas is completely isotropic and the thermodynamically preferred magnetized state.",2006.05247v2 2020-06-17,LiHoF$_4$: Cuboidal Demagnetizing Factor in an Ising Ferromagnet,"The demagnetizing factor has an important effect on the physics of ferromagnets. For cuboidal samples it depends on susceptibility and the historic problem of determining this function continues to generate theoretical and experimental challenges. To test a recent theory, we measure the magnetic susceptibility of the Ising dipolar ferromagnet LiHoF$_4$, using samples of varying aspect ratio, and we reconsider the demagnetizing transformation necessary to obtain the intrinsic material susceptibility. Our experimental results confirm that the microscopic details of the material significantly affect the transformation, as predicted. In particular, we find that the uniaxial Ising spins require a demagnetizing transformation that differs from the one needed for Heisenberg spins and that use of the wrong demagnetizing transformation would result in unacceptably large errors in the measured physical properties of the system. Our results further shed light on the origin of the mysterious `flat' susceptibility of ordered ferromagnets by demonstrating that the intrinsic susceptibility of the ordered ferromagnetic phase is infinite, regardless of sample shape.",2006.10090v1 2020-08-08,Two-magnon frequency-pulling effect in ferromagnetic resonance,"We report the experimental observation in thin films of the hybridization of the uniform ferromagnetic resonance mode with nonuniform magnons as a result of the two-magnon scattering mechanism, leading to a frequency-pulling effect on the ferromagnetic resonance. This effect, when not properly accounted for, leads to a discrepancy in the dependence of the ferromagnetic resonance field on frequency for different field orientations. The frequency-pulling effect is the complement of the broadening of the ferromagnetic resonance lineshape by two-magnon scattering and can be calculated using the same parameters. By accounting for the two-magnon frequency shifts through these means, consistency is achieved in fitting data from in-plane and perpendicular-to-plane resonance conditions.",2008.03423v2 2020-08-11,Itinerant ferromagnetism in the repulsive Hubbard chain with anisotropic odd-wave attraction,"The ground-state properties of the Hubbard chain with on-site repulsion and anisotropic nearest-neighbor attraction are investigated by means of density matrix renormalization group calculations. The non-local attraction acts between fermions of one spin component only, mimicking the effect of p-wave Feshbach resonances in cold-atom systems. We analyze the onset of itinerant ferromagnetism, pinpointing the critical attraction strength where partially and fully ferromagnetic states occur. In the cold-atom setup, where the two (pseudo) spin populations are separately conserved, ferromagnetism occurs with the nucleation of a fully imbalanced band-insulating domain hosting the attractive component only. The size of this domain grows with the attraction strength, therefore increasing the (opposite) imbalance of the other domain, until the two spin components are fully separated. In the presence of a harmonic trap, the ferromagnetic state hosts a partially imbalanced domain in the center with an excess of the attractive component and filling lower than one. This central region is surrounded by fully imbalanced domains, located in the trap tails, hosting only fermions belonging to the other component.",2008.04588v1 2020-08-12,Accelerating the laser-induced demagnetization of a ferromagnetic film by antiferromagnetic order in an adjacent layer,"We study the ultrafast demagnetization of Ni/NiMn and Co/NiMn ferromagnetic/antiferromagnetic bilayer systems after excitation by a laser pulse. We probe the ferromagnetic order of Ni and Co using magnetic circular dichroism in time-resolved pump--probe resonant X-ray reflectivity. Tuning the sample temperature across the antiferromagnetic ordering temperature of the NiMn layer allows to investigate effects induced by the magnetic order of the latter. The presence of antiferromagnetic order in NiMn speeds up the demagnetization of the ferromagnetic layer, which is attributed to bidirectional laser-induced superdiffusive spin currents between the ferromagnetic and the antiferromagnetic layer.",2008.05268v1 2020-09-01,Pseudo spin-valve switch based on ferromagnet/superconductor/ferromagnet trilayer microbridge,"A noticeable magnetoresistive effect has been observed on ferromagnet/superconductor/ferromagnet (FSF) microbridges based on diluted ferromagnetic PdFe alloy containing as small as 1% magnetic atoms. Microstructuring of the FSF trilayers does not destroy the effect: the most pronounced curves were obtained on the smallest bridges of 6-8 um wide and 10-15 um long. Below the superconducting transition we are able to control the critical current of microbridges by switching between P and AP orientations of magnetizations of PdFe layers. The operation of FSF-bridge as a magnetic switch is demonstrated in several regimes providing significant voltage discrimination between digital states or remarkably low bit error rate.",2009.00498v1 2020-09-19,Enhanced Ferromagnetism of CrI3 Bilayer by Self-Intercalation,"Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI3 is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI3. We showed that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI3 bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI3 layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.",2009.09212v1 2020-09-24,Phase diagram of multi-layer ferromagnet system with dipole-dipole interaction,"We investigate various magnetic configurations caused by the dipole-dipole interaction (DDI) in the thin-film magnet with the perpendicular anisotropy under the open boundary conditions. Two different approaches are simulated: one starts from a random magnetic configuration and decreases temperatures step-wisely; the other starts from the saturated out-of-plane ferromagnetic state to evaluate its metastability. As typical patterns of magnetic configuration, five typical configurations are found: an out-of-plane ferromagnetic, in-plane ferromagnetic, vortex, multi-domain, and canted multi-domain states. Notably, the canted multi-domain forms a concentric magnetic-domain-pattern with an in-plane vortex structure, resulting from the open boundary conditions. Concerning to the coercivity, a comparison of the magnetic configurations in both processes reveals that the out-of-plane ferromagnetic state exhibits metastability in the multi-domain state, while not in the vortex state. We also confirm that the so-called Neel-cap magnetic-domain-wall structure, which is originally discussed in the in-plane anisotropy system, appears at the multi-domain state.",2009.11574v1 2020-09-28,Ferromagnetic-electrodes-induced Hall effect in topological Dirac semimetals,"We propose an unconventional type of Hall effect in a topological Dirac semimetal with ferromagnetic electrodes. The topological Dirac semimetal itself has time-reversal symmetry, whereas attached ferromagnetic electrodes break it, causing the large Hall response. This induced Hall effect is a characteristic of the helical surface/edge states that arise in topological materials, such as topological Dirac semimetals or quantum spin Hall insulators. We compute the Hall conductance/resistance and the Hall angle by using a lattice model with four-terminal geometry. For topological Dirac semimetals with four electrodes, the induced Hall effect occurs whether the current electrodes or the voltage electrodes are ferromagnetic. When the spins in electrodes are almost fully polarized, the Hall angle becomes as large as that of quantum Hall states or ideal magnetic Weyl semimetals. We show the robustness of the induced Hall effect against impurities and also discuss the spin injection and spin decay problems. This Hall response can be used to detect whether the magnetizations of the two ferromagnetic electrodes are parallel or antiparallel.",2009.13195v2 2020-09-30,Switching induced by spin Hall effect in an in-plane magnetized ferromagnet with the easy axis parallel to the current,"Magnetization switching in a fine-structured ferromagnet of nanoscale by the spin-transfer torque excited via the spin Hall effect has attracted much attention because it enables us to manipulate the magnetization without directly applying current to the ferromagnet. However, the switching mechanism is still unclear in regard to the ferromagnet having an in-plane easy axis parallel to the current. Here, we develop an analytical theory of the magnetization switching in this type of ferromagnet, and reveal the threshold current formulas for a deterministic switching. It is clarified that the current should be in between a certain range determined by two threshold currents because the spin-transfer torque due to a large current outside the range brings the magnetization in an energetically unstable state, and causes magnetization precession around the hard axis.",2010.00136v1 2020-10-06,Chirality memory stored in magnetic domain walls in the ferromagnetic state of MnP,"Chirality in a helimagnetic structure is determined by the sense of magnetic moment rotation. We found that the chiral information did not disappear even after the phase transition to the high-temperature ferromagnetic phase in a helimagnet MnP. The 2nd harmonic resistivity $\rho^{\rm 2f}$, which reflects the breaking down of mirror symmetry, was found to be almost unchanged after heating the sample above the ferromagnetic transition temperature and cooling it back to the helimagnetic state. The application of a magnetic field along the easy axis in the ferromagnetic state quenched the chirality-induced $\rho^{\rm 2f}$. This indicates that the chirality memory effect originated from the ferromagnetic domain walls.",2010.02491v1 2020-11-05,Engineering the ligand states by surface functionalization: A new way to enhance the ferromagnetism of CrI3,"The newly discovered 2D magnetic materials provide new opportunities for basic physics and device applications. However, their low Curie temperature (TC) is a common weakness. In this paper, by combining magnetic Hamiltonian, Wannier functions and first-principle calculations, we systematically study the magnetic properties of monolayer CrI3 functionalized by halogen. The magnetic exchange coupling (EX) and magnetic anisotropy (MA) are found to increase significantly by X (X=F, Cl and Br) atom adsorption, and increase along with the coverage of X atom. In the frame work of superexchange theory, the enhanced EX can be ascribed to the reduced energy difference and increased hopping strength between Cr d and I p orbitals, due to the states of I ligand are engineered by X adatom. Besides, the X adatom may provide additional ferromagnetic superexchange channel. Finally, the CrI3 that one side is fully adsorbed by F atoms is found to be a room temperature ferromagnetic semiconductor with TC=650 K. Our results not only give an insightful understanding for the enhancement of ferromagnetism of CrI3 by atom adsorption, but also propose a promising way to improve the ferromagnetism of 2D magnetic materials.",2011.02775v1 2021-02-08,Evidence of orbital ferromagnetism in twisted bilayer graphene aligned to hexagonal boron nitride,"We have previously reported ferromagnetism evinced by a large hysteretic anomalous Hall effect in twisted bilayer graphene (tBLG). Subsequent measurements of a quantized Hall resistance and small longitudinal resistance confirmed that this magnetic state is a Chern insulator. Here we report that, when tilting the sample in an external magnetic field, the ferromagnetism is highly anisotropic. Because spin-orbit coupling is negligible in graphene such anisotropy is unlikely to come from spin, but rather favors theories in which the ferromagnetism is orbital. We know of no other case in which ferromagnetism has a purely orbital origin. For an applied in-plane field larger than $5\ \mathrm{T}$, the out-of-plane magnetization is destroyed, suggesting a transition to a new phase.",2102.04039v2 2021-03-09,Self-induced spin-orbit torques in metallic ferromagnets,"We present a phenomenological theory of spin-orbit torques in a metallic ferromagnet with spin-relaxing boundaries. The model is rooted in the coupled diffusion of charge and spin in the bulk of the ferromagnet, where we account for the anomalous Hall effects as well as the anisotropic magnetoresistance in the corresponding constitutive relations for both charge and spin sectors. The diffusion equations are supplemented with suitable boundary conditions reflecting the spin-sink capacity of the environment. In inversion-asymmetric heterostructures, the uncompensated spin accumulation exerts a dissipative torque on the order parameter, giving rise to a current-dependent linewidth in the ferromagnetic resonance with a characteristic angular dependence. We compare our model to recent spin-torque ferromagnetic resonance measurements, illustrating how rich self-induced spin-torque phenomenology can arise even in simple magnetic structures.",2103.05743v2 2021-04-27,Magnon modes as a joint effect of surface ferromagnetism and spin-orbite coupling in CoSi chiral topological semimetal,"CoSi single crystal is a known realization of a chiral topological semimetal with simultaneously broken mirror and inversion symmetries. In addition to the symmetry-induced spin-orbit coupling, surface ferromagnetism is known in nominally diamagnetic CoSi structures, which appears due to the distorted bonds and ordered vacancies near the surface. We experimentally investigate electron transport through a thin CoSi flake at high current density. Surprisingly, we demonstrate $dV/dI(I)$ curves which are qualitatively similar to ones for ferromagnetic multilayers with characteristic $dV/dI$ magnon peaks and unconventional magnetic field evolution of the peaks' positions. We understand these observations as a result of current-induced spin polarization due to the significant spin-orbit coupling in CoSi. Scattering of non-equilibrium spin-polarized carriers within the surface ferromagnetic layer is responsible for the precessing spin-wave excitations, so the observed magnon modes are the joint effect of surface ferromagnetism and spin-orbit coupling in a CoSi chiral topological semimetal. Thus, thin CoSi flakes behave as magnetic conductors with broken inversion symmetry, which is important for different spintronic phenomena.",2104.13252v1 2021-04-28,Emission of electromagnetic radiation due to spin-flip transitions in a ferromagnet,"We theoretically analyze a possibility of electromagnetic wave emission due to electron transitions between spin subbands in a ferromagnet. Different mechanisms of such spin-flip transitions are cousidered. One mechanism is the electron transitions caused by magnetic field of the wave. Another mechanism is due to Rashba spin-orbit interaction. While two mentioned mechanisms exist in a homogeneously magnetized ferromagnet, there are two other mechanisms that exist in non-collinearly magnetized medium. First mechanism is known and is due to the dependence of exchange interaction constant on the quasimomentum of conduction electrons. Second one exists in any non-collinearly magnetized medium. We study these mechanisms in a non-collinear ferromagnet with helicoidal magnetization distribution. The estimations of probabilities of electron transitions due to different mechanisms are made for realistic parameters, and we compare the mechanisms. We also estimate the radiation power and threshold current in a simple model in which spin is injected into the ferromagnet by a spin-polarized electric current through a tunnel barrier.",2104.13943v1 2021-05-05,Strain-induced atomic-scale building blocks for ferromagnetism in epitaxial LaCoO3,"The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself, but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.",2105.01837v1 2021-05-07,Magnetic properties of the itinerant ferromagnet LaCrGe3 under pressure studied by 139La NMR,"$^{139}$La nuclear magnetic resonance (NMR) measurements under pressure ($p = 0-2.64$ GPa) have been carried out to investigate the static and dynamic magnetic properties of the itinerant ferromagnet LaCrGe$_3$. $^{139}$La-NMR spectra for all measured pressures in the ferromagnetically ordered state show a large shift due to the internal field induction $|$$B_{\rm int}$$|$ $\sim$ 4 T at the La site produced by Cr ordered moments. The change in $B_{\rm int}$ by less than 5\% with $p$ up to 2.64~GPa indicates that the Cr 3$d$ moments are robust under pressure. The temperature dependence of NMR shift and $B_{\rm int}$ suggest that the ferromagnetic order develops below $\sim$ 50~K under higher pressures in a magnetic field of $\sim$ 7.2 T. Based on the analysis of NMR data using the self-consistent-renormalization (SCR) theory, the spin fluctuations in the paramagnetic state well above $T_{\rm C}$ are revealed to be three dimensional ferromagnetic throughout the measured $p$ region.",2105.03479v1 2021-05-20,Coexistence of ferromagnetism and spin-orbit coupling by incorporation of platinum in two-dimensional VSe$_2$,"We report on a novel material, namely two-dimensional (2D) V$_{1-x}$Pt$_x$Se$_2$ alloy, exhibiting simultaneously ferromagnetic order and Rashba spin-orbit coupling. While ferromagnetism is absent in 1T-VSe$_2$ due to the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe$_2$ (10-50 %) to form an homogeneous 2D alloy restores ferromagnetic order with Curie temperatures of 6 K for 5 monolayers and 25 K for one monolayer of V$_{0.65}$Pt$_{0.35}$Se$_2$. Moreover, the presence of platinum atoms gives rise to Rashba spin-orbit coupling in (V,Pt)Se$_2$ providing an original platform to study the interplay between ferromagnetism and spin-orbit coupling in the 2D limit.",2105.10022v1 2021-05-29,"Magnetic Impurity Effects on Ferromagnetic Fluctuations in Heavily Overdoped (Bi,Pb)$_2$Sr$_2$Cu$_{1-y}$Fe$_y$O$_{6+δ}$ Cuprates","We investigated Fe-substitution effects on ferromagnetic fluctuations in the superconducting overdoped and non-superconducting heavily overdoped regimes of the Bi-2201 cuprates by the magnetization and electrical-resistivity measurements. It was found that the spin-glass state was induced at low temperatures by the Fe substitution. The Curie constant and the effective Bohr magneton, estimated from the magnetic susceptibility, as well as the dimensionality of the ferromagnetic fluctuations from the resistivity, suggest the enhancement of the ferromagnetic fluctuations owing to the Fe substitution. A ferromagnetic spin-cluster model is proposed in the heavily overdoped regime of Bi-2201, while an antiferromagnetic spin-cluster model has been proposed in the overdoped regime of Bi-2201 [Hiraka et al., Phys. Rev. B 81, 144501 (2010)].",2105.14288v2 2021-06-16,"Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBi2nTe3n+1 (n=2, 3)","MnBi2nTe3n+1(MBT) is the first intrinsic magnetic topological insulator and is promising to hostemergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n >= 4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromag-netic (FM) MBT compounds, including MnBi8Te13and Sb doped MnBi2nTe3n+1(n= 2,3) usingAC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all threecompounds, suggesting its universality among FM MBT. We attribute the origin of the relaxationbehavior to the irreversible domain movements since they only appear below the saturation fieldswhen ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by thelow-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imagedvia our magneto-optical measurements. Finally, we ascribe the rare ""double-peak"" behavior ob-served in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domainformations.",2106.08969v1 2021-06-26,Spin current injection via equal-spin Cooper pairs in ferromagnet/superconductor heterostructures,"Equal-spin Cooper pairs are pivotal building blocks for superconducting spintronics devices. In recent experiments, unusual behavior was observed in ferromagnet/ferromagnet/superconductor devices when a precession of the magnetization was induced by ferromagnetic resonance. By using a non-equilibrium Usadel Green function formalism, we study spin transport for such a setup. We solve for spin-resolved distribution functions and demonstrate that the spin injection process in superconductors is governed by the inverse proximity effect in the superconducting layer. We find that equal-spin Cooper pairs, which are produced by the two misaligned ferromagnetic layers, transport spin inside the S layer. This then results in an increase of the injected spin current below the superconducting critical temperature. Our calculations provide the first evidence of the essential role of equal-spin Cooper pairs on spin-transport properties of S/F devices and pave new avenues for the design of superconducting spintronics devices.",2106.13988v3 2021-07-16,Prediction of 2D ferromagnetism and monovalent europium ions in the EuBr/graphene heterojunctions,"Europium, one of the rare earth elements, exhibits +2 and +3 valence states and has been widely used for magnetic modification of materials. Based on density functional theory calculations, we predict the 2D EuBr/graphene heterojunctions to exhibit metallicity, huge intrinsic-ferromagnetism nearly 7.0 {\mu}B per Eu and the special monovalent Eu ions. Electron localization function (ELF), difference charge densities and Bader charge analyses demonstrate that there are cation-{\pi} interactions between the EuBr films and graphene, which explains the stability of these unusual heterojunctions. Graphene works as substrate to enable the stability of EuBr monolayer crystals where EuBr plays an important role to yield ferromagnetism and enhance metallicity in the heterojunctions. Monte Carlo simulations are used to estimate a Curie temperature of about 7 K, which, together with magnetic configurations, can be further modulated by external strains and charge-carrier doping. In general, our theoretical work predicts the properties of the novel 2D ferromagnetic EuBr/graphene heterojunctions, suggests the possibility of combining the 2D intrinsic-ferromagnetic metal halide crystals and graphene, and opens up a new perspective in next-generation electronic, spintronic devices and high-performance sensors.",2107.08833v2 2021-08-02,Electrically tunable room-temperature ferromagnetism in CrBr$_3$,"The recent discovery of magnetic ordering in two-dimension has lead to colossal efforts to find atomically thin materials that order at high temperatures. However, due to fundamental spin fluctuation in reduced dimension, the room-temperature ferromagnetism remains elusive. Here, we report a dramatic manipulation of magnetic ordering up to room temperature in the monolayer CrBr$_3$, within the first-principles Heisenberg XXZ model. The exchange and anisotropic magnetic interactions are externally modulated by a gate-induced charge carrier doping that triggers a nontrivial phase diagram. High-temperature ferromagnetism is associated with a substantial increase in both effective ferromagnetic exchange and overall magnetic anisotropy under experimentally attainable hole doping. In contrast, electron doping quickly switches the magnetic easy axis. The gate-tuneable room temperature ferromagnetism in CrBr$_3$ presents new possibilities in electrically controlled spintronic and magnetoelectric devices based on atomically thin crystals.",2108.00684v1 2021-10-08,Fermi surface expansion above critical temperature in a Hund ferromagnet,"Using a cluster extension of the dynamical mean-field theory, we show that strongly correlated metals subject to Hund's physics exhibit significant electronic structure modulations above magnetic transition temperatures. In particular, in a ferromagnet having a large local moment due to Hund's coupling (Hund's ferromagnet), the Fermi surface expands even above the Curie temperature ($T_{\rm C}$) as if a spin polarization occurred. Behind this phenomenon, effective ``Hund's physics'' works in momentum space, originating from ferromagnetic fluctuations in the strong coupling regime. The resulting significantly momentum-dependent (spatially nonlocal) electron correlations induce an electronic structure reconstruction involving a Fermi-surface volume change and a redistribution of the momentum-space occupation. Our finding will give a deeper insight into the physics of Hund's ferromagnets above $T_{\rm C}$.",2110.04288v2 2021-11-05,Giant oscillatory Gilbert damping in superconductor/ferromagnet/superconductor junctions,"Interfaces between materials with differently ordered phases present unique opportunities for exotic physical properties, especially the interplay between ferromagnetism and superconductivity in the ferromagnet/superconductor heterostructures. The investigation of zero- and pi-junctions has been of particular interest for both fundamental physical science and emerging technologies. Here, we report the experimental observation of giant oscillatory Gilbert damping in the superconducting Nb/NiFe/Nb junctions with respect to the NiFe thickness. This observation suggests an unconventional spin pumping and relaxation via zero-energy Andreev bound states that exist only in the Nb/NiFe/Nb pi-junctions, but not in the Nb/NiFe/Nb zero-junctions. Our findings could be important for further exploring the exotic physical properties of ferromagnet/superconductor heterostructures, and potential applications of ferromagnet pi-junctions in quantum computing, such as half-quantum flux qubits.",2111.03233v1 2021-11-19,Ferromagnetic and metamagnetic transitions in itinerant electron systems: a microscopic study,"We perform a microscopic study of itinerant ferromagnetic systems. We reveal a very rich phase diagram in the three-dimensional space spanned by the chemical potential, a magnetic field, and temperature beyond the Landau theory analyzed so far. Besides a generic wing structure near a tricritical point upon introducing the magnetic field, we find that an additional wing can be generated close to a quantum critical end point (QCEP) and also even from deeply inside the ferromagnetic phase. A tilting of the wing controls the entropy jump associated with the metamagnetic transition. Ferromagnetic and metamagnetic transitions are usually accompanied by a Lifshitz transition at low temperatures, i.e., a change of Fermi surface topology including the disappearance of the Fermi surface. In particular, the Fermi surface of either spin band vanishes at the QCEP. These rich phase diagrams are understood in terms of the density of states and the breaking of particle-hole symmetry in the presence of a next nearest-neighbor-hopping integral t', which is expected in actual materials. The obtained phase diagrams are discussed in a possible connection to itinerant ferromagnetic systems such as UGe2, UCoAl, ZrZn2, and others including materials exhibiting the magnetocaloric effect.",2111.10068v2 2021-11-30,Nonreciprocity of spin waves in a double layer ferromagnet with Dzyaloshinskii-Moriya interactions,"In this paper, boundary conditions for Landau-Lifshitz equations at the interface between two ferromagnets with different Dzyaloshinskii-Moriya interactions are derived. We calculated and verified the analytical expression for the energy flux density continuity for the spin-wave propagation through the interface between two ferromagnets with/without Dzyaloshinskii-Moriya interactions considering the boundary conditions mentioned. Analytical expressions for reflection, transmission coefficients and the nonreciprocity factor are calculated in the case of spin-wave propagation through a double layer ferromagnet with/without Dzyaloshinskii-Moriya interactions in the first/second layer. Two principally different types of nonreciprocity effects for spin waves are revealed in such a double layer system. The material parameters of a double layer ferromagnet with/without Dzyaloshinskii-Moriya interactions in the first/second layer are found for which the extremely high nonreciprocity factor (>10) is expected according to the results of calculations. The results of the paper deepen the knowledge about the spin-wave propagation control in magnonic devices.",2111.15244v1 2021-12-06,Quantum phases of ferromagnetically coupled dimers on Shastry-Sutherland lattice,"The ground state (gs) of antiferromagnetically coupled dimers on the Shastry-Sutherland lattice (SSL) stabilizes many exotic phases and has been extensively studied. The gs properties of ferromagnetically coupled dimers on SSL are equally important but unexplored. In this model the exchange coupling along the $x$-axis ($J_x$) and $y$-axis ($J_y$) are ferromagnetic and the diagonal exchange coupling ($J$) is antiferromagnetic. In this work we explore the quantum phase diagram of ferromagnetically coupled dimer model numerically using density matrix renormalization group (DMRG) method. We note that in $J_x$-$J_y$ parameter space this model exhibits six interesting phases:(I) stripe $(0,\pi)$, (II) stripe $(\pi,0)$, (III) perfect dimer, (IV) $X$-spiral, (V) $Y$-spiral and (VI) ferromagnetic phase. Phase boundaries of these quantum phases are determined using the correlation functions and gs energies. We also notice the correlation length in this system is less than four lattice units in most of the parameter regimes. The non-collinear behaviour in $X$-spiral and $Y$-spiral phase and the dependence of pitch angles on model parameters are also studied.",2112.02975v1 2022-01-26,Instability of the ferromagnetic quantum critical point in strongly interacting 2D and 3D electron gases with arbitrary spin-orbit splitting,"In this work we revisit itinerant ferromagnetism in 2D and 3D electron gases with arbitrary spin-orbit splitting and strong electron-electron interaction. We identify the resonant scattering processes close to the Fermi surface that are responsible for the instability of the ferromagnetic quantum critical point at low temperatures. In contrast to previous theoretical studies, we show that such processes cannot be fully suppressed even in presence of arbitrary spin-orbit splitting. A fully self-consistent non-perturbative treatment of the electron-electron interaction close to the phase transition shows that these resonant processes always destabilize the ferromagnetic quantum critical point and lead to a first-order phase transition. Characteristic signatures of these processes can be measured via the non-analytic dependence of the spin susceptibility on magnetic field both far away or close to the phase transition.",2201.10995v1 2022-01-31,Anomalous Nernst effect in La0.5Ca0.5Coo3,"We report the occurrence of the anomalous Nernst effect (ANE) in polycrystalline perovskite La0.5Ca0.5CoO3. The sample is ferromagnetic below TC = 147 K and resistivity shows non-metallic behavior above and below the TC with only a small negative magnetoresistance (~2%) around TC. Field dependence of magnetization at 10 K shows large hysteresis with a coercive field of 6 kOe but a small magnetization ~ 0.64 Bohr magneton/Co even in a field of 50 kOe, which indicates the presence of magnetically heterogeneous ground state consisting of ferromagnetic and non-ferromagnetic phases. The field dependence of the Nernst thermopower (Sxy) at low temperatures shows complete saturation but the magnetization does not. This indicates that the ANE in La0.5Ca0.5CoO3 depends only on the transport properties of the ferromagnetic phase, while it is not affected by the non-ferromagnetic phase. Due to the higher value of remnant Sxy, the magnetized polycrystalline sample exhibits ANE in absence of an external magnetic field.",2201.12982v1 2022-03-18,Prediction of novel two-dimensional rare-earth material with room-temperature ferromagnetism and large perpendicular magnetic anisotropy,"Novel 2D ferromagnets with high Curie temperature and large perpendicular magnetic anisotropy are especially attractive owing to the future promising application in modern spintronics, but meanwhile the 2D ferromagnetic materials with high Curie temperature and large perpendicular magnetic anisotropy are rarely reported. Based on density functional theory (DFT) calculations, we predict a new kind of 2D ferromagnetic materials - GdB2N2, which possesses large magnetic moment, high Curie temperature (335 K) and large perpendicular magnetic anisotropy (10.38 meV/f.u.). Biaxial strain ranging from -0.5% to 5% and different concentrations of charge-carrier doping are applied to reveal the influence on the Curie temperature and magnetic anisotropy energy (MAE). Besides, magnetic coupling process within GdB2N2 is found to be via a Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. In summary, our work here predicts a novel 2D rare-earth material GdB2N2, which not only enriches the category of 2D room-temperature ferromagnets, but also proposes a new possibility of combining traditional 2D materials and rare-earth materials to achieve more intriguing magnetic properties, finally it carves out the path for the next-generation spintronic devices and sensors.",2203.10969v1 2022-03-23,Effective decoupling of ferromagnetic sublattices by frustration in Heusler alloys,"Magnetic frustration in ferromagnetic metallic systems is unusual due to the long-range and symmetric nature of the exchange interactions. In this work we prove that it is possible to obtain a highly frustrated ferromagnetic phase in a multi-sublattices cubic structure through a fine tuning of the magnetic interactions. This peculiar state is achieved in Ni-Mn-(In, Sn) Heusler alloys and results in the effective decoupling of their two intertwined ferromagnetic sublattices. One sublattice is ferromagnetic long range ordered below the macroscopic Curie temperature (TC ) whereas the second one remains disordered until a crossover to a polarized state occurs at T << TC . This result points out that a fine engineering of the magnetic interactions in metallic systems can lead to interesting novel and emergent phenomena.",2203.12498v2 2022-04-04,Two-Dimensional Ferromagnetic Half-Metallic Janus V2AsP Monolayer,"Two-dimensional (2D) ferromagnetic materials present promising candidates for spintronic devices, and the half-metallic materials with 100% spin polarization at Fermi energy level are highly desired for many spin-based devices. 2D Janus materials have attracted great attention in recent years due to their excellent properties induced by breaking the symmetry. Here, using the density functional theory, we report that the Janus V2AsP monolayer demonstrates a charming ferromagnetic half-metallic feature. It is dynamically stable in view of the absence of imaginary frequency phonon. The half-metallic gap is about 0.38 eV and the spin splitting of about 1.34eV for the V2AsP monolayer. Interestingly, a tensile strain of 4.9% can induce it to undergo a phase transition from ferromagnetic to anti-ferromagnetic state. Moreover, the Curie temperature (Tc) enhances with the increase of compressive strain. All there appealing properties make the half-metallic Janus V2AsP monolayer a promising material for 2D spintronic applications.",2204.01274v3 2022-04-04,Voltage-driven exchange resonance achieving 100\% mechanical efficiency,"Magnetic resonances driven by current-induced torques are crucial tools to study magnetic materials but are very limited in frequency and mechanical efficiency. We propose an alternative mechanism, voltage-induced torque, to realize high efficiency in generating high-frequency magnetization dynamics. When a ferromagnet-topological insulator-ferromagnet trilayer heterostructure is operated as an adiabatic quantum motor, voltage-induced torque arises from the adiabatic motion of gapped topological electrons on the two interfaces and act oppositely on the two ferromagnetic layers, which can excite the exchange mode where the two ferromagnetic layers precess with a $\pi$-phase difference. The exchange mode resonance, bearing a much higher frequency than the ferromagnetic resonance, is accompanied by topological charge pumping, leading to a sharp peak in electrical admittance at the resonance point. Because the output current is purely adiabatic while dissipative current vanishes identically, the proposed voltage-driven exchange resonance entails a remarkably high mechanical efficiency close to unity, which is impossible in any current-driven systems.",2204.03534v2 2022-04-24,Generating Two-dimensional Ferromagnetic Charge Density Waves via External Fields,"Two-dimensional (2D) ferromagnetic charge density wave (CDW), an exotic quantum state for exploring the intertwining effect between correlated charge and spin orders in 2D limit, has not been discovered in the experiments yet. Here, we propose a feasible strategy to realize 2D ferromagnetic CDWs under external fields, which is demonstrated in monolayer VSe$_2$ using first-principles calculations. Under external tensile strain, two novel ferromagnetic CDWs ($\sqrt{3}$$\times$$\sqrt{3}$ and 2$\times2\sqrt{3}$ CDWs) can be generated, accompanied by distinguishable lattice reconstructions of magnetic V atoms. Remarkably, because the driving forces for generating these two ferromagnetic CDWs are strongly spin-dependent, fundamentally different from that in conventional CDWs, the $\sqrt{3}$$\times$$\sqrt{3}$ and 2$\times2\sqrt{3}$ CDWs can exhibit two dramatically different half-metallic phases under a large strain range, along with either a flat band or a Dirac cone around Fermi level. Our proposed strategy and material demonstration may open a door to generate and manipulate correlation effect between collective charge and spin orders via external fields.",2204.11179v1 2022-05-03,"Carrier Doping Modulates 2D Intrinsic Ferromagnetic Mn2Ge2Te6 Monolayer High Curie Temperature, Large Magnetic Crystal Anisotropy","The Mn2Ge2Te6 shows intrinsic ferromagnetic (FM) order, with Curie temperature (Tc) of 316 K. The FM order origins from superexchange interaction between Mn and Te atoms. Mn2Ge2Te6 is half-metal (HM), and spin-\b{eta} electron is a semiconductor with gap of 1.462 eV. Mn2Ge2Te6 tends in-plane anisotropy (IPA), with magnetic anisotropy energy (MAE) of -13.2 meV/f.u.. The Mn2Ge2Te6 shows good dynamical and thermal stability. Moreover, Mn2Ge2Te6 presents good ferromagnetic and half-metallic stability under charge doping. The carriers doping could effectively tune magnetic and electronic properties. Specifically, the magnetic moment, exchange parameter, and MAE could be efficiently tuned. The total magnetic moment changes linearly with charges doping. The exchange parameters could be controlled by the doping carriers. The carriers doping could modulate MAE to -18.4 (+0.4 e), -0.85 (-1.6 e), 1.31 (-2.4 e) meV/f.u., by changing hybridization between Te atoms' py and pz orbitals. Mn2Ge2Te6 with intrinsic ferromagnetism, high tunable MAE, good stability of ferromagnetism and half-metallicity could help researchers to investigate its wide application in the electronics and spintronics.",2205.01498v1 2022-06-14,Interstitial-induced ferromagnetism in a two-dimensional Wigner crystal,"The two-dimensional Wigner crystal (WC) occurs in the strongly interacting regime ($r_s \gg 1$) of the two-dimensional electron gas (2DEG). The magnetism of a pure WC is determined by tunneling processes that induce multi-spin ring-exchange interactions, resulting in fully polarized ferromagnetism for large enough $r_s$. Recently, Hossain et al. [PNAS 117 (51) 32244-32250] reported the occurrence of a fully polarized ferromagnetic insulator at $r_s \gtrsim 35$ in an AlAs quantum well, but at temperatures orders of magnitude larger than the predicted exchange energies for the pure WC. Here, we analyze the large $r_s$ dynamics of an interstitial defect in the WC, and show that it produces local ferromagnetism with much higher energy scales. Three hopping processes are dominant, which favor a large, fully polarized ferromagnetic polaron. Based on the above results, we speculate concerning the phenomenology of the magnetism near the metal-insulator transition of the 2DEG.",2206.07191v2 2022-07-14,Finite-temperature properties of extended Nagaoka ferromagnetism,"We study finite-temperature properties of a Hubbard model including sites of a particle bath which was proposed as a microscopic model to show itinerant ferromagnetism at finite electron density. We use direct numerical methods, such as exact diagonalization and random vector methods. The temperature dependence of quantities is surveyed in the full range of the temperature. We find that the specific heat has several peaks, which correspond to ordering processes in different energy scales. In particular, magnetic order appears at very low temperature. Depending on the chemical potential of the particle bath and the Coulomb interaction, the system exhibits an itinerant ferromagnetic state or an antiferromagnetic state of the Mott insulator. Microscopically the competition between these two types of orderings causes a peculiar ordering process of local spin correlations. Some local ferromagnetic correlations are found to be robust, which indicates that the ferromagnetic correlation originates from the motion of itinerant electrons in a short-range cluster.",2207.06588v1 2022-07-14,Magnetization dynamics in proximity-coupled superconductor-ferromagnet-superconductor multilayers. Part II,"In this work, we study magnetization dynamics in superconductor-ferromagnet-superconductor thin-film structures. Results of the broad-band ferromagnetic resonance spectroscopy are reported for a large set of samples with varied thickness of both superconducting and ferromagnetic layers in a wide frequency, field, and temperature ranges. Experimentally the one-dimensional anisotropic action of superconducting torque on magnetization dynamics is established; its dependence on thickness of layers is revealed. It is demonstrated that experimental findings support the recently-proposed mechanism of the superconducting torque formation via the interplay between the superconducting kinetic inductance and magnetization precession at superconductor-ferromagnet interfaces.",2207.06751v2 2022-08-15,Absorption of Transverse Spin Current in Ferromagnetic NiCu: Dominance of Bulk Dephasing over Spin-Flip Scattering,"In ferromagnetic metals, transverse spin currents are thought to be absorbed via dephasing -- i.e., destructive interference of spins precessing about the strong exchange field. Yet, due to the ultrashort coherence length of $\approx$1 nm in typical ferromagnetic thin films, it is difficult to distinguish dephasing in the bulk from spin-flip scattering at the interface. Here, to assess which mechanism dominates, we examine transverse spin-current absorption in ferromagnetic NiCu alloy films with reduced exchange fields. We observe that the coherence length increases with decreasing Curie temperature, as weaker dephasing in the film bulk slows down spin absorption. Moreover, nonmagnetic Cu impurities do not diminish the efficiency of spin-transfer torque from the absorbed spin current. Our findings affirm that transverse spin current is predominantly absorbed by dephasing inside the nanometer-thick ferromagnetic metals, even with high impurity contents.",2208.07294v1 2022-08-23,Nematic Tomonaga-Luttinger Liquid Phase in an $S=1/2$ Ferromagnetic-Antiferromagnetic Bond-Alternating Chain,"We numerically investigate the ground-state phase diagram of the $S=1/2$ ferromagnetic-antiferromagnetic bond-alternating chain, in which the ferromagnetic interactions are stronger than the antiferromagnetic ones, and the anisotropies of the former and latter interactions are of the Ising-type and the $XY$-type, respectively. We use various numerical methods, such as the level spectroscopy and phenomenological renormalization-group analyses of the numerical data obtained by the exact diagonalization method, and so on. The resultant phase diagrams contain the ferromagnetic, $XY$1, singlet-dimer, and up-up-down-down phases as well as the nematic Tomonaga-Luttinger liquid (nTLL) phase which appears in a wide region of the interaction parameters. Perturbation calculations from the strong limit of the ferromagnetic interactions reproduce fairly well the numerical results of the phase boundary lines associated with the nTLL phase in the phase diagrams.",2208.10988v2 2022-09-06,High-spin orbital interactions across van der Waals gaps controlling the interlayer ferromagnetism in van der Waals ferromagnets,"We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3 and VI3, to find that high-spin orbital interaction across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2 Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals fer-romagnets.",2209.02233v1 2022-09-22,Neutron Depolarization due to Ferromagnetism and Spin Freezing in CePd$_{1-x}$Rh$_x$,"We report neutron depolarization measurements of the suppression of long-range ferromagnetism and the emergence of magnetic irreversibilities and spin freezing in CePd$_{1-x}$Rh$_x$ around $x^*\approx0.6$. Tracking the temperature versus field history of the neutron depolarization, we find clear signatures of long-range Ising ferromagnetism below a Curie temperature $T_{\rm C}$ for $x=0.4$ and a spin freezing of tiny ferromagnetic clusters below a freezing temperature $T_{\rm F1}$ for $x>x^*$. Under zero-field-cooling/field-heating and for $x>x^*$ a reentrant temperature dependence of the neutron depolarization between $T_{\rm F2}0.1; x_eff>0.08) and the hole concentration is in the order of 10^19 cm^-3.",0201363v1 2002-02-11,Radiation Induced Landau-Lifshitz-Gilbert Damping in Ferromagnets,"The Landau-Lifshitz-Gilbert damping coefficient employed in the analysis of spin wave ferromagnetic resonance is related to the electrical conductivity of the sample. The changing magnetization (with time) radiates electromagnetic fields. The electromagnetic energy is then absorbed by the sample and the resulting heating effect describes magnetic dissipative damping. The ferromagnetic resonance relaxation rate theoretically depends on the geometry (shape and size) of the sample as well as temperature in agreement with experiment.",0202181v1 2002-02-12,Domain structure of superconducting ferromagnets,"In superconducting ferromagnets the equilibrium domain structure is absent in the Meissner state, but appears in the spontaneous vortex phase (the mixed state in zero external magnetic field), though with a period, which can essentially exceed that in normal ferromagnets. Metastable domain walls are possible even in the Meissner state. The domain walls create magnetostatic fields near the sample surface, which can be used for experimental detection of domain walls.",0202193v1 2002-02-22,Spin-polarized light emitting diode using metal/insulator/semiconductor structures,"We have succeeded in growing ferromagnetic metals (Co, Fe, and NiFe)/ Al2O3/ AlGaAs heterostructures with homogeneous and flat interfaces. The electro-luminescence (EL) from the light emitting diode (LED) consisting of the metal/insulator/semiconductor (MIS) structure depends on the magnetization direction of the ferromagnetic electrode at room temperature. This fact shows that a spin-injection from the ferromagnetic metal to the semiconductor is achieved. The spin-injection efficiency is estimated to be the order of 1 % at room temperature.",0202389v1 2002-02-26,Magnetothermopower and magnon-assisted transport in ferromagnetic tunnel junctions,"We present a model of the thermopower in a mesoscopic tunnel junction between two ferromagnetic metals based upon magnon-assisted tunneling processes. In our model, the thermopower is generated in the course of thermal equilibration between two baths of magnons, mediated by electrons. We predict a particularly large thermopower effect in the case of a junction between two half-metallic ferromagnets with antiparallel polarizations, $S_{AP} \sim - (k_B/e)$, in contrast to $S_{P} \approx 0$ for a parallel configuration.",0202475v1 2002-02-27,Spin-dependent Transparency of Ferromagnet/Superconductor Interfaces,"Because the physical interpretation of the spin-polarization of a ferromagnet determined by point-contact Andreev reflection (PCAR) is non-trivial, we have carried out parameter-free calculations of PCAR spectra based upon a scattering-theory formulation of Andreev reflection generalized to spin-polarized systems and a tight-binding linear muffin tin orbital method for calculating the corresponding scattering matrices. PCAR is found to measure the spin-dependent interface transparency rather than the bulk polarization of the ferromagnet which is strongly overestimated by free electron model fitting.",0202515v1 2002-03-05,Ferromagnetism in Diluted Magnetic Semiconductor Heterojunction Systems,"Diluted magnetic semiconductors (DMSs), in which magnetic elements are substituted for a small fraction of host elements in a semiconductor lattice, can become ferromagnetic when doped. In this article we discuss the physics of DMS ferromagnetism in systems with semiconductor heterojunctions. We focus on the mechanism that cause magnetic and magnetoresistive properties to depend on doping profiles, defect distributions, gate voltage, and other system parameters that can in principle be engineered to yield desired results.",0203080v1 2002-03-06,A new mechanism of exchange interaction in ferromagnetic semiconductors,"We propose a new mechanism of indirect exchange interaction, which can be responsible for the ferromagnetic ordering in Mn-doped semiconductors (like GaMnAs) at low carrier concentration. The mechanism is based on the interplay of the hybridization of band states (conduction or valence) with localized impurity (donor or acceptor) states and the direct exchange interaction between localized spins and the band states. The indirect exchange coupling between two impurities occurs when the wavefunctions of the corresponding localized donor (acceptor) states overlap. This coupling is independent of the free carrier concentration and therefore may be responsible for ferromagnetic transition at low or vanishing carrier concentration.",0203119v1 2002-04-04,Generalized boundary conditions for the circuit theory of mesoscopic transport,"The circuit theory of mesoscopic transport provides a unified framework to describe spin-dependent or superconductivity-related phenomena. We extend this theory to hybrid systems of normal metals, ferromagnets and superconductors. Our main result is an expression for the current through an arbitrary contact between two general isotropic ""nodes"", which is suitable to describe the presence of superconducting and ferromagnetic elements in the system, as well as magnetically active interfaces/contacts. In certain cases (weak ferromagnet and magnetic tunnel junction) we derive transparent and simple results for the matrix current.",0204116v1 2002-05-02,Spin battery operated by ferromagnetic resonance,"Precessing ferromagnets are predicted to inject a spin current into adjacent conductors via Ohmic contacts, irrespective of a conductance mismatch with, for example, doped semiconductors. This opens the way to create a pure spin source spin battery by the ferromagnetic resonance. We estimate the spin current and spin bias for different material combinations.",0205028v2 2002-05-08,Epitaxial Growth of an n-type Ferromagnetic Semiconductor CdCr2Se4 on GaAs(001) and GaP(001),"We report the epitaxial growth of CdCr2Se4, an n-type ferromagnetic semiconductor, on both GaAs and GaP(001) substrates, and describe the structural, magnetic and electronic properties. Magnetometry data confirm ferromagnetic order with a Curie temperature of 130 K, as in the bulk material. The magnetization exhibits hysteretic behavior with significant remanence, and an in-plane easy axis with a coercive field of ~125 Oe. Temperature dependent transport data show that the films are semiconducting in character and n-type as grown, with room temperature carrier concentrations of n ~ 1 x 10^18 cm-3.",0205162v1 2002-05-15,Magnetism in a lattice of spinor Bose condensates,"We study the ground state magnetic properties of ferromagnetic spinor Bose-Einstein condensates confined in a deep optical lattices. In the Mott insulator regime, the ``mini-condensates'' at each lattice site behave as mesoscopic spin magnets that can interact with neighboring sites through both the static magnetic dipolar interaction and the light-induced dipolar interaction. We show that such an array of spin magnets can undergo a ferromagnetic or anti-ferromagnetic phase transition under the magnetic dipolar interaction depending on the dimension of the confining optical lattice. The ground-state spin configurations and related magnetic properties are investigated in detail.",0205319v1 2002-05-30,Spin-dependent properties of a two-dimensional electron gas with ferromagnetic gates,"A theoretical prediction of the spin-dependent electron self-energy and in-plane transport of a two-dimensional electron gas in proximity with a ferromagnetic gate is presented. The application of the predicted spin-dependent properties is illustrated by the proposal of a device configuration with two neighboring ferromagnetic gates which produces a magnetoresistance effect on the channel current generated by nonmagnetic source and drain contacts. Specific results are shown for a silicon inversion layer with iron gates. The gate leakage current is found to be beneficial to the spin effects.",0205651v2 2002-06-22,Breakdown of the coexistence of spin-singlet superconductivity and itinerant ferromagnetism,"We discuss the possibility of coexistence of spin-singlet superconductivity and ferromagnetism in a model where the same electrons are assumed responsible for both of them. Our calculations include both zero and finite momentum pairing states with both s-wave and d-wave pairing symmetry. Under the mean-field approximation, the thermodynamic potential of the non-magnetic superconducting (SC) state is shown to be always lower than that of the superconducting ferromagnetic (SF) state. It follows that the spin-singlet SF state is energetically unfavorable, and a spin-triplet SF state is more likely to survive in metals such as UGe$_{2}$ and ZrZn$_{2}$.",0206418v1 2002-07-18,Spin-transfer in diffusive ferromagnet-normal metal systems with spin-flip scattering,"The spin-transfer in biased disordered ferromagnet (F) - normal metal (N) systems is calculated by the diffusion equation. For F1-N2-F2 and N1-F1-N2-F2-N3 spin valves, the effect of spin-flip processes in the normal metal and ferromagnet parts are obtained analytically. Spin-flip in the center metal N2 reduces the spin-transfer, whereas spin-flip in the outer normal metals N1 and N3 can increase it by effectively enhancing the spin polarization of the device.",0207458v1 2002-07-19,Cyclotron Resonance of Itinerant Holes in Ferromagnetic InMnAs/GaSb Heterostructures,"We report the first observation of hole cyclotron resonance (CR) in ferromagnetic InMnAs/GaSb heterostructures both in the high-temperature paramagnetic phase and the low-temperature ferromagnetic phase. We clearly resolve two resonances that exhibit strong temperature dependence in position, linewidth, and intensity. We attribute the two resonances to the so-called fundamental CR transitions expected for delocalized holes in the valence band in the magnetic quantum limit, demonstrating the existence of $p$-like itinerant holes that are describable within the Luttinger-Kohn effective mass theory.",0207485v1 2002-08-21,A key to room-temperature ferromagnetism in Fe-doped ZnO: Cu,"Successful synthesis of room-temperature ferromagnetic semiconductors, Zn$_{1-x}$Fe$_{x}$O, is reported. The essential ingredient in achieving room-temperature ferromagnetism in bulk Zn$_{1-x}$Fe$_{x}$O was found to be additional Cu doping. A transition temperature as high as 550 K was obtained in Zn$_{0.94}$Fe$_{0.05}$Cu$_{0.01}$O; the saturation magnetization at room temperature reached a value of $0.75 \mu_{\rm B}$ per Fe. Large magnetoresistance was also observed below $100 $K.",0208399v1 2002-08-23,Even-odd effects in magnetoresistance of ferromagnetic domain walls,"Difference in density of states for the spin's majority and minority bands in a ferromagnet changes the electrostatic potential along the domains, introducing the discontinuities of the potential at domain boundaries. The value of discontinuity oscillates with number of domains. Discontinuity depends on the positions of domain walls, their motion or collapse of domain walls in applied magnetic field. Large values of magnetoresistance are explained in terms of spin-accumulation. We suggest a new type of domain walls in nanowires of itinerant ferromagnets, in which the magnetization vector changes without rotation. Absence of transverse magnetization components allows considerable spin accumulation assuming the spin relaxation length, L_S, is large enough.",0208463v1 2002-09-10,Canted ferromagnetism in RuSr$_2$GdCu$_2$O$_8$,"First principles calculations using the full-potential linearized augmented plane wave (FLAPW) method including intra-atomic noncollinear magnetism have been performed to determine the magnetic structures of RuSr$_{2}$GdCu$_{2}$O$_{8}$. The magnetism clearly arises from the RuO$_{6}$ octahedra where the moments on neighboring Ru sites order antiferromagnetically but cant perpendicular to the AFM axis - and so induce a weak ferromagnetism. The projected Ru moments along the AFM and FM axes result in magnetic moments of 1.16 and 0.99 $\mu_{B}$ respectively. The results are consistent with the possible coexistence of canted ferromagnetism and superconductivity in the RuSr$_{2}$GdCu$_{2}$O$_{8}$ - inferred from experiments.",0209224v1 2002-09-10,Transport properties of ferromagnet/d-wave superconductor/ferromagnet double junctions,"We investigate transport properties of a trilayer made of a d-wave superconductor connected to two ferromagnetic electrodes. Using Keldysh formalism we show that crossed Andreev reflection and elastic cotunneling exist also with d-wave superconductors. Their properties are controlled by the existence of zero energy states due to the anisotropy of the d-wave pair potential.",0209238v3 2002-09-13,Self-compensation in manganese-doped ferromagnetic semiconductors,"We present a theory of interstitial Mn in Mn-doped ferromagnetic semiconductors. Using density-functional theory, we show that under the non-equilibrium conditions of growth, interstitial Mn is easily formed near the surface by a simple low-energy adsorption pathway. In GaAs, isolated interstitial Mn is an electron donor, each compensating two substitutional Mn acceptors. Within an impurity-band model, partial compensation promotes ferromagnetic order below the metal-insulator transition, with the highest Curie temperature occurring for 0.5 holes per substitutional Mn.",0209329v1 2002-09-18,Stability of ferromagnetism in the Hubbard model on the kagomé lattice,"The Hubbard model on the kagom\'e lattice has highly degenerate ground states (the flat lowest band) in the corresponding single-electron problem and exhibits the so-called flat-band ferromagnetism in the many-electron ground states as was found by Mielke. Here we study the model obtained by adding extra hopping terms to the above model. The lowest single-electron band becomes dispersive, and there is no band gap between the lowest band and the other band. We prove that, at half-filling of the lowest band, the ground states of this perturbed model remain saturated ferromagnetic if the lowest band is nearly flat.",0209423v1 2002-10-04,Current-induced spin-wave excitations in a single ferromagnetic layer,"A new current induced spin-torque transfer effect has been observed in a single ferromagnetic layer without resorting to multilayers. At a specific current density of one polarity injected from a point contact, abrupt resistance changes due to current-induced spin wave excitations have been observed. The critical current at the onset of spin-wave excitations depends linearly on the external field applied perpendicular to the layer. The observed effect is due to current-driven heterogeneity in an otherwise uniform ferromagnetic layer.",0210116v1 2002-12-03,Thermal stability of coupled ferromagnetic and superparamagnetic particles,"We consider a single-domain ferromagnetic particle with uniaxial anisotropy coupled to a single-domain soft ferromagnetic particle (superparamagnetic particle). The problem of thermally agitated magnetization reversal in this case can be reduced to the random magnetization dynamics of the first particle with an effectively larger anisotropy field. The magnetic external field is also altered in a manner that depends on the sign of the coupling and can be either enhanced or suppressed.",0212041v1 2002-12-06,Electron space charge effect on spin injection into semiconductors,"We consider spin polarized transport in a ferromagnet-insulator/semiconductor/insulator-ferromagnet (F1-I-S-I-F2) junction. We find that the spin current is strongly dependent on the spin configurations, the doping and space charge distribution in the semiconductor. When the ferromagnet-semiconductor interface resistance is comparable to the semiconductor resistance, the magnetoresistance ratio of this junction can be greatly enhanced under appropriate doping when the space charge effect in the nonequilibrium transport processes is taken into consideration.",0212133v2 2003-01-24,Classical Infinite-Range-Interaction Heisenberg Ferromagnetic Model: Metastability and Sensitivity to Initial Conditions,"A N-sized inertial classical Heisenberg ferromagnet, which consists in a modification of the well-known standard model, where the spins are replaced by classical rotators, is studied in the limit of infinite-range interactions. The usual canonical-ensemble mean-field solution of the inertial classical $n$-vector ferromagnet (for which $n=3$ recovers the particular Heisenberg model considered herein) is briefly reviewed, showing the well-known second-order phase transition. This Heisenberg model is studied numerically within the microcanonical ensemble, through molecular dynamics.",0301492v1 2003-02-03,Proximity Effect and Spontaneous Vortex Phase in Planar SF-Structures,"The proximity effect in SF structures was examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF-bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arisen in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.",0302028v1 2003-02-21,A model for coexistent superconductivity and ferromagnetism,"We explore various temperature dependencies and thermodynamic quantities of a mean field model of a ferromagnetic-superconducting system. The starting point for this model is based on an s-wave pairing scheme in the singlet channel of the superconducting state and a spontaneously broken symmetry phase in the ferromagnetic state. We show numerically and analytically that a state of coexistence reveals itself and is favored energetically over other possible states, and a simple phase diagram is developed. Finally, a comparison of the specific heat with experiment is shown.",0302449v2 2003-02-25,On the Ordering Instability of Weakly-Interacting Electrons in a Dirty Metal,"In a dirty metal, electron-electron interactions in the spin-triplet channel lead to singular corrections to a variety of physical quantities. We show that these singularities herald the emergence of ferromagnetism. We calculate the effective action for the magnetic moment of weakly-interacting electrons in a dirty metal and show that a state with finite ferromagnetic moment minimizes this effective action. The saddle-point approximation is exact in an appropriate large-N limit. We discuss the physics of the ferromagnetic state with particular regard to thermal fluctuations and localization effects.",0302503v1 2003-03-19,Many-body Green's function theory for thin ferromagnetic anisotropic Heisenberg films: treatment of the exchange anisotropy,"The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier work, on the magnetic properties of thin ferromagnetic films are compared.",0303389v1 2003-04-01,The enhancement of ferromagnetism in uniaxially stressed diluted magnetic semiconductors,"We predict a new mechanism of enhancement of ferromagnetic phase transition temperature $T_c$ in uniaxially stressed diluted magnetic semiconductors (DMS) of p-type. Our prediction is based on comparative studies of both Heisenberg (inherent to undistorted DMS with cubic lattice) and Ising (which can be applied to strongly enough stressed DMS) models in a random field approximation permitting to take into account the spatial inhomogeneity of spin-spin interaction. Our calculations of phase diagrams show that area of parameters for existence of DMS-ferromagnetism in Ising model is much larger than that in Heisenberg model.",0304012v1 2003-04-22,Optical spin transfer in ferromagnetic semiconductors,"Circularly polarized laser pulses that excite electron-hole pairs across the band gap of (III,Mn)V ferromagnetic semiconductors can be used to manipulate and to study collective magnetization dynamics. The initial spin orientation of a photocarrier in a (III,V) semiconductors is determined by the polarization state of the laser. We show that the photocarrier spin can be irreversibly transferred to the collective magnetization, whose dynamics can consequently be flexibly controlled by suitably chosen laser pulses. As illustrations we demonstrate the feasibility of all optical ferromagnetic resonance and optical magnetization reorientation.",0304492v2 2003-05-01,Ultrafast Photoinduced Softening in a III-V Ferromagnetic Semiconductor for Non-thermal Magneto-Optical Recording,"Through time-resolved two-color magneto-optical Kerr spectroscopy we have demonstrated that photogenerated transient carriers decrease the coercivity of ferromagnetic InMnAs at low temperatures. This transient ``softening'' persists only during the carrier lifetime ($\sim$ 2 ps) and returns to its original value as soon as the carriers recombine to disappear. We discuss the origin of this unusual phenomenon in terms of carrier-enhanced ferromagnetic exchange interactions between Mn ions and propose an entirely nonthermal scheme for magnetization reversal.",0305017v2 2003-05-08,Superconducting Transition Temperature in Heterogeneous Ferromagnet-Superconductor Systems,"We study the shift of the the superconducting transition temperature $T_c$ in ferromagnetic-superconducting bi-layers and in a superconducting film supplied a square array of ferromagnetic dots. We find that the transition temperature in these two cases change presumably in opposite direction and that its change is not too small. We extend these results to multilayer structures. We predict that rather small external magnetic field $\sim 10$ Oe can change the transition temperature of the bilayer by 10% .",0305153v1 2003-05-14,Magnetodielectric Effects from Spin Fluctuations in Isostructural Ferromagnetic and Antiferromagnetic Systems,"We report on the effects of spin fluctuations, magnetic ordering, and external magnetic field on the dielectric constant of the ferromagnet SeCuO3, and the antiferromagnet TeCuO3. A model based on the coupling between uniform polarization and the q-dependent spin-spin correlation function is presented to explain the different behaviors for these isostructural compounds. The large magnetocapacitance near the transition temperature in the ferromagnet SeCuO3 suggests routes to enhancing the magnetodielectric response for practical applications.",0305333v1 2003-05-19,Exploration of oxide-based diluted magnetic semiconductors toward transparent spintronics,"A review is given for the recent progress of research in the field of oxide-based diluted magnetic semiconductor (DMS), which was triggered by combinatorial discovery of transparent ferromagnet. The possible advantages of oxide semiconductor as a host of DMS are described in comparison with conventional compound semiconductors. Limits and problems for identifying novel ferromagnetic DMS are described in view of recent reports in this field. Several characterization techniques are proposed in order to eliminate unidentified ferromagnetism of oxide-based DMS (UFO). Perspectives and possible devices are also given.",0305435v1 2003-06-18,A search for ferromagnetism in transition-metal-doped piezoelectric ZnO,"We present the results of a computational study of ZnO in the presence of Co and Mn substitutional impurities. The goal of our work is to identify potential ferromagnetic ground states within the (Zn,Co)O or (Zn,Mn)O material systems that are also good candidates for piezoelectricity. We find that, in contrast to previous results, robust ferromagnetism is not obtained by substitution of Co or Mn on the Zn site, unless additional carriers (holes) are also incorporated. We propose a practical scheme for achieving such $p$-type doping in ZnO.",0306477v1 2003-07-02,Ferromagnetic phase transition and Bose-Einstein condensation in spinor Bose gases,"Phase transitions in spinor Bose gases with ferromagnetic (FM) couplings are studied via mean-field theory. We show that an infinitesimal value of the coupling can induce a FM phase transition at a finite temperature always above the critical temperature of Bose-Einstein condensation. This contrasts sharply with the case of Fermi gases, in which the Stoner coupling $I_s$ can not lead to a FM phase transition unless it is larger than a threshold value $I_0$. The FM coupling also increases the critical temperatures of both the ferromagnetic transition and the Bose-Einstein condensation.",0307063v1 2003-07-10,Capping-induced suppression of annealing in Ga(1-x)Mn(x)As epilayers,"We have studied the effects of capping ferromagnetic Ga(1-x)Mn(x)As epilayers with a thin layer of undoped GaAs, and we find that even a few monolayers of GaAs have a significant effect on the ferromagnetic properties. In particular, the presence of a capping layer only 10 monolayers thick completely suppresses the enhancement of the ferromagnetism associated with low temperature annealing. This result, which demonstrates that the surface of a Ga(1-x)Mn(x)As epilayer strongly affects the defect structure, has important implications for the incorporation of Ga(1-x)Mn(x)As into device heterostructures.",0307255v1 2003-07-12,"Magnetic and microwave properties of (Ni,Co)Fe2O4-ferroelectric and (La,Ca,Sr)MnO3-ferroelectric multilayer structures","Structural, magnetic and ferromagnetic resonance characterization studies have been performed on lay-ered ferromagnetic-ferroelectric oxides that show strong magnetoelectric coupling. The samples contained thick films of ferrites or substituted lanthanum manganites for the ferromagnetic phase and lead zirconate titanate for the ferroelectric phase, and were sintered high temperatures. Results indicate defect free ferrites, but deterioration of manganite parameters due to diffusion at the interface and accounts for poor magnetoelectric coupling in manganite-PZT samples.",0307302v1 2003-07-15,Signatures of pairing mechanisms and order parameters in ferromagnetic superconductors,"Two predictions are made for properties of the ferromagnetic superconductors discovered recently. The first one is that spin-triplet, p-wave pairing in such materials will give the magnons a mass inversely proportional to the square of the magnetization. The second one is based on a specific mechanism for p-wave pairing, and predicts that the observed broad anomaly in the specific heat of URhGe will be resolved into a split transition with increasing sample quality. These predictions will help discriminate between different possible mechanisms for ferromagnetic superconductivity.",0307348v1 2003-07-21,Nuclear Magnetic Resonance in a Ferromagnet-Semiconductor Heterostructure,"We report the observation of nuclear magnetic resonance (NMR) in a ferromagnet-semiconductor heterostructure in the presence of a spin-polarized current. Spin-polarized electrons injected from a metallic ferromagnet generate a large nuclear spin population in a GaAs quantum well by dynamic polarization. The characteristic time for the polarization process is approximately 20 sec, and the nuclear polarization can persist for several minutes after the current is turned off. Resonant depolarization is observed in the presence of an AC magnetic field or when the injection current is modulated at the NMR frequency.",0307512v1 2003-08-23,Half-metallic ferromagnetism and structural stability of zincblende phases of the transition-metal chalcogenides,"An accurate density-functional method is used to study systematically half-metallic ferromagnetism and stability of zincblende phases of 3d-transition-metal chalcogenides. The zincblende CrTe, CrSe, and VTe phases are found to be excellent half-metallic ferromagnets with large half-metallic gaps (up to 0.88 eV). They are mechanically stable and approximately 0.31-0.53 eV per formula unit higher in total energy than the corresponding nickel-arsenide ground-state phases, and therefore would be grown epitaxially in the form of films and layers thick enough for spintronic applications.",0308463v1 2003-09-02,Giant fluctuations of superconducting order parameter in Ferromagnet/superconductor single electron transistors,"Spin dependent transport in a ferromagnet/superconductor/ferromagnet single electron transistor is studied theoretically with spin accumulation, spin relaxation, gap suppression, and charging effects taken into account. A strong dependence of the gap on the magnetic state of the outer electrodes is found, which gives rise to a negative magneto-resistance of up to 100 %. We predict that fluctuations of the spin accumulation due to tunneling of quasi-particles can play such an important role as to cause the island to fluctuate between the superconducting",0309055v3 2003-09-23,External Control of Magnetism in Semiconductors at High Temperature (~100K),"We demonstrate the electrical and optical control of ferromagnetism in semiconductor heterostructures at high temperatures of 100-117 K. The heterostructures consist of Mn delta-doped GaAs and p-type AlGaAs, where the overlap of the hole wavefunction with the Mn delta-doping profile leads to high ferromagnetic transition temperature TC of over 100 K (max. 172 K). We are able to isothermally change the paramagnetic state to the ferromagnetic state and vice versa, by applying a gate electric-field or by light irradiation. The large modulation of TC (DTC ~15 K) at high temperatures (> ~100 K) demonstrated here may pave the way to functional device applications compatible with the present semiconductor technology.",0309532v1 2003-10-02,Dynamic ferromagnetic proximity effect in photoexcited semiconductors,"The spin dynamics of photoexcited carriers in semiconductors in contact with a ferromagnet is treated theoretically and compared with time-dependent Faraday rotation experiments. The long time response of the system is found to be governed by the first tens of picoseconds in which the excited plasma interacts strongly with the intrinsic interface between semiconductor and ferromagnet in spite of the existence of a Schottky barrier in equilibrium.",0310031v1 2003-10-16,Spin Torque and its Relation to Spin Filtering,"The spin torque exerted on a magnetic moment is a reaction to spin filtering when spin-polarized electrons interact with a thin ferromagnetic film. We show that, for certain conditions, a spin transmission resonance (STR) gives rise to a failure of spin filtering. As a consequence, no spin is transfered to the ferromagnet. The condition for STR depends on the incoming energy of electrons and the thickness of the film. For a simple model we find that when the STR condition is satisfied, the ferromagnetic film is transparent to the incoming electrons.",0310392v1 2003-11-11,The internal magnetic field in superconducting ferromagnets,"We have measured the nonlinear response to the ac magnetic field in the superconducting weak ferromagnet Ru-1222, at different regimes of sample cooling which provides unambiguous evidence of the interplay of the domain structure and the vorticity in the superconducting state. This is {\em direct} proof of coexistence of ferromagnetic and superconductive order parameters in high-$T_c$ ruthenocuprates.",0311245v1 2003-11-20,Single-ion versus exchange anisotropy in calculating anisotropic susceptibilities of thin ferromagnetic Heisenberg films within many-body Green's function theory,"We compare transverse and parallel static susceptibilities of in-plane uniaxial anisotropic ferromagnetic Heisenberg films calculated in the framework of many-body Green's function theory using single-ion anisotropies with the previously investigated case of exchange anisotropies. On the basis of the calculated observables (easy and hard axes magnetizations and susceptibilities) no significant differences are found, i. e. it is not possible to propose an experiment that might decide which kind of anisotropy is acting in an actual ferromagnetic film.",0311482v1 2003-12-15,Split transition in ferromagnetic superconductors,"The split superconducting transition of up-spin and down-spin electrons on the background of ferromagnetism is studied within the framework of a recent model that describes the coexistence of ferromagnetism and superconductivity induced by magnetic fluctuations. It is shown that one generically expects the two transitions to be close to one another. This conclusion is discussed in relation to experimental results on URhGe. It is also shown that the magnetic Goldstone modes acquire an interesting structure in the superconducting phase, which can be used as an experimental tool to probe the origin of the superconductivity.",0312378v1 2004-01-05,Enhancement of flux-line pinning in all-oxide superconductor/ferromagnet heterostructures,"We have studied the local critical current density, jc, in the superconductor thin film of bilayer structures consisting of YBa2Cu3O7 and the ferromagnets La2/3Ca1/3MnO3 and SrRuO3, respectively, by means of quantitative magneto-optics. A pronounced hysteresis of jc was observed which is ascribed to the magnetization state of the ferromagnetic layer. The results are discussed within the frame of magnetic vortex - wall interactions.",0401033v1 2004-01-09,Ferromagnetic Transition in One-Dimensional Itinerant Electron Systems,"We use bosonization to derive the effective field theory that properly describes ferromagnetic transition in one-dimensional itinerant electron systems. The resultant theory is shown to have dynamical exponent z=2 at tree leve and upper critical dimension d_c=2. Thus one dimension is below the upper critical dimension of the theory, and the critical behavior of the transition is controlled by an interacting fixed point, which we study via epsilon expansion. Comparisons will be made with the Hertz-Millis theory, which describes the ferromagnetic transition in higher dimensions.",0401149v3 2004-01-24,Normal metal to ferromagnetic superconductor tunneling,"We study the point-contact tunneling between normal metal and ferromagnetic superconductor. In the case of magnon-induced pairing the tunneling conductance is continuous and smooth function of the applied voltage. For small values of the applied voltage the Ohm law holds. We show that one can obtain the magnetization and the superconducting order parameter from the tunneling conduc- tance. In the case of paramagnon-induced superconductivity the tunneling does not depend on the magnetization. We argue that tunneling experiment can unambiguously determine the correct pairing mechanism in the ferromagnetic superconductors.",0401480v2 2004-01-30,Non-quasiparticle states in the core level spectra of ferromagnetic semiconductors and half-metallic ferromagnets,"The Green's functions that determine x-ray spectra are calculated in the s-d exchange model of a saturated conducting ferromagnet in the presence of the core hole. A possibility to observe non-quasiparticle (NQP) states in the core level (x-ray absorption, emission and photoelectron) spectroscopy is demonstrated. It is shown that NQP contributions to resonant x-ray scattering spectra can be considerably enhanced by core hole effects in comparison with those to the density of states.",0401625v2 2004-02-16,Uniform Magnetic Order in a Ferromagnetic-Antiferromagnetic Random Alternating Quantum Spin Chain,"An $S=1/2$ ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg quantum spin chain model is investigated in connection to its realization compound: (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$. The exchange interaction bonds have alternating strong F-AF random bonds and weak uniform AF bonds. Using the quantum Monte Carlo method we have found that the excitation energy gap closes and the uniform AF order becomes critical in the intermediate concentration region. This finding explains the experimental observation of the magnetic transition by considering weak interchain interactions. Present results suggest that the uniform AF order survives even in the presence of randomly located ferromagnetic bonds. This may be a new quantum effect.",0402402v1 2004-03-30,Superfluid vs Ferromagnetic Behaviour in a Bose Gas of Spin-1/2 Atoms,"We study the thermodynamic phases of a gas of spin-1/2 atoms in the Hartree-Fock approximation. Our main result is that, for repulsive or weakly-attractive inter-component interaction strength, the superfluid and ferromagnetic phase transitions occur at the same temperature. For strongly-attractive inter-component interaction strength, however, the ferromagnetic phase transition occurs at a higher temperature than the superfluid phase transition. We also find that the presence of a condensate acts as an effective magnetic field that polarizes the normal cloud. We finally comment on the validity of the Hartree-Fock approximation in describing different phenomena in this system.",0403720v2 2004-04-16,Changes in magnetic scattering anisotropy at a ferromagnetic/superconducting interface,"We show that some metals and alloys (X = Cu, Ag, FeMn, or Cu and Ag combined with each other), sputtered between ferromagnetic Co and superconducting Nb, produce no change in current-perpendicular-to-plane magnetoresistance (CPP-MR) in a carefully designed CPP-spin-valve. In contrast, other metals (Ru or Au) or combinations (Cu or Ag combined with Au, Ru, or FeMn) change the CPP-MR, in some cases even reversing its sign. We ascribe these changes to activation of magnetic scattering anisotropies at a ferromagnetic/superconducting interface, apparently by strong spin-flipping between the Co and Nb layers.",0404396v1 2004-05-19,"Softer than normal, but not as soft as one might think: Spontaneous flux lattices in ferromagnetic spin-triplet superconductors","A theory is developed for the spontaneous vortex lattice that is expected to occur in the ferromagnetic superconductors ZrZn_2, UGe_2, and URhGe, where the superconductivity is likely of spin-triplet nature. The long-wavelength fluctuations of this spontaneous flux lattice are predicted to be huge compared to those of a conventional flux lattice, and to be the same as those for spin-singlet ferromagnetic superconductors. It is shown that these fluctuations lead to unambiguous experimental signatures which may provide the easiest way to observe the spontaneous flux lattice.",0405461v1 2004-07-02,Spin accumulation in ferromagnets,"Using a density matrix formulation for the effective action, we obtain a set of macroscopic equations that describe spin accumulation in a non-homogeneous ferromagnet. We give a new expression for the spin current which extends previous work by taking into account the symmetry of the ferromagnetic state through a careful treatment of the exchange term between the conduction electrons and the magnetization, i.e, d-electrons. We consider a simple application which has been discussed previously and show that in this case spin accumulation is an interface effect confirming earlier results arrived at by different methods.",0407051v1 2004-07-14,Quantum Mechanics of Spin Transfer in Ferromagnetic Multilayers,"We use a quantum mechanical treatment of a ballistic spin current to describe novel aspects of spin transfer to a ferromagnetic multilayer. We demonstrate quantum phenomena from spin transmission resonance (STR) to magnetoelectric spin echo (MESE), depending on the coupling between the magnetic moments in the ferromagnetic thin films. Our calculation reveals new channels through which the zero spin transfer occurs in multilayers: the STR and MESE. We also illustrate that counter-intuitively, a negative spin torque can act initially on the second moment in a bilayer system.",0407365v1 2004-08-10,Quantum coherence in a ferromagnetic metal: time-dependent conductance fluctuations,"Quantum coherence of electrons in ferromagnetic metals is difficult to assess experimentally. We report the first measurements of time-dependent universal conductance fluctuations in ferromagnetic metal (Ni$_{0.8}$Fe$_{0.2}$) nanostructures as a function of temperature and magnetic field strength and orientation. We find that the cooperon contribution to this quantum correction is suppressed, and that domain wall motion can be a source of coherence-enhanced conductance fluctuations. The fluctuations are more strongly temperature dependent than those in normal metals, hinting that an unusual dephasing mechanism may be at work.",0408221v2 2004-08-27,Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions,"We show that the resistivity rho(T) of disordered ferromagnets near, and above, the Curie temperature T_c generically exhibits a stronger anomaly than the scaling-based Fisher-Langer prediction. Treating transport beyond the Boltzmann description, we find that within mean-field theory, d\rho/dT exhibits a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or diluted magnetic semiconductors, whose mobility near T_c is limited by disorder.",0408602v2 2004-09-05,Bias voltage controlled magnetization switch in ferromagnetic semiconductor resonant tunneling diodes,"We predict that the Curie temperature of a ferromagnetic resonant tunneling diode will decrease abruptly, by approximately a factor of two, when the downstream chemical potential falls below the quantum well resonance energy. This property follows from elementary quantum transport theory notions combined with a mean field description of diluted magnetic semiconductor ferromagnetism. We illustrate this effect by solving coupled non-equilibrium Green's function, magnetic mean-field, and electrostatic Poisson equations self-consistently to predict the bias voltage and temperature dependence of the magnetization of a model system.",0409106v2 2004-09-12,"Trends in ferromagnetism, hole localization, and acceptor level depth for Mn substitution in","We examine the intrinsic mechanism of ferromagnetism in dilute magnetic semiconductors by analyzing the trends in the electronic structure as the host is changed from GaN to GaP, GaAs and GaSb, keeping the transition metal impurity fixed. In contrast with earlier interpretations which depended on the host semiconductor, we found that a single mechanism is sufficient to explain the ferromagnetic stabilization energy for the entire series.",0409296v1 2004-09-13,Spin order in the one-dimensional Kondo and Hund lattices,"We study numerically the one-dimensional Kondo and Hund lattices consisting of localized spins interacting antiferro or ferromagnetically with the itinerant electrons, respectively. Using the Density Matrix Renormalization Group we find, for both models and in the small coupling regime, the existence of new magnetic phases where the local spins order forming ferromagnetic islands coupled antiferromagnetically. Furthermore, by increasing the interaction parameter $|J|$ we find that this order evolves toward the ferromagnetic regime through a spiral-like phase with longer characteristic wave lengths. These results shed new light on the zero temperature magnetic phase diagram for these models.",0409332v1 2004-09-16,Weak ferromagnetism of La_{1.99}Sr_{0.01}CuO_4 thin films: evidence for removal of corrugation in CuO_2 plane by epitaxial strain,"The weak ferromagnetism of La_{1.99}Sr_{0.01}CuO_4 epitaxial thin films is investigated using magnetoresistance measurement. While a steplike negative magnetoresistance associated with the weak ferromagnetic transition is clearly observed in the films grown on YAlO_3(001), it is notably suppressed in the films grown on SrTiO_3(100) and (LaAlO_3)_{0.3}(SrAl_{0.5}Ta_{0.5}O_3)_{0.7}(100), and almost disappears in films grown on LaSrAlO_4(001). The strong suppression of the steplike magnetoresistance provides evidence that the CuO_2 planes are much less corrugated in thin films grown on tetragonal substrates, particularly on LaSrAlO_4(001), than in bulk crystals.",0409436v1 2004-10-19,The Meservey-Tedrov effect in FSF double tunneling junctions,"Double tunneling junctions of ferromagnet-superconductor-ferromagnet electrodes (FSF) show a jump in the conductance when a parallel magnetic field reverses the magnetization of one of the ferromagnetic electrodes. This change is generally attributed to the spin-valve effect or to pair breaking in the superconductor because of spin accumulation. In this paper it is shown that the Meservey-Tedrov effect causes a similar change in the conductance since the magnetic field changes the energy spectrum of the quasi-particles in the superconductor. A reversal of the bias reverses the sign in the conductance jump.",0410486v1 2004-10-22,Phenomenological theory of superconductivity near domain walls in ferromagnets,"We develop a phenomenological model of superconductivity near a domain wall in a ferromagnet. In addition to the electromagnetic interaction of the order parameter with the ferromagnetic magnetization, we take into account the possibility of a local enhancement or suppression of superconducting pairing in the vicinity of the wall, and also a non-perfect transparency of the wall to electrons. It is found that the critical temperature of superconductivity near the domain wall might be substantially higher than in the bulk.",0410587v1 2004-11-17,Optical response of a ferromagnetic/DMS hybrid structure,"We investigate the possibility of using local magnetic fields to produce one-dimensional traps in hybrid structures for any quasiparticle possessing spin degree of freedom. We consider a system composed of a diluted magnetic semiconductor quantum well buried below a micron-sized ferromagnetic island. Localized magnetic field is produced by a rectangular ferromagnet in close proximity of a single domain phase. We make quantitative predictions for the optical response of the system as a function of distance between the micromagnet and the quantum well, electronic g-factor, and thickness of the micromagnet.",0411456v1 2004-12-04,"Ferromagnetic Instability in AFe4Sb12 (A = Ca, Sr, and Ba)","Magnetic, transport and thermal properties of AFe4Sb12 (A = Ca, Sr, Ba) are reported. All three compounds show a maximum in both the magnetic susceptibility and thermopower at 50 K, and a large electronic specific heat coefficient of 100 mJ/mol K2. These properties are the characteristics of a nearly ferromagnetic metal. Furthermore, a remanent moment of the order of 10-3muB/Fe was observed below 54, 48, and 40 K for A = Ca, Sr, and Ba, respectively. The volume fraction of the ferromagnetic component was estimated to be 10-20% by zero-field muSR measurements.",0412094v1 2004-12-08,Interplay between ferromagnetism and superconductivity in tunneling currents,"We study tunneling currents in a model consisting of two non-unitary ferromagnetic spin-triplet superconductors separated by a thin insulating layer. We find a novel interplay between ferromagnetism and superconductivity, manifested in the Josephson effect. This offers the possibility of tuning dissipationless currents of charge and spin in a well-defined manner by adjusting the magnetization direction on either side of the junction.",0412193v3 2004-12-15,Effects of edges in spin-1/2 bond-alternating Heisenberg chains: Matrix-product variational approach,"We make a matrix-product variational approach to spin-1/2 ferromagnetic-antiferromagnetic bond-alternating chains with anisotropy on their ferromagnetic bonds, especially under the open boundary condition. The rich phase diagram containing the Haldane, large-D, and two types of Neel phases is well reproduced with only two variational parameters. The on-bond anisotropy has a significant effect on the ferromagnetic coupling between neighboring spins and induces novel edge states peculiar to spin-1/2 chains.",0412390v1 2004-12-17,Mesoscopic bound on anisotropy in itinerant ferromagnets,"We calculate the anisotropy energy of a single-domain ferromagnetic particle in which the only source of anisotropy is the presence of non-magnetic impurities. We find that such anisotropy takes the form of combined easy-axis and easy-plane anisotropies, with random orientations of the axes. Typically the anisotropy energy is of order $N^{1/2} \hbar/tau_{so}$, where $N$ is the number of electrons in the ferromagnetic particle and $\tau_{so}$ is the spin-orbit time.",0412488v1 2005-01-10,Exchange instabilities in electron systems: Bloch versus Stoner Ferromagnetism,"We show that 2D and 3D electron systems with the long-range Coulomb electron-electron interaction could develop ferromagnetic instabilities due to strong exchange effects at low densities. The critical densities in both 2D and 3D systems at which the magnetic instability, which could either be of Stoner type (second-order) or of Bloch type (first-order), are higher than the dispersion instability critical density where effective mass at the Fermi surface diverges. We discuss the theoretical as well as experimental implications of the ferromagnetic instability at low electron densities, particularly in low-disorder semiconductor-based two-dimensional systems.",0501213v2 2005-01-12,Onset of metallic ferromagnetism in a doped spin-orbital chain,"Starting from a spin-orbital model for doped manganites, we investigate a competition between ferromagnetic and antiferromagnetic order in a one-dimensional model at finite temperature. The magnetic and orbital order at half filling support each other and depend on a small antiferromagnetic superexchange between t_{2g} spins and on an alternating Jahn-Teller potential. The crossover to a metallic ferromagnetic phase found at finite doping is partly suppressed by the Jahn-Teller potential which may localize e_g electrons.",0501290v1 2005-01-12,Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures,"We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.",0501298v2 2005-01-25,Novel Josephson Effect in Triplet Superconductor - Ferromagnet - Triplet Superconductor Junctions,"We predict a novel type of Josephson effect to occur in triplet superconductor - ferromagnet - triplet superconductor Josephson junctions. We show that the Josephson current, I_J, exhibits a rich dependence on the relative orientation between the ferromagnetic moment and the d-vectors of the superconductors. This dependence can be used to build several types of Josephson current switches. Moreover, we predict an unconventional temperature dependence of I_J in which I_J changes sign with increasing temperature",0501614v1 2005-01-28,Pseudospin ferromagnetism in double-quantum-wire systems,"We propose that a pseudospin ferromagnetic (i.e inter-wire coherent) state can exist in a system of two parallel wires of finite width in the presence of a perpendicular magnetic field. This novel quantum many-body state appears when the inter wire distance decreases below a certain critical value which depends on the magnetic field. We determine the phase boundary of the ferromagnetic phase by analyzing the softening of the spin-mode velocity using the bosonization approach. We also discuss signatures of this state in tunneling and Coulomb drag experiments.",0501694v2 2005-02-11,Self-generated locality near a ferromagnetic quantum-critical point,"We analyze the behavior of interacting fermions near a ferromagnetic Stoner instability. We show that the Landau damping of the spin susceptibility is a relevant perturbation near a ferromagnetic quantum-critical point (FQCP). We argue that, as the system approaches a FQCP, the fermionic self-energy crosses over from predominantly momentum dependent away from the transition to predominantly frequency dependent in the immediate vicinity of the transition. We argue that due to this self-generated locality, the quasiparticle effective mass does not diverge before a FQCP is reached.",0502302v1 2005-03-14,Exchange coupling induced antiferromagnetic-ferromagnetic transition in $Pr_{0.5}Ca_{0.5}MnO_3/La_{0.5}Ca_{0.5}MnO_3$ superlattices,"Superlattices built from two antiferromagnetic (AFM) charge/orbital order compounds, $Pr_{0.5}Ca_{0.5}MnO_3$ and $La_{0.5}Ca_{0.5}MnO_3$, have been studied as the thickness of $La_{0.5}Ca_{0.5}MnO_3$ ($LCMO$) varied. High structural quality thin films were obtained on $LaAlO_3$ substrates using the pulsed laser deposition technique. An antiferromagnetic-to-ferromagnetic transition, in addition to an enhancement of the coercivity, are observed as the $LCMO$ layer thickness increases. The small shift in the origin of the field-cooled hysteresis loop along the field axis indicates the presence of ferromagnetic and antiferromagnetic phases in the superlattices. We attribute these features to the AFM spin fluctuations at the $Pr_{0.5}Ca_{0.5}MnO_3/La_{0.5}Ca_{0.5}MnO_3$ interfaces resulting from the strain effects.",0503327v1 2005-04-07,Evolution of ferromagnetic circular dichroism coincident with magnetization and anomalous Hall effect in Co-doped rutile TiO2,"Magnetic circular dichroism (MCD) of rutile Ti1-xCoxO2-d is systematically examined with various x and d to reveal a phase diagram for the appearance of ferromagnetism at higher carrier concentration and Co content. The phase diagram exactly matches with that determined from anomalous Hall effect (AHE). The magnetic field dependence of MCD also shows good coincidence with those of the magnetization and AHE. The coincidence of these independent measurements strongly suggests single and intrinsic ferromagnetic origin.",0504167v1 2005-04-07,Magnetic oxide semiconductors,"Magnetic oxide semiconductors, oxide semiconductors doped with transition metal elements, are one of the candidates for a high Curie temperature ferromagnetic semiconductor that is important to realize semiconductor spintronics at room temperature. We review in this paper recent progress of researches on various magnetic oxide semiconductors. The magnetization, magneto-optical effect, and magneto-transport such as anomalous Hall effect are examined from viewpoint of feasibility to evaluate the ferromagnetism. The ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.",0504168v1 2005-04-16,Spontaneous magnetization and structure formation in a spin-1 ferromagnetic Bose-Einstein condensate,"Motivated by recent experiments involving the non-destructive imaging of magnetization of a spin-1 87Rb Bose gas (Higbie et al., cond-mat/0502517), we address the question of how the spontaneous magnetization of a ferromagnetic BEC occurs in a spin-conserving system. Due to competition between the ferromagnetic interaction and the total spin conservation, various spin structures such as staggered magnetic domains, and helical and concentric ring structures are formed, depending on the geometry of the trapping potential.",0504398v1 2005-04-18,Determination of the magnetic domain size in the ferromagnetic superconductor UGe2 by three dimensional neutron depolarization,"Three dimensional neutron depolarization measurements have been carried out on single-crystalline UGe2 between 4 K and 80 K in order to determine the average ferromagnetic domain size d. It is found that below T_C = 52K uniaxial ferromagnetic domains are formed with an estimated magnetic domain size of d = 4 - 5 micrometer.",0504430v1 2005-04-26,Spin filtering through ferromagnetic BiMnO3 tunnel barriers,"We report on experiments of spin filtering through ultra-thin single-crystal layers of the insulating and ferromagnetic oxide BiMnO3 (BMO). The spin polarization of the electrons tunneling from a gold electrode through BMO is analyzed with a counter-electrode of the half-metallic oxide La2/3Sr1/3MnO3 (LSMO). At 3 K we find a 50% change of the tunnel resistances according to whether the magnetizations of BMO and LSMO are parallel or opposite. This effect corresponds to a spin filtering effciency of up to 22%. Our results thus show the potential of complex ferromagnetic insulating oxides for spin filtering and injection.",0504667v1 2005-05-04,Intrinsic Curie temperature bistability in ferromagnetic semiconductor resonant tunneling diodes,"We predict bistability in the Curie temperature-voltage characteristic of double barrier resonant-tunneling structures with dilute ferromagnetic semiconductor quantum wells. Our conclusions are based on simulations of electrostatics and ballistic quantum transport combined with a mean-field theory description of ferromagnetism in dilute magnetic semiconductors.",0505101v2 2005-05-10,Propagating Coherent Acoustic Phonon Wavepackets in InMnAs/GaSb,"We observe pronounced oscillations in the differential reflectivity of a ferromagnetic InMnAs/GaSb heterostructure using two-color pump-probe spectroscopy. Although originally thought to be associated with the ferromagnetism, our studies show that the oscillations instead result from changes in the position and frequency-dependent dielectric function due to the generation of coherent acoustic phonons in the ferromagnetic InMnAs layer and their subsequent propagation into the GaSb. Our theory accurately predicts the experimentally measured oscillation period and decay time as a function of probe wavelength.",0505261v1 2005-06-13,Magnetism via superconductivity in SF proximity structures,"We consider the proximity effects in hybrid superconductor (S) - ferromagnet (F) structures drawing attention to the induced ferromagnetism of the S metal. The analysis is based on a quasiclassical theory of proximity effect for metals in the dirty limit conditions. It is shown that, below the superconducting critical temperature, ferromagnetic correlations extend a distance of order of the superconducting coherence length \xi_{S} into the superconductor, being dependent on the S/F interface parameters. We argue that the properties of mesoscopic SF hybrids may drastically depend upon the magnetic proximity effect, and recent experiments lend support to the model of SF structures where the superconducting and magnetic parameters are tightly coupled.",0506277v1 2005-06-14,Spin-wave softening and Hund's coupling in ferromagnetic manganites,"Using one-orbital model of hole-doped manganites, we show with the help of Holstein-Primakov transformation that finite Hund's coupling is responsible for the spin-wave softening in the ferromagnetic $B$-phase manganites. We obtain an analytical result for the spin-wave spectrum for $\JH\gg t$. In the limit of infinte Hund's coupling, the spectrum is the conventional nearest-neighbor Heisenberg ferromagnetic spin-wave. The $o(t/\JH)$-order correction is negative and thus accounts for the softening near the zone boundary.",0506321v1 2005-06-15,Saturated Ferromagnetism from Statistical Transmutation in Two Dimensions,"The total spin of the ground state is calculated in the U -> infinity Hubbard model with uniform magnetic flux perpendicular to a square lattice, in the absence of Zeeman coupling. It is found that the saturated ferromagnetism emerges in a rather wide region in the space of the flux density \phi and the electron density n_e. In particular, the saturated ferromagnetism at \phi = n_e is induced by the formation of a spin-1/2 boson, which is a composite of an electron and the unit flux quantum.",0506352v3 2005-07-06,A ferromagnetic oxide semiconductor as spin injection electrode in magnetic tunnel junction,"A magnetic tunnel junctions composed of room temperature ferromagnetic semiconductor rutile Ti1-xCoxO2-d and ferromagnetic metal Fe0.1Co0.9 separated by AlOx barrier showed positive tunneling magnetoresistance (TMR) with a ratio of ~11 % at 15 K, indicating that Ti1-xCoxO2-d can be used as a spin injection electrode. The TMR decreased with increasing temperature and vanished above 180 K. TMR action at high temperature is likely prohibited by the inelastic tunneling conduction due to the low quality of the amorphous barrier layer and/or the junction interface.",0507137v1 2005-07-16,Interplay of Magnetic and Superconducting Proximity Effects in FSF Trilayers,"We present theoretical results on the interplay of magnetic and superconducting orders in diffusive ferromagnet-superconductor-ferromagnet trilayers. The induced triplet superconducting correlations throughout the trilayer lead to an induced spin magnetization. We include self-consistency of the order parameter in the superconducting layer at arbitrary temperatures, arbitrary interface transparency, and any relative orientation of the exchange fields in the two ferromagnets. We propose to use the torque on the trilayer in an external magnetic field as a probe of the presence of triplet correlations in the superconducting phase.",0507398v1 2005-07-20,Spin-lattice relaxation phenomena in manganite La0.7Sr0.3MnO3 thin films,"Time-resolved magneto-optics was used to study spin-lattice relaxation dynamics in thin epitaxial La0.7Sr0.3MnO3 films. Two distinct recovery regimes of the ferromagnetic order can be resolved upon photoexcitation, which manifest themselves by two different relaxation times. A pump pulse energy independent spin-lattice relaxation time can be deduced. Due to a weak spin-orbit coupling in manganites this spin-lattice relaxation time is much longer than in ferromagnetic metals. Heat flow into the substrate sets the ultimate recovery speed of the ferromagnetic order and allows for a determination of heat diffusion properties of manganite films.",0507481v1 2005-08-17,Magnetic Composites: Magnonic Excitations vs. Three-Dimensional Structural Periodicity,"This study deals with the spin wave spectrum in magnetic macrostructure (composed of two ferromagnetic materials) showing a 3D periodicity: spherical ferromagnetic grains disposed in the nodes of a simple cubic crystal lattice are embedded in a matrix with different ferromagnetic properties. It is shown that the \textit{magnonic spectrum} of this composite structure exhibits frequency regions \textit{forbidden} for magnon propagation, and the energy gaps are found to be sensitive to the exchange contrast between the constituent materials as well as to the magnetization contrast. The widths of the respective magnonic gaps are studied as functions of parameters characterizing the magnetic structure.",0508390v1 2005-08-24,Double-reversal thickness dependence of critical current in superconductor-ferromagnet-superconductor Josephson junctions,"We report the first experimental observation of the two-node thickness dependence of the critical current in Josephson junctions with a ferromagnetic interlayer. Vanishings of the critical current correspond to transitions into pi-state and back into conventional 0-state. The experimental data allow to extract the superconducting order parameter oscillation period and the pair decay length in the ferromagnet. We develope a theoretical approach based on Usadel equations, which takes into account the spin-flip scattering. Results of numerical calculations are in good agreement with the experimental data.",0508573v1 2005-08-26,Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field,"We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. Application of a magnetic field perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. Below approximately 0.9 K, intermolecular interactions turn this equilibrium state into a ferromagnetically ordered phase. However, long range ferromagnetic correlations nearly disappear for fields larger 5.5 T, possibly suggesting the existence of a quantum critical point.",0508627v1 2005-08-30,Effects of spin current on ferromagnets,"When a spin-polarized current flows through a ferromagnet, the local magnetization receives a spin torque. Two consequences of this spin torque are studied. First, the uniformly magnetized ferromagnet becomes unstable if a sufficiently large current is applied. The characteristics of the instability include spin wave generation and magnetization chaos. Second, the spin torque has profound effects on the structure and dynamics of the magnetic domain wall. A detail analysis on the domain wall mass, kinetic energy and wall depinning threshold is given.",0508735v1 2005-09-15,Ferromagnetic Domain Structure of La0.78Ca0.22MnO3 Single Crystals,"The magneto-optical technique has been employed to observe spontaneous ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The magnetic domain topology was found to be correlated with the intrinsic twin structure of the investigated crystals. With decreasing temperature the regular network of ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range of 70-150 K. The apparent rotation of the domain walls can be understood in terms of the Jahn-Teller deformation of the orthorhombic unit cell, accompanied by additional twinning.",0509423v1 2005-09-16,First principles investigation of the electronic structure of La2MnNiO6: A room-temperature insulating ferromagnet,"Using first principles calculations within DFT based on the full potential APW+lo method, we calculated the electronic and magnetic structures for the ferromagnetic and antiferromagnetic states of La2MnNiO6 and analyzed the site projected density of states and electronic band structures. Our calculations show that the ground state of La2MnNiO6 is ferromagnetic insulating with the magnetization in agreement with Hund's first rule and experimental findings.",0509431v1 2005-09-21,Zero-temperature magnetic-field-induced phase transition between two ordered gapped phases in spin-ladders with ferromagnetic legs,"We suggest that under an increase of a magnetic field a spin-ladder with ferromagnetic legs does not pass without fail through an incommensurate phase but possibly in a straight way turns into a fully polarized ferromagnetic phase. The spin gap remains finite at the transition point. This scenario is demonstrated for the special spin-ladder model with exact singlet-rung ground state. We suppose that this kind of behavior realizes for the spin-ladder material (5IAP)_2CuBr_4*H_2O",0509556v2 2005-09-26,Crossed Andreev reflection as a probe for the pairing symmetry of Ferromagnetic Superconductors,"The coexistence of superconductivity and ferromagnetism has brought about the phenomena of ferromagnetic superconductors. The theory needed to understand the compatibility of such antagonistic phenomena cannot be built until the pairing symmetry of such superconductors is correctly identified. The proper and unambiguous identification of the pairing symmetry of such superconductors is the subject of this paper. This work shows that crossed Andreev reflection can be a very effective tool in order to identify the pairing symmetry of these superconductors.",0509669v2 2005-10-21,Origin of Spontaneous Currents in a Superconductor-Ferromagnet Proximity System,"We have previously shown that a ferromagnet-superconductor heterostructure may possess a spontaneous current circulation parallel to the interface. This current is caused by Andreev bound states in the thin ferromagnetic layer, and can be fully spin-polarized. Here we investigate the total energy of the system in cases where the current either does or does not flow. We show that the current is a true quantum ground state effect, and examine the effect of the current on the different contributions to the total energy.",0510591v1 2005-11-03,Non-equilibrium Magnetic Properties of Single Crystalline La0.7Ca0.3CoO3,"Magnetic and electric properties of a single crystal of La0.7Ca0.3CoO3 have been experimentally studied. The system attains a ferromagnetic spontaneous moment below 170 K and exhibits a re-entrant spin-glass phase below 100 K. In the ordered and the re-entrant phases, the low field magnetic properties are strongly direction dependent, showing considerably higher magnetization values perpendicular than parallel to the c-axis. Magnetic relaxation experiments show that both the ferromagnetic and the re-entrant spin-glass phases are non-equilibrium states, where the system exhibits magnetic aging characteristic of spin-glasses and disordered and frustrated ferromagnets.",0511066v1 2005-11-03,Probable absence of a quadrupolar spin-nematic phase in the bilinear-biquadratic spin-1 chain,"We study numerically the ground-state phase diagram of the bilinear-biquadratic spin-1 chain near the ferromagnetic instability point, where the existence of a gapped or gapless nondimerized quantum nematic phase has been suggested. Our results, obtained by a highly accurate density-matrix renormalization-group (DMRG) calculation are consistent with the view that the order parameter characterizing the dimer phase vanishes only at the point where the system becomes ferromagnetic, although the existence of a gapped or gapless nondimerized phase in a very narrow parameter range between the ferromagnetic and the dimerized regimes cannot be ruled out.",0511079v1 2005-11-17,Collective states of interacting ferromagnetic nanoparticles,"Discontinuous magnetic multilayers [CoFe/Al2O3] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co80Fe20 nanoparticles are embedded in a diamagnetic insulating a-Al2O3 matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.",0511428v1 2005-11-22,Electronic properties of EuB6 in the ferromagnetic regime: Half-metal versus semiconductor,"To understand the halfmetallic ferromagnet EuB6 we use the Kondo lattice model for valence and conduction band. By means of a recently developed many-body theory we calculate the electronic properties in the ferromagnetic regime up to the Curie temperature. The decreasing magnetic order induces a transition from halfmetallic to semiconducting behavior along with a band broadening. We show the temperature dependence of the quasiparticle density of states and the quasiparticle dispersion as well as the effective mass, the number of carriers and the plasma frequency which are in good agreement with the experimental data.",0511536v1 2005-11-22,Fabrication of high quality ferromagnetic Josephson junctions,"We present ferromagnetic Nb/Al2O3/Ni60Cu40/Nb Josephson junctions (SIFS) with an ultrathin Al2O3 tunnel barrier. The junction fabrication was optimized regarding junction insulation and homogeneity of current transport. Using ion-beam-etching and anodic oxidation we defined and insulated the junction mesas. The additional 2 nm thin Cu layer below the ferromagnetic NiCu (SINFS) lowered interface roughness and ensured very homogeneous current transport. A high yield of junctional devices with jc spreads less than 2% was obtained.",0511546v1 2005-12-02,Exchange enhanced anisotropy in ferromagnetic/antiferromagnetic multilayers: dynamic consequences,"The phenomena of exchange anisotropy is well known in terms of static magnetization properties such as enhanced coercivity and magnetization loop shifts. These effects are primarily associated with effective anisotropies introduced into the ferromagnet by exchange coupling with a strongly anisotropic antiferromagnet. These effective anisotropies can be understood as manifestations of a more fundamental exchange induced susceptibility. We show that a consequence of this view is that a class of unusual dynamic effects associated with the exchange susceptibility should also exist. The effects become apparent near the ordering temperature of the antiferromagnet and affect domain wall velocities, domain wall resonances, and precessional switching of the ferromagnet.",0512034v1 2005-12-05,Crossed Andreev reflection-induced magnetoresistance,"We show that very large negative magnetoresistance can be obtained in magnetic trilayers in a current-in-plane geometry owing to the existence of crossed Andreev reflection. This spin-valve consists of a thin superconducting film sandwiched between two ferromagnetic layers whose magnetization is allowed to be either parallelly or antiparallelly aligned. For a suitable choice of structure parameters and nearly fully spin-polarized ferromagnets the magnetoresistance can exceed -80%. Our results are relevant for the design and implementation of spintronic devices exploiting ferromagnet-superconductor structures.",0512090v2 2005-12-13,Mesoscopic anisotropic magnetoconductance fluctuations in ferromagnets,"The conductance of a ferromagnetic particle depends on the relative orientation of the magnetization with respect to the direction of current flow. This phenomenon is known as ""anisotropic magnetoresistance"". Quantum interference leads to an additional, random dependence of the conductance on the magnetization direction. These ""anisotropic magnetoresistance fluctuations"" are caused by spin-orbit scattering, which couples the electron motion to the exchange field in the ferromagnet. We report a calculation of the dependence of the conductance autocorrelation function on the rotation angle of the magnetization direction.",0512287v1 2005-12-20,In-plane ferromagnetism in charge-ordering $Na_{0.55}CoO_2$,"The magnetic and transport properties are systematically studied on the single crystal $Na_{0.55}CoO_2$ with charge ordering and divergency in resistivity below 50 K. A long-range ferromagnetic ordering is observed in susceptibility below 20 K with the magnetic field parallel to Co-O plane, while a negligible behavior is observed with the field perpendicular to the Co-O plane. It definitely gives a direct evidence for the existence of in-plane ferromagnetism below 20 K. The observed magnetoresistance (MR) of 30 % at the field of 6 T at low temperatures indicates an unexpectedly strong spin-charge coupling in triangle lattice systems.",0512507v1 2006-02-04,Spin-transfer-driven ferromagnetic resonance of individual nanomagnets,"We demonstrate a technique that enables ferromagnetic resonance (FMR) measurements of the normal modes for magnetic excitations in individual nanoscale ferromagnets, smaller in volume by a factor of 1000 than can be probed by other methods. The measured peak shapes indicate two regimes of response: simple FMR and phase locking. Studies of the resonance frequencies, amplitudes, and linewidths as a function of microwave power, DC current, and magnetic field provide detailed new information about the exchange, damping, and spin-transfer torques that govern the dynamics in magnetic nanostructures.",0602105v1 2006-02-05,Half-metallic ferromagnetism induced by dynamic electron correlations in VAs,"The electronic structure of the VAs compound in the zinc-blende structure is investigated using a combined density-functional and dynamical mean-field theory approach. Contrary to predictions of a ferromagnetic semiconducting ground state obtained by density-functional calculations, dynamical correlations induce a closing of the gap and produce a half-metallic ferromagnetic state. These results emphasize the importance of dynamic correlations in materials suitable for spintronics.",0602112v2 2006-03-07,Theory of carrier mediated ferromagnetism in dilute magnetic oxides,"We analyze the origin of ferromagnetism as a result of carrier mediation in diluted magnetic oxide semiconductors in the light of the experimental evidence reported in the literature. We propose that a combination of percolation of magnetic polarons at lower temperature and Ruderman-Kittel-Kasuya-Yosida ferromagnetism at higher temperature may be the reason for the very high critical temperatures measured (up to ~700 K).",0603182v2 2006-03-09,Spin Current and Current-Induced Spin Transfer Torque in Ferromagnet-Quantum Dot-Ferromagnet Coupled Systems,"Based on Keldysh's nonequilibrium Green function method, the spin-dependent transport properties in a ferromagnet-quantum dot (QD)-ferromagnet coupled system are investigated. It is shown the spin current shows quite different characteristics from its electrical counterpart, and by changing the relative orientation of both magnetizations, it can change its magnitude even sign. The current-induced spin transfer torque (CISTT) is uncovered to be greatly enhanced when the bias voltage meets with the discrete levels of the QD at resonant positions. The relationship between the CISTT, the electrical current and the spin current is also addressed.",0603235v1 2006-03-10,Model Hamiltonian parameters for half-metallic ferromagnets NiMnSb and CrO2,"Using the recently developed Nth-order muffin-tin-orbital (NMTO) based downfolding technique we revisit the electronic properties of half-metallic ferromagnets, the semi-Heusler NiMnSb and rutile CrO2. The NMTO Wannier orbitals for the Mn-d and Cr-t2g manifolds are constructed and the mechanism of chemical bonding is discussed. The effective hopping Hamiltonian parameters are calculated using a NMTO downfolded basis set. We propose model Hamiltonian parameters with possibly minimal basis sets for both half-metallic ferromagnetic alloys.",0603305v2 2006-03-20,Critical behavior of Griffiths ferromagnets,"From a heuristic calculation of the leading order essential singularity in the distribution of Yang-Lee zeroes, we obtain new scaling relations near the ferromagnetic-Griffiths transition, including the prediction of a discontinuity on the analogue of the critical isotherm. We show that experimental data for the magnetization and heat capacity of $\mathrm{La_{0.7}Ca_{0.3}MnO_3}$ are consistent with these predictions, thus supporting its identification as a Griffiths ferromagnet.",0603533v2 2006-03-29,Exchange Bias with Interacting Random Anti-ferromagnetic Grains,"A model consisting of random interacting anti-ferromagnetic (AF) grains coupled to a ferromagnetic (FM) layer is developed to study the exchange bias phenomenon. This simple model is able to describe several exchange bias behavior observed in real materials. Shifts in hysteresis loops are observed as a function of cooling field and average grain size. We establish a direct relationship between cooling field dependence of exchange bias, coercivity and magnetization state on the AF-FM interface. We also verify that the exchange bias field is inversely proportional to the grain size, and this behavior is independent of the inter-grain interactions, AF/FM coupling and cooling field.",0603780v1 2006-04-06,Origin of ferromagnetism in Cs$_2$AgF$_4$: importance of Ag - F covalency,"The magnetic nature of Cs$_{2}$AgF$_{4}$, an isoelectronic and isostructural analogue of La$_2$CuO$_4$, is analyzed using density functional calculations. The ground state is found to be ferromagnetic and nearly half metallic. We find strong hybridization of Ag-$d$ and F-$p$ states. Substantial moments reside on the F atoms, which is unusual for the halides and reflects the chemistry of the Ag(II) ions in this compound. This provides the mechanism for ferromagnetism, which we find to be itinerant in character, a result of a Stoner instability enhanced by Hund's coupling on the F.",0604167v1 2006-04-21,Dissipationless spin-current between Heisenberg ferromagnets with spin-orbit coupling,"A system exhibiting multiple simultaneously broken symmetries offers the opportunity to influence physical phenomena such as tunneling currents by means of external control parameters. Time-reversal symmetry and inversion symmetry are both absent in ferromagnetic metals with substantial spin-orbit coupling. We here study transport of spin in a system consisting of two ferromagnets with spin-orbit coupling separated by an insulating tunneling junction. A persistent spin-current across the junction is found, which can be controlled in a well-defined manner by external magnetic and electric fields. The behavior of the spin-current for important geometries and limits is studied.",0604510v1 2006-05-08,High resolution determination of ferromagnetic metallic limit in epitaxial La1-xCaxMnO3 films on NdGaO3,"The physical properties of manganites depend strongly on sample morphology, probably due to strain. We investigate the influence of NdGaO3 substrates on the limit of the ferromagnetic-metallic phase field in La1-xCaxMnO3, doping with x=1% resolution. Films with x=0.40 show a metal-insulator transition, but the ferromagnetic volume fraction is half the spin aligned value. Films with x=0.41 are similar but the metal-insulator transition is not always seen. Films with x=0.42, 0.43, 0.44, 0.45 are insulating, and the magnetization is dramatically reduced. The observed phase boundary indicates where to search for enhanced phase separation effects that may be exploited in thin films and devices.",0605187v1 2006-05-12,Domain wall superconductivity in superconductor/ferromagnet bilayers,"We analyze the enhancement of the superconducting critical temperature of superconducting/ferromagnetic bilayers due to the appearance of localized superconducting states in the vicinity of magnetic domain walls in the ferromagnet. We consider the case when the main mechanism of the superconductivity destruction via the proximity effect is the exchange field. We demonstrate that the influence of the domain walls on the superconducting properties of the bilayer may be quite strong if the domain wall thickness is of the order of superconducting coherence length.",0605326v1 2006-05-29,Nuclear spin effect in metallic spin valve,"We study electronic transport through a ferromagnet normal-metal ferromagnet system and we investigate the effect of hyperfine interaction between electrons and nuclei in the normal-metal part. A switching of the magnetization directions of the ferromagnets causes nuclear spins to precess. We show that the effect of this precession on the current through the system is large enough to be observed in experiment.",0605703v2 2006-06-14,Ferromagnetic ground state of the robust charge-ordered manganite Pr(0.5)Ca(0.5MnO(3)obtained by minimal Al-substitution,"We show that minimal disturbance to the robust charge ordered Pr(0.5)Ca(0.5)MnO(3) by 2.5% Al substitution on Mn-site drives the system towards ferromagnetic ground state. The history-dependent coexisting phases observed are explained as an outcome of a hindered first order transition with glass like arrest of kinetics resulting in irreversibility. Consistent with a simple phase diagram having ferromagnetic ground state, it is experimentally shown that these coexisting phases are far from the equilibrium.",0606371v2 2006-06-20,Comment on ''Superconducting decay length in a ferromagnetic metal'',"In the paper "" Superconducting decay length in a ferromagnetic metal"" by Gusakova, Kupriyanov and Golubov [Pis'ma v ZhETF 83, 487 (2006); cond-mat/0605137], the authors claim that they solved the linearized Eilenberger equation in the ferromagnetic region of an S/F heterostructure at arbitrary mean free path. In this comment we show that the solution suggested by the authors is not correct and explain details of the exact solution found by us in an earlier work several years ago (Phys. Rev. B 64, 134506, (2001).",0606528v1 2006-07-03,On-chip detection of ferromagnetic resonance of a single submicron permalloy strip,"We measured ferromagnetic resonance of a single submicron ferromagnetic strip, embedded in an on-chip microwave transmission line device. The method used is based on detection of the oscillating magnetic flux due to the magnetization dynamics, with an inductive pick-up loop. The dependence of the resonance frequency on applied static magnetic field agrees very well with the Kittel formula, demonstrating that the uniform magnetization precession mode is being driven.",0607036v1 2006-07-07,Broken axisymmetry phase of a spin-1 ferromagnetic Bose-Einstein condensate,"A spin-1 ferromagnetic Bose-Einstein condensate subject to a certain magnetic field exhibits a broken-axisymmetry phase in which the magnetization tilts against the applied magnetic field due to the competition between ferromagnetism and linear and quadratic Zeeman effects. The Bogoliubov analysis shows that in this phase two Goldstone modes associated with U(1) and SO(2) symmetry breakings exist, in which phonons and magnons are coupled to restore the two broken symmetries.",0607184v1 2006-07-25,Quantum Hall Ferrimagnetism in lateral quantum dot molecules,"We demonstrate the existance of ferrimagnetic and ferromagnetic phases in a spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall regime. The spin phase diagram is determined from Hartree-Fock Configuration Interaction method as a function of electron numbers N, magnetic field B, Zeeman energy, and tunneling barrier height. The quantum Hall ferrimagnetic phase corresponds to spatially imbalanced spin droplets resulting from strong inter-dot coupling of identical dots. The quantum Hall ferromagnetic phases correspond to ferromagnetic coupling of spin polarization at filling factors between $\nu=2$ and $\nu=1$.",0607638v1 2006-07-27,A possible model to high TC ferromagnetism in Gallium Manganese Nitrides based on resonation properties of impurities in semiconductors,"The high TC ferromagnetism in (Ga,Mn)N were observed and almost all results are approximately similar to the experimental results in (Ga,Mn)As except the value of TC. Though all standard experiments on magnetism clearly support the results, the value is unexpectedly high. This work present and discuss the possibility of high TC ferromagnetism, after brief review of the experimental results. The key speculation to Bosonization method in three dimensions is resembled with the problems in Anderson localization.",0607708v1 2006-09-02,A new diluted magnetic semiconductor: The half-metallic ferromagnet CoTi(1-x)FexSb,"Half-Heusler compounds with 18 valence electrons are semi-conducting. It will be shown that doping with electrons results in half-metallic ferromagnets, similar to the case of diluted semi-conductors. CoTiSb is known to be a semi-conducting Half-Heusler compound. Doping by Fe is expected to result in ferromagnetic order. It was found that Ti can be replaced by up to about 10% Fe while its crystal structure still remains C1b, which was proved by X-ray powder diffraction. SQUID magnetometry revealed a magnetic moment of 0.32 mB per unit cell at 5K.",0609042v1 2006-09-14,Derivation of the Ginzburg-Landau equations of a ferromagnetic p-wave superconductor,"We derive a Ginzburg-Landau free energy for a p-wave ferromagnetic superconductor. The starting point is a microscopic Hamiltonian including a spin generalised BCS term and a Heisenberg exchange term. We find that coexistence of magnetisation and superconductivity depends on the sign of the energy-gradient of the DOS at Fermi level. We also compute the tunneling contribution to the Ginzburg-Landau free energy, and find expressions for the spin-currents and Josephson currents across a tunneling junction separating two ferromagnetic p-wave superconductors.",0609334v1 2006-09-20,Interplay between ferromagnetism and superconductivity in tunneling currents,"We study tunneling currents in a model consisting of two non-unitary ferromagnetic spin-triplet superconductors separated by a thin insulating layer. We find a novel interplay between ferromagnetism and superconductivity, manifested in the Josephson effect. This offers the possibility of tuning dissipationless currents of charge and spin in a well-defined manner by adjusting the magnetization direction on either side of the junction.",0609500v2 2006-09-22,Complete spin polarization of degenerate electrons in semiconductors near ferromagnetic contacts,"We show that spin polarization of electron density in nonmagnetic degenerate semiconductors can achieve 100%. This effect is realized in ferromagnet-semiconductor $FM-n^{+}$-$n$ junctions even at moderate spin selectivity of the $FM-n^{+}$ contact when the electrons are extracted from the heavily doped $n^{+}-$semiconductor into the ferromagnet. We derived a general equation relating spin polarization of the current to that of the electron density in nonmagnetic semiconductors. We found that the effect of the complete spin polarization is achieved near $n^{+}$-$n$ interface when an effective diffusion coefficient goes to zero in this region while the diffusion current remains finite.",0609599v1 2006-10-04,A new kind of vortex pinning in superconductor / ferromagnet nanocomposites,"This paper reports the observation of hysteresis in the vortex pinning in a superconductor / ferromagnetic epitaxial nanocomposite consisting of fcc Gd particles incorporated in a Nb matrix. We show that this hysteretic pinning is associated with magnetic reversal losses in the Gd particles and is fundamentally different in origin to pinning interactions previously observed for ferromagnetic particles or other microstructural features.",0610105v1 2006-10-05,Magnetic properties of Hydrogenated Li and Co doped ZnO nanoparticles,"The effect of hydrogenation on magnetic properties of Zn0.85Co0.05Li0.10O nanoparticles is presented. It was found that the sample hydrided at room temperature (RT) showed weak ferromagnetism (FM) while that hydrided at 400oC showed robust ferromagnetism at room temperature. In both cases reheating the sample at 400oC in air converts it back into paramagnetic state (P) completely. The characterization of samples by X-ray and electron diffraction (ED) showed that room temperature ferromagnetism observed in the samples hydrogenated at RT is intrinsic in nature whereas that observed in the samples hydrogenated at 400oC is partly due to the cobalt metal clusters.",0610145v1 2006-10-13,Ferromagnetism in the Mott insulator Ba2NaOsO6,"Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d^1 ferromagnetic Mott insulator with an ordered moment of ~0.2 Bohr magnetons per formula unit and TC = 6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet groundstate anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.",0610385v2 2006-11-03,Scanning probe imaging of coexistent ferromagnetism and ferroelectricity at room temperature,"Room temperature coexistence of ferromagnetism and ferroelectricity in a thin film of a novel material of nominal composition PbTi0.5Fe0.5O3-d is probed by standard ferroelectric and ferromagnetic hysteresis loop measurements and by scanning probe microscopy of various kinds. Both magnetic domains and ferroelectric domains are observed in the same spatial region of the material, implying phase coexistence in this system. For both order parameters, sample morphology strongly affects roughness of the domain walls.",0611084v1 2006-11-13,Metallic ferromagnetism in the Hubbard model: A rigorous example,"We present the first rigorous example of the Hubbard model in any dimensions which exhibits metallic ferromagnetism. The model is a genuine Hubbard model with short-range hopping and on-site Coulomb repulsion, and has multi single-electron bands. In the limit where the band gap and the Coulomb repulsion become infinite, we prove that the ground states are completely ferromagnetic and at the same time conducting.",0611318v2 2006-11-16,New critical behavior in unconventional ferromagnetic superconductors,"New critical behavior in unconventional superconductors and superfluids is established and described by the Wilson-Fisher renormalization-group method. For certain ordering symmetries a new type of fluctuation-driven first order phase transitions at finite and zero temperature are predicted. The results can be applied to a wide class of ferromagnetic superconducting and superfluid systems, in particular, to itinerant ferromagnets as UGe2 and URhGe.",0611431v1 2006-11-22,Electronic Phase Separation in Manganite/Insulator Interfaces,"By using a realist microscopic model, we study the electric and magnetic properties of the interface between a half metallic manganite and an insulator. We find that the lack of carriers at the interface debilitates the double exchange mechanism, weakening the ferromagnetic coupling between the Mn ions. In this situation the ferromagnetic order of the Mn spins near the interface is unstable against antiferromagnetic CE correlations, and a separation between ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces can occur. We obtain that the insertion of extra layers of undoped manganite at the interface introduces extra carriers which reinforce the double exchange mechanism and suppress antiferromagnetic instabilities.",0611594v1 2006-12-15,Surface phase separation in nanosized charge-ordered manganites,"Recent experiments showed that the robust charge-ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted.",0612391v2 2006-12-27,Interface dominated biferroic La0.6Sr0.4MnO3/0.7Pb(Mg0.33Nb0.66)O3 0.3PbTiO3 epitaxial superlattices,"Superlattices composed of ferromagnetic La0.6Sr0.4MnO3 and ferroelectric 0.7Pb(Mg0,33Nb0.66)O3 0.3(PbTiO3) layers were fabricated on (100) LaAlO3 substrates by pulsed laser deposition technique. Ferromagnetic and frequency independent ferroelectric hysteresis characteristics established the biferroic nature of the superlattices. Influence of magnetic field was observed in tuning the P-E characteristics of the superlattices. Similar effect was observed on application of a high DC electric field to the samples. The nature of the observed ferroelectric properties and their modulation by applied magnetic and electric fields were thus discussed in connection to existence of dielectric passive layers at the ferroelectric/ferromagnetic interface.",0612647v1 2007-02-16,Left-handed Ferromagnet,"The dynamics of the total magnetization in metallic ferromagnet is studied theoretically taking into account the relativistic spin-orbit interaction. It is found that its quantum dynamics is seriously influenced by the band crossings near the Fermi energy, and sometimes the direction of the precession can be reversed from what expected from the commutation relation $[S^{x},S^{y}]= i\hbar S^{z}$ ($h = 2\pi \hbar$: Planck constant), i.e., the left-handed ferromagnet can be realized.",0702379v1 1994-02-16,Ferromagnetic Vacuum and Galactic Magnetic Fields,"Non-abelian gauge theories may have a ferromagnet-like vacuum with a non-zero magnetic field, which also exists at finite temperature. We argue that the formation of the ferromagnet-like vacuum at GUT scales gives rise to a Maxwell magnetic field imprinted on the comoving plasma, and that it is energetically favourable to align the field in different correlation volumes. This results in a coherent magnetic field pertaining the whole universe, with a magnitude $B_{now}\simeq 10^{-14}\G$, which is of the correct size to serve as the seed field for the galactic dynamo.",9402295v1 2003-11-11,Color Ferromagnetism and Quantum Hall states in Quark Matter,"We discuss a possibility of the presence of a stable color ferromagnetic state in SU(2) gauge theory of quark matter; a color magnetic field is spontaneously generated due tothe gluon's dynamics. The state arises between the hadronic state and the color superconducting state when the density of quarks is varied. Although the state has been known to have unstable modes, we show that unstable modes form quantum Hall states, in which the instability disappears. Namely, the quark matter possesses a stable phase with the ferromagnetic state and the quantum Hall state of gluons.",0311136v1 2005-07-11,Competition of Color Ferromagnetic and Superconductive States in a Quark-Gluon System,"The possibility of color ferromagnetism in an SU(2) gauge field model is investigated. The conditions allowing a stable color ferromagnetic state of the quark system in the chromomagnetic field occupying small domains are considered. A phase transition between this state and the color superconducting state is considered. The effect of finite temperature is analyzed.",0507125v1 2005-07-13,Color Ferromagnetic Quark Matter in Neutron Stars,"We show that color ferromagnetic phase of quark matter is energetically more favored than color superconducting phases in neutron stars. Namely, increasing baryon density in neutron stars transforms nuclear matter into the quark matter of the color ferromagnetic phase. Further increase of the density makes the quark matter take the color superconducting phases. We find that a critical mass of the neutron star with such an internal structure is about $1.6M_{\odot}$. We stress that analysis of gluon dynamics is crucial for exploring dense quark matter.",0507151v2 1996-04-08,More on Generalized Heisenberg Ferromagnet Models,"We generalize the integrable Heisenberg ferromagnet model according to each Hermitian symmetric spaces and address various new aspects of the generalized model. Using the first order formalism of generalized spins which are defined on the coadjoint orbits of arbitrary groups, we construct a Lagrangian of the generalized model from which we obtain the Hamiltonian structure explicitly in the case of $CP(N-1)$ orbit. The gauge equivalence between the generalized Heisenberg ferromagnet and the nonlinear Schr\""{o}dinger models is given. Using the equivalence, we find infinitely many conserved integrals of both models.",9604031v1 1997-03-27,Self-dual Chern-Simons Solitons in the Planar Ferromagnet,"We consider a uniaxial planar ferromagnet coupled minimally to an Abelian Chern-Simons gauge field and study self-dual solitons which saturate the Bogomol'nyi bound. We find a rich structure of rotationally symmetric static soliton solutions for various uniform background charge densities. For a given ferromagnet material, the properties of these solitons are controlled only by the external magnetic field and the background charge.",9703193v1 2001-04-02,Quantum Spin Chains and Riemann Zeta Function with Odd Arguments,"Riemann zeta function is an important object of number theory. It was also used for description of disordered systems in statistical mechanics. We show that Riemann zeta function is also useful for the description of integrable model. We study XXX Heisenberg spin 1/2 anti-ferromagnet. We evaluate a probability of formation of a ferromagnetic string in the anti-ferromagnetic ground state in thermodynamics limit. We prove that for short strings the probability can be expressed in terms of Riemann zeta function with odd arguments.",0104008v2 2001-05-15,Evaluation of Integrals Representing Correlations in XXX Heisenberg Spin Chain,"We study XXX Heisenberg spin 1/2 anti-ferromagnet. We evaluate a probability of formation of a ferromagnetic string in the anti-ferromagnetic ground state in thermodynamics limit. We prove that for short strings the probability can be expressed in terms of Riemann zeta function with odd arguments.",0105144v3 1998-06-16,The continuous spin random field model: Ferromagnetic ordering in d>=3,"We investigate the Gibbs-measures of ferromagnetically coupled continuous spins in double-well potentials subjected to a random field (our specific example being the $\phi^4$ theory), showing ferromagnetic ordering in $d\geq 3$ dimensions for weak disorder and large energy barriers. We map the random continuous spin distributions to distributions for an Ising-spin system by means of a single-site coarse-graining method described by local transition kernels. We derive a contour- representation for them with notably positive contour activities and prove their Gibbsianness. This representation is shown to allow for application of the discrete-spin renormalization group developed by Bricmont/Kupiainen implying the result in $d\geq 3$.",9806010v1 2002-09-02,Nonlinear Behavior in Ferromagnetism: Simple Example and Possible Implications,"Two cases of a phenomenological model for ferromagnetism are considered, discrete and continuous. And the relationship, in general, between discrete and continuous models explored. In a similar way to the logistic map behavior, the continuous case is exactly solvable while the discrete one contains the bifurcation route to chaos. Through the ferromagnetic interpretation I comment on the relevance of this to understanding evolution of systems in time, the role of the configuration space in chaotic behavior, and how this understanding may lead to new exotic magnetic phenomena.",0209006v1 2003-04-02,Color Ferromagnetism in Quark Matter,"We show a possibility that there exists a color ferromagnetic state in quark matter, in which a color magnetic field is spontaneously generated. The state arises between the hadronic state and the color superconducting state when the density of quarks is varied.Although the state (Savvidy state) has been known to involve unstablemodes of gluons, we show that the modes compose a quantum Hall state to stabilize the ferromagnetic state. Such a quantum Hall statecan arise only when quarks matter is present. We also show that the order of the phase transition between the state andthe quark gluon plasma is of the first order.",0304005v2 2003-11-16,Nonlinear Phenomena In Ferromagnetic Double Layers,"The non-stationary nonlinear models of magnetostatic waves propagation in layered ferromagnetic structures are developed. This models are based on use of the coupled nonlinear Schrodinger equations for amplitude of a bending around taking into account an electrodynamic coupling between layers. The basic nonlinear processes in such structures, in particular, the effects of self-modulation and solitons formation are studied numerically, in comparison with the similar effects in single ferromagnetic films.",0311078v1 2006-03-29,Ferromagnetic fluid as a model of social impact,"The paper proposes a new model of spin dynamics which can be treated as a model of sociological coupling between individuals. Our approach takes into account two different human features: gregariousness and individuality. We will show how they affect a psychological distance between individuals and how the distance changes the opinion formation in a social group. Apart from its sociological aplications the model displays the variety of other interesting phenomena like self-organizing ferromagnetic state or a second order phase transition and can be studied from different points of view, e.g. as a model of ferromagnetic fluid, complex evolving network or multiplicative random process.",0603248v1 2006-04-05,Thermal entanglement of spins in mean-field clusters,"We determine thermal entanglement in mean field clusters of $N$ spin one-half particles interacting via the anisotropic Heisenberg interaction, with and without external magnetic field. For the $xxx$ cluster in the absence of magnetic field we prove that only the N=2 ferromagnetic cluster shows entanglement. An external magnetic field $B$ can only entangle $xxx$ anti-ferromagnetic clusters in certain regions of the $B-T$ plane. On the other hand, the $xxz$ clusters of size $N>2$ are entangled only when the interaction is ferromagnetic. Detailed dependence of the entanglement on various parameters is investigated in each case.",0604031v2 2007-04-16,Coherent dynamics of domain formation in the Bose Ferromagnet,"We present a theory to describe domain formation observed very recently in a quenched Rb-87 gas, a typical ferromagnetic spinor Bose system. An overlap factor is introduced to characterize the symmetry breaking of M_F=\pm 1 components for the F=1 ferromagnetic condensate. We demonstrate that the domain formation is a co-effect of the quantum coherence and the thermal relaxation. A thermally enhanced quantum-oscillation is observed during the dynamical process of the domain formation. And the spatial separation of domains leads to significant decay of the M_F=0 component fraction in an initial M_F=0 condensate.",0704.1995v2 2007-04-16,"Bound hole states in a ferromagnetic (Ga,Mn)As environment","A numerical technique is developed to solve the Luttinger-Kohn equation for impurity states directly in k-space and is applied to calculate bound hole wave functions in a ferromagnetic (Ga,Mn)As host. The rich properties of the band structure of an arbitrarily strained, ferromagnetic zinc-blende semiconductor yields various features which have direct impact on the detailed shape of a valence band hole bound to an active impurity. The role of strain is discussed on the basis of explicit calculations of bound hole states.",0704.2028v1 2007-04-18,New fluctuation-driven phase transitions and critical phenomena in unconventional superconductors,"Using the renormalization group method, new type of fluctuation-driven first order phase transitions and critical phenomena are predicted for certain classes of ferromagnetic superconductors and superfluids with unconventional (spin-triplet) Cooper pairing. The problem for the quantum phase transitions at extremely low and zero temperatures is also discussed. The results can be applied to a wide class of ferromagnetic superconductive and superfluid systems, in particular, to itinerant ferromagnets as UGe2 and URhGe.",0704.2287v1 2007-04-25,Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements,"Quantized spin excitations in a single ferromagnetic microstrip have been measured using the microwave photovoltage technique. Several kinds of spin wave modes due to different contributions of the dipole-dipole and the exchange interactions are observed. Among them are a series of distinct dipole-exchange spin wave modes, which allow us to determine precisely the subtle spin boundary condition. A comprehensive picture for quantized spin excitations in a ferromagnet with finite size is thereby established. The dispersions of the quantized spin wave modes have two different branches separated by the saturation magnetization.",0704.3397v1 2007-05-07,Two Dimensional Quantum Well of Gluons in Color Ferromagnetic Quark Matter,"We have recently pointed out that color magnetic field is generated in dense quark matter, i.e. color ferromagnetic phase of quark matter. Using light cone quantization, we show that gluons occupying the lowest Landau level under the color magnetic field effectively form a two dimensional quantum well (layer), in which infinitely many zero modes of the gluons are present. We discuss that the zero modes of the gluons form a quantum Hall state by interacting repulsively with each other, just as electrons do in semiconductors. Such a ferromagnetic quark matter with the layer structure of the gluons is a possible origin of extremely strong magnetic field observed in magnetars.",0705.0852v1 2007-05-21,Long range triplet Josephson effect through a ferromagnetic trilayer,"We study the Josephson current through a ferromagnetic trilayer, both in the diffusive and clean limits. For colinear (parallel or antiparallel) magnetizations in the layers, the Josephson current is small due to short range proximity effect in superconductor/ferromagnet structures. For non colinear magnetizations, we determine the conditions for the Josephson current to be dominated by another contribution originating from long range triplet proximity effect.",0705.2929v1 2007-05-27,Kinetic ferromagnetism on a kagome lattice,"We study strongly correlated electrons on a kagome lattice at 1/6 and 1/3 filling. They are described by an extended Hubbard Hamiltonian. We are concerned with the limit |t|< 4$, in units of coherence length $\xi$), ground states are topological in nature -- a ""skyrmion"" in 2D, and a ""hedgehog"" in 3D.",0805.3693v2 2008-05-27,Self-sustained magnetoelectric oscillations in magnetic resonant tunneling structures,"The dynamic interplay of transport, electrostatic, and magnetic effects in the resonant tunneling through ferromagnetic quantum wells is theoretically investigated. It is shown that the carrier-mediated magnetic order in the ferromagnetic region not only induces, but also takes part in intrinsic, robust, and sustainable high-frequency current oscillations over a large window of nominally steady bias voltages. This phenomenon could spawn a new class of quantum electronic devices based on ferromagnetic semiconductors.",0805.4068v1 2008-06-10,Geometrical control of the magnetization direction in high aspect-ratio PdNi ferromagnetic nano-electrodes,"We present a study of electron-beam evaporated Pd0.4Ni0.6 alloy thin films by means of ferromagnetic resonance measurements on extended films of varying thickness and anisotropic magnetoresistance measurements of lithographically patterned high aspect-ratio ferromagnetic electrodes, respectively. The results reveal that the direction of the magnetization strongly depends on the electrode lateral dimensions, transitioning from in-plane magnetization for extended films to out-of-the-plane magnetization for electrode widths below 2-3 microns, reaching 58 degrees off-plane for 100 nm-wide nanoelectrodes.",0806.1744v1 2008-06-24,Spin dynamics in point contacts to single ferromagnetic films,"Excitation of magnons or spin-waves driven by nominally unpolarized transport currents in point contacts of normal and ferromagnetic metals is probed by irradiating the contacts with microwaves. Two characteristic dynamic effects are observed: a rectification of off-resonance microwave current by spin-wave nonlinearities in the point contact conductance, and a resonant stimulation of spin-wave modes in the nano-contact core by the microwave field. These observations provide a direct evidence that the magnetoconductance effects observed are due to GHz spin dynamics at the ferromagnetic interface driven by the spin transfer torque effect of the transport current.",0806.3805v1 2008-06-25,Hidden Orbital Liquid State Within Ferromagnetically Ordered Metallic SrRuO3,"We have experimentally found related anomalies in electrical resistivity, dc and ac magnetic susceptibility, appearing deeply within ferromagnetically ordered state in SrRuO3. Lack of Jahn-Teller distortion in this regime rules out conventional orbital order, forcing one to describe these in terms of an orbital liquid ground state coexisting with ferromagnetic spin order. We suggest that weak spin-orbit coupling in such an unusual state underpins the observed anomalies.",0806.4198v1 2008-08-07,Density of states near a vortex core in ferromagnetic superconductors: Application to STM measurements,"We investigate numerically the local density of states (LDOS) in the vicinity of a vortex core in a ferromagnetic superconductor. Specifically, we investigate how the LDOS is affected by the relative weight of the spin bands in terms of the superconducting pairing, and we also examine the effect of different pairing symmetries for the superconducting order parameter. Our findings are directly related to scanning tunneling microscopy measurements and may thus be highly useful to clarify details of the superconducting pairing in recently discovered ferromagnetic superconductors.",0808.0983v1 2008-08-07,Ferromagnetism and spin polarized charge carriers in In$_{2}$O$_{3}$ thin films,"We present evidence for spin polarized charge carriers in In$_2$O$_3$ films. Both In$_2$O$_3$ and Cr doped In$_2$O$_3$ films exhibit room temperature ferromagnetism after vacuum annealing, with a saturation moment of approximately 0.5 emu/cm$^3$. We used Point Contact Andreev Reflection measurements to directly determine the spin polarization, which was found to be approximately 50$\pm$5% for both compositions. These results are consistent with suggestions that the ferromagnetism observed in certain oxide semiconductors may be carrier mediated.",0808.1123v1 2008-08-27,Quantum Hall to Insulator Transition in the Bilayer Quantum Hall Ferromagnet,"We describe a new phase transition of the bilayer quantum Hall ferromagnet at filling fraction $\nu = 1$. In the presence of static disorder (modeled by a periodic potential), bosonic $S=1/2$ spinons can undergo a superfluid-insulator transition while preserving the ferromagnetic order. The Mott insulating phase has an emergent U(1) photon, and the transition is between Higgs and Coulomb phases of this photon. Physical consequences for charge and counterflow conductivity, and for interlayer tunneling conductance in the presence of quenched disorder are discussed.",0808.3702v1 2008-08-28,Gilbert Damping in Conducting Ferromagnets II: Model Tests of the Torque-Correlation Formula,"We report on a study of Gilbert damping due to particle-hole pair excitations in conducting ferromagnets. We focus on a toy two-band model and on a four-band spherical model which provides an approximate description of ferromagnetic (Ga,Mn)As. These models are sufficiently simple that disorder-ladder-sum vertex corrections to the long-wavelength spin-spin response function can be summed to all orders. An important objective of this study is to assess the reliability of practical approximate expressions which can be combined with electronic structure calculations to estimate Gilbert damping in more complex systems.",0808.3923v1 2008-09-05,Ferromagnetism in Mn Substituted Zirconia: A Density-functional Theory Study,"We study the electronic structure and magnetism of 25% Mn substituted cubic Zirconia (ZrO2) with several homogeneous and heterogeneous doping profiles using density-functional theory calculations. We find that all doping profiles show half-metallic ferromagnetism (HMF), and delta-doping is most energy favorable while homogeneous doping has largest ferromagnetic stabilization energy. Using crystal field theory, we discuss the formation scheme of HMF. Finally, we speculate the potential spintronics applications for Mn doped ZrO2, especially as spin direction controllment.",0809.0971v1 2008-09-16,Stochastic dynamics of magnetization in a ferromagnetic nanoparticle out of equilibrium,"We consider a small metallic particle (quantum dot) where ferromagnetism arises as a consequence of Stoner instability. When the particle is connected to electrodes, exchange of electrons between the particle and the electrodes leads to a temperature- and bias-driven Brownian motion of the direction of the particle magnetization. Under certain conditions this Brownian motion is described by the stochastic Landau-Lifshitz-Gilbert equation. As an example of its application, we calculate the frequency-dependent magnetic susceptibility of the particle in a constant external magnetic field, which is relevant for ferromagnetic resonance measurements.",0809.2611v1 2008-10-02,"Novel critical exponent of magnetization curves near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3 (A = Ca, La0.5Na0.5, and La)","We report a novel critical exponent delta=3/2 of magnetization curves M=H^{1/delta} near the ferromagnetic quantum phase transitions of Sr1-xAxRuO3 (A = Ca, La0.5Na0.5, and La), which the mean field theory of the Ginzburg-Landau-Wilson type fails to reproduce. The effect of dirty ferromagnetic spin fluctuations might be a key.",0810.0343v1 2008-10-15,Inverse proximity effect in superconductor-ferromagnet bilayer structures,"Measurements of the polar Kerr effect using a zero-area-loop Sagnac magnetometer on Pb/Ni and Al/(Co-Pd) proximity-effect bilayers show unambiguous evidence for the ""inverse proximity effect,"" in which the ferromagnet (F) induces a finite magnetization in the superconducting (S) layer. To avoid probing the magnetic effects in the ferromagnet, the superconducting layer was prepared much thicker than the light's optical penetration depth. The sign and size of the effect, as well as its temperature dependence agree with recent predictions by Bergeret et al..",0810.2605v1 2008-10-16,Epitaxial ferromagnetic Fe$_{3}$Si/Si(111) structures with high-quality hetero-interfaces,"To develop silicon-based spintronic devices, we have explored high-quality ferromagnetic Fe$_{3}$Si/silicon (Si) structures. Using low-temperature molecular beam epitaxy at 130 $^circ$C, we realize epitaxial growth of ferromagnetic Fe$_{3}$Si layers on Si (111) with keeping an abrupt interface, and the grown Fe$_{3}$Si layer has the ordered $DO_{3}$ phase. Measurements of magnetic and electrical properties for the Fe$_{3}$Si/Si(111) yield a magnetic moment of ~ 3.16 $mu_{B}$/f.u. at room temperature and a rectifying Schottky-diode behavior with the ideality factor of ~ 1.08, respectively.",0810.2835v1 2008-10-31,Defect-induced ferromagnetism in graphite,"We demonstrate direct evidence for ferromagnetic order at defect structures in highly oriented pyrolytic graphite with magnetic force microscopy at room temperature. Magnetic impurities have been excluded as the origin of the magnetic signal after careful analysis supporting an intrinsic magnetic behavior of carbon-based materials. The observed ferromagnetism has been attributed to originate from unpaired electron spins localized at grain boundaries. Scanning tunneling spectroscopy of grain boundaries showed intense localized states and enhanced charge density compared to bare graphite.",0810.5657v1 2008-11-01,Room Temperature Ferromagnetic Semiconductor Rutile Ti1-xCoxO2-δ Epitaxial Thin Films Grown by Sputtering Method,"Room temperature ferromagnetic semiconductor rutile Ti1-xCoxO2-\delta (101) epitaxial thin films were grown on r-sapphire substrates by a dc sputtering method. Ferromagnetic magnetization, magnetic circular dichroism, and anomalous Hall effect were clearly observed at room temperature in sputter-grown films for the first time. The magnetization value is nearly as large as 3\mu B/Co that is consistent with the high spin state Co2+ in this compound recently established by spectroscopic methods. Consequently, its originally large magneto-optical response is further enhanced.",0811.0073v1 2008-11-13,Peierls Instability Induced Ferromagnetic Insulator at Orbital Order Transition,"The origin of ferromagnetic insulating state of La$_{7/8}$Sr$_{1/8}$MnO$_3$ is investigated. Based on the tight-binding model, it is shown that this state can be attributed to the Peierls instability arisen from the interplay of spin and orbital ordering. The importance of the hole-orbiton-phonon intercoupling in doped manganites is revealed. This picture explains well the recent experimental finding of the reentrance of ferromagnetic metal state at low temperature [Phys. Rev. Lett. 96, 097201 (2006)].",0811.2019v2 2008-11-19,Nonlocal Spin Transport in Lateral Spin Valves with Multiple Ferromagnetic Electrodes,"We study the nonlocal spin transport in a lateral spin valve with multiple ferromagnetic (FM) electrodes. When two current-injecting and two spin current-detecting electrodes are all ferromagnetic, the number of possible nonlocal spin signal states is four at maximum. In reality, this number is reduced, depending on the inter-probe distance and the relative magnitudes of the spin resistances. Our theoretical results are in agreement with recent experiments of spin injection into an Al island, a carbon nanotube, and graphene.",0811.3138v1 2008-11-27,Anisotropic magnetoresistance in ferromagnetic atomic-sized metal contacts,"Recent experiments in ferromagnetic atomic-sized contacts have shown that the anisotropic magnetoresistance (AMR) is greatly enhanced and has an asymmetric angular dependence as compared with that of bulk samples. The origin of these effects is still under debate. In this work we present a theoretical analysis of the AMR in atomic contacts of the 3d ferromagnetic materials. Our results strongly suggest that the anomalous AMR stems from the reduced symmetry of the atomic contact geometries. We also present calculations supporting the idea that the pronounced voltage- and temperature dependence in some experiments can be attributed to impurities near the constrictions.",0811.4491v1 2008-12-02,Ferromagnetism in the Extended Periodic Anderson Model Near Selective Mott Localization,"Motivated by the experimental finding of Ferromagnetism (FM) near non-FL quantum phase transitions (QPT), I investigate FM in an Extended Periodic Anderson Model (EPAM). Using the Frobenius-Perron theorem, a FM {\it metallic} phase near the selective Mott localization is rigorously found. Interesting connections with Tasaki's flat-band ferromagnetism, as well as with the well-known double exchange (DE) mechanism are elucidated. This finding is a higher dimensional generalization of numerical work on the D=1 PAM, and constitutes a robust way to FM metallic behavior in $D>1$. Finally, we discuss a concrete manifestation of our results to $UGe_{2}$.",0812.0472v1 2008-12-04,Phase transitions and ordering of confined dipolar fluids,"We apply a modified mean-field density functional theory to determine the phase behavior of Stockmayer fluids in slitlike pores formed by two walls with identical substrate potentials. Based on the Carnahan-Starling equation of state, a fundamental-measure theory is employed to incorporate the effects of short-ranged hard sphere - like correlations while the long-ranged contributions to the fluid interaction potential are treated perturbatively. The liquid-vapor, ferromagnetic liquid - vapor, and ferromagnetic liquid - isotropic liquid first-order phase separations are investigated. The local orientational structure of the anisotropic and inhomogeneous ferromagnetic liquid phase is also studied. We discuss how the phase diagrams are shifted and distorted upon varying the pore width.",0812.0969v1 2008-12-08,Multi-magnon bound states in easy-axis ferromagnetic zigzag spin chain,"The frustrated spin-1/2 chain with weakly anisotropic ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is studied. We focus on the excitation spectrum and the low-temperature thermodynamics in the ferromagnetic region of the ground state phase diagram. It is shown that the excitation spectrum of the model is characterized by the existence of the multi-magnon bound states. These excitations determine the low-temperature magnetic susceptibility. The energy of the bound magnon complexes is found using the scaling estimates of the perturbation theory and numerical calculations. The relation of the considered model to the edge-sharing cuprate $Li_2CuO_4$ is discussed.",0812.1425v1 2009-01-07,Electrically Controlled Magnetic Memory and Programmable Logic based on Graphene/Ferromagnet Hybrid Structures,"It has been shown that the combining of the electrical effect on the exchange bias field with giant magneto-resistance effect of the graphene/ferromagnet hybrid structures reveals a new non-volatile magnetic random access memory device conception. In such device an electric bias realizes the writing bits instead a magnetic field of remote word line with high energy consumption. Interplay of two graphene mediated exchange bias fields applied to different sides of free ferromagnet results in programable logic operations that depends on specific realization of the structure.",0901.0926v1 2009-01-12,The influence of spin-dependent phases of tunneling electrons on the conductance of a point ferromagnet/isolator/d-wave superconductor contact,"The influence of phase shifts of electron waves passing through and reflected by the potential barrier on the Andreev reflection in a ferromagnet/isolator/d-wave superconductor (FIS) contact is studied. It is found that in a superconductor the surface spin-dependent Andreev bound states inside the superconducting gap are formed as a result of the interference of electron-like and hole-like quasiparticles due to repeated Andreev reflections. The peak in the conductance of the FIS contact at the zero potential for the (110)-oriented superconductor disappears rapidly as the polarization of a ferromagnet increases, whereas for the (100)-oriented superconductor it appears. The physical reason for this behavior of conductance is discussed.",0901.1558v1 2009-01-26,Room temperature ferromagnetism in Cr-doped hydrogenated amorphous Si films,"Ferromagnetism above room temperature was observed in Cr-doped hydrogenated amorphous silicon films deposited by rf magnetron-sputtering. Structure analysis reveals that films are amorphous without any detectable precipitates up to the solubility limit 16 atomic percentages of Cr. Experimental results suggest that hydrogenation has a dramatic influence on magnetic properties, electrical conductivity and carrier concentration in the thin films. Pronounced anomalous Hall effect and magnetization curve both suggest the origin of the ferromagnetism may arises from percolation of magnetic polarons.",0901.4123v1 2009-01-28,Relativistic ferromagnetic magnon at the zigzag edge of graphene,"We study the spin-wave excitations near the zigzag edge of graphene. It is rather interesting that we obtain a single branch of relativistic ferromagnetic magnon due to the presence of the open boundary. Note that magnons in antiferomagnets appear in pairs, while the single brach magnon in ferromagnets does not have relativistic dispersion. Thus, the magnon near the zigzag edge of graphene is a hybrid of both, signaling its intrinsic property as a boundary excitation that must be embedded in a higher dimensional bulk system.",0901.4567v1 2009-02-13,Charge pumping and the colored thermal voltage noise in spin valves,"Spin pumping by a moving magnetization gives rise to an electric voltage over a spin valve. Thermal fluctuations of the magnetization manifest themselves as increased thermal voltage noise with absorption lines at the ferromagnetic resonance frequency and/or zero frequency. The effect depends on the magnetization configuration and can be of the same order of magnitude as the Johnson-Nyquist thermal noise. Measuring colored voltage noise is an alternative to ferromagnetic resonance experiments for nano-scale ferromagnetic circuits.",0902.2389v2 2009-03-15,On the possibility of a long range proximity effect in a ferromagnetic nanoparticle,"We study the proximity effect in a ferromagnetic nanoparticle having a vortex magnetization pattern. We show that for axisymmetric system consisting of a circular particle and a magnetic vortex situated at the center of it no long range superconducting correlations are induced. It means that induced superconductivity is localized in the small area near the superconducting electrode. However, in the real systems axial symmetry can be broken by either a shift of the magnetic vortex from the origin or geometrical anisotropy of the ferromagnetic particle. In this case a long range proximity effect is possible.",0903.2613v1 2009-04-01,Spin-dependent Cooper Pair Phase and Pure Spin Supercurrents in Strongly Polarized Ferromagnets,"We study heterostructures of singlet superconductors (SC) and strongly spin-polarized ferromagnets (sFM) and show that a relative phase arises between the superconducting proximity amplitudes in the two ferromagnetic spin bands. We find a tunable pure spin supercurrent in a sFM contacted with only one SC electrode. We show that Josephson junctions are most effective for a spin polarization $P\sim 0.3$, and that critical currents for positive and negative bias differ for a high transmission Josephson junction, due to a relative phase between single and double pair transmission.",0904.0149v2 2009-04-06,Characterisation of Ferromagnetic Contacts to Carbon Nanotubes,"We present an investigation of different thin-film evaporated ferromagnetic materials for their suitability as electrodes in individual single-wall and multi-wall carbon nanotube-based spin devices. Various electrode shapes made from permalloy (Ni_{81}Fe_{19}), the diluted ferromagnet PdFe, and PdFe/Fe bilayers are studied for both their micromagnetic properties and their contact formation to carbon nanotubes. Suitable devices are tested in low-temperature electron transport measurements, displaying the typical tunneling magnetoresistance of carbon nanotube pseudo spin valves.",0904.0907v2 2009-04-11,Coupled Superconducting Phase and Ferromagnetic Order Parameter Dynamics,"Via a direct coupling between the magnetic order parameter and the singlet Josephson supercurrent, we detect spin-wave resonances, and their dispersion, in ferromagnetic Josephson junctions in which the usual insulating or metallic barrier is replaced with a weak ferromagnet. The coupling arises within the Fraunhofer interferential description of the Josephson effect, because the magnetic layer acts as a time dependent phase plate. A spin-wave resonance at a frequency ws implies a dissipation that is reflected as a depression in the current-voltage curve of the Josephson junction when hbar ws = 2eV. We have thereby performed a resonance experiment on only 10^7 Ni atoms.",0904.1780v1 2009-04-15,Ferromagnetic spin-polaron on complex lattices,"We present a simpler derivation of the exact solution of a spin-polaron in a ferromagnet and generalize it to complex lattices and/or longer range exchange interactions. As a specific example, we analyze a two-dimensional MnO$_2$-like lattice (as in the ferromagnetic layers in LaMnO$_3$) and discuss the properties of the resulting spin-polaron in various regimes. At strong couplings the solution is reminiscent of the Zhang-Rice singlet, however the electronic wavefunction involved in the singlet is dependent on the momentum of the singlet, and multiple bands may appear.",0904.2367v1 2009-05-02,Implications and consequences of ferromagnetism universally exhibited by inorganic nanoparticles,"Occurrence of surface ferromagnetism in inorganic nanoprticles as a universal property not only explains many of the unusual magnetic features of oxidic thin films, but also suggests its possible use in creating new materials, as exemplified by multiferroic BaTiO3 nanoparticles. While the use of Mn-doped ZnO and such materials in spintronics appears doubtful, it is possible to have materials exhibiting coexistence of (bulk) superconductivity with (surface) ferromagnetism.",0905.0183v1 2009-05-04,Room-temperature ferromagnetism in dielectric GaN(Gd),"We present an explanation of recently observed giant magnetic moment and room-temperature ferromagnetism in the dielectric GaN doped with Gd. Our approach uses the polarization mechanism of exchange interaction, which occurs if the d-level of Gd appears in the bandgap close to the valence band edge. Calculated ferromagnetic critical temperature and the value of the magnetic moment well correspond to experimental findings.",0905.0500v1 2009-05-08,The making of ferromagnetic Fe doped ZnO nano-clusters,"In this letter, the authors present a study of the energetics and magnetic interactions in Fe doped ZnO clusters by ab-initio density functional calculations. The results indicate that defects under suitable conditions can induce ferromagnetic interactions between the dopant Fe atoms whereas antiferromagnetic coupling dominates in a neutral defect-free cluster. The calculations also reveal an unusual ionic state of the dopant Fe atom residing at the surface of the cluster, a feature that is important to render the cluster ferromagnetic.",0905.1176v1 2009-05-15,Planar superconductor/ferromagnet hybrids: Anisotropy of resistivity induced by magnetic templates,"We investigated experimentally the transport properties of a superconducting cross-shaped aluminium microbridge fabricated on top of ferromagnetic BaFe12O19 single crystal. It was demonstrated that a one-dimensional domain structure in the ferromagnetic substrate can induce the formation of superconducting channels above magnetic domains. This leads to a giant anisotropy of resistivity of the superconducting bridge, caused by the appearance of continuous paths of supercurrents flowing along domain walls.",0905.2590v1 2009-05-22,Interlayer and interfacial exchange coupling in ferromagnetic metal/semiconductor heterostructures,"We describe a systematic study of the exchange coupling between a magnetically hard metallic ferromagnet (MnAs) and a magnetically soft ferromagnetic semiconductor (GaMnAs) in bilayer and trilayer heterostructures. An exchange spring model of MnAs/GaMnAs bilayers accounts for the variation of the exchange bias field with layer thickness and composition. We also present evidence for hole-mediated interlayer exchange coupling in MnAs/p-GaAs/\GaMnAs trilayers and study the dependence of the exchange bias field on the thickness of the spacer layer.",0905.3691v2 2009-05-28,Spin Hall Current and Spin-transfer Torque in Ferromagnetic Metal,"We theoretically examine the spin-transfer torque in the presence of spin-orbit interaction (SOI) at impurities in a ferromagnetic metal on the basis of linear response theory. We obtained, in addition to the usual spin-transfer torque, a new contributioin $\sim {\bm j}_{\rm SH}^{\phantom{\dagger}} \cdot \nabla {\bm n}$ in the first order in SOI, where ${\bm j}_{\rm SH}^{\phantom{\dagger}}$ is the spin Hall current driven by an external electric field. This is a reaction to inverse spin Hall effect driven by spin motive force in a ferromagnet.",0905.4561v1 2009-06-06,Stable two-dimensional ferromagnets made of regular single-layered lattices of single-molecule nanomagnets on substrates,"We propose that stable two-dimensional (2D) ferromagnets can be made of regular single-layered lattices of single-molecule nanomagnets with enough uniaxial magnetic anisotropy on appropriate substrates by controlling the inter-nanomagnet magnetic interaction. Our Monte Carlo simulated results show that such ideal 2D ferromagnets are thermodynamically stable when the anisotropy is strong enough. If the anisotropy energy equals 80 K, approximately that of the Mn12, the T_c varies from zero to 15 K depending on different inter-nanomagnet coupling constants. Such stable spin systems, experimentally accessible, should be promising for information applications.",0906.1298v1 2009-06-09,Spin-wave theory for dimerized ferromagnetic chains,"We describe a Peierls dimerization which occurs in ferromagnetic spin chains at finite temperature, within the modified spin-wave theory. Usual spin-wave theory is modified by introducing a Lagrange multiplier which enforces a nonmagnetic state at finite temperature. It is shown that this method gives results in excellent agreement with the density--matrix renormalization group applied to transfer matrices for dimerized ferromagnetic chains. We study bond correlation functions and explore the characteristic features of dimerization in the specific heat.",0906.1758v1 2009-06-23,Mass Enhancement and Reentrant Ground State in Magnetic Field,"Three different cases of magnetic field reentrant ground states in heavy fermion systems, URhGe, UGe$_2$ and CeRhIn$_5$ recently studied in Grenoble, are discussed. URhGe is a ferromagnetic superconductor with reentrace of superconductivity under magnetic field ($H$) which is associated to the spin reorientation field. UGe$_2$ is a ferromagnetic superconductor with an enhancement of the upper critical field $H_{c2}$ of superconductivity at its metamagnetic transition between 2 ferromagnetic phases. CeRhIn$_5$ is a superconductor with $H$ reentrant antiferromagnetism. We analyze the links between the $H$ enhancement of the different contributions to the effective mass and the field reentrant phase.",0906.4204v1 2009-06-23,Theory of charge transport in ferromagnetic semiconductor/s-wave superconductor junction,"We study tunneling conductance in ferromagnetic semiconductor/insulator/s-wave superconductor junction where Rashba spin-orbit interaction (RSOI) and exchange field are taken into account in the ferromagnetic semiconductor. We show that normalized conductance at zero voltage has a maximum as a function of RSOI for high transparent interface and finite exchange field. This is because Andreev reflection probability shows a nonmonotonic dependence on RSOI in the presence of the exchange field. On the other hand, for intermediate transparent interface, normalized conductance at zero voltage has a reentrant shape at zero or small exchange field with increasing RSOI but is monotonically increasing by RSOI at large exchange field.",0906.4244v1 2009-06-29,Influence of Demagnetization Effect on Giant Magneto Impedance of soft Ferromagnetic Metal,"The large change in electromagnetic impedance in ferromagnetic state of soft magnetic metals in presence of biasing magnetic field is associated with change in screening of electromagnetic field. The screening depends on the permeability of the metal. Apart from dependence on intrinsic properties of metal the permeability depends on size of the sample. It is observed that the decrease in MI in amorphous ferromagnetic ribbon of Fe40Ni40B20 alloy is large for long sample whereas corresponding change is small for short one with same biasing field. As intrinsic magnetic properties and bias field are same and the demagnetization factor increases with reduction of length of the sample the reduction of MI effect is associated with demagnetization field.",0906.5203v1 2009-06-30,Photoinduced magnetism in the ferromagnetic semiconductors,"We study the enhancement of the magnetic transition temperature $T_c$ due to incident light in ferromagnetic semiconductors such as EuS. The photoexcited carriers mediate an extra ferromagnetic interaction due to the coupling with the localized magnetic moments. The Hamiltonian consists of a Heisenberg model for the localized moments and an interaction term between the photoexcited carriers and the localized moments. The model predicts a small enhancement of the transition temperature in semi-quantitative agreement with the experiments.",0906.5514v1 2009-10-22,Quantum critical behavior of a magnetic quantum paraelectric,"We discuss the interplay between anti-ferromagnetic order and polarization fluctuations in a magnetic quantum paraelectric. Using an action where anti-ferromagnetic order parameter couples to the polarization fluctuations and as well as magnetic field, we derive a set of self consistent equations to study both the temperature and the magnetic field dependence of the static dielectric susceptibility. The temperature dependence of dielectric susceptibility near both the anti-ferromagnetic quantum critical point and ferroelectric quantum critical point are described using scaling arguments. Discussions on achieving various quantum critical points in experiments are also made.",0910.4374v2 2009-10-28,Interacting spin waves in the ferromagnetic Kondo lattice model,"We present an new approach for the ferromagnetic, three-dimensional, translational-symmetric Kondo lattice model which allows us to derive both magnon energies and linewidths (lifetimes) and to study the properties of the ferromagnetic phase at finite temperatures. Both ""anomalous softening"" and ""anomalous damping"" are obtained and discussed. Our method consists of mapping the Kondo lattice model onto an effective Heisenberg model by means of the ""modified RKKY interaction"" and the ""interpolating self-energy approach"". The Heisenberg model is approximatively solved by applying the Dyson-Maleev transformation and using the ""spectral density approach"" with a broadened magnon spectral density.",0910.5478v2 2009-10-30,Interplay of Quantum Criticality and Geometric Frustration in Columbite,"Motivated by CoNb2O6 (belonging to the columbite family of minerals), we theoretically study the physics of quantum ferromagnetic Ising chains coupled anti-ferromagnetically on a triangular lattice in the plane perpendicular to the chain direction. We combine exact solutions of the chain physics with perturbative approximations for the transverse couplings. When the triangular lattice has an isosceles distortion (which occurs in the real material), the T=0 phase diagram is rich with five different states of matter: ferrimagnetic, N\'eel, anti-ferromagnetic, paramagnetic and incommensurate phases, separated by quantum phase transitions. Implications of our results to experiments on CoNb2O6 are discussed.",0911.0038v1 2009-11-09,Triplet contribution to the Josephson current in the nonequilibrium superconductor/ferromagnet/superconductor junction,"The Josephson current through a long s-wave superconductor/weak ferromagnet/s-wave superconductor weak link is studied theoretically in the regime of nonequilibrium spin-dependent occupation of electron states in the ferromagnetic intelayer. While under the considered nonequilibrium condition the standard supercurrent, carried by the singlet part of current-carrying density of states, is not modified, the additional supercurrent flowing via the triplet part of the current-carrying density of states appears. Depending on voltage, controlling the particular form of spin-dependent nonequilibrium in the interlayer, this additional current can enhance or reduce the usual current of the singlet component and also switch the junction between 0- and $\pi$-states.",0911.1646v1 2010-01-14,The phase transition in the localized ferromagnet EuO probed by muSR,"We report results of muon spin rotation measurements performed on the ferromagnetic semiconductor EuO, which is one of the best approximations to a localized ferromagnet. We argue that implanted muons are sensitive to the internal field primarily through a combination of hyperfine and Lorentz fields. The temperature dependences of the internal field and the relaxation rate have been measured and are compared with previous theoretical predictions.",1001.2556v2 2010-01-26,Effect of spin-conserving scattering on Gilbert damping in ferromagnetic semiconductors,"The Gilbert damping in ferromagnetic semiconductors is theoretically investigated based on the $s$-$d$ model. In contrast to the situation in metals, all the spin-conserving scattering in ferromagnetic semiconductors supplies an additional spin relaxation channel due to the momentum dependent effective magnetic field of the spin-orbit coupling, thereby modifies the Gilbert damping. In the presence of a pure spin current, we predict a new contribution due to the interplay of the anisotropic spin-orbit coupling and a pure spin current.",1001.4576v1 2010-02-15,Domain wall motion in ferromagnetic nanowires driven by arbitrary time-dependent fields: An exact result,"We address the dynamics of magnetic domain walls in ferromagnetic nanowires under the influence of external time-dependent magnetic fields. We report a new exact spatiotemporal solution of the Landau-Lifshitz-Gilbert equation for the case of soft ferromagnetic wires and nanostructures with uniaxial anisotropy. The solution holds for applied fields with arbitrary strength and time dependence. We further extend this solution to applied fields slowly varying in space and to multiple domain walls.",1002.2913v1 2010-02-24,Evidence of Ferrimagnetism in Ferromagnetic La0_{67}Ca0_{33}MnO_3 nanoparticle,"The present report is dedicated to show that ferromagnetic La0.67Ca0.33MnO3 (LCMN) particles can be better described in the framework of ferrimagnetic model. To confirm the ferrimagnetic signature in ferromagnetic LCMN particles, the temperature dependence of the inverse of magnetic susceptibility in the paramagnetic state of the samples was taken as a tool of data analysis. The observed ferrimagnetism is understood as an effect of of the core-shell spin structure in LCMN particles.",1002.4574v2 2010-03-23,Supercurrents through half-metallic ferromagnetic CrO$_2$ revisited,"We report on the observation of a supercurrent through the half metallic ferromagnet CrO$_2$ grown on hexagonal Al$_2$O$_3$ (sapphire). The current was observed to flow over a distance of 700 nm between two superconducting amorphous Mo$_{70}$Ge$_{30}$ electrodes which were deposited on the CrO$_2$ film. The critical current $I_c$ increases as function of decreasing temperature. Upon applying an in-plane magnetic field, $I_c$ goes through a maximum at the rather high field of 80 mT. We believe this to be a long range proximity effect in the ferromagnet, carried by odd-frequency pairing correlations.",1003.4446v1 2010-03-31,Half-metallic ferromagnetism in transition-metal doped germanium nitride: A first-principles study,"The electronic and magnetic properties of transition-metal doped $\beta$-Ge3N4 have been studied using first-principles calculations. The results show that the substitutional transition-metal impurities tend to cluster. The V and Cr doped Ge3N4 compounds are ferromagnetic semiconductors, while the compounds with Mn and Fe doping show a half-metallic ferromagnetic character.",1003.5982v2 2010-04-14,Theory of the ferromagnetism in TiCrN solid solutions,"First-principles calculations are used to investigate the magnetic properties of Ti$_{1-x}$Cr$_{x}$N solid solutions. We show that the magnetic interactions between Cr spins that favour antiferromagnetism in CrN is changed upon alloying with TiN leading to the appearance of ferromagnetism in the system at approximately $x \leq 0.50$ in agreement with experimental reports. Furthermore we suggest that this effect originates in an electron density redistribution from Ti to Cr that decreases the polarization of Cr d-states with t$_{2g}$ symmetry while it increases the polarization of Cr d-states with e$_g$ symmetry, both changes working in favour of ferromagnetism.",1004.2337v1 2010-04-15,Ground-state configurations in ferromagnetic nanotori,"Magnetization ground states are studied in toroidal nanomagnets. The energetics associated to the ferromagnetic, vortex and onion-like configurations are explicitly computed. The analysis reveals that the vortex appears to be the most prominent of such states, minimizing total energy in every torus with internal radius $r\gtrsim10\,{\rm nm}$ (for Permalloy). For $r\lesssim10\,{\rm nm}$ the vortex remains the most favorable pattern whenever $R/\ell_{ex}\gtrsim1.5$ ($R$ is the torus external radius and $\ell_{ex}$ is the exchange length), being substituted by the ferromagnetic state whenever $R/\ell_{ex}\lesssim1.5$.",1004.2689v1 2010-06-28,Emergence Of Ferromagnetism In Nanoparticles Of Antiferromagnetic Nd0.4Sr0.6MnO3,"We have investigated the magnetic state of Nd0.4Sr0.6MnO3 with the variation of its grain size down to average nanometric diameter (40 nm). The bulk sample is antiferromagnetic (AFM) in nature. However, on reduction of the grain size, emergence of ferromagnetic behavior is experimentally observed. Linear and nonlinear ac magnetic susceptibility measurements reveal that ferromagnetic nature is enhanced as the grain size is reduced. Large coercivity and bifurcation between zero field cooled and field cooled magnetization curves indicate an intrinsic disorder and large anisotropy in the system. Observed behaviors are attributed to surface disorder as well as to possible pressure effect on nano grains.",1006.5351v1 2010-08-17,Anisotropic magnetic fluctuations in the ferromagnetic superconductor UCoGe studied by angle-resolved ^{59}Co NMR,"We have carried out direction-dependent ^{59}Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.",1008.2837v1 2010-08-25,Upper critical field in superconductors near ferromagnetic quantum critical points; UCoGe,"We study the strong-coupling superconductivity near ferromagnetic quantum critical points, mainly focusing on the upper critical fields $H_{c2}$. Based on our simple model calculations, we discuss experimentally observed unusual behaviors of $H_{c2}$ in a recently discovered ferromagnetic superconductor UCoGe. Especially, the large anisotropy between $H_{c2}\parallel a$-axis and $H_{c2}\parallel c$-axis, and the strong-coupling behaviors in $H_{c2}^{\parallel a}$ are investigated. We also examine effects of non-analytic corrections in the spin susceptibility on the superconductivity, which can arise from effective long range interactions due to particle-hole excitations.",1008.4204v1 2010-09-21,Electron Spin Resonance of the ferromagnetic Kondo lattice CeRuPO,"The spin dynamics of the ferromagnetic Kondo lattice CeRuPO is investigated by Electron Spin Resonance (ESR) at microwave frequencies of 1, 9.4, and 34~GHz. The measured resonance can be ascribed to a rarely observed bulk Ce3+ resonance in a metallic Ce compound and can be followed below the ferromagnetic transition temperature Tc=14 K. At T>Tc the interplay between the RKKY-exchange interaction and the crystal electric field anisotropy determines the ESR parameters. Near Tc the spin relaxation rate is influenced by the critical fluctuations of the order parameter.",1009.4108v1 2010-09-28,Ferromagnetic Behavior of High Purity ZnO nanoparticles,"ZnO nanoparticles with Wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. Nanoparticles are in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization vs temperature, M(T) and isothermal measurements M(H) were determined. The coercive field clearly shows ferromagnetism above room temperature. The chemical synthesis, structural defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior. PACS numbers: 61.46.Hk Nanocrystals, 75.50.Pp Magnetic semiconductors, 81.05.Dz II-VI semiconductors",1009.5641v1 2010-10-01,Thermally induced 0-pi phase transition in Josephson junctions through a ferromagnetic oxide film,"We investigate the Josephson transport through a ferromagnetic oxide film, e.g., La$_2$BaCuO$_5$, theoretically. Using the recursive Green's function technique, we found the formation of a pi-junction in such systems. Moreover the 0-pi phase transition is induced by increasing the temperature. Such ferromagnetic-oxide based Josephson junctions may become an element in the architecture of future quantum computers.",1010.0042v1 2010-10-20,Spin Josephson effect with a single superconductor,"A thin ferromagnetic layer on a bulk equal-spin-pairing triplet superconductor is shown to mediate a Josephson coupling between the spin $\uparrow$ and $\downarrow$ condensates of the superconductor. By deriving analytic expressions for the bound states at the triplet superconductor-ferromagnet interface, we show that this spin Josephson effect establishes an effective anisotropy axis in the ferromagnetic layer. The associated Josephson spin current is predicted to cause a measurable precession of the magnetization about the vector order parameter of the triplet superconductor.",1010.4238v2 2010-11-11,Spin Torque Ferromagnetic Resonance Induced by the Spin Hall Effect,"We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle.",1011.2788v1 2010-11-16,Magnetic field dependence of pairing interaction in ferromagnetic superconductors with triplet pairing,"It is developed a microscopic description of superconductivity in ferromagnetic materials with triplet pairing triggered by the exchange of magnetic fluctuations. Instead widely used paramagnon model we work with phenomenological spectrum of fluctuations in the orthorhombic ferromagnet with strong magnetic anisotropy. Depending of the field orientation parallel or perpendicular to the direction of spontaneous magnetization the effective amplitude of pairing interaction proves to be decreasing or increasing function of magnetic field that allows to explain the drastic difference in magnitudes of upper critical field in these directions.",1011.3753v1 2010-12-21,Itinerant Ferromagnetism in ultracold Fermi gases,"Itinerant ferromagnetism in cold Fermi gases with repulsive interactions is studied applying the Jastrow-Slater approximation generalized to finite polarization and temperature. For two components at zero temperature a second order transition is found at $ak_F\simeq0.90$ compatible with QMC. Thermodynamic functions and observables such as the compressibility and spin susceptibility and the resulting fluctuations in number and spin are calculated. For trapped gases the resulting cloud radii and kinetic energies are calculated and compared to recent experiments. Spin polarized systems are recommended for effective separation of large ferromagnetic domains. Collective modes are predicted and tri-critical points are calculated for multi-component systems.",1012.4569v1 2011-02-13,Dark Energy from ferromagnetic condensation of cosmic magninos,"It is proposed that an ultra-light fermionic species, dubbed cosmic magnino has condensed into a ferromagnetic state in the Universe. The extended structure of domain walls associated with this ferromagnetism accounts for the observed Dark Energy. In modification of the situation with an electron gas, it is proposed that the Stoner criterion is satisfied due to magnetic dipolar repulsion. The cosmological requirements then yeild a lower bound on the magnetic moment of the cosmic magnino. The proposed magnetism is supposed to be associated with a new non-standard electromagnetism. If the magnino is also electrically charged under this electromagnetism, the corresponding oppositely charged heavier species would account partially or entirely for the Dark Matter in the Universe.",1102.2562v1 2011-04-05,Enhancement of the ferromagnetic order of graphite after sulphuric acid treatment,"We have studied the changes in the ferromagnetic behavior of graphite powder and graphite flakes after treatment with diluted sulphuric acid. We show that this kind of acid treatment enhances substantially the ferromagnetic magnetization of virgin graphite micrometer size powder as well as in graphite flakes. The anisotropic magnetoresistance (AMR) amplitude at 300 K measured in a micrometer size thin graphite flake after acid treatment reaches values comparable to polycrystalline cobalt.",1104.0812v1 2011-04-11,Hydrogen mediated ferromagnetism in ZnO single crystals,"We investigated the magnetic properties of hydrogen plasma treated ZnO single crystals by SQUID magnetometry. In agreement with the expected hydrogen penetration depth we found ferromagnetic behavior located at the first 20 nm of the H-treated surface of ZnO with magnetization at saturation up to 6 emu/g at 300 K and Curie temperature T$_c$ $\gtrsim $ 400 K. In the ferromagnetic samples a hydrogen concentration of a few atomic percent in the first 20 nm surface layer was determined by nuclear reaction analysis. The saturation magnetization of H-treated ZnO increases with the concentration of hydrogen.",1104.1899v1 2011-04-15,The ground state energy of the frustrated ferromagnetic spin chain near the transition point,"The one-dimensional quantum spin-1/2 model with nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic interaction is considered. The Hamiltonian is first bosonized by using the linear spin wave approximation, and then is treated by using the Green's function approach. An integral expression of the quantum correction to the classical ground state energy is derived. The critical behavior of the ground state energy in the vicinity of the transition point from the ferromagnetic to the singlet ground state is analyzed by numerical calculation, and the result is $-8\gamma^2$.",1104.2946v1 2011-05-30,A ferromagnetic-like phase transition in new oxychalcogenide HgOCuSe,"We report the synthesis of a new oxychalcogenide HgOCuSe sample. The resistivity decreases as a function of $T^{1.75}$ with decreasing temperature from room temperature down to around 80 K. There exists a very sharp ferromagnetic-like phase transition at around 60 K under a field of $H$ = 100 Oe. Contrary to the usual ferromagnetic materials, the descending and ascending branches of the magnetic hysteresis curve, at 30 K, are reversed in the whole irreversible field range and the reverse irreversibility decreases at 5 K.",1105.5868v1 2011-06-20,Anisotropic supercurrent due to inhomogeneous magnetization in ferromagnet/superconductor junctions,"We investigate transverse charge and spin dc supercurrents in a ferromagnet coupled to a superconductor where the ferromagnet has inhomogeneous magnetic structure. These transverse supercurrents arise from non-trivial structure of the magnetization. The magnetic structure manifested in the transverse charge supercurrent is essentially different from that discussed in the context of anomalous Hall effect, reflecting the disspationless nature of supercurrent. Possible candidates of magnetic structure to verify our prediction are also discussed.",1106.3801v4 2011-06-28,Oxygen adsorption effect on magnetic properties of graphite,"Both experimental and theoretical studies of the magnetic properties of micrographite and nanographite indicate a crucial role of the partial oxidation of graphitic zigzag edges in ferromagnetism. In contrast to total and partial hydrogenation, the oxidation of half of the carbon atoms on the graphite edges transforms the antiferromagnetic exchange interaction between graphite planes and over graphite ribbons to the ferromagnetic interaction. The stability of the ferromagnetism is discussed.",1106.5565v1 2011-07-01,Charge pumping due to triplet vector chirality in ferromagnet/triplet superconductor junctions,"We investigate charge pumping in ferromagnet/triplet superconductor junctions where the magnetization of the ferromagnet is inhomogeneous and dynamical. It is shown that charge current is pumped due to the coupling of the localized spin with triplet vector spin chirality, vector spin chirality formed by the triplet vector of Cooper pairing. Physical mechanism of the charge pumping is also discussed.",1107.0119v2 2011-07-30,Polaronic transport in the ferromagnetic phase of Gd1-xCaxBaCo2O5.53,"Temperature dependent electrical resistivity and thermopower measurements were carried out on Gd1-xCaxBaCo2O5.53 with x varying between 0 and 0.25. Ca subsitution leads to the incorporation of holes (Co4+) into the system that leads to a reduction in resistivity and a stabilisation of the ferromagnetic phase at low temperatures. The temperature dependence of resistivity and thermopower are markedly different in the Ca doped sample, with a dramatic reduction in the resistivity, as compared to that in the pristine sample. The variation in both the resistivity and thermopower with temperature is explained in terms of the transport of polarons in the ferromagnetic phase of Ca doped system.",1108.0059v1 2011-08-01,A non isothermal phase-field model for the ferromagnetic transition,"We propose a model for non isothermal ferromagnetic phase transition based on a phase field approach, in which the phase parameter is related but not identified with the magnetization. The magnetization is split in a paramagnetic and in a ferromagnetic contribution, dependent on a scalar phase parameter and identically null above the Curie temperature. The dynamics of the magnetization below the Curie temperature is governed by the order parameter evolution equation and by a Landau-Lifshitz type equation for the magnetization vector. In the simple situation of a uniaxial magnet it is shown how the order parameter dynamics reproduces the hysteresis effect of the magnetization.",1108.0378v1 2011-08-29,Proximity-induced density-of-states oscillations in a superconductor/strong-ferromagnet system,"We have measured the evolution of the tunneling density of states (DOS) in superconductor/ferromagnet (S/F) bilayers with increasing F-layer thickness, where F in our experiment is the strong ferromagnet Ni. As a function of increasing Ni thickness, we detect multiple oscillations in the DOS at the Fermi energy from differential conductance measurements. The features in the DOS associated with the proximity effect change from normal to inverted twice as the Ni thickness increases from 1 to 5 nm.",1108.5613v1 2011-09-17,Break-junction experiments on the zero-bias anomaly of non-magnetic and ferromagnetically ordered metals,"We have investigated break junctions of normal non-magnetic metals as well as ferromagnets at low temperatures. The point contacts with radii 0.15 - 15 nm showed zero-bias anomalies which can be attributed to Kondo scattering at a single Kondo impurity at the contact or to the switching of a single conducting channel. The Kondo temperatures derived from the width of the anomalies varied between 10 and 1000 K. These results agree well with literature data on atomic-size contacts of the ferromagnets as well as with spear-anvil type contacts on a wide variety of metals.",1109.3774v1 2011-09-20,Hysteresis for ferromagnetism: asymptotics of some 2-scale Landau-Lifshitz model,"We study a 2-scale version of the Landau-Lifshitz system of ferromagnetism, introduced by Starynkevitch to modelize hysteresis: the response of the magnetization is fast compared to a slowly varying applied magnetic fi eld. Taking the exchange term into account, in space dimension 3, we prove that, under some natural stability assumption on the equilibria of the system, the strong solutions follow the dynamics of these equilibria. We also give explicit examples of relevant equilibria and exterior magnetic fields, when the ferromagnetic medium occupies some ellipsoidal domain.",1109.4352v1 2011-12-03,Microwave-induced DC Signal in a Permalloy Thin Strip at Low Applied Magnetic Field,"We investigated the ferromagnetic resonance signals in a polycrystalline permalloy thin strip under in-plane low static magnetic field. A series of DC voltages, which contain ferromagnetic resonance or spin wave resonance signals, were measured by inducing microwave frequencies greater than 10 gigahertz. The resonant signals measured in low magnetic field show different properties from those detected in high field condition. Based on the theory of DC effects in ferromagnetic resonance and the experimental data of anisotropic magnetoresistance, a quantitative model was proposed. We found that the shape anisotropy significantly affects magnetization, and distorts the resonant signals in low field condition.",1112.0610v1 2012-02-22,Transverse Field Ising Ferromagnetism in Mn$_{12}$-acetate-MeOH,"We report measurements of the magnetic susceptibility of single crystals of Mn$_{12}$-acetate-MeOH, a new high-symmetry variant of the original single molecule magnet Mn$_{12}$-acetate. A comparison of these data to theory and to data for the Mn$_{12}$ acetate material shows that Mn$_{12}$-acetate-MeOH is a realization of a transverse-field Ising ferromagnet in contrast to the original Mn$_{12}$ acetate material, in which solvent disorder leads to effects attributed to random field Ising ferromagnetism.",1202.4963v1 2012-02-29,Inverse Spin Hall Effect in Ferromagnetic Metal with Rashba Spin Orbit Coupling,"We report an intrinsic form of the inverse spin Hall effect (ISHE) in ferromagnetic (FM) metal with Rashba spin orbit coupling (RSOC), which is driven by a normal charge current. Unlike the conventional form, the ISHE can be induced without the need for spin current injection from an external source. Our theoretical results show that Hall voltage is generated when the FM moment is perpendicular to the ferromagnetic layer. The polarity of the Hall voltage is reversed upon switching the FM moment to the opposite direction, thus promising a useful readback mechanism for memory or logic applications.",1202.6428v1 2012-03-01,Current-induced resonance in a ferromagnet - antiferromagnet junction,"We calculate the response of a ferromagnet - antiferromagnet junction to a high-frequency magnetic field as a function of the spin-polarized current through the junction. Conditions are choused under which the response is zero in absence of such a current. It is shown that increasing in the current density leads to proportional increase in the resonance frequency and resonant absorption. A principal possibility is indicated of using ferromagnet - antiferromagnet junction as a terahertz radiation detector.",1203.0114v1 2012-03-13,Monopoles in ferromagnetic metals,"The aim of this short review is to give an introduction to monopoles and to present theoretical derivation of two particular monopoles in ferromagnetic metals, a hedgehog monopole and a spin damping monopole. Spin damping monopoles can be generated in simple systems such as a junction of a ferromagnet and a heavy element with strong spin-orbit interaction such as Pt. This monopole is essential in coupling electronics with magnetism, and is thus expected to play an essential role in spintronics.",1203.2709v1 2012-06-13,Ferromagnetism and orbital order in a topological ferroelectric,"We explore via density functional calculations the magnetic doping of a topological ferroelectric as an unconventional route to multiferroicity. Vanadium doping of the layered perovskite La$_{2}$Ti$_{2}$O$_{7}$ largely preserves electric polarization and produces robust ferromagnetic order, hence proper multiferroicity. The marked tendency of dopants to cluster into chains results in an insulating character at generic doping. Ferromagnetism stems from the symmetry breaking of the multi-orbital V system via an unusual ""antiferro""-orbital order, and from the host's low-symmetry layered structure.",1206.2879v2 2012-06-23,"Novel Aharonov-Bohm-like effect: Detectability of the vector potential in a solenoidal configuration with a ferromagnetic core covered by superconducting lead, and surrounded by a thin cylindrical shell of aluminum","The flux as measured by the Josephson effect in a SQUID-like configuration with a ferromagnetic core inserted into its center, is shown to be sensitive to the vector potential arising from the central ferromagnetic core, even when the core is covered with a superconducting material that prevents any magnetic field lines from ever reaching the perimeter of the SQUID-like configuration. This leads to a macroscopic, Aharonov-Bohm-like effect that is observable in an asymmetric hysteresis loop in the response of the SQUID-like configuration to an externally applied magnetic field.",1206.5362v1 2012-09-03,Pressure induced ferromagnetism in antiferromagnetic Fe_1.03Te,"The magnetic properties of Fe1.03Te under hydrostatic pressure up to p ~ 5.7 GPa were investigated by means of muon spin rotation, dc magnetization, and neutron depolarization measurements. With increasing pressure the antiferromagnetic ordering temperature TN decreases continuously from 70 K at ambient pressure towards higher pressures. Surprisingly, the commensurate antiferromagnetic order of FeTe enters a region of incommensurate and dynamical magnetic order before at p ~ 1.7 GPa the system turns ferromagnetic. The ferromagnetic ordering temperature TC increases with increasing pressure.",1209.0383v1 2012-09-09,Temperature-dependent proximity magnetism in Pt,"We experimentally demonstrate the existence of magnetic coupling between two ferromagnets separated by a thin Pt layer. The coupling remains ferromagnetic regardless of the Pt thickness, and exhibits a significant dependence on temperature. Therefore, it cannot be explained by the established mechanisms of magnetic coupling across nonmagnetic spacers. We show that the experimental results are consistent with the presence of magnetism induced in Pt in proximity to ferromagnets, in direct analogy to the well-known proximity effects in superconductivity.",1209.1802v1 2012-09-19,Asymmetry of modal profiles of dipole-exchange spin waves in thin high-magnetic moment metallic ferromagnetic films,"The asymmetry of the modal profiles for dipole-exchange spin waves propagating in in-plane magnetized ferromagnetic films at a right angle to the applied magnetic field has been investigated theoretically. It was found that in the large-magnetic moment ferromagnetic metallic films with typical thicknesses 10-60 nm the fundamental mode of the spectrum is localized at the surface opposite to the surface of localization of the exchange-free Damon-Eshbach surface wave. This anomalous localization of the wave does not affect the non-reciprocity of spin wave excitation by microstrip and coplanar transducers but may be detected in other types of experiments.",1209.4153v1 2012-09-19,Spin-Transfer and Exchange Torques in Ferromagnetic Superconductors,"We consider how superconducting correlations influence spin-transfer torques in ferromagnetic superconductors. It is demonstrated that there is a novel torque arising from particle-hole interference that depends on the U(1) phase associated with the superconducting order parameter. We also show that there is an equilibrium exchange torque between two ferromagnetic superconductors in contact via a normal metal mediated by Andreev states. The latter equilibrium magnetic torque is also sensitive to spin-resolved phase differences in the superconducting order parameters as well as to an externally applied phase difference.",1209.4356v2 2012-09-26,Ferromagnetic InMnAs on InAs Prepared by Ion Implantation and Pulsed Laser Annealing,"Ferromagnetic InMnAs has been prepared by Mn ion implantation and pulsed laser annealing. The InMnAs layer reveals a saturated magnetization of 2.6 mu_B/Mn at 5 K and a perpendicular magnetic anisotropy. The Curie temperature is determined to be 46 K, which is higher than those in previous reports with similar Mn concentrations. Ferromagnetism is further evidenced by the large magnetic circular dichroism.",1209.5865v1 2012-10-12,The role of magnetic polarons in ferromagnetic GdN,"We report an interplay between magnetism and charge transport in the ferromagnetic semiconductor GdN, pointing to the formation of magnetic polarons centred on nitrogen vacancies. The scenario goes some way to resolving a long-standing disagreement between the measured and predicted Curie temperature in GdN. It further constitutes an extension of concepts that relate closely to the behaviour of ferromagnetic semiconductors generally, and EuO in particular.",1210.3441v1 2012-11-06,Spin Wave Scattering in Ferromagnetic Cross,"Spin wave scattering in the right angle ferromagnetic cross was measured. Shape anisotropy defined magnetization ground states at zero biasing magnetic fields. Scattering of the spin waves in the center of ferromagnetic cross is strongly dependent on the amplitude and angle of the biasing magnetic field. Micromagnetic simulations indicate that low in-plane biasing magnetic fields rotate the magnetization of the cross center while the arms stay axially magnetized due to the shape anisotropy. We discuss effect of biasing magnetic fields on the spin wave scattering and approaches to an effective spin wave switch based on the fabricated structure.",1211.1259v1 2013-01-08,Exactly solvable spin-glass models with ferromagnetic couplings: the spherical multi-$p$-spin model in a self-induced field,"We report some results on the quenched disordered Spherical multi-$p$-Spin Model in presence of ferromagnetic couplings. In particular, we present the phase diagrams of some representative cases that schematically describe, in the mean-field approximation, the behavior of most known transitions in glassy materials, including dynamic arrest in super-cooled liquids, amorphous-amorphous transitions and spin-glass transitions. A simplified notation is introduced in order to compute systems properties in terms of an effective, self-induced, field encoding the whole ferromagnetic information.",1301.1475v1 2013-03-07,Spin and Orbital Splitting in Ferromagnetic Contacted Single Wall Carbon Nanotube Devices,"We observed the coulomb blockade phenomena in ferromagnetic contacting single wall semiconducting carbon nanotube devices. No obvious Coulomb peaks shift was observed with existing only the Zeeman splitting at 4K. Combining with other effects, the ferromagnetic leads prevent the orbital spin states splitting with magnetic field up to 2 Tesla at 4K. With increasing magnetic field further, both positive or negative coulomb peaks shift slopes are observed associating with clockwise and anticlockwise orbital state splitting. The strongly suppressed/enhanced of the conductance has been observed associating with the magnetic field induced orbital states splitting/converging.",1303.1580v1 2013-04-19,Spin nematic states in antiferromagnets containing ferromagnetic bonds,"The majority of recent works devoted to spin nematic phases deal with either frustrated magnets or with those described by Hamiltonians with large non-Heisenberg terms. We show in the present study that nonfrustrated antiferromagnets (AFs) containing ferromagnetic (FM) bonds can show nematic phases in a strong magnetic field. Among the particular spin systems discussed are a ladder with FM rungs, two AF layers coupled ferromagnetically, a chain containing alternating AF and FM bonds, and an AF anisotropic spin-1 chain.",1304.5463v1 2013-04-22,Coexistence of p-wave Cooper pairing and ferromagnetism,"A two-band model for coexistence of p-wave superconductivity with localized ferromagnetism is studied using the equation of motion approach. It shows that ferromagnetic and superconducting states enhance each other but in a different way from that of the one-band model. The Curie temperature is not only determined by the exchange interactions between localized spins, but also can be increased with the coupling between electrons and spins, and with the p-wave Cooper-pairing interaction. These results are complementary to those of the one-band model, which suggest that the Curie temperature is unlikely to ever be below the superconducting transition temperature.",1304.5854v1 2013-05-06,Quantum Monte Carlo study of the two-dimensional ferromagnet,"We present Quantum Monte Carlo calculations that probe the paramagnet-ferromagnet phase transition in a two-dimensional Stoner Hamiltonian. With a screened Coulomb interaction we observe a first order ferromagnetic transition for short screening lengths, and a second order transition with a longer screening length, accompanied by a rising critical interaction strength. Finally, we discuss the consequences of our results for an ultracold atomic gas with finite ranged interactions.",1305.1198v1 2013-06-12,Controllable valley and spin transports in ferromagnetic silicene junctions,"We investigate charge, valley, and spin transports in normal/ferromagnetic/normal silicene junc- tion. We show that the charge, valley, and spin conductances in this junction oscillate with the length of the ferromagnetic silicene. It is also found that the current through this junction is valley and spin polarized due to the coupling between valley and spin degrees of freedom, and the valley and spin polarizations can be tuned by local application of a gate voltage. In particular, we find fully valley and spin polarized current by applying the electric field. We also obtain the condition for observing the fully valley and spin polarized current.",1306.2824v1 2013-06-18,Magnetic domain growth in a ferromagnetic Bose-Einstein condensate: Effects of current,"Magnetic domain patterns in a ferromagnetic Bose-Einstein condensate (BEC) show different properties depending on the quadratic Zeeman effect and dissipation. Another important factor that affects domain patterns and domain growth is superfluid flow of atoms. Domain growth in a ferromagnetic BEC with negative quadratic Zeeman energy is characterized by the same growth law as (classical) binary fluid in the inertial hydrodynamic regime. In the absence of the superfluid flow, the domain growth law for negative quadratic Zeeman energy is the same as that of scalar conserved fields such as binary alloys.",1306.4129v2 2013-06-26,"Actuation, propagation, and detection of transverse magnetoelastic waves in ferromagnets","We study propagation of ultrasonic waves through a ferromagnetic medium with special attention to the boundary conditions at the interface with an ultrasonic actuator. In analogy to charge and spin transport in conductors, we formulate the energy transport through the system as a scattering problem. We find that the magneto-elastic coupling leads to a non-vanishing magnetic (elastic) energy accompanying the acoustic (spin) waves with a resonantly enhanced effect around the dispersion relation anti-crossing point. We demonstrate the physics of excitation of magnetization dynamics via acoustic waves injection around the ferromagnetic resonance frequency.",1306.6268v2 2013-10-23,Magnetization Characteristic of Ferromagnetic Thin Strip by Measuring Anisotropic Magnetoresistance and Ferromagnetic Resonance,"The magnetization characteristic in a permalloy thin strip is investigated by electrically measuring the anisotropic magnetoresistance and ferromagnetic resonance in in-plane and out-of-plane configurations. Our results indicate that the magnetization vector can rotate in the film plane as well as out of the film plane by changing the intensity of external magnetic field of certain direction. The magnetization characteristic can be explained by considering demagnetization and magnetic anisotropy. Our method can be used to obtain the demagnetization factor, saturated magnetic moment and the magnetic anisotropy.",1310.6117v1 2013-12-09,Pseudofermion ferromagnetism in the Kondo lattices: a mean-field approach,"Ground state ferromagnetism of the Kondo lattices is investigated within slave fermion approach by Coleman and Andrei within a mean-field approximation in the effective hybridization model. Conditions for formation of both saturated (half-metallic) and non-saturated magnetic state are obtained for various lattices. A description in terms of universal functions which depend only on bare electron density of states (DOS) is presented. A crucial role of the energy dependence of the bare DOS (especially, of DOS peaks) for the small-moment ferromagnetism formation is demonstrated.",1312.2438v2 2013-12-13,Impurity-vacancy complexes and ferromagnetism in doped sesquioxides,"Based on hybrid density-functional calculations, we propose that ferromagnetism in the prototypical bixbyite sesquioxide In$_2$O$_3$ doped with Cr is due to Cr-oxygen vacancy complexes, while isolated Cr cannot support carrier-mediated magnetic coupling. Our proposal is consistent with experimental facts such as the onset of ferromagnetism in O-lean conditions only, the low or vanishing net moment in unintentionally doped material, and its increase upon intentional doping.",1312.3793v2 2013-12-30,Some remarks on the magnetic phase diagram of ferromagnetic superconductor UGe2,"The phase transition to ferromagnetic order in itinerant ferromagnetic superconductor UGe$_2$ as function of pressure shows change of phase transition order. At low temperature and high pressure the transition is of first order and at pressure of about 1.42 GPa the order changes to second. On the basis of Landau expansion of free energy up to the sixth order in magnetization we calculate the phase diagram taking into account the magneto-elastic interaction as the mechanism responsible for this change. We propose a simple Stoner-like dependence of the Curie temperature on the pressure and present the results for measurable thermodynamic quantities as the entropy jump at the first order phase transition.",1312.7773v1 2014-01-14,Defects induced ferromagnetism in plasma-enabled graphene nanopetals,"Ferromagnetism in graphene is fascinating, but it is still a big challenge for practical applications due to the weak magnetization. In order to enhance the magnetization, here, we design plasma-enabled graphene nanopetals with ultra-long defective edges of up to 105 m/g, ultra-dense lattice vacancies and hydrogen chemisorptions. The designed graphene nanopetals display robust ferromagnetism with large saturation magnetization of up to 2 emu/g at 5 K and 1.2 emu/g at room temperatures. This work identifies the plasma-enabled graphene nanopetals as a promising candidate for graphene-based magnetic devices.",1401.3060v1 2014-01-22,Disentangling defect-induced ferromagnetism in SiC,"We present a detailed investigation of the magnetic properties in SiC single crystals bombarded with neon ions. Through careful measuring of the magnetization of virgin and irradiated SiC, we decompose the magnetization of SiC into paramagnetic, superparamagnetic, and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. We qualitatively explain the magnetic properties as a result of the intrinsic clustering tendency of defects.",1401.5576v1 2014-04-08,Darboux Transformation and Exact Solutions of the integrable Heisenberg ferromagnetic equation with self-consistent potentials,"Integrable Heisenberg ferromagnetic equations are an important subclass of integrable systems. The M-XCIX equation is one of a generalizations of the Heisenberg ferromagnetic equation and are integrable. In this paper, the Darboux transformation of the M-XCIX equation is constructed. Using the DT, a 1-soliton solution of the M-XCIX equation is presented.",1404.2270v2 2014-07-03,Imprinting skyrmions in thin films by ferromagnetic and superconducting templates,"Magnetic skyrmions are promising candidates as information carriers in a new generation of memories. How to generate and stabilize skyrmions is essential for their successful application to technology. Here we theoretically demonstrate that arrays of skyrmions can be imprinted in ultrathin ferromagnetic films in large numbers by bringing a magnetic nanostructured template close to the film. Two kind of templates, allowed by present-day nanotechnologies, are studied: arrays of ferromagnetic nanorods and superconducting vortices. Skyrmions are generated when exposing magnetic films to the template fields for short times and remain stable after removing the template.",1407.0928v1 2014-08-05,Self-consistent model of spin accumulation magnetoresistance in ferromagnet-insulator-semiconductor tunnel junctions,"Spin accumulation in a paramagnetic semiconductor due to voltage-biased current tunneling from a polarized ferromagnet is experimentally manifest as a small additional spin-dependent resistance. We describe a rigorous model incorporating the necessary self-consistency between electrochemical potential splitting, spin-dependent injection current, and applied voltage that can be used to simulate this so-called ""3T"" signal as a function of temperature, doping, ferromagnet bulk spin polarization, tunnel barrier features and conduction nonlinearity, and junction voltage bias.",1408.1003v2 2014-09-16,Tuning ferromagnetism at interfaces between insulating perovskite oxides,"We use density functional theory calculations to show that the LaAlO3|SrTiO3 interface between insulating perovskite oxides is borderline in satisfying the Stoner criterion for itinerant ferromagnetism and explore other oxide combinations with a view to satisfying it more amply. The larger lattice parameter of an LaScO3|BaTiO3 interface is found to be less favorable than the greater interface distortion of LaAlO3|CaTiO3. Compared to LaAlO3|SrTiO3, the latter is predicted to exhibit robust magnetism with a larger saturation moment and a higher Curie temperature. Our results provide support for a ""two phase"" picture of coexistent superconductivity and ferromagnetism.",1409.4554v1 2014-11-10,Magnon spectrum in ferromagnets with a skyrmion,"The analysis of the spin wave excitations in two-dimensional isotropic Heisenberg ferromagnet is performed with a single skyrmion in the ground state. We employ the ideas of semiclassical quantization method, duly modified for the use of the lattice model and Maleyev-Dyson boson representation of spin operators. The resulting Schr\""odinger equation for magnons describes the dispersion and wave functions of spin-wave excitations with strictly non-negative spectrum. In contrast to usual ferromagnet, we demonstrate the existence of three zero modes, corresponding to conformal symmetries spontaneously violated by the skyrmion configuration.",1411.2452v2 2014-12-19,"Erratum: Glassy Chimeras Could Be Blind to Quantum Speedup. . . [Phys. Rev. X 4, 021008 (2014)]","Erratum to Phys. Rev. X 4, 021008 (2014): The critical exponent associated with the ferromagnetic susceptibility was computed incorrectly. Furthermore, Ising ferromagnets on the Chimera topology have the same universality class as two-dimensional Ising ferromagnets.",1412.6320v2 2014-12-21,Recovering of superconductivity in S/F bilayers under spin-dependent nonequilibrium quasiparticle distribution,"We study theoretically the influence of spin accumulation on superconductivity in a superconductor/ferromagnet bilayer. It is well-known that the superconductivity in S/F bilayers is suppressed by the proximity to a ferromagnet. The spin accumulation by itself is also a depairing factor. But here we show that creation of the spin accumulation on top of effective exchange depairing, caused by the proximity to a ferromagnet, can lead to an opposite result. The superconductivity can be partially recovered by spin-dependent quasiparticle distribution. The systems with realistic parameters are considered and the possible experimental setup is proposed.",1412.6846v1 2014-12-22,"Thickness dependent Curie temperature and power-law behavior of layering transitions in ferromagnetic classical and quantum thin films described by Ising, XY and Heisenberg models","Ferromagnetic-paramagnetic phase transitions in classical and quantum thin films have been studied up to 50 mono-layers using effective field theory with two-site cluster approximation. Variation of the Curie temperature as a function of film thickness has been examined. The relative shift of the Curie temperature from the corresponding bulk value has been investigated in terms of the shift exponent $\lambda$. We have found that shift exponent $\lambda$ clearly depends on the strength of the ferromagnetic exchange coupling of the surface. Moreover, we have not observed any significant difference between classical and quantum exponents for a particular model.",1412.7189v1 2015-01-02,Non-quasiparticle states in a half-metallic ferromagnet with antiferromagnetic $s-d(f)$ interaction,"Non-quasiparticle (incoherent) states which play an important role in the electronic structure of half-metallic ferromagnets (HMF) are investigated consistently in the case of antiferromagnetic $s-d(f)$ exchange interaction. Their appropriate description in the limit of strong correlations requires a rearrangement of perturbation series in comparison with the usual Dyson equation. This consideration provides a solution of the Kondo problem in the HMF case and can be important for first-principle HMF calculations performed earlier for ferromagnetic $s-d(f)$ interaction.",1501.00345v3 2015-01-24,Magnetic Coupling in Ferromagnetic Semiconductor GaMnAs/AlGaMnAs Bilayer Devices,"We carefully investigated the ferromagnetic coupling in the as-grown and annealed ferromagnetic semiconductor GaMnAs/AlGaMnAs bilayer devices. We observed that the magnetic interaction between the two layers strongly affects the magnetoresistance of the GaMnAs layer with applying out of plane magnetic field. After low temperature annealing, the magnetic easy axis of the AlGaMnAs layer switches from out of plane into in-plane and the interlayer coupling efficiency is reduced from up to 0.6 to less than 0.4. However, the magnetic coupling penetration depth for the annealed device is twice that of the as-grown bilayer device.",1501.05993v1 2015-02-12,Spin-current diode with a ferromagnetic semiconductor,"Diode is a key device in electronics: the charge current can flow through the device under a forward bias, while almost no current flows under a reverse bias. Here we propose a corresponding device in spintronics: the spin-current diode, in which the forward spin current is large but the reversed one is negligible. We show that the lead/ferromagnetic quantum dot/lead system and the lead/ferromagnetic semiconductor/lead junction can work as spin-current diodes. The spin-current diode, a low dissipation device, may have important applications in spintronics, as the conventional charge-current diode does in electronics.",1502.03599v1 2015-03-12,Chirp spectroscopy applied to the characterization of Ferromagnetic Resonance in Magnetic Tunnel Junctions,"Magnetic Tunnel Junction devices find use in several applications based on the exploitation of the Spin-Transfer Torque phenomenon. The Ferromagnetic Resonance curve is a key characteristic of any Magnetic Tunnel Junctions. It is usually characterized both experimentally and numerically by performing a lot of measurements of the magnetic response to a sinusoidal field or current. Here we propose the use of a chirp signal as excitation signal to reconstruct the Ferromagnetic resonance curve with a single measurement/simulation. A micromagnetic comparison of the proposed method with the traditional one is shown.",1503.03719v1 2015-03-17,Disorder effect on magneto-transport on the surface of a topological insulator,"We study the magneto-transport properties on the disordered surface of a topological insulator attached with a ferromagnet/ferromagnet junction. Since, in the surface Dirac Hamiltonian, out-of-plane magnetization induces a mass gap, while in-plane magnetization has a role of the effective vector potential, the mechanism of magneto-transport is different between these two cases. The former is similar to the conventional one in ferromagnetic metals, while the latter is due to the shift of Fermi circles in momentum space. Our numerical calculations show that the magnetoconductance in in-plane configuration is robust against disorder compared to that in out-of-plane configuration.",1503.04953v2 2015-03-23,Spin-Orbit Torques in Two-Dimensional Rashba Ferromagnets,"Magnetization dynamics in single-domain ferromagnets can be triggered by charge current if spin-orbit coupling is sufficiently strong. We apply functional Keldysh theory to investigate Rashba spin-orbit torques in metallic two-dimensional ferromagnets. A reactive, anti-damping-like spin-orbit torque as well as a dissipative, field-like torque are calculated microscopically, to the leading order in the spin-orbit interaction strength. By calculating the first vertex correction we show that the intrinsic anti-damping-like torque vanishes unless the scattering rates are spin-dependent.",1503.06872v2 2015-04-22,Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices,"We present a fabrication scheme called 'fork stamping' optimized for the dry transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic contact electrodes fabricated by standard lithography. We demonstrate the detailed recipes for a residue-free device fabrication and in-situ current annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py) contacts and report preliminary transport characterization and magnetoresistance experiments at cryogenic temperatures. This scheme can directly be used to implement more complex device structures, including multiple gates or superconducting contacts.",1504.05693v1 2015-04-22,Controlled domain wall pinning in ferromagnetic nanowire by nanoparticles stray fields,"We report the results of experimental investigations of controlled domain wall (DW) pinning in a ferromagnetic nanowire (NW) by stray fields of two uniformly magnetized bistable ferromagnetic nanoparticles (NPs) placed on either side of the NW and elongated parallel to the NW axis. We show by magnetic force microscopy measurements that DW pinning strength essentially depends on the orientation of the NP magnetic moments relative to the NW magnetization and can reach 20 mT. We also performed micromagnetic simulations confirming the influence of the magnetostatic interaction of the DW with the NP stray fields on the pinning strength. The possible realization of logic cell with switchable logic function is discussed.",1504.05784v1 2015-04-24,Universal coarsening dynamics of a quenched ferromagnetic spin-1 condensate,"We demonstrate that a quasi-two-dimensional spin-1 condensate quenched to a ferromagnetic phase undergoes universal coarsening in its late time dynamics. The quench can be implemented by a sudden change in the applied magnetic field and, depending on the final value, the ferromagnetic phase has easy-axis (Ising) or easy-plane (XY) symmetry, with different dynamical critical exponents. Our results for the easy-plane phase reveal a fractal domain structure and the crucial role of polar-core spin vortices in the coarsening dynamics.",1504.06404v2 2015-09-08,Anomalous Hall effect driven by dipolar spin waves in uniform ferromagnets,"A new type of anomalous Hall effect is shown to arise from the interaction of conduction electrons with dipolar spin waves in ferromagnets. This effect exists even in homogeneous ferromagnets without relativistic spin-orbit coupling. The leading contribution to the Hall conductivity is proportional to the chiral spin correlation of dynamical spin textures and is physically understood in terms of the skew scattering by dipolar magnons.",1509.02284v1 2015-09-09,Inertial terms to magnetization dynamics in ferromagnetic thin films,"Inertial magnetization dynamics have been predicted at ultrahigh speeds, or frequencies approaching the energy relaxation scale of electrons, in ferromagnetic metals. Here we identify inertial terms to magnetization dynamics in thin Ni$_{79}$Fe$_{21}$ and Co films near room temperature. Effective magnetic fields measured in high-frequency ferromagnetic resonance (115-345 GHz) show an additional stiffening term which is quadratic in frequency and $\sim$ 80 mT at the high frequency limit of our experiment. Our results extend understanding of magnetization dynamics at sub-picosecond time scales.",1509.02836v1 2015-09-11,Construction of van der Waals magnetic tunnel junction using ferromagnetic layered dichalcogenide,"We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe0.25TaS2. The vdW interlayer coupling at the Fe-intercalated plane of Fe0.25TaS2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material through the vdW junction.",1509.03525v1 2015-09-19,Spin torque and Nernst effects in Dzyaloshinskii-Moriya ferromagnets,"We predict that a temperature gradient can induce a magnon-mediated intrinsic torque and a transverse spin current in systems with non-trivial magnon Berry curvature. With the help of a microscopic linear response theory of nonequilibrium magnon-mediated torques and spin currents we identify the interband and intraband components that manifest in ferromagnets with Dzyaloshinskii-Moriya interactions and magnetic textures. To illustrate and assess the importance of such effects, we apply our theory to the magnon-mediated spin Nernst and torque responses in a kagome lattice ferromagnet.",1509.05847v3 2015-11-06,Dynamic Response in a Finite Size Composite Multiferroic Thin Film,"Composite multiferroics, heterostructures of ferromagnetic (FM) and ferroelectric (FE) materials, are characterized by a remarkable magnetoelectric effect at the interface. Previous work has supported the ferromagnetic structure with magnetic spins and the ferroelectric with pseudospins which act as electric dipoles in a microscopic model, coupled with a magnetoelectric interaction [J. Appl. Phys. 118, 124109 (2015)]. In this work, by solving the stochastic Landau-Lifshitz-Gilbert equation, the electric-field-induced magnetization switching in a twisted boundary condition has been studied, and a behavior of domain wall in the ferromagnetic structure is discussed.",1511.01982v2 2015-11-26,Spin textures and spin-wave excitations in doped Dirac-Weyl semimetals,"We study correlations and magnetic textures of localized spins, doped in three-dimensional Dirac semimetals. An effective field theory for magnetic moments is constructed by integrating out the fermionic degrees of freedom. The spin correlation shows a strong anisotropy, originating from spin-momentum locking of Dirac electrons, in addition to the conventional Heisenberg-like ferromagnetic correlation. The anisotropic spin correlation allows topologically nontrivial magnetic excitation textures such as a transient hedgehog state, as well as the ferromagnetic ground state. The spin-wave dispersion in ferromagnetic Weyl semimetal also becomes anisotropic, being less dispersed perpendicular to the magnetization.",1511.08381v1 2016-02-24,Application of the Maximum relative Entropy method to the physics of ferromagnetic materials,"It is known that the Maximum relative Entropy (MrE) method can be used to both update and approximate probability distributions functions in statistical inference problems. In this manuscript, we apply the MrE method to infer magnetic properties of ferromagnetic materials. In addition to comparing our approach to more traditional methodologies based upon the Ising model and Mean Field Theory, we also test the effectiveness of the MrE method on conventionally unexplored ferromagnetic materials with defects.",1603.00068v2 2016-04-02,A Theoretical Prediction on the Intrinsic Half-Metallicity in the Surface-Oxygen-Passivated Cr2N MXene,"Two-dimensional Cr2N MXene as well as the surface-passivated Cr2NF2, Cr2N(OH)2 and Cr2NO2 are investigated by using density functional theory. The Cr2N is an anti-ferromagnetic metal. The F atom or OH group-passivation does not change the anti-ferromagnetic characteristics. However, Cr2NO2 has a ferromagnetic ground state, which is a half-metal. The half-metallicity of Cr2NO2 is still robust when bias is applied to the nanometer sized device. The half-metallicity is intrinsic and does not require atomically clean surfaces. Therefore the stable surface-oxygen-passivated MXene is a good candidate for spintronics.",1604.00510v2 2016-04-05,Strongly Spinor Ferromagnetic Bose Gases,"By studying the zero-temperature and nonzero-temperature phase diagrams of the ferromagnetic spin-1 Bose-Hubbard model under an external magnetic field, we find that the competition between ferromagnetism and the quadratic Zeeman energy yields two superfluid phases, which feature discontinuous first-order phase transitions between them for a strongly spinor Bose gas such as ${}^7$Li, contrary to the corresponding continuum system.",1604.01163v4 2016-04-21,Model for multi-shot all-thermal all-optical switching in ferromagnets,"All optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promis- ing technological applications. However, fundamental understanding of the underlying mechanisms remains elusive. Here we present a new model for multi-shot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of Magnetic Circular Dichroism (MCD). We predict that AOS should be possible with as little as 0.5% of MCD, after a minimum number of laser shots. Finally, we re- produce previous AOS results by simulating the sweeping of a laser beam on an FePtC granular ferromagnetic film.",1604.06441v1 2016-06-24,Current-Induced Instability of a Perpendicular Ferromagnet in Spin Hall Geometry,"We develop a theoretical formula of spin Hall torque in the presence of two ferromagnets. While the direction of the conventional spin Hall torque always points to the in-plane direction, the present system enables to manipulate the torque direction acting on one magnetization by changing the direction of another magnetization. Based on the diffusion equation of the spin accumulation and the Landauer formula, we derive analytical formula of the spin Hall torque. The present model provides a solution to switch a perpendicular ferromagnet deterministically at zero field using the spin Hall effect.",1606.07540v1 2016-06-24,Stochastic modelling of thermal effects on a ferromagnetic nano particle,"In this work, we are interested in the behaviour of a single ferromagnetic mono--domain particle submitted to an external field with a stochastic perturbation. This model is a step toward the mathematical understanding of thermal effects on ferromagnets. In a first part, we discuss modelling issues and propose several ways to integrate a random noise in the deterministic model. Then, among all these approaches, we focus on the more natural one and study its long time behaviour. We prove that the system converges to the unique stable equilibrium of the deterministic model and make precise the L^p rate of the convergence. Finally, we illustrate the theoretical results by numerical simulations.",1606.07708v1 2016-06-29,Damped Topological Magnons in the Kagomé-Lattice Ferromagnets,"We demonstrate that interactions can substantially undermine the free-particle description of magnons in ferromagnets on geometrically frustrated lattices. The anharmonic coupling, facilitated by the Dzyaloshinskii-Moriya interaction, and a highly-degenerate two-magnon continuum yield a strong, non-perturbative damping of the high-energy magnon modes. We provide a detailed account of the effect for the $S=1/2$ ferromagnet on the kagom\'e lattice and propose further experiments.",1606.09249v3 2016-07-13,Easy Axis orientation of Ferromagnetic Films as Described by Third Order Perturbed Heisenberg Hamiltonian,"The magnetic easy axis orientation was studied by the third order perturbed Heisenberg Hamiltonian. Ferromagnetic CoPt/AlN multilayer thin films with number of layers N=11, 16 and 21 synthesized on fused quartz substrates using dc magnetron sputtering technique have been employed as experimental data. According to experimental research performed by some other researchers, easy axis of these fcc structured ferromagnetic films is oriented in the plane of the film at one particular temperature. Average value of out of plane spin component was plotted against temperature in order to determine the spin reorientation temperature. The spin reorientation temperature was highly sensitive to 2nd order magnetic anisotropy constant.",1607.03588v1 2016-07-20,A microwave interferometer of the Michelson-type to improve the dynamic range of broadband ferromagnetic resonance measurements,"We present a Michelson-type microwave interferometer for use in ferromagnetic resonance experiments. The interferometer is capable of broadband operation without manual adjustment of phase delay or amplitude attenuation. A prototype of the design shows significant improvement of the signal-to-noise ratio when compared to non-interferometric ferromagnetic resonance experiments. We demonstrate that this increase in sensitivity can lead to a drastic increase in the data acquisition rate for hard-to-measure thin films that otherwise would require long integration times.",1607.06137v2 2016-09-13,Spin-dependent transport and current modulation in a current-in-plane field-effect transistor,"We propose a current-in-plane spin-valve field-effect transistor (CIP-SV-FET), which is composed of a ferromagnet / nonferromagnet / ferromagnet trilayer structure and a gate electrode. This is a promising device alternative to spin metal-oxide-semiconductor field-effect transistors. Here, we fabricate a ferromagnetic-semiconductor GaMnAs-based CIP-SV-FET and demonstrate its basic operation of the resistance modulation both by the magnetization configuration and by the gate electric field. Furthermore, we present the electric-field-assisted magnetization reversal in this device.",1609.05859v1 2016-10-02,Study of the effect of nearest neighbors on ferromagnetic to paramagnetic phase transition in 2D lattices by Monte Carlo algorithm,"In this work we try to use the Monte Carlo algorithm, metropolis, to study the behavior of 2D magnetic systems; honeycomb, hexagonal and square lattices. In this study we use Ising Model of magnetism, with considering only nearest neighbors in the Hamiltonian and with the Ferromagnetic correlation constant. The code was written in Payton. In this calculation, by comparing the results obtained for all lattices, we will show that the paramagnetic to ferromagnetic phase transition depends on the number of neighbor and it is a direct dependence.",1610.00359v2 2016-10-12,New p- and n-type ferromagnetic semiconductors: Cr-doped BaZn2As2,"We find new ferromagnetic semiconductors, Cr-doped BaZn2As2, by employing a combined method of the density functional theory and the quantum Monte Carlo simulation. Due to a narrow band gap of 0.2 eV in host BaZn2As2 and the different hybridization of 3d orbitals of Cr impurity, the impurity bound states have been induced both below the top of valence band and above the bottom of conduction band. The long-range ferromagnetic coupling between Cr impurities is obtained with both p- and n-type carriers.",1610.03578v1 2016-10-14,DC Magnetization study of Ni$_{1-x}$Rh$_x$ nanoalloy,"Ni$_{1-x}$Rh$_x$ bulk alloys exhibit a ferromagnetic to paramagnetic quantum phase transition (QPT) at the critical concentration $x_c \sim$ 0.37. A spin glass phase arises below the Ferromagnetism by controlling the short range interaction between Ni and Rh atoms. we have synthesized nanoalloys of two different concentrations with two different reaction times by chemical reflux method. From the dc magnetization measurements, existence of ferromagnetism and spin glass phase in these nanoalloys is confirmed. A step like feature in both ZFC and FC curve at certain temperature indicates the existence of the spin glass phase.",1610.04487v1 2017-03-04,Dynamics of domain walls in weak ferromagnets,"It is shown that the total set of equations, which determines the dynamics of the domain bounds (DB) in a weak ferromagnet, has the same type of specific solution as the well-known Walker's solution for ferromagnets. We calculated the functional dependence of the velocity of the DB on the magnetic field, which is described by the obtained solution. This function has a maximum at a finite field and a section of the negative differential mobility of the DB. According to the calculation, the maximum velocity $ c \approx 2 \times 10^6$ cm/sec in YFeO$_3$ is reached at $H_m \approx 4 \times 10^3$ Oe.",1703.01502v1 2017-03-10,A microscopic formulation of dynamical spin injection in ferromagnetic-nonmagnetic heterostructures,"We develop a microscopic formulation of dynamical spin injection in heterostructure comprising nonmagnetic metals in contact with ferromagnets. The spin pumping current is expressed in terms of Green's function of the nonmagnetic metal attached to the ferromagnet where a precessing magnetization is induced. The formulation allows for the inclusion of spin-orbit coupling and disorder. The Green's functions involved in the expression for the current are expressed in real-space lattice coordinates and can thus be efficiently computed using recursive methods.",1703.03775v2 2018-04-04,Theoretical prediction of two-dimensional CrOF sheet as a ferromagnetic semiconductor or a half-metal,"Two-dimensional chromium oxide fluoride CrOF sheet was studied based on density functional theory. The investigation indicates that the CrOF sheet is an intrinsic ferromagnetic semiconductor. The calculated low cleavage energy implies that the ferromagnetic semiconductor can be exfoliated from its bulk form. The corresponding Curie temperature is 150 K. In particular, the Curie temperature increases up to 410 K under hole doping and the CrOF sheet becomes a half-metal. The versatile electronic and magnetic properties indicate that the two-dimensional CrOF sheet can be a promising candidate for next-generation spintronic devices.",1804.01621v1 2010-05-11,Dilatometric Study of LiHoF4 In a Transverse Magnetic Field,"Theoretical and experimental work have not provided a consistent picture of the phase diagram of the nearly ideal Ising ferromagnet LiHoF4 in a transverse magnetic field. Using a newly fabricated capacitive dilatometer, we have investigated the thermal expansion and magnetostriction of LiHoF4 in magnetic fields applied perpendicular to the Ising direction. Critical points for the ferromagnetic phase transition have been determined from both methods in the classical paramagnetic to ferromagnetic regime. Excellent agreement has been found with existing experimental data suggesting that, in this regime, the current theoretical calculations have not entirely captured the physics of this interesting model system.",1005.1935v1 2010-05-12,Proposal for a ferromagnetic multiwell spin oscillator,"The highly nonlinear coupling of transport and magnetic properties in a multiwell heterostructure, which comprises ferromagnetic quantum wells made of diluted magnetic semiconductors, is theoretically investigated. The interplay of resonant tunneling and carrier-mediated ferromagnetism in the magnetic wells induces very robust, self-sustained current and magnetization oscillations. Over a large window of steady bias voltages the spin polarization of the collector current is oscillating between positive and negative values, realizing a spin oscillator device.",1005.2087v1 2013-08-16,Influence of ferromagnetic ordering on Raman scattering in CoS$_2$,"The effects of phonon anharmonicity, phonon-magnon and electron-phonon interactions on the temperature dependence of Raman optical phonon modes are theoretically investigated. Besides of the Klemens result for the phonon width due to anharmonicity, the corresponding lineshift is derived. We argue that the phonon decay into two magnons has very low intensity in ferromagnets with low Curie temperatures. Therefore, the electron interband transitions accompanied with the ferromagnetic ordering are included in considerations to get a good quantitative agreement with experiments.",1308.3571v1 2014-03-19,Spin-orbit coupling effects on spin-dependent inelastic electronic lifetimes in ferromagnets,"For the 3d ferromagnets iron, cobalt and nickel we compute the spin-dependent inelastic electronic lifetimes due to carrier-carrier Coulomb interaction including spin-orbit coupling. We find that the spin-dependent density-of-states at the Fermi energy does not, in general, determine the spin dependence of the lifetimes because of the effective spin-flip transitions allowed by the spin mixing. The majority and minority electron lifetimes computed including spin-orbit coupling for these three 3-d ferromagnets do not differ by more than a factor of 2, and agree with experimental results.",1403.4728v1 2014-03-25,Strong ferromagnetic-dielectric coupling in multiferroic Lu2CoMnO6 single crystals,"We have grown single crystals of multiferroic double-perovskite Lu2CoMnO6 and studied the directional dependence of their magnetic and dielectric properties. The ferromagnetic order emerges below TC ~ 48K along the crystallographic c axis. Dielectric anomaly arises along the b axis at TC, contrary to the polycrystalline work suggesting ferroelectricity along the c axis. Through the strongly coupled ferromagnetic and dielectric states, the highly non-linear variation of both dielectric constant and magnetization was achieved in application of magnetic fields. This concurrent tunability provides an efficient route to manipulation of multiple order parameters in multiferroics.",1403.6236v1 2016-11-03,New applications of the Lambert and generalized Lambert functions to ferromagnetism and quantum mechanics,"The applications of the recent results obtained in the theory of generalized Lambert functions, to the mean field theory of ferromagnetism are presented. As a consequence, all the predictions of the Weiss theory of ferromagnetism can be explicitly and exactly formulated. In several quantum mechanical problems involving delta function potentials, the solutions of the transcendental eigenvalue equations are expressed in terms of two parameters generalized Lambert functions. Some others, till unnoticed examples of eigenvalue equations whose solutions can be written using the Lambert W function, are also presented.",1611.01014v3 2016-11-15,Dipolar ordering and domains in crystals of Mn$_{12}$ Ac molecular magnets,"Dipolar coupling in crystals of the Mn$_{12}$Ac molecular magnet elongated along the anisotropy axis favors ferromagnetic ordering below the mean-field Curie temperature 0.71 K. With the help of Monte Carlo on crystals of up to one million Mn$_{12}$ molecules, it is shown that ordering occurs at 0.36 K. The resulting state is split into ferromagnetic domains with domain walls preferring the diagonal orientation. Domain walls are pinned by the lattice at low temperatures. Making the crystal shorter makes domains finer and smoothens out the singularity at the transition, decreasing susceptibility in the domain state. Hysteresis loops look ferromagnetic for prolate crystals and antiferromagnetic for oblate crystals.",1611.04697v1 2016-11-26,Nagaoka's theorem in the Holstein-Hubbard model,"Nagaoka's theorem on ferromagnetism in the Hubbard model is extended to the Holstein-Hubbard model. This shows that Nagaoka's ferromagnetism is stable even if the electron-phonon interaction is taken into account. We also prove that Nagaoka's ferromagnetism is stable under the influence of the quantized radiation field.",1611.08659v4 2016-11-28,A new class of half-metallic ferromagnets from first principles,"Half-metallic ferromagnetism (HMFM) occurs rarely in materials and yet offers great potential for spintronic devices. Recent experiments suggest a class of compounds with the `ThCr$_{2}$Si$_{2}$' (122) structure -- isostructural and containing elements common with Fe pnictide-based superconductors -- can exhibit HMFM. Here we use $ab$ $initio$ density-functional theory calculations to understand the onset of half-metallicity in this family of materials and explain the appearance of ferromagnetism at a quantum critical point. We also predict new candidate materials with HMFM and high Curie temperatures through A-site alloying.",1611.09422v2 2017-04-11,Ferromagnetic and insulating behavior of LaCoO3 films grown on a (001) SrTiO3 substrate. A simple ionic picture explained ab initio,"This paper shows that the oxygen vacancies observed experimentally in thin films of LaCoO3 subject to tensile strain are thermodynamically stable according to ab initio calculations. By using DFT calculations, we show that oxygen vacancies on the order of 6 % forming chains perpendicular to the (001) direction are more stable than the stoichiometric solution. These lead to magnetic Co2+ ions surrounding the vacancies that couple ferromagnetically. The remaining Co3+ cations in an octahedral environment are non magnetic. The gap leading to a ferromagnetic insulating phase occurs naturally and we provide a simple ionic picture to explain the resulting electronic structure.",1704.03240v1 2017-04-24,Thermoelectric effects in superconductor-ferromagnet tunnel junctions on europium sulfide,"We report on large thermoelectric effects in superconductor-ferromagnet tunnel junctions in proximity contact with the ferromagnetic insulator europium sulfide. The combination of a spin-splitting field and spin-polarized tunnel conductance in these systems breaks the electron-hole symmetry and leads to spin-dependent thermoelectric currents. We show that the exchange splitting induced by the europium sulfide boosts the thermoelectric effect in small applied fields and can therefore eliminate the need to apply large magnetic fields, which might otherwise impede applications in thermometry or cooling.",1704.07241v1 2018-05-09,Crossover of skyrmion and helical modulations in noncentrosymmetric ferromagnets,"The coupling between angular (twisting) and longitudinal modulations arising near the ordering temperature of noncentrosymmetric ferromagnets strongly influences the structure of skyrmion states and their evolution in an applied magnetic field. In the precursor states of cubic helimagnets, a continuous transformation of skyrmion lattices into the saturated state is replaced by the first-order processes accompanied by the formation of multidomain states. Recently the effects imposed by dominant longitudinal modulations have been reported in bulk MnSi and FeGe. Similar phenomena can be observed in the precursor regions of cubic helimagnet epilayers and in easy-plane chiral ferromagnets (e.g. in the hexagonal helimagnet CrNb3S6).",1805.03360v1 2008-07-14,Numerical study of the frustrated ferromagnetic spin-1/2 chain,"The ground state phase diagram of the frustrated ferromagnetic spin-1/2 chain is investigated using the exact diagonalization technique. It is shown that there is a jump in the spontaneous magnetization and the ground state of the system undergos to a phase transition from a ferromagnetic phase to a phase with dimer ordering between next-nearest-neighbor spins. Near the quantum transition point, the critical behavior of the ground state energy is analyzed numerically. Using a practical finite-size scaling approach, the critical exponent of the ground state energy is computed. Our numerical results are in good agreement with the results obtained by other theoretical approaches.",0807.2094v2 2008-07-15,Magnetic properties of the insulating ferromagnetic phase in strained Pr_0.6Ca_0.4MnO_3 thin films,"Bulk magnetization in Pr_{0.6}Ca_{0.4}MnO_3 thin films with tensile (SrTiO_3) and compressive (LaAlO_3) substrate-induced strain is compared to the magnetooptical Kerr effect (MOKE) measurements. In the absence of an external magnetic field, in both films, a stable ferromagnetic insulating majority phase coexists with an antiferromagnetic insulating phase below ~120K. MOKE measurements indicate that at 5K a metastable ferromagnetic metallic (FM) phase is formed at the surface of the stretched film in a magnetic field below 1.1T already, while in the bulk the FM phase starts to form in the field above ~4T in both films.",0807.2312v1 2008-07-16,Nonsinusoidal current-phase relation in strongly ferromagnetic and moderately disordered SFS junctions,"We study the Josephson current in a junction comprising two superconductors linked by a strong ferromagnet in presence of impurities. We focus on a regime where the electron (and hole) motion is ballistic over the exchange length and diffusive on the scale of the weak link length. The current-phase relation is obtained for both two- and three dimensional ferromagnetic weak links. In the clean limit, the possibility of temperature-induced 0- transitions is demonstrated while the corresponding critical current versus temperature dependences are also studied.",0807.2560v1 2008-07-17,Quasi-reversible Magnetoresistance in Exchange Spring Tunnel Junctions,"We report a large, quasi-reversible tunnel magnetoresistance in exchange-biased ferromagnetic semiconductor tunnel junctions wherein a soft ferromagnetic semiconductor (\gma) is exchange coupled to a hard ferromagnetic metal (MnAs). Our observations are consistent with the formation of a region of inhomogeneous magnetization (an ""exchange spring"") within the biased \gma layer. The distinctive tunneling anisotropic magnetoresistance of \gma produces a pronounced sensitivity of the magnetoresistance to the state of the exchange spring.",0807.2885v1 2008-07-30,Mott Relation for Anomalous Hall and Nernst effects in Ga1-xMnxAs Ferromagnetic Semiconductors,"The Mott relation between the electrical and thermoelectric transport coefficients normally holds for phenomena involving scattering. However, the anomalous Hall effect (AHE) in ferromagnets may arise from intrinsic spin-orbit interaction. In this work, we have simultaneously measured AHE and the anomalous Nernst effect (ANE) in Ga1-xMnxAs ferromagnetic semiconductor films, and observed an exceptionally large ANE at zero magnetic field. We further show that AHE and ANE share a common origin and demonstrate the validity of the Mott relation for the anomalous transport phenomena.",0807.4942v2 2009-12-29,Temperature oscillations of magnetization observed in nanofluid ferromagnetic graphite,"We report on unusual magnetic properties observed in the nanofluid room-temperature ferromagnetic graphite (with an average particle size of l=10nm). More precisely, the measured magnetization exhibits a low-temperature anomaly (attributed to manifestation of finite size effects below the quantum temperature) as well as pronounced temperature oscillations above T=50K (attributed to manifestation of the hard-sphere type pair correlations between ferromagnetic particles in the nanofluid).",0912.5350v1 2011-11-23,Rashba spin torque in an ultrathin ferromagnetic metal layer,"In a two-dimensional ferromagnetic metal layer lacking inversion symmetry, the itinerant electrons mediate the interaction between the Rashba spin-orbit interaction and the ferromagnetic order parameter, leading to a Rashba spin torque exerted on the magnetization. Using Keldysh technique, in the presence of both magnetism and a spin-orbit coupling, we derive a spin diffusion equation that provides a coherent description to the diffusive spin dynamics. The characteristics of the spin torque and its implication on magnetization dynamics are discussed in the limits of large and weak spin-orbit coupling.",1111.5466v1 2012-01-04,Quantum Kinetic Theory of Current-Induced Torques in Rashba Ferromagnets,"Motivated by recent experimental studies of thin-film devices containing a single ferromagnetic layer, we develop a quantum kinetic theory of current-induced magnetic torques in Rashba-model ferromagnets. We find that current-induced spin-densities that are responsible for the switching behavior are due most essentially to spin-dependent quasiparticle lifetimes and derive analytic expressions for relevant limits of a simple model. Quantitative model parameter estimates suggest that spin-orbit coupling in the adjacent metal normal magnetic layer plays an essential role in the strength of the switching effect.",1201.0990v1 2012-01-31,Exchange-induced phase separation in Ni-Cu films,"Magneto-structural properties of films of diluted ferromagnetic alloys Ni$_x$Cu$_{1-x}$ in the concentration range $0.7 < x < 1.0$ are studied experimentally. Films deposited by magnetron sputtering show partial phase separation, as evidenced by structural analysis and ferromagnetic resonance measurements. The phase diagram of the Ni$_x$Cu$_{1-x}$ bulk system is obtained using numerical theoretical analysis of the electronic structure, taking into account the inter-atomic exchange interactions. The results confirm the experimentally found partial phase separation, explain it as magnetic in origin, and indicate an additional metastable region connected with the ferromagnetic transition in the system.",1201.6493v1 2012-04-09,Reversible switching of room temperature ferromagnetism in CeO2-Co nanoparticles,"We investigated the reversible ferromagnetic (FM) behavior of pure and Co doped CeO2 nanopowders. The as-sintered samples displayed an increasing paramagnetic contribution upon Co doping. Room temperature FM is obtained simply by performing thermal treatments in vacuum at temperatures as low as 500^{\circ}C and it can be switched off by performing thermal treatments in oxidizing conditions. The FM contribution is enhanced as we increase the time of the thermal treatment in vacuum. Those systematic experiments establish a direct relation between ferromagnetism and oxygen vacancies and open a path for developing materials with tailored properties.",1204.1959v1 2012-04-11,Magnetostatic spin waves and magnetic-wave chaos in ferromagnetic films. II. Numerical simulations of non-linear waves,"A method and some results of numeric simulations of magnetostatic spin waves in ferromagnetic films are exponded, in comparison with the theory earlier presented in arXiv preprint 1204.0200. In particular, roles of films finiteness (edges) and defects in formation of linear and non-linear magnetostatic wave patterns, excitation and evolution of two-dimensional solitons, and chaotic non-linear ferromagnetic resonance are considered.",1204.2423v1 2012-04-28,Nearly itinerant ferromagnetism in CaNi2 and CaNi3,"Single crystals of CaNi2 and CaNi3 were successfully grown out of excess Ca. Both compounds manifest a metallic ground state with enhanced, temperature dependent magnetic susceptibility. The relatively high Stoner factors of Z = 0.79 and Z = 0.87 found for CaNi2 and CaNi3, respectively, reveal their close vicinity to ferromagnetic instabilities. The pronounced field dependence of the magnetic susceptibility of CaNi3 at low temperatures (T < 25 K) suggests strong ferromagnetic fluctuations. A corresponding contribution to the specific heat with a temperature dependence of T^3lnT was also observed.",1204.6355v2 2014-05-30,Diffraction induced Spin Pumping in Normal-Metal/Multiferroic-Helimagnet/Ferromagnet Heterostructures,"Generally the adiabatic quantum pumping phenomenon can be interpreted by the surface integral of the Berry curvature inside the cyclic loop. Spin angular momentum flow without charge current can be pumped out by magnetization precession in ferromagnet-based structures. When an electron is scattered by a helimagnet, spin-dependent diffraction occurs due to the spatial modulation of the spiral. In this work, we consider the charge and spin flow driven by magnetization precession in normal-metal/multiferroic-helimagnet/ferromagnet heterostructures. The pumping behavior is governed by the diffracted states. Gauge dependence in the pumped current was encountered, which does not occur in the static transport properties or pumping behaviors in other systems.",1405.7750v1 2014-06-22,Disorder dependence of the ferromagnetic quantum phase transition,"We quantitatively discuss the influence of quenched disorder on the ferromagnetic quantum phase transition in metals, using a theory that describes the coupling of the magnetization to gapless fermionic excitations. In clean systems, the transition is first order below a tricritical temperature T_tc. Quenched disorder is predicted to suppress T_tc until it vanishes for residual resistivities rho_0 on the order of several microOhmcm for typical quantum ferromagnets. We discuss experiments that allow to distinguish the mechanism considered from other possible realizations of a first-order transition.",1406.5745v2 2015-12-02,Current driven domain wall creation in ferromagnetic nano-wires,"We predict the electrical generation and injection of domain walls into a ferromagnetic nano-wire without the need of an assisting magnetic field. Our theory shows that above a critical current $j_{c}$ domain walls are injected into the nano-wire with a period $T\sim (j-j_{c})^{-1/2}$. In a uniaxial anisotropy geometry this process does not require Dzyaloshinskii-Moria or dipole-dipole interaction and can be done in a simple exchange ferromagnet. We also show that this process and the period exponents are universal and do not depend on the peculiarities of the microscopic Hamiltonian.",1512.00784v2 2015-12-04,Itinerant ferromagnetism in 1D two-component Fermi gases,"We study a one-dimensional two-component atomic Fermi gas with an infinite intercomponent contact repulsion. It is found that adding an attractive resonant odd-wave interaction breaking the rotational symmetry one can make the ground state ferromagnetic. A promising system for the observation of this itinerant ferromagnetic state is a 1D gas of $^{40}$K atoms, where 3D $s$-wave and $p$-wave Feshbach resonances are very close to each other and the 1D confinement significantly reduces the inelastic decay.",1512.01392v1 2015-12-16,Magnetism in Co$_{1-{\rm x}}$Fe$_{\rm x}$Sb$_{3}$ skutterudites from density functional theory,"We have investigated the electronic and magnetic properties of Co$_{1-{\rm x}}$Fe$_{\rm x}$Sb$_{3}$ skutterudites from density functional theory and Monte Carlo simulations. We find that above a certain threshold in the Fe concentration, somewhere between x=0.125 and x=0.25, Co$_{1-{\rm x}}$Fe$_{\rm x}$Sb$_{3}$ is ferromagnetic with an atomic moment which increases asymptotically towards about 1 $\mu_{B}$/Fe and a non-zero Curie temperature which reaches 70 K for FeSb$_{3}$. Ferromagnetism is favored due to a Stoner instability in the electronic structure, where a large density of states at the Fermi-level makes it favorable to form the ferromagnetic ground state.",1512.05124v1 2016-08-12,Magnetotransport signatures of the proximity exchange and spin-orbit couplings in graphene,"Graphene on an insulating ferromagnetic substrate---ferromagnetic insulator or ferromagnetic metal with a tunnel barrier---is expected to exhibit giant proximity exchange and spin-orbit couplings. We use a realistic transport model of charge-disorder scattering and solve the linearized Boltzmann equation numerically exactly for the anisotropic Fermi contours of modified Dirac electrons to find magnetotransport signatures of these proximity effects: proximity anisotropic magnetoresistance, inverse spin-galvanic effect, and the planar Hall resistivity. We establish the corresponding anisotropies due to the exchange and spin-orbit couplings, with respect to the magnetization orientation. We also present parameter maps guiding towards optimal regimes for observing transport magnetoanisotropies in proximity graphene.",1608.03879v1 2017-07-16,Indirect excitation of self-oscillation in perpendicular ferromagnet by spin Hall effect,"A possibility to excite a stable self-oscillation in a perpendicularly magnetized ferromagnet by the spin Hall effect is investigated theoretically. It had been shown that such self-oscillation cannot be stabilized solely by the direct spin torque by the spin Hall effect. Here, we consider adding another ferromagnet, referred to as pinned layer, on the free layer. The pinned layer provides another spin torque through the reflection of the spin current. The study shows that the stable self-oscillation is excited by the additional spin torque when the magnetization in the pinned layer is tilted from the film plane.",1707.04825v1 2017-10-02,Electrical resistivity across the tricriticality in itinerant ferromagnet,"We investigate the discontinuous ferromagnetic phase diagram near tricritical point in UCo 1-x Ru x Al compounds by electrical resistivity measurements. Separation of phases in UCo 0.995 Ru 0.005 Al at ambient pressure and in UCo 0.990 Ru 0.010 Al at pressure of 0.2 GPa and disappearance of ferromagnetism at 0.4 GPa is confirmed. The exponent of temperature dependence of electrical resistivity implies change from Fermi liquid behavior to non-Fermi liquid at 0.2 GPa and reaches minimum at 0.4 GPa. Our results are compared to results obtained on the pure UCoAl and explanation for different exponents is given.",1710.00860v1 2018-01-22,Ferromagnetic domain wall as a nonreciprocal string,"We present a simple model of a domain wall in a thin-film ferromagnet. A domain wall is represented as a nonreciprocal string, on which transverse waves propagate with different speeds in opposite directions. The model has three parameters: mass density, tension, and a gyroscopic constant quantifying the nonreciprocity. We discuss the unusual dynamics of a nonreciprocal string in finite geometry. It agrees well with numerically simulated motion of a ferromagnetic domain wall in a strip of constant width.",1801.07166v4 2018-10-25,Hyperfine interaction in cobalt by high-resolution neutron spectroscopy,"We have investigated the ferromagnetic phase transition of elemental Co by high-resolution neutron backscattering spectroscopy. We monitored the splitting of the nuclear levels by the hyperfine field at the Co nucleus. The energy of this hyperfine splitting is identified as the order parameter of the ferromagnetic phase transition. By measuring the temperature dependence of the energy we determined the critical exponent $\beta = 0.350 \pm 0.002$ and the ferromagnetic Curie temperature of $T_{\text{C}} = 1400$~K. The present result of the critical exponent agrees better with the predicted value (0.367) of the 3-dimensional Heisenberg model than that determined previously by NMR.",1810.10906v1 2019-06-06,Current noise geometrically generated by a driven magnet,"We consider a non-equilibrium cross-response phenomenon, whereby a driven magnetization gives rise to electric shot noise (but no d.c. current). This effect is realized on a nano-scale, with a small metallic ferromagnet which is tunnel-coupled to two normal metal leads. The driving gives rise to a precessing magnetization. The geometrically generated noise is related to a non-equilibrium distribution in the ferromagnet. Our protocol provides a new channel for detecting and characterizing ferromagnetic resonance.",1906.02730v2 2019-06-23,Quantum Ferromagnetic Transition in Clean Dirac Metals,"The ferromagnetic quantum phase transition in clean metals with a negligible spin-orbit interaction is known to be first order due to a coupling of the magnetization to soft fermionic particle-hole excitations. A spin-orbit interaction gives these excitations a mass, suggesting the existence of a ferromagnetic quantum critical point in metals with a strong spin-orbit interaction. We show that this expectation is not borne out in a large class of materials with a Dirac spectrum, since the chirality degree of freedom leads to new soft modes that again render the transition first order.",1906.09605v1 2019-07-06,Angle-resolved broadband ferromagnetic resonance apparatus enabled through a spring-loaded sample mounting manipulator,"Broadband ferromagnetic resonance is a useful technique to determine the magnetic anisotropy and study the magnetization dynamics of magnetic thin films. We report a spring-loaded sample loading manipulator for reliable sample mounting and rotation. The manipulator enables maximum signal, enhances system stability and is particularly useful for fully automated in-plane-field angle-resolved measurements. This angle-resolved broadband ferromagnetic resonance apparatus provides a viable method to study anisotropic damping and weak magnetic anisotropies, both vital for fundamental research and applications.",1907.03097v1 2019-07-19,Nonlocal thermoelectric effects in high-field superconductor-ferromagnet hybrid structures,"We report on the experimental observation of nonlocal spin-dependent thermoelectric effects in superconductor-ferromagnet multiterminal structures. Our samples consist of a thin superconducting aluminum wire with several ferromagnetic tunnel junctions attached to it. When a thermal excitation is applied to one of the junctions in the presence of a Zeeman splitting of the density of states of the superconductor, a thermoelectric current is observed in remote junctions at distances exceeding $10~\mathrm{\mu m}$. The results can be explained by recent theories of coupled spin and heat transport in high-field superconductors.",1907.08441v1 2019-09-06,Families of magnetic semiconductors -- an overview,"The interplay of magnetic and semiconducting properties has been in the focus since more than a half of the century. In this introductory article we briefly review the key properties and functionalities of various magnetic semiconductor families, including europium chalcogenides, chromium spinels, dilute magnetic semiconductors, dilute ferromagnetic semiconductors and insulators, mentioning also sources of non-uniformities in the magnetization distribution, accounting for an apparent high Curie temperature ferromagnetism in many systems. Our survey is carried out from today's perspective of ferromagnetic and antiferromagnetic spintronics as well as of the emerging fields of magnetic topological materials and atomically thin 2D layers.",1909.02999v1 2019-09-11,Quantum statistics of vortices from a dual theory of the XY ferromagnet,"We extend the well-known mapping between the easy-plane ferromagnet and electrostatics in $d=2$ spatial dimensions to dynamical and quantum phenomena in a $d=2+1$ spacetime. Ferromagnetic vortices behave like quantum particles with an electric charge equal to the vortex number and a magnetic flux equal to the transverse spin of the vortex core. Vortices with half-integer core spin exhibit fermionic statistics.",1909.05248v3 2019-10-17,Spin-dependent electronic lenses based on hybrid graphene nanostructures,"We study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localised at different spots depending on their spin. We found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarised electric current can be generated by placing a third contact in the proper place. This opens the possibility to use these effects for fabricating tunable sources of polarized electrons.",1910.07828v1 2019-10-23,Mechanism of ferromagnetic ordering of the Mn chains in CaMnGe$_2$O$_6$ clinopyroxene,"The electronic and magnetic properties of clinopyroxene CaMnGe$_2$O$_6$ were studied using density function calculations within the GGA+U approximation. It is shown that anomalous ferromagnetic ordering of neighboring chains is due to a ""common-enemy"" mechanism. Two antiferromagnetic exchange couplings between nearest neighbours within the Mn-Mn chain and interchain coupling via two GeO$_4$ tetrahedra suppress antiferromagnetic exchange via single GeO$_4$ tetrahedron and stabilize ferromagnetic ordering of Mn chains.",1910.10530v1 2019-10-23,Effect of Demagnetization Method on Remnance Magnetization States in Metallic Ferromagnets,"Parametric plots of the remagnetization versus demagnetization remnances were found for four metallic ferromagnets -- nickel wire, two types of AlNiCo, and samarium cobalt 2:17. These plots, known as Henkel plots, were compared to Wohlfarth's model for noninteracting magnetic particles and several Preisach models. The remagnetization data were taken with a variety of paths to the net zero magnetization state. The resulting Henkel plots exhibit similarities to independent Monte Carlo simulations. The differences can be mostly explained by considering that the magnetization in the metallic ferromagnets occur by domain wall motion.",1910.10648v1 2020-01-10,Long-Range Phonon Spin Transport in Ferromagnet-Nonmagnetic Insulator Heterostructures,"We investigate phonon spin transport in an insulating ferromagnet-nonmagnet-ferromagnet heterostructure. We show that the magnetoelastic interaction between the spins and the phonons leads to nonlocal spin transfer between the magnets. This transfer is mediated by a local phonon spin current and accompanied by a phonon spin accumulation. The spin conductance depends nontrivially on the system size, and decays over millimeter length scales for realistic material parameters, far exceeding the decay lengths of magnonic spin currents.",2001.03462v2 2020-04-16,An extension of the cell-construction method for the flat-band ferromagnetism,"We present an extension of the cell-construction method for the flat-band ferromagnetism. In a rather general setting, we construct Hubbard models with highly degenerate single-electron ground states and obtain a formal representation of these single-electron ground states. By our version of the cell-construction method, various types of flat-band Hubbard models, including the one on line graphs, can be designed and shown to have the unique ferromagnetic ground states when the electron number is equal to the degeneracy of the single-electron ground states.",2004.07516v2 2020-12-02,Piezomagnetism in ferromagnetic superconductors,"The appearance of magnetization during the application of mechanical stress and the creation of elastic deformation during the application of a magnetic field are two fundamental properties of piezomagnetic materials. The symmetry of superconducting ferromagnets UGe_2, URhGe, UCoGe allows piezomagnetism. In addition to conventional piezomagnetic properties occurring in both normal and superconducting states, superconducting state of these piezomagnets has its own specificity. Unlike conventional superconductors, in uranium ferromagnets, the critical transition temperature to the superconducting state changes its value when the direction of the field changes to the opposite.",2012.01207v2 2000-06-09,Strongly reduced gap in the zigzag spin chain with a ferromagnetic interchain coupling,"We study a spin 1/2 Heisenberg zigzag spin chain model near decoupled two chains. Taking into account a symmetry breaking perturbation, we discuss the existence of an energy gap in the ferromagnetic interchain coupling as well as the antiferromagnetic one. In the ferromagnetic model, a marginally relevant fixed line reduces the gap strongly, so that the correlation length becomes an astronomical length scale even in order 1 coupling. This result agrees with density matrix renormalization group results.",0006155v3 2000-06-10,Spin-transport in multi-terminal normal metal - ferromagnet systems with non-collinear magnetizations,"A theory of spin-transport in hybrid normal metal - ferromagnetic electronic circuits is developed, taking into account non-collinear spin-accumulation. Spin-transport through resistive elements is described by 4 conductance parameters. Microscopic expression for these conductances are derived in terms of scattering matrices and explicitly calculated for simple models. The circuit theory is applied to 2-terminal and 3-terminal devices attached to ferromagnetic reservoirs.",0006174v2 2017-05-04,Reduced models for ferromagnetic thin films with periodic surface roughness,"We investigate the influence of periodic surface roughness in thin ferromagnetic films on shape anisotropy and magnetization behavior inside the ferromagnet. Starting from the full micromagnetic energy and using methods of homogenization and $\Gamma$-convergence we derive a two dimensional local reduced model. Investigation of this model provides an insight on the formation mechanism of {\it perpendicular magnetic anisotropy} and uniaxial anisotropy with an {\it arbitrary} preferred direction of magnetization.",1705.01742v1 2017-09-08,Room temperature ferromagnetism in transparent and conducting Mn-doped $SnO_{2}$ thin films,"The magnetization as a function of magnetic field showed hysteretic behavior at room temperature. According to the temperature dependence of the magnetization, the Curie temperature $(T_{C})$ is higher than 350 K. Ferromagnetic Mn-doped tin oxide thin films exhibited low electrical resistivity and high optical transmittance in the visible region (400-800 nm). The coexistence of ferromagnetism, high visible transparency and high electrical conductivity in the Mn-doped $SnO_{2}$ films is expected to be a desirable trait for spintronics devices.",1709.05930v1 2017-11-07,Dzyaloshinskii-Moriya interaction induced extrinsic linewidth broadening of ferromagnetic resonance,"For a thin ferromagnetic film with the Dzyaloshinskii-Moriya interaction (DMI), we derive an expression of the extrinsic ferromagnetic resonance (FMR) linewidth in a quantum mechanical way, taking into account scatterings from structural inhomogeneity. In the presence of the DMI, the magnon dispersion exhibits rich resonant states, especially in small external magnetic fields and strong DMI strength. It is found that the FMR linewidth shows several characteristic features such as a finite linewidth at zero frequency and peaks in the low frequency range.",1711.02263v1 2018-03-13,Ultrafast light switching of ferromagnetism in EuSe,"We demonstrate that light resonant with the bandgap forces the antiferromagnetic semiconducor EuSe to enter ferromagnetic alignment in the picosecond time scale. A photon generates an electron-hole pair, whose electron forms a supergiant spin polaron of magnetic moment of nearly 6,000 Bohr magnetons. By increasing the light intensity, the whole of the sample can be fully magnetized. The key to the novel large photoinduced magnetization mechanism is the huge enhancement of the magnetic susceptibility when both antiferromagnetic and ferromagnetic interactions are present in the material, and are of nearly equal magnitude, as is the case in EuSe.",1803.05038v1 2018-03-28,Quasi-one-dimensional Bose-Einstein Condensation in the Spin-1/2 Ferromagnetic-leg Ladder 3-I-V,"Quantum criticality of the spin-1/2 ferromagnetic-leg ladder 3-I-V [=3-(3-iodophenyl)-1,5-diphenylverdazyl] has been examined with respect to the antiferromagnetic to paramagnetic phase transition near the saturation field $H_{c}$. The phase boundary $T_{c}(H)$ follows the power-law $T_{c}(H)\propto H_{c}-H$ for a wide temperature range. This characteristic behavior is discussed as a quasi-one-dimensional (quasi-1D) Bose-Einstein condensation, which is predicted theoretically for weakly coupled quasi-1D ferromagnets. Thus, 3-I-V provides the first promising candidate for this attractive prediction.",1803.10388v1 2018-08-13,Mechanism for transitions between ferromagnetic and antiferromagnetic orders in $d$-electron metallic magnets,"We propose mechanism for pressure-induced transitions between ferromagnetic and antiferromagnetic phases that relies on a competition between characteristic energy scales ubiquitous among $d$-electron metallic magnetic compounds. Principles behind the mechanism are demonstrated on the example of the minimal two-orbital $p$-$d$ lattice model. We suggest that LaCrGe$_3$, where pressure-induced ferromagnetic-to-antiferromagnetic phase transition has been recently observed, is a promising candidate to realize discussed mechanism.",1808.04109v3 2019-08-07,Robust magnetotransport in disordered ferromagnetic kagome layers with quantum anomalous Hall effect,"The magnetotransport properties of disordered ferromagnetic kagome layers are investigated numerically. We show that a large domain-wall magnetoresistance or negative magnetoresistance can be realized in kagome layered materials (e.g. Fe$_3$Sn$_2$, Co$_3$Sn$_2$S$_2$, and Mn$_3$Sn), which show the quantum anomalous Hall effect. The kagome layers show a strong magnetic anisotropy and a large magnetoresistance depending on their magnetic texture. These domain-wall magnetoresistances are expected to be robust against disorder and observed irrespective of the domain-wall thickness, in contrast to conventional domain-wall magnetoresistance in ferromagnetic metals.",1908.02727v2 2019-11-06,Ferromagnetic Quantum Critical Point in Non-Centrosymmetric Systems,"Ferromagnetic quantum criticality in clean metals has proven elusive due to fermionic soft modes that drive the transition first order. We show that non-centrosymmetric metals with a strong spin-orbit interaction provide a promising class of materials for realizing a ferromagnetic quantum critical point in clean systems. The spin-orbit interaction renders massive the soft modes that interfere with quantum criticality in most materials, while the absence of spatial inversion symmetry precludes the existence of new classes of soft modes that could have the same effect.",1911.02649v2 2019-12-03,Ferromagnetic phase in graphene-based planar heterostructures induced by charged impurity,"The spontaneous self-consistent generation of axial current and corresponding dipole-like magnetic field in a planar electron-positron system similar to graphene and related hetero-structures doped by charged impurity with charge $Z$ is explored. It is shown that this effect takes place for $Z \geq Z^{\ast}$ with $Z^{\ast}$ being a peculiar analogue of the Curie point in ferromagnetics. The properties of induced ferromagnetic state are studied in detail. It is shown also that the arising this way magnetic dipole leads to a significant decrease of the total Casimir (vacuum) energy of the system, which in turn provides its spontaneous generation above the ""Curie point"" $Z \geq Z^{\ast}$.",1912.01696v1 2019-12-10,Ferromagnetically ordered metal in the single-band Hubbard model,"We study a ferromagnetic instability in a single-band Hubbard model on the hypercubic lattice away from half filling. Using dynamical mean-field theory with the continuous-time quantum Monte Carlo simulations based on the segment algorithm, we calculate the magnetic susceptibility in the weak and strong coupling regions systematically. We then find how ferromagnetic fluctuations are enhanced when the interaction strength and density of holes are varied. The efficiency of the double flip updates in the Monte Carlo simulations is also addressed.",1912.05053v1 2019-12-11,Magnetic polaron and antiferro-ferromagnetic transition in doped bilayer CrI$_3$,"Gate-induced magnetic switching in bilayer CrI$_3$ has opened new ways for the design of novel low-power magnetic memories based on van der Waals heterostructures. The proposed switching mechanism seems to be fully dominated by electrostatic doping. Here we explain, by first-principle calculations, the ferromagnetic transition in doped bilayer CrI$_3$. For the case of a very small electron doping, our calculations predict the formation of magnetic polarons (""ferrons"", ""fluctuons"") where the electron is self-locked in a ferromagnetic droplet in an antiferromagnetic insulating matrix. The self-trapping of holes is impossible, at least, within our approximation.",1912.05280v1 2020-03-19,Symplectic ferromagnetism and phase transitions in multi-component fermionic systems,"In this paper, we study the itinerant ferromagnetic phase in multi-component fermionic systems with symplectic (Sp(4), or isomorphically SO(5)) symmetry. Two different microscopic models have been considered and an effective field theory has been proposed to study the critical behavior of the nonmagnetism-magnetism phase transition. It has been shown that such systems exhibit intriguing ferromagnetism and critical behavior that different from those in spin-$\frac 12$ fermionic systems, or in high-spin systems with SU(N) symmetry. An extension of our results to higher spin systems with Sp(2N) symmetry has also been discussed.",2003.08566v1 2020-08-27,Nutation Resonance in Ferromagnets,"The inertial dynamics of magnetization in a ferromagnet is investigated theoretically. The analytically derived dynamic response upon microwave excitation shows two peaks: ferromagnetic and nutation resonances. The exact analytical expressions of frequency and linewidth of the magnetic nutation resonance are deduced from the frequency dependent susceptibility determined by the inertial Landau-Lifshitz-Gilbert equation. The study shows that the dependence of nutation linewidth on the Gilbert precession damping has a minimum, which becomes more expressive with increase of the applied magnetic field.",2008.12221v3 2020-09-02,Skyrmion-antiskyrmion droplets in a chiral ferromagnet,"We find numerically skyrmionic textures with skyrmion number Q=0 in ferromagnets with the Dzyaloshinskii-Moriya interaction and perpendicular anisotropy. These have the form of a skyrmion-antiskyrmion pair and may be called chiral droplets. They are stable in an infinite film as well as in disc-shaped magnetic elements. Droplets are found for values of the parameters close to the transition from the ferromagnetic to the spiral phase. We study their motion under spin-transfer torque. They move in the direction of the spin flow and, thus, their dynamics are drastically different than the Hall dynamics of the standard Q=0 skyrmion.",2009.00890v1 2020-10-05,Localized excitations and scattering of spin waves in ferromagnetic chains containing magnetic nanoclusters,"The spectrum of localized excitations in an anisotropic one-dimensional ferromagnet containing a spin cluster of arbitrary size is found exactly within the framework of the discrete Takeno-Homma model. The boundaries of stability of spin nanoclusters are determined, depending on their size, and the exchange and the anisotropy parameter of the ferromagnet. The problem of the spin waves scattering against nanoclusters is solved and explicit analytical expressions for their reflection and transmission coefficients are obtained. A model of a metamaterial consisting of weakly interacting magnetic molecular nanoclusters and possessing revealed dynamic properties is proposed.",2010.01861v1 2020-11-26,Multicomponent Nonlinear Evolution Equations of the Heisenberg Ferromagnet Type. Local versus Nonlocal Reductions,"The paper is dedicated to a system of matrix nonlinear evolution equations related to a Hermitian symmetric space of the type $\mathbf{A.III}$. The system under consideration extends the $1+1$ dimensional Heisenberg ferromagnet equation in the sense that its Lax pair has a form rather similar to the pair of the original Heisenberg ferromagnet model. We shall present here certain local and nonlocal reductions. A local integrable deformation and some of its reductions will be discussed too.",2011.13437v1 2021-03-10,Anisotropic superconducting spin transport at magnetic interfaces,"We present a theoretical investigation of anisotropic superconducting spin transport at a magnetic interface between a p-wave superconductor and a ferromagnetic insulator. Our formulation describes the ferromagnetic resonance modulations due to spin current generation depending on spin-triplet Cooper pair, including the frequency shift and enhanced Gilbert damping, in a unified manner. We find that the Cooper pair symmetry is detectable from the qualitative behavior of the ferromagnetic resonance modulation. Our theory paves the way toward anisotropic superconducting spintronics.",2103.05871v3 2021-03-15,Ferromagnetic Resonance in Permalloy Metasurfaces,"Permalloy films with one-dimensional (1D) profile modulation of submicron periodicity are fabricated based on commercially available DVD-R discs and studied using ferromagnetic resonance (FMR) method and micromagnetic numerical simulations. The main resonance position shows in-plane angular dependence which is strongly reminiscent of that in ferromagnetic films with uniaxial magnetic anisotropy. The main signal and additional low field lines are attributed to multiple standing spin wave resonances defined by the grating period. The results may present interest in magnetic metamaterials and magnonics applications.",2103.08704v1 2021-03-29,Hydrodynamic model of skyrmions and vorticities in the spin-1 Bose-Einstein condensate in ferromagnetic phase at finite temperatures,"Quantum hydrodynamic of skyrmions in spinor ultracold bosons at the finite temperature is described. The limit regime of BEC appearing at the zero temperature is discussed either. The skyrmions are found as the solution of the spin evolution equation for the ferromagnetic phase of spin-1 ultracold bosons. It is demonstrated that the quantum part of the spin current gives major contribution in the formation of the skyrmions. The interplay between the quantum (spin) vorticity and the classic vorticity is considered for the finite temperatures. Bosons are presented as two fluids associated with the Bose-Einstein condensate and the normal fluid. All effects are considered for the ferromagnetic phase.",2103.15930v1 2021-03-31,Spin charge conversion in Rashba split ferromagnetic interfaces,"We show here theoretically and experimentally that a Rashba-split electron state inside a ferromagnet can efficiently convert a dynamical spin accumulation into an electrical voltage. The effect is understood to stem from the Rashba splitting but with a symmetry linked to the magnetization direction. It is experimentally measured by spin pumping in a CoFeB/MgO structure where it is found to be as efficient as the inverse spin Hall effect at play when Pt replaces MgO, with the extra advantage of not affecting the damping in the ferromagnet.",2103.16867v1 2021-04-08,Ferromagnetism in a Semiconductor with Mobile Carriers via Low-Level Nonmagnetic Doping,"We show that doped cubic iron pyrite, which is a diamagnetic semiconductor, becomes ferromagnetic when $p$-type doped. We furthermore find that this material can exhibit high spin polarization both for tunneling and transport devices. These results are based on first principles electronic structure and transport calculations. This illustrates the use of $p$-type doping without magnetic impurities as a strategy for obtaining ferromagnetic semiconducting behavior, with implications for spintronic applications that require both magnetic ordering and good mobility. This is a combination that has been difficult to achieve by doping semiconductors with magnetic impurities. We show that phosphorus and arsenic may be effective dopants for achieving this behavior.",2104.03877v1 2021-07-07,Fluctuation theorem for spin transport at insulating ferromagnetic junctions,"General relations for nonequilibrium spin transport at a magnetic junction between a normal metal and a ferromagnetic insulator are derived from the quantum fluctuation theorem. They include the extended Onsager relations between the spin conductance and the spin-current noise that hold for nonequilibrium states driven by an external current. These relations, that are valid for a general setup of spin Hall magnetoresistance, provide a comprehensive viewpoint for understanding of unidirectional spin Hall magnetoresistance in insulating ferromagnetic junctions.",2107.03001v3 2021-09-15,Nanograin ferromagnets from non-magnetic bulk materials: the case of gold nanoclusters,"The ferromagnetism of Au nanograins is analysed based on a two-dimensional itinerant lattice model with on-site Coulomb repulsion, many-body spin-orbit interactions, and holding two hybridized bands, one correlated and one uncorrelated. Using periodic boundary conditions in both directions, an exact ferromagnetic ground state is deduced for this non-integrable system by applying special techniques based on positive semidefinite operators.",2109.07297v1 2021-10-15,A micromagnetic theory of skyrmion lifetime in ultrathin ferromagnetic films,"We use the continuum micromagnetic framework to derive the formulas for compact skyrmion lifetime due to thermal noise in ultrathin ferromagnetic films with relatively weak interfacial Dzyaloshinskii-Moriya interaction. In the absence of a saddle point connecting the skyrmion solution to the ferromagnetic state, we interpret the skyrmion collapse event as ``capture by an absorber'' at microscale. This yields an explicit Arrhenius collapse rate with both the barrier height and the prefactor as functions of all the material parameters, as well as the dynamical paths to collapse.",2110.08107v1 2021-12-29,Spin Wave Propagation through Antiferromagnet/Ferromagnet Interface,"We study the problem of controlling spin waves propagation through an antiferromagnet/ferromagnet interface via tuning material parameters. It is done by introducing the degree of sublattice noncompensation of antiferromagnet (DSNA), which is a physical characteristic of finite-thickness interfaces. The DSNA value can be varied by designing interfaces with a particular disorder or curvilinear geometry. We describe a spin-wave propagation through any designed antiferromagnet/ferromagnet interface considering a variable DSNA and appropriate boundary conditions. As a result, we calculate the physical transmittance and reflectance of the spin waves as a function of frequency and show how to control them via the exchange parameters tuning.",2112.14583v1 2021-12-31,Searching for new ferromagnetism precursors in two-dimensional model materials in frame of local force theorem,"In this work we conduct a numerical search of non-trivial mechanisms, leading to new tendencies towards long-range ferromagnetic ordering in two-dimensional materials. For this purpose we employ an original variant of pairwise infinitesimal spin rotations technique to establish the magnetic transition temperature as the rigid function of basic crystal's parameters. It favored the numerical optimization of this function using modified genetic algorithm, designed to harvest local extrema. It resulted in revealing the moderate metallicity, accompanied by essential orbital anisotropy, as the prime configuration, which provides the most favoring conditions to ferromagnetic ordering, related to double-exchange and superexchange mechanisms.",2112.15374v1 2022-01-17,Perpendicularly Polarized Spin Hall Effects Induced by Spin-Dependent Scattering in Ferromagnetic Metals,"Spin currents in ferromagnets afford diverse functionalities. We evaluate the extrinsic spin Hall effects of magnetic impurity scattering in ferromagnetic metals. We show that spin-dependent scattering can provide a high spin current polarized perpendicularly to the magnetization direction and is a dominant mechanism in the moderate-conductivity regime. We find that the superposition of the spin-conserve and spin-flip channels causes the spin currents. These findings suggest that optimizing alloy composition is an effective strategy to control the spin Hall effect.",2201.06222v2 2022-01-27,Temperature Dependence of Magnetocrystalline Anisotropy in Itinerant Ferromagnets,"We theoretically investigated magnetocrystalline anisotropy (MA) at a finite temperature $T$ in ferromagnetic metals. Assuming a Rashba-type ferromagnet with uniaxial MA, we defined the MA constants $K_\mathrm{u}(T)$ derived from several different concepts. Our purpose was to examine the equality between them and to confirm a power law between $K_\mathrm{u}(T)$ and magnetization $M(T)$ in the form of $K_\mathrm{u}(T)/K_\mathrm{u}(0)=[M(T)/M(0)]^\alpha$. We demonstrate that $\alpha$ equals 2 in the itinerant-electron limit and increases with the localized feature of electrons passing through $\alpha=3$, predicted for the single-ion MA in spin models.",2201.11269v1 2022-04-19,On multi-soliton solutions to the Heisenberg ferromagnetic spin chain equation in (2+1)-dimensions,"This paper concentrates on the Heisenberg ferromagnetic spin chain (HFSC) equation in (2+1)-dimensions modelling nonlinear wave propagation in ferromagnetic spin chain. A variable transformation is first employed to reduce the studied equation. And then an associated matrix Riemann-Hilbert problem is built on the real line through analyzing spectral problem of the reduced equation. As a consequence, solving the obtained matrix Riemann-Hilbert problem with the identity jump matrix, corresponding to the reflectionless, the general multi-soliton solutions to the HFSC equation in (2+1)-dimensions are acquired. Specially, the one- and two-soliton solutions are worked out and analyzed graphically.",2204.08679v1 2022-05-14,Magnetization Plateau of the Distorted Diamond Spin Chain with Anisotropic Ferromagnetic Interaction,"The $S=1/2$ distorted diamond spin chain with the anisotropic ferromagnetic interaction is investigated using the numerical diagonalization and the level spectroscopy analysis. It is known that the system exhibits a plateau of the magnetization curve at the 1/3 of the saturation. The present study indicates that as the anisotropy is varied the quantum phase transition occurs between two different mechanisms of the 1/3 magnetization plateau. The phase diagram with respect to the anisotropy and the ferromagnetic coupling is also presented.",2205.07027v1 2022-08-16,Magnetocaloric effect in multilayers studied by membrane-based calorimetry,"We study magnetic multilayers, incorporating dilute ferromagnetic spacers between strongly-ferromagnetic layers exhibiting a proximity-enhanced magnetocaloric effect. Using magnetometry and direct measurements of the adiabatic temperature change based on a nanomembrane-calorimetry, we find that the magnetocaloric effect in the studied multilayer is indeed enhanced compared to that in the bulk spacer material. We develop a phenomenological numerical model of the studied trilayer and find that a long-range exchange interaction through the weakly-ferromagnetic spacer is required to adequately describe the magnetic and magnetocaloric properties of the system.",2208.07761v1 2022-10-11,Intrinsic localized modes in a two-dimensional checkerboard ferromagnetic lattice,"An analytical work on intrinsic localized modes in a two-dimensional Heisenberg ferromagnet on the checkerboard lattice is presented. Taking advantage of an asymptotic method, the governing lattice dynamical equations are reduced to one (2+1) -dimensional nonlinear Schr\""odinger. In our work, we obtain two types of nonlinear localized mode solutions, namely, Brillouin zone center modes and Brillouin zone corner modes. The occurrence conditions for these intrinsic localized modes are given in detail. Especially, we find that the competition between the Dzialozinskii-Moriy interaction and the next-nearest neighbor interaction of the checkerboard ferromagnet has an effect on the local structure of the Brillouin zone corner acoustic mode.",2210.05585v1 2023-01-03,Amplitude representation of Landau-Lifshitz equation and its application to ferromagnetic films,"The purpose of this article is to develop a systematic approach to the solution of the Landau-Lifshitz equation in terms of the magnon wave function {\psi} (r) and apply it to physical phenomena in a thin ferromagnetic film. This article has also a purpose to represent the modern state of art for the properties of ferromagnetic films and the pumping-induced Bose-Einstein condensation of magnons in them at room temperature. Thus, it can be considered as a review on basic principles and the recent advances in the field.",2301.01391v1 2023-01-19,Interfacial Magnon-Mediated Superconductivity in Twisted Bilayer Graphene,"The interfacial coupling between electrons and magnons in adjacent layers can mediate an attractive electron-electron interaction and induce superconductivity. We consider magic-angle twisted bilayer graphene sandwiched between two ferromagnetic insulators to optimize this effect. As a result, magnons induce an interlayer superconducting state characterized by $p$-wave symmetry. We investigate two candidate ferromagnets. The van der Waals ferromagnet CrI$_3$ stands out because it allows compression to tune the superconducting state with an exponential sensitivity. This control adds a new dimension to the tunability of twisted bilayer graphene. Our results open a new path for exploring magnon-induced superconductivity.",2301.07909v1 2023-03-11,The electronic structure of intrinsic magnetic topological insulator MnBi2Te4 quantum wires,"The ferromagnetic and antiferromagnetic nanostructure are crucial for fundamental spintronics devices, motivated by its potential application in spintronics, we theoretically investigate the electronic structure of the ferromagnetic and antiferromagnetic phases of the cylindrical intrinsically magnetic topological insulator $\mathrm{MnBi_{2}Te_{4}}$ quantum wires for both cases. We demonstrate that a few surface states exist between the bulk band gap in the ferromagnetic phase, with only one spin branch. In the antiferromagnetic phase, we show that three coexistent states exist between the energy gaps of the quantum wires.",2303.06303v1 2023-03-31,Stable magnetic levitation of soft ferromagnets for macroscopic quantum mechanics,"We propose a system for passive magnetic levitation and three-dimensional harmonic trapping of soft ferromagnets. Our protocol utilizes the magnetic field gradient for vertical trapping, and the finite size effect of the Meissner effect for horizontal trapping. We provide numerical and analytical estimations of possible mechanical dissipations to show that our system allows high mechanical Q-factors above $ Q > 10^8 $, and quantum control of the levitated object is within reach of current technologies. The utilization of soft ferromagnet's internal collective spin excitation may allow quantum mechanical phenomena with particles as large as the sub-millimeter-scale.",2303.17847v2 2023-04-08,Integral Absorption of Microwave Power by Random-Anisotropy Magnets,"We study analytically and numerically on lattices containing $10^5$ spins, the integral absorption of microwaves by a random-anisotropy magnet, $\int d\omega P(\omega)$. It scales as $D^2_R/J$ on the random-anisotropy strength $D_R$ and the strength of the ferromagnetic exchange $J$ in low-anisotropy amorphous magnetic materials. At high anisotropy and in low-anisotropy materials sintered of sufficiently large ferromagnetic grains, the integral power scales linearly on $D_R$. The maximum bandwidth, combined with the maximum absorption power, is achieved when the amorphous structure factor, or grain size, is of an order of the domain wall thickness in a conventional ferromagnet that is of the order of $(J/D_R)^{1/2}$ lattice spacings.",2304.04121v1 2023-05-22,Stoner ferromagnetism in a momentum-confined interacting 2D electron gas,"In this work we investigate the ground state of a momentum-confined interacting 2D electron gas, a momentum-space analog of an infinite quantum well. The study is performed by combining analytical results with a numerical exact diagonalization procedure. We find a ferromagnetic ground state near a particular electron density and for a range of effective electron (or hole) masses. We argue that this observation may be relevant to the generalized Stoner ferromagnetism recently observed in multilayer graphene systems. The collective magnon excitations exhibit a linear dispersion, which originates from a diverging spin stiffness.",2305.13373v1 2023-05-21,An Alternative Derivation of the Landau-Lifshitz-Gilbert Equation for Saturated Ferromagnets,"The Landau-Lifshitz-Gilbert equation for rigid and saturated ferromagnets is derived using a two-continuum model constructed by H.F. Tiersten for elastic and saturated ferromagnets. The relevant basic laws of physics are applied systematically to the two continua or their combination. The exchange interaction is introduced into the model through surface distributed magnetic couples. This leads to a continuum theory with magnetization gradients in the stored energy density. The saturation condition of the magnetization functions as constraints on the energy density and has implications in the constitutive relations.",2305.18232v1 2023-07-31,Phase diagram of a ferromagnetic semiconductor. The origin of superparamagnetism,"We study the theoretical model of a ferromagnetic semiconductor as a system of randomly distributed Ising spins with a long-range exchange interaction. Using the density-of-states approach, we analytically obtain the magnetic susceptibility and heat capacity over a wide range of temperatures and magnetic fields. It is shown that the finite system of spins in magnetic field less than a certain critical field is in a superparamagnetic state due to thermodynamic fluctuations. The complex phase structure of a ferromagnetic semiconductor is discussed.",2307.16538v2 2023-09-29,Micromagnetics of ferromagnetic/antiferromagnetic nanocomposite materials. Part I: Towards the mesoscopic approach,"In the first of two articles, we present here a novel mesoscopic micromagnetic approach for simulating materials composed of ferromagnetic and antiferromagnetic phases. Starting with the atomistic modeling of quasi one-dimensional systems, we explicitly show how the material parameters for the mesoscopic model of an antiferromagnet can be derived. The comparison between magnetization profiles obtained in atomistic and mesoscopic calculations (using a Heusler alloy as an example) proves the validity of our method. This approach opens up the possibility to recover the details of the magnetization distribution in ferromagnetic/antiferromagnetic materials with the resolution of a few nanometers covering length scales up to several hundreds of nanometers.",2309.17131v1 2023-10-20,HTS Dynamo Flux Pump: The Impact of a Ferromagnetic Stator Substrate,"HTS dynamo magnetic flux pumps are perspective devices for contactless charging the superconductor magnets and coils. In this work, we investigate the influence of a ferromagnetic substrate of a coated conductor used as the pump stator. We use the thin shell model of a coated conductor with a ferromagnetic substrate and show that such a conductor increases the pump-generated voltage if the superconducting layer is between the rotor and the substrate. Chebyshev spectral method is employed for numerical solution. Using simulation results for problems with a given transport current we also derive a simple analytical description for feeding a current to a coil.",2310.13491v1 2024-03-12,Nodal d-wave pairing from spin fluctuations in a thermally disordered anti-ferromagnet,"We consider electron pairing in a two-dimensional thermally disordered itinerant anti-ferromagnet. It is shown that transverse spin fluctuations in such a state can give rise to superconductivity with a sizeable critical temperature $T_c$. Below $T_c$ there is quasi-long-range spin-singlet and $d_{x^2-y^2}$ superconducting order, together with fluctuating triplet order at momentum $(\pi,\pi)$. The singlet pairs we find are tightly bound together, and the pair wavefunction has a purely inter-sublattice structure due to the U(1) spin rotation symmetry of the anti-ferromagnet.",2403.07999v1 2024-03-21,Efecto Hall de espin inverso en peliculas de Nb Mo y Bi por bombeo de espin,"The inverse spin Hall effect used for detection of spin currents was observed by voltage measurements in bilayers of normal metal (NM)/ferromagnetic metal (FM), using Nb, Mo and Bi as normal metal and Permalloy (Py, Ni$_{81}$Fe$_{19}$) as ferromagnetic metal. The spin current was generated by the spin pumping effect with ferromagnetic resonance. The samples were deposited by dc magnetron sputtering at room temperature on Si (001) substrates. The three bilayers of Nb/Py, Mo/Py and Bi/Py had a spin-orbit coupling large enough to observe the voltage generation by spin Hall effect",2403.17976v1 2024-03-27,A Generic Nonlinear Evolution Equation of Magnetic Type II. Particular Solutions,"We consider a matrix nonlinear partial differential equation that generalizes Heisenberg ferromagnet equation. This generalized Heisenberg ferromagnet equation is completely integrable with a linear bundle Lax pair related to the pseudo-unitary algebra. This allows us to explicitly derive particular solutions by using dressing technique. We shall discuss two classes of solutions over constant background: soliton-like solutions and quasi-rational solutions. Both classes have their analogues in the case of the Heisenberg ferromagnet equation related to the same Lie algebra.",2403.18165v1 1997-09-03,The Ferromagnetic Kondo Model for Manganites: Phase Diagram and Charge Segregation Effects,"The phase diagram of the ferromagnetic Kondo model for manganites is investigated using computational techniques. In clusters of dimensions 1 and 2, Monte Carlo simulations in the limit where the localized spins are classical show a rich low temperature phase diagram with three dominant regions: (i) a ferromagnetic phase, (ii) phase separation between hole-poor antiferromagnetic and hole-rich ferromagnetic domains, and (iii) a phase with incommensurate spin correlations. Possible experimental consequences of the regime of phase separation are discussed. Studies using the Lanczos algorithm and the Density Matrix Renormalization Group method applied to chains with localized spin 1/2 (with and without Coulombic repulsion for the mobile electrons) and spin 3/2 degrees of freedom give results in excellent agreement with those in the spin localized classical limit. The Dynamical Mean Field ($D=\infty$) approximation was also applied to the same model. At large Hund coupling phase separation and ferromagnetism were identified, again in good agreement with results in low dimensions. In addition, a Monte Carlo study of spin correlations allowed us to estimate the critical temperature for ferromagnetism $T_c^{FM}$ in 3 dimensional clusters. It is concluded that $T_c^{FM}$ is compatible with current experimental results.",9709029v1 1998-09-21,Influence of Nearest-Neighbor Coulomb Interactions on the Phase Diagram of the Ferromagnetic Kondo Model,"The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the Ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized S=1/2 spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at V=0, the Coulomb interaction breaks the large domains of the two competing phases into small ``islands'' of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure-factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density $n=0.5$, the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of ``charge- ordered ferromagnetism'' may be related with experimental observations of a similar phase by C. H. Chen and S-W. Cheong (Phys. Rev. Lett. 76, 4042 (1996)). Our results reinforce the recently introduction notion (see e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)) that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation/charge-ordering tendencies.",9809281v1 2001-08-09,Extraordinary Hall effect in hybrid ferromagnetic/superconductor (F/S) bilayer,"Extraordinary Hall effect (EHE) in bilayer F/S(N) was investigated theoretically. The conductivity tensor $\sigma_{\alpha\beta}$ is calculated in the Kubo formalism with Green functions found as the solutions of the Gorkov equations. We considered diffuse transport in the ferromagnetic layer, taking into account as a main mechanism of electron resistivity s-d scattering. In this model Gorkov equations for s-electrons in the ferromagnetic layer remain linear and are solved easily. It is shown that Hall field $E^H$ for both F/S and F/N contacts are step-functions of the coordinate perpendicular to the planes of the layers and have zero value in S(N) layer. The Andreev reflection increases the value of Hall constant $R_s$ for F/S case. The value of the Hall constant is $R_H^{F/S} = R_H^{bulk} (\sigma^{\uparrow} + \sigma^{\downarrow})^2 / 4 \sigma^{\uparrow}\sigma^{\downarrow}$, where $\sigma^{\uparrow}$ and $\sigma^{\downarrow}$ are conductivities of electrons with up and down spins, and $R_H^{bulk}$ is the Hall constant in the bulk ferromagnetic metal. In fact, $R_H^{F/S}$ coincides with EHE constant of the bilayer of two ferromagnetic metals with equal thickness and opposite directions of their magnetizations. So we can make a conclusion, that the ideal interface between ferromagnetic metal and superconductor may be considered like a mirror with inversion in spin space.",0108161v2 2003-06-24,First-order transition in the itinerant ferromagnet CoS$_{1.9}$Se$_{0.1}$,"Undoped CoS$_2$ is an isotropic itinerant ferromagnet with a continuous or nearly continuous phase transition at $T_{\rm C} = 122$ K. In the doped CoS$_{1.9}$Se$_{0.1}$ system, the Curie temperature is lowered to $T_{\rm C} = 90$ K, and the transition becomes clearly first order in nature. In particular we find a discontinuous evolution of the spin dynamics as well as strong time relaxation in the ferromagnetic Bragg intensity and small angle neutron scattering in vicinity of the ferromagnetic transition. In the ordered state the long-wavelength spin excitations were found to be conventional ferromagnetic spin-waves with negligible spin-wave gap ($ < 0.04$ meV), indicating that this system is also an excellent isotropic (soft) ferromagnet. In a wide temperature range up to $0.9T_{\rm C}$, the spin-wave stiffness $D(T)$ follows the prediction of the two-magnon interaction theory, $D(T) = D(0)(1 - AT^{5/2})$, with $D(0) = 131.7 \pm 2.8$ meV-\AA$^{2}$. The stiffness, however, does not collapse as $T \to T_{\rm C}$ from below. Instead a quasielastic central peak abruptly develops in the excitation spectrum, quite similar to results found in the colossal magnetoresistance oxides such as (La-Ca)MnO$_3$.",0306618v1 2004-07-20,Synthesis of Colloidal Mn2+:ZnO Quantum Dots and High-TC Ferromagnetic Nanocrystalline Thin Films,"We report the synthesis of colloidal Mn2+-doped ZnO (Mn2+:ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn2+:ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)2 was found to strongly inhibit oxidation of Mn2+ by O2, allowing the synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn2+:ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn2+ with very homogeneous speciation, differing from bulk Mn2+:ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 Bohr magneton/Mn2+ and TC > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn2+:ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.",0407500v1 2005-09-21,Effect of iron-doping on spin-state transition and ferromagnetism in Pr$_{0.5}$Ca$_{0.5}$CoO$_{3-δ}$ cobalt oxides,"Resistivity and dc magnetization measurements were performed for the polycrystalline Pr$_{0.5}$Ca$_{0.5}$Co$_{1-x}$Fe$_{x}$O$_{3-\delta}$ ($x$ = 0, 0.05, 0.10 and 0.15) samples. The as-fabricated samples exhibit ferromagnetic (FM) transition and the transition temperature increases with increasing the iron doping level. Annealing under high oxygen pressure induces a spin-state transition of Co ions in the \emph{iron-free} sample and such transition is reinforced with increasing the annealing oxygen pressure, while the annealing under high oxygen pressure suppresses the ferromagnetic ordering. Contrary to the case of the \emph{iron-free} sample, no spin-state transition is induced by the annealing under high oxygen pressure for the \emph{iron-doped} samples, and the ferromagnetic transition temperature is nearly independent of the annealing procedures. The enhancement of the spin-state transition in the \emph{iron-free} sample after annealing under high oxygen pressure should be attributed to the reduction of the cell volume. The suppression of the spin-state transition by the Fe doping is related to the enlargement of the cell volume and the stronger Fe-O bonds than Co-O bonds. The enhancement of the ferromagnetism by the iron-doping might arise from the ferromagnetic exchange interaction between Fe$^{3+}$ and Co$^{4+}$ through oxygen (Fe$^{3+}$-O-Co$^{4+}$).",0509533v1 2006-06-15,Ferromagnetic -spin glass transition induced by pressure in Gd$_2$Mo$_2$O$_7$,"R$_2$Mo$_2$O$_7$ compounds show a ferromagnetic metal-insulator spin glass transition tuned by the radius of the rare earth ion R$^{3+}$. We have studied Gd$_2$Mo$_2$O$_7$ located on the verge of the transition, by neutron diffraction on a $^{160}$Gd isotopic sample, $\mu$SR and X ray diffraction using the synchrotron radiation. All measurements were done both at ambient and under applied pressure. At ambient pressure, a ferromagnetic state is observed below the Curie temperature (T$_{\rm C}$ = 70 K). The ordered magnetic moments at 1.7 K are parallel and equal to 5.7(5) $\mu_{\rm B}$ and 0.8(2) $\mu_{\rm B}$ for Gd and Mo, respectively. The relaxation rate measured by $\mu$SR evidences strong spin fluctuations below T$_{\rm C}$ and down to the lowest temperature (6.6 K). A spin reorientation occurs in the range 20 K$<$T$<$T$_{\rm C}$. The ferromagnetic state is strongly unstable under pressure. T$_{\rm C}$ sharply decreases (down to 38 K at 1.3 GPa) and Bragg peaks start to coexist with mesoscopic ferromagnetic correlations. The ordered moments decrease under pressure. At 2.7 GPa long range magnetic order completely breaks down. In this spin glass state, Gd-Gd spin correlations remain ferromagnetic with a correlation length limited to the fourth neighbor, and Gd-Mo spin correlations turn to antiferromagnetic. The unique combination of three microscopic probes under pressure provides a detailed description of the magnetic transition, crucial for further theories.",0606420v1 2006-07-28,Atom-by-Atom Substitution of Mn in GaAs and Visualization of their Hole-Mediated Interactions,"The discovery of ferromagnetism in Mn doped GaAs [1] has ignited interest in the development of semiconductor technologies based on electron spin and has led to several proof-of-concept spintronic devices [2-4]. A major hurdle for realistic applications of (Ga,Mn)As, or other dilute magnetic semiconductors, remains their below room-temperature ferromagnetic transition temperature. Enhancing ferromagnetism in semiconductors requires understanding the mechanisms for interaction between magnetic dopants, such as Mn, and identifying the circumstances in which ferromagnetic interactions are maximized [5]. Here we report the use of a novel atom-by-atom substitution technique with the scanning tunnelling microscope (STM) to perform the first controlled atomic scale study of the interactions between isolated Mn acceptors mediated by the electronic states of GaAs. High-resolution STM measurements are used to visualize the GaAs electronic states that participate in the Mn-Mn interaction and to quantify the interaction strengths as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga1-xMnxAs structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples. Our experimental methods also provide a realistic approach to create precise arrangements of single spins as coupled quantum bits for memory or information processing purposes.",0607765v1 2006-11-04,Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites,"What happens to ferromagnetism at the surfaces and interfaces of manganites? With the competition between charge, spin, and orbital degrees of freedom, it is not surprising that the surface behavior may be profoundly different than that of the bulk. Using a powerful combination of two surface probes, tunneling and polarized x-ray interactions, this paper reviews our work on the nature of the electronic and magnetic states at manganite surfaces and interfaces. The general observation is that ferromagnetism is not the lowest energy state at the surface or interface, which results in a suppression or even loss of ferromagnetic order at the surface. Two cases will be discussed ranging from the surface of the quasi-2D bilayer manganite (La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$) to the 3D Perovskite (La$_{2/3}$Sr$_{1/3}$MnO$_3$)/SrTiO$_3$ interface. For the bilayer manganite, that is, ferromagnetic and conducting in the bulk, these probes present clear evidence for an intrinsic insulating non-ferromagnetic surface layer atop adjacent subsurface layers that display the full bulk magnetization. This abrupt intrinsic magnetic interface is attributed to the weak inter-bilayer coupling native to these quasi-two-dimensional materials. This is in marked contrast to the non-layered manganite system (La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrTiO$_3$), whose magnetization near the interface is less than half the bulk value at low temperatures and decreases with increasing temperature at a faster rate than the bulk.",0611112v1 2007-02-15,Theory of Nonequilibrium Spin Transport and Spin Transfer Torque in Superconducting-Ferromagnetic Nanostructures,"Spin transport currents and the spin-transfer torques in voltage-biased superconducting-ferromagnetic nanopillars (SFNFS point contacts) are computed. We develop and implement an algorithm based on the Ricatti formulation of the quasiclassical theory of superconductivity to solve the time-dependent boundary conditions for the nonequilibrium Green's functions for spin transport through the ferromagnetic interfaces. A signature of the nonequilibrium torque is a component perpendicular to the plane spanned by the two ferromagnetic moments. The perpendicular component is absent in normal-metal-ferromagnetic nanopillars (NFNFN) contacts, but is shown to have the same order of magnitude as the in-plane torque for non-equilibrium SFNFS contacts. The out-of-plane torque is due to the rotation of quasiparticle spin by the exchange fields of the ferromagnetic layers. In the ballistic limit the equilibrium torque is related to the spectrum of spin-polarized Andreev bound states, while the {\sl ac} component, for small bias voltages, is determined by the nearly adiabatic dynamics of the Andreev bound states. The nonlinear voltage dependence of the non-equilibrium torque, including the subharmonic gap structure and the high-voltage asymptotics, is attributed to the interplay between multiple Andreev reflections, spin filtering and spin mixing. These properties of spin angular momentum transport may be exploited to control the state of nanomagnets.",0702371v3 2007-05-15,Nonhomogeneous magnetization and superconductivity in superconductor-ferromagnet structures,"We study two different superconductor-ferromagnet (S/F) structures. We consider first a Josephson junction which consists of two S/F bilayers separated by an insulating layer. We show that for an antiparallel alignment of the magnetization in the two F layers the Josephson critical current $I_c$ increases with increasing exchange field $h$. The second system we consider is a S/F structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. Due to the proximity effect not only a singlet but also a triplet component of the superconducting condensate is induced in the ferromagnet. The latter penetrates over the length $\sqrt{D/\epsilon}$ ($D$ is the diffusion coefficient and $\epsilon$ the energy). In the case of temperatures of the order of the Thouless energy this length is comparable to the length of the ferromagnet. This long-range penetration leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature $T_c$. Contrary to the case of the singlet component, the contribution to the conductance due to the odd triplet component is not zero at $T = 0$ and $V = 0$ ($V$ is the voltage) and decays with increasing temperature T in a monotonic way",0705.2174v1 2007-07-19,Quantum transport in noncentrosymmetric superconductors and thermodynamics of ferromagnetic superconductors,"We consider a general Hamiltonian describing coexistence of itinerant ferromagnetism, spin-orbit coupling and mixed spin-singlet/triplet superconducting pairing in the context of mean-field theory. The Hamiltonian is diagonalized and exact eigenvalues are obtained, thus allowing us to write down the coupled gap equations for the different order parameters. Our results may then be applied to any model describing coexistence of any combination of these three phenomena. As a specific application of our results, we consider tunneling between a normal metal and a noncentrosymmetric superconductor with mixed singlet and triplet gaps. The conductance spectrum reveals information about these gaps in addition to how the influence of spin-orbit coupling is manifested. We also consider the coexistence of itinerant ferromagnetism and triplet superconductivity as a model for recently discovered ferromagnetic superconductors. The coupled gap equations are solved self-consistently, and we study the conditions necessary to obtain the coexistent regime of ferromagnetism and superconductivity. Analytical expressions are presented for the order parameters, and we provide an analysis of the free energy to identify the preferred system state. Moreover, we make specific predictions concerning the heat capacity for a ferromagnetic superconductor. In particular, we report a nonuniversal relative jump in the specific heat, depending on the magnetization of the system, at the uppermost superconducting phase transition. [Shortened abstract due to arXiv submission.]",0707.2875v2 2008-01-15,Exchange Interaction and $T_c$ in Alkaline-earth-metal-oxide-based DMS without Magnetic Impurities: First Principle Pseudo-SIC and Monte Carlo Calculation,"The prospects of half-metallic ferromagnetism being induced by the incorporation of C atoms into alkaline-earth-metal-oxides are investigated by the first principle calculation. The origin of the ferromagnetism is discussed through the calculation of the electronic structure and exchange coupling constant by using the pseudo-potential-like self-interaction-corrected local spin density method. The Curie temperature ($T_c$) is also predicted by employing the Monte Carlo simulation. It is shown that by taking the electron self-interaction into account, the half-metallic ferromagnetism induced by C in the host materials is more stabilized in comparison with the standard LDA case, and the C's $2p$ electron states in the bandgap become more localized resulting in the predominance of the short-ranged exchange interaction. While the ferromagnetism in MgO$_{1-x}$C$_x$ is stabilized due to the exchange interaction of the $1st$-nearest neighbor pairs and might be suppressed by the anti-ferromagnetic super-exchange interaction at higher $x$, the ferromagnetism in CaO$_{1-x}$C$_x$, SrO$_{1-x}$C$_x$, and BaO$_{1-x}$C$_x$ is stabilized by both the $1st$- and $2nd$-nearest neighbor pairs, and $T_c$ monotonously increases with the C concentration.",0801.2249v1 2008-12-08,Ferromagnetism and Lattice Distortions in the Perovskite YTiO$_3$,"The thermodynamic properties of the ferromagnetic perovskite YTiO$_3$ are investigated by thermal expansion, magnetostriction, specific heat, and magnetization measurements. The low-temperature spin-wave contribution to the specific heat, as well as an Arrott plot of the magnetization in the vicinity of the Curie temperature $T_C\simeq27$ K, are consistent with a three-dimensional Heisenberg model of ferromagnetism. However, a magnetic contribution to the thermal expansion persists well above $T_C$, which contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a comparison with the corresponding model system EuS. The pressure dependences of $T_C$ and of the spontaneous moment $M_s$ are extracted using thermodynamic relationships. They indicate that ferromagnetism is strengthened by uniaxial pressures $\mathbf{p}\parallel \mathbf{a}$ and is weakened by uniaxial pressures $\mathbf{p}\parallel \mathbf{b},\mathbf{c}$ and hydrostatic pressure. Our results show that the distortion along the $a$- and $b$-axes is further increased by the magnetic transition, confirming that ferromagnetism is favored by a large GdFeO$_3$-type distortion. The c-axis results however do not fit into this simple picture, which may be explained by an additional magnetoelastic effect, possibly related to a Jahn-Teller distortion.",0812.1533v1 2009-02-15,Sudden Critical Current Drops Induced in S/F Structures,"In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_{s} ~ 2 xi_{S}. Nb/Co and Nb/Py (Permalloy Ni_{80}Fe_{20}) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_{F} > 30nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to Tc show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_{F} ~ 50nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.",0902.2572v2 2009-04-30,"Structural, optical, magnetic and electrical properties of Zn1-x Co (x) O thin films","Despite a considerable effort aiming at elucidating the nature of ferromagnetism in ZnO-based magnetic semiconductor, its origin still remains debatable. Although the observation of above room temperature ferromagnetism has been reported frequently in the literature by magnetometry measurement, so far there has been no report on correlated ferromagnetism in magnetic, optical and electrical measurements. In this paper, we investigate systematically the structural, optical, magnetic and electrical properties of Zn1-x Co (x) O:Al thin films prepared by sputtering with x ranging from 0 to 0.33. We show that correlated ferromagnetism is present only in samples with x > 0.25. In contrast, samples with x < 0.2 exhibit weak ferromagnetism only in magnetometry measurement which is absent in optical and electrical measurements. We demonstrate, by systematic electrical transport studies that carrier localization indeed occurs below 20-50 K for samples with x < 0.2; however, this does not lead to the formation of ferromagnetic phase in these samples with an electron concentration in the range of 6 x 10(19) cm(-3) 1 x 10(20) cm(-3). Detailed structural and optical transmission spectroscopy analyses revealed that the anomalous Hall effect observed in samples with x > 0.25 is due to the formation of secondary phases and Co clusters.",0904.4807v1 2009-08-04,Fe-implanted ZnO: Magnetic precipitates versus dilution,"Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present paper we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences and temperatures and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to the Fe concentration and the process temperature are found: 1) Disperse Fe$^{2+}$ and Fe$^{3+}$ at low Fe concentrations and low processing temperatures, 2) FeZn$_2$O$_4$ at very high processing temperatures and 3) an intermediate regime with a co-existence of metallic Fe (Fe$^0$) and ionic Fe (Fe$^{2+}$ and Fe$^{3+}$). Ferromagnetism is only observed in the latter two cases, where inverted ZnFe$_2$O$_4$ and $\alpha$-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier mediated coupling. However, their separation is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison investigations of Fe-implanted epitaxial ZnO thin films are presented.",0908.0405v1 2010-01-20,Theory of Room Temperature Ferromagnet V(TCNE)_x (1.5 < x < 2): Role of Hidden Flat Bands,"Theoretical studies on the possible origin of room temperature ferromagnetism (ferromagnetic once crystallized) in the molecular transition metal complex, V(TCNE)_x (1.521 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te (Tmag < 180 K), the spin-gapless semiconductor Mn2CoAl (Tmag ~720 K), and the ferromagnetic insulators EuO (Tmag ~70 K) and Bi3Cr3O11 (Tmag ~220 K). It is also qualitatively different from known ferrimagnetic insulator/semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this new class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.",1612.05041v1 2019-07-12,Ferromagnetic phase transition in topological crystalline insulator thin films: interplay of anomalous Hall angle and magnetic anisotropy,"In magnetic topological phases of matter, the quantum anomalous Hall (QAH) effect is an emergent phenomenon driven by ferromagnetic doping, magnetic proximity effects and strain engineering. The realization of QAH states with multiple dissipationless edge and surface conduction channels defined by a Chern number $\mathcal{C}\geq1$ was foreseen for the ferromagnetically ordered SnTe class of topological crystalline insulators (TCIs). From magnetotransport measurements on Sn$_{1-x}$Mn$_{x}$Te ($0.00\leq{x}\leq{0.08}$)(111) epitaxial thin films grown by molecular beam epitaxy on BaF$_{2}$ substrates, hole mediated ferromagnetism is observed in samples with $x\geq0.06$ and the highest $T_\mathrm{c}\sim7.5\,\mathrm{K}$ is inferred from an anomalous Hall behavior in Sn$_{0.92}$Mn$_{0.08}$Te. The sizable anomalous Hall angle $\sim$0.3 obtained for Sn$_{0.92}$Mn$_{0.08}$Te is one of the greatest reported for magnetic topological materials. The ferromagnetic ordering with perpendicular magnetic anisotropy, complemented by the inception of anomalous Hall effect in the Sn$_{1-x}$Mn$_{x}$Te layers for a thickness commensurate with the decay length of the top and bottom surface states, points at Sn$_{1-x}$Mn$_{x}$Te as a preferential platform for the realization of QAH states in ferromagnetic TCIs.",1907.05716v1 2019-07-31,Ferromagnetism and its stability from the one-magnon spectrum in twisted bilayer graphene,"We study ferromagnetism and its stability in twisted bilayer graphene. We work with a Hubbard-like interaction that corresponds to the screened Coulomb interaction in a well-defined limit where the Thomas-Fermi screening length $l_\text{TF}$ is much larger than monolayer graphene's lattice spacing $l_g \ll l_\text{TF}$ and much smaller than the Moir\'e super lattice's spacing $ l_\text{TF} \ll l_{\text{Moir\'e}}$. We show that in the perfectly flat band ""chiral"" limit and at filling fractions $\pm 3/4$, the saturated ferromagnetic (spin and valley polarized) states are ideal ground states candidates in the large band-gap limit. By assuming a large enough substrate (hBN) induced sub-lattice potential, the same argument can be applied to filling fractions $\pm 1/4$. We estimate the regime of stability of the ferromagnetic phase around the chiral limit by studying the exactly calculated spectrum of one-magnon excitations. The instability of the ferromagnetic state is signaled by a negative magnon excitation energy. This approach allows us to deform the results of the idealized chiral model (by increasing the bandwidth and/or modified interactions) towards more realistic systems. Furthermore, we use the low energy part of the exact one-magnon spectrum to calculate the spin-stiffness of the Goldstone modes throughout the ferromagnetic phase. The calculated value of spin-stiffness can determine the excitation energy of charged skyrmions.",1907.13633v1 2019-10-16,Controlling Ferromagnetic Ground States and Solitons in Thin Films and Nanowires built from Iron Phthalocyanine Chains,"Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are one-dimensional ferromagnetic chains. We show that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through tuning the crystal phase by switching from organic molecular beam deposition, producing continuous thin films of nanocrystals with controlled orientations, to organic vapour phase deposition, producing ultralong nanowires. Magnetisation measurements are analysed using a suite of concepts and simply stated formulas with broad applicability to all one-dimensional ferromagnetic chains. They show that FePc is best described by a Heisenberg model with a preference for the moments to lie in the molecular planes, where the chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain interactions. The data at large magnetic fields are well-described by the soliton picture, where the dominant (and topologically non-trivial) degrees of freedom are moving one-dimensional magnetic domain walls, which was successful for CsNiF3, and at low temperatures and fields by the super-Curie-Weiss law of 1/(T^2+theta^2) characteristic of nearly one-dimensional xy and Heisenberg ferromagnets. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates as thin films or nanowires, taken together with excellent transistor characteristics reported previously for nanowires of copper and cobalt analogues, makes them potentially useful for magneto-optical and spintronic devices.",1910.07229v1 2020-01-14,2D van der Waals Nanoplatelets with Robust Ferromagnetism,"We have synthesized unique colloidal nanoplatelets of the ferromagnetic two-dimensional (2D) van der Waals material CrI3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The isolated CrI3 nanoplatelets have lateral dimensions of ~25 nm and ensemble thicknesses of only ~4 nm, corresponding to just a few CrI3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering in these nanoplatelets with Curie temperatures similar to those observed in bulk CrI3, despite the strong spatial confinement. These data also show magnetization steps akin to those observed in micron-sized few-layer 2D sheets and associated with concerted spin-reversal of individual CrI3 layers within few-layer van der Waals stacks. Similar data have also been obtained for CrBr3 and anion-alloyed Cr(I1-xBrx)3 nanoplatelets. These results represent the first example of laterally confined 2D van der Waals ferromagnets of any composition. The demonstration of robust ferromagnetism at nanometer lateral dimensions opens new doors for miniaturization in spintronics devices based on van der Waals ferromagnets.",2001.04594v1 2020-02-28,Metallic ferroelectric-ferromagnetic multiferroics in strained EuTiO$_{3-x}$H$_x$,"Polar metals are defined by the coexistence of metallicity and polar crystal structure. They have potential applications in non-linear optics, ferroelectric devices, and quantum devices. Meanwhile, ferroelectric-ferromagnetic (FE-FM) multiferroics display simultaneous ferroelectricity and ferromagnetism, leading to new technologies in information storage. It remains an open question whether metallicity, ferroelectricity, and ferromagnetism can coexist in a single domain of a material. EuTiO$_3$ is actively studied for potential applications in magnetic sensors, memories, magneto-optical devices, and energy conversion devices. It stabilizes at a multi-critical equilibrium and exhibits a rich range of intriguing properties. Here, using the results from hybrid density functional theory calculations, we report metallic FE-FM multiferroics in strain-engineered epitaxial EuTiO$_3$ with H doping. The emergence of the magnetism in polar metals provides a new degree of freedom to control these materials in applications. The underlying mechanism for the coexistence of metallicity, ferroelectricity, and ferromagnetism is discussed. The ferromagnetism in metallic EuTiO$_{3-x}$H$_x$ is explained by the Ruderman-Kittel-Kasuya- Yosida (RKKY) interaction, which agrees with experiments. The coexistence of metallicity and ferroelectricity is allowed because the electrons at the Fermi level are weakly coupled to the ferroelectric distortion. Our results suggest that the combined effect of strain and doping is responsible for achieving EuTiO$_3$-based metallic FE-FM multiferroics and may provide a new way for obtaining metallic FE-FM multiferroics in other materials.",2002.12783v1 2012-05-23,Low-Temperature Properties of Two-Dimensional Ideal Ferromagnets,"The manifestation of the spin-wave interaction in the low-temperature series of the partition function has been investigated extensively over more than seven decades in the case of the three-dimensional ferromagnet. Surprisingly, the same problem regarding ferromagnets in two spatial dimensions, to the best of our knowledge, has never been addressed in a systematic way so far. In the present paper the low-temperature properties of two-dimensional ideal ferromagnets are analyzed within the model-independent method of effective Lagrangians. The low-temperature expansion of the partition function is evaluated up to two-loop order and the general structure of this series is discussed, including the effect of a weak external magnetic field. Our results apply to two-dimensional ideal ferromagnets which exhibit a spontaneously broken spin rotation symmetry O(3) $\to $ O(2) and are defined on a square, honeycomb, triangular or Kagom\'e lattice. Remarkably, the spin-wave interaction only sets in at three-loop order. In particular, there is no interaction term of order $T^3$ in the low-temperature series for the free energy density. This is the analog of the statement that, in the case of three-dimensional ferromagnets, there is no interaction term of order $T^4$ in the free energy density. We also provide a careful discussion of the implications of the Mermin-Wagner theorem in the present context and thereby put our low-temperature expansions on safe grounds.",1205.5293v1 2017-11-30,Room-Temperature Ferromagnetic Topological Phase in CrO$_{2}$: From Tripe Fermions to Weyl Fermions,"Ferromagnetic topological semimetals due to their band topology co-existing with intrinsic magnetization exerted important influences on early study of topological fermions. However, they have not been observed in experiments up to now. In this work, we propose that rutile CrO$_{2}$, a widely used half-metallic ferromagnetic material in magnetic recording taps, exhibits unexpected ferromagnetic topological features. Using first-principles calculations and symmetry analysis, we reveal that rutile CrO$_2$ hosts the triple nodal points in the absence of spin orbital coupling (SOC). By taking into account of SOC, each triple nodal point splits into two Weyl points, which are located on the magnetic axis with four-fold rotational symmetry. Notably, the Fermi arcs and accompanying quasiparticle interference patterns are clearly visible, which facilitate experimental observations. In addition, we find that another experimentally synthesized CrO$_{2}$ in CaCl$_2$ structure, also hosts the topologically nontrivial ferromagnetic phase. Our findings substantially advance the experimental realization of ferromagnetic topological semimetals. More importantly, room-temperature time-reversal-breaking Weyl fermions in CrO$_{2}$ may result in promising applications related to the topological phenomena in industry.",1711.11203v1 2017-11-30,Appearance and disappearance of ferromagnetism in ultra-thin LaMnO$_3$ on SrTiO$_3$ substrate: a viewpoint from first-principles,"The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultra-thin LaMnO$_3$ films on the mostly used SrTiO$_3$ substrate is a long-existing question under debate. Either strain effect or non-stoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [Science \textbf{349}, 716 (2015)], one more mechanism, namely the self-doping due to polar discontinuity, was argued to be the driving force of ferromagnetism beyond the critical thickness. Here systematic first-principles calculations have been performed to check these mechanisms in ultra-thin LaMnO$_3$ films as well as superlattices. Starting from the very precise descriptions of both LaMnO$_3$ and SrTiO$_3$, it is found that the compressive strain is the dominant force for the appearance of ferromagnetism, while the open surface with oxygen vacancies leads to the suppression of ferromagnetism. Within LaMnO$_3$ layers, the charge reconstructions involve many competitive factors and certainly go beyond the intuitive polar catastrophe model established for LaAlO$_3$/SrTiO$_3$ heterostructures. Our study not only explains the long-term puzzle regarding the magnetism of ultra-thin LaMnO$_3$ films, but also shed light on how to overcome the notorious magnetic dead layer in ultra-thin manganites.",1711.11329v1 2018-03-05,Simulation study of ballistic spin-MOSFET devices with ferromagnetic channels based on some Heusler and oxide compounds,"Newly emerged materials from the family of Heuslers and complex oxides exhibit finite bandgaps and ferromagnetic behavior with Curie temperatures much higher than even room temperature. In this work, using the semiclassical top-of-the-barrier FET model, we explore the operation of a spin-MOSFET that utilizes such ferromagnetic semiconductors as channel materials, in addition to ferromagnetic source/drain contacts. Such a device could retain the spin polarization of injected electrons in the channel, the loss of which limits the operation of traditional spin transistors with non-ferromagnetic channels. We examine the operation of four material systems that are currently considered some of the most prominent known ferromagnetic semiconductors, three Heusler-type alloys (Mn2CoAl, CrVZrAl, CoVZrAl) and one from the oxide family (NiFe2O4). We describe their bandstructures by using data from DFT calculations. We investigate under which conditions high spin polarization and significant ION/IOFF ratio, two essential requirements for the spin-MOSFET operation, are both achieved. We show that these particular Heusler channels, in their bulk form, do not have adequate bandgap to provide high ION/IOFF ratios, and have small magnetoconductance compared to state-of-the-art devices. However, with confinement into ultra-narrow sizes down to a few nanometers, and by engineering their spin dependent contact resistances, they could prove promising channel materials for the realization of spin-MOSFET transistor devices that offer combined logic and memory functionalities. Although the main compounds of interest in this paper are Mn2CoAl, CrVZrAl, CoVZrAl, and NiFe2O4 alone, we expect that the insight we provide is relevant to other classes of such materials as well.",1803.01789v1 2018-06-18,Room-temperature ferromagnetism in monolayer WSe2 semiconductor via vanadium dopant,"Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, we report the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe2 monolayer synthesized by chemical vapor deposition. Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~105 at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron microscopy, which implies the possibility of the Ruderman-Kittel-Kasuya-Yoshida interaction (or Zener model) by establishing the long-range ferromagnetic order in V-doped WSe2 monolayer through free hole carriers. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.",1806.06479v2 2019-04-18,Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers. II. Canted magnetization,"It has been suggested by theoretical works that equal spin-triplet Cooper pairs can be generated in Josephson junctions containing both a ferromagnet and a source of spin-orbit coupling. Our recent experimental work suggested that spin-triplet Cooper pairs were not generated by a Pt spin-orbit coupling layer when the ferromagnetic weak link had entirely in-plane anisotropy (N. Satchell and N.O. Birge, Phys. Rev. B 97, 214509 (2018)). Here, we revisit the experiment using Pt again as a source for spin-orbit coupling and a [Co(0.4 nm)/Ni(0.4 nm)]$_{\times8}$/Co(0.4 nm) ferromagnetic weak link with both in-plane and out-of-plane magnetization components (canted magnetization). The canted magnetization more closely matches theoretical predictions than our previous experimental work. Our results suggest that there is no supercurrent contribution in our junctions from equal spin-triplets. In addition, this work includes the first systematic study of supercurrent dependence on Cu interlayer thickness, a common additional layer used to buffer the growth of the ferromagnet and which for Co may significantly improve the growth morphology. We report that the supercurrent in the [Co(0.4 nm)/Ni(0.4 nm)]$_{\times8}$/Co(0.4 nm) ferromagnetic weak links can be enhanced by over two orders of magnitude by tuning the Cu interlayer thickness. This result has important application in superconducting spintronics, where large critical currents are desirable for devices.",1904.08798v2 2019-11-18,"Vector-boson condensates, spin-triplet superfluidity of paired neutral and charged fermions, and $3P_2$ pairing of nucleons","After reminding of properties of the condensate of the complex scalar field in the external uniform magnetic field $H$ focus is made on the study of phases of the complex neutral vector boson fields coupled with magnetic field by the Zeeman coupling and phases of the charged vector boson fields. These systems may behave as nonmagnetic and ferromagnetic superfluids and ordinary and ferromagnetic superconductors. Response of the ferromagnetic superfluid and superconducting systems occupying half of space on the external uniform static magnetic field $H$ is thoroughly studied. Then the spin-triplet pairing of neutral fermions at conserved spin is considered. Novel phases are found. In external magnetic field the phase with zero mean spin proves to be unstable to the formation of a phase with a non-zero spin. For $H>H_{\rm cr 2}$ the spin-triplet pairing and ferromagnetic superfluidity continue to exist above the ""old"" phase transition critical temperature $T_{\rm cr}$. For a certain parameter choice ferromagnetic superfluid phases are formed already for $H=0$, characterized by an own magnetic field $h$. Formation of domains is discussed. Next, spin-triplet pairing of charged fermions is studied. Novel phases are found with a ferromagnetic superconductivity. Then, the 3P$_2$ $nn$ pairing in neutron star matter is studied. Also a 3P$_2$ $pp$ pairing is considered. Numerical estimates are performed in the BCS weak coupling limit and beyond it for the $3P_2$ $nn$ and $pp$ pairings, as well as for the 3S$_1$ $np$ pairing.",1911.07502v2 2019-12-19,Metallic states beyond Tomonaga-Luttinger liquids in one dimension,"In this paper, we propose some new strongly correlated gapless states (or critical states) of spin-1/2 electrons in 1+1-dimensions, such as doped anti-ferromagnetic spin-1/2 Ising chain. We find doped anti-ferromagnetic Ising chain to be a different metallic phase from the doped ferromagnetic Ising chain, despite the two have identical symmetry. The doped anti-ferromagnetic Ising chain has a finite energy gap for all charge-1 fermionic excitations even without pairing caused by attractive interactions, resembling the pseudo-gap phase of underdoped high Tc superconductors. Applying a transverse field to the ferromagnetic and anti-ferromagnetic metallic phases can restore the $Z_2$ symmetry, which gives rise to two distinct critical points despite that the two transitions have exactly the same symmetry breaking pattern. We also propose new chiral metallic states. All those new gapless states are strongly correlated in the sense that they do not belong to the usual Tomonaga-Luttinger phase of fermions, i.e., they cannot be smoothly deformed into the non-interacting fermion systems of the same symmetry. Our non-perturbative results are obtained by noticing that gapless quantum systems have emergent categorical symmetries, i.e., non-invertible gravitational anomalies), which are described by multi-component partition functions that are modular covariant. This allows us to calculate the scaling dimensions and quantum numbers of all the low energy operators for those strongly correlated gapless states. This demonstrates an application of emergent categorical symmetries in determining low energy properties of strongly correlated gapless states, which are hard to obtain otherwise.",1912.09391v3 2020-06-03,First-principles calculation of gate-tunable ferromagnetism in magic-angle twisted bilayer graphene under pressure,"Magic-angle twisted bilayer graphene (MATBG) is notable as a highly tunable platform for investigating strongly correlated phenomena such as high-$T_c$ superconductivity and quantum spin liquids, due to easy control of doping level through gating and sensitive dependence of the magic angle on hydrostatic pressure. Experimental observations of correlated insulating states, unconventional superconductivity and ferromagnetism in MATBG indicate that this system exhibits rich exotic phases. In this work, using density functional theory calculations in conjunction with the effective screening medium method, we find the MATBG under pressure at a twisting angle of $2.88\unicode{xb0}$ and simulate how its electronic states evolve when doping level and out-of-plane electric field are gate-tuned. Our calculations show that, at doping levels between two electrons and four holes per moir\'{e} unit cell, a ferromagnetic solution with spin density localized at AA stacking sites is lower in energy than the nonmagnetic solution. The magnetic moment of this ferromagnetic state decreases with both electron and hole doping and vanishes at four electrons/holes doped per moir\'{e} unit cell. Hybridization between the flat bands at the Fermi level and the surrounding dispersive bands can take place at finite doping. Moreover, upon increasing the out-of-plane electric field at zero doping, a transition from the ferromagnetic state to the nonmagnetic one is seen. We also analyze the interlayer bonding character due to the flat bands via Wannier functions. Finally, we report trivial band topology of the flat bands in the ferromagnetic state at a certain doping level.",2006.02362v1 2021-03-14,Toward a systematic discovery of artificial functional magnetic materials,"Although ferromagnets are found in all kinds of technological applications, only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? To answer this question, in this work the emergence of a ferromagnetic phase upon ion irradiation is systematically investigated both theoretically and experimentally. Quantitative predictions are validated against experimental data from the literature of SiC hosts irradiated with high energy Ne ions and own experiments on low energy Ar ion irradiation of TiO$_2$ hosts. In the high energy regime, a bulk magnetic phase emerges, which is limited by host lattice amorphization, whereas at low ion energies an ultrathin magnetic layer forms at the surface and evolves into full magnetic percolation. Lowering the ion energy, the magnetic layer thickness reduces down to a bilayer, where a perpendicular magnetic anisotropy appears due to magnetic surface states.",2103.08045v3 2021-07-29,Superconducting triplet pairings and anisotropic magnetoresistance effects in ferromagnet/superconductor/ferromagnet double-barrier junctions,"Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder-Tinkham-Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings-that fully determine the magnetoresistance in the normal-conducting state-and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip (""unconventional"") Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double peaks around the singlet-gap energy, reflecting the competition between the singlet and an additionally emerging triplet gap; the latter is an effective superconducting gap that can be ascribed to the formation of triplet Cooper pairs through interfacial spin-flip scatterings (i.e., to the generation of an effective triplet-pairing term in the order parameter). We thoroughly analyze the Andreev reflections' role in connection with superconducting magnetoresistance phenomena, and eventually unravel huge conductance and magnetoresistance magnetoanisotropies-easily exceeding their normal-state counterparts by several orders of magnitude-as another experimentally accessible fingerprint of unconventional Andreev reflections.",2107.13818v2 2021-12-18,Itinerant ferromagnetism in narrow-band metals,"Since its introduction in 1963, the Hubbard model has becomes one of the most popular models used in the literature to study cooperative phenomena in narrow-band metals (ferromagnetism, metal-insulator transitions, charge-density waves, high-T$_c$ superconductivity). Amongst all these cooperative phenomena, the problem of itinerant ferromagnetism in the Hubbard model has the longest history. However, in spite of an impressive research activity in the past, the underlying physics (microscopic mechanisms) that leads to the stabilization of itinerant ferromagnetism in Hubbard model (narrow-band metals) is still far from being understood. In this review we present our numerical results concerning this subject, which have been reached by small cluster exact diagonalization, density matrix renormalization group and quantum Monte Carlo calculations within various extensions of the Hubbard model. Particular attention is paid to a description of crucial mechanisms (interactions) that support the stabilization of the ferromagnetic state, and namely: (i) the long-range hopping, (ii) the correlated hopping, (iii) the long-range Coulomb interaction, (iv) the flat bands and (v) the lattice structure. Most of the presented results have been obtained for the one-dimensional case, but the influence of the increasing dimension of the system on the ferromagnetic state is also intensively discussed.",2112.09913v1 2022-01-16,Evolution from helical to collinear ferromagnetic order of the Eu$^{2+}$ spins in RbEu(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$,"The ground-state magnetic structures of the Eu$^{2+}$ spins in recently discovered RbEu(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$ superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase of Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease of the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the $\mathit{c}$ axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor RbEu(Fe$_{0.95}$Ni$_{0.05}$)$_{4}$As$_{4}$ and the superconducting ferromagnet RbEu(Fe$_{0.93}$Ni$_{0.07}$)$_{4}$As$_{4}$. Such a helical structure transforms into a purely collinear ferromagnetic structure for non-superconducting RbEu(Fe$_{0.91}$Ni$_{0.09}$)$_{4}$As$_{4}$, with all the Eu$^{2+}$ spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the Eu$^{2+}$ spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent Eu$^{2+}$ layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction.",2201.05988v1 2022-03-01,Cu-doping effects on the ferromagnetic semimetal CeAuGe,"We present a study of Cu-substitution effects in 4f-Ce intermetallic compound CeAu1-xCuxGe, with potentially unusual electronic states, in the whole concentration range (x = 0.0 - 1.0). The parent CeAuGe compound, crystallizing in a non-centrosymmetric hexagonal structure, is a ferromagnetic semimetal with Curie temperature 10 K. Cu-doping on Au-site of CeAuGe, CeAu1-xCuxGe, changes the crystal structure from the non-centrosymmetric (P63mc) to centrosymmetric (P63/mmc) space group at the concentration x ~ 0.5, where the c-lattice constant has a maximum value. Magnetic susceptibility and electrical resistivity measurements reveal that all Cu-doped compounds undergo magnetic phase transition near 10 K, with the maximum transition temperature of 12 K for x = 0.5. The neutron powder diffraction experiments show the ferromagnetic ordering of Ce3+ magnetic moments with a value of ~ 1.2 Bohr magneton at 1.8 K, oriented perpendicular to the hexagonal c-axis. By using symmetry analysis, we have found the solutions for the magnetic structure in the ferromagnetic Shubnikov space groups Cmc'21' and P21'/m' for x < 0.5 and x >= 0.5, respectively. Electrical resistivity exhibits a metallic temperature behaviour in all compounds. The resistivity has a local minimum in the paramagnetic state due to Kondo effects at high doping x = 0.8 and 1.0. At the small Cu-doping level, x = 0.2, the resistivity shows a broad feature at the ferromagnetic transition temperature and an additional transition-like peculiarity at 2.5 K in the ferromagnetic state.",2203.00335v1 2022-03-22,Unconventional spin frustration due to two competing ferromagnetic interactions of a spin-1/2 Ising-Heisenberg model on martini and martini-diced lattice,"The spin-1/2 Ising-Heisenberg model on martini and martini-diced lattice is exactly solved using a star-triangle transformation, which affords an exact mapping correspondence to an effective spin-1/2 Ising model on a triangular lattice. The ground-state phase diagram of both investigated quantum spin models display two spontaneously ordered ferromagnetic phases and one macroscopically degenerate disordered phase. In contrast to a classical ferromagnetic phase where the spontaneous magnetization of the Ising as well as Heisenberg spins acquire fully saturated values the spontaneous magnetization of the Heisenberg spins is subject to a quantum reduction to one-third of its saturated value within a quantum ferromagnetic phase. The spontaneous magnetization and logarithmic divergence of the specific heat as the most essential features of both ferromagnetic phases disappear whenever the investigated quantum spin model is driven to the highly degenerate disordered phase. The disordered phase with nonzero residual entropy originates either from a geometric spin frustration caused by antiferromagnetic interactions or more strikingly it may also alternatively arise from a competition of the ferromagnetic Ising and Heisenberg interactions of easy-axis and easy-plane type, respectively. All three available ground states coexist together at a single triple point, around which anomalous magnetic and thermodynamic behavior can be detected.",2203.11582v1 2022-07-05,"Prediction of High Curie Temperature, Large Magnetic Crystal Anisotropy in 2D Ferromagnetic Co$_2$Ge$_2$Te$_6$ Monolayer and Multilayer","The Co$_2$Ge$_2$Te$_6$ shows intrinsic ferromagnetic (FM) order, which origins from superexchange interaction between Co and Te atoms, with higher Curie temperature ($T_c$) of 161 K. Co$_2$Ge$_2$Te$_6$ monolayer (ML) is half-metal (HM), and spin-$\beta$ electron is a semiconductor with gap of 1.311 eV. Co$_2$Ge$_2$Te$_6$ ML tends in-plane anisotropy (IPA), with magnetic anisotropy energy (MAE) of -10.2 meV/f.u.. Co$_2$Ge$_2$Te$_6$ ML shows good dynamical and thermal stability. Most interestingly, bilayers present ferromagnetic half-metallicity independent of the stacking orders. Notley, the multilayers ($N\ge 6$) present ferromagnetic HM, while the magnetoelectronic properties are related with the stacking patterns in thinner multilayers. Moreover, the magnetoelectronic properties are dependent on the stacking orders of bulk. The magnetic order with multilayers is determined by the super-super exchange and weak van der Waals (vdW) interaction. Co$_2$Ge$_2$Te$_6$ with intrinsic ferromagnetism, good stability of ferromagnetism and half-metallicity could help researchers to investigate its wide application in the spintronics.",2207.02024v1 2022-08-31,Surface state mediated ferromagnetism in Mn$_{0.14}$Bi$_{1.86}$Te$_3$ thin films,"A spontaneous ferromagnetic moment can be induced in Bi$_{2}$Te$_{3}$ thin films below a temperature T $\approx$ 16 K by the introduction of Mn dopants. We demonstrate that films grown via molecular beam epitaxy with the stoichiometry Mn$_{0.14}$Bi$_{1.86}$Te$_3$ maintain the crystal structure of pure Bi$_{2}$Te$_{3}$. The van der Waals nature of inter-layer forces in the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ crystal causes lattice mismatch with the underlayer to have a limited effect on the resulting crystal structure, as we demonstrate by thin film growth on tetragonal MgF$_{2}$ (110) and NiF$_{2}$ (110). Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ thin films is enhanced as the Fermi level moves from the bulk conduction band and towards the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Ferromagnetic Mn$_{0.14}$Bi$_{1.86}$Te$_3$/antiferromagnetic NiF$_{2}$ bilayers show evidence that the ferromagnetic moment of the Mn$_{0.14}$Bi$_{1.86}$Te$_3$ film is suppressed, suggesting the existence of an interface effect between the two magnetic layers.",2209.00155v2 2023-02-10,Discovery of Superconductivity in Van der Waals Ferromagnetic Sn$_{0.06}$Cr$_3$Te$_4$,"Coexistence of superconductivity and ferromagnetism, though a very few systems show this, is one of the exciting properties of a solid from the fundamental science point of view. Here, we report on the discovery of superconductivity in the van der Waals ferromagnetic Cr$_3$Te$_4$ that is coexisted with ferromagnetism, induced by the Sn intercalation (Sn$_{0.06}$Cr$_{3}$Te$_4$), below a transition temperature of $T_c$=3.5 K. In addition, despite the magnetism and Meissner effects are anisotropic, the superconductivity observed from the in-plane electrical resistivity ($\rho_{\it{bc}}$) is nearly isotropic between $H\parallel \it{bc}$ and $H\parallel \it{a}$, suggesting separate channels of conduction electrons responsible for the superconductivity and ferromagnetism of this system which is supported by our spin-resolved DFT calculations. We identify two orders increase in the carrier density of Cr$_{3}$Te$_4$ by Sn intercalation compared to the parent Cr$_3$Te$_4$. We think, the enhanced carrier density could be playing a key role for the discovered superconductivity in ferromagnetic Sn$_{0.06}$Cr$_{3}$Te$_4$. We also explored other mechanisms such as the Fermi surface nesting to understand microscopically the superconductivity of this system.",2302.05145v1 2023-03-13,Experimental investigation of the effect of topological insulator on the magnetization dynamics of ferromagnetic metal: $BiSbTe_{1.5}Se_{1.5}$ and $Ni_{80}Fe_{20}$ heterostructure,"We have studied ferromagnetic metal/topological insulator bilayer system to understand magnetization dynamics of ferromagnetic metal (FM) in contact with a topological insulator (TI). At magnetic resonance condition, the precessing magnetization in the metallic ferromagnet ($Ni_{80}Fe_{20}$) injects spin current into the topological insulator ($BiSbTe_{1.5}Se_{1.5}$), a phenomenon known as spin-pumping. Due to the spin pumping effect, fast relaxation in the ferromagnet results in the broadening of ferromagnetic resonance linewidth ($\Delta H$). We evaluated the parameters like effective Gilbert damping coefficient ($\alpha_{eff}$), spin-mixing conductance ($g_{eff}^{\uparrow \downarrow}$) and spin current density ($j_S^0$) to confirm a successful spin injection due to spin-pumping into the $BiSbTe_{1.5}Se_{1.5}$ layer. TIs embody a spin-momentum locked surface state that span the bulk band-gap. It can act differently to the FM magnetization than the other normal metals. To probe the effect of topological surface state, a systematic low temperature study is crucial as surface state of TI dominates at lower temperatures. The exponential growth of $\Delta H$ for all different thickness combination of FM/TI bilayers and effective Gilbert damping coefficient ($\alpha_{eff}$) with lowering temperature confirms the prediction that spin chemical bias generated from spin-pumping induces surface current in TI due to spin-momentum locking. The hump-like feature of magnetic anisotropy field ($H_K$)of the bilayer around 60K suggests that the decrease of interfacial in-plane magnetic anisotropy can result from exchange coupling between the TI surface state and the local moments of FM layer.",2303.07025v2 2023-03-17,Large-area synthesis of ferromagnetic Fe$_{5-x}$GeTe$_{2}$/graphene van der Waals heterostructures with Curie temperature above room temperature,"Van der Waals (vdW) heterostructures combining layered ferromagnets and other two-dimensional (2D) crystals are promising building blocks for the realization of ultra-compact devices with integrated magnetic, electronic and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing to realize highly uniform heterostructures with well-defined interfaces between different 2D layered materials. It also requires that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, we demonstrate large-area growth of Fe$_{5-x}$GeTe$_{2}$/graphene heterostructures achieved by vdW epitaxy of Fe$_{5-x}$GeTe$_{2}$ on epitaxial graphene. Structural characterization confirmed the realization of a continuous vdW heterostructure film with a sharp interface between Fe$_{5-x}$GeTe$_{2}$ and graphene. Magnetic and transport studies revealed that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond non-scalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.",2303.10252v1 2023-05-02,Frustration induced Itinerant Ferromagnetism of Fermions in Optical Lattice,"When the Fermi Hubbard model was first introduced sixty years ago, one of the original motivations was to understand correlation effects in itinerant ferromagnetism. In the past two decades, ultracold Fermi gas in an optical lattice has been used to study the Fermi Hubbard model. However, the metallic ferromagnetic correlation was observed only in a recent experiment using frustrated lattices, and its underlying mechanism is not clear yet. In this letter, we point out that, under the particle--hole transformation, the single-particle ground state can exhibit double degeneracy in such a frustrated lattice. Therefore, the low-energy state exhibits valley degeneracy, reminiscent of multi-orbit physics in ferromagnetic transition metals. The local repulsive interaction leads to the valley Hund's rule, responsible for the observed ferromagnetism. We generalize this mechanism to distorted honeycomb lattices and square lattices with flux. This mechanism was first discussed by M\""uller-Hartmann in a simpler one-dimension model. However, this mechanism has not been widely discussed and has not been related to experimental observations before. Hence, our study not only explains the experimental findings but also enriches our understanding of itinerant ferromagnetism.",2305.01682v1 2023-05-02,Exciton and light induced ferromagnetism from doping a moiré Mott insulator,"Significant efforts have been dedicated to achieving excitonic insulators. In this paper, we explore a new problem of doping excitons into a Mott insulator instead of a band insulator. Specifically, we start with a Mott insulator on a triangular moir\'e superlattice in a transition metal dichalcogenides (TMD) layer and inject excitons by either transferring particles to a different layer or optically pumping electrons from the valence to the conduction band. In both cases, the excitons move in the presence of local spin moments inherited from the Mott insulator. When the Heisenberg spin coupling $J$ is small, the kinetic energy of the excitons decides the magnetism, akin to Nagaoka ferromagnetism in hole-doped Mott insulators. Through density matrix renormalization group (DMRG) calculations, we demonstrate that the spin moments originating from the Mott insulator form $120^\circ$ antiferromagnetic or ferromagnetic order for the two signs of the exciton hoppings over a broad range of exciton densities. Notably, the optical pump case may result in an antiferromagnetic to ferromagnetic transition with increasing exciton density, indicating a potential mechanism for light-induced ferromagnetism. A similar exciton-induced ferromagnetism could be achieved in a moir\'e-monolayer system where the monolayer is electron-doped while the moir\'e Mott insulator is hole-doped. Our works demonstrates a new possibility to engineering magnetism through doping neutral excitons.",2305.01702v2 2023-05-08,Large magnetocaloric effect in the kagome ferromagnet Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$,"Single-crystal growth, magnetic properties, and magnetocaloric effect of the $S = 3/2$ kagome ferromagnet Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ (trigonal, space group: $P\bar{3}c1$) are reported. Magnetization data suggest dominant ferromagnetic intra-plane coupling with a weak anisotropy and the onset of ferromagnetic ordering at $T_{\rm C} \simeq 2.6$ K. Microscopic analysis reveals a very small ratio of interlayer to intralayer ferromagnetic couplings ($J_{\perp}/J \simeq 0.02$). Electron spin resonance data suggest the presence of short-range correlations above $T_{\rm C}$ and confirms quasi-two-dimensional character of the spin system. A large magnetocaloric effect characterized by isothermal entropy change of $-\Delta S_{\rm m}\simeq 31$ J kg$^{-1}$ K$^{-1}$ and adiabatic temperature change of $-\Delta T_{\rm ad}\simeq 9$ K upon a field sweep of 7 T is observed around $T_{\rm C}$. This leads to a large relative cooling power of $RCP \simeq 284$ J kg$^{-1}$. The large magnetocaloric effect, together with negligible hysteresis render Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ a promising material for magnetic refrigeration at low temperatures. The magnetocrystalline anisotropy constant $K \simeq -7.42 \times 10^4$ erg cm$^{-3}$ implies that the compound is an easy-plane type ferromagnet with the hard axis normal to the $ab$-plane, consistent with the magnetization data.",2305.04744v1 2023-07-18,Coexistence of Logarithmic and SdH Quantum Oscillations in Ferromagnetic Cr-doped Tellurium Single Crystals,"We report the synthesis of transition-metal-doped ferromagnetic elemental single-crystal semiconductors with quantum oscillations using the physical vapor transport method. The 7.7 atom% Cr-doped Te crystals (Cr_Te) show ferromagnetism, butterfly-like negative magnetoresistance in the low temperature (< 3.8 K) and low field (< 0.15 T) region, and high Hall mobility, e.g., 1320 cm2 V-1 s-1 at 30 K and 350 cm2 V-1 s-1 at 300 K, implying that Cr_Te crystals are ferromagnetic elemental semiconductors. When B // c // I, the maximum negative MR is -27% at T = 20 K and B = 8 T. In the low temperature semiconducting region, Cr_Te crystals show strong discrete scale invariance dominated logarithmic quantum oscillations when the direction of the magnetic field B is parallel to the [100] crystallographic direction and show Landau quantization dominated Shubnikov-de Haas (SdH) oscillations for B // [210] direction, which suggests the broken rotation symmetry of the Fermi pockets in the Cr_Te crystals. The findings of coexistence of multiple quantum oscillations and ferromagnetism in such an elemental quantum material may inspire more study of narrow bandgap semiconductors with ferromagnetism and quantum phenomena.",2307.09139v1 2023-07-25,Unusual band evolution and persistence of topological surface states in high-T_C magnetic topological insulator,"Understanding the mechanism of ferromagnetism in ferromagnetic topological insulators (TIs) is a key to realize exotic time-reversal-symmetry-broken quantum phases. However, electronic states relevant to the ferromagnetism are highly controversial. Here we report angle-resolved photoemission spectroscopy on (CrxSb1-x)2Te3 thin films, high-Curie-temperature (T_C) ferromagnetic TIs, spanning the non-doped (T_C=0 K) to highly-doped (T_C=192 K) region. We found that, upon Cr doping to Sb2Te3, the bulk valence-band valley exhibits filling-in behavior while retaining band inversion, leading to the formation of a nearly-flat band in high-T_C regime and evolution from a six-petal flower to a Star-of-David Fermi surface. Despite the weakening of spin-orbit coupling with Cr doping, the Dirac-cone state persists up to the highest-T_C sample, and shows a clear magnetic-gap opening below TC accompanied with an unexpected band shift, signifying its strong coupling with spontaneous ferromagnetism. The present result lays the foundation for understanding the interplay between band topology and ferromagnetism in TIs.",2307.13388v1 2023-08-23,Room-Temperature Highly-Tunable Coercivity and Highly-Efficient Nonvolatile Multi-States Magnetization Switching by Small Current in Single 2D Ferromagnet Fe$_3$GaTe$_2$,"Room-temperature electrically-tuned coercivity and nonvolatile multi-states magnetization switching is crucial for next-generation low-power 2D spintronics. However, most methods have limited ability to adjust the coercivity of ferromagnetic systems, and room-temperature electrically-driven magnetization switching shows high critical current density and high power dissipation. Here, highly-tunable coercivity and highly-efficient nonvolatile multi-states magnetization switching are achieved at room temperature in single-material based devices by 2D van der Waals itinerant ferromagnet Fe$_3$GaTe$_2$. The coercivity can be readily tuned up to ~98.06% at 300 K by a tiny in-plane electric field that is 2-5 orders of magnitude smaller than that of other ferromagnetic systems. Moreover, the critical current density and power dissipation for room-temperature magnetization switching in 2D Fe$_3$GaTe$_2$ are down to ~1.7E5 A cm$^{-2}$ and ~4E12 W m$^{-3}$, respectively. Such switching power dissipation is 2-6 orders of magnitude lower than that of other 2D ferromagnetic systems. Meanwhile, multi-states magnetization switching are presented by continuously controlling the current, which can dramatically enhance the information storage capacity and develop new computing methodology. This work opens the avenue for room-temperature electrical control of ferromagnetism and potential applications for vdW-integrated 2D spintronics.",2308.12007v1 2023-09-27,From Stoner to Local Moment Magnetism in Atomically Thin Cr2Te3,"The field of two-dimensional (2D) ferromagnetism has been proliferating over the past few years, with ongoing interests in basic science and potential applications in spintronic technology. However, a high-resolution spectroscopic study of the 2D ferromagnet is still lacking due to the small size and air sensitivity of the exfoliated nanoflakes. Here, we report a thickness-dependent ferromagnetism in epitaxially grown Cr2Te3 thin films and investigate the evolution of the underlying electronic structure by synergistic angle-resolved photoemission spectroscopy, scanning tunneling microscopy, x-ray absorption spectroscopy, and first-principle calculations. A conspicuous ferromagnetic transition from Stoner to Heisenberg-type is directly observed in the atomically thin limit, indicating that dimensionality is a powerful tuning knob to manipulate the novel properties of 2D magnetism. Monolayer Cr2Te3 retains robust ferromagnetism, but with a suppressed Curie temperature, due to the drastic drop in the density of states near the Fermi level. Our results establish atomically thin Cr2Te3 as an excellent platform to explore the dual nature of localized and itinerant ferromagnetism in 2D magnets.",2309.15371v1 2024-03-04,Order-by-disorder without quantum zero-point fluctuations in the pyrochlore Heisenberg ferromagnet with Dzyaloshinskii-Moriya interactions,"Order-by-disorder, whereby fluctuations lift an accidental classical ground state degeneracy to stabilize a subset of ordered states, is a recurrent and prominent theme in the field of frustrated magnetism where magnetic moments, or spins, are subject to competing interactions. Thus far, such a phenomenon has been discussed in systems where the quantum ground state is not a product state. In this circumstance, both thermal and quantum fluctuations act to lift the accidental classical degeneracy, begging the question whether one mechanism of order-by-disorder is possible without the other. In this paper, we present results exposing a novel route to order-by-disorder, without quantum zero-point fluctuations, in a family of ferromagnetic Heisenberg materials with the Dzyaloshinskii-Moriya (DM) interaction as the leading perturbation. We prove that any colinear ferromagnetic state is an exact eigenstate, even in the presence of the anisotropic DM interaction, while thermal fluctuations give rise to a preference in the magnetization direction. Here, we present in detail an example of the pyrochlore Heisenberg ferromagnet with DM interactions. Using both linear spin-wave theory and classical Monte Carlo simulations, we find that the magnetization can spontaneously rotate at intermediate temperature(s) within the colinear ferromagnetic phase when the DM interaction is large. Additionally, we find that in the large DM regime, the ferromagnetic ground state becomes unstable within the framework of non-linear spin-wave theory. Our results show that thermal order-by-disorder is possible even in the absence of quantum zero-point fluctuations.",2403.02391v1 1992-07-15,Ferromagnetism of the 1-D Kondo Lattice Model: A Quantum Monte Carlo Study,"The one dimensional Kondo lattice model is investigated using Quantum Monte Carlo and transfer matrix techniques. In the strong coupling region ferromagnetic ordering is found even at large band fillings. In the weak coupling region the system shows an RKKY like behavior.",9207016v1 1994-01-25,Curie Temperatures for Binary Ising Ferromagnets on the Square Lattice,"High-accuracy Swendsen and Wang Monte Carlo simulations were performed to study the Curie temperature of ferromagnetic, binary Ising systems on the square lattice. Our results are compared with mean-field like approaches. Based on these former theories, we give a new formula to estimate the Curie temperature of the system.",9401052v1 1994-02-03,Curie Temperatures for Three-Dimensional Binary Ising Ferromagnets,"Using the Swendsen and Wang algorithm, high accuracy Monte Carlo simulations were performed to study the concentration dependence of the Curie temperature in binary, ferromagnetic Ising systems on the simple-cubic lattice. Our results are in good agreement with known mean-field like approaches. Based on former theoretical formulas we propose a new way of estimating the Curie temperature of these systems.",9402013v1 1995-05-06,Stochastic Resonance in 3D Ising Ferromagnets,"Finite 3D Ising ferromagnets are studied in periodic magnetic fields both by computer simulations and mean-field theoretical approaches. The phenomenon of stochastic resonance is revealed. The characteristic peak obtained for the correlation function between the external oscillating magnetic field and magnetization versus the temperature of the system, is studied for various external fields and lattice sizes. Excellent agreement between simulation and theoretical results are obtained.",9505025v1 1995-05-26,Stochastic resonance in a system of ferromagnetic particles,"We show that a dispersion of monodomain ferromagnetic particles in a solid phase exhibits stochastic resonance when a driven linearly polarized magnetic field is applied. By using an adiabatic approach, we calculate the power spectrum, the distribution of residence times and the mean first passage time. The behavior of these quantities is similar to their corresponding ones in other systems in which stochastic resonance has also been observed.",9505136v1 1995-06-26,Weak universality in the two dimensional randomly disordered three-state Potts ferromagnet,"For the two dimensional randomly disordered three-state Potts ferromagnet, we find numerically that the critical exponent $\eta$ unchanges with the degree of disorder while those of the correlation length ($\nu$) and magnetic susceptibility ($\gamma$) increase with it continuously. We discuss some consequences of the finding.",9506117v1 1996-03-18,Renormalization-Group Approach to Spin-Wave Theory of Quantum Heisenberg Ferromagnet,"The renormalization-group method is used to analyze the low-temperature behaviour of a two-dimentional, spin-$s$ quantum Heisenberg ferromagnet. A set of recursion equations is derived in an one-loop approximation. The low-temperature asymptotics of the correlation length and the uniform susceptibility are obtained. For small spins ($s= 1/2,1$) the results are essentially different from those in the spin-wave theory.",9603123v1 1996-04-18,Ferromagnetism in an itinerant electron system: Hubbard model on complete graph,"In this work, the ground states of the Hubbard model on complete graph are studied, for a finite lattice size $L$ and arbitrary on-site energy $U$. We construct explicitly the ground states of the system when the number of the electrons $N_e \ge L+1$. In particular, for $N_e=L+1$, the ground state is ferromagnetic with total spin $s_g=(N_e-2)/2$.",9604121v1 1996-05-01,Circular phase of a two dimensional ferromagnet with dipolar interactions,"We make for the first time a large-scale Monte-Carlo simulation of a ferromagnetic Heisenberg model with dipolar interactions on a two dimensional square lattice with open boundaries using an efficient new technique. We find that a phase transition occurs in the model and the ordered phase is characterized by a circular arrangement of the spins.",9604190v1 1996-07-30,Magnetization and overlap distributions of the ferromagnetic Ising model on the Cayley tree,"We analyze the magnetization and the overlap distributions on the ferromagnetic Cayley tree. Two quantities are investigated: the asymptotic scaling of all the moments of the magnetization and overlap distributions, as well as the computation of the fractal dimension of the magnetization and overlap probability measures.",9607215v1 1996-09-20,Variationnal study of ferromagnetism in the t1-t2 Hubbard chain,"A one-dimensional Hubbard model with nearest and (negative) next-nearest neighbour hopping is studied variationally. This allows to exclude saturated ferromagnetism for $U < U_c$. The variational boundary $U_c (n)$ has a minimum at a ``critical density'' $n_c$ and diverges for $n \rightarrow 1$.",9609198v1 1996-11-05,The Generalized Gutzwiller Method for n=>2 Correlated Orbitals: Itinerant Ferromagnetism in eg-bands,"Using the generalized Gutzwiller method we present results on the ferromagnetic behavior of extended Hubbard models with two degenerate eg orbitals. We find significant differences to results obtained from Hartree-Fock theory.",9611031v1 1997-03-04,Dissipative Dynamics of Solitons in Planar Ferromagnets,"Dynamics of magnetic bubbles in planar ferromagnets described by the Landau-Lifshitz equation with dissipation is analyzed. The pure O(3) sigma model has static multisoliton solutions, characterized by a number of parameters. The parameters describe a finite dimensional manifold. A small perturbation of energy functional with respect to the sigma model forces solitons to move. Multisoliton dynamics is effectively reduced to a flow in the parameter space.",9703045v1 1997-10-06,Multi-Magnon Scattering in the Ferromagnetic XXX-Model with Inhomogeneities,"We determine the transition amplitude for multi-magnon scattering induced through an inhomogeneous distribution of the coupling constant in the ferromagnetic XXX-model. The two and three particle amplitudes are explicitely calculated at small momenta. This suggests a rather plausible conjecture also for a formula of the general n-particle amplitude.",9710055v1 1997-10-15,Theoretical study of the Josephson current through a junction including ferromagnetic insulator,"The properties of Josephson junction including a ferromagnetic insulating layer are studied theoretically. The calculated current shows the anomalous dependence on the magnitude of the exchange interaction at the interface. The Josephson junction changes from 0-junction to $\pi$-junction with the increase of the magnitude of the exchange interaction.",9710149v1 1997-10-28,Field-induced quasi-particle coherence effects in certain small magnets,"Small ferromagnets and anti-ferromagnets with an easy-plane anisotropy have a ground to first excited state (tunnel) splitting which is quasi-periodic in the magnitude of a field applied perpendicular to a principal anisotropy axis. The associated oscillations in thermodynamic quantities might be used to prove the existence of a coherent ground state even when the tunnel splitting itself cannot be directly detected.",9710302v1 1998-05-15,Gauge Equivalence between Two-Dimensional Heisenberg Ferromagnets with Single-Site Anisotropy and Zakharov Equations,"Gauge equivalence between the two-dimensional continuous classical Heisenberg ferromagnets(CCHF) of spin 1/2 - the M-I equation with single site anisotropy and the Zakharov equation(ZE) is established for the easy axis case. The anisotropic CCHF is shown to be gauge equivalent to the isotropic CCHF.",9805188v1 1998-10-09,Magnetic Properties of Quantum Ferrimagnetic Spin Chains,"Magnetic susceptibilities of spin-$(S,s)$ ferrimagnetic Heisenberg chains are numerically investigated. It is argued how the ferromagnetic and antiferromagnetic features of quantum ferrimagnets are exhibited as functions of $(S,s)$. Spin-$(S,s)$ ferrimagnetic chains behave like combinations of spin-$(S-s)$ ferromagnetic and spin-$(2s)$ antiferromagnetic chains provided $S=2s$.",9810112v1 1998-10-21,Competition of Ferromagnetic and Antiferromagnetic Order in the Spin-1/2 XXZ Chain at Finite Temperature,"An analytical study is presented of the crossover in the gapless attractive XXZ chain from antiferromagnetic to ferromagnetic behaviour at low to high temperature, respectively. In particular, an analytic formula for the crossover in the long distance asymptotics and explicit results for the nearest-neighbour longitudinal correlation are obtained. We also provide results for the specific heat and magnetic susceptibility for various anisotropies.",9810278v1 1998-11-10,Coupling between electron-spin and ferromagnetic clusters,"We analyze the adiabatic magnetization of ferromagnetic clusters in an intermediate coupling regime, where the anisotropic potential is comparable to other energy scales. We find a non-monotonic behavior of the magnetic susceptibility as a function of coupling with a peak. Coriolis coupling effects are calculated for the first time; they reduce the susceptibility somewhat.",9811126v1 1999-03-09,Skyrmion dynamics in quantum Hall ferromagnets,"Exploring a classical solution of the non-linear sigma model for a quantum Hall ferromagnet, a skyrmion-magnon effective hamiltonian is obtained via the collective coordinates method. Using the Feynman-Vernon functional integral formalism for this model we find the temperature dependent transport coefficients which characterize a single skyrmion dynamics.",9903143v1 1999-04-22,Orbital Dynamics: The Origin of Anomalous Magnon Softening in Ferromagnetic Manganites,"We study the renormalization of magnons by charge and coupled orbital-lattice fluctuations in colossal magnetoresistance compounds. The model considered is an orbitally degenerate double-exchange system coupled to Jahn-Teller active phonons. The modulation of ferromagnetic bonds by low-energy orbital fluctuations is identified as the main origin of the unusual softening of the zone-boundary magnons observed experimentally in manganites.",9904316v2 1999-04-27,Finite temperature phase transition in the two-dimension Randomly Coupled Ferromagnet,"We show using extensive simulation results and physical arguments that an Ising system on a two dimensional square lattice, having interactions of random sign between first neighbors and ferromagnetic interactions between second neighbors, presents a phase transition at a non-zero temperature.",9904397v1 1999-10-03,Hysteretic Response of an Anti-Ferromagnetic Random-Field Ising Model in One Dimension at Zero Temperature,"We consider the hysteretic response of a one-dimensional anti-ferromagnetic random-field Ising model at zero temperature for a uniform bounded distribution of quenched random fields, and present analytic results in a limited range of the applied field.",9910021v1 1999-11-11,Ferromagnetic correlations in quasi-one-dimensional conducting channels,"We propose a model which explains the experimental observation of spontaneous spin polarization of conducting electrons in quasi- one-dimensional GaAs/AlGaAs channels. We show that a ferromagnetic order is a generic property of a quasi-one-dimensional conducting channel embedded in a Wigner crystal. We also discuss gate voltage, magnetic field and temperature dependences of the channel's conductance.",9911175v1 2000-04-25,Multiscaling at ferromagnetic-spin glass transition point of Random Energy Model and complexity,"We calculate moments of free energy's finite size correction for the transition point between ferromagnetic and spin glass phases. We find, that those moments scale with the number of spins with different critical indices, characteristic for the multiscaling. This critical point corresponds to threshold of errorless coding for a gaussian noisy channel. We are give the definition of statistical complexity using this free energy approach.",0004406v1 2000-05-24,Bloch Walls and Macroscopic String States in Bethe's solution of the Heisenberg Ferromagnetic Linear Chain,"We present a calculation of the lowest excited states of the Heisenberg ferromagnet in 1-d for any wave vector. These turn out to be string solutions of Bethe's equations with a macroscopic number of particles in them. These are identified as generalized quantum Bloch wall states, and a simple physical picture provided for the same.",0005397v1 2000-05-25,Quantum critical effects on transition temperature of magnetically mediated p-wave superconductivity,"We determine the behavior of the critical temperature of magnetically mediated p-wave superconductivity near a ferromagnetic quantum critical point in three dimensions, distinguishing universal and non-universal aspects of the result. We find that the transition temperature is non-zero at the critical point, raising the possibility of superconductivity in the ferromagnetic phase.",0005449v2 2000-08-17,Theory of Ferromagnetic Superconductivity,"It is argued that the pairing symmetry realized in a ferromagnetic superconductor UGe$_2$ must be a non-unitary triplet pairing. This particular state is free from the Pauli limitation and can survive under a huge internal molecular filed. To check our identification we examine its basic properties and several experiments are proposed. In particular, the external field is used to raise $T_c$ by controlling the internal spontaneous dipole field.",0008245v1 2000-09-28,"Reply to Comment of Yang, Sun, and Chang on ""Theory of Diluted Magnetic Semiconductor Ferromagnetism""","We reply to the comment cond-mat/0008098 by Yang, Sun, and Chang on our paper ""Theory of Diluted Magnetic Semiconductor Ferromagnetism"", Phys. Rev. Lett. 84, 5628 (2000).",0009453v1 2000-11-29,Dissipationless Spin Transport in Thin Film Ferromagnets,"Metallic thin film ferromagnets generically possess spiral states that carry dissipationless spin currents. We relate the critical values of these supercurrents to micromagnetic material parameters, identify the circumstances under which the supercurrents will be most robust, and propose experiments which could reveal this new collective transport behavior.",0011504v2 2001-01-25,Specific heat of a Fermi system near ferromagnetic quantum phase transition,"We calculate the specific heat for an interacting Fermi system near the ferromagnetic phase transition using the Renormalization Group method. The temperature dependence of the specific heat present for dimension D=3 a logarithmic dependence which shows that the fermionic excitations reaches a non - Fermi behavior. The result is in good agreement with the experimental data obtained recently for $Ni_x Pd_{1-x}$ alloys.",0101392v1 2001-02-06,"Comment on ""Ferromagnetic film on a superconducting substrate""","A superconducting substrate is not able to shrink drastically domains in a ferromagnetic film, contrary to the prediction of Bulaevskii and Chudnovsky [Phys. Rev. B, 63, issue1 (2001)]. This is shown on the basis of the exact solution for the stripe domain structure.",0102102v2 2001-02-13,Resonant electron transmission through a finite quantum spin chain,"Electron transport in a finite one dimensional quantum spin chain (with ferromagnetic exchange) is studied within an $s-d$ exchange Hamiltonian. Spin transfer coefficients strongly depend on the sign of the $s-d$ exchange constant. For a ferromagnetic coupling, they exhibit a novel resonant pattern, reflecting the salient features of the combined electron-spin system. Spin-flip processes are inelastic and feasible at finite voltage or at finite temperature.",0102236v1 2001-03-22,Thermally activated interface motion in a disordered ferromagnet,"We investigate interface motion in disordered ferromagnets by means of Monte Carlo simulations. For small temperatures and driving fields a so-called creep regime is found and the interface velocity obeys an Arrhenius law. We analyze the corresponding energy barrier as well as the field and temperature dependence of the prefactor.",0103464v1 2001-04-13,Dipolar induced anisotropy in the random-field Ising model,"The crystalline ferromagnet with dipole-dipole interactions is studied within the framework of the three-dimensional random-field Ising model. The field distribution function and the analytic expression for temperature dependent free energy are found. The dipolar induced magnetic anisotropy energy of bulk cubic ferromagnetic materials is calculated.",0104246v1 2001-07-12,Theory of electron spectroscopies,"The basic theory of photoemission, inverse photoemission, Auger-electron and appearance-potential spectroscopy is developed within a unified framework starting from Fermi's golden rule. The spin-resolved and temperature-dependent appearance-potential spectroscopy of band-ferromagnetic transition metals is studied in detail. It is shown that the consideration of electron correlations and orbitally resolved transition-matrix elements is essential for a quantitative agreement with experiments for Ni.",0107257v1 2001-08-21,Orbital ferromagnetism and anomalous Hall effect in antiferromagnets on distorted fcc lattice,"The Berry phase due to the spin wavefunction gives rise to the orbital ferromagnetism and anomalous Hall effect in the non-coplanar antiferromagnetic ordered state on face centered cubic (fcc) lattice once the crystal is distorted perpendicular to (1,1,1) or (1,1,0)- plane. The relevance to the real systems $\gamma$-FeMn and NiS$_2$ is also discussed.",0108322v1 2002-02-05,Optical Conductivity of Ferromagnetic Semiconductors,"The dynamical mean field method is used to calculate the frequency and temperature dependent conductivity of dilute magnetic semiconductors. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism.",0202071v1 2002-03-07,A new spinwave expansion for the ordered Kondo lattice,"We present a concise introduction to a new spinwave expansion scheme for magnetically ordered Kondo lattice models. This is motivated by consideration of the ferromagnetically ordered phase of the ""double exchange"" system La(1-x)Ca(x)MnO3. A brief overview is given of the consequences of quantum and thermal fluctuation effects for the magnetic properties of the double exchange ferromagnet.",0203156v1 2002-03-07,Segmented Band Mechanism for Itinerant Ferromagnetism,"We introduce a novel mechanism for itinerant ferromagnetism, which is based on a simple two-band model, and using numerical and analytical methods, we show that the Periodic Anderson Model (PAM) contains this mechanism. We propose that the mechanism, which does not assume an intra-atomic Hund's coupling, is present in both the iron group and some $f$ electron compounds.",0203172v1 2002-03-12,Mermin-Ho vortex in ferromagnetic spinor Bose-Einstein condensates,"The Mermin-Ho and Anderson-Toulouse coreless non-singular vortices are demonstrated to be thermodynamically stable in ferromagnetic spinor Bose-Einstein condensates with the hyperfine state F=1. The phase diagram is established in a plane of the rotation drive vs the total magnetization by comparing the energies for other competing non-axis-symmetric or singular vortices. Their stability is also checked by evaluating collective modes.",0203242v1 2002-03-20,Oscillations of the superconducting critical current in Nb-Cu-Ni-Cu-Nb junctions,"We report on experimental studies of superconductor-ferromagnet layered structures. Strong oscillations of the critical supercurrent were observed with the thickness variation of the ferromagnet. Using known microscopic parameters of Ni, we found reasonable agreement between the period of oscillations and the decay of the measured critical current, and theoretical calculations.",0203408v1 2002-07-05,Gap nodes in the superconducting phase of the itinerant ferromagnet $UGe_2$,"For the UGe$_2$ ferromagnetic superconductor the forms of the order parameter admitted by the crystal symmetry within the strong spin-orbit coupling scheme are written down. For each of the two possible phases existence of gap nodes required by symmetry is discussed and the nodes are found. Some consequences of presence of the nodes, which may be useful for experimental identification of the phases are discussed as well.",0207152v1 2002-08-20,Monte Carlo Study of an Extended 3-State Potts Model on the Triangular Lattice,"By introducing a chiral term into the Hamiltonian of the 3-state Potts model on a triangular lattice additional symmetries are achieved between the clockwise and anticlockwise states and the ferromagnetic state. This model is investigated using Monte Carlo methods. We investigate the full phase diagram and find evidence for a line tricritical points separating the ferromagnetic and antiferromagnetic phases.",0208387v1 2002-08-21,Re-entrant spin glass and magnetoresistance in Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide,"We have investigated the static and dynamic response of magnetic clusters in Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide, where a sequence of magnetic phase transitions, i.e., paramagnetic (PM) to ferromagnetic at T_{C} $\leq$ 270K and ferromagnetic to canted spin glass state at T_f$ $\leq$ 125K is observed.",0208416v1 2002-11-17,Ferromagnetic transition in a double-exchange model,"We calculate the temperature of a ferromagnetic transition in a double-exchange model with classical core spins for arbitrary relation between Hund exchange coupling and electron band width by solving the Dynamical Mean Field Approximation equations.",0211354v3 2002-12-20,Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C,"This article describes a model in which ferromagnetism necessarily accompanies a spin-density-wave lockin transition in the borocarbide structure provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even and n odd. The results account for the magnetic properties of TbNi2B2C, and are also possibly relevant also for those of ErNi2B2C.",0212535v1 2002-12-27,On the Competition between Ferromagnetic and Antiferromagnetic States in Sr$_2$MnMoO$_6,"It is argued that the magnetic behavior of Sr$_2$MnMoO$_6$ is determined by the existence of two total energy minima corresponding to the metallic ferromagnetic and insulating antiferromagnetic states, which may be nearly degenerate depending on the magnitude of the breathing distortion.",0212607v1 2003-04-14,On the Ising-Heisenberg model with the doubly decorated network structure I,"Using an exact mapping transformation method, magnetic properties of a spin-1/2 and spin-1 doubly decorated Ising-Heisenberg model are investigated in detail. By assuming that both interaction parameters are ferromagnetic, we found in addition to an usual ferromagnetic phase another two phases with a nontrivial long-range order.",0304310v1 2003-07-06,Equilibrium current temperature quasi-oscillations in a ferromagnetic loop,"Equilibrium persistent current carried by a small ferromagnet-metal loop is considered. This current is shown to be quasi periodic in temperature at low temperatures. The quasi period is determined mainly by the temperature dependence of the magnetization.",0307128v1 2003-07-18,The Heisenberg antiferromagnet: an explicitly rotational invariant formulation,"A simple derivation of an explicitly rotational invariant Lagrangian describing the dynamics of an antiferromagnetic spin system is presented. The scope of the derived Lagrangian is analysed in the context of schematic models. It is shown that the Lagrangian describes the behaviour of spin systems from the anti-ferromagnetic to the ferromagnetic regimes.",0307461v1 2003-07-22,Ferromagnetism and triplet superconductivity in the two-dimensional Hubbard model,"We review magnetic and superconducting instabilities in the t-t' Hubbard model on the two-dimensional square lattice as obtained with functional one-loop renormalization group techniques. Special emphasis is put on ferromagnetic and triplet superconducting tendencies that could be relevant to the triplet superconductor Sr2RuO4.",0307541v1 2003-07-23,Spin order manipulations in nanostructures of II-VI ferromagnetic semiconductors,"An overview of recent studies on ferromagnetism in Cr- and Mn-based II-VI diluted magnetic semiconductors is presented emphasizing differences in underlying exchange mechanisms. Examples of manipulations with spin ordering by carrier density, dimensionality, light, and electric field are given.",0307573v1 2003-08-31,Theory of Striped Hall Ferromagnets,"We study spin and charge striped states at the half-filled high Landau level in the zero Zeeman energy limit using a Hartree-Fock approximation. It is shown that a ferromagnetic striped Hall state is more stable than the antiferromagnetic striped state or charge striped state. We calculate the collective excitations using the single mode approximation.",0309013v1 2003-10-06,Breakup of a Stoner model for the 2D ferromagnetic quantum critical point,"Re-interpretation of the results by [A. V. Chubukov et. al., Phys. Rev. Lett. 90, 077002 (2003)] leads to the conclusion that ferromagnetic quantum critical point (FQCP) cannot be described by a Stoner model because of a strong interplay between the paramagnetic fluctuations and the Cooper channel, at least in two dimensions.",0310124v1 2003-12-10,Spin Gain Transistor in Ferromagnetic Semiconductors: the Semiconductor Bloch Equations Approach,"Scheme and principle of operation of a spin gain transistor are proposed: a large unmagnetized current creates density sufficient for the ferromagnetic transition; a small magnetized current initiates spontaneous magnetization; large magnetized current is extracted. Therefore spin gain of more than 1000 is predicted. Collective dynamics of spins under Coulomb exchange interaction is described via Semiconductor Bloch Equations.",0312243v1 2004-04-07,Absence of room temperature ferromagnetism in bulk Mn-doped ZnO,"Structural and magnetic properties have been studied for polycrystalline Zn_1-xMn_xO (x=0.02, 0.03, 0.05). Low-temperature (~500 oC) synthesis leaves unreacted starting ZnO and manganese oxides. Contrary to a recent report, no bulk ferromagnetism was observed for single-phase materials synthesized in air at temperatures above 900 oC. Single-phase samples show paramagnetic Curie-Weiss behavior.",0404186v1 2004-06-30,Hysteresis in Ferromagnetic Random Field Ising Model with an Arbitrary Initial State,"We present exact expressions for hysteresis loops in the ferromagnetic random field Ising model in the limit of zero temperature and zero driving frequency for an arbitrary initial state of the model on a Bethe lattice. This work extends earlier results that were restricted to an initial state with all spins pointing parallel to each other.",0406749v1 2004-07-05,Boosting the Curie temperature with correlations in diluted magnetic semiconductors,"We present a quantitative theory for the effects of correlated doping on the ferromagnetism of diluted magnetic semiconductors. It predicts that room temperature ferromagnetism should be possible in homogeneous, but correlated, samples of Mn$_x$Ga$_{1-x}$As. The theory predicts lower critical temperatures for Mn$_x$Ga$_{1-x}$N.",0407101v1 2005-08-09,Geometry and the anomalous Hall effet in ferromagnets,"The geometric ideas underlying the Berry phase and the modern viewpoint of Karplus and Luttinger's theory of the anomalous Hall effect are discussed in an elementary way. We briefly review recent Hall and Nernst experiments which support the dominant role of the KL velocity term in ferromagnets.",0508236v1 2005-12-10,Breaking of Chiral Symmetry and Spontaneous Rotation in a Spinor Bose-Einstein Condensate,"We show that a spin-1 Bose-Einstein condensate with ferromagnetic interactions spontaneously generates a topological spin texture, in which the m = \pm 1 components of the magnetic sublevels form vortices with opposite circulations. This phenomenon originates from an interplay between ferromagnetic interactions and spin conservation.",0512216v1 2006-03-08,Electronic Structure of Nearly Ferromagnetic compound HfZn$_{2}$,"The electronic structure of HfZn$_{2}$ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that HfZn$_{2}$ shows ferromagnetic instability. Large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.",0603203v1 2006-03-11,Electronic Structure and Magnetic Properties of $β$-Ti$_{6}$Sn$_{5}$,"The electronic structure of $\beta$-Ti$_{6}$Sn$_{5}$ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that $\beta$-Ti$_{6}$Sn$_{5}$ is very close to ferromagnetic instability and shows ferromagnetic ordering after rare earth element doping. Large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.",0603313v1 2006-04-21,Dual-frequency ferromagnetic resonance,"We describe a new experimental technique to investigate coupling effects between different layers or modes in ferromagnetic resonance (FMR). Dual FMR frequencies are excited (2-8 GHz) simultaneously and detected selectively in a broadband RF circuit, using lock-in amplifier detection at separate modulation frequencies.",0604519v1 2006-07-28,Anisotropic thermal expansion and uniaxial pressure dependence of superconducting and magnetic transitions in ErNi2B2C,"We present anisotropic thermal expansion measurements on single crystalline ErNi2B2C. All three, superconducting, antiferromagnetic and weak ferromagnetic phase transitions are unambiguously distinguished in the data. Anisotropic uniaxial pressure dependencies of the transitions are estimated based on the Ehrenfest relation, leading to a conclusion, in particular, that weak ferromagnetic state will be suppressed by small, order of several kbar, hydrostatic pressure.",0607762v1 2006-09-20,Ferromagnetism and Superconductivity in Carbon-Based Systems,"In this article we shortly review previous and recently published experimental results that provide evidence for intrinsic, magnetic-impurity-free ferromagnetism and for high-temperature superconductivity in carbon-based materials. The available data suggest that the origin of those phenomena is related to structural disorder and the presence of light elements like hydrogen, oxygen and/or sulfur.",0609497v1 2006-09-29,Negative four-terminal resistance as a probe of crossed Andreev reflection,"We report on the experimental investigation of electronic transport in superconductor-ferromagnet spin-valve structures. Our samples consist of two ferromagnetic iron leads forming planar tunnel contacts to a superconducting aluminum wire. At energies below the superconducting gap, we observe a negative four-probe resistance that can be explained by crossed Andreev reflection.",0609766v1 2006-11-27,Formation of ferromagnetic bulk amorphous Fe40Ni40P14B6 alloys,"Ferromagnetic bulk amorphous Fe40Ni40P14B6 alloy rods with a diameter of 1.2 mm can be prepared by means of a rapid quenching technique. If a fluxing technique is also used, amorphous rods with a diameter as large as 2.5 mm can be synthesized. The critical cooling rate Rc for the glass formation Fe40Ni40P14B6 is estimated to be on the order of 100 K.s-1",0611661v1 2007-01-18,The Berry phase in ferromagnetic spin systems and anomalous Hall Effect,"We have shown that the study of topological aspects of the underlying geometry in a ferromagnetic spin system gives rise to an intrinsic Berry phase. This real space Berry phase arises due to the spin rotations of conducting electrons which can be manifested as a further contribution in anomalous Hall effect.",0701440v1 2007-03-11,Dynamic magnetic response of infinite arrays of ferromagnetic particles,"Recently developed techniques to find the eigenmodes of a ferromagnetic particle of arbitrary shape, as well as the absorption in the presence of an inhomogeneous radio-frequency field, are extended to treat infinite lattices of such particles. The method is applied to analyze the results of recent FMR experiments, and yields substantially good agreement between theory and experiment.",0703273v1 2003-12-26,Possibility of color magnetic superconductivity,"Two aspects of quark matter at high density are addressed: one is color superconductivity and the other is ferromagnetism. We are mainly concerned with the latter and its relation to color superconductivity, which we call ""color magnetic superconductivity"". The relation of ferromagnetism and chiral symmetry restoration is also discussed.",0312351v1 1992-10-28,Thermodynamics of the 3-State Potts Spin Chain,"We demonstrate the relation of the infrared anomaly of conformal field theory with entropy considerations of finite temperature thermodynamics for the 3-state Potts chain. We compute the free energy and compute the low temperature specific heat for both the ferromagnetic and anti-ferromagnetic spin chains, and find the central charges for both.",9210146v2 1996-12-13,The Field Theory of Generalized Ferromagnet on the Hermitian Symmetric Spaces,"We discuss the recent developments in the generalized continuous Heisenberg ferromagnet model formulated as a nonrelativistic field theory defined on the target space of the coadjoint orbits. Hermitian symmetric spaces are special because they provide completely integrable field theories in 1+1 dimension and self-dual Chern-Simons solitons and vortices in 2+1 dimension.",9612132v1 2000-11-23,"Hopf instantons, Chern-Simons vortices, and Heisenberg ferromagnets","The dimensional reduction of the three-dimensional fermion-Chern-Simons model (related to Hopf maps) of Adam et el. is shown to be equivalent to (i) either the static, fixed--chirality sector of our non-relativistic spinor-Chern-Simons model in 2+1 dimensions, (ii) or a particular Heisenberg ferromagnet in the plane.",0011210v1 2003-03-05,Differential geometry of surfaces and Heisenberg ferromagnets,"The relation between differential geometry of surfaces and some Heisenberg ferromagnet models is considered.",0303056v2 2003-04-23,Rigorous Non-Perturbative Ornstein-Zernike Theory for Ising Ferromagnets,"We rigorously derive the Ornstein-Zernike asymptotics of the pair-correlation functions for finite-range Ising ferromagnets in any dimensions and at any temperature above critical.",0304349v1 2006-07-20,Stability for Walls in Ferromagnetic Nanowire,"We study the stability of travelling wall profiles for a one dimensional model of ferromagnetic nanowire submitted to an exterior magnetic field. We prove that these profiles are asymptotically stable modulo a translation-rotation for small applied magnetic fields.",0607499v1 1999-03-01,Mathematical Structure of Magnons in Quantum Ferromagnets,"We provide the mathematical structure and a simple, transparent and rigorous derivation of the magnons as elementary quasi-particle excitations at low temperatures and in the infinite spin limit for a large class of Heisenberg ferromagnets. The magnon canonical variables are obtained as fluctuation operators in the infinite spin limit. Their quantum character is governed by the size of the magnetization.",9903004v1 2000-12-28,On the Ground State of Ferromagnetic Hamiltonians,"It is generally believed that the ground state of the ferromagnetic Heisenberg-Dirac-Van Vleck Hamiltonians acting on s=1/2 spins of a lattice with N sites has the maximal possible value of the total spin S=N/2 and is N+1 times degenerate. We present a rigorous proof of this statement, independent of the lattice dimension and topology.",0012048v1 2002-10-08,Ferromagnetic Antenna and its Application to Generation and Detection of Gravitational Radiation,"A new type of antenna, which we have called Ferromagnetic Antenna, has been considered for Generation and Detection of Gravitational Radiation. A simple experiment, in which gravitational radiation at 10 GHz can be emitted and received in laboratory, is presented.",0210034v4 1997-10-07,Shock waves in one-dimensional Heisenberg ferromagnets,"We use SU(2) coherent state path integral formulation with the stationary phase approximation to investigate, both analytically and numerically, the existence of shock waves in the one- dimensional Heisenberg ferromagnets with anisotropic exchange interaction. As a result we show the existence of shock waves of two types,""bright"" and ""dark"", which can be interpreted as moving magnetic domains.",9710003v1 2007-08-10,Prediction for new magnetoelectric fluorides,"We use symmetry considerations in order to predict new magnetoelectric fluorides. In addition to these magnetoelectric properties, we discuss among these fluorides the ones susceptible to present multiferroic properties. We emphasize that several materials present ferromagnetic properties. This ferromagnetism should enhance the interplay between magnetic and dielectric properties in these materials.",0708.1475v1 2007-10-12,Dynamical instability of the XY spiral state of ferromagnetic condensates,"We calculate the spectrum of collective excitations of the XY spiral state prepared adiabatically or suddenly from a uniform ferromagnetic F=1 condensate. For spiral wavevectors past a critical value, spin wave excitation energies become imaginary indicating a dynamical instability. We construct phase diagrams as functions of spiral wavevector and quadratic Zeeman energy.",0710.2499v1 2008-06-20,Penetration Depth of Transverse Spin Current in Ferromagnetic Metals,"The line width of the ferromagnetic resonance (FMR) spectrum of Cu/CoFeB/Cu/Co/Cu is studied. Analyzing the FMR spectrum by the theory of spin pumping, we determined the penetration depth of the transverse spin current in the Co layer. The obtained penetration depth of Co is 1.7 nm.",0806.3315v1 2008-08-31,Magnetic Monopoles in Ferromagnetic Spin-Triplet Superconductors,"Using the $\phi$-mapping method, we argue that ferromagnetic spin-triplet superconductors allow formation of unstable magnetic monopoles. In particular, we show that the limit points and the bifurcation points of the $\phi$-mapping will serve as the interaction points of these magnetic monopoles.",0809.0120v1 2008-10-21,"Coherent manipulation of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As with successive optical pumping","We report dynamic control of magnetization precession by light alone. A ferromagnetic (Ga,Mn)As epilayer was used for experiments. Amplitude of precession was modulated to a large extent by tuning the time interval between two successive optical pump pulses which induced torques on magnetization through a non-thermal process. Nonlinear effect in precession motion was also discussed.",0810.3728v1 2009-05-23,Influence of a Transport Current on Magnetic Anisotropy in Gyrotropic Ferromagnets,"Current-induced torques are commonly used to manipulate non-collinear magnetization configurations. In this article we discuss current-induced torques present in a certain class of collinear magnetic systems, relating them to current-induced changes in magnetic anisotropy energy. We present a quantitative estimate of their characteristics in uniform strained ferromagnetic (Ga,Mn)As.",0905.3856v1 2009-11-02,Dynamical Toroidal Hopfions in a Ferromagnet with Easy-Axis Anisotropy,"Three-dimensional toroidal precession solitons with a nonzero Hopf index, which uniformly move along the anisotropy axis in a uniaxial ferromagnet, have been found. The structure and existence region of the solitons have been numerically determined by solving the Landau-Lifshitz equation.",0911.0451v1 2010-01-19,The Pairing Symmetry in the Ferromagnetic Superconductor $UGe_{2}$,"We give an extensive treatment of the pairing symmetry in the ferromagnetic superconductor $UGe_{2}$. We show that one can draw important conclusions concerning the superconducting state, considering only the transformation properties of the pairing function, without assumptions about the form of the pairing amplitudes.",1001.3309v1 2010-07-28,Route to ferromagnetism in organic polymers,"Employing a rigorous theoretical method for the construction of exact many-electron ground states we prove that interactions can be employed to tune a bare dispersive band structure such that it develops a flat band. Thereby we show that pentagon chain polymers with electron densities above half filling may be designed to become ferromagnetic or half metallic.",1007.4994v1 2010-08-16,Possible kinetic arrest of the ferromagnetic to anti-ferromagnetic transition in SmCoAsO: The interplay of Sm4f and Co3d spins,The article is withdrawn and its revised version will be submitted later.,1008.2593v3 2010-08-24,Electronic and magnetic properties of the graphene-ferromagnet interface,"The article presents the work on the investigation of the surface structure as well as electronic and magnetic properties of graphene layer on a lattice matched surface of a ferromagnetic material, Ni(111).",1008.3975v1 2011-03-11,Scattering of quasiparticle of spin-triplet pairs in diffusive superconductor-ferromagnetic nanowire-superconductor junction,"We analyze the proximity effect of superconductor/ferromagnet nanostructures and point out that the scattering of quasiparticle of spin-triplet pairs by local magnetic moments leads to large resistance peak slightly below the superconducting transition temperature of the superconductor. Both the temperature and the magnitude of the resistance peak decrease with increasing magnetic field, which agree quite well with the experimental results reported in [Nature Phys. 6, 389 (2010)].",1103.2176v1 2011-05-16,Anisotropic thermoelectric effect in helimagnetic tunnel junctions,"Thermoelectric transport across normal-metal/helical-multiferroic/ferromagnetic heterojunctions is theoretically investigated. We find a anisotropic charge and spin thermopower with a $C_{2v}$ symmetry. The angular dependence on the magnetization orientation of the ferromagnetic layer is substantiated by a phenomenological theory based on the symmetry of the effective spin-orbit interaction induced by the topology of the spiral magnetic order in the multiferroic barrier.",1105.3075v1 2012-11-07,The ferromagnetic order in graphene,"The conditions for spontaneous magnetization in a single graphene sheet are discussed in the context of Mermin-Wagner theorem. It is rigorously proved that at any nonzero temperature the graphene monolayer is nonmagnetic as long as its intrinsic symmetry is preserved. Any electronic fluctuations breaking the pseudospin conservation law can be responsible for the existence of ferromagnetic order inside the graphene structure.",1211.1508v1 2013-02-04,Andreev experiments on superconductor/ferromagnet point contacts,"Andreev reflection is a smart tool to investigate the spin polarisation P of the current through point contacts between a superconductor and a ferromagnet. We compare different models to extract P from experimental data and investigate the dependence of P on different contact parameters.",1302.0660v1 2013-04-15,Enhancement of the superconducting transition temperature of MgB2 by proximity effect of d0 ferromagnet,"Heterostructures of superconducting and ferromagnetic materials are of fundamental interest because of the mutual interaction of antagonistic kinds of ordering at the S-F interface. Normally, the superconducting transition temperature Tc should be strongly suppressed at the S-F interface owing to the penetration of Cooper pairs into the ferromagnetic side. Nevertheless, constructive interactions between S and F orders have been suggested to occur via the modification of ferromagnetic order by the superconducting state. This may induce an inhomogeneous magnetic state, often called a cryptoferromagnetic state, and the relevant domain wall effect, which will lead to a local decrease of the pair-breaking parameter. However, the domain wall effect, even if it exists, is quite subtle from the experimental view point and is normally difficult to observe. Here we show that the defect-related d0 ferromagnetism in MgO and the superconductivity in MgB2 do not antagonize, but rather enhance the superconducting transition temperature Tc to any significant degree. We found in superconducting MgB2-d0 ferromagnetic MgO composites that the superconducting transition proceeds in two steps. The first at the S-F interface, between 110-120 K, then in the rest of the bulk at 39 K, which is the Tc of single phase MgB2 superconductor. Moreover, the additional transition emerges at 60 K at the S-F interface especially in the ferromagnetic side, showing a spin-glass-like magnetic state. Our findings reveal that the proximity effect in the superconductor-d0 ferromagnet heterostructures will provide the knowledge and basis to enhance the Tc value of the existing superconductors.",1304.4105v3 2013-05-09,"Itinerant origin of the ferromagnetic quantum critical point in Fe(Ga,Ge)3","The electronic structure and magnetic properties of FeGa3 and doped FeGa3 are studied using density functional calculations. An itinerant mechanism for ferromagnetism is found both for n-type doping with Ge and also for p-type doping. Boltzmann transport calculations of the thermopower are also reported.",1305.2063v1 2013-10-27,Fundamental limitations of half-metallicicity in spintronic materials,"Zero-point spin fluctuations are shown to strongly influence the ground state of ferromagnetic metals and to impose limitations for the fully spin polarized state assumed in half-metallic ferromagnets, which may influence their applications in spintronics. This phenomenon leads to the low-frequency Stoner excitations and cause strong damping and softening of magnons in magnetoresistive manganites observed experimentally.",1310.7174v1 2014-02-14,Extremely high-resolution measurements of microwave magnetisation dynamics in magnetic thin films and nanostructures,"In this work we discuss the use of interferometric measurement technique to study microwave magnetization dynamics on ferromagnetic nanostructures. We demonstrate that in this way one can resolve features which are impossible to resolve with broadband ferromagnetic resonance and traveling spin wave spectroscopy otherwise.",1402.3459v1 2015-03-02,Andreev Reflection Spectroscopy in Transition Metal Oxides,"Here we review the literature concerning measurement of the Andreev reflection between a superconductor (S) and ferromagnet (F), with particular attention to the case where the ferromagnet is a transition metal oxide. We discuss the practicality of utilisation of the current models for determination of the transport current spin polarisation and examine the evidence for Andreev bound states.",1503.00533v1 2015-04-27,Spontaneous Appearance of Low-dimensional Magnetic Electron System on Semiconductor Nanostructures,"We find that spin-polarized ground states emerge in nanofacets which are self-organized on SiC (0001) surfaces. Our large-scale density-functional calculations reveal that the nanofacet formed by bunching of single bilayer steps generates peculiar carbon dangling bond states localized at but extended along step edges. The flat-band characteristics of those C states cause either ferromagnetic or anti-ferromagnetic chains on covalent semiconductors.",1504.06934v1 2015-06-04,Three magnons in an isotropic $S=1$ ferromagnetic chain as an exactly solvable non-integrable system,"It is shown that a generalization of Bethe Ansatz based on an utilization of {\it degenerative discrete-diffractive} wave functions solves the three-magnon problem for the $S=1$ isotropic ferromagnetic infinite chain. The four-magnon problem is briefly discussed.",1506.01554v2 2015-10-23,Self-consistent approach to magnetic ordering and excited site occupation processes in a two-level system,"Ferromagnetic ordering in a two-level partially excited system is studied in detail. Magnitudes of magnetization (magnetic order parameter) and lattice ordering (excited level occupation number) are calculated self-consistently. The influence of an external magnetic field and excited level gap on the ferromagnetic phase transition is discussed.",1510.06907v1 2016-02-24,The Landau-Lifshitz equation of the ferromagnetic spin chain and Oseen-Frank flow,"In this paper, we consider the Landau-Lifshitz equation of the ferromagnetic spin chain from $\R^2$ to the unit sphere $S^2$ under the general Oseen-Frank energy. We obtain global existence and uniqueness of weak solutions for large energy data; moreover, the number of singular points is finite.",1602.07549v1 2016-07-20,MegaOhm Extraordinary Hall effect in oxidized CoFeB,"We report on development of controllably oxidized CoFeB ferromagnetic films demonstrating the extraordinary Hall effect (EHE) resistivity exceeding 1 Ohmcm and magnetic field sensitivity up to 10^6 Ohm/T. Such EHE resistivity is four orders of magnitude higher than previously observed in ferromagnetic materials, while sensitivity is two orders larger than the best of semiconductors",1607.05923v1 2016-09-26,Bethe Ansatz approach to the three-magnon problem in an infinite $XX$ spin chain with Izing next neighboring ferromagnetic interaction,"Using the traditional and degenerative discrete-diffractive versions of Bethe Ansatz we construct a rude set of three-magnon states in the infinite $XX$ spin chain with Izing next neighboring ferromagnetic term.",1609.07941v1 2016-10-20,Optimal design of mixtures of ferromagnetic interactions,"We provide a general framework for the optimal design of surface energies on networks. We prove sharp bounds for the homogenization of discrete systems describing mixtures of ferromagnetic interactions by constructing optimal microgeometries, and we prove a localization principle which allows to reduce to the periodic setting in the general nonperiodic case.",1610.06455v1 2018-02-10,New Reductions of a Matrix Generalized Heisenberg Ferromagnet Equation,"We present in this report 1+1 dimensional nonlinear partial differential equation integrable through inverse scattering transform. The integrable system under consideration is a pseudo-Hermitian reduction of a matrix generalization of classical 1+1 dimensional Heisenberg ferromagnet equation. We derive recursion operators and describe the integrable hierarchy related to that matrix equation.",1802.03591v1 2018-04-06,Magnon excitations and quantum critical behavior of the ferromagnet U$_4$Ru$_7$Ge$_6$,"We present an extensive study of the ferromagnetic heavy fermion compound U$_4$Ru$_7$Ge$_6$. Measurements of electrical resistivity, specific heat and magnetic properties show that U$_4$Ru$_7$Ge$_6$ orders ferromagnetically at ambient pressure with a Curie temperature $T_{C} = 6.8 \pm 0.3$ K. The low temperature magnetic behavior of this soft ferromagnet is dominated by the excitation of gapless spin-wave modes. Our results on the transport properties of U$_4$Ru$_7$Ge$_6$ under pressures up to $2.49$ GPa suggest that U$_4$Ru$_7$Ge$_6$ has a putative ferromagnetic quantum critical point (QCP) at $P_c \approx 1.7 \pm 0.02$ GPa. In the ordered phase, ferromagnetic magnons scatter the conduction electrons and give rise to a well defined power law temperature dependence in the resistivity. The coefficient of this term is related to the spin-wave stiffness and measurements of the very low temperature resistivity allow to accompany the behavior of this quantity as the the ferromagnetic QCP is approached. We find that the spin-wave stiffness decreases with increasing pressure implying that the transition to the non-magnetic Fermi liquid state is driven by the softening of the magnons. The observed quantum critical behavior of the magnetic stiffness is consistent with the influence of disorder in our system. At quantum criticality ($P = P_c \approx 1.7 \pm 0.02$ GPa), the resistivity shows the behavior expected for an itinerant metallic system near a ferromagnetic QCP.",1804.02420v1 2018-11-26,The mean-field quantum Heisenberg ferromagnet via representation theory,"We use representation theory to write a formula for the magnetisation of the quantum Heisenberg ferromagnet. The core new result is a spectral decomposition of the function $\alpha_k 2^{\alpha_1+\dotsb+\alpha_n}$ where $\alpha_k$ is the number of cycles of length k of a permutation. In the mean-field case, we simplify the formula further, arriving at a closed-form expression for the magnetisation, which allows to analyse the phase transition.",1811.10530v1 2018-11-28,Notes on Integrable Motion of Two Interacting Curves and Two-layer Generalized Heisenberg Ferromagnet Equations,"In the present paper, we study the integrable 2-layer generalized Heisenberg ferromagnet equation (HFE). The relation between this generalized HFE and differential geometry of curves is established. Using this relation we found the geometrical equivalent counterpart of the 2-layer spin system which is the 2-component KdV equation. Finally, the gauge equivalence between these equations is established.",1811.12216v1 2014-03-23,Effect of electron-phonon interactions on Raman line at ferromagnetic ordering,"The theory of Raman scattering in half-metals by optical phonons interacting with conduction electrons is developed. We evaluate the effect of electron-phonon interactions at ferromagnetic ordering in terms of the Boltzmann equation for carriers. The chemical potential is found to decrease with temperature decreasing. Both the linewidth and frequency shift exhibit a dependence on temperature.",1403.5736v1 2016-11-17,Spin breather and rogue formations in a Heisenberg ferromagnetic system,"We construct explicit spin configurations for the breather solution of a one-dimensional Heisenberg ferromagnetic spin system. This corresponds to the breather soliton solution of the gauge equivalent nonlinear Schr\""{o}dinger equation. There are three broad cases, wherein the solution shows distinct behavior. Of particular interest to our study is the rogue behavior in spin field terms.",1611.05615v1 2012-04-04,A compilation of metallic systems that show a quantum ferromagnetic transition,"We provide a compilation of metallic systems in which a low-temperature ferromagnetic or similar transition is observed. Our objective is to demonstrate the universal first-order nature of such transitions in clean systems in two or three spatial dimensions. Please contact the authors with information about omissions, corrections, or any other information.",1204.0873v1 2013-09-22,Ferromagnetism in the one-dimensional Kondo lattice: mean-field approach via Majorana fermion canonical transformation,"Using a canonical transformation it is possible to faithfully represent the Kondo lattice model in terms of Majorana fermions. Studying this representation we discovered an exact mapping between the Kondo lattice Hamiltonian and a Hamiltonian describing three spinless fermions interacting on a lattice. We investigate the effectiveness of this three fermion representation by performing a zero temperature mean-field study of the phase diagram at different couplings and fillings for the one-dimensional case, focusing on the appearance of ferromagnetism. The solutions agree in many respects with the known numerical and analytical results. In particular, in the ferromagnetic region connected to the solution at zero electron density, we have a quantitative agreement on the value of the commensurability parameter discovered in recent DMRG (in one dimension) and DMFT (in infinite dimensions) simulations; furthermore we provide a theoretical justification for it, identifying a symmetry of the Hamiltonian. This ferromagnetic phase is stabilized by the emergence of a spin-selective Kondo insulator that is described quite conveniently by the three spinless fermions. We discovered also a different ferromagnetic phase at high filling and low couplings. This phase resembles the RKKY ferromagnetic phase existing at vanishing filling, but it incorporates much more of the Kondo effect, making it energetically more favorable than the typical spiral (spin ordered) mean field ground states. We believe that this second phase represents a prototype for the strange ferromagnetic tongue identified by numerical simulations inside the paramagnetic dome. At the end of the work we also provide a discussion of possible orders different from the ferromagnetic one. In particular at half-filling, where we obtain as ground state at high coupling the correct Kondo insulating state.",1309.5573v2 2018-01-29,Antiferromagnetic rare region effect in $Pr_{0.5}Ca_{0.5}MnO_3$,"We present evidence of coexistence of electron paramagnetic resonance signal and anti-ferromagnetic resonance signals above the anti-ferromagnetic (AFM) transition ($T_N$) in $Pr_{0.5}Ca_{0.5}MnO_3$. We identify the latter with AFM rare regions within the Griffiths-like phase scenario with the associated temperature scale $T^\ast$ extending above room temperature.",1801.09437v1 2019-03-21,Dispersionless Limits of Integrable Generalized Heisenberg Ferromagnet Equations,"This paper is a continuation of our previous work in which we studied a dispersionless limits of some integrable spin systems (magnetic equations). Now, we shall present dispersionless limits of some integrable generalized Heisenberg ferromagnet equations.",1903.09195v1 2022-03-16,Flux Qubit Based on Hybrid Ferromagnetic-Superconducting Device,"We propose a realization of flux qubit based on the hybrid ferromagnetic-superconducting device where the flux bias is induced purely by vector potential of the vanishing magnetic field. We support our conclusions with theoretical analysis and self-consistent three-dimensional simulations for material specific parameters.",2203.08752v1 2022-06-12,Exchange-enhancement of the ultrafast magnetic order dynamics in antiferromagnets,"We theoretically demonstrate that the ultrafast magnetic order dynamics in antiferromagnets is exchange-enhanced in comparison to their ferromagnetic counterparts. We provide an equation of motion for the magnetic order dynamics validated by computer simulations using atomistic spin dynamics methods. The exchange of angular momentum between sublattices speeds up the dynamics in antiferromagnets, a process absent in ferromagnets.",2206.05783v1 2022-09-09,Spin parity effects in monoaxial chiral ferromagnetic chain,"We present a fully quantum mechanical account of a novel {\it spin parity effect} -- physics which depend sharply on whether the spin quantum number $S$ is half-odd integral or integral, which we find to be present in models of monoaxial chiral ferromagnetic spin chains.",2209.04227v2 2022-11-15,Experimental Observation of a Magnetic Interfacial Effect,"We observed a magnetic interfacial effect due to the coupling between two interfaces of different materials. The interface is compoust of an antiferromagnetic and other quasi-ferromagnetic material. This effect we measured through the ferromagnetic resonance technique without and with electric current.",2211.08130v1 2022-12-15,Triplet odd-frequency superconductivity in hybrid superconductor-ferromagnet structures,"We present an overview of the contributions made by Konstantin B. Efetov and the authors to the field of triplet odd-frequency superconductivity in hybrid ferromagnet-superconductor structures and the repercussions they had on experimental and theoretical research. Konstantin B. Efetov passed away in August 2021. We hope the present short review gives a faithful testimony of an important part of his scientific legacy.",2212.08107v1 2024-01-15,Room-temperature intrinsic ferromagnetism of two-dimensional Na2Cl crystals originated by s- and p-orbitals,"Ferromagnetism, as one of the most valuable properties of materials, has attracted sustained and widespread interest in basic and applied research from ancient compasses to modern electronic devices. Traditionally, intrinsic ferromagnetism has been attributed to the permanent magnetic moment induced by partially filled d- or f-orbitals. However, the development of ferromagnetic materials has been limited by this electronic structure convention. Thus, the identification of additional materials that are not constrained by this conventional rule but also exhibit intrinsic ferromagnetism is highly expected and may impact all the fields based on ferromagnetism. Here, we report the direct observation of room-temperature intrinsic ferromagnetism in two-dimensional (2D) Na2Cl crystals, in which there are only partially filled s- and p-orbitals rather than d- or f-orbitals, using the superconducting quantum interference device (SQUID) and magnetic force microscope (MFM). These Na2Cl crystals formed in reduced graphene oxide (rGO) membranes have an unconventional stoichiometric structure leading to unique electron and spin distributions. And the structure of these 2D Na2Cl crystals, including the Na and Cl sites, is characterized in situ for the first time and directly observed by cryo-electron microscopy (cryo-EM) based on the observed difference in contrast between Na stacked with Cl and single Na. These findings break the conventional rule of intrinsic ferromagnetism and provide new insights into the design of novel magnetic and electronic devices and transistors with a size down to the atomic scale.",2401.07524v1 1993-05-24,Ferromagnetism in the Hubbard Model-- Examples from Models with Degenerate Single-Electron Ground States,"Whether spin-independent Coulomb interaction can be the origin of a realistic ferromagnetism in an itinerant electron system has been an open problem for a long time. Here we study a class of Hubbard models on decorated lattices, which have a special property that the corresponding single-electron Schr\""{o}dinger equation has $N_{\rm d}$-fold degenerate ground states. The degeneracy $N_{\rm d}$ is proportional to the total number of sites $\abs{\Lambda}$. We prove that the ground states of the models exhibit ferromagnetism when the electron filling factor is not more than and sufficiently close to $\rho_0=N_{\rm d}/(2\abs{\Lambda})$, and paramagnetism when the filling factor is sufficiently small. An important feature of the present work is that it provides examples of three dimensional itinerant electron systems which are proved to exhibit ferromagnetism in a finite range of the electron filling factor.",9305026v2 1994-02-11,Macroscopic Quantum Tunneling of a Domain Wall in a Ferromagnetic Metal,"The macroscopic quantum tunneling of a planar domain wall in a ferromagnetic metal is studied by use of an instanton method. Based on the Hubbard model, the effective action of the magnetization is derived within the assumption of slow dependences on space and time. The resulting action is formally similar to that of a ferromagnetic Heisenberg model but with a term non-local in time that describes the dissipation due to the itinerant electron. The crucial difference from the case of the insulator is the presence of the ohmic dissipation even at zero temperature due to the gapless Stoner excitation. The reduction of the tunneling rate due to the dissipation is calculated. The dissipative effect is found to be very large for a thin domain wall with thickness of a few times the lattice spacing, but is negligible for a thick domain wall. The results are discussed in the light of recent experiments on ferromagnets with strong anisotropy. (Submitted to J. Phys. Soc. Jpn)",9402050v1 1994-12-02,Fluctuations and Instabilities of Ferromagnetic Domain Wall pairs in an External Magnetic Field,"Soliton excitations and their stability in anisotropic quasi-1D ferromagnets are analyzed analytically. In the presence of an external magnetic field, the lowest lying topological excitations are shown to be either soliton-soliton or soliton-antisoliton pairs. In ferromagnetic samples of macro- or mesoscopic size, these configurations correspond to twisted or untwisted pairs of Bloch walls. It is shown that the fluctuations around these configurations are governed by the same set of operators. The soliton-antisoliton pair has exactly one unstable mode and thus represents a critical nucleus for thermally activated magnetization reversal in effectively one-dimensional systems. The soliton-soliton pair is stable for small external fields but becomes unstable for large magnetic fields. From the detailed expression of this instability threshold and an analysis of nonlocal demagnetizing effects it is shown that the relative chirality of domain walls can be detected experimentally in thin ferromagnetic films. The static properties of the present model are equivalent to those of a nonlinear sigma-model with anisotropies. In the limit of large hard-axis anisotropy the model reduces to a double sine-Gordon model.",9412009v2 1996-12-27,Low Energy Properties of the Random Spin-1/2 Ferromagnetic-Antiferromagnetic Heisenberg Chain,"The low energy properties of the spin-1/2 random Heisenberg chain with ferromagnetic and antiferromagnetic interactions are studied by means of the density matrix renormalization group (DMRG) and real space renormalization group (RSRG) method for finite chains. The results of the two methods are consistent with each other. The deviation of the gap distribution from that of the random singlet phase and the formation of the large-spin state is observed even for relatively small systems. For a small fraction of the ferromagnetic bond, the effect of the crossover to the random singlet phase on the low temperature susceptibility and specific heat is discussed. The crossover concentration of the ferromagnetic bond is estimated from the numerical data.",9612232v1 1997-01-21,Non-perturbative approaches to magnetism in strongly correlated electron systems,"The microscopic basis for the stability of itinerant ferromagnetism in correlated electron systems is examined. To this end several routes to ferromagnetism are explored, using both rigorous methods valid in arbitrary spatial dimensions, as well as Quantum Monte Carlo investigations in the limit of infinite dimensions (dynamical mean-field theory). In particular we discuss the qualitative and quantitative importance of (i) the direct Heisenberg exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a high spectral density near the band edges caused by an appropriate lattice structure and/or kinetic energy of the electrons. We furnish evidence of the stability of itinerant ferromagnetism in the pure Hubbard model for appropriate lattices at electronic densities not too close to half-filling and large enough $U$. Already a weak direct exchange interaction, as well as band degeneracy, is found to reduce the critical value of $U$ above which ferromagnetism becomes stable considerably. Using similar numerical techniques the Hubbard model with an easy axis is studied to explain metamagnetism in strongly anisotropic antiferromagnets from a unifying microscopic point of view.",9701150v1 1997-03-03,Low density ferromagnetism in Extended Hubbard ( t-t'-U ) Model,"We study the existence of ferromagnetism in one dimensional Hubbard Model with on site interaction and nearest and next nearest neighbor hopping. Using the Hubbard I approximation, a self consistent equation for $$, the particle density with spin $\sigma$ is obtained. We find that ferromagnetism exists over a wide range of values of the next nearest neighbor hopping element t' when on-site interaction U is large. The phase diagram as a function of t'/t and the electron density $$ has been obtained by numerically solving the self consistent equation. It is found that the maximum density at which ferromagnetism can occur, peaks around $t'/t \approx -0.53$. We show that this phenomenon is a result of the peculiar way in which the t' term modifies the density of states.",9703022v1 1997-06-17,Generalized calculation of magnetic coupling constants for Mott-Hubbard insulators: Application to ferromagnetic Cr compounds,"Using a Rayleigh-Schr\""odinger perturbation expansion of multi-band Hubbard models, we present analytic expressions for the super-exchange coupling constants between magnetic transition metal ions of arbitrary separation in Mott-Hubbard insulators. The only restrictions are i) all ligand ions are closed shell anions and ii) all contributing interaction paths are of equal length. For short paths, our results essentially confirm the Goodenough-Kanamori-Anderson rules, yet in general there does not exist any simple rule to predict the sign of the magnetic coupling constants. The most favorable situation for ferromagnetic coupling is found for ions with less than half filled d shells, the (relative) tendency to ferromagnetic coupling increases with increasing path length. As an application, the magnetic interactions of the Cr compounds Rb$_2$CrCl$_4$, CrCl$_3$, CrBr$_3$ and CrI$_3$ are investigated, all of which except CrCl$_3$ are ferromagnets.",9706175v1 1997-12-14,Static and Dynamical Properties of the Ferromagnetic Kondo Model with Direct Antiferromagnetic Coupling Between the localized $t_{2g}$ Electrons,"The phase diagram of the Kondo lattice Hamiltonian with ferromagnetic Hund's coupling in the limit where the spin of the localized $t_{2g}$ electrons is classical is analyzed in one dimension as a function of temperature, electronic density, and a direct antiferromagnetic coupling $J'$ between the localized spins. Studying static and dynamical properties, a behavior that qualitatively resembles experimental results for manganites occurs for $J'$ smaller than 0.11 in units of the $e_g$ hopping amplitude. In particular a coexistence of ferromagnetic and antiferromagnetic excitations is observed at low-hole density in agreement with neutron scattering experiments on $\rm{La_{2-2x}Sr_{1+2x}Mn_2O_7}$ with$x=0.4$. This effect is caused by the recently reported tendency to phase separation between hole-rich ferromagnetic and hole-undoped antiferromagnetic domains in electronic models for manganites. As $J'$ increases metal-insulator transitions are detected by monitoring the optical conductivity and the density of states. The magnetic correlations reveal the existence of spiral phases without long-range order but with fairly large correlation lengths. Indications of charge ordering effects appear in the analysis of charge correlations.",9712152v1 1998-01-31,Evidence for a Genuine Ferromagnetic to Paramagnetic Reentrant Phase Transition in a Potts Spin Glass Model,"uch experimental and theoretical efforts have been devoted in the past twenty years to search for a genuine thermodynamic reentrant phase transition from a ferromagnetic to either a paramagnetic or spin glass phase in disordered ferromagnets. So far, no real system or theoretical model of a short-range spin glass system has been shown to convincingly display such a reentrant transition. We present here results from Migdal-Kadanoff real-space renormalization-group calculations that provide for the first time strong evidence for ferromagnetic to paramagnetic reentrance in Potts spin glasses on hierarchical lattices. Our results imply that there is no fundamental reason ruling out thermodynamic reentrant phase transitions in all non mean-field randomly frustrated systems, and may open the possibility that true reentrance might occur in some yet to be discovered real randomly frustrated materials.",9802003v1 1998-03-06,Spin Polarization and Magneto-Coulomb Oscillations in Ferromagnetic Single Electron Devices,"The magneto-Coulomb oscillation, the single electron repopulation induced by external magnetic field, observed in a ferromagnetic single electron transistor is further examined in various ferromagnetic single electron devices. In case of double- and triple-junction devices made of Ni and Co electrodes, the single electron repopulation always occurs from Ni to Co electrodes with increasing a magnetic field, irrespective of the configurations of the electrodes. The period of the magneto-Coulomb oscillation is proportional to the single electron charging energy. All these features are consistently explained by the mechanism that the Zeeman effect induces changes of the Fermi energy of the ferromagnetic metal having a non-zero spin polarizations. Experimentally determined spin polarizations are negative for both Ni and Co and the magnitude is larger for Ni than Co as expected from band calculations.",9803070v1 1998-05-04,Phase diagram of the anti-ferromagnetic xxz model in the presence of an external magnetic field,"The anisotropic s=1/2 anti-ferromagnetic Heisenberg chain in the presence of an external magnetic field is studied by using the standard quantum renormalization group. We obtain the critical line of the transition from partially magnetized (PM) phase to the saturated ferromagnetic (SFM) phase. The crossover exponent between the PM phase and anti-ferromagnetic Ising (AFI) phase is evaluated. Our results show that the anisotropy($\d$) term is relevant and causes crossover. These results indicate that the standard RG approach yields fairly good values for the critical points and their exponents. The magnetization curve, correlation functions and the ground state energy per site are obtained and compared with the known exact results.",9805024v2 1998-05-21,"Metal-Insulator Transition and Ferromagnetism in the Electron Doped Layered Manganites La2.3-xYxCa0.7Mn2O7 (x=0,0.3,0.5)","Bulk samples of La2.3-xYxCa0.7Mn2O7, x=0,0.3,0.5, with layered perovskite structure have been synthesized and investigated with respect to their electrical, electronic and magnetic properties. It is found that La1.8Y0.5Ca0.7Mn2O7 has tetragonal structure and is a metallic ferromagnet with a magnetic transition temperature of 170 K. The compound shows metallic behavior below 140 K and has a large magnetoresistance (MR) Delta-rho/rho(0)~94% at 100 K at 34 kOe. For x=0 and 0.3 the structure is monoclinic with a suppression of metallicity. For x=0 the material is an ferromagnetic insulator. We observed a large increase in the coefficient of the linear term in specific heat with decreasing x. As far as we are aware, this is the first report of an electron doped manganite showing metal-insulator transition and ferromagnetism.",9805274v1 1998-07-06,Spin polarized tunneling in the half-metallic ferromagnet La0.7Sr0.3MnO3: experiment and theory,"The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La0.7Sr0.3MnO3, and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.",9807084v1 1998-08-18,Itinerant Electron Ferromagnetism in the Quantum Hall Regime,"We report on a study of the temperature and Zeeman-coupling-strength dependence of the one-particle Green's function of a two-dimensional (2D) electron gas at Landau level filling factor $\nu =1$ where the ground state is a strong ferromagnet. Our work places emphasis on the role played by the itinerancy of the electrons, which carry the spin magnetization and on analogies between this system and conventional itinerant electron ferromagnets. We discuss the application to this system of the self-consistent Hartree-Fock approximation, which is analogous to the band theory description of metallic ferromagnetism and fails badly at finite temperatures because it does not account for spin-wave excitations. We go beyond this level by evaluating the one-particle Green's function using a self-energy, which accounts for quasiparticle spin-wave interactions. We report results for the temperature dependence of the spin magnetization, the nuclear spin relaxation rate, and 2D-2D tunneling conductances. Our calculations predict a sharp peak in the tunneling conductance at large bias voltages with strength proportional to temperature. We compare with experiment, where available, and with predictions based on numerical exact diagonalization and other theoretical approaches.",9808186v1 1998-09-03,Sensitivity to temperature perturbations of the ageing states in a re-entrant ferromagnet,"Dynamic magnetic properties and ageing phenomena of the re-entrant ferromagnet (Fe0.20Ni0.80)75P16B6Al3 are investigated by time dependent zero field cooled magnetic relaxation, m (t), measurements. The influence of a temperature cycling (perturbation), +/- *T, (prior the field application) on the relaxation rate is investigated both in the low temperature re-entrant spin glass 'phase' and in the ferromagnetic phase. In the ferromagnetic phase the influence of a positive and a negative temperature cycle (of equal magnitude) on the response is almost the same (symmetric response). The result at lower temperatures, in the RSG 'phase' is asymmetric, with a strongly affected response for positive, and hardly no influence on the response for negative temperature cycles. The behaviour at low temperatures is similar to what is observed in ordinary spin glasses.",9809059v1 1998-09-23,Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice Model : A Qualitative Approach to Oxide Manganites,"The transport properties of the ferromagnetic Kondo lattice model in one dimension are studied via bosonization methods. The antiferromagnetic fluctuations, which normally appear because of the RKKY interactions, are explicitly taken into account as a direct exchange between the ``core'' spins. It is shown that in the paramagnetic regime with the local antiferromagnetic fluctuations, the resistivity decays exponentially as the temperature increases while in the ferromagnetic regime the system is an almost perfect conductor. %A non-perturbative description of localized spin polarons %in the paramagnetic region is obtained. The effect of a weak applied field is discussed to be reduced to the case of the ferromagnetic state leading to band splitting. The qualitative relevance of the results for the problem of the Oxide Manganites is emphasized.",9809309v1 1998-11-25,Metallic ferromagnetism without exchange splitting,"In the band theory of ferromagnetism there is a relative shift in the position of majority and minority spin bands due to the self-consistent field due to opposite spin electrons. In the simplest realization, the Stoner model, the majority and minority spin bands are rigidly shifted with respect to each other. Here we consider models at the opposite extreme, where there is no overall shift of the energy bands. Instead, upon spin polarization one of the bands broadens relative to the other. Ferromagnetism is driven by the resulting gain in kinetic energy. A signature of this class of mechanisms is that a transfer of spectral weight in optical absorption from high to low frequencies occurs upon spin polarization. We show that such models arise from generalized tight binding models that include off-diagonal matrix elements of the Coulomb interaction. For certain parameter ranges it is also found that reentrant ferromagnetism occurs. We examine properties of these models at zero and finite temperatures, and discuss their possible relevance to real materials.",9811346v1 1998-12-09,Spin current in ferromagnet/insulator/superconductor junctions,"A theory of spin polarized tunneling spectroscopy based on a scattering theory is given for tunneling junctions between ferromagnets and d-wave superconductors. The spin filtering effect of an exchange field in the insulator is also treated. We clarify that the properties of the Andreev reflection are largely modified due to a presence of an exchange field in the ferromagnets, and consequently the Andreev reflected quasiparticle shows an evanescent-wave behavior depending on the injection angle of the quasiparticle. Conductance formulas for the spin current as well as the charge current are given as a function of the applied voltage and the spin-polarization in the ferromagnet for arbitrary barrier heights. It is shown that the surface bound states do not contribute to the spin current and that the zero-bias conductance peak expected for a d-wave superconductor splits into two peaks under the influence of the exchange interaction in the insulator.",9812160v3 1999-06-05,Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8),"It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase transition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic insulator (FI) phase. The magnetization shows a sharp jump at the transition field accompanying with a remarkable increase of magnetoresistance. Striction measurements clarified that this transition is associated with the structural change from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic phase. These results evidently show that the FI phase with a pseudo cubic symmetry is more stable in high fields than the FM phase due to the double exchange interaction. The driving force of this transition is explained by the enhancement of the ferromagnetic superexchange interaction induced by an antiferromagnetic type orbital ordering in the pseudo cubic phase, which was recently found in the anomalous X-ray scattering experiments.",9906075v1 1999-07-02,The temperature dependent bandstructure of a ferromagnetic semiconductor film,"The electronic quasiparticle spectrum of a ferromagnetic film is investigated within the framework of the s-f model. Starting from the exact solvable case of a single electron in an otherwise empty conduction band being exchange coupled to a ferromagnetically saturated localized spin system we extend the theory to finite temperatures. Our approach is a moment-conserving decoupling procedure for suitable defined Green functions. The theory for finite temperatures evolves continuously from the exact limiting case. The restriction to zero conduction band occupation may be regarded as a proper model description for ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a simple cubic film cut parallel to the (100) crystal plane, we find some marked correlation effects which depend on the spin of the test electron, on the exchange coupling, and on the temperature of the local-moment system.",9907049v1 1999-07-26,Quantum critical behavior of itinerant ferromagnets,"We investigate the quantum phase transition of itinerant ferromagnets. It is shown that correlation effects in the underlying itinerant electron system lead to singularities in the order parameter field theory that result in an effective long-range interaction between the spin fluctuations. This interaction turns out to be generically {\em antiferromagnetic} for clean systems. In disordered systems analogous correlation effects lead to even stronger singularities. The resulting long-range interaction is, however, generically ferromagnetic. We discuss two possibilities for the ferromagnetic quantum phase transition. In clean systems, the transition is generically of first order, as is experimentally observed in MnSi. However, under certain conditions the transition may be continuous with non-mean field critical behavior. In disordered systems, one finds a very rich phase diagram showing first order and continuous phase transitions and several multicritical points.",9907404v2 1999-08-02,Magnon-Paramagnon Effective Theory of Itinerant Ferromagnets,"The present work is devoted to the derivation of an effective magnon-paramagnon theory starting from a microscopic lattice model of ferromagnetic metals. For some values of the microscopic parameters it reproduces the Heisenberg theory of localized spins. For small magnetization the effective model describes the physics of weak ferromagnets in accordance with the experimental results. It is written in a way which keeps O(3) symmetry manifest,and describes both the order and disordered phases of the system. Analytical expression for the Curie temperature,which takes the magnon fluctuations into account exactly, is obtained. For weak ferromagnets $T_c$ is well below the Stoner's critical temperature and the critical temperature obtained within Moriya's theory.",9908024v3 1999-08-04,Nonlinear Dynamics of Nuclear-Electronic Spin Processes in Ferromagnets,"Spin dynamics is considered in ferromagnets consisting of electron and nuclear subsystems interacting with each other through hyperfine forces. In addition, the ferromagnetic sample is coupled with a resonance electric circuit. Under these conditions, spin relaxation from a strongly nonequilibrium initial state displays several peculiarities absent for the standard set-up in studying spin relaxation. The main feature of the nonlinear spin dynamics considered in this communication is the appearance of ultrafast coherent relaxation, with characteristic relaxation times several orders shorter than the transverse relaxation time $T_2$. This type of coherent spin relaxation can be used for extracting additional information on the intrinsic properties of ferromagnetic materials and also can be employed for different technical applications.",9908072v1 1999-10-11,Suppression of ferromagnetic ordering in doped manganites: Effects of the superexchange interaction,"From a Monte Carlo study of the ferromagnetic Kondo lattice model for doped manganites, including the antiferromagnetic superexchange interaction ($J_{AF}$), we found that the ferromagnetic ordering was suppressed as $J_{AF}$ increased. The ferromagnetic transition temperature $T_c$, as obtained from a mean field fit to the calculated susceptibilities, was found to decrease monotonically with increasing $J_{AF}$. Further, the suppression in $T_c$ scales with the bandwidth narrowing induced by the antiferromagnetic frustration originating from $J_{AF}$. From these results, we propose that the change in the superexchange interaction strength between the $t_{2g}$ electrons of the Mn ions is one of the mechanisms responsible for the suppression in $T_c$ observed in manganites of the type (La$_{0.7-y}$Pr$_{y}$)Ca$_{0.3}$MnO$_3$.",9910152v1 2000-04-18,Disorder and interactions in quantum Hall ferromagnets near $ν=1$,"We report on a finite-size Hartree-Fock study of the competition between disorder and interactions in a two-dimensional electron gas near Landau level filling factor $\nu=1$. The ground state at $\nu=1$ evolves with increasing disorder from a fully spin-polarized ferromagnet with a charge gap, to a partially spin-polarized ferromagnetic Anderson insulator, to a quasi-metallic paramagnet at the critical point between $i=0$ and $i=2$ quantum Hall plateaus. Away from $\nu=1$, the ground state evolves from a ferromagnetic Skyrmion quasiparticle glass, to a conventional quasiparticle glass, and finally to a conventional Anderson insulator. We comment on signatures of these different regimes in low-temperature transport and NMR lineshape and peak position data.",0004319v2 2000-05-03,Current driven switching of magnetic layers,"The switching of magnetic layers is studied under the action of a spin current in a ferromagnetic metal/non-magnetic metal/ferromagnetic metal spin valve. We find that the main contribution to the switching comes from the non-equilibrium exchange interaction between the ferromagnetic layers. This interaction defines the magnetic configuration of the layers with minimum energy and establishes the threshold for a critical switching current. Depending on the direction of the critical current, the interaction changes sign and a given magnetic configuration becomes unstable. To model the time dependence of the switching process, we derive a set of coupled Landau-Lifshitz equations for the ferromagnetic layers. Higher order terms in the non-equilibrium exchange coupling allow the system to evolve to its steady-state configuration.",0005064v2 2000-07-19,Electrical spin injection and detection in a semiconductor. Is it feasible?,"The electrical injection of spin polarized electrons in a semiconductor can be achieved in principle by driving a current from a ferromagnetic metal, where current is known to be significantly spin polarized, into the semiconductor via ohmic conduction. For detection a second ferromagnet can be used as drain. We studied submicron lateral spin valve junctions, based on high mobility InAs/AlSb two-dimensional electron gas (2DEG), with Ni, Co and Permalloy as ferromagnetic electrodes. In the standard geometry it is very difficult to separate true spin injection from other effects, including local Hall effect, anomalous magnetoresistance (AMR) contribution from the ferromagnetic electrodes and weak localization/anti-localization corrections, which can closely mimic the signal expected from spin valve effect. The reduction in size, and the use of a multiterminal non-local geometry allowed us to reduce the unwanted effects to a minimum. Despite all our efforts, we have not been able to observe spin injection. However, we find that this 'negative' result in these systems is actually consistent with theoretical predictions for spin transport in diffusive systems.",0007307v1 2000-07-28,Ferromagnetism from Undressing,"We have recently proposed that superconductivity may be understood as driven by the undressing of quasiparticles as the superconducting state develops. Similarly we propose here that ferromagnetism in metals may be understood as driven by the undressing of quasiparticles as the ferromagnetic state develops. In ferromagnets, the undressing is proposed to occur due to the reduction in $bond$ $charge$ caused by spin polarization, in contrast to superconductors where the undressing is proposed to occur due to the reduction in $site$ $charge$ caused by (hole) pairing. The undressing process manifests itself in the one and two-particle Green's functions as a transfer of spectral weight from high to low frequencies. Hence it should have universal observable consequences in one- and two-particle spectroscopies such as photoemission and optical absorption.",0007454v1 2000-10-24,Quantum critical behavior in disordered itinerant ferromagnets: Logarithmic corrections to scaling,"The quantum critical behavior of disordered itinerant ferromagnets is determined exactly by solving a recently developed effective field theory. It is shown that there are logarithmic corrections to a previous calculation of the critical behavior, and that the exact critical behavior coincides with that found earlier for a phase transition of undetermined nature in disordered interacting electron systems. This confirms a previous suggestion that the unspecified transition should be identified with the ferromagnetic transition. The behavior of the conductivity, the tunneling density of states, and the phase and quasiparticle relaxation rates across the ferromagnetic transition is also calculated.",0010377v2 2000-12-21,Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound,"Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferromagnetic transition appears at $T$ = 3.2 K with small spontaneous magnetization along the $c$ axis, and around the transition temperature large and anisotropic negative magnetoresistance was observed. This compound is the first example which shows ferromagnetic long-range order in a two-dimensional metallic hexagnonal CoO${}_2$ layer.",0012395v1 2001-01-11,Tapping Spin Glasses,"We consider a tapping dynamics, analogous to that in experiments on granular media, on spin glasses and ferromagnets on random thin graphs. Between taps, zero temperature single spin flip dynamics takes the system to a metastable state. Tapping, corresponds to flipping simultaneously any spin with probability $p$. This dynamics leads to a stationary regime with a steady state energy $E(p)$. We analytically solve this dynamics for the one dimensional ferromagnet and $\pm J$ spin glass. Numerical simulations for spin glasses and ferromagnets of higher connectivity are carried out, in particular we find a novel first order transition for the ferromagnetic systems.",0101166v1 2001-01-16,Low Temperature Magnetothermodynamics of Pr{0.7}Ca{0.3}MnO{3},"We present a detailed magnetothermal study of Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$, a perovskite manganite in which an insulator-metal transition can be driven by magnetic field, but also by pressure, visible light, x-rays, or high currents. We find that the field-induced transition is associated with a large release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin wave stiffness than that seen in any other ferromagnetic manganite, which we explain in terms of ferromagnetism among the Pr moments. The Pr ferromagnetism also appears to influence the low temperature thermodynamic phase diagram of this material and the uniquely sensitive metastability of the insulating state.",0101223v1 2001-02-20,"On the Belitz-Kirkpatrick comment on ""Specific heat of a Fermi system near ferromagnetic quantum phase transition"", by I.Grosu, D.Bodea and M.Crisan (cond-mat/0101392)","We reply to Belitz and Kirkpatrick comment cond-mat/0102064, on cond-mat/0101392.",0102354v1 2001-02-20,"Theory of ferromagnetism in (A,Mn)B semiconductors","A brief review of theory of ferromagnetism of dilute magnetic semiconductors of the form (A,Mn)B based on the double exchange model is first given. A systematic investigation of the phenomena extending the current theory is outlined. We begin with an investigation of the regions of instability of the nonmagnetic towards the ferromagnetic state of a system of Mn-atoms doped in AB-type semiconductor. A self-consistent many-body theory of the ferromagnetic state is then developed, going beyond the mean field approaches by including fluctuations of the Mn-spins and the itinerant hole-gas. A functional theory suitable for computation of system properties such as Curie temperature as a function of hole and the Mn-concentration, spin-current, etc. is formulated.",0102361v1 2001-05-10,The temperature-flow renormalization group and the competition between superconductivity and ferromagnetism,"We derive a differential equation for the one-particle-irreducible vertex functions of interacting fermions as a function of the temperature. Formally, these equations correspond to a Wilsonian renormalization group scheme which uses the temperature as an explicit scale parameter. Our novel method allows us to analyze the competition between superconducting and various magnetic Fermi surface instabilities in the one-loop approximation. In particular this includes ferromagnetic fluctuations, which are difficult to treat on an equal footing in conventional Wilsonian momentum space techniques. Applying the scheme to the two-dimensional t-t' Hubbard model we investigate the RG flow of the interactions at the van Hove filling with varying next-nearest neighbor hopping t'. Starting at t'=0 we describe the evolution of the flow to strong coupling from an antiferromagnetic nesting regime over a d-wave regime at moderate t' to a ferromagnetic region at larger absolute values of t'. Upon increasing the particle density in the latter regime the ferromagnetic tendencies are cut off and the leading instability occurs in the triplet superconducting pairing channel.",0105218v1 2001-05-26,Direct Observation of Coexistence of Ferromagnetism and Superconductivity in RuSr2(Gd0.7Ce0.3)2Cu2O10,"Recent reports of the detecting of ferromagnetism and superconductivity in ruthenium-cuprates have aroused great interest. Unfortunately, whether the two antagonistic phenomena coexist in the same space in the compounds remains unresolved. By employing the magneto-optical-imaging technique, ferromagnetism and superconductivity were indeed directly observed to coexist in the same space in RuSr2(Gd0.7Ce0.3)2Cu2O10 within the experimental resolution of ~ 10 (mu)m. The observation sets a length scale limit for models proposed to account for the competition between ferromagnetism and superconductivity, especially d-wave superconductivity, in this interesting class of compounds.",0105510v1 2001-06-05,Phase diagram of the random field Ising model on the Bethe lattice,"The phase diagram of the random field Ising model on the Bethe lattice with a symmetric dichotomous random field is closely investigated with respect to the transition between the ferromagnetic and paramagnetic regime. Refining arguments of Bleher, Ruiz and Zagrebnov [J. Stat. Phys. 93, 33 (1998)] an exact upper bound for the existence of a unique paramagnetic phase is found which considerably improves the earlier results. Several numerical estimates of transition lines between a ferromagnetic and a paramagnetic regime are presented. The obtained results do not coincide with a lower bound for the onset of ferromagnetism proposed by Bruinsma [Phys. Rev. B 30, 289 (1984)]. If the latter one proves correct this would hint to a region of coexistence of stable ferromagnetic phases and a stable paramagnetic phase.",0106074v3 2001-06-12,Steady State Behavior of Mechanically Perturbed Spin Glasses and Ferromagnets,"A zero temperature dynamics of Ising spin glasses and ferromagnets on random graphs of finite connectivity is considered, like granular media these systems have an extensive entropy of metastable states. We consider the problem of what energy a randomly prepared spin system falls to before becoming stuck in a metastable state. We then introduce a tapping mechanism, analogous to that of real experiments on granular media, this tapping, corresponding to flipping simultaneously any spin with probability $p$, leads to stationary regime with a steady state energy $E(p)$. We explicitly solve this problem for the one dimensional ferromagnet and $\pm J$ spin glass and carry out extensive numerical simulations for spin systems of higher connectivity. The link with the density of metastable states at fixed energy and the idea of Edwards that one may construct a thermodynamics with a flat measure over metastable states is discussed. In addition our simulations on the ferromagnetic systems reveal a novel first order transition, whereas the usual thermodynamic transition on these graphs is second order.",0106220v1 2001-06-24,Spontaneous Magnetization of the O(3) Ferromagnet at Low Temperatures,"We investigate the low-temperature behavior of ferromagnets with a spontaneously broken symmetry O(3) $\to$ O(2). The analysis is performed within the perspective of nonrelativistic effective Lagrangians, where the dynamics of the system is formulated in terms of Goldstone bosons. Unlike in a Lorentz-invariant framework (chiral perturbation theory), where loop graphs are suppressed by two powers of momentum, loops involving ferromagnetic spin waves are suppressed by three momentum powers. The leading coefficients of the low-temperature expansion for the partition function are calculated up to order $p^{10}$. In agreement with Dyson's pioneering microscopic analysis of the cubic ferromagnet, we find that, in the spontaneous magnetization, the magnon-magnon interaction starts manifesting itself only at order $T^4$. The striking difference with respect to the low-temperature properties of the O(3) antiferromagnet is discussed from a unified point of view, relying on the effective Lagrangian technique.",0106492v1 2001-07-03,"Sr$_3$CuIrO$_6$, a spin-chain compound with random ferromagnetic-antiferromagnetic interactions","Ac and dc magnetization and heat-capacity (C) measurements performed on the pseudo-one-dimensional compound Sr$_3$CuIrO$_6$ reveal a competition between antiferromagnetic (AF) and ferromagnetic (F) exchange couplings, as evidenced by frequency dependence of ac susceptibility and by the absence of a C anomaly at the magnetic transition. The value of the saturation moment (about 0.35 $\mu_B$/formula unit) is much smaller than expected for ferromagnetism from the two S=1/2 ions (Cu and Ir). Thus, this compound is not a ferromagnet in zero magnetic field, in contrast to earlier beliefs. Of particular importance is the finding that the value of the magnetic ordering temperature is sample dependent, sensitive to synthetic conditions resulting from deviations in oxygen/Cu content. We propose that this compound serves as a unique model system to test theories on random AF-F interaction in a chain system, considering that this competition can be tuned without any chemical substitution.",0107069v1 2001-07-11,Proximity effects at ferromagnet-superconductor interfaces,"We study proximity effects at ferromagnet superconductor interfaces by self-consistent numerical solution of the Bogoliubov-de Gennes equations for the continuum, without any approximations. Our procedures allow us to study systems with long superconducting coherence lengths. We obtain results for the pair potential, the pair amplitude, and the local density of states. We use these results to extract the relevant proximity lengths. We find that the superconducting correlations in the ferromagnet exhibit a damped oscillatory behavior that is reflected in both the pair amplitude and the local density of states. The characteristic length scale of these oscillations is approximately inversely proportional to the exchange field, and is independent of the superconducting coherence length in the range studied. We find the superconducting coherence length to be nearly independent of the ferromagnetic polarization.",0107232v1 2001-08-08,Spin-Glass Phase in the Random Temperature Ising Ferromagnet,"In this paper we study the phase diagram of the disordered Ising ferromagnet. Within the framework of the Gaussian variational approximation it is shown that in systems with a finite value of the disorder in dimensions D=4 and D < 4 the paramagnetic and ferromagnetic phases are separated by a spin-glass phase. The transition from paramagnetic to spin-glass state is continuous (second-order), while the transition between spin-glass and ferromagnetic states is discontinuous (first-order). It is also shown that within the considered approximation there is no replica symmetry breaking in the spin-glass phase. The validity of the Gaussian variational approximation for the present problem is discussed, and we provide a tentative physical interpretation of the results.",0108136v2 2001-08-09,Critical exponents at the ferromagnetic transition in tetrakis(diethylamino)ethylene-C$_{60}$ (TDAE-C$_{60}$),"Critical exponents at the ferromagnetic transition were measured for the first time in an organic ferromagnetic material tetrakis(dimethylamino)ethylene fullerene[60] (TDAE-C$_{60}$). From a complete magnetization-temperature-field data set near $T_{c}=16.1\pm 0.05,$ we determine the susceptibility and magnetization critical exponents $\gamma =1.22\pm 0.02$ and $\beta =0.75 \pm 0.03$ respectively, and the field vs. magnetization exponent at $T_{c}$ of $\delta =2.28\pm 0.14$. Hyperscaling is found to be violated by $\Omega \equiv d^{\prime}-d \approx -1/4$, suggesting that the onset of ferromagnetism can be related to percolation of a particular contact configuration of C$_{60}$ molecular orientations.",0108156v1 2001-08-19,Optical Pumping in Ferromagnet-Semiconductor Heterostructures: Magneto-optics and Spin Transport,"Epitaxial ferromagnetic metal - semiconductor heterostructures are investigated using polarization-dependent electroabsorption measurements on GaAs p-type and n-type Schottky diodes with embedded In1-xGaxAs quantum wells. We have conducted studies as a function of photon energy, bias voltage, magnetic field, and excitation geometry. For optical pumping with circularly polarized light at energies above the band edge of GaAs, photocurrents with spin polarizations on the order of 1 % flow from the semiconductor to the ferromagnet under reverse bias. For optical pumping at normal incidence, this polarization may be enhanced significantly by resonant excitation at the quantum well ground-state. Measurements in a side-pumping geometry, in which the ferromagnet can be saturated in very low magnetic fields, show hysteresis that is also consistent with spin-dependent transport. Magneto-optical effects that influence these measurements are discussed.",0108292v1 2001-09-21,Gate-controlled spin polarized current in ferromagnetic single electron transistors,"We theoretically investigate spin dependent transport in ferromagnetic/normal metal/ferromagnetic single electron transistors by applying master equation calculations using a two dimensional space of states involving spin and charge degrees of freedom. When the magnetizations of ferromagnetic leads are in anti-parallel alignment, the spins accumulate in the island and a difference of chemical potentials of the two spins is built up. This shift in chemical potential acts as charge offset in the island and alternates the gate dependence of spin current. Taking advantage of this effect, one can control the polarization of current up to the polarization of lead by tuning gate voltages.",0109401v1 2001-12-03,Local density of states in superconductor-strong ferromagnet structures,"We study the dependence of the local density of states (LDOS) on coordinates for a superconductor-ferromagnet (S/F) bilayer and a S/F/S structure assuming that the exchange energy h in the ferromagnet is sufficiently large: $% h\tau >>1,$ where $\tau $ is the elastic relaxation time. This limit cannot be described by the Usadel equation and we solve the more general Eilenberger equation. We demonstrate that, in the main approximation in the parameter $% (h\tau)^{-1}$, the proximity effect does not lead to a modification of the LDOS in the S/F system and a non-trivial dependence on coordinates shows up in next orders in $(h\tau) ^{-1}.$ In the S/F/S sandwich the correction to the LDOS is nonzero in the main approximation and depends on the phase difference between the superconductors. We also calculate the superconducting critical temperature $T_{c}$ for the bilayered system and show that it does not depend on the exchange energy of the ferromagnet in the limit of large h and a thick F layer.",0112024v1 2002-02-01,Metal-Insulator Transition and Ferromagnetism in Diluted Magnetic Semiconductors,"We have investigated the interplay between the metal-insulator transition and ferromagnetism in $({\rm III}_{1-x},{\rm Mn}_x){\rm V}$ ferromagnetic semiconductors. Our study is based on a model in which $S=5/2$ Mn local moments are exchange-coupled to band electrons that interact via Coulomb interactions which each other, with ionized Mn acceptors, and with the antisite defects present in these materials. We find quasiparticle participation ratios that are consistent with a metal-insulator transition that occurs in the ferromagnetic state near $x \sim 0.01$. By evaluating the distribution of mean-field exchange coupling strengths at Mn moment sites, we provide evidence in favor of the applicability of small polaron and hole gas models on insulating and metallic sides of the phase transition respectively.",0202021v1 2002-02-07,Ferromagnetism in the Kondo-lattice model,"We propose a modified RKKY-technique to evaluate the magnetic properties of the ferromagnetic Kondo-lattice model. Together with a previously developed selfenergy approach to the conduction electron part of the model we get a closed system of equations which can be solved self-consistently. Ferromagnetism appears for relatively low electron (hole) densities, while it is excluded around half-filling ($n=1$). For small $J$ conventional RKKY ($\sim J^2$) is reproduced; however, with strong deviations already for very moderate exchange couplings. For not too small $n$ a critical $J_c$ is needed to produce ferromagnetism with a finite Curie temperature $T_{\textrm{C}}$, which increases with $J$, then running into a kind of saturation, in order to fall off again and disappearing above an upper critical exchange $J$.",0202114v3 2002-02-20,Transport and triplet superconducting condensate in mesoscopic ferromagnet-superconductor structures,"We calculate the conductance of a superconductor/ferromagnet (S/F) mesoscopic structure in the dirty limit. First we assume that the ferromagnet exhibits a homogeneous magnetization and consider the case that the penetration of the condensate into the F wire is negligible and the case in which the proximity effect is taken into account. It is shown that if the exchange field is large enough, the conductance below the critical temperature $T_C$, is always smaller than the conductance in the normal state. At last, we calculate the conductance for a F/S structure with a local inhomogeneity of the magnetization in the ferromagnet. We demonstrate that a triplet component of the condensate is induced in the F wire.This leads to a increase of the conductance below $T_C$.",0202348v2 2002-03-05,Magnetic relaxation in La0.250Pr0.375Ca0.375MnO3 with varying phase separation,"We have studied the magnetic relaxation properties of the phase-separated manganite compound La0.250Pr0.375Ca0.375MnO3 . A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes - at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and non-ferromagnetic phases.",0203102v1 2002-03-11,High TC ferromagnetism in diluted magnetic semiconducting GaN:Mn films,"Wurtzite GaN:Mn films on sapphire substrates were successfully grown by use of the molecular beam epitaxy (MBE) system. The film has an extremely high Curie temperature of around 940 K, although the Mn concentration is only about 3 ~ 5 %. Magnetization measurements were carried out in magnetic fields parallel to the film surface up to 7 T. The magnetization process shows the coexistence of ferromagnetic and paramagnetic contributions at low temperatures, while the typical ferromagnetic magnetization process is mainly observed at high temperatures because of the decrease of the paramagnetic contributions. The observed transport characteristics show a close relation between the magnetism and the impurity conduction. The double exchange mechanism of the Mn-impurity band is one of the possible models for the high-TC ferromagnetism in GaN:Mn.",0203223v1 2002-03-26,Origin and Properties of the Gap in the Half-Ferromagnetic Heusler Alloys,"We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened KKR method. In the paramagnetic phase the C1_b compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, Z_t, per unit cell, like CoTiSb, are semiconductors, but when Z_t > 18 antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in mu_B is just the difference between Z_t and $2 \times 9$. While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the $sp$ atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.",0203534v3 2002-04-05,Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor,"We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors.",0204140v2 2002-05-25,A possible path to a new class of ferromagnetic and half metallic ferromagnetic materials,"We introduce a path to a possibly new class of magnetic materials whose properties are determined entirely by the presence of a low concentration of specific point defects in their crystal structure. Using model Hamiltonian and ab-initio band structure methods we demonstrate that even large band gap nonmagnetic materials as simple as CaO can exhibit extraordinary properties like half metallic ferromagnetism upon introducing a small concentration of Ca vacancies. We show that such defects will initially bind the introduced charge carriers at neighboring sites and depending on the internal symmetry of the clusters formed by neighboring sites form ""local"" magnetic moments which for concentrations as low as 3% transform this non-magnetic insulator into a half metallic ferromagnet.",0205530v1 2002-05-27,Properties of Ferromagnetic Ga1-xMnxN Films Grown by Ammonia-MBE,"Using ammonia as nitrogen source for molecular beam epitaxy, the GaN-based diluted magnetic semiconductor Ga1-xMnxN is successfully grown with Mn concentration up to x~6.8% and with p-type conductivity. The films have wurtzite structure with substitutional Mn on Ga site in GaN. Magnetization measurements revealed that Ga1-xMnxN is ferromagnetic at temperatures higher than room temperature. The ferromagnetic-paramagnetic transition temperature, Tc, depends on the Mn concentration of the film. At low temperatures, the magnetization increases with increasing of magnetic field, implying that a paramagnetic-like phase coexists with ferromagnetic one. Possible explanations will be proposed for the coexistence of two magnetic phases in the grown films.",0205560v1 2002-06-07,"Charge dynamics and ""ferromagnetism"" of A1-xLaxB6 (A=Ca and Sr)","Ferromagnetism has been reported recently in La-doped alkaline-earth hexaborides, A1-xLaxB6 (A=Ca, Sr, and Ba). We have performed the reflectivity, Hall resistivity, and magnetization measurements of A1-xLaxB6. The results indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (10 micrometer in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A1-xLaxB6 reported so far is neither intrinsic.",0206101v1 2002-06-18,On the probability of ferromagnetic strings in antiferromagnetic spin chains,"We study the probability of formation of ferromagnetic string in the antiferromagnetic spin-1/2 XXZ chain. We show that in the limit of long strings with weak magnetization per site the bosonization technique can be used to address the problem. At zero temperature the obtained probability is Gaussian as a function of the length of the string. At finite but low temperature there is a crossover from the Gaussian behavior at intermediate lengths of strings to the exponential decay for very long strings. Although the weak magnetization per site is a necessary small parameter justifying our results, the extrapolation of obtained results to the case of maximally ferromagnetic strings is in qualitative agreement with known numerics and exact results. The effect of an external magnetic field on the probability of formation of ferromagnetic strings is also studied.",0206353v1 2002-07-06,Spin-polarized Tunneling in Hybrid Metal-Semiconductor Magnetic Tunnel Junctions,"We demonstrate efficient spin-polarized tunneling between a ferromagnetic metal and a ferromagnetic semiconductor with highly mismatched conductivities. This is indicated by a large tunneling magnetoresistance (up to 30%) at low temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a nonmagnetic semiconductor (AlAs). Analysis of the current-voltage characteristics yields detailed information about the asymmetric tunnel barrier. The low temperature conductance-voltage characteristics show a zero bias anomaly and a V^1/2 dependence of the conductance, indicating a correlation gap in the density of states of GaMnAs. These experiments suggest that MnAs/AlAs heterostructures offer well characterized tunnel junctions for high efficiency spin injection into GaAs.",0207178v1 2002-08-02,Ferromagnetic insulating state in manganites:55Mn study,"55Mn NMR was used to study the ferromagnetic insulating state in four different manganites. In all cases the coexistence of two types of regions possessing different NMR spectra and nuclear spin dynamics was found. In the ferromagnetic insulating clusters the hopping frequency fhop of the electron holes is slower comparing to the NMR frequency fres and two lines ascribed to 3+ and 4+ valence states of Mn are observed. The relaxation rate increases rapidly with the increasing temperature, so that these spectra can no longer be observed for temperatures higher than approx 60 K. In ferromagnetic metallic clusters fres < fhop and the motionally narrowed spectrum is observed in a broad temperature interval, as the relaxation remains only moderately fast.",0208033v1 2002-08-06,Spin pumping and magnetization dynamics in metallic multilayers,"We study the magnetization dynamics in thin ferromagnetic films and small ferromagnetic particles in contact with paramagnetic conductors. A moving magnetization vector causes \textquotedblleft pumping\textquotedblright of spins into adjacent nonmagnetic layers. This spin transfer affects the magnetization dynamics similar to the Landau-Lifshitz-Gilbert phenomenology. The additional Gilbert damping is significant for small ferromagnets, when the nonmagnetic layers efficiently relax the injected spins, but the effect is reduced when a spin accumulation build-up in the normal metal opposes the spin pumping. The damping enhancement is governed by (and, in turn, can be used to measure) the mixing conductance or spin-torque parameter of the ferromagnet--normal-metal interface. Our theoretical findings are confirmed by agreement with recent experiments in a variety of multilayer systems.",0208091v2 2002-09-24,Electron paramagnetic resonance studies of the insulating ferromagnetic manganite Nd_0.8Pb_0.2MnO_3 above the transition temperature,"Single crystals of Nd_{1-x}Pb_{x}MnO_{3} with x=0.2 are grown by high temperature solution growth technique using PbO-PbF_{2} flux. Magnetization studies on the samples show a transition to a ferromagnetic state below T_c ~ 125 K and the resistivity measurements show it to be an insulator throughout the temperature range 50 - 300 K. Electron Paramagnetic Resonance studies have been performed for T > T_{C} with a view to comparing the results with those on metallic ferromagnetic manganites. The temperature dependence of various parameters like g-value, linewidth and intensity has been studied in the temperature range 150 - 300 K. It is found that they behave in a manner similar to that exhibited by metallic ferromagnetic manganites.",0209546v1 2002-10-18,Coherent spin valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions,"Coherent quantum transport in ferromagnetic/ semiconductor/ ferromagnetic junctions is studied theoretically within the Landauer framework of ballistic transport. We show that quantum coherence can have unexpected implications for spin injection and that some intuitive spintronic concepts which are founded in semi-classical physics no longer apply: A quantum spin-valve (QSV) effect occurs even in the absence of a net spin polarized current flowing through the device, unlike in the classical regime. The converse effect also arises, i.e. a zero spin-valve signal for a non-vanishing spin-current. We introduce new criteria useful for analyzing quantum and classical spin transport phenomena and the relationships between them. The effects on QSV behavior of spin-dependent electron transmission at the interfaces, interface Schottky barriers, Rashba spin-orbit coupling and temperature, are systematically investigated. While the signature of the QSV is found to be sensitive to temperature, interestingly, that of its converse is not. We argue that the QSV phenomenon can have important implications for the interpretation of spin-injection in quantum spintronic experiments with spin-valve geometries.",0210391v1 2002-10-31,Temperature-dependence of spin-polarized transport in ferromagnet / unconventional superconductor junctions,"Tunneling conductance in ferromagnet / unconventional superconductor junctions is studied theoretically as a function of temperatures and spin-polarization in feromagnets. In d-wave superconductor junctions, the existence of a zero-energy Andreev bound state drastically affects the temperature-dependence of the zero-bias conductance (ZBC). In p-wave triplet superconductor junctions, numerical results show a wide variety in temperature-dependence of the ZBC depending on the direction of the magnetic moment in ferromagnets and the pairing symmetry in superconductors such as $p_{x}$, $p_{y}$ and $p_{x}+ip_{y}$-wave pair potential. The last one is a promising symmetry of Sr$_2$RuO$_4$. From these characteristic features in the conductance, we may obtain the information about the degree of spin-polarization in ferromagnets and the direction of the $d$-vector in triplet superconductors.",0210693v2 2002-12-12,Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement,"The dynamics of magnetization under femtosecond optical excitation is studied in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved magneto-optical Kerr effect measurement with two color probe beams. The transient reflectivity change indicates the rapid rise of the carrier temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a very slow rise of the spin temperature of the order of 500ps is observed. This anomalous behavior originates from the thermal isolation between the charge and spin systems due to the spin polarization of carriers (holes) contributing to ferromagnetism. This constitutes experimental proof of the half-metallic nature of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type mechanism originates from the d-band character of holes.",0212276v2 2002-12-17,Magneto-Optical Spectroscopy of Anatase TiO2 Doped with Co,"Magneto-optical spectroscopy of a transparent ferromagnetic semiconductor, anatase TiO2 doped with Co, is carried out at room temperature. A large magneto-optical response with ferromagnetic field dependence is observed throughout from ultraviolet to visible range and increases with increasing Co content or carrier concentration. The magnitude of magnetic circular dichroism (MCD) per unit thickness has a peak around the absorption edge such a huge value of ~10400 degree/cm at 3.57 eV for a 10 mol% Co-doped specimen. Although the results are not sufficient to prove that the ferromagnetism is in the ordinary framework of diluted magnetic semiconductors, the coexistence of Co impurity and mobile carrier is shown to transform the band structure of host TiO2 to generate ferromagnetism.",0212389v1 2003-01-19,Fermi surface topology and ferromagnetic superconductivity in UGe$_2$,"We consider a Stoner ferromagnet in presence of a quasi-one dimensional Fermi surface in its spin majority band. Assuming a twin $\delta$-function peaked density of state due to the low dimensionality, we computed the single particle self-energy. There appears an additional divergence in the self-energy and hence in the effective mass inside the ferromagnetic phase (besides the standard logarithmic divergence at the Stoner critical point). Since such an effect is purely due to density of states, it might correspond to a first order phase transition making the ferromagnetic phase into two distinct phases. This result is in qualitative agreement with the recent specific heat capacity measurement. We also discuss its relevance to the superconducting state in UGe$_2$.",0301333v1 2003-02-03,Andreev bound states in ferromagnet-superconductor nanostructures,"We discuss the properties of a ferromagnet - superconductor heterostructure on the basis of a Hubbard model featuring exchange splitting in the ferromagnet and electron - electron attraction in the superconductor. We have solved the spin - polarized Hartree - Fock - Gorkov equations together with the Maxwell's equation (Ampere's law) fully self-consistently. We have found that a Proximity Effect - Fulde - Ferrell - Larkin - Ovchinnikov state is realized in such a heterostructure. It manifests itself in an oscillatory behavior of the pairing amplitude in the ferromagnet and spontaneously generated spin polarized current in the ground state. We argue that it is built up from the Andreev bound states, whose energy can be tuned by the exchange splitting and hence can coincide with the Fermi energy giving rise to a current carrying $\pi$-state. We also suggest experiments to verify these predictions.",0302025v1 2003-02-08,Current carrying Andreev bound states in a Superconductor-Ferromagnet proximity system,"We study the ground state properties of a ferromagnet-superconductor heterostructure on the basis of a quasiclassical theory. We have solved the Eilenberger equations together with Maxwell's equation fully self-consistently and found that due to the proximity effect a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) like state is realized in such system. Moreover this state has oscillations of the pairing amplitude in either one or two directions, depending on the exchange splitting and thickness of the ferromagnet. In particular, using semiclassical arguments (Bohr-Sommerfeld quantization rule) we show that owing to the presence of the Andreev bound states in the ferromagnet, a spontaneous current in the ground state is generated as a hallmark of the FFLO state in the direction parallel to the interface. We also discuss the effects of the the elastic disorder and finite transparency of the interface on the properties of the $FFLO$ state in the system.",0302162v1 2003-02-24,Magnon-assisted transport and thermopower in ferromagnet-normal metal tunnel junctions,"We develop a theoretical model of magnon-assisted transport in a mesoscopic tunnel junction between a ferromagnetic metal and a normal (non-magnetic) metal. The current response to a bias voltage is dominated by the contribution of elastic processes rather than magnon-assisted processes and the degree of spin polarization of the current, parameterized by a function $P (\Pi_{\uparrow (\downarrow)},\Pi_{N})$, $0 \leq P \leq 1$, depends on the relative sizes of the majority $\Pi_{\uparrow}$ and minority $\Pi_{\downarrow}$ band Fermi surface in the ferromagnet and of the Fermi surface of the normal metal $\Pi_{N}$. On the other hand, magnon-assisted tunneling gives the dominant contribution to the current response to a temperature difference across the junction. The resulting thermopower is large, $S \sim - (k_B/e) (k_BT/\omega_{D})^{3/2} P (\Pi_{\uparrow (\downarrow)},\Pi_{N})$, where the temperature dependent factor $(k_{B}T/\omega_{D})^{3/2}$ reflects the fractional change in the net magnetization of the ferromagnet due to thermal magnons at temperature $T$ (Bloch's $T^{3/2}$ law) and $\omega_{D}$ is the magnon Debye energy.",0302485v1 2003-03-11,In(1-x)Mn(x)Sb - a new narrow gap ferromagnetic semiconductor,"A narrow-gap ferromagnetic In(1-x)Mn(x)Sb semiconductor alloy was successfully grown by low-temperature molecular beam epitaxy on CdTe/GaAs hybrid substrates. Ferromagnetic order in In(1-x)Mn(x)Sb was unambiguously established by the observation of clear hysteresis loops both in direct magnetization measurements and in the anomalous Hall effect, with Curie temperatures T_C ranging up to 8.5 K. The observed values of T_C agree well with the existing models of carrier-induced ferromagnetism.",0303212v1 2003-04-09,How to make semiconductors ferromagnetic: A first course on spintronics,"The rapidly developing field of ferromagnetism in diluted magnetic semiconductors, where a semiconductor host is magnetically doped by transition metal impurities to produce a ferromagnetic semiconductor (e.g. Ga_{1-x}Mn_xAs with x ~ 1-10 %), is discussed with the emphasis on elucidating the physical mechanisms underlying the magnetic properties. Recent key developments are summarized with critical discussions of the roles of disorder, localization, band structure, defects, and the choice of materials in producing good magnetic quality and high Curie temperature. The correlation between magnetic and transport properties is argued to be a crucial ingredient in developing a full understanding of the properties of ferromagnetic semiconductors.",0304219v1 2003-04-24,Competing Orders and Disorder-induced Insulator to Metal Transition in Manganites,"Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remain the charge/lattice fluctuations instead which are enhanced toward the transition temperature.",0304543v2 2003-05-27,Probing a ferromagnetic critical regime using nonlinear susceptibility,"The second order para-ferromagnetic phase transition in a series of amorphous alloys (Fe{_5}Co{_{50}}Ni{_{17-x}}Cr{_x}B{_{16}}Si{_{12}}) is investigated using nonlinear susceptibility. A simple molecular field treatment for the critical region shows that the third order suceptibility (chi{_3}) diverges on both sides of the transition temperature, and changes sign at T{_C}. This critical behaviour is observed experimentally in this series of amorphous ferromagnets, and the related assymptotic critical exponents are calculated. It is shown that using the proper scaling equations, all the exponents necessary for a complete characterization of the phase transition can be determined using linear and nonlinear susceptiblity measurements alone. Using meticulous nonlinear susceptibility measurements, it is shown that at times chi{_3} can be more sensitive than the linear susceptibility (chi{_1}) in unravelling the magnetism of ferromagnetic spin systems. A new technique for accurately determining T{_C} is discussed, which makes use of the functional form of chi{_3} in the critical region.",0305617v1 2003-06-15,Room-Temperature Ferromagnetism in Co-Doped TiO$_2$ Anatase: Role of Interstitial Co,"TiO$_2$ anatase doped with Co has been recently reported to exhibit room-temperature ferromagnetism. $Ab$ $initio$ study on substitutional Co doping, however, yielded much larger magnetic moment for Co than experiment. Our calculations based on density-functional theory show that the substitutional Co ions incorporated into TiO$_2$ anatase tend to cluster and then the neighboring interstitial tetrahedral sites become energetically favorable for Co to reside, yielding a local environment more like Co$_3$O$_4$ than CoTiO$_3$. The interstitial Co destroys the spin-polarization of the surrounding substitutional Co but enhances the stability of the ferromagnetism significantly. In the absence of carriers, this room-temperature ferromagnetism can only be accounted for by superexchange interaction.",0306389v1 2003-06-18,Hall effect and magnetoresistance in p-type ferromagnetic semiconductors,"Recent works aiming at understanding magnetotransport phenomena in ferromagnetic III-V and II-VI semiconductors are described. Theory of the anomalous Hall effect in p-type magnetic semiconductors is discussed, and the relative role of side-jump and skew-scattering mechanisms assessed for (Ga,Mn)As and (Zn,Mn)Te. It is emphasized that magnetotransport studies of ferromagnetic semiconductors in high magnetic fields make it possible to separate the contributions of the ordinary and anomalous Hall effects, to evaluate the role of the spins in carrier scattering and localization as well as to determine the participation ratio of the ferromagnetic phase near the metal-insulator transition. A sizable negative magnetoresistance in the regime of strong magnetic fields is assigned to the weak localization effect.",0306484v2 2003-06-29,The Origin of Weak Ferromagnetism in CaB6,"We have combined the results of magnetization and Hall effect measurements to conclude that the ferromagnetic moments of lightly doped CaB6 samples display no systematic variation with electron doping level. Removal of the surface with acid etching substantially reduces the measured moment, although the Hall constant and resistivity are unaffected, indicating that the ferromagnetism largely resides on and near the sample surface. Electron microprobe experiments reveal that Fe and Ni are found at the edges of facets and growth steps, and on other surface features introduced during growth. Our results indicate that the weak ferromagnetism previously reported in undoped CaB6 is at least partly extrinsic.",0306709v1 2003-07-10,Axial and off-axial dynamic transitions in uniaxially anisotropic Heisenberg ferromagnet: A comparison,"Uniaxially anisotropic Heisenberg ferromagnet, in the presence of a magnetic fieldvarying sinusoidally in time, is studied by Monte Carlo Simulation. The axial (field applied only along the direction of anisotropy) and off-axial (field applied only along the direction which is prependicular to the direction of anisotropy) dynamic transitions are studied. By studying the distribution of the dynamic order parameter component, it is observed that the axial transition is discontinuous for low anisotropy and becomes continuous in high anisotropy. The off-axial transition is found to be continuous for all values of anisotropy. In the infinite anisotropy limit, both types of transitions are compared with athat observed in an Ising ferromagnet for the same value of the field and frequency. The infinitely anisotropic axial transition and dynamic transition in the Ising ferromagnet occur at different temperatures, whereas the infinitely anisotropic off-axial transition and the equilibrium ferro-para transition in the Ising model occur at the same temperature.",0307225v1 2003-08-07,Spin-Fermion model of $UGe_2$,"It is assumed that U atoms in $UGe_2$ have a number of $f$ electrons appropriate to give them each a spin $s=1$ as well as one extra itinerant electron which may equally well be on one or other U atom. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 fermi operators of itinerant electrons. Applying hydrostatic pressure changes the bandwidths of spin-up and spin-down itinerant electrons in different way, which leads to decreasing of the contribution of the fermions to the magnetization keeping the spin-fermion interaction unchanged. In turn the local spin-fermion interaction leads to ferromagnetic superconductivity. The model accounts, in a quantitative and natural way, for the characteristics of the coexistence of superconductivity and ferromagnetism in $UGe_2$, including many of the key experimental results: metamagnetic transitions, quantum transition from ferromagnetism to ferromagnetic superconductivity, the position of the highest superconducting critical temperature etc.",0308134v1 2003-08-08,Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation,"By coupling a first-principles, spin-density functional calculation with an exact diagonalization study of the Hubbard model, we have searched over various functional groups for the best case for the flat-band ferromagnetism proposed by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic polymers of five-membered rings. The original proposal (poly-aminotriazole) has turned out to be the best case among the materials examined, where the reason why this is so is identified here. We have also found that the ferromagnetism, originally proposed for the half-filled flat band, is stable even when the band filling is varied away from the half-filling. All these make the ferromagnetism proposed here more experimentally inviting.",0308154v1 2003-08-08,Flat-band ferromagnetism proposed for an organic polymer crystal,"Motivated from the flat-band ferromagnetism conceived theoretically for a single chain of five membered rings (polyaminotriazole) by Arita {\it et al.}, [Phys. Rev. Lett. {\bf 88}, 127202 (2002)], we have studied whether the magnetism can indeed occur as a bulk, i.e., in a three-dimensional crystal of the polymer, by means of the spin density functional calculation. We find that the intra-chain ferromagnetism is robust against crystallization as far as the flat band is made half-filled. We have further investigated the actual crystal doped with various compounds, where HF$_2$ is shown to put the system close to the bulk ferromagnetism while stronger anions such as BF$_4$ or PF$_6$ should be promising.",0308156v1 2003-09-15,Theory of Spin Transport Induced by Ferromagnetic Proximity On a Two-Dimensional Electron Gas,"A theory of the proximity effects of the exchange splitting in a ferromagnetic metal on a two dimensional electron gas (2DEG) in a semiconductor is presented. The resulting spin-dependent energy and lifetime in the 2DEG create a marked spin-splitting in the driven in-plane current. The theory of the planar transport allows for current leakage into the ferromagnetic layer through the interface, which leads to a competition between drift and diffusion. The spin-dependent in-plane conductivity of the 2DEG may be exploited to provide a new paradigm for spintronics devices based on planar devices in a field-effect transistor configuration. An illustrative example is provided through the transport theory of a proposed spin-valve which consists of a field-effect transistor configuration with two ferromagnetic gates. Results are provided for two experimentally accessible systems: the silicon inversion layer and the naturally-formed InAs accumulation layer.",0309353v1 2003-09-24,Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films,"We report ferromagnetic and spin wave resonance absorption measurements on high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these films exhibit robust ferromagnetic long-range order, based on the fact that up to seven resonances are detected at low temperatures, and the resonance structure survives to temperatures close to the ferromagnetic transition. On the other hand, we observe a spin wave dispersion which is linear in mode number, in qualitative contrast with the quadratic dispersion expected for homogeneous samples. We perform a detailed numerical analysis of the experimental data and provide analytical calculations to demonstrate that such a linear dispersion is incompatible with uniform magnetic parameters. Our theoretical analysis of the ferromagnetic resonance data, combined with the knowledge that strain-induced anisotropy is definitely present in these films, suggests that a spatially dependent magnetic anisotropy is the most likely reason behind the anomalous behavior observed.",0309566v2 2003-10-03,Formation of mid-gap states and ferromagnetism in semiconducting CaB$_6$,"We present a consistent overall picture of the electronic structure and ferromagnetic interaction in CaB$_6$, based on our joint transport, optical, and tunneling measurements on high-quality {\em defect-controlled} single crystals. Pure CaB$_6$ single crystals, synthesized with 99.9999 %-pure boron, exhibited fully {\em semiconducting} characteristics, such as monotonic resistance for 2--300 K, a tunneling conductance gap, and an optical absorption threshold at 1.0 eV. {\em Boron-related defects} formed in CaB$_6$ single crystals synthesized with 99.9 %-pure boron induced {\em mid-gap states} 0.18 eV below the conduction band and extra free charge carriers, with the transport, optical, and tunneling properties substantially modified. Remarkably, no ferromagnetic signals were detected from single crystals made with 99.9999 %-pure boron, regardless of stoichiometry, whereas those made with 99.9 %-boron exhibited ferromagnetism within a finite range of carrier density. The possible surmise between the electronic state and magnetization will be discussed.",0310068v1 2003-10-06,"Origin of room temperature ferromagnetism in homogeneous (In,Mn)As thin films","The microstructure of (In,Mn)As thin films grown using metalorganic vapor phase epitaxy (MOVPE) was investigated to determine the origin of room temperature ferromagnetism in these films. Transmission electron microscopy (TEM) based techniques were used to investigate phase purity and compositional homogeneity. Microanalysis of an In1-xMnxAs film with x = 0.01 and a Curie temperature of 330 K exhibited a homogeneous distribution of Mn. High Mn concentration films with x = 0.20 exhibited MnAs precipitates within the (In,Mn)As matrix. The analysis indicates that room temperature ferromagnetic, single-phase (In,Mn)As can be formed by MOVPE. The origin of ferromagnetism is attributed to (In,Mn)As solid solution rather than distinct secondary Mn-rich magnetic phase(s).",0310106v1 2003-10-17,Effects of Density of States Asymmetry on Boundary Resistance of Ferromagnetic-Nonferromagnetic Metal Interface,"Asymmetry of conduction of electrons in the spin majority and minority bands of a ferromagnetic metal is well known to produce spin accumulation at the boundary with a normal metal when a current is injected through the interface. However, little emphasis has been put on the density of states (DOS) asymmetry in diffusive ferromagnetic-nonferromagnetic multilayers. We found that if DOS and conduction asymmetry differ, the electric potential in a ferromagnet falls off to its bulk value exponentially on a scale of the spin diffusion length. Therefore the boundary contribution to resistance dramatically depends on whether the potential difference is measured close to the interface or farther than the spin diffusion length from it. This result is not altered by taking surface resistance or spin-flips on the intermetallic boundary into account. Explicit answers for common multilayered spin-valve structures are given.",0310431v1 2003-10-21,Return to return point memory,"We describe a new class of systems exhibiting return point memory (RPM) that are different from those discussed before in the context of ferromagnets. We show numerically that one dimensional random Ising antiferromagnets have RPM, when configurations evolve from a large field. However, RPM is violated when started from some stable configurations at finite field unlike in the ferromagnetic case. This implies that the standard approach to understanding ferromagnetic RPM systems will fail for this case. We also demonstrate RPM with a set of variables that keep track of spin flips at each site. Conventional RPM for the spin configuration is a projection of this result, suggesting that spin flip variables might be a more fundamental representation of the dynamics. We also present a mapping that embeds the antiferromagnetic chain in a two dimensional ferromagnetic model, and prove RPM for spin exchange dynamics in the interior of the chain with this mapping.",0310484v1 2003-11-05,Ferromagnetism from localized deep impurities in magnetic semiconductors,"We propose that localized defects in magnetic semiconductors act as deep impurities and can be described by the Anderson model. Within this model, hybridization of d-orbitals and p-orbitals gives rise to a non-RKKY indirect exchange mechanism, when the localized d-electrons are exchanged through both conduction and valence bands. For semiconductors with indirect band gap the non-RKKY part of exchange integral is antiferromagnetic, which suppresses ferromagnetism. In case of direct band gap, this exchange mechanism can, under certain conditions, lead to enhancement of ferromagnetism. The indirect exchange intergral is much stronger than RKKY, and can be sufficiently long range. Thus, a potentially new class of high-temperature magnetic semiconductors emerges, where doped carriers are not necessary to mediate ferromagnetism. Curie temperatures in such magnetic semiconductors are determined mostly by the interaction between localized impurities, not Zener mechanism. This effect could also be responsible for unusually high Curie temperatures in some magnetic semiconductors with direct band gap, such as GaMnAs.",0311114v1 2003-12-01,A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances,"In considering a novel function in ferromagnetic tunnel junctions consisting of ferromagnet(FM)/barrier/FM junctions, we theoretically investigate multiple valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; that is, (up,up), (up,down), (down,up), (down,down). To obtain such 4-valued conductances, we propose FM1/spin-polarized barrier/FM2 junctions, where the FM1 and FM2 are different ferromagnets, and the barrier has spin dependence. The proposed idea is applied to the case of the barrier having localized spins. Assuming that all the localized spins are pinned parallel to magnetization axes of the FM1 and FM2, 4-valued conductances are explicitly obtained for the case of many localized spins. Furthermore, objectives for an ideal spin-polarized barrier are discussed.",0312019v1 2003-12-11,Design of electron correlation effects in interfaces and nanostructures,"We propose that one of the best grounds for the materials design from the viewpoint of {\it electron correlation} such as ferromagnetism, superconductivity is the atomically controlled nanostructures and heterointerfaces, as theoretically demonstrated here from three examples with first-principles calculations: (i) Band ferromagnetism in a purely organic polymer of five-membered rings, where the flat-band ferromagnetism due to the electron-electron repulsion is proposed. (ii) Metal-induced gap states (MIGS) of about one atomic monolayer thick at insulator/metal heterointerfaces, recently detected experimentally, for which an exciton-mechanism superconductivity is considered. (iii) Alkali-metal doped zeolite, a class of nanostructured host-guest systems, where ferromagnetism has been experimentally discovered, for which a picture of the ""supercrystal"" composed of ""superatoms"" is proposed and Mott-insulator properties are considered. These indicate that design of electron correlation is indeed a promising avenue for nanostructures and heterointerfaces.",0312275v2 2003-12-16,Polaronic Aspects of the two-dimensional Ferromagnetic Kondo Model,"The 2D ferromagnetic Kondo model with classical corespins is studied for a hole doping up to x = 0.125 via unbiased Monte Carlo (MC) simulations. A canonical algorithm for finite temperatures is developed. We show that with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation on a two-dimensional lattice but rather stabilizes individual ferromagnetic polarons. A detailed analysis of unbiased MC results reveals that the polarons can be treated as independent particles. It is found that a simple polaron model perfectly describes the physics of the FM Kondo model. The ferromagnetic polaron picture provides an obvious explanation for the pseudogap in the one-particle spectral function $A_k(\omega)$ observed in the FM Kondo model.",0312394v2 2004-01-05,Influence of s-d scattering on the electron density of states in ferromagnet/superconductor bilayer,"We study the dependence of the electronic density of states (DOS) on the distance from the boundary for a ferromagnet/superconductor bilayer. We calculate the electron density of states in such structure taking into account the two-band model of the ferromagnet (FM) with conducting s and localized d electrons and a simple s-wave superconductor (SC). It is demonstrated that due to the electron s-d scattering in the ferromagnetic layer in the third order of s-d scattering parameter the oscillation of the density of states has larger period and more drastic decrease in comparison with the oscillation period for the electron density of states in the zero order.",0401037v2 2004-01-09,Tailoring ferromagnetic chalcopyrites,"If magnetic semiconductors are ever to find wide application in real spintronic devices, their magnetic and electronic properties will require tailoring in much the same way that band gaps are engineered in conventional semiconductors. Unfortunately, no systematic understanding yet exists of how, or even whether, properties such as Curie temperatures and band gaps are related in magnetic semiconductors. Here we explore theoretically these and other relationships within 64 members of a single materials class, the Mn-doped II-IV-V2 chalcopyrites, three of which are already known experimentally to be ferromagnetic semiconductors. Our first-principles results reveal a variation of magnetic properties across different materials that cannot be explained by either of the two dominant models of ferromagnetism in semiconductors. Based on our results for structural, electronic, and magnetic properties, we identify a small number of new stable chalcopyrites with excellent prospects for ferromagnetism.",0401157v2 2004-02-19,Quantum fluctuation driven first order phase transition in weak ferromagnetic metals,"In a local Fermi liquid (LFL), we show that there is a line of weak first order phase transitions between the ferromagnetic and paramagnetic phases due to purely quantum fluctuations. We predict that an instability towards superconductivity is only possible in the ferromagnetic state. At T=0 we find a point on the phase diagram where all three phases meet and we call this a quantum triple point (QTP). A simple application of the Gibbs phase rule shows that only these three phases can meet at the QTP. This provides a natural explanation of the absence of superconductivity at this point coming from the paramagnetic side of the phase diagram, as observed in the recently discovered ferromagnetic superconductor, $UGe_{2}$.",0402507v1 2004-03-26,Spin screening of magnetic moments in superconductors,"We consider ferromagnetic particles embedded into a superconductor and study the screening of their magnetic moments by the spins of the Cooper pairs in the superconductor. It is shown that a magnetic moment opposite to the one of the ferromagnetic particle is induced in the superconductor. In the case of a small itinerant ferromagnet grain and low temperatures the full screening of the magnetic moment takes place, \textit{% i.e} the absolute value of the total magnetic moment induced in the superconductor is equal to the one of the ferromagnetic particle. In type II superconductors the proposed screening by spins of the conduction electrons can be much stronger than the conventional screening by Meissner currents.",0403658v1 2004-05-02,Spin Dynamics and Multiple Reflections in Ferromagnetic Film in Contact with Normal Metal Layers,"Spin dynamics of a metallic ferromagnetic film imbedded between normal metal layers is studied using the spin-pumping theory of Tserkovnyak et al. [Phys. Rev. Lett. 88, 117601 (2002)]. The scattering matrix for this structure is obtained using a spin-dependent potential with quantum well in the ferromagnetic region. Owing to multiple reflections in the well, the excess Gilbert damping and the gyromagnetic ratio exhibit quantum oscillations as a function of the thickness of the ferromagnetic film. The wavelength of the oscillations is given by the depth of the quantum well. For iron film imbedded between gold layers, the amplitude of the oscillations of the Gilbert damping is in an order of magnitude agreement with the damping observed by Urban et al. [Phys. Rev. Lett. 87, 217204 (2001)]. The results are compared with the linear response theory of Mills [Phys. Rev. B 68, 0144419 (2003)].",0405020v1 2004-05-10,Quantum and thermal effects in the double exchange ferromagnet,"The physics of the ferromagnetic phase of the ``double exchange'' model has been widely discussed in the context of the CMR manganites. Usually, the double exchange ferromagnet is treated is classically, by mapping it onto an effective Heisenberg model. However this mapping does not permit a correct treatment of quantum or thermal fluctuation effects, and the results obtained lack many of the interesting features seen in experiments on the manganites. Here we outline a new analytic approach to systematically evaluating quantum and thermal corrections to the magnetic and electronic properties of the double exchange ferromagnet.",0405189v1 2004-05-15,Positive exchange bias in ferromagnetic La0.67Sr0.33MnO3 / SrRuO3 bilayers,"Epitaxial La0.67Sr0.33MnO3 (LSMO)/ SrRuO3 (SRO) ferromagnetic bilayers have been grown on (001) SrTiO3 (STO) substrates by pulsed laser deposition with atomic layer control. We observe a shift in the magnetic hysteresis loop of the LSMO layer in the same direction as the applied biasing field (positive exchange bias). The effect is not present above the Curie temperature of the SRO layer (), and its magnitude increases rapidly as the temperature is lowered below . The direction of the shift is consistent with an antiferromagnetic exchange coupling between the ferromagnetic LSMO layer and the ferromagnetic SRO layer. We propose that atomic layer charge transfer modifies the electronic state at the interface, resulting in the observed antiferromagnetic interfacial exchange coupling.",0405338v1 2004-06-01,Diagnosis of transport properties in Ferromagnets,"The resistivity model as a function of temperature and ionization energy (doping) is derived with further confinements from spin-disorder scattering in ferromagnetic phase. Magnetization and polaronic effects capture the mechanism of both spin independent and spin-assisted charge transport of ferromagnets, including the newly reported Mn$_x$Ge$_{1-x}$ ferromagnetic semiconductor. The computed $T_{crossover}$ below $T_C$ and carrier density in Ga$_{1-x}$Mn$_x$As system are 8-12 K and 10$^{19}$ cm$^{-3}$, remarkably identical with the experimental values of 10-12 K and 10$^{18}-10^{20}$ cm$^{-3}$ respectively. The calculated charge carriers density for Mn$_x$Ge$_{1-x}$ is 10$^{19}$ cm$^{-3}$, which is also in the same order with the experimental values.",0406030v8 2004-06-23,Carrier induced ferromagnetism in concentrated and diluted local-moment systems,"For modeling the magnetic properties of concentrated and diluted magnetic semiconductors, we use the Kondo-lattice model. The magnetic phase diagram is derived by inspecting the static susceptibility of itinerant band electrons, which are exchange coupled to localized magnetic moments. It turns out that rather low band occupations favour a ferromagnetic ordering of the local moment systems due to an indirect coupling mediated by a spin polarization of the itinerant charge carriers. The disorder in diluted systems is treated by adding a CPA-type concept to the theory. For almost all moment concentrations x, ferromagnetism is possible, however, only for carrier concentrations n distinctly smaller than x. The charge carrier compensation in real magnetic semiconductors (in Ga_{1-x}Mn_{x}As by e.g. antisites) seems to be a necessary condition for getting carrier induced ferromagnetism.",0406569v1 2004-08-10,The role of intra-atomic non-collinear magnetization density in weak ferromagnetism,"We investigate the mechanism behind the breakdown of the compensation of large magnetic moments leading to weak ferromagnetism. For this we use first-principles calculations within density functional theory and we focus on the weak ferromagnetic compound Mn3Sn. Our new implementation allows for an exact treatment of the spin-density matrix and non-collinearity. In order to gain some insight, our results are compared to the ones obtained by using the atomic moment approximation (AMA) and its role is discussed. We find that the appearance of the weak ferromagnetic moment in this compound originates not so much as an effect of spin-orbit coupling as suggested previously from AMA calculations, as from the non-collinearity of the Sn atom magnetization density. This is confirmed by non-collinear calculations in which the SOC effects are neglected.",0408200v1 2004-09-15,Low temperature ferromagnetic properties of the diluted magnetic semiconductor Sb2-xCrxTe3,"We report on magnetic and electrical transport properties of Sb2-xCrxTe3 single crystals with 0 <= x <= 0.095 over temperatures from 2 K to 300 K. A ferromagnetic state develops in these crystals at low temperatures with Curie temperatures that are proportional to x (for x > 0.014), attaining a maximum value of 20 K for x = 0.095. Hysteresis below TC for applied field parallel to the c-axis is observed in both magnetization and Hall effect measurements. Magnetic as well as transport data indicate that Cr takes the 3+ (3d3) valence state, substituting for antimony in the host lattice structure, and does not significantly affect the background hole concentration. Analysis of the anomalous Hall effect reveals that skew scattering is responsible for its presence. These results broaden the scope of ferromagnetism in the V2-VI3 diluted magnetic semiconductors (DMS) and in ferromagnetic DMS structures generally",0409403v1 2004-10-26,Spintronics at Nanoscale: Flat-Band Ferromagnetism in Armchair Nanoribbons and Nanotubes,"We study the electronic correlation effects in armchair nanoribbon and nanotube using weak-coupling approach and non-Abelian density-matrix renormalization-group method. We show that upon appropriate doping, the system exhibits a new type of flat-band ferromagnetism, different from the well-known Milke-Tasaki one. The strongly correlated ground state consists of intrinsic magnetic moments of flat-band states and itinerant carriers of dispersive bands, and the exchange coupling between them yields a ferromagnetism. The resultant ferromagnetic state with metallic conductivity has a potential in spintronics applications at nanoscale.",0410654v2 2004-11-13,Theory of polarized Fermi liquid,"The dispersion law of transverse spin waves known in the Stoner-Hubbard model of itinerant ferromagnetism corresponds to that is well known in more broder and well controlled approach of Fermi-liquid theory. Making use the quantum-field theoretical approach we derive the dispersion law for the transverse spin waves in a weakly polarized Fermi liquid at T=0. Along with the dissipationless part inversely proportional to the polarization it contains also the finite zero-temperature damping. It is shown that similar derivation for ""ferromagnetic Fermi liquid"" taking into consideration the divergency of static transverse susceptibility also leads to the same attenuating spin wave spectrum. Hence, in both cases we deal in fact with spin polarized Fermi liquid but not with isotropic itinerant ferromagnet where the zero temperature atenuation is prohibited by Goldstone theorem. It demonstrates, the troubles of the Fermi liquid formulation of a theory of itinerant ferromagnetic systems.",0411361v1 2004-11-26,Room-temperature ferromagnetism in Sr_(1-x)Y_xCoO_(3-delta) (0.2 < x < 0.25),"We have measured magnetic susceptibility and resistivity of Sr$_{1-x}$Y$_x$CoO$_{3-\delta}$ ($x=$ 0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3, and 0.4), and have found that Sr$_{1-x}$Y$_x$CoO$_{3-\delta}$ is a room temperature ferromagnet with a Curie temperature of 335 K in a narrow compositional range of 0.2 $\leq x\leq$ 0.25. This is the highest transition temperature among perovskite Co oxides. The saturation magnetization for $x=$ 0.225 is 0.25 $\mu_B$/Co at 10 K, which implies that the observed ferromagnetism is a bulk effect. We attribute this ferromagnetism to a peculiar Sr/Y ordering.",0411671v3 2004-12-03,High frequency magnetic permeability of nanocomposite film,"The high frequency magnetic permeability of nanocomposite film consisting of the single-domain spherical ferromagnetic particles in the dielectric matrix is studied. The permeability is assumed to be determined by rotation of the ferromagnetic inclusion magnetic moments around equilibrium direction in AC magnetic field. The composite is modeled by a cubic array of ferromagnetic particles. The magnetic permeability tensor is calculated by solving the Landau-Lifshits-Gilbert equation accounting for the dipole interaction of magnetic particles. The permeability tensor components are found as functions of the frequency, temperature, ferromagnetic inclusions density and magnetic anisotropy. The obtained results show that nanocomposite films could have rather high value of magnetic permeability in the microwave range.",0412073v1 2004-12-03,New Challenges in Cellular Automata Due to Network Geometry - Ferromagnetic Transition Study,"Self-organization to ferromagnetic phase transition in cellular automata of spins governed by stochastic majority rule and when topology between spins is changed, is investigated numerically. Three types of edge rewiring are considered. The algorithm of Watts and Strogatz is applied at each time step to establish a network evolving stochastically (the first network). The preference functions are defined to amplify the role of strongly connected vertices (the second network). Demand to preserve graph connectivity provides the third way of edge rewiring. Each of these processes yields the different network: small-world, scattered nodes with one strongly connected component, and scale-free network when rewiring is properly adjusted. The stochastic majority rule applied to these networks can lead or not to ferromagnetic transition. Classical mean-field transition in case of the first and third network is observed. Scale-free ferromagnetic transition can be observed when rewiring is properly adjusted.",0412082v1 2004-12-09,Coexistence of ferromagnetism and superconductivity near quantum phase transition: The Heisenberg- to Ising-type crossover,"A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transition is analysed and the generic phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet ZrZn2, which sheds light on the proposed phase diagram of this compound. Possible explanation of superconductivity in UGe2 is also discussed.",0412247v1 2004-12-15,Local Environment of Ferromagnetically Ordered Mn in Epitaxial InMnAs,"The magnetic properties of the ferromagnetic semiconductor In0.98Mn0.02As were characterized by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. The Mn exhibits an atomic-like L2,3 absorption spectrum that indicates that the 3d states are highly localized. In addition, a large dichroism at the Mn L2,3 edge was observed from 5-300 K at an applied field of 2T. A calculated spectrum assuming atomic Mn2+ yields the best agreement with the experimental InMnAs spectrum. A comparison of the dichroism spectra of MnAs and InMnAs show clear differences suggesting that the ferromagnetism observed in InMnAs is not due to hexagonal MnAs clusters. The temperature dependence of the dichroism indicates the presence of two ferromagnetic species, one with a transition temperature of 30 K and another with a transition temperature in excess of 300 K. The dichroism spectra are consistent with the assignment of the low temperature species to random substitutional Mn and the high temperature species to Mn near-neighbor pairs.",0412384v1 2004-12-22,"Crystal structures and magnetic order of La{0.5+delta}A{0.5-delta}Mn{0.5+epsilon}Ru{0.5-epsilon}O{3} (A= Ca, Sr, Ba): Possible orbital glass ferromagnetic state","The crystallographic and magnetic properties of La{0.5+delta}A{0.5-delta}Mn{0.5+epsilon}Ru{0.5-epsilon}O{3} (A= Ca, Sr, Ba) were investigated by means of neutron powder diffraction. All studied samples show the orthorhombic perovskite crystal structure, space group Pnma, with regular (Mn,Ru)O{6} octahedra and no chemical ordering of the Mn3+ and Ru4+ ions. Ferromagnetic spin structures were observed below Tc ~ 200-250 K, with an average ordered moment of ~ 1.8-2.0 Bohr magnetons per (Mn,Ru). The observation of long-range ferromagnetism and the absence of orbital ordering are rationalized in terms a strong Mn-Ru hybridization, which may freeze the orbital degree of freedom and broaden the eg valence band, leading to an orbital-glass state with carrier-mediated ferromagnetism.",0412621v1 2005-02-02,Scanning Tunneling Spectroscopy of the superconducting proximity effect in a diluted ferromagnetic alloy,"We studied the proximity effect between a superconductor (Nb) and a diluted ferromagnetic alloy (CuNi) in a bilayer geometry. We measured the local density of states on top of the ferromagnetic layer, which thickness varies on each sample, with a very low temperature Scanning Tunneling Microscope. The measured spectra display a very high homogeneity. The analysis of the experimental data shows the need to take into account an additional scattering mechanism. By including in the Usadel equations the effect of the spin relaxation in the ferromagnetic alloy, we obtain a good description of the experimental data.",0502050v2 2005-02-16,Global phase diagrams of binary dipolar fluid mixtures,"We apply a modified mean-field density functional theory to determine the phase behavior of binary mixtures of Stockmayer fluids whose spherical constituents interact according to Lennard-Jones (LJ) pair potentials with embedded pointlike dipole moments. On the basis of systematic numerical calculations we construct the global phase diagrams of these systems in the three-dimensional thermodynamic space of temperature, pressure, and chemical potential difference of the two components. The vapor-liquid, isotropic liquid - isotropic liquid, isotropic liquid - ferromagnetic liquid, and ferromagnetic liquid - ferromagnetic liquid first-order phase separations are investigated. The loci of the second-order isotropic fluid - ferromagnetic fluid transition are calculated from Landau theory. Liquid-vapor and liquid-liquid critical lines, tricritical lines, triple lines, and lines of critical end points of the binary Stockmayer mixtures are also determined. We discuss how the topology of the phase diagrams change upon varying the strengths of the two dipole moments of the two species as well as their sizes.",0502405v3 2005-02-24,Nonlocal pure spin current injection via quantum pumping and crossed Andreev reflection,"A pure spin current injector is proposed based on adiabatic pumping and crossed normal/Andreev reflection. The device consists of a three-terminal ferromagnet-superconductor-semiconductor system in which the injection of a pure spin current is into the semiconductor which is coupled to the superconductor within a coherence length away from the ferromagnet enabling the phenomena of crossed normal /Andreev reflection to operate. Quantum pumping is induced by adiabatically modulating two independent parameters of the ferromagnetic lead, namely the magnetization strength and the strength of coupling between the ferromagnet and the superconductor. The competition between the normal/Andreev reflection and the crossed normal/Andreev reflection, both induced by pumping, leads to non-local injection of a pure spin current into the semiconductor. The experimental realization of the proposed device is also discussed.",0502589v2 2005-03-01,Berry phase of magnons in textured ferromagnets,"We study the energy spectrum of magnons in a ferromagnet with topologically nontrivial magnetization profile. In the case of inhomogeneous magnetization corresponding to a metastable state of ferromagnet, the spin-wave equation of motion acquires a gauge potential leading to a Berry phase for the magnons propagating along a closed contour. The effect of magnetic anisotropy is crucial for the Berry phase: we show that the anisotropy suppresses its magnitude, which makes the Berry phase observable in some cases, similar to the Aharonov-Bohm effect for electrons. For example, it can be observed in the interference of spin waves propagating in mesoscopic rings. We discuss the effect of domain walls on the interference in ferromagnetic rings, and propose some experiments with a certain geometry of magnetization. We also show that the nonvanishing average topological field acts on the magnons like a uniform magnetic field on electrons. It leads to the quantization of the magnon spectrum in the topological field.",0503013v1 2005-03-17,Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4,"Multiferroic materials, which reveal magnetic and electric order, are in the focus of recent solid state research. Especially the simultaneous occurrence of ferroelectricity and ferromagnetism, combined with an intimate coupling of magnetization and polarization via magneto-capacitive effects, could pave the way for a new generation of electronic devices. Here we present measurements on a simple cubic spinel with unusual properties: It shows ferromagnetic order and simultaneously relaxor ferroelectricity, i.e. a ferroelectric cluster state, reached by a smeared-out phase transition, both with sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature the magneto-capacitive coupling, characterized by a variation of the dielectric constant in an external magnetic field, reaches colossal values of nearly 500%. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments, but here is found to impede long-range order of the structural degrees of freedom.",0503434v1 2005-04-01,Coherent effects in double-barrier ferromagnet/superconductor/ferromagnet junctions,"Coherent quantum transport in ferromagnet/superconductor/ferromagnet (FSF) double-barrier junctions is studied. Analytic expressions for charge and spin conductance spectra are derived for the general case of insulating interfaces (from metallic to tunnel limit), the Fermi velocity mismatch, and for parallel (P) and antiparallel (AP) alignment of the electrode magnetizations. We focus on two characteristic features of finite size and coherency: subgap electronic transport, and oscillations of the differential conductance. Periodic vanishing of the Andreev reflection at the energies of geometrical resonances above the superconducting gap is a striking consequence of the quasiparticle interference. In contrast with the case of incoherent transport, a non-trivial spin-polarization without the excess spin accumulation is found for the AP alignment.",0504029v1 2005-04-07,Observation of spin-wave characteristics in the two-dimensional ferromagnetic ordering of in-plane spins,"The role of dipolar interactions and anisotropy are important to obtain, otherwise forbidden, ferromagnetic ordering at finite temperature for ions arranged in two-dimensional (2D) arrays (monolayers). Here we demonstrate that conventional low temperature magnetometry and polarized neutron scattering measurements can be performed to study ferromagnetic ordering of in-plane spins in 2D systems using a multilayer stack of non-interacting monolayers of gadolinium ions. The spontaneous magnetization is absent in the heterogenous magnetic phase observed here and the saturation value of the net magnetization was found to depend on the applied magnetic field. The net magnetization rises exponentially with lowering temperature and then reaches saturation following a $T\ln(\beta T)$ dependence. These findings verify predictions of the spin-wave theory of 2D in-plane spin system with ferromagnetic interaction and will initiate further theoretical development.",0504155v1 2005-04-15,"Competition between ferromagnetism and spin glass: the key for large magnetoresistance in oxygen deficient perovskites SrCo1-xMxO3-d (M = Nb, Ru)","The magnetic and magnetotransport properties of the oxygen deficient perovskites, SrCo1-xMxO3-d with M = Nb and Ru, were investigated. Both Nb- and Ru-substituted cobaltites are weak ferromagnets, with transition temperatures Tm of 130-150 K and 130-180 K, respectively, and both exhibit a spin glass behavior at temperatures below Tf = 80-90 K. It is demonstrated that there exists a strong competition between ferromagnetism and spin glass state, where Co4+ induces ferromagnetism, whereas Nb or Ru substitution at the cobalt sites induces magnetic disorder, and this particular magnetic behavior is the origin of large negative magnetoresistance of these oxides, reaching up to 30% at 5 K in 7 T. The differences between Nb- and Ru-substituted cobaltites are discussed on the basis of the different electronic configuration of niobium and ruthenium cations.",0504379v1 2005-04-27,The effect of stripe domain structure on dynamic permeability of thin ferromagnetic films with out-of-plane uniaxial anisotropy,"The permeability is calculated for a thin ferromagnetic film with the stripe domain structure and out-of-plane uniaxial magnetic anisotropy. Analytical expressions for the frequency dependence of components of permeability tensor are derived with the use of the Smit-Beljers method, with the thickness of domain walls and the domain wall motion being neglected. The effect of the domain width and the angle between the anisotropy axis and the film plane on the frequency dependence of the permeability is analyzed. General equations relating the static permeability components and the ferromagnetic resonance frequencies are found. The results of the approach are applied to the derivation of the constraint for the microwave permeability of thin ferromagnetic films. The analysis of the constraint as a function of the axis deviation angle, the domain aspect ratio and the damping parameter allows the conditions to be found for maximal microwave permeability. The results obtained may be useful in connection with the problem of developing high-permeable microwave magnetic materials.",0504710v1 2005-05-15,"Ion Irradiation Control of Ferromagnetism in (Ga,Mn)As","We report on a promising approach to the artificial modification of ferromagnetic properties in (Ga,Mn)As using a Ga$^+$ focused ion beam (FIB) technique. The ferromagnetic properties of (Ga,Mn)As such as magnetic anisotropy and Curie temperature can be controlled using Ga$^+$ ion irradiation, originating from a change in hole concentration and the corresponding systematic variation in exchange interaction between Mn spins. This change in hole concentration is also verified using micro-Raman spectroscopy. We envisage that this approach offers a means of modifying the ferromagnetic properties of magnetic semiconductors on the micro- or nano-meter scale.",0505365v1 2005-05-24,Appearance of room temperature ferromagnetism in Cu-doped TiO$_{2-δ}$ films,"In recent years there has been an intense search for room temperature ferromagnetism in doped dilute semiconductors, which have many potentially applications in spintronics and optoelectronics. We report here the unexpected observation of significant room temperature ferromagnetism in a semiconductor doped with nonmagnetic impurities, Cu-doped TiO$_2$ thin films grown by Pulsed Laser Deposition. The magnetic moment, calculated from the magnetization curves, resulted surprisingly large, about 1.5 $\mu_B$ per Cu atom. A large magnetic moment was also obtained from ab initio calculations using the supercell method for TiO$_2$ with Cu impurities, but only if an oxygen vacancy in the nearest-neighbour shell of Cu was present. This result suggests that the role of oxygen vacancies is crucial for the appearance of ferromagnetism. The calculations also predict that Cu doping favours the formation of oxygen vacancies.",0505602v2 2005-06-01,Stability of thermodynamic and dynamical order in a system of globally coupled rotors,"A system of globally coupled rotors is studied in a unified framework of microcanonical and canonical ensembles. We consider the Fokker-Planck equation governing the time evolution of the system, and examine various stationary as well as non-stationary solutions. The canonical distribution, describing equilibrium, provides a stationary solution also in the microcanonical ensemble, which leads to order in a system with ferromagnetic coupling at low temperatures. On the other hand, the microcanonical ensemble admits additional stationary and non-stationary solutions; the latter allows dynamical order, characterized by multiple degrees of clustering, for both ferromagnetic and antiferromagnetic interactions. We present a detailed stability analysis of these solutions: In a ferromagnetic system, the canonical distribution is observed stable down to a certain temperature, which tends to get lower as the number of Fourier components of the perturbed distribution is increased in the analysis. The non-stationary solution remains neutrally stable below the critical temperature, indicating inequivalence between the two ensembles. For antiferromagnetic systems, all solutions are found to be neutrally stable at all temperatures, suggesting that dynamical ordering is relatively easy to observe at low temperatures compared with ferromagnetic systems.",0506014v1 2005-06-02,Odd Triplet Superconductivity and Related Phenomena in Superconductor-Ferromagnet Structures,"We consider novel unusual effects in superconductor-ferromagnet (S/F) structures. In particular we analyze the triplet component (TC) of the condensate generated in those systems.This component is odd in frequency and even in the momentum, which makes it insensitive to non-magnetic impurities. If the exchange field is not homogeneous in the system the triplet component is not destroyed even by a strong exchange field and can penetrate the ferromagnet over long distances. Some other effects considered here and caused by the proximity effect are: enhancement of the Josephson current due to the presence of the ferromagnet, induction of a magnetic moment in superconductors resulting in a screening of the magnetic moment, formation of periodic magnetic structures due to the influence of the superconductor, etc. We compare the theoretical predictions with existing experiments.",0506047v2 2005-06-06,Unusual transport properties of ferromagnetic Heusler alloy Co$_2$TiSn,"We report results of magnetization, zero field resistivity and magnetoresistance measurements in ferromagnetic Heusler alloy Co$_2$TiSn. There is a striking change in the character of electron transport as the system undergoes the paramagnetic to ferromagnetic transition. In the paramagnetic state the nature of the electron transport is like that of a semiconductor and this changes abruptly to metallic behaviour at the onset of ferromagnetic ordering. Application of external magnetic field tends to suppress this semiconducting like transport leading to a negative magnetoresistance which reaches a peak in the vicinity of Curie temperature. Comparison is made with the similar unusual behaviour observed in other systems including UNiSn and manganites.",0506126v1 2005-06-06,Nanoscale ferromagnet-superconductor-ferromagnet switches controlled by magnetization orientation,"We study clean ferromagnet-superconductor-ferromagnet (FSF) nanostructures in which the magnetization of the F layers can be parallel (P) or antiparallel (AP). We consider the case where the thickness of the S layer is of order of the coherence length, with thinner F layers. We find that reversing the direction of the magnetization in one of the F layers leads in general to drastic changes in the superconductor's state. Under a wide variety of conditions, the AP geometry favors superconductivity. Magnetization reversal in one of the F layers can lead to the superconductivity turning on and off, or to switching between different states. Our results are obtained via self consistent solution of the Bogoliubov-de Gennes equations and evaluation of the condensation energies of the system.",0506147v1 2005-06-15,IPA-CuCl$_3$: a S=1/2 Ladder with Ferromagnetic Rungs,"The spin gap material IPA-CuCl3 has been extensively studied as a ferromagnetic-antiferromagnetic bondalternating S = 1/2 chain. This description of the system was derived from structural considerations and bulk measurements. New inelastic neutron scattering experiments reveal a totally different picture: IPA-CuCl3 consists of weakly coupled spin ladders with antiferromagnetic legs and ferromagnetic rungs. The ladders run perpendicular to the originally supposed bondalternating chain direction. The ferromagnetic rungs make this system equivalent to a Haldane S = 1 antiferromagnet. With a gap energy of 1.17(1) meV, a zone-boundary energy of 4.1(1) meV, and almost no magnetic anisotropy, IPA-CuCl3 may the best Haldane-gap material yet, in terms of suitability for neutron scattering studies in high magnetic fields.",0506382v1 2005-08-05,Triplet Pairing Superconductivity Induced by Short-Range Ferromagnetic Correlations in Sr$_2$RuO$_4$,"The microscopic origin of triplet superconductivity in Sr$_2$RuO$_4$ is discussed, paying attention to the role of Coulomb interaction, $U_{pp}$, at the O site. It is shown on the $d$-$p$ model that $U_{pp}$ induces a ferromagnetic exchange interaction between ""d-electrons"" (molecular orbital with $d_{xy}$-symmetry) at adjacent Ru sites, leading to short-range ferromagnetic correlations and promoting Cooper pairing with ($\sin p_x\pm\ii\sin p_y$)-symmetry on the $\gamma$-band. The reason why such ferromagnetic correlations work effectively may be traced back to the fact that the level of 4d-electrons at Ru sites is relatively low and located near that of $2p$-electrons at O sites.",0508147v1 2005-08-30,Cooperative Phenomenon of Ferromagnetism and Unconventional Superconductivity in UGe$_2$: A ^73^Ge-NQR Study under Pressure,"We report on a cooperative phenomenon of ferromagnetism and unconventional superconductivity (SC) in UGe$_2$ through the measurements of $^{73}$Ge nuclear-quadrupole-resonance (NQR) under pressure ($P$). The NQR spectra evidenced phase separation into ferromagnetic and paramagnetic phases in the vicinity of $P_c\sim 1.5$ GPa, pointing to a first-order transition. The measurements of nuclear-spin-lattice-relaxation-rate $1/T_1$ revealed that SC emerges under the background of ferromagnetism, but not of the paramagnetic phase.",0508731v1 2005-09-01,Ferromagnetic resonance study of polycrystalline Cobalt ultrathin films,"We present room temperature ferromagnetic resonance (FMR) studies of polycrystalline ||Pt/10 nm Cu/t Co/10 nm Cu/Pt|| films as a function of Co layer thickness (1 < t < 10 nm) grown by evaporation and magnetron sputtering. FMR was studied with a high frequency broadband coplanar waveguide (up to 25 GHz) using a flip-chip method. The resonance field and the linewidth were measured as a function of the ferromagnetic layer thickness. The evaporated films exhibit a lower magnetization density (Ms = 1131 emu/cm^3) compared to the sputtered films (Ms= 1333 emu/cm^3), with practically equal perpendicular surface anisotropy (Ks ~ -0.5 erg/cm^2). For both series of films, a strong increase of the linewidth was observed for Co layer thickness below 3 nm. For films with a ferromagnetic layer thinner than 4 nm, the damping of the sputtered films is larger than that of the evaporated films. The thickness dependence of the linewidth can be understood in term of the spin pumping effect, from which the interface spin mixing conductance g^{\uparrow\downarrow}S^{-1} is deduced.",0509036v1 2005-09-06,Inverse spin switch effects in Ferromagnet / Superconductor hybrids with strong ferromagnets,"In F/S/F trilayers where the magnetization directions of the F layers can be controlled separately, it has theoretically been predicted that the antiparallel (AP) configuration can have a higher superconducting transition temperature T_c than the parallel configuration. This is the so-called spin switch, which also experimentally has been found for the case of weak ferromagnets. Here we show that strong ferromagnets yield the opposite effect. We study the transport properties of F/S/F trilayers with F = Ni_0.80Fe_0.20 (Permalloy, Py) and S = Nb, structured in strips of different sizes. Using two different thicknesses for the Py layers, we can switch in a well-defined way between the AP- and P-configurations. In the superconducting transition we find a clear increase of the resistance in the AP-state. We ascribe this to enhanced reflection of spin-polarized quasiparticles at the S/F interfaces which leads to a stronger suppression of superconductivity on the S-side.",0509156v1 2005-09-23,Clustering in disordered ferromagnets: The Curie temperature in diluted magnetic semiconductors,"We theoretically investigate impurity correlation and magnetic clustering effects on the long-range ferromagnetic ordering in diluted magnetic semiconductors, such as $\textrm{Ga}_{1-x}\textrm{Mn}_{x}\textrm{As}$, using analytical arguments and direct Monte Carlo simulations. We obtain an analytic formula for the ferromagnetic transition temperature $T_{c}$ which becomes asymptotically exact in the strongly disordered, highly dilute (i.e. small $x$) regime. We establish that impurity correlations have only small effects on $T_{c}$ with the neutrally correlated random disorder producing the nominally highest $T_{c}$. We find that the ferromagnetic order is approached from the high temperature paramagnetic side through a random magnetic clustering phenomenon consistent with the percolation transition scenario.",0509614v2 2005-09-28,Effect of weak disorder on the ground state of uniaxial dipolar spin systems in the upper critical dimension,"Extensive Monte Carlo simulations are used to investigate the stability of the ferromagnetic ground state in three-dimensional systems of Ising dipoles with added quenched disorder. These systems model the collective ferromagnetic order observed in various systems with dipolar long-range interactions. The uniaxial dipolar spins are arranged on a face-centred cubic lattice with periodic boundary conditions. Finite-size scaling relations for the pure dipolar ferromagnetic system are derived by a renormalisation group calculation. These functions include logarithmic corrections to the expected mean field behaviour since the system is in its upper critical dimension. Scaled data confirm the validity of the finite-size scaling description and results are compared with subsequent analysis of weakly disordered systems. A disorder-temperature phase diagram displays the preservation of the ferromagnetic ground state with the addition of small amounts of disorder, suggesting the irrelevance of weak disorder in these systems.",0509751v1 2005-10-14,Ferromagnetism Induced by Uniaxial Pressure in the Itinerant Metamagnet Sr3Ru2O7,"We report a uniaxial-pressure study on the magnetisation of single crystals of the bilayer perovskite Sr3Ru2O7, a metamagnet close to a ferromagnetic instability. We observed that the application of a uniaxial pressure parallel to the c-axis induces ferromagnetic ordering with a Curie temperature of about 80 K and critical pressures of about 4 kbar or higher. This value for the critical pressure is even higher than the value previously reported (~ 1 kbar), which might be attributed to the difference of the impurity level. Below the critical pressure parallel to the c-axis, the metamagnetic field appears to hardly change. We have also found that uniaxial pressures perpendicular to the c-axis, in contrast, do not induce ferromagnetism, but shift the metamagnetic field to higher fields.",0510383v1 2005-10-28,Negative Refraction in Ferromagnet/Superconductor Superlattices,"Negative refraction, which reverses many fundamental aspects of classical optics, can be obtained in systems with negative magnetic permeability and negative dielectric permittivity. This Letter documents an experimental realization of negative refraction at millimeter waves, finite magnetic fields and cryogenic temperatures utilizing a multilayer stack of ferromagnetic and superconducting thin films. In the present case the superconducting YBa_2Cu_3O_7 layers provide negative permittivity while negative permeability is achieved via ferromagnetic (La:Sr)MnO_3 layers for frequencies and magnetic fields close to the ferromagnetic resonance. In these superlattices the refractive index can be switched between positive and negative regions using external magnetic field as tuning parameter.",0510777v1 2005-11-17,One-Dimensional Transition Metal-Benzene Sandwich Polymers: Possible Ideal Conductors for Spin Transport,"We investigate the electronic and magnetic properties of the proposed one-dimensional transition metal (TM=Sc, Ti, V, Cr, and Mn)-benzene (Bz) sandwich polymers by means of density functional calculations. [V(Bz)]$_{\infty}$ is found to be a quasi-half-metallic ferromagnet and half-metallic ferromagnetism is predicted for [Mn(Bz)]$_{\infty}$. Moreover, we show that stretching the [TM(Bz)]$_{\infty}$ polymers could have dramatic effects on their electronic and magnetic properties. The elongated [V(Bz)]$_{\infty}$ displays half-metallic behavior, and [Mn(Bz)]$_{\infty}$ stretched to a certain degree becomes an antiferromagnetic insulator. The possibilities to stabilize the ferromagnetic order in [V(Bz)]$_{\infty}$ and [Mn(Bz)]$_{\infty}$ polymers at finite temperature are discussed. We suggest that the hexagonal bundles composed by these polymers might display intrachain ferromagnetic order at finite temperature by introducing interchain exchange coupling.",0511417v1 2005-12-08,Modulation of the Curie Temperature in Ferromagnetic/Ferroelectric Hybrid Double Quantum Wells,"We propose a ferromagnetic/ferroelectric hybrid double quantum well structure, and present an investigation of the Curie temperature (Tc) modulation in this quantum structure. The combined effects of applied electric fields and spontaneous electric polarization are considered for a system that consists of a Mn \delta-doped well, a barrier, and a p-type ferroelectric well. We calculate the change in the envelope functions of carriers at the lowest energy sub-band, resulting from applied electric fields and switching the dipole polarization. By reversing the depolarizing field, we can achieve two different ferromagnetic transition temperatures of the ferromagnetic quantum well in a fixed applied electric field. The Curie temperature strongly depends on the position of the Mn \delta-doped layer and the polarization strength of the ferroelectric well.",0512166v2 2005-12-14,Spin-wave dispersion in orbitally ordered La(0.5)Sr(1.5)MnO(4),"The magnon dispersion in the charge, orbital and spin ordered phase in La(0.5)Sr(1.5)MnO(4) has been studied by means of inelastic neutron scattering. We find an excellent agreement with a magnetic interaction model basing on the CE-type superstructure. The magnetic excitations are dominated by ferromagnetic exchange parameters revealing a nearly-one dimensional character at high energies. The nearest neighbor ferromagnetic interaction in La(0.5)Sr(1.5)MnO(4) is significantly larger than the one in the metallic ferromagnetically ordered manganites. The large ferromagnetic interaction in the charge/orbital ordered phase appears to be essential for the capability of manganites to switch between metallic and insulating phases.",0512305v1 2005-12-29,Current-induced magnetization dynamics in disordered itinerant ferromagnets,"Current-driven magnetization dynamics in ferromagnetic metals are studied in a self-consistent adiabatic local-density approximation in the presence of spin-conserving and spin-dephasing impurity scattering. Based on a quantum kinetic equation, we derive Gilbert damping and spin-transfer torques entering the Landau-Lifshitz equation to linear order in frequency and wave vector. Gilbert damping and a current-driven dissipative torque scale identically and compete, with the result that a steady current-driven domain-wall motion is insensitive to spin dephasing in the limit of weak ferromagnetism. A uniform magnetization is found to be much more stable against spin torques in the itinerant than in the \textit{s}-\textit{d} model for ferromagnetism. A dynamic spin-transfer torque reminiscent of the spin pumping in multilayers is identified and shown to govern the current-induced domain-wall distortion.",0512715v4 2006-01-04,Prospect for room temperature tunneling anisotropic magnetoresistance effect: density of states anisotropies in CoPt systems,"Tunneling anisotropic magnetoresistance (TAMR) effect, discovered recently in (Ga,Mn)As ferromagnetic semiconductors, arises from spin-orbit coupling and reflects the dependence of the tunneling density of states in a ferromagnetic layer on orientation of the magnetic moment. Based on ab initio relativistic calculations of the anisotropy in the density of states we predict sizable TAMR effects in room-temperature metallic ferromagnets. This opens prospect for new spintronic devices with a simpler geometry as these do not require antiferromagnetically coupled contacts on either side of the tunnel junction. We focus on several model systems ranging from simple hcp-Co to more complex ferromagnetic structures with enhanced spin-orbit coupling, namely bulk and thin film L1$_0$-CoPt ordered alloys and a monatomic-Co chain at a Pt surface step edge. Reliability of the predicted density of states anisotropies is confirmed by comparing quantitatively our ab initio results for the magnetocrystalline anisotropies in these systems with experimental data.",0601071v1 2006-01-07,Temperature dependent bilayer ferromagnetism in Sr3Ru2O7,"The Ruthenium based perovskites exhibit a wide variety of interesting collective phenomena related to magnetism originating from the Ru 4d electrons. Much remains unknown concerning the nature of magnetic fluctuations and excitations in these systems. We present results of detailed inelastic neutron scattering measurements of Sr3Ru2O7 as a function of temperature, probing the ferromagnetic fluctuations of the bilayer structure. A magnetic response is clearly visible for a range of temperatures, T = 3.8 K up to T = 100 K, and for energy transfers between 2 and 14 meV. These measurements indicate that the ferromagnetic fluctuations manifest in the bilayer structure factor persist to surprisingly large temperatures. This behavior may be related to the proximity of the system in zero magnetic field to the metamagnetic/ferromagnetic transition.",0601134v1 2006-01-22,Borderline magnetism in Sr4Ru3O10: Impact of dilute La and Ca doping on itinerant ferromagnetism and metamagnetism,"An investigation of La and Ca doped Sr4Ru3O10, featuring a coexistence of interlayer ferromagnetism and intralayer metamagnetism, is presented. La doping readily changes magnetism between ferromagnetism and metamagnetism by tuning the density of states. It also results in different Curie temperatures for the c-axis and the basal plane, highlighting a rare spin-orbit coupling with the crystal field states. In contrast, Ca doping enhances the c-axis ferromagnetism and the magnetic anisotropy. La doping also induces a dimensional crossover in the interlayer transport whereas Ca doping exhibits a tunneling magnetoresistance and an extraordinary T3/2-dependence of the resisitivity. The drastic changes caused by the dilute doping demonstrate a rare borderline magnetism that is delicately linked to the interplay of the density of states and spin-orbit coupling.",0601498v1 2006-01-25,Aging dynamics of ferromagnetic and reentrant spin glass phases in stage-2 Cu$_{0.80}$C$_{0.20}$Cl$_{2}$ graphite intercalation compound,"Aging dynamics of a reentrant ferromagnet stage-2 Cu$_{0.8}$Co$_{0.2}$Cl$_{2}$ graphite intercalation compound has been studied using DC magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures $T_{c}$ ($\approx 8.7$ K) and $T_{RSG}$ ($\approx 3.3$ K). The relaxation rate $S_{ZFC}(t)$ exhibits a characteristic peak at $t_{cr}$ below $T_{c}$. The peak time $t_{cr}$ as a function of temperature $T$ shows a local maximum around 5.5 K, reflecting a frustrated nature of the ferromagnetic phase. It drastically increases with decreasing temperature below $T_{RSG}$. The spin configuration imprinted at the stop and wait process at a stop temperature $T_{s}$ ($ 1/3$). However, these processes and the corresponding shot noise increase are suppressed in narrow wires, so that charge transport experiments measuring the Fano factor $F_{\uparrow \to \uparrow \downarrow}$ in a ferromagnet/SO-coupled-wire/paramagnet setup also quantify the degree of phase-coherence of transported spin--we predict a one-to-one correspondence between the magnitude of the spin polarization vector and $F_{\uparrow \to \uparrow \downarrow}$.",0604187v2 2006-04-20,Suppression of ferromagnetism in CeSi_1.81 under temperature and pressure,"We have studied the pressure dependence of the magnetization of single crystalline CeSi_1.81. At ambient pressure ferromagnetism develops below T_C = 9.5 Below ~ 5 K an additional shoulder in low-field hysteresis loops and a metamagnetic crossover around 4 T suggest the appearance of an additional magnetic modulation to the ferromagnetic state. The suppression of the magnetic order in CeSi_1.81 as function of temperature at ambient pressure and as function of pressure at low temperature are in remarkable qualitative agreement. The continuous suppression of the ordered moment at p ~ 13.1 kbar suggests the existence of a ferromagnetic quantum critical point in this material.",0604470v1 2006-04-27,Ferromagnetic - spin glass transition induced by pressure in the geometrically frustrated pyrochlore (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$,"We have studied (Tb$_{1-x}$La$_x$)$_2$Mo$_2$O$_7$ pyrochlores by neutron diffraction and $\mu$SR at ambient and under applied pressure. Substitution of Tb for La expands the lattice and induces a change from a spin-glass like state ({\itshape x}=0) to a non collinear ferromagnet ({\itshape x}=0.2). In the ferromagnetic structure, the Tb moments orient close to their local anisotropy axis as for an ordered spin ice, while the Mo moments orient close to the net moment. The $\mu$SR dynamical relaxation rate shows a cusp-like peak at the Curie temperature T$_C$ and a broad anomaly at T$^\star$$<$T$_C$, suggesting a second transition of local or dynamical nature. Under pressure, the long range order breaks down and a spin glass-like state is recovered. The whole set of data provide a microscopic picture of the spin correlations and fluctuations in the region of the ferromagnetic-spin glass threshold.",0604627v1 2006-05-07,Phase Diagrams of Ferromagnet-Superconductor Multilayers with Misaligned Exchange Fields,"We study the influence of misalignment of the ferromagnetic exchange field on the equilibrium properties of hybrid structures, composed of superconducting (S) and ferromagnetic (F) parts. In particular, we study numerically the superconducting critical temperature Tc in F-S-F trilayers and in F-S-F-S-F Josephson junctions as a function of the misalignment angle theta of the ferromagnetic magnetization. We discuss the corresponding phase diagrams for these hybrid structures. For the Josephson junctions, a transition between the zero-phase and the pi-phase ground state as a function of theta takes place under certain conditions. Within the quasiclassical Green's function technique in the diffusive limit, we introduce a fast and effective method for calculating Tc in such multilayer structures.",0605172v1 2006-05-21,High-temperature ferromagnetism of $sp$ electrons in narrow impurity bands: Application to CaB$_6$,"Ferromagnetism with high Curie temperature $T_c$, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high $T_c$ within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering $T_c$ below its value given by Stoner theory. These ideas are applied to CaB$_6$ and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B$_2$ and O$_2$ dimers might be exploited in suitable structures containing these elements.",0605495v1 2006-05-26,Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature,"The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB respectively. Furthermore a clear correlation between the magnetisation per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetisation on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers.",0605654v1 2006-05-26,Collective Modes and Skyrmion Excitations in Graphene SU(4) Quantum Hall Ferromagnets,"Graphene exhibits quantum Hall ferromagnetism in which an approximate SU(4) symmetry involving spin and valley degrees of freedom is spontaneously broken. We construct a set of integer and fractional quantum Hall states that break the SU(4) spin/valley symmetry, and study their neutral and charged excitations. Several properties of these ferromagnets can be evaluated analytically in the SU(4) symmetric limit, including the full collective mode spectrum at integer fillings. By constructing explicit wave functions we show that the lowest energy skyrmion states carry charge $\pm 1$ for {\em any} integer filling, and that skyrmions are the lowest energy charged excitations for graphene Landau level index $|n| \le 3$. We also show that the skyrmion lattice states which occur near integer filling factors support four gapless collective mode branches in the presence of full SU(4) symmetry. Comparisons are made with the more familiar SU(2) quantum Hall ferromagnets studied previously.",0605666v3 2006-05-29,Disturbance of spin equilibrium by current through the interface of noncollinear ferromagnets,"Boundary conditions are derived that determine the penetration of spin current through an interface of two non-collinear ferromagnets with an arbitrary angle between their magnetization vectors. We start from the well-known transformation properties of an electron spin wave functions under the rotation of a quantization axis. It allows directly find the connection between partial electric current densities for different spin subbands of the ferromagnets. No spin scattering is assumed in the near interface region, so that spin conservation takes place when electron intersects the boundary. The continuity conditions are found for partial chemical potential differences in the situation. Spatial distribution of nonequilibrium electron magnetizations is calculated under the spin current flowing through a contact of two semi-infinite ferromagnets. The distribution describes the spin accumulation effect by current and corresponding shift of the potential drop at the interface. These effects appear strongly dependent on the relation between spin contact resistances at the interface.",0605694v2 2006-06-20,Comparison between ab-initio and phenomenological modeling of the exchange couplings in diluted magnetic semiconductors: the case of $Zn_{1-x}Cr_{x}Te$,"Using a recently developed semi-analytical method (Self-Consistent Local RPA or SC-LRPA) we study the stability of the ferromagnetic phase in diluted magnetic systems where the exchange coupling between magnetic impurities are of RKKY form. A short discussion of the relevance of these calculations with respect to the ferromagnetism observed in diluted ferromagnetic materials is provided. Then, within a two step approach, we study ferromagnetism in $Zn_{1-x}Cr_{x}Te$. In the first step of our study, we calculate the magnetic couplings between Mn impurities within the LDA. In the second step, we diagonalize the resulting effective Heisenberg Hamiltonian using the SC-LRPA. We also compare, when available, our calculations with Monte Carlo simulations and experimental measurements.",0606523v1 2006-06-20,Magnetic Percolation and the Phase Diagram of the Disordered RKKY model,"We consider ferromagnetism in spatially randomly located magnetic moments, as in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect exchange RKKY interaction. We obtain via Monte Carlo the magnetic phase diagram as a function of the impurity moment density $n_{i}$ and the relative carrier concentration $n_{c}/n_{i}$. As evidenced by the diverging correlation length and magnetic susceptibility, the boundary between ferromagnetic (FM) and non-ferromagnetic (NF) phases constitutes a line of zero temperature critical points which can be viewed as a magnetic percolation transition. In the dilute limit, we find that bulk ferromagnetism vanishes for $n_{c}/n_{i}>.1$. We also incorporate the local antiferromagnetic direct superexchange interaction between nearest neighbor impurities, and examine the impact of a damping factor in the RKKY range function.",0606532v2 2006-07-11,Density of Yang-Lee zeros for the Ising ferromagnet,"The densities of Yang-Lee zeros for the Ising ferromagnet on the $L\times L$ square lattice are evaluated from the exact grand partition functions ($L=3\sim16$). The properties of the density of Yang-Lee zeros are discussed as a function of temperature $T$ and system size $L$. The three different classes of phase transitions for the Ising ferromagnet, first-order phase transition, second-order phase transition, and Yang-Lee edge singularity, are clearly distinguished by estimating the magnetic scaling exponent $y_h$ from the densities of zeros for finite-size systems. The divergence of the density of zeros at Yang-Lee edge in high temperatures (Yang-Lee edge singularity), which has been detected only by the series expansion until now for the square-lattice Ising ferromagnet, is obtained from the finite-size data. The identification of the orders of phase transitions in small systems is also discussed using the density of Yang-Lee zeros.",0607257v1 2006-07-26,"Angle-dependent magnetotransport in cubic and tetragonal ferromagnets: Application to (001)- and (113)A-oriented (Ga,Mn)As","General expressions for the longitudinal and transverse resistivities of single-crystalline cubic and tetragonal ferromagnets are derived from a series expansion of the resistivity tensor with respect to the magnetization orientation. They are applied to strained (Ga,Mn)As films, grown on (001)- and (113)A-oriented GaAs substrates, where the resistivities are theoretically and experimentally studied for magnetic fields rotated within various planes parallel and perpendicular to the sample surface. We are able to model the measured angular dependences of the resistivities within the framework of a single ferromagnetic domain, calculating the field-dependent orientation of the magnetization by numerically minimizing the free-enthalpy density. Angle-dependent magnetotransport measurements are shown to be a powerful tool for probing both anisotropic magnetoresistance and magnetic anisotropy. The anisotropy parameters of the (Ga,Mn)As films inferred from the magnetotransport measurements agree with those obtained by ferromagnetic resonance measurements within a factor of two.",0607679v1 2006-08-01,Josephson current in a superconductor-ferromagnet junction with two non-collinear magnetic domains,"We study the Josephson effect in a superconductor--ferromagnet--superconductor (SFS) junction with ferromagnetic domains of non-collinear magnetization. As a model for our study we consider a diffusive junction with two ferromagnetic domains along the junction. The superconductor is assumed to be close to the critical temperature $T_c$, and the linearized Usadel equations predict a sinusoidal current-phase relation. We find analytically the critical current as a function of domain lengths and of the angle between the orientations of their magnetizations. As a function of those parameters, the junction may undergo transitions between 0 and $\pi$ phases. We find that the presence of domains reduces the range of junction lengths at which the $\pi$ phase is observed. For the junction with two domains of the same length, the $\pi$ phase totally disappears as soon as the misorientation angle exceeds $\pi/2$. We further comment on possible implication of our results for experimentally observable 0--$\pi$ transitions in SFS junctions.",0608009v3 2006-08-24,Observation of standard spin-switch effects in F/S/F trilayers with a strong ferromagnet,"We have measured the superconducting transition temperature T_c of F/S/F trilayers using Permalloy (Py=Ni_84 Fe_16) as a strongly polarized ferromagnetic material. For a parallel (P) or anti-parallel (AP) alignment of the magnetization directions of the outer ferromagnets, we observe a T_c difference as large as 20 mK, with a stronger suppression of superconductivity in the P state than in the AP state. This behavior is opposite to the recent observations of Rusanov et al., Phys. Rev. B 73, 060505 (2006) in Py/Nb/Py trilayers, but is consistent with earlier results on trilayers with Ni or CuNi alloy as the ferromagnetic material.",0608545v1 2006-09-13,Tunneling currents in ferromagnetic systems with multiple broken symmetries,"SHORTENED ABSTRACT: A system exhibiting multiple simultaneously broken symmetries offers the opportunity to influence physical phenomena such as tunneling currents by means of external control parameters. In this paper, we consider the broken SU(2) (internal spin) symmetry of ferromagnetic systems coexisting with \textit{i)} the broken U(1) symmetry of superconductors and \textit{ii)} the broken spatial inversion symmetry induced by a Rashba term in a spin-orbit coupling Hamiltonian. In order to study the effect of these broken symmetries, we consider tunneling currents that arise in two different systems; tunneling junctions consisting of non-unitary spin-triplet ferromagnetic superconductors and junctions consisting of ferromagnets with spin-orbit coupling.",0609314v2 2006-09-14,Role of coexisting ferromagnetic and antiferromagnetic phases on the magnetocaloric effect in metamagnetic Tb2Ni2Sn,"We report the anomalous magnetocaloric behavior in the polycrystalline compound Tb2Ni2Sn. The magnetization measurements show that this compound shows multiple magnetic transitions, which are attributed to the coexistence of ferromagnetic and antiferromagnetic phases at low temperatures. With increase in field and temperature, the compound undergoes a metamagnetic transition to a ferromagnetic state. In the temperature range where the antiferromagnetic phase is dominant, it exhibits inverse (negative) magnetocaloric effect. At temperatures close to the Neel temperature, the compound shows positive magnetocaloric effect. Below the critical field needed for the metamagnetic transition, the temperature variation of the magnetocaloric effect is seen to be correlated with the ferromagnetic fraction.",0609335v2 2006-09-20,Ferromagnetism below 10 K in Mn doped BiTe,"Ferromagnetism is observed below 10 K in [Bi0.75Te0.125Mn0.125]Te. This material has the BiTe structure, which is made from the stacking of two Te-Bi-Te-Bi-Te blocks and one Bi-Bi block per unit cell. Crystal structure analysis shows that Mn is localized in the Bi2 blocks, and is accompanied by an equal amount of TeBi anti-site occupancy in the Bi2Te3 blocks. These TeBi anti-site defects greatly enhance the Mn solubility. This is demonstrated by comparison of the [Bi1-xMnx]Te and [Bi1-2xTexMnx]Te series; in the former, the solubility is limited to x = 0.067, while the latter has xmax = 0.125. The magnetism in [Bi1-xMnx]Te changes little with x, while that for [Bi1-2xTexMnx]Te shows a clear variation, leading to ferromagnetism for x > 0.067. Magnetic hysteresis and the anomalous Hall Effect are observed for the ferromagnetic samples.",0609519v1 2006-10-03,Enhancement of ferromagnetism by nickel doping in the 112 cobaltite EuBaCo2O5.50,"The study of the ordered oxygen deficient perovskite EuBaCo2-xNixO5.50 shows that the doping of cobalt sites by nickel induces a strong ferromagnetic component at low temperature in the antiferromagnetic matrix of EuBaCo2O5.50. This system exhibits indeed phase separation, i.e. consists of ferromagnetic domains embedded in the antiferromagnetic matrix of EuBaCo2O5.50. Besides, a magnetic transition is observed for the first time at 40K in the undoped and nickel doped phases, which can be attributed to the ferromagnetic ordering of the Eu3+ moments below this temperature. Moreover sharp ultra magnetization multisteps are observed below 5K, characteristic of motion of domain walls in a strong pinning system and very different from any metamagnetic transition.",0610064v1 2006-10-10,Spin-transfer in an open ferromagnetic layer: from negative damping to effective temperature,"Spin-transfer is a typical spintronics effect that allows a ferromagnetic layer to be switched by spin-injection. Most of the experimental results about spin transfer are described on the basis of the Landau-Lifshitz-Gilbert equation of the magnetization, in which additional current-dependent damping factors are added, and can be positive or negative. The origin of the damping can be investigated further by performing stochastic experiments, like one shot relaxation experiments under spin-injection in the activation regime of the magnetization. In this regime, the N\'eel-Brown activation law is observed which leads to the introduction of a current-dependent effective temperature. In order to justify the introduction of these counterintuitive parameters (effective temperature and negative damping), a detailed thermokinetic analysis of the different sub-systems involved is performed. We propose a thermokinetic description of the different forms of energy exchanged between the electric and the ferromagnetic sub-systems at a Normal/Ferromagnetic junction. The corresponding Fokker Planck equations, including relaxations, are derived. The damping coefficients are studied in terms of Onsager-Casimir transport coefficients, with the help of the reciprocity relations. The effective temperature is deduced in the activation regime.",0610264v1 2006-10-12,Universal Scaling Behavior of Anomalous Hall Effect and Anomalous Nernst Effect in Itinerant Ferromagnets,"Anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) in a variety of ferromagnetic metals including pure metals, oxides, and chalcogenides, are studied to obtain unified understandings of their origins. We show a universal scaling behavior of anomalous Hall conductivity $\sigma_{xy}$ as a function of longitudinal conductivity $\sigma_{xx}$ over five orders of magnitude, which is well explained by a recent theory of the AHE taking into account both the intrinsic and extrinsic contributions. ANE is closely related with AHE and provides us with further information about the low-temperature electronic state of itinerant ferromagnets. Temperature dependence of transverse Peltier coefficient $\alpha_{xy}$ shows an almost similar behavior among various ferromagnets, and this behavior is in good agreement quantitatively with that expected from the Mott rule.",0610324v1 2006-10-16,Inelastic scattering in ferromagnetic and antiferromagnetic metal spintronics,"We use a ferromagnetic voltage probe model to study the influence of inelastic scattering on giant magnetoresistance and current-induced torques in ferromagnetic and antiferromagnetic metal spin valves. The model is based on the Green's function formulation of transport theory and represents spin-dependent and spin-independent inelastic scatterers by interior voltage probes that are constrained to carry respectively no charge current and no spin or charge current. We find that giant magnetoresistance and spin transfer torques in ferromagnetic metal spin valve structures survive arbitrarily strong spin-independent inelastic scattering, while the recently predicted analogous phenomena in antiferromagnetic metal spin valves are partially suppressed. We use toy-model numerical calculations to estimate spacer layer thickness requirements for room temperature operation of antiferromagnetic metal spin valves.",0610417v1 2006-10-31,Spin-transfer torque in magnetic multilayer nanopillars,"We consider a quasi one-dimensional configuration consisting of two small pieces of ferromagnetic material separated by a metallic one and contacted by two metallic leads. A spin-polarized current is injected from one lead. Our goal is to investigate the correlation induced between the magnetizations of the two ferromagnets by spin-transfer torque. This torque results from the interaction between the magnetizations and the spin polarization of the current. We discuss the dynamics of a single ferromagnet, the extension to the case of two ferromagnets, and give some estimates for the parameters based on experiments.",0610878v1 2006-11-01,A review of the electronic and magnetic properties of tetrahedrally bonded half-metallic ferromagnets,"The emergence of the field of spintronics brought half-metallic ferromagnets to the center of scientific research. A lot of interest was focused on newly created transition-metal pnictides (such as CrAs) and chalcogenides (such as CrTe) in the metastable zinc-blende lattice structure. These compounds were found to present the advantage of high Curie temperature values in addition to their structural similarity to semiconductors. Significant theoretical activity has been devoted to the study of the electronic and magnetic properties of these compounds in an effort to achieve a better control of their experimental behaviour in realistic applications. This review is devoted to an overview of the studies of these compounds, with emphasis on theoretical results, covering their bulk properties (electronic structure, magnetism, stability of the zinc-blende phase, stability of ferromagnetism) as well as low-dimensional structures (surfaces, interfaces, nanodots and transition-metal delta-doped semiconductors) and phenomena that can possibly destroy the half-metallic property, like structural distortions or defects.",0611006v1 2006-11-02,Order from Disorder in Graphene Quantum Hall Ferromagnet,"Valley-polarized quantum Hall states in graphene are described by a Heisenberg O(3) ferromagnet model, with the ordering type controlled by the strength and sign of valley anisotropy. A mechanism resulting from electron coupling to strain-induced gauge field, giving leading contribution to the anisotropy, is described in terms of an effective random magnetic field aligned with the ferromagnet z axis. We argue that such random field stabilizes the XY ferromagnet state, which is a coherent equal-weight mixture of the $K$ and $K'$ valley states. Other implications such as the Berezinskii-Kosterlitz-Thouless ordering transition and topological defects with half-integer charge are discussed.",0611062v1 2006-11-07,Phase diagram of the $p$-spin-interacting spin glass with ferromagnetic bias and a transverse field in the infinite-$p$ limit,"The phase diagram of the $p$-spin-interacting spin glass model in a transverse field is investigated in the limit $p \to \infty$ under the presence of ferromagnetic bias. Using the replica method and the static approximation, we show that the phase diagram consists of four phases: Quantum paramagnetic, classical paramagnetic, ferromagnetic, and spin-glass phases. We also show that the static approximation is valid in the ferromagnetic phase in the limit $p \to \infty$ by using the large-$p$ expansion. Since the same approximation is already known to be valid in other phases, we conclude that the obtained phase diagram is exact.",0611168v2 2006-11-09,Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets,"We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05 films to their magnetic properties. The formation of ferromagnetic nanometer sized inclusions in a defect free Ge matrix fabricated by low temperature molecular beam epitaxy is observed down to substrate temperatures T_S as low as 70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn_5Ge_3. The volume and amount of these precipitates decreases with decreasing T_S. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of Mn_5Ge_3 at approximately room temperature and a lower, T_S dependent blocking temperature T_B the magnetic properties are dominated by superparamagnetism of the Mn_5Ge_3 precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic for a metastable state.",0611241v1 2006-11-24,Magnetic and transport properties of a one dimensional frustrated t-J model for vanadate nanotubes,"We propose a one-dimensional model consisting of a chain with a t-J Hamiltonian coupled to a Heisenberg chain in a frustrated geometry to describe the appearance of the ferromagnetic phase which has been experimentally observed in vanadate nanotubes. This model contains a mechanism of frustration suppressed by doping suggested by L. Krusin-Elbaum, et al. [Nature 431, 672 (2004)]. We study, using numerical techniques in small clusters, the relation between magnetic order and transport properties in the proposed model, and we perform a detailed comparison of the physical properties of this model with those of the ferromagnetic Kondo lattice model. For this comparison, a number of results for the latter model, obtained using the same numerical techniques, will be provided to complement those results already available in the literature. We conclude that it does not appear to be a true ferromagnetic order in the proposed model, but rather an incommensurate ferrimagnetic one, and contrary to what happens in the ferromagnetic Kondo lattice model, electronic transport is somewhat suppressed by this ferrimagnetic order.",0611603v2 2007-01-06,Sequential tunneling and shot noise in ferromagnet/normal-metal/ferromagnet double tunnel junctions,"The tunneling through a ferromagnet/normal metal/ferromagnet double junction in the Coulomb blockade regime is studied, assuming that the spin relaxation time of electron in the central metallic island is sufficiently large. Using the master equation, the current, the tunnel magnetoresistance (TMR), and the current noise spectrum have been calculated for devices of different parameters. It was shown that the interplay between spin and charge correlations strongly depends on the asymmetry of measured device. The charge correlation makes both the chemical potential shift, which describes the spin accumulation in the central island, and the TMR oscillated with the same period as the Coulomb staircase in current-voltage characteristics. This effect is smeared by the temperature. The spin correlation may cause an enhancement of noise at finite frequencies, while the zero frequency noise is still always sub-poissonian. The gate voltage causes an oscillation of not only conductance, but also TMR and noise.",0701123v1 2007-01-11,Bulk ferromagnetism and large changes in photoluminescence intensity by magnetic field in $β$-Ga$_2$O$_3$,"In this letter we report observation of room temperature ferromagnetism in bulk Ga$_2$O$_3$ . We also observe large (10-60%) increase/decrease in photoluminescence. In the red(700 nm wavelength)/blue(500 nm), with the application of small magnetic field(0.4 Tesla). We argue, that ferromagnetism occurs entirely due to chains of oxygen(O(3) sites, Fig. 5) vacancies. We propose a simple model to explain, origin and location of moment, formation of ferromagnetic dislocation needles, and strong increase/decrease ofred/blue photoluminescence intensity with magnetic field.",0701232v1 2007-01-29,Ferromagnetic 0-pi Josephson junctions,"We present a study on low-$T_c$ superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions. SIFS junctions have gained considerable interest in recent years because they show a number of interesting properties for future classical and quantum computing devices. We optimized the fabrication process of these junctions to achieve a homogeneous current transport, ending up with high-quality samples. Depending on the thickness of the ferromagnetic layer and on temperature, the SIFS junctions are in the ground state with a phase drop either 0 or $\pi$. By using a ferromagnetic layer with variable step-like thickness along the junction, we obtained a so-called 0-$\pi$ Josephson junction, in which 0 and $\pi$ ground states compete with each other. At a certain temperature the 0 and $\pi$ parts of the junction are perfectly symmetric, i.e. the absolute critical current densities are equal. In this case the degenerate ground state corresponds to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum $\Phi_0$.",0701693v2 2007-01-29,Intrinsic anomalous Hall effect in ferromagnetic metals studied by the multi d-orbital tight-binding model,"To elucidate the origin of anomalous Hall effect (AHE) in ferromagnetic transition metals, we study the intrinsic AHE based on a multi-orbital (xz,yz)-tight-binding model. We find that a large anomalous velocity comes from the off-diagonal (inter-orbital) hopping. By this reason, the present model shows a large intrinsic anomalous Hall conductivity (AHC) which is compatible with typical experimental values in ferromagnets [100-1000 [1/\Omega cm]], without necessity to assume a special band structure at the Fermi level. In good metals where resistivity \rho is small, the intrinsic AHC is constant (dissipation-less) as found by Karplus and Luttinger. In bad metals, however, we find that the AHC is proportional to \rho^{-2} when \hbar/2\tau is larger than the minimum band-splitting measured from the Fermi level. This crossover behavior of the intrinsic AHE, which was first derived in J. Phys. Soc. Jpn. 63 (1994) 2627, is recently observed in various ferromagnetic metals universally by A. Asamitsu et al. We also stress that the present (xz,yz)-tight binding model shows a huge spin Hall effect in a paramagnetic state.",0701696v2 1998-11-25,Dynamical Generation of the Primordial Magnetic Field by Ferromagnetic Domain Walls,"The spontaneous generation of uniform magnetic condensate in $QED_3$ gives rise to ferromagnetic domain walls at the electroweak phase transition. These ferromagnetic domain walls are caracterized by vanishing effective surface energy density avoiding, thus, the domain wall problem. Moreover we find that the domain walls generate a magnetic field $B \simeq 10^{24} Gauss$ at the electroweak scale which account for the seed field in the so called dynamo mechanism for the cosmological primordial magnetic field. We find that the annihilation processes of walls with size $R \simeq 10^5 Km$ could release an energy of order $10^{52} erg$ indicating the invisible ferromagnetic walls as possible compact sources of Gamma Ray Bursts.",9811221v1 2000-02-04,Magnetic instability of quark matter,"Spontaneous magnetization of quark liquid is examined. It is pointed out that quark liquid has potential to be ferromagnetic at rather low densities.",0002014v1 2003-05-20,Two-Dimensional Magnetic Resonance Tomographic Microscopy using Ferromagnetic Probes,"We introduce the concept of computerized tomographic microscopy in magnetic resonance imaging using the magnetic fields and field gradients from a ferromagnetic probe. We investigate a configuration where a two-dimensional sample is under the influence of a large static polarizing field, a small perpendicular radio-frequency field, and a magnetic field from a ferromagnetic sphere. We demonstrate that, despite the non-uniform and non-linear nature of the fields from a microscopic magnetic sphere, the concepts of computerized tomography can be applied to obtain proper image reconstruction from the original spectral data by sequentially varying the relative sample-sphere angular orientation. The analysis shows that the recent proposal for atomic resolution magnetic resonance imaging of discrete periodic crystal lattice planes using ferromagnetic probes can also be extended to two-dimensional imaging of non-crystalline samples with resolution ranging from micrometer to Angstrom scales.",0305084v1 2005-03-01,Large Magnetic Moments of Arsenic-Doped Mn Clusters and their Relevance to Mn-Doped III-V Semiconductor Ferromagnetism,"We report electronic and magnetic structure of arsenic-doped manganese clusters from density-functional theory using generalized gradient approximation for the exchange-correlation energy. We find that arsenic stabilizes manganese clusters, though the ferromagnetic coupling between Mn atoms are found only in Mn$_2$As and Mn$_4$As clusters with magnetic moments 9 $\mu_B$ and 17 $\mu_B$, respectively. For all other sizes, $x=$ 3, 5-10, Mn$_x$As clusters show ferrimagnetic coupling. It is suggested that, if grown during the low temperature MBE, the giant magnetic moments due to ferromagnetic coupling in Mn$_2$As and Mn$_4$As clusters could play a role on the ferromagnetism and on the variation observed in the Curie temperature of Mn-doped III-V semiconductors.",0503009v1 2007-02-23,Quantum annealing of the random-field Ising model by transverse ferromagnetic interactions,"We introduce transverse ferromagnetic interactions, in addition to a simple transverse field, to quantum annealing of the random-field Ising model to accelerate convergence toward the target ground state. The conventional approach using only the transverse-field term is known to be plagued by slow convergence when the true ground state has strong ferromagnetic characteristics for the random-field Ising model. The transverse ferromagnetic interactions are shown to improve the performance significantly in such cases. This conclusion is drawn from the analyses of the energy eigenvalues of instantaneous stationary states as well as by the very fast algorithm of Bethe-type mean-field annealing adopted to quantum systems. The present study highlights the importance of a flexible choice of the type of quantum fluctuations to achieve the best possible performance in quantum annealing. The existence of such flexibility is an outstanding advantage of quantum annealing over simulated annealing.",0702214v1 2007-05-01,Criticality in inhomogeneous magnetic systems: Application to quantum ferromagnets,"We consider a $\phi^4$-theory with a position-dependent distance from the critical point. One realization of this model is a classical ferromagnet subject to non-uniform mechanical stress. We find a sharp phase transition where the envelope of the local magnetization vanishes uniformly. The first-order transition in a quantum ferromagnet also remains sharp. The universal mechanism leading to a tricritical point in an itinerant quantum ferromagnet is suppressed, and in principle one can recover a quantum critical point with mean-field exponents. Observable consequences of these results are discussed.",0705.0152v2 2007-05-14,Nanoscale ferromagnetism in non-magnetic doped semiconductors,"While ferromagnetism at relatively high temperatures is seen in diluted magnetic semiconductors such as Ga_(1-x)Mn_(x)As, doped semiconductors without magnetic ions have not shown evidence for ferromagnetism. Using a generalized disordered Hubbard model designed to characterize hydrogenic centers in semiconductors, we find that such systems may also exhibit a ferromagnetic ground state, at least on the nanoscale. This is found most clearly in a regime inaccessible to bulk systems, but attainable in quantum dots as well as heterostructures. We present numerical results demonstrating the occurrence of high spin ground states in both lattice and positionally disordered systems. We examine how the magnetic phases are affected by characteristics of real doped semiconductors, such as positional disorder and electron-hole asymmetry.",0705.2038v3 2007-05-17,Ferromagnetism in nanoscale BiFeO3,"A remarkably high saturation magnetization of ~0.4mu_B/Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4-40 nm) multiferroic BiFeO_3 which in bulk form exhibits weak magnetization (~0.02mu_B/Fe) and an antiferromagnetic order. The magnetic hysteresis loops, however, exhibit exchange bias as well as vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, like in bulk BiFeO_3, both the calorimetric and dielectric permittivity data in nanoscale BiFeO_3 exhibit characteristic features at the magnetic transition point. These features establish formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO_3.",0705.2498v1 2007-06-12,Josephson current in a superconductor -- ferromagnet -- superconductor junction with in-plane ferromagnetic domains,"We study a diffusive superconductor--ferromagnet--superconductor (SFS) junction with in-plane ferromagnetic domains. Close to the superconducting transition temperature, we describe the proximity effect in the junction with the linearized Usadel equations. We find that properties of such a junction depend on the size of the domains relative to the magnetic coherence length. In the case of large domains, the junction exhibits transitions to the $\pi$ state, similarly to a single-domain SFS junction. In the case of small domains, the magnetization effectively averages out, and the junction is always in the zero state, similarly to a superconductor--normal metal--superconductor (SNS) junction. In both those regimes, the influence of domain walls may be approximately described as an effective spin-flip scattering. We also study the inhomogeneous distribution of the local current density in the junction. Close to the 0--$\pi$ transitions, the directions of the critical current may be opposite in the vicinity of the domain wall and in the middle of the domains.",0706.1638v1 2007-06-21,Quantum and thermal fluctuations in a two-dimensional correlated band ferromagnet -- Goldstone-mode preserving investigation with self-energy and vertex corrections,"Ferromagnetism in the t-t' Hubbard model is investigated on a square lattice. Correlation effects in the form of self-energy and vertex corrections are systematically incorporated within a spin-rotationally-symmetric scheme which explicitly preserves the Goldstone mode and is therefore in accord with the Mermin-Wagner theorem. Interplay of band dispersion and correlation effects on ferromagnetic-state stability are highlighted with respect to both long- and short-wavelength fluctuations, which are shown to have substantially different behaviour. Our approach provides a novel understanding of the enhancement of ferromagnetism near van Hove filling for t'~0.5 in terms of strongly suppressed saddle-point contribution to the destabilizing exchange part of spin stiffness. Finite-temperature electron spin dynamics is investigated directly in terms of spectral-weight transfer across the Fermi energy due to electron-magnon coupling. Relevant in the context of recent magnetization measurements on ultrathin films, the role of strong thermal spin fluctuations in low dimensions is highlighted, in the anisotropy-stabilized ordered state, by determining the thermal decay of magnetization and T_c within a renormalized spin-fluctuation theory.",0706.3088v1 2007-06-21,Itinerant ferromagnetism in the multiorbital Hubbard model: a dynamical mean-field study,"In order to resolve the long-standing issue of how the itinerant ferromagnetism is affected by the lattice structure and Hund's coupling, we have compared various three-dimensional lattice structures in the single- and multiorbital Hubbard models with the dynamical mean-field theory with an improved quantum Monte Carlo algorithm that preserves the spin-SU(2) symmetry. The result indicates that {\it both} the lattice structure and the d-orbital degeneracy are essential for the ferromagnetism in the parameter region representing a transition metal. Specifically, (a) Hund's coupling, despite the common belief, is important, which is here identified to come from particle-hole scatterings, and (b) the ferromagnetism is a correlation effect (outside the Stoner picture) as indicated from the band-filling dependence.",0706.3109v1 2007-06-22,Visualization of ferromagnetic domains in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C single crystals: Weak ferromagnetism and its coexistence with superconductivity,"The magnetic flux structure in the basal plane, (001), of single crystals of superconducting (R = Er) and non-superconducting (R = Tb) RNi$_2$B$_2$C was studied by high resolution Bitter decoration at temperatures below $T_c$ (superconducting transition) and/or $T_N$ (antiferromagnetic transition). For both materials two sets of domain boundaries, in \{110\} and \{100\} planes, were observed. The temperature ranges in which the \{100\} domain boundaries were observed in TbNi$_2$B$_2$C and ErNi$_2$B$_2$C coincide with the weak ferromagnetic (WFM) ordering in these materials. On the other hand, the \{110\} twin boundaries - the antiferromagnetic domain boundaries - were observed in both compounds below $T_N$. The possibility of interpretation of \{100\} boundaries as Bloch domain walls in the weakly ferromagnetic phase, for $T < T_{WFM} < T_N$ (TbNi$_2$B$_2$C) or $T < T_{WFM} < T_N < T_c$ (ErNi$_2$B$_2$C) is discussed.",0706.3340v1 2007-06-25,Odd-frequency Pairs and Josephson Current through a Strong Ferromagnet,"We study Josephson current in superconductor / diffusive ferromagnet /superconductor junctions by using the recursive Green function method. When the exchange potential in a ferromagnet is sufficiently large as compared to the pair potential in a superconductor, an ensemble average of Josephson current is much smaller than its mesoscopic fluctuations. The Josephson current vanishes when the exchange potential is extremely large so that a ferromagnet is half-metallic. Spin-flip scattering at junction interfaces drastically changes the characteristic behavior of Josephson current. In addition to spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half metal. Such equal-spin pairs have an unusual symmetry property called odd-frequency symmetry and carry the Josephson current through a half metal. The penetration of odd-frequency pairs into a half metal enhances the low energy quasiparticle density of states, which could be detected experimentally by scanning tunneling spectroscopy. We will also show that odd-frequency pairs in a half metal cause a nonmonotonic temperature dependence of the critical Josephson current.",0706.3542v2 2007-06-26,Magnetic confinement of the superconducting condensate in superconductor/ferromagnet hybrid composites,"The influence of an inhomogeneous magnetic field on the magnetoresistance of thin Al films, used in different superconductor/ferromagnet hybrids, has been investigated. Two contrasting magnetic textures with out-of-plane magnetization are explored, namely (i) a plain film in a multidomain state and (ii) an array of micro-sized dots. The stray fields of the ferromagnetic structures confine the superconducting condensate and, accordingly, modify the condition for the nucleation of superconductivity. By switching between different magnetic states of the ferromagnet, this confinement can be tuned at will, hereby reversibly changing the dependence of the critical temperature Tc on an external magnetic field H. In particular, the continuous evolution from a conventional linear Tc(H) dependence with a single maximum to a reentrant superconducting phase boundary with multiple Tc peaks has been demonstrated.",0706.3803v1 2007-07-04,Ferromagnetic behaviour in the strongly interacting two-component Bose gas,"We investigate the low temperature behaviour of the integrable 1D two-component spinor Bose gas using the thermodynamic Bethe ansatz. We find that for strong coupling the characteristics of the thermodynamics at low temperatures are quantitatively affected by the spin ferromagnetic states, which are described by an effective ferromagnetic Heisenberg chain. The free energy, specific heat, susceptibility and local pair correlation function are calculated for various physical regimes in terms of temperature and interaction strength. These thermodynamic properties reveal spin effects which are significantly different than those of the spinless Bose gas. The zero-field susceptibility for finite strong repulsion exceeds that of a free spin paramagnet. The critical exponents of the specific heat $c_v \sim T^{1/2}$ and the susceptibility $\chi \sim T^{-2}$ are indicative of the ferromagnetic signature of the two-component spinor Bose gas. Our analytic results are consistent with general arguments by Eisenberg and Lieb for polarized spinor bosons.",0707.0613v2 2007-07-04,Detection of the magneto-structural phase coexistence in MnAs epilayers at a very early stage,"We report on the appearance of magnetic stripes in MnAs/GaAs(100) epilayers at temperatures well below the ferromagnetic transition of the system. The study has been performed by ferromagnetic resonance experiments (FMR) on MnAs epilayers grown on (100) and (111) GaAs substrates. The FMR spectra of the MnAs/GaAs(100) samples at 180 K reveal the appearance of zones of different magnetic behavior with respect to the low-temperature homogeneous ferromagnetic phase. The angular and the temperature dependence of the spectra serve us to detect the inter-growth of the non-magnetic phase into the ferromagnetic phase at a very early stage of the process. The experimental data show that the new phase nucleates in a self-arranged array of stripes in MnAs/GaAs(100) thin films while it grows randomly in the same films grown on GaAs(111).",0707.0703v1 2007-07-10,Vortex rings in ferromagnets,"Vortex ring solutions are presented for the Landau-Lifshitz equation, which models the dynamics of a three-dimensional ferromagnet. The vortex rings propagate at constant speed along their symmetry axis and are characterized by the integer-valued Hopf charge. They are stable to axial perturbations, but it is demonstrated that an easy axis anisotropy results in an instability to perturbations which break the axial symmetry. The unstable mode corresponds to a migration of spin flips around the vortex ring that leads to a pinching instability and ultimately the collapse of the vortex ring. It is found that this instability does not exist for an isotropic ferromagnet. Similarities between vortex rings in ferromagnets and vortons in cosmology are noted.",0707.1383v1 2007-07-11,Spin-Dependent Ringing and Beats in a Quantum Dot System,"We report spin-dependent quantum coherent oscillations (ringing) and beats of the total and spin currents flowing through a quantum dot with Zeeman split levels. The spin dependent transport is calculated via nonequilibrium Green function in the transient after a bias voltage is turned on at t=0. The dot is coupled to two electrodes that can be ferromagnetic or nonmagnetic. In the ferromagnetic case both parallel and antiparallel alignments are considered. The coherent oscillation and beat frequencies are controlled via the Zeeman energy E_Z. In particular, for E_Z=0 no beats are observed and the spin current is zero for nonmagnetic leads. In the ferromagnetic case a finite spin current is found for E_Z=0. The effects of temperature are also analyzed. We observe that with increasing temperature the ringing response and the beats tend to disappear. Additionally, the spin current goes to zero for nonmagnetic leads, remaining finite in the ferromagnetic case. The tunnel magnetoresistance (TMR) also reveals quantum coherent oscillations and beats, and it attains negative values for small enough temperatures and short times.",0707.1631v2 2007-07-21,Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides,"Room-temperature ferromagnetism has been observed in the nanoparticles (7 - 30 nm dia) of nonmagnetic oxides such as CeO2, Al2O3, ZnO, In2O3 and SnO2. The saturated magnetic moments in CeO_2 and Al_2O_3 nanoparticles are comparable to those observed in transition metal doped wide band semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be due to the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanopartilces of metal oxides",0707.3183v1 2007-08-02,Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature,"The alpha-beta magneto-structural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 K to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an alpha-beta phase coexistence and, more important, for the stabilization of the ferromagnetic alpha-phase at higher temperature than in bulk. We explain the premature appearance of the beta-phase at 275 K and the persistence of the ferromagnetic alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature.",0708.0350v1 2007-08-08,"Magnetic properties of Gd_xY_{1-x}Fe_2Zn_{20}: dilute, large, $\textbf {S}$ moments in a nearly ferromagnetic Fermi liquid","Single crystals of the dilute, rare earth bearing, pseudo-ternary series, Gd_xY_{1-x}Fe_2Zn_{20} were grown out of Zn-rich solution. Measurements of magnetization, resistivity and heat capacity on Gd_xY_{1-x}Fe_2Zn_{20} samples reveal ferromagnetic order of Gd^{3+} local moments across virtually the whole series ($x \geq 0.02$). The magnetic properties of this series, including the ferromagnetic ordering, the reduced saturated moments at base temperature, the deviation of the susceptibilities from Curie-Weiss law and the anomalies in the resistivity, are understood within the frame work of dilute, $\textbf {S}$ moments (Gd^{3+}) embedded in a nearly ferromagnetic Fermi liquid (YFe_2Zn_{20}). The s-d model is employed to further explain the variation of $T_{\mathrm{C}}$ with x as well as the temperature dependences of of the susceptibilities.",0708.1170v1 2007-08-26,Weak ferromagnetism and spin glass state with nano-sized nickel carbide,"Ni3C nanoparticles of about 40 nm have been studied experimentally to exhibit weak ferromagnetic (FM), spin-glass (SG) and paramagnetic (PM) properties. The freezing temperature of the SG phase at zero applied field is determined as, TF0 ~ 11.0 K. At T > TF0, a very weak ferromagnetism has been observed over a PM background. The Curie temperature, TC, is shown to exceed 300 K and the ferromagnetism at 300 K is determined as about 0.02 emu/g (~6.7*10^{-4}mu_B per Ni3C formula unit) by subtracting the background paramagnetism. An anomalous dip appears in the temperature dependent coercivity, HC(T), near the freezing temperature, TF0. It reflects a distortedly reduced coercivity in the M(H) hysteresis loop measured at T = TF0 with the applied sweeping field around H = 0. This is attributable to the exchange coupling effect between the SG and the weak FM phases. The possible origin of the magnetic moments that account for the observed FM, SG and PM properties is discussed.",0708.3468v2 2007-09-13,Ferromagnetism in 2p Light Element-Doped II-oxide and III-nitride Semiconductors,"II-oxide and III-nitride semiconductors doped by nonmagnetic 2p light elements are investigated as potential dilute magnetic semiconductors (DMS). Based on our first-principle calculations, nitrogen doped ZnO, carbon doped ZnO, and carbon doped AlN are predicted to be ferromagnetic. The ferromagnetism of such DMS materials can be attributed to a p-d exchange-like p-p coupling interaction which is derived from the similar symmetry and wave function between the impurity (p-like t_2) and valence (p) states. We also propose a co-doping mechanism, using beryllium and nitrogen as dopants in ZnO, to enhance the ferromagnetic coupling and to increase the solubility and activity.",0709.2059v1 2007-09-22,Quantum-Mechanically Induced Asymmetry in the Phase Diagrams of Spin-Glass Systems,"The spin-1/2 quantum Heisenberg model is studied in all spatial dimensions d by renormalization-group theory. Strongly asymmetric phase diagrams in temperature and antiferromagnetic bond probability p are obtained in dimensions d \geq 3. The asymmetry at high temperatures approaching the pure ferromagnetic and antiferromagnetic systems disappears as d is increased. However, the asymmetry at low but finite temperatures remains in all dimensions, with the antiferromagnetic phase receding to the ferromagnetic phase. A finite-temperature second-order phase boundary directly between the ferromagnetic and antiferromagnetic phases occurs in d \geq 6, resulting in a new multicritical point at its meeting with the boundaries to the paramagnetic phase. In d=3,4,5, a paramagnetic phase reaching zero temperature intervenes asymmetrically between the ferromagnetic and reentrant antiferromagnetic phases. There is no spin-glass phase in any dimension.",0709.3589v2 2007-10-10,Microwave photovoltage and photoresistance effects in ferromagnetic microstrips,"We investigate the dc electric response induced by ferromagnetic resonance in ferromagnetic Permalloy (Ni80Fe20) microstrips. The resulting magnetization precession alters the angle of the magnetization with respect to both dc and rf current. Consequently the time averaged anisotropic magnetoresistance (AMR) changes (photoresistance). At the same time the time-dependent AMR oscillation rectifies a part of the rf current and induces a dc voltage (photovoltage). A phenomenological approach to magnetoresistance is used to describe the distinct characteristics of the photoresistance and photovoltage with a consistent formalism, which is found in excellent agreement with experiments performed on in-plane magnetized ferromagnetic microstrips. Application of the microwave photovoltage effect for rf magnetic field sensing is discussed.",0710.1974v3 2007-10-22,Fe-doping-induced evolution of charge-orbital ordering in a bicritical-state manganite,"Impurity effects on the stability of a ferromagnetic metallic state in a bicritical-state manganite, (La0.7Pr0.3)0.65Ca0.35MnO3, on the verge of metal-insulator transition have been investigated by substituting a variety of transition-metal atoms for Mn ones. Among them, Fe doping exhibits the exceptional ability to dramatically decrease the ferromagnetic transition temperature. Systematic studies on the magnetotransport properties and x-ray diffraction for the Fe-doped crystals have revealed that charge-orbital ordering evolves down to low temperatures, which strongly suppresses the ferromagnetic metallic state. The observed glassy magnetic and transport properties as well as diffuse phase transition can be attributed to the phase-separated state where short-range charge-orbital-ordered clusters are embedded in the ferromagnetic metallic matrix. Such a behavior in the Fe-doped manganites form a marked contrast to the Cr-doping effects on charge-orbital-ordered manganites known as impurity-induced collapse of charge-orbital ordering.",0710.4069v1 2007-11-07,Soft x-ray magnetic circular dichroism study of Ca_1-xSr_xRuO_3 across the ferromagnetic quantum phase transition,"Ca_1-xSr_xRuO_3, which is ferromagnetic for Sr concentration x > 0.3, has been studied by x-ray magnetic circular dichroism (XMCD) in Ru 3p and O 1s core-level x-ray absorption. XMCD signals appear at x ~ 0.3 and monotonically increases with x in the ferromagnetic phase. While the monotonic increase of the XMCD signals with x is of a typical Stoner-type, the absence of appreciable change in the spectral line shapes of both the Ru 3p and O 1s XMCD spectra indicate that the itinerant-electron ferromagnetism in Ca_1-xSr_xRuO_3 is influenced by strong electron correlation.",0711.1000v2 2007-12-10,Chirality tunneling and quantum dynamics for domain walls in mesoscopic ferromagnets,"We studied the quantum dynamics of ferromagnetic domain walls (topological kink-type solitons) in one dimensional ferromagnetic spin chains. We show that the tunneling probability does not depend on the number of spins in a domain wall; thus, this probability can be large even for a domain wall containing a large number of spins. We also predict that there is a strong interplay between the tunneling of a wall from one lattice site to another (tunneling of the kink coordinate) and the tunneling of the kink topological charge (so-called chirality). Both of these elementary processes are suppressed for kinks in one-dimensional ferromagnets with half-integer spin. The dispersion law (i.e., the domain wall energy versus momentum) is essentially different for chains with either integer or half-integer spins. The predicted quantum effects could be observed for mesoscopic magnetic structures, e.g., chains of magnetic clusters, large-spin molecules, or nanosize magnetic dots.",0712.1392v1 2008-01-08,Relevance of ferromagnetic correlations for the Electron Spin Resonance in Kondo lattice systems,"Electron Spin Resonance (ESR) measurements of the ferromagnetic Kondo lattice system CeRuPO show a well defined ESR signal which is related to the magnetic properties of the Ce3+ moment. In contrast, no ESR signal could be observed in the antiferromagnetic homologue CeOsPO. Additionally, we detect an ESR signal in a further ferromagnetic Yb compound, YbRh, while it was absent in a number of Ce or Yb intermetallic compounds with dominant antiferromagnetic exchange, independently of the presence of a strong Kondo interaction or the proximity to a (quantum) critical point. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity of a quantum critical point, but seems to be connected to the presence of ferromagnetic fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature as observed in the heavy fermion metal YbRh2Si2 but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.",0801.1191v1 2008-01-11,Melting of magnetic correlations in charge-orbital ordered La(0.5)Sr(1.5)MnO(4) : competition of ferro and antiferromagnetic states,"The magnetic correlations in the charge- and orbital-ordered manganite La(0.5)Sr(1.5)MnO(4) have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined CE-type magnetic structure with the corresponding magnons a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, La(0.5)Sr(1.5)MnO(4) exhibits a dispersion of ferromagnetic correlations which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.",0801.1787v1 2008-01-17,Unusual upper critical field of the ferromagnetic superconductor UCoGe,"We report upper critical field $B_{c2}(T)$ measurements on a single-crystalline sample of the ferromagnetic superconductor UCoGe. $B_{c2}(0)$ obtained for fields applied along the orthorhombic axes exceeds the Pauli limit for $B \parallel a,b$ and shows a strong anisotropy $B_{c2}^{a} \simeq B_{c2}^{b} \gg B_{c2} ^{c}$. This provide evidence for an equal spin pairing state and a superconducting gap function of axial symmetry with point nodes along the c axis, which is also the direction of the uniaxial ferromagnetic moment $m_0 = 0.07 \mu_{B}$. An unusual curvature or kink is observed in the temperature variation of $B_{c2}$, which possibly indicates UCoGe is a two-band ferromagnetic superconductor.",0801.2628v1 2008-01-17,Exotic (anti)ferromagnetism in single crystals of Pr6Ni2Si3,"The ternary intermetallic compound Pr6Ni2Si3, is a member of a structure series of compounds based on a triangular structure where the number of Pr atoms in the prism cross section can be systematically varied. Pr6Ni2Si3 contains two distinct Pr lattice sites which result in complex interactions between the magnetic ions. Extensive measurements of specific heat and magnetization on single crystal samples indicate that Pr6Ni2Si3 orders with both a ferromagnet and an antiferromagnet component, with ordering temperatures of 39.6 K and ~ 32 K, respectively. The ferromagnetic component // c-axis is accompanied by a large hysteresis, and the antiferromagnetic component,_|_ c-axis is accompanied by a spin-flop-type transition. More detailed measurements, of the vector magnetization, indicate that the ferromagnetic and the antiferromagnetic order appear independent of each other. These results not only clarify the behavior of Pr6Ni2Si3 itself, but also of the other members of the structure series, Pr5Ni2Si3 and Pr15Ni7Si10.",0801.2704v1 2008-01-24,Ferromagnet proximity effects and magnetoresistance of bilayer graphene,"A drastic modification of electronic band structure is predicted in bilayer graphene when it is placed between two ferromagnetic insulators. Due to the exchange interaction with the proximate ferromagnet, the electronic energy dispersion in the graphene channel strongly depends on the magnetization orientation of two ferromagnetic layers, $\mathbf{M_{1}}$ and $\mathbf{M_{2}} $. While the parallel configuration $\mathbf{M_{1}}= \mathbf{M_{2}}$ leads to simple spin splitting of both conduction and valence bands, an energy gap is induced as soon as the angle $\theta$ between $\mathbf{M_{1}}$ and $% \mathbf{M_{2}}$ becomes non-zero with the maximum achieved at $\theta=\pi$ (i.e., antiparallel alignment). Consequently, bilayer graphene may exhibit a sizable magnetoresistive effect in the current-in-plane configuration. A rough estimate suggests the resistance changes on the order of tens of percent at room temperature. This effect is expected to become more pronounced as the temperatures decreases.",0801.3829v2 2008-02-21,Extended Scaling for the high dimension and square lattice Ising Ferromagnets,"In the high dimension (mean field) limit the susceptibility and the second moment correlation length of the Ising ferromagnet depend on temperature as chi(T)=tau^{-1} and xi(T)=T^{-1/2}tau^{-1/2} exactly over the entire temperature range above the critical temperature T_c, with the scaling variable tau=(T-T_c)/T. For finite dimension ferromagnets temperature dependent effective exponents can be defined over all T using the same expressions. For the canonical two dimensional square lattice Ising ferromagnet it is shown that compact ""extended scaling"" expressions analogous to the high dimensional limit forms give accurate approximations to the true temperature dependencies, again over the entire temperature range from T_c to infinity. Within this approach there is no cross-over temperature in finite dimensions above which mean-field-like behavior sets in.",0802.3011v1 2008-03-09,Doping-dependent Phase Diagram of LaO{\it M}As ({\it M}=V--Cu) and Electron-type Superconductivity near Ferromagnetic Instability,"By first-principles calculations, we present a doping-dependent phase diagram of LaO{\it M}As ({\it M}=V--Cu) family. It is characterized as antiferromagnetic semiconductor around LaOMnAs side and ferromagnetic metal around LaOCoAs. Both LaOFeAs and LaONiAs, where superconductivity were discovered, are located at the borderline of magnetic phases. Extensive Fermi surface analysis suggests that the observed superconductivity is of electron-type in its origin. We discuss possible pairing mechanisms in the context of competing ferromagnetic phases found in this work and the ferromagnetic spin fluctuations.",0803.1282v3 2008-03-29,Tunnel-barrier-enhanced dc voltage signals induced by magnetization dynamics in magnetic tunnel junctions,"We theoretically study the recently observed tunnel-barrier-enhanced dc voltage signals generated by magnetization precession in magnetic tunnel junctions. While the spin pumping is suppressed by the high tunneling impedance, two complimentary processes are predicted to result in a sizable voltage generation in ferromagnet (F)|insulator (I)|normal-metal (N) and F|I|F junctions, with one ferromagnet being resonantly excited. Magnetic dynamics in F|I|F systems induces a robust charge pumping, translating into voltage in open circuits. In addition, dynamics in a single ferromagnetic layer develops longitudinal spin accumulation inside the ferromagnet. A tunnel barrier then acts as a nonintrusive probe that converts the spin accumulation into a measurable voltage. Neither of the proposed mechanisms suffers from spin relaxation, which is typically fast on the scale of the exponentially slow tunneling rates. The longitudinal spin-accumulation buildup, however, is very sensitive to the phenomenological ingredients of the spin-relaxation picture.",0803.4228v2 2008-04-02,Origin and control of high-temperature ferromagnetism in semiconductors,"The extensive experimental and computational search for multifunctional materials has resulted in the development of semiconductor and oxide systems, such as (Ga,Mn)N, (Zn,Cr)Te, and HfO2, which exhibit surprisingly stable ferromagnetic signatures despite having a small or nominally zero concentration of magnetic elements. Here, we show that the ferromagnetism of (Zn,Cr)Te, and the associated magnetooptical and magnetotransport functionalities, are dominated by the formation of Cr-rich (Zn,Cr)Te metallic nanocrystals embedded in the Cr-poor (Zn,Cr)Te matrix. Importantly, the formation of these nanocrystals can be controlled by manipulating the charge state of Cr ions during the epitaxy. The findings provide insight into the origin of ferromagnetism in a broad range of semiconductors and oxides, and indicate possible functionalities of these composite systems. Furthermore, they demonstrate a bottom-up method for self-organized nanostructure fabrication that is applicable to any system in which the charge state of a constituent depends on the Fermi-level position in the host semiconductor.",0804.0322v1 2008-04-11,Room temperature soft ferromagnetism in the nanocrystalline form of YCo2 - a well-known bulk Pauli paramagnet,"The Laves phase compound, YCo2, is a well-known exchange-enahnced Pauli paramagnet. We report here that, in the nanocrystalline form, this compound interestingly is an itinerant ferromagnet at room temperature with a low coercive-field. The magnitude of the saturation moment (about 1 Bohr-magneton per formula unit) is large enough to infer that the ferromagnetism is not a surface phenomenon in these nanocrystallites. Since these ferromagnetic nanocrystallines are easy to synthesize with a stable form in air, one can explore applications, particularly where hysteresis is a disadvantage.",0804.1926v2 2008-05-05,"Orbital degeneracy, Hund's coupling, and band ferromagnetism: effective quantum parameter, suppression of quantum corrections, and enhanced stability","An effective quantum parameter is obtained for the band ferromagnet in terms of orbital degeneracy and Hund's coupling. This quantum parameter determines, in analogy with 1/N for the generalized Hubbard model and 1/S for quantum spin systems, the strength of quantum corrections to spin stiffness and spin-wave energies. Quantum corrections are obtained by incorporating correlation effects in the form of self-energy and vertex corrections within a spin-rotationally-symmetric approach in which the Goldstone mode is explicitly preserved order by order. It is shown that even a relatively small Hund's coupling is rather efficient in strongly suppressing quantum corrections, especially for large N, resulting in strongly enhanced stability of the ferromagnetic state. This mechanism for the enhancement of ferromagnetism by Hund's coupling implicitly involves a subtle interplay of lattice, dimensionality, band dispersion, spectral distribution, and band filling effects.",0805.0470v1 2008-05-20,A theory of ferromagnetism by Ettore Majorana,"We present and analyze in detail an unknown theory of ferromagnetism developed by Ettore Majorana as early as the beginnings of 1930s, substantially different in the methods employed from the well-known Heisenberg theory of 1928 (and from later formulations by Bloch and others). Similarly to this, however, it describes successfully the main features of ferromagnetism, although the key equation for the spontaneous mean magnetization and the expression for the Curie temperature are different from those deduced in the Heisenberg theory (and in the original phenomenological Weiss theory). The theory presented here contains also a peculiar prediction for the number of nearest neighbors required to realize ferromagnetism, which avoids the corresponding arbitrary assumption made by Heisenberg on the basis of known (at that time) experimental observations. Some applications of the theory (linear chain, triangular chain, etc.) are, as well, considered.",0805.3057v1 2008-06-17,"Coexistence of ferromagnetism and superconductivity: the role of kinetic interactions, kinetic correlations, and external pressure","We use the Hubbard type model to describe the coexistence between superconductivity (SC) and ferromagnetism (F). Our Hamiltonian contains single-site and two-site interactions. All inter-site interactions will have included the inter-site kinetic correlation: $$, within the Hartree-Fock approximation. To obtain the SC transition temperature $T_{SC}$ and Curie temperature $T_{C}$ we use the Green's functions method. The numerical results show that the singlet SC is eliminated by F, but the triplet SC is either enhanced or depleted by F, depending on the carrier concentration and direction of a superconducting spin pair with respect to magnetization. The kinetic correlation is capable of creating superconductivity. We find that the ferromagnetism created by change of the bandwidth can coexist with singlet superconductivity. In the case of triplet superconductivity the ferromagnetism creates different critical SC temperatures for the $A_{1}$ and $A_{2}$ phase (the pair's spin parallel and antiparallel to magnetization, respectively).",0806.2774v1 2008-06-27,Field-dependent AC susceptibility of itinerant ferromagnets,"Whereas dc measurements of magnetic susceptibility, $\chi$, fail to distinguish between local and weak itinerant ferromagnets, radio-frequency (rf) measurements of $\chi$ in the ferromagnetic state show dramatic differences between the two. We present sensitive tunnel-diode resonator measurements of $\chi$ in the weak itinerant ferromagnet ZrZn$_2$ at a frequency of 23 MHz. Below Curie temperature, $T_C \approx 26$ K, the susceptibility is seen to increase and pass through a broad maximum at approximately 15 K in zero applied dc magnetic field. Application of a magnetic field reduces the amplitude of the maximum and shifts it to lower temperatures. The existence and evolution this maximum with applied field is not predicted by either the Stoner or self-consistent renormalized (SCR) spin fluctuations theories. For temperatures below $T_C$ both theories derive a zero-field limit expression for $\chi$. We propose a semi-phenomenological model that considers the effect of the internal field from the polarized fraction of the conduction band on the remaining, unpolarized conduction band electrons. The developed model accurate describes the experimental data.",0806.4612v1 2008-08-04,Measuring entropy generated by spin-transfer,"An experimental protocol is presented that allows the entropy generated by spin-transfer to be measured. The effect of a strong spin-polarized current injected on a ferromagnetic nanostructure is investigated with focusing on the quasi-static equilibrium states of a ferromagnetic single domain. The samples are single contacted Ni nanowires obtained by electrodeposition in a nanoporous template. The thermal susceptibility of the magnetoresistance is measured as a function of the magnetic field for different values of the current injected through the wire. This quantity is related to the thermal magnetic susceptibility of the ferromagnetic wire through the anisotropic magnetoresistance. The ferromagnetic entropy generated by the current injection is deduced thanks to a thermodynamic Maxwell relation. This study shows that the effect of the spin-transfer in our samples results in the generation of incoherent excitations instead of rotation of the magnetization.",0808.0463v1 2008-08-14,Half-metallic ferromagnetism in binary compounds of alkali metals with nitrogen: Ab initio calculations,"The first-principles full-potential linearized augmented plane-wave method based on density functional theory is used to investigate electronic structure and magnetic properties of hypothetical binary compounds of I$^{A}$ subgroup elements with nitrogen (LiN, NaN, KN and RbN) in assumed three types of cristalline structure (rock salt, wurtzite and zinc-blende). We find that, due to the spin polarized \textit{p} orbitas of N, all four compounds are half-metallic ferromagnets with wide energy bandgaps (up to 2.0 eV). The calculated total magnetic moment in all investigated compounds for all three types of crystal structure is exactly 2.00 $\mu_{\text{B}}$ per formula unit. The predicted half-metallicity is robust with respect to lattice-constant contraction. In all the cases ferromagnetic phase is energetically favored with respect to the paramagnetic one. The mechanism leading to half-metallic ferromagnetism and synthesis possibilities are discussed.",0808.1951v2 2008-08-22,Ferromagnetism in transparent Mn(II)-doped indium tin oxide films prepared by sol-gel process,"We observe remarkably strong room temperature ferromagnetism (~1.5 Bohr Magneton/Mn) in optically transparent Mn(II)-doped indium tin oxide (ITO) films. The nanocrystalline films with average grain size 10-22 nm and thickness 150-350 nm are prepared by sol-gel coating technique on sodalime silica glass substrate. The ferromagnetic property is, of course, weak for films deposited on pure silica glass substrate. The structural parameters of the films appear to be governing the magnetic property strongly which vary appreciably depending on the substrate. The observation of room temperature ferromagnetism in transparent conducting ITO films may find a plethora of applications in the area of magneto-optics.",0808.3063v1 2008-08-27,Entanglement and quantum phase transition in alternating XY spin chain with next-nearest neighbour interactions,"By using the method of density-matrix renormalization-group to solve the different spin-spin correlation functions, the nearest-neighbouring entanglement(NNE) and next-nearest-neighbouring entanglement(NNNE) of one-dimensional alternating Heisenberg XY spin chain is investigated in the presence of alternating nearest neighbour interactions of exchange couplings, external magnetic fields and next-nearest neighbouring interactions. For dimerized ferromagnetic spin chain, NNNE appears only above the critical dimerized interaction, meanwhile, the dimerized interaction effects quantum phase transition point and improves NNNE to a large value. We also study the effect of ferromagnetic or antiferromagnetic next-nearest neighboring (NNN) interactions on the dynamics of NNE and NNNE. The ferromagnetic NNN interaction increases and shrinks NNE below and above critical frustrated interaction respectively, while the antiferromagnetic NNN interaction always decreases NNE. The antiferromagnetic NNN interaction results to a larger value of NNNE in comparison to the case when the NNN interaction is ferromagnetic.",0808.3686v1 2008-09-09,Sign reversal of ac Josephson current in a ferromagnetic Josephson junction,"The ac Josephson effect in a ferromagnetic Josephson junction, which is composed of two superconductors separated by a ferromagnetic metal (FM), is studied by a tunneling Hamiltonian and Green's function method. We obtain two types of superconducting phase dependent current, i.e., Josephson current and quasiparticle-pair-interference current (QPIC). These currents change their signs with thickness of the FM layer due to the 0-$\pi$ transition characteristic to the ferromagnetic Josephson junction. As a function of applied voltage, the Josephson critical current shows a logarithmic divergence called the Riedel peak at the gap voltage, while the QPIC shows a discontinuous jump. The Riedel peak reverses due to the 0-$\pi$ transition and disappears near the 0-$\pi$ transition point. The discontinuous jump in the QPIC also represents similar behaviors to the Riedel peak. These results are in contrast to the conventional ones.",0809.1470v2 2008-10-07,Interface Ferromagnetism in (110)-Oriented La0.7Sr0.3MnO3/SrTiO3 Ultrathin Superlattices,"We explore manganite interface magnetism in epitaxially grown La0.7Sr0.3MnO3(LSMO)/SrTiO3 ultrathin superlattices (SL) along (110) orientation. we show that robust ferromagnetism persists down to four monolayers LSMO(MLs) (~1.1 nm in thickness), of which 50% Mn is at the interface state. Above eight MLs, the magnetic moment is nearly saturated to the theoretical value of 3.7u_B, with an estimated interface moment of 3.2u_B. In comparison to (100)-oriented SLs which were previously shown to have a spin canted ground state, (110)-oriented SLs exhibit stronger low-dimensional ferromagnetism and better metallicity, suggesting a ferromagnetic interface spin state well suited for all-oxide spintronic devices. The underlining mechanism is qualitatively discussed.",0810.1309v2 2008-10-22,Strong asymmetry of microwave absorption by bi-layer conducting ferromagnetic films in the microstrip-line based broadband ferromagnetic resonance,"Peculiarities of ferromagnetic resonance response of conducting magnetic bi-layer films of nanometric thicknesses excited by microstrip microwave transducers have been studied theoretically. Strong asymmetry of the response has been found. Depending on the order of layers with respect to the transducer either the first higher-order standing spin wave mode, or the fundamental mode shows the largest response. Film conductivity and lowered symmetry of microwave fields of such transducers are responsible for this behavior. Amplitude of which mode is larger also depends on the driving frequency. This effect is explained as shielding of the asymmetric transducer field by eddy currents in the films. This shielding remains very efficient for films with thicknesses well below the microwave skin depth. This effect may be useful for studying buried magnetic interfaces and should be accounted for in future development of broadband inductive ferromagnetic resonance methods.",0810.4020v1 2008-10-28,Nitrogen defects and ferromagnetism of Cr-doped AlN diluted magnetic semiconductor from first principles,"High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high temperature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V_N). Calculated results are in agreement with experimental observations and facts of real Cr-doped AlN samples and their synthesis. Our first-principles results are useful to elucidating the mechanism for the ferromagnetism and exploring high-performance Cr-doped AlN diluted magnetic semiconductors.",0810.5048v1 2008-11-13,Strong-disorder paramagnetic-ferromagnetic fixed point in the square-lattice +- J Ising model,"We consider the random-bond +- J Ising model on a square lattice as a function of the temperature T and of the disorder parameter p (p=1 corresponds to the pure Ising model). We investigate the critical behavior along the paramagnetic-ferromagnetic transition line at low temperatures, below the temperature of the multicritical Nishimori point at T*= 0.9527(1), p*=0.89083(3). We present finite-size scaling analyses of Monte Carlo results at two temperature values, T=0.645 and T=0.5. The results show that the paramagnetic-ferromagnetic transition line is reentrant for TT*. Our results for the critical exponents are consistent with the hyperscaling relation 2 beta/nu - eta = d - 2 = 0.",0811.2101v3 2008-11-28,Proximity-driven source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor voltage-biased Josephson junction,"We theoretically investigate an opportunity to implement a source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor (SFS) voltage-biased junction in the regime of essential proximity effect and calculate the current flowing through the probe electrode tunnel coupled to the ferromagnetic interlayer region. It is shown that while the polarization of the dc current component is generally small in case of weak exchange field of the ferromagnet, there is an ac component of the current in the system. This ac current is highly spin-polarized and entirely originated from the non-equilibrium proximity effect in the interlayer. The frequency of the current is controlled by the voltage applied to SFS junction. We discuss a possibility to obtain a source of coherent ac currents with a certain phase shift between them by tunnel coupling two probe electrodes at different locations of the interlayer region.",0811.4659v1 2009-01-09,Critical Current Behavior in Josephson Junctions with the Weak Ferromagnet PdNi,"We have studied the variation of critical current in Superconductor/Ferromagnet/Superconductor (S/F/S) Josephson Junctions as a function of ferromagnet thickness using a weakly-ferromagnetic alloy Pd82Ni12. Measurements were performed for the thickness range 32 to 100 nm, over which the critical current density decreases by five orders of magnitude. The critical current density oscillates with a period of 12.9 +/- 0.3 nm, and decays over a characteristic length of 8.0 +/- 0.5 nm. There is no evidence of a crossover to a slower decay, which might indicate the presence of long-range spin-triplet pair correlations. We discuss possible reasons for their absence, including the possibility of strong spin flip scattering in PdNi.",0901.1321v1 2009-01-10,Enhanced Ferromagnetic Ordering in GdBaCo$_{2}$O$_{5.5+δ}$ Films on SrTiO3 (001) Substrate,"The authors investigated the structure and properties of GdBaCo$_{2}$O$_{5.5+\delta}$ thin films epitaxially grown on SrTiO$_{3}$ (001) single crystal substrates. The thin films were found to have a notable remnant magnetization above room temperature, which is much higher than that observed in bulk material. Transmission electron microscopy and x-ray diffraction patterns reveal that phase separation occurs in these films, and the phase responsible for the enhanced ferromagnetic order is $a$-oriented. The enhanced ferromagnetic order is attributed to the enhanced orbital order of Co$^{3+}$ in CoO$_{5}$ pyramids, and the disappearance of ferromagnetic to antiferromagnetic transition is explained the stabilization of higher spin state of Co$^{3+}$ in CoO$_{6}$ octahedra.",0901.1388v1 2009-01-14,What is the Mechanism Underlying 3-D Heisenberg-like Ferromagnetism Across the Compositional Metal-Insulator Transition in La1-xCaxMnO3 (0.18 < xc < 0.22)?,"Detailed measurements of the magnetic and transport properties of the two La1-xCaxMnO3 (x = 0.18, x = 0.20) single crystals straddling the compositional metal-insulator transition boundary (0.18 < xc < 0.22) are summarized. The analysis of magnetization/susceptibility data reveals the occurrence of a second order/continuous ferromagnetic-paramagnetic phase transition described not only by nearest neighbour, 3-D Heisenberg model exponents (gamma = 1.387, beta = 0.365, delta = 4.783), but also with comparable values of the critical amplitudes in both the insulating and the metallic samples. These results support the assertion that double exchange cannot be the underlying mechanism supporting ferromagnetism in this composition range, and arguments are presented that the relevant interaction is ferromagnetic super exchange modulated by proximity to the orbitally ordered to disordered transition.",0901.1926v1 2009-01-27,Ferromagnetism in armchair graphene nanoribbon,"Due to the weak spin-orbit interaction and the peculiar relativistic dispersion in graphene, there are exciting proposals to build spin qubits in graphene nanoribbons with armchair boundaries. However, the mutual interactions between electrons are neglected in most studies so far and thus motivate us to investigate the role of electronic correlations in armchair graphene nanoribbon by both analytical and numerical methods. Here we show that the inclusion of mutual repulsions leads to drastic changes and the ground state turns ferromagnetic in a range of carrier concentrations. Our findings highlight the crucial importance of the electron-electron interaction and its subtle interplay with boundary topology in graphene nanoribbons. Furthermore, since the ferromagnetic properties sensitively depends on the carrier concentration, it can be manipulated at ease by electric gates. The resultant ferromagnetic state with metallic conductivity is not only surprising from an academic viewpoint, but also has potential applications in spintronics at nanoscale.",0901.4177v1 2009-02-02,Ferromagnetic Kondo Effect at Nanocontacts,"Magnetic impurities bridging nanocontacts and break junctions of nearly magnetic metals may lead to permanent moments, analogous to the giant moments well known in the bulk case. A numerical renormalization group (NRG) study shows that, contrary to mean field based expectations, a permanent moment never arises within an Anderson model, which invariably leads to strong Kondo screening. By including in the model an additional ferromagnetic exchange coupling between leads and impurity, the NRG may instead stabilize a permanent moment through a ferromagnetic Kondo effect. The resulting state is a rotationally invariant spin, which differs profoundly from mean field. A sign inversion of the zero-bias anomaly and other spectroscopic signatures of the switch from regular to ferromagnetic Kondo are outlined.",0902.0254v1 2009-03-12,Observation of localized ferromagnetic resonance in a continuous ferromagnetic film via magnetic resonance force microscopy,"We present Magnetic Resonance Force Microscopy (MRFM) measurements of Ferromagnetic Resonance (FMR) in a 50 nm thick permalloy film, tilted with respect to the direction of the external magnetic field. At small probe-sample distances the MRFM spectrum breaks up into multiple modes, which we identify as local ferromagnetic resonances confined by the magnetic field of the MRFM tip. Micromagnetic simulations support this identification of the modes and show they are stabilized in the region where the dipolar tip field has a component anti-parallel to the applied field.",0903.2285v1 2009-04-17,Ferromagnetism and Electronic Structures of Nonstoichiometric Heusler-Alloy Fe_3-xMn_xSi Epilayers Grown on Ge(111),"For the study of ferromagnetic materials which are compatible with group-IV semiconductor spintronics, we demonstrate control of the ferromagnetic properties of Heusler-alloys Fe_3-xMn_xSi epitaxially grown on Ge(111) by tuning the Mn composition x. Interestingly, we obtain L2_1-ordered structures even for nonstoichiometric atomic compositions. The Curie temperature of the epilayers with x ~ 0.6 exceeds 300 K. Theoretical calculations indicate that the electronic structures of the nonstoichiometric Fe_3-xMn_xSi alloys become half-metallic for 0.75 < x < 1.5. We discuss the possibility of room-temperature ferromagnetic Fe_3-xMn_xSi/Ge epilayers with high spin polarization.",0904.2610v1 2009-04-29,Magnetism in tunable quantum rings,"We have studied the spin structure of circular four-electron quantum rings using tunable confinement potentials. The calculations were done using the exact diagonalization method. Our results indicate that ringlike systems can have oscillatory flips between ferromagnetic and antiferromagnetic behaviour as a function of the magnetic field. Furthermore, at constant external magnetic fields there were seen similar oscillatory changes between ferromagnetism and antiferromagnetism when the system parameters were changed. Ackording to our results, the magnetism of quantum rings could be tuned by system parameters.We have studied the spin structure of circular four-electron quantum rings using tunable confinement potentials. The calculations were done using the exact diagonalization method. Our results indicate that ringlike systems can have oscillatory flips between ferromagnetic and antiferromagnetic behaviour as a function of the magnetic field. Furthermore, at constant external magnetic fields there were seen similar oscillatory changes between ferromagnetism and antiferromagnetism when the system parameters were changed. According to our results, the magnetism of quantum rings could be tuned by system parameters.",0904.4580v3 2009-04-30,A microscopic model for current-induced switching of magnetization for half-metallic leads,"We study the behaviour of the magnetization in a half-metallic ferromagnet/nonmagnetic insulator/ferromagnetic metal/paramagnetic metal (FM1/NI/FM2/PM) tunnel junction. It is calculated self-consistently within the nonequilibrium Keldysh formalism. The magnetic regions are treated as band ferromagnets and are described by the single-band Hubbard model. We developed a nonequilibrium spectral density approach to solve the Hubbard model approximately in the switching magnet. By applying a voltage to the junction it is possible to switch between antiparallel (AP) and parallel (P) alignment of the magnetizations of the two ferromagnets. The transition from AP to P occurs for positive voltages while the inverse transition from P to AP can be induced by negative voltages only. This behaviour is in agreement with the Slonczewski model of current-induced switching and appears self-consistently within the model, i.e. without using half-classical methods like the Landau-Lifshitz-Gilbert equation.",0904.4821v1 2009-05-04,Ferromagnetism in Carbon doped Zinc Oxide Systems,"We report spin polarized density functional calculations of ferromagnetic properties of a series of ZnO clusters and solid containing one or two substitutional carbon impurities. We analyze the eigen value spectra, spin densities and molecular orbitals, and induced magnetic moments for ZnC, Zn$_{2}$C, Zn$_{2}$OC, carbon substituted clusters Zn$_{n}$O$_{n}$ (n=3--10, 12) and ZnO solid. The results show that the doping induces magnetic moment of the $\sim$2 $\mu_{B}$ in all the cases. All the systems with two carbon impurities show ferromagnetic interaction except when the carbon atoms share the same Zn atom as the nearest neighbor. This ferromagnetic interaction is predominantly mediated via $\pi$ bonds in ring structures and through $\pi$ and $\sigma$ bonds in three dimensional structure. The calculations also show that the interaction is significantly enhanced in solid, bringing out the role of dimensionality of Zn-O network connecting two carbon atoms.",0905.0366v1 2009-05-20,Coexistence of Weak Ferromagnetism and Ferroelectricity in the High Pressure LiNbO3-type Phase of FeTiO3,"We report the magnetic and electrical characteristics of a polycrystalline specimen of FeTiO3 synthesized at high pressure that is isostructural with acentric LiNbO3 (LBO). Piezoresponse force microscopy, optical second harmonic generation, and magnetometry demonstrate that FeTiO3-II is ferroelectric at and below room temperature and weakly ferromagnetic below ~120 K. These results validate symmetry-based materials design criteria and first principles calculations of coexistence between ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LBO structure. The high-pressure form of FeTiO3 stands out as a rare example of a ferroelectric exhibiting weak ferromagnetism generated by a Dzyaloshinskii-Moriya interaction.",0905.3332v1 2009-08-05,Non-DMS related ferromagnetism in transition metal doped zinc oxide,"We review pitfalls in recent efforts to make a conventional semiconductor, namely ZnO, ferromagnetic by means of doping with transition metal ions. Since the solubility of those elements is rather low, formation of secondary phases and the creation of defects upon low temperature processing can lead to unwanted magnetic effects. Among others, ion implantation is a method of doping, which is highly suited for the investigation of those effects. By focussing mainly on Fe, Co or Ni implanted ZnO single crystals we show that there are manifold sources for ferromagnetism in this material which can easily be confused with the formation of a ferromagnetic diluted magnetic semiconductor (DMS). We will focus on metallic as well as oxide precipitates and the difficulties of their identification.",0908.0645v1 2009-08-09,Microscopic Model versus Systematic Low-Energy Effective Field Theory for a Doped Quantum Ferromagnet,"We consider a microscopic model for a doped quantum ferromagnet as a test case for the systematic low-energy effective field theory for magnons and holes, which is constructed in complete analogy to the case of quantum antiferromagnets. In contrast to antiferromagnets, for which the effective field theory approach can be tested only numerically, in the ferromagnetic case both the microscopic and the effective theory can be solved analytically. In this way the low-energy parameters of the effective theory are determined exactly by matching to the underlying microscopic model. The low-energy behavior at half-filling as well as in the single- and two-hole sectors is described exactly by the systematic low-energy effective field theory. In particular, for weakly bound two-hole states the effective field theory even works beyond perturbation theory. This lends strong support to the quantitative success of the systematic low-energy effective field theory method not only in the ferromagnetic but also in the physically most interesting antiferromagnetic case.",0908.1225v1 2009-08-27,Magnetic groundstates in a correlated two orbital Hubbard model,"We examine the orbital and magnetic order of the two orbital Hubbard model within dynamical mean field theory. The model describes the low energy physics of a partially filled $e_g$-band as can be found in some transition metal compounds. The model shows antiferromagnetic as well as ferromagnetic phases. For stabilizing ferromagnetism we find that Hund's coupling is particularly important. Quarter filling represents a very special situation in the phase diagram, where the coupling of spin, charge, and orbital degrees of freedom are involved. Exactly at quarter filling we find a metal insulator transition (MIT) between two almost fully polarized ferromagnetic states. This MIT can be tuned by changing the local interaction strength and seems to be a first order transition at zero temperature. Apart from these ferromagnetic states we were also able to stabilize antiferromagnetic and charge ordered phases at quarter filling, depending on the interaction parameters.",0908.3990v2 2009-08-27,Spontaneous spin current due to triplet superconductor--ferromagnet interfaces,"We examine the appearance of a spontaneous bulk spin current in a triplet superconductor in contact with a metallic ferromagnet. The spin current results from the spin-flip of Cooper pairs upon reflection from the interface with the ferromagnet, and is shown to display strong similarities to the spontaneous charge current in a Josephson junction. We express the spin current in terms of the Andreev reflection coefficients, which are derived by the construction of the quasiclassical scattering wavefunctions. The dependence of the spin current upon a number of parameters is investigated, in particular the orientation of the magnetic moment of the ferromagnet, the exchange splitting, the temperature, and the orbital pairing state of the triplet superconductor.",0908.4065v2 2009-09-09,Magnetic phase diagram of a spin-1 condensate in two dimensions with dipole interaction,"Several new features arise in the ground-state phase diagram of a spin-1 condensate trapped in an optical trap when the magnetic dipole interaction between the atoms is taken into account along with confinement and spin precession. The boundaries between the regions of ferromagnetic and polar phases move as the dipole strength is varied and the ferromagnetic phases can be modulated. The magnetization of the ferromagnetic phase perpendicular to the field becomes modulated as a helix winding around the magnetic field direction, with a wavelength inversely proportional to the dipole strength. This modulation should be observable for current experimental parameters in $^{87}$Rb. Hence the much-sought supersolid state, with broken continuous translation invariance in one direction and broken global U(1) invariance, occurs generically as a metastable state in this system as a result of dipole interaction. The ferromagnetic state parallel to the applied magnetic field becomes striped in a finite system at strong dipolar coupling.",0909.1751v2 2009-10-07,Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe,"Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field H_c2. H_c2 for H // b-axis (H_c2^b) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of H_c2^a shows upward curvature with a low temperature value exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H // b-axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H_c2^b curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.",0910.1157v1 2009-11-02,Double-Exchange Ferromagnetism and Orbital-Fluctuation-Induced Superconductivity in Cubic Uranium Compounds,"A double-exchange mechanism for the emergence of ferromagnetism in cubic uranium compounds is proposed on the basis of a $j$-$j$ coupling scheme. The idea is {\it orbital-dependent duality} of $5f$ electrons concerning itinerant $\Gamma_8^-$ and localized $\Gamma_7^-$ states in the cubic structure. Since orbital degree of freedom is still active in the ferromagnetic phase, orbital-related quantum critical phenomenon is expected to appear. In fact, odd-parity p-wave pairing compatible with ferromagnetism is found in the vicinity of an orbital ordered phase. Furthermore, even-parity d-wave pairing with significant odd-frequency components is obtained. A possibility to observe such exotic superconductivity in manganites is also discussed briefly.",0911.0250v1 2009-11-02,Giant anomalous Hall resistivity of the room temperature ferromagnet Fe3Sn2 - a frustrated metal with the kagome-bilayer structure,"We have investigated magnetic and transport properties of the {\it kagom\'{e}-bilayer} ferromagnet Fe$_{3}$Sn$_{2}$. A soft ferromagnetism and a large anomalous Hall effect are observed. The saturated Hall resistivity of Fe$_{3}$Sn$_{2}$ is 3.2 $\mu\Omega$cm at 300 K, which is almost 20 times higher than that of typical itinerant-ferromagnets such as Fe and Ni. The anomalous Hall coefficient $R_{{\rm s}}$ is 6.7$\times10^{-9}$ $\Omega$cm/G at 300 K, which is three orders of magnitude larger than that of pure Fe. $R_{{\rm s}}$ obeys an unconventional scaling to the longitudinal resistivity, $\rho_{xx}$, of $R_{{\rm s}} \propto \rho_{xx}^{3.3}$. Such a relationship cannot be explained by the skew and/or side-jump mechanisms and indicates that the origin of the anomalous Hall effect in the frustrated magnet Fe$_{3}$Sn$_{2}$ is indeed extraordinary.",0911.0289v1 2009-11-13,Electrically driven magnetism on a Pd thin film,"Using first-principles density functional calculations we demonstrate that ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd metal thin-film surface through application of an external electric field. As free charges are either accumulated or depleted at the Pd surface to screen the applied electric field there is a corresponding change in the surface density of states. This change can be made sufficient for the Fermi-level density of states to satisfy the Stoner criterion, driving a transition locally at the surface from a paramagnetic state to an itinerant ferromagnetic state above a critical applied electric field, Ec. Furthermore, due to the second-order nature of this transition, the surface magnetization of the ferromagnetic state just above the transition exhibits a substantial dependence on electric field, as the result of an enhanced magnetoelectric susceptibility. Using a linearized Stoner model we explain the occurrence of the itinerant ferromagnetism and demonstrate that the magnetic moment on the Pd surface follows a square-root variation with electric field consistent with our first-principles calculations.",0911.2678v1 2009-11-22,Phase transition in a one-dimensional Ising ferromagnet at zero-temperature under Glauber dynamics with a synchronous updating mode,"In the past decade low-temperature Glauber dynamics for the one-dimensional Ising system has been several times observed experimentally and occurred to be one of the most important theoretical approaches in a field of molecular nanomagnets. On the other hand, it has been shown recently that Glauber dynamics with the Metropolis flipping probability for the zero-temperature Ising ferromagnet under synchronous updating can lead surprisingly to the antiferromagnetic steady state. In this paper the generalized class of Glauber dynamics at zero-temperature will be considered and the relaxation into the ground state, after a quench from high temperature, will be investigated. Using Monte Carlo simulations and a mean field approach, discontinuous phase transition between ferromagnetic and antiferromagnetic phases for a one-dimensional ferromagnet will be shown.",0911.4242v3 2010-01-09,Correlation induced half-metallicity in a ferromagnetic single-layered compound: Sr$_2$CoO$_4$,"The electronic and magnetic properties of Sr$_2$CoO$_4$ compound have been studied using $\emph{ab initio}$ electronic structure calculations. As opposed to GGA calculation, which gives ferromagnetic metallic solution, GGA+$U$ calculations provide two kind of ferromagnetic solutions: (i) half-metallic and (ii) metallic. The half-metallic solution is a ground state of the system and the metallic one is a metastable state. The strong hybridization between Co 3$d$ and O 2$p$ orbitals decides the electronic and magnetic properties of the compound. The total magnetic moment per formula unit is found to be $\sim$ 3 $\mu_B$ ($S$ = 3/2). Our calculations give the magnetocrystalline anisotropy energy of $\sim$ 2.7 meV, which provides a good description of experimentally observed large magnetocrystalline anisotropy. The Heisenberg exchange parameters up to fourth nearest neighbours are also calculated. The mean-field theory gives the $T_C$ = 887 K. The possible physical implications of the ferromagnetic half-metallic ground state are also discussed.",1001.1405v1 2010-01-18,Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders,"Using the Density Matrix Renormalization Group and two-leg ladders, we investigate an electronic two-orbital Hubbard model including plaquette diagonal hopping amplitudes. Our goal is to search for regimes where charges added to the undoped state form pairs, presumably a precursor of a superconducting state.For the electronic density $\rho=2$, i.e. the undoped limit, our investigations show a robust $(\pi,0)$ antiferromagnetic ground state, as in previous investigations. Doping away from $\rho=2$ and for large values of the Hund coupling $J$, a ferromagnetic region is found to be stable. Moreover, when the interorbital on-site Hubbard repulsion is smaller than the Hund coupling, i.e. for $U'2 nm, and highlight an additional short-period (0.76 nm) oscillation for tAu<3 nm in Nb/Au/Co. This short-period oscillation can be explained in terms of a damped RKKY-like oscillation of the spin-polarization in Au. The robustness of the long-period oscillation against the substitution of Co for Fe suggests that it is intrinsic to the Au(111) layer on Nb, and may represent a new form of quantum interference in very clean trilayer systems.",1003.1181v1 2010-03-23,Intrinsic Room Temperature Ferromagnetism in Boron-doped ZnO,"We report room temperature ferromagnetism in boron-doped ZnO both experimentally and theoretically. The single phase Zn1-xBxO films deposited under high oxygen pressure by pulsed-laser deposition show ferromagnetic behavior at room temperature. The saturation magnetization increases monotonously from 0 to 1.5 emu/cm3 with the increasing of B component x from 0 to 6.8%. The first-principles calculations based on density functional theory demonstrate that the ferromagnetism in B-doped ZnO originates from the induced magnetic moments of oxygen atoms in the nearest neighbor sites to the B-Zn vacancy pair. The calculated total magnetic moment increasing tendency with B component is well consistent with experiments.",1003.4423v1 2010-04-05,The suppression of hidden order and onset of ferromagnetism in URu2Si2 via Re substitution,"Substitution of Re for Ru in the heavy fermion compound URu2Si2 suppresses the hidden order transition and gives rise to ferromagnetism at higher concentrations. The hidden order transition of URu(2-x)Re(x)Si2, tracked via specific heat and electrical resistivity measurements, decreases in temperature and broadens, and is no longer observed for x>0.1. A critical scaling analysis of the bulk magnetization indicates that the ferromagnetic ordering temperature and ordered moment are suppressed continuously towards zero at a critical concentration of x = 0.15, accompanied by the additional suppression of the critical exponents gamma and (delta-1) towards zero. This unusual trend appears to reflect the underlying interplay between Kondo and ferromagnetic interactions, and perhaps the proximity of the hidden order phase.",1004.0704v1 2010-04-30,Epitaxial integration of the intrinsic ferromagnetic semiconductor GdN with silicon technology,"A major challenge for the next generation of spintronics devices is the implementation of ferromagnetic-semiconductor thin films as spin injectors and detectors. Spin-polarised carrier injection cannot be accomplished efficiently from metals, and coupled with the rarity of intrinsic ferromagnetic semiconductors this has driven intensive study of diluted magnetic semiconductors. Chief among these is the doped III-V compound (Ga,Mn)As. These materials suffer from a number of drawbacks; they (i) require magnetic-ion doping well above the solubility limit, and (ii) must be hole doped to above the degenerate limit, preventing independent control of the carrier concentration and charge sign. Here we demonstrate the first epitaxial growth of a recently-characterised intrinsic ferromagnetic semiconductor, GdN, on silicon substrates, providing an essential step on the way to integrate new spintronics functionalities into Si-based technology. The films have been characterised as regards their growth toward fully relaxed GdN, the density and mobility of their carriers, and their magnetic behaviour.",1004.5428v1 2010-06-01,The superconducting ferromagnet UCoGe,"The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T_C = 3 K and a superconductor with a transition temperature T_s = 0.6 K. We review its basic thermal, magnetic - on the macro and microscopic scale - and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T_s = 0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.",1006.0065v1 2010-06-02,Magnetization pinning in conducting films demonstrated using broadband ferromagnetic resonance,"The broadband microstrip ferromagnetic resonance technique has been applied for detection and characterization of a magnetic inhomogeneity in a film sample. In the case of a 100nm thick Permalloy film an additional magnetically depleted top sub-layer, practically unidentifiable by the conventional ferromagnetic resonance setup, has been detected and characterized. These results have been confirmed by Brillouin light scattering spectroscopy revealing the fact that the optical properties of the additional sub-layer do not differ much from those of the bulk of the film. Subsequent characterization of a large number of other presumably single-layer films with thicknesses in the range 30-100nm using the same ferromagnetic resonance technique also revealed the same effect.",1006.0331v1 2010-07-04,Transport Properties in Ferromagnetic Josephson Junction between Triplet Superconductors,"Charge and spin Josephson currents in a ballistic superconductor-ferromagnet-superconductor junction with spin-triplet pairing symmetry are studied using the quasiclassical Eilenberger equation. The gap vector of superconductors has an arbitrary relative angle with respect to magnetization of the ferromagnetic layer. We clarify the effects of the thickness of ferromagnetic layer and magnitude of the magnetization on the Josephson charge and spin currents. We find that 0-\pi transition can occur except for the case that the exchange field and d-vector are in nearly perpendicular configuration. We also show how spin current flows due to misorientation between the exchange field and d-vector.",1007.0529v2 2010-07-05,Laser-induced Spin Dynamics in Metallic Multilayers,"Electronic excitations in a ferromagnet can trigger ultrafast spin dynamics with potential applications in a speed increase in magnetic recording. The project investigates ultrafast magnetization dynamics, which is driven in metallic layers by ballistic hot electrons. In a ferromagnet these electrons induce a change in the absolute value of the magnetization M through spin-dependent scattering. If the electrons are spin-polarized, scattering at the interface of a noble metal and a ferromagnet results in spin-transfer torque and hence modifies the direction of M. To reveal the underlying mechanisms, we study model systems, which are realized by layers of Au with its large ballistic mean free path and Fe as an itinerant ferromagnet. We aim at understanding ultrafast demagnetization and femtosecond all-optical generation of spin transfer torque effects (excitation of hot electrons with a femtosecond laser). The launched dynamics are probed by magneto-optics in a time-resolved experiment.",1007.0726v1 2010-07-15,Full spin switch effect for the superconducting current in a superconductor/ferromagnet thin film heterostructure,"Superconductor/ferromagnet (S/F) proximity effect theory predicts that the superconducting critical temperature of the F1/F2/S or F1/S/F2 trilayers for the parallel orientation of the F1 and F2 magnetizations is smaller than for the antiparallel one. This suggests a possibility of a controlled switching between the superconducting and normal states in the S layer. Here, using the spin switch design F1/F2/S theoretically proposed by Oh et al. [Appl. Phys. Lett. 71, 2376 (1997)], that comprises a ferromagnetic bilayer separated by a non-magnetic metallic spacer layer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have successfully realized a full spin switch effect for the superconducting current.",1007.2511v1 2010-07-21,Exchange anisotropy pinning of a standing spin wave mode,"Standing spin waves in a thin film are used as sensitive probes of interface pinning induced by an antiferromagnet through exchange anisotropy. Using coplanar waveguide ferromagnetic resonance, pinning of the lowest energy spin wave thickness mode in Ni(80)Fe(20)/Ir(25)Mn(75) exchange biased bilayers was studied for a range of IrMn thicknesses. We show that pinning of the standing mode can be used to amplify, relative to the fundamental resonance, frequency shifts associated with exchange bias. The shifts provide a unique `fingerprint' of the exchange bias and can be interpreted in terms of an effective ferromagnetic film thickness and ferromagnet/antiferromagnet interface anisotropy. Thermal effects are studied for ultra-thin antiferromagnetic Ir(25)Mn(75) thicknesses, and the onset of bias is correlated with changes in the pinning fields. The pinning strength magnitude is found to grow with cooling of the sample, while the effective ferromagnetic film thickness simultaneously decreases. These results suggest that exchange bias involves some deformation of magnetic order in the interface region.",1007.3577v1 2010-08-03,Absence of weak antilocalization in ferromagnetic films,"We present magnetoresistance measurements performed on ultrathin films of amorphous Ni and Fe. In these films the Curie temperature drops to zero at small thickness, making it possible to study the effect of ferromagnetism on localization. We find that non-ferromagnetic films are characterized by positive magnetoresistance. This is interpreted as resulting from weak antilocalization due to strong Bychkov-Rashba spin orbit scattering. As the films become ferromagnetic the magnetoresistance changes sign and becomes negative. We analyze our data to identify the individual contributions of weak localization, weak antilocalization and anisotropic magnetoresistance and conclude that the magnetic order suppresses the influence of spin-orbit effects on localization phenomena in agreement with theoretical predictions.",1008.0456v1 2010-08-10,Transition from a ferromagnetic insulating to a ferromagnetic metallic state in nanoparticles of Nd0.8Sr0.2MnO3 : Study of the electronic - and magneto - transport properties,"A detailed investigation of the electronic - and magneto - transport properties of Nd0.8Sr0.2MnO3 with the variation of grain size (down to 42 nm) is presented here. Interestingly, we observe that the ferromagnetic insulating state is suppressed and a metallic state is stabilized as the grain size of the sample is reduced. As a result, metal insulator transition is observed in this low doped manganite which is insulating in nature in its bulk form. Destabilization of polaronic order in the ferromagnetic insulating state due to enhanced surface disorder on grain size reduction has been attributed to this effect. A phenomenological model has been proposed to represent the concept of destabilization of polaron formation in the surface region of the nano grains. Resistivity and magnetoresistance data have been carefully analyzed employing different suitable models. Electrical third harmonic resistance has been measured to directly probe the electrical nonlinearity in the samples.",1008.1693v1 2010-08-13,Unusual signatures of the ferromagnetic transition in the heavy Fermion compound UMn$_2$Al$_{20}$,"Magnetic susceptibility results for single crystals of the new cubic compounds UT$_2$Al$_{20}$ (T=Mn, V, and Mo) are reported. Magnetization, specific heat, resistivity, and neutron diffraction results for a single crystal and neutron diffraction and inelastic spectra for a powder sample are reported for UMn$_2$Al$_{20}$. For T = V and Mo, temperature independent Pauli paramagnetism is observed. For UMn$_2$Al$_{20}$, a ferromagnetic transition is observed in the magnetic susceptibility at $T_c$ = 20 K. The specific heat anomaly at $T_c$ is very weak while no anomaly in the resistivity is seen at $T_c$. We discuss two possible origins for this behavior of UMn$_2$Al$_{20}$: moderately small moment itinerant ferromagnetism, or induced local moment ferromagnetism.",1008.2235v1 2010-08-19,Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions,"We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson junctions with high critical current densities, large normal resistance times area products, high quality factors, and very good spatial uniformity. For these junctions a transition from 0- to \pi-coupling is observed for a thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The magnetic field dependence of the \pi-coupled junctions demonstrates good spatial homogeneity of the tunneling barrier and ferromagnetic interlayer. Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane anisotropy and large saturation magnetization, indicating negligible dead layers at the interfaces. A careful analysis of Fiske modes provides information on the junction quality factor and the relevant damping mechanisms up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at low frequencies, the damping is dominated by the finite surface resistance of the junction electrodes at high frequencies. High quality factors of up to 30 around 200 GHz have been achieved. Our analysis shows that the fabricated junctions are promising for applications in superconducting quantum circuits or quantum tunneling experiments.",1008.3341v1 2010-08-23,Self-magnetic compensation and Exchange Bias in ferromagnetic Samarium systems,"For Sm(3+) ions in a vast majority of metallic systems, the following interesting scenario has been conjured up for long, namely, a magnetic lattice of tiny self (spin-orbital) compensated 4f-moments exchange coupled (and phase reversed) to the polarization in the conduction band. We report here the identification of a self-compensation behavior in a variety of ferromagnetic Sm intermetallics via the fingerprint of a shift in the magnetic hysteresis (M-H) loop from the origin. Such an attribute, designated as exchange bias in the context of ferromagnetic/antiferromagnetic multilayers, accords these compounds a potential for niche applications in spintronics. We also present results on magnetic compensation behavior on small Gd doping (2.5 atomic percent) in one of the Sm ferromagnets (viz. SmCu(4)Pd). The doped system responds like a pseudo-ferrimagnet and it displays a characteristic left-shifted linear M-H plot for an antiferromagnet.",1008.3779v1 2010-08-26,Modulation of the ferromagnetic insulating phase in Pr0.8Ca0.2MnO3 by Co substitution,"Ferromagnetic insulator Pr0.8Ca0.2Mn1-yCoyO3 (0 <= y <= 0.7) thin films were epitaxially grown on (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (100) substrates by pulsed laser deposition. To probe the ferromagnetic insulator state of hole-doped manganites, the Co content dependences of the structural, magnetic, and transport properties were studied. Variation of lattice constant by the substitution of Co ions is well reproduced considering that divalent and trivalent Co ions substitute for Mn ions at the perovskite B-sites. For 0 <= y <= 0.3, the Curie temperature, saturation magnetization, and magnetoresistance increase with increasing Co content, retaining the insulating properties. Detailed analyses of transport and magnetic properties indicate the contribution of both double exchange and superexchange interactions to the appearance of the ferromagnetic insulating phase.",1009.1524v2 2010-09-14,Uncompensated magnetization and exchange-bias field in La$_{0.7}$Sr$_{0.3}$MnO$_3$/YMnO$_3$ bilayers: The influence of the ferromagnetic layer,"We studied the magnetic behavior of bilayers of multiferroic and nominally antiferromagnetic o-YMnO$_3$ (375~nm thick) and ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.67}$Ca$_{0.33}$MnO$_3$ ($8 \ldots 225~$nm), in particular the vertical magnetization shift $M_E$ and exchange bias field $H_E$ for different thickness and magnetic dilution of the ferromagnetic layer at different temperatures and cooling fields. We have found very large $M_E$ shifts equivalent to up to 100\% of the saturation value of the o-YMO layer alone. The overall behavior indicates that the properties of the ferromagnetic layer contribute substantially to the $M_E$ shift and that this does not correlate straightforwardly with the measured exchange bias field $H_E$.",1009.2619v1 2010-09-19,Effect of spin transfer torque on the magnetic domain wall ferromagnetic resonance frequency in the nanowires,"We investigate the influence of the domain wall ferromagnetic resonance frequency on the spin transfer torque in a ferromagnetic nanowire. By employing micromagnetic simulations with the spin transfer torque, we find that the domain wall resonance frequency decreases with increasing spin polarized current density, when there is no change in the resonance frequency of the domain itself. Surprisingly, the variation of the resonance frequency is remarkable (> 1.6 GHz) with the spin transfer torque even though the domain wall is pinned. Since the presented domain wall ferromagnetic resonance study has been performed for the pinned domain wall, the contributions of extrinsic defects are excluded. It is strong advantages of the present study, since the effects of extrinsic pinning sites are inevitable in the imaging or transport measurements.",1009.3618v1 2010-09-23,Ferromagnetic order in aged Co-doped TiO2 anatase nanopowders,"Oxide based diluted magnetic semiconductor (DMS) materials have been a subject of increasing interest due to reports of room temperature ferromagnetism in several systems and their potential use in the development of spintronic devices. However, concerns on the stability of the magnetic properties of different DMS systems have been raised. Their magnetic moment is often unstable, vanishing with a characteristic decay time of weeks or months, which precludes the development of real applications. This paper reports on the ferromagnetic properties of two-year-aged Ti1-xCoxO2-{\delta} reduced anatase nanopowders with different Co contents (0.03=0.22 whereupon the Eu ions order ferromagnetically. Density functional theory based calculations reproduce the observed experimental findings consistently. We discuss in detail the coexistence of superconductivity and magnetism in a tiny region of the phase space and comment on the competition of ferromagnetism and superconductivity in the title compound.",1011.4481v2 2010-12-02,Estimating the Hubbard repulsion sufficient for the onset of nearly-flat-band ferromagnetism,"We consider nearly-flat-band Hubbard models of a ferromagnet, that is the models that are weak perturbations of those flat-band Hubbard models whose ground state is ferromagnetic for any nonzero strength $U$ of the Hubbard repulsion. In contrast to the flat-band case, in the nearly-flat-band case the ground state, being paramagnetic for $U$ in a vicinity of zero, turns into a ferromagnetic one only if $U$ exceeds some nonzero threshold value $U_{th}$. We address the question whether $U_{th}$ of the considered models is in a physical range, therefore we attempt at obtaining possibly good estimates of the threshold value $U_{th}$. A rigorous method proposed by Tasaki is extended and the resulting estimates are compared with small-system, finite-size scaling results obtained for open- and periodic-boundary conditions. Contrary to suggestions in literature, we find the latter conditions particularly useful for our task.",1012.0443v1 2010-12-06,Room temperature ferromagnetism and anomalous Hall effect in Si$_{1-x}$Mn$_x$ ($x\approx 0.35$) alloys,"A detailed study of the magnetic and transport properties of Si$_{1-x}$Mn$_x$ ($x\approx 0.35$) films is presented. We observe the anomalous Hall effect (AHE) in these films up to room temperature. The results of the magnetic measurements and the AHE data are consistent and demonstrate the existence of long-range ferromagnetic (FM) order in the systems under study. A correlation of the AHE and the magnetic properties of Si$_{1-x}$Mn$_x$ ($x\approx 0.35$) films with their conductivity and substrate type is shown. A theoretical model based on the idea of a two-phase magnetic material, in which molecular clusters with localized magnetic moments are embedded in the matrix of a weak itinerant ferromagnet, is discussed. The long-range ferromagnetic order at high temperatures is mainly due to the Stoner enhancement of the exchange coupling between clusters through thermal spin fluctuations (paramagnons) in the matrix. Theoretical predictions and experimental data are in good qualitative agreement.",1012.1172v1 2010-12-07,Magnetoelectric coupling at the interface of BiFeO3/La0.7Sr0.3MnO3 multilayers,"Electric-field controlled exchange bias in a heterostructure composed of the ferromagnetic manganite La0.7Sr0.3MO3 and the ferroelectric antiferromagnetic BiFeO3 has recently been demonstrated experimentally. By means of a microscopic model Hamiltonian we provide a possible explanation of the origin of this magnetoelectric coupling. We find, in agreement with experimental results, a net ferromagnetic moment at the BiFeO3 interface. The induced ferromagnetic moment is the result of the competition between the e_g-electrons double exchange and the t_2g-spins antiferromagnetic superexchange that dominate in bulk BiFeO3. The balance of these simultaneous ferromagnetic and antiferromagnetic tendencies is strongly affected by the interfacial electronic charge density which, in turn, can be controlled by the BiFeO3 ferroelectric polarization.",1012.1448v1 2010-12-09,Spin-Currents and Spin-Pumping Forces for Spintronics,"A general definition of the Spintronics concept of spin-pumping is proposed as generalized forces conjugated to the spin degrees of freedom in the framework of the theory of mesoscopic non-equilibrium thermodynamics. It is shown that at least three different kinds of spin-pumping forces and associated spin-currents can be defined in the most simple spintronics system (the Ferromagnetic/Non-Ferromagnetic metal interface). Furthermore, the generalized force associated to the ferromagnetic collective variable is also introduced in an equal footing, in order to describe the coexistence of the spin of the conduction electrons (paramagnetic spins attached to $s$-band electrons) and the ferromagnetic-order parameter. The dynamical coupling between these two kinds of magnetic degrees of freedom is presented, and interpreted in terms of spin-transfer effects.",1012.2083v1 2010-12-20,Conduction Electron Scattering and Spin-Flipping at Sputtered Co/Ni Interfaces,"Current-perpendicular-to-plane magnetoresistance (CPP-MR) measurements let us quantify conduction electron scattering and spin-flipping at a sputtered ferromagnetic/ferromagnetic (F1/F2 = Co/Ni) interface, with important consequences for CPP-MR and spin-torque experiments with perpendicular anisotropy. We use ferromagnetically coupled ([Ni/Co]xn)Ni multilayers, and Py-based, symmetric double exchange-biased spin-valves (DEBSVs) containing inserts of ferromagnetically coupled ([Co/Ni]xn)Co or ([Ni/Co]xn)Ni multilayers, to derive Co/Ni interface specific resistances AR(Co/Ni)(Up) = 0.03 (+0.02)(-0.03) f-ohm-m^2 and AR(Co/Ni)(down) = 1.00 +/- 0.07 f-ohm-m^2, and interface spin-flipping parameter delta(Co/Ni) = 0.35 +/- 0.05. The specific resistances are consistent with our no-free-parameter calculations for an interface thickness between 2 and 4 monolayers (ML) that is compatible with expectations.",1012.4388v1 2011-01-05,"Non-thermal laser induced precession of magnetization in ferromagnetic semiconductor (Ga,Mn)As","Non-thermal laser induced spin excitations, recently discovered in conventional oxide and metal ferromagnets, open unprecedented opportunities for research and applications of ultrafast optical manipulation of magnetic systems. Ferromagnetic semiconductors, and (Ga,Mn)As in particular, should represent ideal systems for exploring this new field. Remarkably, the presence of non-thermal effects has remained one of the outstanding unresolved problems in the research of ferromagnetic semiconductors to date. Here we demonstrate that coherent magnetization dynamics can be excited in (Ga,Mn)As non-thermally by a transfer of angular momentum from circularly polarized femtosecond laser pulses and by a combination of non-thermal and thermal effects due to a transfer of energy from laser pulses. The thermal effects can be completely suppressed in piezo-electrically controlled samples. Our work is based on pump-and-probe measurements in a large set of (Ga,Mn)As epilayers and on systematic analysis of circular and linear magneto-optical coefficients. We provide microscopic theoretical interpretation of the experimental results.",1101.1049v1 2011-01-10,A story of high-temperature ferromagnetism in semiconductors,"The comprehensive search for multifunctional materials has resulted in the discovery of semiconductors and oxides showing ferromagnetic features persisting to room temperature. In this tutorial review the methods of synthesis of these materials, as well as the application of element-specific nano-analytic tools, particularly involving synchrotron radiation and electron microscopy, are described and shown to reveal the presence of nano-scale phase separations. Various means to control the aggregation of magnetic cations are discussed together with the mechanisms accounting for ferromagnetism of either condensed or diluted magnetic semiconductors. Finally, the question of whether high temperature ferromagnetism is possible in semiconductors not containing magnetic ions is touched upon.",1101.1981v1 2011-01-28,"Pairing, Ferromagnetism, and Condensation of a normal spin-1 Bose gas","We theoretically study the stability of a normal, spin disordered, homogenous spin-1 Bose gas against ferromagnetism, pairing, and condensation through a Random Phase Approximation which includes exchange (RPA-X). Repulsive spin-independent interactions stabilize the normal state against both ferromagnetism and pairing, and for typical interaction strengths leads to a direct transition from an unordered normal state to a fully ordered single particle condensate. Atoms with much larger spin-dependent interaction may experience a transition to a ferromagnetic normal state or a paired superfluid, but, within the RPA-X, there is no instability towards a normal state with spontaneous nematic order. We analyze the role of the quadratic Zeeman effect and finite system size.",1101.5639v1 2011-02-01,Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases,"We analyze the properties of a single impurity immersed in a Fermi sea. At positive energy and scattering lengths, we show that the system possesses a well-defined but metastable excitation, the repulsive polaron, and we calculate its energy, quasiparticle residue and effective mass. From a thermodynamic argument we obtain the number of particles in the dressing cloud, illustrating the repulsive character of the polaron. Identifying the important 2- and 3-body decay channels, we furthermore calculate the lifetime of the repulsive polaron. The stability conditions for the formation of fully spin polarized (ferromagnetic) domains are then examined for a binary mixture of atoms with a general mass ratio. Our results indicate that mass imbalance lowers the critical interaction strength for phase-separation, but that very short quasiparticle decay times will complicate the experimental observation of itinerant ferromagnetism. Finally, we present the spectral function of the impurity for various coupling strengths and momenta.",1102.0121v2 2011-02-27,Phase Transition of Generalized Ferromagnetic Potts Model - Effect of Invisible States -,"We investigate the nature of the phase transition of the ferromagnetic Potts model with invisible states. The ferromagnetic Potts model with invisible states can be regarded as straightforward extension of the standard ferromagnetic Potts model. The invisible states contribute the entropy, however they do not affect the internal energy. They also do not change the symmetry which breaks at the transition temperature. The invisible states stimulate a first-order phase transition. We confirm that the first-order phase transition with spontaneous $q$-fold symmetry breaking for $q=2,3$, and 4 takes place even on two-dimensional lattice by Monte Carlo simulation. We also find that the transition temperature decreases and the latent heat increases as the number of invisible states increases.",1102.5475v1 2011-02-28,Ferromagnetic resonance in epitaxial films: Effects of lattice strains and voltage control via ferroelectric substrate,"The phenomenon of ferromagnetic resonance (FMR) provides fundamental information on the physics of magnetic materials and lies at the heart of a variety of signal processing microwave devices. Here we demonstrate theoretically that substrate-induced lattice strains may change the FMR frequency of an epitaxial ferromagnetic film dramatically, leading to ultralow and ultrahigh resonance frequencies at room temperature. Remarkably, the FMR frequency varies with the epitaxial strain nonmonotonically, reaching minimum at a critical strain corresponding to the strain-induced spin reorientation transition. Furthermore, by coupling the ferromagnetic film to a ferroelectric substrate, it becomes possible to achieve an efficient voltage control of FMR parameters. In contrast to previous studies, we found that the tunability of FMR frequency varies with the applied electric field and strongly increases at critical field intensity. The revealed features open up wide opportunities for the development of advanced tunable magnetoelectric devices based on strained nanomagnets.",1102.5722v1 2011-03-01,Josephson effect and spin-triplet pairing correlations in SF1F2S junctions,"We study theoretically the Josephson effect and pairing correlations in planar SF1F2S junctions that consist of conventional superconductors (S) connected by two metallic monodomain ferromagnets (F1 and F2) with transparent interfaces. We obtain both spin-singlet and -triplet pair amplitudes and the Josephson current-phase relations for arbitrary orientation of the magnetizations using the self-consistent solutions of Eilenberger equations in the clean limit and for a moderate disorder in ferromagnets. We find that the long-range spin-triplet correlations cannot prevail in symmetric junctions with equal ferromagnetic layers. Surprisingly, the long-range spin-triplet correlations give the dominant second harmonic in the Josephson current-phase relation of highly asymmetric SF1F2S junctions. The effect is robust against moderate disorder and variations in the layers thickness and exchange energy of ferromagnets.",1103.0293v2 2011-03-01,Two-orbital Kondo effect in quantum dot coupled to ferromagnetic leads,"We study the Kondo effect of a two-orbital vertical quantum dot (QD) coupled to two ferromagnetic leads by employing an equation of motion method. When the ferromagnetic leads are coupled with parallel spin polarization, we find three peaks in the single-particle excitation spectra. The middle one is the Kondo resonance caused by the orbital degrees of freedom. In magnetic fields, the Kondo effect vanishes. However, at a certain magnetic field new two-fold degenerate states arise and the Kondo effect emerges there. In contrast, when the ferromagnetic leads are coupled with antiparallel spin polarization, the Kondo effect caused by the spin (orbital) degrees of freedom survives (is suppressed) in magnetic fields. We investigate the field dependence of the conductance in the parallel and antiparallel spin polarizations of the leads and find that the conductance changes noticeably in magnetic fields.",1103.0304v2 2011-03-05,Three-dimensional quantum phase diagram of the exact ground states of a mixture of two species of spin-1 Bose gases with interspecies spin exchange,"We find nearly all the exact ground states of a mixture of two species of spin-1 atoms with both interspecies and intraspecies spin exchanges in absence of a magnetic field. The quantum phase diagram in the three-dimensional parameter space and its two-dimensional cross sections are described. The boundaries where the ground states are either continuous or discontinuous are determined, with the latter identified as where quantum phase transitions take place. The two species are always disentangled if the interspecies spin coupling is ferromagnetic or zero. Quantum phase transitions occur when the interspecies spin coupling varies between antiferromagtic and zero or ferromagnetic while the two intraspecies spin couplings both remain ferromagnetic. On the other hand, by tuning the interspecies spin coupling from zero to antiferromagnetic and then back to zero, one can circumvent the quantum phase transition due to sign change of the intraspecies spin coupling of a single species, which is spin-decoupled with the other species with ferromagnetic intraspecies spin coupling. Overall speaking, interplay among interspecies and two intraspecies spin exchanges significantly enriches quantum phases of spinor atomic gases.",1103.1023v1 2011-03-08,Exact diagonalization study of the tunable edge magnetism in graphene,"The tunable magnetism at graphene edges with lengths of up to 48 unit cells is analyzed by an exact diagonalization technique. For this we use a generalized interacting one-dimensional model which can be tuned continuously from a limit describing graphene zigzag edge states with a ferromagnetic phase, to a limit equivalent to a Hubbard chain, which does not allow ferromagnetism. This analysis sheds light onto the question why the edge states have a ferromagnetic ground state, while a usual one-dimensional metal does not. Essentially we find that there are two important features of edge states: (a) umklapp processes are completely forbidden for edge states; this allows a spin-polarized ground state. (b) the strong momentum dependence of the effective interaction vertex for edge states gives rise to a regime of partial spin-polarization and a second order phase transition between a standard paramagnetic Luttinger liquid and ferromagnetic Luttinger liquid.",1103.1645v1 2011-03-11,Hysteresis and nucleation in condensed matter,"The physical origin of hysteresis in condensed matter had not been previously identified. The current ""science of hysteresis"" is useful, but limited by phenomenological modeling. This article fills the void by revealing the exclusive cause of the hysteresis in structural, ferromagnetic and ferroelectric phase transitions, as well as upon magnetization in magnetic fields and polarization in electric fields. This exclusive cause is nucleation lags. The lags are inevitable due to the nucleation specifics, far from the classical ""random fluctuation"" model. A major assumption that spin orientation is determined by the orientation of its carrier explains why ferromagnetic transitions and magnetization in magnetic fields materialize by structural rearrangements at interfaces, as well as why magnetization by ""rotation"" is impossible. Formation of the structural and ferromagnetic hysteresis loops is considered in detail.",1103.2194v1 2011-03-22,Tracking defect-induced ferromagnetism in GaN:Gd,"We report on the magnetic properties of GaN:Gd layers grown by molecular beam epitaxy (MBE). A poor reproducibility with respect to the magnetic properties is found in these samples. Our results show strong indications that defects with a concentration of the order of 10^19 cm^-3 might play an important role for the magnetic properties. Positron annihilation spectroscopy does not support the suggested connection between the ferromagnetism and the Ga vacancy in GaN:Gd. Oxygen co-doping of GaN:Gd promotes ferromagnetism at room temperature and points to a role of oxygen for mediating ferromagnetic interactions in Gd doped GaN.",1103.4256v2 2011-04-04,FMR and voltage induced transport in normal metal-ferromagnet-superconductor trilayers,"We study the subgap spin and charge transport in normal metal-ferromagnet-superconductor trilayers induced by bias voltage and/or magnetization precession. Transport properties are discussed in terms of time-dependent scattering theory. We assume the superconducting gap is small on the energy scales set by the Fermi energy and the ferromagnetic exchange splitting, and compute the non-equilibrium charge and spin current response to first order in precession frequency, in the presence of a finite applied voltage. We find that the voltage-induced instantaneous charge current and longitudinal spin current are unaffected by the precessing magnetization, while the pumped transverse spin current is determined by spin-dependent conductances and details of the electron-hole scattering matrix. A simplified expression for the transverse spin current is derived for structures where the ferromagnet is longer than the transverse spin coherence length.",1104.0552v1 2011-04-13,Unusual ferromagnetic superexchange in CdVO3: The role of Cd,"A microscopic magnetic model of the low-pressure modification of CdVO3 is established, based on density functional theory (DFT) band-structure calculations, magnetization measurements, and quantum Monte-Carlo simulations. This compound is a rare example of a quasi-one-dimensional spin-1/2 system showing exclusively ferromagnetic exchange. The spin lattice of CdVO3 entails zigzag chains with an effective intrachain coupling J ~ -90 K and interchain couplings of Jc ~ -18 K and Ja ~ -3 K. Quantum fluctuations are partially suppressed by the sizable interchain coupling Jc that leads to an intermediate regime between one-dimensional and two-dimensional ferromagnetic systems. Apart from the peculiar spin model, CdVO3 features an unusual mechanism of ferromagnetic superexchange. The couplings largely originate from Cd 5s states mediating hoppings between half-filled and empty 3d states of V+4.",1104.2495v2 2011-04-14,Aggregation of ferromagnetic and paramagnetic atoms at edges of graphenes and graphite,"In this work, we report that when ferromagnetic metals (Fe, Co and Ni) are thermally evaporated onto n-layer graphenes and graphite, a metal nanowire and adjacent nanogaps can be found along the edges regardless of its zigzag or armchair structure. Similar features can also be observed for paramagnetic metals, such as Mn, Al and Pd. Meanwhile, metal nanowires and adjacent nanogaps can not be found for diamagnetic metals (Au and Ag). An external magnetic field during the evaporation of metals can make these unique features disappear for ferromagnetic and paramagnetic metal; and the morphologies of diamagnetic metal do not change after the application of an external magnetic field. We discuss the possible reasons for these novel and interesting results, which include possible one dimensional ferromagnets along the edge and edge-related binding energy.",1104.2666v1 2011-05-10,Spontaneous magnon decays in planar ferromagnet,"We predict that spin-waves in an easy-plane ferromagnet have a finite lifetime at zero temperature due to spontaneous decays. In zero field the damping is determined by three-magnon decay processes, whereas decays in the two-particle channel dominate in a transverse magnetic field. Explicit calculations of the magnon damping are performed in the framework of the spin-wave theory for the $XXZ$ square-lattice ferromagnet with an anisotropy parameter $\lambda<1$. In zero magnetic field the decays occur for $\lambda^*<\lambda<1$ with $\lambda^*\approx 1/7$. We also discuss possibility of experimental observation of the predicted effect in a number of ferromagnetic insulators.",1105.1893v1 2011-05-17,Superconductivity and ferromagnetism in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$,"Superconductivity and ferromagnetism are two antagonistic cooperative phenomena, which makes it difficult for them to coexist. Here we demonstrate experimentally that they do coexist in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ with $0.2\leq x\leq0.4$, in which superconductivity is associated with Fe-3$d$ electrons and ferromagnetism comes from the long-range ordering of Eu-4$f$ moments via Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The coexistence is featured by large saturated ferromagnetic moments, high and comparable superconducting and magnetic transition temperatures, and broad coexistence ranges in temperature and field. We ascribe this unusual phenomenon to the robustness of superconductivity as well as the multi-orbital characters of iron pnictides.",1105.3255v1 2011-06-01,The ground state phase diagram of the diluted ferromagnetic Kondo-lattice model,"We investigate the existence of several (anti-)ferromagnetic phases in the diluted ferromagnetic Kondo-lattice model, i.e. ferromagnetic coupling of local moment and electron spin. To do this we use a coherent potential approximation (CPA) with a dynamical alloy analogy. For the CPA we need effective potentials, which we get first from a mean-field approximation. To improve this treatment we use in the next step a more appropriate moment conserving decoupling approach and compare both methods. The different magnetic phases are modelled by defining two magnetic sublattices. As a result we present zero-temperature phase diagrams according to the important model parameters and different dilutions.",1106.0181v1 2011-07-05,Critical current measurements in superconductor - ferromagnet - superconductor junctions of $YBa_2Cu_3O_y$-$SrRuO_3$-$YBa_2Cu_3O_y$: No evidence for a dominant proximity induced triplet superconductivity in the ferromagnetic barrier,"Transport measurements in ramp-type junctions of $YBa_2Cu_3O_y-SrRuO_3-YBa_2Cu_3O_y$ with $T_c$ values of either 80-90 K or 60-70 K are reported. In both type of junctions but without a barrier (""shorts""), the supercurrent densities at 4.2 K reached 7.5 and 3.5 MA/cm$^2$, respectively, indicating the high quality of the fabrication process. Plots of the critical current versus thickness of the ferromagnetic barrier at 4.2 K show exponential decays with decay lengths of 1.1 nm for the 90 K phase and 1.4 nm for the 60 K phase, which are much shorter than the relevant coherence lengths $\xi_F\sim 5-6$ nm or $\xi_N\sim$16 nm of $SrRuO_3$. We thus conclude that there is no dominant proximity induced triplet superconductivity in the ferromagnet in our junctions.",1107.0808v1 2011-07-13,Size of stripe domains in a superconducting ferromagnet,"In a superconducting ferromagnet, the superconducting state appears in the ferromagnetic phase where usually a domain structure has already developed. We study the influence of the superconducting screening currents on a stripe structure with out-of-plane magnetization, in a film of arbitrary thickness. We find that superconductivity always induces a shrinkage of the domains, and there is a critical value of penetration depth below which a mono-domain structure is more stable than the periodic one. Furthermore we investigate the possible different effects of singlet and triplet superconductivity on the domain width, as well as the conditions for the existence of vortices in the domains. The obtained results are then discussed in light of the experimental data of superconducting ferromagnets URhGe, UGe2, and UCoGe.",1107.2608v1 2011-07-21,Superconductivity and Ferromagnetism in Oxide Interface Structures: Possibility of Finite Momentum Pairing,"We introduce a model to explain the observed ferromagnetism and superconductivity in LAO/STO oxide interface structures. Due to the polar catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface layer. We argue that this charge localizes and orders ferromagnetically via exchange with the conduction electrons. Ordinarily this ferromagnetism would destroy superconductivity, but due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming an FFLO-type condensate of Cooper pairs at finite momentum, which is surprisingly robust in the presence of strong disorder.",1107.4352v3 2011-08-11,Spin- and band-ferromagnetism in trilayer graphene,"We study the ground state properties of an ABA-stacked trilayer graphene. The low energy band structure can be described by a combination of both a linear and a quadratic particle-hole symmetric dispersions, reminiscent of monolayer- and bilayer-graphene, respectively. The multi-band structure offers more channels for instability towards ferromagnetism when the Coulomb interaction is taken into account. Indeed, if one associates a pseudo-spin 1/2 degree of freedom to the bands (parabolic/linear), it is possible to realize also a band-ferromagnetic state, where there is a shift in the energy bands, since they fill up differently. By using a variational procedure, we compute the exchange energies for all possible variational ground states and identify the parameter space for the occurrence of spin- and band-ferromagnetic instabilities as a function of doping and interaction strength.",1108.2445v2 2011-08-22,Ferromagnetic Quantum Criticality in the Quasi-One-Dimensional Heavy Fermion Metal YbNi4P2,"We present a new Kondo-lattice system, YbNi4P2, which is a clean heavy-fermion metal with a severely reduced ferromagnetic ordering temperature at T_C=0.17K, evidenced by distinct anomalies in susceptibility, specific-heat, and resistivity measurements. The ferromagnetic nature of the transition, with only a small ordered moment of ~0.05mu_B, is established by a diverging susceptibility at T_C with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi4P2 is a stoichiometric system with a quasi-one-dimensional crystal and electronic structure and strong correlation effects which dominate the low temperature properties. This is reflected by a stronger-than-logarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above T_C which cannot be explained by any current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and make this an interesting material for further in-depth investigations.",1108.4274v1 2011-09-06,Low Field Magnetic Susceptibility and the Hidden Order Transition in URu2Si2,"Despite several decades of both experimental and theoretical work the nature of the hidden order transition at THO = 17.5 K in URu2Si2 remains enigmatic. We report here low field DC magnetization as well as AC susceptibility measurements performed on single crystals with field along the c-axis. A peak in the first order AC susceptibility is observed at T=16 K in close proximity to the well known change in slope at THO=17.5 K. Third order susceptibility measurements on the other hand reveal a signature only at 16 K with no discernible change at THO. However, both transitions are visible in the fifth order susceptibility. A signature at this lower transition, as a spontaneous ferromagnetic signal, also appears in low field DC magnetization measurements. A similar but larger ferromagnetic response appears at T~35 K. Both these ferromagnetic signatures are suppressed in a small field (B ~ 40 Gauss ||c-axis). The close proximity of the lower signature to the hidden order transition and the simultaneous presence and suppression of a ferromagnetic signature at 35 K could be due to the removal of degeneracy in the complex order-parameter of the hidden order transition due a ""symmetry breaking field"", at least locally within the crystals of URu2Si2.",1109.1038v1 2011-09-16,Superconductor-insulator-ferromagnet-superconductor Josephson junction: From the dirty to the clean limit,"The proximity effect and the Josephson current in a superconductor-insulator-ferromagnet-superconductor (SIFS) junction are investigated within the framework of the quasiclassical Eilenberger equations. This investigation allows us to compare the dirty and the clean limits, to investigate an arbitrary impurity scattering, and to determine the applicability limits of the Usadel equations for such structures. The role of different types of the FS interface is analyzed. It is shown that the decay length and the spatial oscillation period of the Eilenberger function may exhibit a nonmonotonic dependence on the properties of the ferromagnetic layer such as exchange field or electron mean free path. The results of our calculations are applied to the interpretation of experimentally observed dependencies of the critical current density on the ferromagnet thickness in Josephson junctions containing a Ni layer with an arbitrary scattering.",1109.3658v1 2011-10-04,Low-temperature thermodynamics of the classical frustrated ferromagnetic chain in magnetic field,"Low-temperature magnetization curves of the classical frustrated ferromagnetic chain in the external magnetic field near the transition point between the ferromagnetic and the helical phases is studied. It is shown that the calculation of the partition function in the scaling limit reduces to the solution of the Schr\""{o}dinger equation of the special form for the quantum particle. It is proposed that the magnetization of the classical model in the ferromagnetic part of the phase diagram including the transition point defines the universal scaling function which is valid for quantum model as well. Explicit analytical formulae for the magnetization are given in the limiting cases of low and high magnetic fields. The influence of the easy-axis anisotropy on the magnetic properties of the model is studied. It is shown that even small anisotropy essentially changes the behavior of the susceptibility in the vicinity of the transition point.",1110.0771v2 2011-10-10,First-principles characterization of ferromagnetism in N-doped SrTiO3 and BaTiO3,"The spin-polarization and magnetic coupling character of N-doped SrTiO3 and BaTiO3 are studied through first-principles calculations. The substitutional N doping at O sites leads to a half-metallic property and produces a magnetic moment of 1.0 {\mu}B. The magnetic interaction between the nearest and next-nearest N dopants results in a strong ferromagnetic coupling. When the distance between the N dopants is larger than 7 {\AA}, the ground state of the system tends to be paramagnetic. A nitrogen-concentration threshold to produce the ferromagnetism is estimated. The calculated results give a good explanation for the experimentally observed ferromagnetism in N-doped SrTiO3 and BaTiO3.",1110.2138v2 2011-10-17,Crossed Andreev reflection and spin-resolved non-local electron transport,"The phenomenon of crossed Andreev reflection (CAR) is known to play a key role in non-local electron transport across three-terminal normal-superconducting-normal (NSN) devices. Here we review our general theory of non-local charge transport in three-terminal disordered ferromagnet-superconductor-ferromagnet (FSF) structures. We demonstrate that CAR is highly sensitive to electron spins and yields a rich variety of properties of non-local conductance which we describe non-perturbatively at arbitrary voltages, temperature, degree of disorder, spin-dependent interface transmissions and their polarizations. We demonstrate that magnetic effects have different implications: While strong exchange field suppresses disorder-induced electron interference in ferromagnetic electrodes, spin-sensitive electron scattering at SF interfaces can drive the total non-local conductance negative at sufficiently low energies. At higher energies magnetic effects become less important and the non-local resistance behaves similarly to the non-magnetic case. Our results can be applied to multi-terminal hybrid structures with normal, ferromagnetic and half-metallic electrodes and can be directly tested in future experiments.",1110.3654v1 2011-12-05,Magnetotransport in the Kondo model with ferromagnetic exchange interaction,"We consider the transport properties in an applied magnetic field of the spin S=1/2 Kondo model with ferromagnetic exchange coupling to electronic reservoirs, a description relevant for the strong coupling limit of underscreened spin S=1 Kondo impurities. Because the ferromagnetic Kondo interaction is marginally irrelevant, perturbative methods should prove accurate down to low energies. For the purpose of this study, we use a combination of Majorana diagrammatic theory with Density Matrix Numerical Renormalization Group simulations. In the standard case of antiferromagnetic Kondo exchange, we first show that our technique recovers previously obtained results for the T-matrix and spin relaxation at weak coupling (above the Kondo temperature). Considering then the ferromagnetic case, we demonstrate how the low-energy Kondo anomaly splits for arbitrary small values of the Zeeman energy, in contrast to fully screened Kondo impurities near the strong coupling Fermi liquid fixed point, and in agreement with recent experimental findings for spin S=1 molecular quantum dots.",1112.0925v2 2011-12-07,Exchange bias of mu-metal thin films,"The exchange bias of the soft ferromagnet mu-metal, Ni77Fe14Cu5Mo4, with the metallic antiferromagnet Fe50Mn50 has been studied as a function of ferromagnet thickness and buffer layer material. Mu-metal exhibits classic exchange bias behavior: the exchange bias (HEB) and coercive fields scale inversely with the ferromagnet's thickness, with HEB varying as the cosine of the in-plane applied field angle. While the exchange bias, coercivity, and exchange energy are greatest when the buffer layer material is (111) oriented Cu, amorphous Ta buffers allow the mu-metal to retain more of its soft magnetic character. The ability to preserve soft ferromagnetic behavior in an exchange biased heterostructure may be useful for low field sensing and other device applications.",1112.1604v1 2011-12-08,Photovoltaic Effect on Molecule Coupled Ferromagnetic Films of a Magnetic Tunnel Junction,"Economical solar energy conversion to electricity can be boosted by the discovery of fundamentally new photovoltaic mechanism, and a suitable system to realize it with commonly available materials like iron (Fe) and nickel (Ni). This paper reports the observation of photovoltaic effect on a molecular spintronics device, composed of magnetic tunnel junction (MTJ) and organometallic molecular clusters (OMCs). A prefabricated MTJ with exposed side edges, after enabling the bridging of OMC channels between its two ferromagnetic films, exhibited following phenomenon (i) dramatic increase in exchange coupling, (ii) 3-6 orders current suppression and (iii) photovoltaic effect. This paper focuses on the photovoltaic effect. Control experiments on isolated ferromagnetic films suggested that OMCs neither affected the magnetic properties nor produced any photovoltaic effect; photovoltaic effect was only observed on the ferromagnetic films serving as magnetic electrodes in a MTJ. Present paper invites further investigation of the similar photovoltaic effect on other combinations of MTJs and promising magnetic molecules, like single molecular magnets, organometallic clusters and porphyrins. This research can lead to mass producible and economical spin photovoltaic devices.",1112.1879v2 2011-12-19,Signature of the long range triplet proximity effect in the density of states,"We study the impact of the long-range spin-triplet proximity effect on the density of states (DOS) in planar SF1F2S Josephson junctions that consist of conventional superconductors (S) connected by two metallic monodomain ferromagnets (F1 and F2) with transparent interfaces. We determine the electronic DOS in F layers and the Josephson current for arbitrary orientation of the magnetizations using the solutions of Eilenberger equations in the clean limit and for a moderate disorder in ferromagnets. We find that fully developed long-range proximity effect can occur in highly asymmetric ferromagnetic bilayer Josephson junctions with orthogonal magnetizations. The effect manifests itself as an enhancement in DOS, and as a dominant second harmonic in the Josephson current-phase relation. Distinctive variation of DOS in ferromagnets with the angle between magnetizations is experimentally observable by tunneling spectroscopy. This can provide an unambiguous signature of the long-range spin-triplet proximity effect.",1112.4450v1 2012-02-02,Towards precise measurement of oscillatory domain wall by ferromagnetic Josephson junction,"We theoretically propose a principle for precise measurement of oscillatory domain wall (DW) by a ferromagnetic Josephson junction, which is composed of a ferromagnetic wire with DW and two superconducting electrodes. The current-voltage curve exhibits stepwise structures, only when DW oscillates in the ferromagnetic wire. The voltage step appears at V = n(\hbar/2e)\omega_DW with the fundamental constant \hbar/e, integer number n, and the DW frequency \omega_DW. Since V can be determined in the order of 10^9 accuracy, the oscillatory DW will be measured more precisely than present status by conventional method.",1202.0382v2 2012-02-16,Disorder promotes ferromagnetism: Rounding of the quantum phase transition in Sr_{1-x}Ca_xRuO_3,"The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr1-xCaxRuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.",1202.3810v1 2012-02-26,Understanding ferromagnetism in Cr-based 3d-5d double perovskites,"Ferromagnetism in Cr-based double perovskites is analyzed using effective model as well as simulation approach. Starting from a microscopic model proposed recently for this class of double perovskites, an effective spin-only model is derived in the limit of large exchange coupling at the B-site. Analytic expressions for the resultant exchange coupling is derived in certain limiting cases. The behaviour of this exchange is used to provide a plausible explanation for the enhanced ferromagnetic tendency as also the enigmatic increase in Curie temperature observed in Cr-based DP-s, in the series Sr$_{2}$CrWO$_{6}$,Sr$_{2}$CrReO$_{6}$ to Sr$_{2}$CrOsO$_{6}$. The superexchange between neighbouring B and B' sites is found to play a crucial role both in stabilizing ferromagnetism, especially in the latter two compounds, as well as increasing the T$_{c}$.",1202.5735v3 2012-03-14,Oxygen vacancy enhanced room temperature ferromagnetism in Al-doped MgO nanoparticles,"We have investigated the room temperature ferromagnetic order that develops in Al-substituted magnesium oxide, Mg(Al)O, nanoparticles with Al fractions of up to 5 at.%. All samples, including undoped MgO nanoparticles, exhibit room temperature ferromagnetism, with the saturation magnetization reaching a maximum of 0.023 emu/g at 2 at.% of Al. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in both doped and undoped MgO nanoparticles, with the vacancy concentration increasing upon vacuum annealing of Mg(Al)O, resulting in two-fold enhancement of the saturation magnetization for 2 at.% Al-doped MgO. Our results suggest that the oxygen vacancies are largely responsible for room temperature ferromagnetism in MgO.",1203.2991v3 2012-03-28,Ferromagnetic condensation in high density hadronic matter,"We investigate the occurrence of a ferromagnetic phase transition in high density hadronic matter (e.g., in the interior of a neutron star). This could be induced by a four fermion interaction analogous to the one which is responsible for chiral symmetry breaking in the Nambu-Jona-Lasinio model, to which it is related through a Fierz transformation. Flavor SU(2) and flavor SU(3) quark matter are considered. A second order phase transition is predicted at densities about 5 times the normal nuclear matter density, a magnetization of the order of $10^{16}$ gauss being expected. It is also found that in flavor SU(3) quark matter, a first order transition from the so-called 2 flavor super-conducting (2SC) phase to the ferromagnetic phase arises. The color-flavor-locked (CFL) phase may be completely hidden by the ferromagnetic phase.",1203.6272v2 2012-06-15,Single-parameter quantum charge and spin pumping in armchair graphene nanoribbons,"We investigate quantum charge and spin pumping in armchair graphene nanoribbons under a single ac gate voltage connected with nonmagnetic/ferromagnetic leads via the nonequilibrium Green's function method. In the case of nonmagnetic leads, where only part of the nanoribbon is subject to an ac gate voltage to break the left-right spatial symmetry, we discover that peaks of the charge pumping current appear at the Fermi energies around the subband edges in the ac-field-free region of the nanoribbon. In the case of ferromagnetic leads with the lead magnetizations being antiparallel to break the left-right symmetry, similar peaks appear in the spin pumping current when the Fermi energies are around the edges of the the majority-spin subbands in the ferromagnetic leads. All these peaks originate from the pronounced symmetry breaking in the transmissions with energies being around the corresponding subband edges. Moreover, we predict a {\em pure} spin current in the case of ferromagnetic leads with the whole graphene nanoribbon under an ac gate voltage. The ac-field-strength and -frequency dependences of the pumping current are also investigated with the underlying physics revealed.",1206.3435v1 2012-06-26,The coexistence of superconductivity and ferromagnetism in nano-scale metallic grains,"A nano-scale metallic grain in which the single-particle dynamics are chaotic is described by the so-called universal Hamiltonian. This Hamiltonian includes a superconducting pairing term and a ferromagnetic exchange term that compete with each other: pairing correlations favor minimal ground-state spin, while the exchange interaction favors maximal spin polarization. Of particular interest is the fluctuation-dominated regime where the bulk pairing gap is comparable to or smaller than the single-particle mean level spacing and the Bardeen-Cooper-Schrieffer theory of superconductivity breaks down. Superconductivity and ferromagnetism can coexist in this regime. We identify signatures of the competition between superconductivity and ferromagnetism in a number of quantities: ground-state spin, conductance fluctuations when the grain is weakly coupled to external leads and the thermodynamic properties of the grain, such as heat capacity and spin susceptibility.",1206.6059v2 2012-06-28,Spin-orbit coupled transport and spin torque in a ferromagnetic heterostructure,"Ferromagnetic heterostructures provide an ideal platform to explore the nature of spin-orbit torques arising from the interplay mediated by itinerant electrons between a Rashba-type spin-orbit coupling and a ferromagnetic exchange interaction. For such a prototypic system, we develop a set of coupled diffusion equations to describe the diffusive spin dynamics and spin-orbit torques. We characterize the spin torque and its two prominent--out-of-plane and in-plane--components for a wide range of relative strength between the Rashba coupling and ferromagnetic exchange. The symmetry and angular dependence of the spin torque emerging from our simple Rashba model is in an agreement with experiments. The spin diffusion equation can be generalized to incorporate dynamic effect such as spin pumping and magnetic damping.",1206.6726v2 2012-07-18,Interface Ferromagnetism in a SrMnO3/LaMnO3 Superlattice,"Resonant soft x-ray absorption measurements at the O K edge on a SrMnO3/LaMnO3 superlattice show a shoulder at the energy of doped holes, which corresponds to the main peak of resonant scattering from the modulation in the doped hole density. Scattering line shape at the Mn L3,2 edges has a strong variation below the ferromagnetic transition temperature. This variation has a period equal to half the superlattice superperiod and follows the development of the ferromagnetic moment, pointing to a ferromagnetic phase developing at the interfaces. It occurs at the resonant energies for Mn3+ and Mn4+ valences. A model for these observations is presented, which includes a double-exchange two-site orbital and the variation with temperature of the hopping frequency tij between the two sites.",1207.4522v1 2012-07-25,Thermodynamics of Two-Dimensional Ideal Ferromagnets - Three-Loop Analysis,"Within the effective Lagrangian framework, we explicitly evaluate the partition function of two-dimensional ideal ferromagnets up to three loops at low temperatures and in the presence of a weak external magnetic field. The low-temperature series for the free energy density, energy density, heat capacity, entropy density, as well as the magnetization are given and their range of validity is critically examined in view of the Mermin-Wagner theorem. The calculation involves the renormalization and numerical evaluation of a particular three-loop graph which is discussed in detail. Interestingly, in the low-temperature series for the two-dimensional ideal ferromagnet, the spin-wave interaction manifests itself in the form of logarithmic terms. In the free energy density the leading such term is of order $T^4 \ln T$ -- remarkably, in the case of the three-dimensional ideal ferromagnet no logarithmic terms arise in the low-temperature series. While the present study demonstrates that it is straightforward to consider effects up to three-loop order in the effective field theory framework, this precision seems to be far beyond the reach of microscopic methods such as modified spin-wave theory.",1207.5937v1 2012-07-30,Ferromagnetic state in the one-dimensional Kondo lattice model,"In our recent study, Phys. Rev. Lett. 108 086402 (2012), we have revealed the intriguing properties of the ferromagnetic state in the Kondo lattice model with antiferromagnetic coupling in infinite dimensions: within the ferromagnetic metallic phase, the minority conduction electrons form a gap at the Fermi energy and do not participate in transport irrespective of interaction strength and filling. This half-metallic state is referred to as a spin-selective Kondo insulator. We here show that the spin-selective Kondo insulator can also be realized in the one-dimensional Kondo lattice model by studying static and dynamical quantities with the density matrix renormalization group. The emergence of the spin selective Kondo insulator both in one and infinite dimensions certainly demonstrates that this mechanism is quite general and ubiquitous for the ferromagnetic state in the Kondo lattice model irrespective of the dimensionality of the system.",1207.6835v2 2012-07-31,Effects of Magnetovolume and Spin-orbit Coupling in the Ferromagnetic Cubic Perovskite BaRuO3,"BaRuO3 having five different crystal structures has been synthesized by varying the pressure while sintering. Contrary to the other phases being nonmagnetic, the cubic perovskite phase synthesized recently shows an itinerant ferromagnetic character. We investigated this ferromagnetic BaRuO3 using first principles calculations. A few van Hove singularities appear around the Fermi energy, causing unusually high magnetovolume effects of $\Delta M/\Delta a$ ~ 4.3 $\mu_B$/\AA as well as a Stoner instability [IN(0) ~ 1.2]. At the optimized lattice parameter a, the magnetic moment M is 1.01 $\mu_B$ in the local spin density approximation. When spin-orbit coupling is included, the topologies of some Fermi surfaces are altered, and the net moment is reduced by 10% to a value very close to the experimentally observed value of ~ 0.8 $\mu_B$. Our results indicate that this ferromagnetism is induced by the Stoner instability, but the combined effects of the p-d hybridization, the magnetovolume, and the spin-orbit coupling determine the net moment. In addition, we briefly discuss the results of the tight-binding Wannier function technique.",1207.7145v2 2012-08-02,Anomalous Fermi level behavior in GaMnAs at the onset of ferromagnetism,"We present the systematic study of the resonant tunneling spectroscopy on a series of ferromagnetic-semiconductor Ga1-xMnxAs with the Mn content x from ~0.01 to 3.2%. The Fermi level of Ga1-xMnxAs exists in the band gap in the whole x region. The Fermi level is closest to the valence band (VB) at x=1.0% corresponding to the onset of ferromagnetism near the metal-insulator transition (MIT), but it moves away from the VB as x increasing or decreasing from 1.0%. This anomalous behavior of the Fermi level indicates that the ferromagnetism and MIT emerge in the Mn-derived impurity band.",1208.0575v2 2012-08-07,Temperature-controlled interlayer exchange coupling in strong/weak ferromagnetic multilayers: a thermo-magnetic Curie-switch,"We investigate a novel type of interlayer exchange coupling based on driving a strong/weak/strong ferromagnetic tri-layer through the Curie point of the weakly ferromagnetic spacer, with the exchange coupling between the strongly ferromagnetic outer layers that can be switched, on and off, or varied continuously in magnitude by controlling the temperature of the material. We use Ni-Cu alloy of varied composition as the spacer material and model the effects of proximity-induced magnetism and the interlayer exchange coupling through the spacer from first principles, taking into account not only thermal spin-disorder but also the dependence of the atomic moment of Ni on the nearest-neighbor concentration of the non-magnetic Cu. We propose and demonstrate a gradient-composition spacer, with a lower Ni-concentration at the interfaces, for greatly improved effective-exchange uniformity and significantly improved thermo-magnetic switching in the structure. The reported magnetic multilayer materials can form the base for a variety of novel magnetic devices, such as sensors, oscillators, and memory elements based on thermo-magnetic Curie-switching in the device.",1208.1493v1 2012-09-03,"Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As","We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow a n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs.",1209.0250v1 2012-09-13,Artificial ferroelectricity due to anomalous Hall effect in magnetic tunnel junctions,"We theoretically investigated Anomalous Hall Effect (AHE) and Spin Hall Effect (SHE) transversally to the insulating spacer O, in magnetic tunnel junctions of the form F/O/F where F are ferromagnetic layers and O represents a tunnel barrier. We considered the case of purely ballistic (quantum mechanical) transport, taking into account the assymetric scattering due to spin-orbit interaction in the tunnel barrier. AHE and SHE in the considered case have a surface nature due to proximity effect. Their amplitude is in first order of the scattering potential. This contrasts with ferromagnetic metals wherein these effect are in second (side-jump scattering) and third (skew scattering) order on these potentials. The value of AHE voltage in insulating spacer may be much larger than in metallic ferromagnetic electrodes. For the antiparallel orientation of the magnetizations in the two F-electrodes, a spontaneous Hall voltage exists even at zero applied voltage. Therefore an insulating spacer sandwiched between two ferromagnetic layers can be considered as exhibiting a spontaneous ferroelectricity.",1209.2872v1 2012-09-24,Magnetic-coupling-dependent spin-triplet supercurrents in helimagnet/ferromagnet Josephson junctions,"The experimental achievements during the past year in demonstrating the existence of long-ranged spin-triplet supercurrents in ferromagnets proximity coupled to singlet superconductors open up the possibility for new interesting physics and applications [for a review, see M. Eschrig, Phys. Today 64(1), 43 (2011)]. Our group reported the injection of triplet supercurrents into a magnetically uniform ferromagnet (Co) by sandwiching it between two helimagnet/superconductor (Ho/Nb) bilayers to form a Nb/Ho/Co/Ho/Nb-type Josephson device. In the function of the Ho layer thicknesses, the supercurrent was found to modulate in a complex way that seemed to depend on the magnetic structure of Ho. To understand this unusual behavior, we have theoretically studied the properties of an ideal Josephson device with a helimagnet/ferromagnet/helimagnet (HM/F/HM) barrier in the clean limit using the Eilenberger equation; we show, in particular, that the maximum triplet supercurrent that can pass across the barrier will depend non-monotonically on the thicknesses of the HM layers if the HM and F layers are magnetically exchange coupled at their interface.",1209.5355v1 2012-10-03,Easy-axis ferromagnetic chain on a metallic surface,"The phases and excitation spectrum of an easy-axis ferromagnetic chain of S=1/2 magnetic impurities built on the top of a clean metallic surface are studied. As a function of the (Kondo) coupling to the metallic surface and at low temperatures, the spin chain exhibits a quantum phase transition from an Ising ferromagnetic phase with long-range order to a paramagnetic phase where quantum fluctuations destroy the magnetic order. In the paramagnetic phase, the system consists of a chain of Kondo-singlets where the impurities are completely screened by the metallic host. In the ferromagnetic phase, the excitations above the Ising gap are damped magnons, with a finite lifetime arising due to the coupling to the substrate. We discuss the experimental consequences of our results to spin-polarized electron energy loss spectroscopy (SPEELS), and we finally analyze possible extensions to spin chains with S>1/2.",1210.1195v1 2012-10-17,Ferromagnetic Quantum Critical Point in Heavy-Fermion Iron Oxypnictide Ce(Ru$_{1-x}$Fe$_x$)PO,"We have performed ^{31}P-NMR measurements on Ce(Ru_{1-x}Fe_{x})PO in order to investigate ferromagnetic (FM) quantum criticality, since heavy-fermion (HF) ferromagnet CeRuPO with a two-dimensional structure turns to a HF paramagnet by an isovalent Fe-substitution for Ru. We found that Ce(Ru_{0.15}Fe_{0.85})PO shows critical fluctuations down to \sim 0.3 K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe-substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at x \sim 0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru_{0.15}Fe_{0.85})PO share the similarity with those in YbRh_{2}Si_{2}, where the local criticality of $f$ electrons has been discussed. We reveal that Ce(Ru_{1-x}Fe_{x})PO is a new system to study FM quantum criticality in HF compounds.",1210.4604v1 2012-11-28,Gate voltage controlled electronic transport through a ferromagnet/normal/ferromagnet junction on the surface of a topological insulator,"We investigate the electronic transport properties of a ferromagnet/normal/ferromagnet junction on the surface of a topological insulator with a gate voltage exerted on the normal segment. It is found that the conductance oscillates with the width of normal segment and gate voltage, and the maximum of conductance gradually decreases while the minimum of conductance approaches zero as the width increases. The conductance can be controlled by tuning the gate voltage like a spin field-effect transistor. It is found that the magnetoresistance ratio can be very large, and can also be negative owing to the anomalous transport. In addition, when there exists a magnetization component in the surface plane, it is shown that only the component parallel to the junction interface has an influence on the conductance.",1211.6511v1 2012-12-27,Interacting spin-droplets and magnetic properties of a low-density two-dimensional electron gas,"We argue that the magnetic susceptibility data, Refs. 1-3, for the low-density two-dimensional (2D) silicon-based electron gas indicate that magnetically active electrons are localised in spin-droplets. The droplets exist in both the insulating and metallic phases, and interact ferromagnetically, forming an effective 2D Heisenberg ferromagnet. Comparing the data with known analytical and numerical results for a 2D Heisenberg ferromagnet, we determine that JS^2 \approx 0.6K, where S is the spin of the droplet and J is the ferromagnetic exchange constant between droplets. We further argue that most likely S=1 with four electrons occupying each droplet on average. We discuss the dependence of the magnetic susceptibility and the specific heat on the external magnetic field, which follows from the model, and hence we suggest further experimental tests of the model.",1212.6438v1 2012-12-31,Electrical and magnetic transport study on strain driven ferromagnetic insulating thin film of low doped manganite,"In this paper we have created a strain driven single crystal like ferromagnetic insulating (FMI) state in a PLD grown thin film of low doped LCMO(X = 0.15)on NGO(100) substrate and make a thorough study of strain effects on the electric and magnetic transport of this film. We have studied and compared the FMI state, ferromagnetic transition temperature (TC), ferromagnetic insulating temperature (TFMI) and the resistivity of the film in details with bulk single crystals of X=0.18 to 0.22 doping region. We have found that TFMI and the localisation length of the carriers are increased and there is also a decrease in the Coulomb gap. The magneto transport behavior of the film also differs from the bulk single crystals and the magnetoresistance of the sample is nearly 20 to 75% with the application of the applied field(0 to 10 T) and it falls up to 5 to 40% below a certain temperature and the TC of the film increases to higher temperature with the increasing field. The film also shows anisotropic magnetoresistance as large as 20% depending on applied magnetic field direction.",1212.6858v1 2013-01-31,Current-field diagram of magnetic states of a surface spin valve in a point contact with a single ferromagnetic film,"We present a study of the influence of an external magnetic field H and an electric current I on the spin-valve (SV) effect between a ferromagnetic thin film (F) and a sharp tip of a nonmagnetic metal (N). To explain our observations, we propose a model of a local surface SV which is formed in such a N/F contact. In this model, a ferromagnetic cluster at the N/F interface plays the role of the free layer in this SV. This cluster exhibits a larger coercive field than the bulk of the ferromagnetic film, presumably due to its nanoscale nature. Finally, we construct a magnetic state diagram of the surface SV as a function of I and H.",1301.7635v1 2013-02-27,Specific heat and thermal conductivity of ferromagnetic magnons in Yttrium Iron Garnet,"The specific heat and thermal conductivity of the insulating ferrimagnet Y$_3$Fe$_5$O$_{12}$ (Yttrium Iron Garnet, YIG) single crystal were measured down to 50 mK. The ferromagnetic magnon specific heat $C$$_m$ shows a characteristic $T^{1.5}$ dependence down to 0.77 K. Below 0.77 K, a downward deviation is observed, which is attributed to the magnetic dipole-dipole interaction with typical magnitude of 10$^{-4}$ eV. The ferromagnetic magnon thermal conductivity $\kappa_m$ does not show the characteristic $T^2$ dependence below 0.8 K. To fit the $\kappa_m$ data, both magnetic defect scattering effect and dipole-dipole interaction are taken into account. These results complete our understanding of the thermodynamic and thermal transport properties of the low-lying ferromagnetic magnons.",1302.6739v1 2013-03-18,Theory of spin pumping through an interacting quantum dot tunnel coupled to a ferromagnet with time-dependent magnetization,"We investigate two schemes for pumping spin adiabatically from a ferromagnet through an interacting quantum dot into a normal lead, which exploit the possibility to vary in time the ferromagnet's magnetization, either its amplitude or its direction. For this purpose, we extend a diagrammatic real-time technique for pumping to situations in which the leads' properties are time dependent. In the first scheme, the time-dependent magnetization amplitude is combined with a time-dependent level position of the quantum dot to establish both a charge and a spin current. The second scheme uses a uniform rotation of the ferromagnet's magnetization direction to generate a pure spin current without a charge current. We discuss the influence of an interaction-induced exchange field on the pumping characteristics.",1303.4184v2 2013-04-08,Temperature dependence of spin polarization in ferromagnetic metals using lateral spin valves,"A high reproducibility in the performance of cobalt/copper and permalloy/copper lateral spin valves with transparent contacts is obtained by optimizing the interface quality and the purity of copper. This allows us to study comprehensively the spin injection properties of both ferromagnetic materials, as well as the spin transport properties of copper, which are not affected by the used ferromagnetic material, leading to long spin diffusion lengths. Spin polarizations of permalloy and cobalt are obtained as a function of temperature. Analysis of the temperature dependence of both the spin polarization and conductivity of permalloy using the standard two-channel model for ferromagnetic metals suggests that a correction factor of ~2 is needed for the spin polarization values obtained by lateral spin valve experiments.",1304.2123v3 2013-04-24,Long-range spin current driven by superconducting phase difference in a Josephson junction with double layer ferromagnets,"We theoretically study spin current through ferromagnet (F) in a Josephson junction composed of s-wave superconductors and two layers of ferromagnets. Using quasiclassical theory, we show that the long-range spin current can be driven by the superconducting phase difference without voltage drop. The origin of this spin current is due to spin-triplet Cooper pairs (STCs) formed by electrons of equal-spin, which are induced by proximity effect inside the F. We find that the spin current carried by the STCs exhibits long-range propagation in the F even where the Josephson charge current is practically zero. We also show that this spin current persists over a remarkably longer distance than the ordinary spin current carried by spin polarized conduction electrons in the F. Our results thus indicate the promising potential of Josephson junctions based on multilayer ferromagnets for spintronics applications with long-range propagating spin current.",1304.6452v1 2013-04-26,Strain-induced ferromagnetism in antiferromagnetic LuMnO3 thin films,"Single phase and strained LuMnO3 thin films are discovered to display co-existing ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism (approx. 1 muB), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly-strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth managanite thin films as promising candidate systems for new multifunctional devices.",1304.7200v1 2013-05-23,Realization of stripes and slabs in two and three dimensions,"We consider Ising models in two and three dimensions with nearest neighbor ferromagnetic interactions and long range, power law decaying, antiferromagnetic interactions. If the strength of the ferromagnetic coupling J is larger than a critical value J_c, then the ground state is homogeneous and ferromagnetic. As the critical value is approached from smaller values of J, it is believed that the ground state consists of a periodic array of stripes (d=2) or slabs (d=3), all of the same size and alternating magnetization. Here we prove rigorously that the ground state energy per site converges to that of the optimal periodic striped/slabbed state, in the limit that J tends to the ferromagnetic transition point. While this theorem does not prove rigorously that the ground state is precisely striped/slabbed, it does prove that in any suitably large box the ground state is striped/slabbed with high probability.",1305.5323v1 2013-05-23,In situ observation of strongly interacting ferromagnetic domains in a shaken optical lattice,"Solid state systems derive their richness from the interplay between interparticle interactions and novel band structures that deviate from those of free particles. Strongly interacting systems, where both of these phenomena are of equal importance, exhibit a variety of theoretically interesting and practically useful phases. Systems of ultracold atoms are rapidly emerging as precise and controllable simulators, and it is precisely in this strongly interacting regime where simulation is the most useful. Here we demonstrate how to hybridize Bloch bands in optical lattices to introduce long-range ferromagnetic order in an itinerant atomic system. We find spontaneously broken symmetry for bosons with a double-well dispersion condensing into one of two distinct minima, which we identify with spin-up and spin-down. The density dynamics following a rapid quench to the ferromagnetic state confirm quantum interference between the two states as the mechanism for symmetry breaking. Unlike spinor condensates, where interaction is driven by small spin-dependent differences in scattering length, our interactions scale with the scattering length itself, leading to domains which equilibrate rapidly and develop sharp boundaries characteristic of a strongly interacting ferromagnet.",1305.5487v1 2013-05-27,Thermal entanglement in the mixed three spin XXZ Heisenberg Model on a triangular cell,"We numerically investigate thermal entanglement of the spins (1/2,1) and (1/2,1/2) in the three-mixed (1/2,1,1/2) anisotropic Heisenberg XXZ spin system on a simple triangular cell under an inhomogeneous magnetic field. We show that the external magnetic field induces strong plateau formation in pairwise thermal entanglement for fixed parameters of Hamiltonian in the case of the ferromagnetic and anti-ferromagnetic interactions. We also observe an unexpected critical point at finite temperature in the thermal entanglement of the spins (1/2,1) for antiferromagnetic case while the entanglement of the spins (1/2,1) for ferromagnetic case and the entanglement of the spins (1/2,1/2) for both ferromagnetic and antiferromagnetic cases almost decays exponentially to zero with increasing temperature. The critical point in entanglement of the spins (1/2,1) for antiferromagnetic case may be signature of the quantum phase transition at finite temperature.",1305.6230v2 2013-05-27,Understanding the metamagnetic transition and magnetic behavior of Ni48Co6Mn26Al20 polycrystalline ribbons,"In this work we demonstrate that the polycrystalline ribbons of (Ni48Co6)Mn26Al20 with B2 structure at room temperature show a magnetic behavior with competing magnetic exchange interactions leading to frozen disorders at low temperatures. It is established that by considering the presence of both antiferromagnetic and ferromagnetic sublattices, we can explain the observed magnetic behavior including the metamagnetic transition observed in these samples. From the Arrott plots, the N\'eel temperature of (Ni48Co6)Mn26Al20 is deduced to be ~170 K and the broad ferro to para like magnetic phase transition is observed at ~ 200 K. Based on N\'eel theory, a cluster model is used to explain the presence of ferromagnetic and anti-ferromagnetic clusters in the studied ribbons. Formation of ferromagnetic clusters can be understood in terms of positive exchange interactions among the Mn atoms that are neighboring to Co atoms which are located on the Ni sites.",1305.6255v3 2013-05-28,Emergence of ferromagnetism and Jahn-Teller distortion in low Cr-substituted LaMnO3,"The emergence of a ferromagnetic component in $LaMnO_{3}$ with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment strength for the Mn and Cr are proportional to each other and follows the macroscopic magnetization. The net ferromagnetic components of $Cr^{3+}$ and $Mn^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of $Cr^{3+}$ ions up to x = 0.15 does not decrease the Jahn-Teller (JT) distortion and consequently does not significantly affect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.",1305.6486v1 2013-05-29,Theory of Self-Induced Vortex State in Ferromagnetic Superconductors,"We have developed the quasi-classical formalism for a self-induced vortex state in ferromagnetic superconductors. By combining the spatially averaged approximation of the superconductivity with the phenomenological form of the ferromagnetism, we demonstrate the thermodynamic properties of the p-wave triplet equal-spin-pairing state with the spontaneous vortex lattice. The occurrence condition for the self-induced vortex state is discussed within this formalism. The comparison of the calculated quantities with the observed ones indicates that the self-induced vortex state indeed occurs in UCoGe, and its pairing symmetry seems to be the A2-type with a small spin difference in the gap magnitudes. In the vicinity of the ferromagnetic quantum critical point with a large uniform susceptibility, it is possible to exhibit the type-II to the type-I transition in the magnetization process.",1305.6835v1 2013-05-30,Suppression of ferromagnetism in the LaVxCr1-xGe3 system,"We report synthesis of hexagonal LaVxCr1-xGe3 (x = 0-0.21, 1.00) single crystals, and present a systematic study of this series by measurements of temperature and field dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for x = 0-0.21, and the system manifests a strong axial anisotropy in its ordered state. The decrease of the saturated moment and the effective moment per Cr with the increase of V-concentration suggests this is an itinerant ferromagnetic system. The Curie temperature declines monotonically as the V-concentration increases. Single crystalline samples could only be grown for x-values up to 0.21 for which the transition temperature was suppressed down to 36 K. Although we could not fully suppress TC via V-substitution, for x = 0.16, we performed magnetization measurements under pressure. The ferromagnetic state is suppressed under pressure at an initial rate of dTC/dp = -11.7 K/GPa and vanishes by 3.3 GPa.",1305.7258v1 2013-06-01,Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets,"We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important short-coming of previous work within the framework of Usadel theory. We show that non-magnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin-current as a function of temperature.",1306.0141v1 2013-07-04,Electronic transport in ferromagnetic barriers on the surface of a topological insulator with $δ$ doping,"We investigate electron transporting through a two-dimensional ferromagnetic/normal/ferromagnetic tunnel junction on the surface of a three-dimensional topological insulator with taking into $\delta$ doping account. It is found that the conductance oscillates with the Fermi energy, the position and the aptitude of the $\delta$ doping. Also the conductance depends sensitively on the direction of the magnetization of the two ferromagnets, which originate from the control of the spin flow due to spin-momentum locked. It is found that the conductance is the maximum at the parallel configuration while it is minimum at the antiparallel configuration and vice versa, which may stem from the half wave loss due to the electron wave entering through the antiparallel configuration. These characters are very helpful for making new types of magnetoresistance devices due to the practical applications.",1307.1283v1 2013-07-12,Angular and Linear Momentum of Excited Ferromagnets,"The angular momentum vector of a Heisenberg ferromagnet with isotropic exchange interaction is conserved, while under uniaxial crystalline anisotropy the projection of the total spin along the easy axis is a constant of motion. Using Noether's theorem, we prove that these conservation laws persist in the presence of dipole-dipole interactions. However, spin and orbital angular momentum are not conserved separately. We also define the linear momentum of ferromagnetic textures. We illustrate the general principles with special reference to the spin transfer torques and identify the emergence of a non-adiabatic effective field acting on domain walls in ferromagnetic insulators.",1307.3432v1 2013-07-16,Anomalous Hall effect in two-phase semiconductor structures: the crucial role of ferromagnetic inclusions,"The Hall effect in InMnAs layers with MnAs inclusions of 20-50 nm in size is studied both theoretically and experimentally. We find that the anomalous Hall effect can be explained by the Lorentz force caused by the magnetic field of ferromagnetic inclusions and by an inhomogeneous distribution of the current density in the layer. The hysteretic dependence of the average magnetization of ferromagnetic inclusions on an external magnetic field results in a hysteretic dependence of RH(Hext). Thus we show the possibility of a hysteretic RH(Hext) dependence (i.e. observation of the anomalous Hall effect) in thin conductive layers with ferromagnetic inclusions in the absence of carriers spin polarization.",1307.4225v1 2013-07-16,Phase diagram with enhanced spin-glass region of mixed Ising / XY magnets LiHo$_{x}$Er$_{1-x}$F$_{4}$,"We present the experimental phase diagram of LiHo$_x$Er$_{1-x}$F$_4$, a dilution series of dipolar-coupled model magnets.The phase diagram was determined using a combination of AC susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF$_4$ and the XY anti-ferromagnet LiErF$_4$ have been identified. Below $x=0.86$ an embedded spin-glass phase is observed, where a spin-glass exists within the ferromagnetic structure. Below $x=0.57$ an Ising spin-glass is observed consisting of frozen needle-like clusters. For $x \sim 0.3 - 0.1$ an antiferromagnetically coupled spin-glass occurs. A reduction of $T_C(x)$ for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHo$_x$Y$_{1-x}$F$_4$.",1307.4271v1 2013-07-25,Reentrant Superconductivity and Superconducting Critical Temperature Oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 Grown on Cobalt Oxide,"Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the magnetization-orientation of one of the ferromagnetic layers. In the present paper we demonstrate that this is possible, keeping the superconducting behavior of the core structure undisturbed.",1307.6737v1 2013-10-13,Spin-wave-induced corrections to the electronic density of states in metallic ferromagnets,"We calculate the correction to the electronic density of states in a disordered ferromagnetic metal induced by spin-wave mediated interaction between the electrons. Our calculation is valid for the case that the exchange splitting in the ferromagnet is much smaller than the Fermi energy, but we make no assumption on the relative magnitude of the exchange splitting and the elastic electronic scattering time. In the ""clean limit"", where the exchange splitting is much larger than the electronic scattering rate, we find a correction with a T^{d/2} temperature dependence, where d is the effective dimensionality of the ferromagnet. In the opposite ""dirty limit"" the density-of-states correction is a non-monotonous function of energy and temperature.",1310.3511v1 2013-10-23,Exchange-dominated Standing Spin Wave Excitations under microwave irradiation in Ni80Fe20 Thin Films,"We investigated the microwave-assisted DC voltages of ferromagnetic resonances and exchangedominated standing spin wave excitations in two different in-plane magnetized permalloy thin films via homodyne detection. The line shapes of ferromagnetic resonance spectra and the dispersion curves of ferromagnetic resonance and standing spin wave are in agreement of previous studies, while further investigations of DC voltage spectra for these two excitations reveal that 1. unlike ferromagnetic resonance signals, the anti-symmetrical line shapes of standing spin wave excitations are not depend on the electromagnetic relative phase of assisted microwave, and 2. linewidths of their DC voltage spectra are distinct. The complicated spin dynamics of standing spin wave is consequently discussed by applying Landau-Lifshitz-Gilbert equation in term of exchange interaction.",1310.6108v1 2013-10-29,Ferromagnetic Order Analysis of Fe- and FeO-modified Graphene-nano-ribbon,"Graphene-nano-ribbon (GNR) is very attractive for ultra-high density spintronics devices. For checking a capability of self-induced ferromagnetic order, Fe- and FeO-modified GNR were analyzed based on the density functional theory. Model unit cells were [C32H2Fe2] and [C32H2Fe2O2]. Calculated results show the most stable spin state to be Sz=8/2 in [C32H2Fe2], whereas Sz=6/2 in [C32H2Fe2O2]. Magnetic moment M of Fe in [C32H2Fe2] was 3.65{\mu}B, which could be explained based on the Hund-rule considering donated charge to carbon to be M*=3.67{\mu}B . There is a capability of ferromagnetic Fe spin array through an interaction with carbon {\pi}-conjugated spin system. There shows a long-range super-exchange order in [C32H2Fe2O2]. Optimized atomic configuration gave the typical 90 degree super-exchange coupling between Fe-3d and O-2p orbit. Magnetic moment of Fe by DFT was 2.58{\mu}B, whereas super-exchange model considering donated charge to oxygen gave an estimated magnetic moment to be 2.60{\mu}B. Sign of magnetic moment of Fe and O are all up-spin each other. We could expect ferromagnetic long-range-order. Band structure was analyzed in FeO-modified case, which suggested half-metal like behavior. We can expect several spintronics applications as like a spin filter.",1310.7740v1 2013-11-01,Magnetism in nanoparticle LaCoO3,"LaCoO3 (LCO) nanoparticles were synthesized and their magnetic and structural properties were examined using SQUID magnetometery and neutron diffraction. The nanoparticles exhibit ferromagnetic long-range order beginning at T_C approximately 87K that persists to low temperatures. This behavior is contrasted with the ferromagnetism of bulk LCO, which also starts at T_C approximately 87K but is suppressed below a second transition at T_o approximately 37K, due to a structural phase transition. The ferromagnetism in both systems is attributed to the tensile stress from particle surfaces and impurity phase interfaces. This stress locally increases the Co-O-Co bond angle gamma, and competes with the thermal contraction of the lattice. It has recently been shown that LCO loses long-range ferromagnetic order when gamma decreases below the critical value gamma_c = 162.8 degrees. Consistent with this model, we show that gamma in nanoparticles remains larger than gamma_c at low temperatures, likely a consequence of all spins being in close proximity to surfaces or interfaces.",1311.0240v1 2013-11-02,Stimulation of a Singlet Superconductivity in SFS Weak Links by Spin-Exchange Scattering of Cooper Pairs,"Josephson junctions with a ferromagnetic metal weak link reveal a very strong decrease of the critical current compared to a normal metal weak link. We demonstrate that in the ballistic regime the presence of a small region with a non-collinear magnetization near the center of a ferromagnetic weak link restores the critical current inherent to the normal metal. The above effect can be stimulated by additional electrical bias of the magnetic gate which induces a local electron depletion of ferromagnetic barrier. The underlying physics of the effect is the interference phenomena due to the magnetic scattering of the Cooper pair, which reverses its total momentum in the ferromagnet and thus compensates the phase gain before and after the spin-reversed scattering. In contrast with the widely discussed triplet long ranged proximity effect we elucidate a new singlet long ranged proximity effect. This phenomenon opens a way to easily control the properties of SFS junctions and inversely to manipulate the magnetic moment via the Josephson current.",1311.0369v2 2013-12-03,Stoner instability revisited: Emergence of local quantum criticality?,"We revisit Stoner instability, an old problem but in a modern point of view. An idea is to extract out dynamics of directional fluctuations of spins explicitly, resorting to the CP$^{1}$ representation and integrating over their amplitude fluctuations. As a result, we derive an effective field theory for ferromagnetic quantum phase transitions in terms of bosonic spinons and fermionic holons. We show that this effective field theory reproduces overdamped spin dynamics in a paramagnetic Fermi liquid and magnon spectrum in a ferromagnetic Fermi liquid. An interesting observation is that the velocity of spinons becomes zero, approaching the ferromagnetic quantum critical point, which implies emergence of local quantum criticality. Based on this scenario, we predict the $\omega/T$ scaling behavior near ferromagnetic quantum criticality beyond the conventional scenario of the weak-coupling approach.",1312.0807v2 2013-12-03,Reconsidering the possibility of room temperature ferromagnetism in Mn doped Zirconium oxide,"The possibility to induce long range ferromagnetic order by doping oxides with transition metal ions has become a very exciting challenge in the last decade. Theoretically, it has been claimed that Mn doped ZrO$_2$ could be a very promising spintronic candidate and that high critical temperatures could be already achieved even for a low Mn concentration. Some experiments have reported room temperature ferromagnetism (RT-FM) whilst some others only paramagnetism. When observed, the nature of RT-FM appears to be controversial and not clearly understood. In this study, we propose to clarify and shed light on some of theses existing issues. A detailed study of the critical temperatures and low energy magnetic excitations in Mn doped ZrO$_2$ is performed. We show that the Curie temperatures were largely overestimated previously, due to the inadequate treatment of both thermal and transverse fluctuations, and disorder. It appears that the Mn-Mn couplings can not explain the observed RT-FM. We argue, that this can be attributed to the interaction between large moments induced in the vicinity of the manganese. This is similar to the non-magnetic defect induced ferromagnetism reported in oxides, semiconductors and graphene/graphite.",1312.0808v1 2013-12-11,New Green function approach describing the ferromagnetic state in the simple Hubbard model,"We consider a Hubbard model with occupation dependent hopping integrals. Using the Hartree-Fock (H-F) approximation and the new Green function approach with inter-site kinetic averages included, we analyze the influence of the correlated hopping on ferromagnetic ordering. The influence of correlated hopping on the nonlocal quasiparticle energies and corresponding k-dependent spectral weights is included. In addition we obtain the shift of the spin bands which is a major factor in creating spontaneous ferromagnetism. At some parameters of the model the correlated hopping effect is strong enough to achieve saturated ferromagnetism. This state may be obtained at the asymmetric density of states (DOS) and the Fermi energy located in the region of large spectral weight near the band edge. The results are compared with DMFT-Quantum Monte Carlo calculations and with the Hubbard III approximation which includes the correlated hopping effect.",1312.3150v1 2013-12-13,Full Counting Statistics of Generic Spin Entangler with Quantum Dot-Ferromagnet detectors,"Entanglement between spatially separated electrons in nanoscale transport is a fundamental property, yet to be demonstrated experimentally. Here we propose and analyse theoretically the transport statistics of a generic spin entangler coupled to a hybrid quantum dot-ferromagnet detector system. We show that the full distribution of charges arriving at the ferromagnetic terminals provide complete information on the spin state of the particles emitted by the entangler. This provides means for spin entanglement detection via electrical current correlations, with optimal measurement strategies depending on the a priori knowledge of ferromagnet polarization and spin-flip rates in the detector dots. The scheme is exemplified by applying it to Andreev and triple dot entanglers.",1312.3982v1 2014-01-02,Evidence for Majorana bound states in transport properties of hybrid structures based on helical liquids,"Majorana bound states can emerge as zero-energy modes at the edge of a two-dimensional topological insulator in proximity to an ordinary s-wave superconductor. The presence of an additional ferromagnetic domain close to the superconductor can lead to their localization. We consider both N-S and S-N-S junctions based on helical liquids and study their spectral properties for arbitrary ferromagnetic scatterers in the normal region. Thereby, we explicitly compute Andreev wave-functions at zero energy. We show under which conditions these states form localized Majorana bound states in N-S and S-N-S junctions. Interestingly, we can identify Majorana-specific signatures in the transport properties of N-S junctions and the Andreev bound levels of S-N-S junctions that are robust against external perturbations. We illustrate these findings with the example of a ferromagnetic double barrier (i.e. a quantum dot) close to the N-S boundaries.",1401.0507v2 2014-01-20,Topological insulator in junction with ferromagnets: quantum Hall effects,"The ferromagnet-topological insulator-ferromagnet (FM-TI-FM) junction exhibits thermal and electrical quantum Hall effects. The generated Hall voltage and transverse temperature gradient can be controlled by the directions of magnetizations in the FM leads, which inspires the use of FM-TI-FM junctions as electrical and as heat switches in spintronic devices. Thermal and electrical Hall coefficients are calculated as functions of the magnetization directions in ferromagnets and the spin-relaxation time in TI. Both the Hall voltage and the transverse temperature gradient decrease but are not completely suppressed even at very short spin-relaxation times. The Hall coefficients turn out to be independent of the spin-relaxation time for symmetric configuration of FM leads.",1401.4986v3 2014-01-22,Coexistence of Superconductivity and Ferromagnetism in P-doped EuFe2As2,"The magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.15 has been determined using element specific x-ray resonant magnetic scattering. Combining magnetic, thermodynamic and scattering measurements, we conclude that the long range ferromagnetic order of the Eu2+ moments aligned primarily along the c axis coexists with the bulk superconductivity at zero field. At an applied magnetic field >= 0.6 T, superconductivity still coexists with the ferromagnetic Eu2+ moments which are polarized along the field direction. We propose a spontaneous vortex state for the coexistence of superconductivity and ferromagnetism in EuFe2(As0.85P0.15)2.",1401.5463v1 2014-01-29,Nonreciprocal Dispersion of Spin Waves in Ferromagnetic Thin Films Covered with a Finite-Conductivity Metal,"We study the effect of one-side metallization of a uniform ferromagnetic thin film on its spin-wave dispersion relation in the Damon-Eshbach geometry. Due to the finite conductivity of the metallic cover layer on the ferromagnetic film the spin-wave dispersion relation may be nonreciprocal only in a limited wave-vector range. We provide an approximate analytical solution for the spin-wave frequency, discuss its validity and compare it with numerical results. The dispersion is analyzed systematically by varying the parameters of the ferromagnetic film, the metal cover layer and the value of the external magnetic field. The conclusions drawn from this analysis allow us to define a structure based on a 30 nm thick CoFeB film with an experimentally accessible nonreciprocal dispersion relation in a relatively wide wave-vector range.",1401.7454v1 2014-01-29,A first order phase transition and self-organizing states in unidomain ferromagnet,"Unidomain ferromagnet during the first order phase transition induced by a magnetic field directed along the axis of anisotropy, has been analyzed. It is shown that there is continuous spectrum of stationary states, which depends of relation between acting field value and the demagnetisation field. At addition action of the periodic field perpendicular to the main magnetic field with a certain frequency, there are dynamic equilibrium state, ie ""self-organizing states"" of ferromagnet, when the entropy increase connected with dissipation is compensated by the negative entropy flow due to the periodic field. It is shown that under these conditions, by varying the frequency of the periodic field, we can control the self-organising system, ie decrease or increase the system energy and, correspondingly, change the direction of magnetisation in ferromagnet.",1401.7521v1 2014-04-03,Enhanced DC Spin Pumping into a Fluctuating Ferromagnet near Tc,"A linear-response formulation of the dc spin pumping, i.e., a spin injection from a precessing ferromagnet into an adjacent spin sink, is developed in view of describing many-body effects caused by spin fluctuations in the spin sink. It is shown that, when an itinerant ferromagnet near Tc is used as the spin sink, the spin pumping is largely increased owing to the fluctuation enhancement of the spin conductance across the precessing ferromagnet/spin sink interface. As an example, the enhanced spin pumping from permalloy into nickel palladium alloy (Tc ~ 20K) is analyzed by means of a self-consistent renormalization scheme, and it is predicted that the enhancement can be as large as tenfold.",1404.0768v2 2014-04-12,New room temperature multiferroics on the base of single-phase nanostructured perovskites,"The theoretical description of the nanostructured Pb(Fe1/2Ta1/2)x(Zr1/2Ti1/2)1-xO3 (PFTx-PZT(1-x)) and Pb(Fe1/2Nb1/2)x(Zr1/2Ti1/2)1-xO3 (PFNx-PZT(1-x)) intriguing ferromagnetic, ferroelectric and magnetoelectric properties at temperatures higher than 100 K are absent to date. The goal of this work is to propose the theoretical description of the physical nature and the mechanisms of the aforementioned properties, including room temperature ferromagnetism, phase diagram dependence on the composition x with a special attention to the multiferroic properties at room temperature, including anomalous large value of magnetoelectric coefficient. The comparison of the developed theory with experiments establishing the boundaries between paraelectric, ferroelectric, paramagnetic, antiferromagnetic, ferromagnetic and magnetoelectric phases, as well as the characteristic features of ferroelectric domain switching by magnetic field are performed and discussed. The experimentally established absence of ferromagnetic phase in PFN, PFT and in the solid solution of PFN with PbTiO3 (PFNx-PT(1-x)) was explained in the framework of the proposed theory.",1404.3267v1 2014-04-29,Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques,"Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the active region of a spin torque oscillator driven by spin orbit torques. Our work demonstrates that magnetization self-oscillations can be excited in a one-dimensional magnetic system and that dimensions of the active region of spin torque oscillators can be extended beyond the nanometer length scale.",1404.7262v2 2014-07-02,Induced Ferromagnetism at BiFeO3/YBa2Cu3O7 Interfaces,"Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO3/YBa2Cu3O7 interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO3, we are able to show that a metallic ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interaction strengths, from the first-principles calculations, into a resultant generic phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the presence of a hysteresis loop in field-dependent magnetization data. The emergence of interfacial ferromagnetism should have implications to electronic and transport properties.",1407.0722v1 2014-07-12,Role of dimensionality in spontaneous magnon decay: easy-plane ferromagnet,"We calculate magnon lifetime in an easy-plane ferromagnet on a tetragonal lattice in transverse magnetic field. At zero temperature magnons are unstable with respect to spontaneous decay into two other magnons. Varying ratio of intrachain to interchain exchanges in this model we consider the effect of dimensionality on spontaneous magnon decay. The strongest magnon damping is found in the quasi-one-dimensional case for momenta near the Brillouin zone boundary. The sign of a weak interchain coupling has a little effect on the magnon decay rate. The obtained theoretical results suggest possibility of experimental observation of spontaneous magnon decay in a quasi-one-dimensional ferromagnet CsNiF$_3$. We also find an interesting enhancement of the magnon decay rate for a three-dimensional ferromagnet. The effect is present only for the nearest-neighbor model and is related to effective dimensionality reduction in the two-magnon continuum.",1407.3402v1 2014-07-20,Half-metallic magnetism and the search for better spin valves,"We use a previously proposed theory for the temperature dependence of tunneling magnetoresistance to shed light on ongoing efforts to optimize spin valves. First we show that a mechanism in which spin valve performance at finite temperatures is limited by uncorrelated thermal fluctuations of magnetization orientations on opposite sides of a tunnel junction is in good agreement with recent studies of the temperature-dependent magnetoresistance of high quality tunnel junctions with MgO barriers. Using this insight, we propose a simple formula which captures the advantages for spin-valve optimization of using materials with a high spin polarization of Fermi-level tunneling electrons, and of using materials with high ferromagnetic transition temperatures. We conclude that half-metallic ferromagnets can yield better spin-value performance than current elemental transition metal ferromagnet/MgO systems only if their ferromagnetic transition temperatures exceed $\sim 950~\mathrm{K}$.",1407.5240v1 2014-08-25,Electrical detection of ferromagnetic resonance in ferromagnet/n-GaAs heterostructures by tunneling anisotropic magnetoresistance,"We observe a dc voltage peak at ferromagnetic resonance (FMR) in samples consisting of a single ferromagnetic (FM) layer grown epitaxially on the $\mathit{n-}$GaAs (001) surface. The FMR peak is detected as an interfacial voltage with a symmetric line shape and is present in samples based on various FM/$n$-GaAs hetrostructures, including Co$_{2}$MnSi/$n$-GaAs, Co$_{2}$FeSi/$n$-GaAs and Fe/$n$-GaAs. We show that the interface bias voltage dependence of the FMR signal is identical to that of the tunneling anisotropic magnetoresistance (TAMR) over most of the bias range. Furthermore, we show how the precessing magnetization yields a dc FMR signal through the TAMR effect and how the TAMR phenomenon can be used to predict the angular dependence of the FMR signal. This TAMR-induced FMR peak can be observed under conditions where no spin accumulation is present and no spin-polarized current flows in the semiconductor.",1408.5842v1 2014-08-26,Ferromagnetic and Nematic Non-Fermi Liquids in Spin-Orbit Coupled Two-Dimensional Fermi Gases,"We study the fate of a two-dimensional system of interacting fermions with Rashba spin-orbit coupling in the dilute limit. The interactions are strongly renormalized at low densities, and give rise to various fermionic liquid crystalline phases, including a spin-density wave, an in-plane ferromagnet, and a non-magnetic nematic phase, even in the weak coupling limit. The nature of the ground state in the low-density limit depends on the range of the interactions: for short range interactions it is the ferromagnet, while for dipolar interactions the nematic phase is favored. Interestingly, the ferromagnetic and nematic phases exhibit strong deviations from Fermi liquid theory, due to the scattering of the Fermionic quasi-particles off long-wavelength collective modes. Thus, we argue that a system of interacting fermions with Rashba dispersion is generically a non-Fermi liquid at low densities.",1408.6250v3 2014-09-18,Interference effects induced by Andreev bound states in a hybrid nanostructure composed by a quantum dot coupled to ferromagnetic and superconductor leads,"In this work, it is considered a nanostructure composed by a quantum dot coupled to two ferromagnets and a superconductor. The transport properties of this system are studied within a generalized mean-field approximation taking into account proximity effects and spin-flip correlations within the quantum dot. It is shown that the zero-bias transmittance for the co-tunneling between the ferromagnetic leads presents a dip whose height depends on the relative orientation of the magnetizations. When the superconductor is coupled to the system, electron-hole correlations between different spin states leads to a resonance in the place of the dip appearing in the transmittance. Such an effect is accompanied by two anti-resonances explained by a leakage of conduction channels from the co-tunneling to the Andreev transport. In the non-equilibrium regime, correlations within the quantum dot introduce a dependence of the resonance condition on the finite bias applied to the ferromagnetic leads. However, it is still possible to observe signatures of the same interference effect in the electrical current.",1409.5390v1 2014-10-02,Strong coupling between a permalloy ferromagnetic contact and a helical edge channel in a narrow HgTe quantum well,"We experimentally investigate spin-polarized electron transport between a permalloy ferromagnet and the edge of a two-dimensional electron system with band inversion, realized in a narrow, 8~nm wide HgTe quantum well. In zero magnetic field, we observe strong asymmetry of the edge potential distribution with respect to the ferromagnetic ground lead. This result indicates, that the helical edge channel, specific for the structures with band inversion even at the conductive bulk, is strongly coupled to the ferromagnetic side contact, possibly due to the effects of proximity magnetization. It allows selective and spin-sensitive contacting of helical edge states.",1410.0585v5 2014-10-12,Differences and analogies between quantum chromodynamics and ferromagnets,"The low-energy physics of quantum chromodynamics (QCD) and ferromagnets is dominated by Goldstone bosons. While the effective theory of QCD - chiral perturbation theory - is well established in the particle physics community, the systematic studies of ferromagnetic systems within the effective Lagrangian framework are not well-known. We analyze the low-temperature properties of ferromagnets in one, two and three space dimensions up to three-loop order in the effective expansion, i.e., beyond the accuracy of any previous results obtained with conventional condensed matter methods. In particular, in the nonrelativistic domain, the effective method perfectly works in one space dimension.",1410.3107v1 2014-10-30,Critical aspects of three-dimensional anisotropic spin-glass models,"We study the $\pm J$ three-dimensional Ising model with a longitudinal anisotropic bond randomness on the simple cubic lattice. The random exchange interaction is applied only in the $z$ direction, whereas in the other two directions, $xy$ - planes, we consider ferromagnetic exchange. By implementing an effective parallel tempering scheme, we outline the phase diagram of the model and compare it to the corresponding isotropic one, as well as to a previously studied anisotropic (transverse) case. We present a detailed finite-size scaling analysis of the ferromagnetic - paramagnetic and spin glass - paramagnetic transition lines, and we also discuss the ferromagnetic - spin glass transition regime. We conclude that the present model shares the same universality classes with the isotropic model, but at the symmetric point has a considerably higher transition temperature from the spin-glass state to the paramagnetic phase. Our data for the ferromagnetic - spin glass transition line are supporting a forward behavior in contrast to the reentrant behavior of the isotropic model.",1410.8397v2 2014-11-12,Coexistence of ferromagnetism and superconductivity in CeO$_{0.3}$F$_{0.7}$BiS$_{2}$,"Bulk magnetization, transport and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, CeO$_{0.3}$F$_{0.7}$BiS$_{2}$. Ferromagnetism develops below T$_{FM}$ = 6.54(8) K and superconductivity is found to coexist with the ferromagnetic state below T$_{SC}$ ~ 4.5 K. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the c-axis to the ab-plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed.",1411.3394v1 2014-11-23,Gate controllable spin pumping in graphene via rotating magnetization,"We investigate pure spin pumping in graphene by imposing a ferromagnet (F) with rotating magnetization on top of it. Using the generalized scattering approach for adiabatic spin pumping, we obtain the spin current pumped through magnetic graphene to a neighboring normal (N) region. The spin current can be easily controlled by gate voltages and under certain conditions, becomes sufficiently large to be measurable in current experimental setups. In fact it reaches a maximum value when one of the spins are completely filtered due to the vanishing density of states of the corresponding spin species in the ferromagnetic part. Considering an N|F|N structure with a finite ferromagnetic region, it is found that in contrast to the metallic ferromagnetic materials the transverse spin coherence length can be comparable to the length of F denoted by $L$. Subsequently, due to the quantum interferences, the spin current becomes oscillatory function of $JL/\hbar v_F$ in which $J$ is the spin splitting inside F. Finally we show, originated from the controllability of pumped spin into two different normal sides, a profound spin battery effect can be seen in the hybrid N|F|N device.",1411.6261v1 2014-12-01,Experimental Verification of Van Vleck Nature of Long-Range Ferromagnetic Order in Vanadium-Doped Three-Dimensional Topological Insulator Sb$_{2}$Te$_{3}$,"We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in vanadium (V)-doped topological insulator Sb$_2$Te$_3$ has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L$_3$ and L$_2$) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T=10K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from onset of FM order. Crystal field analysis indicates that in V-doped Sb$_2$Te$_3$, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.",1412.0636v1 2014-12-01,Dissipation due to pure spin-current generated by spin pumping,"Based on spin-dependent transport theory and thermodynamics, we develop a generalized theory of the Joule heating in the presence of a spin current. Along with the conventional Joule heating consisting of an electric current and electrochemical potential, it is found that the spin current and spin accumulation give an additional dissipation because the spin-dependent scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a change of entropy. The theory is applied to investigate the dissipation due to pure spin-current generated by spin pumping across a ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from an interface because the spin pumping is a transfer of both the spin angular momentum and the energy from the ferromagnet to conduction electrons near the interface. It is found that the dissipation is proportional to the enhancement of the Gilbert damping constant by spin pumping.",1412.0688v1 2014-12-03,"Eddy current interactions in a Ferromagnet-Normal metal bilayer structure, and its impact on ferromagnetic resonance lineshapes","We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnet-normal metal (FM/NM) bilayer structures. Eddy-current effects are usually neglected for NM layer thicknesses below the microwave (MW) skin depth (approx. 800 nm for Au at 10 GHz). However, we show that in much thinner NM layers (10-100 nm of Au or Cu) they induce a phase shift in the FMR excitation when the MW driving field has a component perpendicular to the sample plane. This results in a strong asymmetry of the measured absorption lines. In contrast to typical eddy-current effects, the asymmetry is larger for thinner NM layers and is tunable through changing the sample geometry and the NM layer thickness.",1412.1385v3 2014-12-15,A Minimal tight-binding model for ferromagnetic canted bilayer manganites,"Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ $(x=0.38)$, transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO$_2$ planes and the mixing of the $|x^2 - y^2>$ and $|3z^2 - r^2>$ Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of $k_z$ dispersion are included.",1412.4728v1 2014-12-16,Non-uniform magnetic system driven by non-magnetic ion substitution in CaRu1-xScxO3:Two-component analysis,"We have studied magnetic and transport properties in polycrystalline CaRu1-xScxO3 for 0 =< x =< 0.20 in order to clarify the substitution effects of a non-magnetic trivalent ion. We find that a ferromagnetic transition with Tc = 30 K is observed in Sc-substituted samples. The composition dependence of the Curie-Weiss temperature implies that the magnetic susceptibility has a paramagnetic contribution with negative theta and a ferromagnetic contribution with positive theta. The field dependence of magnetization at 2 K is also understood as a summation of the ferromagnetic and paramagnetic components. These results suggest that CaRu1-xScxO3 is a non-uniform magnetic system. The relationship between the ferromagnetic ordering and the transport properties is also discussed.",1412.4947v1 2014-12-19,Reentrant superconductivity in URhGe,"There is presented a phenomenological description of phase diagram of ferromagnet superconductor URhGe. In frame of the Landau phenomenological theory it was found that phase transition between anisotropic ferromagnetic and paramagnetic states under strong enough magnetic field perpendicular to direction of easy magnetization changes from the second to the first order type. It is shown that magnetic susceptibility corresponding to longitudinal magnetic fluctuations strongly increases in vicinity of the first order transition stimulating reentrance of superconducting state. The reentrant superconductivity observed near the first order transition line at temperatures about twice lower than the tricritical point temperature exists both in ferromagnet and paramagnet state. The critical temperature of transition to superconducting state falls down at intersection with line of ferromagnet-paramagnet phase transition.",1412.6289v2 2015-01-03,Exact results for itinerant ferromagnetism in a $t_{2g}$ orbital system on cubic and square lattices,"We study itinerant ferromagnetism in a $t_{2g}$ multi-orbital Hubbard system in the cubic lattice, which consists of three planar oriented orbital bands of $d_{xy}$, $d_{yz}$, and $d_{zx}$. Electrons in each orbital band can only move within a two-dimensional plane in the three-dimensional lattice parallel to the corresponding orbital orientation. Electrons of different orbitals interact through the on-site multi-orbital interactions including Hund's coupling. The strong coupling limit is considered in which there are no doubly occupied orbitals but multiple on-site occupations are allowed. We show that, in the case in which there is one and only one hole for each orbital band in each layer parallel to the orbital orientation, the ground state is a fully spin-polarized itinerant ferromagnetic state, which is unique apart from the trivial spin degeneracy. When the lattice is reduced into a single two-dimensional layer, the $d_{zx}$ and $d_{yz}$ bands become quasi-one-dimensional while the $d_{xy}$ band remains two-dimensional. The ground state ferromagnetism also appears in the strong-coupling limit as a generalization of the double exchange mechanism. Possible applications to the systems of SrRuO$_3$ and LaAlO$_3$/SrTiO$_3$ interface are discussed.",1501.00536v2 2015-01-06,Sensing magnetic nanoparticles using nano-confined ferromagnetic resonances in a magnonic crystal,"We demonstrate the use of the magnetic-field-dependence of highly spatially confined, GHz-frequency ferromagnetic resonances in a ferromagnetic nanostructure for the detection of adsorbed magnetic nanoparticles. This is achieved in a large area magnonic crystal consisting of a thin ferromagnetic film containing a periodic array of closely spaced, nano-scale anti-dots. Stray fields from nanoparticles within the anti-dots modify resonant dynamic magnetisation modes in the surrounding magnonic crystal, generating easily measurable resonance peak shifts. The shifts are comparable to the resonance linewidths for high anti-dot filling fractions with their signs and magnitudes dependent upon the modes' localisations (in agreement with micromagnetic simulation results). This is a highly encouraging result for the development of frequency-based nanoparticle detectors for high speed nano-scale biosensing.",1501.01171v1 2015-01-08,Antisite Disorder-induced Exchange Bias Effect in Multiferroic Y2CoMnO6,"Exchange bias effect in the ferromagnetic double perovskite compound Y$_2$CoMnO$_6$, which is also a multiferroic, is reported. The exchange bias, observed below 8~K, is explained as arising due to the interface effect between the ferromagnetic and antiferromagnetic clusters created by {\it antisite} disorder in this material. Below 8~K, prominent ferromagnetic hysteresis with metamagnetic ""steps"" and significant coercive field, $H_c \approx$ 10~kOe are observed in this compound which has a $T_c \approx$ 75~K. A model based on growth of ferromagnetic domains overcoming the elastic energy of structurally pinned magnetic interfaces, which closely resembles martensitic-like transitions, is adapted to explain the observed effects. The role of {\it antisite} disorder in creating the domain structure leading to exchange bias effect is highlighted in the present work.",1501.01766v1 2015-01-16,La displacement driven Double-exchange like mediation in Titanium $d_{xy}$ ferromagnetism at the LaAlO$_3$/SrTiO$_3$,"The epitaxial atomistic interfaces of two insulating oxides, LaAlO$_3$ (LAO)/SrTiO$_3$ (STO), have attracted great interest owing to rich emergent phenomena \cite{hwang12:nmat,coey13:mrs,ohtomo04:} such as interface metallicity \cite{ohtomo04:,nakagawa06:}, thickness dependent insulator-metal transition \cite{huijben06:nmat}, superconductivity \cite{N.Reyren08312007}, ferromagnetism \cite{brinkman.07:natmat}, and even their coexistence \cite{bert11:nphys,li11:nphys,dikin11:prl}. However, the physics origin of ferromagnetic ordering in the $n$-type LAO/STO interface is in debate. Here we propose that the polar distortion of La atom can ignite the ferromagnetism at the interface even without oxygen vacancy. The induced hybridization between La $d_{z^2}$ and O $p_{x,y}$ states can mediate double-exchange like interaction between Ti $d_{xy}$ electrons. We further suggest that the structural and electrical modification of the outermost surface of LAO or switching the polarization direction of ferroelectric overlayers on LAO/STO can promote such La displacement.",1501.03877v1 2015-01-30,Microscopic properties of degradation-free capped GdN thin films studied by Electron Spin Resonance,"The microscopic magnetic properties of high-quality GdN thin films have been investigated by electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements. Detailed temperature dependence ESR measurements have shown the existence of two ferromagnetic components at lower temperatures which was not clear from the previous magnetization measurements. The temperature, where the resonance shift occurs for the major ferromagnetic component, seems to be consistent with the Curie temperature obtained from the previous magnetization measurement. On the other hand, the divergence of line width is observed around 57 K for the minor ferromagnetic component. The magnetic anisotropies of GdN thin films have been obtained by the analysis of FMR angular dependence observed at 4.2 K. Combining the X-ray diffraction results, the correlation between the magnetic anisotropies and the lattice constants is discussed.",1501.07672v1 2015-02-10,Metallic Quantum Ferromagnets,"This review gives an overview of the quantum phase transition (QPT) problem in metallic ferromagnets, discussing both experimental and theoretical aspects. These QPTs can be classified with respect to the presence and strength of quenched disorder: Clean systems generically show a discontinuous, or first-order, QPT from the ferromagnetic state to a paramagnetic one as a function of some control parameter, as predicted by theory. Disordered systems are much more complicated, depending on the disorder strength and the distance from the QPT. In many disordered materials the QPT is continuous, or second order, and Griffiths-phase effects coexist with QPT singularities near the transition. In other systems the transition from the ferromagnetic state at low temperatures is to a different type of long-range order, such as an antiferromagnetic or a spin-density-wave state. In still other materials a transition to a state with glass-like spin dynamics is suspected. The review provides a comprehensive discussion of the current understanding of these various transitions, and of the relation between experimental and theoretical developments.",1502.02898v2 2015-02-16,Electrical manipulation of a ferromagnet by an antiferromagnet,"We demonstrate that an antiferromagnet can be employed for a highly efficient electrical manipulation of a ferromagnet. In our study we use an electrical detection technique of the ferromagnetic resonance driven by an in-plane ac-current in a NiFe/IrMn bilayer. At room temperature, we observe antidamping-like spin torque acting on the NiFe ferromagnet, generated by the in-plane current driven through the IrMn antiferromagnet. A large enhancement of the torque, characterized by an effective spin-Hall angle exceeding most heavy transition metals, correlates with the presence of the exchange-bias field at the NiFe/IrMn interface. It highlights that, in addition to strong spin-orbit coupling, the antiferromagnetic order in IrMn governs the observed phenomenon.",1502.04570v2 2015-02-23,The nature of Itinerant Ferromagnetism of SrRuO3 : A DFT+DMFT Study,"We have investigated the temperature (T)-dependent evolution of electronic structures and magnetic properties of an itinerant ferromagnet SrRuO3, employing the combined scheme of the density functional theory and the dynamical mean-field theory (DFT+DMFT). The inclusion of finite dynamical correlation effects beyond the DFT well describes not only the incoherent hump structure observed in the photoemission experiment but also the T-dependent magnetic properties in accordance with experiments. We have shown that the magnetization of SrRuO3 evolves with the Stoner behavior below the Curie temperature (Tc), reflecting the weak itinerant ferromagnetic behavior, but the local residual magnetic moment persists even above Tc, indicating the local magnetic moment behavior. We suggest that the ferromagnetism of SrRuO3 has dual nature of both weak and local moment limits, even though the magnetism of SrRuO3 is more itinerant than that of Fe.",1502.06322v1 2015-02-24,Coarsening dynamics driven by vortex-antivortex annihilation in ferromagnetic Bose-Einstein condensates,"In ferromagnetic Bose-Einstein condensates (BECs), the quadratic Zeeman effect controls magnetic anisotropy, which affects on magnetic domain pattern formation. While the longitudinal magnetization is dominant (similar to the Ising model) for a negative quadratic Zeeman energy, the transverse magnetization is dominant (similar to the XY model) for a positive one. When the quadratic Zeeman energy is positive, the coarsening dynamics is driven by vortex-antivortex annihilation in the same way as the XY model. However, due to superfluid flow of atoms, there exist several combinations of vortex-antivortex pairs in ferromagnetic BECs, which makes the coarsening dynamics more complicated than that of the XY model. We propose a revised domain growth law, which is based on the growth law of the two-dimensional XY model, for a two-dimensional ferromagnetic BEC with a positive quadratic Zeeman energy.",1502.06673v3 2015-03-11,Analytic results for the Casimir free energy between ferromagnetic metals,"We derive perturbation analytic expressions for the Casimir free energy and entropy between two dissimilar ferromagnetic plates which are applicale at arbitrarily low temperature. The dielectric properties of metals are described using either the nondissipative plasma model or the Drude model taking into account the dissipation of free charge carriers. Both cases of constant and frequency-dependent magnetic permeability are considered. It is shown that for ferromagnetic metals described by the plasma model the Casimir entropy goes to zero when the temperature vanishes, i.e., the Nernst heat theorem is satisfied. For ferromagnetic metals with perfect crystal lattices described by the Drude model the Casimir entropy goes to a nonzero constant depending on the parameters of a system with vanishing temperature, i.e., the Nernst heat theorem is violated. This constant can be positive which is quite different from the earlier investigated case of two nonmagnetic metals.",1503.03470v2 2015-03-22,Nitrogen-induced Ferromagnetism in BaO,"Density functional theory with local spin density approximation has been used to propose possible room temperature ferromagnetism in N-doped NaCl-type BaO. Pristine BaO is a wide bandgap semiconductor, however, N induces a large density of states at the Fermi level in the nonmagnetic state, which suggests magnetic instability within the Stoner mean field model. The spin-polarized calculations show that N-doped BaO is a true half- metal, where N has a large magnetic moment, which is mainly localized around the N atoms and a small polarization at the O sites is also observed. The origin of magnetism is linked to the electronic structure. The ferromagnetic(FM) and antiferromagnetic (AFM) coupling between the N atoms in BaO reveal that doping N atoms have a FM ground state, and the calculated transition temperature ($T_{C}$), within the Heisenberg mean field theory, theorizes possible room temperature FM in N-doped BaO. Nitrogen also induces ferromagnetism when doping occurs at surface O site and has a smaller defect formation energy than the bulk N-doped BaO. The magnetism of N-doped BaO is also compared with Co-doped BaO, and we believe that N has a greater potential for tuning magnetism in BaO than Co.",1503.06455v2 2015-03-29,Low-non-linearity spin-torque oscillations driven by ferromagnetic nanocontacts,"Spin-torque oscillators are strong candidates as nano-scale microwave generators and detectors. However, because of large amplitude-phase coupling (non-linearity), phase noise is enhanced over other linear auto-oscillators. One way to reduce nonlinearity is to use ferromagnetic layers as a resonator and excite them at localized spots, making a resonator-excitor pair. We investigated the excitation of oscillations in dipole-coupled ferromagnetic layers, driven by localized current at ferromagnetic nano-contacts. Oscillations possessed properties of optical-mode spin-waves and at low field ($\approx$200 Oe) had high frequency (15 GHz), a moderate precession amplitude (2--3$^\circ$), and a narrow spectral linewidth ($<$3 MHz) due to localized excitation at nano-contacts. Micromagnetic simulation showed emission of resonator's characteristic optical-mode spin-waves from disturbances generated by domain-wall oscillations at nano-contacts.",1503.08408v2 2015-03-31,Spin Hall magnetoresistance in metallic bilayers,"Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find nearly a ten-fold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator (FI) systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity with temperature. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the HM/FM interface.",1503.08903v2 2015-04-01,Surface Transport in the ν=0 Quantum Hall Ferromagnetic State in the Organic Dirac Fermion System,"We discuss the surface magnetotransport in the quantum Hall (QH) ferromagnetic state expected in the organic Dirac fermion system \alpha-(BEDT-TTF)_2I_3. The QH ferromagnetic state is one of the possible \nu=0 QH states in the two-dimensional Dirac fermion system resulting from the degeneracy breaking of the n=0 Landau level. It is characterized by the helical edge state. We have studied the interlayer surface transport via helical edge state in the multilayer QH ferromagnet, in which the bulk region is insulating. We have clarified that the surface conductivity is much less than e^2/h and decreases as the magnetic field is tilted to the normal direction of the side surface. These features explain the observed interlayer magnetoresistance in \alpha-(BEDT-TTF)_2I_3.",1504.00270v1 2015-04-03,Spin-flipping with Holmium: Case study of proximity effect in superconductor/ferromagnet/superconductor heterostructures,"Superconductor/ferromagnet/superconductor heterostructures exhibit a so-called long-range proximity effect provided some layers of conical magnet Holmium are included in the respective interface regions. The Ho layers lead to a spin-flip process at the interface generating equal-spin spin-triplet pairing correlations in the ferromagnet. These equal-spin spin-triplet pairing correlations penetrate much further into the heterostructure compared to the spin-singlet and unequal-spin spin-triplet correlations which occur in the absence of Ho. Here we present calculations of this effect based on the spin-dependent microscopic Bogoliubov-de Gennes equations solved within a tight-binding model in the clean limit. The influence of the ferromagnet and conical magnet layer thickness on the induced equal-spin spin-triplet pairing correlations is obtained and compared to available experimental data. It is shown that, in agreement with experiment, a critical minimum thickness of conical magnet layers has to be present in order to observe a sizeable amount of equal-spin spin-triplet pairing correlations.",1504.00816v1 2015-04-09,Spin-transfer torque effects in the dynamic forced response of the magnetization of nanoscale ferromagnets in superimposed ac and dc bias fields in the presence of thermal agitation,"Spin-transfer torque (STT) effects on the stationary forced response of nanoscale ferromagnets subject to thermal fluctuations and driven by an ac magnetic field of arbitrary strength and direction are investigated via a generic nanopillar model of a spin-torque device comprising two ferromagnetic strata representing the free and fixed layers and a nonmagnetic conducting spacer all sandwiched between two ohmic contacts. The STT effects are treated via the Brown magnetic Langevin equation generalized to include the Slonczewski STT term thereby extending the statistical moment method [Y. P. Kalmykov et al., Phys. Rev. B 88, 144406 (2013)] to the forced response of the most general version of the nanopillar model. The dynamic susceptibility, nonlinear frequency-dependent dc magnetization, dynamic magnetic hysteresis loops, etc. are then evaluated highlighting STT effects on both the low-frequency thermal relaxation processes and the high-frequency ferromagnetic resonance, etc., demonstrating a pronounced dependence of these on the spin polarization current and facilitating interpretation of STT experiments.",1504.02319v2 2015-04-10,Enhancement of the Anti-Damping Spin Torque Efficacy of Platinum by Interface Modification,"We report a strong enhancement of the efficacy of the spin Hall effect (SHE) of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer by the insertion of $\approx$ 0.5 nm layer of Hf between a Pt film and a thin, < 2 nm, Fe$_{60}$Co$_{20}$B$_{20}$ ferromagnetic layer. This enhancement is quantified by measurement of the switching current density when the ferromagnetic layer is the free electrode in a magnetic tunnel junction. The results are explained as the suppression of spin pumping through a substantial decrease in the effective spin-mixing conductance of the interface, but without a concomitant reduction of the ferromagnet\' s absorption of the SHE generated spin current.",1504.02806v1 2015-05-01,Exposure of the Hidden Anti-Ferromagnetism in Paramagnetic CdSe:Mn Nanocrystals,"We present theoretical and experimental investigations of the magnetism of paramagnetic semiconductor CdSe:Mn nanocrystals and propose an efficient approach to the exposure and analysis of the underlying anti-ferromagnetic interactions between magnetic ions therein. A key advance made here is the build-up of an analysis method with the exploitation of group theory technique that allows us to distinguish the anti-ferromagnetic interactions between aggregative Mn2+ ions from the overall pronounced paramagnetism of magnetic ion doped semiconductor nanocrystals. By using the method, we clearly reveal and identify the signatures of anti-ferromagnetism from the measured temperature dependent magnetisms, and furthermore determine the average number of Mn2+ ions and the fraction of aggregative ones in the measured CdSe:Mn nanocrystals.",1505.00126v1 2015-05-05,Spin pumping from a ferromagnet into a hopping insulator: the role of resonant absorption of magnons,"Motivated by recent experiments on spin pumping from a ferromagnet into organic materials in which the charge transport is due to hopping, we study theoretically the generation and propagation of spin current in a hopping insulator. Unlike metals, the spin polarization at the boundary with ferromagnet is created as a result of magnon absorption within pairs of localized states and it spreads following the current-currying resistor network (although the charge current is absent). We consider a classic resonant mechanism of the ac absorption in insulators and adapt it to the absorption of magnons. A strong enhancement of pumping efficiency is predicted when the Zeeman splitting of the localized states in external magnetic field is equal to the frequency of ferromagnetic resonance. Under this condition the absorption of a magnon takes place within individual sites.",1505.01211v1 2015-05-06,Magnetoresistance control in granular Zn/1-x-y/Cd/x/Mn/y/GeAs/2/ nanocomposite ferromagnetic semiconductors,"We present studies of structural, magnetic and electrical properties of Zn/1-x-y/Cd/x/Mn/y/GeAs/2/ nanocomposite ferromagnetic semiconductor samples with changeable chemical composition. The presence of MnAs clusters induces in the studied alloy room temperature ferromagnetism with the Curie temperature, TC, around 305 K. The chemical composition of the chalcopyrite matrix controls the geometrical parameters of the clusters inducing different magnetoresistance effects in the crystals. The presence of ferromagnetic clusters in the alloy induces either negative or positive magnetoresistance with different values. The Cd-content allows a change of magnetoresistance sign in our samples from negative (for x = 0.85) to positive (for x = 0.12). The negative magnetoresistance present in the samples with x = 0.85 is observed at temperatures T < 25 K with maximum values of about -32% at T = 1.4 K and B = 13 T, strongly depending on the Mn content, y. The positive magnetoresistance present in the samples with x = 0.12 is observed with maximum values not exceeding 50% at B =13 T and T = 4.3 K, changing with the Mn content, y.",1505.01357v1 2015-05-08,Itinerant Ferromagnetism and $p+ip'$ Superconductivity in Doped Bilayer Silicene,"We study the electronic instabilities of doped bilayer silicene using the random phase approximation. In contrast to the singlet $d+id'$ superconductivity at the low doping region, we find that the system is an itinerant ferromagnet in the narrow doping regions around the Van Hove singularities, and a triplet $p+ip'$ superconductor in the vicinity of these regions. Adding a weak Kane-Mele spin-orbit coupling to the system further singles out the time-reversal invariant equal-spin helical $p+ip'$ pairing as the leading instability. The triplet pairing identified here is driven by the ferromagnetic fluctuations, which become strong and enhance the superconducting critical temperature remarkably near the phase boundaries between ferromagnetism and superconductivity.",1505.01971v1 2015-05-18,Evidence on the macroscopic length scale spin coherence for the edge currents in a narrow HgTe quantum well,"We experimentally investigate spin-polarized electron transport between two ferromagnetic contacts, placed at the edge of a two-dimensional electron system with band inversion. The system is realized in a narrow (8~nm) HgTe quantum well, the ferromagnetic side contacts are formed from a pre-magnetized permalloy film. In zero magnetic field, we find a significant edge current contribution to the transport between two ferromagnetic contacts. We experimentally demonstrate that this transport is sensitive to the mutual orientation of the magnetization directions of two 200~$\mu$m-spaced ferromagnetic leads. This is a direct experimental evidence on the spin-coherent edge transport over the macroscopic distances. Thus, the spin is extremely robust at the edge of a two-dimensional electron system with band inversion, confirming the helical spin-resolved nature of edge currents.",1505.04535v1 2015-05-21,Ferromagnetism in Cr-doped topological insulator TlSbTe2,"We have synthesized a new ferromagnetic topological insulator by doping Cr to the ternary topological-insulator material TlSbTe2. Single crystals of Tl_{1-x}Cr_{x}SbTe2 were grown by a melting method and it was found that Cr can be incorporated into the TlSbTe2 matrix only within the solubility limit of about 1%. The Curie temperature \theta_c was found to increase with the Cr content but remained relatively low, with the maximum value of about 4 K. The easy axis was identified to be the c-axis and the saturation moment was 2.8 \mu_B (Bohr magneton) at 1.8 K. The in-plane resistivity of all the samples studied showed metallic behavior with p-type carriers. Shubnikov-de Hass (SdH) oscillations were observed in samples with the Cr-doping level of up to 0.76%. We also tried to induce ferromagnetism in TlBiTe2 by doping Cr, but no ferromagnetism was observed in Cr-doped TlBiTe2 crystals within the solubility limit of Cr which turned out to be also about 1%.",1505.05631v1 2015-05-26,High-$T_c$ $d^{0}$ ferromagnetism in a doped Mott insulator: the case of hydrogenated epitaxial graphene on SiC(0001),"We show that the $d^{0}$ ferromagnetism with high Curie temperature ($T_c$) can be achieved in the electron doped hydrogenated epitaxial graphene on some certain SiC substrates through first-principles calculations. The pristine systems are found to be a Mott insulator independent of SiC polytypes (2$H$, 4$H$ or 6$H$) which, however, play a significant role in the modulation of magnetic interaction. Carrier doping enhances the ferromagnetic coupling due to the double exchange mechanism and thus realizes the phase transition from antiferromagnetism to ferromagnetism. A $T_c$ of around 400 K is predicted on the 2$H$-SiC. We employ a non-degenerate Hubbard model to demonstrate how the SiC affects the interfacial magnetism in intra-atomic Coulomb repulsion and intersite hopping interactions.",1505.06823v1 2015-05-28,Incommensurate spin density wave at a ferromagnetic quantum critical point in a three-dimensional parabolic semimetal,"We explore the ferromagnetic quantum critical point in a three-dimensional semimetallic system with upward- and downward-dispersing bands touching at the Fermi level. Evaluating the static spin susceptibility to leading order in the coupling between the fermions and the fluctuating ferromagnetic order parameter, we find that the ferromagnetic quantum critical point is masked by an incommensurate, longitudinal spin density wave phase. We first analyze an idealized model which, despite having strong spin-orbit coupling, still possesses O(3) rotational symmetry generated by the total angular momentum operator. In this case, the direction of the incommensurate spin density wave propagation can point anywhere, while the magnetic moment is aligned along the direction of propagation. Including symmetry-allowed anisotropies in the fermion dispersion and the coupling to the order parameter field, however, the ordering wavevector instead breaks a discrete symmetry and aligns along either the [111] or [100] direction, depending on the signs and magnitudes of these two types of anisotropy.",1505.07685v1 2015-06-01,Magnetism in Na-filled Fe-based skutterudites,"The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. Here we investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe4Sb12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prior work. In accord with prior work, the magnetization is overestimated, as expected for a material near an itinerant ferromagnetic quantum critical point. NaFe4P12 also shows a ferromagnetic instability at the density functional level, but this instability is much weaker than that of NaFe4Sb12, possibly placing it on the paramagnetic side of the quantum critical point. NaFe4As12 shows intermediate behavior. We also present results for skutterudite FeSb3, which is a metastable phase that has been reported in thin film form.",1506.00514v1 2015-06-02,Screened moments and absence of ferromagnetism in FeAl,"While the stoichiometric intermetallic compound FeAl is found to be paramagnetic in experiment, standard band-theory approaches predict the material to be ferromagnetic. We show that this discrepancy can be overcome by a better treatment of electronic correlations with density functional plus dynamical mean field theory. Our results show no ferromagnetism down to 100 K and since the susceptibility is decreasing at the lowest temperatures studied we also do not expect ferromagnetism at even lower temperatures. This behavior is found to originate from temporal quantum fluctuations that screen short-lived local magnetic moments of 1.6 $\mu_B$ on Fe.",1506.00908v2 2015-06-09,Spin dynamics in a Curie-switch,"Ferromagnetic resonance properties of F$_1$/f/F$_2$/AF multilayers, where weakly ferromagnetic spacer f is sandwiched between strongly ferromagnetic layers F$_1$ and F$_2$, with F$_1$ being magnetically soft and F$_2$ - magnetically hard due to exchange pinning to antiferromagnetic layer AF, are investigated. Spacer-mediated exchange coupling is shown to strongly affect the resonance fields of both F$_1$ and F$_2$ layers. Our theoretical calculations as well as measurements show that the key magnetic parameters of the spacer, which govern the ferromagnetic resonance in F$_1$/f/F$_2$/AF, are the magnetic exchange length ($\Lambda$), effective saturation magnetization at $T=0$ $(m_0)$, and effective Curie temperature ($T_{\text{C}}^{\text{eff}}$). The values of these key parameters are deduced from the experimental data for multilayers with f = Ni$_x$Cu$_{100-x}$, for the key ranges in Ni-concentration ($x=54\div70$ at. %) and spacer thickness ($d=3\div 6$ nm). The results obtained provide a deeper insight into thermally-controlled spin precession and switching in magnetic nanostructures, with potential applications in spin-based oscillators and memory devices.",1506.02928v1 2015-06-23,Monte Carlo Methods for the Ferromagnetic Potts Model Using Factor Graph Duality,"Normal factor graph duality offers new possibilities for Monte Carlo algorithms in graphical models. Specifically, we consider the problem of estimating the partition function of the ferromagnetic Ising and Potts models by Monte Carlo methods, which are known to work well at high temperatures, but to fail at low temperatures. We propose Monte Carlo methods (uniform sampling and importance sampling) in the dual normal factor graph, and demonstrate that they behave differently: they work particularly well at low temperatures. By comparing the relative error in estimating the partition function, we show that the proposed importance sampling algorithm significantly outperforms the state-of-the-art deterministic and Monte Carlo methods. For the ferromagnetic Ising model in an external field, we show the equivalence between the valid configurations in the dual normal factor graph and the terms that appear in the high-temperature series expansion of the partition function. Following this result, we discuss connections with Jerrum-Sinclair's polynomial randomized approximation scheme (the subgraphs-world process) for evaluating the partition function of ferromagnetic",1506.07044v4 2015-06-25,Quantum Hall ferromagnets and transport properties of buckled Dirac materials,"We study the ground states and low-energy excitations of a generic Dirac material with spin-orbit coupling and a buckling structure in the presence of a perpendicular magnetic field. The ground states can be classified into three types under different conditions: SU(2), easy-plane, and Ising quantum Hall ferromagnets. For the SU(2) and the easy-plane quantum Hall ferromagnets there are goldstone modes in the collective excitations, while all the modes are gapped in an Ising-type ground state. We compare the Ising quantum Hall ferromagnet with that of bilayer graphene and present the domain wall solution at finite temperatures. We then specify the phase transitions and transport gaps in silicene in Landau levels 0 and 1. The phase diagram strongly depends on the magnetic field and the dielectric constant. We note that there exists triple points in the phase diagrams in Landau level N = 1 that could be observed in experiments.",1506.07896v1 2015-07-03,Magnetization switching by spin-orbit torque in an antiferromagnet/ferromagnet bilayer system,"Spin-orbit torque (SOT)-induced magnetization switching shows promise for realizing ultrafast and reliable spintronics devices. Bipolar switching of perpendicular magnetization via SOT is achieved under an in-plane magnetic field collinear with an applied current. Typical structures studied so far comprise a nonmagnet/ferromagnet (NM/FM) bilayer, where the spin Hall effect in the NM is responsible for the switching. Here we show that an antiferromagnet/ferromagnet (AFM/FM) bilayer system also exhibits a SOT large enough to switch the magnetization of FM. In this material system, thanks to the exchange-bias effect of the AFM, we observe the switching under no applied field by using an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer with a perpendicular easy axis. Furthermore, tailoring the stack achieves a memristor-like behaviour where a portion of the reversed magnetization can by controlled in an analogue manner. The AFM/FM system is thus a promising building block for SOT devices as well as providing an attractive pathway towards neuromorphic computing.",1507.00888v1 2015-07-21,Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases,"Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases, but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.",1507.05907v1 2015-07-23,Ba{0.4}Rb{0.6}Mn2As2: A Prototype Half-Metallic Ferromagnet,"Half-metallic ferromagnetism (FM) in single-crystal Ba{0.39(1)}Rb{0.61(1)}Mn2As2 below its Curie temperature TC = 103(2) K is reported. The magnetization M versus applied magnetic field H isotherm data at 1.8 K show complete polarization of the itinerant doped-hole magnetic moments that are introduced by substituting Rb for Ba. The material exhibits extremely soft FM, with unobservably small remanent magnetization and coercive field. Surprisingly, and contrary to typical itinerant FMs, the M(H) data follow the Arrott-plot paradigm that is based on a mean-field theory of local-moment FMs. The in-plane electrical resistivity data are fitted well by an activated-T^2 expression for T < TC, whereas the data sharply deviate from this model for T > TC. Hence the activated-T^2 resistivity model is an excellent diagnostic for determining the onset of half-metallic FM in this compound, which in turn demonstrates the presence of a strong correlation between the electronic transport and magnetic properties of the material. Together with previous data on 40% hole-doped Ba{0.6}K{0.4}Mn2As2, these measurements establish 61%-doped Ba{0.39}Rb{0.61}Mn2As2 as a prototype for a new class of half-metallic ferromagnets in which all the itinerant carriers in the material are ferromagnetic.",1507.06679v1 2015-08-05,Inverse proximity effect at superconductor-ferromagnet interfaces: Evidence for induced triplet pairing in the superconductor,"Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coherence peaks and, in some cases, a zero-bias conductance peak. As the field increases to the magnetic saturation of LCMO where the magnetization is homogeneous, the spectra become more BCS-like and the critical temperature of the NbN increases, implying a reduced proximity effect. Our results therefore suggest that triplet-pairing correlations are also induced in the S side of an S-F bilayer.",1508.01070v1 2015-08-05,Comparison of the ferromagnetic Blume-Emery-Griffiths model and the AF spin-1 longitudinal Ising model at low temperature,"We derive the exact Helmholtz free energy (HFE) of the standard and staggered one-dimensional Blume-Emery-Griffiths (BEG) model in the presence of an external longitudinal magnetic field. We discuss in detail the thermodynamic behavior of the ferromagnetic version of the model, which exhibits magnetic field-dependent plateaux in the $z$-component of its magnetization at low temperatures. We also study the behavior of its specific heat and entropy, both per site, at finite temperature. The degeneracy of the ground state, at $T=0$, along the lines that separate distinct phases in the phase diagram of the ferromagnetic BEG model is calculated, extending the study of the phase diagram of the spin-1 antiferromagnetic (AF) Ising model in[S.M. de Souza and M.T. Thomaz, J. of Mag. and Mag. Mat. {354} (2014) 205]. We explore the implications of the equality of phase diagrams, at $T=0$, of the ferromagnetic BEG model with $\frac{K}{|J|} = -2$ and of the spin-1 AF Ising model for $\frac{D}{|J|} > \frac{1}{2}$.",1508.01245v1 2015-08-14,Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductor ErNi2B2C,"We present a local probe study of the magnetic superconductor, ErNi$_2$B$_2$C, using magnetic force microscopy at sub-Kelvin temperatures. ErNi$_2$B$_2$C is an ideal system to explore the effects of concomitant superconductivity and ferromagnetism. At 500 mK, far below the transition to a weakly ferromagnetic state, we directly observe a structured magnetic background on the micrometer scale. We determine spatially resolved absolute values of the magnetic penetration depth $\lambda$ and study its temperature dependence as the system undergoes magnetic phase transitions from paramagnetic to antiferromagnetic, and to weak ferromagnetic, all within the superconducting regime. In addition, we estimate the absolute pinning force of Abrikosov vortices, which shows a position- and temperature dependence as well, and discuss the possibility of the purported spontaneous vortex formation.",1508.03462v1 2015-09-03,Investigation of ferromagnetic domain behavior and phase transition at nanoscale in bilayer manganites,"Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x=0.38$, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curie temperature within the $a-b$ plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. The ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.",1509.01106v1 2015-09-06,Study of spin dynamics and damping on the magnetic nanowire arrays with various nanowire widths,"We investigate the spin dynamics including Gilbert damping in the ferromagnetic nanowire arrays. We have measured the ferromagnetic resonance of ferromagnetic nanowire arrays using vector-network analyzer ferromagnetic resonance (VNA-FMR) and analyzed the results with the micromagnetic simulations. We find excellent agreement between the experimental VNA-FMR spectra and micromagnetic simulations result for various applied magnetic fields. We find that the demagnetization factor for longitudinal conditions, Nz (Ny) increases (decreases) as decreasing the nanowire width in the micromagnetic simulations. For the transverse magnetic field, Nz (Ny) increases (decreases) as increasing the nanowire width. We also find that the Gilbert damping constant increases from 0.018 to 0.051 as the increasing nanowire width for the transverse case, while it is almost constant as 0.021 for the longitudinal case.",1509.01807v1 2015-09-11,"Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb","We investigate the crystal structure, transport and magnetic properties of Fe-doped ferromagnetic semiconductor (Al1-x,Fex)Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al1-x,Fex)Sb thin films with x lower or equal to 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al1-x,Fex)Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (TC) of 40 K. In the (Al1-x,Fex)Sb thin film with x = 14%, a sudden drop of mobility and TC was observed, which may be due to microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices.",1509.03430v1 2015-09-22,Field-effect Modulation of Anomalous Hall Effect in Diluted Ferromagnetic Topological Insulator Epitaxial Films,"High quality chromium (Cr) doped three-dimensional topological insulator (TI) Sb2Te3 films are grown via molecular beam epitaxy on heat-treated insulating SrTiO3(111) substrates. We report that the Dirac surface states are insensitive to Cr doping, and a perfect robust long-range ferromagnetic order is unveiled in epitaxial Sb2-xCrxTe3 films. The anomalous Hall effect is modulated by applying a bottom gate, contrary to the ferromagnetism in conventional diluted magnetic semiconductors (DMSs), here the coercivity field is not significantly changed with decreasing carrier density. Carrier-independent ferromagnetism heralds Sb2-xCrxTe3 films as the base candidate TI material to realize the quantum anomalous Hall (QAH) effect. These results also indicate the potential of controlling anomalous Hall voltage in future TI-based magneto-electronics and spintronics.",1509.06608v1 2015-10-04,Tunneling Magnetoresistance in Junctions Composed of Ferromagnets and Time-Reversal Invariant Topological Superconductors,"Tunneling Magnetoresistance between two ferrromagnets is an issue of fundamental importance in spintronics. In this work, we show that tunneling magnetoresistance can also emerge in junctions composed of ferromagnets and time-reversal invariant topological superconductors without spin-rotation symmetry. Here the physical origin is that when the spin-polarization direction of injected electron from the ferromagnet lying in the same plane of the spin-polarization direction of Majorana zero modes, the electron will undergo a perfect spin-equal Andreev reflection, while injected electrons with other spin-polarization direction will be partially Andreev reflected and partially normal reflected, which consequently have a lower conductance, and therefore, the magnetoresistance effect emerges. Compared to conventional magnetic tunnel junctions, an unprecedented advantage of the junctions studied here is that arbitrary high tunneling magnetoresistance can be obtained even the magnetization of the ferromagnets are weak and the insulating tunneling barriers are featureless. Our findings provide a new fascinating mechanism to obtain high tunneling magnetoresistance.",1510.00901v2 2015-10-07,Towards ferromagnetic quantum criticality in FeGa3-xGex: 71Ga NQR as a zero field microscopic probe,"71Ga NQR, magnetization and specific heat measurements have been performed on polycrystalline Ge doped FeGa3 samples. A crossover from an insulator to a correlated local moment metal in the low doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, xC=0.15, (1/T1T) exhibits a pronounced T4/3 power law over two orders of magnitude in temperature which indicates 3D quantum critical ferromagnetic fluctuations. Furthermore, for the ordered x = 0.2 sample (TC = 6 K) 1/T1T could be fitted well in the frame of Moriya's SCR theory for weakly FM systems with 1/T1T proportional to chi. In contrast to this, the low doping regime nicely displays local moment behavior where 1/T1T proportional to chi2 is valid. For temperature tending to zero the Sommerfeld ratio = (C/T) is enhanced (70 mJ/mole-K2 for x = 0.1) which indicates the formation of heavy 3d-electrons.",1510.01974v1 2015-10-07,Spontaneous Liquid Crystal and Ferromagnetic Ordering of Colloidal Magnetic Nanoplates,"Ferrofluids are familiar as colloidal suspensions of ferromagnetic nanoparticles in aqueous or organic solvents. The dispersed particles are randomly oriented but their moments become aligned if a magnetic field is applied, producing a variety of exotic and useful magneto-mechanical effects. A longstanding interest and challenge has been to make such suspensions macroscopically ferromagnetic, that is having uniform magnetic alignment in absence of a field. Here we report a fluid suspension of magnetic nanoplates which spontaneously aligns into an equilibrium nematic liquid crystal phase that is also macroscopically ferromagnetic. Its zero-field magnetization produces distinctive magnetic self-interaction effects, including liquid crystal textures of fluid block domains arranged in closed flux loops, and makes this phase highly sensitive, with it dramatically changing shape even in the Earth's magnetic field.",1510.02161v2 2015-10-26,Doping-Induced Ferromagnetism and Possible Triplet Pairing in d4 Mott Insulators,"We study the effects of electron doping in Mott insulators containing d^4 ions such as Ru4+, Os4+, Rh5+, and Ir5+ with J=0 singlet ground state. Depending on the strength of the spin-orbit coupling, the undoped systems are either nonmagnetic or host an unusual, excitonic magnetism arising from a condensation of the excited J=1 triplet states of t_2g^4. We find that the interaction between J-excitons and doped carriers strongly supports ferromagnetism, converting both the nonmagnetic and antiferromagnetic phases of the parent insulator into a ferromagnetic metal, and further to a nonmagnetic metal. Close to the ferromagnetic phase, the low-energy spin response is dominated by intense paramagnon excitations that may act as mediators of a triplet pairing.",1510.07508v2 2015-10-28,Magnetic Impurity in a Tonks Gas of Fermions,"In this letter we consider a magnetic impurity in a one-dimensional spin-$1/2$ Fermi gas with infinitely strong repulsive interaction between fermions. We rigorously prove that, independent of whether the magnetic coupling between impurity and fermions is ferromagnetic or anti-ferromagnetic, the ground state is always a fully polarized ferromagnetic state for the itinerant fermions. This ferromagnetism can be understood as a cooperative effect of avoiding frustration of magnetic coupling during fermion hopping and the large spin degeneracy of a fermion Tonks gas. By numerically diagonalizing a finite size system, we show that the spin gap first increases linearly with magnetic coupling strength in the weak coupling regime, while decreases in the strong coupling regime. Our results show that a magnetic impurity in a strongly correlated gas can exhibit effect different from the Kondo effect as in a weakly correlated Fermi liquid.",1510.08303v2 2015-11-07,Quantum triangular ice in the easy-axis ferromagnetic phase,"We use spin wave theory to investigate the ground state properties of the $Z_2$-invariant quantum XXZ model on the triangular lattice in the ferromagnetic phase. The Hamiltonian comprises nearest and next-nearest-neighbour Ising couplings, external magnetic fields, and a $Z_2$-invariant ferromagnetic coupling. We show that quantum fluctuations are suppressed in this system, hence linear spin wave theory gives reasonable estimates of the ground state thermodynamic properties. Our results show that, at half-filling (zero magnetic fields), the spontaneous breaking of $Z_2$ symmetry leads to a ferromagnetic phase whose energy spectrum is gapped at all excitations with a maxon dispersion at $\mathbf{k}=0$. This is in sharp contrast to rotational invariant systems with a vanishing phonon dispersion. We show that the $\mathbf{k}=0$ mode enhances the estimated values of the thermodynamic quantities. We obtain the trends of the particle density and the condensate fraction. The density of states and the dynamical structure factors exhibit fascinating peaks at unusual wave vectors, which should be of interest.",1511.02311v3 2016-01-14,Topological textures and their bifurcation processes in 2D ferromagnetic thin films,"In this paper, by the use of the topological current theory, the topological structures and the dynamic processes in thin-film ferromagnetic systems are investigated directly from viewpoint of topology. It is found that the topological charge of a thin-film ferromagnetic system can be changed by annihilation or creation processes of opposite polarized vortex-antivortex pairs taking place at space-time singularities of the normalized magnetization vector field of the system, the variation of the topological charge is integer and can further be expressed in terms of the Hopf indices and Brouwer degrees of the magnetization vector field around the singularities. Moreover, the change of the topological charge of the system is crucial to vortex core reversal processes in ferromagnetic thin films. With the help of the topological current theory and implicit function theorem, the processes of vortex merging, splitting as well as vortex core",1601.03487v1 2016-01-18,Incommensurate spin-density-wave antiferromagnetism in CeRu$_2$Al$_2$B,"The newly discovered Ising-type ferromagnet CeRu$_2$Al$_2$B exhibits an additional phase transition at $T_\textrm{N}$ = 14.2 K before entering the ferromagnetic ground state at $T_\textrm{C}$ = 12.8 K. We clarify the nature of this transition through high resolution neutron diffraction measurements. The data reveal the presence of a longitudinal incommensurate spin-density wave (SDW) in the temperature range of $T_\textrm{C}0$ the switching fields are essentially independent of $J$ but the frequency of the optical eigenmode of the bilayer and the linewidth of the acoustical and optical eigenmode are directly sensitive to the coupling. We derive a corpus of compact analytical expressions to analyze these frequencies, their linewidth and discuss the meaning thereof. We illustrate this corpus on a system mimicking the fixed layers of a magnetic tunnel junction meant for spin torque applications.",1604.06332v1 2016-04-29,Magneto-Elastic Coupling in a potential ferromagnetic 2D Atomic Crystal,"Cr2Ge2Te6 has been of interest for decades, as it is one of only a few naturally forming ferromagnetic semiconductors. Recently, this material has been revisited due to its potential as a 2 dimensional semiconducting ferromagnet and a substrate to induce anomalous quantum Hall states in topological insulators. However, many relevant properties of Cr2Ge2Te6 still remain poorly understood, especially the spin-phonon coupling crucial to spintronic, multiferrioc, thermal conductivity, magnetic proximity and the establishment of long range order on the nanoscale. We explore the interplay between the lattice and magnetism through high resolution micro-Raman scattering measurements over the temperature range from 10 K to 325 K. Strong spin-phonon coupling effects are confirmed from multiple aspects: two low energy modes splits in the ferromagnetic phase, magnetic quasielastic scattering in paramagnetic phase, the phonon energies of three modes show clear upturn below Tc, and the phonon linewidths change dramatically below Tc as well. Our results provide the first demonstration of spin-phonon coupling in a potential 2 dimensional atomic crystal.",1604.08745v1 2016-06-02,Band structure and spin texture of Bi$_2$Se$_3$/3d ferromagnetic metal interface,"The spin-helical surface states in three-dimensional topological insulator (TI), such as Bi2Se3, are predicted to have superior efficiency in converting charge current into spin polarization. This property is said to be responsible for the giant spin-orbit torques observed in ferromagnetic metal/TI structures. In this work, using first-principles and model tight-binding calculations, we investigate the interface between the topological insulator Bi2Se3 and 3d-transition ferromagnetic metals Ni and Co. We find that the difference in the work functions of the topological insulator and the ferromagnetic metals shift the topological surface states down about 0.5 eV below the Fermi energy where the hybridization of these surface states with the metal bands destroys their helical spin structure. The band alignment of Bi2Se3 and Ni (Co) places the Fermi energy far in the conduction band of bulk Bi2Se3, where the spin of the carriers is aligned with the magnetization in the metal. Our results indicate that the topological surface states are unlikely to be responsible for the huge spin-orbit torque effect observed experimentally in these systems.",1606.00763v1 2016-06-24,Tunable magnon Weyl points in ferromagnetic pyrochlores,"The dispersion relations of magnons in ferromagnetic pyrochlores with Dzyaloshinskii-Moriya interaction is shown to possess Weyl points, i.\,e., pairs of topological nontrivial crossings of two magnon branches with opposite topological charge. As a consequence of their topological nature, their projections onto a surface are connected by magnon arcs, thereby resembling closely Fermi arcs of electronic Weyl semimetals. On top of this, the positions of the Weyl points in reciprocal space can be tuned widely by an external magnetic field: rotated within the surface plane, the Weyl points and magnon arcs are rotated as well; tilting the magnetic field out-of-plane shifts the Weyl points toward the center $\bar{\Gamma}$ of the surface Brillouin zone. The theory is valid for the class of ferromagnetic pyrochlores, i.\,e., three-dimensional extensions of topological magnon insulators on kagome lattices. In this Letter, we focus on the $(111)$ surface, identify candidates of established ferromagnetic pyrochlores which apply to the considered spin model, and suggest experiments for the detection of the topological features.",1606.07612v1 2016-07-13,Surface state dominated spin-charge current conversion in topological insulator/ferromagnetic insulator heterostructures,"We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films -- Bi2Se3 and (Bi,Sb)2Te3 -- deposited by molecular beam epitaxy on yttrium iron garnet thin films. By systematically varying the Bi2Se3 film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (lambda_IREE), increases dramatically as the film thickness is increased from 2 quintuple layers, saturating above 6 quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological insulator/ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of YIG/(BiSb)2Te3 heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and lambda_IREE.",1607.03872v1 2016-07-15,Chiral Phonons and Electrical Resistivity of Ferromagnetic Metals at Low Temperatures,"Ferromagnetism is an exciting phase of matter exhibiting strongly correlated electron behavior and a standard example of spontaneously broken rotational symmetry: below the Curie temperature, atomic magnets in an isotropic single-domain ferromagnetic metal align along a spontaneously chosen direction. The scattering of conduction electrons from thermal perturbations to this spin order, together with electron-electron collisions, mark the material electrical behavior at low temperatures, where the resistivity varies mostly quadratically with the temperature. Around liquid-helium temperatures however, an interesting phenomenon occurs, giving rise to an extra \emph{linear} contribution to the variation of the electrical resistivity with temperature, whose theoretical explanation has encountered problems for a long time. Here I introduce a spin-flip scattering mechanism of conduction electrons in ferromagnetic metals arising from their interaction with the internal magnetic induction and mediated by chiral modes of the crystal lattice vibrations carrying spin 1. This mechanism is able to explain the above anomaly and give a good account of the spin-lattice relaxation times of iron, cobalt and nickel at room temperatures.",1607.04585v2 2016-07-21,Collapse of ferromagnetism and Fermi surface instability near reentrant superconductivity of URhGe,"We present thermoelectric power and resistivity measurements in the ferromagnetic superconductor URhGe for magnetic field applied along the hard magnetization b axis of the orthorhombic crystal. Reentrant superconductivity is observed near the the spin reorientation transition at $H_{R}$=12.75 T, where a first order transition from the ferromagnetic to the polarized paramagnetic state occurs. Special focus is given to the longitudinal configuration, where both electric and heat current are parallel to the applied field. The validity of the Fermi-liquid $T^2$ dependence of the resistivity through $H_R$ demonstrates clearly that no quantum critical point occurs at $H_R$. Thus the ferromagnetic transition line at $H_R$ becomes first order implying the existence of a tricritical point at finite temperature. The enhancement of magnetic fluctuations in the vicinity of the tricritical point stimulates the reentrance of superconductivity. The abrupt sign change observed in the thermoelectric power with the thermal gradient applied along the b axis together with the strong anomalies in the other directions is a definitive macroscopic evidence that in addition a significant change of the Fermi surface appears through $H_R$.",1607.06399v1 2016-07-29,Weak de-localization in graphene on a ferromagnetic insulating film,"Graphene has been predicted to develop a magnetic moment by proximity effect when placed on a ferromagnetic film, a promise that could open exciting possibilities in the fields of spintronics and magnetic data recording. In this work, we study in detail the interplay between the magnetoresistance of graphene and the magnetization of an underlying ferromagnetic insulating film. A clear correlation between both magnitudes is observed but we find, through a careful modelling of the magnetization and the weak localization measurements, that such correspondence can be explained by the effects of the magnetic stray fields arising from the ferromagnetic insulator. Our results emphasize the complexity arising at the interface between magnetic and two-dimensional materials.",1607.08772v1 2016-10-12,Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure,"We investigate a spin-electricity conversion effect in a topological insulator/ferromagnet heterostructure. In the spin-momentum-locked surface state, an electric current generates nonequilibrium spin accumulation, which causes a spin-orbit torque that acts on the ferromagnet. When spins in the ferromagnet are completely parallel to the accumulated spin, this spin-orbit torque is zero. In the presence of spin excitations, however, a coupling between magnons and electrons enables us to obtain a nonvanishing torque. In this paper, we consider a model of the heterostructure in which a three-dimensional magnon gas is coupled with a two-dimensional massless Dirac electron system at the interface. We calculate the torque induced by an electric field, which can be interpreted as a magnon spin current, up to the lowest order of the electron-magnon interaction. We derive the expressions for high and low temperatures and estimate the order of magnitude of the induced spin current for realistic materials at room temperature.",1610.03636v3 2016-10-21,Spin transport and dynamics in all-oxide perovskite La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ bilayers probed by ferromagnetic resonance,"Thin films of perovskite oxides offer the possibility of combining emerging concepts of strongly correlated electron phenomena and spin current in magnetic devices. However, spin transport and magnetization dynamics in these complex oxide materials are not well understood. Here, we experimentally quantify spin transport parameters and magnetization damping in epitaxial perovskite ferromagnet/paramagnet bilayers of La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ (LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO thickness dependence of Gilbert damping, we estimate a short spin diffusion length of $\lesssim$1 nm in SRO and an interfacial spin-mixing conductance comparable to other ferromagnet/paramagnetic-metal bilayers. Moreover, we find that anisotropic non-Gilbert damping due to two-magnon scattering also increases with the addition of SRO. Our results demonstrate LSMO/SRO as a spin-source/spin-sink system that may be a foundation for examining spin-current transport in various perovskite heterostructures.",1610.06661v1 2017-02-10,Orbital and spin order in spin-orbit coupled $d^1$ and $d^2$ double perovskites,"We consider strongly spin-orbit coupled double perovskites A$_2$BB'O$_6$ with B' magnetic ions in either $d^1$ or $d^2$ electronic configuration and non-magnetic B ions. We provide insights into several experimental puzzles, such as the predominance of ferromagnetism in $d^1$ versus antiferromagnetism in $d^2$ systems, the appearance of negative Curie-Weiss temperatures for ferromagnetic materials, and the size of effective magnetic moments. We develop and solve a microscopic model with both spin and orbital degrees of freedom within the Mott insulating regime at finite temperature using mean field theory. The interplay between anisotropic orbital degrees of freedom and spin-orbit coupling results in complex ground states in both $d^1$ and $d^2$ systems. We show that the ordering of orbital degrees of freedom in $d^1$ systems results in coplanar canted ferromagnetic and 4-sublattice antiferromagnetic structures. In $d^2$ systems we find additional colinear antiferromagnetic and ferromagnetic phases not appearing in $d^1$ systems. At finite temperatures, we find that orbital ordering driven by both superexchange and Coulomb interactions may occur at much higher temperatures compared to magnetic order and leads to distinct deviations from Curie-Weiss law.",1702.03199v1 2017-02-12,"Micromagnetic study of a feasibility of the magnetic anisotropy engineering in nano-structured epitaxial films of (III,Mn)V ferromagnetic semiconductors","The attainability of modification of the apparent magnetic anisotropy in (III,Mn)V ferromagnetic semiconductors is probed by means of the finite-elements-based modelling. The most representative case of (Ga,Mn)As and its in-plane uniaxial anisotropy is investigated. The hysteresis loops of the continuous films of a ferromagnetic semiconductor as well as films structured with the elliptic antidots are modelled for various eccentricity, orientation, and separation of the anti dots. The effect of anti-dots on the magnetic anisotropy is confirmed but overall is found to be very weak. The subsequent modelling for (Ga,Mn)As film with the elliptic dots comprising of metallic NiFe shows much stronger effect, revealing switching of the magnetic moment in the ferromagnetic semiconductor governed by the switching behavior of the metallic inclusions.",1702.03509v1 2017-03-25,$d^0$ half-metallic ferromagnetism in CaN and CaAs pnictides: An ab initio study,"Conventional magnetism occurs in systems which contain transition metals or rare earth ions with partially filled $d$ or $f$ shells. It is theoretically predicted that compounds of groups IA and IIA with IV and V, in some structural phases, are ferromagnetic half-metals which made them new candidates for spintronics applications. Employing density functional theory (DFT) we investigate magnetism in binary compounds CaN and CaAs. Regarding the structure of analogous magnetic materials and experimental results of CaAs synthesis, we have considered two cubic structures: rocksalt (RS) and zincblende (ZB), and four hexagonal structures: NiAs, wurtzite (WZ), anti-NiAs, and NaO. The calculated results show that CaN in cubic, NiAs, and wurtzite structures, and CaAs only in zincblende phase have ferromagnetic ground states with a magnetic moment of $1\mu _B$. Electronic structure analysis of these materials indicates that magnetism originates from anion $p$ states. Existence of flat $p$ bands and consequently high density of states at the Fermi level of magnetic structures gives rise to Stoner spin splitting and spontaneous ferromagnetism.",1703.08691v1 2017-03-28,Coarsening dynamics of an isotropic ferromagnetic superfluid,"In zero magnetic field the ground state manifold of a ferromagnetic spin-1 condensate is SO(3) and exhibits $\mathbb{Z}_2$ vortices as topological defects. We investigate the phase ordering dynamics of this system after being quenched into this ferromagnetic phase from a zero temperature unmagnetized phase. Following the quench, we observe the ordering of both magnetic and gauge domains. We find that these domains grow diffusively, i.e. with domain size $L(t)\sim t^{1/2}$, and exhibit dynamic scale invariance. The coarsening dynamics progresses as $\mathbb{Z}_2$ vortices annihilate, however we find that at finite energy a number of these vortices persist in small clumps without influencing magnetic or gauge order. We consider the influence of a small non-zero magnetic field, which reduces the ground state symmetry, and show that this sets a critical length scale such that when the domains reach this size the system dynamically transitions in order parameter and scaling behaviour from an isotropic to an anisotropic ferromagnetic superfluid.",1703.09360v2 2017-03-29,A new route to enhance the ferromagnetic transition temperature in diluted magnetic semiconductors,"We investigate the magnetic and the transport properties of diluted magnetic semiconductors using a spin-fermion Monte-Carlo method on a 3D lattice in the intermediate coupling regime. The ferromagnetic transition temperature $T_c$ shows an optimization behavior, first increases and then decreases, with respect to the absolute carrier density $p_{abs}$ for a given magnetic impurity concentration $x$, as seen in the experiment. Our calculations also show an insulator-metal-insulator transition across the optimum $p_{abs}$ where the $T_c$ is maximum. Remarkably, the optimum $p_{abs}$ values lie in a narrow range around 0.11 for all $x$ values and the ferromagnetic $T_c$ increases with $x$. We explain our results using the polaron percolation mechanism and outline a new route to enhance the ferromagnetic transition temperature in experiments.",1703.10129v1 2018-02-01,An analytical computation of magnetic field generated from a cylinder ferromagnet,"An analytical formulation to compute a magnetic field generated from an uniformly magnetized cylinder ferromagnet is developed. Exact solutions of the magnetic field generated from the magnetization pointing in an arbitrary direction are derived, which are applicable both inside and outside the ferromagnet. The validities of the present formulas are confirmed by comparing them with demagnetization coefficients estimated in earlier works. The results will be useful for designing practical applications, such as high-density magnetic recording and microwave generators, where nanostructured ferromagnets are coupled to each other through the dipole interactions and show cooperative phenomena such as synchronization. As an example, the magnetic field generated from a spin torque oscillator for magnetic recording based on microwave assisted magnetization reversal is studied.",1802.00384v1 2018-02-19,Borderline Magnetism: How Does Adding Magnesium to Paramagnetic CeCo$_3$ Make a 450 K Ferromagnet with Large Magnetic Anisotropy?,"A recent experimental study (Phys. Rev. Appl. 9, 024023, 2018) on paramagnetic CeCo$_3$ finds that Magnesium alloying induces a ferromagnetic transition with intrinsic properties large enough for permanent magnet applications. Here we explain these surprising results \textit{via} a first principles study of the electronic structure and magnetism of Magnesium-alloyed CeCo$_3$. We find the origin of this Magnesium-induced ferromagnetic transition to be Stoner physics - the substantial increase in the Fermi-level density-of-states $N(E_F)$ with Mg alloying. Our calculations suggest that both Ce and Co atoms are important for generating large magnetic anisotropy suggesting the viability of Co-3$d$, and Ce-4$f$ interaction for the generation of magnetic anisotropy in magnetic materials. These results offer a new route to the discovery of ferromagnetic materials and provide fundamental insight into the magnetic properties of these alloys",1802.06747v2 2018-02-21,Spin inversion in graphene spin valves by gate-tunable magnetic proximity effect at one-dimensional contacts,"Graphene has remarkable opportunities for spintronics due to its high mobility and long spin diffusion length, especially when encapsulated in hexagonal boron nitride (h-BN). Here, for the first time, we demonstrate gate-tunable spin transport in such encapsulated graphene-based spin valves with one-dimensional (1D) ferromagnetic edge contacts. An electrostatic backgate tunes the Fermi level of graphene to probe different energy levels of the spin-polarized density of states (DOS) of the 1D ferromagnetic contact, which interact through a magnetic proximity effect (MPE) that induces ferromagnetism in graphene. In contrast to conventional spin valves, where switching between high- and low-resistance configuration requires magnetization reversal by an applied magnetic field or a high-density spin-polarized current, we provide an alternative path with the gate-controlled spin inversion in graphene. The resulting tunable MPE employing a simple ferromagnetic metal holds promise for spintronic devices and to realize exotic topological states, from quantum spin Hall and quantum anomalous Hall effects, to Majorana fermions and skyrmions.",1802.07790v1 2018-02-28,The effects of thermal and correlated noise on magnons in a quantum ferromagnet,"The dynamics and thermal equilibrium of spin waves (magnons) in a quantum ferromagnet as well as the macroscopic magnetisation are investigated. Thermal noise due to an interaction with lattice phonons and the effects of spatial correlations in the noise are considered. We first present a Markovian master equation approach with analytical solutions for any homogeneous spatial correlation function of the noise. We find that spatially correlated noise increases the decay rate of magnons with low wave vectors to their thermal equilibrium, which also leads to a faster decay of the ferromagnet's magnetisation to its steady-state value. For long correlation lengths and higher temperature we find that additionally there is a component of the magnetisation which decays very slowly, due to a reduced decay rate of fast magnons. This effect could be useful for fast and noise-protected quantum or classical information transfer and magnonics. We further compare ferromagnetic and antiferromagnetic behaviour in noisy environments and find qualitatively similar behaviour in Ohmic but fundamentally different behaviour in super-Ohmic environments.",1802.10258v2 2018-04-02,Spin torques and magnetic texture dynamics driven by the supercurrent in superconductor/ferromagnet structures,"We introduce the general formalism to describe spin torques induced by the supercurrents injected from the adjacent superconducting electrodes into the spin-textured ferromagnets. By considering the adiabatic limit for the equal-spin superconducting correlations in the ferromagnet we show that the supercurrent can generate both the field-like spin transfer torque and the spin-orbital torque. These dissipationless spin torques are expressed through the current-induced corrections to the effective field derived from the system energy. The general formalism is applied to show that the supercurrent can either shift or move the magnetic domain walls depending on their structure and the type of spin-orbital interaction in the system. These results can be used for the prediction and interpretation of the experiments studying magnetic texture dynamics in superconductor/ferromagnet/superconductor Josephson junctions and other hybrid structures.",1804.00718v2 2018-04-04,Direct observation of the magnetic proximity effect in amorphous exchange-spring magnets by neutron reflectometry,"In this letter we report a direct observation of a magnetic proximity effect in an amorphous thin film exchange-spring magnet by the use of neutron reflectometry. The exchange-spring magnet is a trilayer consisting of two ferromagnetic layers with high $T_c$'s separated by a ferromagnetic layer, which is engineered to have a significantly lower $T_c$ than the embedding layers. This enables us to measure magnetization depth profiles at which the low $T_c$ material is in a ferromagnetic or paramagnetic state, while the embedding layers are ferromagnetic. A clear proximity effect is observed 7 K above the $T_c$ of the embedded layer, with a range extending 50 $\unicode{xC5}$.",1804.01479v2 2018-04-17,Ultrasensitive multi-mode ESR probed ferromagnetic two-level system of $Mn^{4+}$ impurity ion in the insulated $MnO_6$ complex of $SrLaAlO_4$ at $20~mK$,"Ultrasnsitive multi-mode electron spin resonance spectroscopy in the $SrLaAlO_4$ dielectric resonator at $20~mK$ reveals ferromagnetic states of $Mn^{4+}$ impurity ion. The formation of ferromagnetic states in the $MnO_6$ complex implies to oxygen deficiency of this multi-valence $Mn^{4+}$ ion. Experiment results supports that an intricate electronic hybridization in $MnO_6$ structural instability is related to Pseudo Jahn-Teller effect. Measured dipolar hyperfine structure parameter of nucleus is $P_{\scriptscriptstyle\parallel} =-3.7\times 10^{-4}~cm^{-1}$. Mean inverse third power of the electron distance is $\langle r_q^{-3}\rangle=3.325~a.u.$ assuming nuclear electric quadruple moment $Q=+0.33(1)~barn$. In such a state, giant g-factor is observed due to magneto (ferromagnetic) impedance taking in to account a two-level system on the adiabatic-potential-energy-surface. The spins exhibited parity is opposite in the interaction of highest-occupied-molecular-orbital and lowest-unoccupied-molecular-orbital coupling.",1804.06320v1 2018-04-19,Absence of ferromagnetism in VSe2 caused by its charge density wave phase,"In this study we present a detailed ab initio analysis of the magnetic properties of VSe2 . Ab initio calculations in the so-called 1T structure yield a ferromagnetic phase as most stable, with a magnetic moment of about 0.6 {\mu} B /V. According to our calculations this ferromagnetic state is on the verge of instability. We have modeled ab initio the charge density wave state reported in the literature. This introduces a periodic lattice distortion leading to a supercell with periodicity 4a x 4a x 3c (4a x 4a for the monolayer) in which we have fully relaxed the atomic positions. We demonstrate that this structural rearrangement causes a strong reduction in the density of states at the Fermi level and the ground state of the system becomes non-magnetic for the bulk. In the monolayer limit the rearrangement induces a Peierls distortion causing an energy gap opening at the Fermi level and the quenching of ferromagnetism.",1804.07102v2 2018-04-19,Ferromagnetic phases due to competing short- and long-range interactions in the spin-S Blume-Capel model,"We broaden the study of the statistical physics of the spin-$S$ Blume-Capel model with ferromagnetic mean-field interactions $J$ in competition with short-range antiferromagnetic interactions $K$ in a linear chain in the thermodynamic limit. This work describes the critical behavior of the model when the $S$ takes a half integer and an integer value. In both cases the phase diagrams exhibit new ferromagnetic phases (for certain values of $K$) enclosed by branches emerging from the first-order frontiers of the pure ferromagnetic model. For finite temperatures the complex topologies were obtained by numerical minimization of the free energy.",1804.07228v1 2018-04-20,Theory of tunneling spectroscopy of normal metal/ferromagnet/spin-triplet superconductor junctions,"We study the tunneling conductance of a ballistic normal metal / ferromagnet / spin-triplet superconductor junction using the extended Blonder-Tinkham-Klapwijk formalism as a model for a $c$-axis oriented Au / SrRuO$_{3}$ / Sr$_{2}$RuO$_{4}$ junction. We compare chiral $p$-wave (CPW) and helical $p$-wave (HPW) pair potentials, combined with ferromagnet magnetization directions parallel and perpendicular to the interface. For fixed $\theta_{M}$, where $\theta_{M}$ is a direction of magnetization in the ferromagnet measured from the $c$-axis, the tunneling conductance of CPW and HPW clearly show different voltage dependencies. It is found that the cases where the $d$-vector is perpendicular to the magnetization direction (CPW with $\theta_{M} = \pi/2$ and HPW with $\theta_{M} = 0$) are identical. The obtained results serve as a guide to determine the pairing symmetry of the spin-triplet superconductor Sr$_{2}$RuO$_{4}$.",1804.07678v2 2018-11-06,Electrically-Tunable High Curie Temperature Two-Dimensional Ferromagnetism in Van der Waals Layered Crystals,"Identifying intrinsic low-dimensional ferromagnets with high transition temperature and electrically tunable magnetism is crucial for the development of miniaturized spintronics and magnetoelectrics. Recently long-range 2D ferromagnetism was observed in van der Waals crystals CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, however their Curie temperature is significantly lowered when reducing down to monolayer/few layers. Herein, using renormalized spin-wave theory and first-principles electronic structure theory, we present a theoretical study of electrically tunable 2D ferromagnetism in van der Waals layered CrSBr and CrSeBr semiconductors with high Curie temperature of ~150K and sizable band gap. High transition temperature is attributed to strong anion-mediated superexchange interaction and a sizable spin-wave excitation gap due to large exchange and single-ion anisotropy. Remarkably, hole and electron doping can switch magnetization easy axis from in-plane to out-of-plane direction. These unique characteristics establish monolayer CrSBr and CrSeBr as promising platform for realizing 2D spintronics and magnetoelectrics such as 2D spin field effect transistor.",1811.02674v2 2018-11-28,Interfacial spin glass mediated unusual spontaneous exchange bias effect in epitaxial self-assembled La0.7Sr0.3MnO3:NiO nanocomposite thin films,"Zero field cooled spontaneous exchange bias (SEB) is observed in epitaxial La0.7Sr0.3MnO3 (LSMO):NiO self-assembled nanocomposite thin films grown on (001) SrTiO3 single crystal substrate by pulsed laser deposition. SEB is displayed by the novel asymmetry in the hysteresis loop of the composite film along with the field cooled conventional exchange bias (CEB) effect. The training effect shows that exchange bias relaxation is disorder mediated. It is revealed from DC magnetization results that such nanocomposite film divulges spin glass like behaviour, which is arising due to competing ferromagnetic and antiferromagnetic inteactions at the assorted interfaces of ferromagnetic LSMO and antiferromagnetic NiO. Also corroborated by x-ray magnetic circular dichroism (XMCD) measurements, we conclude that SEB is mainly originating due to ferromagnetic coupling of unstable interfacial antiferromagnetic spin due to NiO with the ferromagnetic LSMO at the disordered interface. These results of self-assembled thin films provide a useful input to realize and understand microscopic origin of SEB for device application.",1811.11461v1 2018-11-29,Control of magnetism in bilayer CrI$_{3}$ by an external electric field,"Recently intrinsic ferromagnetism in two-dimensional(2D) van der Waals materials was discovered [1, 2, 3]. A monolayer of Chromiun triiodide(CrI3) is ferromagnetic while a bilayer structure was reported to be anti-ferro magnetic, moreover an external electric field changes its magnetic phase [4]. We have studied the two found in nature stackings of CrI3 bilayers and found that indeed the magnetic phase of one of them can be tuned by an external electric field while the other remains ferromagnetic. We simulate those results with ab initio calculations and explain them with a simple model based on a rigid shift of the bands associated with different spins. The model can be applied to similar van der Waal stacked insulating bilayer anti-ferromagnets.",1811.11910v1 2019-05-02,Anomalous spin Hall angle of a metallic ferromagnet determined by a multiterminal spin injection/detection device,"We report on the determination of the anomalous spin Hall angle in the ferromagnetic metal alloy cobalt-iron (Co$_{25}$Fe$_{75}$, CoFe). This is accomplished by measuring the spin injection/detection efficiency in a multiterminal device with nanowires of platinum (Pt) and CoFe deposited onto the magnetic insulator yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG). Applying a spin-resistor model to our multiterminal spin transport data, we determine the magnon conductivity in YIG, the spin conductance at the YIG/CoFe interface and finally the anomalous spin Hall angle of CoFe as a function of its spin diffusion length in a single device. Our experiments clearly reveal a negative anomalous spin Hall angle of the ferromagnetic metal CoFe, but a vanishing ordinary spin Hall angle. This is in contrast to the results reported for the ferromagnetic metals Co and permalloy.",1905.00663v1 2019-05-09,Evolution of nematic and ferromagnetic ordering in suspensions of magnetic nanoplatelets,"Suspensions of magnetic nanoplatelets in isotropic solvents are very interesting examples of ferrofluids. It has been shown that above a certain concentration {\Phi}NI such suspensions form a ferromagnetic nematic phase, which makes this system a unique example of a dipolar fluid. The formation of a nematic phase is driven by anisotropic electrostatic and long-range dipolar magnetic interactions. Here, we present studies of the evolution of short range positional and orientational magnetic order in the suspensions with volume fractions below and above {\Phi}NI, using small angle neutron scattering (SANS). The results show that in the absence of an external magnetic field, short range positional and orientational order already exist at relatively low volume fractions. Polarized SANS revealed that the contribution of ferromagnetic ordering to the formation of the nematic phase is significant. The ferromagnetic correlations can be qualitatively explained by a simple model, which takes into account anisotropic screened electrostatic and dipolar magnetic interactions.",1905.03572v1 2019-05-28,Spin-Orbit Torque in a Single Ferromagnetic Layer with Large Spin-Orbit Coupling,"Spin-orbit torque in heavy metal/ferromagnet heterostructures with broken spatial inversion symmetry provides an efficient mechanism for manipulating magnetization using a charge current. Here, we report the presence of a spin torque in a single ferromagnetic layer in both asymmetric MgO/Fe0.8Mn0.2 and symmetric MgO/Fe0.8Mn0.2/MgO structures, which manifests itself in the form of an effective field transverse to the charge current. The current to effective field conversion efficiency, which is characterized using both the nonlinear magnetoresistance and second-order planar Hall effect methods, is comparable to the efficiency in typical heavy metal/ferromagnet bilayers. We argue that the torque is caused by spin rotation in the vicinity of the surface via impurity scattering in the presence of a strong spin-orbit coupling. Instead of cancelling off with each other, the torques from the top and bottom surfaces simply add up, leading to a fairly large net torque, which is readily observed experimentally.",1905.11565v1 2020-07-02,An Origin of Dzyaloshinskii-Moriya Interaction at Graphene-Ferromagnet Interfaces Due to the Intralayer RKKY/BR Interaction,"We present a theory of both the itinerant carrier-mediated RKKY interaction and the virtual excitations-mediated Bloembergen-Rowland (BR) interaction between magnetic moments in graphene induced by proximity effect with a ferromagnetic film. We show that the RKKY/BR interaction consists of the Heisenberg, Ising, and Dzyaloshinskii-Moriya (DM) terms. In the case of the nearest distance, we estimate the DM term from the RKKY/BR interaction is about 0.13 meV for the graphene/Co interface, which is consistent with the experimental result of DM interaction $0.16 \pm 0.05$ meV. Our calculations indicate that the intralayer RKKY/BR interaction may be a possible physical origin of the DM interaction in the graphene-ferromagnet interface. This work provides a new perspective to comprehend the DM interaction in graphene/ferromagnet systems.",2007.02690v1 2020-07-07,Current Fluctuations Driven by Ferromagnetic and Antiferromagnetic Resonance,"We consider electron transport in ferromagnets or antiferromagnets sandwiched between metals. When spins in the magnetic materials precess, they emit currents into the surrounding conductors. Generally, adiabatic pumping in mesoscopic systems also enhances current fluctuations. We generalize the description of current fluctuations driven by spin dynamics in three ways using scattering theory. First, our theory describes a general junction with any given electron scattering properties. Second, we consider antiferromagnets as well as ferromagnets. Third, we treat multiterminal devices. Using shot noise-induced current fluctuations to reveal antiferromagnetic resonance appears to be easier than using them to reveal ferromagnetic resonance. The origin of this result is that the associated energies are much higher as compared to the thermal energy. The thermal energy governs the Johnson-Nyquist that is independent of the spin dynamics. We give results for various junctions, such as ballistic and disordered contacts. Finally, we discuss experimental consequences.",2007.03257v2 2020-07-08,Pressure-induced suppression of ferromagnetism in CePd$_2$P$_2$,"The correlated electron material CePd$_2$P$_2$ crystallizes in the ThCr$_2$Si$_2$ structure and orders ferromagnetically at 29 K. Lai et al. [Phys. Rev. B 97, 224406 (2018)] found evidence for a ferromagnetic quantum critical point induced by chemical compression via substitution of Ni for Pd. However, disorder effects due to the chemical substitution interfere with a simple analysis of the possible critical behavior. In the present work, we examine the temperature - pressure - magnetic field phase diagram of single crystalline CePd$_2$P$_2$ to 25 GPa using a combination of resistivity, magnetic susceptibility, and x-ray diffraction measurements. We find that the ferromagnetism appears to be destroyed near 12 GPa, without any change in the crystal structure.",2007.04165v1 2020-07-17,Entangling the vibrational modes of two massive ferromagnetic spheres using cavity magnomechanics,"We present a scheme to entangle the vibrational phonon modes of two massive ferromagnetic spheres in a dual-cavity magnomechanical system. In each cavity, a microwave cavity mode couples to a magnon mode (spin wave) via the magnetic dipole interaction, and the latter further couples to a deformation phonon mode of the ferromagnetic sphere via a nonlinear magnetostrictive interaction. We show that by directly driving the magnon mode with a red-detuned microwave field to activate the magnomechanical anti-Stokes process a cavity-magnon-phonon state-swap interaction can be realized. Therefore, if the two cavities are further driven by a two-mode squeezed vacuum field, the quantum correlation of the driving fields is successively transferred to the two magnon modes and subsequently to the two phonon modes, i.e., the two ferromagnetic spheres become remotely entangled. Our work demonstrates that cavity magnomechanical systems allow to prepare quantum entangled states at a more massive scale than currently possible with other schemes.",2007.09083v3 2010-05-28,Controlling the Local Spin-Polarization at the Organic-Ferromagnetic Interface,"By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments we show how to manipulate the local spin-polarization of a ferromagnetic surface by creating a complex energy dependent magnetic structure. We demonstrate this novel effect by adsorbing organic molecules containing pi(pz)-electrons onto a ferromagnetic surface, in which the hybridization of the out-of-plane pz atomic type orbitals with the d-states of the metal leads to the inversion of the spin-polarization at the organic site due to a pz - d Zener exchange type mechanism. As a key result, we demonstrate that it is possible to selectively inject spin-up and spin-down electrons from the same ferromagnetic surface, an effect which can be exploited in future spintronic devices.",1005.5411v1 2013-08-18,Pulsed Laser Deposition of High-Quality Thin Films of the Insulating Ferromagnet EuS,"High-quality thin films of the ferromagnetic-insulator europium(II) sulfide (EuS) were fabricated by pulsed laser deposition on Al2O3 (0001) and Si (100) substrates. A single orientation was obtained with the [100] planes parallel to the substrates, with atomic-scale smoothness indicates a near-ideal surface topography. The films exhibit uniform ferromagnetism below 15.9 K, with a substantial component of the magnetization perpendicular to the plane of the films. Optimization of the growth condition also yielded truly insulating films with immeasurably large resistance. This combination of magnetic and electric properties open the gate for novel devices that require a true ferromagnetic insulator.",1308.3820v2 2013-08-20,Dynamic-Symmetry-Breaking Breathing and Spreading Transitions in Ferromagnetic Film Irradiated by Spherical Electromagnetic Wave,"The dynamical responses of a ferromagnetic film to a propagating spherical electromagnetic wave passing through it are studied by Monte Carlo simulation of two dimensional Ising ferromagnet. For a fixed set of values of the frequency and wavelength of the spherical EM wave, and depending on the values of amplitude of the EM wave and temperature of the system, three different modes are identified. The static pinned mode, the localised dynamical breathing mode and extended dynamical spreading mode are observed. The nonequilibrium dynamical-symmetry-breaking breathing and spreading phase transitions are also observed and the transition temperatures are obtained as functions of the amplitude of the magnetic field of EM wave. A comprehensive phase diagram is drawn. The boundaries of breathing and spreading transitions merge eventually at the equilibrium transition temperature for two dimensional Ising ferromagnet as the value of the amplitude of the magnetic field becomes vanishingly small.",1308.4277v1 2014-03-03,Ferromagnetic SrRuO3 thin-film deposition on a spin-triplet superconductor Sr2RuO4 with highly conducting interface,"Ferromagnetic SrRuO3 thin films are deposited on the ab-surface of single crystals of the spin-triplet superconductor Sr2RuO4 as substrates using pulsed laser deposition. The films are under a severe in-plane compressive strain. Nevertheless, the films exhibit ferromagnetic order with the easy axis along the c-direction below the Curie temperature of 158 K. The electrical transport reveals that the SrRuO3/Sr2RuO4 interface is highly conducting, in contrast with the interface between other normal-metals and the ab-surface of Sr2RuO4. Our results will stimulate the investigations on proximity effects between a ferromagnet and a spin-triplet superconductor.",1403.0345v2 2016-11-09,Effect of nanostructure layout on spin pumping phenomena in antiferromagnet/ nonmagnetic metal/ ferromagnet multilayered stacks,"In this work we focus on magnetic relaxation in Mn$_{80}$Ir$_{20}$(12 nm)/ Cu(6 nm)/ Py($d_\mathrm{F}$) antiferromagnet/Cu/ferromagnet (AFM/Cu/FM) multilayers with different thickness of the ferromagnetic permalloy layer. An effective FM-AFM interaction mediated via the conduction electrons in the nonmagnetic Cu spacer -- the spin-pumping effect -- is detected as an increase in the linewidth of the ferromagnetic resonance (FMR) spectra and a shift of the resonant magnetic field. We further find experimentally that the spin-pumping-induced contribution to the linewidth is inversely proportional to the thickness of the Py layer. We show that this thickness dependence likely originates from the dissipative dynamics of the free and localized spins in the AFM layer. The results obtained could be used for tailoring the dissipative properties of spintronic devices incorporating antiferromagnetic layers.",1611.02865v1 2017-01-17,Optical patterning of magnetic domains and defects in ferromagnetic liquid crystal colloids,"A promising approach in designing composite materials with unusual physical behavior combines solid nanostructures and orientationally ordered soft matter at the mesoscale. Such composites not only inherit properties of their constituents but also can exhibit emergent behavior, such as ferromagnetic ordering of colloidal metal nanoparticles forming mesoscopic magnetization domains when dispersed in a nematic liquid crystal. Here we demonstrate the optical patterning of domain structures and topological defects in such ferromagnetic liquid crystal colloids, which allows for altering their response to magnetic fields. Our findings reveal the nature of the defects in this soft matter system which is different as compared to non-polar nematics and ferromagnets alike.",1701.04840v1 2017-01-27,Control of superconductivity with a single ferromagnetic layer in niobium/erbium bilayers,"Superconducting spintronics in hybrid superconductor/ferromagnet (S-F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, $T_c$, of the S-layer can be controlled by the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using field history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both $T_c$ and the shape of the resistive transition, R(T), to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the effective exchange field of the ferromagnet and link this, using conventional S-F theory, to the suppression of $T_c$. We observe stepped features in the R(T) which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description.",1701.08065v2 2017-04-01,Asymmetric isolated skyrmions in polar magnets with easy-plane anisotropy,"We introduce a new class of isolated magnetic skyrmions emerging within tilted ferromagnetic phases of polar magnets with easy-plane anisotropy. The asymmetric magnetic structure of these skyrmions is associated with an intricate pattern of the energy density, which exhibits positive and negative asymptotics with respect to the surrounding state with a ferromagnetic moment tilted away from the polar axis. Correspondingly, the skyrmion-skyrmion interaction has an anisotropic character and can be either attractive or repulsive depending on the relative orientation of the skyrmion pair. We investigate the stability of these novel asymmetric skyrmions against the elliptical cone state and follow their transformation into axisymmetric skyrmions, when the tilted ferromagnetic moment of the host phase is reduced. Our theory gives clear directions for experimental studies of isolated asymmetric skyrmions and their clusters embedded in tilted ferromagnetic phases.",1704.00100v1 2017-04-11,Tuning Interfacial Ferromagnetism in LaNiO$_3$/CaMnO$_3$ Superlattices by Stabilizing Non-Equilibrium Crystal Symmetry,"Perovskite oxide heterostructures offer an important path forward for stabilizing and controlling low-dimensional magnetism. One of the guiding design principles for these materials systems is octahedral connectivity. In superlattices composed of perovskites with different crystal symmetries, variation of the relative ratio of the constituent layers as well as the individual layer thicknesses gives rise to non-equilibrium crystal symmetries that, in turn, lead to unprecedented control of interfacial ferromagnetism. We have found that in superlattices of CaMnO$_3$ (CMO) and LaNiO$_3$ (LNO), interfacial ferromagnetism can be modulated by a factor of three depending on LNO and CMO layer thicknesses as well as their relative ratio. Such an effect is only possible due to the non-equilibrium crystal symmetries at the interfaces and can be understood in terms of the anisotropy of the exchange interactions and modifications in the interfacial Ni-O-Mn and Mn-O-Mn bond angles and lengths with increasing LNO layer thickness. These results demonstrate the potential of engineering non-equilibrium crystal symmetries in designing ferromagnetism.",1704.03154v2 2017-04-28,Single-domain ferromagnet of noncentrosymmetric uniaxial magnetic ions and magnetoelectric interaction,"The feasibility of a single-domain ferromagnet based on uniaxial magnetic ions was examined. For a noncentrosymmetric uniaxial magnetic ion of magnetic moment at a site of local electric dipole moment p, it is unknown to date whether the magnetic moment prefers to be parallel or antiparallel to the electric dipole moment. The nature of this magnetoelectric interaction was probed in terms of analogical reasoning based on the Rashba effect and density functional calculations. We show that the magnetic and electric dipole moments prefer an antiparallel arrangement, predict that Fe-doped CaZnOS is a single-domain ferromagnet like a bar magnet, and find the probable cause for the ferromagnetism and weak magnetization hysteresis in Fe-doped hexagonal ZnO and ZnS at very low dopant concentrations.",1704.08940v1 2018-05-15,Gate-tunable magnetism of C adatoms on graphene,"We have performed density functional theory calculations of graphene decorated with carbon adatoms, which bind at the bridge site of a C--C bond. Earlier studies have shown that the C adatoms have magnetic moments and have suggested the possibility of ferromagnetism with high Curie temperature. Here we propose to use a gate voltage to fine tune the magnetic moments from zero to 1$\mu_B$ while changing the magnetic coupling from antiferromagnetism to ferromagnetism and again to antiferromagnetism. These results are rationalized within the Stoner and RKKY models. When the SCAN meta-GGA correction is used, the magnetic moments for zero gate voltage are reduced and the Stoner band ferromagnetism is slightly weakened in the ferromagnetic region.",1805.06044v2 2018-05-22,Loss of Hall Conductivity Quantization in a Non-Hermitian Quantum Anomalous Hall Insulator,"Recent work has extended topological band theory to open, non-Hermitian Hamiltonians, yet little is understood about how non-Hermiticity alters the topological quantization of associated observables. We address this problem by studying the quantum anomalous Hall effect (QAHE) generated in the Dirac surface states of a 3D time-reversal-invariant topological insulator (TI) that is proximity-coupled to a metallic ferromagnet. By constructing a contact self-energy for the ferromagnet, we show that in addition to generating a mass gap in the surface spectrum, the ferromagnet can introduce a non-Hermitian broadening term, which can obscure the mass gap in the spectral function. We calculate the Hall conductivity for the effective non-Hermitian Hamiltonian describing the heterostructure and show that it is no longer quantized despite being classified as a Chern insulator based on non-Hermitian topological band theory. Our results indicate that the QAHE will be challenging to experimentally observe in ferromagnet-TI heterostructures due to the finite lifetime of quasi-particles at the interface.",1805.08892v2 2018-05-25,"Suppression of ferromagnetic order by Ag-doping: A neutron scattering investigation on Ce2(Pd1-xAgx)2In (x = 0.20, 0.50)","The ground state magnetic behaviour of Ce2(Pd0.8Ag0.2)2In and Ce2(Pd0.5Ag0.5)2In, found in the ferromagnetic branch of Ce2Pd2In, has been investigated by neutron powder diffraction at low temperature. Ce2(Pd0.8Ag0.2)2In is characterized by a ferromagnetic structure with the Ce moments aligned along the c-axis and values of 0.96(2) mB. The compound retains the P4/mbm throughout the magnetic transition, although the magnetic ordering is accompanied by a significant decrease of the lattice strain along [00l], suggesting a magnetostructural contribution. The magnetic behaviour of Ce2(Pd0.5Ag0.5)2In is very different; this compound exhibits an extremely reduced magnetic scattering contribution in the diffraction pattern, that can be ascribed to a different kind of ferromagnetic ordering, with extremely reduced magnetic moments (~ 0.1 mB) aligned along [0l0]. These results point to a competition between different types of magnetic correlations induced by Ag-substitution, giving rise to a magnetically frustrated scenario in Ce2(Pd0.5Ag0.5)2In.",1805.10023v1 2008-07-02,Numerical Studies of Quantum Hall Ferromagnetism in Two-Subband Systems,"We carry out a numerical study of the quantum Hall ferromagnetism in a two-subband system using a set of experimental parameters in a recently experiment [X. C. Zhang, I. Martin, and H. W. Jiang, Phys. Rev. B \textbf{74}, 073301 (2006)]. Employing the self-consistence local density approximation for growth direction wave function and the Hartree-Fock theory for the pseudospin anisotropy energy, we are able to account for the easy-axis and easy-plane quantum Hall ferromagnetism observed at total filling factor $\nu = 3$ and $\nu= 4$, respectively. Our study provides some insight of how the anisotropy energy, which highly depends upon the distribution of growth direction wave functions, determines the symmetry of the quantum Hall ferromagnets.",0807.0297v1 2008-07-23,First order ferromagnetic phase transition in the low electronic density regime of a biased graphene bilayer,"The phase diagram of a biased graphene bilayer is computed and the existence of a ferromagnetic phase is discussed both in the critical on-site interaction $U_{c}$ versus doping density and versus temperature. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is hole like in one plane and electron like in the other. We give evidence for a \emph{first-order} phase transition between paramagnetic and ferromagnetic phases induced by doping at zero temperature.",0807.3623v1 2009-12-07,High temperature ferromagnetism of Li-doped vanadium oxide nanotubes,"The nature of a puzzling high temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high temperature ferromagnetism of vanadium oxide nanotubes.",0912.1237v1 2009-12-23,Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe,"Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe ($T_{\rm Curie} \sim 2.5$ K and $T_{\rm SC}$ $\sim$ 0.6 K) is reported from $^{59}$Co nuclear quadrupole resonance (NQR). The $^{59}$Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while nuclear spin-lattice relaxation rate $1/T_1$ in the ferromagnetic (FM) phase decreases below $T_{\rm SC}$ due to the opening of the superconducting(SC) gap. The SC state was found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the $^{59}$Co-NQR spectrum around $T_{\rm Curie}$ show that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order.",0912.4545v1 2011-11-04,Phase diagram of UGe2. Whether there are quantum phase transitions ?,"The phase diagram of several itinerant ferromagnets reveals the common feature. The phase transition temperature decreases with pressure increase and reaches zero value at some critical pressure $P_c$ such that at low enough temperatures one can expect critical behavior specific for quantum phase transition. It is not the case, however. Being the second order at ambient pressure the transition from paramagnetic to ferromagnetic state at high pressures - low temperatures is transformed to the discontinuous jump. We discuss the magneto-elastic mechanism of development of the first order type instability at the phase transition to the ferromagnet state in strongly anisotropic ferromagnet UGe2. Using the parameters characterizing the properties of UGe$_2$ we argue the effectiveness of this mechanism transforming the very weak first order type transition to the really observable one.",1111.1089v1 2011-11-04,Diffusive spin dynamics in ferromagnetic thin films with a Rashba interaction,"In a ferromagnetic metal layer, the coupled charge and spin diffusion equations are obtained in the presence of both Rashba spin-orbit interaction and magnetism. The mis-alignment between the magnetization and the non-equilibrium spin density induced by the Rashba field gives rise to Rashba spin torque acting on the ferromagnetic order parameter. In a general form, we find that the Rashba torque consists of both in-plane and out-of-plane components, ie $\bm{T}=T_{\bot}\hat{\bm{y}}\times{\hat{\bm m}}+T_{\parallel}{\hat{\bm m}}\times({\hat{\bm y}}\times{\hat{\bm m}})$. Numerical simulations on a two dimensional nano-wire discuss the impact of diffusion on the Rashba torque, which reveals a large enhancement to the ratio $T_{\parallel}/T_{\bot}$ for thin wires. Our theory provides an explanation to the mechanism that drives the magnetization switching in a single ferromagnet as observed in the recent experiments.",1111.1216v2 2011-11-14,Nematic phase and phase separation near saturation field in frustrated ferromagnets,"We study the effects of quantum fluctuations in magnetic properties of quantum frustrated ferromagnets in a magnetic field. It is shown that a non-classical phase or a phase separation appears due to quantum fluctuations below the saturation field in a parameter range close to the classical zero-field phase boundary between ferromagnetic and antiferromagnetic phases, for the case that the classical antiferromagnetic state is not an eigenstate of the quantum model. As an example to which this argument is applicable, we study the S=1/2 J1-J2 Heisenberg model with ferromagnetic J1 (J1<0) on the bcc lattice using a dilute Bose gas approach. For -1.50097 < J1/J2 < -1.389, magnons form f-wave two-magnon bound states, leading to a spin nematic phase, and for -1.389 < J1/J2 < -0.48 a canted coplanar antiferromagnetic phase appears accompanied with a phase separation below the saturation field.",1111.3184v3 2011-11-17,"Theory of magnetization precession induced by a picosecond strain pulse in ferromagnetic semiconductor (Ga,Mn)As","A theoretical model of the coherent precession of magnetization excited by a picosecond acoustic pulse in a ferromagnetic semiconductor layer of (Ga,Mn)As is developed. The short strain pulse injected into the ferromagnetic layer modifies the magnetocrystalline anisotropy resulting in a tilt of the equilibrium orientation of magnetization and subsequent magnetization precession. We derive a quantitative model of this effect using the Landau-Lifshitz equation for the magnetization that is precessing in the time-dependent effective magnetic field. After developing the general formalism, we then provide a numerical analysis for a certain structure and two typical experimental geometries in which an external magnetic field is applied either along the hard or the easy magnetization axis. As a result we identify three main factors, which determine the precession amplitude: the magnetocrystalline anisotropy of the ferromagnetic layer, its thickness, and the strain pulse parameters.",1111.4043v1 2011-12-31,"Ferromagnetic Ordering in Carbon Nanotubes, Incorporated in Diamond Single Crystals","The physical origin of the mechanism of the formation of ferromagnetic ordering in carbon nanotubes (NTs), produced by high energy ion beam modification of diamond single crystals in $\langle{110}\rangle$ and $\langle{111}\rangle$ directions has been found. It is concluded from analysis of experimental results on ferromagnetic spin wave resonance observed, that the only $\pi$-electronic subsystem of given NTs is responsible for the appearance of ferromagnetism. It is determined by asymmetry in spin density distribution in Su-Schrieffer-Heeger (SSH) topological soliton lattice. The formation of SSH topological soliton lattice is considered in the frames of generalized SSH-model of organic conductors, in which $\pi$-electronic subsystem is represented being to be 1D quantum Fermi liquid. The phenomenon of formation of uncompensated antiferromagnetic ordering coexisting with superconductivity at room temperature in carbon nanotubes, produced by high energy ion beam modification of diamond single crystals in $\langle{100}\rangle$ direction is argued.",1201.0285v3 2012-01-05,Thermoelectricity of the Ferromagnetic Superconductor UCoGe,"UCoGe exhibits superconductivity in the presence of ferromagnetism. When a field is applied along the b axis (perpendicular to the easy axis), ferromagnetism is weakened and superconductivity is enhanced. This enhancement has been attributed to an increase in coupling as observed in the enhanced effective mass produced by the critical fluctuations as the ferromagnetic transition is strongly suppressed. However it is also important to know if and how the Fermi surface changes near the critical point. Here we report measurements of the thermoelectricity of UCoGe which reveal a low carrier density metal. Under magnetic field applied along the b axis, a sharp peak is observed in the thermopower of UCoGe at H*=11.1T and low temperature which becomes broader at higher temperatures. At higher field, the thermopower changes sign which suggests a modification of the Fermi Surface. We analyze these results using a topological change in Fermi surface and show that this can explain both the thermopower and the enhanced superconductivity.",1201.1147v1 2012-01-14,Non-local spin valve in Van der Pauw cross geometry with four ferromagnetic electrodes,"We consider a non-local spin valve in a Van der Pauw cross geometry with four ferromagnetic electrodes. Two antiparallel ferromagnets are used as (charge) source and drain while the detector circuit involves measuring the voltage between two collinear ferromagnets with parallel or antiparallel magnetizations. We find a potentially large increase of the non-local spin voltage. The setup displays several additional interesting properties: (i) infinite GMR for the non-local resistance (if a symmetry requirement for the device is met); (ii) ON-OFF switch effect, when the injector electrodes are parallel instead of antiparallel; (iii) insensitivity to offset voltages. The device can additionally be used as a Direct Spin Hall Effect probe and as a reprogrammable magneto-logic gate implementing basic operations (NOR, NAND, inverter, AND, OR,...).",1201.2982v1 2012-01-20,Modified Spin Wave Analysis of Low Temperature Properties of Spin-1/2 Frustrated Ferromagnetic Ladder,"Low temperature properties of the spin-1/2 frustrated ladder with ferromagnetic rungs and legs, and two different antiferromagnetic next nearest neighbor interaction are investigated using the modified spin wave approximation in the region with ferromagnetic ground state. The temperature dependence of the magnetic susceptibility and magnetic structure factors is calculated. The results are consistent with the numerical exact diagonalization results in the intermediate temperature range. Below this temperature range, the finite size effect is significant in the numerical diagonalization results, while the modified spin wave approximation gives more reliable results. The low temperature properties near the limit of the stability of the ferromagnetic ground state are also discussed.",1201.4242v1 2012-04-04,Electron transport in a ferromagnetic/normal/ferromagnetic tunnel junction based on the surface of a topological insulator,"We theoretically study the electron transport properties in a ferromagnetic/normal/ferromagnetic tunnel junction, which is deposited on the top of a topological surface. The conductance at the parallel (\textbf{P}) configuration can be much bigger than that at the antiparallel (\textbf{AP}) configuration. Compared \textbf{P} with \textbf{AP} configuration, there exists a shift of phase which can be tuned by gate voltage. We find that the exchange field weakly affects the conductance of carriers for \textbf{P} configuration but can dramatically suppress the conductance of carriers for \textbf{AP} configuration. This controllable electron transport implies anomalous magnetoresistance in this topological spin valve, which may contribute to the development of spintronics . In addition, we find that there is a Fabry-Perot-like electron interference.",1204.0956v2 2012-04-05,The Magnon Spectrum in the Domain Ferromagnetic State of Antisite Disordered Double Perovskites,"In their ideal structure, double perovskites like Sr_2FeMoO_6 have alternating Fe and Mo along each cubic axes, and a homogeneous ferromagnetic metallic ground state. Imperfect annealing leads to the formation of structural domains. The moments on mislocated Fe atoms that adjoin each other across the domain boundary have an antiferromagnetic coupling between them. This leads to a peculiar magnetic state, with ferromagnetic domains coupled antiferromagnetically. At short distance the system exhibits ferromagnetic correlation while at large lengthscales the net moment is strongly suppressed due to inter-domain cancellation. We provide a detailed description of the spin wave excitations of this complex magnetic state, obtained within a 1/S expansion, for progressively higher degree of mislocation, i.e., antisite disorder. At a given wavevector the magnons propagate at multiple energies, related, crudely, to `domain confined' modes with which they have large overlap. We provide a qualitative understanding of the trend observed with growing antisite disorder, and contrast these results to the much broader spectrum that one obtains for uncorrelated antisites.",1204.1194v1 2012-04-06,Andreev current induced by ferromagnetic resonance,"We study charge transport through a metallic dot coupled to a superconducting and a ferromagnetic lead with a precessing magnetization due to ferromagnetic resonance. Using the quasiclassical theory, we find that the magnetization precession induces a dc current in the subgap regime even in the absence of a bias voltage. This effect is due to the rectification of the ac spin currents at the interface with the ferromagnet; it exists in the absence of spin current in the superconductor. When the dot is strongly coupled to the superconductor, we find a strong enhancement in a wide range of parameters as compared to the induced current in the normal state.",1204.1491v1 2012-04-09,Free Energy Distribution Function of a Random Ising ferromagnet,"We study the free energy distribution function of weakly disordered Ising ferromagnet in terms of the D-dimensional random temperature Ginzburg-Landau Hamiltonian. It is shown that besides the usual Gaussian ""body"" this distribution function exhibits non-Gaussian tails both in the paramagnetic and in the ferromagnetic phases. Explicit asymptotic expressions for these tails are derived. It is demonstrated that the tails are strongly asymmetric: the left tail (for large negative values of the free energy) is much more slow than the right one (for large positive values of the free energy). It is argued that in the critical point the free energy of the random Ising ferromagnet in dimensions D<4 is described by a non-trivial universal distribution function being non self-averaging",1204.1831v1 2012-04-10,Complex itinerant ferromagnetism in noncentrosymmetric Cr11Ge19,"The noncentrosymmetric ferromagnet Cr11Ge19 has been investigated by electrical transport, AC and DC magnetization, heat capacity, x-ray diffraction, resonant ultrasound spectroscopy, and first principles electronic structure calculations. Complex itinerant ferromagnetism in this material is indicated by nonlinearity in conventional Arrott plots, unusual behavior of AC susceptibility, and a weak heat capacity anomaly near the Curie temperature (88 K). The inclusion of spin wave excitations was found to be important in modeling the low temperature heat capacity. The temperature dependence of the elastic moduli and lattice constants, including negative thermal expansion along the c axis at low temperatures, indicate strong magneto-elastic coupling in this system. Calculations show strong evidence for itinerant ferromagnetism and suggest a noncollinear ground state may be expected.",1204.2254v2 2012-04-13,Quantum corrections of the biquadratic interaction in the 1D spin-1/2 frustrated ferromagnetic systems,"Quantum corrections of the biquadratic interaction in the 1D spin-1/2 frustrated ferromagnetic Heisenberg model are studied. The biquadratic interaction for spin-1/2 chains is eliminated and transformed to the quadratic interaction. Doing a numerical experiment, new insight as to how the classical phases get modified on the inclusion of quantum fluctuations is provided. Observed results suggest the existence of an intermediate region in the ground state phase diagram of the frustrated ferromagnetic spin-1/2 chains with combination of dimer and chiral orders. In addition, from the quantum entanglement view point, differences between quantum phases are also obtained. The nearest neighbor spins never be entangled in the frustrated ferromagnetic chains but are entangled up to the Majumdar-Ghosh point in the frustrated antiferromagnetic chains. On the other hand, the next nearest neighbor spins in the mentioned intermediate region are entangled.",1204.2905v1 2012-04-18,Universal low-temperature magnetic properties of the classical and quantum dimerized ferromagnetic spin chain,"Low-temperature magnetic properties of both classical and quantum dimerized ferromagnetic spin chains are studied. It is shown that at low temperatures the classical dimerized model reduces to the classical uniform model with the effective exchange integral $J_{0}=J(1-\delta^{2})$, where $\delta$ is the dimerization parameter. The partition function and spin correlation function are calculated by means of mapping to the continuum limit, which is justified at low temperatures. In the continuum limit the calculation of the partition function and spin correlation function is reduced to the eigenvalue problem of quantum rotator in gravitational field. Quantum model is studied using Dyson-Maleev representation of the spin operators. It is shown that in the long-wavelength limit the Hamiltonian of the quantum dimerized chain reduces to that of the uniform ferromagnetic chain with the effective exchange integral $J_{0}=J(1-\delta^{2})$. This fact implies that the known equivalence of the low-temperature magnetic properties of classical and quantum ferromagnetic chains remains for the dimerized chains. The considered model can be generalized to include the next-neighbor antiferromagnetic interaction.",1204.4076v2 2012-04-26,Ballistic spin filtering across the ferromagnetic-semiconductor interface,"The ballistic spin-filter effect from a ferromagnetic metal into a semiconductor has theoretically been studied with an intention of detecting the spin polarizability of density of states in FM layer at a higher energy level. The physical model for the ballistic spin filtering across the interface between ferromagnetic metals and semiconductor superlattice is developed by exciting the spin polarized electrons into n-type AlAs/GaAs superlattice layer at a much higher energy level and then ballistically tunneling through the barrier into the ferromagnetic film. Since both the helicity-modulated and static photocurrent responses are experimentally measurable quantities, the physical quantity of interest, the relative asymmetry of spin-polarized tunneling conductance, could be extracted experimentally in a more straightforward way, as compared with previous models. The present physical model serves guidance for studying spin detection with advanced performance in the future.",1204.5826v1 2014-05-01,Spin injection from a ferromagnet into a semiconductor in the case of a rough interface,"The effect of the interface roughness on the spin injection from a ferromagnet into a semiconductor is studied theoretically. Even a small interface irregularity can lead to a significant enhancement of the injection efficiency. When a typical size of the irregularity, a, is within a domain lambda_F << a << lambda_N, where lambda_F and lambda_N are the spin-diffusion lengths in the ferromagnet and semiconductor, respectively, the geometrical enhancement factor is ~lambda_N. The origin of the enhancement is the modification of the local electric field on small scales ~a near the interface. We demonstrate the effect of enhancement by considering a number of analytically solvable examples of injection through curved ferromagnet-semiconductor interfaces. For a generic curved interface the enhancement factor is ~lambda_N / R, where R is the local radius of curvature.",1405.0059v1 2014-05-12,Carrier transport properties of the Group-IV ferromagnetic semiconductor Ge1-xFex with and without boron doping,"We have investigated the transport and magnetic properties of group-IV ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3 %) with and without boron doping grown by molecular beam epitaxy (MBE). In order to accurately measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (~5 nm) were used as substrates. Owing to the low Fe content, the hole concentration and mobility in the Ge1-xFex films were exactly estimated by Hall measurements because the anomalous Hall effect in these films was found to be negligibly small. By boron doping, we increased the hole concentration in Ge1-xFex from ~1018 cm-3 to ~1020 cm-3 (x = 1.0%) and to ~1019 cm-3 (x = 2.3%), but no correlation was observed between the hole concentration and magnetic properties. This result presents a contrast to the hole-induced ferromagnetism in III-V ferromagnetic semiconductors.",1405.2764v1 2014-05-19,Controllable generation of a spin-triplet supercurrent in a Josephson spin-valve,"It has been predicted theoretically that an unconventional odd-frequency spin-triplet component of superconducting order parameter can be induced in multilayered ferromagnetic structures with non-collinear magnetization. In this work we study experimentally nano-scale devices, in which a ferromagnetic spin valve is embedded into a Josephson junction. We demonstrate two ways of in-situ analysis of such Josephson spin valves: via magnetoresistance measurements and via in-situ magnetometry based on flux quantization in the junction. We observe that supercurrent through the device depends on the relative orientation of magnetization of the two ferromagnetic layers and is enhanced in the non-collinear state of the spin valve. This provides a direct prove of controllable generation of the spin-triplet superconducting component in a ferromagnet.",1405.4754v1 2014-05-19,Ferromagnetic phase in the polarized two-species bosonic Hubbard Model,"We recently studied a doped two-dimensional bosonic Hubbard model with two hard-core species, with different masses, using quantum Monte Carlo simulations [Phys. Rev. B 88, 161101(R) (2013)]. Upon doping away from half-filling, we find several distinct phases, including a phase-separated ferromagnet with Mott behavior for the heavy species and both Mott insulating and superfluid behaviors for the light species. Introducing polarization, an imbalance in the population between species, we find a fully phase-separated ferromagnet. This phase exists for a broad range of temperatures and polarizations. By using finite size scaling of the susceptibility, we find a critical exponent which is consistent with the two-dimensional Ising universality class. Significantly, since the global entropy of this phase is higher than that of the ferromagnetic phase with single species, its experimental observation in cold atoms may be feasible.",1405.4849v2 2014-05-22,Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO$_{1-x}$F$_{x}$BiS$_{2}$ revealed by Ce $L_3$-edge x-ray absorption spectroscopy,"We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO$_{1-x}$F$_x$BiS$_2$, in which the superconductivity of the BiS$_2$ layer and the ferromagnetism of the CeO$_{1-x}$F$_x$ layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS$_2$ exhibits coexistence of $4f^1$ (Ce$^{3+}$) and $4f^0$ (Ce$^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F-doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS$_2$ layer and the ferromagnetic CeO$_{1-x}$F$_x$ layer.",1405.5639v1 2014-05-22,Giant magnetoelectric effect in magnetic tunnel junctions coupled to an electromagnetic environment,"We study the magnetization dynamics in ferromagnet$\mid$insulator$\mid$ferromagnet and ferromagnet$\mid$insulator$\mid$normal metal ultra-small tunnel junctions, and the associated voltage drop in the presence of an electromagnetic environment assisting the tunneling processes. We show that the environment strongly affects the resulting voltage, which becomes a highly non-linear function of the precession cone angle $\theta$. We find that voltages comparable to the driving frequency $\omega$ can be reached even for small precession cone angles $\theta$, in stark contrast to the case where the environment is absent. Such an effect could be useful for the detection local magnetization precessions in textured ferromagnets or, conversely, for probing the environment via the magnetization dynamics.",1405.5744v2 2014-06-05,"Magnetotransport in ferromagnetic Mn5Ge3, Mn5Ge3C0.8, and Mn5Si3C0.8 thin films","The electrical resistivity, anisotropic magnetoresistance (AMR), and anomalous Hall effect of ferromagnetic Mn5Ge3, Mn5Ge3C0.8, and Mn5Si3C0.8 thin films has been investigated. The data show a behavior characteristic for a ferromagnetic metal, with a linear increase of the anomalous Hall coefficient with Curie temperature. While for ferromagnetic Mn5Si3C0.8 the normal Hall coefficient R0 and the AMR ratio are independent of temperature, these parameters strongly increase with temperature for the germanide films. This difference is attributed to the different hybridization of electronic states in the materials due different lattice parameters and different atomic configurations (Ge vs. Si metalloid). The concomitant sign change of R0 and the AMR ratio with temperature observed for the germanide films is discussed in a two-current model indicating an electron-like minority-spin transport at low temperatures.",1406.1442v1 2014-06-10,Multiferroic interfaces composed of d0 perovskites oxides,"We investigate the electronic, ferroelectric and magnetic properties of KTaO3/PbTiO3 interfaces by using conventional density functional theory (DFT) and advanced DFT such as hybrid functional HSE06. We show that doped holes in valence bands or electrons in conduction bands give rise to ferromagnetism at the interfaces. The ferromagnetic states are ground states for both hole-doped (p-type) and electron-doped (n-type) interfaces by comparison with their corresponding nonmagnetic and antiferromagnetic states. Carriers (holes or electrons) concentrate near the interface to screen the polarization charge and thus the concentration of carrier varies with the ferroelectric polarization. Furthermore, the interface magnetization, which is nearly proportional to the concentration of carrier, can be tuned by ferroelectric polarization reversal, leading to strong intrinsic magnetoelectric effects at the interface of originally nonmagnetic KTaO3 and PbTiO3. Interestingly, a ferromagnetic-nonmagnetic transition tuned by an applied electric field can be realized at the p-type interface. This suggests an illuminating approach to multiferroic materials beyond conventional single-phase multiferroics and multi-phase multiferroics such as ferroelectric/ferromagnet heterostructures. The KTaO3/PbTiO3 interfaces may be promising in future multiferroic devices applications.",1406.2595v2 2014-06-16,The suppression of Curie temperature by Sr doping in diluted ferromagnetic semiconductor (La1-xSrx)(Zn1-yMny)AsO,"(La1-xSrx)(Zn1-yMny)AsO is a two dimensional diluted ferromagnetic semiconductor that has the advantage of decoupled charge and spin doping. The substitution of Sr2+ for La3+ and Mn2+ for Zn2+ into the parent semiconductor LaZnAsO introduces hole carriers and spins, respectively. This advantage enables us to investigate the influence of carrier doping on the ferromagnetic ordered state through the control of Sr concentrations in (La1-xSrx)(Zn0.9Mn0.1)AsO. 10 % Sr doping results in a ferromagnetic ordering below TC ~ 30 K. Increasing Sr concentration up to 30 % heavily suppresses the Curie temperature and saturation moments. Neutron scattering measurements indicate that no structural transition occurs for (La0.9Sr0.1)(Zn0.9Mn0.1)AsO below 300 K.",1406.4439v1 2015-12-20,Piezo Voltage Controlled Planar Hall Effect Devices,"The electrical control of the magnetization switching in ferromagnets is highly desired for future spintronic applications. Here we report on hybrid piezoelectric (PZT) /ferromagnetic (Co2FeAl) devices in which the planar Hall voltage in the ferromagnetic layer is tuned solely by piezo voltages. The change of planar Hall voltage is associated with magnetization switching through 90 in the plane under piezo voltages. Room temperature magnetic NOT and NOR gates are demonstrated based on the piezo voltage controlled Co2FeAl planar Hall effect devices without the external magnetic field. Our demonstration may lead to the realization of both information storage and processing using ferromagnetic materials.",1512.06374v2 2015-12-29,Effect of External Magnetic Field on the Co-existence of Superconductivity and Antiferromagnetism in rare Earth Nickle Borocarbides (RNi2B2C),"In this paper we have studied the effect of external magnetic field in the co-existing phase of superconducting and anti-ferromagnetism of rare earth nickel borocarbides. The anti-ferromagnetism in these systems might have originated due to both localized 'f' electrons as well as itinerant electrons which are responsible for conduction. On the other hand, superconductivity is due to spin density wave, arising out of Fermi surface instability. The anti-ferromagnetism order is mostly influenced by hybridization of the 'f' electron with the conduction electron. Here we have obtained the dependence of superconducting energy gap as well as staggered magnetic field on temperature T and energy $\epsilon_k$ in a framework based on mean field Hamiltonian using double time electron Green's function. We have shown in our calculation the effect of external magnetic field on superconducting and anti-ferromagnetic order parameter for $YNi_2B_2C$ in the presence of hybridization. The ratio of the calculated effective gap and $T_C$ is close to BCS value which agrees quite well with experimental results.",1512.08702v1 2016-05-17,Self-current induced spin-orbit torque in FeMn/Pt multilayers,"Extensive efforts have been devoted to the study of spin-orbit torque in ferromagnetic metal/heavy metal bilayers and exploitation of it for magnetization switching using an in-plane current. As the spin-orbit torque is inversely proportional to the thickness of the ferromagnetic layer, sizable effect has only been realized in bilayers with an ultrathin ferromagnetic layer. Here we demonstrate that, by stacking ultrathin Pt and FeMn alternately, both ferromagnetic properties and current induced spin-orbit torque can be achieved in FeMn/Pt multilayers without any constraint on its total thickness. The critical behavior of these multilayers follows closely three-dimensional Heisenberg model with a finite Curie temperature distribution. The spin torque effective field is about 4 times larger than that of NiFe/Pt bilayer with a same equivalent NiFe thickness. The self-current generated spin torque is able to switch the magnetization reversibly without the need for an external field or a thick heavy metal layer. The removal of both thickness constraint and necessity of using an adjacent heavy metal layer opens new possibilities for exploiting spin-orbit torque for practical applications.",1605.05135v1 2016-05-18,Neutron depolarization imaging of the hydrostatic pressure dependence of inhomogeneous ferromagnets,"The investigation of fragile and potentially inhomogeneous forms of ferromagnetic order under extreme conditions, such as low temperatures and high pressures, is of central interest for areas such as geophysics, correlated electron systems, as well as the optimization of materials synthesis for applications where particular material properties are required. We report neutron depolarization imaging measurements on the weak ferromagnet Ni$_3$Al under pressures up to 10kbar using a Cu:Be clamp cell. Using a polychromatic neutron beam with wavelengths $\lambda\geq$ 4\AA in combination with $^3$He neutron spin filter cells as polarizer and analyzer we were able to track differences of the pressure response in inhomogeneous samples by virtue of high resolution neutron depolarization imaging. This provides spatially resolved and non-destructive access to the pressure dependence of the magnetic properties of inhomogeneous ferromagnetic materials.",1605.05459v1 2016-05-21,Magnetism and Superconductivity in Ferromagnetic Heavy Fermion System UCoGe under In-plane Magnetic Fields,"We study the ferromagnetic superconductor UCoGe at ambient pressure under $ab$-plane magnetic fields $\vec{H}$ which are perpendicular to the ferromagnetic easy axis. It is shown that, by taking into account the Dyaloshinskii-Moriya interaction arising from the zigzag chain crystal structure of UCoGe, we can qualitatively explain the experimentally observed in-plane anisotropy for critical magnetic fields of the paramagnetic transition. Because of this strong dependence on the magnetic field direction, upper critical fields of superconductivity, which is mediated by ferromagnetic spin fluctuations, also become strongly anisotropic. The experimental observation of ""S-shaped"" $H_{c2}\parallel b$-axis is qualitatively explained as a result of enhancement of the spin fluctuations due to decreased Curie temperature by the $b$-axis magnetic field. We also show that the S-shaped $H_{c2}$ is accompanied by a rotation of the $d$-vector, which would be a key to understand the experiments not only at ambient pressure but also under pressure.",1605.06625v1 2016-05-23,An effective mean field theory for the coexistence of anti-ferromagnetism and superconductivity: Applications to iron-based superconductors and cold Bose-Fermi atomic mixtures,"We study an effective fermion model on a square lattice to investigate the cooperation and competition of superconductivity and anti-ferromagnetism. In addition to particle tunneling and on-site interaction, a bosonic excitation mediated attractive interaction is also included in the model. We assume that the attractive interaction is mediated by spin fluctuations and excitations of Bose-Einstein condensation (BEC) in electronic systems and Bose-Fermi mixtures on optical lattices, respectively. Using an effective mean-field theory to treat both superconductivity and anti-ferromagnetism at equal footing, we study the model within the Landau energy functional approach and a linearized theory. Within our approaches, we find possible co-existence of superconductivity and anti-ferromagnetism for both electronic and cold-atomic models. Our linearized theory shows while spin fluctuations favor d-wave superconductivity and BEC excitations favor s-wave superconductivity.",1605.07073v3 2016-05-29,A New Ferromagnetic Superconductor: CsEuFe$_4$As$_4$,"Superconductivity (SC) and ferromagnetism (FM) are in general antagonistic, which makes their coexistence very rare. Following our recent discovery of robust coexistence of SC and FM in RbEuFe$_4$As$_4$ [Y. Liu et al., arXiv: 1605.04396 (2016)], here we report another example of such a coexistence in its sister compound CsEuFe$_4$As$_4$, synthesized for the first time. The new material exhibits bulk SC at 35.2 K and Eu$^{2+}$-spin ferromagnetic ordering at 15.5 K, demonstrating that it is a new robust ferromagnetic superconductor.",1605.09007v1 2016-05-31,Pure spin current devices based on ferromagnetic topological insulators,"Two-dimensional topological insulators possess two counter propagating edge channels with op- posite spin direction. Recent experimental progress allowed to create ferromagnetic topological insulators realizing a quantum anomalous Hall (QAH) state. In the QAH state one of the two edge channels disappears due to the strong ferromagnetic exchange field. We investigate heterostuctures of topological insulators and ferromagnetic topological insulators by means of numerical transport calculations. We show that spin current flow in such heterostructures can be controlled with high fidelity. Specifically, we propose spintronic devices that are capable of creating, switching and de- tecting pure spin currents using the same technology. In these devices electrical currents are directly converted into spin currents, allowing a high conversion efficiency. Energy independent transport properties in combination with large bulk gaps in some topological insulator materials may allow operation even at room temperature.",1605.09590v1 2016-08-03,Magnetoanisotropic Josephson effect due to interfacial spin-orbit fields in superconductor/ferromagnet/superconductor junctions,"We study theoretically the effects of interfacial Rashba and Dresselhaus spin-orbit coupling in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions---with allowing for tunneling barriers between the layers---by solving the Bogoljubov-de Gennes equation for a realistic heterostructure and applying the Furusaki-Tsukada technique to calculate the electric current at a finite temperature. The presence of spin-orbit couplings leads to out- and in-plane magnetoanisotropies of the Josephson current, which are giant in comparison to current magnetoanisotropies in similar normal-state ferromagnet/normal metal (F/N) junctions. Especially huge anisotropies appear in the vicinity of $ 0 $-$ \pi $ transitions, caused by the exchange-split bands in the ferromagnetic metal layer. We also show that the direction of the Josephson critical current can be controlled (inducing $ 0 $-$ \pi $ transitions) by the strength of the spin-orbit coupling and, more crucial, by the orientation of the magnetization. Such a control can bring new functionalities into Josephson junction devices.",1608.01218v2 2016-08-08,Biaxial-stress-driven full spin polarization in ferromagnetic hexagonal chromium telluride,"It is important to spintronics to achieve fully-spin-polarized magnetic materials that are stable and can be easily fabricated. Here, through systematical density-functional-theory investigations, we achieve high and even full spin polarization for carriers in the ground-state phase of CrTe by applying tensile biaxial stress. The resulting strain is tensile in the xy plane and compressive in the z axis. With the in-plane tensile strain increasing, the ferromagnetic order is stable against antiferromagnetic fluctuations, and a half-metallic ferromagnetism is achieved at an in-plane strain of 4.8%. With the spin-orbit coupling taken into account, the spin polarization is equivalent to 97% at the electronic phase transition point, and then becomes 100.0% at the in-plane strain of 6.0%. These make us believe that the full-spin-polarized ferromagnetism in this stable and easily-realizable hexagonal phase could be realized soon, and applied in spintronics.",1608.02405v3 2016-12-10,Devil's Staircase Continuum in the Chiral Clock Spin Glass with Competing Ferromagnetic-Antiferromagnetic and Left-Right Chiral Interactions,"The chiral clock spin-glass model with q=5 states, with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations, is studied in d=3 spatial dimensions by renormalization-group theory. The global phase diagram is calculated in temperature, antiferromagnetic bond concentration p, random chirality strength, and right-chirality concentration c. The system has a ferromagnetic phase, a multitude of different chiral phases, a chiral spin-glass phase, and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil's staircases, where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase, bordered by fragments of regular and temperature-inverted devil's staircases, are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos.",1612.03330v2 2016-12-21,Quantum Griffiths phase inside the ferromagnetic phase of Ni$_{1-x}$V$_x$,"We study by means of bulk and local probes the d-metal alloy Ni$_{1-x}$V$_x$ close to the quantum critical concentration, $x_c \approx 11.6\%$, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly $x$-dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition.",1612.07207v2 2016-12-21,Chirality transitions in frustrated ferromagnetic spin chains: a link with the gradient theory of phase transitions,"We study chirality transitions in frustrated ferromagnetic spin chains, in view of a possible connection with the theory of Liquid Crystals. A variational approach to the study of these systems has been recently proposed by Cicalese and Solombrino, focusing close to the helimagnet-ferromagnet transition point corresponding to the critical value of the frustration parameter $\alpha=4$. We reformulate this problem for any $\alpha\geq0$ in the framework of surface energies in nonconvex discrete systems with nearest neighbors ferromagnetic and next-to-nearest neighbors antiferromagnetic interactions and we link it to the gradient theory of phase transitions, by showing a uniform equivalence by $\Gamma$-convergence on $[0,4]$ with Modica-Mortola type functionals.",1612.07262v4 2017-06-07,Density-controlled quantum Hall ferromagnetic transition in a two-dimensional hole system,"Quantum Hall ferromagnetic transitions are typically achieved by increasing the Zeeman energy through in-situ sample rotation, while transitions in systems with pseudo-spin indices can be induced by gate control. We report here a gate-controlled quantum Hall ferromagnetic transition between two real spin states in a conventional two-dimensional system without any in-plane magnetic field. We show that the ratio of the Zeeman splitting to the cyclotron gap in a Ge two-dimensional hole system increases with decreasing density owing to inter-carrier interactions. Below a critical density of $\sim2.4\times 10^{10}$ cm$^{-2}$, this ratio grows greater than $1$, resulting in a ferromagnetic ground state at filling factor $\nu=2$. At the critical density, a resistance peak due to the formation of microscopic domains of opposite spin orientations is observed. Such gate-controlled spin-polarizations in the quantum Hall regime opens the door to realizing Majorana modes using two-dimensional systems in conventional, low-spin-orbit-coupling semiconductors.",1706.02044v1 2017-06-13,Generalized quasiclassical theory of the long-range proximity effect and spontaneous currents in superconducting heterostructures with strong ferromagnets,"We present the generalized quasiclassical theory of the long-range superconducting proximity effect in heterostructures with strong ferromagnets, where the exchange splitting is of the order of Fermi energy. In the ferromagnet the propagation of spin-triplet Cooper pairs residing on the spin-split Fermi surfaces is shown to be governed by the spin-dependent Abelian gauge field which results either from the spin-orbital coupling or from the magnetic texture. The additional gauge field enters into the quasiclassical equations in superposition with the usual electromagnetic vector potential and results in the generation of spontaneous superconducting currents and phase shifts in various geometries which provide the sources of long-range spin-triplet correlations. We derive the Usadel equations and boundary conditions for the strong ferromagnet and consider several generic examples of the Josephson systems supporting spontaneous currents.",1706.04239v2 2017-06-15,First order magneto-structural transition and magnetocaloric effect in MnNiGe$_{0.9}$Ga$_{0.1}$,"The first order magneto-structural transition ($T_t\simeq95$ K) and magnetocaloric effect in MnNiGe$_{0.9}$Ga$_{0.1}$ are studied via powder x-ray diffraction and magnetization measurements. Temperature dependent x-ray diffraction measurements reveal that the magneto-structural transition remains incomplete down to 23 K, resulting in a coexistence of antiferromagnetic and ferromagnetic phases at low temperatures. The fraction of the high temperature Ni$_2$In-type hexagonal ferromagnetic and low temperature TiNiSi-type orthorhombic antiferromagnetic phases is estimated to be $\sim 40\%$ and $\sim 60\%$, respectively at 23 K. The ferromagnetic phase fraction increases with increasing field which is found to be in non-equilibrium state and gives rise to a weak re-entrant transition while warming under field-cooled condition. It shows a large inverse magnetocaloric effect across the magneto-structural transition and a conventional magnetocaloric effect across the second order paramagnetic to ferromagnetic transition. The relative cooling power which characterizes the performance of a magnetic refrigerant material is found to be reasonably high compared to the other reported magnetocaloric alloys.",1706.04740v1 2017-06-26,Magnetic Proximity Effect in Pt/CoFe2O4 Bilayers,"We observe the magnetic proximity effect (MPE) in Pt/CoFe2O4 bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe2O4. By demonstrating MPE in Pt/CoFe2O4, these results establish the spinel ferrite family as a promising material for MPE and spin manipulation via proximity exchange fields.",1706.08473v1 2017-07-10,Theory of in-plane current induced spin torque in metal/ferromagnet bilayers,"Using a semiclassical approach that simultaneously incorporates the spin Hall effect (SHE), spin diffusion, quantum well states, and interface spin-orbit coupling (SOC), we address the interplay of these mechanisms as the origin of the in-plane current induced spin torque observed in the normal metal/ferromagnetic metal bilayer thin films. Focusing on the bilayers with a ferromagnet much thinner than its spin diffusion length, such as Pt/Co with $\sim 10$nm thickness, our approach addresses simultaneously the two contributions to the spin torque, namely the spin-transfer torque (SHE-STT) due to SHE induced spin injection, and the spin-orbit torque (SOT) due to SOC induced spin accumulation. The SOC produces an effective magnetic field at the interface, hence it modifies the angular momentum conservation expected for the SHE-STT. The SHE induced spin voltage and the interface spin current are mutually dependent, hence are solved in a self-consistent manner. In addition, the spin transport mediated by the quantum well states may be responsible for the experimentally observed rapid variation of the spin torque with respect to the thickness of the ferromagnet.",1707.02771v1 2017-07-13,Ferromagnetism in the Hubbard model with a gapless nearly-flat band,"We present a version of the Hubbard model with a gapless nearly-flat lowest band which exhibits ferromagnetism in two or more dimensions. The model is defined on a lattice obtained by placing a site on each edge of the hypercubic lattice, and electron hopping is assumed to be only between nearest and next nearest neighbor sites. The lattice, where all the sites are identical, is simple, and the corresponding single-electron band structure, where two cosine-type bands touch without an energy gap, is also simple. We prove that the ground state of the model is unique and ferromagnetic at half-filling of the lower band, if the lower band is nearly flat and the strength of on-site repulsion is larger than a certain value which is independent of the lattice size. This is the first example of ferromagnetism in three dimensional non-singular models with a gapless band structure.",1707.03944v2 2017-10-11,Weak ferromagnetism and glassy state in kappa-(BEDT-TTF)2Hg(SCN)2Br,"Since the first observation of weak ferromagnetism in the charge-transfer salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840 (1992)], no further evidence of ferromagnetism in this class of organic materials has been reported. Here we present static and dynamic spin susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the molecular dimers suppress long-range order, forming a spin-glass ground state in the insulating phase.",1710.04028v3 2017-10-25,Dimensionality driven enhancement of ferromagnetic superconductivity in URhGe,"In most unconventional superconductors, like the high-Tc cuprates, iron pnictides, or heavy fermion systems, superconductivity emerges in the proximity of an electronic instability. Identifying unambiguously the pairing mechanism remains nevertheless an enormous challenge. Among these systems, the orthorhombic uranium ferromagnetic superconductors have a unique position, notably because magnetic fields couple directly to ferromagnetic order, leading to the fascinating discovery of the re-emergence of superconductivity in URhGe at high field. Here we show that uniaxial stress is a remarkable tool allowing fine-tuning of the pairing strength. With a relatively small stress, the superconducting phase diagram is spectacularly modified, with a merging of the low and high field superconducting states and a significant enhancement of superconductivity. The superconducting critical temperature increases both at zero field and under field, reaching 1K, more than twice higher than at ambient pressure. The enhancement of superconductivity is directly related to a change of the magnetic dimensionality with an increase of the transverse magnetic susceptibility, demonstrating that in addition to the Ising-type longitudinal ferromagnetic fluctuations, transverse magnetic fluctuations also play an important role in the superconducting pairing.",1710.09097v1 2017-12-13,Comparison of dynamic behavior of ferroelectric and ferromagnetic nematic suspensions,"The pioneering realization of suspensions of ferroelectric nanoparticles in a nematic host was one of excellent contributions of Yuri Reznikov to the science of liquid crystals [Y. Reznikov et al., Appl. Phys. Lett. 82 (2003) 191]. This achievement created great excitement as entailed the enhancement of materials dielectric properties and increase of the phase transition temperature between nematic and isotropic phase. In this contribution, we examine the spectrum of fundamental fluctuations in one of his ferroelectric suspensions by dynamic light scattering measurements, which gives insight into the coupling between the particles and the orientation of the liquid crystalline matrix. We set side by side these results with the equivalent ones obtained for the case of stable suspensions of ferromagnetic nanoplatelets in a nematic matrix showing macroscopic ferromagnetic ordering. The underlying origin of the difference between the particle-matrix coupling observed in both cases is discussed and its effect on the orientational fluctuations spectrum is compared.",1712.04668v1 2017-12-14,Theoretical prediction of a two-dimensional intrinsic double-metal ferromagnetic semiconductor MnCoO4,"A two-dimensional double-metal oxide MnCoO4 was predicted to be an intrinsic ferromagnetic semiconductor by using density functional theory. The low cleavage energy 0.36 Jm-2, which is similar to that of graphene, indicates that it can be easily exfoliated. The bulk structure has an antiferromagnetic ground state while the ferromagnetic configuration is the ground state against two antiferromagnetic and three ferrimagnetic configurations in the two-dimensional structure. The spin flip gaps for valence and conduction bands are 0.41 and 0.10 eV calculated with the HSE06 density functional, which are much larger than the thermal energy at room temperature. The Curie temperature obtained from the Monte Carlo simulation is 40 K. Under 9% tensile strain, the spin flip gaps increase largely so that the spin flip can be suppressed. The direct antiferromagnetic coupling between the Mn and Co atoms reduces largely while the indirect ferromagnetic couplings between two Mn or two Co atoms mediated by the O atoms do not decrease much in the stretched structure. The Curie temperature increases to 230 K, higher than the dry ice temperature. Moreover, phonon dispersion indicates that the MnCoO4 is also stable under the tensile stain. Therefore, two-dimensional MnCoO4 could be a good candidate for low-dimensional spintronics.",1712.05071v1 2018-01-08,Direct observation of double exchange in ferromagnetic La$_{0.7}$Sr$_{0.3}$CoO$_3$ by broadband ellipsometry,"We present results of our broadband ellipsometry measurements of the optical response of ferromagnetic La$_{0.7}$Sr$_{0.3}$CoO$_3$. Our data show that the ferromagnetic transition is accompanied by a transfer of optical spectral weight from an absorption band centered at 1.5 eV to a narrow component of the Drude-like peak. The associated reduction of the intraband kinetic energy is significantly larger than $k_{\rm B}T_c$, confirming that the double exchange plays a major role in the ferromagnetism of doped cobaltites. In conjunction with results of recent theoretical studies, the temperature dependence of the Drude-like peak suggests that the double exchange is mediated by $t_{2g}$ orbitals.",1801.02424v2 2018-01-10,The hidden magnetization in ferromagnetic material: Miamagnetism,"This paper presents the hidden magnetization features of ferromagnetic materials: called miamagnetism. As we know, we have several forms of magnetization: the diamagnetism, the paramagnetism, the ferromagnetism etc. The main character of the diamagnetism is that its magnetic susceptibility is negative (from -1e-9 for gas and -1e-6 for liquid and solid to -1 for superconducting materials of type I) and it is not less than -1 unless for special materials like metamaterials at high frequencies. The miamagnetism has the character that the magnetic susceptibility can reach at low frequencies a negative value of -155 of magnitude leading to a negative permeability. We can not see it because it is hidden by the ferromagnetic character which has a high positive magnetic susceptibility. We use the discrete Fourier transform to illustrate this hidden character and the hysteresis model can be represented only by harmonics of (2n+1)f0 of magnitude. This magnetization follows a Boltzmann distribution for the modulus of theses harmonics.",1801.03479v1 2018-01-30,Critical behavior of two-dimensional intrinsically ferromagnetic semiconductor CrI3,"CrI3, which belongs to a rare category of two-dimensional (2D) ferromagnetic semiconductors, is of great interest for spintronic device applications. Unlike CrCl3 whose magnetism presents a 2D-Heisenberg behavior, CrI3 exhibits a larger van der Waals gap, smaller cleavage energy, and stronger magnetic anisotropy which could lead to a 3D magnetic characteristic. Hence, we investigate the critical behavior of CrI3 in the vicinity of magnetic transition. We use the modified Arrott plot and Kouvel-Fisher method, and conduct critical isotherm analysis to estimate the critical exponents near the ferromagnetic phase transition. This shows that the magnetism of CrI3 follows the crossover behavior of a 3D-Ising model with mean field type interactions where the critical exponents \b{eta}, {\gamma}, and {\delta} are 0.323, 0.835, and 3.585, respectively, at the Curie temperature of 64 K. We propose the crossover behavior can be attributed to the strong uniaxial anisotropy and inevitable interlayer coupling. Our experiment demonstrates the applicability of crossover behavior to a 2D ferromagnetic semiconductor.",1801.09878v1 2018-09-10,Electrodynamics of Josephson junctions containing strong ferromagnets,"Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization can penetrate thicker ferromagnetic barriers compared to the singlet component. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics.",1809.03301v1 2018-09-13,Enhanced magnetoelectric effect of exactly solved spin-electron model on a doubly decorated square lattice in vicinity of a continuous phase transition,"Magnetoelectric properties of a coupled spin-electron model on a doubly decorated square lattice in an external electric field applied along the crystallographic axis [11] are rigorously examined with the help of generalized decoration-iteration transformation. The phase diagram, spontaneous magnetization and electric polarization are exactly calculated and their dependencies are comprehensively investigated under a concurrent influence of temperature and electric field. It is found that the electric field mostly stabilizes at zero temperature the spontaneous antiferromagnetic order with respect to the ferromagnetic one. At finite temperatures the external electric field gradually suppresses a spontaneous ferromagnetic (antiferromagnetic) order emergent close to a quarter (half) filling. An enhanced magnetoelectric response is detectable in vicinity of a continuous phase transition at which the spontaneous magnetization vanishes and the electric polarization displays a weak-type singularity. It is demonstrated that reentrant phase transitions of the ferromagnetic or antiferromagnetic phase may be induced at moderate values of the electric field, which simultaneously produces a sharp kink in a critical line of the ferromagnetic phase nearby a quarter filling.",1809.04889v1 2018-09-21,Defect Induced Room Temperature Ferromagnetism in Methylammonium Lead Iodide Perovskite,"The defect tolerance nature of organic-inorganic hybrid perovskite is reflected from its stupendous growth in photovoltaic performances. The presence of lattice defect can manipulate or even gives rise to some exceptional properties which otherwise would have remained unseen. One of such properties reported in this article is the experimental observation of defect mediated room temperature ferromagnetism in methylammonium lead halide perovskite for the very first time, ably supported by ab-initio calculations. Theoretical analysis predicts the ferromagnetism principally arises from the iodide vacancies in the orthorhombic and cubic crystal phases but not in the tetragonal phase. The low temperature (100 K) ferromagnetic hysteresis loop was stable even at a high temperature of 380 K substantiating the fact that the origin of magnetism embedded in its defective nature.",1809.07957v1 2018-10-02,Spin-dependent (inverse) spin Hall effect in Co$_{60}$Fe$_{20}$B$_{20}$,"In ferromagnetic metals, the interconversion of spin and charge currents via the spin Hall effect and its inverse can depend on the angle between the ferromagnets magnetization and the spin current polarization direction. Here, such a spin-dependent (inverse) spin Hall effect is found in the ferromagnetic alloy Co$_{60}$Fe$_{20}$B$_{20}$. In a nonlocal magnon transport experiment, Co$_{60}$Fe$_{20}$B$_{20}$ is used to both excite and detect magnonic spin currents flowing in the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. We find that the signal amplitude is significantly modulated by tuning the direction of the Co$_{60}$Fe$_{20}$B$_{20}$ magnetization. We design a sample structure that completely prevents direct magnonic coupling between the ferromagnets. Thus, we can identify unambiguously an intrinsic electronic origin of the observed effect.",1810.01227v3 2018-10-03,Induced Energy Gap in Finite-Sized Superconductor/Ferromagnet Hybrids,"We theoretically study self-consistent proximity effects in finite-sized systems consisting of ferromagnet ($\rm F$) layers coupled to an $s$-wave superconductor ($\rm S$). We consider both $\rm SF_1F_2$ and $\rm SH$ nanostructures, where the $\rm F_1 F_2$ bilayers are uniformly magnetized, and the ferromagnetic $\rm H$ layer possesses a helical magnetization profile. We find that when the $\rm F_1 F_2$ layers are weakly ferromagnetic, a hard gap can emerge when the relative magnetization directions are rotated from parallel to antiparallel. Moreover, the gap is most prominent when the thicknesses of $\rm F_1$ and $\rm F_2$ satisfy $\rm d_{F1}\leq d_{F2}$, respectively. For the $\rm SH$ configuration, increasing the spatial rotation period of the exchange field can enhance the induced hard gap. Our investigations reveal that the origin of these findings can be correlated with the propagation of quasiparticles with wavevectors directed along the interface. To further clarify the source of the induced energy gap, we also examine the spatial and energy resolved density of states, as well as the spin-singlet, and spin-triplet superconducting correlations, using experimentally accessible parameter values. Our findings can be beneficial for designing magnetic hybrid structures where a tunable superconducting hard gap is needed.",1810.01886v1 2018-10-19,Ferromagnetic nodal-line metal in monolayer {\em h}-InC,"Based on first-principles calculations, we predict a new two-dimensional ferromagnetic material that exhibits exotic Fermi surface topology. We show that monolayer hexagonal indium carbide ({\em h}-InC) is thermodynamically and dynamically stable, and it energetically favors the ferromagnetic ordering of spins. The perfectly planar geometry in two dimensions, together with ferromagnetism, gives rise to a unique opportunity to encounter intriguing electronic properties, captured in the Fermi surface and band topology. We show that multiple nodal lines coexist in momentum space, accompanied by the electron and hole pockets that touch each other linearly at the nodal lines. Inclusion of spin-orbit coupling enriches the magnetic and electronic properties of {\em h}-InC. Spin-orbit coupling leads to an easy-plane type magnetocrystalline anisotropy, and the nodal lines can be tuned into topological nodal points, contingent upon the magnetization direction. Symmetry analysis and a tight-binding model are provided to explain the nodal structure of the bands. Our findings suggest {\em h}-InC as a new venue for supporting carbon-based magnetism and exotic band topology in two dimensions.",1810.08563v2 2019-06-18,Two Dimensional Ferromagnetic Semiconductor: Monolayer CrGeS$_3$,"Recently, two-dimensional ferromagnetic semiconductors have been an important class of materials for many potential applications in spintronic devices. Based on density functional theory, we systematically explore the magnetic and electronic properties of CrGeS$_3$ with the monolayer structures. The comparison of total energy between different magnetic states ensures the ferromagnetic ground state of monolayer CrGeS$_3$. It is also shown that ferromagnetic and semiconducting properties are exhibited in monolayer CrGeS$_3$ with the magnetic moment of 3 $\mu_{B}$ for each Cr atom, donated mainly by the intense $dp$$\sigma$-hybridization of Cr $e_g$-S $p$. There are the bandgap of 0.70 eV of spin-up state in the monolayer structure when 0.77 eV in spin-down state. The global gap is 0.34 eV (2.21 eV by using HSE06 functional), which originates from bonding $dp\sigma$ hybridized states of Cr $e_g$-S $p$ and unoccupied Cr $t_{2g}$-Ge $p$ hybridization. Besides, we estimate that the monolayer CrGeS$_3$ possesses the Curie temperature of 161 K by mean-field theory.",1906.07396v1 2019-06-27,Magnetic proximity effect induced FMR frequency enhancement in {Py/FeMn} bilayers,"Ferromagnetic resonance (FMR) in exchange-coupled ferromagnet-antiferromagnet (FM/AFM) bilayers commonly shows a moderate increase in the resonance frequency owing to the induced unidirectional anisotropy. Here we report a large FMR frequency enhancement toward the sub-THz range observed in Py/FeMn with ultrathin AFM FeMn. The effect is connected with a sizable induced magnetic moment in FeMn caused by the magnetic proximity effect from the Py layer. The observed FMR properties are explained as due to the competing intrinsic antiferromagnetic order and the ferromagnetic proximity effect in nanometer thin FeMn. Our results show that combining materials with strong and weak anti/ferromagnetic ordering can potentially close the notoriously difficult GHz-THz gap important for high-speed spintronic applications.",1906.11688v2 2019-06-27,Perspectives of Electrically generated spin currents in ferromagnetic materials,"Spin-orbit coupling enables charge currents to give rise to spin currents and vice versa, which has applications in non-volatile magnetic memories, miniature microwave oscillators, thermoelectric converters and Terahertz devices. In the past two decades, a considerable amount of research has focused on electrical spin current generation in different types of nonmagnetic materials. However, electrical spin current generation in ferromagnetic materials has only recently been actively investigated. Due to the additional symmetry breaking by the magnetization, ferromagnetic materials generate spin currents with different orientations of spin direction from those observed in nonmagnetic materials. Studies centered on ferromagnets where spin-orbit coupling plays an important role in transport open new possibilities to generate and detect spin currents. We summarize recent developments on this subject and discuss unanswered questions in this emerging field.",1906.11772v1 2019-07-21,Layer Dependence of Stacking Order in Nonencapsulated Few-layer CrI3,"Long-range magnetic orders in atomically thin ferromagnetic CrI3 give rise to new fascinating physics and application perspectives. The physical properties of two-dimensional (2D) ferromagnetism CrI3 are significantly influenced by interlayer spacing and stacking order, which are sensitive to the hydrostatic pressure and external environments. However, there remains debate on the stacking order at low temperature. Here, we study the interlayer coupling and stacking order of non-encapsulated 2-5 layer and bulk CrI3 at 10 K by Raman spectroscopy; demonstrate a rhombohedral stacking in both antiferromagnetic and ferromagnetic CrI3. The opposite helicity dependence of Ag and Eg modes arising from phonon symmetry further validate the rhombohedral stacking. An anomalous temperature-dependent behavior is observed due to spin-phonon coupling below 60 K. Our work provides insights into the interlayer coupling and stacking orders of 2D ferromagnetic materials.",1907.08886v2 2019-07-23,Indication of Ferromagnetic Quantum Critical Point in Kondo Lattice CeRh$_6$Ge$_4$,"We report resistivity measurements under pressure for Kondo-lattice ferromagnet CeRh$_6$Ge$_4$, and present that a quantum ferromagnetic (FM) phase transition is easily achieved. In most clean metallic ferromagnets, a quantum critical point (QCP) at zero field is avoided by changing the FM transition to a discontinuous transition or to an antiferromagnetic transition. In CeRh$_6$Ge$_4$, to the contrary, the Curie temperature of 2.5 K decreases continuously as increasing pressure without any clear signature that the transition changes to first order. The obvious non Fermi liquid behavior is observed in the vicinity of the quantum FM phase transition. The experimental data do not contradict a picture in which CeRh$_6$Ge$_4$ shows the FM QCP at zero field. Band structure calculation suggests the unusual electronic state of CeRh$_6$Ge$_4$ among Ce-based Kondo lattices. CeRh$_6$Ge$_4$ deserves further investigations and will be a key material to understand the matter of the FM QCP.",1907.09802v2 2019-07-30,Understanding the ferromagnetic insulating state in Cr doped VO$_2$,"Experimentally Cr doping in the rutile phase of VO$_2$ is found to stabilize a charge ordered ferromagnetic insulating state in the doping range of 10\% to 20\%. In this work, we investigated its origin at 12.5\% Cr doping using a combination of ab-initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab-initio band structure onto a tight-binding Hamiltonian allows one to calculate the energy gain from different exchange pathways. This gain is quantified in this work for the first time and the role of charge ordering in stabilizing a ferromagnetic insulating state is understood.",1907.12803v2 2019-07-31,Carrier free long-range magnetism in Mo doped one quintuple layer Bi2Te3 and Sb2Te3,"One of the keys to the realization of Quantum Anomalous Hall effect (QAHE) is long range ferromagnetism, which is only experimentally realized in Cr or V doped (Bi,Sb)2Te3 system. Both elements are 3d transition metals and 4d transition metals are found to be ineffective to produce long range ferromagnetism in Bi2Se3. Still, whether long range ferromagnetism can be realized by magnetic doping of 4d elements is an open question. Based on density functional theory calculations, we predict that long range ferromagnetism can be realized in Mo doped Bi2Te3 and Sb2Te3, which are semiconducting. The coupling strength is comparable with that of Cr doped Bi2Te3 and Sb2Te3. Therefore, Mo doped Bi2Te3 and Sb2Te3 or their alloys can be new systems to realize diluted magnetic semiconductors and QAHE.",1907.13282v1 2019-09-04,Proximity-induced ferromagnetism and chemical reactivity in few layers VSe2 heterostructures,"Among Transition-Metal Dichalcogenides, mono and few-layers thick VSe2 has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of Fe/VSe2 heterostructure by combining element sensitive absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk VSe2 do not show any signature of an intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the Fe/VSe2 interface and its relation with interfacial magnetic coupling.",1909.01713v1 2019-09-11,Spin dynamics of the planar kagome lattice ferromagnet with four-site ring exchange processes,"By means of quantum Monte Carlo simulations, combined with a stochastic analytic continuation, we examine the spin dynamics of the spin-1/2 planar (XY) ferromagnet on the kagome lattice with additional four-site ring exchange terms. Such exchange processes were previously considered to lead into an extended $Z_2$ quantum spin liquid phase beyond a quantum critical point from the XY-ferromagnet. We examine the dynamical spin structure factor in the non-magnetic regime and probe for signatures of spin fractionalization. Furthermore, we contrast our findings and the corresponding energy scales of the excitation gaps in the ring exchange model to those emerging in a related Balents-Fisher-Girvin model with a $Z_2$ quantum spin liquid phase, and monitor the softening of the magnon mode upon approaching the quantum critical point from the XY-ferromagnetic regime.",1909.04877v1 2019-09-16,Onset of ferromagnetism for strongly correlated electrons in one-dimensional chains,"The existence of the Nagaoka ferromagnetism is examined in the context of the one-dimensional $U=\infty$ Hubbard model. We construct the exact quantum partition function to describe the physics of such a regime. Our calculation reveals that, while the ground state in an open chain is always spin-degenerate, in a finite size closed chain with at least one vacancy, the ground state can only be ferromagnetic when the number of electrons is less or equal to three. Our results shed more light on a very recent experimental verification of Nagaoka ferromagnetism in a quantum dot set up.",1909.07349v3 2019-09-30,Phase diagram for ensembles of random close packed Ising-like dipoles as a function of texturation,"We study random close packed systems of magnetic spheres by Monte Carlo simulations in order to estimate their phase diagram. The uniaxial anisotropy of the spheres makes each of them behave as a single Ising dipole along a fixed easy axis. We explore the phase diagram in terms of the temperature and the degree of alignment (or texturation) among the easy axes of all spheres. This degree of alignment ranges from the textured case (all easy axes pointing along a common direction) to the non-textured case (randomly distributed easy axes). In the former case we find long-range ferromagnetic order at low temperature but, as the degree of alignment is diminished below a certain threshold, the ferromagnetic phase gives way to a spin-glass phase. This spin-glass phase is similar to the one previously found in other dipolar systems with strong frozen disorder. The transition between ferromagnetism and spin-glass passes through a narrow intermediate phase with quasi-long-range ferromagnetic order.",1909.13573v1 2019-10-21,Dynamics of an elliptical ferromagnetic skyrmion driven by the spin-orbit torque,"Magnetic skyrmion is a promising building block for developing information storage and computing devices. It can be stabilized in a ferromagnetic thin film with the Dzyaloshinskii-Moriya interaction (DMI). The moving ferromagnetic skyrmion may show the skyrmion Hall effect, that is, the skyrmion shows a transverse shift when it is driven by a spin current. Here, we numerically and theoretically study the current-driven dynamics of a ferromagnetic nanoscale skyrmion in the presence of the anisotropic DMI, where the skyrmion has an elliptical shape. The skyrmion Hall effect of the elliptical skyrmion is investigated. It is found that the skyrmion Hall angle can be controlled by tuning the profile of elliptical skyrmion. Our results reveal the relation between the skyrmion shape and the skyrmion Hall effect, which could be useful for building skyrmion-based spintronic devices with preferred skyrmion Hall angle. Also, our results provide a method for the minimization of skyrmion Hall angle for applications based on in-line motion of skyrmions.",1910.09341v2 2019-10-25,Local minima in disordered mean-field ferromagnets,"We consider the complexity of random ferromagnetic landscapes on the hypercube $\{\pm 1\}^N$ given by Ising models on the complete graph with i.i.d. non-negative edge-weights. This includes, in particular, the case of Bernoulli disorder corresponding to the Ising model on a dense random graph $\mathcal G(N,p)$. Previous results had shown that, with high probability as $N\to\infty$, the gradient search (energy-lowering) algorithm, initialized uniformly at random, converges to one of the homogeneous global minima (all-plus or all-minus). Here, we devise two modified algorithms tailored to explore the landscape at near-zero magnetizations (where the effect of the ferromagnetic drift is minimized). With these, we numerically verify the landscape complexity of random ferromagnets, finding a diverging number of (1-spin-flip-stable) local minima as $N\to\infty$. We then investigate some of the properties of these local minima (e.g., typical energy and magnetization) and compare to the situation where the edge-weights are drawn from a heavy-tailed distribution.",1910.11862v1 2019-10-29,Steady one-dimensional domain wall motion in biaxial ferromagnets: mapping of the Landau-Lifshitz equation to the sine-Gordon equation,"Motivated by the difference between the dynamics of magnetization textures in ferromagnets and antiferromagnets, the Landau-Lifshitz equation of motion is explored. A typical one-dimensional domain wall in a bulk ferromagnet with biaxial magnetic anisotropy is considered. In the framework of Walker-type of solutions of steady-state ferromagnetic domain wall motion, the reduction of the non-linear Landau-Lifshitz equation to a Lorentz-invariant sine-Gordon equation typical for antiferromagnets is formally possible for velocities lower than a critical velocity of the topological soliton. The velocity dependence of the domain wall energy and the domain wall width are expressed in the relativistic-like form in the limit of large ratio of the easy-plane/easy-axis anisotropy constants. It is shown that the mapping of the Landau-Lifshitz equation of motion to the sine-Gordon equation can be performed only by going beyond the steady-motion Walker-type of solutions.",1910.13266v2 2020-01-04,Persistent currents and spin torque caused by percolated quantum spin Hall state,"Motivated by recent experiments, we investigate the quantum spin Hall state in 2D topological insulator/ferromagnetic metal planar junctions by means of a tight-binding model and linear response theory. We demonstrate that whether the edge state Dirac cone is submerged into the ferromagnetic subbands and the direction of the magnetization dramatically affect (i) how the edge state percolates into the ferromagnet, and (ii) the spin-momentum locking of the edge state. Laminar flows of room temperature persistent charge and spin currents near the interface are uncovered. In addition, the current-induced spin polarization at the edge of the 2D topological insulator is found to be dramatically enhanced near the impurities. The current-induced spin polarization in the ferromagnet is mainly polarized in the out-of-plane direction ${\hat{\bf z}}$, rendering a current-induced spin torque that is predominantly field-like $\propto {\bf S}\times{\hat{\bf z}}$.",2001.01081v3 2020-01-09,High-Temperature Ferromagnetic Semiconductors: Janus Monolayer Vanadium Trihalides,"Two-dimensional (2D) intrinsic ferromagnetic semiconductors are expected to stand out in the spintronic field. Recently, the monolayer VI$_{3}$ has been experimentally synthesized but the weak ferromagnetism and low Curie temperature ($T_C$) limit its potential application. Here we report that the Janus structure can elevate the $T_C$ to 240 K. And it is discussed that the reason for high $T_C$ in Janus structure originates from the lower virtual exchange gap between $t_{2g}$ and $e_{g}$ states of nearest-neighbor V atoms. Besides, $T_C$ can be further substantially enhanced by tensile strain due to the increasing ferromagnetism driven by rapidly quenched direct exchange interaction. Our work supports a feasible approach to enhance Curie temperature of monolayer VI$_{3}$ and unveils novel stable intrinsic FM semiconductors for realistic applications in spintronics.",2001.02846v2 2020-01-15,Quantum critical point in the itinerant ferromagnet Ni$_{1-x}$Rh$_x$,"We report a chemical substitution-induced ferromagnetic quantum critical point in polycrystalline Ni$_{1-x}$Rh$_x$ alloys. Through magnetization and muon spin relaxation measurements, we show that the ferromagnetic ordering temperature is suppressed continuously to zero at $x_{crit} = 0.375$ while the magnetic volume fraction remains 100% up to $x_{crit}$, pointing to a second order transition. Non-Fermi liquid behavior is observed close to $x_{crit}$, where the electronic specific heat $C_{el}/T$ diverges logarithmically, while immediately above $x_{crit}$ the volume thermal expansion coefficient $\alpha_{V}/T$ and the Gr\""uneisen ratio $\Gamma = \alpha_{V}/C_{el}$ both diverge logarithmically in the low temperature limit, further indication of a ferromagnetic quantum critical point in Ni$_{1-x}$Rh$_x$.",2001.05533v1 2020-02-06,Spin-orbit torque switching of perpendicular magnetization in ferromagnetic trilayers,"In ferromagnetic trilayers, a spin-orbit-induced spin current can have a spin polarization of which direction is deviated from that for the spin Hall effect. Recently, magnetization switching in ferromagnetic trilayers has been proposed and confirmed by the experiments. In this work, we theoretically and numerically investigate the switching current required for perpendicular magnetization switching in ferromagnetic trilayers. We confirm that the tilted spin polarization enables field-free deterministic switching at a lower current than conventional spin-orbit torque or spin-transfer torque switching, offering a possibility for high-density and low-power spin-orbit torque devices. Moreover, we provide analytical expressions of the switching current for an arbitrary spin polarization direction, which will be useful to design spin-orbit torque devices and to interpret spin-orbit torque switching experiments.",2002.02132v1 2020-02-17,Study of polycrystalline bulk Sr$_3$OsO$_6$ double-perovskite insulator: comparison with 1000 K ferromagnetic epitaxial films,"Polycrystalline Sr$_3$OsO$_6$, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 $^\circ$C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr$_3$OsO$_6$ toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr$_3$OsO$_6$ films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr$_3$OsO$_6$ is under the influence of interfaces. A specific heat capacity of 39.6(9) 10$^{-3}$ J mol$^{-1}$ K$^{-2}$ is found at low temperatures ($<$17 K). This value is remarkably high, suggesting the presence of possible fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr$_3$OsO$_6$ share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.",2002.06743v1 2020-02-26,Extreme narrow magnetic domain walls in U ferromagnets: The UCoGa case,"Surface magnetic domains of a UCoGa single crystal during magnetization/demagnetization processes in increasing/decreasing magnetic fields were investigated by means of magnetic-force-microscopy (MFM) images at low temperatures. The observed domain structure is typical for a ferromagnet with strong uniaxial anisotropy. The evolution of magnetic domains during cooling of the crystal below TC has also been manifested by MFM images. Analysis of the available data reveals that the high uniaxial magnetocrystalline energy in combination with the relatively small ferromagnetic exchange interaction in UCoGa gives rise to the formation of very narrow domain walls formed by the pairs of the nearest U neighbor ions with antiparallel magnetic moments within the basal plane. Since the very high anisotropy energy is a common feature of the majority of the uniaxial U ferromagnets, analogous domain-wall properties are expected for all these materials.",2002.11517v1 2020-02-26,Control of spin dynamics in artificial honeycomb spin-ice-based nanodisks,"We report the experimental and theoretical characterization of the angular-dependent spin dynamics in arrays of ferromagnetic nanodisks arranged on a honeycomb lattice. The magnetic field and microwave frequency dependence, measured by broadband ferromagnetic resonance, reveal a rich spectrum of modes that is strongly affected by the microstate of the network. Based on symmetry arguments with respect to the external field, we show that certain parts of the ferromagnetic network contribute to the detected signal. A comparison of the experimental data with micromagnetic simulations reveals that different subsections of the lattice predominantly contribute to the high-frequency response of the array. This is confirmed by optical characterizations using microfocused Brillouin light scattering. Furthermore, we find indications that nucleation and annihilation of vortex-like magnetization configurations in the low-field range affect the dynamics, which is different from clusters of ferromagnetic nanoellipses. Our work opens up new perspectives for designing magnonic devices that combine geometric frustration in gyrotropic vortex crystals at low frequencies with magnonic crystals at high frequencies.",2002.11694v1 2020-04-06,High Curie Temperature Ferromagnetism and High Hole Mobility in Tensile Strained Mn-doped SiGe Thin Films,"Diluted magnetic semiconductors (DMSs) based on group-IV materials are desirable for spintronic devices compatible with current silicon technology. In this work, amorphous Mn-doped SiGe thin films were first fabricated on Ge substrates by radio frequency magnetron sputtering and then crystallized by rapid thermal annealing (RTA). After the RTA, the samples became ferromagnetic (FM) semiconductors, in which the Curie temperature increased with increasing Mn doping concentration and reached 280 K with 5% Mn concentration. The data suggest that the ferromagnetism came from the hole-mediated process and was enhanced by the tensile strain in the SiGe crystals. Meanwhile, the Hall effect measurement up to 33 T to eliminate the influence of anomalous Hall effect (AHE) reveals that the hole mobility of the annealed samples was greatly enhanced and the maximal value was ~1000 cm2/Vs, owing to the tensile strain-induced band structure modulation. The Mn-doped SiGe thin films with high Curie temperature ferromagnetism and high hole mobility may provide a promising platform for semiconductor spintronics.",2004.02676v1 2020-04-09,Hysteresis-free magnetization reversal of exchange-coupled bilayers with finite magnetic anisotropy,"Exchange-coupled structures consisting of ferromagnetic and ferrimagnetic layers become technologically more and more important. We show experimentally the occurrence of completely reversible, hysteresis-free minor loops of [Co(0.2 nm)/Ni(0.4 nm)/Pt(0.6 nm)]$_N$ multilayers exchange-coupled to a 20 nm thick ferrimagnetic Tb$_{28}$Co$_{14}$Fe$_{58}$ layer, acting as hard magnetic pinning layer. Furthermore, we present detailed theoretical investigations by means of micromagnetic simulations and most important a purely analytical derivation for the condition of the occurrence of full reversibility in magnetization reversal. Hysteresis-free loops always occur if a domain wall is formed during the reversal of the ferromagnetic layer and generates an intrinsic hard-axis bias field that overcomes the magnetic anisotropy field of the ferromagnetic layer. The derived condition further reveals that the magnetic anisotropy and the bulk exchange of both layers, as well as the exchange coupling strength and the thickness of the ferromagnetic layer play an important role for its reversibility.",2004.04419v1 2020-04-11,Strain engineering of ferromagnetic-graphene-ferroelectric nanostructures,"We calculated a spin-polarized conductance in the almost unexplored nanostructure ""high temperature ferromagnetic insulator/ graphene/ ferroelectric film"" with a special attention to the impact of electric polarization rotation in a strained multiaxial ferroelectric film. The rotation and value of polarization vector are controlled by a misfit strain. We proposed a phenomenological model, which takes into account the shift of the Dirac point due to the proximity of ferromagnetic insulator and uses the Landauer formula for the conductivity of the graphene channel. We derived analytical expressions, which show that the strain-dependent ferroelectric polarization governs the concentration of two-dimensional charge carriers and Fermi level in graphene in a self-consistent way. We demonstrate the realistic opportunity to control the spin-polarized conductance of graphene by a misfit strain (""strain engineering"") at room and higher temperatures in the nanostructures CoFeO4/graphene/PZT and Y3Fe5O12/graphene/PZT. Obtained results open the possibilities for the applications of ferromagnetic/graphene/ferroelectric nanostructures as non-volatile spin filters and spin valves.",2004.05338v1 2020-04-21,Pairing symmetry in monolayer of orthorhombic CoSb,"Ferromagnetism and superconductivity are generally considered to be antagonistic phenomena in condensed matter physics. Here, we theoretically study the interplay between the ferromagnetic and superconducting orders in a recent discovered monolayered CoSb superconductor with an orthorhombic symmetry and net magnetization, and demonstrate the pairing symmetry of CoSb as a candidate of non-unitary superconductor with time-reversal symmetry breaking. By performing the group theory analysis and the first-principles calculations, the superconducting order parameter is suggested to be a triplet pairing with the irreducible representation of $^3B_{2u}$, which displays intriguing nodal points and non-zero periodic modulation of Cooper pair spin polarization on the Fermi surface topologies. These findings not only provide a significant theoretical insight into the coexistence of superconductivity and ferromagnetism, but also reveal the exotic spin polarized Cooper pairing driven by ferromagnetic spin fluctuations in a triplet superconductor.",2004.10200v2 2021-01-29,Magnetization Reversal Mechanism in Exchange-Biased Spring-like Thin-Film Composite,"Development of modern spintronic devices requires materials exhibiting specific magnetic effects. In this paper, we investigate a magnetization reversal mechanism in a [Co/Pdx]7/CoO/[Co/Pdy]7 thin-film composite where an antiferromagnet is sandwiched between a hard and a soft ferromagnets with different coercivities. The antiferromagnet/ferromagnet interfaces give rise to the exchange bias effect. The application of soft and hard ferromagnetic films causes exchange-spring-like behavior while the choice of the Co/Pd multilayers provides large out-of-plane magnetic anisotropy. We observed that the magnitude and the sign of the exchange bias anisotropy field are related to the arrangement of the magnetic moments in the antiferromagnetic layer. This ordering is induced by the spin orientation present in neighboring ferromagnetic films which is, in turn, dependent on the orientation and strength of the external magnetic field.",2101.12495v1 2017-05-22,Interaction between magnetic moments and itinerant carriers in d0 ferromagnetic SiC,"Elucidating the interaction between magnetic moments and itinerant carriers is an important step to spintronic applications. Here, we investigate magnetic and transport properties in d0 ferromagnetic SiC single crystals prepared by postimplantation pulsed laser annealing. Magnetic moments are contributed by the p states of carbon atoms, but their magnetic circular dichroism is different from that in semi-insulating SiC samples. The anomalous Hall effect and negative magnetoresistance indicate the influence of d0 spin order on free carriers. The ferromagnetism is relatively weak in N-implanted SiC compared with that in Al-implanted SiC after annealing. The results suggest that d0 magnetic moments and itinerant carriers can interact with each other, which will facilitate the development of SiC spintronic devices with d0 ferromagnetism.",1705.07793v1 2017-05-26,Static and Dynamic Magnetic Properties of FeMn/Pt Multilayers,"Recently we have demonstrated the presence of spin-orbit toque in FeMn/Pt multilayers which, in combination with the anisotropy field, is able to rotate its magnetization consecutively from 0o to 360o without any external field. Here, we report on an investigation of static and dynamic magnetic properties of FeMn/Pt multilayers using combined techniques of magnetometry, ferromagnetic resonance, inverse spin Hall effect and spin Hall magnetoresistance measurements. The FeMn/Pt multilayer was found to exhibit ferromagnetic properties, and its temperature dependence of saturation magnetization can be fitted well using a phenomenological model by including a finite distribution in Curie temperature due to subtle thickness variations across the multilayer samples. The non-uniformity in static magnetic properties is also manifested in the ferromagnetic resonance spectra, which typically exhibit a broad resonance peak. A damping parameter of around 0.106 is derived from the frequency dependence of ferromagnetic resonance linewidth, which is comparable to the reported values for other types of Pt-based multilayers. Clear inverse spin Hall signals and spin Hall magnetoresistance have been observed in all samples below the Curie temperature, which corroborate the strong spin-orbit torque effect observed previously.",1705.09423v1 2017-09-15,"Interplay between localization and magnetism in (Ga,Mn)As and (In,Mn)As","Ion implantation of Mn combined with pulsed laser melting is employed to obtain two representative compounds of dilute ferromagnetic semiconductors (DFSs): Ga1-xMnxAs and In1-xMnxAs. In contrast to films deposited by the widely used molecular beam epitaxy, neither Mn interstitials nor As antisites are present in samples prepared by the method employed here. Under these conditions the influence of localization on the hole-mediated ferromagnetism is examined in two DFSs with a differing strength of p-d coupling. On the insulating side of the transition, ferromagnetic signatures persist to higher temperatures in In1-xMnxAs compared to Ga1-xMnxAs with the same Mn concentration x. This substantiates theoretical suggestions that stronger p-d coupling results in an enhanced contribution to localization, which reduces hole-mediated ferromagnetism. Furthermore, the findings support strongly the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions even on the insulator side of the metal-insulator transition.",1709.05043v1 2017-09-24,Why rare-earth ferromagnets are so rare: insights from the p-wave Kondo model,"Magnetic exchange in Kondo lattice systems is of the Ruderman-Kittel-Kasuya-Yosida type, whose sign depends on the Fermi wave vector, $k_F$ . In the simplest setting, for small $k_F$ , the interaction is predominately ferromagnetic, whereas it turns more antiferromagnetic with growing $k_F$. It is remarkable that even though $k_F$ varies vastly among the rare-earth systems, an overwhelming majority of lanthanide magnets are in fact antiferromagnets. To address this puzzle, we investigate the effects of a p-wave form factor for the Kondo coupling pertinent to nearly all rare-earth intermetallics. We show that this leads to interference effects which for small kF are destructive, greatly reducing the size of the RKKY interaction in the cases where ferromagnetism would otherwise be strongest. By contrast, for large $k_F$, constructive interference can enhance antiferromagnetic exchange. Based on this, we propose a new route for designing ferromagnetic rare-earth magnets.",1709.08241v1 2017-11-07,Critical scaling of the mutual information in two-dimensional disordered Ising models,"Renyi Mutual information (RMI), computed from second Renyi entropies, can identify classical phase transitions from their finite-size scaling at the critical points. We apply this technique to examine the presence or absence of finite temperature phase transitions in various two-dimensional models on a square lattice, which are extensions of the conventional Ising model by adding a quenched disorder. When the quenched disorder causes the nearest neighbor bonds to be both ferromagnetic and antiferromagnetic, (a) a spin glass phase exists only at zero temperature, and (b) a ferromagnetic phase exists at a finite temperature when the antiferromagnetic bond distributions are sufficiently dilute. Furthermore, finite temperature paramagnetic-ferromagnetic transitions can also occur when the disordered bonds involve only ferromagnetic couplings of random strengths. In our numerical simulations, the ""zero temperature only"" phase transitions are identified when there is no consistent finite-size scaling of the RMI curves, while for finite temperature critical points, the curves can identify the critical temperature $T_c$ by their crossings at $T_c$ and $2\, T_c $.",1711.02352v2 2017-11-10,Quantum Spin Liquid with Even Ising Gauge Field Structure on Kagome Lattice,"Employing large-scale quantum Monte Carlo simulations, we study the extended $XXZ$ model on the kagome lattice. A $\mathbb Z_2$ quantum spin liquid phase with effective even Ising gauge field structure emerges from the delicate balance among three symmetry-breaking phases including stripe solid, staggered solid and ferromagnet. This $\mathbb{Z}_2$ spin liquid is stabilized by an extended interaction related to the Rokhsar-Kivelson potential in the quantum dimer model limit. The phase transitions from the staggered solid to a spin liquid or ferromagnet are found to be first order and so is the transition between the stripe solid and ferromagnet. However, the transition between a spin liquid and ferromagnet is found to be continuous and belongs to the 3D $XY^*$ universality class associated with the condensation of spinons. The transition between a spin liquid and stripe solid appears to be continuous and associated with the condensation of visons.",1711.03679v2 2017-11-29,Pressure-induced ferromagnetism due to an anisotropic electronic topological transition in Fe1.08Te,"A rapid and anisotropic modification of the Fermi-surface shape can be associated with abrupt changes in crystalline lattice geometry or in the magnetic state of a material. In this study we show that such an electronic topological transition is at the basis of the formation of an unusual pressure-induced tetragonal ferromagnetic phase in Fe$_{1.08}$Te. Around 2 GPa, the orthorhombic and incommensurate antiferromagnetic ground-state of Fe$_{1.08}$Te is transformed upon increasing pressure into a tetragonal ferromagnetic state via a conventional first-order transition. On the other hand, an isostructural transition takes place from the paramagnetic high-temperature state into the ferromagnetic phase as a rare case of a `type 0' transformation with anisotropic properties. Electronic-structure calculations in combination with electrical resistivity, magnetization, and x-ray diffraction experiments show that the electronic system of Fe$_{1.08}$Te is instable with respect to profound topological transitions that can drive fundamental changes of the lattice anisotropy and the associated magnetic order.",1711.10745v1 2018-03-14,Electronically Mediated Magnetic Anisotropy in Vibrating Magnetic Molecules,"We address the electronically induced anisotropy field acting on a spin moment comprised in a vibrating magnetic molecule located in the junction between ferromagnetic metals. Under weak coupling between the electrons and molecular vibrations, the nature of the anisotropy can be changed from favoring a high spin (easy axis) magnetic moment to a low spin (easy plane) by applying a temperature difference or a voltage bias across the junction. For unequal spin-polarizations in the ferromagnetic metals it is shown that the character of the anisotropy is essentially determined by the properties of the weaker ferromagnet. By increasing the temperature in this metal, or introducing a voltage bias, its influence can be suppressed such that the dominant contribution to the anisotropy is interchanged to the stronger ferromagnet. With increasing coupling strength between the molecular vibrations and the electrons, the nature of the anisotropy is locked into favoring easy plane magnetism.",1803.05480v1 2018-03-29,Giant resonant nonlinear damping in nanoscale ferromagnets,"Magnetic damping is a key metric for emerging technologies based on magnetic nanoparticles, such as spin torque memory and high-resolution biomagnetic imaging. Despite its importance, understanding of magnetic dissipation in nanoscale ferromagnets remains elusive, and the damping is often treated as a phenomenological constant. Here we report the discovery of a giant frequency-dependent nonlinear damping that strongly alters the response of a nanoscale ferromagnet to spin torque and microwave magnetic field. This novel damping mechanism originates from three-magnon scattering that is strongly enhanced by geometric confinement of magnons in the nanomagnet. We show that the giant nonlinear damping can invert the effect of spin torque on a nanomagnet leading to a surprising current-induced enhancement of damping by an antidamping torque. Our work advances understanding of magnetic dynamics in nanoscale ferromagnets and spin torque devices.",1803.10925v1 2018-06-08,Anomalous Hall magnetoresistance in a ferromagnet,"The anomalous Hall effect, observed in conducting ferromagnets with broken time-reversal symmetry, offers the possibility to couple spin and orbital degrees of freedom of electrons in ferromagnets. In addition to charge, the anomalous Hall effect also leads to spin accumulation at the surfaces perpendicular to both the current and magnetization direction. Here we experimentally demonstrate that the spin accumulation, subsequent spin backflow, and spin-charge conversion can give rise to a different type of spin current related magnetoresistance, dubbed here as the anomalous Hall magnetoresistance, which has the same angular dependence as the recently discovered spin Hall magnetoresistance. The anomalous Hall magnetoresistance is observed in four types of samples: co-sputtered (Fe1-xMnx)0.6Pt0.4, Fe1-xMnx and Pt multilayer, Fe1-xMnx with x = 0.17 to 0.65 and Fe, and analyzed using the drift-diffusion model. Our results provide an alternative route to study charge-spin conversion in ferromagnets and to exploit it for potential spintronic applications.",1806.03083v1 2018-06-14,Tailoring ferromagnetism through electrically controlled morphology,"Converse magnetoelectric coupling in artificial multiferroics is generally modelled through three possible mechanisms: charge transfer, strain mediated or ion migration. Here we demonstrate a novel and highly reliable approach, where electrically controlled morphological modifications control the ferromagnetic response of a magnetoelectric heterostructure, specifically FexMn1-x ferromagnetic films on ferroelectric PMN-PT (001) substrates. The ferroelectric PMN-PT substrates present, in correspondence to electrical switching, fully reversible morphological changes at the surface, to which correspond reproducible modifications of the ferromagnetic response of the FexMn1-x films. Topographic analysis by atomic force microscopy shows the formation of surface cracks after application of a positive electric field up to 6 kV/cm, which disappear after application of negative voltage of the same amplitude. In-operando x-ray magnetic circular dichroic spectroscopy at Fe edge in FexMn1-x layers shows local variations of dichroic signal up to a factor 2.5 as a function of the electrically-driven morphological state. These findings highlight the role of morphology and surface topography as a key aspect in magnetoelectric coupling, whose proof of electrically reversible modification of the magnetic response adds a new possibility in the design of multiferroic heterostructures with electrically controlled functionalities.",1806.05414v1 2018-06-21,Spin triplet superconducting proximity effect in a ferromagnetic semiconductor,"Conventional spin-singlet superconductivity that deeply penetrates into ferromagnets is typically killed by the exchange interaction, which destroys the spin-singlet pairs. Under certain circumstances, however, superconductivity survives this interaction by adopting the pairing behavior of spin triplets. The necessary conditions for the emergence of triplet pairs are well-understood, owing to significant developments in theoretical frameworks and experiments. The long-term challenges to inducing superconductivity in magnetic semiconductors, however, involve difficulties in observing the finite supercurrent, even though the generation of superconductivity in host materials has been well-established and extensively examined. Here, we show the first evidence of proximity-induced superconductivity in a ferromagnetic semiconductor (In, Fe)As. The supercurrent reached a distance scale of $\sim 1~\mu$m, which is comparable to the proximity range in two-dimensional electrons at surfaces of pure InAs. Given the long range of its proximity effects and its response to magnetic fields, we conclude that spin-triplet pairing is dominant in proximity superconductivity. Therefore, this progress in ferromagnetic semiconductors is a breakthrough in semiconductor physics involving unconventional superconducting pairing.",1806.08035v1 2018-06-22,Observation of a pressure-induced transition from interlayer ferromagnetism to intralayer antiferromagnetism in Sr4Ru3O10,"Sr4Ru3O10 is a Ruddlesden-Popper compound with triple Ru-O perovskite layers separated by Sr-O alkali layers. This compound presents a rare coexistence of interlayer (c-axis) ferromagnetism and intralayer (basal-plane) metamagnetism at ambient pressure. Here we report the observation of pressure-induced, intralayer itinerant antiferromagnetism arising from the interlayer ferromagnetism. The application of modest hydrostatic pressure generates an anisotropy that causes a flattening and a tilting of RuO6 octahedra. All magnetic and transport results from this study indicate these lattice distortions diminish the c-axis ferromagnetism and basal-plane metamagnetism, and induce a basal-plane antiferromagnetic state. The unusually large magnetoelastic coupling and pressure tunability of Sr4Ru3O10 makes it a unique model system for studies of itinerant magnetism.",1806.08466v1 2018-06-23,Magnetic and magnetoresistive behavior of the ferromagnetic heavy fermion YbNi$_2$,"We present a study on the magnetic susceptibility $\chi(T)$ and electrical resistance, as a function of temperature and magnetic field $R(T,H)$, of the ferromagnetic heavy fermion YbNi$_2$. The X-ray diffraction analysis shows that the synthesized polycrystalline samples crystallizes in the cubic Laves phase structure C15, with a spatial group $Fd\overline{3}m$. The magnetic measurements indicate a ferromagnetic behavior with transition temperature at 9 K. The electrical resistance is metallic-like at high temperatures and no signature of Kondo effect was observed. In the ferromagnetic state, the electrical resistance can be justified by electron-magnon scattering considering the existence of an energy gap in the magnonic spectrum. The energy gap was determined for various applied magnetic fields. Magnetoresistance as a function of applied magnetic field, subtracted from the $R(T,H)$ curves at several temperatures, is negative from 2 K until about 40 K for all applied magnetic fields. The negative magnetoresistance originates from the suppression of magnetic disorder by the magnetic field.",1806.08881v1 2018-07-07,Two-Dimensional Multiferroic Semiconductors with Coexisting Ferroelectricity and Ferromagnetism,"Low-dimensional multiferroicity, though highly scarce in nature, has attracted great attention due to both fundamental and technological interests. Using first-principles density functional theory, we show that ferromagnetism and ferroelectricity can coexist in monolayer transition metal phosphorus chalcogenides (TMPCs) - CuMP$_2$X$_6$ (M=Cr, V; X=S, Se). These van der Waals layered materials represent a class of 2D multiferroic semiconductors that simultaneously possess ferroelectric and ferromagnetic orders. In these monolayer materials, Cu atoms spontaneously move away from the center atomic plane, giving rise to nontrivial electric dipole moment along the plane normal. In addition, their ferromagnetism originates from indirect exchange interaction between Cr/V atoms, while their out-of-plane ferroelectricity suggests the possibility of controlling electric polarization by external vertical electric field. Monolayer semiconducting TMPCs thus provide a solid-state 2D materials platform for realizing 2D nanoscale switches and memory devices patterned with top and bottom electrodes.",1807.02717v1 2018-07-19,Three-Dimensional Quantum Anomalous Hall Effect in Ferromagnetic Insulators,"The quantum anomalous Hall effect (QAHE) hosts the dissipationless chiral edge states associated with the nonzero Chern number, providing potentially significant applications in future spintronics. The QAHE usually occurs in a two-dimensional (2D) system with time-reversal symmetry breaking. In this work, we propose that the QAHE can exist in three-dimensional (3D) ferromagnetic insulators. By imposing inversion symmetry, we develop the topological constraints dictating the appearance of 3D QAHE based on the parity analysis at the time-reversal invariant points in reciprocal space. Moreover, using first-principles calculations, we identify that 3D QAHE can be realized in a family of intrinsic ferromagnetic insulating oxides, including layered and non-layered compounds that share a centrosymmetric structure with space group $R\bar{3}m$ (No. 166). The Hall conductivity is quantized to be $-\frac{3e^2}{hc}$ with the lattice constant $c$ along $c$-axis. The chiral surface sheet states are clearly visible and uniquely distributed on the surfaces that are parallel to the magnetic moment. Our findings open a promising pathway to realize the QAHE in 3D ferromagnetic insulators.",1807.07262v1 2018-07-30,Topological spin excitations in honeycomb ferromagnet CrI$_3$,"In two dimensional honeycomb ferromagnets, bosonic magnon quasiparticles (spin waves) may either behave as massless Dirac fermions or form topologically protected edge states. The key ingredient defining their nature is the next-nearest neighbor Dzyaloshinskii-Moriya (DM) interaction that breaks the inversion symmetry of the lattice and discriminates chirality of the associated spin-wave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet CrI$_3$ ($T_C=61$ K) have two distinctive bands of ferromagnetic excitations separated by a $\sim$4 meV gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with DM interaction, thus providing experimental evidence that spin waves in CrI$_3$ can have robust topological properties useful for dissipationless spintronic applications.",1807.11452v3 2018-08-18,Electric-field controlled superconductor-ferromagnetic insulator transition,"How to control collectively ordered electronic states is a core interest of condensed matter physics. We report an electric field controlled reversible transition from superconductor to ferromagnetic insulator in (Li,Fe)OHFeSe thin flake using solid ion conductor as the gate dielectric. By driving Li ions into and out of the (Li,Fe)OHFeSe thin flake with electric field, we obtained a dome-shaped superconducting region with optimal Tc ~ 43 K, which is separated by a quantum critical point from ferromagnetically insulating phase. The ferromagnetism arises from the long range order of the interstitial Fe ions expelled from the (Li,Fe)OH layers by Li injection. The device can reversibly manipulate collectively ordered electronic states and stabilize new metastable structures by electric field.",1808.06051v1 2018-08-24,Tunable room-temperature ferromagnetism in the SiC monolayer,"It is essential to explore two-dimensional (2D) material with magnetic ordering in new generation spintronic devices. Particularly, the seeking of room-temperature 2D ferromagnetic (FM) materials is a hot topic of current research. Here, we study magnetism of the Mn-doped and electron-doped SiC monolayer using first-principle calculations. For the Mn-doped SiC monolayer, we find that either electron or hole could mediate the ferromagnetism in the system and the Curie temperature ($T_C$) can be improved by appropriate carrier doping. The codoping strategy is also discussed on improving $T_C$. The transition between antiferromagnetic and FM phase can be found by strain engineering. The $T_C$ is improved above room temperature (RT) under the strain larger than $0.06$. Moreover, the Mn-doped SiC monolayer develops half-metal at the strain range of $0.05-0.1$. On the other hand, the direct electron doping can induce ferromagnetism due to the van Hove singularity in density of states of the conduction band edge of the SiC monolayer. The $T_C$ is found to be around RT. These fascinating controllable electronic and magnetic properties are desired for spintronic applications.",1808.07995v2 2018-12-12,Room-temperature ferromagnetic insulating state in highly cation-ordered epitaxial oxide double perovskite,"Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, since ferromagnetism generally accompanies metallicity, FMIs are innately rare to find in nature. Here, novel room-temperature FMI films are epitaxially synthesized by deliberate control of the ratio of two B-site cations in the double perovskite Sr2FeReO6. In contrast to the known ferromagnetic metallic phase in stoichiometric Sr2FeReO6, a FMI state with a high Curie temperature (Tc~400 K) and a large saturation magnetization (MS~1.8 {\mu}B/f.u.) is found in highly cation-ordered Fe-rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe3+-Fe3+ and Fe3+-Re6+ bonding states, which originate from the excess Fe. The emerging FMI state by controlling cations in the epitaxial oxide perovskites opens the door to developing novel oxide quantum materials & heterostructures.",1812.04953v1 2018-12-14,Current Control of Magnetism in Two-Dimensional Fe3GeTe2,"The recent discovery of magnetism in two-dimensional van der Waals systems opens the door to discovering exciting physics. We investigate how a current can control the ferromagnetic properties of such materials. Using symmetry arguments, we identify a recently realized system in which the current-induced spin torque is particularly simple and powerful. In Fe3GeTe2, a single parameter determines the strength of the spin-orbit torque for a uniform magnetization. The spin-orbit torque acts as an effective out-of-equilibrium free energy. The contribution of the spin-orbit torque to the effective free energy introduces new in-plane magnetic anisotropies to the system. Therefore, we can tune the system from an easy-axis ferromagnet via an easy-plane ferromagnet to another easy-axis ferromagnet with increasing current density. This finding enables unprecedented control and provides the possibility to study the Berezinskii-Kosterlitz-Thouless phase transition in the 2D XY model and its associated critical exponents.",1812.06096v2 2018-12-17,Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy,"Non-coplanar spin textures with scalar spin chirality can generate effective magnetic field that deflects the motion of charge carriers, resulting in topological Hall effect (THE), a powerful probe of the ground state and low-energy excitations of correlated systems. However, spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy has not been considered in prior studies. Herein, we report direct evidence of universal spin chirality fluctuation by probing the THE above the transition temperatures in two different ferromagnetic ultra-thin films, SrRuO$_3$ and V doped Sb$_2$Te$_3$. The temperature, magnetic field, thickness, and carrier type dependences of the THE signal, along with our Monte-Carlo simulations, unambiguously demonstrate that the spin chirality fluctuation is a universal phenomenon in two-dimensional Ising ferromagnets. Our discovery opens a new paradigm of exploring the spin chirality with topological Hall transport in two-dimensional magnets and beyond",1812.07005v2 2019-01-04,Weak ferromagnetic insulator with huge coercivity in monoclinic double perovskite La2CuIrO6,"Insulating ferromagnets with high TC are required for many new magnetic devices. More complexity arises when strongly correlated 3d ions coexist with strongly spin-orbit coupled 5d ones in a double perovskite. Here, we perform the structural, magnetic, and density functional theory study of such double perovskite La2CuIrO6. A new P21/n polymorph is found according to the comprehensive analysis of x-ray, Raman scattering and phonon spectrum. The magnetization reveals a weak ferromagnetic (FM) transition at TC = 62 K and short range FM order in higher temperature range. A huge coercivity is found as high as HC~11.96 kOe at 10K, which in combination with the negative trapped field results in the magnetization reversal in the zero field cooling measurement. The first principle calculations confirm the observed FM state and suggest La2CuIrO6 of this polymorph is a Mott insulating ferromagnet assisted by the spin-orbit coupling.",1901.01106v4 2019-01-09,Anisotropic magnetic entropy change in Cr$_2$X$_2$Te$_6$ (X = Si and Ge),"Intrinsic, two-dimensional (2D) ferromagnetic semiconductors are an important class of materials for spintronics applications. Cr$_2$X$_2$Te$_6$ (X = Si and Ge) semiconductors show 2D Ising-like ferromagnetism, which is preserved in few-layer devices. The maximum magnetic entropy change associated with the critical properties around the ferromagnetic transition for Cr$_2$Si$_2$Te$_6$ $-\Delta S_M^{max} \sim$ 5.05 J kg$^{-1}$ K$^{-1}$ is much larger than $-\Delta S_M^{max} \sim$ 2.64 J kg$^{-1}$ K$^{-1}$ for Cr$_2$Ge$_2$Te$_6$ with an out-of-plane field change of 5 T. The rescaled $-\Delta S_M(T,H)$ curves collapse onto a universal curve independent of temperature and field for both materials. This indicates similar critical behavior and 2D Ising magnetism, confirming the magnetocrystalline anisotropy that could preserve the long-range ferromagnetism in few-layers of Cr$_2$X$_2$Te$_6$.",1901.02876v1 2019-01-13,Intrinsic spin currents in ferromagnets,"First principles calculations show that electric fields applied to ferromagnets generate spin currents flowing perpendicularly to the electric field. Reduced symmetry in these ferromagnets enables a wide variety of such spin currents. However, the total spin current is approximately the sum of a magnetization-independent spin Hall current and an anisotropic spin anomalous Hall current. Intrinsic spin currents are not subject to dephasing, enabling their spin polarizations to be misaligned with the magnetization, which enables the magnetization-independent spin Hall effect. The spin Hall conductivity and spin anomalous Hall conductivities of transition metal ferromagnets are comparable to those found in heavy metals, opening new avenues for efficient spin current generation in spintronic devices.",1901.04022v2 2019-01-14,Spin polarization and orbital effects in superconductor-ferromagnet structures,"We study theoretically spontaneous currents and magnetic field induced in a superconductor-ferromagnet (S-F) bilayer due to direct and inverse proximity effects. The induced currents {are Meissner currents that appear even in the absence of an external magnetic field due to the magnetic moment in the ferromagnet }and {to the magnetization } in the superconductor . The latter is induced by the inverse proximity effect over a distance of the order of the superconducting correlation length $\xi _{S}$. On the other hand the magnetic induction $B$, caused by Meissner currents, penetrates the S film over the London length $\lambda _{S}$. Even though $\lambda _{S}$ usually exceeds considerably the correlation length, the amplitude and sign of $B$ at distances much larger than $\xi _{S}$ depends crucially on the strength of the exchange energy in the ferromagnet and on the magnetic moment induced in the in the S layer.",1901.04446v2 2019-01-19,Concentration of multi-overlaps for random ferromagnetic spin models,"We consider ferromagnetic spin models on dilute random graphs and prove that, with suitable one-body infinitesimal perturbations added to the Hamiltonian, the multi-overlaps concentrate for all temperatures, both with respect to the thermal Gibbs average and the quenched randomness. Results of this nature have been known only for the lowest order overlaps, at high temperature or on the Nishimori line. Here we treat all multi-overlaps by a non-trivial application of Griffiths-Kelly-Sherman correlation inequalities. Our results apply in particular to the pure and mixed p-spin ferromagnets on random dilute Erdoes-R\'enyi hypergraphs. On physical grounds one expects that multi-overlap concentration directly implies the correctness of the cavity (or replica symmetric) formula for the pressure. The proof of this formula for the general p-spin ferromagnet on a random dilute hypergraph remains an open problem.",1901.06521v1 2019-01-27,Strain-Induced Room-Temperature Ferromagnetic Semiconductors with Large Anomalous Hall Conductivity in Two-Dimensional Cr2Ge2Se6,"By density functional theory calculations, we predict a stable two-dimensional (2D) ferromagnetic semiconductor Cr$_2$Ge$_2$Se$_6$, where the Curie temperature $T$$_c$ can be dramatically enhanced beyond room temperature by applying a few percent strain. In addition, the anomalous Hall conductivity in 2D Cr$_2$Ge$_2$Se$_6$ and Cr$_2$Ge$_2$Te$_6$ is predicted to be comparable to that in ferromagnetic metals of Fe and Ni, and is an order of magnitude larger than that in diluted magnetic semiconductor Ga(Mn,As). Based on superexchange interactions, the enhanced $T$$_c$ in 2D Cr$_2$Ge$_2$Se$_6$ by strain can be understood by the decreased energy difference between 3$d$ orbitals of Cr and 4$p$ orbitals of Se. Our finding highlights the microscopic mechanism to obtain the room temperature ferromagnetic semiconductors by strain.",1901.09306v2 2019-01-30,Magnetization direction dependent spin Hall effect in 3d ferromagnets,"We have studied the intrinsic spin Hall conductivity of 3d transition metal ferromagnets using first-principles calculations. We find the spin Hall conductivity of bcc Fe and fcc Ni, prototypes of ferromagnetic systems, depends on the direction of magnetization. The spin Hall conductivity of electrons with their spin orientation orthogonal to the magnetization are found to be larger than that when the two are parallel. For example, the former can be more than four times larger than the latter in bcc Fe. Such a difference arises due to the anisotropy of the spin current operator in the spinor space: Its expectation value with the Bloch states depends on the relative angle between the conduction electron spin and the magnetization. A simple analytical form is developed to describe the relation between the spinor states, with respect to the magnetization direction, and the Berry and spin Berry curvatures. The model can account for the characteristics found in the calculations. These results show that ferromagnets can be used to generate spin current and its magnitude can be controlled by the magnetization direction.",1901.10740v2 2019-02-11,Interplay of electronic and spin degrees in ferromagnetic SrRuO$_3$: anomalous softening of magnon gap and stiffness,"The magnon dispersion of ferromagnetic SrRuO$_3$ was studied by inelastic neutron scattering experiments on single crystals as function of temperature. Even at low temperature the magnon modes exhibit substantial broadening pointing to strong interaction with charge carriers. We find an anomalous temperature dependence of both the magnon gap and the magnon stiffness, which soften upon cooling in the ferromagnetic phase. Both effects trace the temperature dependence of the anomalous Hall effect. We argue that these results show that Weyl points and the anomalous Hall effect can directly influence the spin dynamics in metallic ferromagnets.",1902.04036v1 2019-02-20,Direct photoluminescence probing of ferromagnetism in monolayer two-dimensional CrBr3,"Atomically thin magnets are the key element to build up spintronics based on two-dimensional materials. The surface nature of two-dimensional ferromagnet opens up opportunities to improve the device performance efficiently. Here, we report the intrinsic ferromagnetism in atomically thin monolayer CrBr3, directly probed by polarization resolved magneto-photoluminescence. The spontaneous magnetization persists in monolayer CrBr3 with a Curie temperature of 34 K. The development of magnons by the thermal excitation is in line with the spin-wave theory. We attribute the layer-number dependent hysteresis loops in thick layers to the magnetic domain structures. As a stable monolayer material in air, CrBr3 provides a convenient platform for fundamental physics and pushes the potential applications of the two-dimensional ferromagnetism.",1902.07446v1 2019-03-03,Goos-Hanchen Shift of a Spin-Wave Beam at the Interface Between Two Ferromagnets,"Spin waves are promising information carriers which can be used in modern magnonic devices, characterized by higher performance and lower energy consumption than presently used electronic circuits. However, before practical application of spin waves, the efficient control over spin wave amplitude and phase needs to be developed. We analyze analytically reflection and refraction of the spin waves at the interface between two ferromagnetic materials. In the model we consider the system consisting of two semi-infinite ferromagnetic media, separated by the ultra-narrow interface region with the magnetic anisotropy. We have found the Goos-Hanchen shift for spin waves in transmission and reflection, and performed detailed investigations of its dependence on the anisotropy at the interface and materials surrounding the interface. We have demonstrated possibility of obtaining Goos-Hanchen shift of several wavelengths in reflection for realistic material parameters. That proves the possibility for change of the spin waves phase in ferromagnetic materials at subwavelength distances, which can be regarded as a metasurface for magnonics.",1903.00861v1 2019-03-22,Spin-mechanics with levitating ferromagnetic particles,"We propose and demonstrate first steps towards schemes where the librational mode of levitating ferromagnets is strongly coupled to the electronic spin of Nitrogen-Vacancy (NV) centers in diamond. Experimentally, we levitate ferromagnets in a Paul trap and employ magnetic fields to attain oscillation frequencies in the hundreds of kHz range with Q factors close to $10^4$. These librational frequencies largely exceed the decoherence rate of NV centers in typical CVD grown diamonds offering prospects for sideband resolved operation. We also prepare and levitate composite diamond-ferromagnet particles and demonstrate both coherent spin control of the NV centers and read-out of the particle libration using the NV spin. Our results will find applications in ultra-sensitive gyroscopy and bring levitating objects a step closer to spin-mechanical experiments at the quantum level.",1903.09699v3 2019-03-24,Theory of the magnetic domains phases in ferromagnetic superconductors,"Recently discovered superconducting P-doped EuFe$_2$As$_2$ compounds reveal the situation when the superconducting critical temperature substantially exceeds the ferromagnetic transition temperature. The main mechanism of the interplay between magnetism and superconductivity occurs to be an electromagnetic one and a short period magnetic domain structure was observed just below Curie temperature [Stolyarov et al., Sci. Adv. \textbf{4}, eaat1061 (2018)]. We elaborate a theory of such transition and demonstrate how the initial sinusoidal magnetic structure gradually transforms into a soliton-like domain one. Further cooling may trigger a first-order transition from the short-period domain Meissner phase to the self-induced ferromagnetic vortex state and we calculate the parameters of this transition. The size of the domains in the vortex state is basically the same as in the normal ferromagnet, but with the domain walls which should generate the set of vortices perpendicular to the vortices in the domains.",1903.09953v1 2019-03-28,Non-Hermitian Weyl Physics in Topological Insulator Ferromagnet Junctions,"We introduce and investigate material junctions as a generic and tuneable electronic platform for the realization of exotic non-Hermitian (NH) topological states of matter, where the NH character is induced by the surface self-energy of a thermal reservoir. As a conceptually rich and immediately experimentally realizable example, we consider a three-dimensional topological insulator (TI) coupled to a ferromagnetic lead. Remarkably, the symmetry protected TI is promoted in a dissipative fashion to a non-symmetry protected NH Weyl phase with no direct Hermitian counterpart and which exhibits robustness against any perturbation. The transition between a gapped phase and the NH Weyl phase may be readily tuned experimentally with the magnetization direction of the ferromagnetic lead. Given the robustness of this exotic nodal phase, our general analysis also applies to, e.g., a two-dimensional electron gas close to criticality in proximity to a ferromagnetic lead. There, the predicted bulk Fermi arcs are directly amenable to surface spectroscopy methods such as angle-resolved photoemission spectroscopy.",1903.12187v2 2019-04-08,"YBa2Cu3O7/LaXMnO3 (X: Ca, Sr) based Superconductor/Ferromagnet/Superconductor junctions with memory functionality","Complex oxides exhibit a variety of unusual physical properties, which can be used for designing novel electronic devices. Here we fabricate and study experimentally nano-scale Superconductor/ Ferromagnet/Superconductor junctions with the high-Tc cuprate superconductor YBa2Cu3O7 and the colossal magnetoresistive (CMR) manganite ferromagnets LaXMnO3 (X: Ca or Sr). We demonstrate that in a broad temperature range the magnetization of a manganite nanoparticle, forming the junction interface, switches abruptly in a mono-domain manner. The CMR phenomenon translates the magnetization loop into a hysteretic magnetoresistance loop. The latter facilitates a memory functionality of such a junction with just a single CMR ferromagnetic layer. The orientation of the magnetization (stored information) can be read out by simply measuring the junction resistance in an applied magnetic field. The CMR facilitates a large read-out signal in a small applied field. We argue that such a simple single layer CMR junction can operate as a memory cell both in the superconducting state at cryogenic temperatures and in the normal state up to room temperature.",1904.03951v1 2019-04-09,Manipulation of Magnetic Skyrmions by Superconducting Vortices in Ferromagnet-Superconductor Heterostructures,"Dynamics of magnetic skyrmions in hybrid ferromagnetic films harbors novel physical phenomena and holds promise for technological applications. In this work, we discuss the behavior of magnetic skyrmions when coupled to superconducting vortices in a ferromagnet-superconductor heterostructure. We use numerical simulations and analytic arguments to reveal broader possibilities for manipulating the skyrmion-vortex dynamic correlations in the hybrid system, that are not possible in its separated constituents. We explore the thresholds of particular dynamic phases, and quantify the phase diagram as a function of the relevant material parameters, applied current and induced magnetic torques. Finally, we demonstrate the broad and precise tunability of the skyrmion Hall-angle in presence of vortices, with respect to currents applied to either or both the superconductor and the ferromagnet within the heterostructure.",1904.04537v3 2019-04-18,Electrical control of magnetization in S/F/S junctions on a 3D topological insulator,"Strong dependence of the Josephson energy on the magnetization orientation in Josephson junctions with ferromagnetic interlayers and spin-orbit coupling opens a way to control magnetization by Josephson current or Josephson phase. Here we investigate the perspectives of magnetization control in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions on the surface of a 3D topological insulator hosting Dirac quasiparticles. Due to the spin-momentum locking of these Dirac quasiparticles a strong dependence of the Josephson current-phase relation on the magnetization orientation is realized. It is demonstrated that this can lead to splitting of the ferromagnet's easy-axis in the voltage driven regime. We show that such a splitting can lead to stabilization of an unconventional four-fold degenerate ferromagnetic state.",1904.09009v2 2019-04-24,Two-Dimensional Room Temperature Ferromagnetic Semiconductors with Quantum Anomalous Hall Effect,"To obtain room temperature ferromagnetic semiconductors is one of big challenges in science, and also premises essentially to realize room temperature quantum anomalous Hall effect (QAHE), both of which are quite expected for a long time. Here we report that, based on first-principles calculations, PdBr3, PtBr3, PdI3 and PtI3 monolayers are ferromagnetic semiconductors with possible high temperature QAHE. Monte Carlo simulations give Curie temperatures 350 K and 375 K for PdBr3 and PtBr3 monolayers, respectively. These two-dimensional (2D) materials are Chern insulators. The band gaps of PdBr3 and PtBr3 are 58.7 meV and 28.1 meV with GGA and 100.8 meV and 45 meV with HSE06, respectively, quite well in favor of observing room temperature QAHE. The large band gaps were unveiled from multi-orbital electron correlations. By carefully checking the stability, PdBr3, PtBr3, and PtI3 monolayers are all predicted to be feasible in experiment. The present work sheds new light on developing promising spintronic devices by using the room temperature ferromagnetic semiconductors, and dissipationless devices by applying room temperature QAHE.",1904.11357v4 2019-04-29,Dependence of the magnetic interactions in MoS$_2$ monolayer on Mn-doping configurations,"Understanding the magnetic properties of the various Mn doping configurations that can be encountered in $2H$-MoS$_2$ monolayer could be beneficial for its use in spintronics. Using density functional theory plus Hubbard U (DFT$+$U) approach, we study how a single isolated, double- and triple-substitution configurations of Mn atoms within a MoS$_2$ monolayer could contribute to its total magnetization. We find that the doping-configuration plays a critical role in stabilizing a ferromagnetic state in a Mn-doped MoS$_2$ monolayer. Indeed, the Mn-Mn magnetic interaction is found to be ferromagntic and strong for Mn in equidistant substitution positions where the separation average range of 6-11 {\AA}. The strongest ferromagnetic interaction is found when substitutions are in second nearest neighbors Mo-sites of the armchair chain. Clustering is energetically favorable but it strongly reduces the ferromagnetic exchange energies. Our results suggest that ordering the Mn dopants on MoS$_2$ monolayer is needed to increase its potential ferromagnetism.",1904.12905v2 2019-08-17,Ferromagnetism in the SU($n$) Hubbard model with a nearly flat band,"We present rigorous results for the SU($n$) Fermi-Hubbard model on the railroad-trestle lattice. We first study the model with a flat band at the bottom of the single-particle spectrum and prove that the ground states exhibit SU($n$) ferromagnetism when the total fermion number is the same as the number of unit cells. We then perturb the model by adding extra hopping terms and make the flat band dispersive. Under the same filling condition, it is proved that the ground states of the perturbed model remain SU($n$) ferromagnetic when the bottom band is nearly flat. This is the first rigorous example of the ferromagnetism in nonsingular SU($n$) Hubbard models in which both the single-particle density of states and the on-site repulsive interaction are finite.",1908.06286v5 2019-11-01,Giant unidirectional magnetoresistance in topological insulator -- ferromagnetic semiconductor heterostructures,"The unidirectional magnetoresistance (UMR) is one of the most complex spin-dependent transport phenomena in ferromagnet/non-magnet bilayers, which involves spin injection and accumulation due to the spin Hall effect (SHE) or Rashba-Edelstein effect (REE), spin-dependent scattering, and magnon scattering at the interface or in the bulk of the ferromagnet. While UMR in metallic bilayers has been studied extensively in very recent years, its magnitude is as small as 10$^-$$^5$, which is too small for practical applications. Here, we demonstrate a giant UMR effect in a heterostructure of BiSb topological insulator -- GaMnAs ferromagnetic semiconductor. We obtained a large UMR ratio of 1.1%, and found that this giant UMR is governed not by the giant magnetoresistance (GMR)-like spin-dependent scattering, but by magnon emission/absorption and strong spin-disorder scattering in the GaMnAs layer. Our results provide new insight into the complex physics of UMR, as well as a strategy for enhancing its magnitude for device applications.",1911.00247v1 2019-11-25,Fe$_{5-x}$Ge$_2$Te$_2$ -- A new exfoliable itinerant ferromagnet with high Curie temperature and large perpendicular magnetic anisotropy,"Layered van der Waals (vdW) crystals with intrinsic magnetic properties such as high Curie temperature (TC) and large perpendicular magnetic anisotropy (PMA) are key to the development and application of spintronic devices. The ferromagnetic vdW metal Fe3-xGeTe2 (FGT) has gained prominence recently due to its high TC (220 K) and strong PMA. Here, we introduce a new metallic vdW ferromagnets, Fe5-xGe2Te2 or FG2T, which was successfully synthesized and fully characterized. FG2T is a metal that orders ferromagnetically with a very sharp transition at 250 K (bulk and single crystal thin flakes) and shows large PMA, as found by both experimental and computational studies. This work enables novel heterostructure devices with near room temperature capabilities by using FG2T as spin injector.",1911.11259v1 2019-12-03,Stable and Metastable Kinetic Ferromagnetism on a Ring,"Performing an exact diagonalization of the effective spin problem, a ferromagnetic ground state of kinetic origin is shown to emerge in a system of $N$ strongly correlated electrons on a $L$-site ring ($L > N$). This phenomenon is brought about by the quantum necklace statistics originated from the no double occupancy constraint leading to a fractional shifted electron momentum quantization. As a consequence of such special energy level distribution, the kinetic ferromagnetism is stable only for $N=3$. For odd $N>3$ the fully polarized FM state energy is only a local minimum but it is protected by a finite energy barrier that inhibits one spin-flip processes. The metastable ferromagnetic state survives perturbations of small magnitude opening up a possibility of being experimentally observed by an appropriate tuning of the interdot tunneling amplitudes in currently available quantum dot arrays.",1912.01535v1 2019-12-16,Magnon-mediated superconductivity on the surface of a topological insulator,"We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions can be enhanced by coupling the topological insulator asymmetrically to the two sublattices of the antiferromagnet. This effect is further amplified by next nearest neighbor frustration in the antiferromagnetic insulator. The enhancement makes the induced interactions significantly stronger in the antiferromagnetic case, as compared to the ferromagnetic case. These results indicate that an uncompensated antiferromagnetic interface might be a better candidate than a ferromagnetic interface for proximity-induced magnon-mediated superconductivity on the surface of a topological insulator.",1912.07607v2 2019-12-23,Peculiarities of electronic transport and magnetic state in half-metallic ferromagnetic and spin gapless semiconducting Heusler alloys,"A brief survey of experimental and theoretical studies of half-metallic ferromagnets (HMFs) and spin gapless semiconductors is given, the possible candidates being the X$_2$YZ (X = Mn, Fe, Co; Y = Ti, V, Cr, Mn, Fe, Co, Ni; Z = Al, Si, Ga, Ge, In, Sn, Sb) Heusler alloys. The data on the electrical resistivity, normal and anomalous Hall Effect, and magnetic properties are presented. It is shown that the Co$_2$FeZ alloys demonstrate properties of conventional ferromagnets, the HMF properties being also manifested at the variation of the Z-component. The Fe$_2$YAl and Mn$_2$YAl alloys show at the variation of the Y-component both metallic and semiconducting electronic characteristics, the magnetic properties, changing from the ferromagnetic to compensated ferrimagnetic state. The HMF and spin gapless semiconductor states are supposed to exist in these Heusler alloys systems.",1912.10771v1 2019-12-27,Ferromagnetic state above room temperature in a proximitized topological Dirac semimetal,"We report an above-room-temperature ferromagnetic state realized in a proximitized Dirac semimetal, which is fabricated by growing typical Dirac semimetal Cd$_3$As$_2$ films on a ferromagnetic garnet with strong perpendicular magnetization. Observed anomalous Hall conductivity with substantially large Hall angles is found to be almost proportional to magnetization and opposite in sign to it. Theoretical calculations based on first-principles electronic structure also demonstrate that the Fermi-level dependent anomalous Hall conductivity reflects the Berry curvature originating in the split Weyl nodes. The present Dirac-semimetal/ferromagnetic-insulator heterostructure will provide a novel platform for exploring Weyl-node transport phenomena and spintronic functions lately proposed for topological semimetals.",1912.11964v1 2020-03-15,Magnetic critical behavior of the van der Waals Fe5GeTe2 crystal with near room temperature ferromagnetism,"The van der Waals ferromagnet Fe5GeTe2 has a Curie temperature TC of about 270 K, which can be raised above room temperature by tuning the Fe deficiency content. To achieve insights into its ferromagnetic exchange, we have studied the critical behavior by measuring the magnetization in bulk Fe5GeTe2 crystal around the ferromagnetic to paramagnetic phase transition. The analysis of the magnetization by employing various techniques including the modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis achieved a set of reliable critical exponents with TC = 273.7 K, beta = 0.3457, gamma = 1.40617, and delta = 5.021, suggesting a three-dimensional magnetic exchange with the distance decaying as J(r) ~ (r)$^-4.916, which is close to that of a three-dimensional Heisenberg model with long-range magnetic coupling.",2003.06825v1 2020-03-18,Directed transport of suspended ferromagnetic nanoparticles under both gradient and uniform magnetic fields,"The suspended ferromagnetic particles subjected to the gradient and uniform magnetic fields experience both the translational force generated by the field gradient and the rotational torque generated by the fields strengths. Although the uniform field does not contribute to the force, it nevertheless influences the translational motion of these particles. This occurs because the translational force depends on the direction of the particle magnetization, which in turn depends on the fields strengths. To study this influence, a minimal set of equations describing the coupled translational and rotational motions of nanosized ferromagnetic particles is introduced and solved in the low Reynolds number approximation. Trajectory analysis reveals that, depending on the initial positions of nanoparticles, there exist four regimes of their directed transport. The intervals of initial positions that correspond to different dynamical regimes are determined, their dependence on the uniform magnetic field is established, and strong impact of this field on the directed transport is demonstrated. The ability and efficiency of the uniform magnetic field to control the separation of suspended ferromagnetic nanoparticles is also discussed.",2003.08498v2 2020-03-20,Phase Diagram of the Dynamics of a Precessing Qubit under Quantum Measurement,"We study the phase transitions induced by sequentially measuring a single qubit precessing under an external transverse magnetic field. Under projective quantum measurement, the probability distribution of the measurement outcomes can be mapped exactly to the thermodynamic probability distribution of a one-dimensional Ising model, whose coupling can be varied by the magnetic field from ferromagnetic to anti-ferromagnetic. For the general case of sequential quantum measurement,we develop a fast and exact algorithm to calculate the probability distribution function of the ferromagnetic order and anti-ferromagnetic order, and a phase diagram is obtained in the parameter space spanned by the measurement strength and magnetic field strength. The mapping to a long-range interacting Ising model is obtained in the limit of small measurement strength. Full counting statistical approach is applied to understand the phase diagram, and to make connections with the topological phase transition that is characterized by the braid group. This work deepens the understanding of phase transitions induced by quantum measurement, and may provide a new method to characterize and steer the quantum evolution.",2003.09056v1 2020-05-20,Ferromagnetism and giant magnetoresistance in zinc-blende FeAs monolayers embedded in semiconductor structures,"Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds embedded with a regular interval in a semiconductor InAs matrix, which resembles the crystal structure of Fe-based superconductors. Contrary to the case of Fe-based pnictides, these FeAs/InAs superlattices (SLs) exhibit ferromagnetism, whose Curie temperature (Tc) increases rapidly with decreasing the InAs interval thickness t_InAs (Tc ~ t_InAs^-3), and an extremely large magnetoresistance up to 500% that is tunable by a gate voltage. Our first principles calculations reveal the important role of disordered positions of Fe atoms in the establishment of ferromagnetism in these quasi-2D FeAs-based SLs. These unique features mark the FeAs/InAs SLs as promising structures for spintronic applications.",2005.10181v2 2020-05-20,Correlating magnetic structure and magnetotransport in semimetal thin films of Eu$_{1-x}$Sm$_x$TiO$_3$,"We report on the evolution of the average and depth-dependent magnetic order in thin film samples of biaxially stressed and electron-doped EuTiO$_3$ for samples across a doping range $<$0.1 to 7.8 $\times 10^{20}$ cm$^{-3}$. Under an applied in-plane magnetic field, the G-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated Eu$^{2+}$ moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.",2005.10339v1 2020-05-22,Pressure-induced suppression of ferromagnetism in the itinerant ferromagnet LaCrSb$_3$,"We have performed an extensive pressure-dependent structural, spectroscopic, and electrical transport study of LaCrSb$_3$. The ferromagnetic phase (T$_C$ = 120 K at p = 0 GPa) is fully suppressed by p = 26.5 GPa and the Cr-moment decreases steadily with increasing pressure. The unit cell volume decreases smoothly up to p = 55 GPa. We find that the bulk modulus and suppression of the magnetism are in good agreement with theoretical predictions, but the Cr-moment decreases smoothly with pressure, in contrast to steplike drops predicted by theory. The ferromagnetic ordering temperature appears to be driven by the Cr-moment.",2005.11356v1 2020-05-30,Ferromagnetic MnSn monolayer epitaxially grown on silicon substrate,"Two-dimensional (2D) ferromagnetic materials have been exhibiting promising potential in applications, such as spintronics devices. To grow epitaxial magnetic films on silicon substrate, in the single-layer limit, is practically important but challenging. In this study, we realized the epitaxial growth of MnSn monolayer on Si(111) substrate, with an atomically thin Sn/Si(111)-$2\sqrt{3}\times2\sqrt{3}$- buffer layer, and controlled the MnSn thickness with atomic-layer precision. We discovered the ferromagnetism in MnSn monolayer with the Curie temperature (Tc) of ~54 K. As the MnSn film is grown to 4 monolayers, Tc increases accordingly to ~235 K. The lattice of the epitaxial MnSn monolayer as well as the Sn/Si(111)-$2\sqrt{3}\times2\sqrt{3}$ is perfectly compatible with silicon, and thus an sharp interface is formed between MnSn, Sn and Si. This system provides a new platform for exploring the 2D ferromagnetism, integrating magnetic monolayers into silicon-based technology, and engineering the spintronics heterostructures.",2006.00333v1 2020-06-01,Dynamical quantum phase transitions in a spin chain with deconfined quantum critical points,"We analytically and numerically study the Loschmidt echo and the dynamical order parameters in a spin chain with a deconfined phase transition between a dimerized state and a ferromagnetic phase. For quenches from a dimerized state to a ferromagnetic phase, we find that the model can exhibit a dynamical quantum phase transition characterized by an associating dimerized order parameters. In particular, when quenching the system from the Majumdar-Ghosh state to the ferromagnetic Ising state, we find an exact mapping into the classical Ising chain for a quench from the paramagnetic phase to the classical Ising phase by analytically calculating the Loschmidt echo and the dynamical order parameters. By contrast, for quenches from a ferromagnetic state to a dimerized state, the system relaxes very fast so that the dynamical quantum transition may only exist in a short time scale. We reveal that the dynamical quantum phase transition can occur in systems with two broken symmetry phases and the quench dynamics may be independent on equilibrium phase transitions.",2006.00726v2 2020-06-29,Ultrafast optically induced ferromagnetic state in an elemental antiferromagnet,"We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L$_3$ resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.",2006.16061v4 2020-08-21,Confinement effect enhanced Stoner ferromagnetic instability in monolayer 1T-VSe2,"Monolayer 1T-VSe2 has been reported as a room-temperature ferromagnet. In this work, by using first-principles calculations, we unveil that the ferromagnetism in monolayer 1T-VSe2 is originated from its intrinsic huge Stoner instability enhanced by the confinement effect, which can eliminate the interlayer coupling, and lead to a drastic increase of the density of states at the Fermi level due to the presence of Van Hove singularity. Our calculations also demonstrate that the Stoner instability is very sensitive to the interlayer distance. These results provide a useful route to modulate the nonmagnetic to ferromagnetic transition in few-layers or bulk 1T-VSe2, which also shed light on the enhancement of its Curie temperature by enlarging the interlayer distance.",2008.09562v1 2020-08-25,Kondo holes in the 2D itinerant Ising ferromagnet Fe3GeTe2,"Heavy fermion (HF) states emerge in correlated quantum materials due to the interplay between localized magnetic moments and itinerant electrons, but rarely appear in 3d-electron systems due to high itinerancy of d-electrons. Here, an anomalous enhancement of Kondo screening is observed at the Kondo hole of local Fe vacancies in Fe3GeTe2 which is a recently discovered 3d-HF system featuring of Kondo lattice and two-dimensional itinerant ferromagnetism. An itinerant Kondo-Ising model is established to reproduce the experimental results which provides insight of the competition between Ising ferromagnetism and Kondo screening. This work explains the microscopic origin of the d-electron HF states and inspires study of the enriched quantum many-body phenomena with Kondo holes in Ising ferromagnets.",2008.10842v2 2020-08-25,A topological current divider,"We study the transport properties of a hybrid junction made of a ferromagnetic lead in electrical connection with the helical edge modes of a two-dimensional topological insulator. In this system, the time reversal symmetry, which characterizes the ballistic edge modes of the topological insulator, is explicitly broken inside the ferromagnetic region. This conflict situation generates unusual transport phenomena at the interface which are the manifestation of the interplay between the spin polarization of the injected current and the spin-momentum locking mechanism operating inside the topological insulator. We show that the spin polarized current originated in the ferromagnetic region is asymmetrically divided in spatially separated branch currents sustained by edge channels with different helicity inside the topological insulator. The above findings provide the working principle of a topological current divider in which the relative intensity of the branch currents is determined by the polarization of the incoming current. We discuss the relevance of this effect in spintronics where, for instance, it offers an alternative way to measure the current polarization generated by a ferromagnetic electrode.",2008.10905v1 2020-09-07,Tuneable Magneto-Resistance by Severe Plastic Deformation,"Bulk metallic samples were synthesized from different binary powder mixtures consisting of elemental Cu, Co, and Fe using severe plastic deformation. Small particles of the ferromagnetic phase originate in the conductive Cu phase, either by incomplete dissolution or by segregation phenomena during the deformation process. These small particles are known to give rise to granular giant magnetoresistance. Taking advantage of the simple production process, it is possible to perform a systematic study on the influence of processing parameters and material compositions on the magneto-resistance. Furthermore, it is feasible to tune the magnetoresistive behavior as a function of the specimens chemical composition. It was found that specimens of low ferromagnetic content show an almost isotropic drop in resistance in a magnetic field. With increasing ferromagnetic content, percolating ferromagnetic phases cause an anisotropy of the magnetoresistance. By changing the parameters of the high pressure torsion process, i.e., sample size, deformation temperature, and strain rate, it is possible to tailor the magnitude of giant magneto-resistance. A decrease in room temperature resistivity of approx. 3.5% was found for a bulk specimen containing an approximately equiatomic fraction of Co and Cu.",2009.02952v1 2020-09-08,Electrical detection of magnetic circular dichroism: application to magnetic microscopy in ultra-thin ferromagnetic films,"Imaging the magnetic configuration of thin-films has been a long-standing area of research. Since a few years, the emergence of two-dimensional ferromagnetic materials calls for innovation in the field of magnetic imaging. As the magnetic moments are extremely small, standard techniques like SQUID, torque magnetometry, magnetic force microscopy and Kerr effect microscopy are challenging and often lead to the detection of parasitic magnetic contributions or spurious effects. In this work, we report a new magnetic microscopy technique based on the combination of magnetic circular dichroism and Seebeck effect in semiconductor/ferromagnet bilayers. We implement this method with perpendicularly magnetized (Co/Pt) multilayers sputtered on Ge (111). We further show that the electrical detection of MCD is more sensitive than the Kerr magnetometry, especially in the ultra-thin film regime, which makes it particularly promising for the study of emergent two-dimensional ferromagnetic materials.",2009.03982v1 2020-09-30,Imprint from ferromagnetic skyrmions in an antiferromagnet via exchange bias,"Magnetic skyrmions are topological spin textures holding great potential as nanoscale information carriers. Recently, skyrmions have been predicted in antiferromagnets, with key advantages in terms of stability, size, and dynamical properties over their ferromagnetic analogs. However, their experimental demonstration is still lacking. Here, we show the imprint from ferromagnetic skyrmions into a thin film of an IrMn antiferromagnet, at room temperature and zero external magnetic field, using exchange-bias. Using high-spatial-resolution x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM), we observed the imprinted spin textures within the IrMn from the XMCD signal of the uncompensated Mn spins at the interface with the ferromagnet. This result opens up a path for logic and memory devices based on skyrmion manipulation in antiferromagnets.",2009.14796v2 2020-10-08,Chiral Hinge Magnons in Second-Order Topological Magnon Insulators,"When interacting spins in condensed matter order ferromagnetically, their ground state wave function is topologically trivial. Nonetheless, in two dimensions, the ferromagnetic state can support spin excitations with nontrivial topology, an exotic state known as topological magnon insulator (TMI). Here, we theoretically unveil and numerically confirm a novel ferromagnetic state in three dimensions dubbed second-order TMI, whose hallmarks are excitations at its hinges, where facets intersect. Since ferromagnetism naturally comes with broken time-reversal symmetry, the hinge magnons are chiral, rendering backscattering impossible. Hence, they trace out a three-dimensional path about the sample unimpeded by defects and are topologically protected by the spectral gap. They are remarkably robust against disorder and simultaneously highly tunable by atomic-level engineering of the sample termination. Our findings empower magnonics with the tools of higher-order topology, a promising route to combine low-energy information transfer free of Joule heating with three-dimensional vertical integration.",2010.04142v1 2020-10-09,Electronic and magnetic properties of $α$-FeGe$_2$ films embedded in vertical spin valve devices,"We studied metastable $\alpha$-FeGe$_2$, a novel layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic Fe$_3$Si and Co$_2$FeSi electrodes. For both types of electrodes, spin valve operation is demonstrated with a metallic transport behavior of the $\alpha$-FeGe$_2$ spacer layer. The spin valve signals are found to increase both with temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110~K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.",2010.04453v2 2020-10-15,"Microscopic mechanism of high-temperature ferromagnetism in Fe, Mn, and Cr-doped InSb, InAs, and GaSb magnetic semiconductors","In recent experiments, high Curie temperatures Tc above room temperature were reported in ferromagnetic semiconductors Fe-doped GaSb and InSb, while low Tc between 20 K to 90 K were observed in some other semiconductors with the same crystal structure, including Fe-doped InAs and Mn-doped GaSb, InSb, and InAs. Here we study systematically the origin of high temperature ferromagnetism in Fe, Mn, Cr-doped GaSb, InSb, and InAs magnetic semiconductors by combining the methods of density functional theory and quantum Monte Carlo. In the diluted impurity limit, the calculations show that the impurities Fe, Mn, and Cr have similar magnetic correlations in the same semiconductors. Our results suggest that high (low) Tc obtained in these experiments mainly comes from high (low) impurity concentrations. In addition, our calculations predict the ferromagnetic semiconductors of Cr-doped InSb, InAs, and GaSb that may have possibly high Tc. Our results show that the origin of high Tc in (Ga,Fe)Sb and (In,Fe)Sb is not due to the carrier induced mechanism because Fe3+ does not introduce carriers.",2010.07454v1 2020-11-20,Crystal growth and metallic ferromagnetism induced by electron doping in FeSb$_2$,"In order to study the metallic ferromagnetism induced by electron doping in the narrow-gab semiconductor FeSb$_2$, single crystals of FeSb$_2$, Fe$_{1-x}$Co$_x$Sb$_2$ ($0 \le x \le 0.5$) and FeSb$_{2-y}$Te$_y$ ($0 \le y \le 0.4$), were grown by a simplified self-flux method. From powder x-ray diffraction (XRD) patterns, wavelength-dispersive x-ray spectroscopy (WDX) and x-ray Laue diffraction, pure and doped high-quality single crystals, within the selected solubility range, show only the orthorhombic $Pnnm$ structure of FeSb$_2$ with a monotonic change in lattice parameters with increasing the doping level. In consistence with the model of nearly ferromagnetic small-gap semiconductor, the energy gap of FeSb$_2$ Pauli paramagnet gradually collapses by electron doping before it closes at about $x$ or $y$ = 0.15 and subsequent itinerant electron anisotropic ferromagnetic states are observed with higher doping levels. A magnetic phase diagram is established and discussed in view of proposed theoretical scenarios.",2011.10343v2 2020-11-23,Quantum Oscillations in the Field-induced Ferromagnetic State of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$,"The intrinsic antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$ undergoes a metamagnetic transition in a c-axis magnetic field. It has been predicted that ferromagnetic MnBi$_{2}$Te$_{4}$ is an ideal Weyl semimetal with a single pair of Weyl nodes. Here we report measurements of quantum oscillations detected in the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$, where Sb substitution tunes the majority carriers from electrons to holes. Single frequency Shubnikov-de Haas oscillations were observed in a wide range of Sb concentrations (0.54 $\leq$ x $\leq$ 1.21). The evolution of the oscillation frequency and the effective mass shows reasonable agreement with the Weyl semimetal band-structure of ferromagnetic MnBi$_{2}$Te$_{4}$ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure sensitively depends on magnetism.",2011.11798v1 2021-02-21,Direct and inverse spin-orbit torques in antiferromagnetic and ferromagnetic FeRh/W(001),"We use \textit{ab-initio} calculations to investigate spin-orbit torques (SOTs) in FeRh(001) deposited on W(100). Since FeRh undergoes a ferromagnetic-antiferromagnetic phase transition close to room temperature, we consider both phases of FeRh. In the antiferromagnetic case we find that the effective magnetic field of the even torque is staggered and therefore ideal to induce magnetization dynamics or to switch the antiferromagnet (AFM). At the antiferromagnetic resonance the inverse SOT induces a current density, which can be determined from the SOT. In the ferromagnetic case our calculations predict both even and odd components of the SOT, which can also be used to describe the ac and dc currents induced at the ferromagnetic resonance. For comparison we compute the SOTs in the c($2\times 2$) AFM state of Fe/W(001).",2102.10598v1 2021-02-22,Crossover between short and long range proximity effects in SFS junctions with Ni-based ferromagnets,"We study Superconductor/Ferromagnet/Superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in Pt1-xNix with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices.",2102.10927v1 2021-03-11,Flatband-Induced Itinerant Ferromagnetism in RbCo$_2$Se$_2$,"$A$Co$_2$Se$_2$ ($A$=K,Rb,Cs) is a homologue of the iron-based superconductor, $A$Fe$_2$Se$_2$. From a comprehensive study of RbCo$_2$Se$_2$ via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principle calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin-splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a $d_{x^2-y^2}$ flatband near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this $d_{x^2-y^2}$ flatband to be a common feature in the band structures of both iron-chalcogenides and iron-pnictides, accessible via heavy electron doping.",2103.06528v1 2021-03-30,Large voltage-tunable spin valve based on a double quantum dot,"We study the spin-dependent transport properties of a spin valve based on a double quantum dot. Each quantum dot is assumed to be strongly coupled to its own ferromagnetic lead, while the coupling between the dots is relatively weak. The current flowing through the system is determined within the perturbation theory in the hopping between the dots, whereas the spectrum of a quantum dot-ferromagnetic lead subsystem is determined by means of the numerical renormalization group method. The spin-dependent charge fluctuations between ferromagnets and quantum dots generate an effective exchange field, which splits the double dot levels. Such field can be controlled, separately for each quantum dot, by the gate voltages or by changing the magnetic configuration of external leads. We demonstrate that the considered double quantum dot spin valve setup exhibits enhanced magnetoresistive properties, including both normal and inverse tunnel magnetoresistance. We also show that this system allows for the generation of highly spin-polarized currents, which can be controlled by purely electrical means. The considered double quantum dot with ferromagnetic contacts can thus serve as an efficient voltage-tunable spin valve characterized by high output parameters.",2103.16059v1 2021-04-08,Anisotropic c-f hybridization in the ferromagnetic quantum critical metal CeRh$_6$Ge$_4$,"Heavy fermion compounds exhibiting a ferromagnetic quantum critical point have attracted considerable interest. Common to two known cases, i.e., CeRh$_6$Ge$_4$ and YbNi$_4$P$_2$, is that the 4f moments reside along chains with a large inter-chain distance, exhibiting strong magnetic anisotropy that was proposed to be vital for the ferromagnetic quantum criticality. Here we report an angle-resolved photoemission study on CeRh6Ge4, where we observe sharp momentum-dependent 4f bands and clear bending of the conduction bands near the Fermi level, indicating considerable hybridization between conduction and 4f electrons. The extracted hybridization strength is anisotropic in momentum space and is obviously stronger along the Ce chain direction. The hybridized 4f bands persist up to high temperatures, and the evolution of their intensity shows clear band dependence. Our results provide spectroscopic evidence for anisotropic hybridization between conduction and 4f electrons in CeRh$_6$Ge$_4$, which could be important for understanding the electronic origin of the ferromagnetic quantum criticality.",2104.03600v1 2021-05-03,Giant anomalous Hall conductivity in the itinerant ferromagnet LaCrSb3 and the effect of f-electrons,"Itinerant ferromagnets constitute an important class of materials wherein spin-polarization can affect the electric transport properties in nontrivial ways. One such phenomenon is anomalous Hall effect which depends on the details of the band structure such as the amount of band crossings in the valence band of the ferromagnet. Here, we have found extraordinary anomalous Hall effect in an itinerant ferromagnetic metal LaCrSb3. The rather two-dimensional nature of the magnetic subunit imparts large anisotropic anomalous Hall conductivity of 1250 S/cm at 2K. Our investigations suggest that a strong Berry curvature by abundant momentum-space crossings and narrow energy-gap openings are the primary sources of the anomalous Hall conductivity. An important observation is the existence of quasi-dispersionless bands in LaCrSb3 which is now known to increase the anomalous Hall conductivity. After introducing f-electrons, anomalous Hall conductivity experiences more than two-fold increase and reaches 2900 S/cm in NdCrSb3.",2105.00906v1 2021-06-10,Room-temperature spin injection and spin-to-charge conversion in a ferromagnetic semiconductor / topological insulator heterostructure,"Spin injection using ferromagnetic semiconductors at room temperature is a building block for the realization of spin-functional semiconductor devices. Nevertheless, this has been very challenging due to the lack of reliable room-temperature ferromagnetism in well-known group IV and III-V based semiconductors. Here, we demonstrate room-temperature spin injection by using spin pumping in a (Ga,Fe)Sb / BiSb heterostructure, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature (TC) and BiSb is a topological insulator (TI). Despite the very small magnetization of (Ga,Fe)Sb at room temperature (45 emu/cc), we are able to detect spin injection from (Ga,Fe)Sb by utilizing the inverse spin Hall effect (ISHE) in the topological surface states of BiSb with a large inverse spin Hall angle of 2.5. Our study provides the first demonstration of spin injection as well as spin-to-charge conversion at room temperature in a FMS/TI heterostructure.",2106.05902v1 2021-06-23,"Magnetic anisotropy in uranium monosulfide, probed by the magnetic torque measurements","We have studied the magnetic torque in uranium monosulfide (US) single crystals to explore the magnetic anisotropy in this material. Uranium monosulfide crystallizes in cubic, NaCl-type of crystal structure and exhibits the largest magneto-crystalline anisotropy observed in cubic systems. By performing detailed torque measurements we observe a strongly anisotropic behavior in the paramagnetic phase due to crystal defects and quadrupolar pair interactions. Our studies also confirm the presence of a large anisotropy in the ferromagnetic state of the US system with the <100>, <111>, and <110> directions being hard, easy, and intermediate axis, respectively. Furthermore, the anisotropy in the paramagnetic phase shows similar characteristics to the anisotropy observed in the ferromagnetic phase, as characterized by second and fourth rank susceptibility terms. The similarity of the anisotropic behaviors in paramagnetic and ferromagnetic phases is the consequence of strong magneto-elastic properties in this system, which possibly lead to the rhombohedral structural distortion, not only in the ferromagnetic phase but also in the paramagnetic phase (induced by applied magnetic fiield).",2106.12464v1 2021-07-09,Two-dimensional ferromagnetic spin-orbital excitations in the honeycomb VI$_{3}$,"VI$_{3}$ is a ferromagnet with planar honeycomb sheets of bonded V$^{3+}$ ions held together by van der Waals forces. We apply neutron spectroscopy to measure the two dimensional ($J/J_{c} \approx 17$) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ($\Delta \approx k_{B} T_{c} \approx$ 55 K) and dispersive excitations. We apply a multi-level spin wave formalism to describe the spectra in terms of two coexisting domains hosting differing V$^{3+}$ orbital ground states built from contrasting distorted octahedral environments. This analysis fits a common nearest neighbor in-plane exchange coupling ($J$=-8.6 $\pm$ 0.3 meV) between V$^{3+}$ sites. The distorted local crystalline electric field combined with spin-orbit coupling provides the needed magnetic anisotropy for spatially long-ranged two-dimensional ferromagnetism in VI$_{3}$.",2107.04311v1 2021-07-14,"Phase-manipulation-induced Majorana Mode and Braiding Realization in Iron-based Superconductor Fe(Te,Se)","Recent experiment reported the evidence of dispersing one-dimensional Majorana mode trapped by the crystalline domain walls in FeSe$_{0.45}$Te$_{0.55}$. Here, we perform the first-principles calculations to show that iron atoms in the domain wall spontaneously form the ferromagnetic order in line with orientation of the wall. The ferromagnetism can impose a $\pi$ phase difference between the domain-wall-separated surface superconducting regimes under the appropriate width and magnetization of the wall. Accordingly, the topological surface superconducting state of FeSe$_{0.45}$Te$_{0.55}$ can give rise to one-dimensional Majorana modes trapped by the wall. More interestingly, we further propose a surface junction in the form of FeSe$_{0.45}$Te$_{0.55}$/ferromagnet/FeSe$_{0.45}$Te$_{0.55}$, which can be adopted to create and fuse the Majorana zero modes through controlling the width or magnetization of the interior ferromagnetic barrier. The braiding and readout of Majorana zero modes can be realized by the designed device. Such surface junction has the potential application in the superconducting topological quantum computation.",2107.06558v3 2021-07-23,Green's function formalism for nonlocal elliptical magnon transport,"We develop a non-equilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreover, the spin state of the ferromagnet becomes squeezed in the high-anisotropy regime. We show that the squeezing may be globally reduced by the application of a local spin bias.",2107.11072v2 2021-07-28,"Controlling phase transition in monolayer metal diiodides XI$_{2}$ (X: Fe, Co, and Ni) by carrier doping","We applied the generalized Bloch theorem to verify the ground state (most stable state) in monolayer metal diiodides 1T-XI$_{2}$ (X: Fe, Co, and Ni), a family of metal dihalides, using the first-principles calculations. The ground state, which can be ferromagnetic, antiferromagnetic, or spiral state, was specified by a wavevector in the primitive unit cell. While the ground state of FeI$_{2}$ is ferromagnetic, the spiral state becomes the ground state for CoI$_{2}$ and NiI$_{2}$. Since the multiferroic behavior in the metal dihalide can be preserved by the spiral structure, we believe that CoI$_{2}$ and NiI$_{2}$ are promising multiferroic materials in the most stable state. When the lattice parameter increases, we also show that the ground state of NiI$_{2}$ changes to a ferromagnetic state while others still keep their initial ground states. For the last discussion, we revealed the phase transition manipulated by hole-electron doping due to the spin-spin competition between the ferromagnetic superexchange and the antiferromagnetic direct exchange. These results convince us that metal diiodides have many benefits for future spintronic devices.",2107.13287v1 2021-07-30,Emergence of Ferromagnetism Through the Metal-Insulator Transition in Undoped Indium Tin Oxide Films,"We present a detailed study of the emergence of bulk ferromagnetism in low carrier density samples of undoped indium tin oxide (ITO). We used annealing to increase the density of oxygen vacancies and change sample morphology without introducing impurities through the metal insulator transition (MIT). We utilized a novel and highly sensitive ""Corbino-disk torque magnetometry"" technique to simultaneously measure the thermodynamic and transport effects of magnetism on the same sample after successive annealing. With increased sample granularity, carrier density increased, the sample became more metallic, and ferromagnetism appeared as resistance approached the MIT. Ferromagnetism was observed through the detection of magnetization hysteresis, anomalous Hall effect (AHE), and hysteretic magnetoresistance. A sign change of the AHE as the MIT is approached may elucidate the interplay between the impurity band and the conduction band in the weakly insulating side of the MIT.",2107.14463v1 2021-08-02,Antiferromagnetic order of ferromagnetically coupled dimers in the double pyrovanadate CaCoV$_2$O$_7$,"Magnetic properties of the pyrovanadate CaCoV$_2$O$_7$ have been studied by means of the bulk magnetization and neutron powder diffraction measurements. Magnetic susceptibility in the paramagnetic phase shows Curie-Weiss behavior with negative Weiss temperature $\Theta \simeq -22.5$ K, indicating dominant antiferromagnetic interactions. At $T_{\rm N} = 3.44$ K, CaCoV$_2$O$_7$ shows antiferromagnetic order, accompanied by a weak net ferromagnetic moment of $\sim 0.05$~$\mu_{\rm B}/{\rm Co}^{2+}$. Neutron powder diffraction confirms the formation of antiferromagnetic order below $T_{\rm N}$. It was further confirmed from the magnetic structure determination that the two Co$^{2+}$ ions in the adjacent edge-sharing octahedra have almost parallel (ferromagnetic) spin arrangement, indicative of a formation of a ferromagnetic spin dimer. The antiferromagnetic order is, in turn, stabilized by sizable inter-dimer antiferromagnetic interactions.",2108.00715v1 2021-08-26,Pressure-induced collapse of ferromagnetism in Nickel,"Transition metals, Fe, Co and Ni, are the canonical systems for studying the effect of external perturbations on ferromagnetism. Among these, Ni stands out as it undergoes no structural phase transition under pressure. Here we have investigated the long-debated issue of pressure-induced magnetisation drop in Ni from first-principles. Our calculations confirm an abrupt quenching of magnetisation at high pressures, not associated with any structural phase transition. We find that the pressure substantially enhances the crystal field splitting of Ni-$3d$ orbitals, driving the system towards a new metallic phase violating the Stoner Criterion for ferromagnetic ordering. Analysing the charge populations in each spin channel, we show that the next nearest neighbour interactions play a crucial role in quenching ferromagnetic ordering in Ni and materials alike.",2108.11977v1 2021-09-20,"Electron Spin Resonance of the itinerant ferromagnets LaCrGe3, CeCrGe3 and PrCrGe3","We report Electron Spin Resonance of the itinerant ferromagnets LaCrGe3, CeCrGe3, and PrCrGe3. These compounds show well defined and very similar spectra of itinerant Cr 3d spins in the paramagnetic temperature region. Upon cooling and crossing the Cr-ferromagnetic ordering (below around 90 K) strong spectral structures start to dominate the resonance spectra in a quite different manner in the three compounds. In the Ce- and Pr-compounds the resonance is only visible in the paramagnetic region whereas in the La-compound the resonance can be followed far below the ferromagnetic ordering temperature. This behavior will be discussed in terms of the specific interplay between the 4f and 3d magnetism which appears quite remarkable since CeCrGe3 displays heavy fermion behavior even in the magnetically ordered state.",2109.09341v1 2021-09-30,Theoretical prediction of Curie temperature in two-dimensional ferromagnetic monolayer,"Theoretical prediction of Curie temperature (TC) is of vital importance for designing the spintronic devices in two-dimensional (2D) ferromagnetic materials. Herein, based on the extensive investigation of Monte Carlo simulations, we summary and propose an improved method to estimate TC more precisely, which includes the different contributions of multiple near-neighbor interactions. Taking monolayer CrI3 as an example, the trends of TC with biaxial strain are investigated via Monte Carlo simulations, mean-field formulas and our method. Besides, our method is not only accurate and convenient to predicting the TC in 2D ferromagnetic honeycomb lattice CrI3 but it can be extended for predicting the TC of other 2D lattices. Our work paves the way to accelerate the prediction and discovery of novel 2D ferromagnets for spintronic applications.",2109.14802v2 2021-10-04,Lumped circuit model for inductive antenna spin-wave transducers,"We derive a lumped circuit model for inductive antenna spin-wave transducers in the vicinity of a ferromagnetic medium. The model considers the antenna's Ohmic resistance, its inductance, as well as the additional inductance due to the excitation of ferromagnetic resonance or spin waves in the ferromagnetic medium. As an example, the additional inductance is discussed for a wire antenna on top of a ferromagnetic waveguide, a structure that is characteristic for many magnonic devices and experiments. The model is used to assess the scaling properties and the energy efficiency of inductive antennas. Issues related to scaling antenna transducers to the nanoscale and possible solutions are also addressed.",2110.01318v3 2021-10-04,Anomalous temperature dependence of phonon pumping by ferromagnetic resonance in Co/Pd multilayers with perpendicular anisotropy,"We demonstrate the pumping of phonons by ferromagnetic resonance in a series of [Co(0.8 nm)/Pd(1.5 nm)]$_n$ multilayers ($n =$ 6, 11, 15, and 20) with large magnetostriction and perpendicular magnetic anisotropy. The effect is shown using broadband ferromagnetic resonance over a range of temperatures (10 to 300 K), where a resonant damping enhancement is observed at frequencies corresponding to standing wave phonons across the multilayer. The strength of this effect is enhanced by approximately a factor of 4 at 10 K compared to room temperature, which is anomalous in the sense that the temperature dependence of the magnetostriction predicts an enhancement that is less than a factor of 2. Lastly, we demonstrate that the damping enhancement is correlated with a shift in the ferromagnetic resonance field as predicted quantitatively from linear response theory.",2110.01714v1 2021-10-14,Ferromagnetic Negative Charge-Transfer Insulator: from Theoretical Proposal to Material Realization,"Here we propose another type of ferromagnetic semiconductors: ferromagnetic negative charge-transfer insulator (FNCTI). In FNCTI, the negative charge-transfer states strongly enhance the ferromagnetic (FM) exchange interactions and the orbital hybridization gap permits the magnetic molecular orbitals as the underlying magnetic units rather than local atomic orbitals. Thus the FM exchange interactions are rather strong and decay slowly due to the large spearding of magnetic molecular orbitals. This is distinct from the superexchange mechanism where FM exchange interactions are quite weak as summarized in the well-known Goodenough-Kanamori-Anderson semi-empirical rules.Through first-principle calculations with the hybrid functional, PbO-type CrAs monolayer is mapped out to be a FNCTI, which possesses a band gap $\sim$ 0.35 eV, FM nearest-/next-nearest-neighbor exchange coupling strength $\sim$ 57/40 meV, and a high $T_c$ $\sim$ 1500 K respectively. It is believed that the existence of FNCTI validates the long-pending hypothesis by D. I. Khomskii and G. A. Sawatzky in 1997 [Solid State Commun. 102, 87 (1997)].",2110.07372v2 2021-10-20,Nonlinear response induced by Ferromagnetism in a Noncentrosymmetric Kondo Lattice system,"Recently, nonlinear responses have been actively studied in both experiments and theory. Particularly interesting are inversion-symmetry broken systems, where an even-order nonlinear electrical conductivity can be nonzero, resulting in nonreciprocity. Second-order nonlinear conductivities attract much attention because of their sensitivity to detect inversion-symmetry breaking in materials and their functionalities. However, while the nonlinear response has been actively studied in noninteracting systems for a long time, the nonlinear response in strongly correlated materials is still poorly understood. This paper analyzes the nonlinear conductivity in a correlated noncentrosymmetric system, namely a Kondo lattice system with Rashba type spin-orbit coupling. We mainly focus on the ferromagnetic phase, in which the second-order nonlinear conductivity becomes finite. Remarkably, we find that the second-order conductivity becomes only finite perpendicular to the ferromagnetic magnetization and has a strong spin dependence; due to a gap at the Fermi energy for one spin direction, the linear and nonlinear conductivity is only finite for the ungapped spin direction. Finally, we analyze sign changes in the nonlinear conductivity, which can be explained by a combination of correlation effects and the energetic shift due to the occurring ferromagnetism.",2110.10496v1 2021-10-20,Spin Fluctuations Yield zT Enhancement in Ferromagnets,"Thermal fluctuation of local magnetization in magnetic metals intercoupled with charge carriers and phonons offers a path to enhance thermoelectric performance. The thermopower enhancement by spin fluctuations (SF) has been observed before. However, the crucial evidence for enhancing thermoelectric-figure-of-merit (zT) by SF has not been reported until now. Here we report evidence for such enhancement in the ferromagnetic CrTe. The SF leads to nearly 80% zT enhancement in ferromagnetic CrTe near and below TC~335 K. The ferromagnetism in CrTe is originated from the collective electronic and localized magnetic moments. The field-dependent transport properties demonstrate the profound impact of SF on the electrons and phonons. The SF simultaneously enhances the thermopower and reduces the thermal conductivity. Under an external magnetic field, the enhancement in thermopower is suppressed, and the thermal conductivity is enhanced, evidencing the existence of a strong SF near and below TC. The anomalous thermoelectric transport properties are analyzed based on theoretical models, and a good agreement with experimental data is found. Furthermore, the detailed analysis proves an insignificant impact from spin-wave contribution to the transport properties. This study contributes to the fundamental understanding of spin fluctuation for designing high-performance spin-driven thermoelectric materials.",2110.10798v1 2021-10-25,Superconducting triplet rim currents in a spin-textured ferromagnetic disk,"Since the discovery of the long-range superconducting proximity effect, the interaction between spin-triplet Cooper pairs and magnetic structures such as domain walls and vortices has been the subject of intense theoretical discussions, while the relevant experiments remain scarce. We have developed nanostructured Josephson junctions with highly controllable spin texture, based on a disk-shaped Nb/Co bilayer. Here, the vortex magnetization of Co and the Cooper pairs of Nb conspire to induce long-range triplet (LRT) superconductivity in the ferromagnet. Surprisingly, the LRT correlations emerge in highly localized (sub-80 nm) channels at the rim of the ferromagnet, despite its trivial band structure. We show that these robust rim currents arise from the magnetization texture acting as an effective spin-orbit coupling, which results in spin accumulation at the bilayer-vacuum boundary. Lastly, we demonstrate that by altering the spin texture of a single ferromagnet, both $0$ and $\pi$-channels can be realized in the same device.",2110.13035v2 2021-11-06,Tuning the electronic band structure in a kagome ferromagnetic metal via magnetization,"Materials with zero energy band gap display intriguing properties including high sensitivity of the electronic band structure to external stimulus such as pressure or magnetic field. An interesting candidate for zero energy band gap are Weyl nodes at the Fermi level EF. A prerequisite for the existence of Weyl nodes is to either have inversion or time reversal symmetry broken. Weyl nodes in systems with broken time reversal symmetry are ideal to realize the tunability of the electronic band structure by magnetic field. Theoretically, it has been shown that in ferromagnetic Weyl materials, the band structure is dependent upon the magnetization direction and thus the electronic bands can be tuned by controlling the magnetization direction. Here, we demonstrate tuning of the band structure in a kagome Weyl ferromagnetic metal Fe3Sn2 with magnetization and magnetic field. Owing to spin-orbit coupling, we observe changes in the band structure depending on the magnetization direction that amount to a decrease in the carrier density by a factor of four when the magnetization lies in the kagome plane as compared to when the magnetization is along the c axis. Our discovery opens a way for tuning the carrier density in ferromagnetic materials.",2111.03826v1 2021-11-22,Excitation and reception of magnetostatic surface spin waves in thin conducting ferromagnetic films by coplanar microwave antennas. Part I: Theory,"A fully self-consistent model for the excitation and reception of magnetostatic surface waves in thin ferromagnetic films by a set of coplanar antennas was developed and implemented numerically. The model assumes that the ferromagnetic film is highly conducting and is interfaced with non-magnetic metallic films, but is also suitable for modeling magneto-insulating films. Perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction can be included at both interfaces of the ferromagnetic layer. The model calculates the coupling impedances between the different strips constituting the coplanar antennas. In some situations, this leads to a frequency non-reciprocity between counter-propagating waves even in the case of no asymmetry in the spin-wave dispersion relation. Several intermediate results of the model were checked numerically and the final output of the model, given as the scattering parameters, $S_{11}$, $S_{12}$, and $S_{21}$ of the antenna system, were in good agreement with previous experimental studies.",2111.11106v3 2021-12-01,Electric Field Control of Magnetism of Mn dimer supported on Carbon-doped-h-BN surface,"Using density functional theory we show that the interaction between two Mn atoms can be tuned from anti-ferromagnetic (AFM) to ferromagnetic (FM) state by creating charge disproportion between the two on a 2D surface. The non-metallic planar heterostructures, the 2D surface, in our work is designed by doping carbon hexagon rings in a hexagonal boron nitride (h-BN) sheet. In addition, we show that an external electric field can be used to control the charge disproportion and hence the magnetism. In fact, our calculations demonstrate that the magnetic states of the dimer can be switched from AFM to FM or vice versa in an external electric field. The origin of this magnetic switching is explained using the charge transfer from (or to) the Mn dimer to (or from) the 2D material. The switching between anti-ferromagnetic to ferromagnetic states can be useful for future spintronic applications.",2112.00638v1 2021-12-07,Nanometers-thick Ferromagnetic Surface Produced by Laser Cutting of Diamond,"In this work, we demonstrate that cutting diamond crystals with a laser (532~nm wavelength, 0.5~mJ energy, 200~ns pulse duration at 15~kHz) produces a $\lesssim 20~$nm thick surface layer with magnetic order at room temperature. We have measured the magnetic moment of five natural and six CVD diamond crystals of different size, nitrogen content and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300~K is observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals are much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.",2112.03671v2 2021-12-11,Itinerant ferromagnetism of a dipolar Fermi gas with Raman-induced spin-orbit coupling,"We elucidate the itinerant ferromagnetism of a dipolar Fermi gas with a Raman-induced spin-orbit coupling by investigating the exotic phase diagrams at zero and finite temperature. It is revealed that the dipolar interaction along with spin-orbit coupling can corroborate the formation of ferromagnetism and the Raman coupling adversely eliminates the tendency to this ferromagnetism transition, which greatly transcends the general understanding of this subject with contact interaction only. We explore the ground states through the density and spin-flip distribution in momentum space, which exhibits novel degeneracy at strong Raman coupling indicated by a non-zero entropy at zero temperature. We calculate the transition temperatures well within the reach of an experimental system when altering the dipolar and spin-orbit coupling strength, which paves a way to the further experimental realization.",2112.05898v2 2021-12-18,Zeroth-Order Nucleation Transition under Nanoscale Phase Separation,"Materials with nanoscale phase separation are considered. A system representing a heterophase mixture of ferromagnetic and paramagnetic phases is studied. After averaging over phase configurations, a renormalized Hamiltonian is derived describing the coexisting phases. The system is characterized by direct and exchange interactions and an external magnetic field. The properties of the system are studied numerically. The stability conditions define the stable state of the system. At a temperature of zero, the system is in a pure ferromagnetic state. However, at finite temperature, for some interaction parameters, the system can exhibit a zeroth-order nucleation transition between the pure ferromagnetic phase and the mixed state with coexisting ferromagnetic and paramagnetic phases. At the nucleation transition, the finite concentration of the paramagnetic phase appears via a jump.",2112.09953v1 2021-12-21,Nonreciprocal spin-wave dynamics in Pt/Co/W/Co/Pt multilayers,"We present a detailed study of the spin-wave dynamics in single Pt/Co/W and double Pt/Co/W/Co/Pt ferromagnetic layer systems. The dispersion of spin waves was measured by wavevector-resolved Brillouin light scattering spectroscopy while the in-plane and out-of-plane magnetization curves were measured by alternating gradient field magnetometry. The interfacial Dzyaloshinskii-Moriya interaction induced nonreciprocal dispersion relation was demonstrated for both single and double ferromagnetic layers and explicated by numerical simulations and theoretical formulas. The results indicate the crucial role of the order of layers deposition on the magnetic parameters. A significant difference between the perpendicular magnetic anisotropy constant in double ferromagnetic layer systems conduces to the decline of the interlayer interactions and different dispersion relations for the spin-wave modes. Our study provides a significant contribution to the realization of the multifunctional nonreciprocal magnonic devices based on ultrathin ferromagnetic/heavy-metal layer systems.",2112.11206v1 2022-01-05,Absence of a critical nematic phase in the vicinity of the $\rm {SU}(3)$ ferromagnetic point for the one-dimensional spin-1 bilinear-biquadratic model,"The absence of a critical nematic phase in the vicinity of the $\rm {SU}(3)$ ferromagnetic point for the one-dimensional spin-1 bilinear-biquadratic model is demonstrated by means of the tensor network algorithms. As it turns out, the phase transition from the ferromagnetic phase to the dimerized phase at the $\rm {SU}(3)$ ferromagnetic point is direct, but not of the first-order. The transition point features highly degenerate ground states, which are scale but not conformally invariant, with the fractal dimension being equal to 2. The conceptual developments in effective field theories - the fractal dimension and the counting rule of the Goldstone modes - play a pivotal role in clarifying the numerical artifacts arising from the finiteness of the bond dimension in the tensor network simulations, which are attributed to a proximity effect to a highly entangled scale or conformally invariant ground state.",2201.01434v2 2022-01-28,Itinerant ferromagnetism in a spin-fermion model for diluted spin systems,"We investigate the itinerant ferromagnetism using a diluted spin-fermion model, derived from a repulsive Hubbard model, where itinerant fermions are coupled antiferromagnetically to auxiliary fields in a three-dimensional simple cubic lattice. We focus, in particular, on understanding the spin-dependent transport properties of the itinerant fermions in the impurity band by taking positional disorder of the auxiliary fields into account. For on-site repulsion $U$ $\sim$ bandwidth the density of the itinerant carriers confined to the impurity band, play a key role in determining the kinetic energy of the system and consequently the carrier spin polarization. Our semi-classical Monte Carlo calculations show that the ferromagnetic transition temperature of the carrier spins indeed shows an optimization behavior with the carrier density. We calculate the transport properties in details to establish a one-to-one correspondence between the magnetic and transport properties of the carriers. Our results obtained beyond the perturbative regime are significant for understanding the ferromagnetism in diluted magnetic semiconductors.",2201.12182v1 2022-02-15,"Theoretical Investigation of Structural, Electronic Properties and Half-Metallic Ferromagnetism in Ti doped CaS","In this research paper, we investigated the structural, electronic, and magnetic features of titanium atom substituting calciumatom in rock salt structure of CaS to explore the new dilute magnetic semiconductor compounds. The calculations are carried out using the full potential linearized augmented plane wave (FP-LAPW) method based on spin-polarized density functional theory, implemented in WIEN2k code. The generalized gradient approximation and Tran-Balaha modified Becke-Johnson exchange potential. The stability of Ti doped CaS in ferromagnetic state is provided by the total energy released from the optimized structures and defect formation energies. The classical model of Heisenberg is employed to estimate Curie temperature of these compounds. It is found that the room temperature ferromagnetism is achieved at low concentrations. The studied materials exhibit half metallic ferromagnetic demeanor. The half metallic gaps (GHM) are the extremely significant factors to consider for spintronic applications. The insertion of impurity significantly decreased the value of GHM due the broadening of 3d Ti states in the gap of the minority spin. Furthermore, to evaluate the effects of the exchange splitting process, the pd exchange splitting and the exchange constants are predicted.",2202.06947v1 2022-02-28,Suppression of ferromagnetism and influence of disorder in silicon-substituted CeRh6Ge4,"We report a study of isoelectronic chemical substitution in the recently discovered quantum critical ferromagnet CeRh$_6$Ge$_4$. Upon silicon-doping, the ferromagnetic ordering temperature of CeRh$_6$(Ge$_{1-x}$Si$_x$)$_4$ is continuously suppressed, and no transition is observed beyond $x_c$$\approx$0.125. Non-Fermi liquid behavior with $C/T \propto$log($T^*/T$) is observed close to $x_c$, indicating the existence of strong quantum fluctuations, while the $T$-linear behavior observed upon pressurizing the parent compound is absent in the resistivity, which appears to be a consequence of the disorder induced by silicon doping. Our findings provide evidence for the role played by disorder on the unusual ferromagnetic quantum criticality in CeRh$_6$Ge$_4$, and provides further evidence for understanding the origin of this behavior.",2202.13582v1 2022-03-09,Ferromagnetism in layered metallic Fe1/4TaS2 in the presence of conventional and Dirac carriers,"In this paper we present the microscopic origin of the ferromagnetism of Fe0.25TaS2 and its finite-temperature magnetic properties. We first obtain the band structures of Fe0.25TaS2 by the first-principles calculations and find that both conventional and Dirac carriers coexist in metallic Fe0.25TaS2. Accordingly, considering the spin-orbit coupling of Fe 3d ion, we derive an effective RKKY-type Hamiltonian between Fe spins in the presence of both the conventional parabolic-dispersion and the Dirac linear-dispersion carriers, which contains a Heisenberg-like, an Ising-like and an XY-like term. In addition, we obtain the ferromagnetic Curie temperature Tc by using the cluster self-consistent field method. Our results could address not only the high ferromagnetic Curie temperature, but also the large magnetic anisotropy in FexTaS2.",2203.04505v1 2022-03-24,Giant electromagnetic proximity effect in superconductor/ferromagnet superlattices,"We show that in superlattices with alternating superconducting (S) and ferromagnetic (F) layers the spontaneous magnetic field induced in the superconducting layers due to the electromagnetic proximity effect becomes dramatically enhanced compared to the previously studied S/F bilayers. The effect reveals itself for the in-plane orientation of the magnetic moments both for ferromagnetic and anti-ferromagnetic ordering of the moments in the F layers. In the finite size samples the magnetic field decays from the sample surface towards the bulk of the structure, and the decay length strongly depends on the relative orientation of the sample surface, the layers planes and magnetic moments in the F layers. The obtained results provide additional insights into experimental data on the neutron scattering in Nb/Gd superlattices.",2203.13003v1 2022-03-31,High-temperature ferromagnetism in two-dimensional material MnSn originated from interlayer coupling,"MnSn monolayer synthesized recently is a novel two-dimensional ferromagnetic material with a hexagonal lattice, in which three Mn atom come together to form a trimer, making it remarkably different from other magnetic two-dimensional materials. Most impressively, there happens a sharp increase of Curie temperature from 54 K to 225 K when the number of layers increase from 1 to 3. However, no quantitative explanation is reported in previous studies. Herein, by means of first-principle calculations method and Monte carlo method, we demonstrate that strong interlayer ferromagnetic coupling is the essential role in enhancing its critical temperature, which act as a magnetic field to stabilize the ferromagnetism in the MnSn multilayers.",2203.17126v1 2022-04-04,Phase-field modeling of paramagnetic austenite-ferromagnetic martensite transformation coupled with mechanics and micromagnetics,"A three-dimensional phase-field model is proposed for simulating the magnetic martensitic phase transformation. The model considers a paramagnetic cubic austenite to ferromagnetic tetragonal martensite transition, as it occurs in magnetic Heusler alloys like Ni2 MnGa, and is based on a Landau 2-3-4 polynomial with temperature dependent coefficients. The paramagnetic-ferromagnetic transition is recaptured by interpolating the micromagnetic energy as a function of the order parameter for the ferroelastic domains. The model is numerically implemented in real space by finite element (FE) method. FE simulations in the martensitic state show that the model is capable to correctly recapture the ferroelastic and -magnetic microstructures, as well as the influence of external stimuli. Simulation results indicate that the paramagnetic austenite to ferromagnetic martensite transition shifts towards higher temperatures when a magnetic field or compressive stress is applied. The dependence of the phase transition temperature shift on the strength of the external stimulus is uncovered as well. Simulation of the phase transition in magnetocaloric materials is of high interest for the development of energy-efficient magnetocaloric cooling devices.",2204.01308v1 2022-04-06,Unconventional ferromagnetism and spin-triplet superconductivity in the imbalanced Kagome-lattice Hubbard model,"Unconventional ferromagnetism and superconductivity in the imbalanced kagome-lattice Hubbard model are investigated by the mean-field theory and determinant quantum Monte Carlo method. Due to the asymmetric band structure of kagome lattice, the spin-$z$ ferromagnetic order intrinsically exists in the system, which is first enhanced by the interaction, and then continuously destructed after reaching a maximum at a moderate interaction strength. In contrast, the $xy$-plane ferromagnetism develops only above a critical interaction, which is estimated to be $U_c/t=3.65 \pm 0.05$ by finite-size scaling. We further verify the nature of the above transverse magnetic transition, and demonstrate it belongs to the three-dimensional $XY$ universality class. Finally, we study the superconducting property, and reveal the possible superconducting state has a triplet $f$-wave pairing symmetry. Our results uncover the exotic quantum states induced by the interactions on kagome lattice, and provide important insights regarding the interplay between electronic correlations and geometry frustrations.",2204.02554v1 2022-04-22,A short-circuited coplanar waveguide for low-temperature single-port ferromagnetic resonance spectroscopy set-up to probe the magnetic properties of ferromagnetic thin films,"A coplanar waveguide shorted in one end is proposed, designed, and implemented successfully to measure the properties of magnetic thin films as a part of the vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy set-up. Its simple structure, potential applications and easy installation inside the cryostat chamber made it advantageous especially for low-temperature measurements. It provides a wide band of frequencies in the gigahertz range essential for FMR measurements. Our spectroscopy set-up with short-circuited coplanar waveguide has been used to extract Gilbert damping coefficient and effective magnetization values for standard ferromagnetic thin films like Py and Co. The thickness and temperature dependent studies of those magnetic parameters have also been done here for the afore mentioned magnetic samples.",2204.10596v2 2022-04-22,Emerging research landscape of altermagnetism,"Magnetism is one of the largest, most fundamental, and technologically most relevant fields of condensed-matter physics. Traditionally, two basic magnetic phases have been considered -- ferromagnetism and antiferromagnetism. The breaking of the time-reversal symmetry and spin splitting of the electronic states by the magnetization in ferromagnets underpins a range of macroscopic responses in this extensively explored and exploited type of magnets. By comparison, antiferromagnets have vanishing net magnetization. This Perspective reflects on recent observations of materials hosting an intriguing ferromagnetic-antiferromagnetic dichotomy, in which spin-split spectra and macroscopic observables, akin to ferromagnets, are accompanied by antiparallel magnetic order with vanishing magnetization, typical of antiferromagnets. An unconventional non-relativistic symmetry-group formalism offers a resolution of this apparent contradiction by delimiting a third basic magnetic phase, dubbed altermagnetism. Our Perspective starts with an overview of the still emerging unique phenomenology of the phase, and of the wide array of altermagnetic material candidates. In the main part of the article, we illustrate how altermagnetism can enrich our understanding of overarching condensed-matter physics concepts, and have impact on prominent condensed-matter research areas.",2204.10844v1 2022-05-31,Electrical measurement of the Spin Hall Effect isotropy in a ferromagnet,"The spin-dependent transport properties of paramagnetic metals are roughly invariant under rotation. By contrast, in ferromagnetic materials the magnetization breaks the rotational symmetry, and thus the spin Hall effect is expected to become anisotropic. Here, using a specific design of lateral spin valves, we measure electrically the spin Hall Effect anisotropy in NiCu and NiPd, both in their ferromagnetic and paramagnetic phases. We show that the appearance of the ferromagnetic order does not lead to a sizeable anisotropy of the spin charge interconversion in these materials.",2205.15651v2 2022-06-14,Sign-reversed anomalous Nernst effect in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$: the role of Fe vacancies,"Anomalous Nernst effect, as a thermal partner of anomalous Hall effect, is particularly sensitive to the Berry curvature anomaly near the Fermi level, and has been used to probe the topological nature of quantum materials. In this work, we report the observation of both effects in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$ with tunable Fe vacancies. With decreasing Fe vacancies, the anomalous Hall conductivity evolves as a function of the longitudinal conductivity from the hopping region to the region where the intrinsic Berry curvature contribution dominates. Concomitant evolutions in the anomalous Nernst signal and the anomalous off-diagonal thermoelectric coefficient are observed below the Curie temperature, displaying a unique sign change caused by the Fe vacancies. Combining these results with first-principles calculations, we argue that the Fe-vacancy concentration plays a unique role in simultaneously tuning the chemical potential and ferromagnetism, which in turn controls the Berry curvature contribution in this family of ferromagnetic topological semimetals.",2206.06545v1 2022-06-29,Ferromagnetism in Magic-angle Twisted Bilayer Graphene: A Monte Carlo Study,"Ferromagnetism emerges when the Moire superlattice formed by stacking two graphene monolayers in a magic twist angle are filled with integer number electrons. This work investigates the ferromagnetism based on the Ising models for a triangular lattice at one-quarter filling, a square lattice at half filling and a Kagome lattice at three-quarters filling of electrons. The temperature dependent heat capacity, magnetic susceptibility, energy and magnetization curves are calculated at zero magnetic field with a Monte Carlo method, leading to derive the phase transition temperatures, T_c=0.76, 1.33 and 4.75 K, respectively. Magnetization curves at finite magnetic field show strong hysteresis at temperature below 0.5 K for all the fillings considered, indicating the ferromagnetism of the system; the results are in agreement with experimental observations.",2206.14432v1 2022-07-15,Anisotropic field response of specific heat for a ferromagnetic superconductor UCoGe in magnetic fields,"Magnetic-field-angle-resolved specific heat and magnetization measurements were conducted on a ferromagnetic superconductor UCoGe with remarkable anisotropic upper critical field $H_{\rm c2}$. Although $H_{\rm c2}$ reaches a high magnetic field ($\sim 20$~T) along the $b$ axis, it is small ($\sim~0.6$~T) when a magnetic field is applied along the magnetic easy $c$-axis. This study indicates that the specific heat is abruptly suppressed when the magnetic field is applied toward the $c$ axis from the $a$ and $b$ axes in the ferromagnetic state. The field response of density of states (DOS) is anisotropic, relative to the $c$ axis, and its angle dependence is slightly singular. The Ising-type magnetic anisotropy of the ferromagnetic state is dominant even in the anisotropic reinforced superconducting state. These facts indicate that the suppression of DOS may closely relate to the superconducting state. We theoretically analyze these findings together with URhGe and UTe$_2$ by highlighting the common and distinctive features among three compounds.",2207.07266v2 2022-07-15,Engineering a skyrmion crystal in ferromagnetic/antiferromagnetic bilayers free from the DM interaction,"We theoretically propose a new stabilization mechanism of a skyrmion crystal (SkX) in a bilayer triangular lattice system consisting of the ferromagnetic and the antiferromagnetic layers. By performing variational calculations and Monte Carlo simulations in a complementary way, we find that a magnetic frustration between the ferromagnetic and antiferromagnetic layers is a source of a finite-Q spiral state and the SkX in the strong interlayer coupling regime. We also show that the degree of frustration is related to the interlayer exchange interaction. The stronger interlayer coupling tends to make the effect of frustration larger, which results in the stabilization of the SkX. The present results not only provide a way of engineering the SkX in the ferromagnetic/antiferromagnetic domain and heterostructure but also imply the possibility of the SkX based on interorbital frustration scenario.",2207.07375v1 2022-07-19,Nature of visons in the perturbed ferromagnetic and antiferromagnetic Kitaev honeycomb models,"The Kitaev honeycomb model hosts a fascinating fractionalized state of matter featuring emergent Majorana fermions and a vison particle that carries the flux of an emergent gauge field. In the exactly solvable model these visons are static but certain perturbations can induce their motion. We show that the nature of the vison motion induced by a Zeeman field is sharply distinct in the ferromagnetic vs the antiferromagnetic Kitaev models. Namely, in the ferromagnetic model the vison has a trivial non-projective translational symmetry, whereas in the antiferromagnetic Kitaev model it has a projective translational symmetry with $\pi$-flux per unit cell. The vison band of the ferromagnetic case has zero Berry curvature, and no associated intrinsic contribution to the thermal Hall effect. In contrast, in the antiferromagnetic case there are two gapped vison bands with opposite Chern numbers and an associated intrinsic vison contribution to the thermal Hall effect. We discuss these findings in the light of the physics of the spin liquid candidate $\alpha$-RuCl$_3$.",2207.09492v2 2022-08-19,Strongly correlated itinerant magnetism on the boundary of superconductivity in a magnetic transition metal dichalcogenide,"Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal NiTa$_4$Se$_8$ (or Ni$_{1/4}$TaSe$_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and \textit{ab initio} calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about 0.7$\mu_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. Remarkably, electron and hole Fermi surfaces are associated with opposite spin polarization. We discuss the implications of this feature on the superconductivity that emerges near itinerant ferromagnetism in this material, including the possibility of spin-polarized superconductivity.",2208.09475v1 2022-09-04,Spin-Polarisation measurement using NbN-Insulator-Ferromagnet Tunnel Junction with oxidized barrier,"We report a two-step process for the fabrication of superconductor-insulator-normal metal tunnel junctions using NbN as the superconducting electrode and its surface oxide as the insulating tunnel barrier, and investigate its efficacy in measuring spin-polarisation of ferromagnets using the Meservey-Tedrow technique. We observe that for NbN film thickness below 10 nm, under the application of parallel magnetic field, the superconducting density of states show clear ""Zeeman"" splitting into spin-up and spin-down sub-bands. Tunnelling measurements on devices where ferromagnetic Co is used as the normal electrode shows that these devices can be used to reliably measure spin polarisation of a ferromagnet at temperatures up to 1.6 K. The simplicity of our fabrication process, and the ability to perform spin-polarisation measurements at 4He temperatures makes NbN a very attractive candidate for spin polarisation measurements.",2209.01626v1 2022-09-08,Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3,"We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered an exclusive domain of antiferromagnets.",2209.03725v2 2022-09-13,Spin-flip induced superfluidity in a ring of spinful hard-core bosons,"The t - J Hamiltonian of the spinful hard-core bosonic ring in the Nagaoka limit is solved. The energy spectrum becomes quantized due to presence of spin, where each energy level corresponds to a cyclic permutation state of the spin chains. The ground state is true ferromagnetic when the ring contains N = 2, 3 spinful hard-core bosons; for all other N it is a mixture of the ferromagnetic and non-ferromagnetic states. This behaviour is different from the fermionic ring, where ground state is true ferromagnetic only for N = 3. It is shown that the intrinsic spin generated gauge fields are analogous to the synthetic gauge fields generated by rotation of either the condensate or the confining potential. It is argued that the low lying excited levels of the spin flipped states intrinsically support the superfluidity. Possible ways to experimentally verify these results are also discussed.",2209.06149v1 2022-09-16,Observation of thermodynamics originating from a mixed-spin ferromagnetic chain,"We present a model compound that forms a mixed-spin ferromagnetic chain. Our material design, based on the organic radicals, affords a verdazyl-based complex (p-Py-V)2[Mn(hfac)2]. The molecular orbital calculations of the compound indicate the formation of a mixed spin-(1/2, 1/2, 5/2) ferromagnetic chain. The temperature dependence of magnetic susceptibility reveals its ferromagnetic behavior. The magnetic specific heat exhibits a double-peak structure and indicates a phase transition at the low-temperature peak. The observed characteristics are explained using the quantum Monte Carlo calculations. Furthermore, the modified spin-wave theory verifies that the double-peak structure of the specific heat significantly reflects the relative ration of the acoustic excitation band and the optical excitation gap.",2209.07681v1 2022-10-01,Nonlinear features of the superconductor--ferromagnet--superconductor $\varphi_0$ Josephson junction in ferromagnetic resonance region,"We demonstrate the manifestations of the nonlinear features in magnetic dynamics and IV-characteristics of the $\varphi_0$ Josephson junction in the ferromagnetic resonance region. We show that at small values of system parameters, namely, damping, spin-orbit interaction, and Josephson to magnetic energy ratio, the magnetic dynamics is reduced to the dynamics of the scalar Duffing oscillator, driven by the Josephson oscillations. The role of increasing superconducting current in the resonance region is clarified. Shifting of the ferromagnetic resonant frequency and the reversal of its damping dependence due to nonlinearity are demonstrated by the full Landau-Lifshitz-Gilbert-Josephson system of equations, and in its different approximations. Finally, we demonstrate the negative differential resistance in the IV--characteristics, and its correlation with the foldover effect.",2210.00366v1 2022-10-21,Spin-Dependent High-Order Topological Insulator and Two Types of Distinct Corner Modes in Monolayer FeSe/GdClO Heterostructure,"We propose that a spin-dependent second-order topological insulator can be realized in monolayer FeSe/GdClO heterostructure, in which substrate GdClO helps to stabilize and enhance the antiferromagnetic order in FeSe. The second-order topological insulator is free from spin-orbit coupling and in-plane magnetic field. We also find that there exist two types of distinct corner modes residing in intersections of two ferromagnetic edges and two antiferromagnetic edges, respectively. The underlying physics for ferromagnetic corner mode follows a sublattice-chirality-kink picture. More interestingly, ferromagnetic corner mode shows spin-dependent property, which is also robust against spin-orbit coupling. Unexpectedly, antiferromagnetic corner mode can be taken as a typical emergent and hierarchical phenomenon from an array of ferromagnetic corner modes. Remarkably, antiferromagnetic corner modes violate general kink picture and can be understood as bound states of a one-dimensional Schrodinger equation under a connected potential well. Our findings not only provide a promising second-order topological insulator in electronic materials, but uncover some new properties of corner modes in high-order topological insulator.",2210.11813v1 2022-10-27,Electron-phonon mediated spin-flip as driving mechanism for ultrafast magnetization dynamics in 3$d$ ferromagnets,"Despite intense experimental effort, theoretical proposals and modeling approaches, a lack of consensus exists about the intrinsic mechanisms driving ultrafast magnetization dynamics in 3$d$ ferromagnets. In this work, we find evidence of electron-phonon mediated spin-flip as the driving mechanism for the ultrafast magnetization dynamics in all three 3$d$ ferromagnets; nickel, iron and cobalt. We use a microscopic three temperature model with parameters calculated from first-principles, which has been validated by direct comparison to the electron and lattice dynamics extracted from previous experiments. By direct comparison to the experimentally measured magnetization dynamics for different laser fluence, we determine the spin-flip probability of each material. In contrast to previous findings but in agreement to ab-initio predictions, we find that relatively small values of the spin-flip probability enable ultrafast demagnetization in all three 3$d$ ferromagnets.",2210.15269v1 2022-11-29,Semiconducting Electronic Structure of the Ferromagnetic Spinel $\mathbf{Hg}\mathbf{Cr}_2\mathbf{Se}_4$ Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy,"We study the electronic structure of the ferromagnetic spinel $\mathrm{Hg}\mathrm{Cr}_2\mathrm{Se}_4$ by soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and first-principles calculations. While a theoretical study has predicted that this material is a magnetic Weyl semimetal, SX-ARPES measurements give direct evidence for a semiconducting state in the ferromagnetic phase. Band calculations based on the density functional theory with hybrid functionals reproduce the experimentally determined band gap value, and the calculated band dispersion matches well with ARPES experiments. We conclude that the theoretical prediction of a Weyl semimetal state in $\mathrm{Hg}\mathrm{Cr}_2\mathrm{Se}_4$ underestimates the band gap, and this material is a ferromagnetic semiconductor.",2211.15884v2 2022-12-08,Impact of a ferromagnetic insulating barrier in magnetic tunnel junctions,"We investigate spin-dependent conductance across a magnetic tunnel junction (MTJ) including a ferromagnetic insulating barrier. The MTJ consists of two half-metallic ferromagnetic La2/3Sr1/3MnO3 (LSMO) manganites as electrodes and La2NiMnO6 (LNMO) double perovskite as a ferromagnetic insulating barrier. The resistance of the junction is strongly dependent not only on the orientation of the magnetic moments in LSMO electrodes, but also on the direction of the magnetization of the LNMO barrier with respect to that of LSMO. The ratio of tunnel magnetoresistance reaches a maximum value of 24% at 10 K, and it decreases with temperature until it completely disappears above the critical temperature of LNMO at 280 K. The tunneling process is described using a mechanism which involves both empty and filled eg states of the LNMO barrier acting as a spin-filter. A magnetic insulating barrier is an interesting path for achieving room temperature magnetoresistance in oxide-based heterostructures.",2212.04416v1 2022-12-13,Non-local correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets,"We present an ab initio method for eletcronic structure calculations, which accounts for the interaction of electrons and magnons in ferromagnets. While it is based on a many body perturbation theory we approximate numerically complex quantities with quantities from time dependent density functional theory. This results in a simple and affordable algorithm which allows us to consider more complex materials than those usually studied in this context ($3d$ ferromagnets) while still being able to account for the non-locality of the self energy. Furthermore, our approach allows for a relatively simple way to incorporate self-consistency. Our results are in a good agreement with experimental and theoretical findings for iron and nickel. Especially the experimental exchange splitting of nickel is predicted accurately within our theory. Additionally, we study the halfmetallic ferromagnet NiMnSb concerning its non-qusiparticle states appearing in the bandgap due to spin-flip excitations.",2212.06420v3 2023-01-09,X-ray detected ferromagnetic resonance techniques for the study of magnetization dynamics,"Element-specific spectroscopies using synchrotron-radiation can provide unique insights into materials properties. The recently developed technique of X-ray detected ferromagnetic resonance (XFMR) allows studying the magnetization dynamics of magnetic spin structures. Magnetic sensitivity in XFMR is obtained from the X-ray magnetic circular dichroism (XMCD) effect, where the phase of the magnetization precession of each magnetic layer with respect to the exciting radio frequency is obtained using stroboscopic probing of the spin precession. Measurement of both amplitude and phase response in the magnetic layers as a function of bias field can give a clear signature of spin-transfer torque (STT) coupling between ferromagnetic layers due to spin pumping. Over the last few years, there have been new developments utilizing X-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz frequency range. The techniques of diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) provide novel ways for the probing of the dynamics of chiral and multilayered magnetic materials, thereby opening up new pathways for the development of high-density and low-energy consumption data processing solutions.",2301.03256v1 2023-02-11,Room Temperature d$^0$ Ferromagnetism in Carbon Doped LaH$_3$: Insights From Density Functional Theory Simulations,"Employing the state-of-the-art Density Functional Theory with both GGA and hybrid HSE06 functional along with the incorporation of spin-orbit coupling, we have engineered stable room temperature ferromagnetism in non-magnetic LaH$_3$ through C substitution at octahedral and tetrahedral H sites where the induced magnetic moment is mostly contributed by 2p orbital of C atom. It is interesting that the magnetic signature is switched on with an impurity concentration as low as 1.04 at% with a magnetic moment of $\approx$ 1.0 $\mu_B$ per impurity, where the localized behavior of the 2p states of C along with significant exchange splitting energy can be attributed as the origin of the induced magnetic moment. The verification of the Stoner criterion in the material further confirmed the onset of ferromagnetism in the system, and the computed Curie temperature is found to be well above room temperature. Reduced formation energy and requirement of lower impurity concentration ensure practical feasibility towards a spintronic device where room temperature ferromagnetism is established from the non-magnetic host and the dopant.",2302.05625v1 2023-03-02,"Synthesis and physical properties of a new layered ferromagnet, Cr1.21Te2","Single crystals of a new layered compound, Cr1.21Te2, was synthesized via a vapor transport method. The crystal structure and physical properties were characterized by single crystal and powder x-ray diffraction, temperature- and field-dependent magnetization, zero-field heat capacity and angle-resolved photoemission spectroscopy. Cr1.21Te2, containing two Cr sites, crystalizes in a trigonal structure with a space group P-3 (No. 147). The Cr site in the interstitial layer is partially occupied. Physical property characterizations indicate that Cr1.21Te2 is metallic with hole pockets at the Fermi energy and undergoes a ferromagnetic phase transition at ~173 K. The magnetic moments align along the c-axis in the ferromagnetic state. Based on low temperature magnetization, the spin stiffness constant D and spin excitation gap $\Delta$ were estimated according to Bloch's law to be D = 99 $\pm$ 24 meV $\r{A}^2$ and $\Delta$ = 0.46 $\pm$ 0.33 meV, suggesting its possible application as a low dimensional ferromagnet.",2303.00956v1 2023-04-19,Jahn-Teller distortion driven ferromagnetism in a perovskite fluoride monolayer,"The Jahn-Teller distortion and the resulting orbital order usually cause some fascinating correlated electronic behaviors, and generally lead to antiferromagnetism in perovskite bulks. Here we demonstrate that the Jahn-Teller distortion present in the perovskite fluoride KCrF$_3$ bulk can be retained to the two-dimensional limit, resulting in a staggered orbital order and ferromagnetism in the perovskite monolayer. Octahedral tilt and rotation distortion also appear in the ground-state structure of the perovskite monolayer, which have minor effects on the electronic and magnetic properties with respect to the Jahn-Teller distortion. In addition, in the prototype phase without structural distortion, the partial occupation of the $e_g$ orbitals leads to a ferromagnetic metallic state. This work facilitates the design of two-dimensional ferromagnets and functional properties based on Jahn-Teller distortion and orbital order",2304.09359v1 2023-04-19,Cavity magnomechanical coupling with coupled magnon modes in a synthetic antiferromagnet,"On-chip cavity magnomechanics is an emerging field exploring acoustic and magnonic functionalities of various ferromagnetic materials and structures using strongly confined phonons. It is expected that such cavity magnomechanics can be extended to multilayer ferromagnets, especially synthetic antiferromagnets (SAFs) that exhibit zero net magnetization through interlayer exchange coupling. However, the conventional theoretical framework for a single ferromagnet cannot be used directly because of the antiferromagnetic magnetization dynamics associated with the interlayer exchange coupling. In this paper, we theoretically investigate phonon-magnon coupling with a three-layer SAF. Our formulation of the phonon-magnon coupling constants reveals that the acoustic (optical) magnon mode dominantly couples to the cavity phonon when the magnetization angles in the two ferromagnetic layers are antiparallel (orthogonal). Moreover, numerical calculations including the effects of dipole-dipole interactions and in-plane uniaxial magnetic anisotropy allow us to predict phonon frequency shifts and linewidth broadening that can be detected in experiments. These theoretical insights would greatly help us to make a strategy for bringing the system into the strong coupling regime and to devise novel control protocols in analogy to cavity quantum electrodynamics and cavity optomechanics.",2304.09458v2 2023-04-22,Entanglement entropy for scale-invariant states: universal finite-size scaling,"A universal finite system-size scaling analysis of the entanglement entropy is presented for highly degenerate ground states arising from spontaneous symmetry breaking with type-B Goldstone modes in exactly solvable one-dimensional quantum many-body systems. These states appear to be scale-invariant, but not conformally invariant. Our findings are based on a physical argument, imposing three constraints on the entanglement entropy, in addition to further confirmation from an asymptotic analysis of the entanglement entropy for the ${\rm SU}(2)$ spin-$1/2$ ferromagnetic states. The resulting universal scaling form is demonstrated for three fundamental models -- the ${\rm SU}(2)$ spin-$s$ Heisenberg ferromagnetic model, the ${\rm SU}(N+1)$ ferromagnetic model, and the staggered ${\rm SU}(3)$ spin-1 ferromagnetic biquadratic model. The results point towards a classification for distinct types of scale-invariant states, relevant to a complete classification of quantum states of matter.",2304.11339v1 2023-05-09,Nagaoka ferromagnetism in doped Hubbard models in optical lattices,"The search for ferromagnetism in the Hubbard model has been a problem of outstanding interest since Nagaoka's original proposal in 1966. Recent advances in quantum simulation have today enabled the study of tunable doped Hubbard models in ultracold atomic systems. Here, we examine a realistic variant of such a model wherein any second electron on a single lattice site is weakly bound compared to the first one. Employing large-scale density-matrix renormalization group calculations, we establish the existence of high-spin ground states on square and triangular lattices, analyze the microscopic mechanisms behind their origin, and investigate the interplay between ferromagnetism and other competing orders, such as stripes. Our results also explain$\unicode{x2014}$and shed new light on$\unicode{x2014}$the intriguing observations of ferromagnetic correlations in recent optical-lattice experiments.",2305.05683v1 2023-05-10,Electron doping induced stable ferromagnetism in two-dimensional GdI_3 monolayer,"As a two-dimensional material with a hollow hexatomic ring structure, N\'eel-type anti-ferromagnetic (AFM) GdI3 can be used as a theoretical model to study the effect of electron doping. Based on first-principles calculations, we find that the Fermi surface nesting occurs when more than 1/3 electron per Gd is doped, resulting in the failure to obtain a stable ferromagnetic (FM) state. More interestingly, GdI3 with appropriate Mg/Ca doping (1/6 Mg/Ca per Gd) turns to be half-metallic FM state. This AFM-FM transition results from the transfer of doped electrons to the spatially expanded Gd-5d orbital, which leads to the FM coupling of local half-full Gd-4f electrons through 5d-4f hybridization. Moreover, the shortened Gd-Gd length is the key to the formation of the stable ferromagnetic coupling. Our method provides new insights into obtaining stable FM materials from AFM materials.",2305.05872v1 2023-05-31,Pressure evolution of electronic structure and magnetism in the layered van der Waals ferromagnet CrGeTe$_3$,"Layered van der Waals ferromagnets, which preserve their magnetic properties down to exfoliated monolayers, are fueling an abundance of fundamental research and nanoscale device demonstration. CrGeTe$_3$ is a prime example for this class of materials. Its temperature-pressure phase diagram features an insulator-to-metal transition and a significant increase of ferromagnetic Curie-Weiss temperatures upon entering the metallic state. We use density functional theory to understand the magnetic exchange interactions in CrGeTe$_3$ at ambient and elevated pressure. We calculate Heisenberg exchange couplings, which provide the correct ferromagnetic ground state and explain the experimentally observed pressure dependence of magnetism in CrGeTe$_3$. Furthermore, we combine density functional theory with dynamical mean field theory to investigate the effects of electronic correlations and the nature of the high pressure metallic state in CrGeTe$_3$.",2306.00081v2 2023-06-01,Ferromagnetic phase transitions in $SU(N)$,"We study the thermodynamics of a non-abelian ferromagnet consisting of ""atoms"" each carrying a fundamental representation of $SU(N)$, coupled with long-range two-body quadratic interactions. We uncover a rich structure of phase transitions from non-magnetized, global $SU(N)$-invariant states to magnetized ones breaking global invariance to $SU(N-1) \times U(1)$. Phases can coexist, one being stable and the other metastable, and the transition between states involves latent heat exchange, unlike in usual $SU(2)$ ferromagnets. Coupling the system to an external non-abelian magnetic field further enriches the phase structure, leading to additional phases. The system manifests hysteresis phenomena both in the magnetic field, as in usual ferromagnets, and in the temperature, in analogy to supercooled water. Potential applications are in fundamental situations or as a phenomenological model.",2306.01051v3 2023-06-23,Spin transport between polarized Fermi gases near the ferromagnetic phase transition,"We theoretically study the spin current between two polarized Fermi gases with repulsive interactions near the itinerant ferromagnetic phase transition. We consider a two-terminal model where the left reservoir is fixed to be fully polarized while the polarization of the right reservoir is tuned through a fictitious magnetic field defined by the chemical-potential difference between different atomic hyperfine states. We calculate the spectra of the spin-flip susceptibility function, which displays a magnon dispersion emerging from the Stoner continuum at low momentum in the ferromagnetic phase. Based on the spin-flip susceptibility and using Keldysh Green's function formalism, we investigate the spin current induced by quasiparticle and spin-flip tunneling processes, respectively, and show their dependence on the polarization bias between two reservoirs. The one-body (quasiparticle) tunneling demonstrates a linear dependence with respect to the polarization bias. In contrast, the spin-flip process manifests a predominantly cubic dependence on the bias. While indicating an enhanced magnon tunneling in the strong-coupling regime, our results also demonstrate a characteristic behavior around the critical repulsive strength for ferromagnetic phase transition at low temperatures.",2306.13536v2 2023-07-08,Magnon influence on the superconducting density of states in superconductor$-$ferromagnetic-insulator bilayers,"Superconductor$-$ferromagnetic-insulator heterostructures are paradigmatic systems for studying the mutual influence of superconductivity and magnetism via proximity effects. In particular, spin-split superconductivity is realized in such structures. Recent experiments and theories demonstrate a rich variety of transport phenomena occurring in devices based on such heterostructures that suggest direct applications in thermoelectricity, low-dissipative spintronics, radiation detection, and sensing. In this work we investigate the influence of the electron-magnon interaction at the superconductor$-$ferromagnetic-insulator interface on the spin-split superconductivity. It is predicted that due to the magnon-mediated electron spin-flip processes the spin-split quasiparticle branches are partially mixed and reconstructed, and the BCS-like spin-split shape of the superconducting density of states, which is typical for superconductors in the effective exchange field, is strongly modified. An odd-frequency superconducting order parameter admixture to the leading singlet order parameter is also found. These findings expand the physical picture of spin-split superconductivity beyond the mean-field description of the ferromagnet exchange field.",2307.03954v2 2023-07-10,Porous CrO$_2$: a ferromagnetic half-metallic member in sparse hollandite oxide family,"A stable polymorph of CrO$_2$ is predicted using PBE+U method. The porous material is isostructural with $\alpha$-MnO$_2$ making it the second transition metal oxide in sparse hollandite group of materials. However, unlike the anti-ferromagnetic semiconducting character of the $\alpha$-MnO$_2$, it is found to be a ferromagnetic half-metal. At Fermi level, the hole pocket has ample contribution from O-2$p$ orbital, though, the electron pocket is mostly contributed by Cr-3$d_{xy}$ and Cr-3d$_{x^2-y^2}$. A combination of negative charge transfer through orbital mixing and extended anti-bonding state near Fermi level is responsible for the half-metallic ferromagnetic character of the structure. A comparative study of rutile and hollandite CrO$_2$ and hollandite MnO$_2$ structures delineate the interplay between structural, electronic and magnetic properties. The material shows a robust magnetic character under hydrothermal pressure, as well as, the band topology is conserved under uniaxial strain. Moderate magneto-crystalline anisotropy is observed and it shows a correspondence with the anisotropy of elastic constants.",2307.04584v1 2023-07-20,Superconductivity Induced Ferromagnetism In The Presence of Spin-Orbit Coupling,"We investigate the behavior of magnetic impurities placed on the surface of superconductor thin films with spin-orbit coupling. Our study reveals long-range interactions between the impurities, which decay according to a power law, mediated by the supercurrents. Importantly, these interactions possess a ferromagnetic component when considering the influence of the electromagnetic field, leading to the parallel alignment of the magnetic moments in the case of two impurities. In a Bravais lattice of magnetic impurities, superconductivity facilitates the establishment of ferromagnetic order within specific parameter ranges. These findings challenge the conventional understanding that ferromagnetism and superconductivity are mutually exclusive phenomena. Our theoretical framework provides a plausible explanation for the recently observed remanent flux in iron-based superconductors, particularly Fe(Se,Te).",2307.10723v3 2023-07-29,Unveiling Exotic Magnetic Phases in Fibonacci Quasicrystalline Stacking of Ferromagnetic Layers through Machine Learning,"In this study, we conduct a comprehensive theoretical analysis of a Fibonacci quasicrystalline stacking of ferromagnetic layers, potentially realizable using van der Waals magnetic materials. We construct a model of this magnetic heterostructure, which includes up to second neighbor interlayer magnetic interactions, that displays a complex relationship between geometric frustration and magnetic order in this quasicrystalline system. To navigate the parameter space and identify distinct magnetic phases, we employ a machine learning approach, which proves to be a powerful tool in revealing the complex magnetic behavior of this system. We offer a thorough description of the magnetic phase diagram as a function of the model parameters. Notably, we discover among other collinear and non-collinear phases, a unique ferromagnetic alternating helical phase. In this non-collinear quasiperiodic ferromagnetic configuration the magnetization decreases logarithmically with the stack height.",2307.16052v1 2023-08-06,Ferromagnetic half levitation of LK-99-like synthetic samples,"We successfully synthesized polycrystalline LK-99-like ceramic samples with a solid-state-sintering method. Powder X-ray diffraction shows that the main contents are $\mathrm{Pb_{10-x}Cu_x(PO_4)_6O}$ and $\mathrm{Cu_2S}$, consistent with recent reports [arXiv:2307.12037; arXiv:2308.01192]. In some small flaky fragments, we successfully observed ``half levitation'' atop a $\mathrm{Nd_2Fe_{14}B}$ magnet. Using magnetization measurements on such small pieces, as well as on a large piece which does not exhibit the half levitation, we show that the samples ubiquitously contain weak yet definitive soft ferromagnetic components. We argue that, together with the pronounced shape anisotropy of the small fragments, the soft ferromagnetism is sufficient to explain the observed half levitation in strong vertical magnetic fields. Our measurements do not indicate the presence of the Meissner effect, nor zero resistance, in our samples, leading us to believe that our samples do not exhibit superconductivity. The precise chemical composition and the physics behind the ferromagnetic component remain outstanding questions to be addressed in future research.",2308.03110v2 2023-08-21,Two-orbital spin-fermion model study of ferromagnetism in honeycomb lattice,"The spin-fermion model was previously successful to describe the complex phase diagrams of colossal magnetoresistive manganites and iron-based superconductors. In recent years, two-dimensional magnets have rapidly raised up as a new attractive branch of quantum materials, which are theoretically described based on classical spin models in most studies. Alternatively, here the two-orbital spin-fermion model is established as a uniform scenario to describe the ferromagnetism in a two-dimensional honeycomb lattice. This model connects the magnetic interactions with the electronic structures. Then the continuous tuning of magnetism in these honeycomb lattices can be predicted, based on a general phase diagram. The electron/hole doping, from the empty $e_{g}$ to half-filled $e_{g}$ limit, is studied as a benchmark. Our Monte Carlo result finds that the ferromagnetic $T_{C}$ reaches the maximum at the quarter-filled case. In other regions, the linear relationship between $T_{C}$ and doping concentration provides a theoretical guideline for the experimental modulations of two-dimensional ferromagnetism tuned by ionic liquid or electrical gating.",2308.10535v1 2023-08-21,Photoinduced topological phase transitions in Kitaev-Heisenberg honeycomb ferromagnets with the Dzyaloshinskii-Moriya interaction,"We theoretically study topological properties of Floquet magnon in a laser-irradiated Kitaev-Heisenberg honeycomb ferromagnet with the Dzyaloshinskii-Moriya interaction by means of the Floquet-Bloch theory. It is found that the Kitaev-Heisenberg ferromagnet can reveal two topological phases with different Chern numbers when it is irradiated by a circular-polarized light laser. Our results show that the topological phase of the system can be switched from one topological phase to another one via varying the light intensity. The intrinsic DMI plays a crucial role in the occurrence of photoinduced topological phase transition. It is shown that the sign reversal of the thermal hall conductivity is an important indicator on photoinduced topological phase transitions in the Kitaev-Heisenberg honeycomb ferromagnet.",2308.11077v1 2023-08-30,Impact of epitaxial strain relaxation on ferromagnetism in a freestanding La2/3Sr1/3MnO3 membrane,"Manganite perovskites host emerging physical properties of strongly-correlated electrons with charge, spin, and lattice degrees of freedom. Using epitaxial lift-off technique, we report enhancement of saturation magnetization and ferromagnetic transition temperature of the freestanding La2/3Sr1/3MnO3 membrane compared with the as-grown film on SrTiO3 substrate involving lateral tensile strain. Structural analysis reveals shrinkage of unit-cell volume by tensile strain relaxation in the freestanding membrane, which causes enhancement of the ferromagnetic interaction. The impact of the microscopic lattice deformation on the ferromagnetism of La2/3Sr1/3MnO3 indicates a high potential of this material for flexible electronics application with intriguing functionalities in strongly-correlated electron systems.",2308.16045v1 2023-09-12,Non-constant ground configurations in the disordered ferromagnet,"The disordered ferromagnet is a disordered version of the ferromagnetic Ising model in which the coupling constants are non-negative quenched random. A ground configuration is an infinite-volume configuration whose energy cannot be reduced by finite modifications. It is a long-standing challenge to ascertain whether the disordered ferromagnet on the $\mathbb{Z}^D$ lattice admits non-constant ground configurations. We answer this affirmatively in dimensions $D\ge 4$, when the coupling constants are sampled independently from a sufficiently concentrated distribution. The obtained ground configurations are further shown to be translation-covariant with respect to $\mathbb{Z}^{D-1}$ translations of the disorder. Our result is proved by showing that the finite-volume interface formed by Dobrushin boundary conditions is localized, and converges to an infinite-volume interface. This may be expressed in purely combinatorial terms, as a result on the fluctuations of certain minimal cutsets in the lattice $\mathbb{Z}^D$ endowed with independent edge capacities.",2309.06437v2 2023-09-16,Investigation of the Anomalous and Topological Hall Effects in Layered Monoclinic Ferromagnet Cr$_{2.76}$Te$_4$,"We studied the electrical transport, Hall effect, and magnetic properties of monoclinic layered ferromagnet Cr$_{2.76}$Te$_4$. Our studies demonstrate Cr$_{2.76}$Te$_4$ to be a soft ferromagnet with strong magnetocrystalline anisotropy. Below 50 K, the system shows an antiferromagnetic-like transition. Interestingly, between 50 and 150 K, we observe fluctuating magnetic moments between in-plane and out-of-plane orientations, leading to non-coplanar spin structure. On the other hand, the electrical resistivity data suggest it to be metallic throughout the measured temperature range, except a $kink$ at around 50 K due to AFM ordering. The Rhodes-Wohlfarth ratio $\frac{\mu_{eff}}{\mu_{s}}=1.89 (>1)$ calculated from our magnetic studies confirms that Cr$_{2.76}$Te$_4$ is an itinerant ferromagnet. Large anomalous Hall effect has been observed due to the skew-scattering of impurities and the topological Hall effect has been observed due to non-coplanar spin-structure in the presence of strong magnetocrystalline anisotropy. We examined the mechanism of anomalous Hall effect by employing the first principles calculations.",2309.08898v1 2023-09-16,Electronic and Topological Properties of a Topological Insulator Thin Film Sandwiched between Ferromagnetic Insulators,"We consider a thin film of a topological insulator (TI) sandwiched between two ferromagnetic (FM) layers. The system is additionally under an external gate voltage. The surface electron states of TI are magnetized due to the magnetic proximity effect to the ferromagnetic layers. The magnetization of ferromagnetic layers can be changed by applying an external magnetic field or by varying thickness of the topological insulator (owing to the interlayer exchange coupling). The change in the magnetic configuration of the system affects the transport properties of the surface electronic states. Using the Green function formalism, we calculate spin polarization, anomalous Hall effect, and magnetoresistance of the system. We show, among others, that by tuning the gate voltage and magnetizations of the top and bottom FM layers, one can observe the topological transition to the anomalous quantum Hall state.",2309.09014v1 2023-09-26,Orbital Pumping by Magnetization Dynamics in Ferromagnets,"We show that dynamics of the magnetization in ferromagnets can pump the orbital angular momentum, which we denote by orbital pumping. This is the reciprocal phenomenon to the orbital torque that induces magnetization dynamics by the orbital angular momentum in non-equilibrium. The orbital pumping is analogous to the spin pumping established in spintronics but requires the spin-orbit coupling for the orbital angular momentum to interact with the magnetization. We develop a formalism that describes the generation of the orbital angular momentum by magnetization dynamics within the adiabatic perturbation theory. Based on this, we perform first-principles calculation of the orbital pumping in prototypical $3d$ ferromagnets, Fe, Co, and Ni. The results show that the ratio between the orbital pumping and the spin pumping ranges from 5 to 15 percents, being smallest in Fe and largest in Ni. This implies that ferromagnetic Ni is a good candidate for measuring the orbital pumping. Implications of our results on experiments are also discussed.",2309.14817v2 2023-09-27,Goodenough-Kanamori-Anderson high-temperature ferromagnetism in tetragonal transition-metal xenes,"Seminal Goodenough-Kanamori-Anderson (GKA) rules provide the inceptive understanding of the superexchange interaction of two magnetic metal ions bridged with an anion, and suggest fostered ferromagnetic interaction for orthogonal bridging bonds. However, there are no examples of two-dimensional (2D) materials with structure that optimizes the GKA arguments towards enhanced ferromagnetism and its critical temperature. Here we reveal that an ideally planar GKA ferromagnetism is indeed stable in selected tetragonal transition-metal xenes (tTMXs), with Curie temperature above 300~K found in CrC and MnC. We provide the general orbitally-resolved analysis of magnetic interactions that supports the claims and sheds light at the mechanisms dominating the magnetic exchange process in these structures. With recent advent of epitaxially-grown tetragonal 2D materials, our findings earmark tTMXs for facilitated spintronic and magnonic applications, or as a desirable magnetic constituent of functional 2D heterostructures.",2309.15445v1 2023-10-13,Magnon spin capacitor,"In this work we show that a magnon spin capacitor can be realized at a junction between two exchange coupled ferromagnets. In this junction, the buildup of magnon spin over the junction is coupled to the difference in magnon chemical potential, realizing the magnon spin analogue of an electrical capacitor. The relation between magnon spin and magnon chemical potential difference directly follows from considering the magnon density-density interaction between the two ferromagnets. We analyse the junction in detail by considering spin injection and detection from normal metal leads, the tunneling current across the junction and magnon decay within the ferromagnet, showing that such a structure realizes a magnon spin capacitor in series with a spin resistor. Choosing yttrium iron garnet as the ferromagnet, we numerically calculate the capacitance, which ranges from picofarad to microfarad, depending on the area of the junction. We therefore conclude that the magnon spin capacitor could directly be of use in applications.",2310.09064v3 2023-11-18,Design of spin-orbital-textures in ferromagnetic/topological insulator interfaces,"Spin-orbital textures in topological insulators due to the spin locking with the electron momentum, play an important role in spintronic phenomena that arise from the interplay between charge and spin degrees of freedom. We have explored interfaces between a ferromagnetic system (CrI$_3$) and a topological insulator (Bi$_2$Se$_3$) that allow the manipulation of spin-orbital textures. Within an {\it ab initio} approach we have extracted the spin-orbital-textures dependence of experimentally achievable interface designs. The presence of the ferromagnetic system introduces anisotropic transport of the electronic spin and charge. From a parameterized Hamiltonian model we capture the anisotropic backscattering behavior, showing its extension to other ferromagnetic/topological insulator interfaces. We verified that the van der Waals TI/MI interface is an excellent platform for controlling the spin degree of freedom arising from topological states, providing a rich family of unconventional spin texture configurations.",2311.11084v1 2023-11-21,Influence of interlayer exchange coupling on ultrafast laser-induced magnetization reversal in ferromagnetic spin valves,"In this study, we explore the influence of interlayer exchange coupling on magnetization reversal triggered by femtosecond laser pulses in ferromagnetic spin valves. Our experiments, focused on femtosecond laser-induced magnetization reversal, methodically vary the thickness of the copper (Cu) spacer layer. We identify a critical Cu thickness threshold at 2.32 nm. Above this threshold, a stable reversed magnetic domain is consistently generated upon exposure to a single laser pulse. Conversely, with a Cu spacer thinner than 2.32 nm, the observed magnetization reversal from parallel (P) to anti-parallel (AP) states occurs only under continuous laser irradiation. Once the laser is stopped, the magnetic configuration relaxes back to its initial P state, influenced by ferromagnetic exchange coupling. This research enhances our understanding of the mechanisms that drive optically induced ultrafast magnetization reversal in ferromagnetic spin valves.",2311.12753v2 2023-11-27,Gilbert damping in two-dimensional metallic anti-ferromagnets,"A finite spin life-time of conduction electrons may dominate Gilbert damping of two-dimensional metallic anti-ferromagnets or anti-ferromagnet/metal heterostructures. We investigate the Gilbert damping tensor for a typical low-energy model of a metallic anti-ferromagnet system with honeycomb magnetic lattice and Rashba spin-orbit coupling for conduction electrons. We distinguish three regimes of spin relaxation: exchange-dominated relaxation for weak spin-orbit coupling strength, Elliot-Yafet relaxation for moderate spin-orbit coupling, and Dyakonov-Perel relaxation for strong spin-orbit coupling. We show, however, that the latter regime takes place only for the in-plane Gilbert damping component. We also show that anisotropy of Gilbert damping persists for any finite spin-orbit interaction strength provided we consider no spatial variation of the N\'eel vector. Isotropic Gilbert damping is restored only if the electron spin-orbit length is larger than the magnon wavelength. Our theory applies to MnPS3 monolayer on Pt or to similar systems.",2311.16268v2 2023-12-12,Interacting Floquet topological magnons in laser-irradiated Heisenberg honeycomb ferromagnets,"When a Heisenberg honeycomb ferromagnet is irradiated by high frequency circularly polarized light, the underlying uncharged magnons acquire a time dependent Aharonov Casher phase, which makes it a Floquet topological magnon insulator. In this context, we investigate the many body interaction effects of Floquet magnons in laser irradiated Heisenberg honeycomb ferromagnets with ocontaining Dzyaloshinskii Moriya interaction under the application of circularly polarized off resonant light. We demonstrate that the quantum ferromagnet systems periodically laser driven exhibits temperature driven topological phase transitions due to Floquet magnon magnon interactions. The thermal Hall effect of Floquet magnons serves as a prominent signature for detecting these many body effects near the critical point, enabling experimental investigation into this phenomenon. Our study complements the lack of previous theoretical works that the topological phase transition of the Floquet magnon under the linear spin wave approximation is only tunable by the light field. Our study presents a novel approach for constructing Floquet topological phases in periodically driven quantum magnet systems that goes beyond the limitations of the linear spin wave theory. We provide numerical results based on the well known van der Waals quantum magnet CrX3 (X=F, Cl, Br, and I), calling for experimental implementation.",2312.06929v1 2023-12-28,Anomalous exchange bias effect in ferromagnetic VI3 flakes,"The exchange bias (EB) effect, pivotal in magnetic data storage and sensing devices, has been observed not only in interfacial regions but also in intrinsic ferromagnetic materials. Here, we've uncovered a robust and stable exchange bias effect within the layered van der Waals (vdW) ferromagnet VI3 employing magnetic circular dichroism microscopy. At 10 K, we observed a significant exchange field of approximately 0.1 T, accompanied by random shifts (positive or negative relative to zero magnetic field) after zero-field cooling. Notably, this effect is effectively controllable after field cooling, with shift direction opposing the applied magnetic field. The presence of strong magnetic anisotropic energy within VI3 results in larger coercivity-bound magnetic domains. These domains dictate the neighboring ferromagnetic alignment and induce shifts in the hysteresis loop. Our study not only contributes to comprehending fundamental nanoscale magnetic interactions but also sheds light on emergent phenomena within layered van der Waals magnets.",2312.16899v1 2023-12-29,Prediction of Magnetic State of UO2 within Hubbard-corrected Density-Functional Theory: A self-consistent approach,"The magnetic state of UO$_2$ was determined experimentally to be anti-ferromagnetic. Starting from this experimental fact, researchers have calculated other properties within the Hubbard-corrected density-functional theory, DFT+U. Up to now, the Hubbard parameters for UO$_2$ were usually so chosen that the calculations give good results for some experimental data. Also, to our knowledge there exists no valid theoretical research report on the energetically stable magnetic state of this system. In present work, employing the new method which is based on density-functional perturbation theory, we have determined self-consistently the Hubbard parameters and ground-state energies for UO$_2$ crystal in both ferromagnetic and anti-ferromagnetic configurations, and the calculated results show that UO$_2$ crystal energetically favors an anti-ferromagnetic state with a small energy difference. In all the calculations the PBE-sol approximation was used for the exchange-correlation energy functional.",2401.00864v1 2024-01-07,Deformation of a planar ferromagnetic elastic ribbon,"While extensive studies have been conducted on purely elastic ribbons, in this paper we explore the influence of magnetisation on the deformation of planar ferromagnetic elastic ribbons. We begin the investigation by deriving the leading-order magnetic energy associated with a curved planar ferromagnetic elastic ribbon. The sum of the magnetic and the elastic energy is the total energy of the ribbon. We derive the equilibrium equations by taking the first variation of the total energy. We then systematically determine and analyse solutions to these equilibrium equations under various canonical boundary conditions. We also analyse the stability of the equilibrium solutions. Comparing our findings with the well-studied Euler's Elastica provides insights into the magnetic effects on the deformation behaviour of elastic ribbons. Our analysis contributes to a deeper understanding of the interplay between magnetisation and the mechanical response of planar ferromagnetic structures, and offers valuable insights for both theoretical and practical applications.",2401.03447v1 2024-01-13,Field theory of collinear and noncollinear magnetic order,"These lecture notes from the 2023 Summer ""School Principles and Applications of Symmetry in Magnetism"" introduce the reader to the classical field theory of ferromagnets and antiferromagnets.",2401.07171v1 2024-01-23,Superfluid transition of a ferromagnetic Bose gas,"The strongly ferromagnetic spin-1 Bose-Einstein condensate (BEC) has recently been realized with atomic $^{7}$Li. It was predicted that a strong ferromagnetic interaction can drive the normal gas into a magnetized phase at a temperature above the superfluid transition, and $^{7}$Li likely satisfies the criterion. We re-examine this theoretical proposal employing the two-particle-irreducible (2PI) effective potential, and conclude that there exists no stable normal magnetized phase for a dilute ferromagnetic Bose gas. For $^{7}$Li, we predict that the normal gas undergoes a joint first order transition and jump directly into a state with finite condensate density and magnetization. We estimate the size of the first order jump, and examine how a partial spin polarization in the initial sample affects the first order transition. We propose a qualitative phase diagram at fixed temperature for the trapped gas.",2401.12541v1 2024-01-31,Giant Anomalous Hall and Nernst Effects in a Heavy Fermion Ferromagnet,"The anomalous Hall and Nernst effects describe the voltage drop perpendicular to an applied current and temperature gradient due to the magnetization of a magnetic material. These effects can be utilized to measure the Berry curvature at the Fermi energy, and have potential applications in future electronic devices and thermoelectric energy conversion. In this paper, we report giant anomalous Hall conductivity and anomalous Nernst coefficient, as high as about 1000 $\Omega^{-1}$ cm$^{-1}$ and 10 $\mu$V K$^{-1}$, respectively, in a heavy fermion ferromagnet, CeCrGe$_3$. This compound uniquely manifests strong hybridization between the 4$f$ and conduction electrons, leading to a Kondo lattice state in the presence of ferromagnetic order. Unlike conventional topological semimetals in which the electron correlation is weak, CeCrGe$_3$ manifests a strong Berry curvature field of the heavy fermion with an extremely low Fermi energy. Our findings pave the way for exploring correlation-driven topological responses in a ferromagnetic Kondo lattice environment.",2401.17624v1 2024-02-02,Mechanism of ferromagnetism enhancement in a La$_{2/3}$ Sr$_{1/3}$ MnO$_3$ membrane released from epitaxial strain,"Recent studies have shown that the magnetic properties of the ferromagnetic perovskite oxide La$_{2/3}$ Sr$_{1/3}$ MnO$_3$ (LSMO) grown on an SrTiO3 (STO) substrate, such as its magnetic moment and Curie temperature, can be improved by releasing the film from the substrate. However, the microscopic origin of this enhancement is not yet well understood. In this study, we use synchrotron radiation measurements to investigate the mechanism of ferromagnetism enhancement in an LSMO membrane released from an STO substrate by dissolving a water-soluble Sr$_4$Al$_2$O$_7$ buffer layer. Using resonant photoemission spectroscopy on the as-grown LSMO film and LSMO membrane, we elucidate that the strain release from the STO substrate enhances the itineracy of the Mn-3d electrons via p-d hybridization, and this strengthens the double-exchange interaction. The reinforcement of the double-exchange interaction, in turn, improves the ferromagnetism of LSMO.",2402.01179v1 2024-02-02,Coexistence of Ferromagnetism and Antiferromagnetic Dimers in Topological Insulators,"The addition of magnetic impurities in topological insulators can drive ferromagnetic order that leads to novel quantum anomalous Hall transport well below the Curie temperature. The fragility of the quantized regime has been ascribed to the random nature of the magnetic moment distribution. Here, we refine this hypothesis by using inelastic neutron scattering and density-functional theory calculations to show that two antagonistic components define the magnetism in Mn-substituted SnTe, thereby limiting the effectiveness of dilute magnetic TIs. One component is strongly bound antiferromagnetic dimers that compete with ferromagnetic order. The other component consists of undimerized moments where ferromagnetic order develops via long-range interactions.",2402.01531v1 2024-03-04,Half-Metallic Ferromagnetic Weyl Fermions Related to Dynamic Correlations in the Zinc-blende Compound VAs,"The realization of 100\% polarized topological Weyl fermions in half-metallic ferromagnets is of particular importance for fundamental research and spintronic applications. Here, we theoretically investigate the electronic and topological properties of the zinc-blende compound VAs, which was deemed as a half-metallic ferromagnet related to dynamic correlations. Based on the combination of density functional theory and dynamical mean field theory, we uncover that the half-metallic ferromagnet VAs exhibit attractive Weyl semimetallic behaviors with twelve pairs of Weyl points, which are very close to the Fermi level. Meanwhile, we also investigate the magnetization-dependent topological properties; the results show that the change of magnetization directions only slightly affects the positions of Weyl points, which is attributed to the weak spin-orbital coupling effects. The topological surface states of VAs projected on semi-infinite (001) and (111) surfaces are investigated. The Fermi arcs of all Weyl points are clearly visible on the projected Fermi surfaces. Our findings suggest that VAs is a fully spin-polarized Weyl semimetal with many-body correlated effects for spintronic applications.",2403.02157v2 2024-03-18,Numerical observation of $\mathrm{SU}(N)$ Nagaoka ferromagnetism,"We provide numerical evidence of the Nagaoka's theorem in the $\mathrm{SU}(N)$ Fermi-Hubbard model on various cluster geometries, such as the square, the honeycomb and the triangular lattices. In particular, by diagonalizing several finite-size clusters, we show that for one hole away from filling $1/N$, the itinerant ferromagnetism arises for $U$ (the positive on-site interaction) larger than $U_c$ (the value at the transition), which strongly depends on the coordination number $z$ and on $N$, the number of degenerate orbitals, that we vary from $N=2$ to $N=6$ in our simulations. We prove that $U_c$ is a non decreasing function of $N$. In addition, we find that the lattice dependency is rooted in the kinetic energy of the hole. We find that large coordination numbers $z$ lower the value of $U_c$. Complementary, we explore the effect of long-range hopping on the appearance of itinerant ferromagnetism and demonstrate that it acts as an increased coordination number, protecting the ferromagnetic phase at small $U$. Finally, both the effects of the presence of some additional holes and of the finite size of the clusters are briefly discussed.",2403.11588v1 2024-03-19,Acoustoelectric non-local spin wave power detector for studying magnon-phonon coupling,"We have developed a simple detection scheme to study spin waves excited by surface acoustic wave (SAW) in ferromagnetic thin films. Metallic antennas made of Ta and a ferromagnetic element are placed along the SAW propagation path. The SAW excites spin waves in the ferromagnetic element and induces acoustoelectric current in the antennas, which are detected as a DC voltage. The DC voltage takes an extremum at the spin wave resonance condition, which demonstrates that the antenna acts as a non-local spin wave detector. The antennas placed before and after the ferromagnetic element along the SAW propagation path can probe spin wave excitation from reflected and transmitted SAWs, respectively. Interestingly, we find characteristics of spin wave excitations that are different for the reflected and transmitted SAWs: the former excites spin waves with larger frequency with broader linewidth compared to the latter. The results show that the antennas act as a non-local spin wave power detector and can be used to map out the spin wave spectra in a unique way, providing insights into the magnon-phonon coupling in magnetic nanostructures fabricated on phononic SAW devices.",2403.12745v1 2024-03-20,Dipole-dipole-interaction-induced entanglement between two-dimensional ferromagnets,"We investigate the viability of dipole-dipole interaction as a means of entangling two distant ferromagnets. To this end we make use of the Bogoliubov transformation as a symplectic transformation. We show that the coupling of the uniform magnon modes can be expressed using four squeezing parameters which we interpret in terms of hybridization, one-mode and two-mode squeezing. We utilize the expansion in terms of the squeezing parameters to obtain an analytic formula for the entanglement in the magnon ground state using the logarithmic negativity as entanglement measure. Our investigation predicts that for infinitely large two-dimensional ferromagnets, the dipole-dipole interaction does not lead to significant long-range entanglement. However, in the case of finite ferromagnets, finite entanglement can be expected",2403.13354v1 2001-12-03,Theory of proximity effect in superconductor/ferromagnet heterostructures,"We present a microscopic theory of proximity effect in the ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures where S is s-wave low-T_c superconductor and F's are layers of 3d transition ferromagnetic metal. Our approach is based on the solution of Gor'kov equations for the normal and anomalous Green's functions together with a self-consistent evaluation of the superconducting order parameter. We take into account the elastic spin-conserving scattering of the electrons assuming s-wave scattering in the S layer and s-d scattering in the F layers. In accordance with the previous quasiclassical theories, we found that due to exchange field in the ferromagnet the anomalous Green's function F(z) exhibits the damping oscillations in the F-layer as a function of distance z from the S/F interface. In the given model a half of period of oscillations is determined by the length \xi_m^0 = \pi v_F/E_ex, where v_F is the Fermi velocity and E_ex is the exchange field, while damping is governed by the length l_0 = (1/l_{\uparrow} + 1/l_{\downarrow})^{-1} with l_{\uparrow} and l_{\downarrow} being spin-dependent mean free paths in the ferromagnet. The superconducting transition temperature T_c(d_F) of the F/S/F trilayer shows the damping oscillations as a function of the F-layer thickness d_F with period \xi_F = \pi/\sqrt{m E_ex}, where m is the effective electron mass. We show that strong spin-conserving scattering either in the superconductor or in the ferromagnet significantly suppresses these oscillations. The calculated T_c(d_F) dependences are compared with existing experimental data for Fe/Nb/Fe trilayers and Nb/Co multilayers.",0112034v3 2009-01-14,Theory of superconducting and magnetic proximity effect in S$\mid$F structures with inhomogeneous magnetization textures and spin-active interfaces,"We present a study of the proximity effect and the inverse proximity effect in a superconductor$\mid$ferromagnet bilayer, taking into account several important factors which mostly have been ignored in the literature so far. These include spin-dependent interfacial phase shifts (spin-DIPS) and inhomogeneous textures of the magnetization in the ferromagnetic layer, both of which are expected to be present in real experimental samples. Our approach is numerical, allowing us to access the full proximity effect regime. In Part I of this work, we study the superconducting proximity effect and the resulting local density of states in an inhomogeneous ferromagnet with a non-trivial magnetic texture. Our two main results in Part I are a study of how Bloch and N\'eel domain walls affect the proximity-induced superconducting correlations and a study of the superconducting proximity effect in a conical ferromagnet. The latter topic should be relevant for the ferromagnet Ho, which was recently used in an experiment to demonstrate the possibility to generate and sustain long-range triplet superconducting correlations. In Part II of this work, we investigate the inverse proximity effect with emphasis on the induced magnetization in the superconducting region as a result of the ""leakage"" from the ferromagnetic region. It is shown that the presence of spin-DIPS modify conclusions obtained previously in the literature with regard to the induced magnetization in the superconducting region. In particular, we find that the spin-DIPS can trigger an anti-screening effect of the magnetization, leading to an induced magnetization in the superconducting region with \textit{the same sign} as in the proximity ferromagnet.",0901.1961v1 2012-07-13,Cationic vacancy induced room-temperature ferromagnetism in transparent conducting anatase Ti_{1-x}Ta_xO_2 (x~0.05) thin films,"We report room-temperature ferromagnetism in highly conducting transparent anatase Ti1-xTaxO2 (x~0.05) thin films grown by pulsed laser deposition on LaAlO3 substrates. Rutherford backscattering spectrometry (RBS), x-ray diffraction (XRD), proton induced x-ray emission (PIXE), x-ray absorption spectroscopy (XAS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicated negligible magnetic contaminants in the films. The presence of ferromagnetism with concomitant large carrier densities was determined by a combination of superconducting quantum interference device (SQUID) magnetometry, electrical transport measurements, soft x-ray magnetic circular dichroism (SXMCD), XAS, and optical magnetic circular dichroism (OMCD) and was supported by first-principle calculations. SXMCD and XAS measurements revealed a 90% contribution to ferromagnetism from the Ti ions and a 10% contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites though carrier activation was only 50% at 5% Ta concentration implying compensation by cationic defects. The role of Ti vacancy and Ti3+ was studied via XAS and x-ray photoemission spectroscopy (XPS) respectively. It was found that in films with strong ferromagnetism, the Ti vacancy signal was strong while Ti3+ signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localised magnetic moments, Ti vacancy sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to ferromagnetism in wide-band-gap semiconducting oxides without any magnetic elements.",1207.3156v1 2013-11-19,Ferromagnetic Potts Model: Refined #BIS-hardness and Related Results,"Recent results establish for 2-spin antiferromagnetic systems that the computational complexity of approximating the partition function on graphs of maximum degree D undergoes a phase transition that coincides with the uniqueness phase transition on the infinite D-regular tree. For the ferromagnetic Potts model we investigate whether analogous hardness results hold. Goldberg and Jerrum showed that approximating the partition function of the ferromagnetic Potts model is at least as hard as approximating the number of independent sets in bipartite graphs (#BIS-hardness). We improve this hardness result by establishing it for bipartite graphs of maximum degree D. We first present a detailed picture for the phase diagram for the infinite D-regular tree, giving a refined picture of its first-order phase transition and establishing the critical temperature for the coexistence of the disordered and ordered phases. We then prove for all temperatures below this critical temperature that it is #BIS-hard to approximate the partition function on bipartite graphs of maximum degree D. As a corollary, it is #BIS-hard to approximate the number of k-colorings on bipartite graphs of maximum degree D when k <= D/(2 ln D). The #BIS-hardness result for the ferromagnetic Potts model uses random bipartite regular graphs as a gadget in the reduction. The analysis of these random graphs relies on recent connections between the maxima of the expectation of their partition function, attractive fixpoints of the associated tree recursions, and induced matrix norms. We extend these connections to random regular graphs for all ferromagnetic models and establish the Bethe prediction for every ferromagnetic spin system on random regular graphs. We also prove for the ferromagnetic Potts model that the Swendsen-Wang algorithm is torpidly mixing on random D-regular graphs at the critical temperature for large q.",1311.4839v3 2016-07-22,Magnon-induced long-range correlations and their neutron-scattering signature in quantum magnets,"We consider the coupling of the magnons in both quantum ferromagnets and antiferromagnets to the longitudinal order-parameter fluctuations, and the resulting nonanalytic behavior of the longitudinal susceptibility. In classical magnets it is well known that long-range correlations induced by the magnons lead to a singular wave-number dependence of the form $1/k^{4-d}$ in all dimensions 2\Delta_{\rm AF}$), in agreement with [Eur. Phys. J. B {\bf 25}, 373 (2002)]. If a third spin polarized electrode is connected to a superconductor we find that $\Delta_{\rm F}-\Delta_{\rm AF}$ can change sign as the transparency of the third electrode increases. This can be understood from the fact that the superconducting order parameter is reduced if pair correlations among the ferromagnetic electrodes increase. If the two ferromagnetic electrodes are within a finite distance we find Friedel oscillations in the Gorkov function but we still obtain $\Delta_{\rm F} > \Delta_{\rm AF}$.",0204610v2 2002-07-16,Magnetic and Transport Properties of La$_{0.7}$Sr$_{0.3}$Co$_{1-y}$Mn$_y$O$_3$,"While both La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$CoO$_3$ are ferromagnets with metallic conductivity below $T_\mathrm{c}$ (360K and 220K, respectively), partial substitutions of Co by Mn in La$_{0.7}$Sr$_{0.3}$Co$_{1-y}$Mn$_y$O$_3$ ($y < 0.1$) drastically suppress the ferromagnetic long-range order pre-established by Co-O-Co double exchange and tune the conductivity towards insulating behavior. Since Mn-O-Mn double-exchange interactions are avoidable at low Mn-substitution levels, the deterioration of ferromagnetism and conductivity thus provides evidence for no Mn-O-Co double exchange (but antiferromagnetic superexchange) in the present system. At $y = 0.1$, the ferromagnetism is no longer observed; the system becomes an insulating spin-glass with $T_\mathrm{g}\approx 62$K as estimated from the ac-susceptibility data using the conventional critical slowing-down scaling law. With further substitution ($y\geq 0.3$), the ferromagnetism is recovered ($T_\mathrm{c} = 165$K for $y = 0.3$ and 200K for $y = 0.5$) while the resistivity continues increasing and exhibits insulating behavior. These results indicate that the substitution is not simply a mixture of La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$CoO$_3$, but produces a more complicated scenario of spin states, interactions and disorder.",0207380v1 2002-09-23,Giant magnetothermopower of magnon-assisted transport in ferromagnetic tunnel junctions,"We present a theoretical description of the thermopower due to magnon-assisted tunneling in a mesoscopic tunnel junction between two ferromagnetic metals. The thermopower is generated in the course of thermal equilibration between two baths of magnons, mediated by electrons. For a junction between two ferromagnets with antiparallel polarizations, the ability of magnon-assisted tunneling to create thermopower $S_{AP}$ depends on the difference between the size $\Pi_{\uparrow, \downarrow}$ of the majority and minority band Fermi surfaces and it is proportional to a temperature dependent factor $(k_{B}T/\omega_{D})^{3/2}$ where $\omega_{D}$ is the magnon Debye energy. The latter factor reflects the fractional change in the net magnetization of the reservoirs due to thermal magnons at temperature $T$ (Bloch's $T^{3/2}$ law). In contrast, the contribution of magnon-assisted tunneling to the thermopower $S_P$ of a junction with parallel polarizations is negligible. As the relative polarizations of ferromagnetic layers can be manipulated by an external magnetic field, a large difference $\Delta S = S_{AP} - S_P \approx S_{AP} \sim - (k_B/e) f (\Pi_{\uparrow},\Pi_{\downarrow}) (k_BT/\omega_{D})^{3/2}$ results in a magnetothermopower effect. This magnetothermopower effect becomes giant in the extreme case of a junction between two half-metallic ferromagnets, $\Delta S \sim - k_B/e$.",0209520v1 2002-12-16,Significant suppression of weak ferromagnetism in (La${}_{1.8}$Eu${}_{0.2}$)CuO${}_4$,"The magnetic structure of (La${}_{1.8}$Eu${}_{0.2}$)CuO${}_4$ has been studied by magnetization measurements of single crystals, which show antiferromagnetic long-range order below $T_N$ = 265 K and a structural phase transition at $T_s$ = 130 K. At $T_s < T < T_N$, the Cu spin susceptibility exhibits almost the same behavior as that of La${}_2$CuO${}_4$ in the low-temperature orthorhombic phase, which indicates the existence of finite spin canting out of the CuO${}_2$ plane. At $T < T_s$, the magnitude of the weak-ferromagnetic moment induced by the spin canting is suppressed approximately by 70{%}. This significant suppression of the weak-ferromagnetic moment is carefully compared with the theoretical analysis of weak ferromagnetism by Stein {\it et al.} (Phys. Rev. B {\bf 53}, 775 (1996)), in which the magnitude of weak-ferromagnetic moments strongly depend on the crystallographic symmetry. Based on such comparison, below $T_s$ (La${}_{1.8}$Eu${}_{0.2}$)CuO${}_4$ is in the low-temperature less-orthorhombic phase with a space group of $Pccn$. We also discuss the possible magnetic structure of the pure low-temperature tetragonal phase with space group $P4_2/{ncm}$, which is relevant for rare-earth and alkaline-earth ions co-doped La${}_2$CuO${}_4$.",0212360v2 2003-03-10,"Electronic structure, exchange interactions and Curie temperature in diluted III-V magnetic semiconductors: (GaCr)As, (GaMn)As, (GaFe)As","We complete our earlier (Phys. Rev. B, {\bf 66}, 134435 (2002)) study of the electronic structure, exchange interactions and Curie temperature in (GaMn)As and extend the study to two other diluted magnetic semiconductors (GaCr)As and (GaFe)As. Four concentrations of the 3d impurities are studied: 25%, 12.5%, 6.25%, 3.125%. (GaCr)As and (GaMn)As are found to possess a number of similar features. Both are semi-metallic and ferromagnetic, with similar properties of the interatomic exchange interactions and the same scale of the Curie temperature. In both systems the presence of the charge carriers is crucial for establishing the ferromagnetic order. An important difference between two systems is in the character of the dependence on the variation of the number of carriers. The ferromagnetism in (GaMn)As is found to be very sensitive to the presence of the donor defects, like As$_{\rm Ga}$ antisites. On the other hand, the Curie temperature of (GaCr)As depends rather weakly on the presence of this type of defects but decreases strongly with decreasing number of electrons. We find the exchange interactions between 3d atoms that make a major contribution into the ferromagnetism of (GaCr)As and (GaMn)As and propose an exchange path responsible for these interactions. The properties of (GaFe)As are found to differ crucially from the properties of (GaCr)As and (GaMn)As. (GaFe)As does not show a trend to ferromagnetism and is not half-metallic that makes this system unsuitable for the use in spintronic semiconductor devices.",0303172v3 2003-08-26,Ferromagnetic materials in the zinc-blende structure,"New materials are currently sought for use in spintronics applications. Ferromagnetic materials with half metallic properties are valuable in this respect. Here we present the electronic structure and magnetic properties of binary compounds consisting of 3d transition metals and group V elements viz. P, Sb and As in the zinc-blende structure. We demonstrate that compounds of V, Cr and Mn show half metallic behavior for appropriate lattice constants. By comparing the total energies in the ferromagnetic and antiferromagnetic structures, we have ascertained that the ferromagnetic phase is stable over the antiferromagnetic one. Of the different compounds studied, the Cr based systems exhibit the strongest interatomic exchange interactions, and are hence predicted to have the highest critical temperatures. Also, we predict that VAs under certain growth conditions should be a semiconducting ferromagnet. Moreover, critical temperatures of selected half metallic compounds have been estimated from mean field theory and Monte Carlo simulations using parameters obtained from a {\it ab-initio} non-collinear, tight binding linearized muffin-tin orbital method. From a simple model, we calculate the reflectance from an ideal MnAs/InAs interface considering the band structures of MnAs and InAs. Finally we present results on the relative stabilities of MnAs and CrSb compounds in the NiAs and zinc-blende structures, and suggest a parameter space in substrate lattice spacings for when the zinc-blende structure is expected to be stable.",0308524v1 2003-09-22,Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes,"While most calculations on the properties of the ferromagnetic semiconductor GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn$_{Ga}$), we investigate here whether alternate lattice sites are favored and what the magnetic consequences of this might be. Under As-rich (Ga-poor) conditions prevalent at growth, we find that the formation energies are lower for Mn$_{Ga}$ over interstitial Mn (Mn$_i$).As the Fermi energy is shifted towards the valence band maximum via external $p$-doping, the formation energy of Mn$_i$ is reduced relative to Mn$_{Ga}$. Furthermore, under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn are strongly enhanced over what is possible under bulk growth conditions. The high concentration of Mn attained under epitaxial growth of p-type material opens the possibility of Mn atoms forming small clusters. We consider various types of clusters, including the Coulomb-stabilized clusters involving two Mn$_{Ga}$ and one Mn$_i$. While isolated Mn$_i$ are hole killers (donors), and therefore destroy ferromagnetism,complexes such as Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$) are found to be more stable than complexes involving Mn$_{Ga}$-Mn$_{Ga}$-Mn$_{Ga}$. The former complexes exhibit partial or total quenching of holes, yet Mn$_i$ in these complexes provide a channel for a ferromagnetic arrangement of the spins on the two Mn$_{Ga}$ within the complex. This suggests that ferromagnetism in Mn doped GaAs arises both from holes due to isolated Mn$_{Ga}$ as well as from strongly Coulomb stabilized Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$ clusters.",0309502v1 2004-09-09,Nonlocal magnetization dynamics in ferromagnetic heterostructures,"Two complementary effects modify the GHz magnetization dynamics of nanoscale heterostructures of ferromagnetic and normal materials relative to those of the isolated magnetic constituents: On the one hand, a time-dependent ferromagnetic magnetization pumps a spin angular-momentum flow into adjacent materials and, on the other hand, spin angular momentum is transferred between ferromagnets by an applied bias, causing mutual torques on the magnetizations. These phenomena are manifestly nonlocal: they are governed by the entire spin-coherent region that is limited in size by spin-flip relaxation processes. We review recent progress in understanding the magnetization dynamics in ferromagnetic heterostructures from first principles, focusing on the role of spin pumping in layered structures. The main body of the theory is semiclassical and based on a mean-field Stoner or spin-density--functional picture, but quantum-size effects and the role of electron-electron correlations are also discussed. A growing number of experiments support the theoretical predictions. The formalism should be useful to understand the physics and to engineer the characteristics of small devices such as magnetic random-access memory elements.",0409242v3 2004-09-12,Stochastic Resonance and Nonequilibrium Dynamic Phase Transition of Ising Spin System Driven by a Joint External Field,"We studied the dynamic response and stochastic resonance of kinetic Ising spin system (ISS), subject to the joint external field of weak sinusoidal modulation and stochastic white-noise, through solving the mean-field equation of motion based on Glauber dynamics. The periodically driven stochastic ISS shows the occurrence of characteristic stochastic resonance as well as nonequilibrium dynamic phase transition (NDPT) when the frequency and amplitude h0 of driving field, the temperature t of the system and noise intensity D attain a specific accordance in quantity. There exist in the system two typical dynamic phases, referred to as dynamic disordered paramagnetic and ordered ferromagnetic phases respectively, corresponding to zero and unit dynamic order parameter. We also figured out the NDPT boundary surface of the system which separates the dynamic paramagnetic and dynamic ferromagnetic phase in the 3D parameter space of h0~t~D. An intriguing dynamical ferromagnetic phase with an intermediate order parameter at 0.66 was revealed for the first time in the ISS subject to the perturbation of a joint determinant and stochastic field. Our primary result indicates that the intermediate order dynamical ferromagnetic phase is dynamic metastable in nature and owns a peculiar characteristic in its stability and response to external driving field when compared with fully order dynamic ferromagnetic phase.",0409306v1 2004-12-20,Ferromagnetism in Fe-substituted spinel semiconductor ZnGa$_2$O$_4$,"Motivated by the recent experimental observation of long range ferromagnetic order at a relatively high temperature of 200K in the Fe-doped ZnGa$_2$O$_4$ semiconducting spinel, we propose a possible mechanism for the observed ferromagnetism in this system. We show, supported by band structure calculations, how a model similar to the double exchange model can be written down for this system and calculate the ground state phase diagram for the two cases where Fe is doped either at the tetrahedral position or at the octahedral position. We find that in both cases such a model can account for a stable ferromagnetic phase in a wide range of parameter space. We also argue that in the limit of high Fe$^{2+}$ concentration at the tetrahedral positions a description in terms of a two band model is essential. The two $e_g$ orbitals and the hopping between them play a crucial role in stabilizing the ferromagnetic phase in this limit. The case when Fe is doped simultaneously at both the tetrahedral and the octahedral position is also discussed.",0412530v3 2005-01-19,Ferromagnetic Resonance in selected nanostructural materials designed for technological applications,"During the past ten years nanostructures have been subject of active research. Fabrication of such systems follows well developed methods. The increase in the number of materials available for research and applications requires that the methods of their characterization be even more precise then before. Thin film structures have many advantages for technological applications because of compatibility with integrated circuit design. The magnetoimpedance, MI (change of impedance of a ferromagnet on application of a field) in 3-layered structures consisting of two magnetic layers separated by a non-magnetic conductive layer has been predicted to show high MI. In many cases the experimental values of MI effect are smaller than the theoretical predictions. Therefore, more careful characterization of the samples is a must. Accordingly, the first part of the present research deals with a ferromagnetic resonance, FMR, study of thin films and multilayers containing Fe20Ni80 layered nanocomponents. The second system proposed for ferromagnetic resonance study consists of Co/GdCo multilayers prepared by rf-sputtering. It was chosen as a model system both for convenience and in view of possible applications. The third group of magnetic materials for FMR characterization consists of powders: commercial polystyrene beads (Dynabeads-480) and CoNi powders with nanoscale particle dimensions. These particles have many biomedical applications. FMR and microwave absorption in micron size powders have been studied previously. More recently new methods of small particle fabrication have been developed. Therefore their characterization by microwave methods is highly desirable.",0501449v1 2005-08-29,"Magnetic Properties of J-J-J' Quantum Heisenberg Chains with Spin S=1/2, 1, 3/2 and 2 in a Magnetic Field","By means of the density matrix renormalization group (DMRG) method, the magnetic properties of the J-J-J$^{\prime}$ quantum Heisenberg chains with spin $S=1/2$, 1, 3/2 and 2 in the ground states are investigated in the presence of a magnetic field. Two different cases are considered: (a) when $J$ is antiferromagnetic and $J^{\prime}$ is ferromagnetic (i.e. the AF-AF-F chain), the system is a ferrimagnet. The plateaus of the magnetization are observed. It is found that the width of the plateaus decreases with increasing the ferromagnetic coupling, and disappears when $% J^{\prime}/J$ passes over a critical value. The saturated field is observed to be independent of the ferromagnetic coupling; (b) when $J$ is ferromagnetic and $J^{\prime}$ is antiferromagnetic (i.e. the F-F-AF chain), the system becomes an antiferromagnet. The plateaus of the magnetization are also seen. The width of the plateaus decreases with decreasing the antiferromagnetic coupling, and disappears when $J^{\prime}/J $ passes over a critical value. Though the ground state properties are quite different, the magnetization plateaus in both cases tend to disappear when the ferromagnetic coupling becomes more dominant. Besides, no fundamental difference between the systems with spin half-integer and integer has been found.",0508685v1 2005-12-21,Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors,"We calculate Curie temperatures and study the stability of ferromagnetism in diluted magnetic materials, taking as a model for the exchange between magnetic impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor t range term representing the effects of superexchange. To properly include effects of spin and thermal fluctuations as well as geometric disorder, we solve the effective Heisenberg Hamiltonian by means of a recently developed semi-analytical approach. This approach, ``self-consistent local Random Phase Approximation (SC-L RPA)'', is explained. We show that previous mean-field treatments, which have been widely used in the literature, largely overestimate both the Curie temperatures and the stability of ferromagnetism as a function of carrier density. The discr epancy when compared to the current approach was that effects of frustration in RKKY oscillations had been strongly underestimated by such simple mea n-field theories. We argue that the use, as is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in diluted III-V systems is inconsistent with the observation of ferromagnetism over a wide region of itinerant carrier densities. This may be puzzling when compared to the apparent success of calculations based on {\it ab-initio} estimates of the coupling; we propose a resolution to this issue by taking RKKY-like interactions between resonant states close to the Fermi level.",0512540v1 2006-02-14,Can a non-ideal metal ferromagnet inject spin into a semiconductor with 100% efficiency without a tunnel barrier?,"Current understanding of spin injection tells us that a metal ferromagnet can inject spin into a semiconductor with 100% efficiency if either the ferromagnet is an ideal half metal with 100% spin polarization, or there exists a suitable tunnel barrier at the interface. In this paper, we show that, at absolute zero temperature, 100% spin injection efficiency from a non-ideal metal ferromagnet into a semiconductor quantum wire can be reached at certain injection energies, without a tunnel barrier, provided there is an axial magnetic field along the direction of current flow as well as a spin orbit interaction in the semiconductor. At these injection energies, spin is injected only from the majority spin band of the ferromagnetic contact, resulting in 100% spin injection efficiency. This happens because of the presence of antiresonances in the transmission coefficient of the minority spins when their incident energies coincide with Zeeman energy states in the quantum wire. At absolute zero and below a critical value of the axial magnetic field, there are two distinct Zeeman energy states and therefore two injection energies at which ideal spin filtering is possible; above the critical magnetic field there is only one such injection energy. The spin injection efficiency rapidly decreases as the temperature increases. The rate of decrease is slower when the magnetic field is above the critical value. The appropriate choice of semiconductor materials and structures necessary to maintain a large spin injection efficiency at elevated temperatures is discussed.",0602335v1 2006-06-16,Spin transfer in an antiferromagnet,"An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. In this paper we demonstrate transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density (~10^12 A/m^2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias can be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications.",0606462v1 2003-08-05,Ferromagnetic Ordering of Energy Levels,"We study a natural conjecture regarding ferromagnetic ordering of energy levels in the Heisenberg model which complements the Lieb-Mattis Theorem of 1962 for antiferromagnets: for ferromagnetic Heisenberg models the lowest energies in each subspace of fixed total spin are strictly ordered according to the total spin, with the lowest, i.e., the ground state, belonging to the maximal total spin subspace. Our main result is a proof of this conjecture for the spin-1/2 Heisenberg XXX and XXZ ferromagnets in one dimension. Our proof has two main ingredients. The first is an extension of a result of Koma and Nachtergaele which shows that monotonicity as a function of the total spin follows from the monotonicity of the ground state energy in each total spin subspace as a function of the length of the chain. For the second part of the proof we use the Temperley-Lieb algebra to calculate, in a suitable basis, the matrix elements of the Hamiltonian restricted to each subspace of the highest weight vectors with a given total spin. We then show that the positivity properties of these matrix elements imply the necessary monotonicity in the volume. Our method also shows that the first excited state of the XXX ferromagnet on any finite tree has one less than maximal total spin.",0308006v2 2003-10-30,Competition of Two Terms in Exchange Hamiltonian for magnetic substances,"The unjustifiable or wrong in the previous magnetism theories has been indicated in this paper. For a N electrons system with Heisenberg exchange integral, the correct exchange Hamilton should be of two terms, not only one as in the previous magnetism theories. The role of the minor term in the exchange Hamilton was considered. Based on the principle of superposition of state, the eigenstate of the system with Heisenberg exchange integral, the sum of a positive term and a negative term, and the energy (relative to exchange interaction) eigenvalue were attained. When the positive term is equal to the absolute value of the negative term, the system is in the spin glass state, the probabilities of parallel and antiparallel arrange for every pair of spins of electron of nearest neighbours in the system are equal. When the positive term is not equal to the absolute value of the negative term,the probabilities are not equal, and there coexist the ferromagnetic states and spin glass or antiferromagnetic state and spin glass,when ferromagnetic states and spin glass or antiferromagnetic state and spin glass coexist, the energy of the system is lower than that when only ferromagnetic states or antiferromagnetic state exists as in previous theory. Weiss ferromagnetic state or Neel anti ferromagnetic state is just a special state as the positive term is equal to zero or negative term is equal to zero.",0310178v1 2007-07-11,Changeover from Glassy ferromagnetism of the orbital domain state to long range ferromagnetic ordering in La{_{0.9}}Sr_{0.1}MnO{_3},"An attempt is made to resolve the controversy related to the low temperature phase (ground state) of the low-doped ferromagnetic (FM)- insulator(I) manganite through bulk magnetic measurements on La${_{0.9}}$Sr$_{0.1}$MnO${_3}$ sample. It is shown that the FM phase, formed out of well defined transition in the low-doped system, becomes inhomogeneous with decrease in temperature. This inhomogeniety is considered to be an outcome of the formation of orbital domain state of e$_g$-electrons having hole rich (metallic) walls separating the hole deficient (insulating) regions. The resulting complexity brings in metastability and glassy behaviour within the FM phase at low temperature, however, with no resemblance to spin glass, cluster glass or reentrant phases. It shows ageing effect without memory but magnetic relaxation shows signatures of inter-cluster interaction. The energy landscape picture of this glassy phase is described in terms of hierarchical model. Further, it is shown that this inhomogeneity disappear in La${_{0.9}}$Sr$_{0.1}$MnO$_{3.08}$ where, the orbital domain state is destroyed by self doping resulting in reduction of Mn$^{3+}$ and hence e$_g$-electrons. The ferromagnetic phase of the non-stoichiometric sample, does not show glassy behaviour. It neither follows 'hierarchical model' nor 'droplet model' generally used to explain glassy or inhomogeneous systems. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferromagnet.",0707.1555v1 2008-04-10,Electrical transport and ferromagnetism in Ga1-xMnxAs synthesized by ion implantation and pulsed-laser melting,"We present a detailed investigation of the magnetic and magnetotransport properties of thin films of ferromagnetic Ga1-xMnxAs synthesized using ion implantation and pulsed-laser melting (II-PLM). The field and temperature-dependent magnetization, magnetic anisotropy, temperature-dependent resistivity, magnetoresistance, and Hall effect of II-PLM Ga1-xMnxAs films have all of the characteristic signatures of the strong p-d interaction of holes and Mn ions observed in the dilute hole-mediated ferromagnetic phase. The ferromagnetic and electrical transport properties of II-PLM films correspond to the peak substitutional Mn concentration meaning that the non-uniform Mn depth distribution is unimportant in determining the film properties. Good quantitative agreement is found with films grown by low temperature molecular beam epitaxy (LT-MBE) and having the similar substitutional Mn_Ga composition. Additionally, we demonstrate that II-PLM Ga1-xMnxAs films are free from interstitial Mn_I because of the high temperature processing. At high Mn implantation doses the kinetics of solute redistribution during solidification alone determine the maximum resulting Mn_Ga concentration. Uniaxial anisotropy between in-plane [-110]and [110] directions is present in II-PLM Ga1-xMnxAs giving evidence for this being an intrinsic property of the carrier-mediated ferromagnetic phase.",0804.1612v1 2008-04-21,Local electronic structure of Cr in the II-VI diluted ferromagnetic semiconductor Zn$_{1-x}$Cr$_x$Te,"The electronic structure of the Cr ions in the diluted ferromagnetic semiconductor Zn$_{1-x}$Cr$_x$Te ($x=0.03$ and 0.15) thin films has been investigated using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). Magnetic-field ($H$) and temperature ($T$) dependences of the Cr $2p$ XMCD spectra well correspond to the magnetization measured by a SQUID magnetometer. The line shape of the Cr $2p$ XMCD spectra is independent of $H$, $T$, and $x$, indicating that the ferromagnetism is originated from the same electronic states of the Cr ion. Cluster-model analysis indicates that although there are two or more kinds of Cr ions in the Zn$_{1-x}$Cr$_x$Te samples, the ferromagnetic XMCD signal is originated from Cr ions substituted for the Zn site. The Cr 3d partial density of states extracted using Cr $2p \to 3d$ resonant PES shows a broad feature near the top of the valence band, suggesting strong $s$,$p$-$d$ hybridization. No density of states is detected at the Fermi level, consistent with their insulating behavior. Based on these findings, we conclude that double exchange mechanism cannot explain the ferromagnetism in Zn$_{1-x}$Cr$_{x}$Te.",0804.3272v1 2008-05-05,The role of interface transparency and spin-dependent scattering in diffusive ferromagnet/superconductor heterostructures,"We present a numerical study of the density of states in a ferromagnet/superconductor junction and the Josephson current in a superconductor/ferromagnet/superconductor junction in the diffusive limit by solving the Usadel equation with Nazarov's boundary conditions. Our calculations are valid for an arbitrary interface transparency and arbitrary spin-dependent scattering rate, which allows us to explore the entire proximity-effect regime. We first investigate how the proximity-induced anomalous Green's function affects the density of states in the ferromagnet for several values of the exchange field. In each case, we consider the effect of the barrier transparency and allow for various concentrations of magnetic impurities. In particular, we address how the zero-energy peak and minigap observed in experiments may be understood in terms of the interplay between the singlet and triplet anomalous Green's function and their dependence on the concentration of magnetic impurities. We also investigate the role of the barrier transparency and spin-flip scattering in a superconductor/ferromagnet/superconductor junction. We suggest that such diffusive Josephson junctions with large residual values of the supercurrent at the 0-$\pi$ transition, where the first harmonic term in the current vanishes, may be used as efficient supercurrent-switching devices. It is also found that uniaxial spin-flip scattering has very different effect on the 0-$\pi$ transition points depending on whether one regards the width- or temperature-dependence of the current.",0805.0533v1 2008-10-02,Voltage controlled inversion of magnetic anisotropy in a ferromagnetic thin film at room temperature,"The control of magnetic properties by means of an electric field is an important aspect in magnetism and magnetoelectronics. We here utilize magnetoelastic coupling in ferromagnetic/piezoelectric hybrids to realize a voltage control of magnetization orientation at room temperature. The samples consist of polycrystalline nickel thin films evaporated onto piezoelectric actuators. The magnetic properties of these multifunctional hybrids are investigated at room temperature as a function of the voltage controlled stress exerted by the actuator on the Ni film. Ferromagnetic resonance spectroscopy shows that the magnetic easy axis in the Ni film plane is rotated by 90 degree upon changing the polarity of the voltage Vp applied to the actuator. In other words, the in-plane uniaxial magnetic anisotropy of the Ni film can be inverted via the application of an appropriate voltage Vp. Using SQUID magnetometry, the evolution of the magnetization vector is recorded as a function of Vp and of the external magnetic field. Changing Vp allows to reversibly adjust the magnetization orientation in the Ni film plane within a range of approximately 70 degree. All magnetometry data can be quantitatively understood in terms of the magnetic free energy determined from the ferromagnetic resonance experiments. These results demonstrate that magnetoelastic coupling in hybrid structures indeed is a viable option to control magnetization orientation in technologically relevant ferromagnetic thin films at room temperature.",0810.0389v1 2009-04-07,Thermoelectrical manipulation of nanomagnets,"We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermo-electronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermo-electric manipulation of nanomagnets). Supporting experimental evidence of such thermally controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel-to-antiparallel switching causes magneto-resistance oscillations whose frequency can be controlled by proper biasing from essentially DC to GHz. We discuss in detail an experimental realization of a device that can operate as a thermo-magneto-resistive switch or oscillator.",0904.1156v2 2009-07-31,Effects of Disorder and Interactions in the Quantum Hall Ferromagnet,"This work treats the effects of disorder and interactions in a quantum Hall ferromagnet, which is realized in a two-dimensional electron gas (2DEG) in a perpendicular magnetic field at Landau level filling factor equal one. We study the problem by projecting the original fermionic Hamiltonian into magnon states, which behave as bosons in the vicinity of the ferromagnetic ground state. The approach permits the reformulation of a strongly interacting model into a non-interacting one. The latter is a non-perturbative scheme that consists in treating the two-particle neutral excitations of the electron system as a bosonic single-particle. Indeed, the employment of bosonization facilitates the inclusion of disorder in the study of the system. It has been shown previously that disorder may drive a quantum phase transition in the Hall ferromagnet. However, such studies have been either carried out in the framework of nonlinear sigma model, as an effective low-energy theory, or included the long-range Coulomb interaction in a quantum description only up to the Hartree-Fock level. Here, we establish the occurrence of a disorder-driven quantum phase transition from a ferromagnetic 2DEG to a spin glass phase by taking into account interactions between electrons up to the random phase approximation level in a fully quantum description.",0907.5513v1 2009-08-30,Measurement of spin memory lengths in PdNi and PdFe ferromagnetic alloys,"Weakly ferromagnetic alloys are being used by several groups in the study of superconducting/ferromagnetic hybrid systems. Because spin-flip and spin-orbit scattering in such alloys disrupt the penetration of pair correlations into the ferromagnetic material, it is desirable to have a direct measurement of the spin memory length in such alloys. We have measured the spin memory length at 4.2 K in sputtered Pd0.88Ni0.12 and Pd0.987Fe0.013 alloys using methods based on current-perpendicular-to-plane giant magnetoresistance. The alloys are incorporated into hybrid spin valves of various types, and the spin memory length is determined by fits of the Valet-Fert spin-transport equations to data of magnetoresistance vs. alloy thickness. For the case of PdNi alloy, the resulting values of the spin memory length are lsf(PdNi) = 2.8 +/- 0.5 nm and 5.4 +/- 0.6 nm, depending on whether or not the PdNi is exchange biased by an adjacent Permalloy layer. For PdFe, the spin memory length is somewhat longer, lsf(PdFe) = 9.6 +/- 2 nm, consistent with earlier measurements indicating lower spin-orbit scattering in that material. Unfortunately, even the longer spin memory length in PdFe may not be long enough to facilitate observation of spin-triplet superconducting correlations predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of magnetic inhomogeneity.",0908.4375v1 2009-09-29,Spin-polarized Josephson current in SFS junctions with inhomogeneous magnetization,"SHORTENED ABSTRACT: We study numerically the properties of spin- and charge-transport in a nanoscale diffusive superconductor$\mid$ferromagnet$\mid$superconductor junction when the magnetization texture is non-uniform. Specifically, we incorporate the presence of a Bloch/Neel domain walls and conical ferromagnetism, including spin-active interfaces. The superconducting leads are assumed to be of s-wave type. We investigate how the 0-$\pi$ transition is influenced by the inhomogeneous magnetization texture and focus on the particular case where the charge-current vanishes while the spin-current is non-zero. In the case of a Bloch/Neel domain-wall, the spin-current can be seen only for one component of the spin polarization, whereas in the case of conical ferromagnetism the spin-current has the three components. We explain all of these results in terms of the interplay between the triplet anomalous Green's function induced in the ferromagnetic region and the local direction of the magnetization vector in the ferromagnet. Interestingly, we find that the spin-current exhibits discontinuous jumps at the 0--$\pi$ transition points of the critical charge-current. We explain this result in terms of the different symmetry obeyed by the current-phase relation when comparing the charge- and spin-current. Specifically, we find that whereas the charge-current obeys the well-known relation $I_c(\phi) = -I_c(2\pi-\phi)$, the spin-current satisfies $I_s(\phi) = I_s(2\pi-\phi)$, where $\phi$ is the superconducting phase difference.",0909.5427v1 2009-11-02,Direct Search for a Ferromagnetic Phase in a Heavily Overdoped Nonsuperconducting Copper Oxide,"The doping of charge carriers into the CuO2 planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La2-xSrxCuO4 using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping.",0911.0407v3 2011-01-01,Current-voltage characteristics of tunnel Josephson junctions with a ferromagnetic interlayer,"We present a quantitative study of the current-voltage characteristics (CVC) of diffusive superconductor/ insulator/ ferromagnet/ superconductor (SIFS) tunnel Josephson junctions. In order to obtain the CVC we calculate the density of states (DOS) in the F/S bilayer for arbitrary length of the ferromagnetic layer, using quasiclassical theory. For a ferromagnetic layer thickness larger than the characteristic penetration depth of the superconducting condensate into the F layer, we find an analytical expression which agrees with the DOS obtained from a self-consistent numerical method. We discuss general properties of the DOS and its dependence on the parameters of the ferromagnetic layer. In particular we focus our analysis on the DOS oscillations at the Fermi energy. Using the numerically obtained DOS we calculate the corresponding CVC and discuss their properties. Finally, we use CVC to calculate the macroscopic quantum tunneling (MQT) escape rate for the current biased SIFS junctions by taking into account the dissipative correction due to the quasiparticle tunneling. We show that the influence of the quasiparticle dissipation on the macroscopic quantum dynamics of SIFS junctions is small, which is an advantage of SIFS junctions for superconducting qubits applications.",1101.0361v2 2011-02-10,An effective quantum parameter for strongly correlated metallic ferromagnets,"The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.",1102.2115v1 2011-02-22,Incoherent Effect of Fe and Ni Substitutions in the Ferromagnetic-Insulator La0.6Bi0.4MnO3+d,"A comparative study of the effect of Fe and Ni doping on the bismuth based perovskite La0.6Bi0.4MnO3.1, a projected spintronics magnetic semiconductor has been carried out. The doped systems show an expressive change in magnetic ordering temperature. However, the shifts in ferromagnetic transition (TC) of these doped phases are in opposite direction with respect to the parent phase TC of 115 K. The Ni-doped phase shows an increase in TC ~200 K, whereas the Fe-doped phase exhibits a downward shift to TC~95 K. Moreover, the Fe-doped is hard-type whereas the Ni-doped compound is soft-type ferromagnet. It is observed that the materials are semiconducting in the ferromagnetic phase with activation energies of 77 & 82 meV for Fe & Ni-doped phases respectively. In the presence of external magnetic field of 7 Tesla, they exhibit minor changes in the resistivity behaviours and the maximum isothermal magnetoresistance is around -20 % at 125 K for the Ni-phase. The results are explained on the basis of electronic phase separation and competing ferromagnetic and antiferromagnetic interactions between the various mixed valence cations.",1102.4471v2 2011-03-21,Spontaneous Magnetization of an Ideal Ferromagnet: Beyond Dyson's Analysis,"Using the low-energy effective field theory for magnons, we systematically evaluate the partition function of the O(3) ferromagnet up to three loops. Dyson, in his pioneering microscopic analysis of the Heisenberg model, showed that the spin-wave interaction starts manifesting itself in the low-temperature expansion of the spontaneous magnetization of an ideal ferromagnet only at order $T^4$. Although several authors tried to go beyond Dyson's result, to the best of our knowledge, a fully systematic and rigorous investigation of higher order terms induced by the spin-wave interaction, has never been achieved. As we demonstrate in the present paper, it is straightforward to evaluate the partition function of an ideal ferromagnet beyond Dyson's analysis, using effective Lagrangian techniques. In particular, we show that the next-to-leading contribution to the spontaneous magnetization resulting from the spin-wave interaction already sets in at order $T^{9/2}$ -- in contrast to all claims that have appeared before in the literature. Remarkably, the corresponding coefficient is completely determined by the leading-order effective Lagrangian and is thus independent of the anisotropies of the cubic lattice. We also consider even higher-order corrections and thereby solve -- once and for all -- the question of how the spin-wave interaction in an ideal ferromagnet manifests itself in the spontaneous magnetization beyond the Dyson term.",1103.4110v1 2011-05-31,Weak ferromagnetism and magnetic phase transitions in Gd$_2$CuO$_4$,"We report a polarised neutron study of the magnetic structures and phase trasitions in \gdc\ in low magnetic fields. These experiments have been complemented by integrated intensity measurements with unpolarised neutrons in zero field. Polarised neutron flipping ratio measurements have been made with magnetic fields $H = 0.05$, 0.10 and 0.5 T in the temperature range 4-20 K. These have enabled us to deduce that the anomalous temperature behaviour of the coherent magnetic scattering from the Cu sublattice, which shows sharp intensity minima at \Tcn1 $\approx 18$K and \Tcn2 $\approx 8$ K, is due to cross-overs in the sign of the interaction between strongly coupled, weakly ferromagnetic, CuO$_2$ layers. At \Tcn1\ the coupling changes from ferromagnetic to anti-ferromagnetic and long-range order between layers is temporarily lost. \Tcn2\ is the temperature at which the Gd moments order and a further reorganisation of the interlayer order takes place. The weak ferromagnetism of the CuO layers is found to be due to a small rotation of the Cu moments in the same direction as that in which their coordinating oxygen squares rotate in the tetragonal to orthorhombic distortion of the crystal structure. Further analysis of the flipping ratio measurements has enabled us to model the magnetic structures of the zero-field and the field-induced phases of Gd$_2$CuO$_4$.",1105.6196v1 2011-06-03,Iron-based n-type electron-induced ferromagnetic semiconductor,"Carrier-induced ferromagnetic semiconductors (FMSs) have been intensively studied for decades as they have novel functionalities that cannot be achieved with conventional metallic materials. These include the ability to control magnetism by electrical gating or light irradiation, while fully inheriting the advantages of semiconductor materials such as band engineering. Prototype FMSs such as (In,Mn)As or (Ga,Mn)As, however, are always p-type, making it difficult to be used in real spin devices. This is because manganese (Mn) atoms in those materials work as local magnetic moments and acceptors that provide holes for carrier-mediated ferromagnetism. Here we show that by introducing iron (Fe) into InAs, it is possible to fabricate a new FMS with the ability to control ferromagnetism by both Fe and independent carrier doping. Despite the general belief that the tetrahedral Fe-As bonding is antiferromagnetic, we demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS, a missing piece of semiconductor spintronics for decades. This achievement opens the way to realise novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs.",1106.0561v3 2011-07-09,Ferroelectricity and ferromagnetism in EuTiO3 nanowires,"We predicted the ferroelectric-ferromagnetic multiferroic properties of EuTiO3 nanowires and generated the phase diagrams in coordinates of temperature and wire radii. The calculations were performed within the Landau-Ginzburg-Devonshire theory with phenomenological parameters extracted from tabulated experimental data and first principles calculations. Since bulk EuTiO3 is antiferromagnetic at temperatures lower than 5.5 K and paraelectric at all temperatures, our goal was to investigate the possibility of inducing the ferroelectric and ferromagnetic properties of EuTiO3 by reducing the bulk to nanosystems. Our results indicate that ferroelectric spontaneous polarization of ~0.1-0.5C/m2 is induced in EuTiO3 nanowires due to the intrinsic surface stress, which is inversely proportional to the nanowire radius. The spontaneous polarization exists at temperatures lower than 300 K, for the wire radius less than 1 nm and typical surface stress coefficients ~ 15 N/m. Due to the strong biquadratic magnetoelectric coupling, the spontaneous polarization in turn induces the ferromagnetic phase at temperatures lower than 30 K for 2 nm nanowire, and at temperatures lower than 10 K for 4 nm nanowire in EuTiO3. Thus we predicted that the EuTiO3 nanowires can be the new ferroelectric-ferromagnetic multiferroic.",1107.1785v1 2011-07-12,Approximation algorithms for two-state anti-ferromagnetic spin systems on bounded degree graphs,"In a seminal paper (Weitz, 2006), Weitz gave a deterministic fully polynomial approximation scheme for count- ing exponentially weighted independent sets (equivalently, approximating the partition function of the hard-core model from statistical physics) on graphs of degree at most d, up to the critical activity for the uniqueness of the Gibbs measure on the infinite d-regular tree. More recently Sly (Sly, 2010) showed that this is optimal in the sense that if there is an FPRAS for the hard-core partition function on graphs of maximum degree d for activities larger than the critical activity on the infinite d-regular tree then NP = RP. In this paper, we extend Weitz's approach to derive a deterministic fully polynomial approx- imation scheme for the partition function of the anti-ferromagnetic Ising model with arbitrary field on graphs of maximum degree d, up to the corresponding critical point on the d-regular tree. The main ingredient of our result is a proof that for two-state anti-ferromagnetic spin systems on the d-regular tree, weak spatial mixing implies strong spatial mixing. This in turn uses a message-decay argument which extends a similar approach proposed recently for the hard-core model by Restrepo et al (Restrepo et al, 2011) to the case of the anti-ferromagnetic Ising model with arbitrary field. By a standard correspondence, these results translate to arbitrary two-state anti-ferromagnetic spin systems with soft constraints.",1107.2368v3 2011-07-22,Interaction of Josephson and magnetic oscillations in Josephson tunnel junctions with a ferromagnetic layer,"We study the dynamics of Josephson junctions with a thin ferromagnetic layer F [superconductor-ferromagnet-insulator-ferromagnet-superconductor (SFIFS) junctions]. In such junctions, the phase difference $\phi$ of the superconductors and magnetization $M$ in the F layer are two dynamic parameters coupled to each other. We derive equations describing the dynamics of these two parameters and formulate the conditions of validity. The coupled Josephson plasma waves and oscillations of the magnetization $M$ affect the form of the current-voltage ($I$-$V$) characteristics in the presence of a weak magnetic field (Fiske steps). We calculate the modified Fiske steps and show that the magnetic degree of freedom not only changes the form of the Fiske steps but also the overall view of the $I$-$V$ curve (new peaks related to the magnetic resonance appear). The $I$-$V$ characteristics are shown for different lengths of the junction including those which correspond to the current experimental situation. We also calculate the power $P$ absorbed in the system if a microwave radiation with an ac in-plane magnetic field is applied (magnetic resonance). The derived formula for the power $P$ essentially differs from the one which describes the power absorption in an isolated ferromagnetic film. In particular, this formula describes the peaks related to the excitation of standing plasma waves as well as the peak associated with the magnetic resonance.",1107.4493v2 2012-07-10,"Successive spin glass, cluster ferromagnetic, and superparamagnetic transitions in RuSr2Y1.5Ce0.5Cu2O10 complex magneto-superconductor","We report structural, DC magnetization, detailed linear/non-linear AC susceptibility, (with applied frequency and amplitude) isothermal and thermoremanent magnetization (TRM) behavior for RuSr2Y1.5Ce0.5Cu2O10 (YRu-1222) magneto-superconductor to understand its complex magnetism. Studied sample is synthesized through the novel solid state High Pressure (6 GPa) High Temperature (1450oC) (HPHT) technique. The compound is crystallized in tetragonal structure with space group I4/mmm (No. 139). DC magnetic susceptibility shows that studied YRu-1222 is magneto-superconducting with Ru spins magnetic ordering at around 110 K and superconductivity (SC) in the Cu-O2 planes below ~ 30 K. Frequency and field dependent detailed AC magnetic susceptibility measurements confirms the spin-glass (SG) behavior with homogeneous/non-homogeneous ferromagnetic (FM) clusters in this system. Variation of cusp position with applied AC frequency follows the famous Vogel-Fulcher law, which is commonly accepted feature for spin-glass (SG) system with homogeneous/non-homogeneous ferromagnetic clusters embedded in spin-glass (SG) matrix. Above the freezing temperature (Tf), first and third harmonics AC susceptibility analysis indicated possibility of the co-existence of spin cluster ferromagnetism with superparamagnetism (SPM). The M-H loops at low temperature exhibit the ferromagnetic behavior with rather small coercive field (Hc) and remnant magnetization (Mr). Summarily, the magnetic (DC and AC) susceptibility measurements and their analysis have enabled us to unearth the complex magnetism in terms of successive SG-FM-SPM transitions with temperature.",1207.2219v1 2013-01-29,Spin/orbit moment imbalance in the near-zero moment ferromagnetic semiconductor SmN,"SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetisations found in any ferromagnetic material. The near-zero moment is a result of the nearly equal and opposing spin and orbital moments in the 6H5/2 ground state of the Sm3+ ion, which leads finally to a nearly complete cancellation for an ion in the SmN ferromagnetic state. Here we explore the spin alignment in this compound with X-ray magnetic circular dichroism at the Sm L2,3 edges. The spectral shapes are in qualitative agreement with computed spectra based on an LSDA+U (local spin density approximation with Hubbard-U corrections) band structure, though there remain differences in detail which we associate with the anomalous branching ratio in rare-earth L edges. The sign of the spectra determine that in a magnetic field the Sm 4f spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4f orbital moment and antiparallel to the spin moment. Further measurements on very thin (1.5 nm) SmN layers embedded in GdN show the opposite alignment due to a strong Gd-Sm exchange, suggesting that the SmN moment might be further reduced by about 0.5 % Gd substitution.",1301.6829v1 2014-01-03,Detection of small exchange fields in S/F structures,"Ferromagnetic materials with exchange fields E_ex smaller or of the order of the superconducting gap Delta are important for applications of corresponding (s-wave) superconductor/ ferromagnet/ superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small E_ex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal - ferromagnet - insulator - superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=E_ex. Thus measuring the subgap conductance one can reliably determine small E_ex < Delta. In the opposite case E_ex > Delta one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=E_ex. This peak can be only visible for small enough exchange fields of the order of few Delta.",1401.0646v2 2014-01-09,FMR Study of Co/Ti Bilayer Thin Films,". We focused on the interaction between two ferromagnetic cobalt layers through a non-magnetic titanium layer. The magnetic properties of the structure were characterized by ferromagnetic resonance technique (FMR). The data were collected as a function of non-magnetic titanium layer thickness. Co/Ti multilayer (Ti (50 {\AA})/Co(45 {\AA})/Ti(2-40 {\AA})/Co(40 {\AA})/Ti(100 {\AA}))films were grown onto naturally oxidized p-type single crystal Si (100) substrate at UHV condition with magnetron sputtering system at room temperature. The thickness of Ti spacer layer ranges from 2 to 40 {\AA} with 2 {\AA} steps. We did not observe usual optic and acoustic modes; instead we had two broad overlapped peaks for the films ranged from 6 {\AA} to 40 {\AA}. One interesting result was the high anisotropic resonance field values for these films. Exchange coupling between ferromagnetic layers causes shift on resonance field values but these shifts in our samples were much larger than expected. This large anisotropic behavior is not clear at the moment. Our theoretical model was not able to determine a value for the exchange coupling parameter. One reason can be the close thickness values for Co sublayers. The other reason can be the Ti non-magnetic layer. If titanium did not grow layer by layer on cobalt, the cobalt ferromagnetic layers may behave as a single layer. As a result one cannot observe exchange interaction between ferromagnetic layers through non-magnetic spacer.",1401.1924v1 2014-02-22,Ferromagnetism and Spin-dependent Transport in n-type Mn-Bi2Te3 Thin Films,"We describe a detailed study of the structural, magnetic, and magneto-transport properties of single-crystal, n-type, Mn-doped Bi2Te3 thin films grown by molecular beam epitaxy. With increasing Mn concentration, the crystal structure changes from the tetradymite structure of the Bi2Te3 parent crystal at low Mn concentrations towards a BiTe phase in the (Bi2Te3)m(Bi2)n homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temperature in the range 13.8 K - 17 K in films with 2 % - 10 % Mn concentration. Magnetization hysteresis loops reveal that the magnetic easy axis is along the c-axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68 nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain wall scattering. Ordinary Hall effect measurements show that the carrier density is n-type, increases with Mn doping, and is high enough (> 2.8 x 10^{13} cm^{-2}) to place the chemical potential in the conduction band. Thus, the observed ferromagnetism is likely associated with both bulk and surface states. Surprisingly, the Curie temperature does not show any clear dependence on the carrier density but does increase with Mn concentration. Our results suggest that the ferromagnetism probed in these Mn-doped Bi2Te3 films is not mediated by carriers in the conduction band or in an impurity band.",1402.5546v1 2014-02-24,Raman studies of nearly half-metallic ferromagnet CoS2,"We measured the Raman spectra of ferromagnetic nearly half metal CoS2 in a broad temperature range. All five Raman active modes Ag, Eg, Tg(1), Tg(2) and Tg(3) were observed. The magnetic ordering is indicated by a change of the temperature dependences of the frequency and the line width of Ag and T g(2) modes at the Curie point. The temperature dependence of the frequencies and linewidths of the Ag, Eg, Tg(1), T g(2) modes in the paramagnetic phase can be described in the framework of the Klemens approach. Hardening of the Tg(2), Tg(1) and A g modes on cooling can be unambiguously seen in the ferromagnetic phase. The linewidths of Tg(2) and Ag modes behave a natural way at low exciting laser power (decrease with decreasing temperature) in the ferromagnetic phase. At high exciting laser power the corresponding linewidths increase at temperature decreasing below the Curie temperature. Then as can be seen the line width of Ag mode reaches a maxima at about 80K. This intriging feature probably signifies a specific channel of the optical phonon decay in the ferromagnetic phase of CoS2. Tentative explanations of some of the observed effects are given, taking into account the nearly half metallic nature of CoS2.",1402.5785v1 2014-07-02,Spin Waves in Ferromagnetic Insulators Coupled via a Normal Metal,"Herein, we study the spin-wave dispersion and dissipation in a ferromagnetic insulator--normal metal--ferromagnetic insulator system. Long-range dynamic coupling because of spin pumping and spin transfer lead to collective magnetic excitations in the two thin-film ferromagnets. In addition, the dynamic dipolar field contributes to the interlayer coupling. By solving the Landau-Lifshitz-Gilbert-Slonczewski equation for macrospin excitations and the exchange-dipole volume as well as surface spin waves, we compute the effect of the dynamic coupling on the resonance frequencies and linewidths of the various modes. The long-wavelength modes may couple acoustically or optically. In the absence of spin-memory loss in the normal metal, the spin-pumping-induced Gilbert damping enhancement of the acoustic mode vanishes, whereas the optical mode acquires a significant Gilbert damping enhancement, comparable to that of a system attached to a perfect spin sink. The dynamic coupling is reduced for short-wavelength spin waves, and there is no synchronization. For intermediate wavelengths, the coupling can be increased by the dipolar field such that the modes in the two ferromagnetic insulators can couple despite possible small frequency asymmetries. The surface waves induced by an easy-axis surface anisotropy exhibit much greater Gilbert damping enhancement. These modes also may acoustically or optically couple, but they are unaffected by thickness asymmetries.",1407.0635v1 2014-09-03,Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas,"We compute the finite-temperature phase diagram of a pseudospin-$1/2$ Bose gas with contact interactions, using two complementary methods: the random phase approximation (RPA) and self-consistent Hartree-Fock theory. We show that the inter-spin interactions, which break the (pseudo) spin-rotational symmetry of the Hamiltonian, generally lead to the appearance of a magnetically ordered phase at temperatures above the superfluid transition. In three dimensions, we predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong repulsive/attractive inter-species interactions respectively. The normal easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in electronic systems. For the case of inter-spin attraction, we also discuss the possibility of a \textit{bosonic} analog of the Cooper paired phase. This state is shown to significantly lose in energy to the transverse ferromagnet in three dimensions, but is more energetically competitive in lower dimensions. Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba and Dresselhaus-type couplings (as recently realized in experiment), we investigate the possibility of stripe ordering in the normal phase. Within our approximations however, we do not find an instability towards stripe formation, suggesting that the stripe order melts below the condensation temperature, which is consistent with the experimental observations of Ji \textit{et al.} [Ji \textit{et al.}, Nature Physics \textbf{10}, 314 (2014)].",1409.1242v2 2014-09-16,Distribution of critical temperature at Anderson localization,"Based on a local mean-field theory approach at Anderson localization, we find a distribution function of critical temperature from that of disorder. An essential point of this local mean-field theory approach is that the information of the wave-function multifractality is introduced. The distribution function of the Kondo temperature ($T_{K}$) shows a power-law tail in the limit of $T_{K} \rightarrow 0$ regardless of the Kondo coupling constant. We also find that the distribution function of the ferromagnetic transition temperature ($T_{c}$) gives a power-law behavior in the limit of $T_{c} \rightarrow 0$ when an interaction parameter for ferromagnetic instability lies below a critical value. However, the $T_{c}$ distribution function stops the power-law increasing behavior in the $T_{c} \rightarrow 0$ limit and vanishes beyond the critical interaction parameter inside the ferromagnetic phase. These results imply that the typical Kondo temperature given by a geometric average always vanishes due to finite density of the distribution function in the $T_{K} \rightarrow 0$ limit while the typical ferromagnetic transition temperature shows a phase transition at the critical interaction parameter. We propose that the typical transition temperature serves a criterion for quantum Griffiths phenomena vs. smeared transitions: Quantum Griffiths phenomena occur above the typical value of the critical temperature while smeared phase transitions result at low temperatures below the typical transition temperature. We speculate that the ferromagnetic transition at Anderson localization shows the evolution from quantum Griffiths phenomena to smeared transitions around the critical interaction parameter at low temperatures.",1409.4531v3 2014-10-14,Coherent coupling between ferromagnetic magnon and superconducting qubit,"Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending in macroscopic dimensions. Magnon is a quantum of an elementary excitation in the ordered spin system, such as ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic insulators has been extensively studied and widely applied for decades in the contexts of ferromagnetic resonance, and more recently of Bose-Einstein condensation as well as spintronics. Moreover, towards hybrid systems for quantum memories and transducers, coupling of magnons and microwave photons in a resonator have been investigated. However, quantum-state manipulation at the single-magnon level has remained elusive because of the lack of anharmonic element in the system. Here we demonstrate coherent coupling between a magnon excitation in a millimetre-sized ferromagnetic sphere and a superconducting qubit, where the interaction is mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we find a tunable magnon-qubit coupling scheme utilising a parametric drive with a microwave. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and thus opens a new discipline of quantum magnonics.",1410.3781v1 2014-10-30,The effects of Co3O4 on the Structure and Unusual Magnetism of LaCoO3,"Bulk La_wCoO3 particles with w=1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La2O3 and Co3O4. The resulting particles are characterized as LaCoO3 crystals interfaced with a crystalline Co3O4 phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO3 phase resulting from the Co3O4 content, but magnetization data indicate that the amount of Co3O4 strongly affects the ferromagnetic ordering at the interfaces below T_C ~89K. In addition to ferromagnetic long-range order, LaCoO3 exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe < H < 5 kOe is fit to a combination of a power law ((T-T_C)/T_C)^beta behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent beta=0.63 +- 0.02 is consistent with 2D (surface) ordering. Increased Co3O4 correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ~ 40K is a consequence of the lattice reaching a critical rhombahedral distortion as T is decreased for core regions far from the Co3O4 interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.",1410.8284v1 2014-12-22,Polarized neutron diffraction and X-ray magnetic circular dichroism study of Ru doped UCoGe,"We report on microscopic study of the ferromagnetism enhancement in the Ru doped ferromagnetic superconductor (FM SC) UCoGe. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge were prepared. Both single crystals were investigated by polarized neutron diffraction (PND) at low temperatures in magnetic fields and the latter one also by the soft X-ray magnetic circular dichroism (XMCD) method. UCo0.96Ru0.03Ge and UCo0.88Ru0.12Ge have been found ordering ferromagnetically below the Curie temperature TC = 6 K and 8.5 K, respectively, which are considerable higher than TC = 3 K of UCoGe. The increase of TC is accompanied by enhancement of the spontaneous moment to ms = 0.11 mB/f.u. and ms = 0.21 mB/f.u., respectively. The analysis of the PND results assigns the ferromagnetism enhancement mainly to the growth of the orbital part of uranium 5 f moment. In contrast to the published results of PND study of the parent UCoGe, we have found parallel orientation of the U and Co moments in both doped compounds. Evolution of magnetic characteristics with Ru concentration is discussed within a scenario considering the varying 5 f-ligand hybridization as the key mechanism.",1412.7087v2 2015-01-13,Strain-Induced Extrinsic High-Temperature Ferromagnetism in the Fe-Doped Hexagonal Barium Titanate,"Diluted magnetic semiconductors possessing intrinsic static magnetism at high temperatures represent a promising class of multifunctional materials with high application potential in spintronics and magneto-optics. In the hexagonal Fe-doped diluted magnetic oxide, 6H-BaTiO$_{3-\delta}$, room-temperature ferromagnetism has been previously reported. Ferromagnetism is broadly accepted as an intrinsic property of this material, despite its unusual dependence on doping concentration and processing conditions. However, the here reported combination of bulk magnetization and complementary in-depth local-probe electron spin resonance and muon spin relaxation measurements, challenges this conjecture. While a ferromagnetic transition occurs around 700 K, it does so only in additionally annealed samples and is accompanied by an extremely small average value of the ordered magnetic moment. Furthermore, several additional magnetic instabilities are detected at lower temperatures. These coincide with electronic instabilities of the Fe-doped 3C-BaTiO$_{3-\delta}$ pseudocubic polymorph. Moreover, the distribution of iron dopants with frozen magnetic moments is found to be non-uniform. Our results demonstrate that the intricate static magnetism of the hexagonal phase is not intrinsic, but rather stems from sparse strain-induced pseudocubic regions. We point out the vital role of internal strain in establishing defect ferromagnetism in systems with competing structural phases.",1501.02938v1 2015-02-11,Anisotropic defect-induced ferromagnetism and transport in Gd-doped GaN two-dimensional electron gasses,"Here we report on the effect of rare earth Gd-doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular beam epitaxy and consist of AlN/GaN heterostructures where Gd is delta-doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane (c-axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for in-plane configuration than for out-of-plane, which is indicative of exchange coupled spins locked along the polar c-axis. The sample with the lowest Gd concentration (2.3 $\times$ $10^{14}$ cm$^{-2}$) exhibits a saturation magnetization of 41.1 $\mu_B/Gd^{3+}$ at 5 K revealing that the Gd ion spins (7 ${\mu}_B$) alone do not account for the magnetization. Surprisingly, control samples grown without any Gd display inconsistent magnetic properties; in some control samples weak ferromagnetism is observed and in others paramagnetism. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies non-linearly with the magnetic field, but does not track the magnetization indicating the lack of coupling between the ferromagnetic phase and the conduction band electrons within the 2DEG.",1502.03478v2 2015-02-23,Swendsen-Wang Algorithm on the Mean-Field Potts Model,"We study the $q$-state ferromagnetic Potts model on the $n$-vertex complete graph known as the mean-field (Curie-Weiss) model. We analyze the Swendsen-Wang algorithm which is a Markov chain that utilizes the random cluster representation for the ferromagnetic Potts model to recolor large sets of vertices in one step and potentially overcomes obstacles that inhibit single-site Glauber dynamics. Long et al. studied the case $q=2$, the Swendsen-Wang algorithm for the mean-field ferromagnetic Ising model, and showed that the mixing time satisfies: (i) $\Theta(1)$ for $\beta<\beta_c$, (ii) $\Theta(n^{1/4})$ for $\beta=\beta_c$, (iii) $\Theta(\log n)$ for $\beta>\beta_c$, where $\beta_c$ is the critical temperature for the ordered/disordered phase transition. In contrast, for $q\geq 3$ there are two critical temperatures $0<\beta_u<\beta_{rc}$ that are relevant. We prove that the mixing time of the Swendsen-Wang algorithm for the ferromagnetic Potts model on the $n$-vertex complete graph satisfies: (i) $\Theta(1)$ for $\beta<\beta_u$, (ii) $\Theta(n^{1/3})$ for $\beta=\beta_u$, (iii) $\exp(n^{\Omega(1)})$ for $\beta_u<\beta<\beta_{rc}$, and (iv) $\Theta(\log{n})$ for $\beta\geq\beta_{rc}$. These results complement refined results of Cuff et al. on the mixing time of the Glauber dynamics for the ferromagnetic Potts model.",1502.06593v4 2015-04-08,Selective interlayer ferromagnetic coupling between the Cu spins in YBa$_2$ Cu$_3$ O$_{7-x}$ grown on top of La$_{0.7}$ Ca$_{0.3}$ MnO$_3$,"Studies to date on ferromagnet/d-wave superconductor heterostructures focus mainly on the effects at or near the interfaces while the response of bulk properties to heterostructuring is overlooked. Here we use resonant soft x-ray scattering spectroscopy to reveal a novel c-axis ferromagnetic coupling between the in-plane Cu spins in YBa$_2$ Cu$_3$ O$_{7-x}$ (YBCO) superconductor when it is grown on top of ferromagnetic La$_{0.7}$ Ca$_{0.3}$ MnO$_3$ (LCMO) manganite layer. This coupling, present in both normal and superconducting states of YBCO, is sensitive to the interfacial termination such that it is only observed in bilayers with MnO_2but not with La$_{0.7}$ Ca$_{0.3}$ interfacial termination. Such contrasting behaviors, we propose, are due to distinct energetic of CuO chain and CuO$_2$ plane at the La$_{0.7}$ Ca$_{0.3}$ and MnO$_2$ terminated interfaces respectively, therefore influencing the transfer of spin-polarized electrons from manganite to cuprate differently. Our findings suggest that the superconducting/ferromagnetic bilayers with proper interfacial engineering can be good candidates for searching the theorized Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state in cuprates and studying the competing quantum orders in highly correlated electron systems.",1504.01820v2 2015-05-29,Impact of oxygen doping and oxidation state of iron on the electronic and magnetic properties of BaFeO$_{3-δ},"We studied structural, electronic and magnetic properties of a cubic perovskite BaFeO$_{3-\delta}$ ($0 \le \delta \le 0.5$) within the density functional theory using a generalized gradient approximation and a GGA+U method. According to our calculations, BaFeO$_3$ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with extremely high Curie temperature ($T_C$) of 700 - 900 K. However, a such estimate of $T_C$ disagrees with all available experiments, which report that $T_C$ of the BaFeO$_3$ and undoped BaFeO$_{3-\delta}$ films varies between 111 K and 235 K or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for BaFeO$_3$, we concluded that the presence of oxygen vacancies in our model enables a good agreement. Thus, the relatively low $T_C$ measured in BaFeO$_3$ can be explained by oxygen vacancies intrinsically presented in the material. Since iron species near the O vacancy change their oxidation state from $4+$ to $3+$, the interaction between Fe$^{4+}$ and Fe$^{3+}$, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing $\delta$ in BaFeO$_{3-\delta}$, its $T_C$ decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO$_{3-\delta}$ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above the room temperature.",1505.07942v1 2015-09-17,Controllable 0-pi Josephson junctions containing a ferromagnetic spin valve,"Superconductivity and ferromagnetism are antagonistic forms of order, and rarely coexist. Many interesting new phenomena occur, however, in hybrid superconducting/ferromagnetic systems. For example, a Josephson junction containing a ferromagnetic material can exhibit an intrinsic phase shift of pi in its ground state for certain thicknesses of the material. Such ""pi-junctions"" were first realized experimentally in 2001, and have been proposed as circuit elements for both high-speed classical superconducting computing and for quantum computing. Here we demonstrate experimentally that the phase state of a Josephson junction containing two ferromagnetic layers can be toggled between 0 and pi by changing the relative orientation of the two magnetizations. These controllable 0-pi junctions have immediate applications in cryogenic memory where they serve as a necessary component to an ultra-low power superconducting computer. Such a fully superconducting computer is estimated to be orders of magnitude more energy-efficient than current semiconductor-based supercomputers. Phase controllable junctions also open up new possibilities for superconducting circuit elements such as superconducting ""programmable logic,"" where they could function in superconducting analogs to field-programmable gate arrays.",1509.05368v1 2016-01-08,Spin-glass like dynamics of ferromagnetic clusters in La$_{0.75}$Ba$_{0.25}$CoO$_3$,"We report the magnetization study of the compound La$_{0.75}$Ba$_{0.25}$CoO$_3$ where Ba$^{2+}$ doping is just above the critical limit for percolation of ferromagnetic clusters. The field cooled (FC) and zero field cooled (ZFC) magnetization exhibit a thermomagnetic irreversibility and the ac susceptibility show a frequency dependent peak at the ferromagnetic ordering temperature (T$_C$$\approx$203~K) of the clusters. These features indicate about the presence of a non-equilibrium state below T$_C$. In the non-equilibrium state, the dynamic scaling of the imaginary part of ac susceptibility and the static scaling of the nonlinear susceptibility clearly establish a spin-glass like cooperative freezing of ferromagnetic clusters at 200.9(2)~K. The existence of spin-glass like freezing of ferromagnetic clusters is further substantiated by the ZFC aging and memory experiments. We also observe certain dynamical features which are not present in a typical spin-glass, such as, initial magnetization after ZFC aging first increases and then decreases with the wait time and an imperfect recovery of relaxation in negative temperature cycling experiments. This imperfect recovery transforms to perfect recovery on concurrent field cycling. Our analysis suggests that these additional dynamical features have their origin in inter-cluster exchange interaction and cluster size distribution. The inter-cluster exchange interaction above the magnetic percolation gives a superferromagnetic state in some granular thin films but our results show the absence of typical superferromagnetic like state in La$_{0.75}$Ba$_{0.25}$CoO$_3$.",1601.02009v1 2016-01-15,X-ray absorption study of the ferromagnetic Cu moment at the $\mathbf{{YBa_2Cu_3O_7}/{La_{2/3}Ca_{1/3}MnO_3}}$ interface and the variation of its exchange interaction with the Mn moment,"With x-ray absorption spectroscopy and polarized neutron reflectometry we studied how the magnetic proximity effect at the interface between the cuprate high-$T_C$ superconductor $\mathrm{YBa_2Cu_3O_7}$ (YBCO) and the ferromagnet $\mathrm{La_{2/3}Ca_{1/3}MnO_3}$ (LCMO) is related to the electronic and magnetic properties of the LCMO layers. In particular, we explored how the magnitude of the ferromagnetic Cu moment on the YBCO side depends on the strength of the antiferromagnetic (AF) exchange coupling with the Mn moment on the LCMO side. We found that the Cu moment remains sizeable if the AF coupling with the Mn moments is strongly reduced or even entirely suppressed. The ferromagnetic order of the Cu moments thus seems to be intrinsic to the interfacial CuO$_2$ planes and related to a weakly ferromagnetic intra-planar exchange interaction. The latter is discussed in terms of the partial occupation of the Cu $3d_{3z^2-r^2}$ orbitals, which occurs in the context of the so-called orbital reconstruction of the interfacial Cu ions.",1601.03844v2 2016-07-01,The Chiral Potts Spin Glass in d=2 and 3 Dimensions,"The chiral spin-glass Potts system with q=3 states is studied in d=2 and 3 spatial dimensions by renormalization-group theory and the global phase diagrams are calculated in temperature, chirality concentration p, and chirality-breaking concentration c, with determination of phase chaos and phase-boundary chaos. In d=3, the system has ferromagnetic, left-chiral, right-chiral, chiral spin-glass, and disordered phases. The phase boundaries to the ferromagnetic, left- and right-chiral phases show, differently, an unusual, fibrous patchwork (microreentrances) of all four (ferromagnetic, left-chiral, right-chiral, chiral spin-glass) ordered ordered phases, especially in the multicritical region. The chaotic behavior of the interactions, under scale change, are determined in the chiral spin-glass phase and on the boundary between the chiral spin-glass and disordered phases, showing Lyapunov exponents in magnitudes reversed from the usual ferromagnetic-antiferromagnetic spin-glass systems. At low temperatures, the boundaries of the left- and right-chiral phases become thresholded in p and c. In the d=2, the chiral spin-glass system does not have a spin-glass phase, consistently with the lower-critical dimension of ferromagnetic-antiferromagnetic spin glasses. The left- and right-chirally ordered phases show reentrance in chirality concentration p.",1607.00112v2 2017-02-27,Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers,"In a number of recent experiments, it has been shown that femtosecond laser pulses can control magnetization on picosecond timescales, which is at least an order of magnitude faster compared to conventional magnetization dynamics. Among these demonstrations, one material system (GdFeCo ferromagnetic films) is particularly interesting, as deterministic toggle-switching of the magnetic order has been achieved without the need of any symmetry breaking magnetic field. This phenomenon is often referred to as all optical switching (AOS). However, so far, GdFeCo remains the only material system where such deterministic switching has been observed. When extended to ferromagnetic systems, which are of greater interest in many technological applications, only a partial effect can be achieved, which in turn requires repeated laser pulses for full switching. However, such repeated pulsing is not only energy hungry, it also negates the speed advantage of AOS. Motivated by this problem, we have developed a general method for single-shot, picosecond timescale, complete all optical switching of ferromagnetic materials. We demonstrate that in exchange-coupled layers of Co/Pt and GdFeCo, single shot, switching of the ferromagnetic Co/Pt layer is achieved within 7 picoseconds after irradiation by a femtosecond laser pulse. We believe that this approach will greatly expand the range of materials and applications for ultrafast magnetic switching.",1702.08491v3 2018-04-12,"The melilite-type compound (Sr$_{1-x}$,$A_x$)$_2$MnGe$_2$S$_6$O ($A$=K, La) being a room temperature ferromagnetic semiconductor","The seeking of room temperature ferromagnetic semiconductors, which take advantages of both the charge and spin degrees of freedom of electrons to realize a variety of functionalities in devices integrated with electronic, optical, and magnetic storage properties, has been a long-term goal of scientists and engineers. Here, by using the spin-polarized density functional theory calculations, we predict a new series of high temperature ferromagnetic semiconductors based on the melilite-type oxysulfide Sr$_2$MnGe$_2$S$_6$O through hole (K) and electron (La) doping. Due to the lack of strong antiferromagnetic superexchange between Mn ions, the weak antiferromagnetic order in the parent compound Sr$_2$MnGe$_2$S$_6$O can be suppressed easily by charge doping with either $p$-type or $n$-type carriers, giving rise to the expected ferromagnetic order. At a doping concentration of 25%, both the hole-doped and electron-doped compounds can achieve a Curie temperature ($T_\text{c}$) above 300 K. The underlying mechanism is analyzed. Our study provides an effective approach for exploring new types of high temperature ferromagnetic semiconductors.",1804.04519v1 2018-11-02,Spinning superconductors and ferromagnets,"When a magnetic field is applied to a ferromagnetic body it starts to spin (Einstein-de Haas effect). This demonstrates the intimate connection between the electron's magnetic moment $\mu_B=e\hbar/2m_ec$, associated with its spin angular momentum $S=\hbar/2$, and ferromagnetism. When a magnetic field is applied to a superconducting body it also starts to spin (gyromagnetic effect), and when a normal metal in a magnetic field becomes superconducting and expels the magnetic field (Meissner effect) the body also starts to spin. Yet according to the conventional theory of superconductivity the electron's spin only role is to label states, and the electron's magnetic moment plays no role in superconductivity. Instead, within the unconventional theory of hole superconductivity, the electron's spin and associated magnetic moment play a fundamental role in superconductivity. Just like in ferromagnets the magnetization of superconductors is predicted to result from an aggregation of magnetic moments with angular momenta $\hbar/2$. This gives rise to a ""Spin Meissner effect"", the existence of a spin current in the ground state of superconductors. The theory explains how a superconducting body starts spinning when it expels magnetic fields, it provides a dynamical explanation for the Meissner effect, and it explains how supercurrents stop without dissipation, all of which we argue the conventional theory fails to explain. Essential elements of the theory of hole superconductivity are that superconductivity is driven by lowering of kinetic energy, which we have also proposed is true for ferromagnets], that the normal state charge carriers in superconducting materials are holes, and that the spin-orbit interaction plays a key role in superconductivity. The theory is proposed to apply to all superconductors.",1811.03938v1 2018-11-23,Broadening Frequency Range of a Ferromagnetic Axion Haloscope with Strongly Coupled Cavity-Magnon Polaritons,"With the axion being a prime candidate for dark matter, there has been some recent interest in direct detection through a so called `Ferromagnetic haloscope.' Such devices exploit the coupling between axions and electrons in the form of collective spin excitations of magnetic materials with the readout through a microwave cavity. Here, we present a new, general, theoretical treatment of such experiments in a Hamiltonian formulation for strongly coupled magnons and photons, which hybridise as cavity-magnon polaritons. Such strongly coupled systems have an extended measurable dispersive regime. Thus, we extend the analysis and operation of such experiments into the dispersive regime, which allows any ferromagnetic haloscope to achieve improved bandwidth with respect to the axion mass parameter space. This experiment was implemented in a cryogenic setup, and initial search results are presented setting laboratory limits on the axion-electron coupling strength of $g_{aee}>3.7\times10^{-9}$ in the range $33.79~\mu$eV$< m_a<33.94~\mu$eV with $95\%$ confidence. The potential bandwidth of the Ferromagnetic haloscope was calculated to be in two bands, the first of about $1$GHz around $8.24$GHz (or $4.1~\mu$eV mass range around $34.1~\mu$eV) and the second of about $1.6$GHz around $10$GHz ($6.6~\mu$eV mass range around $41.4~\mu$eV). Frequency tuning may also be easily achieved via an external magnetic field which changes the ferromagnetic resonant frequency with respect to the cavity frequency. The requirements necessary for future improvements to reach the DFSZ axion model band are discussed in the paper.",1811.09348v3 2013-08-25,Enhanced ferromagnetic moment in Co-doped BiFeO3 thin films studied by soft X-ray circular dichroism,"BiFeO$_3$ (BFO) shows both ferroelectricity and magnetic ordering at room temperature but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferromagnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe$_{1-x}$Co$_x$O$_3$ ({\it x} = 0 to 0.30) (BFCO) thin films grown on LaAlO$_3$(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe$^{3+}$ and Co$^{3+}$ states. The XMCD results showed that the Fe ions show ferromagnetism while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe$^{3+}$ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe$^{3+}$ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic $\gamma$-Fe$_2$O$_3$ which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe$^{3+}$ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe$^{3+}$ ions.",1308.5672v1 2014-03-03,Pressure-induced ferromagnetism with strong Ising-type anisotropy in YbCu$_2$Si$_2$,"We report dc magnetic measurements on YbCu$_2$Si$_2$ at pressures above 10 GPa using a miniature ceramic anvil cell. YbCu$_2$Si$_2$ shows a pressure-induced transition from the non-magnetic to a magnetic phase at 8 GPa. We find a spontaneous dc magnetization in the pressure-induced phase above 9.4 GPa. The pressure dependence of the ferromagnetic transition temperature T_C and the spontaneous magnetic moment m_0 at 2.0 K have been determined. The value of m_0 in the present macroscopic measurement is less than half of that determined via Mossbauer experiment. The difference may be attributed to spatial phase separation between the ferromagnetic and paramagnetic phases. This separation suggests that the pressure-induced phase boundary between the paramagnetic and ferromagnetic states is of first order. Further, we have studied the magnetic anisotropy in the pressure-induced ferromagnetic state. The effect of pressure on the magnetization with magnetic field along the magnetic easy $c$-axis is much larger than for field along the hard $a$-axis in the tetragonal structure. The pressure-induced phase has strong Ising-type uniaxial anisotropy, consistent with the two crystal electric field (CEF) models proposed for YbCu$_2$Si$_2$.",1403.0290v2 2016-11-01,Thermoelectric properties of a ferromagnet-superconductor hybrid junction: Role of interfacial Rashba spin-orbit interaction,"We investigate thermoelectric properties of a ferromagnet-superconductor hybrid structure with Rashba spin-orbit interaction and delta function potential barrier at the interfacial layer. The exponential rise of thermal conductance with temperature manifests a cross-over temperature scale separating two opposite behaviors of it with the change of polarization in the ferromagnet whereas the inclusion of interfacial Rashba spin-orbit field results in a non-monotonic behavior of it with the strength of Rashba field. We employ scattering matrix approach to determine the amplitudes of all the scattering processes possible at the interface to explain the thermoelectric properties of the device. We examine Seebeck effect and show that higher thermopower can be achieved when the polarization of the ferromagnet tends towards the half-metallic limit. It can be enhanced even for lower polarization in presence of the finite potential barrier. In presence of interfacial Rashba spin-orbit interaction, Seebeck coefficient rises with the increase of barrier strength and polarization at weak or moderate interfacial Rashba field. From the application perspective, we compute the figure of merit and show that $zT\sim 4-5$ with higher polarization of the ferromagnet both in absence and presence of weak or moderate Rashba spin-orbit interaction along with the scalar potential barrier.",1611.00353v3 2016-11-03,Tricritical wings and modulated magnetic phases in LaCrGe$ _{3}$ under pressure,"We determined on the temperature-pressure-magnetic field ($T$-$p$-$H$) phase diagram of the ferromagnet LaCrGe$_3$ from electrical resistivity measurements on single crystals. In ferromagnetic systems, quantum criticality is avoided either by a change of the transition order, becoming of the first order at a tricritical point, or by the appearance of modulated magnetic phases. In the first case, the application of a magnetic field reveals a wing-structure phase diagram as seen in itinerant ferromagnets such as ZrZn$_2$ and UGe$_2$. In the second case, no tricritical wings have been observed so far. Our investigation of LaCrGe$_3$ reveals a double-wing structure indicating strong similarities with ZrZn$_2$ and UGe$_2$. But, unlike these, simpler systems, LaCrGe$_3$ is thought to exhibit a modulated magnetic phase under pressure which already precludes it from a pressure-driven paramagnetic-ferromagnetic quantum phase transition in zero field. As a result, the $T$-$p$-$H$ phase diagram of LaCrGe$_3$ shows both the wing structure as well as the appearance of new magnetic phases, providing the first example of this new possibility for the phase diagram of metallic quantum ferromagnets.",1611.01212v1 2017-04-26,Ultra-small moment incommensurate spin density wave order masking a ferromagnetic quantum critical point in NbFe$_2$,"In the metallic magnet Nb$_{1-y}$Fe$_{2+y}$, the low temperature threshold of ferromagnetism can be investigated by varying the Fe excess $y$ within a narrow homogeneity range. We use elastic neutron scattering to track the evolution of magnetic order from Fe-rich, ferromagnetic Nb$_{0.981}$Fe$_{2.019}$ to approximately stoichiometric NbFe$_2$, in which we can, for the first time, characterise a long-wavelength spin density wave state burying a ferromagnetic quantum critical point. The associated ordering wavevector $\mathbf{q}_{\rm SDW}=$(0,0,$l_{\rm SDW}$) is found to depend significantly on $y$ and $T$, staying finite but decreasing as the ferromagnetic state is approached. The phase diagram follows a two order-parameter Landau theory, for which all the coefficients can now be determined. Our findings suggest that the emergence of SDW order cannot be attributed to band structure effects alone. They indicate a common microscopic origin of both types of magnetic order and provide strong constraints on related theoretical scenarios based on, e.g., quantum order by disorder.",1704.08379v2 2008-07-09,High temperature ferromagnetism in Co-implanted TiO2 rutile,"We report on structural, magnetic and electronic properties of Co-implanted TiO2 rutile single crystals for different implantation doses. Strong ferromagnetism at room temperature and above is observed in TiO2 rutile plates after cobalt ion implantation, with magnetic parameters depending on the cobalt implantation dose. While the structural data indicate the presence of metallic cobalt clusters, the multiplet structure of the Co L3 edge in the XAS spectra gives clear evidence for a substitutional Co 2+ state. The detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted TiO2 plates. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the rutile phase, and the second one is a superparamagnetic phase originates from the formation of metallic cobalt clusters in the implanted region. Using x-ray resonant magnetic scattering, the element specific magnetization of cobalt, oxygen and titanium in Co-implanted TiO2 single crystals are investigated. Magnetic dichroism was observed at the Co L edges as well as at the O K edge. The interaction mechanism, which leads to ferromagnetic ordering of substituted cobalt ions in the host matrix, is also discussed.",0807.1555v1 2011-11-13,Thermal spin pumping and magnon-phonon-mediated spin-Seebeck effect,"The spin-Seebeck effect (SSE) in ferromagnetic metals and insulators has been investigated systematically by means of the inverse spin-Hall effect (ISHE) in paramagnetic metals. The SSE generates a spin voltage as a result of a temperature gradient in a ferromagnet, which injects a spin current into an attached paramagnetic metal. In the paramagnet, this spin current is converted into an electric field due to the ISHE, enabling the electric detection of the SSE. The observation of the SSE is performed in longitudinal and transverse configurations consisting of a ferromagnet/paramagnet hybrid structure, where thermally generated spin currents flowing parallel and perpendicular to the temperature gradient are detected, respectively. Our results explain the SSE in terms of a two-step process: (1) the temperature gradient creates a non-equilibrium state in the ferromagnet governed by both magnon and phonon propagations and (2) the non-equilibrium between magnons in the ferromagnet and electrons in the paramagnet at the contact interface leads to ""thermal spin pumping"" and the ISHE signal. The non-equilibrium state of metallic magnets (e.g. Ni81Fe19) under a temperature gradient is governed mainly by the phonons in the sample and the substrate, while in insulating magnets (e.g. Y3Fe5O12) both magnon and phonon propagations appear to be important. The phonon-mediated non-equilibrium that drives the thermal spin pumping is confirmed also by temperature-dependent measurements, giving rise to a giant enhancement of the SSE signals at low temperatures.",1111.3036v2 2011-11-30,Correlation Decay up to Uniqueness in Spin Systems,"We give a complete characterization of the two-state anti-ferromagnetic spin systems which are of strong spatial mixing on general graphs. We show that a two-state anti-ferromagnetic spin system is of strong spatial mixing on all graphs of maximum degree at most $\Delta$ if and only if the system has a unique Gibbs measure on infinite regular trees of degree up to $\Delta$, where $\Delta$ can be either bounded or unbounded. As a consequence, there exists an FPTAS for the partition function of a two-state anti-ferromagnetic spin system on graphs of maximum degree at most $\Delta$ when the uniqueness condition is satisfied on infinite regular trees of degree up to $\Delta$. In particular, an FPTAS exists for arbitrary graphs if the uniqueness is satisfied on all infinite regular trees. This covers as special cases all previous algorithmic results for two-state anti-ferromagnetic systems on general-structure graphs. Combining with the FPRAS for two-state ferromagnetic spin systems of Jerrum-Sinclair and Goldberg-Jerrum-Paterson, and the hardness results of Sly-Sun and independently of Galanis-Stefankovic-Vigoda, this gives a complete classification, except at the phase transition boundary, of the approximability of all two-state spin systems, on either degree-bounded families of graphs or family of all graphs.",1111.7064v3 2016-05-10,Photoenhanced spin/valley polarization and tunneling magnetoresistance in ferromagnetic-normal-ferromagnetic silicene junction,"We theoretically demonstrate a simple way to significantly enhance the spin/valley polarizations and tunnel- ing magnetoresistnace (TMR) in a ferromagnetic-normal-ferromagnetic (FNF) silicene junction by applying a circularly polarized light in off-resonant regime to the second ferromagnetic (FM) region. We show that the fully spin-polarized current can be realized in certain ranges of light intensity. Increasing the incident energy in the presence of light will induce a transition of perfect spin polarization from positive to negative or vice versa depending on magnetic configuration (parallel or anti-parallel) of FNF junction. Additionally, under a circularly polarized light, valley polarization is very sensitive to electric field and the perfect valley polarization can be achieved even when staggered electric field is much smaller than exchange field. The most important result we would like to emphasize in this paper is that the perfect spin polarization and 100% TMR induced by a circularly polarized light are completely independent of barrier height in normal region. Furthermore, the sign reversal of TMR can be observed when the polarized direction of light is changed. A condition for observing the 100% TMR is also reported. Our results are expected to be informative for real applications of FNF silicene junction, especially in spintronics.",1605.02829v1 2016-05-14,Superconductivity and Ferromagnetism in Hole-Doped RbEuFe$_4$As$_4$,"We discover a robust coexistence of superconductivity and ferromagnetism in an iron arsenide RbEuFe$_4$As$_4$. The new material crystallizes in an intergrowth structure of RbFe$_2$As$_2$ and EuFe$_2$As$_2$, such that the Eu sublattice turns out to be primitive instead of being body-centered in EuFe$_2$As$_2$. The FeAs layers, featured by asymmetric As coordinations, are hole doped due to charge homogenization. Our combined measurements of electrical transport, magnetization and heat capacity unambiguously and consistently indicate bulk superconductivity at 36.5 K in the FeAs layers and ferromagnetism at 15 K in the Eu sublattice. Interestingly, the Eu-spin ferromagnetic ordering belongs to a rare third-order transition, according to the Ehrenfest classification of phase transition. We also identify an additional anomaly at $\sim$ 5 K, which is possibly associated with the interplay between superconductivity and ferromagnetism.",1605.04396v3 2017-07-20,Optical manifestation of the Stoner ferromagnetic transition in 2D electron systems,"We perform a magneto-optical study of a two-dimensional electron systems (2DES) in the regime of the Stoner ferromagnetic instability for even quantum Hall filling factors on Mg$_x$Zn$_{1-x}$O/ZnO heterostructures. Under conditions of Landau-level crossing, caused by enhanced spin susceptibility in combination with the tilting of the magnetic field, the transition between two rivaling phases- paramagnetic and ferromagnetic- is traced in terms of optical spectra reconstruction. Synchronous sharp transformations are observed both in the photoluminescence structure and parameters of collective excitations upon transition from paramagnetic to ferromagnetic ordering. Based on these measurements, a phase diagram is constructed in terms of the 2D electron density and tilt angle of the magnetic field. Apart from stable paramagnetic and ferromagnetic phases, an instability region is found at intermediate parameters with the Stoner transition occurring at $\nu\approx 2$. The spin configuration in all cases is unambiguously determined by means of inelastic light scattering by spin-sensitive collective excitations. One indicator of the spin ordering is the intra-Landau-level spin exciton, which acquires a large spectral weight in the ferromagnetic phases. The other - is an abrupt energy shift of the intersubband charge density excitation due to change in the many-particle energy contribution upon spin rearrangement. From our analysis of photoluminescence and light scattering data, we estimate the ratio of surface areas occupied by the domains of the two phases in the vicinity of a transition point. In addition, the thermal smearing of a phase transition is characterized.",1707.06421v1 2018-09-21,Hybrid superlattices of graphene and hexagonal boron nitride: A ferromagnetic semiconductor at room temperature,"Carbon (C) doped hexagonal boron nitride (hBN) has been experimentally reported to be ferromagnetic at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work we derive a mechanistic understanding of ferromagnetism with graphene islands in hBN from first principles and mean-field Hubbard model. We find a general property, that in bipartite lattices where the sublattices differ in on-site energies, as in hBN, the ordering between local magnetic moments can be substantial and predominantly anti-ferromagnetic (AFM) if they are embedded in the same sublattice, unless dominated by Mott like inter-sublattice spin separation due to strong localization. The dominant AFM order is rooted at spin resolved spatial separation of lone pairs of nitrogen (N) and back transferred electrons on boron (B) due to Coulomb repulsion thus essentially implying a super-exchange pathway. Subsequently we propose a class of ferri-magnetically ordered inter-penetrating super-lattices of magnetic graphene islands in hBN, which can be chosen to be a ferromagnetic semiconductor or a half-metal, and retain a net non-zero magnetic moment at room temperature.",1809.08270v3 2018-10-29,Ferromagnetism and spin excitations in topological Hubbard models with a flatband,"We study the spin-1 excitation spectra of the flatband ferromagnetic phases in interacting topological insulators. As a paradigm, we consider a quarter filled square lattice Hubbard model whose free part is the $\pi$ flux state with topologically nontrivial and nearly-flat electron bands, which can realize either the Chern or $Z_2$ Hubbard model. By using the numerical exact diagonalization method with a projection onto the nearly-flat band, we obtain the ferromagnetic spin-1 excitation spectra for both the Chern and $Z_2$ Hubbard models, consisting of spin waves and Stoner continuum. The spectra exhibit quite distinct dispersions for both cases, in particular the spin wave is gapless for the Chern Hubbard model, while gapped for the $Z_2$ Hubbard model. Remarkably, in both cases, the nonflatness of the free electron bands introduces dips in the lower boundary of the Stoner continuum. It significantly renormalizes the energies of the spin waves around these dips downward and leads to roton-like spin excitations. We elaborate that it is the softening of the roton-like modes that destabilizes the ferromagnetic phase, and determine the parameter region where the ferromagnetic phase is stable.",1810.11986v2 2019-06-17,Imaging uncompensated moments and exchange-biased emergent ferromagnetism in FeRh thin films,"Uncompensated moments in antiferromagnets are responsible for exchange bias in antiferromagnet/ferromagnet heterostructures; however, they are difficult to directly detect because any signal they contribute is typically overwhelmed by the ferromagnetic layer. We use magneto-thermal microscopy to image uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a 1st-order phase transition to a ferromagnetic state near 100~$^\circ$C. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (10-15~$^\circ$C) temperature range near $T_C$. In the AF phase below $T_C$, we image both pinned UMs, which cause local vertical exchange bias, and unpinned UMs, which exhibit an enhanced coercive field that reflects exchange-coupling to the AF bulk. Near $T_C$, where AF and FM order coexist, we find that the emergent FM order is exchange-coupled to the bulk N\'eel order. This exchange coupling leads to the nucleation of unusual configurations in which different FM domains are pinned parallel, antiparallel, and perpendicular to the applied magnetic field before suddenly collapsing into a state uniformly parallel to the field.",1906.07243v1 2019-06-26,In Quest of a Ferromagnetic Insulator -- Structure Controlled Magnetism in Mg-Ti-O Thin Films,"Ferromagnetic insulator thin films can convey information by spin waves, avoiding charge displacement and Eddy current losses. The sparsity of high-temperature insulating ferromagnetic materials hinders the development of spin wave based devices. Stoichiometric magnesium titanate, MgTiO$_3$, has an electronic-energy-band structure in which all bands are either full or empty, being a paramagnetic insulator. The MgTiO$_3$ ilmenite consists of ordered octahedra and cation network in which one third of the octahedra are vacant, one third host magnesium and one third titanium. By giving up these characteristics, a rich variety of different magnetic structures can be formed. Our experiments and electronic-energy-band-structure computations show that the magnetic and electric properties of Mg-Ti-O films can drastically be changed and controlled by Mg- and Ti-cation arrangement and abundancy in the octahedra. Insulating titanium- and semiconducting magnesium-rich films were ferromagnetic up to elevated temperatures. The presence and origin of ferromagnetic insulating phase in the films is not apparent - the expectation, based on the well-established rules set by Goodenough and Kanamori, is paramagnetic or antiferromagnetic ordering. We show that ferro- and paramagnetic phases, possessing the same stoichiometry, can be obtained by merely rearranging the cations, thus allowing defect-free interfaces in multilayer structures.",1906.10975v1 2019-09-09,Ferromagnetic resonance assisted optomechanical magnetometer,"The resonant enhancement of mechanical and optical interaction in optomechanical cavities enables their use as extremely sensitive displacement and force detectors. In this work we demonstrate a hybrid magnetometer that exploits the coupling between the resonant excitation of spin waves in a ferromagnetic insulator and the resonant excitation of the breathing mechanical modes of a glass microsphere deposited on top. The interaction is mediated by magnetostriction in the ferromagnetic material and the consequent mechanical driving of the microsphere. The magnetometer response thus relies on the spectral overlap between the ferromagnetic resonance and the mechanical modes of the sphere, leading to a peak sensitivity better than 900 pT Hz$^{-1/2}$ at 206 MHz when the overlap is maximized. By externally tuning the ferromagnetic resonance frequency with a static magnetic field we demonstrate sensitivity values at resonance around a few nT Hz$^{-1/2}$ up to the GHz range. Our results show that our hybrid system can be used to build high-speed sensor of oscillating magnetic fields.",1909.03924v4 2019-10-28,"Realization of an intrinsic, ferromagnetic topological state in MnBi8Te13","The interplay between topology and magnetism is essential for realizing novel topological states including the axion insulator, the magnetic Weyl semimetal, etc. An intrinsically ferromagnetic topological material with only the topological bands at the charge neutrality energy has so far remained elusive. By rationally designing the natural heterostructure consisting of [MnBi2Te4] septuple layers and [Bi2Te3] quintuple layers, we report MnBi8Te13 as the first intrinsic ferromagnetic topological material with clean low-energy band structure. Based on the thermodynamic, transport and neutron diffraction measurements, our data show that despite the adjacent [MnBi2Te4] being 44.1 {\AA} apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. Our first-principles calculations and angle-resolved photoemission spectroscopy measurements further demonstrate that MnBi8Te13 is an intrinsic ferromagnetic axion state. Therefore, MnBi8Te13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized topological magnetoelectric effect.",1910.12847v2 2020-01-09,"Hybridization between the ligand $p$ band and Fe-3$d$ orbitals in the p-type ferromagnetic semiconductor (Ga,Fe)Sb","(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature ($T_{\rm C}$) is above room temperature. However, the origin of ferromagnetism with high $T_{\rm C}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of (Ga$_{0.95}$,Fe$_{0.05}$)Sb including the Fe-3$d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in (Ga$_{0.95}$,Fe$_{0.05}$)Sb observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The Fe-3$d$ resonant ARPES spectra demonstrate that the Fe-3$d$ IB crosses the Fermi level ($E_{\rm F}$) and hybridizes with the VB of GaSb. These observations indicate that the VB structure of (Ga$_{0.95}$,Fe$_{0.05}$)Sb is consistent with that of the IB model which is based on double-exchange interaction between the localized 3$d$ electrons of the magnetic impurities. The results indicate that the ferromagnetism in (Ga,Fe)Sb is formed by the hybridization of the Fe-3$d$ IB with the ligand $p$ band of GaSb.",2001.02895v1 2020-02-02,Broadband Magnetoresistance in Ferromagnetic and Paramagnetic Samples of La0.7Ca0.3-xSrxMnO3,"We have studied the room-temperature magnetoimpedance of paramagnetic (x = 0.06) and ferromagnetic (x = 0.1) samples in La0.7Ca0.3-xSrxMnO3 series using a radio-frequency impedance analyzer and also microwave power absorption using a network analyzer. In both measurements, samples were enclosed tightly inside a copper stripcoil and impedance or reflection coefficient of this copper stripcoil was measured as a function of the applied magnetic field for different frequencies of current (f = 0.1 to 2.5 GHz). The direction of the applied magnetic field was perpendicular to the alternating magnetic field produced by the coil. In the ferromagnetic sample (x = 0.1), magnetoresistance shows a peak around zero field for lower frequencies but a peak appears at H away from the origin at higher frequencies. The position of the peak shifts towards higher fields with increasing frequency. A similar trend is also found for the paramagnetic sample (x = 0.06) but the peak occurs at a higher field compared to the ferromagnetic sample for the same frequency. Microwave power absorption also shows features similar to magnetoresistance. Line shape analysis of the data was performed by fitting the data to a Lorentzian function. It is concluded that the observed features are imprints of ferromagnetic resonance in x = 0.1 and paramagnetic resonance in x = 0.06 samples.",2002.00356v1 2020-02-12,Artificial Multiferroics and Enhanced Magnetoelectric Effect in van der Waals Heterostructures,"Multiferroic materials with coupled ferroelectric and ferromagnetic properties are important for multifunctional devices due to their potential ability of controlling magnetism via electric field, and vice versa. The recent discoveries of two-dimensional ferromagnetic and ferroelectric materials have ignited tremendous research interest and aroused hope to search for two-dimensional multiferroics. However, intrinsic two-dimensional multiferroic materials and, particularly, those with strong magnetoelectric couplings are still rare to date. In this paper, using first-principles simulations, we propose artificial two-dimensional multiferroics via a van der Waals heterostructure formed by ferromagnetic bilayer chromium triiodide (CrI3) and ferroelectric monolayer Sc2CO2. In addition to the coexistence of ferromagnetism and ferroelectricity, our calculations show that, by switching the electric polarization of Sc2CO2, we can tune the interlayer magnetic couplings of bilayer CrI3 between ferromagnetic and antiferromagnetic states. We further reveal that such a strong magnetoelectric effect is from a dramatic change of the band alignment induced by the strong build-in electric polarization in Sc2CO2 and the subsequent change of the interlayer magnetic coupling of bilayer CrI3. These artificial multiferroics and enhanced magnetoelectric effect give rise to realizing multifunctional nanoelectronics by van der Waals heterostructures.",2002.05213v1 2021-01-12,Van der Waals Ferromagnetic Josephson Junctions,"Superconductor-ferromagnet (S-F) interfaces in two-dimensional (2D) heterostructures present a unique opportunity to study the interplay between superconductivity and ferromagnetism. The realization of such nanoscale heterostructures in van der Waals (vdW) crystals remains largely unexplored due to the challenge of making an atomically-sharp interface from their layered structures. Here, we build a vdW ferromagnetic Josephson junction (JJ) by inserting a few-layer ferromagnetic insulator Cr2Ge2Te6 into two layers of superconductor NbSe2. Owing to the remanent magnetic moment of the barrier, the critical current and the corresponding junction resistance exhibit a hysteretic and oscillatory behavior against in-plane magnetic fields, manifesting itself as a strong Josephson coupling state. Through the control of this hysteresis, we can effectively trace the magnetic properties of atomic Cr2Ge2Te6 in response to the external magnetic field. Also, we observe a central minimum of critical current in some thick JJ devices, evidencing the coexistence of 0 and {\pi} phase coupling in the junction region. Our study paves the way to exploring the sensitive probes of weak magnetism and multifunctional building blocks for phase-related superconducting circuits with the use of vdW heterostructures.",2101.04323v1 2017-09-07,Electron Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures,"Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO3/SrTiO3 (001) heterostructures. Using a combination of element-specific X-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO3. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron over-accumulation. In turn, by controlling the doping of the LaMnO3, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO3 films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.",1709.02035v1 2017-11-15,Phase diagrams and multistep condensations of spin-1 bosonic gases in optical lattices,"Motivated by recent experimental processes, we systemically investigate strongly correlated spin-1 ultracold bosons trapped in a three-dimensional optical lattice in the presence of an external magnetic field. Based on a recently developed bosonic dynamical mean-field theory (BDMFT), we map out complete phase diagrams of the system for both antiferromagnetic and ferromagnetic interactions, where various phases are found as a result of the interplay of spin-dependent interaction and quadratic Zeeman energy. For antiferromagnetic interactions, the system demonstrates competing magnetic orders, including nematic, spin-singlet and ferromagnetic insulating phase, depending on longitudinal magnetization, whereas, for ferromagnetic case, a ferromagnetic-to-nematic-insulating phase transition is observed for small quadratic Zeeman energy, and the insulating phase demonstrates the nematic order for large Zeeman energy. Interestingly, at low magnetic field and finite temperature, we find an abnormal multi-step condensation of the strongly correlated superfluid, i.e. the critical condensing temperature of the $m_F=-1$ component with antiferromagnetic interactions demonstrates an increase with longitudinal magnetization, while, for ferromagnetic case, the Zeeman component $m_F = 0$ demonstrates a local minimum for the critical condensing temperature, in contrast to weakly interacting cases.",1711.05470v2 2017-11-28,Nanoclustering phase competition induces the resistivity hump in colossal magnetoresistive manganites,"Using a two-band double-exchange model with Jahn-Teller lattice distortions and super-exchange interactions, supplemented by quenched disorder, at electron density $n=0.65$, we explicitly demonstrate the coexistence of the $n$ = 1/2-type ($\pi, \pi$) charge-ordered and the ferromagnetic nanoclusters above the ferromagnetic transition temperature $T_{\rm c}$ in colossal magnetoresistive (CMR) manganites. The resistivity increases due to the enhancement of the volume fraction of the charge-ordered and the ferromagnetic nanoclusters with decreasing the temperature down to $T_{\rm c}$. The ferromagnetic nanoclusters start to grow and merge, and the volume fraction of the charge-ordered nanoclusters decreases below $T_{\rm c}$, leading to the sharp drop in the resistivity. By applying a small external magnetic field $h$, we show that the resistivity above $T_{\rm c}$ increases, as compared with the case when $h=0$, a fact which further confirms the coexistence of the charge-ordered and the ferromagnetic nanoclusters. In addition, we show that the volume fraction of the charge-ordered nanoclusters decreases with increasing the bandwidth and consequently the resistivity hump diminishes for large bandwidth manganites, in good qualitative agreement with experiments. The obtained insights from our calculations provide a complete pathway to understand the phase competition in CMR manganites.",1711.10211v1 2018-03-06,Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe$_3$GeTe$_2$,"Material research has been a major driving force in the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices; identifying new magnetic materials is key to better device performance and new device paradigm. The advent of two-dimensional van der Waals crystals creates new possibilities. This family of materials retain their chemical stability and structural integrity down to monolayers and, being atomically thin, are readily tuned by various kinds of gate modulation. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr$_2$Ge$_2$Te$_6$ and CrI$_3$ at low temperatures. Here, we developed a new device fabrication technique, and successfully isolated monolayers from layered metallic magnet Fe$_3$GeTe$_2$ for magnetotransport study. We found that the itinerant ferromagnetism persists in Fe$_3$GeTe$_2$ down to monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, $T_c$, is suppressed in pristine Fe$_3$GeTe$_2$ thin flakes. An ionic gate, however, dramatically raises the $T_c$ up to room temperature, significantly higher than the bulk $T_c$ of 205 Kelvin. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe$_3$GeTe$_2$ opens up opportunities for potential voltage-controlled magnetoelectronics based on atomically thin van der Waals crystals.",1803.02038v1 2018-06-18,Anomalous Nernst effect beyond the magnetization scaling relation in the ferromagnetic Heusler compound Co$_2$MnGa,"Applying a temperature gradient in a magnetic material generates a voltage that is perpendicular to both the heat flow and the magnetization. This is the anomalous Nernst effect (ANE) which was thought to be proportional to the value of the magnetization for a long time. However, more generally, the ANE has been predicted to originate from a net Berry curvature of all bands near the Fermi level. Subsequently, a large anomalous Nernst thermopower has recently been observed in topological materials with no net magnetization but large net Berry curvature around E$_F$. These experiments clearly fall outside the scope of the conventional magnetization-model of the ANE, but a significant question remains: Can the value of the ANE in topological ferromagnets exceed the highest values observed in conventional ferromagnets? Here, we report a remarkably high anomalous Nernst thermopower value of ~6.0 \mu V/K at 1 T in the ferromagnetic topological Heusler compound Co$_2$MnGa at room temperature, which is around 7-times larger than any anomalous Nernst thermopower value ever reported for a conventional ferromagnet. Combined electrical, thermoelectric and first-principles calculations reveal that this high value of the ANE arises from a large net Berry curvature near the Fermi level associated with nodal lines and Weyl points.",1806.06753v1 2018-08-22,Coexistence of ferromagnetic and stripe-type antiferromagnetic spin fluctuations in YFe$_2$Ge$_2$,"We report neutron scattering measurements of single-crystalline YFe$_2$Ge$_2$ in the normal state, which has the same crystal structure to the 122 family of iron pnictide superconductors. YFe$_2$Ge$_2$ does not exhibit long range magnetic order, but exhibits strong spin fluctuations. Like the iron pnictides, YFe$_2$Ge$_2$ displays anisotropic stripe-type antiferromagnetic spin fluctuations at ($\pi$, $0$, $\pi$). More interesting, however, is the observation of strong spin fluctuations at the in-plane ferromagnetic wavevector ($0$, $0$, $\pi$). These ferromagnetic spin fluctuations are isotropic in the ($H$, $K$) plane, whose intensity exceeds that of stripe spin fluctuations. Both the ferromagnetic and stripe spin fluctuations remain gapless down to the lowest measured energies. Our results naturally explain the absence of magnetic order in YFe$_2$Ge$_2$ and also imply that the ferromagnetic correlations may be a key ingredient for iron-based materials.",1808.07262v1 2018-12-04,Anomalous orbital moment in the ferromagnetic phase of the Sr4Ru3O10,"The coupling of spin and orbital degrees of freedom in the trilayer Sr4Ru3O10 sets a long-standing puzzle, due to the peculiar anisotropic coexistence of out-of-plane ferromagnetism and in-plane metamagnetism. Recently, the induced magnetic structure by in-plane applied fields has been investigated by means of spin-polarized neutron diffraction, which allowed to extract a substantial orbital component of the magnetic densities at Ru sites. It has been argued that the latter is at the origin of the evident layer dependent magnetic anisotropy, where the inner layers carry larger magnetic moments than the outer ones. We present a spin-polarized neutron diffraction study in order to characterize the nature of the ferromagnetic state of Sr4Ru3O10, in the presence of a magnetic field applied along the c-axis. The components of the magnetic densities at the Ru sites reveal a vanishing contribution of the orbital magnetic moment which is unexpected for a material system where orbital and spin degeneracy are lifted by spin-orbit coupling and ferromagnetism. We employ a model that includes the Coulomb interaction and spin-orbit coupling at the Ru site to address the origin of the suppression of the orbital magnetic moment. The emerging scenario is that of non-local orbital degrees of freedom playing a significant role in the ferromagnetic phase, with the Coulomb interaction that is crucial to make anti-aligned orbital moments at short distance resulting in a ground state with vanishing local orbital moments.",1812.01359v2 2019-01-08,Ferromagnetic ordering along the hard axis in the Kondo lattice YbIr3Ge7,"Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this work, we report the discovery of a new ferromagnetic Kondo lattice compound, YbIr3Ge7. Like almost all ferromagnetic Kondo lattice systems, YbIr3Ge7 shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. YbIr3Ge7 is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. AC magnetic susceptibility, magnetization, and specific heat measurements show that YbIr3Ge7 has a Kondo temperature TK = 14 K and a Curie temperature TC = 2.4 K. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard axis ordering is unlikely. On the other hand, the broad second-order phase transition at TC favors a fluctuation-induced mechanism.",1901.02489v1 2019-01-31,Fundamental Spin Interactions Underlying the Magnetic Anisotropy in the Kitaev Ferromagnet CrI$_3$,"We lay the foundation for determining the microscopic spin interactions in two-dimensional (2D) ferromagnets by combining angle-dependent ferromagnetic resonance (FMR) experiments on high quality CrI$_3$ single crystals with theoretical modeling based on symmetries. We discover that the Kitaev interaction is the strongest in this material with $K \sim -5.2$ meV, 25 times larger than the Heisenberg exchange $J \sim -0.2$ meV, and responsible for opening the $\sim$5 meV gap at the Dirac points in the spin-wave dispersion. Furthermore, we find that the symmetric off-diagonal anisotropy $\Gamma \sim -67.5$ $\mu$eV, though small, is crucial for opening a $\sim$0.3 meV gap in the magnon spectrum at the zone center and stabilizing ferromagnetism in the 2D limit. The high resolution of the FMR data further reveals a $\mu$eV-scale quadrupolar contribution to the $S=3/2$ magnetism. Our identification of the underlying exchange anisotropies opens paths toward 2D ferromagnets with higher $T_\text{C}$ as well as magnetically frustrated quantum spin liquids based on Kitaev physics.",1902.00077v2 2019-02-21,Asymmetric Ferromagnetic Criticality in Pyrochlore Ferromagnet Lu$_2$V$_2$O$_7$,"Critical phenomenon at the phase transition reveals the universal and long-distance properties of the criticality. We study the ferromagnetic criticality of the pyrochlore magnet Lu$_2$V$_2$O$_7$ at the ferromagnetic transition ${T_\text{c}\approx 70\, \text{K}}$ from the isotherms of magnetization $M(H)$ via an iteration process and the Kouvel-Fisher method. The critical exponents associated with the transition are determined as ${\beta = 0.32(1)}$, ${\gamma = 1.41(1)}$, and ${\delta = 5.38}$. The validity of these critical exponents is further verified by scaling all the $M(H)$ data in the vicinity of $T_\text{c}$ onto two universal curves in the plot of $M/|\varepsilon|^\beta$ versus $H/|\varepsilon|^{\beta+\gamma}$, where ${\varepsilon = T/T_\text{c} -1}$. The obtained $\beta$ and $\gamma$ values show asymmetric behaviors on the ${T < T_\text{c}}$ and the ${T > T_\text{c}}$ sides, and are consistent with the predicted values of 3D Ising and cubic universality classes, respectively. This makes Lu$_2$V$_2$O$_7$ a rare example in which the critical behaviors associated with a ferromagnetic transition belong to different universality classes. We describe the observed criticality from the Ginzburg-Landau theory with the quartic cubic anisotropy that microscopically originates from the anti-symmetric Dzyaloshinskii-Moriya interaction as revealed by recent magnon thermal Hall effect and theoretical investigations.",1902.07924v1 2019-03-19,Spin-polarized Correlated Insulator and Superconductor in Twisted Double Bilayer Graphene,"Ferromagnetism and superconductivity typically compete with each other since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors. Only a handful of ferromagnetic superconductors are known in heavy fermion systems, where many-body electron interactions promoted by the narrow energy bands play a key role in stabilizing these emergent states. Recently, interaction-driven superconductivity and ferromagnetism have been demonstrated as separate phenomena in different density regimes of flat bands enabled by graphene moire superlattices. Combining superconductivity and magnetism in a single ground state may lead to more exotic quantum phases. Here, employing van der Waals heterostructures of twisted double bilayer graphene (TDBG), we realize a flat electron band that is tunable by perpendicular electric fields. Similar to the magic angle twisted bilayer graphene, TDBG exhibits energy gaps at the half and quarter filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the ferromagnetic half-filled insulator, superconductivity emerges with a critical temperature controlled by both density and electric fields. We observe that the in-plane magnetic field enhances the superconductivity in the low field regime, suggesting spin-polarized electron pairing. Spin-polarized superconducting states discovered in TDBG provide a new route to engineering interaction-driven topological superconductivity.",1903.08130v2 2019-03-26,Magnetic competition induced colossal magnetoresistance in n-type HgCr2Se4 under high pressures,"The n-type HgCr2Se4 exhibits a sharp semiconductor-to-metal transition (SMT) in resistivity accompanying the ferromagnetic order at TC = 106 K. Here, we investigate the effects of pressure and magnetic field on the concomitant SMT and ferromagnetic order by measuring resistivity, dc and ac magnetic susceptibility, as well as single-crystal neutron diffraction under various pressures up to 8 GPa and magnetic fields up to 8 T. Our results demonstrate that the ferromagnetic metallic ground state of n-type HgCr2Se4 is destabilized and gradually replaced by an antiferromagnetic, most likely a spiral magnetic, and insulating ground state upon the application of high pressure. On the other hand, the application of external magnetic fields can restore the ferromagnetic metallic state again at high pressures, resulting in a colossal magnetoresistance (CMR) as high as ~ 3 * 10^11 % under 5 T and 2 K at 4 GPa. The present study demonstrates that n-type HgCr2Se4 is located at a peculiar critical point where the balance of competion between ferromagnetic and antiferromagnetic interactions can be easily tipped by the external stimuli, providing a new platform for achieving CMR in a single-valent system.",1903.10689v1 2019-04-11,Nagaoka ferromagnetism observed in a quantum dot plaquette,"Engineered, highly-controllable quantum systems hold promise as simulators of emergent physics beyond the capabilities of classical computers. An important problem in many-body physics is itinerant magnetism, which originates purely from long-range interactions of free electrons and whose existence in real systems has been subject to debate for decades. Here we use a quantum simulator consisting of a four-site square plaquette of quantum dots to demonstrate Nagaoka ferromagnetism. This form of itinerant magnetism has been rigorously studied theoretically but has remained unattainable in experiment. We load the plaquette with three electrons and demonstrate the predicted emergence of spontaneous ferromagnetic correlations through pairwise measurements of spin. We find the ferromagnetic ground state is remarkably robust to engineered disorder in the on-site potentials and can induce a transition to the low-spin state by changing the plaquette topology to an open chain. This demonstration of Nagaoka ferromagnetism highlights that quantum simulators can be used to study physical phenomena that have not yet been observed in any system before. The work also constitutes an important step towards large-scale quantum dot simulators of correlated electron systems.",1904.05680v3 2019-08-19,Coherent Epitaxial Semiconductor-Ferromagnetic Insulator InAs/EuS Interfaces: Band Alignment and Magnetic Structure,"Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field and local proximitized magnetic exchange. In this work, we present lattice matched hybrid epitaxy of semiconductor - ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure as well as their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the bandgap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs. Induced moments in the adjacent InAs layers were not detected although our ab initio calculations indicate a small exchange field in the InAs layer. This work presents a step towards realizing high quality semiconductor - ferromagnetic insulator hybrids, which is a critical requirement for development of various quantum and spintronic applications without external magnetic fields.",1908.07096v1 2019-11-03,Optically Driven Magnetic Phase Transition of Monolayer RuCl3,"Strong light-matter interactions within nanoscale structures offer the possibility of optically controlling material properties. Motivated by the recent discovery of intrinsic long-range magnetic order in two-dimensional materials, which allows for the creation of novel magnetic devices of unprecedented small size, we predict that light can couple with magnetism and efficiently tune magnetic orders of monolayer ruthenium trichloride (RuCl3). First-principles calculations show that both free carriers and optically excited electron-hole pairs can switch monolayer RuCl3 from the proximate spin-liquid phase to a stable ferromagnetic phase. Specifically, a moderate electron-hole pair density (on the order of 10^13 cm-2) can significantly stabilize the ferromagnetic phase by 10 meV/f.u. in comparison to the zigzag phase, so that the predicted ferromagnetism can be driven by optical pumping experiments. Analysis shows that this magnetic phase transition is driven by a combined effect of doping-induced lattice strain and itinerant ferromagnetism. According to the Ising-model calculation, we find that the Curie temperature of the ferromagnetic phase can be increased significantly by raising carrier or electron-hole pair density. This enhanced opto-magnetic effect opens new opportunities to manipulate two-dimensional magnetism through non-contact, optical approaches.",1911.00979v1 2019-11-08,Effect of strain on magnetic and orbital ordering of LaSrCrO$_3$/LaSrMnO$_3$ heterostructures,"We investigate the effect of strain and film thickness on the orbital and magnetic properties of LaSrCrO$_3$ (LSCO)/LaSrMnO$_3$ (LSMO) heterostructures using bulk magnetometry, soft X-ray magnetic spectroscopy, first-principles density functional theory, high-resolution electron microscopy and X-ray diffraction. We observe an anti-parallel ordering of the magnetic moments between the ferromagnetic LSMO layers and the LSCO spacers leading to a strain-independent ferromagnetic ground state of the LSCO/LSMO heterostructures for LSMO layers as thin as 2 unit cells. As the LSMO thickness is increased, a net ferromagnetic state is maintained, however, the average magnetic moment per Mn is found to be dependent on the magnitude of the substrate-induced strain. The differences in the magnetic responses are related to preferential occupation of the Mn $x^2-y^2$ (in-plane) d-orbitals for tensile strain and $3z^2-r^2$ (out-of-plane) orbitals under compressive strain leading to competing ferromagnetic and anti-ferromagnetic exchange interactions within the LSMO layers. These results underscore the relative contributions of orbital, structural and spin degree of freedom and their tunability in atomically-thin crystalline complex oxide layers.",1911.03007v2 2019-12-10,Josephson current through a ferromagnetic bilayer: Beyond the quasiclassical approximation,"Based on the Bogoliubov-de Gennes equations, we provide an exact numerical solution for the critical current of Josephson junctions with a composite ferromagnetic bilayer. We demonstrate that for the antiparallel orientation of the magnetic moments of the bilayer, the presence of a potential barrier at the bilayer interface results in large oscillations of the critical current as a function of ferromagnet thickness and/or exchange field. Because of this, and remarkably, in the range of small exchange field and thicknesses, the magnetism leads to the increase of the critical current. This effect is well pronounced at low temperature but disappears near $T_c$. If the potential barrier is replaced by a spin-active barrier at the bilayer interface the conventional 0-$\pi$ transition, similar to the case of an uniform ferromagnetic Josephson junction, is observed. Strikingly, for a parallel orientation of the magnetic moments of the bilayer, the presence of the spin-active barrier restores the anomalous behavior---potential barrier in the antiparallel case. These behaviors result from the resonant tunneling of Cooper pairs across the composite barrier---an effect related to the spin-dependent Fermi vector in the presence of the ferromagnets' exchange field.",1912.04447v2 2020-06-23,Avoided quantum criticality and cluster-glass formation in itinerant ferromagnet Sr$_{1-x}$(La$_{0.5}$K$_{0.5}$)$_x$RuO$_3$,"We demonstrate that the cluster-glass state emerges as ferromagnetic quantum criticality is avoided in itinerant ferromagnet Sr1-x(La0.5K0.5)xRuO3. In this compound, the ferromagnetic order is suppressed by increasing x, and then disappears at the critical concentration: x=0.5. In this x range, the present study reveals that no prominent feature is ascribed to the quantum critical fluctuations in specific heat. Instead, ac magnetic susceptibility exhibits a broad peak due to spontaneous spin freezing, and the peak temperature depends significantly on the frequency of the applied ac magnetic field. Furthermore, specific heat is enhanced within a wide temperature range, whereas specific heat shows no salient anomaly associated with spin freezing. These features are characteristics of the formation of cluster-glass; in particular, the observed frequency variations in ac magnetic susceptibility are well described by the Vogel-Fulcher law. We compare the features concerning the suppression of the ferromagnetic order in this doped compound with those in isostructural Ca- and La-doped SrRuO3, and suggest that a local correlated disorder effect and the very small coherence of itinerant Ru 4d electrons are responsible for the cluster-glass formation instead of the quantum phase transition in Sr1-x(La0.5K0.5)xRuO3.",2006.12981v1 2020-06-28,Tuning the magneto-electrical properties of multiferroic multilayers through interface strain and disorder,"Artificially engineered superlattices were designed and fabricated to induce different growth mechanisms and structural characteristics. DC sputtering was used to grow ferromagnetic (La$_{0.8}$Ba$_{0.2}$MnO$_3$) / ferroelectric (Ba$_{0.25}$Sr$_{0.75}$TiO$_3$ or BaTiO$_3$) superlattices. We systematically modified the thickness of the ferromagnetic layer to analyze dimensional and structural effects on the superlattices with different structural characteristics. The crystalline structure was characterized by X-Ray diffraction and transmission electron microscopy. The magnetic and electronic properties were investigated by SQUID magnetometry and resistance measurements. The results show that both strain and structural disorder can significantly affect the physical properties of the systems. Compressive strain tends to increase the competition between the magnetic interactions decreasing the ferromagnetism of the samples and the localization of the charge carrier through the electron-phonon interaction. Tensile strain reduces the charge carrier localization, increasing the ferromagnetic transition temperature. Structural defects have a stronger influence on the magnetic properties than on the transport properties, reducing the ferromagnetic transition temperature while increasing the magnetic hardness of the superlattices. These results help to further understand the role of strain and interface effects in the magnetic and transport properties of manganite based multiferroic systems.",2006.15748v1 2020-08-08,Tuning interchain ferromagnetic instability in A2Cr3As3 ternary arsenides by chemical pressure and uniaxial strain,"We analyze the effects of chemical pressure induced by alkali metal substitution and uniaxial strain on magnetism in the A2Cr3As3 (A = Na, K, Rb, Cs) family of ternary arsenides with quasi-one dimensional structure. Within the framework of the density functional theory, we predict that the non-magnetic phase is very close to a 3D collinear ferrimagnetic state, which realizes in the regime of moderate correlations, such tendency being common to all the members of the family with very small variations due to the different interchain ferromagnetic coupling. We uncover that the stability of such interchain ferromagnetic coupling has a non-monotonic behavior with increasing the cation size, being critically related to the degree of structural distortions which is parametrized by the Cr-As-Cr bonding angles along the chain direction. In particular, we demonstrate that it is boosted in the case of the Rb, in agreement with recent experiments. We also show that uniaxial strain is a viable tool to tune the non-magnetic phase towards an interchain ferromagnetic instability. The modifcation of the shape of the Cr triangles within the unit cell favors the formation of a net magnetization within the chain and of a ferromagnetic coupling among the chains. This study can provide relevant insights about the interplay between superconductivity and magnetism in this class of materials.",2008.03552v4 2020-08-31,Electronic properties in itinerant ferromagnet SrRu$_{1-x}$Ti$_x$O$_3$,"Here, we study the electrical transport and specific heat in 4$d$ based ferromagnetic material SrRuO$_3$ and its Ti substituted SrRu$_{1-x}$Ti$_x$O$_3$ series ($x$ $\le$ 0.7). The SrRuO$_3$ is a metal and shows itinerant ferromagnetism with transition temperature $T_c$ $\sim$ 160 K. The nonmagnetic Ti$^{4+}$ (3$d^0$) substitution would not only weaken the active Ru-O-Ru channel but is also expected to tune the electronic density and electron correlation effect. A metal to insulator transition has been observed around $x$ $\sim$ 0.4. The nature of charge transport in paramagnetic-metallic state ($x$ $\leq$ 0.4) and in insulating state ($x$ $>$ 0.4) follows modified Mott's variable range hopping model. In ferromagnetic-metallic state, resistivity shows a $T^2$ dependence below $T_c$ which though modifies to $T^{3/2}$ dependence at low temperature. In Ti substituted samples, temperature range for $T^{3/2}$ dependence extends to higher temperature. Interestingly, this $T^{3/2}$ dependence dominates in whole ferromagnetic regime in presence of magnetic field. This evolution of electronic transport behavior can be explained within the framework of Fermi liquid theory and electron-magnon scattering mechanism. The negative magnetoresistance exhibits a hysteresis and a crossover between negative and positive value with magnetic field which is connected with magnetic behavior in series. The decreasing electronic coefficient of specific heat with $x$ supports the increasing insulating behavior in present series. We calculate a high Kadowaki-Woods ratio ($x$ $\leq$ 0.3) for SrRuO$_3$ which increases with substitution concentration. This signifies an increasing electronic correlation effect with substitution concentration.",2009.00076v1 2020-09-07,Spin pumping in d-wave superconductor/ferromagnet hybrids,"Spin-pumping across ferromagnet/superconductor (F/S) interfaces has attracted much attention lately. Yet the focus has been mainly on s-wave superconductors-based systems whereas (high-temperature) d-wave superconductors such as YBa2Cu3O7-d (YBCO) have received scarce attention despite their fundamental and technological interest. Here we use wideband ferromagnetic resonance to study spin-pumping effects in bilayers that combine a soft metallic Ni80Fe20 (Py) ferromagnet and YBCO. We evaluate the spin conductance in YBCO by analyzing the magnetization dynamics in Py. We find that the Gilbert damping exhibits a drastic drop as the heterostructures are cooled across the normal-superconducting transition and then, depending on the S/F interface morphology, either stays constant or shows a strong upturn. This unique behavior is explained considering quasiparticle density of states at the YBCO surface, and is a direct consequence of zero-gap nodes for particular directions in the momentum space. Besides showing the fingerprint of d-wave superconductivity in spin-pumping, our results demonstrate the potential of high-temperature superconductors for fine tuning of the magnetization dynamics in ferromagnets using k-space degrees of freedom of d-wave/F interfaces.",2009.03196v3 2020-09-14,"Minority-Spin Impurity Band in n-Type (In,Fe)As: A Materials Perspective for Ferromagnetic Semiconductors","Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream p-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in n-type FMSs is theoretically non-trivial. Soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) is a powerful approach to examine the mechanism of carrier-induced ferromagnetism in FMSs. Here our SX-ARPES study on the prototypical n-type FMS (In,Fe)As reveals the entire band structure including the Fe-3d impurity bands (IBs) and the host InAs ones, and provides direct evidence for electron occupation of the InAs-derived conduction band (CB). A minority-spin Fe-3d IB is found to be located just below the conduction-band minimum (CBM). The IB is formed by the hybridization of the unoccupied Fe-3d states with the occupied CBM of InAs in a spin-dependent way, resulting in the large spin polarization of CB. The band structure with the IB is varied with band filling, which cannot be explained by the rigid-band picture, suggesting a unified picture for realization of carrier-induced ferromagnetism in FMS materials.",2009.06285v1 2020-10-02,Spontaneous vortex state in a superconductor/ferromagnet nanocomposite,"The mechanism of the interplay between superconductivity and magnetism is one of the intriguing and challenging problems in physics. Theory has predicted that the ferromagnetic order can coexist with the superconducting order in the form of a spontaneous vortex phase in which magnetic vortices nucleate in the absence of an external field. However, there has been no rigorous demonstration of spontaneous vortices by bulk magnetic measurements. Here we show the results of experimental observations of spontaneous vortices using a superconductor/ferromagnet fractal nanocomposite, in which superconducting MgB2 and ferromagnetic nanograins are dispersedly embedded in the normal matrix to realize the remote electromagnetic interaction and also to induce a long-range Josephson coupling. We found from bulk magnetization measurements that the sample with nonzero remanent magnetization exhibits the magnetic behaviors which are fully consistent with a spontaneous vortex scenario predicted theoretically for magnetic inclusions in a superconducting material. The resulting spontaneous vortex state is in equilibrium and coexists surprisingly with a Meissner state (complete shielding of an external magnetic field). The present observation not only reveals the evolution process of the spontaneous vortices in superconductor/ferromagnet hybrids, but it also sheds light on the role of the fractal disorder and structural heterogeneity on the vortex nucleation under the influence of Josephson superconducting currents.",2010.00842v1 2020-10-07,Apparent Ferromagnetism in Exfoliated Ultra-thin Pyrite Sheets,"Experimental evidence for ferromagnetic ordering in isotropic atomically thin two-dimensional crystals has been missing until a bilayer Cr2Ge2Te6, and a three-atom thick monolayer CrI3 are shown to retain ferromagnetic ordering at finite temperatures. Here, we demonstrate successful isolation of a non-van der Waals type ultra-thin nanosheet of FeS2 derived from naturally occurring pyrite mineral (FeS2) by means of liquid-phase exfoliation. Structural characterizations imply that (111) oriented sheets are predominant and is supported theoretically by means of density functional theory surface energy calculations. Spin-polarized density theory calculations further predicted that (111) oriented three-atom thick pyrite sheet has a stable ferromagnetic ground state different from its diamagnetic bulk counterpart. This theoretical finding is evaluated experimentally employing low temperature superconducting quantum interference device measurements and observed an anomalous ferromagnetic kind of behavior.",2010.03113v2 2020-10-22,Skyrmion zoo in graphene at charge neutrality in a strong magnetic field,"As a consequence of the approximate spin-valley symmetry in graphene, the ground state of electrons in graphene at charge neutrality is a particular SU(4) quantum-Hall ferromagnet to minimize their exchange energy. If only the Coulomb interaction is taken into account, this ferromagnet can appeal either to the spin degree of freedom or equivalently to the valley pseudo-spin degree of freedom. This freedom in choice is then limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is given by the Zeeman effect that orients the spin in the direction of the magnetic field. In addition, there are also valley symmetry breaking terms that can arise from short-range interactions or electron-phonon couplings. Here, we build upon the phase diagram, which has been obtained by Kharitonov [Phys. Rev. B \textbf{85}, 155439 (2012)], in order to identify the different skyrmions that are compatible with these types of quantum-Hall ferromagnets. Similarly to the ferromagnets, the skyrmions at charge neutrality are described by the $\text{Gr}(2,4)$ Grassmannian at the center, which allows us to construct the skyrmion spinors. The different skyrmion types are then obtained by minimizing their energy within a variational approach, with respect to the remaining free parameters that are not fixed by the requirement that the skyrmion at large distances from their center must be compatible with the ferromagnetic background. We show that the different skyrmion types have a clear signature in the local, sublattice-resolved, spin magnetization, which is in principle accessible in scanning-tunneling microscopy and spectroscopy.",2010.11830v2 2020-11-27,d0 Ferromagnetism in Li-doped ZnO Compounds,"Recently, d0 ferromagnetic materials have been projected as one of the promising novel materials for spintronics applications. In this work, we have studied Li-doped ZnO compounds, i.e. Zn1-xLixO (x=0, 0.02, 0.04, and 0.06) samples, prepared by the solid-state reaction route method. From the study of crystal structure using X-ray diffraction (XRD) patterns, it is evident that the prepared materials have been formed in a single-phase of the hexagonal wurtzite structure. The refinement of the XRD patterns suggests that there are very small changes in the lattice parameters upon Li-incorporation in ZnO. The average crystallite size (SC), estimated from XRD patterns was found to be in the range of 35-50 nm. The microstructural study by scanning electron microscope reveals the uniform morphology of the grains of the order of 50-70 nm. The energy dispersive spectrum indicates that no unwanted ferromagnetic impurities have crept into the final prepared samples. The measurement of the temperature (T) variation of magnetization (M) with SQUID magnetometer indicates that undoped ZnO exhibits diamagnetic property but all Li-doped compounds exhibit room-temperature ferromagnetism and with a magnetic irreversibility behavior between zero-field cooled and field cooled M-T data. From the magnetization versus field measurements at 3 and 300 K, it is observed that Li-doped samples exhibit ferromagnetic loops with ultra-soft coercivity (~50 Oe) and with a maximum saturation magnetization of 0.10 emu/gm for x= 0.02 sample, which decreases with the increase in Li concentration.",2011.13713v1 2021-03-18,Giant spin Hall angle in the Heusler alloy Weyl ferromagnet Co$_2$MnGa,"Weyl semimetals are playing a major role in condensed matter physics due to exotic topological properties, and their coexistence with ferromagnetism may lead to enhanced spin-related phenomena. Here, the inverse spin Hall effect (ISHE) in the ferromagnetic Weyl-semimetal Heusler alloy Co$_2$MnGa was investigated at room temperature by means of electrical spin injection in lateral spin valve structures. Spin transport properties such as spin polarization and spin diffusion length in this material were precisely extracted in order to estimate the spin Hall angle $\theta_{\textrm{SH}}$, which was found to be $-0.19\pm0.04$ and is among the highest reported for a ferromagnet. Although this value is on the same order of magnitude of known heavy metals, the significantly higher resistivity of Co$_2$MnGa implies an improvement on the magnitude of detection voltages, while its ferromagnetic nature allows controlling the intensity of SHE through the magnetization direction. It was also shown that Onsager's reciprocity does not hold for this system, which is in part attributable to a different spin-dependent Hall conductivity for spin-up and spin-down carriers.",2103.10188v1 2021-04-02,The atlas of ferroicity in two dimensional MGeX3 family: room-temperature ferromagnetic half metals and unexpected ferroelectricity and ferroelasticity,"Two-dimensional (2D) ferromagnetic and ferroelectric materials attract unprecedented attention due to the spontaneous-symmetry-breaking induced novel properties and multifarious potential applications. Here we systematically investigate a large family (148) of 2D MGeX3 (M = metal elements, X = O/S/Se/Te) by means of the high-throughput first-principles calculations, and focus on their possible ferroic properties including ferromagnetism, ferroelectricity, and ferroelasticity. We discover eight stable 2D ferromagnets including five semiconductors and three half-metals, 21 2D antiferromagnets, and 11 stable 2D ferroelectric semiconductors including two multiferroic materials. Particularly, MnGeSe3 and MnGeTe3 are predicted to be room-temperature 2D ferromagnetic half metals with Tc of 490 and 308 K, respectively. It is probably for the first time that ferroelectricity is uncovered in 2D MGeX3 family, which derives from the spontaneous symmetry breaking induced by unexpected displacements of Ge-Ge atomic pairs, and we also reveal that the electric polarizations are in proportion to the ratio of electronegativity of X and M atoms, and IVB group metal elements are highly favored for 2D ferroelectricity. Magnetic tunnel junction and water-splitting photocatalyst based on 2D ferroic MGeX3 are proposed as examples of wide potential applications. The atlas of ferroicity in 2D MGeX3 materials will spur great interest in experimental studies and would lead to diverse applications.",2104.00865v1 2021-04-09,"Spatially modulated, orbital selective ferromagnetism in La$_5$Co$_2$Ge$_3$","We present density functional theory calculations for low-$T_c$ metallic ferromagnet La$_5$Co$_2$Ge$_3$ at ambient and applied pressures. Our investigations reveal that the system is a quasi-one-dimensional ferromagnet with a peculiar coexistence of two different orbital-selective magnetic moments at two crystallographically inequivalent cobalt atoms, Co1 and Co2. Namely, due to different crystal-field splitting, the magnetic moment of Co1 atoms predominantly derives from $d_{xz}$ orbital whereas of Co2 atoms from $d_{xy}$ orbital. Consequently, Co1 and Co2 atoms develop unequal net magnetic moments, a feature that gives rise to a periodic, spatial modulation of magnetization along crystallographic $c$-direction. The amplitude of the spatial modulation, small at ambient pressure, drastically increases with applied pressure, until Co2 atoms become nonmagnetic. With the help of a toy model mimicking found orbital-selective ferromagnetic order, we demonstrate that the increasing amplitude of spatial modulation provides a consistent interpretation to the recently observed resistivity anomaly emerging at applied pressure identified as the appearance of the {\it new state}. Although, proposed here structural origin of the spatial modulation of magnetic moments in La$_5$Co$_2$Ge$_3$ is an alternative one to the advocated for this material ferromagnetic quantum criticality avoidance, the effects of quantum fluctuations can still play an important role at pressure larger than up-to-date measured 5GPa.",2104.04245v2 2021-06-14,Quantum Kibble-Zurek mechanism: Kink correlations after a quench in the quantum Ising chain,"The transverse field in the quantum Ising chain is linearly ramped from the para- to the ferromagnetic phase across the quantum critical point at a rate characterized by a quench time $\tau_Q$. We calculate a connected kink-kink correlator in the final state at zero transverse field. The correlator is a sum of two terms: a negative (anti-bunching) Gaussian that depends on the Kibble-Zurek (KZ) correlation length only and a positive term that depends on a second longer scale of length. The second length is made longer by dephasing of the state excited near the critical point during the following ramp across the ferromagnetic phase. This interpretation is corroborated by considering a linear ramp that is halted in the ferromagnetic phase for a finite waiting time and then continued at the same rate as before the halt. The extra time available for dephasing increases the second scale of length that asymptotically grows linearly with the waiting time. The dephasing also suppresses magnitude of the second term making it negligible for waiting times much longer than $\tau_Q$. The same dephasing can be obtained with a smooth ramp that slows down in the ferromagnetic phase. Assuming sufficient dephasing we obtain also higher order kink correlators and the ferromagnetic correlation function.",2106.07335v3 2021-06-15,Long-Range Orbital Magnetoelectric Torque in Ferromagnets,"While it is often assumed that the orbital response is suppressed and short-ranged due to strong crystal field potential and orbital quenching, we show that the orbital magnetoelectric response can be remarkably long-ranged in ferromagnets. In a bilayer consisting of a nonmagnet and a ferromagnet, spin injection from the interface results in spin accumulation and torque in the ferromagnet, which rapidly oscillate and decay by spin dephasing. In contrast, we find that even when an external electric field is applied only on the nonmagnet, we find substantially long-ranged orbital magnetoelectric response in the FM, which can go far beyond the spin dephasing length. This unusual feature is attributed to nearly degenerate orbital characters imposed by the crystal symmetry, which form hotspots for the intrinsic orbital response. Because only the states near the hotspots contribute dominantly, the induced orbital angular momentum does not exhibit destructive interference among states with different momentum as in the case of the spin dephasing. This gives rise to a distinct type of orbital torque on the magnetization, increasing with the thickness of the ferromagnet. Such behavior may serve as critical long-sought evidence of orbital transport to be directly tested in experiments. Our findings open the possibility of using long-range orbital magnetoelectric effect in orbitronic device applications.",2106.07928v2 2021-07-16,Long-Range Superconducting Proximity Effect in Nickel Nanowires,"When a ferromagnet is placed in contact with a superconductor, owing to incompatible spin order, the Cooper pairs from the superconductor cannot survive more than one or two nanometers inside the ferromagnet. This is confirmed in the measurements of ferromagnetic nickel (Ni) nanowires contacted by superconducting niobium (Nb) leads. However, when a 3 nm thick copper oxide (CuO) buffer layer made by exposing an evaporated or a sputtered 3 nm Cu film to air, is inserted between the Nb electrodes and the Ni wire, the spatial extent of the superconducting proximity range is dramatically increased from 2 to a few tens of nanometers. Scanning transmission electron microscope study confirms the formation of a 3 nm thick CuO layer when an evaporated Cu film is exposed to air. Magnetization measurements of such a 3 nm CuO film on a SiO2/Si substrate and on Nb/SiO2/Si show clear evidence of ferromagnetism. One way to understand the long-range proximity effect in the Ni nanowire is that the CuO buffer layer with ferromagnetism facilitates the conversion of singlet superconductivity in Nb into triplet supercurrent along the Ni nanowires.",2107.08018v3 2021-10-18,Air Stable and Layer Dependent Ferromagnetism in Atomically Thin van der Waals CrPS$_{4}$,"Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI$_{3}$, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air stable and layer dependent ferromagnetic (FM) van der Waals crystal, CrPS$_{4}$, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics.",2110.09061v1 2021-11-02,Evidence for anisotropic spin-triplet Andreev reflection at the 2D van der Waals ferromagnet/superconductor interface,"Fundamental symmetry breaking and relativistic spin-orbit coupling give rise to fascinating phenomena in quantum materials. Of particular interest are the interfaces between ferromagnets and common s-wave superconductors, where the emergent spin-orbit fields support elusive spin-triplet superconductivity, crucial for superconducting spintronics and topologically-protected Majorana bound states. Here, we report the observation of large magnetoresistances at the interface between a quasi-two-dimensional van der Waals ferromagnet Fe0.29TaS2 and a conventional s-wave superconductor NbN, which provides the possible experimental evidence for the spin triplet Andreev reflection and induced spin-triplet superconductivity at ferromagnet/superconductor interface arising from Rashba spin-orbit coupling. The temperature, voltage, and interfacial barrier dependences of the magnetoresistance further support the induced spin-triplet superconductivity and spin-triplet Andreev reflection. This discovery, together with the impressive advances in two-dimensional van der Waals ferromagnets, opens an important opportunity to design and probe superconducting interfaces with exotic properties.",2111.01360v2 2021-11-09,Tunable intrinsic ferromagnetic topological phases in bulk van der Waals crystal MnSb6Te10,"Intrinsic ferromagnetism is a crucial ingredient to realize quantum anomalous Hall effect in quasi two dimensional materials, thus the search of intrinsic ferromagnetic topological materials is one of the most concerned issues in the field of topological phases of matter. In this work, combining magnetotransport measurements, first principles calculations, and angle-resolved photoemission spectroscopy studies, we find that in MnSb6Te10, the n = 2 member of the MnSb2Te4/(Sb2Te3)n family, the strong magnetic competition realizes a fragile ferromagnetic ground state, which whereas easily enters into ferrimagnetic and the Z_2 antiferromagnetic topological insulator phase with warming to higher temperature. Interestingly, the system stays in an inversion-symmetry-protected axion insulator phase in the ferromagnetic ground state as well as in the external magnetic field driven spin-polarized FM phase and can be converted into a Weyl semimetal with multiple Weyl nodes in the valence bands with hole doping, which are manifested by the measured notable intrinsic anomalous Hall effect. Our work thus provides an intrinsic magnetic topological material which is highly tunable into versatile topological phases by temperature, magnetic field, as well as carrier doping.",2111.04973v1 2021-11-11,High temperature ideal Weyl semimetal phase and Chern insulator phase in ferromagnetic BaEuNiOsO6 and its (111) (BaEuNiOsO6)/(BaTiO3)10 superlattice,"Weyl semimetals (WSMs) have recently stimulated intensive interest because they exhibit fascinating physical properties and also promise exciting technological applications. So far, however, the few conrmed magnetic WSMs generally have a large number of Weyl points either located away from the Fermi level (EF ) or shrouded by nontopological Fermi surface pockets. Based on first principles density functional theory calculations, we establish cubic double perovskite BaEuNiOsO6 to be a high Curie temperature (Tc) ferromagnetic WSM with magnetization along the [111] direction, just two pairs of Weyl points at the EF and Tc = 325 K. The strong ferromagnetism is attributed to the strong ferromagnetic Ni 3d-Eu 4f-Os 5d coupling induced by the substitution of half of Ba atoms with Eu atoms in double perovskite Ba2NiOsO6. Moreover, the momentum separation of one Weyl point pair is large, thus giving rise to not only a long (001) surface Fermi arc but also large anomalous Hall conductivity. Intriguingly, as a unique physical result of a ferromagnetic WSM, the (111) BaEuNiOsO6 monolayer superlattice (BaEuNiOsO6)/(BaTiO3)10, being its (111) quantum-well structure, is found to be a high temperature (Tc = 210 K) Chern insulator with a large band gap of 190 meV. Therefore, cubic double perovskite BaEuNiOsO6 will provide a superior high temperature material plotform for exploring fundamental physics of Weyl fermions and its (111) monolayer superlattices will offer a high temperature magnetic topological insulator for studying exotic quantum phenomena such as quantum anomalous Hall effect.",2111.06035v1 2021-11-29,Superconductivity coexisting with ferromagnetism in a quasi-one dimensional non-centrosymmetric (TaSe$_4$)$_3$I,"Low-dimensional materials with broken inversion symmetry and strong spin-orbit coupling can give rise to fascinating quantum phases and phase transitions. Here we report coexistence of superconductivity and ferromagnetism below 2.5\,K in the quasi-one dimensional crystals of non-centrosymmetric (TaSe$_4$)$_3$I (space group: $P\bar{4}2_1c$). The unique phase is a direct consequence of inversion symmetry breaking as the same material also stabilizes in a centro-symmetric structure (space group: $P4/mnc$) where it behaves like a non-magnetic insulator. The coexistence here upfront contradicts the popular belief that superconductivity and ferromagnetism are two apparently antagonistic phenomena. Notably, here, for the first time, we have clearly detected Meissner effect in the superconducting state despite the coexisting ferromagnetic order. The coexistence of superconductivity and ferromagnetism projects non-centrosymmetric (TaSe$_4$)$_3$I as a host for complex ground states of quantum matter including possible unconventional superconductivity with elusive spin-triplet pairing.",2111.14525v2 2021-12-08,Tailoring Magnetic Exchange Interactions in Ferromagnet-Intercalated MnBi2Te4 Superlattices,"The intrinsic magnetic topological insulator MnBi2Te4 (MBT) has provided a platform for the successful realization of exotic quantum phenomena. To broaden the horizons of MBT-based material systems, we intercalate ferromagnetic MnTe layers to construct the [(MBT)(MnTe)m]N superlattices by molecular beam epitaxy. The effective incorporation of ferromagnetic spacers mediates the anti-ferromagnetic interlayer coupling among the MBT layers through the exchange spring effect at the MBT/MnTe hetero-interfaces. Moreover, the precise control of the MnTe thickness enables the modulation of relative strengths among the constituent magnetic orders, leading to tunable magnetoelectric responses, while the superlattice periodicity serves as an additional tuning parameter to tailor the spin configurations of the synthesized multi-layers. Our results demonstrate the advantages of superlattice engineering for optimizing the magnetic interactions in MBT-family systems, and the ferromagnet-intercalated strategy opens up new avenues in magnetic topological insulator structural design and spintronic applications.",2112.04303v1 2021-12-17,"Gate-controlled proximity magnetoresistance in In1-xGaxAs/(Ga,Fe)Sb bilayer heterostructures","The magnetic proximity effect (MPE), ferromagnetic coupling at the interface of magnetically dissimilar layers, attracts much attention as a promising pathway for introducing ferromagnetism into a high-mobility non-magnetic conducting channel. Recently, our group found giant proximity magnetoresistance (PMR), which is caused by MPE at an interface between a non-magnetic semiconductor InAs quantum well (QW) layer and a ferromagnetic semiconductor (Ga,Fe)Sb layer. The MPE in the non-magnetic semiconductor can be modulated by applying a gate voltage and controlling the penetration of the electron wavefunction in the InAs QW into the neighboring insulating ferromagnetic (Ga,Fe)Sb layer. However, optimal conditions to obtain strong MPE at the InAs/(Ga,Fe)Sb interface have not been clarified. In this paper, we systematically investigate the PMR properties of In1-xGaxAs (x = 0%, 5%, 7.5%, and 10%) / (Ga,Fe)Sb bilayer semiconductor heterostructures under a wide range of gate voltage. The inclusion of Ga alters the electronic structures of the InAs thin film, in particular changing the effective mass and the QW potential of electron carriers. Our experimental results and theoretical analysis of the PMR in these In1-xGaxAs/(Ga,Fe)Sb heterostructures show that the MPE depends not only on the degree of penetration of the electron wavefunction into (Ga,Fe)Sb but also on the electron density. These findings help us to unveil the microscopic mechanism of MPE in semiconductor-based non-magnetic/ferromagnetic heterojunctions.",2112.09286v1 2021-12-19,The origin of insulating and non-ferromagnetic SrRuO3 monolayers,"The electro-magnetic properties of ultrathin epitaxial ruthenate films have long been the subject of debate. Here we combine experimental with theoretical investigations of (SrTiO3)5-(SrRuO3)n-(SrTiO3)5 (STO5-SROn-STO5) heterostructures with n = 1 and 2 unit cells, including extensive atomic-resolution scanning-transmission-electron-microscopy imaging, electron-energy-loss-spectroscopy chemical mapping, as well as transport and magneto-transport measurements. The experimental data demonstrate that the STO5-SRO2-STO5 heterostructure is stoichiometric, metallic, and ferromagnetic with TC ~ 128 K, even though it lacks the characteristic bulk-SRO octahedral tilts and matches the cubic STO structure. In contrast, the STO5-SRO1-STO5 heterostructure features Ru-Ti intermixing in the RuO2 layer, also without octahedral tilts, but is accompanied by a loss of metallicity and ferromagnetism. Density-functional-theory calculations show that stoichiometric n = 1 and n = 2 heterostructures are metallic and ferromagnetic with no octahedral tilts, while non-stoichiometry in the Ru sublattice in the n = 1 case opens an energy gap and induces antiferromagnetic ordering. Thus, the results indicate that the observed non-stoichiometry is the cause of the observed loss of metallicity and ferromagnetism in the n = 1 case.",2112.10076v1 2021-12-30,Non-uniform magnetization profile in ferromagnetic heterostructures leading to topological Hall effect like signatures,"Anomalous Hall effect (AHE), which arises when a current is passed through a ferromagnetic material subjected to a perpendicular magnetic field, is proportional to the magnetization of the sample.Additional hump-like features in AHE are often attributed to the presence of non trivial spin textures leading to topological Hall effect (THE). However, several recent reports have emphasized in context of ferromagnetic SrRuO$_3$ based heterostructures that the sample inhomogeneity can also result in THE-like features. In order to investigate this issue in general for any ferromagnetic heterostructure, we have considered a phenomenological model to calculate the changes in the shape of hysteresis loop due to various interfacial effects. These changes in the magnetization have been accounted for by considering that the interdomain magnetic coupling parameter ($\alpha$) varies exponentially with the distance from the interface along the growth direction of the heterostructure. In case of symmetric interfaces on both sides of a ferromagnet, we have considered the variation of $\alpha$ as a Gaussian function. We have found that the additional AHE contribution due to the net change in magnetization in such cases are akin to experimentally observed THE, even though we have not considered any topological quantity explicitly in our model. Thus, we propose another situation with nonuniform magnetization profile that may be used to explain additional features in AHE, which might not necessarily be intrinsic THE.",2112.15020v1 2022-01-26,Coexisting Kondo hybridization and itinerant f-electron ferromagnetism in UGe2,"Kondo hybridization in partially filled f-electron systems conveys significant amount of electronic states sharply near the Fermi energy leading to various instabilities from superconductivity to exotic electronic orders. UGe2 is a 5f heavy fermion system, where the Kondo hybridization is interrupted by the formation of two ferromagnetic phases below a 2nd order transition Tc ~ 52 K and a crossover transition Tx ~ 32 K. These two ferromagnetic phases are concomitantly related to a spin-triplet superconductivity that only emerges and persists inside the magnetically ordered phase at high pressure. The origin of the two ferromagnetic phases and how they form within a Kondo-lattice remain ambiguous. Using scanning tunneling microscopy and spectroscopy, we probe the spatial electronic states in the UGe2 as a function of temperature. We find a Kondo resonance and sharp 5f-electron states near the chemical potential that form at high temperatures above Tc in accordance with our density functional theory (DFT) + Gutzwiller calculations. As temperature is lowered below Tc, the resonance narrows and eventually splits below Tx dumping itinerant f-electron spectral weight right at the Fermi energy. Our findings suggest a Stoner mechanism forming the highly polarized ferromagnetic phase below Tx that itself sets the stage for the emergence of unconventional superconductivity at high pressure.",2201.11189v2 2022-01-31,Magnetostriction in microwave synthesized La0.5Ba0.5CoO3,"A single-phase polycrystalline La0.5Ba0.5CoO3-d sample was synthesized by microwave irradiation within 20 minutes of processing time and its structural, magnetic, electrical, and magnetostrictive properties were investigated. While the temperature dependence of field-cooled magnetization (M) in a field of H = 0.5 kOe indicates the onset of ferromagnetic transition at TC = 177 K, irreversibility between the zero field-cooled and field cooled M(T) persists even at H = 3 kOe. M(H) at 10 K does not saturate at the maximum available field and has a much smaller value (0.87 {\mu}B/Co in a field of 50 kOe) than 1.9 {\mu}B/Co expected for spin-only contribution from intermediate Co3+ and Co4+ spins. The resistivity shows insulating behavior down to 10 K and only a small magnetoresistance (~ 2% for H = 70 kOe) occurs around TC. All these results suggest a magnetically heterogeneous ground state with weakly interacting ferromagnetic clusters coexisting with a non-ferromagnetic phase. The length of the sample expands in the direction of the applied magnetic field (positive magnetostriction) and does not show saturation even at 50 kOe. The magnetostriction has a maximum value (= 252 ppm) at 10 K and it decreases with increasing temperature. The smaller value of magnetostriction compared to the available data on La0.5Sr0.5CoO3 suggests that non-ferromagnetic matrix is most likely antiferromagnetic and it restrains the field-induced expansion of ferromagnetic clusters in the microwave synthesized La0.5Ba0.5CoO3-d sample.",2201.12979v1 2022-02-01,1T-FeS$_2$$:$ a new type of two-dimensional metallic ferromagnet,"Discovery of intrinsic two-dimensional (2D) magnetic materials is crucial for understanding the fundamentals of 2D magnetism and realizing next-generation magnetoelectronic and magneto-optical devices. Although significant efforts have been devoted to identifying 2D magnetism by exfoliating bulk magnetic layered materials, seldom studies are performed to synthesize ultra-thin magnetic materials directly for non-layered magnetic materials. Here, we report the successful synthesis of a new type of theoretically proposed 2D metallic ferromagnet 1T FeS2, through the molten-salt-assisted chemical vapor deposition (CVD) method. The long-range 2D ferromagnetic order is confirmed by the observation of a large anomalous Hall effect (AHE) and a hysteretic magnetoresistance. The experimentally detected out-of-plane ferromagnetic ordering is theoretically suported with Stoner criterion. Our findings open up new possibilities to search novel 2D ferromagnets in non-layered compounds and render opportunities for realizing realistic ultra-thin spintronic devices.",2202.00252v1 2022-03-01,Ultrafast enhancement of interfacial exchange coupling in ferromagnetic bilayer,"Fast spin manipulation in magnetic heterostructures, where magnetic interactions between different materials often define the functionality of devices, is a key issue in the development of ultrafast spintronics. Although recently developed optical approaches such as ultrafast spin-transfer and spin-orbit torques open new pathways to fast spin manipulation, these processes do not fully utilize the unique possibilities offered by interfacial magnetic coupling effects in ferromagnetic multilayer systems. Here, we experimentally demonstrate ultrafast photo-enhanced interfacial exchange interactions in the ferromagnetic Co$_2$FeAl/(Ga,Mn)As system at low laser fluence levels. The excitation efficiency of Co$_2$FeAl with the (Ga,Mn)As layer is 30-40 times higher than the case with the GaAs layer at 5 K due to a photo-enhanced exchange coupling interaction via photoexcited charge transfer between the two ferromagnetic layers. In addition, the coherent spin precessions persist to room temperature, excluding the drive of photo-enhanced magnetization in the (Ga,Mn)As layer and indicating a proximity-effect-related optical excitation mechanism. The results highlight the importance of considering the range of interfacial exchange interactions in ferromagnetic heterostructures and how these magnetic coupling effects can be utilized for ultrafast, low-power spin manipulation.",2203.00293v3 2022-04-14,Piezoelectric ferromagnetism in Janus monolayer YBrI: a first-principle prediction,"Coexistence of intrinsic ferromagnetism and piezoelectricity, namely piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI is a PFM, which is dynamically, mechanically and thermally stable. Electronic correlation effects on physical properties of YBrI are investigated by using generalized gradient approximation plus $U$ (GGA+$U$) approach. For out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and the valley splitting is larger than 82 meV in considered $U$ range. The anomalous valley Hall effect (AVHE) can be achieved under an in-plane electric field. However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM) semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of YBrI is always in-plane with magnetic anisotropy energy (MAE) from 0.309 meV to 0.237 meV ($U$=0.0 eV to 3.0 eV). However, the magnetization can be adjusted from the in-plane to off-plane direction by external magnetic field, and then lead to the occurrence of valley polarization. Moreover, missing centrosymmetry along with mirror symmetry breaking results in both in-plane and out-of-plane piezoelectricity in YBrI monolayer. At a typical $U$=2.0 eV, the $d_{11}$ is predicted to be -5.61 pm/V, which is higher than or compared with ones of other two-dimensional (2D) known materials. The electronic and piezoelectric properties of YBrI can be effectively tuned by applying a biaxial strain. For example, tensile strain can enhance valley splitting and $d_{11}$ (absolute value). The predicted Curie temperature of YBrI is higher than those of experimentally synthesized 2D ferromagnetic materials $\mathrm{CrI_3}$ and $\mathrm{Cr_2Ge_2Te_6}$.",2204.06713v1 2022-07-08,Measuring the magnon-photon coupling in shaped ferromagnets: tuning of the resonance frequency,"Cavity photons and ferromagnetic spins excitations can exchange information coherently in hybrid architectures, at speeds set by their mutual coupling strength. Speed enhancement is usually achieved by optimizing the geometry of the electromagnetic cavity. Here we show that the geometry of the ferromagnet plays also an important role, by setting the fundamental frequency of the magnonic resonator. Using focused ion beam patterning, we vary the aspect ratio of different Permalloy samples reaching operation frequencies above 10 GHz while working at low external magnetic fields. Additionally, we perform broad band ferromagnetic resonance measurements and cavity experiments that demonstrate that the magnon-photon coupling strength can be estimated using either open transmission lines or resonant cavities, yielding very good agreement. Finally, we describe a simple theoretical framework based on electromagnetic and micromagnetic simulations that successfully accounts for the experimental results. This approach can be used to design hybrid quantum systems exploiting whatsoever magnetostatic mode excited in ferromagnets of arbitrary size and shape and to tune their operation conditions.",2207.03829v3 2022-09-13,Emergence of mesoscale quantum phase transitions in a ferromagnet,"Mesoscale patterns as observed, e.g., in ferromagnets, ferroelectrics, superconductors, mono-molecular films, or block-copolymers, reflect spatial variations of a pertinent order parameter at length- and time-scales that may be described classically. This raises the question for the relevance of mesoscale patterns near zero temperature phase transitions, also known as quantum phase transitions (QPTs). Here we report the magnetic susceptibility of LiHoF$_4$ -- a dipolar Ising ferromagnet -- near a well-understood transverse-field quantum critical point (TF-QCP). When tilting the magnetic field away from the hard axis such that the Ising symmetry is always broken, a line of well-defined phase transitions emerges from the TF-QCP characteristic of an additional symmetry breaking, in stark contrast to a crossover expected microscopically. We show that a continuous suppression of ferromagnetic domains, representing a breaking of translation symmetry on mesoscopic scales in an environment of broken magnetic Ising symmetry on microscopic scales, is in excellent qualitative and quantitative agreement with the field- and temperature dependence of the susceptibility and the magnetic phase diagram of LiHoF$_4$ under tilted field. This identifies a new type of phase transition that may be referred to as mesocale quantum criticality, which emanates from the text-book example of a microscopic ferromagnetic TF-QCP. Our results establish the surroundings of QPTs as a regime of mesoscale pattern formation, where non-analytical quantum dynamics and materials properties without classical analogue may be expected.",2209.06224v1 2022-09-20,Modeling of 3D Printable Electrical Machineries Ferromagnetic Parts,"The electrical machinery core is formed with a ferromagnetic material that offers high magnetic properties. As ferromagnetic materials have high relative magnetic permeability, they are important in the formation of electromagnetic device cores. Conventional subtractive and powder metallurgy methods for fabrication electrical machineries offer significant core losses and reduce magnetic flux density and magnetic permeability. With the advancement of technology, the limitation of the traditional process can be overcome by using the additive manufacturing process. Hence, this paper proposes a 3D printable model of two types of single-phase transformers, referred to as E-I shape and U-I shape transformers respectively. Possibilities of designing the electrical machinery part which has a ferromagnetic core are investigated. The efficiency of the transformers is evaluated in terms of magnetic flux density distribution and volumetric loss density based on the results of a large number of Finite element simulation methods under various operating situations on COMSOL. The performance of various ferromagnetic materials such as Soft Iron (Fe) and Ferrite (Fe2O3) on the transformer core is evaluated. This analysis reveals that if U-I shaped transformer can be made from 3D printing, it will be the best feasible structure for higher operating frequency.",2209.09894v1 2022-10-20,Increased localization of Majorana modes in antiferromagnetic chains on superconductors,"Magnet-superconductor hybrid (MSH) systems are a key platform for custom-designed topological superconductors. Ideally, the ends of a one-dimensional MSH structure will host Majorana zero-modes (MZMs), the fundamental unit of topological quantum computing. However, some of the experiments with ferromagnetic chains show a more complicated picture. Due to tiny gap sizes and hence long coherence lengths MZMs might hybridize and lose their topological protection. Recent experiments on a niobium surface have shown that both ferromagnetic and antiferromagnetic chains may be engineered, with the magnetic order depending on the crystallographic direction of the chain. While ferromagnetic chains are well understood, antiferromagnetic chains are less so. Here we study two models inspired by the niobium surface: a minimal model to elucidate the general topological properties of antiferromagnetic chains, and an extended model to more closely simulate a real system by mimicking the proximity effect. We find that in general for antiferromagnetic chains the topological gap is larger than for ferromagnetic ones and thus coherence lengths are shorter for antiferromagnetic chains, yielding more pronounced localization of MZMs in these chains. While topological phases for both ferromagnetic and antiferromagnetic chains both depend on the magnetic moment of the adatoms and the chemical potential, we find that antiferromagnetic chains also have a strong dependence on the magnitude of Rashba spin-orbit coupling at the surface.",2210.11587v2 2022-12-08,Ferromagnetic instability in itinerant fcc lattice electron systems with higher order van Hove singularities: Functional renormalization group study,"We investigate the possibility of ferromagnetic ordering in the non-degenerate Hubbard model on the face-centered cubic lattice within the functional renormalization group technique using temperature as a scale parameter. We assume the relations between nearest, next-nearest, and next-next-nearest hopping parameters providing higher order (giant) van Hove singularity of the density of states. The ferromagnetic instability formation with lowering temperature is described consistently in the one-loop approximation for a one-particle irreducible vertex of two-particle electron interaction. The chemical potential versus temperature phase diagrams are calculated. We find ferromagnetic order only for sufficiently strong divergence of the density of states and fillings in the vicinity of van Hove singularity. The obtained Curie temperature is more than an order of magnitude smaller than the results of the random-phase approximation. The main origin of the suppression of ferromagnetism is the screening of interaction in the particle-particle channel. We also do not find the pronounced tendency towards incommensurate order when the Fermi level is moved away from a van Hove singularity, such that the first order quantum phase transitions from the ferro- to paramagnetic phase are obtained.",2212.04135v2 2023-02-13,Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films,"The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts have been devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, we have systematically investigated the spin state in epitaxial LaCoO3 thin films to clarify the mechanism of strain induced ferromagnetism using element-specific x-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, we unambiguously demonstrate that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) are practically a low spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high spin and low spin states with a ratio close to 1:3. From the identification of this spin state ratio, we infer that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films.",2302.06063v1 2023-02-23,GaTe-Assisted CVD Growth of Ultrathin Large-Scale 2D Ferromagnetic Cr5Te8,"Recently, 2D Cr5Te8 has been successfully synthesized experimentally and has attracted widespread research interest due to its intriguing magnetic properties, such as hard magnetism with strong perpendicular anisotropy. However, exploring new methods for growing ultrathin 2D Cr5Te8 with a larger scale and their controllable synthesis remain challenging. Herein, the synthesis of ultrathin 2D ferromagnetic Cr5Te8 nanosheets by chemical vapor deposition (CVD) using GaTe as growth source is reported, whose size is up to ~160 {\mu}m and the thickness is lower to only 5 nm. The GaTe promotes the concentration of effective Te atoms to facilitate the direction of the synthesis reaction, enabling the rapid lateral growth rate. As a result, the synthesis of ultrathin, large-scale 2D ferromagnetic Cr5Te8 was achieved. By precisely adjusting the growth temperature and the source-substrate distance (Dss), the lateral size of the Cr5Te8 nanosheets can be tuned from a few to ~164 {\mu}m. Furthermore, magnetic property measurement system (MPMS) suggested that Cr5Te8 nanosheets possess intense out-of-plane ferromagnetism and the Curie temperature exhibits a monotonic increase from 163 to 175 K as the Cr5Te8 thickness. This work not only paves a way for the controllable growth of ultrathin, large-scale 2D ferromagnetic crystalline, but also provides a new platform for the spintronics and the practical application of magnetic memory devices.",2302.12169v2 2023-04-12,Co-existence of charge density wave and anti-ferromagnetic coupling in the spin-chain compound Ba$_6$Cr$_2$S$_{10}$,"Here we have performed detailed first principles calculations for the electronic structure and magnetic properties of Ba$_6$Cr$_2$S$_{10}$ to study the origin of the anti-ferromagnetic exchange interaction between spins on Cr ions for the spin-chain compound Ba$_6$Cr$_2$S$_{10}$ synthesised recently. Most importantly, we have found the co-existence of a charge density wave phase along one line and an anti-ferromagnetic spin chain along another. The dimerization of sulfur atoms loosely bonded with Ba atoms drives the system into an insulating state owing to the formation of charge density wave. Meanwhile, the small size of the effective Hubbard $U$ parameter ($\sim 0.5$ eV) due to electrostatic screening mainly accounts for the anti-ferromagnetic ground state. This co-existence equips us with a platform to tune the charge and spin degrees of freedom independently. Moreover, there exists a next-nearest-neighbouring anti-ferromagnetic interaction along the chain, which could bring forward spin frustration and hence quantum spin liquid.",2304.06156v3 2023-06-13,Circuit QED detection of induced two-fold anisotropic pairing in a hybrid superconductor-ferromagnet bilayer,"Hybrid systems represent one of the frontiers in the study of unconventional superconductivity and are a promising platform to realize topological superconducting states. Owing to their mesoscopic dimensions, these materials are challenging to probe using many conventional measurement techniques, and require new experimental probes to successfully characterize. In this work, we develop a probe that enables us to measure the superfluid density of micron-size superconductors using microwave techniques drawn from circuit quantum electrodynamics (cQED). We apply this technique to a paradigmatic hybrid system, the superconductor/ferromagnet bilayer, and find that the proximity-induced superfluid density is two-fold anisotropic within the plane of the sample and exhibits power law temperature-scaling which is indicative of a nodal superconducting state. These experimental results are consistent with the theoretically predicted signatures of induced triplet pairing with a nodal $p$-wave order parameter. Moreover, we unexpectedly observe drastic modifications to the microwave response at frequencies near the ferromagnetic resonance, suggesting a coupling between the spin dynamics and induced superconducting order in the ferromagnetic layer. Our results offer new insights into the unconventional superconducting states induced in superconductor/ferromagnet heterostructures and simultaneously establish a new avenue for the study of fragile unconventional superconductivity in low-dimensional materials such as van der Waals heterostructures.",2306.08043v1 2023-07-24,Temperature and thickness dependence of the thermal conductivity in 2D ferromagnet Fe$_3$GeTe$_2$,"The emergence of symmetry-breaking orders such as ferromagnetism and the weak interlayer bonding in van der Waals materials, offers a unique platform to engineer novel heterostructures and tune transport properties like thermal conductivity. Here, we report the experimental and theoretical study of the cross-plane thermal conductivity, $\kappa_\perp$, of the van der Waals 2D ferromagnet Fe$_3$GeTe$_2$. We observe a non-monotonic increase of $\kappa_\perp$ with the thickness and a large suppression in artificially-stacked layers, indicating a diffusive transport regime with ballistic contributions. These results are supported by the theoretical analyses of the accumulated thermal conductivity, which show an important contribution of phonons with mean free paths between 10 and 200 nm. Moreover, our experiments show a reduction of the $\kappa_\perp$ in the low-temperature ferromagnetic phase occurring at the magnetic transition. The calculations show that this reduction in $\kappa_\perp$ is associated with a decrease in the group velocities of the acoustic phonons and an increase in the phonon-phonon scattering of the Raman modes that couple to the magnetic phase. These results demonstrate the potential of van der Waals ferromagnets for thermal transport engineering.",2307.12863v1 2023-11-09,Electrically induced angular momentum flow between separated ferromagnets,"Converting angular momentum between different degrees of freedom within a magnetic material results from a dynamic interplay between electrons, magnons and phonons. This interplay is pivotal to implementing spintronic device concepts that rely on spin angular momentum transport. We establish a new concept for long-range angular momentum transport that further allows to address and isolate the magnonic contribution to angular momentum transport in a nanostructured metallic ferromagnet. To this end, we electrically excite and detect spin transport between two parallel and electrically insulated ferromagnetic metal strips on top of a diamagnetic substrate. Charge-to-spin current conversion within the ferromagnetic strip generates electronic spin angular momentum that is transferred to magnons via electron-magnon coupling. We observe a finite angular momentum flow to the second ferromagnetic strip across a diamagnetic substrate over micron distances, which is electrically detected in the second strip by the inverse charge-to-spin current conversion process. We discuss phononic and dipolar interactions as the likely cause to transfer angular momentum between the two strips. Moreover, our work provides the experimental basis to separate the electronic and magnonic spin transport and thereby paves the way towards magnonic device concepts that do not rely on magnetic insulators.",2311.05290v1 2023-12-20,Singular Hall response from a correlated ferromagnetic flat nodal-line semimetal,"Topological quantum phases have been largely understood in weakly correlated systems, which have identified various quantum phenomena such as spin Hall effect, protected transport of helical fermions, and topological superconductivity. Robust ferromagnetic order in correlated topological materials particularly attracts attention, as it can provide a versatile platform for novel quantum devices. Here, we report singular Hall response arising from a unique band structure of flat topological nodal lines in combination with electron correlation in an itinerant, van der Waals ferromagnetic semimetal, Fe3GaTe2, with a high Curie temperature of Tc=360 K. High anomalous Hall conductivity violating the conventional scaling, resistivity upturn at low temperature, and a large Sommerfeld coefficient are observed in Fe3GaTe2, which implies heavy fermion features in this ferromagnetic topological material. Our circular dichroism in angle-resolved photoemission spectroscopy and theoretical calculations support the original electronic features in the material. Thus, low-dimensional Fe3GaTe2 with electronic correlation, topology, and room-temperature ferromagnetic order appears to be a promising candidate for robust quantum devices.",2312.12889v1 2024-01-25,Thermal and Magnetoelastic Properties of the van der Waals Ferromagnet Fe$_{3-δ}$GeTe$_2$: Anisotropic Spontaneous Magnetostriction and Ferromagnetic Magnon Excitations,"By determining the lattice parameters as a function of temperature of the hexagonal van der Waals ferromagnet Fe$_{2.92(1)}$Ge$_{1.02(3)}$Te$_2$ we obtain the temperature dependence of the spontaneous in-plane magnetostriction in the ferromagnetic and the linear thermal expansion coefficients in the paramagnetic state. The spontaneous magnetostriction is clearly seen in the temperature dependence of the in-plane lattice parameter $a(T)$, but less well pronounced perpendicular to the planes along $c$. Below $T_{\rm C}$ the spontaneous magnetostriction follows the square of the magnetization and leads to an expansion of the hexagonal layers. Extrapolating to $T\rightarrow$ 0~K we obtain a spontaneous in-plane saturation magnetostriction of $\lambda_{{\rm sp},a}(T \rightarrow 0) \approx-220 ~\times~10^{-6}$. In the paramagnetic state the linear thermal expansion coefficients amount to 13.9(1)$\times$10$^{-6}$~K$^{-1}$ and to 23.2(2)$\times$10$^{-6}$~K$^{-1}$ for the in-plane and out-of-plane direction, respectively, indicating a linear volume thermal expansion coefficient of 50.8(4)$\times$10$^{-6}$K$^{-1}$ which we use to estimate the volume thermal expansion contribution to the heat capacity determined at constant pressure. A Sommerfeld-type linear term in the low-temperature heat capacities can be quantitatively ascribed to 2dim ferromagnetic magnon excitations.",2401.14050v1 2024-02-24,Observation of the In-plane Anomalous Hall Effect induced by Octupole in Magnetization Space,"The Anomalous Hall Effect (AHE) manifests as a transverse voltage proportional to magnetization in ferromagnetic materials under the application of a charge current, being an indispensable tool for probing magnetism, especially in nanoscale devices. However, the AHE primarily sensitizes to out-of-plane magnetization, thereby hindering its capacity to discern the in-plane magnetization, a characteristic prevalent in ferromagnetic films. Here we challenge this conventional understanding by demonstrating the in-plane magnetization-induced AHE in iron and nickel, two ubiquitous ferromagnets. This observation of the in-plane AHE is remarkable as it contradicts existing theories that forbid such phenomena in cubic crystal systems. We trace the origin of this unanticipated phenomenon to a hitherto unconsidered octupole of the anomalous Hall conductivity in the magnetization space, a mechanism we propose could enable the detection of in-plane AHE in a wide range of ferromagnetic materials. This work realizes the in-plane AHE in common ferromagnets by exploiting the anomalous Hall conductivity octupole, revealing a new physical origin of the AHE and promising to revolutionize the design of magnetic devices and sensors.",2402.15741v1 2024-03-13,Chiral spin state and nematic ferromagnet in the spin-1 Kitaev-$Γ$ model,"The higher-spin Kitaev magnets, in which the Kitaev interaction and off-diagonal exchange couplings are overwhelmingly large, have emerged as a fertile avenue to explore exotic phases and unusual excitations. In this work, we study the quantum phase diagram of the spin-1 Kitaev-$\Gamma$ model on the honeycomb lattice using density-matrix renormalization group. It harbours six distinct phases and the intriguing findings are three magnetically ordered phases in which both time-reversal symmetry and lattice symmetry albeit of different sort are broken spontaneously. The chiral spin state originates from the order-by-disorder effect and exhibits an almost saturated scalar spin chirality at the quantum level. Depending on the relative strength of the two interactions, it also features columnar or plaquette valence-bond-solid-like pattern as a consequence of the translational symmetry breaking. In parallel, the nematic ferromagnets are situated at ferromagnetic Kitaev side and possess small but finite ferromagnetic ordering. The lattice-rotational symmetry breaking enforces nonequivalent bond energy along one of the three bonds. Although the intrinsic difference between the two nematic ferromagnets remains elusive, the discontinuities in the von Neumann entropy, hexagonal plaquette operator, and Wilson loop operator convincingly suggest that they are separated via a first-order phase transition.",2403.08382v1 2007-05-11,Electrical transport and optical studies of ferromagnetic Cobalt doped ZnO nanoparticles exhibiting a metal-insulator transition,"The observed correlation of oxygen vacancies and room temperature ferromagnetic ordering in Co doped ZnO1-o nanoparticles reported earlier (Naeem et al Nanotechnology 17, 2675-2680) has been further explored by transport and optical measurements. In these particles room temperature ferromagnetic ordering had been observed to occur only after annealing in forming gas. In the current work the optical properties have been studied by diffuse reflection spectroscopy in the UV-Vis region and the band gap of the Co doped compositions has been found to decrease with Co addition. Reflections minima are observed at the energies characteristic of Co+2 d-d (tethrahedral symmetry) crystal field transitions, further establishing the presence of Co in substitutional sites. Electrical transport measurements on palletized samples of the nanoparticles show that the effect of a forming gas is to strongly decrease the resistivity with increasing Co concentration. For the air annealed and non-ferromagnetic samples the variation in the resistivity as a function of Co content are opposite to those observed in the particles prepared in forming gas. The ferromagnetic samples exhibit an apparent change from insulator to metal with increasing temperatures for T>380K and this change becomes more pronounced with increasing Co content. The magnetic and resistive behaviors are correlated by considering the model by Calderon et al [M. J. Calderon and S. D. Sarma, Annals of Physics 2007 (Accepted doi: 10.1016/j.aop.2007.01.010] where the ferromagnetism changes from being mediated by polarons in the low temperature insulating region to being mediated by the carriers released from the weakly bound states in the higher temperature metallic region.",0705.1593v3 2011-05-18,"Influence of B - site Disorder in $La_{0.5}Ca_{0.5}Mn_{1-x}B_{x}O_{3}$ (B = Fe, Ru, Al and Ga) Manganites","We have investigated the influence of B - site doping on the crystal and magnetic structure in $La_{0.5}Ca_{0.5}Mn_{1-x}B_{x}O_{3}$ (B= Fe, Ru, Al and Ga) compounds using neutron diffraction, SANS, magnetization and resistivity techniques. The B - site doped samples are isostructural and possess an orthorhombic structure in \textit{Pnma} space group at 300K. A structural transition from orthorhombic to monoclinic is found to precede the magnetic transition to CE - type antiferromagnetic state in few of these samples. On doping with Fe, charge and orbitally ordered CE - type antiferromagnetic state is suppressed, followed by the growth in ferromagnetic insulating phase in $0.02\leq x\leq0.06$ compounds. At higher Fe doping in $x>0.06$, the ferromagnetic state is also suppressed and no evidence of long range magnetic ordering is observed. In Ru doped samples $(0.01\leq x\leq0.05)$, the ferromagnetic metallic state is favored at $T{}_{C}\approx200K$ and $T_{MI}\approx125K$ and no significant change in $T_{C}$ and $T_{MI}$ as a function of Ru doping is found. In contrast, with non magnetic Al substitution for $0.01\leq x\leq0.03$, the charge ordered CE - type antiferromagnetic state coexists with the ferromagnetic metallic phase. With further increase in Al doping $(0.05\leq x\leq0.07)$, both CE - type antiferromagnetic and ferromagnetic phases are gradually suppressed. This behavior is accompanied by the evolution of A - type antiferromagnetic insulating state. Eventually, at higher Al doping $(0.10\leq x\leq0.13)$, this phase is also suppressed and signature of spin glass like transition are evident in M(T). Likewise, substitution with Ga is observed to induce similar effects as described for Al doped samples. The presence of short ranged ferromagnetic ordering has been further explored using SANS measurements in few of the selected samples.",1105.3572v1 2011-06-07,Nonperturbative Effects on the Ferromagnetic Transition in Repulsive Fermi Gases,"It is generally believed that a dilute spin-1/2 Fermi gas with repulsive interactions can undergo a ferromagnetic phase transition to a spin-polarized state at a critical gas parameter $(k_{\rm F}a)_c$. Previous theoretical predictions of the ferromagnetic phase transition have been based on the perturbation theory, which treats the gas parameter as a small number. On the other hand, Belitz, Kirkpatrick, and Vojta (BKV) have argued that the phase transition in clean itinerant ferromagnets is generically of first order at low temperatures, due to the correlation effects that lead to a nonanalytic term in the free energy. The second-order perturbation theory predicts a first-order phase transition at $(k_{\rm F}a)_c=1.054$, consistent with the BKV argument. However, since the critical gas parameter is expected to be of order O(1), perturbative predictions may be unreliable. In this paper we study the nonperturbative effects on the ferromagnetic phase transition by summing the particle-particle ladder diagrams to all orders in the gas parameter. We consider a universal repulsive Fermi gas where the effective range effects can be neglected, which can be realized in a two-component Fermi gas of $^6$Li atoms by using a nonadiabatic field switch to the upper branch of a Feshbach resonance with a positive s-wave scattering length. Our theory predicts a second-order phase transition, which indicates that ferromagnetic transition in dilute Fermi gases is possibly a counterexample to the BKV argument. The predicted critical gas parameter $(k_{\rm F}a)_c=0.858$ is in good agreement with the recent quantum Monte Carlo result $(k_{\rm F}a)_c=0.86$ for a nearly zero-range potential [S. Pilati, \emph{et al}., Phys. Rev. Lett. {\bf 105}, 030405 (2010)]. We also compare the spin susceptibility with the quantum Monte Carlo result and find good agreement.",1106.1345v4 2011-07-08,Study of spin glass and cluster ferromagnetism in RuSr2Eu1.4Ce0.6Cu2O10-δ magneto Superconductor,"We report DC magnetization, detailed systematic linear and nonlinear AC magnetic susceptibility and transport for a single phase RuSr2Eu1.4Ce0.6Cu2O10-{\delta} (EuRu-1222) magneto-superconductor. The studied sample is synthesized through standard solid state reaction route, which is crystallized in single phase tetragonal structure with space group I4/mmm. DC magnetic susceptibility measurements revealed that the studied EuRu-1222 is a magneto-superconductor with Ru spins ordering at around 110 K and superconductivity in the Cu-O2 planes below \approx 30 K. Temperature dependence of AC susceptibility with different frequency and amplitude variations confirms spin-glass behavior with cluster ferromagnetism of the system. Change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted feature for a spin-glass (SG) system with ferromagnetic clusters. The third harmonic of AC susceptibility (\c{hi}3) shows that the system undergoes a spin glass transition below 80 K. Superconducting transition temperature (Tc) onset and {\rho} = 0 are seen at around 30 and 18 K without any applied field and the same decreases to 10 and 2 K under 130 kOe applied field. Also low fields isothermal (MH) suggests that ferromagnetic clusters are embedded in spin-glass (SG) matrix. The magnetization vs. applied field (MH) loops exhibited ferromagnetic (FM) like behavior with rather small coercive fields. Detailed AC magnetic susceptibility measurements are carried out to unearth the short range magnetic correlations. These results support the spin-glass (SG) formation followed by ferromagnetic clustering effects at low temperatures. Our detailed magnetization and magneto transport results will undoubtedly contribute to current understanding of the complex magnetism of the EuRu-1222 system.",1107.1612v2 2012-06-13,"Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator","The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical and spectroscopic measurements of \MBS thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range 15 K $\leq T \leq 100$ K, accompanied by a suppressed density of surface states at the Dirac point. Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high temperature ferromagnetism seen in surface studies, revealing instead a low temperature ferromagnetic phase at $T \lesssim 5$ K. The absence of both a magneto-optical Kerr effect and anomalous Hall effect suggests that this low temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.",1206.2908v2 2012-12-05,Depth profile of the ferromagnetic order in a YBa$_2$Cu$_3$O$_7$ / La$_{2/3}$Ca$_{1/3}$MnO$_3$ superlattice on a LSAT substrate: a polarized neutron reflectometry study,"Using polarized neutron reflectometry (PNR) we have investigated a YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the high structural quality of the superlattice and the substrate, the specular reflectivity signal extends with a high signal-to-background ratio beyond the fourth order superlattice Bragg peak. This allows us to obtain more detailed and reliable information about the magnetic depth profile than in previous PNR studies on similar superlattices that were partially impeded by problems related to the low temperature structural transitions of the SrTiO3 substrates. In agreement with the previous reports, our PNR data reveal a strong magnetic proximity effect showing that the depth profile of the magnetic potential differs significantly from the one of the nuclear potential that is given by the YBCO and LCMO layer thickness. We present fits of the PNR data using different simple block-like models for which either a ferromagnetic moment is induced on the YBCO side of the interfaces or the ferromagnetic order is suppressed on the LCMO side. We show that a good agreement with the PNR data and with the average magnetization as obtained from dc magnetization data can only be obtained with the latter model where a so-called depleted layer with a strongly suppressed ferromagnetic moment develops on the LCMO side of the interfaces. The models with an induced ferromagnetic moment on the YBCO side fail to reproduce the details of the higher order superlattice Bragg peaks and yield a wrong magnitude of the average magnetization. We also show that the PNR data are still consistent with the small, ferromagnetic Cu moment of 0.25muB that was previously identified with x-ray magnetic circular dichroism and x-ray resonant magnetic reflectometry measurements on the same superlattice.",1212.0986v1 2013-12-02,Double ellipsoidal Fermi surface model of the normal state of ferromagnetic superconductors,"We model the normal state of ferromagnetic superconductors with two general ellipsoidal Fermi surfaces (FSs), one for each spin projection $\sigma=\{\uparrow,\downarrow\}$, each with its ferromagnetically split chemical potential $\mu_{\sigma}$ and its three distinct single particle effective masses, $\{m_{i\sigma}\}$, the geometric mean of which is $m_{\sigma}$. We study this model in the presence of an arbitrarily oriented magnetic induction, ${\bf B}=\mu_{0}{\bf H}+{\bf M_{0}}$, where ${\bf M_{0}}$ includes the Ising-like spontaneous ferromagnetic order, which for URhGe is in the $c$-axis direction above the superconducting transition temperature $T_c$. We assume the low-$T$ total particle density $\Sigma_{\sigma} n_{\sigma}({\bf B})$ to be independent of ${\bf B}$, and obtain a self-consistent asymptotic expansion for $\sum_{\sigma}\Pi^{3/2}_{\sigma}({\bf B})$ in even powers of ${\bf B}$, where $\Pi_{\sigma}({\bf B})=m_{\sigma}({\bf B})\mu_{\sigma}({\bf B})$. We assume that the $\mu_{\sigma}({\bf B})$ are linear in ${\bf B}$ for both spins due to the Zeeman interaction and that the remaining even ${\bf B}$ dependence in the $\Pi_{\sigma}({\bf B})$ arises only from $m_{\downarrow}({\bf B})$. Our analogous expression for the Sommerfeld constant $\gamma({\bf B})$ leads to good fits to the $\gamma({\bf H})$ data of Aoki and Flouquet [J. Phys. Soc. Jpn. \textbf{81}, 011003 (2012)] obtained for the ferromagnetic superconductor URhGe in the ferromagnetic, non-superconducting phase, with the applied magnetic field ${\bf H}$ along each of the three crystallographic directions. We discuss this model in terms of the reentrant superconducting properties of URhGe and UCoGe. This model can be generalized to an arbitrary number of ellipsoidal FSs.",1312.0644v2 2015-04-22,Gradual localization of 5f states in orthorhombic UTX ferromagnets - polarized neutron diffraction study of Ru substituted UCoGe,"We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 K and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment mspont. = 0.11 mB/f.u. and mspont. = 0.21 mB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07mB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also by soft X-ray magnetic circular dichroism (XMCD). The analysis of the PND results indicates that the observed enhancement of ferromagnetism is mainly due to the growth of the orbital part of the uranium 5f moment mL(U), reflecting a gradual localization of the 5f electrons with Ru substitution. In addition, the parallel orientation of the U and Co moments has been established in both substituted compounds. The results are discussed and compared with related isostructural ferromagnetic UTX compounds (T - transition metals, X - Si, Ge) in the context of a varying degree of the 5f-ligand hybridization.",1504.05645v1 2016-10-03,Spiral order from orientationally correlated random bonds in classical XY models,"We discuss the stability of ferromagnetic long-range order in three-dimensional classical XY ferromagnets upon substitution of a small subset of equally oriented bonds by impurity bonds, on which the ferromagnetic exchange J_perp > 0 is replaced by a strong antiferromagnetic coupling J_imp < 0. In the presence of a single impurity bond, once the absolute value of the frustrating coupling J_imp < 0 exceeds a threshold J_c > 0, the ground state becomes two-fold degenerate, corresponding to either clockwise or anticlockwise canting of the spins in the vicinity of the impurity bond. In the presence of a small concentration of impurity bonds, the effective low-energy Hamiltonian is that of Ising variables encoding the sense of rotation of the local canting around the impurities. Those degrees of freedom interact through a dipolar interaction mediated by spin waves. A ferromagnetic Ising ground state indicates the instability of the XY ferromagnet towards a spiral state with a wave vector proportional to the concentration of impurity bonds. To analyze under which circumstances such a ground state arises, we study first impurities forming superlattices. For a subclass of those, we can rigorously establish the existence of spiral order. For another class of superlattices, the Ising variables order ferromagnetically in planes perpendicular to the orientation of impurity bonds, but antiferromagnetically parallel to it, which results in a fan-like XY ground state. Second, we consider the case when the impurity bonds are randomly distributed on the three-dimensional host lattice according to a Poisson process. We show the phenomenon of spiral order by disorder with an ordering wave vector proportional to the impurity concentration. The analytical predictions are confirmed by Monte Carlo simulations and are relevant for magnetic materials such as YBaCuFeO_5.",1610.00784v4 2017-03-14,Thermodynamics of frustrated ferromagnetic spin-$1/2$ Heisenberg chains: The role of inter-chain coupling,"The thermodynamics of coupled frustrated ferromagnetic chains is studied within a spin-rotation-invariant Green's function approach. We consider an isotropic Heisenberg spin-half system with a ferromagnetic in-chain coupling $J_1<0$ between nearest neighbors and a frustrating antiferromagnetic next-nearest neighbor in-chain coupling $J_2>0$. We focus on moderate strength of frustration $J_2 < |J_1|/4$ such that the in-chain spin-spin correlations are predominantly ferromagnetic. We consider two inter-chain couplings (ICs) $J_{\perp,y}$ and $J_{\perp,z}$, corresponding to the two axis perpendicular to the chain, where ferromagnetic as well as antiferromagnetic ICs are taken into account. We discuss the influence of frustration on the ground-state properties for antiferromagnetic ICs, where the ground state is of quantum nature. The major part of our study is devoted to the finite-temperature properties. We calculate the critical temperature $T_{c}$ as a function of the competing exchange couplings $J_{2},J_{\perp,y}, J_{\perp,z}$. We find that for fixed ICs $T_c$ monotonically decreases with increasing frustration $J_2$, where as $J_2 \to |J_1|/4$ the $T_c(J_2)$-curve drops down rapidly. To characterize the magnetic ordering below and above $T_c$ we calculate the spin-spin correlation functions $\langle {\bf S}_0 {\bf S}_{\bf R} \rangle$, the magnetic order parameter $M$, the uniform static susceptibility $\chi_0$ as well as the correlation length $\xi$. Moreover, we discuss the specific heat $C_V$ and the temperature dependence of the excitation spectrum $\omega_{\mathbf{q}}$. As $J_2 \to |J_1|/4$ some unusual frustration-induced features were found, such as an increase of the in-chain spin stiffness (in case of ferromagnetic ICs) or of the in-chain spin-wave velocity (in case of antiferromagnetic ICs) with growing temperature.",1703.04573v1 2018-02-22,Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit,"Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $\alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.",1802.08152v1 2014-03-11,Spin-polaron band of heavy carriersin the heavy-fermion ferromagnetic superconductor UGe2,"In strongly correlated materials, cooperative behavior of the electrons causes a variety of quantum ordered states that may, in some cases, coexist. It has long been believed, however, that such coexistence among ferromagnetic ordering, superconductivity and heavy-fermion behavior is impossible, as the first supports parallel spin alignment while the conventional understanding of the latter two phenomena assumes spin-singlet or anti-parallel spins. This understanding has recently been challenged by an increasing number of observations in uranium systems (UGe2, URhGe, UIr and UCoGe) in which superconductivity is found within a ferromagnetic state and, more fundamentally, both ordering phenomena are exhibited by the same set of heavy 5f electrons. Since the coexistence of superconductivity and ferromagnetism is at odds with the standard theory of phonon-mediated spin-singlet superconductivity, it requires an alternative pairing mechanism, in which electrons are bound into spin-triplet pairs by spin fluctuations. Within the heavy-fermion scenario, this alternative mechanism assumes that the magnetism has a band character and that said band forms from heavy quasiparticles composed of f electrons. This band is expected to be responsible for all three phenomena although its nature and the nature of those heavy quasiparticles still remains unclear. Here we report spectroscopic evidence for the formation in UGe2 of subnanometer-sized spin polarons whose dynamics we follow into the paramagnetic and ferromagnetic phases. These spin polarons behave as heavy carriers and thus may serve as heavy quasiparticles made of 5f electrons; once coherence is established, they form a narrow spin-polaron band which thus provides a natural reconciliation of itinerant ferromagnetism with spin-triplet superconductivity and heavy-fermion behavior.",1403.2597v1 2018-01-23,Current-driven domain wall dynamics in ferromagnetic layers synthetically exchange-coupled by a spacer: A micromagnetic study,"The current-driven domain wall motion along two exchange-coupled ferromagnetic layers with perpendicular anisotropy is studied by means of micromagnetic simulations and compared to the conventional case of a single ferromagnetic layer. Our results, where only the lower ferromagnetic layer is subjected to the interfacial Dzyaloshinskii-Moriya interaction and to the spin Hall effect, indicate that the domain walls can be synchronously driven in the presence of a strong interlayer exchange coupling, and that the velocity is significantly enhanced due to the antiferromagnetic exchange coupling as compared with the single-layer case. On the contrary, when the coupling is of ferromagnetic nature, the velocity is reduced. We provide a full micromagnetic characterization of the current-driven motion in these multilayers, both in the absence and in the presence of longitudinal fields, and the results are explained based on a one-dimensional model. The interfacial Dzyaloshinskii-Moriya interaction, only necessary in this lower layer, gives the required chirality to the magnetization textures, while the interlayer exchange coupling favors the synchronous movement of the coupled walls by a dragging mechanism, without significant tilting of the domain wall plane. Finally, the domain wall dynamics along curved strips is also evaluated. These results indicate that the antiferromagnetic coupling between the ferromagnetic layers mitigates the tilting of the walls, which suggest these systems to achieve efficient and highly packed displacement of trains of walls for spintronics devices. A study, taking into account defects and thermal fluctuations, allows to analyze the validity range of these claims.",1801.07432v1 2018-10-22,Tuning a binary ferromagnet into a multi-state synapse with spin-orbit torque induced plasticity,"Inspired by ion-dominated synaptic plasticity in human brain, artificial synapses for neuromorphic computing adopt charge-related quantities as their weights. Despite the existing charge derived synaptic emulations, schemes of controlling electron spins in ferromagnetic devices have also attracted considerable interest due to their advantages of low energy consumption, unlimited endurance, and favorable CMOS compatibility. However, a generally applicable method of tuning a binary ferromagnet into a multi-state memory with pure spin-dominated synaptic plasticity in the absence of an external magnetic field is still missing. Here, we show how synaptic plasticity of a perpendicular ferromagnetic FM1 layer can be obtained when it is interlayer-exchange-coupled by another in-plane ferromagnetic FM2 layer, where a magnetic-field-free current-driven multi-state magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin-orbit torque. We use current pulses to set the perpendicular magnetization state which acts as the synapse weight, and demonstrate spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing-dependent plasticity. This functionality is made possible by the action of the in-plane interlayer exchange coupling field which leads to broadened, multi-state magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in-plane field component tuning the efficiency of the spin-orbit-torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside our Pt/FM1/Ta/FM2 structure with inherent in-plane coupling field is therefore tuned into a multi-state perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing.",1810.09064v1 2018-10-24,Long spin coherence length and bulk-like spin-orbit torque in ferrimagnetic multilayers,"Ferromagnetic spintronics has been a main focus as it offers non-volatile memory and logic applications through current-induced spin-transfer torques. Enabling wider applications of such magnetic devices requires a lower switching current for a smaller cell while keeping the thermal stability of magnetic cells for non-volatility. As the cell size reduces, however, it becomes extremely difficult to meet this requirement with ferromagnets because spin-transfer torque for ferromagnets is a surface torque due to rapid spin dephasing, leading to the 1/ferromagnet-thickness dependence of the spin-torque efficiency. Requirement of a larger switching current for a thicker and thus more thermally stable ferromagnetic cell is the fundamental obstacle for high-density non-volatile applications with ferromagnets. Theories predicted that antiferromagnets have a long spin coherence length due to the staggered spin order on an atomic scale, thereby resolving the above fundamental limitation. Despite several spin-torque experiments on antiferromagnets and ferrimagnetic alloys, this prediction has remained unexplored. Here we report a long spin coherence length and associated bulk-like-torque characteristic in an antiferromagnetically coupled ferrimagnetic multilayer. We find that a transverse spin current can pass through > 10 nm-thick ferrimagnetic Co/Tb multilayers whereas it is entirely absorbed by 1 nm-thick ferromagnetic Co/Ni multilayer. We also find that the switching efficiency of Co/Tb multilayers partially reflects a bulk-like-torque characteristic as it increases with the ferrimagnet-thickness up to 8 nm and then decreases, in clear contrast to 1/thickness-dependence of Co/Ni multilayers. Our results on antiferromagnetically coupled systems will invigorate researches towards energy-efficient spintronic technologies.",1810.10404v1 2020-02-19,Ion-Irradiation-Induced Cobalt/Cobalt Oxide Heterostructures: Printing 3D Interfaces,"Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, conversion between charge and spin currents and vice-versa. These interfacial phenomena play crucial roles for magnetic data storage and transfer applications, which require forming FM nano-structures embedded in non-ferromagnetic matrices. Here, we investigate the possiblity of creating such nano-structures by ion-irradiation. We study the effect of lateral confinement on the ion-irradiation-induced reduction of non-magnetic metal oxides (e.g., antiferro- or paramagnetic) to form ferromagnetic metals. Our findings are later exploited to form 3-dimensional magnetic interfaces between Co, CoO and Pt by spatially-selective irradiation of CoO/Pt multilayers. We demonstrate that the mechanical displacement of the O atoms plays a crucial role during the reduction from insulating, non-ferromagnetic cobalt oxides to metallic cobalt. Metallic cobalt yields both perpendicular magnetic anisotropy in the generated Co/Pt nano-structures, and, at low temperatures, exchange bias at vertical interfaces between Co and CoO. If pushed to the limit of ion-irradiation technology, this approach could, in principle, enable the creation of densely-packed, atomic scale ferromagnetic point-contact spin-torque oscillator (STO) networks, or conductive channels for current-confined-path based current perpendicular-to-plane giant magnetoresistance read-heads.",2002.08090v1 2018-07-04,Phase Boundary Exchange Coupling in the Mixed Magnetic Phase Regime of a Pd-doped FeRh Epilayer,"Spin-wave resonance measurements were performed in the mixed magnetic phase regime of a Pd-doped FeRh epilayer that appears as the first-order ferromagnetic-antiferromagnetic phase transition takes place. It is seen that the measured value of the exchange stiffness is suppressed throughout the measurement range when compared to the expected value of the fully ferromagnetic regime, extracted via the independent means of a measurement of the Curie point, for only slight changes in the ferromagnetic volume fraction. This behavior is attributed to the influence of the antiferromagnetic phase: inspired by previous experiments that show ferromagnetism to be most persistent at the surfaces and interfaces of FeRh thin films, we modelled the antiferromagnetic phase as forming a thin layer in the middle of the epilayer through which the two ferromagnetic layers are coupled up to a certain critical thickness. The development of this exchange stiffness is then consistent with that expected from the development of an exchange coupling across the magnetic phase boundary, as a consequence of a thickness dependent phase transition taking place in the antiferromagnetic regions and is supported by complimentary computer simulations of atomistic spin-dynamics. The development of the Gilbert damping parameter extracted from the ferromagnetic resonance investigations is consistent with this picture.",1807.01615v6 2019-01-02,Magnetization reorientation due to the superconducting transition in heavy-metal heterostructures,"Recent theoretical and experimental work has demonstrated how the superconducting critical temperature ($T_c$) can be modified by rotating the magnetization of a single homogeneous ferromagnet proximity-coupled to the superconducting layer. This occurs when the superconductor and ferromagnet are separated by a thin heavy normal metal that provides an enhanced interfacial Rashba spin-orbit interaction. We consider the reciprocal effect: magnetization reorientation driven by the superconducting phase transition. We solve the tight-binding Bogoliubov-de Gennes equations on a lattice self-consistently and find that the relative angle between the spin-orbit field and the magnetization gives rise to a contribution in the free energy even in the normal state due to band-structure effects. For temperatures below $T_c$, superconductivity gives rise to a competing contribution. By lowering the temperature, in addition to reorientation of the favored magnetization direction from in-plane to out-of-plane, a $\pi/4$ in-plane rotation for thicker ferromagnetic layers is possible. Furthermore, computation of $T_c$ of the structure in the ballistic limit shows a dependence on the in-plane orientation of the magnetization, in contrast to our previous result on the diffusive limit. This finding is relevant with respect to thin-film heterostructures since these are likely to be in the ballistic regime of transport. Finally, we discuss the experimental feasibility of observing the magnetic anisotropy induced by the superconducting transition when other magnetic anisotropies, such as the shape anisotropy for a ferromagnetic film, are taken into account. Our work suggests that the superconducting condensation energy in principle can trigger a reorientation of the magnetization of a thin-film ferromagnet upon lowering the temperature below $T_c$, in particular for ferromagnets with weak magnetic anisotropies.",1901.00501v3 2019-03-08,The critical temperature of the 2D-Ising model through Deep Learning Autoencoders,"We investigate deep learning autoencoders for the unsupervised recognition of phase transitions in physical systems formulated on a lattice. We focus our investigation on the 2-dimensional ferromagnetic Ising model and then test the application of the autoencoder on the anti-ferromagnetic Ising model. We use spin configurations produced for the 2-dimensional ferromagnetic and anti-ferromagnetic Ising model in zero external magnetic field. For the ferromagnetic Ising model, we study numerically the relation between one latent variable extracted from the autoencoder to the critical temperature $T_c$. The proposed autoencoder reveals the two phases, one for which the spins are ordered and the other for which spins are disordered, reflecting the restoration of the $\mathbb{Z}_2$ symmetry as the temperature increases. We provide a finite volume analysis for a sequence of increasing lattice sizes. For the largest volume studied, the transition between the two phases occurs very close to the theoretically extracted critical temperature. We define as a quasi-order parameter the absolute average latent variable ${\tilde z}$, which enables us to predict the critical temperature. One can define a latent susceptibility and use it to quantify the value of the critical temperature $T_c(L)$ at different lattice sizes and that these values suffer from only small finite scaling effects. We demonstrate that $T_c(L)$ extrapolates to the known theoretical value as $L \to \infty$ suggesting that the autoencoder can also be used to extract the critical temperature of the phase transition to an adequate precision. Subsequently, we test the application of the autoencoder on the anti-ferromagnetic Ising model, demonstrating that the proposed network can detect the phase transition successfully in a similar way.",1903.03506v2 2019-04-02,Relationship between charge redistribution and ferromagnetism at the heterointerface between perovskite oxides LaNiO$_3$ and LaMnO$_3$,"To investigate the relationship between the charge redistribution and ferromagnetism at the heterointerface between perovskite transition-metal oxides LaNiO$_3$ (LNO) and LaMnO$_3$ (LMO), we performed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) measurements. In the LNO/LMO heterostructures with asymmetric charge redistribution, the electrons donated from Mn to Ni ions are confined within one monolayer (ML) of LNO at the interface, whereas holes are distributed over 3-4 ML on the LMO side. A detailed analysis of the Ni-$L_{2,3}$ and Mn-$L_{2,3}$ XMCD spectra reveals that Ni magnetization is induced only by the Ni$^{2+}$ ions in the 1 ML LNO adjacent to the interface, while the magnetization of Mn ions is increased in the 3-4 ML LMO of the interfacial region. The characteristic length scale of the emergent (increased) interfacial ferromagnetism of the LNO (LMO) layers is in good agreement with that of the charge distribution across the interface, indicating a close relationship between the charge redistribution due to the interfacial charge transfer and the ferromagnetism of the LNO/LMO interface. Furthermore, the XMCD spectra clearly demonstrate that the vectors of induced magnetization of both ions are aligned ferromagnetically, suggesting that the delicate balance between the exchange interactions occurring inside each layer and across the interface may induce the canted ferromagnetism of Ni$^{2+}$ ions, resulting in weak magnetization in the 1 ML LNO adjacent to the interface.",1904.01335v1 2022-03-18,Quantum anomalous Hall effect and electric-field-induced topological phase transition in AB-stacked MoTe${}_2$/WSe${}_2$ moiré heterobilayers,"We propose a new mechanism to explain the quantum anomalous Hall (QAH) effect and the electric-field-induced topological phase transition in AB-stacked MoTe${}_2$/WSe${}_2$ moir\'e heterobilayers at $\nu=1$ hole filling. We suggest that the Chern band of the QAH state is generated from an intrinsic band inversion composed of the highest two moir\'e hole bands with opposite valley numbers and a gap opening induced by two Coulomb-interaction-driven magnetic orders. These magnetic orders, including an in-plane $120^{\circ}$-N\'eel order and an in-plane ferromagnetic order, interact with moir\'e bands via corresponding in-plane exchange fields. The N\'eel order ensures the insulating gap, the ferromagnetic order induces the non-zero Chern number, and both orders contribute to time-reversal symmetry breaking. The N\'eel order is acquired from the Hartree-Fock exchange interaction, and the formation of ferromagnetic order is attributed to interlayer-exciton condensation and exciton ferromagnetism. The exciton ferromagnetism can be demonstrated by excitonic Bose-Hubbard physics and Berezinskii-Kosterlitz-Thouless transition. In low electric fields, the equilibrium state is a Mott-insulator state. At a certain electric field, a correlated insulating state composed of the hole-occupied band and the exciton condensate becomes a new thermodynamically stable phase, and the topological phase transition occurs as the ferromagnetic order emerges. The consistency between the present theory and experimental observations is discussed. Experimental observations, including the spin-polarized/valley-coherent nature of the QAH state, the absence of charge gap closure at the topological phase transition, the canted spin texture, and the insulator-to-metal transition, are interpreted by the mechanism.",2203.10088v4 2022-07-10,Quantum phase transition from a paramagnetic Anderson insulating state to a ferromagnetic many-body localized state via an intermediate ferromagnetic metallic phase,"Effects of electron correlations on Anderson insulators have been one of the central themes for recent two decades, suggesting that the Anderson insulating phase turns into a novel insulating state referred to as many body localization (MBL). However, the role of spin degrees of freedom in this dynamical phase transition still remains unclarified as a function of the interaction strength. In this study, we perform real-space spin-resolved Hartree-Fock-Anderson simulations to investigate metal-insulator transitions above a critical disorder strength in three spatial dimensions, where all single-particle states are Anderson-localized without interactions. Here, relatively weak correlations below the Mott regime are taken into account in the mean-field fashion but disorder effects are introduced essentially exactly. We find two types of single-particle mobility edges, where the multifractal spectrum of the interaction-driven low-energy mobility edge deviates from that of the high-energy one smoothly connected with the multifractal spectrum of the metal-insulator transition without interactions. We show that the weakly interacting insulating phase remains to be a paramagnetic Anderson insulating state up to the temperature of the order of the band width. On the other hand, we uncover that the relatively strongly interacting insulating phase still below the Mott regime is ferromagnetic, which turns into a ferromagnetic metallic state at a critical temperature much lower than the order of the bandwidth. Based on all these results, we propose a quantum phase transition from a paramagnetic Anderson insulating state to a ferromagnetic MBL insulating phase via an intermediate ferromagnetic metallic state, which intervenes between these two insulators at the Fermi energy.",2207.04370v1 2022-11-03,Mn(Pt$_{1-x}$Pd$_{x}$)$_5$P: Isovalent Tuning of Mn Sublattice Magnetic Order,"We report the growth and characterization of MnPd$_5$P, a ferromagnet with T$_C$ $\approx$ 295 K, and conduct a substitutional study with its antiferromagnetic analogue MnPt$_5$P. We grow single crystals of MnPd$_5$P and Mn(Pt$_{1-x}$Pd$_x$)$_5$P by adding Mn into (Pt$_{1-x}$Pd$_{x}$)-P based melts. All compounds in the family adopt the layered anti-CeCoIn$_5$ structure with space group P4/mmm, and EDS and XRD results indicate that MnPt$_5$P and MnPd$_5$P form a solid solution. Based on magnetization and resistance data, we construct a T-x phase diagram for Mn(Pt$_{1-x}$Pd$_x$)$_5$P and demonstrate the antiferromagnetic order found in MnPt$_5$P is extraordinarily sensitive to Pd substitution. At low Pd fractions (x $<$ 0.010), the single antiferromagnetic transition in pure MnPt$_5$P splits into a higher temperature ferromagnetic transition followed on cooling by a lower temperature ferromagnetic to antiferromagnetic transition and then by a re-entrant antiferromagnetic to ferromagnetic transition at lower temperatures. The antiferromagnetic region makes up a bubble that persists to x $\approx$ 0.009 for T $\approx$ 150 K, with all samples x $<$ 0.009 recovering their initial ferromagnetic state with further cooling to base temperature. Over the same low x range we find a non-monotonic change in the room temperature unit cell volume, further suggesting that pure MnPt$_5$P is close to an instability. Once x $>$ 0.010, Mn(Pt$_{1-x}$Pd$_x$)$_5$P undergoes a single ferromagnetic transition. The Curie temperature increases rapidly with x, rising from T$_C$ $\approx$ 197 K at x = 0.013 to a maximum of T$_C$ $\approx$ 312 K for x $\approx$ 0.62, and then falls back to T$_C$ $\approx$ 295 K for pure MnPd$_5$P (x = 1). Given that Pt and Pd are isoelectronic, this work raises questions as to the origin of the extreme sensitivity of the magnetic ground state in MnPt$_5$P upon introducing Pd.",2211.01818v1 2023-05-23,Nano-Patterned Magnetic Edges in CrGeTe3 for Quasi 1-D Spintronic Devices,"The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices.",2305.14431v1 2023-07-20,Observation of long-range ferromagnetism via anomalous supercurrents in a spin-orbit coupled superconductor,"Conventional superconductors naturally disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. In superconductors with strong Rashba spin-orbit coupling, impurity magnetic moments induce supercurrents through the spin-galvanic effect. As a result, long-range ferromagnetic interaction among the impurity moments may be mediated through such anomalous supercurrents in a similar fashion as in itinerant ferromagnets. Fe(Se,Te) is such a superconductor with topological surface bands, previously shown to exhibit quantum anomalous vortices around impurity spins. Here, we take advantage of the flux sensitivity of scanning superconducting quantum interference devices to investigate superconducting Fe(Se,Te) in the regime where supercurrents around impurities overlap. We find homogeneous remanent flux patterns after applying a supercurrent through the sample. The patterns are consistent with anomalous edge and bulk supercurrents generated by in-plane magnetization, which occur above a current threshold and follow hysteresis loops reminiscent of those of a ferromagnet. Similar long-range magnetic orders can be generated by Meissner current under a small out-of-plane magnetic field. The magnetization weakens with increasing temperature and disappears after thermal cycling to above superconducting critical temperature; further suggesting superconductivity is central to establishing and maintaining the magnetic order. These observations demonstrate surface anomalous supercurrents as a mediator for ferromagnetism in a spin-orbit coupled superconductor, which may potentially be utilized for low-power cryogenic memory.",2307.10722v2 1993-07-20,Thouless number and spin diffusion in quantum Heisenberg ferromagnets,"Using an analogy between the conductivity tensor of electronic systems and the spin stiffness tensor of spin systems, we introduce the concept of the Thouless number $g_0$ and the dimensionless frequency dependent conductance $g( \omega )$ for quantum spin models. It is shown that spin diffusion implies the vanishing of the Drude peak of $g ( \omega )$, and that the spin diffusion coefficient $D_s$ is proportional to $g_0$. We develop a new method based on the Thouless number to calculate $D_s$, and present results for $D_s$ in the nearest-neighbor quantum Heisenberg ferromagnet at infinite temperatures for arbitrary dimension $d$ and spin $S$.",9307042v1 1993-10-28,Quantum Ferromagnetism and Phase Transitions in Double-Layer Quantum Hall Systems,"Double layer quantum Hall systems have interesting properties associated with interlayer correlations. At $\nu =1/m$ where $m$ is an odd integer they exhibit spontaneous symmetry breaking equivalent to that of spin $1/2$ easy-plane ferromagnets, with the layer degree of freedom playing the role of spin. We explore the rich variety of quantum and finite temperature phase transitions in these systems. In particular, we show that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnetic order.",9310071v1 1993-11-15,Quantum Heisenberg Chain with Long-Range Ferromagnetic Interactions at Low Temperature,"A modified spin-wave theory is applied to the one-dimensional quantum Heisenberg model with long-range ferromagnetic interactions. Low-temperature properties of this model are investigated. The susceptibility and the specific heat are calculated; the relation between their behaviors and strength of the long-range interactions is obtained. This model includes both the Haldane-Shastry model and the nearest-neighbor Heisenberg model; the corresponding results in this paper are in agreement with the solutions of both the models. It is shown that there exists an ordering transition in the region where the model has longer-range interactions than the HS model. The critical temperature is estimated.",9311034v1 1994-07-07,Scaling properties of the ferromagnetic state in the Hubbard model,"A numerical scaling analysis is used to show that Nagaoka's ferromagnetic state in two-dimensional Hubbard model with one hole is supersede by an antiferromagnetic (AF) state with a discontinuous jump in the total spin due to the AF coupling as the Hubbard $U$ is made finite. The same applies to the two-hole system, which has a spiral spin structure. We can show, via the scaling, that the crossover to an AF state is a precursor of a pathological coalescence of states having the minimum spin and Nagaoka's state at $U=\infty$ in the thermodynamic limit.",9407036v1 1994-08-19,Quantum description of spherical spins,"The spherical model for spins describes ferromagnetic phase transitions well, but it fails at low temperatures. A quantum version of the spherical model is proposed. It does not induce qualitative changes near the phase transition. However, it produces a physical low temperature behavior. The entropy is non-negative. Model parameters can be adapted to the description of real quantum spins. Several applications are discussed. Zero-temperature quantum phase transitions are analyzed for a ferromagnet and a spin glass in a transversal field. Their crossover exponents are presented.",9408055v3 1994-10-19,Random Frustration in two-dimensional spin-1/2 Heisenberg antiferromagnet,"The square lattice spin-1/2 antiferromagnet containing a dilute concentration, $\delta$, of randomly placed ferromagnetic nearest-neighbor bonds is studied at low-temperature via non-linear $\sigma$-model techniques and by exact diagonalization. We generally find that long-range N\' eel order is destroyed above a critical strength in the defective ferromagnetic exchange coupling-constant given by $|K_c|/J\sim \delta^{-1/2}$. We also observe large statistical fluctuations both in the spin-stiffness and in the antiferromagnetic structure-factor near this critical point, suggesting the onset of a spin-glass phase.",9410075v2 1994-12-14,Robustness of a local Fermi Liquid against Ferromagnetism and Phase Separation,"We study the properties of Fermi Liquids with the microscopic constraint of a local self-energy. In this case the forward scattering sum-rule imposes strong limitations on the Fermi-Liquid parameters, which rule out any Pomeranchek instabilities. For both attractive and repulsive interactions, ferromagnetism and phase separation are suppressed. Superconductivity is possible in an s-wave channel only. We also study the approach to the metal-insulator transition, and find a Wilson ratio approaching 2. This ratio and other properties of Sr_{1-x}La_xTiO_3 are all consistent with the local Fermi Liquid scenario.",9412068v1 1995-01-06,"Universal Finite-Size Scaling Function of the Ferromagnetic Heisenberg Chain in a Magnetic Field,","The finite-size scaling function of the magnetization of the ferromagnetic Heisenberg chain is argued to be universal with respect to the magnitude of the spin. The finite-size scaling function is given explicitly by an analytical calculation in the classical limit $S=\infty.$ The universality is checked for $S=1/2$ and $1$ by means of numerical calculations. Critical exponents are obtained as well. It is concluded that this universal scaling function originates in the universal behavior of the correlation function.",9501013v1 1995-01-20,Bounds on the mass gap of the ferromagnetic XXZ chain,"We prove rigorous lower and upper bounds for the mass gap of the ferromagnetic spin 1/2 XXZ chain. The two bounds coincide asymptotically in the Ising limit $\Delta\to\infty$. Near the isotropic point, $\Delta=1$, the estimates are good enough to determine the critical behaviour of the mass gap unambiguously. The derivation does not rely on exact solutions.",9501098v1 1995-02-13,Griffiths singularity in the two dimensional random bond disordered Ising ferromagnet,"For the two dimensional random bond disordered Ising ferromagnet, we measured bulk data of the magnetic susceptibility ($\chi$) and correlation length ($\xi$) up to $\xi \simeq 536$, with the use of a novel finite size scaling Monte Carlo technique. Our data are exclusively consistent with normal power-law critical behaviors with only one singular point at criticality, disproving the existence of Griffiths singularity even in an extremely deep scaling region. The critical exponents of $\chi$ and $\xi$ increase continuously with the strength of disorder.",9502053v2 1995-06-20,Universal Finite-Size Scaling Function of the Ferromagnetic Heisenberg Chain in a Magnetic Field. II,"The finite-size scaling function of the nonlinear susceptibility of the ferromagnetic Heisenberg chain is given explicitly. It is conjectured that the scaling function is universal for any values of $S$. The conjecture is based on the exact solution of the nonlinear susceptibility for $S=\infty$, and numerical calculations for $S=1/2$ and $S=1$.",9506087v1 1995-10-13,Density Matrix Renormalization Group Study of the Spin 1/2 Heisenberg Ladder with Antiferromagnetic Legs and Ferromagnetic Rungs,"The ground state and low lying excitation of the spin 1/2 Heisenberg ladder with antiferromagnetic leg ($J$) and ferromagnetic rung ($-\lambda J, \lambda >0$) interaction is studied by means of the density matrix renormalization group method. It is found that the state remains in the Haldane phase even for small $\lambda \sim 0.02$ suggesting the continuous transition to the gapless phase at $\lambda = 0$. The critical behavior for small $\lambda$ is studied by the finite size scaling analysis. The result is consistent with the recent field theoretical prediction.",9510071v1 1995-10-18,Zero field muon spin lattice relaxation rate in a Heisenberg ferromagnet at low temperature,"We provide a theoretical framework to compute the zero field muon spin relaxation rate of a Heisenberg ferromagnet at low temperature. We use the linear spin wave approximation. The rate, which is a measure of the spin lattice relaxation induced by the magnetic fluctuations along the easy axis, allows one to estimate the magnon stiffness constant.",9510096v1 1995-12-02,The Magnus Force on Skyrmions in Ferromagnets and the Quantum Hall Effect,"The topological solitons, or ``skyrmions'', in a planar ferromagnet experience a Magnus force proportional to the product of their velocity and the surrounding magnetization. It has been suggested that the charged quasiparticles near filling factor $\nu=1$ in the $GaAs$ quantum Hall effect are skyrmions. If so we might expect this spin-induced Magnus force to act on the quasiparticles in addition to the Lorentz force they experience because of their charge. We show that this is not the case, and that the Magnus and Lorentz forces are merely different descriptions of the same physical effect.",9512010v1 1995-12-08,Ferromagnetism in the Hubbard model on the square lattice: Improved instability criterion for the Nagaoka state,"The instability of the fully polarized ferromagnetic state (Nagaoka state) with respect to single spin flips is re-examined for the Hubbard model on the square lattice with a large family of variational wave functions which include correlation effects of the majority spins in the vicinity of the flipped spin. We find a critical hole density of $\delta_{cr} = 0.251$ for $U = \infty$ and a critical coupling of $U_{cr} = 77.7 t$. Both values improve previous variational results considerably.",9512060v1 1995-12-19,Finite-Size Scaling and Universality in the Spin 1 Quantum XY Chain,"The spin-1 XY chain in a transverse field is studied using finite-size scaling. The ground state phase diagram displays a paramagnetic, an ordered ferromagnetic and an ordered oscillatory phase. The paramagnetic-ferromagnetic transition line belongs to the universality class of the 2D Ising model. Along this line, universality is confirmed for the finite-size scaling functions of several correlation lengths and for the conformal operator content.",9512137v1 1996-01-03,Quantum critical behavior of disordered itinerant ferromagnets,"The quantum ferromagnetic transition at zero temperature in disordered itinerant electron systems is considered. Nonmagnetic quenched disorder leads to diffusive electron dynamics that induces an effective long-range interaction between the spin or order parameter fluctuations of the form r^{2-2d}, with d the spatial dimension. This leads to unusual scaling behavior at the quantum critical point, which is determined exactly. In three-dimensional systems the quantum critical exponents are substantially different from their finite temperature counterparts, a difference that should be easily observable. Experiments to check these predictions are proposed.",9601008v2 1996-01-22,Finite size corrections on the boundary between the spin-glass and the ferromagnetic phases of Derrida's model,"The boundary line between the ferromagnetic and the spin-glass phases was investigated. Finite size corrections to the free energy and magnetization were calculated. The situation coincides with the case in the information theory, when the transmission rate equals to the capacity of channel.",9601092v3 1996-02-01,Magnetic coupling in mesoscopic metal/ferromagnet layered systems,"We consider a mesoscopic mechanism of the exchange interaction in a system of alternating ferromagnetic/nonmagnetic metallic layers. In the case of small mesoscopic samples the sign and the amplitude of the exchange energy turn out to be random sample specific quantities. They can be changed by applying an external magnetic field, by attaching to the system superconducting electrodes with different phases of the superconducting order parameter and by changing the chemical potential of electrons in the matal with the help of a gate. In the case of square or cubic geometries of the nonmagnetic layer at low temperature the variance of the exchange energy turns out to be sample size independent.",9602005v1 1996-02-21,Universal low-temperature properties of quantum and classical ferromagnetic chains,"We identify the critical theory controlling the universal, low temperature, macroscopic properties of both quantum and classical ferromagnetic chains. The theory is the quantum mechanics of a single rotor. The mapping leads to an efficient method for computing scaling functions to high accuracy.",9602114v2 1996-03-26,Spin Diffusion in 2D XY Ferromagnet with Dipolar Interaction,"In the ordered phase of 2D XY ferromagnet the dipolar interaction between spins induces a strong, relevant interaction between spin-waves. We study quasi-excitations of the interacting spin-wave 'liquid' in the long wavelength limit. We employ the Janssen-De-Dominicis method for classical Langevin equation to find the transformation of the spin-wave excitation into a new soft-mode excitation in the intermediate range of wavelengths; and into an anomalous anisotropic diffusion mode excitation at long wavelengths. The dissipation of a spin-wave at short wavelengths is found to be highly anisotropic.",9603160v1 1996-03-26,Low density ferromagnetism in the Hubbard model,"A single-band Hubbard model with nearest and next-nearest neighbour hopping is studied for $d=1$, 2, 3, using both analytical and numerical techniques. In one dimension, saturated ferromagnetism is found above a critical value of $U$ for a band structure with two minima and for small and intermediate densities. This is an extension of a scenario recently proposed by M\""uller--Hartmann. For three dimensions and non-pathological band structures, it is proven that such a scenario does not work.",9603163v1 1996-03-30,"Spectral Flow and Vortex Dynamics in Superfluids, Superconductors and Ferromagnets","We discuss the nondissipative Magnus-type force acting on linear defects in Fermi systems, such as Abrikosov vortices in superconductors, singular and continuous vortices in superfluid phases of $^3$He, magnetic vortices and skyrmions in ferromagnets. Spectral flow of fermion zero modes in the vortex core gives an essential contribution to the nondissipative force, which is confirmed in Manchester experiments on rotating $^3$He-B. The spectral flow effect is closely related to the angular momentum paradox.",9603197v1 1996-05-08,Many skyrmion wave functions and skyrmion statistics in quantum Hall ferromagnets,"We determine the charge and statistical angle of skyrmions in quantum Hall ferromagnets by performing Berry phase calculations based on the microscopic variational wave functions for many-skyrmion states. We find, in contradiction to a recent claim by Dziarmaga, that both the charge and the statistical angle of a skyrmion are independent of its spin (size), and are identical to those of Laughlin quasiparticles at the same filling factor. We discuss some subtleties in the use of these variational wave functions.",9605054v1 1996-06-20,Low Temperature Magnetic Properties of the Double Exchange Model,"We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener to explain ferromagnetism has unexpected properties when there is more than one itinerant electron. We find that, in general, the many-body ground state of the DE model is {\it not} globally FM ordered (except for special filled-shell cases). Also, the low energy excitations of this model are distinct from spin wave excitations in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.",9606148v1 1996-07-22,The annealed positions of ferromagnetic bonds doped into a 2D antiferromagnet,"We determine where a given concentration of ferromagnetic (FM) bonds doped into a square lattice antiferromagnet must go to minimize the system's total magnetic energy. We find (i) an infinite degeneracy of ground--state arrangements of FM bonds that correspond to completely unfrustrated configurations for classical spins, and (ii) this degeneracy is lifted when quantum fluctuations are included, and phase separated ground states, such as periodic arrays of stripes of FM bonds, are found. A discussion of the application of these ideas to doped cuprate high $T_c$ superconductors with annealed disorder is presented.",9607154v1 1996-07-30,An extension of the characteristic angle method to the easy-plane spin-3/2 ferromagnet,"The Characteristic Angle (CA) method [Lei Zhou and Ruibao Tao, J. Phys. A, {\bf 27} 5599] developed previously for the easy-plane spin-1 magnetic systems has been successfully extended to the spin-3/2 case. A compact form of the CA spin-3/2 operator transformation is given, then the ground state energy, the magnon dispersion relation and the spontaneous magnetization are discussed for an easy-plane spin-3/2 ferromagnet by using the CA method. Comparisons with the old theoretical methods are made in the end.",9607218v1 1996-09-05,Magnetization switching in nanoscale ferromagnetic grains: simulations with heterogeneous nucleation,"We present results obtained with various types of heterogeneous nucleation in a kinetic Ising model of magnetization switching in single-domain ferromagnetic nanoparticles. We investigate the effect of the presence of the system boundary and make comparison with simulations on periodic lattices. We also study systems with bulk disorder and compare how two different types of disorder influence the switching behavior.",9609056v1 1996-09-26,Ferromagnetism in the two dimensional t-t' Hubbard model at the Van Hove density,"Using an improved version of the projection quantum Monte Carlo technique, we study the square-lattice Hubbard model with nearest-neighbor hopping t and next-nearest-neighbor hopping t', by simulation of lattices with up to 20 X 20 sites. For a given R=2t'/t, we consider that filling which leads to a singular density of states of the noninteracting problem. For repulsive interactions, we find an itinerant ferromagnet (antiferromagnet) for R=0.94 (R=0.2). This is consistent with the prediction of the T-matrix approximation, which sums the most singular set of diagrams.",9609253v1 1996-10-25,Protracted Screening in the Periodic Anderson Model,"The asymmetric infinite-dimensional periodic Anderson model is examined with a quantum Monte Carlo simulation. For small conduction band filling, we find a severe reduction in the Kondo scale, compared to the impurity value, as well as protracted spin screening consistent with some recent controversial photoemission experiments. The Kondo screening drives a ferromagnetic transition when the conduction band is quarter-filled and both the RKKY and superexchange favor antiferromagnetism. We also find RKKY-driven ferromagnetic and antiferromagnetic transitions.",9610188v1 1996-11-05,Chiral Metal as a Heisenberg Ferromagnet,"The two dimensional surface of an integer quantum hall multilayer is mapped onto a Heisenberg spin-chain with ferromagnetic coupling. Using this mapping it is shown non-perturbatively that the surface states constitute a very anisotropic metal in the infinite size limit. For multilayers of finite size, two diffusive mesoscopic regimes are identified and the conductance fluctuations are calculated perturbatively for both. The Heisenberg spin-chain representation is used to study the directed wave problem and the exact result is obtained that the mean-square deflection of a directed wave grows as the square root of the propagation of the distance.",9611035v1 1996-12-07,Resistivity due to a Domain Wall in Ferromagnetic Metal,"The resistivity due to a domain wall in ferromagnetic metallic wire is calculated based on the linear response theory. The interaction between conduction electrons and the wall is expressed in terms of a classical gauge field which is introduced by the local gauge transformation in the electron spin space. It is shown that the wall contributes to the decoherence of electrons and that this quantum correction can dominate over the Boltzmann resisitivity, leading to a decrease of resisitivity by nucleation of a wall. The conductance fluctuation due to the motion of the wall is also investigated. The results are compared with recent experiments.",9612067v1 1996-12-18,Interplay of Mott Transition and Ferromagnetism in the Orbitally Degenerate Hubbard Model,"A slave boson representation for the degenerate Hubbard model is introduced. The location of the metal to insulator transition that occurs at commensurate densities is shown to depend weakly on the band degeneracy M. The relative weights of the Hubbard sub-bands depend strongly on M, as well as the magnetic properties. It is also shown that a sizable Hund's rule coupling is required in order to have a ferromagnetic instability appearing. The metal to insulator transition driven by an increase in temperature is a strong function of it.",9612172v1 1997-02-12,Tilting instability and other anomalies in the flux-lattice in some magnetic superconductors,"The flux-line lattice in the compound $ErNi_2B_2C$, which has a tendency to ferromagnetic order in the a-b plane is studied with external magnetic field direction close to the c-axis. We show the existence of an instability where the direction of flux-lines spontaneously tilts away from that of the applied field near the onset of ferromagnetic order. The enhanced fluctuations in the flux lattice and the square flux lattice recently observed are explained and further experiments suggested.",9702104v1 1997-02-25,Low-Temperature Scaling Regime of Random Ferromagnetic-Antiferromagnetic Spin Chains,"Using the Continuous Time Quantum Monte Carlo Loop algorithm, we calculate the temperature dependence of the uniform susceptibility, and the specific heat of a spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings, down to very low temperatures. Our data show a consistent scaling behavior in both quantities and support strongly the conjecture drawn from the approximative real-space renormalization group treatment. A statistical analysis scheme is developed which will be useful for the search scaling behavior in numerical and experimental data of random spin chains.",9702215v1 1997-03-04,Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9},"Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions, we study the effect of iron disproportionation on the magnetic ordering, and describe the first-order magnetic transition occurring in the perovskite Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of the exchange integrals for the antiferromagntic and ferromagnetic interactions, in agreement with previous works on kindered compounds. Spin-wave theory yields a magnon spectrum with a gapless antiferromagnetic mode together with two gapped ferromagnetic ones.",9703044v1 1997-03-06,Ferromagnetic spin fluctuation induced superconductivity in Sr_2RuO_4,"We propose a quantitative model for triplet superconductivity in Sr_2RuO_4 based on first principles calculations for the electronic structure and magnetic susceptibility. The superconductivity is due to ferromagnetic spin fluctuations, that are strong at small wave vectors. The calculated effective mass renormalization, renormalized susceptibility, and superconducting critical temperature are all in good agreement with experiment. The most stable superconducting solution has order parameters of comparable magnitude on all three sheets of the Fermi surface. We also propose an explanation of the discrepancy between the Fermi surface topology observed in the photoemission and in de Haas-van Alphen experiments.",9703068v2 1997-03-07,Quantum Nucleation in a Ferromagnetic Film Placed in a Magnetic Field at an Arbitrary Angle,"We study the quantum nucleation in a thin ferromagnetic film placed in a magnetic field at an arbitrary angle. The dependence of the quantum nucleation and the temperature of the crossover from thermal to quantum regime on the direction and the strength of the applied field are presented. It is found that the maximal value of the rate and that of the crossover temperature are obtained at a some angle with the magnetic field, not in the direction of the applied field opposite to the initial easy axis.",9703069v1 1997-03-14,High Temperature Thermodynamics of the Ferromagnetic Kondo-Lattice Model,"We present a high temperature series expansion for the ferromagnetic Kondo lattice model in the large coupling limit, which is used to model CMR perovskites. Our results show the expected cross-over to Curie-Wei{\ss} behavior at a temperature related to the bandwidth. Estimates for the magnetic transition temperatures are in the experimentally observed range. The compressibility shows that the high temperature charge excitations can be modeled by spinless fermions. The CMR effect itself, however, warrants the inclusion of dynamic effects and cannot be explained by a static calculation.",9703146v1 1997-03-24,A theoretical investigation of the specific heat of superlattices in a magnetic field,"We analyze the specific heat variations as a function of an external magnetic field of a simple model of superlattice that includes (i) in--plane ferromagnetic exchange, (ii) interplane ferromagnetic exchange, (iii) dipolar interactions, (iv) magnetocristalline anisotropy. The calculations are carried out at the spin wave level. The interplay between the existence of a canting transition and the anisotropy of the structure generates non trivial behavior for the spin wave contribution to the low temperature specific heat as a function of an external magnetic field when dipolar interactions and magnetocristalline anisotropy are taken into account.",9703202v1 1997-05-16,"Superconducting, ferromagnetic and antiferromagnetic phases in the t-t' Hubbard model","We apply a renormalization group approach to the determination of the phase diagram of the t-t' Hubbard model at the Van Hove filling, as function of t'/t, for small values of U/t. The model presents ferromagnetic, antiferromagnetic and d-wave superconducting phases. Antiferromagnetism and d-wave superconductivity arise from the same interactions, and compete in the same region of parameter space.",9705165v1 1997-05-19,Nucleation Theory of Magnetization Switching in Nanoscale Ferromagnets,"A nucleation picture of magnetization switching in single-domain ferromagnetic nanoparticles with high local anisotropy is discussed. Relevant aspects of nucleation theory are presented, stressing the effects of the particle size on the switching dynamics. The theory is illustrated by Monte Carlo simulations and compared with experiments on single particles.",9705189v1 1997-06-02,Phase Separation in Electronic Models for Manganites,"The Kondo lattice Hamiltonian with ferromagnetic Hund's coupling as a model for manganites is investigated. The classical limit for the spin of the (localized) $t_{2g}$ electrons is analyzed on lattices of dimension 1,2,3 and $\infty$ using several numerical methods. The phase diagram at low temperature is presented. A regime is identified where phase separation occurs between hole undoped antiferromagnetic and hole-rich ferromagnetic regions. Experimental consequences of this novel regime are discussed. Regions of incommensurate spin correlations have also been found. Estimations of the critical temperature in 3D are compatible with experiments.",9706014v2 1997-07-13,Stochastic Model and Equivalent Ferromagnetic Spin Chain with Alternation,"We investigate a non-equilibrium reaction-diffusion model and equivalent ferromagnetic spin 1/2 XY spin chain with alternating coupling constant. The exact energy spectrum and the n-point hole correlations are considered with the help of the Jordan-Wigner fermionization and the inter-particle distribution function method. Although the Hamiltonian has no explicit translational symmetry, the translational invariance is recovered after long time due to the diffusion. We see the scaling relations for the concentration and the two-point function in finite size analysis.",9707137v1 1997-08-01,Double exchange magnets: Spin-dynamics in the paramagnetic phase,"The electronic structure of perovskite manganese oxides is investigated in terms of a Kondo lattice model with ferromagnetic Hund coupling and antiferromagnetic exchange between $t_{2g}$-spins using a finite temperature diagonalization technique. Results for the dynamic structure factor are consistent with recent neutron scattering experiments for the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ . The susceptibility shows Curie-Weiss behaviour and is used to derive a phase diagram. In the paramagnetic phase carriers are characterized as ferromagnetic polarons in an antiferromagnetic spin liquid.",9708007v1 1997-08-11,Stationary Properties of a Randomly Driven Ising Ferromagnet,"We consider the behavior of an Ising ferromagnet obeying the Glauber dynamics under the influence of a fast switching, random external field. Analytic results for the stationary state are presented in mean-field approximation, exhibiting a novel type of first order phase transition related to dynamic freezing. Monte Carlo simulations performed on a quadratic lattice indicate that many features of the mean field theory may survive the presence of fluctuations.",9708073v1 1997-10-21,Two-Dimensional Quantum Ferromagnets,"We present 1/N Schwinger boson and quantum Monte Carlo calculations of the magnetization and NMR relaxation rate for the two-dimensional ferromagnetic Heisenberg model representing a quantum Hall system at filling factor nu=1. Comparing the analytic and numerical calculations, we find that the SU(N) version of Schwinger boson theory gives accurate results for the magnetization at low temperatures, whereas the O(N) model works well at higher temperatures.",9710220v2 1997-10-27,Thermodynamic Properties of Heisenberg Ferrimagnetic Spin Chains: Ferromagnetic-Antiferromagnetic Crossover,"We study thermodynamic properties of the one-dimensional Heisenberg ferrimagnet with antiferromagnetically exchange-coupled two kinds of spins 1 and 1/2. The specific heat and the magnetic susceptibility are calculated employing a modified spin-wave theory as well as a quantum Monte Carlo method. The specific heat is in proportion to $T^{1/2}$ at low enough temperatures but shows a Schottky-like peak at mid temperatures. The susceptibility diverges as $T^{-2}$. We reveal that at low temperatures the model is regarded as a ferromagnet, while at mid temperatures it behaves like a gapped antiferromagnet.",9710291v2 1997-11-17,Suppressed Coherence due to Orbital Correlations in the Ferromagnetically Ordered Metallic Phase of Mn Compounds,"Small Drude weight $D$ together with small specific heat coefficient $\gamma$ observed in the ferromagnetic phase of R$_{1-x}$A$_x$MnO$_3$ (R=La, Pr, Nd, Sm; A=Ca, Sr, Ba) are analyzed in terms of a proximity effect of the Mott insulator. The scaling theory for the metal-insulator transition with the critical enhancement of orbital correlations toward the staggered ordering of two $e_g$ orbitals such as $3x^2-r^2$ and $3y^2-r^2$ symmetries may lead to the critical exponents of $D \propto \delta^{u}$ and $\gamma \propto \delta^v$ with $u=3/2$ and $v=0$. The result agrees with the experimental indications.",9711153v1 1997-12-25,Interacting Boson Theory of the Magnetization Process of the Spin-1/2 Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chain,"The low temperature magnetization process of the ferromagnetic-antiferromagnetic Heisenberg chain is studied using the interacting boson approximation. In the low field regime and near the saturation field, the spin wave excitations are approximated by the $\delta$ function boson gas for which the Bethe ansatz solution is available. The finite temperature properties are calculated by solving the integral equation numerically. The comparison is made with Monte Carlo calculation and the limit of the applicability of the present approximation is discussed.",9712289v1 1998-01-05,Shadow Bands and Tunneling Magnetoresistance in Itinerant Electron Ferromagnets,"In itinerant electron ferromagnets spectral weight is transferred at finite temperatures from quasiparticle peaks located at majority and minority-spin band energies to shadow-band peaks. For a given Bloch wavevector and band index, the majority-spin shadow-band peak is located near the minority-spin quasiparticle energy and the minority-spin shadow-band peak is located near the majority-spin quasiparticle energy. This property can explain much of the temperature dependence seen in the magnetoresistance of magnetic tunnel junctions.",9801023v1 1998-01-13,Domain walls and the conductivity of mesoscopic ferromagnets,"Quantum interference phenomena in the conductivity of mesoscopic ferromagnets are considered, particularly with regard to the effects of geometric phases acquired by electrons propagating through regions of spatially varying magnetization (due, e.g., to magnetic domain walls). Weak localization and electron-electron interaction quantum corrections to the conductivity and universal conductance fluctuations are discussed. Experiments are proposed for multiply-connected geometries that should reveal conductance oscillations with variations of the profile of the magnetization.",9801113v1 1998-01-20,1/N expansion for two-dimensional quantum ferromagnets,"The magnetization of a two-dimensional ferromagnetic Heisenberg model, which represents a quantum Hall system at filling factor nu=1, is calculated employing a large N Schwinger boson approach. Corrections of order 1/N to the mean field (N=infinity) results for both the SU(N) and the O(N) generalization of the bosonized model are presented. The calculations are discussed in detail and the results are compared with quantum Monte Carlo simulations as well as with recent experiments. The SU(N) model describes both Monte Carlo and experimental data well at low temperatures, whereas the O(N) model is much better at moderate and high temperatures.",9801203v1 1998-01-30,Thermally assisted macroscopic quantum tunneling of a ferromagnetic particle in a magnetic field at an arbitrary angle,"At finite temperature we study the quantum tunneling of magnetization for a small ferromagnetic particle with the biaxial symmetry placed in a magnetic field at an arbitrary angle. We present numerical WKB exponent below the crossover temperature in which the quantum tunneling is affected by the thermal activation, and the approximate form of the WKB exponent around the crossover region. The effect of quantum fluctuations on the thermal activation rate beyond the crossover regime are discussed.",9801319v1 1998-02-13,Antiferromagnetic interlayer exchange coupling across an amorphous metallic spacer layer,"By means of magneto-optical Kerr effect we observe for the first time antiferromagnetic coupling between ferromagnetic layers across an amorphous metallic spacer layer. Biquadratic coupling occurs at the transition from a ferromagnetically to an antiferromagnetically coupled region. Scanning tunneling microscopy images of all involved layers are used to extract thickness fluctuations and to verify the amorphous state of the spacer. The observed antiferromagnetic coupling behavior is explained by RKKY interaction taking into account the amorphous structure of the spacer material.",9802148v1 1998-03-12,Improved Composite-Boson Theory of Quantum Hall Ferromagnets and Skyrmions without Sigma Models,"We propose an improved composite-boson theory of quantum Hall ferromagnets, where the field operator describes solely the physical degrees of freedom representing the deviation from the ground state. In this scheme skyrmions appear merely as generic excitations confined in the lowest Landau level. We evaluate the excitation energy of one skyrmion. Our theoretical estimation accounts for the activation-energy data due to Schmeller et al. remarkably well.",9803146v2 1998-04-03,Spherical Model for Anisotropic Ferromagnetic Films,"The corrections to the Curie temperature T_c of a ferromagnetic film consisting of N layers are calculated for N \gg 1 for the model of D-component classical spin vectors in the limit D \to \infty, which is exactly soluble and close to the spherical model. The present approach accounts, however, for the magnetic anisotropy playing the crucial role in the crossover from 3 to 2 dimensions in magnetic films. In the spatially inhomogeneous case with free boundary conditions the D=\infty model is nonequivalent to the standard spherical one and always leads to the diminishing of T_c(N) relative to the bulk.",9804043v2 1998-04-06,Folding Transitions of the Square-Diagonal Lattice,"We address the problem of ""phantom"" folding of the tethered membrane modelled by the two-dimensional square lattice, with bonds on the edges and diagonals of each face. Introducing bending rigidities $K_1$ and $K_2$ for respectively long and short bonds, we derive the complete phase diagram of the model, using transfer matrix calculations. The latter displays two transition curves, one corresponding to a first order (ferromagnetic) folding transition, and the other to a continuous (anti-ferromagnetic) unfolding transition.",9804073v1 1998-04-21,"The randomly driven Ising ferromagnet, Part I: General formalism and mean field theory","We consider the behavior of an Ising ferromagnet obeying the Glauber dynamics under the influence of a fast switching, random external field. After introducing a general formalism for describing such systems, we consider here the mean-field theory. A novel type of first order phase transition related to spontaneous symmetry breaking and dynamic freezing is found. The non-equilibrium stationary state has a complex structure, which changes as a function of parameters from a singular-continuous distribution with Euclidean or fractal support to an absolutely continuous one.",9804230v1 1998-04-21,"The randomly driven Ising ferromagnet, Part II: One and two dimensions","We consider the behavior of an Ising ferromagnet obeying the Glauber dynamics under the influence of a fast switching, random external field. In Part I, we introduced a general formalism for describing such systems and presented the mean field theory. In this article we derive results for the one dimensional case, which can be only partially solved. Monte Carlo simulations performed on a square lattice indicate that the main features of the mean field theory survive the presence of strong fluctuations.",9804233v1 1998-04-28,Interlayer Magnetic Coupling and the Quantum Hall Effect in Multilayer Electron Systems,"We study the effect that the electron-electron interaction has on the properties of a multilayer electron system. We consider the case corresponding to filling factor unity in each layer. We find that as a function of the sample parameters the system has ferromagnetic, canted antiferromagnetic or paramagnetic interlayer spin correlations. These three ground states are QHE phases, because of the existence of a finite activation energy. In the ferromagnetic phase the gap is due to the intrawell exchange energy, whereas in the paramagnetic phase the gap appears due to the spatial modulation of the interwell coherence.",9804298v1 1998-04-30,Manganites at Low Temperatures and Light Doping:Band Approach and Percolation,"A tight-band model is employed for the $e_{2g}$-orbitals in manganites. It is shown that a large intraatomic Hund's coupling, $J_H$ and the resulting double-exchange mechanism lead to antiferromagnetic ordering along one of the cubic axis stabilized by the cooperative JT effect which further decreases the band energy of electrons. As a result, LaMnO$_3$ is a band insulator built of 2D ferromagnetic layers. The critical concentration $x_c\simeq 0.16$, for onset of ferromagnetic and metallic behavior at low temperatures in La$_{1-x}$Sr$_x$MnO$_3$ and the phase transition are treated in the percolation approach.",9804328v1 1998-05-05,Integral Relaxation Time of Single-Domain Ferromagnetic Particles,"The integral relaxation time \tau_{int} of thermoactivating noninteracting single-domain ferromagnetic particles is calculated analytically in the geometry with a magnetic field H applied parallel to the easy axis. It is shown that the drastic deviation of \tau_{int}^{-1} from the lowest eigenvalue of the Fokker-Planck equation \Lambda_1 at low temperatures, starting from some critical value of H, is the consequence of the depletion of the upper potential well. In these conditions the integral relaxation time consists of two competing contributions corresponding to the overbarrier and intrawell relaxation processes.",9805056v2 1998-05-12,The Two Dimensional XY Spin Glass with Ferromagnetic Next-Nearest-Neighbour Interactions,"The random-bond XY spin glass with ferromagnetic next-nearest-neighbour interactions is studied on a square lattice by Monte Carlo simulations. We find strong evidence for a finite-temperature spin glass transition at $T_c\approx 1.1$. We also give estimates for the spin glass critical exponents for different values of the strength of the nearest-neighbour interaction. Our results are consistent with universal behaviour.",9805141v1 1998-05-24,Thermally assisted domain walls quantum tunneling at the high temperature range,"A theoretical and numerical investigations of the quantum tunneling of the domain walls in ferromagnets and weak ferromagnets was performed taking into account the interaction between walls and thermal excitations of a crystal. The numerical method for calculations of the probability of a thermally stimulated quantum depinning as the function of temperature at the wide temperature range has been evolved.",9805304v2 1998-06-06,Magnons and skyrmions in fractional Hall ferromagnets,"Recent experiments have established a qualitative difference between the magnetization temperature-dependences $M(T)$ of quantum Hall ferromagnets at integer and fractional filling factors. We explain this difference in terms of the relative energies of collective magnon and particle-hole excitations in the two cases. Analytic calculations for hard-core model systems are used to demonstrate that, in the fractional case, interactions suppress the magnetization at finite temperatures and that particle-hole excitations rather than long-wavelength magnons control $M(T)$ at low $T$.",9806087v1 1998-06-28,Combination of Ferromagnetic and Antiferromagnetic Features in Heisenberg Ferrimagnets,"We investigate the thermodynamic properties of Heisenberg ferrimagnetic mixed-spin chains both numerically and analytically with particular emphasis on the combination of ferromagnetic and antiferromagnetic features. Employing a new density-matrix renormalization-group technique as well as a quantum Monte Carlo method, we reveal the overall thermal behavior: At very low temperatures, the specific heat and the magnetic susceptibility times temperature behave like $T^{1/2}$ and $T^{-1}$, respectively, whereas at intermediate temperatures, they exhibit a Schottky-like peak and a minimum, respectively. Developing the modified spin-wave theory, we complement the numerical findings and give a precise estimate of the low-temperature behavior.",9806344v1 1998-07-09,Magnitoelastic interaction and long-range magnetic ordering in two-dimesional ferromagnetics,"The influence of magnitoelastic (ME) interaction on the stabilization of long-range magnetic order (LMO) in the two-dimensional easy-plane ferromagnetic is investigated in this work. The account of ME exchange results in the root dispersion law of magnons and appearance of ME gap in the spectra of elementary excitations. Such a behavior of the spectra testifies to the stabilization of LMO and finite Curie's temperature.",9807129v1 1998-07-14,Spin-spin interaction and magnetic state of 2-D Wigner crystal,"It is demonstrated that there is anti-ferromagnetic spin-spin interaction between nearest electrons in the 2D Wigner crystal. This is also valid for the Wigner liquid - the state with destroyed long-range order but preserved short-range one. We calculate the value of the anti-ferromagnetic interaction (both analytically and numerically) and discuss a possible magnetic state of the Wigner crystal. This state can be strongly influenced by the spin-Peierls mechanism.",9807195v1 1998-08-03,Thermodynamics of Random Ferromagnetic Antiferromagnetic Spin-1/2 Chains,"Using the quantum Monte Carlo Loop algorithm, we calculate the temperature dependence of the uniform susceptibility, the specific heat, the correlation length, the generalized staggered susceptibility and magnetization of a spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings, down to very low temperatures. Our data show a consistent scaling behavior in all the quantities and support strongly the conjecture drawn from the approximate real-space renormalization group treatment.A statistical analysis scheme is developed which will be useful for the search of scaling behavior in numerical and experimental data of random spin chains.",9808027v1 1998-08-05,Magnetization bound for classical spin models on graphs,"In this paper we prove the existence of phase transitions at finite temperature for O(n) classical ferromagnetic spin models on infrared finite graphs. Infrared finite graphs are infinite graphs with $\lim {m\to 0^+} {\bar Tr (L+m)^{-1} < \infty$, where $L$ is the Laplacian operator of the graph. The ferromagnetic couplings are only requested to be bounded by two positive constants. The proof, inspired by the classical result of Fr\""ohlich, Simon and Spencer on lattices, is given through a rigorous bound on the average magnetization. The result holds for $n\ge 1$ and it includes as a particular case the Ising model.",9808046v1 1998-08-25,Spin polarized tunneling in ferromagnet/unconventional superconductor junctions,"We study tunneling in ferromagnet/unconventional superconductor (F/S) junctions. We include the effects of spin polarization, interfacial resistance, and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev reflection (AR) at the F/S interface, governing tunneling at low bias voltage, is strongly modified by these parameters. The conductance exhibits a very wide variety of features as a function of applied voltage.",9808285v2 1998-08-26,Orbital Ordering Induced Antiferromagnetic Spin Fluctuations in the Ferromagnetic State of 50 % Hole-doped Manganites: Pr{1/2}Sr{1/2}MnO3 and Nd{1/2}Sr{1/2}MnO3,"We report inelastic neutron scattering results on the spin dynamics in the 50 % hole-doped manganites, Pr{1/2}Sr{1/2}MnO3 and Nd{1/2}Sr{1/2}MnO3. In the paramagnetic phase, these systems exhibit a ridge-type diffuse scattering, indicating a two-dimensional ferromagnetic (FM) correlation. With decreasing temperature, the systems enter the FM state, but simultaneously they develop A-type antiferromagnetic spin fluctuations. The spin wave dispersion in the FM state is strongly anisotropic. These behaviors can be consistently interpreted from a viewpoint of the underlying orbital ordering.",9808286v1 1998-09-02,Anomalous ferromagnetic spin fluctuations in an antiferromagnetic insulator Pr_{1-x}Ca_{x}MnO_{3},"The high temperature paramagnetic state in an antiferromagnetic (AFM) insulator Pr_{1-x}Ca_{x}MnO_{3} is characterized by the ferromagnetic (FM) spin fluctuations with an anomalously small energy scale. The FM fluctuations show a precipitous decrease of the intensity at the charge ordering temperature T_{CO}, but persist below T_{CO}, and vanish at the AFM transition temperature T_{N}. These results demonstrate the importance of the spin ordering for the complete switching of the FM fluctuation in doped manganites.",9809033v1 1998-09-18,Prediction of Ferromagnetic Ground State of NaCl-type FeN,"Ab-initio results for structural and electronic properties of NaCl-type FeN are presented in a framework of plane-wave and ultrasoft pseudopotentials. Competition among different magnetic ordering is examined. We find the ferromagnetic phase stable overall. Stabilization over the unpolarized phase is obtained by splitting one flat t_2g-type band crossing the Fermi energy. A comparison with CrN is considered. We find large differences in the properties of the two systems that can be addressed to the smaller ionicity and magnetization of FeN.",9809256v2 1998-09-25,Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic Films,"The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within Heisenberg spin models. Analytic expressions for the temperature dependent anisotropy are derived from which it is seen that the reduced magnetization in the film surface at finite temperatures plays a crucial role for this transition. Detailed phase diagrams in the temperature-thickness plane are calculated.",9809347v1 1998-09-30,Localization of spinwaves in the quantum Hall ferromagnet,"The quantum Hall ferromagnet at filling fraction $\nu = 1$ has some unusual properties due to the remarkable identity between the topological density of a spin distortion and the associated electrical charge density. We investigate the localization of spinwaves by coupling to a scalar disorder potential via their topological density. A low energy description of the system in terms of a non-linear sigma model of unitary supermatrices is derived. All states of this model are localized in two dimensions. A possible experimental signature of these effects in photo-luminescence is suggested.",9809406v1 1998-10-01,Improved Composite-Boson Theory of Monolayer and Bilayer Quantum Hall Ferromagnets,"An improved composite-boson theory of quantum Hall ferromagnets is formulated both for the monolayer and bilayer systems. In this scheme the field operator describes solely the physical degrees of freedom representing the deviation from the ground state. Skyrmions are charged excitations confined to the lowest Landau level. By evaluating the excitation energy of one skyrmion in the interlayer-coherent phase it is shown that the bilayer QH state becomes stabler as the interlayer density difference becomes larger.",9810003v1 1998-10-09,Zero Temperature Phases of the Electron Gas,"The stability of different phases of the three-dimensional non-relativistic electron gas is analyzed using stochastic methods. With decreasing density, we observe a {\it continuous} transition from the paramagnetic to the ferromagnetic fluid, with an intermediate stability range ($25\pm 5 \leq r_s\leq 35 \pm 5$) for the {\it partially} spin-polarized liquid. The freezing transition into a ferromagnetic Wigner crystal occurs at $r_s=65 \pm 10$. We discuss the relative stability of different magnetic structures in the solid phase, as well as the possibility of disordered phases.",9810126v1 1998-11-11,Density of Zeros on the Lee-Yang circle obtained from magnetization data of a two-dimensional Ising ferromagnet,"In order to provide an experimental access to the statistical theory of Lee and Yang (1952) the density function g(theta) of zeros on the Lee-Yang circle has been determined for the first time by analyzing isothermal magnetization data, m(H), of the Ising ferromagnet FeCl2 in axial magnetic fields, H, at temperatures 34 < T < 99K. The validity of our approach is demonstrated by the perfect agreement of magnetic specific heat data as calculated from g(theta) and m(H) via Maxwells relation, respectively. Moreover, the correct in-plane exchange constant of FeCl2 emerges from the series expansion of m(H) involved in the analysis.",9811157v1 1998-11-13,Spin-accumulation in small ferromagnetic double barrier junctions,"The non-equilibrium spin accumulation in ferromagnetic double barrier junctions is shown to govern the transport in small structures. Transport properties of such systems are described by a generalization of the theory of the Coulomb blockade. The spin accumulation enhances the magnetoresistance. The transient non-linear transport properties are predicted to provide a unique experimental evidence of the spin-accumulation in the form of a reversed current on time scales of the order of the spin-flip relaxation time.",9811179v1 1998-12-08,Crossover properties from random percolation to frustrated percolation,"We investigate the crossover properties of the frustrated percolation model on a two-dimensional square lattice, with asymmetric distribution of ferromagnetic and antiferromagnetic interactions. We determine the critical exponents nu, gamma and beta of the percolation transition of the model, for various values of the density of antiferromagnetic interactions pi in the range 00.",9812118v1 1998-12-11,Spin-Pseudospin Coherence and CP$^{3}$ Skyrmions in Bilayer Quantum Hall Ferromagnets,"We analyze bilayer quantum Hall ferromagnets, whose underlying symmetry group is SU(4). Spin-pseudospin coherence develops spontaneously when the total electron density is low enough. Quasiparticles are CP^3 skyrmions. One skyrmion induces charge modulations on both of the two layers. At the filling factor$\nu =2/m$ one elementary excitation consists of a pair of skyrmions and its charge is $2e/m$. Recent experimental data due to Sawada et al. [Phys. Rev. Lett. {\bf 80}, 4534 (1998)] support this conclusion.",9812188v2 1998-12-14,Non-equilibrium spin accumulation in ferromagnetic single-electron transistors,"We study transport in ferromagnetic single-electron transistors. The non- equilibrium spin accumulation on the island caused by a finite current through the system is described by a generalized theory of the Coulomb blockade. It enhances the tunnel magnetoresistance and has a drastic effect on the time- dependent transport properties. A transient decay of the spin accumulation may reverse the electric current on time scales of the order of the spin-flip relaxation time. This can be used as an experimental signature of the non- equilibrium spin accumulation.",9812224v1 1998-12-30,First order transitions and multicritical points in weak itinerant ferromagnets,"It is shown that the phase transition in low-T_c clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical endpoint. A semi-quantitative discussion is given in terms of recent experiments on MnSi, and predictions for other experiments are made.",9812420v2 1999-01-06,Thickness dependent Curie temperatures of ferromagnetic Heisenberg films,"We develop a procedure for calculating the magnetic properties of a ferromagnetic Heisenberg film with single-ion anisotropy which is valid for arbitrary spin and film thickness. Applied to sc(100) and fcc(100) films with spin S=7/2 the theory yields the layer dependent magnetizations and Curie temperatures of films of various thicknesses making it possible to investigate magnetic properties of films at the interesting 2D-3D transition.",9901036v1 1999-01-18,Kondo effect in a quantum critical ferromagnet,"We study the Heisenberg ferromagnetic spin chain coupled with a boundary impurity. Via Bethe ansatz solution, it is found that (i) for J>0, the impurity spin behaves as a diamagnetic center and is completely screened by 2S bulk spins in the ground state, no matter how large the impurity spin is; (ii) The specific heat of the local composite (impurity plus 2S bulk spins which form bound state with it) shows a simple power law $C_{loc}\sim T^{3/2}$. (iii)For J<0, the impurity is locked into the critical behavior of the bulk. Possible phenomena in higher dimensions are discussed.",9901167v2 1999-01-21,Possible Existence Of Topological Excitations In Quantum Spin Models In Low Dimensions,"The possibility of existence of topological excitations in the anisotropic quantum Heisenberg model in one and two spatial dimensions is studied using coherent state method. It is found that a part of the Wess-Zumino term contributes to the partition function, as a topological term for ferromagnets in the long wavelength limit in both one and two dimensions. In particular, the XY limit of the two-dimensional anisotropic ferromagnet is shown to retain the topological excitations, as expected from the quantum Kosterlitz-Thouless scenario.",9901209v1 1999-01-22,Skyrmions and Quantum Hall Ferromagnets in Improved Composite-Boson Theory,"An improved composite-boson theory of quantum Hall ferromagnets is proposed. It is tightly related with the microscopic wave-function theory. The characteristic feature is that the field operator describes solely the physical degrees of freedom representing the deviation from the ground state. It presents a powerful tool to analyze excited states within the \LLLd. Excitations include a Goldstone mode and nonlocal topological solitons. Solitons are vortices and Skyrmions carrying the U(1) and SU(2) topological charges, respectively. Their classical configurations are derived from their microscopic wave functions. The activation energy of one Skyrmion is estimated, which explains experimental data remarkably well.",9901236v1 1999-01-29,Theory of optical conductivity in doped manganites,"The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory. As these materials are cooled below the Curie temperature, the colossal magnetoressitance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by exchange interaction with the localized Mn spins during the transition to the low temperature ferromagnetic phase.",9901340v2 1999-02-15,Ballistic and diffuse transport through a ferromagnetic domain wall,"We study transport through ballistic and diffuse ferromagnetic domain walls in a two-band Stoner model with a rotating magnetization direction. For a ballistic domain wall, the change in the conductance due to the domain wall scattering is obtained from an adiabatic approximation valid when the length of the domain wall is much longer than the Fermi wavelength. In diffuse systems, the change in the resistivity is calculated using a diagrammatic technique to the lowest order in the domain wall scattering and taking into account spin-dependent scattering lifetimes and screening of the domain wall potential.",9902210v2 1999-02-18,Dephasing Effects by Ferromagnetic Boundary on Resistivity in Disordered Metallic Layer,"The resistivity of disordered metallic layer sandwiched by two ferromagnetic layers at low-temperature is investigated theoretically. It is shown that the magnetic field acting at the interface does not affect the classical Boltzmann resistivity but causes a dephasing among electrons in the presence of the spin-orbit interaction, suppressing the anti-localization due to the spin-orbit interaction. The dephasing turns out to be stronger in the case where the magnetization of the two layers is parallel, contributing to a positive magnetoresistance close to a switching field at low temperature.",9902256v1 1999-02-19,Orbital Degree of Freedom and Phase Separation in Ferromagnetic Manganites at Finite Temperatures,"The spin and orbital phase diagram for perovskite manganites are investigated as a function of temperature and hole concentration. The superexchange and double exchange interactions dominate the ferromagnetic phases in the low and high concentration regions of doped holes, respectively.The two interactions favor different orbital states each other. Between the phases, two interactions compete with each other and the phase separation appears in the wide range of temperature and hole concentration. The anisotropy of the orbital space causes discontinuous changes of the orbital state and promotes the phase separation. The relation between the phase separation and the stripe- and sheet-type charge segregation is discussed.",9902266v1 1999-03-02,Theory of the Josephson effect in superconductor / ferromagnet / superconductor junctions,"To reveal the influence of the exchange interaction on the Josephson effect, the d.c. Josephson current in unconventional-superconductor / ferromagnet / unconventional-superconductor junctions is studied. When the two superconductors have d-wave symmetry, the Josephson current is drastically suppressed with the increase of the exchange interaction. On the other hand, when the two superconductors have different parities, e.g. s-wave and p-wave, an enhancement of the Josephson current is obtained with the increment of the exchange interaction due to the break-down of SU(2) symmetry in the spin space.",9903034v3 1999-03-05,Barkhausen avalanches in anisotropic ferromagnets with $180^\circ$ domain walls,"We show that Barkhausen noise in two-dimensional disordered ferromagnets with extended domain walls is characterized by the avalanche size exponent $\tau_s =1.54$ at low disorder. With increasing disorder the characteristic domain size is reduced relative to the system size due to nucleation of new domains and a dynamic phase transition occurs to the scaling behavior with $\tau_s=1.30$. The exponents decrease at finite driving rate. The results agree with recently observed behavior in amorphous Metglas and Fe-Co-B ribbons when the applied anisotropic stress is varied.",9903090v2 1999-03-13,Colossal magnetooptical conductivity in doped manganites,"We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance (CMR) itself, the corresponding magnetooptical effect is explained by the dissociation of localized bipolarons into mobile polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations.",9903217v1 1999-03-15,Giant Mutual Proximity Effects in Ferromagnetic/Superconducting Nanostructures,"A strong mutual influence of superconductors (S) and ferromagnetic (F) conductors in hybrid F/S (Ni/Al) nanostructures is observed. The magnitude of a proximity-induced conductance on the F-side is more than two orders larger than that predicted by theory. Re-entrance of the superconductors to the normal state reciprocated by changes on the F-side has been found in low applied magnetic fields with new peaks in the differential resistance as an effect of the saturation magnetisation. An analysis has been developed providing a base for a numerical description of the system. PACS numbers: 74.50.+r, 74.80.Fp, 85.30St",9903237v1 1999-03-20,Comparison of superconductivity in Sr_2RuO_4 and copper oxides,"To compare the superconductivity in strongly correlated electron systems with the antiferromagnetic fluctuations in the copper oxides and with the ferromagnetic fluctuations in Sr_2RuO_4 a t-J-I model is proposed. The antiferromagnetic coupling J results in the superconducting state of d_{x^2-y^2} symmetry and the ferromagnetic coupling constant I results in the spin-triplet p-type state. The difference in the gap anisotropies provides the large difference in T_c values, for the typical values of the coupling constants: T_c of order of 1K for the ruthenate and T_c of order of 100K for the cuprates.",9903314v1 1999-04-08,Magnetic and quadrupolar order in a one-dimensional ferromagnet with cubic crystal-field anisotropy,"The zero temperature phase diagram of a one-dimensional S=2 Heisenberg ferromagnet with single-ion cubic anisotropy is studied numerically using the density-matrix renormalization group method. Evidence is found that although the model does not involve quadrupolar couplings, there is a purely quadrupolar phase for large values of the anisotropy. The phase transition between the magnetic and quadrupolar phases is continuous and it seems to be characterized by Ising critical exponents.",9904109v1 1999-04-17,Tunneling Via Individual Electronic States in Ferromagnetic Nanoparticles,"We measure electron tunneling via discrete energy levels in ferromagnetic cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due to magnetic anisotropy, the energy of each tunneling resonance shifts as an applied magnetic field rotates the particle's magnetic moment. We see both spin-increasing and decreasing tunneling transitions, but we do not observe the spin degeneracy at small magnetic fields seen previously in non-magnetic materials. The tunneling spectrum is denser than predicted for independent electrons, possibly due to spin-wave excitations.",9904248v2 1999-04-23,Shot noise in ferromagnetic single electron tunneling devices,"Frequency dependent current noise in ferromagnetic double junctions with Coulomb blockade is studied theoretically in the limit of sequential tunneling. Two different relaxation processes are found in the correlations between spin polarized tunneling currents; low frequency spin fluctuations and high frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lead to a negative differential resistance. We also show that large spin noise activated in the range of negative differential resistance gives rise to a significant enhancement of the current noise.",9904341v1 1999-05-08,H_c_3 for a thin-film superconductor with a ferromagnetic dot,"We investigate the effect of a ferromagnetic dot on a thin-film superconductor. We use a real-space method to solve the linearized Ginzburg-Landau equation in order to find the upper critical field, H_c_3. We show that H_c_3 is crucially dependent on dot composition and geometry, and may be significantly greater than H_c_2. H_c_3 is maximally enhanced when (1) the dot saturation magnetization is large, (2) the ratio of dot thickness to dot diameter is of order one, and (3) the dot thickness is large.",9905107v1 1999-05-14,Scaling and finte-size-scaling in the two dimensional random-coupling Ising ferromagnet,"It is shown by Monte Carlo method that the finite size scaling (FSS) holds in the two dimensional random-coupled Ising ferromagnet. It is also demonstrated that the form of universal FSS function constructed via novel FSS scheme depends on the strength of the random coupling for strongly disordered cases. Monte Carlo measurements of thermodynamic (infinite volume limit) data of the correlation length ($\xi$) up to $\xi \simeq 200$ along with measurements of the fourth order cumulant ratio (Binder's ratio) at criticality are reported and analyzed in view of two competing scenarios. It is demonstrated that the data are almost exclusively consistent with the scenario of weak universality.",9905202v1 1999-05-24,Soliton-Magnon Scattering in Two-Dimensional Isotropic Ferromagnets,"It is studied the scattering of magnons by the 2d topological Belavin-Polyakov soliton in isotropic ferromagnet. Analytical solutions of the scattering problem are constructed: (i) exactly for any magnon wave vectors for the partial wave with the azimuthal number m=1 (translational mode), and (ii) in the long- and short-wave limits for the rest modes. The magnon mode frequencies are found for the finite size magnets. An effective equation of the soliton motion is constructed. The magnon density of states, connected with the soliton-magnon interaction, is found in a long-wave approximation.",9905348v1 1999-05-27,Influence of spin-flip scattering on the stability of ferromagnetism in a two-band Hubbard model,"We investigate the influence of an interband exchange interaction on magnetism in a two-band Hubbard model. Our main emphasis lies on spin-flip scattering which is often neglected but is neccessary to retain the full rotational symmetry of the Hamiltonian. We find a striking dependence of the magnetization on the interband exchange coupling constant J and a substantial suppression of ferromagnetic order for a large range of values of J. The onset of an RKKY-like magnetic ordering mechanism is also observed.",9905396v1 1999-06-01,Two-dimensional behavior of the sublattice magnetization in three-dimensional Ising antiferromagnets,"A three-dimensional layered Ising-Antiferromagnet with a ferromagnetic intra-layer coupling to z neighbors, zJ > 0, and an antiferromagnetic interlayer coupling to z' neighbors, z'J' < 0, is investigated by Monte Carlo simulations on a hexagonal lattice. The physical nature of the anomalous temperature bahavior of the sublattice magnetizations, which is found for certain values of r=zJ/z'J' and z' in magnetic fields is explained in terms of successive phase transitions. They take place on the ferromagnetic 2-dimensional spin-down sublattice at T = T_c^{2d}, smeared by a finite stabilizing molecular field, and on both antiferromagnetically coupled sublattices at T_c^{3d} > T_c^{2d}.",9906003v1 1999-06-07,p-wave and d-wave superconductivity in quasi-two-dimensional metals,"We compare predictions of the mean-field theory of superconductivity for nearly antiferromagnetic and nearly ferromagnetic metals in two dimensions. The calculations are based on a parametrization of the effective interaction arising from the exchange of magnetic fluctuations. The Eliashberg equations for the transition temperature are solved including the full momentum dependence of the self-energy. The results show that for comparable parameters d-wave singlet pairing in nearly antiferromagnetic metals is generally much stronger than p-wave triplet pairing in nearly ferromagnetic metals in quasi two dimensions. The relevance to the layered materials, and in particular Sr2RuO4 that exhibits p-wave triplet pairing, is discussed.",9906091v1 1999-06-09,The anomalous metallic ferromagnetic state of Sr doped manganites,"We deduce a model relevant for the anomalous metallic state of Sr doped manganites at low temperatures within the ferromagnetic phase. It provides a natural explanation to several anomalous features observed experimentally, such as the vanishing Drude contribution in optical conductivity, the pseudo-gap in the density of states, an the unusual dispersion observed in photoemission.",9906131v2 1999-06-26,Ferromagnetism and superstructure in Ca_{1-x}La_xB_6,"We critically investigate the model of a doped excitonic insulator, which recently has been invoked to explain some experimental properties of the ferromagnetic state in Ca_{1-x}La_xB_6. We demonstrate that the ground state of this model is intrinsically unstable towards the appearance of a superstructure. In addition, the model would lead to a phase separation regime and the domain structure which may be prevented by the Coulomb forces only. Recent experiments indicate that a superstructure may indeed show up in this material.",9906401v1 1999-06-29,Quantitative Model of Large Magnetostrain Effect in Ferromagnetic Shape Memory Alloys,"A quantitative model describing large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni2MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties for a similar type materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Poisson's rule. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni2MnGa.",9906433v1 1999-07-08,Diffusion Thermopower of Ferromagnetic Transition metals,"This paper discusses a simple calculation for the diffusion thermopower of a transition metal ferromagnet. The main result of this calculation is that unlike in the case of a free-electron model, the diffusion thermopower S and its derivative with respect to temperature dS/dT have opposite signs. The results of this calculation agree qualitatively with experimental results in the high temperature region, where the diffusion thermopower is dominant.",9907116v3 1999-07-27,Triplet superconductivity in a one-dimensional ferromagnetic t-J model,"In this paper we study the ground state phase diagram of a one-dimensional $t-U-J$ model, at half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy, a phase with gapless charge and massive spin excitations, characterized by the coexistence of triplet superconducting ($TS$) and spin density wave ($SDW^{z}$) instabilities is realized in the ground state. With reduction of the bandwidth, a transition into an insulating phase showing properties of the spin-1/2 XY model takes place. In the case of weakly anisotropic antiferromagnetic exchange the system shows a long range dimerized (Peierls) ordering in the ground state. The complete weak-coupling phase diagram of the model, including effects of the on-site Hubbard interaction, is obtained.",9907415v1 1999-07-29,Calculation Method for the Three-Dimensional Ising Ferromagnet Thermodynamics within the Frames of $ρ^6$ Model,"Calculation of thermodynamic functions of the three-dimensional Ising ferromagnet above and below critical temperature is performed in the approximation of sixfold basis distribution ($\rho^6$ model). Comparison with the results for the $\rho^4$ model indicates that dependence of the thermodynamic functions on the renormalization group parameter $s$ becomes weaker. The optimal interval of the renormalization group parameter values is determined.",9907468v2 1999-08-03,Diffusion in disordered lattices and related Heisenberg ferromagnets,"We study the diffusion of classical hard-core particles in disordered lattices within the formalism of a quantum spin representation. This analogy enables an exact treatment of non-instantaneous correlation functions at finite particle densities in terms of single spin excitations in disordered ferromagnetic backgrounds. Applications to diluted chains and percolation clusters are discussed. It is found that density fluctuations in the former exhibit a stretched exponential decay while an anomalous power law asymptotic decay is conjectured for the latter.",9908051v1 1999-08-04,Large Magnetoresistance Ratio in Ferromagnetic Single-Electron Transistors in the Strong Tunneling Regime,"We study transport through a ferromagnetic single-electron transistor. The resistance is represented as a path integral, so that systems where the tunnel resistances are smaller than the quantum resistance can be investigated. Beyond the low order sequential tunneling and co-tunneling regimes, a large magnetoresistance ratio at sufficiently low temperatures is found. In the opposite limit, when the thermal energy is larger than the charging energy, the magnetoresistance ratio is only slightly enhanced.",9908061v2 1999-08-26,Percolative phase separation induced by nonuniformly distributed excess oxygens,"The zero-field $^{139}$La and $^{55}$Mn nuclear magnetic resonances were studied in $\rm La_{0.8}Ca_{0.2}MnO_{3+\delta}$ with different oxygen stoichiometry $\delta$. The signal intensity, peak frequency and line broadening of the $^{139}$La NMR spectrum show that excess oxygens have a tendency to concentrate and establish local ferromagnetic ordering around themselves. These connect the previously existed ferromagnetic clusters embedded in the antiferromagnetic host, resulting in percolative conduction paths. This phase separation is not a charge segregation type, but a electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides a direct evidence that phase separation is one source of colossal magnetoresistance effect.",9908371v1 1999-08-31,Spin resolved Andreev reflection in ferromagnet-superconductor junctions with Zeeman splitting,"Andreev reflection in ferromagnet-superconductor junctions is derived in a regime in which Zeeman splitting dominates the response of the superconductor to an applied magnetic field. Spin-up and spin-down Andreev reflections are shown to be resolved as voltage is increased. In the metallic limit, the transition from Andreev to tunnel conductivity in the spin-up channels has a non trivial behavior when spin polarization is increased. The conductance is asymmetric in a voltage reversal.",9908465v1 1999-09-09,Application of the Projected Dynamics Method to an Anisotropic Heisenberg Model,"The Projected Dynamics method was originally developed to study metastable decay in ferromagnetic discrete spin models. Here, we apply it to a classical, continuous Heisenberg model with anisotropic ferromagnetic interactions, which evolves under a Monte Carlo dynamic. The anisotropy is sufficiently large to allow comparison with the Ising model. We describe the Projected Dynamics method and how to apply it to this continuous-spin system. We also discuss how to extract metastable lifetimes and how to extrapolate from small systems to larger systems.",9909142v1 1999-09-10,Time Evolution of Spin Waves,"A rigorous derivation of macroscopic spin-wave equations is demonstrated. We introduce a macroscopic mean-field limit and derive the so-called Landau-Lifshitz equations for spin waves. We first discuss the ferromagnetic Heisenberg model at T=0 and finally extend our analysis to general spin hamiltonians for the same class of ferromagnetic ground states.",9909146v1 1999-09-14,Lro in Lattice Systems of Linear Classical and Quantum Oscillators. Strong N-N Pair Quadratic Interaction,"For systems of one-component interacting oscillators on the d-dimensional lattice, d>1, whose potential energy besides a large nearest-neighbour (n-n) ferromagnetic translation-invariant quadratic term contains small non-nearest-neighbour translation invariant term, an existence of a ferromagnetic long-range order for two valued lattice spins, equal to a sign of oscillator variables, is established for sufficiently large magnitude g of the n-n interaction with the help of the Peierls type contour bound. The Ruelle superstability bound is used for a derivation of the contour bound.",9909207v1 1999-09-15,The magnetism of the t-t' Hubbard model,"The magnetic properties of the t-t' Hubbard Model in the two dimensional square lattice are studied within an unrestricted Hartree-Fock approximation in real space. The interplay between antiferromagnetism, ferromagnetism, phase separation and inhomogeneous magnetic textures is studied. It is shown that, at sufficiently large values of t'/t, a rich fenomenology is to be expected between the antiferromagnetic phase at half filling and the ferromagnetic phase a lower fillings.",9909224v1 1999-09-16,"Ferromagnetic, A-type, and Charge-Ordered CE-type States in Doped Manganites using Jahn-Teller Phonons","The two-orbital model for manganites with both non-cooperative and cooperative Jahn-Teller phonons is studied at hole density x=0.5 using Monte Carlo techniques. The phase diagram is obtained varying the electron-phonon coupling and the $\rm t_{2g}$-spins exchange. The insulating CE-type charge- and orbital-ordered state with the $z$-axis charge $stacking$ observed in narrow-bandwidth manganites is stabilized in the simulations. Its charge gap $\Delta_{\rm CO}$ is much larger than the critical temperature $\rm k_B T_{CO}$. Metallic-like A-type and ferromagnetic states are also obtained in the same framework, and the phase boundaries among them have first-order characteristics.",9909254v1 1999-09-21,Electronic Correlations in Manganites,"The influence of local electronic correlations on the properties of colossal magnetoresistance manganites is investigated. To this end, a ferromagnetic two-band Kondo lattice model is supplemented with the local Coulomb repulsion missing in this model, and is analyzed within dynamical mean-field theory. Results for the spectral function, optical conductivity, and the paramagnetic-to-ferromagnetic phase transition show that electronic correlations have drastic effects and may explain some experimental observations.",9909311v2 1999-10-12,Hysteresis in One-Dimensional Anti-Ferromagnetic Random-Field Ising Model at Zero-Temperature,"We analyse hysteresis in a one-dimensional anti-ferromagnetic random field Ising model at zero-temperature. The random field is taken to have a rectangular distribution of width $2 \Delta$ centered about the origin. A uniform applied field is varied slowly from $-\infty$ to $+\infty$ and back. Analytic expression for the hysteresis loop is obtained in the case $\Delta \le |J|$, where $|J|$ is the strength of the nearest neighbor interaction.",9910169v1 1999-10-14,Giant 1/f noise in perovskite manganites: evidence of the percolation threshold,"We discovered an unprecedented magnitude of the 1/f noise near the Curie temperature Tc in low-Tc manganites. The scaling behavior of the 1/f noise and the resistance provides strong evidence of the percolation nature of the ferromagnetic transition in the polycrystalline samples. The step-like changes of the resistance with temperature, observed for single crystals, suggest that the size of the ferromagnetic domains depends on the size of crystallites.",9910220v1 1999-11-23,First and second order ferromagnetic transition at T=0 in a 1D itinerant system,"We consider a modified version of the one-dimensional Hubbard model, the t1-t2 Hubbard chain, which includes an additional next-nearest-neighbor hopping. It has been shown that at weak coupling this model has a Luttinger liquid phase or a spin liquid phase depending upon the ratio of t2 to t1. Additionally if the on-site interaction U is large enough, the ground state is fully polarized. Using exact diagonalization and the density-matrix renormalization group, we show that the transition to the ferromagnetic phase is either of first or second order depending on whether the Luttinger liquid or spin liquid is being destabilized. Since we work at T=0, the second order transition is a quantum magnetic critical point.",9911361v1 1999-11-24,Exactly solvable spin ladder model with degenerate ferromagnetic and singlet states,"We study the spin ladder model with interactions between spins on neighboring rungs. The model Hamiltonian with the exact singlet ground state degenerated with ferromagnetic state is obtained. The singlet ground state wave function has a special recurrent form and depends on two parameters. Spin correlations in the singlet ground state show double-spiral structure with period of spirals equals to the system size. For special values of parameters they have exponential decay. The spectrum of the model is gapless and there are asymptotically degenerated excited states for special values of parameters in the thermodynamic limit.",9911394v1 1999-12-02,Charge and orbital ordering in underdoped La1-xSrxMnO3,"We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0 < x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At x=1/8, several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous ferromagnetic state. The present calculation indicates that a ferromagnetic state with a charge modulation along the c-axis which is consistent with the experiment by Yamada et al. might be responsible for the anomalous behavior around x = 1/8.",9912021v1 1999-12-06,Finite-size scaling corrections in two-dimensional Ising and Potts ferromagnets,"Finite-size corrections to scaling of critical correlation lengths and free energies of Ising and three-state Potts ferromagnets are analysed by numerical methods, on strips of width $N$ sites of square, triangular and honeycomb lattices. Strong evidence is given that the amplitudes of the ``analytical'' correction terms, $N^{-2}$, are identically zero for triangular-- and honeycomb Ising systems. For Potts spins, our results are broadly consistent with this lattice-dependent pattern of cancellations, though for correlation lengths non-vanishing (albeit rather small) amplitudes cannot be entirely ruled out.",9912090v1 1999-12-07,Theory of Spin polarized Tunneling in Superconducting Sr2RuO4,"A theory of tunneling conductance in ferromagnetic metal/insulator/triplet - supercondcutor junctions is presented for unitary and non-unitary spin triplet pairing states which are promising candidates for the superconducting paring symmetry of Sr2RuO4. As the magnitude of the exchange interaction in the ferromagnetic metal is increased, the conductance for the unitary pairing state below the energy gap is reduced in contrast to the case for the non-unitary pairing state.",9912100v1 1999-12-20,Ferromagnetic Nanowires with Superconducting Electrodes,"The proximity effect in mesoscopic ferromagnet/superconductor ($FS$) Ni/Al structures of various geometries was studied experimentally on both $F$- and $S$-sides of the structures. Samples with a wide range of interface transparency were fabricated. The dependence of the effect on $FS$ interface transparency was investigated. The amplitude of this effect was found to be larger than expected from classical theory of proximity effect. Preliminary experiments showed no phase-sensitive oscillations in Andreev interferometer geometry. Various theoretical models are discussed.",9912370v1 1999-12-22,Observation of domain wall resistivity in $\rm SrRuO_3$,"$\rm SrRuO_3$ is an itinerant ferromagnet with $T_c \sim 150 \rm K$. When $\rm SrRuO_3$ is cooled through $T_c$ in zero applied magnetic field, a stripe domain structure appears whose orientation is uniquely determined by the large uniaxial magnetocrystalline anisotropy. We find that the ferromagnetic domain walls clearly enhance the resisitivity of $\rm SrRuO_3$ and that the enhancement has different temperature dependence for currents parallel and perpendicular to the domain walls. We discuss possible interpretations of our results.",9912404v1 2000-01-11,The temperature dependent behaviour of surface states in ferromagnetic semiconductors,"We present a model calculation for the temperature dependent behaviour of surface states on a ferromagnetic local-moment film. The film is described within the s-f model featuring local magnetic moments being exchange coupled to the itinerant conduction electrons. The surface states are generated by modifying the hopping in the vicinity of the surface of the film. In the calculation for the temperature dependent behaviour of the surface states we are able to reproduce both Stoner-like and spin-mixing behaviour in agreement with recent (inverse) photoemission data on the temperature dependent behaviour of a Gd(0001) surface state.",0001141v1 2000-01-13,"Comment on ""Observation of Spin Injection at a Ferromagnet-Semiconductor Interface, by P.R. Hammar et al","In a recent Letter Hammar et al. claim the observation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an important observation, since, despite considerable effort of several groups, all attempts to realize spin-injection into a 2DEG using purely electrical measurements have failed sofar. However, in my opinion the claim made is not correct, and the observed behaviour can be explained by a combination of a magneto resistance (Hall) effect (e.g. generated by the fringe magnetic fields present at the edges of the ferromagnetic electrode), with a {\it spin-independent} rectification effect due to the presence of a metal- semiconductor junction.",0001187v1 2000-01-13,Scaling exponents for Barkhausen avalanches in polycrystals and amorphous ferromagnets,"We investigate the scaling properties of the Barkhausen effect, recording the noise in several soft ferromagnetic materials: polycrystals with different grain sizes and amorphous alloys. We measure the Barkhausen avalanche distributions and determine the scaling exponents. In the limit of vanishing external field rate, we can group the samples in two distinct classes, characterized by exponents \tau = 1.50 \pm 0.05 or \tau = 1.27 \pm 0.03, for the avalanche size distributions. We interpret these results in terms of the depinning transition of domain walls and obtain an expression relating the cutoff of the distributions to the demagnetizing factor which is in quantitative agreement with experiments.",0001191v1 2000-01-17,Anderson-Mott Transition in a Magnetic Field: Corrections to Scaling,"It is shown that the Anderson-Mott metal-insulator transition of paramagnetic, interacting disordered electrons in an external magnetic field is in the same universality class as the transition from a ferromagnetic metal to a ferromagnetic insulator discussed recently. As a consequence, large corrections to scaling exist in the magnetic-field universality class, which have been neglected in previous theoretical descriptions. The nature and consequences of these corrections to scaling are discussed.",0001232v2 2000-02-16,Ferromagnetism and Canted Spin Phase in AlAs/GaMnAs Single Quantum Wells: Monte Carlo Simulation,"The magnetic order resulting from a confinement-adapted Ruderman-Kittel-Kasuya-Yosida indirect exchange between magnetic moments in the metallic phase of a AlAs/Ga(1-x)Mn(x)As quantum well is studied by Monte Carlo simulation. This coupling mechanism involves magnetic moments and carriers (holes), both coming from the same Mn(2+) ions. It leads to a paramagnetic, a ferromagnetic, or a canted spin phase, depending on the carrier concentration, and on the magnetic layer width. It is shown that high transition temperatures may be obtained.",0002254v1 2000-03-29,Ultrafast spin dynamics and critical behavior in half-metallic ferromagnet : Sr_2FeMoO_6,"Ultrafast spin dynamics in ferromagnetic half-metallic compound Sr_2FeMoO_6 is investigated by pump-probe measurements of magneto-optical Kerr effect. Half-metallic nature of this material gives rise to anomalous thermal insulation between spins and electrons, and allows us to pursue the spin dynamics from a few to several hundred picoseconds after the optical excitation. The optically detected magnetization dynamics clearly shows the crossover from microscopic photoinduced demagnetization to macroscopic critical behavior with universal power law divergence of relaxation time for wide dynamical critical region.",0003467v3 2000-03-30,Resistance Jumps and Hysteresis in Ferromagnetic Wires,"A phenomenological approach based on the principle of minimum heat generation is proposed for understanding experiments for metallic wires with a ferromagnetic domain wall controlled by a magnetic field. This approach elucidates the origin of negative jump and of hysteresis in the magnetoresistance experiments. To justify our approach, we computed the transmission probability of a single electron through a wire. Under certain conditions, the presence of a domain wall enhances the transmission through an impurity potential. Combining this result with the principle of minimum heat generation explains the counterintuitive negative resistance jump.",0003485v2 2000-05-02,Two-Dimensional Modeling of Soft Ferromagnetic Films,"We examine the response of a soft ferromagnetic film to an in-plane applied magnetic field. Our theory, based on asymptotic analysis of the micromagnetic energy in the thin-film limit, proceeds in two steps: first we determine the magnetic charge density by solving a convex variational problem; then we construct an associated magnetization field using a robust numerical method. Experimental results show good agreement with the theory. Our analysis is consistent with prior work by van den Berg and by Bryant and Suhl, but it goes much further; in particular it applies even for large fields which penetrate the sample.",0005057v1 2000-05-27,Complex orbital state in manganites,"The $e_g$-orbital states with complex coefficients of the linear combination of $x^2-y^2$ and $3z^2-r^2$ are studied for the ferromagnetic state in doped manganites. Especially the focus is put on the competition among uniform complex, staggered complex, and real orbital states. As the hole-doping $x$ increases, the real, the canted complex, and the staggered complex orbital states appears successively. Uniform complex state analoguous to Nagaoka ferromagnet does not appear. These complex states can be expressed as a resonating state among the planer orbitals as the orbital liquid, accompanied by no Jahn-Teller distortion.",0005482v1 2000-07-11,Spin diffusion in doped semiconductors: the role of Coulomb interactions,"We examine the effect of the Coulomb interaction on the mobility and diffusion of spin packets in doped semiconductors. We find that the diffusion constant is reduced, relative to its non-interacting value, by the combined effect of Coulomb-enhanced spin susceptibility and spin Coulomb drag. In ferromagnetic semiconductors, the spin diffusion constant vanishes at the ferromagnetic transition temperature.",0007197v4 2000-07-25,Indication of the ferromagnetic instability in a dilute two-dimensional electron system,"The magnetic field B_c, in which the electrons become fully spin-polarized, is found to be proportional to the deviation of the electron density from the zero-field metal-insulator transition in a two-dimensional electron system in silicon. The tendency of B_c to vanish at a finite electron density suggests a ferromagnetic instability in this strongly correlated electron system.",0007402v2 2000-08-01,Fate of Zero-Temperature Ising Ferromagnets,"We investigate the relaxation of homogeneous Ising ferromagnets on finite lattices with zero-temperature spin-flip dynamics. On the square lattice, a frozen two-stripe state is apparently reached approximately 1/4 of the time, while the ground state is reached otherwise. The asymptotic relaxation is characterized by two distinct time scales, with the longer stemming from the influence of a long-lived diagonal stripe ``defect''. In greater than two dimensions, the probability to reach the ground state rapidly vanishes as the size increases and the system typically ends up wandering forever within an iso-energy set of stochastically ``blinking'' metastable states.",0008019v1 2000-08-07,Features of Hopping Integral and Unconventional Orbital Modes for Quantum Skyrmions in Heisenberg Ferromagnet,"We use the coherent state approximation for the skyrmion in the isotropic quantum Heisenberg ferromagnet to obtain analytical expression for skyrmion hopping integral as a function of distance. It appears the skyrmion hopping is restricted by distances larger than its effective diameter. Some puzzling properties of the orbital skyrmionic modes are discussed. Spin distribution in these states is obtained and effect of quantum spin contraction is demonstrated.The principal possibility of lowering the skyrmion energy due to a bonding-like state formation is illustrated.",0008104v2 2000-08-16,Effects of Fe doping in La1/2Ca1/2MnO3,"The effect of Fe doping in the Mn site on the magnetic, transport and structural properties of polycrystalline La1/2Ca1/2MnO3 was studied. Doping with low Fe concentration (< 10%) strongly affects electrical transport and magnetization. Long range charge order is disrupted even for the lowest doping level studied (~2%). For Fe concentration up to 5% a ferromagnetic state develops at low temperature with metallic like conduction and thermal hysteresis. In this range, the Curie temperature decreases monotonously as a function of Fe doping. Insulating behavior and a sudden depression of the ferromagnetic state is observed by further Fe doping.",0008236v1 2000-08-18,Dynamic critical behavior of disordered 3D Heisenberg ferromagnets,"The influence of a strong disorder on the critical dynamics of 3D Heisenberg ferromagnet was investigated by high-resolution quasielastic neutron scattering. A new disorder-induced dynamical critical behavior was found, characterized by its critical exponent z=2.3. The dynamical scaling function appeared mainly unaffected by disorder but its description led to a new parameterization of the Renormalization Group theory, as in the case of pure Nickel.",0008267v1 2000-08-21,Bag Formation in Quantum Hall Ferromagnets,"Charged skyrmions or spin-textures in the quantum Hall ferromagnet at filling factor nu=1 are reinvestigated using the Hartree-Fock method in the lowest Landau level approximation. It is shown that the single Slater determinant with the minimum energy in the unit charge sector is always of the hedgehog form. It is observed that the magnetization vector's length deviates locally from unity, i.e. a bag is formed which accommodates the excess charge. In terms of a gradient expansion for extended spin-textures a novel O(3) type of effective action is presented, which takes bag formation into account.",0008297v1 2000-08-29,Transport Anomalies and Marginal Fermi-Liquid Effects at a Quantum Critical Point,"The conductivity and the tunneling density of states of disordered itinerant electrons in the vicinity of a ferromagnetic transition at low temperature are discussed. Critical fluctuations lead to nonanalytic frequency and temperature dependences that are distinct from the usual long-time tail effects in a disordered Fermi liquid. The crossover between these two types of behavior is proposed as an experimental check of recent theories of the quantum ferromagnetic critical behavior. In addition, the quasiparticle properties at criticality are shown to be those of a marginal Fermi liquid.",0008431v2 2000-09-04,Observation of anomalous single-magnon scattering in half-metallic ferromagnets by chemical pressure control,"Temperature variation of resistivity and specific heat have been measured for prototypical half-metallic ferromagnets, R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have found variation of the temperature scalings in the resistivity from T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in terms of the anomalous single-magnon scattering (AMS) process in the half-metallic system.",0009035v1 2000-10-05,Oscillations of Andreev states in clean ferromagnetic films,"We investigate the influence of the exchange field on the Andreev bound states in a ferromagnetic (F) film backed on one side by a superconductor (S). Our model accounts for diffusive reflection at the outer surface and possible backscattering at the FS-interface. Phase shifting of the Andreev level by the exchange field results in an oscillatory behaviour of the density of states of F as a function of the layer thickness. We show that our results agree quantitatively with recent experiments.",0010089v1 2000-10-06,Static critical behavior of the ferromagnetic transition in LaMnO3.14 manganite,"The ferromagnetic phase transition in LaMnO3.14 is investigated by measuring the dc magnetization as a function of magnetic field and temperature. Modified Arrott plot and Kouvel Fisher analysis yield estimates for the critical exponents beta, and gama, with values between that predicted for the Heisenberg model and mean field theory. At low fields we found an anomalous small value of beta, indicating that the critical behavior is influenced by the range of magnetic fields used.",0010108v1 2000-10-16,Spin waves in ultrathin ferromagnetic overlayers,"The influence of a non-magnetic metallic substrate on the spin wave excitations in ultrathin ferromagnetic overlayers is investigated for different crystalline orientations. We show that spin wave dumping in these systems occur due to the tunneling of holes from the substrate into the overlayer, and that the spin wave energies may be considerably affected by the exchange coupling mediated by the substrate.",0010215v1 2000-10-28,Theory of Tunneling Spectroscopy in Ferromagnetic Nanoparticles,"We present a theory of the low-energy excitations of a ferromagnetic metal nanoparticle. In addition to the particle-hole excitations, which occur in a paramagnetic metal nanoparticle, we predict a branch of excitations involving the magnetization-orientation collective coordinate. Tunneling matrix elements are in general sizable for several different collective states associated with the same band configuration. We point out that the average change in ground state spin per added electron differs from non-interacting quasiparticle expectations, and that the change in the spin-polarization, due to Zeeman coupling, is strongly influenced by Coulomb blockade physics.",0010469v1 2000-11-03,Critical exponents of ferromagnetic Ising model on fractal lattices,"We review the value of the critical exponents $\nu^{-1}$, $\beta/\nu$, and $\gamma/\nu$ of ferromagnetic Ising model on fractal lattices of Hausdorff dimension between one and three. They are obtained by Monte Carlo simulation with the help of Wolff algorithm. The results are accurate enough to show that the hyperscaling law $d_f=2\beta/\nu+\gamma/\nu$ is satisfied in non-integer dimension. Nevertheless, the discrepancy between the simulation results and the $\epsilon$-expansion studies suggests that the strong universality should be adapted for the fractal lattices.",0011060v1 2000-11-07,Electron Spin Flip Relaxation by One Magnon Processes: Application to the Gadolinium surface band,"The ""s-f model"", also known as the ferromagnetic Kondo lattice, contains a description of band electrons coupled to localized spins which is an appropriate description of the magnetic part of the low-energy physics of Gd metal. Here the model is used to estimate the lifetime broadening of the minority spin component of the surface electron band in ferromagnetic gadolinium metal at temperatures below the Curie temperature. The low temperature result 0.10 eV agrees nicely with a measurement by Fedorov et al.",0011121v2 2000-11-10,Electronic pressure on ferromagnetic domain wall,"The scattering of the eletron by a domain wall in a nano-wire is studied perturbatively to the lowest order. The correction to the thermodaynamic potential of the electron system due to the scattering is calculated from the phase shift. The wall profile is determined by taking account of this correction, and the result indicates that the wall in a ferromagnet with small exchange coupling can be squeezed to be very thin to lower the electron energy.",0011177v1 2000-11-22,Coexistence of 0 and $π$ states in Josephson junctions,"New modes of the magnetic flux penetration in SQUIDs with ferromagnetic junctions are predicted theoretically. Characteristic multinode anharmonicity of the current-phase relation implies the coexistence of stable and metastable 0 and $\pi$ states for a large interval of strength of the ferromagnetic barrier influence at low temperature. As a consequence, the coexistence of integer and half-integer fluxoid configurations appears in both rf and dc SQUIDs, and generates two flux jumps per one external flux quantum. In dc SQUIDs, new flux jumps are manifested as two dips in the critical current dependence on the external magnetic flux.",0011378v1 2000-11-24,"Jahn-Teller, Charge and Magnetic Ordering in half-doped Manganese Oxides","The phase diagram of half-doped manganite systems of formula A_{0.5}A'_{0.5}MnO_3 is investigated within a single-orbital model incorporating magnetic double-exchange and superexchange, together with intersite Coulomb and electron-lattice interactions. Strong Jahn-Teller and breathing mode deformations compete together and result in shear lattice deformations. The latters stabilize the charge-ordered CE-type phase, which undergo first-order transitions with temperature or magnetic field to either Ferromagnetic metallic or Paramagnetic insulating phases. An essential feature is the self-consistent screening of Coulomb and electron-phonon interactions in the ferromagnetic phase.",0011419v1 2000-12-15,History Dependent Phenomena in the Transverse Ising Ferroglass: the Free Energy Landscape,"In this paper we investigate the relationship between glassy and ferromagnetic phases in disordered Ising ferromagnets in the presence of transverse magnetic fields, $\Gamma$. Iterative mean field simulations probe the free energy landscape and suggest the existence of a glass transition as a function of $\Gamma$ which is distinct from the Curie temperature. New experimental field-cooled and zero-field-cooled data on LiHo$_x$Y$_{1-x}$F$_4$ provide support for our theoretical picture.",0012298v2 2000-12-18,The phase-separated states in antiferromagnetic semiconductors with polarizable lattice,"The possibility of the slab or stripe phase separation (alternating ferromagnetic highly- conductive and insulating antiferromagnetic layers) is proved for isotropic degenerate antiferromagnetic semiconductors. This type of phase separation competes with the droplet phase separation (ferromagnetic droplets in the antiferromagnetic host or vice versa). The interaction of electrons with optical phonons alone cannot cause phase-separated state with alternating highly-conductive and insulating regions but it stabilizes the magnetic phase separation. The magnetostriction deformation of the lattice in the phase-separated state is investigated.",0012321v1 2000-12-19,Material-specific spin filtering in ferromagnet/superconductor ballistic nanojunctions,"We study spin-dependent electronic transport across ferromagnet/superconductor ballistic junctions modeled using tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. We show that by accurately modeling the band structure of the bulk materials, one can reproduce the measured differential conductance of Cu/Pb nanocontacts. In contrast the differential conductance of Co/Pb contacts can only be reproduced if an enhanced magnetic moment is present at the interface.",0012352v1 2000-12-27,Test of Bell's Inequality using the Spin Filter Effect in Ferromagnetic Semiconductor Micro-structures,"A theoretical proposal for testing Bell's inequality in mesoscopic systems is presented. We show that the entanglement of two electron spins can be detected in the spin filter effect in the mesoscopic semiconductor / ferromagnetic semiconductor / semiconductor junction. The current-current correlation function is calculated by use of the quantum scattering theory and we compare it with the local hidden variable theory. We also discuss the influence of an imperfect spin filter and derive the condition to see the violation of Bell's inequality experimentally.",0012475v2 2001-01-17,Negative Magnetoresistance Produced by Hall Fluctuations in a Ferromagnetic Domain Structure,"We present a model for a negative magnetoresistance (MR) that would develop in a material with many ferromagnetic domains even if the individual domains have no magnetoresistance and even if there is no boundary resistance. The negative MR is due to a classical current-distortion effect arising from spatial variations in the Hall conductivity, combined with a change in domain structure due to an applied magnetic field. The negative MR can exceed 1000% if the product of the carrier relaxation time and the internal magnetic field due to spontaneous magnetization is sufficiently large.",0101268v2 2001-01-22,Orbital Correlations in the Ferromagnetic Half-Metal $CrO_{2},"We deduce a model relevant for the description of the ferromagnetic half-metal Chromium dioxide ($CrO_{2}$), widely used in magnetic recording technology. The model describes the effect of dynamical, local orbital correlations arising from local quantum chemistry of the material. A finite temperature solution of the model in $d=\infty$ provides a natural explanation of the optical response, photoemission, resistivity and the large Woods-Saxon ratio observed in experiments. Our study confirms the important role of many body dynamical correlation effects for a proper understanding of the metallic phase of $CrO_{2}$.",0101335v1 2001-02-11,Antiferromagnetic Interlayer Coupling in Ferromagnetic Semiconductor EuS/PbS(001) Superlattices,"Antiferromagnetic coupling between ferromagnetic layers has been observed for the first time in all-semiconductor superlattice structure EuS/PbS(001), by neutron scattering and magnetization measurements. Spin-dependent superlattice band structure effects are invoked to explain the possible origin and the strength of the observed coupling.",0102194v2 2001-02-14,Nagaoka ferromagnetism in the two-dimensional infinite-U Hubbard model,"We present different numerical calculations based on variational quantum Monte Carlo simulations supporting a ferromagnetic ground-state for finite and small hole densities in the two-dimensional infinite-$U$ Hubbard model. Moreover, by studying the energies of different total spin sectors, these calculations strongly suggest that the paramagnetic phase is unstable against a phase with a partial polarization for large hole densities $\delta \sim 0.40$ with evidence for a second-order transition to the paramagnetic large doping phase.",0102249v1 2001-02-26,Dynamical Properties in the Bilayer Quantum Hall Ferromagnet,"The spectral functions of the pseudospin correlation functions in the bilayer quantum Hall system at \nu=1 are investigated numerically, where the pseudospin describes the layer degrees of freedom. In the pseudospin-ferromagnetic phase, the lowest-energy excitation branch is closely connected with the ground state through the fluctuations of pseudospin S_y and S_z, and it plays a significant role on the tunneling properties in this system. For the system with very small tunneling amplitude and layer separation smaller than the critical one, the system-size dependence of calculated spectral function A_{y z} suggests the superfluidity on the tunneling current in the absence of impurities.",0102476v1 2001-03-05,"Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As","We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into account. We relate our quantum-mechanical calculation to the classical micromagnetic energy functional and determine anisotropy energies and exchange constants. We find that the exchange constant is substantially enhanced compared to the case of a parabolic heavy-hole-band model.",0103116v2 2001-03-06,Short-time dynamics of random-bond Potts ferromagnet with continuous self-dual quenched disorders,"We present Monte Carlo simulation results of random-bond Potts ferromagnet with the Olson-Young self-dual distribution of quenched disorders in two-dimensions. By exploring the short-time scaling dynamics, we find universal power-law critical behavior of the magnetization and Binder cumulant at the critical point, and thus obtain estimates of the dynamic exponent $z$ and magnetic exponent $\eta$, as well as the exponent $\theta$. Our special attention is paid to the dynamic process for the $q$=8 Potts model.",0103130v1 2001-03-16,Critical dynamics and universality of the random-bond Potts ferromagnet with tri-distributed quenched disorders,"Critical behavior in short-time dynamics is investigated by a Monte Carlo study for the random-bond Potts ferromagnet with a trinary distribution of quenched disorders on two-dimensional triangular lattices. The dynamic scaling is verified and applied to estimate critical exponents $\theta$, $z$ and $\beta/\nu$ for several realizations of the trinary distribution. Our critical scaling analysis strongly indicates that the bond randomness influences the critical universality.",0103355v2 2001-03-30,A Model for Ferromagnetic Nanograins with Discrete Electronic States,"We propose a simple phenomenological model for an ultrasmall ferromagnetic grain, formulated in terms of the grain's discrete energy levels. We compare the model's predictions with recent measurements of the discrete tunneling spectrum through such a grain. The model can qualitatively account for the observed features if we assume (i) that the anisotropy energy varies among different eigenstates of one grain, and (ii) that nonequilibrium spin accumulation occurs.",0103626v1 2001-04-18,Resistance spikes and domain wall loops in Ising quantum Hall ferromagnets,"We explain the recent observation of resistance spikes and hysteretic transport properties in Ising quantum Hall ferromagnets in terms of the unique physics of their domain walls. Self-consistent RPA/Hartree-Fock theory is applied to microscopically determine properties of the ground state and domain-wall excitations. In these systems domain wall loops support one-dimensional electron systems with an effective mass comparable to the bare electron mass and may carry charge. Our theory is able to account quantitatively for the experimental Ising critical temperature and to explain characteristics of the resistive hysteresis loops.",0104334v1 2001-05-01,Storage Capacity of Two-dimensional Neural Networks,"We investigate the maximum number of embedded patterns in the two-dimensional Hopfield model. The grand state energies of two specific network states, namely, the energies of the pure-ferromagnetic state and the state of specific one stored pattern are calculated exactly in terms of the correlation function of the ferromagnetic Ising model. We also investigate the energy landscape around them by computer simulations. Taking into account the qualitative features of the phase diagrams obtained by Nishimori, Whyte and Sherrington [Phys. Rev. E {\bf 51}, 3628 (1995)], we conclude that the network cannot retrieve more than three patterns.",0105008v2 2001-05-03,Freezing in random graph ferromagnets,"Using T=0 Monte Carlo and simulated annealing simulation, we study the energy relaxation of ferromagnetic Ising and Potts models on random graphs. In addition to the expected exponential decay to a zero energy ground state, a range of connectivities for which there is power law relaxation and freezing to a metastable state is found. For some connectivities this freezing persists even using simulated annealing to find the ground state. The freezing is caused by dynamic frustration in the graphs, and is a feature of the local search-nature of the Monte Carlo dynamics used. The implications of the freezing on agent-based complex systems models are briefly considered.",0105061v2 2001-05-09,Hot Electron Magnetotransport in a Spin-Valve Transistor at Finite temperatures,"The hot electron magnetotransport in a spin-valve transistor has been theoretically explored at finite temperatures. We have explored the parallel and anti-parallel collector current changing the relative spin orientation of the ferromagnetic layers at finite temperatures. In this model calculations, hot electron energy redistribution due to spatial inhomogeneity of Schottky barrier heights and hot electron spin polarization in the ferromagnetic layer at finite temperatures have been taken into account. The results of this model calculations accord with the experimental data semi-quantitative manner. We therefore suggest that both effects remarked above should be taken into account substantially when one explores the hot electron magnetotransport in a spin-valve system transistor at finite temperatures.",0105190v1 2001-05-12,Weak Ferromagnetism and Excitonic Condensates,"We investigate a model of excitonic ordering (i.e electron-hole pair condensation) appropriate for the divalent hexaborides. We show that the inclusion of imperfectly nested electron hole Fermi surfaces can lead to the formation of an undoped excitonic metal phase. In addition, we find that weak ferromagnetism with compensated moments arises as a result of gapless excitations. We study the effect of the low lying excitations on the density of states, Fermi surface topology and optical conductivity and compare to available experimental data.",0105245v1 2001-05-17,Rhodium Doped Manganites : Ferromagnetism and Metallicity,"The possibility to induce ferromagnetism and insulator to metal transitions in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown for the first time. Colossal magnetoresistance (CMR) properties are evidenced for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to induce such properties is compared to the results obtained by chromium and ruthenium doping. Models are proposed to explain this behavior.",0105347v1 2001-05-24,Shapes and textures of ferromagnetic liquid droplets,"Theoretical calculations, computer simulations and experiments indicate the possible existence of a ferromagnetic liquid state. Should such a state exist, demagnetization effects would force a nontrivial magnetization texture governed by the shape of the liquid droplet. Since liquid droplets are deformable, the droplet shape couples to the magnetization texture. This paper solves the joint shape/texture problem subject to the assumption of cylindrical droplet symmetry. The shape undergoes a change in topology from spherical to toroidal as exchange energy grows or surface tension decreases.",0105467v2 2001-05-26,Time-resolved ferromagnetic resonance in epitaxial Fe1-xCox films,"Magnetodynamics in epitaxial Fe1-xCox films on GaAs (100) are studied using time-resolved ferromagnetic resonance, in which the free precession of the magnetization after an impulsive excitation is measured using the polar Kerr effect. The sample is rotated with respect to the static and pulsed field directions, providing a complete mapping of the free energy surface and characteristic relaxation times. The magnetic response can be simulated with a simple coherent rotation model except in the immediate vicinity of switching fields. Bulk and surface anisotropies are identified, and unusual dynamics associated with the coexistence of cubic and uniaxial anisotropies are observed.",0105513v1 2001-06-05,Emptiness Formation Probability for the One-Dimensional Isotropic XY Model,"We study a correlation function for the one-dimensional isotropic ${XY}$ model (${XX0}$ model), which is called the Emptiness Formation Probability (EFP). It is the probability of the formation of a ferromagnetic string in the anti-ferromagnetic ground state. Using the expression of the EFP as a Toeplitz determinant, we discuss its asymptotic behaviors. We also compare the analytical results with numerical calculations as the density-matrix renormalization group and the quantum Monte-Carlo method.",0106062v2 2001-06-14,Non-equilibrium Relaxation Study of Ferromagnetic Transition in Double-Exchange Systems,"Ferromagnetic transition in double-exchange systems is studied by non-equilibrium relaxation technique combined with Monte Carlo calculations. Critical temperature and critical exponents are estimated from relaxation of the magnetic moment. The results are consistent with the previous Monte Carlo results in thermal equilibrium. The exponents estimated by these independent techniques suggest that the universality class of this transition is the same as that of short-range interaction models but is different from the mean-field one.",0106266v1 2001-06-19,Crystallographic structure of ultrathin Fe films on Cu(100),"We report bcc-like crystal structures in 2-4 ML Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The non-pseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.",0106370v1 2001-06-28,Superconductivity Near Ferromagnetism in MgCNi3,"An unusual quasi-two-dimensional heavy band mass van Hove singularity (vHs) lies very near the Fermi energy in MgCNi3, recently reported to superconduct at 8.5 K. This compound is strongly exchange enhanced and is unstable to ferromagnetism upon hole doping with 12% Mg --> Na or Li. The 1/4-depleted fcc (frustrated) Ni sublattice and lack of Fermi surface nesting argues against competing antiferromagnetic and charge density wave instabilities. We identify an essentially infinite mass along the M-Gamma line, leading to quasi-two-dimensionality of this vHs may promote unconventional p-wave pairing that could coexist with superconductivity.",0106583v1 2001-07-03,Random phase approximation for multi-band Hubbard models,"We derive the random-phase approximation for spin excitations in general multi-band Hubbard models, starting from a collinear ferromagnetic Hartree-Fock ground state. The results are compared with those of a recently introduced variational many-body approach to spin-waves in itinerant ferromagnets. As we exemplify for Hubbard models with one and two bands, the two approaches lead to qualitatively different results. The discrepancies can be traced back to the fact that the Hartree-Fock theory fails to describe properly the local moments which naturally arise in a correlated-electron theory.",0107050v1 2001-07-03,Persistence in One-dimensional Ising Models with Parallel Dynamics,"We study persistence in one-dimensional ferromagnetic and anti-ferromagnetic nearest-neighbor Ising models with parallel dynamics. The probability P(t) that a given spin has not flipped up to time t, when the system evolves from an initial random configuration, decays as P(t) \sim 1/t^theta_p with theta_p \simeq 0.75 numerically. A mapping to the dynamics of two decoupled A+A \to 0 models yields theta_p = 3/4 exactly. A finite size scaling analysis clarifies the nature of dynamical scaling in the distribution of persistent sites obtained under this dynamics.",0107053v1 2001-07-05,Magnetization in quasiperiodic magnetic multilayers with biquadratic exchange and uniaxial anisotropy,"A theoretical study is made of the magnetization versus applied field curves of ferromagnetic/nonmagnetic multilayers constructed according to a Fibonacci quasiperiodic sequence. The ferromagnetic films are assumed to have uniaxial anisotropy and are coupled by both bilinear and biquadratic effective exchange. The effects of quasiperiodicity in the magnetic phases are illustrated numerically for Fe/Cr systems.",0107127v1 2001-07-12,Theory of Angular Magnetoresistance in CPP spin valves,"The resistance of CPP spin valve is a continuous function of the angle $\theta $ between the magnetizations of both ferromagnets. We use the cicuit theory for non-collinear magnetoelectronics to compute the angular magnetoresistance of CPP spin valves taking the spin accumulation in the ferromagnetic layers into account.",0107250v1 2001-07-27,"Noncollinear Ferromagnetism in (III,Mn)V Semiconductors","We investigate the stability of the collinear ferromagnetic state in kinetic exchange models for (III,Mn)V semiconductors with randomly distributed Mn ions >. Our results suggest that {\em noncollinear ferromagnetism} is commom to these semiconductor systems. The instability of the collinear state is due to long-ranged fluctuations invloving a large fraction of the localized magnetic moments. We address conditions that favor the occurrence of noncollinear groundstates and discuss unusual behavior that we predict for the temperature and field dependence of its saturation magnetization.",0107573v2 2001-07-28,Ferromagnetic-spin glass transition in four-dimensional random-bond Ising model,"The four-dimensional +-J random-bond Ising model is studied using ground-state calculations. System sizes up to N=6^4 spins are considered. Here it is found that the ferromagnetic-spin glass transition occurs at a critical concentration p_c=0.28(1) of the antiferromagnetic bonds, which is comparable to values found previously by high-temperature series expansions. The transition is characterized by a correlation-length exponent \nu=1.0(1) and an order-parameter exponent \beta=0.4(1). Thus, this transition is in a different universality class from four-dimensional bond percolation, where \nu=0.678(50) and \beta=0.639(20).",0107583v1 2001-07-30,Spin Torques in Ferromagnetic/Normal Metal Structures,"Recent theories of spin-current-induced magnetization reversal are formulated in terms of a spin-mixing conductance $G^{mix}$. We evaluate $G^{mix}$ from first-principles for a number of (dis)ordered interfaces between magnetic and non-magnetic materials. In multi-terminal devices, the magnetization direction of a one side of a tunnel junction or a ferromagnetic insulator can ideally be switched with negligible charge current dissipation.",0107589v1 2001-08-01,Correlation between magnetic and transport properties of phase separated La$_{0.5}$Ca$_{0.5}$MnO$_{3}$,"The effect of low magnetic fields on the magnetic and electrical transport properties of polycrystalline samples of the phase separated compound La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is studied. The results are interpreted in the framework of the field induced ferromagnetic fraction enlargement mechanism. A fraction expansion coefficient af, which relates the ferromagnetic fraction f with the applied field H, was obtained. A phenomenological model to understand the enlargement mechanism is worked out.",0108026v2 2001-08-07,Fictitious fluxes in doped antiferromagnets,"In a tight binding model of charged spin-1/2 electrons on a square lattice, a fully polarized ferromagnetic spin configuration generates an apparent U(1) flux given by $2\pi$ times the skyrmion charge density of the ferromagnetic order parameter. We show here that for an antiferromagnet, there are two ``fictitious'' magnetic fields, one staggered and one unstaggered. The staggered topological flux per unit cell can be varied between $-\pi\le\Phi\le\pi$ with a negligible change in the value of the effective nearest neighbor coupling constant whereas the magnitude of the unstaggered flux is strongly coupled to the magnitude of the second neighbor effective coupling.",0108113v1 2001-08-20,Magnetic Field Scaling of the Conductance of Epitaxial Cuprate-Manganite Bilayers,"Conductance-voltage characteristics of epitaxial interfaces between oxide ferromagnets and oxide superconductors have been measured as a function of temperature and magnetic field. Their functional form is similar to that predicted by theories of transport across nearly transparent contacts between highly spin-polarized ferromagnets and d-wave superconductors. However, their magnetic field dependencies scale in striking and unusual ways, challenging our current understanding. Existing theories fail to account for apparent nonequilibium effects that are natural for spin injection in such geometries.",0108313v1 2001-08-29,Magnetic Semiconductors are Frustrated Ferromagnets,"Starting from microscopic and symmetry considerations, we derive the Hamiltonian describing the exchange interaction between the localized Mn spins and the valence band holes in $Ga_{1-x}Mn_x As$. We find that due to the strong spin-orbit coupling in the valence band, this exchange interaction has a rather complex structure and generates a highly anisotropic effective interaction between the Mn spins. The corresponding ground state has a finite ferromagnetic magnetization but is intrinsically spin-disordered even at zero temperature.",0108477v1 2001-09-04,Proteinlike behavior of a spin system near the transition between ferromagnet and spin glass,"A simple spin system is studied as an analog for proteins. We investigate how the introduction of randomness and frustration into the system effects the designability and stability of ground state configurations. We observe that the spin system exhibits protein-like behavior in the vicinity of the transition between ferromagnet and spin glass. Our results illuminate some guiding principles in protein evolution.",0109044v1 2001-09-12,Dynamic Monte Carlo Simulations for a Square-Lattice Ising Ferromagnet with a Phonon Heat Bath,"We derive a direct connection between Monte Carlo time and physical time in terms of physical parameters, using a quantum Hamiltonian with a $d$-dimensional phonon heat bath interacting with a square-lattice Ising ferromagnet. Based on the calculated transition rates, we perform dynamic Monte Carlo simulations using absorbing Markov chains to measure the lifetimes of the metastable state at low temperatures. We also calculate the lifetimes analytically using absorbing Markov chains. The phonon dynamic gives field-dependent prefactors in the lifetimes at low temperatures, that are different from the piecewise field-independent prefactors obtained from the Glauber dynamic.",0109214v1 2001-09-14,Singular Effects of Impurities near the Ferromagnetic Quantum-Critical Point,"Systematic theoretical results for the effects of a dilute concentration of magnetic impurities on the thermodynamic and transport properties in the region around the quantum critical point of a ferromagnetic transition are obtained. In the quasi-classical regime, the dynamical spin fluctuations enhance the Kondo temperature. This energy scale decreases rapidly in the quantum fluctuation regime, where the properties are those of a line of critical points of the multichannel Kondo problem with the number of channels increasing as the critical point is approached, except at unattainably low temperatures where a single channel wins out.",0109276v2 2001-10-02,Global phase diagram of bilayer quantum Hall ferromagnets,"We present a microscopic study of the interlayer spacing d versus in-plane magnetic field $B_\parallel$ phase diagram for bilayer quantum Hall (QH) pseudo-ferromagnets. In addition to the interlayer charge balanced commensurate and incommensurate states analyzed previously, we address the corresponding interlayer charge unbalanced ""canted"" QH states. We predict a large anomaly in the bilayer capacitance at the canting transition and the formation of dipole stripe domains with periods exceeding 1 micron in the canted state.",0110049v1 2001-10-11,Electrical Spin Injection in a Ferromagnetic / Tunnel Barrier/ Semiconductor Heterostructure,"We demonstrate experimentally the electrical ballistic electron spin injection from a ferromagnetic metal / tunnel barrier contact into a semiconductor III-V heterostructure. We introduce the Oblique Hanle Effect technique for reliable optical measurement of the degree of injected spin polarization. In a CoFe / Al2O3 / GaAs / (Al,Ga)As heterostructure we observed injected spin polarization in excess of 8 % at 80K.",0110240v1 2001-10-16,"Spin Waves, Phase Separation, and Interphase Boundaries in Double Exchange Magnets","We study a classical double exchange magnet with direct antiferromagnetic superexchange coupling, J, between the localized spins. It is shown that the de-stabilization of the ferromagnetic ground state with increasing J leads to phase separation; the latter always preempts the spin-wave instability (softening of the magnon spectrum). It is also found that the boundaries separating the ferromagnetic and antiferromagnetic areas of the sample tend to be abrupt.",0110322v2 2001-10-19,Magnetic properties of 3d-impurities substituted in GaAs,"We have calculated the magnetic properties of substituted 3d-impurities (Cr-Ni) in a GaAs host by means of first principles electronic structure calculations. We provide a novel model explaining the ferromagnetic long rang order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism is shown to be due to delocalized spin-uncompensated As dangling bond electrons. Besides the quantitative prediction of the magnetic moments, our model provides an understanding of the halfmetallicity, and the raise of the critical temperature with the impurity concentration.",0110405v1 2001-10-22,Anisotropic spin and charge transport in Rashba Hamiltonian,"We explore spin and charge transport phenomena in two dimensional electron gas in presence of Rashba spin-orbit coupling connected to two ideal Ferromagnetic leads. In particular we show through a combination of analytical and numerical calculation that the spin polarization which is transported depends on the Magnetization direction of ferromagnet even if the magnetization of both FM's are parallel.Conductance is also shown to be anisotropic. These anisotrpies present in spin and charge transport are a consequence of breaking of rotational invariance due to Rashba spin-orbit interaction and are present irrespective of the Hamiltonian considered being an effective mass Hamiltonian or tight binding model Hamiltonian.",0110440v1 2001-10-22,Total Energy Studies for Ferromagnetic Nickel: What is the Optimum Combination of the Multi-band Gutzwiller Method and Density Functional Theory?,"The multi-band Gutzwiller method, combined with calculations based on density functional theory, is employed to study total energy curves of the ferromagnetic ground state of Ni. A new method is presented which allows flow of charge between d and s, p type orbitals in an approximate way. Further it is emphasized that the missing repulsive contribution to the total energy at large magnetic moments can be estimated from an analysis of specific DFT calculations.",0110450v1 2001-10-22,Dimensional crossover and driven interfaces in disordered ferromagnets,"We study the depinning transition of driven interfaces in thin ferromagnetic films driven by external magnetic fields. Approaching the transition point the correlation length increases with decreasing driving. If the correlation length becomes of the order of the film thickness a crossover to two dimensional behavior occurs. From the corresponding scaling analysis we determine the exponents characterizing the transition of the three dimensional system.",0110456v1 2001-10-22,First-principles study of band offsets in ferromagnetic semiconductor heterojunctions,"We report valence and conduction band alignments and offsets for heterojunctions between CdCr2Se4, an n-type ferromagnetic semiconductor, and the non-magnetic materials Si and GaAs, evaluated using density functional theory. We explore numerically the impact of different interface features on the type of band alignment and the magnitude of the offsets. For example, we find it is energetically favorable to deplete Cr atoms from the layers at the interface; this also leads to band alignments smaller in magnitude compared to those obtained for Cr-rich interfaces and ideal for electrical spin-injection into either Si or Ga-terminated GaAs substrates.",0110474v1 2001-10-24,Topological Nature of Anomalous Hall Effect in Ferromagnet,"The anomalous Hall effect in two-dimensional ferromagnets is discussed to be the physical realization of the parity anomaly in (2+1)D, and the band crossing points behave as the topological singularity in the Brillouin zone. This appears as the sharp peaks and the sign changes of the transverse conductance $\sigma_{xy}$ as a function of the Fermi energy and/or the magnetization. The relevance to the experiments including the three dimensional systems is also discussed.",0110504v1 2001-11-03,"Mean-field approach to ferromagnetism in (III,Mn)V diluted magnetic semiconductors at low carrier densities","We present a detailed study, within the mean-field approximation, of an impurity band model for III-V diluted magnetic semiconductors. Such a model should be relevant at low carrier densities, below and near the metal-insulator transition. Positional disorder of the magnetic impurities inside the host semiconductor is shown to have observable consequences for the shape of the magnetization curve. Below the critical temperature the magnetization is spatially inhomogeneous, leading to very unusual temperature dependence of the average magnetization as well as specific heat. Disorder is also found to enhance the ferromagnetic transition temperature. Unusual spin and charge transport is implied.",0111045v1 2001-11-26,Ferromagnetic transition in a double-exchange system containing impurities in the Dynamical Mean Field Approximation,"We formulate the Dynamical Mean Field Approximation equations for the double-exchange system with quenched disorder for arbitrary relation between Hund exchange coupling and electron band width. Close to the ferromagnetic-paramagnetic transition point the DMFA equations can be reduced to the ordinary mean field equation of Curie-Weiss type. We solve the equation to find the transition temperature and present the magnetic phase diagram of the system.",0111491v4 2001-11-29,Positive cross-correlations induced by ferromagnetic contacts,"Due to the Fermionic nature of carriers, correlations between electric currents flowing through two different contacts attached to a conductor present a negative sign. Possibility for positive cross-correlations has been demonstrated in hybrid normal/superconductor structures under certain conditions. In this paper we show that positive cross-correlations can be induced, if not already present, in such structures by employing ferromagnetic leads with magnetizations aligned anti-parallel to each other. We consider three-terminal hybrid structures and calculate the mean-square correlations of current fluctuations as a function of the bias voltage at finite temperature.",0111564v2 2001-12-11,Conductance of Rashba spin-split systems with ferromagnetic contacts,"We study theoretically the conductance of heterostructures with ferromagnetic conductors (F) and a two dimensional electron gas with Rashba spin-orbit interaction (R) using the Landauer-B\""{u}ttiker formalism. Assuming a one-dimensional model, we first find the $S$-matrix for the FR interface. This result is then applied to different devices such as a FRF structure, first suggested by Datta and Das[Appl. Phys. Lett. 56, 665 (1990)]. We find analytic results for the conductance for the case of collinear magnetization.",0112175v1 2001-12-13,Magnon softening and damping in the ferromagnetic manganites due to orbital correlations,"We present a theory for spin excitations in ferromagnetic metallic manganites and demonstrate that orbital fluctuations have strong effects on the magnon dynamics in the case these compounds are close to a transition to an orbital ordered state. In particular we show that the scattering of the spin excitations by low-lying orbital modes with cubic symmetry causes both the magnon softening and damping observed experimentally.",0112252v2 2001-12-17,Ferromagnetic Phase Transition in Barabasi-Albert Networks,"Ising spins put onto a Barabasi-Albert scale-free network show an effective phase transition from ferromagnetism to paramagnetism upon heating, with an effective critical temperature increasing as the logarithm of the system size. Starting with all spins up and upon equilibration pinning the few most-connected spins down nucleates the phase with most of the spins down.",0112312v1 2001-12-18,"Spins, charges and currents at Domain Walls in a Quantum Hall Ising Ferromagnet","We study spin textures in a quantum Hall Ising ferromagnet. Domain walls between ferro and unpolarized states at $\nu=2$ are analyzed with a functional theory supported by a microscopic calculation. In a neutral wall, Hartree repulsion prevents the appearance of a fan phase provoked by a negative stiffness. For a charged system, electrons become trapped as solitons at the domain wall. The size and energy of the solitons are determined by both Hartree and spin-orbit interactions. Finally, we discuss how electrical transport takes place through the domain wall.",0112334v3 2001-12-20,Ferromagnetic and antiferromagnetic spin fluctuations and superconductivity in the hcp-phase of Fe,"High purity iron, which transforms into the hcp phase under pressure, has recently been reported to be superconducing in the pressure range 150-300 kBar [shim]. The electronic structure and the electron-phonon coupling ($\lambda_{ph}$) are calculated for hcp iron at different volumes. A parameter-free theory for calculating the coupling constants $\lambda_{sf}$ from ferromagnetic (FM) and antiferromagnetic (AFM) spin fluctuations is developed. The calculated $\lambda_{sf}$ are sufficiently large to explain superconductivity especially from FM fluctuations. The results indicate that superconductivity mediated by spin fluctuations is more likely than from electron-phonon interaction.",0112382v1 2001-12-21,Theory of Ferromagnetism in Doped Excitonic Condensates,"Nesting in a semimetal can lead to an excitonic insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. In this paper we present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.",0112420v1 2001-12-26,Spin-polarized tunneling in ferromagnetic double barrier junctions,"Spin-polarized tunneling in FMS/M/FMS double tunnel junctions where FMSs are ferromagnetic semiconductor layers and M is a metal spacer is studied theoretically within the single-site coherent potential approximation (CPA). The exchange interaction between a conduction electron and localized moment of the magnetic ion is treated in the framework of the s-f model. The spin polarization in the FMS layers is observed to oscillates as a function of the number of atomic planes in the spacer layer. Amplitude of these oscillations decreases with increasing the exchange interaction in FMS layers.",0112464v1 2002-01-09,Mean-field model of the ferromagnetic ordering in the superconducting phase of ErNi_2B_2C,"A mean-field model explaining most of the details in the magnetic phase diagram of ErNi_2B_2C is presented. The low-temperature magnetic properties are found to be dominated by the appearance of long-period commensurate structures. The stable structure at low temperatures and zero field is found to have a period of 40 layers along the a direction, and upon cooling it undergoes a first-order transition at T_C = 2.3 K to a different 40-layered structure having a net ferromagnetic component of about 0.4 mu_B/Er. The neutron-diffraction patterns predicted by the two 40-layered structures, above and below T_C, are in agreement with the observations of Choi et al.",0201133v1 2002-01-16,Why ferromagnetic semiconductors?,"Rapid development of information technologies originates from the exponential increase in the density of information that can be processed, stored, and transfer by the unit area of relevant devices. There is, however, a growing amount of evidences that the progress achieved in this way approaches its limits. Various novel ideas put forward to circumvent barriers ahead are described. Particular attention is paid to those concepts which propose to exploit electron or nuclear spins as the information carriers. Here, ferromagnetic semiconductors of III-V or II-VI compounds containing a sizable concentration of transition metals appear as outstanding spintronic materials.",0201279v1 2002-01-18,Electrons in a ferromagnetic metal with a domain wall,"We present theoretical description of conduction electrons interacting with a domain wall in ferromagnetic metals. The description takes into account interaction between electrons. Within the semiclassical approximation we calculate the spin and charge distributions, particularly their modification by the domain wall. In the same approximation we calculate local transport characteristics, including relaxation times and charge and spin conductivities. It is shown that these parameters are significantly modified near the wall and this modification depends on electron-electron interaction.",0201338v1 2002-01-23,"Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors","The effect of disorder on transport and magnetization in ferromagnetic III-V semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that Coulomb-induced correlations of the defect positions are crucial for the transport and magnetic properties of these highly compensated materials. We employ Monte Carlo simulations to obtain the correlated defect distributions. Exact diagonalization gives reasonable results for the spectrum of valence-band holes and the metal-insulator transition only for correlated disorder. Finally, we show that the mean-field magnetization also depends crucially on defect correlations.",0201411v2 2002-02-01,Nonlinear spin-polarized transport through a ferromagnetic domain wall,"A domain wall separating two oppositely magnetized regions in a ferromagnetic semiconductor exhibits, under appropriate conditions, strongly nonlinear I-V characteristics similar to those of a p-n diode. We study these characteristics as functions of wall width and temperature. As the width increases or the temperature decreases, direct tunneling between the majority spin bands decreases the effectiveness of the diode. This has important implications for the zero-field quenched resistance of magnetic semiconductors and for the design of a recently proposed spin transistor.",0202002v1 2002-02-04,Coexistence of spin-triplet superconductivity and ferromagnetism induced by the local Hund's rule exchange,"We characterize the coexistence of itinerant ferromagnetism and spin-triplet superconductivity within a single mechanism involving local (Hund's rule) exchange among $d$ electrons. The ratio of transition temperatures and the spin anisotropy of the superconducting gap is estimated for $ZrZn_2$. The $A$ phase is stable in very low applied and molecular fields, whereas the $A1$ phases persists in higher fields. A small residual magnetic moment is present below the Stoner threshold in the superconducting phase.",0202043v1 2002-02-14,Orbital Zeeman effect: Signature of a massive spin wave mode in ferromagnetism,"By deriving the quantum hydrodynamic equations for an isotropic single-band ferromagnet in an arbitrary magnetic field, we find that a massive mode recently predicted splits under the action of the field. The splitting is a peculiarity of charged fermions and is linear in the field to leading order in $q$ bearing resemblance to the Zeeman effect in this limit, and providing a clear signature for the experimental observation of this mode.",0202225v1 2002-02-26,Enhanced Granular Magnetoresistance due to Ferromagnetic Layers,"Giant magnetoresistance (GMR) of sequentially evaporated Fe-Ag structures have been investigated. Direct experimental evidence is given that inserting ferromagnetic layers into a granular structure significantly enhances the magnetoresistance. The increase of the GMR effect is attributed to spin polarization effects. The large enhancement (up to more than a fourfold value) and the linear variation of the GMR in low magnetic fields are explained by scattering of the spin polarized conduction electrons on paramagnetic grains.",0202464v1 2002-02-28,Anisotropy of ultra-thin ferromagnetic films and the spin reorientation transition,"The influence of uniaxial anisotropy and the dipole interaction on the direction of the magnetization of ultra-thin ferromagnetic films in the ground-state is studied. The ground-state energy can be expressed in terms of anisotropy constants which are calculated in detail as function of the system parameters and the film thickness. In particular non-collinear spin arrangements are taken into account. Conditions for the appearance of a spin reorientation transition are given and analytic results for the width of the canted phase and its shift in applied magnetic fields associated with this transition are derived.",0202534v1 2002-03-12,Tunnel magnetoresistance and interfacial electronic state,"We study the relation between tunnel magnetoresistance (TMR) and interfacial electronic states modified by magnetic impurities introduced at the interface of the ferromagnetic tunnel junctions, by making use of the periodic Anderson model and the linear response theory. It is indicated that the TMR ratio is strongly reduced depending on the position of the $d$-levels of impurities, based on reduction in the spin-dependent $s$-electron tunneling in the majority spin state. The results are compared with experimental results for Cr-dusted ferromagnetic tunnel junctions, and also with results for metallic multilayers for which similar reduction in giant magnetoresistance has been reported.",0203247v1 2002-03-13,Ferromagnetic imprinting of spin polarization in a semiconductor,"We present a theory of the imprinting of the electron spin coherence and population in an n-doped semiconductor which forms a junction with a ferromagnet. The reflection of non-equilibrium semiconductor electrons at the interface provides a mechanism to manipulate the spin polarization vector. In the case of unpolarized excitation, this ballistic effect produces spontaneous electron spin coherence and nuclear polarization in the semiconductor, as recently observed by time-resolved Faraday rotation experiments. We investigate the dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor and suggest control mechanisms for possible device applications.",0203285v1 2002-03-18,Ultra-efficient Cooling in Ferromagnet-Superconductor Microrefrigerators,"A promising scheme for electron microrefrigeration based on ferromagnet-superconductor contacts is presented. In this setup, cooling power densities up to 600 nW/$\mu$m$^2$ can be achieved leading to electronic temperature reductions largely exceeding those obtained with existing superconductor-normal metal tunnel contacts. Half-metallic CrO$_2$/Al bilayers are indicated as ideal candidates for the implementation of the device.",0203355v1 2002-04-05,Mesoscopic effects in superconductor-ferromagnet-superconductor junctions,"We show that at zero temperature the supercurrent through the superconductor - ferromagnetic metal - superconductor junctions does not decay exponentially with the thickness $L$ of the junction. At large $L$ it has a random sample-specific sign which can change with a change in temperature. In the case of mesoscopic junctions the phase of the order parameter in the ground state is a random sample-specific quantity. In the case of junctions of large area the ground state phase difference is $\pm \pi/2$.",0204123v1 2002-04-05,Atomic correlations in itinerant ferromagnets: quasi-particle bands of nickel,"We measure the band structure of nickel along various high-symmetry lines of the bulk Brillouin zone with angle-resolved photoelectron spectroscopy. The Gutzwiller theory for a nine-band Hubbard model whose tight-binding parameters are obtained from non-magnetic density-functional theory resolves most of the long-standing discrepancies between experiment and theory on nickel. Thereby we support the view of itinerant ferromagnetism as induced by atomic correlations.",0204142v1 2002-04-16,Spin accumulation in ferromagnetic single-electron transistors in the cotunneling regime,"We propose a new method of direct detection of spin accumulation, which overcomes problems of previous measurement schemes. A spin dependent current in a single-electron transistor with ferromagnetic electrodes leads to spin accumulation on the metallic island. The resulting spin-splitting of the electrochemical potentials of the island, because of an additional shift by the charging energy, can be detected from the spacing between two resonances in the current-voltage characteristics. The results were obtained in the framework of a real-time diagrammatic approach which allows to study higher order (co-)tunneling processes in the strong nonequlibrium situation.",0204354v1 2002-04-21,Mean field solution of the Ising model on a Barabasi-Albert network,"The mean field solution of the Ising model on a Barabasi-Albert scale-free network with ferromagnetic coupling between linked spins is presented. The critical temperature $T_c$ for the ferromagnetic to paramagnetic phase transition (Curie temperature) is infinite and the effective critical temperature for a finite size system increases as the logarithm of the system size in agreement with recent numerical results of Aleksiejuk, Holyst and Stauffer.",0204455v4 2002-04-25,Nonsinusoidal current-phase relation in SFS Josephson junctions,"Various types of the current-phase relation I(phi) in superconductor-ferromagnet-superconductor (SFS) point contacts and planar double-barrier junctions are studied within the quasiclassical theory in the limit of thin diffusive ferromagnetic interlayers. The physical mechanisms leading to highly nontrivial I(phi) dependence are identified by studying the spectral supercurrent density. These mechanisms are also responsible for the 0-pi transition in SFS Josephson junctions.",0204568v2 2002-05-13,Effect of Subband Landau Level Coupling to the Linearly Dispersing Collective Mode in a Quantum Hall Ferromagnet,"In a recent experiment (Phys. Rev. Lett. {\bf 87}, 036903 (2001)), Spielman et al observed a linearly dispersing collective mode in quantum Hall ferromagnet. While it qualitatively agrees with the Goldstone mode dispersion at small wave vector, the experimental mode velocity is slower than that calculated by previous theories by a factor about 0.55. A better agreement with the experimental data may possibly be achieved by taking the subband Landau level coupling into account due to the finiteness of the layer thickness. A novel coupling of quantum fluctuation to the tunneling is briefly discussed.",0205250v2 2002-05-22,Universal angular magnetoresistance and spin torque in ferromagnetic/normal metal hybrids,"The electrical resistance of ferromagnetic/normal-metal (F/N) heterostructures depends on the nature of the junctions which may be tunnel barriers, point contacts, or intermetallic interfaces. For all junction types, the resistance of disordered F/N/F perpendicular spin valves as a function of the angle between magnetization vectors is shown to obey a simple universal law. The spin-current induced magnetization torque can be measured by the angular magnetoresistance of these spin valves. The results are generalized to arbitrary magnetoelectronic circuits.",0205453v1 2002-05-23,Magnetic-domain-controlled vortex pinning in a superconductor/ferromagnet bilayer,"Vortex pinning in a type-II superconducting Pb film covering a Co/Pt multilayer with perpendicular magnetic anisotropy is investigated. Different stable magnetic domain patterns like band and bubble domains can be created in the Co/Pt multilayer, clearly influencing the vortex pinning in the superconducting Pb layer. Most effective pinning is observed for the bubble domain state. We demonstrate that the pinning properties of the superconductor/ferromagnet bilayer can be controlled by tuning the size, density and magnetization direction of the bubbles.",0205470v1 2002-06-01,From classical to quantum spintronics: Theory of coherent spin injection and spin valve phenomena,"We present a theory of coherent quantum transport in ferromagnetic/ non-magnetic/ ferromagnetic heterojunctions. We predict quantum coherence to give rise to a quantum spin valve effect that, unlike its familiar classical analog, occurs even in the absence of a net spin current through the heterostructure. Thus the relationship between spin and charge transport is qualitatively different in the presence of quantum interference than in the (semi)classical regime. This has important implications for the design of quantum coherent spintronic devices and the interpretation of experiments.",0206009v1 2002-06-24,Derivatives and inequalities for order parameters in the Ising spin glass,"Identities and inequalities are proved for the order parameters, correlation functions and their derivatives of the Ising spin glass. The results serve as additional evidence that the ferromagnetic phase is composed of two regions, one with strong ferromagnetic ordering and the other with the effects of disorder dominant. The Nishimori line marks a crossover between these two regions.",0206438v2 2002-06-24,Temperature-dependent quasiparticle band structure of the ferromagnetic semiconductor EuS,"We present calculations for the temperature-dependent electronic structure of the ferromagnetic semiconductor EuS. A combination of a many-body evaluation of a multiband Kondo-lattice model and a first-principles T=0--bandstructure calculation (tight-binding linear muffin-tin orbital (TB-LMTO)) is used to get realistic information about temperature- and correlation effects in the EuS energy spectrum. The combined method strictly avoids double-counting of any relevant interaction. Results for EuS are presented in terms of spectral densities, quasiparticle band structures and quasiparticle densities of states, and that over the entire temperature range.",0206449v1 2002-06-25,Order parameter symmetry in ferromagnetic superconductors,"We analyze the symmetry and the nodal structure of the superconducting order parameter in a cubic ferromagnet, such as ZrZn$_2$. We demonstrate how the order parameter symmetry evolves when the electromagnetic interaction of the conduction electrons with the internal magnetic induction and the spin-orbit coupling are taken into account. These interactions break the cubic symmetry and lift the degeneracy of the order parameter. It is shown that the order parameter which appears immediately below the critical temperature has two components, and its symmetry is described by {\em co-representations} of the magnetic point groups. This allows us to make predictions about the location of the gap nodes.",0206487v1 2002-06-30,Spatially Resolved Dynamics of Localized Spin-Wave Modes in Ferromagnetic Wires,"We have observed localized spin-wave modes in individual thin-film ferromagnetic wires using time-resolved Kerr microscopy as a micron-scale spectroscopic probe. The localization is due to the partial demagnetization of a wire when an external field is applied in the plane of the film and perpendicular to the long axis of the wire. Spatially-resolved spectra demonstrate the existence of distinct modes at the edges of a rectangular wire. Spectral images clearly show the crossover of the two edge modes into a single mode in low applied fields, in agreement with the results of micromagnetic simulations.",0207022v1 2002-07-02,Unveiling Order behind Complexity: Coexistence of Ferromagnetism and Bose-Einstein Condensation,"We present an algebraic framework for identifying the order parameter and the possible phases of quantum systems that is based on identifying the local dimension $N$ of the quantum operators and using the SU(N) group representing the generators of generalized spin-particle mappings. We illustrate this for $N$=3 by presenting for any spatial dimension the exact solution of the bilinear-biquadratic $S$=1 quantum Heisenberg model at a high symmetry point. Through this solution we rigorously show that itinerant ferromagnetism and Bose-Einstein condensation may coexist.",0207073v1 2002-07-11,Heavy quasiparticles in the ferromagnetic superconductor ZrZn2,"We report a study of the de Haas-van Alphen effect in the normal state of the ferromagnetic superconductor ZrZn2. Our results are generally consistent with an LMTO band structure calculation which predicts four exchange-split Fermi surface sheets. Quasiparticle effective masses are enhanced by a factor of about 4.9 implying a strong coupling to magnetic excitations or phonons. Our measurements provide insight in to the mechanism for superconductivity and unusual thermodynamic properties of ZrZn2.",0207285v1 2002-07-13,Using single quantum states as spin filters to study spin polarization in ferromagnets,"By measuring electron tunneling between a ferromagnet and individual energy levels in an aluminum quantum dot, we show how spin-resolved quantum states can be used as filters to determine spin-dependent tunneling rates. We also observe magnetic-field-dependent shifts in the magnet's electrochemical potential relative to the dot's energy levels. The shifts vary between samples and are generally smaller than expected from the magnet's spin-polarized density of states. We suggest that they are affected by field-dependent charge redistribution at the magnetic interface.",0207333v1 2002-07-16,Ferromagnetic Luttinger Liquids,"We study weak itinerant ferromagnetism in one-dimensional Fermi systems using perturbation theory and bosonization. We find that longitudinal spin fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the Fermi velocity. This leads to a large anomalous dimension in the spin-channel and strong algebraic singularities in the single-particle spectral function and in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F, where 2 Delta is the exchange splitting.",0207383v1 2002-07-19,Fluctuation-Driven Quantum Phase Transitions in Clean Itinerant Ferromagnets,"The quantum phase transition in clean itinerant ferromagnets is analyzed. It is shown that soft particle-hole modes invalidate Hertz's mean-field theory for $d \leq 3$. A renormalized mean-field theory predicts a fluctuation-induced first order transition for $1 < d \leq 3$, whose stability is analyzed by renormalization group techniques. Depending on microscopic parameter values, the first order transition can be stable, or be pre-empted by a fluctuation-induced second order transition. The critical behavior at the latter is determined. The results are in agreement with recent experiments.",0207470v2 2002-07-23,Magnetically mediated superconductivity: Crossover from cubic to tetragonal lattice,"We compare predictions of the mean-field theory of superconductivity for nearly antiferromagnetic and nearly ferromagnetic metals for cubic and tetragonal lattices. The calculations are based on the parametrization of an effective interaction arising from the exchange of magnetic fluctuations and assume that a single band is relevant for superconductivity. The results show that for comparable model parameters, the robustness of magnetic pairing increases gradually as one goes from a cubic structure to a more and more anisotropic tetragonal structure either on the border of antiferromagnetism or ferromagnetism.",0207556v1 2002-07-26,Ballistic versus diffusive magnetoresistance of a magnetic point contact,"The quasiclassical theory of a nanosize point contacts (PC) between two ferromagnets is developed. The maximum available magnetoresistance values in PC are calculated for ballistic versus diffusive transport through the area of a contact. In the ballistic regime the magnetoresistance in excess of few hundreds percents is obtained for the iron-group ferromagnets. The necessary conditions for realization of so large magnetoresistance in PC, and the experimental results by Garcia et al are discussed",0207648v1 2002-08-02,Axisymmetric versus Non-axisymmetric Vortices in Spinor Bose-Einstein Condensates,"The structure and stability of various vortices in F=1 spinor Bose-Einstein condensates are investigated by solving the extended Gross-Pitaevskii equation under rotation. We perform an extensive search for stable vortices, considering both axisymmetric and non-axisymmetric vortices and covering a wide range of ferromagnetic and antiferromagnetic interactions. The topological defect called Mermin-Ho (Anderson-Toulouse) vortex is shown to be stable for ferromagnetic case. The phase diagram is established in a plane of external rotation Omega vs total magnetization M by comparing the free energies of possible vortices. It is shown that there are qualitative differences between axisymmetric and non-axisymmetric vortices which are manifested in the Omega- and M-dependences.",0208032v1 2002-08-08,Metallic Continuum Quantum Ferromagnets at Finite Temperature,"We study via renormalization group (RG) and large N methods the problem of continuum SU(N) quantum Heisenberg ferromagnets (QHF) coupled to gapless electrons. We establish the phase diagram of the dissipative problem and investigate the changes in the Curie temperature, magnetization, and magnetic correlation length due to dissipation and both thermal and quantum fluctuations. We show that the interplay between the topological term (Berry's phase) and dissipation leads to non-trivial effects for the finite temperature critical behavior.",0208185v2 2002-08-13,Ferromagnetism in Cr-doped GaN: A First Principles Calculation,"Otherwise antiferromagnetic chromium is shown to couple ferromagnetically when doped into GaN irrespective of whether the host is a crystal or a cluster. The results on the doped clusters and crystals are obtained from density functional theory based molecular orbital theory and linearized muffin tin orbital tight binding supercell band structure method respectively. The calculated half metallic behavior of Cr-doped GaN crystal combined with the observed room temperature Curie point make it an ideal candidate for spintronics applications.",0208258v1 2002-08-14,Plasticity of Ferromagnets near the Curie Point,"We propose an explanation of the anomalous growth of plasticity in ferromagnets near the Curie point. We demonstrate that this effect is caused by spin-dependent detachment of dislocations from obstacles under an influence of the internal magnetic field. Magnetization fluctuations grow in the vicinity of the Curie point, yield an increase of the detachment probability and, hence, an increase of the plasticity. We apply this model for a description of the temperature behaviour of the critical stress in nickel and of the microhardness of gadolinium. An external magnetic field suppresses the magnetization fluctuations and, hence may suppress the above singularities.",0208271v1 2002-08-16,Possible symmetries of the superconducting order parameter in a hexagonal ferromagnet,"We study the order parameter symmetry in a hexagonal crystal with co-existing superconductivity and ferromagnetism. An experimental example is provided by carbon-based materials, such as graphite-sulfur composites, in which an evidence of such co-existence has been recently discovered. The presence of a non-zero magnetization in the normal phase brings about considerable changes in the symmetry classification of superconducting states, compared to the non-magnetic case.",0208339v2 2002-08-27,Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets,"Using a model of nonmagnetic impurity potential, we have examined the behavior of planar vortex solutions in the classical two-dimensional XY ferromagnets in the presence of a spin vacancy localized out of the vortex core. Our results show that a spinless atom impurity gives rise to an effective potential that repels the vortex structure.",0208523v1 2002-08-30,Exact calculation of the skyrmion lifetime in a ferromagnetic Bose condensate,"The tunneling rate of a skyrmion in ferromagnetic spin-1/2 Bose condensates through an off-centered potential barrier is calculated exactly with the periodic instanton method. The prefactor is shown to depend on the chemical potential of the core atoms, at which level the atom tunnels. Our results can be readily extended to estimate the lifetime of other topological excitations in the condensate, such as vortices and monopoles.",0208593v1 2002-08-30,Spin Waves in Random Spin Chains,"We study quantum spin-1/2 Heisenberg ferromagnetic chains with dilute, random antiferromagnetic impurity bonds with modified spin-wave theory. By describing thermal excitations in the language of spin waves, we successfully observe a low-temperature Curie susceptibility due to formation of large spin clusters first predicted by the real-space renormalization-group approach, as well as a crossover to a pure ferromagnetic spin chain behavior at intermediate and high temperatures. We compare our results of the modified spin-wave theory to quantum Monte Carlo simulations.",0208607v1 2002-09-03,"Defect correlations, metal-insulator transition, and magnetic order in ferromagnetic semiconductors","Diluted ferromagnetic III-V semiconductors typically show a high degree of compensation. Compensation is connected to the presence of comparable densities of charged defects of either sign. This naturally leads to the development of strong correlations between defect positions during growth and annealing. We show that these correlations are required to understand the experimentally observed transport and magnetic properties as well as the persistence of the energy gap upon doping with magnetic ions.",0209055v1 2002-09-05,Orbital Kondo Effect in $CrO_{2}$: A LSDA+DMFT Study,"Motivated by a collection of experimental results indicating the strongly correlated nature of the ferromagnetic metallic state of $CrO_{2}$, we present results based on a combination of the actual bandstructure [3] with dynamical mean-field theory (DMFT) for the multi-orbital case. In striking contrast with LSDA(+U) [3] and model many-body approaches [14], much better semiquantitative agreement with (i) recent photoemission results, (ii) domain of applicability of the half-metal concept, and (iii) thermodynamic and dc transport data, is obtained within a single picture. Our approach has broad applications for the detailed first principles investigation of other transition metal oxide-based half-metallic ferromagnets.",0209132v1 2002-09-10,Hidden Quantum Critical Point in a Ferromagnetic Superconductor,"We consider a coexistence phase of both Ferromagnetism and superconductivity and solve the self-consistent mean-field equations at zero temperature. The superconducting gap is shown to vanish at the Stoner point whereas the magnetization doesn't. This indicates that the para-Ferro quantum critical point becomes a hidden critical point. The effective mass in such a phase gets enhanced whereas the spin wave stiffness is reduced as compared to the pure FM phase. The spin wave stiffness remains finite even at the para-Ferro quantum critical point.",0209237v1 2002-09-11,Current and Noise in a FM/quantum dot/FM System,"Using the Keldysh nonequilibrium technique we calculate current, noise and Fano factor in a ferromagnetic(FM)-quantum dot-ferromagnetic(FM) system with Coulomb interaction and spin-flip scattering in the dot. The lead polarizations are considered in both parallel P and antiparallel AP alignments. We show that spin-flip can increase both AP-current and AP-noise, while the P-current and P-noise are almost insensible to it. This fact leads to a suppression of the tunnelling magnetoresistance with increasing spin-flip rate.",0209263v1 2002-09-17,Possible phase separated bulk superconductivity in Ru0.9Sr2YCu2.1O7.9,"The Ru0.9Sr2YCu2.1O7.9 sample being synthesized by high-pressure high-temperature solid-state reaction underwent (weak) ferromagnetic transition at 150 K followed by superconducting transition at 30 K. It showed clear Meissner signal in the field-cooled process up to external magnetic field of a few hundred Oe (~300 Oe). The magnetic susceptibility data and magnetic hysteresis data could be explained assuming that the sample was a macroscopic mixture of a superconductor and a (weak) ferromagnet. Any anomalous behaviour implying the coexistence of superconductivity and magnetism on a microscopic scale was not observed.",0209376v1 2002-09-17,Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator,"Neutron scattering was used to determine the spin structure and the magnon spectrum of the Mott--Hubbard insulator YTiO$_3$. The magnetic structure is complex, comprising substantial G-type and A-type antiferromagnetic components in addition to the predominant ferromagnetic component. The magnon spectrum, on the other hand, is gapless and nearly isotropic. We show that these findings are inconsistent with the orbitally ordered states thus far proposed for YTiO$_3$ and discuss general implications for a theoretical description of exchange interactions in orbitally degenerate systems.",0209400v1 2002-09-18,Microphase separation in Pr0.67Ca0.33MnO3 by small angle neutron scattering,"We have evidenced by small angle neutron scattering at low temperature the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in Pr0.67Ca0.33MnO3. The results are compared to those obtained in Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF respectively. Quantitative analysis shows that the small angle scattering is not due to a mesoscopic mixing but to a nanoscopic electronic and magnetic ''red cabbage'' structure, in which the ferromagnetic phase exists in form of thin layers in the AF matrix (stripes or 2D ''sheets'').",0209426v1 2002-09-18,Quantum behavior of Orbitals in Ferromagnetic Titanates: Novel Orderings and Excitations,"We investigate the collective behavior of orbital angular momentum in the spin ferromagnetic state of a Mott insulator with $t_{2g}$ orbital degeneracy. The frustrated nature of the interactions leads to an infinite degeneracy of classical states. Quantum effects select four distinct orbital orderings. Two of them have a quadrupolar order, while the other states show in addition weak orbital magnetism. Specific predictions are made for neutron scattering experiments which might help to identify the orbital order in YTiO$_3$ and to detect the elementary orbital excitations.",0209434v1 2002-09-20,Spontaneous spin polarized tunneling current through a quantum dot array,"We show theoretically that a strongly spin-polarized current can be generated in semiconductors by taking advantage of the ferromagnetic phase of a quantum dot array (QDA). A Hubbard model with coupling to leads is used to study the tunneling current of the QDA system as a function of gate voltage. Due to the weak interdot coupling and strong Coulomb repulsion, it is found that a ferromagnetic phase exists in QDA within a window of gate voltage. Therefore QDA can be used as a spin filter to detect and control spin states in quantum information devices.",0209499v1 2002-09-23,Rigorous Proof of Pseudospin Ferromagnetism in Two-Component Bosonic Systems with Component-Independent Interactions,"For a two-component bosonic system, the components can be mapped onto a pseudo-spin degree of freedom with spin quantum number S=1/2. We provide a rigorous proof that for a wide-range of real Hamiltonians with component independent mass and interaction, the ground state is a ferromagnetic state with pseudospin fully polarized. The spin-wave excitations are studied and found to have quadratic dispersion relations at long wave length.",0209529v1 2002-09-27,Local Density of States in the Antiferromagnetic and Ferromagnetic Kondo Models,"Based on a simple approximation scheme we have computed the local density of states (LDOS) of the antiferromagnetic and ferromagnetic Kondo models for the full range of band occupations and coupling strengths. For both models the LDOS with its full energy dependence has not been calculated before. Arguments are given for the results to be qualitatively trustworthy despite the simplicity of the approximation scheme.",0209639v1 2002-10-26,Dynamic exchange coupling in magnetic bilayers,"A long-ranged dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance (FMR) and explained by an adiabatic spin-pump theory. In FMR the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the FMR line width when the resonance fields are well apart, and results in a dramatic line-width narrowing when the FMR fields approach each other.",0210588v2 2002-10-29,First order superconducting transition near a ferromagnetic quantum critical point,"We address the issue of how triplet superconductivity emerges in an electronic system near a ferromagnetic quantum critical point (FQCP). Previous studies found that the superconducting transition is of second order, and Tc is strongly reduced near the FQCP due to pair-breaking effects from thermal spin fluctuations. In contrast, we demonstrate that near the FQCP, the system avoids pair-breaking effects by undergoing a first order transition at a much larger Tc. A second order superconducting transition emerges only at some distance from the FQCP.",0210653v1 2002-11-08,Spin Motion in Electron Transmission through Ultrathin Ferromagnetic Films Accessed by Photoelectron Spectroscopy,"Ab initio and model calculations demonstrate that the spin motion of electrons transmitted through ferromagnetic films can be analyzed in detail by means of angle- and spin-resolved core-level photoelectron spectroscopy. The spin motion appears as precession of the photoelectron spin polarization around and as relaxation towards the magnetization direction. In a systematic study for ultrathin Fe films on Pd(001) we elucidate its dependence on the Fe film thickness and on the Fe electronic structure. In addition to elastic and inelastic scattering, the effect of band gaps on the spin motion is addressed in particular.",0211163v1 2002-11-11,Spin-dependent electron transport through a ferromagnetic domain wall,"We present a theoretical study of spin-dependent transport through a ferromagnetic domain wall. With an increase of the number of components of the exchange coupling, we have observed that the variance of the conductance becomes half. As the strength of the domain wall magnetization is increased, negative magnetoresistance is also observed.",0211203v1 2002-11-21,Parametric quantum spin pump,"We investigate a non-adiabatic parametric quantum pump consists of a nonmagnetic scattering region connected by two ferromagnetic leads. The presence of ferromagnetic leads allows electrons with different spins to experience different potential landscape. Using this effect we propose a quantum spin pump that drives spin-up electrons to flow in one direction and spin-down electrons to flow in opposite direction. As a result, the spin pump can deliver a spin current with vanishing charge current.",0211460v1 2002-12-05,Phenomenological theory of spinor Bose-Einstein condensates,"A phenomenological model is proposed to describe the behavior of spinor Bose-Einstein condensates. In the absence of hyperfine spin-spin interactions, Bose-Einstein condensation leads to a spontaneous magnetization at the same transition temperature. This is the so-called Bose-Einstein ferromagnetism. Including the hyperfine spin interactions, the phase diagram of the spinor condensate in an optical trap is studied and the Gross-Pitaevskii equation is extended. The possibility of checking for the existence of the Bose-Einstein ferromagnetism experimentally is also discussed.",0212114v3 2002-12-18,Paramagnetic-ferromagnetic transition in a double-exchange model,"We study paramagnetic - ferromagnetic transition due to exchange interaction between classical localized magnetic moments and conduction electrons. We formulate the Dynamical Mean Field Approximation equations for arbitrary electron dispersion law, concentration and relation between exchange coupling and the electron band width. Solving these equations we find explicit formula for the transition temperature $T_c$. We present the results of calculations of the $T_c$ for the semi-circular electron density of states.",0212450v2 2003-01-07,Order parameter oscillations in Fe/Ag/Bi2Sr2CaCu2O{8+delta} tunnel junctions,"We have performed temperature dependent tunneling conductance spectroscopy on Fe/Ag/Bi2Sr2CaCu2O8 (BSCCO) planar junctions. The multilayered Fe counterelectrode was designed to probe the proximity region of the ab-plane of BSCCO. The spectra manifested a coherent oscillatory behavior with magnitude and sign dependent on the energy, decaying with increasing distance from the junction barrier, in conjunction with the theoretical predictions involving d-wave superconductors coupled with ferromagnets. The conductance oscillates in antiphase at E = 0 and E = +/-Delta. Spectral features characteristic to a broken time-reversal pairing symmetry are detected and they do not depend on the geometrical characteristics of the ferromagnetic film.",0301081v1 2003-01-16,Observation of soft magnetorotons in bilayer quantum Hall ferromagnets,"Inelastic light scattering measurements of low-lying collective excitations of electron double layers in the quantum Hall state at total filling nu_T=1 reveal a deep magnetoroton in the dispersion of charge-density excitations across the tunneling gap. The roton softens and sharpens markedly when the phase boundary for transitions to highly correlated compressible states is approached. The findings are interpreted with Hartree-Fock evaluations that link soft magnetorotons to enhanced excitonic Coulomb interactions and to quantum phase transitions in the ferromagnetic bilayers.",0301295v1 2003-01-24,Quantized Anomalous Hall Effect in Two-Dimensional Ferromagnets - Quantum Hall Effect from Metal -,"We study the effect of disorder on the anomalous Hall effect (AHE) in two-dimensional ferromagnets. The topological nature of AHE leads to the integer quantum Hall effect from a metal, i.e., the quantization of $\sigma_{xy}$ induced by the localization except for the few extended states carrying Chern number. Extensive numerical study on a model reveals that Pruisken's two-parameter scaling theory holds even when the system has no gap with the overlapping multibands and without the uniform magnetic field. Therefore the condition for the quantized AHE is given only by the Hall conductivity $\sigma_{xy}$ without the quantum correction, i.e., $|\sigma_{xy}| > e^2/(2h)$.",0301482v1 2003-01-31,Non-collinear magnetism in iron at high pressures,"Using a first principles based, magnetic tight-binding total energy model, the magnetization energy and moments are computed for various ordered spin configurations in the high pressure polymorphs of iron (fcc, or $\gamma$-Fe, and hcp, or $\epsilon$-Fe), as well ferromagnetic bcc iron ($\alpha$-Fe). For hcp, a non-collinear, antiferromagnetic, spin configuration that minimizes unfavorable ferromagnetic nearest neighbor ordering is the lowest energy state and is more stable than non-magnetic $\epsilon$ iron up to about 75 GPa. Accounting for non-collinear magnetism yields better agreement with the experimental equation of state, in contrast to the non-magnetic equation of state, which is in poor agreement with experiment below 50 GPa.",0301615v1 2003-02-13,Exact diagonalization study of domain structure in integer filling factor quantum Hall ferromagnets,"Opposite spin Landau levels in a quantum well can be brought into coincidence by tilting the magnetic field away from normal orientation. We demonstrate that the magnetotransport anomaly at integer filling factors that was recently discovered by Pan {\it et al} is due to such a coincidence. By performing exact diagonalization calculations using microscopically evaluated effective electron-electron interactions, we are able to establish that the electronic ground state at coincidence is an Ising quantum Hall ferromagnet and that the low energy excitations correspond to the formation of a domain wall.",0302271v1 2003-02-24,First order transition from correlated electron semiconductor to ferromagnetic metal in single crystalline FeSi1-xGex,"The phase diagram of FeSi1-xGex, obtained from magnetic, thermal and transport measurements on single crystals, shows a first-order transition from a correlated electron semiconductor to a ferromagnetic metal at a critical concentration, x ~ 0.25. The gap of the insulating phase strongly decreases with x. The specific heat coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with a correlation induced insulator-metal transition in conjunction with disorder on the Si/Ge ligand site.",0302497v1 2003-03-02,Andreev conductance of a domain wall,"At low temperatures, the transport through a superconductor-ferromagnet tunnel interface is due to tunneling of electrons in pairs. Exchange field of a monodomain ferromagnet aligns electron spins and suppresses the two electron tunneling. The presence of the domain walls at the SF interface strongly enhances the subgap current. The Andreev conductance is proven to be proportional to the total length of domain walls at the SF interface.",0303014v1 2003-03-10,Critical Behavior of the Ferromagnetic Ising Model on a Sierpinski Carpet: Monte Carlo Renormalization Group Study,"We perform a Monte Carlo Renormalization Group analysis of the critical behavior of the ferromagnetic Ising model on a Sierpi\'nski fractal with Hausdorff dimension $d_f\simeq 1.8928$. This method is shown to be relevant to the calculation of the critical temperature $T_c$ and the magnetic eigen-exponent $y_h$ on such structures. On the other hand, scaling corrections hinder the calculation of the temperature eigen-exponent $y_t$. At last, the results are shown to be consistent with a finite size scaling analysis.",0303173v1 2003-03-19,Long Range Coherent Manipulation of Nuclear Spins in Quantum Hall Ferromagnet,"A coherent superposition of many nuclear spin states can be prepared and manipulated via the hyperfine interaction with the electronic spins by varying the Landau level filling factor through the gate voltage in appropriately designed Quantum Hall Ferromagnet. During the manipulation periods the 2D electron system forms spatially large Skyrmionic spin textures, where many nuclear spins follow locally the electron spin polarization. It is shown that the collective spin rotation of a single spin texture is gapless in the limit of zero Zeeman splitting, and may dominate the nuclear spins relaxation and decoherence processes in the quantum well.",0303394v1 2003-03-29,Modified self-consistent harmonic approximation and its application to two-dimensional easy-axis quantum ferromagnets,"In the paper we describe the modification of self-consistent harmonic approximation for quantum S=1 systems. This method has a number of advantages in comparison with usual SCHA. We apply the method to two-dimensional ferromagnets with easy-axis exchange or single-site anisotropy. The results are in good agreement with Monte-Carlo simulations and pure-quantum SCHA.",0303619v2 2003-04-04,Co-ordination between Rashba spin-orbital interaction and space charge effect and enhanced spin injection into semiconductors,"We consider the effect of the Rashba spin-orbital interaction and space charge in a ferromagnet-insulator/semiconductor/insulator-ferromagnet junction where the spin current is severely affected by the doping, band structure and charge screening in the semiconductor. In diffusion region, if the the resistance of the tunneling barriers is comparable to the semiconductor resistance, the magnetoresistance of this junction can be greatly enhanced under appropriate doping by the co-ordination between the Rashba effect and screened Coulomb interaction in the nonequilibrium transport processes within Hartree approximation.",0304117v1 2003-04-17,Competing Ground States in Triple-layered Sr4Ru3O10: Verging on Itinerant Ferromagnetism with Critical Fluctuations,"Sr4Ru3O10 is characterized by a sharp metamagnetic transition and ferromagnetic behavior occurring within the basal plane and along the c-axis, respectively. Resistivity at magnetic field, B, exhibits low-frequency quantum oscillations when B||c-axis and large magnetoresistivity accompanied by critical fluctuations driven by the metamagnetism when B^c-axis. The complex behavior evidenced in resistivity, magnetization and specific heat presented is not characteristic of any obvious ground states, and points to an exotic state that shows a delicate balance between fluctuations and order.",0304411v1 2003-04-17,Spontaneous Pseudospin Spiral Order in Bilayer Quantum Hall Systems,"Using exact diagonalization of bilayer quantum Hall systems at total filling factor $\nu_T=1$ in the torus geometry, we show that there is a new long-range interlayer phase coherence due to spontaneous pseudospin spiral order at interlayer distances larger than the critical value at which the pseudospin ferromagnetic order is destroyed. We emphasize the distinction between the interlayer phase coherence and the pseudospin ferromagnetic order.",0304417v2 2003-04-23,Critical Resistance of the Quantum Hall Ferromagnet in AlAs 2D Electrons,"Magnetic transitions in AlAs two-dimensional electrons give rise to sharp resistance spikes within the quantum Hall effect. Such spikes are likely caused by carrier scattering at magnetic domain walls below the Curie temperature. We report a critical behavior in the temperature dependence of the spike width and amplitude, from which we deduce the Curie temperature of the quantum Hall ferromagnet. Our data also reveal that the Curie temperature increases monotonically with carrier density.",0304507v2 2003-05-02,Monte Carlo Study of Doping Change and Disorder Effect on Double Exchange Ferromagnetism,"Phase diagram and critical properties are studied for three-dimensional double exchange model with and without quenched disorder. Employing the Monte Carlo method and the systematic analysis on the finite-size effect, we estimate the Curie temperature and the critical exponent as functions of the doping concentration and the strength of the random potential. The Curie temperature well scales to the kinetic energy of electrons in the ground state as expected for this kinetics-driven ferromagnetism. The universality class of this transition is described by the short-range Heisenberg fixed point. The results are compared with the experimental results in the colossal magnetoresistance manganites.",0305029v1 2003-05-15,Inhomogeneous Ferromagnetism and Unconventional Charge Dynamics in Disordered Double Exchange Magnets,"We solve the double exchange model in the presence of arbitrary substitutional disorder by using a self consistently generated effective Hamiltonian for the spin degrees of freedom. The magnetic properties are studied through classical Monte Carlo while the effective exchange, $D_{ij}$, are calculated by solving the disordered fermion problem, and renormalised self-consistently with increasing temperature. We present exact results on the conductivity, magnetoresistance, optical response and `real space' structure of the inhomogeneous ferromagnetic state, and compare our results with charge dynamics in disordered La_{1-x}Sr_xMnO_3. The large sizes, ${\cal O} (10^3)$, accessible within our method allows a complete, controlled calculation on the disordered strongly interacting problem.",0305352v1 2003-05-20,Yang-Lee zeros of the one-dimensional Q-state Potts model,"The distributions of the Yang-Lee zeros of the ferromagnetic and antiferromagnetic Q-state Potts models in one dimension are studied for arbitrary Q and temperature. The Yang-Lee zeros of the Potts antiferromagnet have been fully investigated for the first time. The distributions of the Yang-Lee zeros show a variety of different shapes. Some of the Yang-Lee zeros lie on the positive real axis even for T>0. For the ferromagnetic model this happens only for Q<1, while there exist some zeros of the antiferromagnetic model on the positive real axis both for Q<1 and for Q>1.",0305456v1 2003-05-21,From Bubble to Skyrmion: Dynamic Transformation Mediated by a Strong Magnetic Tip,"Skyrmions in thin metallic ferromagnetic films are stable due to competition between the RKKY interaction and uniaxial magnetic anisotropy. We mimic the RKKY interaction by the next-nearest-neighbors ferromagnetic and antiferromagnetic exchange interactions. We demonstrate analytically and numerically dissipative transformation of a bubble created by a strong cylindrical magnetic tip into a stable Skyrmion.",0305487v1 2003-05-23,Little-Parks effect and multiquanta vortices in a hybrid superconductor--ferromagnet system,"Within the phenomenological Ginzburg-Landau theory we investigate the phase diagram of a thin superconducting film with ferromagnetic nanoparticles. We study the oscillatory dependence of the critical temperature on an external magnetic field similar to the Little-Parks effect and formation of multiquantum vortex structures. The structure of a superconducting state is studied both analytically and numerically.",0305551v1 2003-06-02,Ferromagnetism in Fe-doped Ba6Ge25 Chiral Clathrate,"We have successfully synthesized a Ba6Ge25 clathrate, substituting 3 Fe per formula unit by Ge. This chiral clathrate has Ge sites forming a framework of closed cages and helical tunnel networks. Fe atoms randomly occupy these sites, and exhibit high-spin magnetic moments. A ferromagnetic transition is observed with Tc = 170 K, the highest observed Tc for a magnetic clathrate. However, the magnetic phase is significantly disordered, and exhibits a transformation to a re-entrant spin glass phase. This system has a number of features in common with other dilute magnetic semiconductors.",0306044v1 2003-06-04,Competition of Orbital Antiferromagnetism With Q-Triplet-Pairing in the Ferromagnetic Superconductor UGe$_2$,"Within a multicomponent BCS-like framework we study the coexistence and competition of various CDW and SC order parameters in the presence of a strong ferromagnetic background. We find that the competition of unconventional CDW, also called orbital antiferromagnet, with SC at finite momentum (${\bf Q}$-triplet pairing) shows unique characteristics like an extreme sensitivity on the deviations from nesting. We argue that pressure in UGe$_2$ creates deviations from the nesting and report a phase diagram in qualitative agreement with the observed behavior of UGe$_2$.",0306106v1 2003-06-10,Determining the current polarization in Al/Co nanostructured point contacts,"We present a study of the Andreev reflections in superconductor/ferromagnet nanostructured point contacts. The experimental data are analyzed in the frame of a model with two spin-dependent transmission coefficients for the majority and minority charge carriers in the ferromagnet. This model consistently describes the whole set of conductance measurements as a function of voltage, temperature, and magnetic field. The ensemble of our results shows that the degree of spin polarization of the current can be unambiguously determined using Andreev physics.",0306241v1 2003-06-10,Anisotropic susceptibilities of thin ferromagnetic films within many-body Green's function theory,"Transverse and parallel static susceptibilities of in-plane uniaxial anisotropic ferromagnetic films are calculated within many-body Green's function theory on the basis of an Heisenberg model. The importance of collective magnetic excitations in particular in the paramagnetic regime are stressed by comparing with mean field calculations. The paper generalizes the work of Jensen et al. [1] to the multilayer case and to spins with S>1/2.",0306243v2 2003-06-19,Griffiths phase in diluted magnetic semiconductors,"We study the effects of disorder in the vicinity of the ferromagnetic transition in a diluted magnetic semiconductor in the strongly localized regime. We derive an effective polaron Hamiltonian, which leads to the Griffiths phase above the ferromagnetic transition point. The Griffiths-McCoy effects yield non-perturbative contributions to the dynamic susceptibility. We explicitly derive the long-time susceptibility, which has a pseudo-scaling form, with the dynamic critical exponent being expressed through the percolation indices.",0306488v2 2003-06-20,Quantum dynamics of tunneling between ferromagnets,"We study the Josephson-like spin currents between two ferromagnetic metals by deriving the effective action of the junction. A DC spin Josephson current with the full O(3) symmetry is obtained. By the analogy to the superconducting junctions, we have shown that a time-independent uniform magnetic field can serve as the source of the AC spin Josephson effect. That is, the spin current becomes a periodic function of the time with the period proportional to the inverse of the magnitude of the external magnetic field.",0306518v2 2003-06-20,Andreev bound states for superconducting-ferromagnetic box,"Within the microscopic Bogoliubov--de Gennes (BdG) formalism an exact quantization condition for Andreev bound states of the ferromagnetic-superconducting hybrid systems of box geometry is derived and a semi-classical formula for the density of states is obtained. The semi-classical formula is shown to agree with the exact result, even when the exchange field $h$, is much larger than the superconductor order parameter, provided $h$ is small compared with the Fermi energy.",0306523v1 2003-06-26,Thermodynamic properties of a classical d-dimensional spin-S Heisenberg ferromagnet with long-range interactions via the spectral density method,"The thermodynamic properties of a classical d-dimensional spin-S Heisenberg ferromagnet, with long-range interactions decaying as $r^{-p}$ and in the presence of an external magnetic field, is investigated by means of the spectral density method in the framework of classical statistical mechanics. We find that long-range order exists at finite temperature for $dd$ with $d>2$, consistently with known theorems. Besides, the related critical temperature is determined and a study of the critical properties is performed.",0306673v1 2003-06-27,The coexistence of p-wave spin triplet superconductivity and itinerant ferromagnetism,"A model for coexistence of p-wave spin-triplet superconductivity (SC) and itinerant ferromagnetism (FM) is presented. The Hamiltonian can be diagonalized by using the so(5) algebraic coherent state. We obtain the coupling equations of the magnetic exchange energy and superconducting gaps through the double-time Green function. It is found that the ferromagnetisation gives rise to the phase transitions of p-wave superconducting states or superfluid of $^{3}He$.",0306691v1 2003-07-01,Spin clustering and ferromagnetic couplings in a dilute magnetic semiconductor,"Ferromagnetic couplings in spin clusters are shown to be strongly enhanced compared to those for an ordered impurity arrangement, even for the same spin separation and hole doping. The consequent energy-enhancement of the cluster-localized spin-wave modes indicates a potentially significant role of positional Mn disorder in enhancing $T_{\rm C}$. Within a simple model involving two spin-excitation energy scales corresponding to weakly and strongly coupled spins, the temperature dependence of magnetization is found to be in good agreement with the SQUID magnetization data for Ga$_{1-x}$Mn$_x$As.",0307009v1 2003-07-14,First Principles Calculation of Anomalous Hall Conductivity in Ferromagnetic bcc Fe,"We perform a first principles calculation of the anomalous Hall effect in ferromagnetic bcc Fe. Our theory identifies an intrinsic contribution to the anomalous Hall conductivity and relates it to the k-space Berry phase of occupied Bloch states. The theory is able to account for both dc and magneto-optical Hall conductivities with no adjustable parameters.",0307337v2 2003-07-16,Induced four fold anisotropy and bias in compensated NiFe/FeMn double layers,"A vector spin model is used to show how frustrations within a multisublattice antiferromagnet such as FeMn can lead to four-fold magnetic anisotropies acting on an exchange coupled ferromagnetic film. Possibilities for the existence of exchange bias are examined and shown to exist for the case of weak chemical disorder at the interface in an otherwise perfect structure. A sensitive dependence on interlayer exchange is found for anisotropies acting on the ferromagnet through the exchange coupling, and we show that a wide range of anisotropies can appear even for a perfect crystalline structure with an ideally flat interface.",0307384v1 2003-07-29,Metamorphosis of the Quantum Hall Ferromagnet at $ν= 2/5$,"We report on the dramatic evolution of the quantum Hall ferromagnet in the fractional quantum Hall regime at $\nu = 2/5$ filling. A large enhancement in the characteristic timescale gives rise to a dynamical transition into a novel quantized Hall state. The observed Hall state is determined to be a zero-temperature phase distinct from the spin-polarized and spin-unpolarized $\nu = 2/5$ fractional quantum Hall states. It is characterized by a strong temperature dependence and puzzling correlation between temperature and time.",0307711v1 2003-07-31,Ferromagnetism in the Strong Hybridization Regime of the Periodic Anderson Model,"We determine exactly the ground state of the one-dimensional periodic Anderson model (PAM) in the strong hybridization regime. In this regime, the low energy sector of the PAM maps into an effective Hamiltonian that has a ferromagnetic ground state for any electron density between half and three quarters filling. This rigorous result proves the existence of a new magnetic state that was excluded in the previous analysis of the mixed valence systems.",0308002v1 2003-08-08,Numerical Investigation on Asymmetric Bilayer System at Integer Filling Factor,"Deformation of the easy-axis ferromagnetic state in asymmetric bilayer systems are investigated numerically. Using the exact diagonalization the easy-axis to easy-plane ferromagnetic transition at total filling factor 3 or 4 is investigated. At still higher filling, novel stripe state in which stripes are aligned in the vertical direction occurs. The Hartree-Fock energies of relevant ordered states are calculated and compared.",0308158v2 2003-09-03,Superconducting Vortices and Elliptical Ferromagnetic Textures,"In this article an analytical and numerical study of superconducting thin film with ferromagnetic textures of elliptical geometries in close proximity is presented. The screening currents induced in the superconductor due to the magnetic texture are calculated. Close to the superconducting transition temperature $T_c$ the spontaneous creation of superconducting vortices becomes energy favorable depending on the value of the magnetization and the geometrical quantities of the magnetic texture. The creation of vortices by elliptic dots is more energy favorable than those created by circular ones. The superconductor covered by elliptic dots array exhibits anisotropic transport properties.",0309090v1 2003-09-10,Non-monotonic angular magnetoresistance in asymmetric spin valves,"The electric resistance of ferromagnet/normal-metal/ferromagnet perpendicular spin valves depends on the relative angle between the magnetization directions. In contrast to common wisdom, this angular magnetoresistance is found to be not necessarily a monotone function of the angle. The parameter dependence of the global resistance minimum at finite angles is studied and the conditions for experimental observation are specified.",0309252v2 2003-09-15,Spiral phase and phase separation of the double exchange model in the large-S limit,"The phase diagram of the double exchange model is studied in the large-S limit at zero temperature in two and three dimensions. We find that the spiral state has lower energy than the canted antiferromagnetic state in the region between the antiferromagnetic phase and the ferromagnetic phase. At small doping, the spiral phase is unstable against phase separation due to its negative compressibility. When the Hund coupling is small, the system separates into spiral regions and antiferromagnetic regions. When the Hund coupling is large, the spiral phase disappears completely and the system separates into ferromagnetic regions and antiferromagnetic regions.",0309346v1 2003-09-22,Spin textures in slowly rotating Bose-Einstein Condensates,"Slowly rotating spin-1 Bose-Einstein condensates are studied through a variational approach based upon lowest Landau level calculus. The author finds that in a gas with ferromagnetic interactions, such as $^{87}$Rb, angular momentum is predominantly carried by clusters of two different types of skyrmion textures in the spin-vector order parameter. Conversely, in a gas with antiferromagnetic interactions, such as $^{23}$Na, angular momentum is carried by $\pi$-disclinations in the nematic order parameter which arises from spin fluctuations. For experimentally relevant parameters, the cores of these $\pi$-disclinations are ferromagnetic, and can be imaged with polarized light.",0309511v1 2003-10-02,Nonquasiparticle states in half-metallic ferromagnet NiMnSb,"Nonquasiparticle states above the Fermi energy are studied by first-principle dynamical mean field calculations for a prototype half-metallic ferromagnet, NiMnSb. We present a quantitative evaluation of the spectral weight of this characteristic feature and discuss the possible experimental investigation (BIS, NMR, STM and Andreev reflection) to clarify the existence of these states.",0310032v1 2003-10-07,One dimensional gapless magnons in a single anisotropic ferromagnetic nanolayer,"Gapless magnons in a plane ferromagnet with normal axis anisotropy are shown to exist besides the usual gapped modes that affect spin dependent transport properties only above a finite temperature. These magnons are one dimensional objects, in the sense that they are localized inside the domain walls that form in the film. They may play an essential role in the spin dependent scattering processes even down to very low temperatures.",0310159v1 2003-10-08,Quantum phase transitions of magnetic rotons,"Due to weak spin-orbit coupling, the magnetic excitations of an itinerant ferromagnet become magnetic rotons, excitations with degenerate minima on a hypersphere at finite wavevector. Using self-consistent Hartree and renormalization group calculations, we study weak fluctuation-driven first-order quantum phase transitions, a quantum tricritical point controlled by anisotropy and the non-Fermi liquid behavior associated with the large phase volume of magnetic rotons. We propose that magnetic rotons are essential for the description of the anomalous high-pressure behavior of the itinerant helical ferromagnet MnSi.",0310186v2 2003-10-10,Magnetocrystalline anisotropy and orbital polarization in ferromagnetic transition metals,"The magnetocrystalline anisotropy energies (MAEs) of the ferromagnetic metals bcc Fe, fcc and hcp Co, and fcc Ni have been calculated by using the {\it ab initio} tight-binding method. Disentangling the strong correlation among the $d$ orbitals with the Hamiltonian in the local spin-density approximation, we have investigated the orbital polarizations induced by the Hubbard $U$ and Racah $B$. The experimental MAE of fcc Ni is found with the value of $U$ close to that determined from experiments and used in other theories. With the optimized values of $U$ and $J$, both the MAEs and the orbital moments for Fe and Co are in close agreement with experiment.",0310249v1 2003-11-04,Coexistence of ferromagnetism and singlet superconductivity via kinetic exchange,"We propose a novel mechanism for the coexistence of metallic ferromagnetism and singlet superconductivity assuming that the magnetic instability is due to kinetic exchange. Within this scenario, the unpaired electrons which contribute to the magnetization have a positive feedback on the gain of the kinetic energy in the coexisting phase by undressing the effective mass of the carriers involved into the pairing. The evolution of the magnetization and pairing amplitude, and the phase diagram are first analyzed for a generic kinetic exchange model and then are determined within a specific case with spin dependent bond-charge occupation.",0311063v1 2003-11-11,Massive Spin Collective Mode in Quantum Hall Ferromagnet,"It is shown that the collective spin rotation of a single Skyrmion in quantum Hall ferromagnet can be regarded as precession of the entire spin texture in the external magnetic field, with an effective moment of inertia which becomes infinite in the zero g-factor limit. This low-lying spin excitation may dramatically enhance the nuclear spin relaxation rate via the hyperfine interaction in the quantum well slightly away from filling factor equal one.",0311239v1 2003-11-14,Large anisotropy in the paramagnetic susceptibility of SrRuO3 films,"By using the extraordinary Hall effect in SrRuO3 films we performed sensitive measurements of the paramagnetic susceptibility in this itinerant ferromagnet, from Tc (~ 150 K) to 300 K. These measurements, combined with measurements of magnetoresistance, reveal that the susceptibility, which is almost isotropic at 300 K, becomes highly anisotropic as the temperature is lowered, diverging along a single crystallographic direction in the vicinity of Tc. The results provide a striking manifestation of the effect of exceptionally large magnetocrystalline anisotropy in the paramagnetic state of a 4d itinerant ferromagnet.",0311341v1 2003-11-18,Quantum-critical behavior of itinerant ferromagnets,"We study the stability of the Quantum Critical Point (QCP) for itinerant ferromagnets commonly described by the Hertz-Millis-Moriya (HMM) theory. We argue that in $D \leq 3$, long-range spatial correlations associated with the Landau damping of the order parameter field generate a universal {\it negative}, non-analytic $|q|^{(D+1)/2}$ contribution to the static magnetic susceptibility $\chi_s (q, 0)$, which makes HMM theory unstable. We argue that the actual transition is either towards incommensurate ordering, or first order. We also show that singular corrections are specific to the spin problem, while charge susceptibility remains analytic at criticality.",0311420v1 2003-11-24,A first principles study of wurtzite-structure MnO,"We present results of a density functional theory study of MnO in the wurtzite structure. Our motivation is provided by recent experiments reporting ferromagnetism in Mn-doped wurtzite structure ZnO. We find that wurtzite MnO a) is not strongly energetically disfavored as compared with the ground state rocksalt MnO, b) shows strong magnetostructural coupling and c) has a piezoelectric response that is larger than that of ZnO. These predictions augur well for the creation of ferromagnetic piezoelectric semiconductor based on Mn-doped ZnO.",0311529v1 2003-12-09,Structural and magnetic properties of transition metal substituted ZnO,"Structural and magnetic properties have been studied for polycrystalline Zn_{1-x}TM_xO, where TM (transition metal ions) = Mn, Fe, and Co. No bulk ferromagnetism was observed for single-phase materials, contrary to the existing theories. Single-phase samples demonstrate paramagnetic Curie-Weiss behavior with antiferromagnetic interactions, similar to other diluted magnetic semiconductors. Non-optimal synthesis conditions lead to formation of second phases that are responsible for spin-glass behavior (ZnMnO_3 impurity for Zn_{1-x}Mn_xO (S. Kolesnik et al., J. Supercond.: Incorp. Novel Magn. 15, 251 (2002)) or high-temperature ferromagnetic ordering (Co metal for Zn_{1-x}Co_xO with the Curie temperature T_C > 800 K or (Zn,Fe)_3O_4 for Zn_{1-x}Fe_xO with T_C = 440 K).",0312233v1 2003-12-15,Phonon Mechanism of the Ferromagnetic Transition in La1-xSrxMnO3,"Temperature dependence of longitudinal optical phonons with oxygen character was measured in La1-xSrxMnO3 (x=0.2, 0.3) by inelastic neutron scattering in the (1 0 0) cubic direction. The zone center mode intensity is consistent with the Debye-Waller factor. However the intensity of the bond-stretching phonons half way to the zone boundary and near the zone boundary decreases dramatically as the temperature increases through the ferromagnetic (FM) transition. We found evidence that the lost phonon spectral weight might shift into polaron scattering at the same wavevectors. The temperature evolution starts well below the onset of the FM transition suggesting that the transition is driven by phonon renormalization rather than by magnetic fluctuations.",0312368v1 2003-12-31,Excitation of Spin Waves in Superconducting Ferromagnets,"This Letter presents a theoretical analysis of propagation of spin waves in a superconducting ferromagnet. The surface impedance was calculated for the case when the magnetization is normal to the sample surface. We found the frequencies at which the impedance and the power absorption have singularities related to the spin wave propagation, and determined the form of these singularities. With a suitable choice of parameters, there is a frequency interval in which two propagating spin waves of the same circular polarization are generated, one of them having a negative group velocity.",0312720v1 2004-01-14,Origin of asymmetric reversal modes in ferromagnetic/antiferromagnetic multilayers,"Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e. g. by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a non uniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.",0401232v1 2004-01-16,Magnetic field induced phase transitions in spin ladders with ferromagnetic legs,"We study the phase diagram of spin ladders with ferromagnetic legs under the influence of a symmetry breaking magnetic field in the weak coupling effective field theory by bosonization. For antiferromagnetic interleg coupling we identify two phase transitions introduced by the external magnetic field. In order to establish the universality of the phases we supplement the bosonization approach by results from a strong coupling (rung) expansion and from spin wave analysis.",0401301v1 2004-01-22,Asymptotically Exact Solution for Superconductivity near Ferromagnetic Criticality,"We analyze an asymptotically exact solution for the transition temperature of p-wave superconductivity near ferromagnetic criticality on the basis of the three-dimensional electron systems in which scattering processes are dominated by exchange interactions with small momentum transfers. Taking into account all Feynman diagrams in the gap equation, we show that vertex corrections neglected in the conventional Eliashberg's formalism enhance the dynamical retarded effect of the pairing interaction, and raise the superconducting transition temperature significantly, though they just give subleading corrections to properties of the normal state.",0401412v2 2004-01-29,Review on Giant Magnetoelectric effects in Oxide ferromagnetic/ferroelectric Layered Structures,"The synthesis of layered ferrite-lead titanate zirconate (PZT) and lanthanum nanganite-PZT and the observation of giant magneto-electric interactions are discussed. The ferrites used in our studies included pure and Zn substituted cobalt-, nickel- and lithium ferrites. Ferromagnetic manganites used included both calcium and strontium substituted systems. The samples were prepared from 10-40 mm thick films obtained by tape-casting. Our studies showed strong ME coupling in manganite-PZT and cobalt zinc ferrite-PZT, and a giant ME effect in nickel zinc ferrite-PZT. We found evidence for Zn assisted enhancement in ME coefficients that were attributed to low anisotropy and high permeability that resulted in favorable magneto-mechanical coupling in the composites.",0401607v1 2004-02-04,Ferromagnetism mediated by few electrons in a semimagnetic quantum dot,"A (II,Mn)VI diluted magnetic semiconductor quantum dot with an integer number of electrons controlled with a gate voltage is considered. We show that a single conduction band electron is able to induce a spontaneous collective magnetization of the Mn spins, overcoming the short range antiferromagnetic interactions. The carrier mediated ferromagnetism in the dot survives at temperatures above 1 Kelvin, two orders of magnitude larger than the Curie temperature for the same material in bulk. The magnetic behavior of the dot depends dramatically on the parity of the number of injected electrons.",0402140v2 2004-02-12,Spin Dynamics of Double-Exchange Manganites with Magnetic Frustration,"This work examines the effects of magnetic frustration due to competing ferromagnetic and antiferromagnetic Heisenberg interactions on the spin dynamics of the double-exchange model. When the local moments are non-colinear, a charge-density wave forms because the electrons prefer to sit on lines of sites that are coupled ferromagnetically. With increasing hopping energy, the local spins become aligned and the average spin-wave stiffness increases. Phase separation is found only within a narrow range of hopping energies. Results of this work are applied to the field-induced jump in the spin-wave stiffness observed in the manganite Pr$_{1-x}$Ca$_x$MnO$_3$ with $0.3 \le x \le 0.4$.",0402344v1 2004-02-16,Triplet superconductivity vs. easy-plane ferromagnetism in a 1D itinerant electron system with transverse spin anisotropy,"In this paper we study the ground state phase diagram of a one-dimensional $t-U-J$ model, at half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy, a phase with gapless charge and massive spin excitations, characterized by the coexistence of triplet superconducting and spin density wave instabilities is realized in the ground state. With reduction of the bandwidth, a transition into an insulating phase showing properties of the spin-1/2 XY model takes place.",0402405v1 2004-02-16,Numerical simulations of two dimensional magnetic domain patterns,"I show that a model for the interaction of magnetic domains that includes a short range ferromagnetic and a long range dipolar anti-ferromagnetic interaction reproduces very well many characteristic features of two-dimensional magnetic domain patterns. In particular bubble and stripe phases are obtained, along with polygonal and labyrinthine morphologies. In addition, two puzzling phenomena, namely the so called `memory effect' and the `topological melting' observed experimentally are also qualitatively described. Very similar phenomenology is found in the case in which the model is changed to be represented by the Swift-Hohenberg equation driven by an external orienting field.",0402406v2 2004-02-26,"Zinc-blende CaP, CaAs and CaSb as half-metals: A new route to magnetism in calcium compounds","Existence of ferromagnetism in bulk calcium compounds is discovered theoretically. First-principles calculations of calcium phosphide, calcium arsenide and calcium antimonide in the zinc-blende structure have been performed to show the half-metallic ground state in each optimized stable structure. Magnetism comes from spin-polarization of electrons in $p$-orbitals of P, As or Sb and $d$-orbitals of calcium atoms. The half-metallicity is analogous to the half-metallic zinc-blende compounds, {\it e.g.} CrAs or CrSb, but the predicted compounds become ferromagnetic without transition metals. In (In$_{1-x}$Ca$_x$)Sb, the magnetism remains to be stable in a range of the doping rate ($x>0.8$).",0402641v1 2004-02-28,Crossover in the Structure Between Bloch and Linear Domain Walls,"Near the Curie temperature of a ferromagnet the form of a domain wall changes from the Bloch type to (asymptotically) the linear Zhirnov wall. Unlike the simple 180 degree rotation of the magnetization vector in a Bloch wall, its absolute value diminishes near the center of the wall. This leads to a decrease of the total transverse component of the exchange field inside the wall and to an increase of mistracking of the spins of the electrons traversing the wall. This mechanism may help explain large magnetoresistance of domain walls in thin nanowires, as the Curie temperatures of low-dimensional nanostructures are known to be lower than in bulk ferromagnets while the anisotropy energy stays virtually unchanged.",0403006v1 2004-03-02,"Ferromagnetic EuS films: Magnetic stability, electronic structure and magnetic surface states","We present the temperature and layer dependent electronic structure of a 20-layer EuS(100)-film using a combination of first-principles and model calculation, the latter based on the ferromagnetic Kondo-lattice. The calculated thickness-dependent Curie temperature agrees very well with experimental data. The projected 5d-bandstructure is at finite temperatures strongly influenced by damping effects due to spin exchange processes. Spin-split unoccupied 5d-surfaces states are found with a Stoner-like collapsing for increasing temperature towards the Curie point and with an exponential decay of spectral weight with increasing distance from the surface.",0403075v1 2004-03-03,"Anti-ferromagnetism, spin-phonon interaction and the local-density approximation in high-T$_C$ superconductors","Results from different sets of band calculations for undoped and doped HgBa$_2$CuO$_4$ show that small changes in localization can lead to very different ground states. The normal LDA results are compared with 'modified' LDA results, in which different linearization energies make the O-p band more localized. The ground states in the normal calculations are far from the anti-ferromagnetic ones, while nearly AFM states are found in the modified calculations. The proximity of an AFM state in the doped system leads to increased $\lambda_{sf}$, and the modified band structure has favorable conditions for spin-phonon coupling and superconductivity mediated by spin fluctuations.",0403103v1 2004-03-03,Antiferromagnetism at the YBa2Cu3O7 / La2/3Ca1/3MnO3 interface,"The magnetic properties of a series of YBa2Cu3O7-x/La2/3Ca1/3MnO3 (YBCO/LC1/3MO) superlattices grown by dc sputtering at high oxygen pressures (3.5 mbar) show the expected ferromagnetic behaviour. However, field cooled hysteresis loops at low temperature show the unexpected existence of exchange bias, effect associated with the existence of ferromagnetic/antiferromagnetic (F/AF) interfaces. The blocking temperature (TB) is found thickness dependent and the exchange bias field (HEB) is found inversely proportional to the FM layer thickness, as expected. The presence of an AF material is probably associated to interface disorder and Mn valence shift towards Mn4+.",0403113v1 2004-03-18,Low Losses Left Handed Materials Using Metallic Magnetic Cylinders,"We discuss materials based on arrays of metallic magnetic cylindrical structures near ferromagnetic resonance with applied magnetic fields at microwave frequencies. We have found that the materials have a negative refraction index when the appropriate structure is chosen. Numerical FDTD simulations were performed, after a very large number of geometries were swept. The simulations reveal that only ferromagnetic cylinders, with diameters of 0.1 cm and 0.5 cm apart, and with periodic or random configurations, are left-handed materials with very small losses; i.e. with transmitivity practically unity or no losses.",0403479v1 2004-03-19,"Current-driven Magnetization Reversal in a Ferromagnetic Semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As Tunnel Junction","Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 um^2 device revealed that magnetization switching occurs at low critical current densities of 1.1 - 2.2 x 10^5 A/cm^2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.",0403500v3 2004-03-24,Direct evidence for ferromagnetic spin polarization in gold nanoparticles,"We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using X-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms estimated by XMCD shows a good agreement with the results obtained by conventional magnetometry. This result is evidence of intrinsic spin polarization in nano-sized gold.",0403600v1 2004-04-01,Semiclassical dynamics of domain walls in the one-dimensional Ising ferromagnet in a transverse field,"We investigate analytically and numerically the dynamics of domain walls in a spin chain with ferromagnetic Ising interaction and subject to an external magnetic field perpendicular to the easy magnetization axis (transverse field Ising model). The analytical results obtained within the continuum approximation and numerical simulations performed for discrete classical model are used to analyze the quantum properties of domain walls using the semiclassical approximation. We show that the domain wall spectrum shows a band structure consisting of 2$S$ non-intersecting zones.",0404014v1 2004-04-05,Nonequilibrium Superconductivity near Spin Active Interfaces,"The Riccati formulation of the quasiclassical theory of nonequilibrium superconductors is developed for spin-dependent scattering near magnetic interfaces. We derive boundary conditions for the Riccati distribution functions at a spin-active interface that are applicable to a wide range of nonequilibrium transport properties of hybrid systems of superconducting and magnetic materials. We use these results to calculate spin, charge and heat transport for normal metal-ferromagnet-superconductor (NFS) and superconducting-ferromagnetic-superconducting (SFS) point contacts.",0404074v1 2004-04-07,Electric Field Effect in Diluted Magnetic Insulator Anatase Co:TiO2,"An external electric field induced reversible modulation of room temperature magnetic moment is achieved in an epitaxial and insulating thin film of dilutely cobalt-doped anatase TiO2. This first demonstration of electric field effect in any oxide based diluted ferromagnet is realized in a high quality epitaxial heterostructure of PbZr0.2Ti0.8O3/Co:TiO2/SrRuO3 grown on (001) LaAlO3. The observed effect, which is about 15% in strength in a given heterostructure, can be modulated over several cycles. Possible mechanisms for electric field induced modulation of insulating ferromagnetism are discussed.",0404172v1 2004-04-14,Investigation of the field-treated magnetic state in Gd5Ge4,"At low temperatures, the intermetallic compound Gd$_5$Ge$_4$ shows a sharp field-induced transition into a ferromagnetic state around 25 kOe of applied field. The material remains in the field treated ferromagnetic state even when the magnetic field is removed. We have investigated the character of this field-treated state by magnetization and heat capacity measurements. The nature of the magnetization and the heat capacity are found to be different above and below a charactersitic temperature Tirr = 25 K.",0404302v1 2004-04-19,High Curie temperatures in ferromagnetic Cr-doped AlN thin films,"Al1-xCrxN thin films with $0.02\le x \le 0.1$ were deposited by reactive co-sputtering onto c-plane (001) sapphire. Room-temperature ferromagnetism with a coercive field of 85 Oe was observed in samples with chromium contents as low as x = 0.027 (2.7%). With increasing Cr content the mean magnetic moment is strongly suppressed, with a maximum saturation moment of 0.62 and 0.71 $\mu_B$ per Cr atom at 300 and 50 K respectively. We show that the Curie temperature of Al1-xCrxN for x = 0.027 is greater than 900 K.",0404436v1 2004-04-21,Hund's rule and metallic ferromagnetism,"We study tight-binding models of itinerant electrons in two different bands, with effective on-site interactions expressing Coulomb repulsion and Hund's rule. We prove that, for sufficiently large on-site exchange anisotropy, all ground states show metallic ferromagnetism: They exhibit a macroscopic magnetization, a macroscopic fraction of the electrons is spatially delocalized, and there is no energy gap for kinetic excitations.",0404483v2 2004-05-13,Precessional dynamics of elemental moments in a ferromagnetic alloy,"We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps.",0405295v1 2004-05-14,Calculating the Curie Temperature reliably in diluted III-V ferromagnetic semiconductors,"We present a semi-analytic theory for the Curie temperature in diluted magnetic semi-conductors that treats disorder effects exactly in the effective Heisenberg Hamiltonian, and spin fluctuations within a local RPA. The exchange couplings are taken from concentration dependent {\it ab initio} estimates. The theory gives very good agreement with published data for well-annealed samples of Mn$_x$Ga$_{1-x}$As. We predict the critical temperatures for Mn$_x$Ga$_{1-x}$N lower than in doped GaAs, despite the stronger nearest-neighbour ferromagnetic coupling. We also predict the dependence on the hole concentration.",0405322v1 2004-05-22,"Oxygen stoichiometry, crystal structure, and magnetism in La$_{0.5}$Sr$_{0.5}$CoO$_{3-δ}$","We have prepared a series of polycrystalline samples La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$} with $0 < \delta \le 0.21$ and characterized their oxygen content, crystal structure, and magnetic properties. While the fully oxygenated samples are good ferromagnets, samples with larger $\delta$ values display increasingly broad magnetic transitions. The saturation magnetization at 5 K falls rapidly as $\delta$ increases. First principles electronic structure calculations provide insights into the magnetic behavior of the fully oxygenated compound, and the manner in which ferromagnetic ordering is affected by increasing oxygen non-stoichiometry.",0405535v2 2004-05-27,Excitation of magnetostatic spin waves in ferromagnetic films,"The torque equation of nonlinear spin dynamics is considered in the magnetostatic approximation. In this framework, exact expressions for propagator of linear magnetostatic waves in ferromagnetic film between two antennas and corresponding mutual impedance of the antennas are derived, under conditions of uniform but arbitrarily oriented static magnetization and arbitrary anisotropy. The results imply also full description of spectrum of the waves.",0405640v1 2004-06-21,Spin-Polarized Transprot through Double Quantum Dots,"We investigate spin-polarized transport phenomena through double quantum dots coupled to ferromagnetic leads in series. By means of the slave-boson mean-field approximation, we calculate the conductance in the Kondo regime for two different configurations of the leads: spin-polarization of two ferromagnetic leads is parallel or anti-parallel. It is found that transport shows some remarkable properties depending on the tunneling strength between two dots. These properties are explained in terms of the Kondo resonances in the local density of states.",0406490v1 2004-06-22,Spiral Exchange Interaction in Diluted Magnetic Semiconductor Junction,"We studied the exchange interactions mediated by polarized itinerant carriers in diluted magnetic semiconductor (DMS) junction. In contrast to the ordinary RKKY oscillations, the induced moment possesses an interesting spiral motion, accompanied with angular oscillations. The spiral motion remains robust in the entire $T cubic anisotropy. A change in the magnetic easy axis from [100] to [110] with increasing temperature can be explained by the reduced contribution of <100> cubic anisotropy to the magnetic properties above the transition temperature of the (Ga,Mn)As matrix.",0410022v1 2004-10-13,"Structure, Tranport and Magnetism of La(0.67)Ca(0.33)Mn(1-x)Ta(x)O(3)","For the first time,we report the effect of pentavalent subtitution at the Mn site of La(0.67)Ca(0.33)MnO(3). The ferromagnetic -metallic ground state modifies to a cluster-glass insulator beyond x=0.03. The observed suppression in the transition temperatures (39K/at.%) is the largest reported for the Mn site subtituted manganites. We argue that charge state(valence) of the substituent has a dominant role in affecting the ferromagnetic - metallic ground state of CMR manganites.",0410329v1 2004-10-13,Tricritical behavior in itinerant quantum ferromagnets,"It is shown that the peculiar features observed in the low-temperature phase diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple mean-field theory. The nature of the ferromagnetic transition changes from second order to first order at a tricritical point, and in a small external magnetic field surfaces of first-order transitions emerge which terminate in quantum critical points. This field dependence of the phase diagram follows directly from the existence of the tricritical point. The quantum critical behavior in a nonzero field is calculated exactly.",0410344v1 2004-10-20,Static properties and spin dynamics of the ferromagnetic spin-1 Bose gas in magnetic field,"Properties of spin-1 Bose gases with ferromagnetic interaction in the presence of a nonzero magnetic field are studied. The equation of state and thermodynamic quantities are worked out with the help of a mean-field approximation. The phase diagram besides Bose-Einstein condensation contains a first order transition where two values of the magnetization coexist. The dynamics is investigated with the help of the Random Phase Approximation. The soft mode corresponding to the critical point of the magnetic phase transition is found to behave like in conventional theory.",0410501v1 2004-10-26,Superconducting pi qubit with a ferromagnetic Josephson junction,"Solid-state qubits have the potential for the large-scale integration and for the flexibility of layout for quantum computing. However, their short decoherence time due to the coupling to the environment remains an important problem to be overcome. We propose a new superconducting qubit which incorporates a spin-electronic device: the qubit consists of a superconducting ring with a ferromagnetic pi junction which has a metallic contact and a normal Josephson junction with an insulating barrier. Thus, a quantum coherent two-level state is formed without an external magnetic field. This feature and the simple structure of the qubit make it possible to reduce its size leading to a long decoherence time.",0410658v1 2004-10-31,Lateral length scales in exchange bias,"When a ferromagnet is in proximity with an antiferromagnet, lateral length scales such as the respective magnetic domain sizes drastically affect the exchange bias. Bilayers of FeF2 and either Ni, Co or Fe are studied using SQUID and spatially resolved MOKE. When the antiferromagnetic domains are larger than or comparable to the ferromagnetic domains, a local, non-averaging exchange bias is observed. This gives rise to unusual and tunable magnetic hysteresis curves.",0411014v1 2004-11-04,Localised magnetic excitations of coupled impurities in a transverse Ising ferromagnet,"A Green's function formalism is used to calculate the spectrum of excitations of two neighboring impurities implanted in a semi-infinite ferromagnetic. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field and results are given for the effect of the exchange coupling, position and orientation of the impurities on the spectra of localized spin wave modes.",0411119v1 2004-11-11,Ferromagnetic-superconducting hybrid films and their possible applications: A direct study in a model combinatorial film,"Model combinatorial films (CFs) which host a pure superconductor adjacent to a ferromagneticsuperconducting hybrid film (HF) are manufactured for the study of the influence of ferromagnetic nanoparticles (FNs) on the nucleation of superconductivity. Careful resistance measurements were performed simultaneously on two different sites of the CFs. Enhancement of superconductivity and magnetic memory effects were observed only on the hybrid site of the CFs but were absent on their purely superconducting part. Our results give direct proof that the FNs modulate the superconducting order parameter in an efficient and controlled way giving us the possibility of miscellaneous practical applications.",0411311v1 2004-11-18,Hanle Effect in Transport through Quantum Dots Coupled to Ferromagnetic Leads,"We suggest a series of transport experiments on spin precession in quantum dots coupled to one or two ferromagnetic leads. Dot spin states are created by spin injection and analyzed via the linear conductance through the dot, while an applied magnetic field gives rise to the Hanle effect. Such a Hanle experiment can be used to determine the spin lifetime in the quantum dot, to measure the spin injection efficiency into the dot, as well as proving the existence of intrinsic spin precession which is driven by the Coulomb interaction.",0411477v2 2004-12-01,Magnetic phase separation in EuB6 detected by muon spin rotation,"We report results of the first muon-spin rotation measurements performed on the low carrier density ferromagnet EuB$_6$. The ferromagnetic state is reached via two magnetic transitions at $T_{\rm m}=16.5$ K and $T_{\rm c}=12.6$ K. Two distinct components are resolved in the muon data, one oscillatory and one non-oscillatory, which arise from different types of magnetic environment, and we have followed the temperature dependence of these components in detail. These results provide evidence for magnetic phase separation and can be interpreted in terms of the gradual coalescing of magnetic polarons.",0412019v1 2004-12-02,Integrating Magnetism into Semiconductor Electronics,"Vision of ferromagnet/semiconductor hybrid as a strongly coupled but flexible spin system is presented. We analyze the experiments and argue that contrary to the common sense the nonmagnetic semiconductor plays a crucial role in manipulating of ferromagnetism. The magnetism of the hybrid (magnetic hysteresis loop and the orientation of magnetization vector in space) is tuned both optically and electrically with the help of semiconductor. As a result the hybrid represents an elementary magnetic storage with electronic record and readout",0412054v2 2004-12-22,Ballistic anisotropic magnetoresistance,"Electronic transport in ferromagnetic ballistic conductors is predicted to exhibit ballistic anisotropic magnetoresistance (BAMR) - a change in the ballistic conductance with the direction of magnetization. This phenomenon originates from the effect of the spin-orbit interaction on the electronic band structure which leads to a change in the number of bands crossing the Fermi energy when the magnetization direction changes. We illustrate the significance of this phenomenon by performing ab-initio calculations of the ballistic conductance in ferromagnetic Ni and Fe nanowires which display a sizable BAMR when the magnetization changes direction from parallel to perpendicular to the wire axis.",0412637v1 2005-01-07,Nonequilibrium quantum phase transition in itinerant electron systems,"We study the effect of the voltage bias on the ferromagnetic phase transition in a one-dimensional itinerant electron system. The applied voltage drives the system into a nonequilibrium steady state with a non-zero electric current. The bias changes the universality class of the second order ferromagnetic transition. While the equilibrium transition belongs to the universality class of the uniaxial ferroelectric, we find the mean-field behavior near the nonequilibrium critical point.",0501133v2 2005-01-19,Ferromagnetic 0-pi Junctions as Classical Spins,"The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson junctions shows a spontaneous half quantum vortex, sustained by a supercurrent of undetermined sign. This supercurrent flows in the electrode of a Josephson junction used as a detector and produces a phi(0)/4 shift in its magnetic diffraction pattern. We have measured the statistics of the positive or negative sign shift occurring at the superconducting transition of such a junction. The randomness of the shift sign, the reproducibility of its magnitude and the possibility of achieving exact flux compensation upon field cooling: all these features show that 0-pi junctions behave as classical spins, just as magnetic nanoparticles with uniaxial anisotropy.",0501459v1 2005-01-19,Maximal entanglement of two spinor Bose-Einstein condensates,"Starting with two weakly-coupled anti-ferromagnetic spinor condensates, we show that by changing the sign of the coefficient of the spin interaction, $U_{2}$, via an optically-induced Feshbach resonance one can create an entangled state consisting of two anti-correlated ferromagnetic condensates. This state is maximally entangled and a generalization of the Bell state from two anti-correlated spin-1/2 particles to two anti-correlated spin$-N/2$ atomic samples, where $N$ is the total number of atoms.",0501471v1 2005-01-20,Observation of magnetoelectric behavior at room temperature in Pb(Fe1-xTix)O3,"The coexistence of ferroelectric and ferromagnetic properties at room temperature is very rarely observed. We have been successful in converting ferroelectric PbTiO3 into a magnetoelectric material by partly substituting Fe at the Ti site. The Pb(Fe1-xTix)O3 system exhibits ferroelectric and ferromagnetic ordering at room temperature. Even more remarkably, our results demonstrate a coupling between the two order parameters. Hence it could be a futuristic material to provide cost effective and simple path for designing novel electromagnetic devices.",0501489v1 2005-01-20,Magnetoresistance of mesoscopic granular ferromagnets,"We have performed magnetoresistance (MR) measurements of granular ferromagnets having lateral dimensions smaller than 0.5 $\mu$m and containing a small number of grains (down to about 100). Compared to macroscopic samples, these granular samples exhibit unusually large saturation fields and MR amplitudes. In addition, the evolution of the magnetoresistance curve as the intergrain distance decreases is qualitatively different than that of large samples. We discuss these results and suggest that they reflect a transition from percolation to quasi single-channel dominated transport.",0501508v1 2005-01-21,Density of states anomalies in hybrid superconductor-ferromagnet-normal metal structures,"The results of calculations of the spatially-resolved density of states (DoS) in an S(F/N) bilayer are presented (S is a superconductor, F is a metallic ferromagnet, N is a normal metal) within quasiclassical theory in the dirty limit. Analytical solutions are obtained in the case of thin F, N layers which demonstrate the peculiar features of DoS in this system. The dependencies of the minigap and the DoS peak positions on the exchange energy and parameters of the layers are studied numerically",0501514v1 2005-01-24,Spin transport and magnetoresistance in F/S/F spin valves,"We consider spin transport and spin relaxation in superconductors using the quasiclassical theory of superconductivity. We include spin relaxation due to spin-orbit interaction as well as magnetic impurities, and show that the energy dependence of the spin-flip rate is different for these two mechanisms. In ferromagnet-superconductor-ferromagnet systems made of Co and Al, interface resistances can be small compared to bulk resistances. This simplifies the description of transport in Co/Al/Co spin valves, for which we numerically calculate the temperature and Al length dependence of the magnetoresistance.",0501566v1 2005-02-23,Quantum Monte Carlo Simulations on S=1/2 antiferromagnetic-Ferromagnetic Random Alternating spin chain,"The S=1/2 Heisenberg chain with bond alternation and randomness of antiferromagnetic (AFM) and ferromagnetic (FM) interactions is investigated by quantum Monte Carlo simulations of loop/cluster algorithm. Our results have shown interesting finite temperature magnetic properties of this model. The relevance of our study to the observed results of the material (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$ is discussed.",0502543v1 2005-02-24,Tailoring Fe/Ag Superparamagnetic Composites by Multilayer Deposition,"The magnetic properties of Fe/Ag granular multilayers were examined by SQUID magnetization and Mossbauer spectroscopy measurements. Very thin (0.2 nm) discontinuous Fe layers show superparamagnetic properties that can be tailored by the thickness of both the magnetic and the spacer layers. The role of magnetic interactions was studied in novel heterostructures of superparamagnetic and ferromagnetic layers and the specific contribution of the ferromagnetic layers to the low field magnetic susceptibility was identified.",0502578v1 2005-03-05,Pressure dependence of superconducting and magnetic critical temperatures in the ruthenocuprates,"We study the dependence on filling and pressure of the superconducting and ferromagnetic critical temperatures of the ruthenocuprates, within the two-band model. At zero pressure, we find separate regions of coexistence of superconductivity and ferromagnetism as a function of filling, with contiguous regions merging together as pressure increases. As a function of pressure, a stronger enhancement of the magnetic phase results in a reduced pressure effect on the superconducting critical temperature. Comparison with recent experiments on the determination of the critical temperatures as a function of the pressure is also discussed.",0503119v1 2005-03-13,Magnetism in C60 Films Induced by Proton Irradiation,"It is shown that polycrystalline fullerene thin films on hydrogen passivated Si(111) substrates irradiated by 2 MeV protons displays ferromagnetic-like behavior at 5 K. At 300 K both the pristine and the irradiated film show diamagnetic behavior. Magnetization data in the temperature range 2 - 300 K, in 1 Tesla applied field, for the irradiated film shows much stronger temperature dependence compared to the pristine film. Possible origin of ferromagnetic-like signals in the irradiated films are discussed.",0503315v2 2005-03-14,Surface spin waves in superconducting and insulating ferromagnets,"Surface magnetization waves are studied on a semi-infinite magnetic medium in the perpendicular geometry. Both superconducting and insulating ferromagnets are considered. Exchange and dipole energies are taken into account, as well as retardation effects. At large wave vectors, the spectrum for a superconductor and insulator is the same, though for the former the branch is terminated much earlier than for the latter due to excitation of plasmons. At small wave vectors, the surface wave is more robust in the superconductor since it is separated from the bulk continuum by a finite gap.",0503340v1 2005-03-17,High Temperature Ferromagnetism in GaAs-based Heterostructures with Mn Delta Doping,"We show that suitably-designed magnetic semiconductor heterostructures consisting of Mn delta-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the 2-dimensional hole gas wavefunction, realized remarkably high ferromagnetic transition temperatures (TC). Significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low temperature annealing led to high TC up to 250 K. The heterostructure with high TC exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.",0503444v1 2005-04-04,Transition from Ferromagnetism to Antiferromagnetism in Ga$_{1-x}$Mn$_x$N,"Using density functional theory, we study the magnetic stability of the Ga$_{1-x}$Mn$_x$N alloy system. We show that unlike Ga$_{1-x}$Mn$_x$As, which shows only ferromagnetic (FM) phase, Ga$_{1-x}$Mn$_x$N can be stable in either FM or antiferromagnetic phases depending on the alloy concentration. The magnetic order can also be altered by applying pressure or with charge compensation. A unified model is used to explain these behaviors.",0504083v1 2005-04-06,Non-collinear single-electron spin-valve transistors,"We study interaction effects on transport through a small metallic cluster connected to two ferromagnetic leads (a single-electron spin-valve transistor) in the ""orthodox model"" for the Coulomb blockade. The non-local exchange between the spin accumulation on the island and the ferromagnetic leads is shown to affect the transport properties such as the electric current and spin-transfer torque as a function of the magnetic configuration, gate voltage, and applied magnetic field.",0504137v1 2005-04-15,Tuning the ferromagnetic properties of hydrogenated GaMnAs,"Hydrogenation and posthydrogenation annealings have been used as a very efficient tool to tune the hole density over a wide range, at fixed magnetic moment concentration, in thin GaMnAs layers. Reduction of the hole density resulted in strong modifications of their ferromagnetic properties. In particular, we observed in magnetotransport experiments the decrease of the Curie temperature, along with modifications of the magnetic anisotropy, a behavior consistent with the mean-field theory.",0504387v2 2005-04-21,Sum rules for X-ray magnetic circular dichroism spectra in strongly correlated ferromagnets,"It is proven that the sum rules for X-ray magnetic dichroism (XMCD) spectra that are used to separate spin and orbital contributions to the magnetic moment are formally correct for an arbitrary strength of electron-electron interactions. However, their practical application for strongly correlated systems can become complicated due to the spectral density weight spreading over a broad energy interval. Relevance of incoherent spectral density for the XMCD sum rules is illustrated by a simple model of a ferromagnet with orbital degrees of freedom.",0504549v2 2005-04-25,Reentrant spin-glass transition in a dilute magnet,"We have performed a large scale Monte Carlo simulation of a dilute classical Heisenberg model with ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor interactions. We found that the model reproduces a reentrant spin-glass transition. That is, as the temperature is decreased, the magnetization increases rapidly below a certain temperature, reaches a maximum value, then ceases at some lower temperature. The low temperature phase was suggested to be a spin-glass phase that is characterized by ferromagnetic clusters.",0504615v1 2005-04-26,Green's function theory of quasi-two-dimensional spin-half Heisenberg ferromagnets: stacked square versus stacked kagomé lattice,"We consider the thermodynamic properties of the quasi-two-dimensional spin-half Heisenberg ferromagnet on the stacked square and the stacked kagom\'e lattices by using the spin-rotation-invariant Green's function method. We calculate the critical temperature $T_C$, the uniform static susceptibility $\chi$, the correlation lengths $\xi_\nu$ and the magnetization $M$ and investigate the short-range order above $T_C$. We find that $T_C$ and $M$ at $T>0$ are smaller for the stacked kagom\'e lattice which we attribute to frustration effects becoming relevant at finite temperatures.",0504662v2 2005-04-26,Spin-Polarized Standing Waves in the Fermi Surface of a Ferromagnetic Thin Film,"The spin-selective electron reflection at a ferromagnetic-paramagnetic interface is investigated using Fe films on a W(110) substrate. Angle-resolved photoemission of the majority and minority Fermi surfaces of the Fe film is used to probe standing wave formation. Intense quantum well states resulting from interfacial reflection are observed exclusively for majority states. Such high spin polarization is explained by the Fermi surface topology of the connecting substrate, and we argue that Fe/W is a particularly suitable interface for that purpose.",0504670v1 2005-05-10,Griffiths singularity in the random Ising ferromagnet,"The explicit form of the Griffiths singularity in the random ferromagnetic Ising model in external magnetic field is derived. In terms of the continuous random temperature Ginzburg-Landau Hamiltonian it is shown that in the paramagnetic phase away from the critical point the free energy as the function of the external magnetic field h in the limit h -> 0 has the essential singularity of the form $\exp[-(const)h^{-D/3}]$ (where 1$5K) this transition is quite smooth in nature. Comparing with the recently observed similar behaviour in manganites showing colossal magnetoresistance and magnetocaloric material Gd$_5$Ge$_4$ we argue that such magnetization step is a generalized feature of a disorder influenced first order phase transition.",0505609v1 2005-05-31,Spontaneous currents and half-integer Shapiro steps in Superconductor-Ferromagnet-Superconductor 0-pi Josephson junctions,"We study Superconductor-Ferromagnet-Superconductor (Nb-Cu0.47 Ni0.53-Nb) Josephson junctions with spatial variations in the barrier thickness. Critical current vs. magnetic flux diffraction patterns indicate that the critical current density changes sign along the width of the junctions, creating interfaces between 0 and pi junction regions around which spontaneous currents can circulate. Shapiro steps observed at half-integer Josephson voltages can be explained by the phase-locking of the spontaneous circulating currents to the applied rf modulation.",0506003v1 2005-06-03,Half-metallic ferromagnets for magnetic tunnel junctions,"Using theoretical arguments, we show that, in order to exploit half-metallic ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to eliminate interface states at the Fermi level within the half-metallic gap; contrary to this, no such problem arises in giant magnetoresistance elements. Moreover, based on an a priori understanding of the electronic structure, we propose an antiferromagnetically coupled TMR element, in which interface states are eliminated, as a paradigm of materials design from first principles. Our conclusions are supported by ab-initio calculations.",0506079v1 2005-06-24,Complete spin extraction from semiconductors near ferromagnet-semiconductor interfaces,"We show that spin polarization of electrons in nonmagnetic semiconductors near specially tailored ferromagnet-semiconductor junctions can achieve 100%. This effect is realized even at moderate spin injection coefficients of the contact when these coefficients only weakly depend on the current. The effect of complete spin extraction occurs at relatively strong electric fields and arises from a reduction of spin penetration length due to the drift of electrons from a semiconductor towards the spin-selective tunnel junction.",0506625v1 2005-06-24,Off-perturbative states in disordered systems,"The systematic approach for the off-perturbative calculations in disordered systems is developed. The proposed scheme is applied for the random temperature and the random field ferromagnetic Ising models. It is shown that away from the critical point, in the paramagnetic phase of the random temperature model, and in the ferromagnetic phase of the random field one, the free energy contains non-analytic contributions which have the form of essential singularities. It is demonstrated that these contributions appear due to localized in space instanton-like excitations.",0506633v2 2005-07-04,Appearance of an inhomogeneous superconducting state in La{0.67}Sr{0.33}MnO{3}-YBa{2}Cu{3}O{7}-La{0.67}Sr{0.33}MnO{3} trilayers,"An experimental study of proximity effect in La{0.67}Sr{0.33}MnO3 - YBa2Cu3O7 - La{0.67}Sr{0.33}MnO3 trilayers is reported. Transport measurements on these samples show clear oscillations in critical current (I_c) as the thickness of La{0.67}Sr{0.33}MnO3 layers (d_F) is scanned from ~ 50 A to ~ 1100 A. In the light of existing theories of ferromagnet-superconductor (FM-SC) heterostructures, this observation suggests a long range proximity effect in the manganite, modulated by it's weak exchange energy (~ 2 meV). The observed modulation of the magnetic coupling between the ferromagnetic LSMO layers as a function of d_F, also suggests an oscillatory behavior of the SC order parameter near the FM-SC interface.",0507073v1 2005-07-05,Critical aging of a ferromagnetic system from a completely ordered state,"We adapt the non-linear $\sigma$ model to study the nonequilibrium critical dynamics of O(n) symmetric ferromagnetic system. Using the renormalization group analysis in $d=2+\epsilon$ dimensions we investigate the pure relaxation of the system starting from a completely ordered state. We find that the average magnetization obeys the long-time scaling behavior almost immediately after the system starts to evolve while the correlation and response functions demonstrate scaling behavior which is typical for aging phenomena. The corresponding fluctuation-dissipation ratio is computed to first order in $\epsilon$ and the relation between transverse and longitudinal fluctuations is discussed.",0507112v2 2005-07-08,Pairing symmetry signatures of T1 in superconducting ferromagnets,"We study the nuclear relaxation rate 1/T1 as a function of temperature for a superconducting-ferromagnetic coexistent system using a p-wave triplet model for the superconducting pairing symmetry. This calculation is contrasted with a singlet s-wave one done previously, and we see for the s-wave case that there is a Hebel-Slichter peak, albeit reduced due to the magnetization, and no peak for the p-wave case. We then compare these results to a nuclear relaxation rate experiment on UGe2 to determine the possible pairing symmetry signatures in that material. It is seen that the experimental data is inconclusive to rule out the possibility of s-wave pairing in $UGe_{2}$.",0507212v1 2005-07-11,Electron-induced stabilization of ferromagnetism in GaGdN,"Using {\it ab initio} band structure calculations and symmetry arguments, we show that the magnetic property of Ga$_{1-x}$Gd$_x$N is drastically different from TM-doped GaN. The coupling between Gd atoms in the alloy is antiferromagnetic, but the ferromagnetic phase can be stabilized by introducing electrons. Furthermore, we propose a model that may explain the recently observed colossal magnetic moments in this system, based on the polarization of donor electrons.",0507257v1 2005-07-15,Tunnel magnetoresistance of quantum dots coupled to ferromagnetic leads in the sequential and cotunneling regimes,"We study electronic transport through quantum dots weakly coupled to ferromagnetic leads with collinear magnetization directions. Tunneling contributions of first and second order in the tunnel-coupling strength are taken into account. We analyze the tunnel magnetoresistance (TMR) for all combinations of linear and nonlinear response, at or off resonance, with an even or odd dot-electron number. Different mechanisms for transport and spin accumulation the various regimes give rise to different TMR behavior.",0507357v1 2005-07-28,Optical study of interactions in a d-electron Kondo lattice with ferromagnetism,"We report on a comprehensive optical, transport and thermodynamic study of the Zintl compound Yb$_{14}$MnSb$_{11}$, demonstrating that it is the first ferromagnetic Kondo lattice compound in the underscreened limit. We propose a scenerio whereby the combination of Kondo and Jahn-Teller effects provides a consistent explanation of both transport and optical data.",0507693v1 2005-08-01,The Ruthenocuprates - Natural Superconductor-Ferromagnet Multilayers,"The recently discovered ruthenocuprates have attracted great interest because of the microscopic coexistence of superconducting and ferromagnetic order. Typically, these materials become magnetically ordered at temperatures around 125-145 K and superconductivity sets in between 15 and 50 K. While superconductivity arises in the CuO2 layers the RuO2 layers in between order magnetically. In this paper we summarize some of the crystallographic, magnetic and superconducting properties of the ruthenocuprates, as obtained from investigations on polycrystalline samples as well as single crystals.",0508044v1 2005-08-02,Numerical simulation of magnetization process in antiferromagnetic ferromagnetic bilayer with compensated interface,"The properties of antiferromagnetic ferromagnetic bilayer have been studied using self-consistent mean-field approximation for Heisenberg Hamiltonian. The perpendicular exchange coupling has been revealed in a bilayer with a compensated interface. For a uniform antiferromagnetic film a symmetrical hysteresis loop has been calculated, because the transverse instability develops within the antiferromagnetic film at certain critical value of external magnetic field. On the other hand, shifted hysteresis loop with a finite exchange bias field has been obtained for a non-uniform antiferromagnetic film consisting of various domains with perpendicular directions of the easy anisotropy axes.",0508069v1 2005-08-03,Electronic Mechanism for the Coexistence of Ferroelectricity and Ferromagnetism,"We study the strong coupling limit of a two-band Hubbard Hamiltonian that also includes an inter-orbital on-site repulsive interaction $U_{ab}$. When the two bands have opposite parity and are quarter filled, we prove that the ground state is simultaneously ferromagnetic and ferroelectric for infinite intra-orbital Coulomb interactions $U_{aa}$ and $U_{bb}$. We also show that this coexistence leads to a singular magnetoelectric effect.",0508113v2 2005-08-04,Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCr$_2$S$_4$ from first principles,"We have studied the dielectric properties of the ferromagnetic spinel CdCr$_2$S$_4$ from first principles. Zone-center phonons and Born effective charges were calculated by frozen-phonon and Berry phase techniques within LSDA+U. We find that all infrared-active phonons are quite stable within the cubic space group. The calculated static dielectric constant agrees well with previous measurements. These results suggest that the recently observed anomalous dielectric behavior in CdCr$_2$S$_4$ is not due to the softening of a polar mode. We suggest further experiments to clarify this point.",0508136v1 2005-08-07,Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors,"We develop a theory of the magnetization dynamics triggered by ultrafast optical excitation of ferromagnetic semiconductors. We describe the effects of the strong carrier spin relaxation on the nonlinear optical response by using the Lindblad semigroup method. We demonstrate magnetization control during femtosecond timescales via the interplay between circularly polarized optical excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin interactions. Our results show a light-induced magnetization precession and relaxation for the duration of the optical pulse.",0508178v1 2005-08-19,Vortex-antivortex dynamics and field-polarity-dependent flux creep in hybrid superconductor/ferromagnet nanostructures,"Vortex-antivortex arrays that are created in a type-II superconducting film by lattices of ferromagnetic dots with perpendicular anisotropy have been investigated. The highest critical current is shifted to a nonzero value of the applied field, and current-voltage characteristics show different regimes of vortex motion even in zero-applied field due to the presence of the vortex-antivortex arrays. Creep of interstitial vortices is observed at low driving forces. This flux creep is strongly field-polarity dependent.",0508459v1 2005-08-23,Switching magnetization of nano-scale ferromagnetic particle using non-local spin injection,"We have performed non-local spin injection into a nano-scale ferromagnetic particle configured in a lateral spin valve structure to switch its magnetization only by spin current. The non-local spin injection aligns the magnetization of the particle parallel to the magnetization of the spin injector. The responsible spin current for switching is estimated from the experiment to be about 200 $\mu$A, which is reasonable compared with the values obtained for conventional pillar structures. Interestingly the switching always occurs from anti-parallel to parallel in the particle/injector magnetic configurations, whereas no opposite switching is observed. Possible reasons for this discrepancy are discussed.",0508559v1 2005-08-23,Direct measurement of charge transfer phenomena at ferromagnetic/superconducting oxide interfaces,"YBa2Cu3O7-x/La0.67Ca0.33MnO3 ferromagnetic/superconducting interfaces are analyzed by scanning transmission electron microscopy and electron energy loss spectroscopy with monolayer resolution. We demonstrate that extensive charge transfer occurs between the manganite and the superconductor, in a manner similar to modulation-doped semiconductors, which explains the reduced critical temperatures of heterostructures. This behavior is not seen with insulating PrBa2Cu3O7 layers. Furthermore, we confirm directly that holes in the YBa2Cu3O7-x are located on the CuO2 planes.",0508564v2 2005-08-26,Kinetic arrest of the first order ferromagnetic to antiferromagnetic transition in Ce(Fe$_{0.96}$Ru$_{0.04}$)$_2$ : formation of a magnetic-glass,"We present results of dc magnetization and magnetic relaxation study showing the kinetic arrest of a first order ferromagnetic to antiferromagnetic transition in Ce(Fe$_{0.96}$Ru$_{0.04}$)$_2$. This leads to the formation of a non-ergodic glass-like magnetic state. The onset of the magnetic-glass transformation is tracked through the slowing down of the magnetization dynamics. This glassy state is formed with the assistance of an external magnetic field and this is distinctly different from the well known 'spin-glass' state.",0508625v1 2005-08-29,Interlayer Transport in Bilayer Quantum Hall Systems,"Bilayer quantum Hall systems have a broken symmetry ground state at filling factor $\nu=1$ which can be viewed either as an excitonic superfluid or as a pseudospin ferromagnet. We present a theory of inter-layer transport in quantum Hall bilayers that highlights remarkable similarities and critical differences between transport in Josephson junction and ferromagnetic metal spin-transfer devices. Our theory is able to explain the size of the large but finite low bias interlayer conductance and the voltage width of this collective transport anomaly.",0508702v2 2005-09-02,Phonon spectrum of the ferromagnetic superconductor UGe$_2$ : Consequences for the specific heat,"We report inelastic neutron scattering measurements of the phonon spectrum of the pressure-induced ferromagnetic superconductor UGe$_{2}$. No changes of the spectrum were found on cooling down to low temperature. The phonon contribution to the specific heat was estimated from a fit to our data. The excess specific heat previously noted at around $T_{x} \approx$ 30 K is not due to phonons but is well described by the temperature dependence of the magnetic order parameter at the molecular field level.",0509052v2 2005-09-10,Microscopic Theory of Skyrmions in Quantum Hall Ferromagnets,"We present a microscopic theory of skyrmions in the monolayer quantum Hall ferromagnet. It is a peculiar feature of the system that the number density and the spin density are entangled intrinsically as dictated by the W$%_{\infty}$ algebra. The skyrmion and antiskyrmion states are constructed as W$_{\infty }$-rotated states of the hole-excited and electron-excited states, respectively. They are spin textures accompanied with density modulation that decreases the Coulomb energy. We calculate their excitation energy as a function of the Zeeman gap and compared the result with experimental data.",0509262v1 2005-09-19,Spiral vortices in a 2D ferromagnet,"We present a study of a new class of exact solutions having a form of spiral vortices for an isotropic two-dimensional Heisenberg ferromagnet using a continuum theory and direct numerical simulations of the spin system on a square lattice. We find their features issued from the conservation laws and describe their interaction. Reasons behind the formation of the proper spin configurations on a square lattice are investigated.",0509481v1 2005-10-03,Thermoelectric effects in strongly interacting quantum dot coupled to ferromagnetic leads,"We study thermoelectric effects in Kondo correlated quantum dot coupled to ferromagnetic electrodes by calculating conductance, thermopower and thermal conductance in the Kondo regime. We also study the effect of the asymmetry in the coupling to the leads, which has important consequences for anti-parallel magnetization configuration. We discuss the thermoelectric figure of merit, tunnel magnetoresistance and violation of the Wiedemann-Franz law in this system. The results agree with recently measured thermopower of the quantum dot defined in a two dimensional electron gas.",0510036v1 2005-10-05,Gated Spin Transport through an Individual Single Wall Carbon Nanotube,"Hysteretic switching in the magnetoresistance of short-channel, ferromagnetically contacted individual single wall carbon nanotubes is observed, providing strong evidence for nanotube spin transport. By varying the voltage on a capacitively coupled gate, the magnetoresistance can be reproducibly modified between +10% and -15%. The results are explained in terms of wave vector matching of the spin polarized electron states at the ferromagnetic / nanotube interfaces.",0510112v1 2005-10-08,Slater-Pauling Rule and Curie-Temperature of Co$_2$-based Heusler compounds,"A concept is presented serving to guide in the search for new materials with high spin polarization. It is shown that the magnetic moment of half-metallic ferromagnets can be calculated from the generalized Slater-Pauling rule. Further, it was found empirically that the Curie temperature of Co$_2$ based Heusler compounds can be estimated from a seemingly linear dependence on the magnetic moment. As a successful application of these simple rules, it was found that Co$_2$FeSi is, actually, the half-metallic ferromagnet exhibiting the highest magnetic moment and the highest Curie temperature measured for a Heusler compound.",0510210v1 2005-10-11,Non-damping magnetization oscillations in a single-domain ferromagnet,"Non-damped oscillations of the magnetization vector of a ferromagnetic system subject to a spin polarized current and an external magnetic field are studied theoretically by solving the Landau-Lifshitz-Gilbert equation. It is shown that the frequency and amplitude of such oscillations can be controlled by means of an applied magnetic field and a spin current. The possibility of injection of the oscillating spin current into a non-magnetic system is also discussed.",0510280v1 2005-10-15,From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions,"We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 nm to the atomic scale and eventually to the tunneling regime by means of electromigration. These devices are mechanically stable against magnetostriction and magnetostatic effects. We measure magnetoresistances ~ 0.3% for 100*30 nm^2 constrictions, increasing to a maximum of 80% for atomic-scale widths. These results are consistent with a geometrically-constrained domain wall trapped at the constriction. For the devices in the tunneling regime we observe large fluctuations in MR, between -10 and 85%.",0510410v1 2005-10-18,Anomalous Hall effect in ferromagnetic disordered metals,"The anomalous Hall effect in disordered band ferromagnets is considered in the framework of quantum transport theory. A microscopic model of electrons in a random potential of identical impurities including spin-orbit coupling is used. The Hall conductivity is calculated from the Kubo formula for both, the skew scattering and the side-jump mechanisms. The recently discussed Berry phase induced Hall current is also evaluated within the model. The effect of strong impurity scattering is analyzed and it is found to affect the ratio of the non-diagonal (Hall) and diagonal components of the conductivity as well as the relative importance of different mechanisms.",0510481v1 2005-10-25,Resonant peak in the density of states in the normal metal / diffusive ferromagnet / superconductor junctions,"The conditions for the formation of zero-energy peak in the density of states (DOS) in the normal metal / insulator / diffusive ferromagnet / insulator / s-wave superconductor (N/I/DF/I/S) junctions are studied by solving the Usadel equations. The DOS of the DF is calculated in various regimes for different magnitudes of the resistance, Thouless energy and the exchange field of the DF, as well as for various resistances of the insulating barriers. The conditions for the DOS peak are formulated for the cases of weak proximity effect (large resistance of the DF/S interface) and strong proximity effect (small resistance of the DF/S interface).",0510657v1 2005-10-25,Resonant proximity effect in normal metal / diffusive ferromagnet / superconductor junctions,"Resonant proximity effect in the normal metal / insulator / diffusive ferromagnet / insulator / $s$- and d-wave superconductor (N/I/DF/I/S) junctions is studied for various regimes by solving the Usadel equation with the generalized boundary conditions. It is shown that the resonant proximity effect originating from the exchange field in DF layer strongly modifies the tunneling conductance and density of states.",0510665v1 2005-11-02,Biferroic YCrO3,"YCrO3 which has a monoclinic structure, shows weak ferromagnetism below 140 K (TN) and a ferroelectric transition at 473 K accompanied by hysteresis. We have determined the structure and energetics of YCrO3 with ferromagnetic and antiferromagnetic ordering by means of first-principles density functional theory calculations, based on pseudopotentials and a plane wave basis. The non-centrosymmetric monoclinic structure is found to be lower in energy than the orthorhombic structure, supporting the biferroic nature of YCrO3.",0511045v1 2005-11-07,Current-induced switching in single ferromagnetic layer nanopillar junctions,"Current induced magnetization dynamics in asymmetric Cu/Co/Cu single magnetic layer nanopillars has been studied experimentally at room temperature and in low magnetic fields applied perpendicular to the thin film plane. In sub-100 nm junctions produced using a nanostencil process a bistable state with two distinct resistance values is observed. Current sweeps at fixed applied fields reveal hysteretic and abrupt transitions between these two resistance states. The current induced resistance change is 0.5%, a factor of 5 greater than the anisotropic magnetoresistance (AMR) effect. We present an experimentally obtained low field phase diagram of current induced magnetization dynamics in single ferromagnetic layer pillar junctions.",0511144v1 2005-11-21,Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations,"Non-equilibrium Green's function technique has been used to calculate spin-dependent electronic transport through a quantum dot in the Kondo regime. The dot is described by the Anderson Hamiltonian and is coupled either symmetrically or asymmetrically to ferromagnetic leads, whose magnetic moments are noncollinear. It is shown that the splitting of the zero bias Kondo anomaly in differential conductance decreases monotonically with increasing angle between magnetizations, and for antiparallel configuration it vanishes in the symmetrical case while remains finite in the asymmetrical one.",0511506v1 2005-11-23,Phase separation in the trapped spinor gases with anisotropic spin-spin interaction,"We investigate the effect of the anisotropic spin-spin interaction on the ground state density distribution of the one dimensional spin-1 bosonic gases within a modified Gross-Pitaevskii theory both in the weakly interaction regime and in the Tonks-Girardeau (TG) regime. We find that for ferromagnetic spinor gas the phase separation occurs even for weak anisotropy of the spin-spin interaction, which becomes more and more obvious and the component of $m_F=0$ diminishes as the anisotropy increases. However, no phase separation is found for anti-ferromagnetic spinor gas in both regimes.",0511581v2 2005-12-13,Thermal collapse of spin-polarization in half-metallic ferromagnets,"The temperature dependence of the magnetization and spin-polarization at the Fermi level is investigated for half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab-initio calculations. The thermal properties are studied by mapping ab-initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.",0512277v1 2005-12-13,First order phase transitions in Ferromagnet/Superconductor layered structures,"We study the thermodynamics of clean structures composed of superconductor (S) and ferromagnet (F) layers and consisting of one or more SFS junctions. We use fully self consistent numerical methods to compute the condensation free energies of the possible order parameter configurations as a function of temperature $T$. As $T$ varies, we find that there are phase transitions between states characterized by different junction configurations (denoted as ``0'' or ``$\pi$'' according to the phase difference of the order parameter in consecutive S layers). We show that these transitions are of first order. We calculate the associated latent heats and find them to be measurable.",0512285v1 2005-12-19,Experimental Evidence for Crossed Andreev Reflection,"We report on electronic transport properties of mesoscopic superconductor-ferromagnet spin-valve structures. Two ferromagnetic iron leads form planar tunnel contacts to a superconducting aluminum wire, where the distance of the two contacts is of the order of the coherence length of the aluminum. We observe a negative non-local resistance which can be explained by crossed Andreev reflection, a process where an electron incident from one of the leads gets reflected as a hole into the other, thereby creating a pair of spatially separated, entangled particles.",0512445v1 2005-12-19,Aspects of the FM Kondo Model: From Unbiased MC Simulations to Back-of-an-Envelope Explanations,"Effective models are derived from the ferromagnetic Kondo lattice model with classical corespins, which greatly reduce the numerical effort. Results for these models are presented. They indicate that double exchange gives the correct order of magnitude and the correct doping dependence of the Curie temperature. Furthermore, we find that the jump in the particle density previously interpreted as phase separation is rather explained by ferromagnetic polarons.",0512462v1 2005-12-19,Spin injection into a short DNA chain,"Quantun spin transport through a short DNA chain connected to ferromagnetic electrodes has been investigated by the transfer matrix method. We describe the system by a tight-binding model where the parameters are extracted from the experimental data and realistic metal energy bands. For ferromagnetic iron electrodes, the magnetoresistance of a 30-basepair Poly(G)-Poly(C) DNA is found to be lower than 10% at a bias of < 4 V, but can rach up to 20% at a bias of 5 V. In the presence of the spin-flip mechanism, the magnetoresistance is significantly enhanced when the spin-flip coupling is weak but as the coupling becomes stronger the decreasing magnetoresistance develops an oscillatory behavior.",0512473v1 2005-12-27,Electronic Structure and Bulk Spin Valve Behavior in Ca$_3$Ru$_2$O$_7$,"We report density functional calculations of the magnetic properties and Fermiology of Ca$_3$Ru$_2$O$_7$. The ground state consists of ferromagnetic bilayers, stacked antiferromagnetically. The bilayers are almost but not exactly half-metallic. In the ferromagnetic state opposite spin polarizations are found for in-plane and out-of-plane transport. Relatively high out of plane conductivity is found for the majority spin, which is relatively weakly conductive in-plane. In the ground state in-plane quantities are essentially the same, but the out of plane transport is strongly reduced.",0512673v1 2005-12-28,Correlation effects in the valence bands of ferromagnetic semiconductor EuS,"We present a many body analysis of the multi-band Kondo lattice model. The study is then combined with the first principles TB-LMTO band structure calculations, in order to investigate the temperature dependent correlation effects in the 3$\textit{p}$ valence bands of the ferromagnetic semiconductor EuS. Some of the physical properties of interest like the quasi-particle density of states (Q-DOS), spectral density (SD) and quasi-particle band structure (Q-BS) are calculated and discussed. Therewith, we propose a spin resolved ARPES of the valence bands of EuS to be performed.",0512688v1 2005-12-30,A Tunable Anomalous Hall Effect in a Non-Ferromagnetic System,"We measure the low-field Hall resistivity of a magnetically-doped two-dimensional electron gas as a function of temperature and electrically-gated carrier density. Comparing these results with the carrier density extracted from Shubnikov-de Haas oscillations reveals an excess Hall resistivity that increases with decreasing temperature. This excess Hall resistivity qualitatively tracks the paramagnetic polarization of the sample, in analogy to the ferromagnetic anomalous Hall effect. The data are consistent with skew-scattering of carriers by disorder near the crossover to localization.",0512730v1 2006-01-10,Short-time behavior of a classical ferromagnet with double-exchange interaction,"We investigate the critical dynamics of a classical ferromagnet on the simple cubic lattice with double-exchange interaction. Estimates for the dynamic critical exponents $z$ and $\theta$ are obtained using short-time Monte Carlo simulations. We also estimate the static critical exponents $\nu$ and $\beta$ studying the behavior of the samples at an early time. Our results are in good agreement with available estimates and support the assertion that this model and the classical Heisenberg model belong to the same universality class.",0601208v1 2006-01-12,Random phase approximation for the 1D anti-ferromagnetic Heisenberg model,"The Hartree-Fock-RPA approach is applied to the 1D anti-ferromagnetic Heisenberg model in the Jordan-Wigner representation. Somewhat contrary to expectation, this leads to reasonable results for spectral functions and sum rules in the symmetry unbroken phase. In a preliminary application of Self-Consistent RPA to finite size chains strongly improved results are obtained.",0601264v4 2006-01-17,Microwave Response and Spin Waves in Superconducting Ferromagnets,"Excitation of spin waves is considered in a superconducting ferromagnetic slab with the equilibrium magnetization both perpendicular and parallel to the surface. The surface impedance is calculated and its behavior near propagation thresholds is analyzed. Influence of non-zero magnetic induction at the surface is considered in various cases. The results provide a basis for investigation of materials with coexisting superconductivity and magnetism by microwave response measurements.",0601386v2 2006-01-24,Attractive electron-electron interaction induced by geometric phase in a Bloch band,"We investigate electron pairing in the presence of the Berry curvature field that ubiquitously exists in ferromagnetic metals with spin-orbit coupling. We show that a sufficiently strong Berry curvature field on the Fermi surface can transform a repulsive interaction between electrons into an attractive one in the p-wave channel. We also reveal a topological possibility for turning an attractive s-wave interaction into one in the p-wave channel, even if the Berry curvature field only exists inside the Fermi surface (circle). We speculate that these novel mechanism might be relevant to the recently discovered ferromagnetic superconductors such as UGe$_{2}$ and URhGe.",0601531v2 2006-02-03,Low Ghz loss in sputtered epitaxial Fe,"We show that sputtered, pure epitaxial iron films can have high-frequency loss as low as, or lower than, any known metallic ferromagnetic heterostructure. Minimum 34 Ghz ferromagnetic resonance (FMR) linewidths of 41 Oe are demonstrated, some ~ 5-10 % lower than the previous minimum reported for molecular beam epitaxially (MBE) deposited Fe. Intrinsic and extrinsic damping have been separated over 0-40 Ghz, giving a lower bound for intrinsic LL(G) relaxation rates of lambda or G = 85 MHz (alpha = 0.0027) and extrinsic 50 Mhz. Swept frequency measurements indicate the potential for integrated frequency domain devices with Q>100 at 30-40 Ghz.",0602094v1 2006-02-06,Thermoelectric effects in a strongly interacting quantum dot coupled to ferromagnetic leads,"We study thermoelectric effects in Kondo correlated quantum dot coupled to ferromagnetic electrodes by calculating thermopower S in the Kondo regime as function of on-dot energy level and temperature. The system is represented by the Anderson model and the results agree well with those recently measured for a quantum dot coupled to nonmagnetic leads. For magnetic electrodes one observes marked dependence of S on the degree of their polarization.",0602121v1 2006-02-06,Colossal magnetocapacitance and colossal magnetoresistance in HgCr2S4,"We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly enhanced in the region below 60 - 80 K, which can be ascribed to polar relaxational dynamics triggered by the onset of ferromagnetic correlations. In addition, the observation of polarization hysteresis curves indicates the development of ferroelectric order below about 70 K. Moreover, our investigations in external magnetic fields up to 5 T reveal the simultaneous occurrence of magnetocapacitance and magnetoresistance of truly colossal magnitudes in this material.",0602126v1 2006-02-10,Low-frequency vortex dynamic susceptibility and relaxation in mesoscopic ferromagnetic dots,"Vortex dynamics in a restricted geometry is considered for a magnetic system consisting of ferromagnetic cylindrical dots. To describe the vortex dynamic susceptibility and relaxation the equation of motion for the vortex center position is applied. The dependencies of the vortex dynamic susceptibility and resonance linewidth on geometrical parameters are calculated. A new method of extracting damping parameter from the vortex low-frequency resonance peaks is proposed and applied for interpretation of resonance data on FeNi circular dots.",0602279v1 2006-02-17,Ferromagnetic and superconducting instabilities in graphite,"We review our recent work on magnetic properties of graphite and related carbon materials. The results demonstrate that a structural disorder, topological defects, as well as adsorbed foreign atoms can be responsible for the occurrence of both ferromagnetic and superconducting patches in graphitic structures.",0602413v2 2006-02-28,On magnetic susceptibility of a spin-S impurity in nearly ferromagnetic Fermi liquid,"We present the renormalization group analysis for the problem of a spin-S impurity in nearly ferromagnetic Fermi liquid. We evaluate the renormalization group function that governs the temperature behavior of the invariant charge to the second order of both weak and strong coupling expansions. It allows us to determine behavior of the zero field magnetic susceptibility of impurity at low and high temperatures. We predict that derivative of the susceptibility with temperature should always have the maximum.",0602650v1 2006-03-06,Edge states generated by spin-orbit coupling at domain walls in magnetic semiconductors,"Electronic states localized at domain walls between ferromagnetically ordered phases in two-dimensional electron systems are generated by moderate spin-orbit coupling. The spin carried by these states depends on the slope of the magnetic background at the domain wall. The number of localized states is determined by a real space topological number, and spin perpendicular to the ferromagnetic order accumulates in these localized states at domain walls. These trapped states may be observed in experiments that probe either spin density or conduction paths in quantum wells.",0603145v3 2006-03-13,Phase transitions in random magnetic bilayer,"The influence of random interlayer exchange on the phase states of the simplest magnetic heterostructure consisting of two ferromagnetic Ising layers with large interaction radius is studied. It is shown that such system can exist in three magnetic phases: ferromagnetic, antiferromagnetic and ferrimagnetic. The possible phase diagrams and temperature dependencies of thermodynamic parameters are described. The regions of existence of the magnetic phases in external magnetic field are determined at zero temperature.",0603347v3 2006-03-14,Fingerprints of the Magnetic Polaron in Nonequilibrium Electron Transport through a Quantum Wire Coupled to a Ferromagnetic Spin Chain,"We study nonequilibrium quantum transport through a mesoscopic wire coupled via local exchange to a ferromagnetic spin chain. Using the Keldysh formalism in the self-consistent Born approximation, we identify fingerprints of the magnetic polaron state formed by hybridization of electronic and magnon states. Because of its low decoherence rate, we find coherent transport signals. Both elastic and inelastic peaks of the differential conductance are discussed as a function of external magnetic fields, the polarization of the leads and the electronic level spacing of the wire.",0603374v2 2006-03-15,Dynamic hysteresis from zigzag domain walls,"We investigate dynamic hysteresis in ferromagnetic thin films with zigzag domain walls. We introduce a discrete model describing the motion of a wall in a disordered ferromagnet with in-plane magnetization, driven by an external magnetic field, considering the effects of dipolar interactions and anisotropy. We analyze the effects of external field frequency and temperature on the coercive field by Monte Carlo simulations, and find a good agreement with the experimental data reported in literature for Fe/GaAs films. This implies that dynamic hysteresis in this case can be explained by a single propagating domain wall model without invoking domain nucleation.",0603406v2 2006-03-15,"Dephasing in (Ga,Mn)As nanowires and rings","To understand quantum mechanical transport in ferromagnetic semiconductor the knowledge of basic material properties like phase coherence length and corresponding dephasing mechanism are indispensable ingredients. The lack of observable quantum phenomena prevented experimental access to these quantities so far. Here we report about the observations of universal conductance fluctuations in ferromagnetic (Ga,Mn)As. The analysis of the length and temperature dependence of the fluctuations reveals a T^{-1} dependence of the dephasing time.",0603418v1 2006-03-20,Triplet superconductivity in a 1D itinerant electron system with transverse spin anisotropy,"In this paper we study the ground state phase diagram of a one-dimensional t-J-U model away from half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy a phase with gapless charge and massive spin excitations, characterized by the coexistence of triplet superconducting and spin density wave instabilities is realized in the ground state. With increasing ferromagnetic exchange transitions into a ferrometallic and then a spin gapped triplet superconducting phase take place.",0603504v1 2006-03-21,Huge nonequilibrium magnetoresistance in hybrid superconducting spin valves,"A hybrid ferromagnet-superconductor spin valve is proposed. Its operation relies on the interplay between nonequilibrium transport and proximity-induced exchange coupling in superconductors. Huge tunnel magnetoresistance values as large as some 10^6% can be achieved in suitable ferromagnet-superconductor combinations under proper voltage biasing. The controllable spin-filter nature of the structure combined with its intrinsic simplicity make this setup attractive for low-temperature spintronic applications where reduced power dissipation is an additional requirement.",0603555v1 2006-03-30,Dynamics of a quantum phase transition in the random Ising model,"A quantum phase transition from paramagnetic to ferromagnetic phase is driven by a time-dependent external magnetic field. For any rate of the transition the evolution is non-adiabatic and finite density of defects is excited in the ferromagnetic state. The density of excitations has only logarithmic dependence on the transition rate. This is much weaker than any usual power law scaling predicted for pure systems by the Kibble-Zurek mechanism.",0603814v2 2006-04-13,Spin-wave instability for parallel pumping in ferromagnetic thin films under oblique field,"Spin-wave instability for parallel pumping is studied theoretically. The spin-wave instability threshold is calculated in ferromagnetic thin films under oblique field which has an oblique angle to the film plane. The butterfly curve of the threshold usually has a cusp at a certain value of the static external field. While the static field value of the cusp point varies as the oblique angle changes, the general properties of the butterfly curve show little noteworthy change. For very thin films, however, multiple cusps of the butterfly curve can appear due to different standing spin-wave modes, which indicate a novel feature for thin films under oblique field.",0604329v1 2006-04-13,Magnetic order and valency at La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ interfaces,"We report on first principles calculations of the electronic structure of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ junction with two possible types of interface terminations. We find that the La$_{0.7}$Sr$_{0.3}$O/TiO$_{2}$ interface preserves the interlayer ferromagnetic coupling between the interface MnO$_{2}$ layer and the bulk. The other interface, MnO$_{2}$/SrO, favours antiferromagnetic coupling with the bulk. By inserting two unit cells of undoped LaMnO$_{3}$ at the interface the ferromagnetism is recovered. This is understood in terms of the doping level and the mobility of carriers near the interface.",0604343v1 2006-04-26,Time-domain measurement of driven ferromagnetic resonance,"We present a time-resolved measurement of magnetization dynamics during ferromagnetic resonance (FMR) in a single layer of Ni81Fe19. Small-angle (<1 deg.) precession of elemental Ni, Fe moments could be measured directly and quantitatively using time-resolved x-ray magnetic circular dichroism (XMCD) in transmission. The high temporal and rotational sensitivity of of this technique has allowed characterization of the phase and amplitude of driven FMR motion at 2.3 GHz, verifying basic expectations for a driven resonance.",0604610v1 2006-05-08,Microscopic Calculation of Spin Torques in Disordered Ferromagnets,"Effects of conduction electrons on magnetization dynamics, represented by spin torques, are calculated microscopically in the first order in spatial gradient and time derivative of magnetization. Special attention is paid to the so-called $\beta$-term and the Gilbert damping, $\alpha$, in the presence of electrons' spin-relaxation processes, which are modeled by quenched magnetic (and spin-orbit) impurities. The obtained results such as $\alpha \ne \beta$ hold for localized as well as itinerant ferromagnetism.",0605186v1 2006-05-09,Influence of band structure effects on domain-wall resistance in diluted ferromagnetic semiconductors,"Intrinsic domain-wall resistance (DWR) in (Ga,Mn)As is studied theoretically and compared to experimental results. The recently developed model of spin transport in diluted ferromagnetic semiconductors [Van Dorpe et al., Phys. Rev. B 72, 205322 (2005)] is employed. The model combines the disorder-free Landauer-B\""uttiker formalism with the tight-binding description of the host band structure. The obtained results show how much the spherical 4x4 kp model [Nguyen, Shchelushkin, and Brataas, cond-mat/0601436] overestimates DWR in the adiabatic limit, and reveal the dependence of DWR on the magnetization profile and crystallographic orientation of the wall.",0605230v1 2006-05-10,Field-Induced Quantum Criticality of Systems with Ferromagnetically Coupled Structural Spin Units,"The field-induced quantum criticality of compounds with ferromagnetically coupled structural spin units (as dimers and ladders) is explored by applying Wilson's renormalization group framework to an appropriate effective action. We determine the low-temperature phase boundary and the behavior of relevant quantities decreasing the temperature with the applied magnetic field fixed at its quantum critical point value. In this context, a plausible interpretation of some recent experimental results is also suggested.",0605274v1 2006-05-12,The magnetic reorientation transition in thin ferromagnetic films treated by many-body Green's function theory,"This contribution describes the reorientation of the magnetization of thin ferromagnetic Heisenberg films as function of the temperature and/or an external field. Working in a rotating frame allows an exact treatment of the single-ion anisotropy when going to higher-order Green's functions. Terms due to the exchange interaction are treated by a generalized Tyablikov (RPA) decoupling.",0605331v1 2006-05-12,Surfactant assisted synthesis of Co and Li doped ZnO nanocrystalline samples showing room temperature ferromagnetism,"We have developed a simple, surfactant assisted synthesis route for the preparation, in gram quantities, of Co and Li doped ZnO nanocrystalline samples showing robust room temperature ferromagnetism. Our studies show that RTF is intrinsic to Zn0.85Co0.05Li0.10O and not due to any segregated secondary phases. In addition, it has been shown that the defects play an important role in activating RTF in these oxide systems. This also provide an explanation for the widely varying results observed in the literature. The method can also be extended for the synthesis of other transition metal doped ZnO.",0605332v1 2006-05-12,Large domain wall resistance in self-organised manganite film,"The electrical resistance of magnetic domain walls in ferromagnetic metallic manganites can be enhanced to 10-12 Ohm.m2 by patterning nanoconstrictions [J. Appl. Phys. 89, 6955 (2001)]. We show equally large enhancements in a phase separated La0.60Ca0.40MnO3 manganite film without recourse to nanopatterning. The domain walls were measured in the current-perpendicular-to-the-plane (CPP) geometry between ferromagnetic metallic La0.70Ca0.30MnO3 electrodes patterned like magnetic tunnel junctions.",0605341v2 2006-05-23,Intrinsic vs. extrinsic anomalous Hall effect in ferromagnets,"A unified theory of the anomalous Hall effect (AHE) is presented for multi-band ferromagnetic metallic systems with dilute impurities. In the clean limit, the AHE is mostly due to the extrinsic skew-scattering. When the Fermi level is located around anti-crossing of band dispersions split by spin-orbit interaction, the intrinsic AHE to be calculated ab initio is resonantly enhanced by its non-perturbative nature, revealing the extrinsic-to-intrinsic crossover which occurs when the relaxation rate is comparable to the spin-orbit interaction energy.",0605580v2 2006-05-31,"Spin injection from perpendicular magnetized ferromagnetic $δ$-MnGa into (Al,Ga)As heterostructures","Electrical spin injection from ferromagnetic $\delta$-MnGa into an (Al,Ga)As p-i-n light emitting diode (LED) is demonstrated. The $\delta$-MnGa layers show strong perpendicular magnetocrystalline anisotropy, enabling detection of spin injection at remanence without an applied magnetic field. The bias and temperature dependence of the spin injection are found to be qualitatively similar to Fe-based spin LED devices. A Hanle effect is observed and demonstrates complete depolarization of spins in the semiconductor in a transverse magnetic field.",0606013v1 2006-06-07,Detection of current-induced spins by ferromagnetic contacts,"Detection of current-induced spin accumulation via ferromagnetic contacts is discussed. Onsager's relations forbid that in a two-probe configuration spins excited by currents in time-reversal symmetric systems can be detected by switching the magnetization of a ferromangetic detector contact. Nevertheless, current-induced spins can be transferred as a torque to a contact magnetization and affect the charge currents in many-terminal configurations. We demonstrate the general concepts by solving the microscopic transport equations for the diffuse Rashba system with magnetic contacts.",0606193v2 2006-06-13,Peculiar behavior of the electrical resistivity of MnSi at the ferromagnetic phase transition,"The electrical resistivity of a single crystal of MnSi was measured across its ferromagnetic phase transition line at ambient and high pressures. Sharp peaks of the temperature coefficient of resistivity characterize the transition line. Analysis of these data shows that at pressures to ~0.35 GPa these peaks have fine structure, revealing a shoulder at ~ 0.5 K above the peak. It is symptomatic that this structure disappears at pressures higher than ~0.35 GPa, which was identified earlier as a tricritical point",0606324v1 2006-06-21,Current-excited magnetization dynamics in narrow ferromagnetic wires,"We investigate the current-excited magnetization dynamics in a narrow ferromagnetic Permalloy wire by means of Lorentz microscopy, together with the results of simultaneous transport measurements. A detailed structural evolution of the magnetization is presented as a function of the applied current density. Local structural deformation, bidirectional displacement, and magnetization reversal are found below the Curie temperature with increasing the current density. We discuss probable mechanisms of observed features of the current-excited magnetization dynamics.",0606547v1 2006-07-03,Magnetic effects on the phase transitions in unconventional superconductors,"A new critical behavior in unconventional superconductors and superfluids is established and described by the Wilson-Fisher renormalization-group method. For certain symmetries of order a new type of fluctuation-driven first order phase transitions at finite and zero temperature are predicted. The results can be applied to a wide class of ferromagnetic superconductive and superfluid systems, in particular, to itinerant ferromagnets as UGe_2 and URhGe.",0607057v2 2006-07-23,Non-collinear Spin Valve Effect in Ferromagnetic Semiconductor Trilayers,"We report the observation of the spin valve effect in (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayer devices. Magnetoresistance measurements carried out in the current in plane geometry reveal positive magnetoresistance peaks when the two ferromagnetic layers are magnetized orthogonal to each other. Measurements carried out for different post-growth annealing conditions and spacer layer thickness suggest that the positive magnetoresistance peaks originate in a noncollinear spin valve effect due to spin-dependent scattering that is believed to occur primarily at interfaces.",0607580v2 2006-08-01,"Structural and magnetic properties of ZnO:TM (TM: Co,Mn) nanopowders","We report on the structural and magnetic characterization of Co0.1Zn0.9O and Mn0.1Zn0.9O nanopowders obtained by a soft chemistry route. We show that those samples fired at low temperatures display a ferromagnetic interaction that can not be attributed to the presence of impurities. A magnetic aging mechanism is observed, reflecting the key role played by defects in the stabilization of ferromagnetism in this kind of diluted magnetic semiconductors.",0608014v1 2006-08-16,Quantum-size effect and tunneling magnetoresistance in ferromagnetic-semiconductor quantum heterostructures,"We report on the resonant tunneling effect and the increase of tunneling magnetoresistance (TMR) induced by it in ferromagnetic-semiconductor GaMnAs quantum-well heterostructures. The observed quantum levels of the GaMnAs quantum well were successfully explained by the valence-band kp model with the p-d exchange interaction. It was also found that the Fermi level of the electrode injecting carriers is important to observe resonant tunneling in this system.",0608357v1 2006-08-19,Spin dynamics in molecular ring nanomagnets: Significant effect of acoustic phonons and magnetic anisotropies,"The nuclear spin-lattice relaxation rate 1/T_1_ is calculated for magnetic ring clusters by fully diagonalizing their microscopic spin Hamiltonians. Whether the nearest-neighbor exchange interaction J is ferromagnetic or antiferromagnetic, 1/T_1_ versus temperature T in ring nanomagnets may be peaked at around k_B_T=|J| provided the lifetime broadening of discrete energy levels is in proportion to T^3^. Experimental findings for ferromagnetic and antiferromagnetic Cu^II^ rings are reproduced with crucial contributions of magnetic anisotropies as well as acoustic phonons.",0608438v1 2006-08-22,Clustering induced suppression of ferromagnetism in diluted magnets,"Ferromagnetism in diluted magnets in the compensated regime p << x is shown to be suppressed by the formation of impurity spin clusters. The majority bulk spin couplings are shown to be considerably weakened by the preferential accumulation of holes in spin clusters, resulting in low-energy magnon softening and enhanced low-temperature decay of magnetic order. A locally self-consistent magnon renormalization analysis of spin dynamics shows that although strong intra-cluster correlations tend to prolong global order, T_c is still reduced compared to the ordered case.",0608474v2 2006-09-01,Thermally excited spin-current in metals with embedded ferromagnetic nanoclusters,"We show that a thermally excited spin-current naturally appears in metals with embedded ferromagnetic nanoclusters. When such materials are subjected to a magnetic field, a spin current can be generated by a temperature gradient across the sample as a signature of electron-hole symmetry breaking in a metal due to the electron spin-flip scattering from polarised magnetic moments. Such a spin current can be observed via a giant magneto-thermopower which tracks the polarisation state of the magnetic subsystem and is proportional to the magnetoresistance. Our theory explains the recent experiment on Co clusters in copper by S. Serrano-Guisan \textit{et al} [Nature Materials AOP, doi:10.1038/nmat1713 (2006)]",0609012v1 2006-09-06,"Magnitude and crystalline anisotropy of hole magnetization in (Ga,Mn)As","Theory of hole magnetization Mc in zinc-blende diluted ferromagnetic semiconductors is developed relaxing the spherical approximation of earlier approaches. The theory is employed to determine Mc for (Ga,Mn)As over a wide range of hole concentrations and a number of crystallographic orientations of Mn magnetization. It is found that anisotropy of Mc is practically negligible but the obtained magnitude of Mc is significantly greater than that determined in the spherical approximation. Its sign and value compares favorably with the results of available magnetization measurements and ferromagnetic resonance studies.",0609128v1 2006-09-19,$π$-Electron Ferromagnetism in Metal Free Carbon Probed by Soft X-Ray Dichroism,"Elemental carbon represents a fundamental building block of matter and the possibility of ferromagnetic order in carbon attracted widespread attention. However, the origin of magnetic order in such a light element is only poorly understood and has puzzled researchers. We present a spectromicroscopy study at room temperature of proton irradiated metal free carbon using the elemental and chemical specificity of x-ray magnetic circular dichroism (XMCD). We demonstrate that the magnetic order in the investigated system originates only from the carbon $\pi$-electron system.",0609478v3 2006-10-06,Significant enhancement of room temperature ferromagnetism in surfactant coated polycrystalline Mn doped ZnO particles,"We report a surfactant assisted synthesis of Mn doped ZnO polycrystalline samples showing robust room temperature ferromagnetism as characterized by X-ray diffraction analysis, Transmission Electron Microscopy, Electron Paramagnetic Resonance and DC magnetization mearurements. This surfactant assisted synthesis method, developed by us, is found to be highly reproducible. Further, it can also be extended to the synthesis of other transition metal doped ZnO.",0610170v1 2006-10-09,Single-domain versus two-domain configuration in thin ferromagnetic prisms,"Thin ferromagnetic elements in the form of rectangular prisms are theoretically investigated in order to study the transition from single-domain to two-domain state, with changing the in-plane aspect ratio p. We address two main questions: first, how general is the transition; second, how the critical value p_c depends on the physical parameters. We use two complementary methods: discrete-lattice calculations and a micromagnetic continuum approach. Ultrathin films do not appear to split in two domains. Instead, thicker films may undergo the above transition. We have used the continuum approach to analyze recent Magnetic Force Microscopy observations in 30 nm-thick patterned Permalloy elements, finding a good agreement for p_c.",0610233v1 2006-10-11,0-pi transition in SFS junctions with strongly spin-dependent scattering,"We develop theory of proximity effect in a superconductor - GMR alloy - superconductor trilayers, which takes into account strong spin dependence of electron scattering of compositional disorder in a diluted ferromagnetic alloy. We show that in such a system the critical current oscillations as the function of the thickness of the ferromagnetic layer, with the period of $v_{F}/2I$, decay exponentially with the characteristic length of the order of the mean free path.",0610299v3 2006-10-19,"Magneto-resistive memory in ferromagnetic (Ga,Mn)As nanostructures","We show a novel magneto-resistive effect that appears in lithographically shaped, three-arm nanostructure, fabricated from ferromagnetic (Ga,Mn)As layers. The effect, related to a rearrangement of magnetic domain walls between different pairs of arms in the structure, reveals as a dependence of zero-field resistance on the direction of previously applied magnetic field. This effect could allow designing devices with unique switching and memory properties.",0610535v1 2006-10-23,Scaling and universality in the aging kinetics of the two-dimensional clock model,"We study numerically the aging dynamics of the two-dimensional p-state clock model after a quench from an infinite temperature to the ferromagnetic phase or to the Kosterlitz-Thouless phase. The system exhibits the general scaling behavior characteristic of non-disordered coarsening systems. For quenches to the ferromagnetic phase, the value of the dynamical exponents, suggests that the model belongs to the Ising-type universality class. Specifically, for the integrated response function $\chi (t,s)\simeq s^{-a_\chi}f(t/s)$, we find $a_\chi $ consistent with the value $a_\chi =0.28$ found in the two-dimensional Ising model.",0610615v1 2006-10-24,Flux-flow induced giant magnetoresistance in all-amorphous superconductor-ferromagnet hybrids,"We present magnetoresistance measurements on all-amorphous ferromagnet (F) / superconductor (S) heterostructures. The F/S/F trilayers show large magnetoresistance peaks in a small field range around the coercive field of the F layers, at temperatures within and below the superconducting transition. This is interpreted as flux flow of weakly pinned vortices induced by the stray field of Bloch magnetic domains in the F layers. Bilayers show much smaller effects, implying that the Bloch walls of the F-layers in the trilayer line up and focus the stray fields. The data are used to discuss the expected minimum F-layer thickness needed to nucleate vortices.",0610667v1 2006-11-09,Super-Aging in two-dimensional random ferromagnets,"We study the aging properties, in particular the two-time autocorrelations, of the two-dimensional randomly diluted Ising ferromagnet below the critical temperature via Monte-Carlo simulations. We find that the autocorrelation function displays additive aging $C(t,t_w)=C_{st}(t)+C_{ag}(t,t_w)$, where the stationary part $C_{st}$ decays algebraically. The aging part shows anomalous scaling $C_{ag}(t,t_w)={\cal C}(h(t)/h(t_w))$, where $h(u)$ is a non-homogeneous function excluding a $t/t_w$ scaling.",0611246v1 2006-11-10,Spin transport through a single self-assembled InAs quantum dot with ferromagnetic leads,"We have fabricated a lateral double barrier magnetic tunnel junction (MTJ) which consists of a single self-assembled InAs quantum dot (QD) with ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance (TMR) effect, which is evidence for spin transport through a single semiconductor QD. The TMR ratio and the curve shapes are varied by changing the gate voltage.",0611269v2 2006-11-10,Nuclear spin ferromagnetic phase transition in an interacting 2D electron gas,"Electrons in a two-dimensional semiconducting heterostructure interact with nuclear spins via the hyperfine interaction. Using a a Kondo lattice formulation of the electron-nuclear spin interaction, we show that the nuclear spin system within an interacting two-dimensional electron gas undergoes a ferromagnetic phase transition at finite temperatures. We find that electron-electron interactions and non-Fermi liquid behavior substantially enhance the nuclear spin Curie temperature into the $mK$ range with decreasing electron density.",0611292v2 2006-11-22,Octupolar order in the multiple spin exchange model on a triangular lattice,"We show how a gapless spin liquid with hidden octupolar order arises in an applied magnetic field, in a model applicable to thin films of 3He with competing ferromagnetic and antiferromagnetic (cyclic) exchange interactions. Evidence is also presented for nematic -- i.e., quadrupolar -- correlations bordering on ferromagnetism in the absence of a magnetic field.",0611575v2 2006-11-26,Critical cooling rate for the glass formation of ferromagnetic Fe40Ni40P14B6 alloy,"Bulk ferromagnetic amorphous Fe-Ni-P-B alloys in rod shape were formed by a rapid solidification technique. The largest amorphous specimen prepared had a diameter of ~2.5 mm and the corresponding cooling rate for the glass formation of this alloy system in our experiment can be estimate to be around 492.4 K/s by the method of finite-difference numerical calculation. This value is on the same order of magnitude as the critical cooling rate Rc of Fe40Ni40P14B6 alloy estimated by the method of constructing the continuous-cooling-transformation (CCT) curve. It is indicated that the heterophase impurities have been eliminated well in our experiment.",0611651v2 2006-11-30,Theory of Brillouin Light Scattering from Ferromagnetic Nanospheres,"We develop the theory of Brillouin light scattering (BLS) from spin wave modes in ferromagnetic nanospheres, within a framework that incorporates the spatial variation of the optical fields within the sphere. Our recent theory of exchange dipole spin wave modes of the sphere provides us with eigenvectors. When properly normalized, these eigenvectors allow calculation of the absolute cross section of various modes which contribute to BLS spectrum. We then present explicit calculation of the BLS spectrum associated with the first few dipole/exchange spin wave modes with emphasis on their relative intensity.",0611785v1 2006-12-02,Magneto-optical and magnetotransport properties of heavily Mn-doped GaMnAs,"We have studied the magneto-optical and magnetotrasnport properties of GaMnAs thin films with high Mn concentrations (x= 12.2 - 21.3%) grown by molecular-beam epitaxy. These heavily Mn-doped GaMnAs films were formed by decreasing the growth temperature to as low as 150-190 degree C and by reducing the film thickness to 10 nm in order to prevent precipitation of hexagonal MnAs clusters. Magnetic circular dichroism (MCD) and anomalous Hall effect measurements indicate that these GaMnAs films have the nature of intrinsic ferromagnetic semiconductors with high ferromagnetic transition temperature up to 170 K.",0612055v1 2006-12-19,Characteristic temperatures of exchange biased systems,"Characteristic temperatures in ferromagnetic - antiferromagnetic exchange biased systems are analyzed. In addition to usual blocking temperature of exchange bias $T_{B}$, and the N\'{e}el temperature of an antiferromagnet $T_{N}$, the inducing temperature $T_{ind}$, i.e., the temperature, at which the direction of exchange anisotropy is established, has been recently proposed. We demonstrate that this temperature is in general case different from $T_{B}$ and $T_{N}$. Physics and experimental approaches to measure the inducing temperature are discussed. Measurements of $T_{ind}$, in addition to $T_{B}$, and $T_{N}$, provide important information about exchange interactions in ferromagnetic - antiferromagnetic heterostructures.",0612500v1 2006-12-30,Low relaxation rate in a low-Z alloy of iron,"The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined by the intrinsic (Gilbert) relaxation rate \emph{$G$}. For many years, pure iron (Fe) has had the lowest known value of $G=\textrm{57 Mhz}$ for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy with vanadium (V) possesses values of $G$ which are significantly reduced, to 35$\pm$5 Mhz at 27% V. The result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the atomic number $Z$.",0701004v1 2007-02-24,Quantum spin tunneling of magnetization in small ferromagnetic particles,"A new quantum approach is presented that can account for the description of small ferromagnetic particles magnetization tunneling. An estimate of the saturation value of an external applied magnetic field along the easy axis is obtained. An analytic expression for the tunneling factor in the absence of an external magnetic field is deduced from the present approach that also allows to obtain the crossover temperature characterizing the regime where tunneling is dominated by the quantum effects.",0702572v1 2007-03-05,Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion,"With a variational three--body calculation we study the role of the interplay between the onsite Coulomb, Hund's rule, and superexchange interactions on the spinwave excitation spectrum of itinerant ferromagnets. We show that correlations between a Fermi sea electron--hole pair and a magnon result in a very pronounced zone boundary softening and strong deviations from the Heisenberg spinwave dispersion. We show that this spin dynamics depends sensitively on the Coulomb and exchange interactions and discuss its possible relevance to experiments in the manganites.",0703099v2 2007-03-09,"Crystal structure, magnetism, and bonding of the hexagonal compounds Pd$_{1.63}$Mn$_{0.37}$Si and Pd$_{1.82}$Mn$_{0.18}$Ge related to the Fe$_2$P structure","We have used single crystal X-ray diffraction methods to establish the crystal structures of a compound in the Pd-Mn-Si system and in the Pd-Mn-Ge system. The title compounds have structures related to the Fe$_2$P structure type and are ferromagnetic with Curie temperatures above the room temperature. Density functional electronic structure calculations help to understand the nature of the local moment ferromagnetism in these compounds. However neither the electronic structure calculations nor the magnetic measurements provide any evidence of half-metallic behavior.",0703240v1 2007-03-27,Critical Scattering and Dynamical Scaling in an Heisenberg Ferromagnet Neutron Spin Echo versus Renormalization Group Theory,"High resolution Neutron Spin Echo (NSE) spectroscopy was used to investigate the dynamics of an 3D Heisenberg ferromagnet in the exchange-controlled regime over a broad range of temperatures and momentum transfer. These results allow for the first time an extensive comparison between the experimental dynamical critical behavior and the predictions of the Renormalization Group (RG) theory. The agreement is exhaustive and surprising as the RG theory accounts not only for the critical relaxation but also for the shape crossover towards an exponential diffusive relaxation when moving from the critical to the hydrodynamic regime above $T_C$.",0703702v1 2007-03-30,Multicomponent Bright Solitons in F = 2 Spinor Bose-Einstein Condensates,"We study soliton solutions for the Gross--Pitaevskii equation of the spinor Bose--Einstein condensates with hyperfine spin F=2 in one-dimension. Analyses are made in two ways: by assuming single-mode amplitudes and by generalizing Hirota's direct method for multi-components. We obtain one-solitons of single-peak type in the ferromagnetic, polar and cyclic states, respectively. Moreover, twin-peak type solitons both in the ferromagnetic and the polar state are found.",0703805v3 2006-03-28,Remote-control and clustering of physical computations using the XML-RPC protocol and the open-Mosix system,"The applications of the remote control of physical simulations performed in clustered computers running under an open-Mosix system are presented. Results from the simulation of a 2-dimensional ferromagnetic system of spins in the Ising scheme are provided. Basic parameters of a simulated hysteresis loop like coercivity and exchange bias due to pinning of ferromagnetic spins are given. The paper describes in physicists terminology a cost effective solution which utilizes an XML-RPC protocol (Extensible Markup Language - Remote Procedure Calling) and standard C++ and Python languages.",0603111v1 1992-10-23,The Savvidy ``ferromagnetic vacuum'' in three-dimensional lattice gauge theory,"The vacuum effective potential of three-dimensional SU(2) lattice gauge theory in an applied color-magnetic field is computed over a wide range of field strengths. The background field is induced by an external current, as in continuum field theory. Scaling and finite volume effects are analyzed systematically. The first evidence from lattice simulations is obtained of the existence of a nontrivial minimum in the effective potential. This supports a ``ferromagnetic'' picture of gluon condensation, proposed by Savvidy on the basis of a one-loop calculation in (3+1)-dimensional QCD.",9210028v1 1996-02-05,Anti-Ferromagnetic Condensate in Yang-Mills Theory,"SU(2) gauge theory with competing interactions is shown to possess a rich phase structure with anti-ferromagnetic vacua. It is argued that the phase boundaries persist in the weak coupling limit suggesting the existence of different renormalized continuum theories for QCD.",9602003v2 1996-11-02,Is trivial the antiferromagnetic RP(2) model in four dimensions?,"We study the antiferromagnetic RP(2) model in four dimensions. We find a second order transition with two order parameters, one ferromagnetic and the other antiferromagnetic. The antiferromagnetic sector has mean-field critical exponents and a renormalized coupling which goes to zero in the continuum limit. The exponents of the ferromagnetic channel are not the mean-field ones, but the difference can be interpreted as logarithmic corrections. We perform a detailed analysis of these corrections and conclude the triviality of the continuum limit of this model.",9611003v1 1997-03-05,Ferromagnetism of Axion Domain Wall,"We show that axion domain wall is ferromagnetic in the universe with nonvanishing baryon number or lepton number. It is caused by protons and electrons bounded to the domain wall with their spins aligned. These bound states arise due to attractive potentials generated through pseudo-vector couplings between the fermions and the axion. Using a model of hadronic axions we predict existence of a primordial magnetic field with strength $10^{-13}$ Gauss at recombination.",9703237v1 1999-10-25,Ferromagnetism of quark liquid,"Usually it is believed that the Hartree-Fock state of quark matter is a Fermi gas state with no polarisation of spins. We examine the possibility of the polarised quark liquid interacting with the one-gluon-exchange interaction. It is suggested that the Hartree-Fock state shows a spontaneous magnetic instability at low densities through the same mechanism as the appearance of ferromagnetism in electron gas. Metastability of the polarised quark liquid is also discussed.",9910470v1 2000-07-12,No Primordial Magnetic Field from Domain Walls,"It is pointed out that, contrary to some claims in the literature, the domain walls cannot be a source of a correlated at large scales primordial magnetic field, even if the fermionic modes bound on the wall had ferromagnetic properties. In a particular model with massive (2+1) dimensional fermions bound to a domain wall, previously claimed to exhibit a ferromagnetic behavior, it is explicitly shown that the fermionic system in fact has properties of a normal diamagnetic with the susceptibility vanishing at high temperature.",0007123v2 2005-08-20,Color Ferromagnetism of Quark Matter ; a Possible Origin of Strong Magnetic Field in Magnetars,"We show a possibility that strong ``magnetic field'' $\sim 10^{15} $G is produced by color ferromagnetic quark matter in neutron stars. In the quark matter a color magnetic field is generated spontaneously owing to Savvidy mechanism and a gluon condensate arises for the stabilization of the field. Since the quark matter is electrically charged in the neutron stars, the rotation of the quarks around the color magnetic field produces the strong ``magnetic field''.",0508219v1 2006-09-06,Possible Ferromagnetism in the Large N(c) and N(f) limit of quark matter,"We consider high density quark matter in the large $\N_c$ and $\N_f$ limit with $\N_f/\N_c$ fixed. In this limit, the color superconductivity disappears. We discuss that the chiral density wave state is also absent in the limit, if we assume the existence of the non-perturbative magnetic screening effect as indicated by recent lattice study. We argue that ferromagnetism can become a candidate for the ground state if quarks are massive.",0609060v4 1994-08-24,Equations for Correlation Functions of Eight-Vertex Model: Ferromagnetic and Disordered Phases,"The Kyoto group (Jimbo, Miwa, Nakayashikiet al.) showed that the partition function and correlation funtions of the eight-vertex model in antiferromagnetic phases can be calculated using simple analytical properties of the $R$-matrix. We extend these methods to ferromagnetic and disordered phases. We use Baxter's symmetries to obtain appropriate parametrizations of the $R$-matrix and to substantiate the validity of the analytical approach for these phases. These symmetries allow one to relate correlation functions in different phases.",9408131v2 1996-06-25,Gauge Theory Model -- quark dynamics and anti-ferromagnets,"Two-dimensional QED with N-flavor fermions serves as a model of quark dynamics in QCD as well as an effective theory of an anti-ferromagnetic spin chain. It is reduced to N-degree quantum mechanics in which a potential is self-consistently determined by the Schr\""odinger equation itself.",9606167v1 1996-12-06,Self-Dual Chern-Simons Solitons and Generalized Heisenberg Ferromagnet Models,"We consider the (2+1)-dimensional gauged Heisenberg ferromagnet model coupled with the Chern-Simons gauge fields. Self-dual Chern-Simons solitons, the static zero energy solution saturating Bogomol'nyi bounds, are shown to exist when the generalized spin variable is valued in the Hermitian symmetric spaces G/H. By gauging the maximal torus subgroup of H, we obtain self-dual solitons which satisfy vortex-type nonlinear equations thereby extending the two dimensional instantons in a nontrivial way. An explicit example for the CP(N) case is given.",9612063v1 2001-04-03,Smirnov-type integral formulae for correlation functions of the bulk/boundary XXZ model in the anti-ferromagnetic regime,"Presented are the integral solutions to the quantum Knizhnik-Zamolodchikov equations for the correlation functions of both the bulk and boundary XXZ models in the anti-ferromagnetic regime. The difference equations can be derived from Smirnov-type master equations for correlation functions on the basis of the CTM bootstrap. Our integral solutions with an appropriate choice of the integral kernel reproduce the formulae previously obtained by using the bosonization of the vertex operators of the quantum affine algebra $U_q (\hat{\mathfrak{sl}_2})$.",0104018v1 2003-09-09,Some topological issues for ferromagnets and fluids,"We analyze the canonical structure of a continuum model of ferromagnets and clarify known difficulties in defining a momentum density. The moments of the momentum density corresponding to volume-preserving coordinate transformations can be defined, but a nonsingular definition of the other moments requires an enlargement of the phase space which illuminates a close relation to fluid mechanics. We also discuss the nontrivial connectivity of the phase space for two and three dimensions and show how this feature can be incorporated in the quantum theory, working out the two-dimensional case in some detail.",0309087v1 2005-04-01,Fifth-neighbor spin-spin correlator for the anti-ferromagnetic Heisenberg chain,"We study the generating function of the spin-spin correlation functions in the ground state of the anti-ferromagnetic spin-1/2 Heisenberg chain without magnetic field. We have found its fundamental functional relations from those for general correlation functions, which originate in the quantum Knizhink-Zamolodchikov equation. Using these relations, we have calculated the explicit form of the generating functions up to n=6. Accordingly we could obtain the spin-spin correlator up to k=5.",0504008v3 2005-09-06,A Bethe Ansatz Study of Free Energy and Excitation Spectrum for Even Spin Fateev Zamolodchikov Model,"A Bethe Ansatz study of a self dual Z_N spin model is undertaken for even spin system. One has to solve a coupled system of Bethe Ansatz Equations (BAE) involving zeroes of two families of transfer matrices. A numerical study on finite size lattices is done for identification of elementary excitations over the Ferromagnetic and Antiferromagnetic ground states. The free energies for both Ferromagnetic and Antiferromagnetic ground states and dispersion relation for elementary excitations are found.",0509045v1 2005-09-20,Large-n excitations in the ferromagnetic Ising field theory in a small magnetic field: mass spectrum and decay widths,"In presence of a small magnetic field h, the elementary excitations in the scaling two-dimensional Ising model are studied perturbatively in h in the ferromagnetic phase. For excitations with large numbers n, the mass spectrum is obtained in the first order in h. The decay widths of excitations with energies above the stability threshold are calculated in the leading h^3-order.",0509149v1 2001-01-22,Winterbottom Construction for Finite Range Ferromagnetic Models: An L_1 Approach,"We provide a rigorous microscopic derivation of the thermodynamic description of equilibrium crystal shapes in the presence of a substrate, first studied by Winterbottom. We consider finite range ferromagnetic Ising models with pair interactions in dimensions greater or equal to 3, and model the substrate by a finite-range boundary magnetic field acting on the spins close to the bottom wall of the box.",0101174v1 2004-01-27,An efficient geometric integrator for thermostatted anti-/ferromagnetic models,"(Anti)-/ferromagnetic Heisenberg spin models arise from discretization of Landau-Lifshitz models in micromagnetic modelling. In many applications it is essential to study the behavior of the system at a fixed temperature. A formulation for thermostatted spin dynamics was given by Bulgac and Kusnetsov which incorporates a complicated nonlinear dissipation/driving term while preserving spin length. It is essential to properly model this term in simulation, and simplified schemes give poor numerical performance, e.g. requiring an excessively small timestep for stable integration. In this paper we present an efficient, structure-preserving method for thermostatted spin dynamics.",0401369v1 2006-07-20,A preconditioning strategy for microwave susceptibility in ferromagnets,"3D numerical simulations of ferromagnetic materials can be compared with experimental results via microwave susceptibility. In this paper, an optimised computation of this microwave susceptibility for large meshes is proposed. The microwave susceptibility is obtained by linearisation of the Landau and Lifchitz equations near equilibrium states and the linear systems to be solved are very ill-conditionned. Solutions are computed using the Conjugate Gradient method for the Normal equation (CGN Method). An efficient preconditioner is developed consisting of a projection and an approximation of an ``exact'' preconditioner in the set of circulant matrices. Control of the condition number due to the preconditioning and evolution of the singular value decomposition are shown in the results.",0607493v1 2006-09-08,Control of travelling walls in a ferromagnetic nanowire,"We investigate the problem of controlling the magnetic moment in a ferromagnetic nanowire submitted to an external magnetic field in the direction of the nanowire. The system is modeled with the one dimensional Landau-Lifschitz equation. In the absence of control, there exist particular solutions, which happen to be relevant for practical issues, called travelling walls. In this paper, we prove that it is possible to move from a given travelling wall profile to any other one, by acting on the external magnetic field. The control laws are simple and explicit, and the resulting trajectories are shown to be stable.",0609231v2 2003-02-28,A Polymer Expansion for the Quantum Heisenberg Ferromagnet Wave Function,"A polymer expansion is given for the Quantum Heisenberg Ferromagnet wave function. Working on a finite lattice, one is dealing entirely with algebraic identities; there is no question of convergence. The conjecture to be pursued in further work is that effects of large polymers are small. This is relevant to the question of the utility of the expansion and its possible extension to the infinite volume. In themselves the constructions of the present paper are neat and elegant and have surprising simplicity.",0302067v2 2003-03-13,A Polymer Expansion for the Random Walk on the Permutation Group Associated to the Quantum Heisenberg Ferromagnet,"For a long time one has associated to the Quantum Heisenberg Ferromagnet on a lattice, a random walk on the permutation group of the lattice vertices. We here present a polymer expansion for the solution of the heat equation coupled to the random walk. We work on a finite lattice, there is no question of convergence. We leave to future work bounding terms in the expansion necessary to extend the result to an infinite lattice.",0303036v1 2003-07-24,The large-spin asymptotics of the ferromagnetic XXZ chain,"We present new results and give a concise review of recent previous results on the asymptotics for large spin of the low-lying spectrum of the ferromagnetic XXZ Heisenberg chain with kink boundary conditions. Our main interest is to gain detailed information on the interface ground states of this model and the low-lying excitations above them. The new and most detailed results are obtained using a rigorous version of bosonization, which can be interpreted as a quantum central limit theorem.",0307051v1 2001-07-25,Magnetization Reversal in Ferromagnetic Film Through Solitons by Electromagnetic Field,"We study the reversal of magnetization in an isotropic ferromagnetic film free from charges by exposing it to a circularly polarized electromagnetic (EM) field. The magnetization excitations obtained in the form of line and lump solitons of the completely integrable modified KP-II equation which is derived using a reductive perturbation method from the set of coupled Landau-Lifshitz and Maxwell equations. It is observed that when the polarization of the EM-field is reversed followed by a rotation, for every $ {\pi \over 2} $-degrees, the magnetization is reversed.",0107058v1 2003-08-14,Dynamic Domains in Strongly Driven Ferromagnetic Films,"The spatiotemporal structure formation problem is investigated in the region far above the transverse ferromagnetic resonance instability. The investigations are based on the dissipative Landau-Lifshitz equation and have been performed on a model which takes external fields, isotropic exchange fields, anisotropy fields and the demagnetizing part of the dipolar field into consideration. The numerical simulations for these models exhibit stationary domain structure in the rotating frame. Employing analytical methods and simplifying the model, certain features, such as the magnetization within the domains and the proportion of the system in each domain, are described analytically.",0308017v1 2000-10-05,Ferromagnetism of Nuclear Matter in the Relativistic Approach,"We study the spin-polarization mechanism in the highly dense nuclear matter with the relativistic mean-field approach. In the relativistic Hartree-Fock framework we find that there are two kinds of spin-spin interaction channels, which are the axial-vector and tensor exchange ones. If each interaction is strong and different sign, the system loses the spherical symmetry and holds the spin-polarization in the high-density region. When the axial-vector interaction is negative enough, the system holds ferromagnetism.",0010018v2 2004-06-02,Ultra-cold Neutron Production in Anti-ferromagnetic Oxygen Solid,"Spin waves, or magnons, in the anti-ferromagnetic $\alpha$ phase of solid oxygen provide a novel mechanism for ultra-cold neutron (UCN) production. Magnons dominate the energy exchange mechanisms for cold neutrons and UCN in solid $\alpha$-oxygen, much in the same way as do phonons in solid deuterium superthermal UCN sources. We present calculations of UCN production and upscattering rates in S-O$_2$. The results indicate that S-O$_2$ is potentially a much more efficient UCN source material than solid deuterium.",0406004v1 2006-04-24,Ferromagnetic instabilities in neutron matter at finite temperature with the Gogny interaction,"The properties of spin polarized neutron matter are studied both at zero and finite temperature using the D1 and the D1P parameterizations of the Gogny interaction. The results show two different behaviors: whereas the D1P force exhibits a ferromagnetic transition at a density of $\rho_c \sim 1.31$ fm$^{-3}$ whose onset increases with temperature, no sign of such a transition is found for D1 at any density and temperature, in agreement with recent microscopic calculations.",0604058v2 2003-02-14,"Retarded Electromagnetic Interaction and Origin of Non-linear Phenomena in Ferroelectrics, Ferromagnetics and Optics","The non-linear relations between polarization strength and electric field strength for ferroelectrics, as well as magnetization strength and magnetic field strength for ferromagnetics, can be achieved by introducing retarded electromagnetic interactions. The electric and magnetic hysteretic loops, as well as polarization and magnetization curves, can be described well based on soft mode theory in the alternating electromagnetic fields of low and medium frequencies. The fundamental phenomenological formula in non-linear optics can also be obtained in high frequency fields. The results show that all of these non-linear phenomena originate from retarded electromagnetic interaction.",0302048v1 2006-09-22,Application of atomic magnetometry in magnetic particle detection,"We demonstrate the detection of magnetic particles carried by water in a continuous flow using an atomic magnetic gradiometer. Studies on three types of magnetic particles are presented: a single cobalt particle (diameter ~150 um, multi-domain), a suspension of superparamagnetic magnetite particles (diameter \~1 um), and ferromagnetic cobalt nanoparticles (diameter ~10 nm, 120 kA/m magnetization). Estimated detection limits are 20 um diameter for a single cobalt particle at a water flow rate 30 ml/min, 5x10^3 magnetite particles at 160 ml/min, and 50 pl for the specific ferromagnetic fluid at 130 ml/min. Possible applications of our method are discussed.",0609197v1 2004-04-05,Logarithmic divergence of the block entanglement entropy for the ferromagnetic Heisenberg model,"Recent studies have shown that logarithmic divergence of entanglement entropy as function of size of a subsystem is a signature of criticality in quantum models. We demonstrate that the ground state entanglement entropy of $ n$ sites for ferromagnetic Heisenberg spin-1/2 chain of the length $L$ in a sector with fixed magnetization $y$ per site grows as ${1/2}\log_{2} \frac{n(L-n)}{L}C(y)$, where $C(y)=2\pi e({1/4}-y^{2})$",0404026v1 2004-07-09,Thermal entanglement of spins in a nonuniform magnetic field,"We study the effect of inhomogeneities in the magnetic field on the thermal entanglement of a two spin system. We show that in the ferromagnetic case a very small inhomogeneity is capable to produce large values of thermal entanglement. This shows that the absence of entanglement in the ferromagnetic Heisenberg system is highly unstable against inhomogeneoity of magnetic fields which is inevitably present in any solid state realization of qubits.",0407073v3 2004-12-15,Thermal effects on quantum communication through spin chains,"We study the effect of thermal fluctuations in a recently proposed protocol for transmission of unknown quantum states through quantum spin chains. We develop a low temperature expansion for general spin chains. We then apply this formalism to study exactly thermal effects on short spin chains of four spins. We show that optimal times for extraction of output states are almost independent of the temperature which lowers only the fidelity of the channel. Moreover we show that thermal effects are smaller in the anti-ferromagnetic chains than the ferromagnetic ones.",0412115v2 1999-07-22,Multi-Field Integrable Systems Related to WKI-Type Eigenvalue Problems,"Higher flows of the Heisenberg ferromagnet equation and the Wadati-Konno-Ichikawa equation are generalized into multi-component systems on the basis of the Lax formulation. It is shown that there is a correspondence between the multi-component systems through a gauge transformation. An integrable semi-discretization of the multi-component higher Heisenberg ferromagnet system is obtained.",9907018v1 2007-04-05,Correlation functions and excitation spectrum of the frustrated ferromagnetic spin-1/2 chain in an external magnetic field,"Magnetic field effects on the one-dimensional frustrated ferromagnetic chain are studied by means of effective field theory approaches in combination with numerical calculations utilizing Lanczos diagonalization and the density matrix renormalization group method. The nature of the ground state is shown to change from a spin-density-wave region to a nematic-like one upon approaching the saturation magnetization. The excitation spectrum is analyzed and the behavior of the single spin-flip excitation gap is studied in detail, including the emergent finite-size corrections.",0704.0764v2 2007-04-09,Electron Transport in Nanogranular Ferromagnets,"We study electronic transport properties of ferromagnetic nanoparticle arrays and nanodomain materials near the Curie temperature in the limit of weak coupling between the grains. We calculate the conductivity in the Ohmic and non-Ohmic regimes and estimate the magnetoresistance jump in the resistivity at the transition temperature. The results are applicable for many emerging materials, including artificially self-assembled nanoparticle arrays and a certain class of manganites, where localization effects within the clusters can be neglected.",0704.1167v2 2007-04-22,Sm2Al: Another ferromagnet with spin-orbital compensation?,"We report the observation of a compensation point in the temperature dependence of magnetization data of polycrystalline Sm2Al. Magnetization measurements show that this compound magnetically orders at about 150 K. Below this temperature, the magnetization data shows a compensation point, which shifts with field. Hysteresis loops obtained below the compensation temperature show that the compound possesses exchange anisotropy. Both the exchange anisotropy field and the coercive field are found to be quite large and comparable to those of the classical spin-orbit compensated ferromagnet (Sm,Gd)Al2. The heat capacity data also suggest that there are similarities between (Sm,Gd)Al2 and Sm2Al.",0704.2864v1 2007-05-04,Theory of a Magnetically-Controlled Quantum-Dot Spin Transistor,"We examine transport through a quantum dot coupled to three ferromagnetic leads in the regime of weak tunnel coupling. A finite source-drain voltage generates a nonequilibrium spin on the otherwise non-magnetic quantum dot. This spin accumulation leads to magnetoresistance. A ferromagnetic but current-free base electrode influences the quantum-dot spin via incoherent spin-flip processes and coherent spin precession. As the dot spin determines the conductance of the device, this allows for a purely magnetic transistor-like operation. We analyze the effect of both types of processes on the electric current in different geometries.",0705.0648v2 2007-05-25,Ferrimagnetism and antiferromagnetism in half-metallic Heusler alloys,"Half-metallic Heusler alloys are among the most promising materials for future applications in spintronic devices. Although most Heusler alloys are ferromagnets, ferrimagnetic or antiferromagnetic (also called fully-compensated ferrimagnetic) alloys would be more desirable for applications due to the lower external fields. Ferrimagnetism can be either found in perfect Heusler compounds or achieved through the creation of defects in ferromagnetic Heusler alloys.",0705.3731v1 2007-06-02,Thermodynamic Properties of Ferromagnetic Mott- Insulators GaV4S8,"We present the results of the magnetic and specific heat measurements on V4 tetrahedral-cluster compound GaV4S8 between 2 to 300K. We find two transitions related to a structural change at 42K followed by ferromagnetic order at 12K on cooling. Remarkably similar properties were previously reported for the cluster compounds of Mo4. These compounds show an extremely high density of low energy excitations in their electronic properties. We explain this behavior in a cluster compound as due to the reduction of coulomb repulsion among electrons that occupy highly degenerate orbits of different clusters.",0706.0265v1 2007-06-02,Giant magnetoresistance in nanoscale ferromagnetic heterocontacts,"A quasiclassical theory of giant magnetoresistance in nanoscale point contacts between different ferromagnetic metals is developed. The contacts were sorted by three types of mutual positions of the conduction spin-subband bottoms which are shifted one against another by the exchange interaction. A model of linear domain wall has been used to account for the finite contact length. The magnetoresistance is plotted against the size of the nanocontact. In heterocontacts the magnetoresistance effect turned out to be not only negative, as usual, but can be positive as well. Relevance of the results to existing experiments on GMR in point heterocontacts is discussed.",0706.0266v1 2007-06-12,Theory of the Pseudospin Resonance in Semiconductor Bilayers,"The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic resonance mode of a ferromagnet. We present a theory of the dependence of the energy and the damping of this mode on layer separation $d$. Based on these results, we discuss the possibility of realizing transport-current driven pseudospin-transfer oscillators in semiconductors.",0706.1702v1 2007-06-14,"Magnetic patterning of (Ga,Mn)As by hydrogen passivation","We present an original method to magnetically pattern thin layers of (Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the hole density, and thereby locally suppress the carrier-mediated ferromagnetic phase. The sample surface is thus maintained continuous, and the minimal structure size is of about 200 nm. In micron-sized ferromagnetic dots fabricated by hydrogen passivation on perpendicularly magnetized layers, the switching fields can be maintained closer to the continuous film coercivity, compared to dots made by usual dry etch techniques.",0706.2138v1 2007-07-12,Bistability in superconducting rings containing an inhomogeneous Josephson junction,"We investigate the magnetic response of a superconducting Nb ring containing a ferromagnetic PdNi Josephson junction and a tunnel junction in parallel. A doubling of the switching frequency is observed within certain intervals of the external magnetic field. Assuming sinusoidal current-phase relations of both junctions our model of a dc-SQUID embedded within a superconducting ring explains this feature by a sequence of current reversals in the ferromagnetic section of the junction in these field intervals. The switching anomalies are induced by the coupling between the magnetic fluxes in the two superconducting loops.",0707.1791v1 2007-07-16,Ferromagnetism of an all-carbon composite composed of a carbon nanowire nside a single-walled carbon nanotube,"Using the first-principles spin density functional approach, we have studied magnetism of a new type of all-carbon nanomaterials, i.e., the carbon nanowires inserted into the single-walled carbon nanotubes. It is found that if the 1D carbon nanowire density is not too higher, the ferromagnetic ground state will be more stable than the antiferromagnetic one, which is caused by weak coupling between the 1D carbon nanowire and the single-walled carbon nanotube. Also, both dimerization of the carbon nanowire and carbon vacancy on the tube-wall are found to enhance the magnetic moment of the composite.",0707.2386v2 2007-07-17,Subnanosecond switching of local spin-exchange coupled to ferromagnets,"The dynamics of a single spin embedded in a the tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Using time-dependent equation of motion for the spin under influence of the spin-polarized tunneling current, it is shown that the magnetic field induced by bias voltage pulses allows for sub-nanosecond switching of the local spin and the possibility of spin reversal is illustrated. Furthermore, it is shown that the time-evolution of the Larmor frequency sharply peaks around the spin-flip event, and it is argued that this feature can be used as an indicator for the spin-flip.",0707.2600v2 2007-07-25,Exchange shift of stripe domains in antiferromagnetically coupled multilayers,"Antiferromagnetically coupled multilayers with perpendicular anisotropy, as [CoPt]/Ru, Co/Ir, Fe/Au, display ferromagnetic stripe phases as the ground states. It is theoretically shown that the antiferromagnetic interlayer exchange causes a relative shift of domains in adjacent layers. This ``exchange shift'' is responsible for several recently observed effects: an anomalous broadening of domain walls, the formation of so-called ``tiger-tail'' patterns, and a ``mixed state'' of antiferromagnetic and ferromagnetic domains in [CoPt]/Ru multilayers. The derived analitical relations between the values of the shift and the strength of antiferromagnetic coupling provide an effective method for a quantitative determination of the interlayer exchange interactions.",0707.3765v1 2007-07-26,"Room temperature Ferromagnetism in Th1-xFexO2-d (x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25) nanoparticles","Nanocrystalline (Th1-xFex)O2-d particles with different Fe concentrations (x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25) have been prepared by a gel combustion method. Rietveld refinement analyses of X-ray diffraction data revealed the formation of an impurity free cubic type Th1-xFexO2-d structure up to x = 0.20. This observation is further confirmed from the detailed studies conducted on 10 at. percent Fe doped ThO2 using high-resolution transmission electron microscopy (HRTEM) imaging and indexing of the selected-area electron diffraction (SAED) patterns. DC Magnetization studies as a function field indicate that they are ferromagnetic with Curie temperature (Tc) well above room temperature.",0707.3883v1 2007-07-27,"Photo-induced precession of magnetization in ferromagnetic (Ga,Mn)As","Precession of magnetization induced by pulsed optical excitation is observed in a ferromagnetic semiconductor (Ga,Mn)As by time-resolved magneto-optical measurements. It appears as complicated oscillations of polarization plane of linearly-polarized probe pulses, but is reproduced by gyromagnetic theory incorporating an impulsive change in an effective magnetic field due to changes in magnetic anisotropy. It is inferred from the shape of the impulse that the changes in anisotropy result from non-equilibrium carrier population: cooling of hot photo-carriers and subsequent annihilation of photo-carriers.",0707.4055v2 2007-07-28,Performances of a Newly High Sensitive Trilayer F/Cu/F GMI Sensor,"We have selected stress-annealed nanocrystalline Fe-based ribbons for ferromagnetic/copper/ferromagnetic sensors exhibiting high magneto-impedance ratio. Longitudinal magneto-impedance reaches 400% at 60 kHz and longitudinal magneto-resistance increases up to 1300% around 200 kHz.",0707.4232v2 2007-08-02,Simultaneous ferromagnetic metal-semiconductor transition in electron-doped EuO,"We present a general framework to describe the simultaneous para-to-ferromagnetic and semiconductor-to-metal transition in electron-doped EuO. The theory correctly describes detailed experimental features of the conductivity and of the magnetization, in particular the doping dependence of the Curie temperature. The existence of correlation-induced local moments on the impurity sites is essential for this description.",0708.0416v2 2007-08-06,Multi-magnon bound states in the frustrated ferromagnetic 1D chain,"We study a one-dimensional Heisenberg chain with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest neighbor interactions in magnetic field. Starting from the fully polarized high-field state, we calculate the dispersions of the lowest-lying $n$-magnon excitations and the saturation field (n=2,3,4). We show that the lowest-lying excitations are always bound multi-magnon states with a total momentum of $\pi$ except for a small parameter range. We argue that bose condensation of the bound $n$ magnons leads to novel Tomonaga-Luttinger liquids with multi-polar correlations; nematic and triatic ordered liquids correspond to n=2 and n=3.",0708.0701v1 2007-08-14,"Random telegraph noise from magnetic nanoclusters in the ferromagnetic semiconductor (Ga,Mn)As","Measurements of the low frequency electrical noise in the ferromagnetic semiconductor (Ga,Mn)As reveal an enhanced integrated noise at low temperature. For moderate localization, we find a 1/f normalized power spectrum density over the entire range of temperatures studied (4.2K < T < 70K). However, for stronger localization and a high density of Mn interstitials, we observe Lorentzian noise spectra accompanied by random telegraph noise. Magnetic field dependence and annealing studies suggest that interstitial Mn defects couple with substitutional Mn atoms to form nanoscale magnetic clusters characterized by a net moment of about 20 Bohr magnetons whose fluctuations modulate hole transport.",0708.1895v1 2007-08-15,Spin Valve Effect in Self-exchange Biased Ferromagnetic Metal/Semiconductor Bilayers,"We report magnetization and magetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures comprised of MnAs/(Ga,Mn)As bilayers. Our measurements show that the (metallic) MnAs and (semiconducting) (Ga,Mn)As layers are exchange coupled, re- sulting in an exchange biasing of the magnetically softer (Ga,Mn)As layer that weakens with layer thickness. Magnetoresistance measurements in the current-perpendicular-to-the-plane geometry show a spin valve effect in these self-exchange biased bilayers. Similar measurements in MnAs/p- GaAs/(Ga,Mn)As trilayers show that the exchange coupling diminishes with spatial separation between the layers.",0708.2092v1 2007-08-17,Observation of Ferromagnetic Clusters in Bi0.125Ca0.875MnO3,"The electron doped manganite system, Bi0.125Ca0.875MnO3, exhibits large bulk magnetization of unknown origin. To select amongst possible magnetic ordering models, we have conducted temperature and magnetic field dependent small-angle neutron scattering measurements. Nontrivial spin structure has been revealed. Ferromagnetic spin clusters form in the antiferromagnetic background when temperature is decreased to Tc~108K. With a further reduction in temperature or the application of external magnetic field, the clusters begin to form in larger numbers, which gives an overall enhancement of magnetization below Tc.",0708.2300v1 2007-08-27,Weakly anisotropic frustrated zigzag spin chain,"The frustrated spin-1/2 model with weakly anisotropic ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is studied with use of variational mean-field approach, scaling estimates of the infrared divergencies in the perturbation theory and finite-size calculations. The ground state phase diagram of this model contains three phases: the ferromagnetic phase, the commensurate spin-liquid phase and the incommensurate phase. The non-trivial behavior of the boundaries between these phases and the character of the phase transitions in case of weak anisotropy are determined.",0708.3555v1 2007-08-30,Reentrant transitions of Ising-Heisenberg ferromagnet on a triangular lattice with diamond-like decorations,"The mixed spin-1/2 and spin-1 Ising-Heisenberg ferromagnet on the decorated triangular lattice consisting of inter-connected diamonds is investigated within the framework of an exact decoration-iteration mapping transformation. It is shown that the diamond-like decoration by a couple of the Heisenberg spins gives rise to a diverse critical behaviour including reentrant phase transitions with two consecutive critical points.",0708.4103v1 2007-08-30,Quadratic operators used in deducing exact ground states for correlated systems: ferromagnetism at half filling provided by a dispersive band,"Quadratic operators are used in transforming the model Hamiltonian (H) of one correlated and dispersive band in an unique positive semidefinite form coopting both the kinetic and interacting part of H. The expression is used in deducing exact ground states which are minimum energy eigenstates only of the full Hamiltonian. It is shown in this frame that at half filling, also dispersive bands can provide ferromagnetism in exact terms by correlation effects .",0708.4117v1 2007-09-19,Spin-dependent tunneling and Coulomb blockade in ferromagnetic nanoparticles,"We review studies on spin-dependent tunneling phenomena in systems containing ferromagnetic nanoparticles. We discuss preparation methods of assembling nanoparticles as well as the mechanisms and results of spin-dependent transport properties. The emphasis of this review is on characteristic spin-dependent tunneling phenomena such as enhanced tunnel magnetoresistance (TMR) due to co-tunneling in the Coulomb blockade regime and sign changes of the TMR due to spin accumulation in nanoparticles.",0709.2940v1 2007-09-22,Ferromagnetic behavior in magnetized plasmas,"We consider a low-temperature plasma within a newly developed MHD Fluid model. In addition to the standard terms, the electron spin, quantum particle dispersion and degeneracy effects are included. It turns out that the electron spin properties can give rise to Ferromagnetic behavior in certain regimes. If additional conditions are fulfilled, a homogenous magnetized plasma can even be unstable. This happen in the low-temperature high-density regime, when the magnetic properties associated with the spin can overcome the stabilizing effects of the thermal and Fermi pressure, to cause a Jeans like instability.",0709.3575v1 2007-09-24,Magnetoresistance in an all-manganite heterostructure,"We study the magnetic and transport properties of all-manganite heterostructures consisting of ferromagnetic metallic electrodes separated by an antiferromagnetic barrier. We find that the magnetic ordering in the barrier is influenced by the relative orientation of the electrodes magnetization producing a large difference in resistance between the parallel and antiparallel orientations of the ferromagnetic layers. The external application of a magnetic field in a parallel configuration also leads to large magnetoresistance.",0709.3720v1 2007-09-26,Single crystal growth and anisotropy of CeRuPO,"We report on the single crystal growth of the ferromagnetic Kondo lattice system CeRuPO using a Sn flux method. Magnetic susceptibility and electrical resistivity measurements indicate strong anisotropy of this structurally layered compound. They evidence that the magnetic moments order ferromagnetically along the c-direction of the tetragonal unit cell, whereas the crystal electric field (CEF) anisotropy favors the ab-plane. Therefore, CeRuPO presents the unusual case within rare earth systems, where the anisotropy of the interionic exchange interaction overcomes the single ion anisotropy due to the CEF interaction.",0709.4144v2 2007-10-04,Time dilation of a bound half-fluxon pair in a long Josephson junction with a ferromagnetic insulator,"The fluxon dynamics in a long Josephson junction with a ferromagnetic insulating layer is investigated. It is found that the Josephson phase obeys a double sine-Gordon equation involving a bound pi fluxon solution, and the internal oscillations of the bound pair acting as a clock exhibit Lorentz reductions in their frequencies regarded as a relativistic effect in the time domain, i.e., time dilation. This is the complement to the Lorentz contraction of fluxons with no clock. A possible observation scheme is also discussed.",0710.0947v1 2007-10-06,Modulated phases in magnetic models frustrated by long-range interactions,"We study an Ising model in one dimension with short range ferromagnetic and long range (power law) antiferromagnetic interactions. We show that the zero temperature phase diagram in a (longitudinal) field H involves a sequence of up and down domains whose size varies continuously with H, between -H_c and H_c which represent the edge of the ferromagnetic up and down phases. The implications of long range interaction in many body systems are discussed.",0710.1341v1 2007-10-11,Direct current voltage induced by microwave signal in a ferromagnetic wire,"Experimental results of rectification of a constant wave radio frequency (RF) current flowing in a single-layered ferromagnetic wire are presented. We show that a detailed external magnetic field dependence of the RF current induced a direct-current voltage spectrum. The mechanism of the rectification is discussed in a term of the spin transfer torque, and the rectification is closely related to resonant spin wave excitation with the assistant of the spin-polarized RF current. The micromagnetic simulation taking into account the spin transfer torque provides strong evidence which supports the generation of spin wave excitation by the RF current.",0710.2172v1 2007-10-13,Rotation of easy axis in training effect and recovery of exchange bias in ferromagnet/antiferromagnet bilayers,"For ferromagnet/antiferromagnet bilayers, rotation of the easy axis has been \textit{for the first time} observed during measurements of training effect and the recovery of exchange bias using FeNi/FeMn system. These salient phenomena strongly suggest irreversible motion of antiferromagnet spins during subsequent measurements of hysteresis loops. It is found that the rotation of the easy axis can partly account for the training effect and the recovery of the exchange bias.",0710.2594v1 2007-10-15,Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films,"Ferromagnetic resonance has been used to study the magnetic properties and magnetization dynamics of polycrystalline Fe$_{1-x}$V$_{x}$ alloy films with $0\leq x < 0.7$. Films were produced by co-sputtering from separate Fe and V targets, leading to a composition gradient across a Si substrate. FMR studies were conducted at room temperature with a broadband coplanar waveguide at frequencies up to 50 GHz using the flip-chip method. The effective demagnetization field $4 \pi M_{\mathrm{eff}}$ and the Gilbert damping parameter $\alpha$ have been determined as a function of V concentration. The results are compared to those of epitaxial FeV films.",0710.2826v2 2007-11-05,Magnetization easy-axis in martensitic Heusler alloys estimated by strain measurements under magnetic-field,"We study the temperature dependence of strain under constant magnetic-fields in Ni-Mn based ferromagnetic Heusler alloys in the form Ni-Mn-$X$ ($X$: Ga, In, Sn, Sb) which undergo a martensitic transformation. We discuss the influence of the applied magnetic-field on the nucleation of ferromagnetic martensite and extract information on the easy-axis of magnetization in the martensitic state.",0711.0639v1 2007-11-08,Ferroelectrically induced weak-ferromagnetism in a single-phase multiferroic by design,"We present a strategy to design structures for which a polar lattice distortion induces weak ferromagnetism. We identify a large class of multiferroic oxides as potential realizations and use density-functional theory to screen several promising candidates. By elucidating the interplay between the polarization and the Dzyaloshinskii-Moriya vector, we show how the direction of the magnetization can be switched between 180$^{\circ}$ symmetry equivalent states with an applied electric field.",0711.1331v2 2007-11-19,Diluted antiferromagnet in a ferromagnetic enviroment,"The question of robustness of a network under random ``attacks'' is treated in the framework of critical phenomena. The persistence of spontaneous magnetization of a ferromagnetic system to the random inclusion of antiferromagnetic interactions is investigated. After examing the static properties of the quenched version (in respect to the random antiferromagnetic interactions) of the model, the persistence of the magnetization is analysed also in the annealed approximation, and the difference in the results are discussed.",0711.3016v1 2007-11-21,"Electron-electron interaction in 2D and 1D ferromagnetic (Ga,Mn)As","We investigated the magnetotransport in high quality ferromagnetic (Ga,Mn)As films and wires. At low temperature the conductivity decreases with decreasing temperature without saturation down to 20 mK. Here we show, that the conductivity decrease follows a ln($T/T_0$) dependency in 2D films and a $-1/\sqrt{T}$ dependency in 1D wires and is independent of an applied magnetic field. This behavior can be explained by the theory of electron-electron interaction.",0711.3278v2 2007-11-29,On the question of ferromagnetism in alkali metal thin films,"Electronic and magnetic structure of $(100)$ films of K and Cs are calculated within the plane-wave projector augmented wave (PAW) formalism of the density functional theory (DFT) using both local spin density approximation (LSDA) and the PW91 generalized gradient approximation (GGA). Only a 6 layer Cs film is found to have a ferromagnetic (FM) state which is degenerate with a paramagnetic (PM) state within the accuracy of these calculations. This is at variance with the results obtained from a finite thickness uniform jellium model (UJM). Implications of these results for the experiments on transition metal doped alkali metal thin films and bulk hosts are discussed.",0711.4744v1 2007-11-30,Large voltage from spin pumping in magnetic tunnel junctions,"We studied the response of a ferromagnet-insulator-normal metal tunnel structure under an external oscillating radio frequency (R.F.) magnetic field. The D. C. voltage across the junction is calculated and is found not to decrease despite the high resistance of the junction; instead, it is of the order of $\mu V$ to $100\mu V$, much larger than the experimentally observed value (100 nano-V) in the ""strong coupled"" ohmic ferromagnet-normal metal bilayers. This is consistent with recent experimental results in tunnel structures, where the voltage is larger than $\mu V$s. The damping and loss of an external RF field in this structure is calculated.",0711.4939v1 2007-12-06,Two-band ferromagnetic Kondo-lattice model for local-moment half-metals,"We introduce a two-band Kondo-lattice model to describe ferromagnetic half-metals with local magnetic moments. In a model study, the electronic and magnetic properties are presented by temperature dependent magnetization curves, band-structures, spin polarizations and plasma frequencies. These are obtained from numerically evaluated equations, based on the single-electron Green functions. We show that the mutual influence between the itinerant electrons and the local magnetic moments is responsible for several phase transitions of the half-metals, namely first and second order magnetic phase transitions, as well as half-metal to semiconductor and half-metal to semimetal transitions.",0712.0905v1 2007-12-17,Proximity effect-assisted absorption of spin currents in superconductors,"The injection of pure spin current into superconductors by the dynamics of a ferromagnetic contact is studied theoretically. Taking into account suppression of the order parameter at the interfaces (inverse proximity effect) and the energy-dependence of spin-flip scattering, we determine the temperature-dependent ferromagnetic resonance linewidth broadening. Our results agree with recent experiments in Nb|permalloy bilayers [C. Bell et al., arXiv:cond-mat/0702461].",0712.2814v1 2007-12-19,Energy spectrum of bound-spinons in the quantum Ising spin-chain ferromagnet,"We study the excitation energy spectrum in the S=1/2 ferromagnetic Ising spin chain with the easy axis z in a magnetic field h={h_x,0,h_z}. According to Wu and McCoy's scenario of weak confinement, the fermionic spinon excitations (kinks), being free at h_z = 0 in the ordered phase, are coupled into bosonic bound states at arbitrary small h_z. We calculate the energy spectrum of such excitations in the leading order in small h_z, using different perturbative methods developed for the similar problem in the Ising field theory.",0712.3189v1 2007-12-20,Spin-dependent diffraction at ferromagnetic/spin spiral interface,"Spin-dependent transport is investigated in ballistic regime through the interface between a ferromagnet and a spin spiral. We show that spin-dependent interferences lead to a new type of diffraction called ""spin-diffraction"". It is shown that this spin-diffraction leads to local spin and electrical currents along the interface. This study also shows that in highly non homogeneous magnetic configuration (non adiabatic limit), the contribution of the diffracted electrons is crucial to describe spin transport in such structures.",0712.3351v1 2008-01-14,The Transmission Property of the Discrete Heisenberg Ferromagnetic Spin Chain,"We present a mechanism for displaying the transmission property of the discrete Heisenberg ferromagnetic spin chain (DHF) via a geometric approach. By the aid of a discrete nonlinear Schr\""odinger-like equation which is the discrete gauge equivalent to the DHF, we show that the determination of transmitting coefficients in the transmission problem is always bistable. Thus a definite algorithm and general stochastic algorithms are presented. A new invariant periodic phenomenon of the non-transmitting behavior for the DHF, with a large probability, is revealed by an adoption of various stochastic algorithms.",0801.2060v1 2008-01-14,Weak Ferromagnetic Exchange and Anomalous Specific Heat in ZnCu3(OH)6Cl2,"Experimental evidence for a plethora of low energy spin excitations in the spin-1/2 kagome antiferromagnet ZnCu3(OH)6Cl2 may be understandable in terms of an extended Fermi surface of spinons coupled to a U(1) gauge field. We carry out variational calculations to examine the possibility that such a state may be energetically viable. A Gutzwiller-projected wavefunction reproduces the dimerization of a kagome strip found previously by DMRG. Application to the full kagome lattice shows that the inclusion of a small ferromagnetic next-nearest-neighbor interaction favors a ground state with a spinon Fermi surface.",0801.2138v2 2008-01-15,Broadband ferromagnetic resonance of Ni81Fe19 wires using a rectifying effect,"The broadband ferromagnetic resonance measurement using the rectifying effect of Ni81Fe19 wire has been investigated. One wire is deposited on the center strip line of the coplanar waveguide (CPW) and the other one deposited between two strip lines of CPW. The method is based on the detection of the magnetoresistance oscillation due to the magnetization dynamics induced by the radio frequency field. The magnetic field dependences of the resonance frequency and the rectification spectrum are presented and analytically interpreted on the standpoint of a uniform magnetization precession model.",0801.2203v1 2008-01-18,Selective Spin Injection Controlled by Electrical way in Ferromagnet/Quantum Dot/Semiconductor system,"Selective and large polarization of current injected into semiconductor (SC) is predicted in Ferromagnet (FM)/Quantum Dot (QD)/SC system by varying the gate voltage above the Kondo temperature. In addition, spin-dependent Kondo effect is also revealed below Kondo temperature. It is found that Kondo resonances for up spin state is suppressed with increasing of the polarization P of the FM lead. While the down one is enhanced. The Kondo peak for up spin is disappear at P=1.",0801.2922v1 2008-01-25,Ferromagnetism and Kondo Insulator Behavior in the Disordered Periodic Anderson Model,"The effect of binary alloy disorder on the ferromagnetic phases of f-electron materials is studied within the periodic Anderson model. We find that disorder in the conduction band can drastically enhance the Curie temperature due to an increase of the local f-moment. The effect may be explained qualitatively and even quantitatively by a simple theoretical ansatz. The emergence of an alloy Kondo insulator at non-integer filling is also pointed out.",0801.3934v1 2008-01-25,"Optimization of parameters of nanostructure for study inverse proximity effects on ""superconductor-ferromagnetic"" interface using Polarized Neutron Reflectometry in enhanced standing wave regime","This work is devoted to experimental study of influence of superconductivity (S) on ferromagnetism (FM) (inverse proximity effects) with the help of Polarized Neutron Reflectivity. Combining meausurements of specular and diffuse intensities it is possible to obtain full picture of magnetization change in S/FM layered systems like magnetization rotation, domain state formation, inducing of magnetization in S layer, etc. To increase weak magnetic signal we propose to use enhanced neutron standing wave regime (e.g. waveguides). Choose of materials, optimization of thicknesses of layers, estimation of roughnesses influence is presented in this work.",0801.3967v1 2008-01-29,Comparative study of ordered and disordered Y1-xSrxCoO3-d,"We have succeeded in preparing A-site ordered- and disordered-Y1/4Sr3/4CoO3-d with various oxygen deficiencies delta, and have made comparative study of their structural and physical properties. In the A-site ordered structure, oxygen vacancies order, and d = 0.34 sample shows a weak ferromagnetic transition beyond 300 K. On the other hand, in the A-site disordered structure, no oxygen vacancy ordering is observed, and d = 0.16 sample shows a ferromagnetic metallic transition around 150 K. A-site disordering destroys the orderings of oxygen-vacancies and orbitals, leading to the strong modification of the electronic phases.",0801.4445v1 2008-02-08,Fractional periodicity and magnetism of extended quantum rings,"The magnetic properties and nature of the persistent current in small flux-penetrated $t-t'-U$ rings are investigated. An effective rigid-rotator description is formulated for this system, which coincides with a transition to a ferromagnetic state in the model. The criteria for the onset of effective rigid rotation is given. The model is used to understand continuum model ground-state solutions for a 2D few-particle hard-wall quantum dot, where ferromagnetic solutions are found even without the Zeeman coupling to spin. After the onset of effective rigid rotation, a 97--98% correspondence can be determined between the lattice model and continuum model eigenstate results.",0802.1108v1 2008-02-19,Small dimensional microstrips embedded with ferromagnetic layers: Numerical simulations and experimental results,"We use a numerical electromagnetic simulation software to investigate a filtering device consisting of a small dimensional microstrips embedded with a thin layer of ferromagnetic material and we compare our results to experimental results. We are able to show good correlation of simulation versus experiment for the magnitude of insertion loss and phase shift. The microstrips considered have dimensions on the order of the skin depth of the conductor and hence the field distribution is not easily calculated by analytic methods. We show that numerical simulation methods provide an accurate means of characterizing these structures.",0802.2599v1 2008-02-26,Electronic Transport Properties of the Ising Quantum Hall Ferromagnet in a Si Quantum Well,"Magnetotransport properties are investigated for a high mobility Si two dimensional electron systems in the vicinity of a Landau level crossing point. At low temperatures, the resistance peak having a strong anisotropy shows large hysteresis which is attributed to Ising quantum Hall ferromagnetism. The peak is split into two peaks in the paramagnetic regime. A mean field calculation for the peak positions indicates that electron scattering is strong when the pseudospin is partially polarized. We also study the current-voltage characteristics which exhibit a wide voltage plateau.",0802.3756v2 2008-03-03,Laser-induced Precession of Magnetization in GaMnAs,"We report on the photo-induced precession of the ferromagnetically coupled Mn spins in (Ga,Mn)As, which is observed even with no external magnetic field applied. We concentrate on various experimental aspects of the time-resolved magneto-optical Kerr effect (TR-MOKE) technique that can be used to clarify the origin of the detected signals. We show that the measured data typically consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in the sample.",0803.0320v1 2008-03-05,Distribution of magnetic domain pinning fields in GaMnAs ferromagnetic films,"Using the angular dependence of the planar Hall effect in GaMnAs ferromagnetic films, we were able to determine the distribution of magnetic domain pinning fields in this material. Interestingly, there is a major difference between the pinning field distribution in as-grown and in annealed films, the former showing a strikingly narrower distribution than the latter. This conspicuous difference can be attributed to the degree of non-uniformity of magnetic anisotropy in both types of films. This finding provides a better understanding of the magnetic domain landscape in GaMnAs that has been the subject of intense debate.",0803.0714v1 2008-03-06,Finite frequency noise of a superconductor/ferromagnet quantum point contact,"We have calculated the finite-frequency current noise of a superconductor-ferromagnet quantum point contact (SF QPC). This signal is qualitatively affected by the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon reflection on the QPC. For a weakly transparent QPC, noise steps appear at frequencies or voltages determined directly by the SDIPS. These steps can occur at experimentally accessible temperatures and frequencies. Finite frequency noise is thus a promising tool to characterize the scattering properties of a SF QPC.",0803.0854v2 2008-03-14,Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations: effects due to electron-phonon coupling,"Spin-polarized transport through a quantum dot strongly coupled to ferromagnetic electrodes with non-collinear magnetic moments is analyzed theoretically in terms of the non-equilibrium Green function formalism. Electrons in the dot are assumed to be coupled to a phonon bath. The influence of electron-phonon coupling on tunnelling current, linear and nonlinear conductance, and on tunnel magnetoresistance is studied in detail. Variation of the main Kondo peaks and phonon satellites with the angle between magnetic moments of the leads is analyzed.",0803.2152v1 2008-03-15,Spin diode based on a single-wall carbon nanotube,"Electronic transport through a single-wall metallic carbon nanotube weakly coupled to one ferromagnetic and one nonmagnetic lead is analyzed in the sequential tunneling limit. It is shown that both the spin and charge currents flowing through such systems are highly asymmetric with respect to the bias reversal. As a consequence, nanotubes coupled to one nonmagnetic and one ferromagnetic lead can be effectively used as spin diodes whose functionality can be additionally controlled by a gate voltage.",0803.2279v1 2008-03-17,The influence of spin-dependent phases of tunneling electrons on the conductance of a point ferromagnet/isolator/superconductor contact,"The Andreev reflection probability for a ferromagnet/isolator/superconductor (FIS) contact at the arbitrary spin-dependent amplitudes of the electron waves transmitted through and reflected from the potential barrier is found. It is shown that Andreev reflection probabilities of electron and hole excitations in the FIS contact are different. The energy levels of Andreev bound states are found. The ballistic conductance of the point FIS contact is calculated.",0803.2499v2 2008-03-18,The Blume-Emery-Griffiths Spin Glass and Inverted Tricritical Points,"The Blume-Emery-Griffiths spin glass is studied by renormalization-group theory in d=3. The boundary between the ferromagnetic and paramagnetic phases has first-order and two types of second-order segments. This topology includes an inverted tricritical point, first-order transitions replacing second-order transitions as temperature is lowered. The phase diagrams show disconnected spin-glass regions, spin-glass and paramagnetic reentrances, and complete reentrance, where the spin-glass phase replaces the ferromagnet as temperature is lowered for all chemical potentials.",0803.2720v2 2008-03-26,Unusual spin-wave population in nickel after femtosecond laser pulse excitation,"The spin-wave relaxation mechanisms after intense laser excitation in ferromagnetic nickel films are investigated with all-optical pump-probe experiments. Uniform precession (Kittel mode), Damon-Eshbach surface modes and perpendicular standing spin waves can be identified by their dispersion f(H). However, different to other ferromagnets f(H) deviates from the expected behavior. Namely, a mode discontinuity is observed, that can be attributed to a non-linear process. Above a critical field the power spectrum reveals a redistribution of the energy within the spin-wave spectrum populated.",0803.3686v2 2008-04-04,Inhomogeneous Gilbert damping from impurities and electron-electron interactions,"We present a unified theory of magnetic damping in itinerant electron ferromagnets at order $q^2$ including electron-electron interactions and disorder scattering. We show that the Gilbert damping coefficient can be expressed in terms of the spin conductivity, leading to a Matthiessen-type formula in which disorder and interaction contributions are additive. In a weak ferromagnet regime, electron-electron interactions lead to a strong enhancement of the Gilbert damping.",0804.0820v2 2008-04-05,Integrable Isotropic Geometrical Flows and Heisenberg Ferromagnets,"Geometrical flows (GF) play an important role in modern mathematics and physics. In this letter we have considered some integrable isotropic GF -- Ricci flows (RF) and mean curvature flows (MCF) -- which are related with integrable Heisenberg ferromagnets. In 2+1 dimensions, these GF have a singularity at $t=t_{0}$.",0804.0837v1 2008-04-17,Non-analytic behavior of 2D itinerant ferromagnets,"We consider an ordered ferromagnet in the vicinity of a T=0 transition into a paramagnet. We show that the free energy and the transverse and longitudinal static susceptibilities contain non-analyticities which destroy a continuous second-order transition. Depending on the parameters, the transition either becomes first-order, or occurs via an intermediate spiral phase.",0804.2736v1 2008-04-17,Conductivity behavior of La$_{0.75}$Ca$_{0.25}$MnO$_{3}$ in vicinity of ferromagnetic-paramagnetic transition studied with single current pulses,"Temperature and current dependences of resistivity of bulk La$_{0.75}$Ca$_{0.25}$MnO$_{3}$ sample grown by the floating-zone method were studied using single ramp pulses of current. It is found that near the Curie temperature $T_C$ the sample resistance depends substantially on current magnitude. The observed features can be determined by inhomogeneous Joule overheating due to mixed phase state of manganites near the ferromagnetic-paramagnetic transition and percolation character of this transition.",0804.2806v1 2008-04-28,Criticality in diluted ferromagnet,"We perform a detailed study of the critical behavior of the mean field diluted Ising ferromagnet by analytical and numerical tools. We obtain self-averaging for the magnetization and write down an expansion for the free energy close to the critical line. The scaling of the magnetization is also rigorously obtained and compared with extensive Monte Carlo simulations. We explain the transition from an ergodic region to a non trivial phase by commutativity breaking of the infinite volume limit and a suitable vanishing field. We find full agreement among theory, simulations and previous results.",0804.4503v1 2008-05-08,Theory of huge tunneling magnetoresistance in graphene,"We investigate theoretically the spin-independent tunneling magnetoresistance effect in a graphene monolayer modulated by two parallel ferromagnets deposited on a dielectric layer. For the parallel magnetization configuration, Klein tunneling can be observed in the transmission spectrum, but at specific oblique incident angles. For the antiparallel magnetization configuration, the transmission can be blocked by the magnetic-electric barrier provided by the ferromagnets. Such a transmission discrepancy results in a tremendous magnetoresistance ratio and can be tuned by the inclusion of an electric barrier.",0805.1105v1 2008-05-08,"Electronic Structure, Phonons and Dielectric Anomaly in Ferromagnetic Insulating Double Perovskite La2NiMnO6","Using first-principles density functional calculations, we study the electronic and magnetic properties of ferromagnetic insulating double-perovskite compound La2NiMnO6, which has been reported to exhibit interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals existence of very soft infra-red active phonons that couple strongly with spins at the Ni and Mn sites through modification of the super-exchange interaction. We suggest that these modes are the origin for observed dielectric anomaly in La2NiMnO6.",0805.1112v1 2008-05-09,"Spin dynamics in (III,Mn)V ferromagnetic semiconductors: the role of correlations","We address the role of correlations between spin and charge degrees of freedom on the dynamical properties of ferromagnetic systems governed by the magnetic exchange interaction between itinerant and localized spins. For this we introduce a general theory that treats quantum fluctuations beyond the Random Phase Approximation based on a correlation expansion of the Green's function equations of motion. We calculate the spin susceptibility, spin--wave excitation spectrum, and magnetization precession damping. We find that correlations strongly affect the magnitude and carrier concentration dependence of the spin stiffness and magnetization Gilbert damping.",0805.1320v2 2008-06-10,Exact degenerate ground states for the F-AF spin chain with bond alternation,"We investigate the $J_1$-$J_2$ spin chain consisting of spins with magnitude $\frac12$. The nearest-neighbor and the next-nearest-neighbor exchange interactions are ferromagnetic and antiferromagnetic, respectively, and induce strong frustration. Both these interactions involve the bond alternation. We find exact solutions for all the degenerate ground states on the phase boundary of the ferromagnetic phase. The degeneracy remains irrespective of two parameters representing the bond alternation. The exact solutions are of closed forms for no bond alternation and of recursion formulae in general. The exact solutions are applicable to the $\Delta$ chain as a special case.",0806.1604v2 2008-06-17,Structural and angular dependence of coercivity and magnetic remanence of electrochemical ferromagnetic nanowires,"A novel method for controlling nanowire magnetic properties and growth from filling time profile is presented. The wires are grown with an electrodeposition method (""Template synthesis"") with a wide selection of pore diameters. We show that stray-fields presence in ferromagnetic nanowires are entirely dependent on the nanowire diameter. Besides a crossover effect in the reversal mechanism is observed with change in diameter. In this work, theory and experiment agree and confirm that according to the variety of hysteresis loop measured, about four ranges of values of pore diameter control the orientation of nanowire magnetization easy axis with respect to the geometrical axis.",0806.2826v1 2008-08-06,Surface Magnetoelectric Effect in Ferromagnetic Metal Films,"A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001) and Co(0001) films in the presence of external electric field. The effect originates from spin-dependent screening of the electric field which leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. These results are of considerable interest in the area of electrically-controlled magnetism and magnetoelectric phenomena.",0808.0841v1 2008-08-09,Gilbert Damping in Conducting Ferromagnets I: Kohn-Sham Theory and Atomic-Scale Inhomogeneity,"We derive an approximate expression for the Gilbert damping coefficient \alpha_G of itinerant electron ferromagnets which is based on their description in terms of spin-density-functional-theory (SDFT) and Kohn-Sham quasiparticle orbitals. We argue for an expression in which the coupling of magnetization fluctuations to particle-hole transitions is weighted by the spin-dependent part of the theory's exchange-correlation potential, a quantity which has large spatial variations on an atomic length scale. Our SDFT result for \alpha_G is closely related to the previously proposed spin-torque correlation-function expression.",0808.1373v1 2008-08-20,"Anharmonic effect on lattice distortion, orbital ordering and magnetic properties in Cs2AgF4","We develop the cluster self-consistent field method incorporating both electronic and lattice degrees of freedom to study the origin of ferromagnetism in Cs$_{2}$AgF$_{4}$. After self-consistently determining the harmonic and anharmonic Jahn-Teller distortions, we show that the anharmonic distortion stabilizes the staggered x$^{2}$-z$^{2}$/y$^{2}$-z$^{2}$ orbital and ferromagnetic ground state, rather than the antiferromagnetic one. The amplitudes of lattice distortions, Q$_{2}$ and Q$_{3}$, the magnetic coupling strengthes, J$_{x,y}$, and the magnetic moment, are in good agreement with the experimental observation.",0808.2759v1 2008-08-21,"GaMnAs: layers, wires and dots","Thin layers of GaMnAs ferromagnetic semiconductor grown by molecular beam epitaxy on GaAs(001) substrates were studied. To improve their magnetic properties the post-growth annealing procedures were applied, using the surface passivation layers of amorphous arsenic. This post growth treatment effectively increases the ferromagnetic-to-paramagnetic phase transition temperature in GaMnAs, and provides surface-rich MnAs layer which can be used for formation of low dimensional structures such as superlattices. If the surface rich MnAs layer consists of MnAs dots, then it is possible to grow Mn-doped GaAs nanowires",0808.2887v1 2008-08-27,"Coherent control of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As","We report single-color, time resolved magneto-optical measurements in ferromagnetic semiconductor (Ga,Mn)As. We demonstrate coherent optical control of the magnetization precession by applying two successive ultrashort laser pulses. The magnetic field and temperature dependent experiments reveal the collective Mn-moment nature of the oscillatory part of the time-dependent Kerr rotation, as well as contributions to the magneto-optical signal that are not connected with the magnetization dynamics.",0808.3738v2 2008-08-29,Spin-wave mediated quantum corrections to the conductivity in thin ferromagnetic gadolinium films,"We present a study of quantum corrections to the conductivity of thin ferromagnetic gadolinium films. In situ magneto-transport measurements were performed on a series of thin films with thickness d < 135A. For sheet resistances R0 < 4011 Ohm and temperatures T < 30K, we observe a linear temperature dependence of the conductivity in addition to the logarithmic temperature dependence expected from well known quantum corrections in two dimensions. We show that such a linear T-dependence can arise from a spin-wave mediated Altshuler-Aronov type correction.",0808.4103v1 2008-09-07,Magnetic properties of Co-doped TiO2 anatase nanopowders,"This letter reports on the magnetic properties of Ti1-xCoxO2 anatase phase nanopowders with different Co contents. It is shown that oxygen vacancies play a fundamental role in promoting the long-range ferromagnetic order in the material studied, in addition to the transition-metal doping. Furthermore, the results allow ruling out the premise of a strict connection between Co clustering and the ferromagnetism observed in the Co:TiO2 anatase system.",0809.1256v2 2008-09-08,Electronic inhomogeneity in EuO: Possibility of magnetic polaron states,"We have observed the spatial inhomogeneity of the electronic structure of a single-crystalline electron-doped EuO thin film with ferromagnetic ordering by employing infrared magneto-optical imaging with synchrotron radiation. The uniform paramagnetic electronic structure changes to a uniform ferromagnetic structure via an inhomogeneous state with decreasing temperature and increasing magnetic field slightly above the ordering temperature. One possibility of the origin of the inhomogeneity is the appearance of magnetic polaron states.",0809.1299v1 2008-09-09,Size Dependence of Current Spin Polarization Through Superconductor/Ferromagnet Nanocontacts,"The spin polarization P of the transport current through the interface between superconducting Al and ferromagnetic Fe is determined by means of Andreev reflection at nanostructured point contacts. We observe a systematic decrease of P with decreasing contact resistance. Our data provide evidence for the reduction of P by spin-orbit scattering and thus establish a link between density-of-states and transport spin polarizations.",0809.1576v1 2008-09-11,Ferromagnetic ordering in dilute magnetic dielectrics with and without free carriers,"The state of art in the theoretical and experimental studies of transition metal doped oxides (dilute magnetic dielectrics) is reviewed. The available data show that the generic non-equilibrium state of oxide films doped with magnetic impurities may either favor ferromagnetism with high Curie temperature or result in highly inhomogeneous state without long-range magnetic order. In both case concomitant defects (vacancies, interstitial ions play crucial part.",0809.2005v1 2008-09-18,The importance of local band effects for ferromagnetism in hole doped La$_2$CuO$_4$,"Band calculations for supercells of La$_{(2-x)}$Ba$_x$CuO$_4$ show that the rigid band model for doping is less adequate than what is commonly assumed. In particular, weak ferromagnetism (FM) can appear locally around clusters of high Ba concentration. The clustering is important at large dilution and averaged models for magnetism, such as the virtual crystal approximation, are unable to stabilize magnetic moments. These results give a support to the idea that weak FM can be the cause of the destruction of superconductivity at high hole doping.",0809.3246v1 2008-09-26,Mott Insulators in the Strong Spin-Orbit Coupling Limit: From Heisenberg to a Quantum Compass and Kitaev Models,"We study the magnetic interactions in Mott-Hubbard systems with partially filled $t_{2g}$-levels and with strong spin-orbit coupling. The latter entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians that extrapolate from the Heisenberg to a quantum compass model depending on the lattice geometry. This gives way to ""engineer"" in such Mott insulators an exactly solvable spin model by Kitaev relevant for quantum computation. We, finally, explain ""weak"" ferromagnetism, with an anomalously large ferromagnetic moment, in Sr$_2$IrO$_4$.",0809.4658v2 2008-09-28,Coherent terahertz needle radiation emission mechanism from nanostructured ferromagnetic metal films excited by femtosecond laser pulses,"A coherent terahertz needle radiation emission mechanism is proposed. This mechanism can be realized in the experiments on femtosecond laser pulses interacting with surface of nanostructured ferromagnetic metal film. The proposed mechanism is based on exciting of coupled coherent phonon-magnon modes on a nanostructured metal surface by laser femtosecond pulse. The value of the Poynting vector for the terahertz radiation is estimated.",0809.4868v1 2008-10-06,Spin polarized current and Andreev transmission in planar superconducting/ferromagnetic Nb/Ni junctions,"We have measured the tunnelling current in Nb/Nb$_x$O$_y$/Ni planar tunnel junctions at different temperatures. The junctions are in the intermediate transparency regime. We have extracted the current polarization of the metal/ferromagnet junction without applying a magnetic field. We have used a simple theoretical model, that provides consistent fitting parameters for the whole range of temperatures analyzed. We have also been able to gain insight into the microscopic structure of the oxide barriers of our junctions.%",0810.0929v1 2008-10-19,Spin-triplet f-wave pairing due to three-site cyclic-exchange ferromagnetic interactions,"Ferromagnetiam and superconductivity in a two-dimensional triangular-lattice Hubbard model are studied using the density-matrix renormalization group method. We propose a mechanism of the {\it f}-wave spin-triplet pairing derived from the three-site cyclic-exchange ferromagnetic interactions. We point out that a triangular network of hopping integrals, which is required for the three-site cyclic hopping processes, is contained in the (possibly) spin-triplet superconducting systems, such as Bechgaard salts (TMTSF)$_2$X, cobalt oxide Na$_{0.35}$CoO$_2$$\cdot$1.3H$_2$O, and layered perovskite Sr$_2$RuO$_4$.",0810.3387v1 2008-10-23,Magnetic moment manipulation by a Josephson current,"We consider a Josephson junction where the weak-link is formed by a non-centrosymmetric ferromagnet. In such a junction, the superconducting current acts as a direct driving force on the magnetic moment. We show that the a.c. Josephson effect generates a magnetic precession providing then a feedback to the current. Magnetic dynamics result in several anomalies of current-phase relations (second harmonic, dissipative current) which are strongly enhanced near the ferromagnetic resonance frequency.",0810.4286v3 2008-10-28,Ferromagnetism and increased ionicity in epitaxially grown TbMnO3 films,"Thin films of TbMnO3 have been grown on SrTiO3 substrates. The films grow under compressive strain and are only partially clamped to the substrate. This produces remarkable changes in the magnetic properties and, unlike the bulk material, the films display ferromagnetic interactions below the ordering temperature of ~40K. X-ray photoemission measurements in the films show that the Mn-3s splitting is 0.3eV larger than that of the bulk. Ab initio embedded cluster calculations yield Mn-3s splittings that are in agreement with the experiment and reveal that the larger observed values are due to a larger ionicity of the films.",0810.5137v1 2008-11-18,Quasiparticles in the Kondo lattice model at partial fillings of the conduction band,"We study the spectral properties of the one-dimensional Kondo lattice model as function of the exchange coupling, the band filling, and the quasimomentum in the ferromagnetic and paramagnetic phase. Using the density-matrix renormalization group method, we compute the dispersion relation of the quasiparticles, their lifetimes, and the Z-factor. As a main result, we provide evidence for the existence of the spinpolaron at partial band fillings. We find that the quasiparticle lifetime differs by orders of magnitude between the ferromagnetic and paramagnetic phase and depends strongly on the quasimomentum.",0811.3000v1 2008-11-28,Bit storage by $360^\circ$ domain walls in ferromagnetic nanorings,"We propose a design for the magnetic memory cell which allows an efficient storage, recording, and readout of information on the basis of thin film ferromagnetic nanorings. The information bit is represented by the polarity of a stable 360$^\circ$ domain wall introduced into the ring. Switching between the two magnetization states is achieved by the current applied to a wire passing through the ring, whereby the $360^\circ$ domain wall splits into two charged $180^\circ$ walls, which then move to the opposite extreme of the ring to recombine into a $360^\circ$ wall of the opposite polarity.",0811.4663v1 2008-12-02,Independent magnetization behavior of a ferromagnetic metal/semiconductor hybrid system,"We report the discovery of an effect where two ferromagnetic materials, one semiconductor ((Ga,Mn)As) and one metal (permalloy), can be directly deposited on each other and still switch their magnetization independently. We use this independent magnetization behavior to create various resistance states dependent on the magnetization direction of the individual layers. At zero magnetic field a two layer device can reach up to four non-volatile resistance states.",0812.0455v1 2008-12-03,Observation of ferromagnetic resonance in strontium ruthenate (SrRuO3),"We report the observation of ferromagnetic resonance (FMR) in SrRuO3 using the time-resolved magneto-optical Kerr effect. The FMR oscillations in the time-domain appear in response to a sudden, optically induced change in the direction of easy-axis anistropy. The high FMR frequency, 250 GHz, and large Gilbert damping parameter, alpha ~ 1, are consistent with strong spin-orbit coupling. We find that the parameters associated with the magnetization dynamics, including alpha, have a non-monotonic temperature dependence, suggestive of a link to the anomalous Hall effect.",0812.0832v1 2008-12-12,Entanglement generation and perfect state transfer in ferromagnetic qubit chains,"We propose to use ferromagnetic systems for entanglement generation and distribution together with perfect state transfer between distant parties in a qubit chain. The scheme relies on an effective 2-qubit dynamics, realized by leaving two empty sites in a uniformly filled chain. This allows long-range interacting qubit chains to serve as quantum channels for both tasks with optimal performances. Remarkably, the entanglement between sender and receiver sites is independent of both the transmission distance and of system size. This property opens new perspectives for short and mid-range quantum communication with qubit chains.",0812.2404v2 2009-01-14,Interface coupling properties and reflection of bulk spin waves from biaxial multilayer ferromagnetic media,"The reflection coefficient of bulk spin waves from multilayer ferromagnetic structure with periodically modulated parameters of exchange interaction, uniaxial and rhombic magnetic anisotropy and saturation magnetization is calculated with a non-ideal coupling between layers. The strong dependence of spin-wave reflection coefficient on frequency, magnetic field and parameter of interfacial coupling is revealed. It allows changing the reflection intensity from 0 to 1 by changing either only external magnetic field value or frequency. The proposed model of boundary conditions gives the opportunity to take into account the quality of interfaces when studying the reflection processes of spin waves in multilayer structures.",0901.2012v1 2009-01-15,Vortex stabilization in magnetic trilayer dots,"The magnetization reversal and spin structure in circular Co/insulator/Ni80Fe20 trilayer dots has been investigated numerically. The effect of dipolar coupling between a soft ferromagnetic Permalloy (Py=Ni80Fe20) layer and a hard ferromagnetic Cobalt layer inside one stack is studied. We find either a stabilization or even a triggering of the vortex state in the Py layer due to the magnetic stray field of the Co layer, while the Co magnetization remains in a single-domain state. Furthermore, for thin Py layers a 360 deg-domain wall is observed. We construct a phase diagram, where regions of vortex stabilization, triggering, and occurrence of a 360 deg domain wall are marked.",0901.2209v1 2009-01-16,Cr-doping effect on the orbital fluctuation of heavily doped Nd1-xSrxMnO3 (x ~ 0.625),"We have investigated the Cr-doping effect of Nd0.375Sr0.625MnO3 near the phase boundary between the x2-y2 and 3z2-r2 orbital ordered states, where a ferromagnetic correlation and concomitant large magnetoresistance are observed owing to orbital fluctuation. Cr-doping steeply suppresses the ferromagnetic correlation and magnetoresistance in Nd0.375Sr0.625Mn1-yCryO3 with 0 < y < 0.05, while they reappear in 0.05 < y < 0.10. Such a reentrant behavior implies that a phase boundary is located at y = 0.05, or a phase crossover occurs across y = 0.05.",0901.2419v1 2009-01-19,Magnetism in Mn delta-doped cubic GaN: density-functional theory studies,"The magnetism in 12.5% and 25% Mn delta-doped cubic GaN has been investigated using the density-functional theory calculations. The results show that the single-layer delta-doping and half-delta-doping structures show robust ground state half-metallic ferromagnetism (HMF), and the double-layer delta-doping structure shows robust ground state antiferromagnetism (AFM) with large spin-flip energy of 479.0 meV per Mn-Mn pair. The delta-doping structures show enhanced two-dimensional magnetism. We discuss the origin of the HMF using a simple crystal field model. Finally, we discuss the antiferromagnet/ferromagnet heterostructure based on Mn doped GaN.",0901.2820v1 2009-01-19,The ground state magnetic phase diagram of the ferromagnetic Kondo-lattice model,"The magnetic ground state phase diagram of the ferromagnetic Kondo-lattice model is constructed by calculating internal energies of all possible bipartite magnetic configurations of the simple cubic lattice explicitly. This is done in one dimension (1D), 2D and 3D for a local moment of S = 3/2. By assuming saturation in the local moment system we are able to treat all appearing higher local correlation functions within an equation of motion approach exactly. A simple explanation for the obtained phase diagram in terms of bandwidth reduction is given. Regions of phase separation are determined from the internal energy curves by an explicit Maxwell construction.",0901.2855v1 2009-01-21,Frustrated Ising Model on a Diamond Hierarchical Lattice,"A frustrated Ising model on a diamond hierarchical lattice is studied. We obtain the exact partition function of this model and calculate the transition temperature, specific heat, entropy, magnetization, and ferromagnetic correlation function. Depending on the magnitude of a parameter giving the frustration, there exist three types of ground states: ferromagnetic, classical spin-liquid with highly developed short-range order, and paramagnetic. The dependence of the zero-temperature entropy on the frustration parameter has an infinite number of steps. The temperature dependence of the specific heat exhibits many peaks with decreasing temperature and entropy loss. The dominant spin configurations at low temperatures are also specified.",0901.3181v2 2009-02-06,Spin torque from tunneling through impurities in a magnetic tunnel junction,"We calculate the contribution to the spin transfer torque from sequential tunneling through impurities in a magnetic tunnel junction. For a junction with weakly polarized ferromagnetic contacts, the torque is found to be in the plane spanned by the magnetizations of the ferromagnetic contacts and proportional to $\sin \theta$, where $\theta$ is the angle between the magnetic moments. If the polarization is larger, the torque acquires a significant out-of-plane component and a different dependence on $\theta$.",0902.1150v2 2009-02-07,Implementing Quantum Gates using the Ferromagnetic Spin-J XXZ Chain with Kink Boundary Conditions,"We demonstrate an implementation scheme for constructing quantum gates using unitary evolutions of the one-dimensional spin-J ferromagnetic XXZ chain. We present numerical results based on simulations of the chain using the time-dependent DMRG method and techniques from optimal control theory. Using only a few control parameters, we find that it is possible to implement one- and two-qubit gates on a system of spin-3/2 XXZ chains, such as Not, Hadamard, Pi-8, Phase, and C-Not, with fidelity levels exceeding 99%.",0902.1276v2 2009-02-11,A Blume-Capel Ising Ferromagnet with Annealed Vacancies on a Hierarchical Lattice,"A dilute Ising ferromagnet is considered in this study using renormalization group techniques with a hierarchical lattice. A series of phase diagrams have been produced that probe the effects of varying the temperature and concentration of nonmagnetic impurities. Each phase diagram corresponds to a different strength for the internal coupling coefficients on our lattice. Phases have been interpreted and critical exponents calculated for the higher order transitions.",0902.1971v2 2009-02-19,Effective mass suppression in a ferromagnetic two-dimensional electron liquid,"We present numerical calculations of the electron effective mass in an interacting, ferromagnetic, two-dimensional electron system. We consider quantum interaction effects associated with the charge-density fluctuation induced many-body vertex corrections. Our theory, which is free of adjustable parameters, reveals that the effective mass is suppressed (relative to its band value) in the strong coupling limit, in good agreement with the results of recent experimental measurements.",0902.3437v2 2009-02-25,"Tunneling magnetoresistance in (La,Pr,Ca)MnO3 nanobridges","The manganite (La,Pr,Ca)MnO3 is well known for its micrometer scale phase separation into coexisting ferromagnetic metallic and antiferromagnetic insulating (AFI) regions. Fabricating bridges with widths smaller than the phase separation length scale has allowed us to probe the magnetic properties of individual phase separated regions. We observe tunneling magnetoresistance across naturally occurring AFI tunnel barriers separating adjacent ferromagnetic regions spanning the width of the bridges. Further, near the Curie temperature, a magnetic field induced metal-to-insulator transition among a discrete number of regions within the narrow bridges gives rise to abrupt and colossal low-field magnetoresistance steps at well defined switching fields.",0902.4386v1 2009-03-07,Shot noise of a ferromagnetic nanowire with a domain wall,"We study shot noise of the spin-polarized current in a diffusive ferromagnetic nanowire which contains a ballistic domain wall. We find that the existence of a short domain wall influences strongly the shot noise for sufficiently high spin-polarization of the wire. Compared to the situation of the absence of the domain wall, the shot noise can be reduced or enhanced depending on the length of the domain wall and its relative conductance.",0903.1360v1 2009-03-17,Electronic transport in ferromagnetic conductors with inhomogeneous magnetic order parameter -- domain-wall resistance,"We microscopically derive transport equations for the conduction electrons in ferromagnetic materials with an inhomogeneous magnetization profile. Our quantum kinetic approach includes elastic scattering and anisotropic spin-flip scattering at magnetic impurities. In the diffusive limit, we calculate the resistance through a domain wall and find that the domain-wall resistance can be positive or negative. In the limit of long domain walls we derive analytical expressions and compare them with existing works, which used less general models or different theoretical frameworks.",0903.3033v2 2009-03-23,Spatial distribution of Gaussian fluctuations of the molecular field and magnetization in the pyramid-like Ising nanoscopic system interacting with the substrate,"We study thermodynamic properties of an Ising model of a ferromagnetic nanoscopic pyramid deposited onto a ferromagnetic bulk substrate. The influence of the interaction between the pyramid and the substrate is calculated in terms of the equilibrium reduced-state (density) operator used for description of thermodynamic properties of nanoscopic systems. The spatial distribution of the fluctuations of molecular field and magnetization in the nanoscopic pyramid is obtained in the Gaussian fluctuations approximation. Experimental consequences for the magnetic force measurements are briefly discussed.",0903.3659v1 2009-03-23,Multi-terminal spin-dependent transport in ballistic carbon nanotubes,"We study theoretically nonlocal spin transport in a ballistic carbon nanotube contacted to two ferromagnetic leads and two normal-metal leads. When the magnetizations of the two ferromagnets are changed from a parallel to an antiparallel configuration, the circuit shows a hysteretic behavior which is specific to the few-channel regime. In the coherent limit, the amplitude of the magnetic signals is strongly enhanced due to resonance effects occurring inside the nanotube. Our calculations pave the way for experiments on low-dimensional nonlocal spin transport, which should give results remarkably different from the experiments realized so far in the multichannel diffusive incoherent regime.",0903.3813v1 2009-04-02,Spin-Fluctuation Drag Thermopower of Nearly Ferromagnetic Metals,"We investigate theoretically the Seebeck effect in materials close to a ferromagnetic quantum critical point to explain anomalous behaviour at low temperatures. It is found that the main effect of spin fluctuations is to enhance the coefficient of the leading $T$-linear term, and a quantum critical behaviour characterized by a spin-fluctuation temperature appears in the temperature dependence of correction terms as in the specific heat.",0904.0287v2 2009-04-17,Novel magnetic properties of graphene: Presence of both ferromagnetic and antiferromagnetic features and other aspects,"Investigations of the magnetic properties of graphenes prepared by different methods reveal that dominant ferromagnetic interactions coexist along with antiferromagnetic interactions in all the samples. Thus, all the graphene samples exhibit room-temperature magnetic hysteresis. The magnetic properties depend on the number of layers and the sample area, small values of both favoring larger magnetization. Molecular charge-transfer affects the magnetic properties of graphene, interaction with a donor molecule such as tetrathiafulvalene having greater effect than an electron-withdrawing molecule such as tetracyanoethylene",0904.2739v1 2009-04-23,"Crystal growth, structure and ferromagnetic properties of a Ce3Pt23Si11 single crystal","A high-quality single crystal of Ce3Pt23Si11 has been grown using the Czochralski method. The crystal structure is presented and the chemical composition has been checked using an electron microprobe analyzer. Measurements of the electrical resistivity and magnetic susceptibility performed at low temperature show a ferromagnetic transition at Tc = 0.44 K.",0904.3720v1 2009-04-27,Surface magnetization in non-doped ZnO nanostructures,"We have investigated the magnetic properties of non-doped ZnO nanostructures by using {\it ab initio} total energy calculations. Contrary to many proposals that ferromagnetism in non-doped semiconductors should be induced by intrinsic point defects, we show that ferromagnetism in nanostructured materials should be mediated by extended defects such as surfaces and grain boundaries. This kind of defects create delocalized, spin polarized states that should be able to warrant long-range magnetic interactions.",0904.4147v1 2009-05-13,Orientation dependence of the Schottky barrier height for La$_0.6$Sr$_0.4$MnO$_3$/SrTiO$_3$ heterojunctions,"The authors report on the crystallographic orientation dependence of the Schottky properties for heterojunctions between a half-metallic ferromagnet La$_0.6$Sr$_0.4$MnO$_3$ (LSMO) and Nb-doped SrTiO3 semiconductor. The Schottky barrier height determined by in situ photoemission measurements is independent for the substrate orientations (001) and (110), while the magnetic properties of LSMO (110) films are more enhanced than for (001) films. These results suggest that the performance of magnetic devices based on ferromagnetic manganite is improved by using (110)-oriented substrates.",0905.2018v1 2009-05-13,Itinerant ferromagnetism in an atomic Fermi gas: Influence of population imbalance,"We investigate ferromagnetic ordering in an itinerant ultracold atomic Fermi gas with repulsive interactions and population imbalance. In a spatially uniform system, we show that at zero temperature the transition to the itinerant magnetic phase transforms from first to second order with increasing population imbalance. Drawing on these results, we elucidate the phases present in a trapped geometry, finding three characteristic types of behavior with changing population imbalance. Finally, we outline the potential experimental implications of the findings.",0905.2036v2 2009-05-13,Magnetic and transport properties of n-type Fe-doped In2O3 ferromagnetic thin films,"Room temperature ferromagnetism was observed in n-type Fe-doped In2O3 thin films deposited on c-cut sapphire substrates by pulsed laser deposition. Structure, magnetism, composition, and transport studies indicated that Fe occupied the In sites of the In2O3 lattice rather than formed any metallic Fe or other magnetic impurity phases. Magnetic moments of films were proved to be intrinsic and showed to have a strong dependence on the carrier densities which depended on the Fe concentration and its valance state as well as oxygen pressure.",0905.2083v1 2009-05-14,A compact apparatus for studies of element and phase-resolved ferromagnetic resonance,"We present a compact sample holder equipped with electromagnets and high frequency transmission lines; the sample holder is intended for combined x-ray magnetic circular dichroism (XMCD) and ferromagnetic resonance measurements (FMR). Time-resolved measurements of resonant x-ray detected FMR during forced precession are enabled by use of a rf excitation that is phase-locked to the storage ring bunch clock. Several applications of the combined XMCD + FMR technique are presented, demonstrating the flexibility of the experimental design.",0905.2353v1 2009-05-21,Freezing into Stripe States in Two-Dimensional Ferromagnets and Crossing Probabilities in Critical Percolation,"When a two-dimensional Ising ferromagnet is quenched from above the critical temperature to zero temperature, the system eventually converges to either a ground state (all spins aligned) or an infinitely long-lived metastable stripe state. By applying results from percolation theory, we analytically determine the probability to reach the stripe state as a function of the aspect ratio and the form of the boundary conditions. These predictions agree with simulation results. Our approach generally applies to coarsening dynamics of non-conserved scalar fields in two dimensions.",0905.3521v2 2009-05-26,Proximity effect in atomic-scaled hybrid superconductor/ferromagnet structures: crucial role of electron spectra,"We study the influence of the configuration of the majority and minority spin subbands of electron spectra on the properties of atomic-scaled superconductor-ferromagnet S-F-S and F-S-F hybrid structures. At low temperatures, the S/F/S junction is either a 0 or junction depending on the energy shift between S and F materials and the anisotropy of the Fermi surfaces. We found that the spin switch effect in F/S/F system can be reversed if the minority spin electron spectra in F metal is of the hole-like type.",0905.4212v1 2009-06-02,Effect of annealing on the magnetic and superconducting properties of single-crystalline UCoGe,"Single-crystals of the new ferromagnetic superconductor UCoGe have been grown. The quality of as-grown samples can be significantly improved by a heat-treatment procedure, which increases the residual resistance ratio (RRR) from ~5 to ~30. Magnetization and resistivity measurements show the annealed samples have a sharp ferromagnetic transition with a Curie temperature T_C is 2.8 K. The ordered moment of 0.06 mu_B is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature T_s = 0.65 K.",0906.0497v1 2009-06-03,Effects of ferromagnetic fluctuations on the electric and thermal transport properties in NaxCoO2,"We investigate the electronic states and the transport properties of the CoO2 plane in NaxCoO2 on the basis of the two-dimensional triangular lattice 11-band d-p model by using the fluctuation exchange approximation, where we consider the Coulomb interaction between the t2g electrons on a Co site. It is found that all of the effective mass of quasiparticles, the resistivity, the thermoelectric power and the uniform spin susceptibility increase with increasing x for x > 0.6. This implies that the ferromagnetic fluctuations play significant roles on determining the electronic states. These results are qualitatively consistent with the experiments.",0906.0618v1 2009-06-18,Metamagnetic phase transition in the Ising plus Dzyaloshinskii-Moriya model,"We study the 1D ferromagnetic Ising (spin-1/2) model with the Dzyaloshinskii-Moriya (DM) interaction. We analyze the low energy excitation spectrum and the ground state magnetic phase diagram using the Lanczos method. The DM interaction-dependency is calculated for the low-energy excitation spectrum, spiral order parameter and spin-spin correlation functions. We show that a metamagnetic quantum phase transition occurs between the ferromagnetic and spiral phases. The existence of the metamagnetic phase transition is confirmed, using the variational matrix product states approach.",0906.3443v1 2009-06-18,Temperature-dependent resistivity of ferromagnetic GaMnAs: Interplay between impurity scattering and many-body effects,"The static conductivity of the dilute magnetic semiconductor GaMnAs is calculated using the memory function formalism and time-dependent density-functional theory to account for impurity scattering and to treat Hartree and exchange interactions within the hole gas. We find that the Coulomb scattering off the charged impurities alone is not sufficient to explain the experimentally observed drop in resistivity below the ferromagnetic transition temperature: the often overlooked scattering off the fluctuations of localized spins is shown to play a significant role.",0906.3526v1 2009-06-22,Temperature dependent electrical resistivity of a single strand of ferromagnetic single crystalline nanowire,"We have measured the electrical resistivity of a single strand of a ferromagnetic Ni nanowire of diameter 55 nm using a 4-probe method in the temperature range 3 K-300 K. The wire used is chemically pure and is a high quality oriented single crystalline sample in which the temperature independent residual resistivity is determined predominantly by surface scattering. Precise evaluation of the temperature dependent resistivity ($\rho$) allowed us to identify quantitatively the electron-phonon contribution (characterized by a Debye temperature $\theta_R$) as well as the spin-wave contribution which is significantly suppressed upon size reduction.",0906.3903v1 2009-06-26,Conductance oscillation due to the geometrical resonance in FNS double junctions,"We theoretically analyzed the Andreev reflection in ferromagnetic metal/nonmagnetic metal/superconductor double junctions with special attention to the electron interference effect in the nonmagnetic metal layer. We showed that the conductance oscillates as a function of the bias voltage due to the geometrical resonance. We found that the exchange field, and therefore the spin polarization of the ferromagnetic metal can be determined from the period of the conductance oscillation, which is proportional to the square root of the exchange field.",0906.4863v1 2009-06-29,Clamping of ferroelectric and ferromagnetic domain walls in conical spiral magnets,"We study the structure of domain walls in multiferroic magnets with the conical spiral ordering. We formulate a simple spin model which has a conical spiral ground state in absence of magnetic anisotropies. We find a transition from the regime where ferromagnetic and ferroelectric domain walls are clamped to the regime where they are decoupled and derive a continuum model describing rotation of the spiral plane at the domain wall. The importance of these results for the switching phenomena observed in CoCr2O4 is discussed.",0906.5298v1 2009-06-30,Survival of short-range order in the Ising model on negatively curved surfaces,"We examine the ordering behavior of the ferromagnetic Ising lattice model defined on a surface with a constant negative curvature. Small-sized ferromagnetic domains are observed to exist at temperatures far greater than the critical temperature, at which the inner core region of the lattice undergoes a mean-field phase transition. The survival of short-range order at such high temperatures can be attributed to strong boundary-spin contributions to the ordering mechanism, as a result of which boundary effects remain active even within the thermodynamic limit. Our results are consistent with the previous finding of disorder-free Griffiths phase that is stable at temperatures lower than the mean-field critical temperature.",0906.5523v1 2009-07-16,Anomalous magnetoresistance on the topological surface,"We investigate charge transport in two-dimensional ferromagnet/feromagnet junction on a topological insulator. The conductance across the interface depends sensitively on the directions of the magnetizations of the two ferromagnets, showing anomalous behaviors compared with the conventional spin-valve. It is found that the conductance depends strongly on the in-plane direction of the magnetization. Moreover, in sharp contrast to the conventional magnetoresistance effect, in the p-n junction, the conductance at the parallel configuration is much smaller than that at the antiparallel configuration. This stems from the way how the wavefunctions connect between both sides.",0907.2810v2 2009-07-27,Ferromagnetism in the Hubbard model with Topological/Non-Topological Flat Bands,"We introduce and study two classes of Hubbard models with magnetic flux or with spin-orbit coupling, which have a flat lowest band separated from other bands by a nonzero gap. We study the Chern number of the flat bands, and find that it is zero for the first class but can be nontrivial in the second. We also prove that the introduction of on-site Coulomb repulsion leads to ferromagnetism in both the classes.",0907.4564v3 2009-08-05,"Femtosecond Coherent Control of Spin with Light in (Ga,Mn)As ferromagnets","Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near--ultraviolet ($\sim$3eV), coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the \{111\} equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. We predict nonthermal magnetization control by tuning the laser frequency and polarization direction. Our mechanism explains recent ultrafast pump--probe experiments.",0908.0707v1 2009-08-10,Phase diagram of an anisotropic frustrated ferromagnetic spin-1/2 chain in a magnetic field: a density matrix renormalization group study,"We study the phase diagram of a frustrated spin-1/2 ferromagnetic chain with anisotropic exchange interactions in an external magnetic field, using the density matrix renormalization group method. We show that an easy-axis anisotropy enhances the tendency towards multimagnon bound states, while an easy-plane anisotropy favors chirally ordered phases. In particular, a moderate easy-plane anisotropy gives rise to a quantum phase transition at intermediate magnetization. We argue that this transition is related to the finite-field phase transition experimentally observed in the spin-1/2 compound LiCuVO_4.",0908.1281v2 2009-08-25,The magnetism of wurtzite CoO nanoclusters,"The possibility that the apparent room temperature ferromagnetism, often measured in Co-doped ZnO, is due to uncompensated spins at the surface of wurtzite CoO nanoclusters is investigated by means of a combination of density functional theory and Monte Carlo simulations. We find that the critical temperature extracted from the specific heat systematically drops as the cluster size is reduced, regardless of the particular cluster shape. Furthermore the presence of defects, in the form of missing magnetic sites, further reduces $T_\mathrm{C}$. This suggests that even a spinodal decomposed phase is unlikely to sustain room temperature ferromagnetism in ZnO:Co.",0908.3632v1 2009-08-25,The Canted Spiral: An Exact Ground State of XXZ Zigzag Ladders,"We derive the exact ground states for a one dimensional family of $S=1/2$ XXZ Hamiltonians on the zigzag ladder. These states exhibit true long range spiral order that spontaneously breaks the U(1) invariance of the Hamiltonian. Besides breaking a continuous symmetry in $d=1$, this spiral ordering has a ferromagnetic component along the symmetry axis that can take any value between zero and full saturation. In this sense, our canted spiral solutions are a generalization of the SU(2) Heisenberg ferromagnet to non-zero ordering wave-vectors of the transverse spin components. We extend this result to the $d=2$ anisotropic triangular lattice.",0908.3639v1 2009-08-25,Non-equilibrium magnetization dynamics of ferromagnetically coupled Kondo spins,"An analytical description of non-equilibrium phenomena in interacting quantum systems is rarely possible. Here we present one example where such a description can be achieved, namely the ferromagnetic Kondo model. In equilibrium, this model is tractable via perturbative renormalization-group techniques. We employ a recently developed extension of the flow-equation method to calculate the non-equilibrium decay of the local magnetization at zero temperature. The flow equations admit analytical solutions which become exact at short and long times, in the latter case revealing that the system always retains a memory of its initial state.",0908.3647v1 2009-09-16,Magneto-Optical Modulation Ellipsometry,"The paper investigates the ellipsometric method of measuring the magneto-optical parameter and optical constants in one experiment at affixed angle of the light incidence in ferromagnetics. The influence of the magnetization modulation on the change of ellipsometrical angles is shown. The method is considered for the geometries of polar, meridional and equatorial reflection Kerr effects. The formulae relating to the measured signals in polarizer-subject-analyzer (PSA) circuit with four magneto-optical and optical constants illustrated by the example of ferromagnetic Ni are given.",0909.2977v1 2009-09-16,Intrinsic ferromagnetic impurity phases in SmFeAsO1-xFx detected by muSR,"We report about muSR measurements on SmFeAsO1-xFx which helped us to identify the signature of diluted ferromagnetic inclusions, ubiquitous in the iron pnictides. These impurities are characterized by a Curie temperature close to room temperature and they seem responsible for a non negligible magnetic relaxation of the implanted muons, that should not be confused with intrinsic pnictide properties.",0909.3004v1 2009-11-08,Nonequilibrium dynamical ferromagnetism of interacting Single-Molecule Magnets,"We propose a nonequilibrium Monte Carlo (MC) approach to explore nonequilibrium dynamical ferromagnetism of interacting single molecule magnets (SMMs). Both quantum spin tunneling and thermally activated spin reversal are successfully implemented in the same MC simulation framework. Applied to a typical example, this simulation method satisfactorily reproduces experimental magnetization curves with experimental parameters. Our results show that both quantum and classical effects are essential to determine the hysteresis behaviors. This method is effective and reliable to gain deep insights into SMMs.",0911.1539v1 2009-11-08,Elusive Glassy Phase in the Random Field Ising Model,"We consider the random field Ising model and show rigorously that the spin glass susceptibility at equilibrium is always bounded by the ferromagnetic susceptibility, and therefore that no spin glass phase can be present at equilibrium out of the ferromagnet critical line. When the magnetization is, however, fixed to values smaller than the equilibrium one, a glassy phase can exist, as we show explicitly on the Bethe lattice.",0911.1551v2 2009-11-30,Ferromagnetism in the Blume-Emery-Griffiths model on finite-size Cayley tree,"The ferromagnetic properties of the spin-1 BEG model on finite-size Cayley tree are investigated using the exact recursion method. The spontaneous magnetization of the system is studied in detail for different values of the reduced crystal-field interaction D/J, and it is found that there is an unusual behavior (anti-Curie temperature) when D/J>2.0. We also obtain the Curie temperature of this finite-size system. When the system size is large enough, our results will fit well with that in the thermodynamic limit.",0911.5586v1 2010-01-05,Slow Cooling of an Ising Ferromagnet,"A ferromagnetic Ising chain which is endowed with a single-spin-flip Glauber dynamics is investigated. For an arbitrary annealing protocol, we derive an exact integral equation for the domain wall density. This integral equation admits an asymptotic solution in the limit of extremely slow cooling. For instance, we extract an asymptotic of the density of domain walls at the end of the cooling procedure when the temperature vanishes. Slow annealing is usually studied using a Kibble-Zurek argument; in our setting, this argument leads to approximate predictions which are in good agreement with exact asymptotics.",1001.0741v1 2010-01-12,Dynamics of random dipoles : chaos {\it vs} ferromagnetism,"The microcanonical dynamics of an ensemble of random magnetic dipoles in a needle has been investigated. Analyzing magnetic reversal times, a transition between a chaotic paramagnetic phase and an integrable ferromagnetic phase has been numerically found. In particular, a simple criterium for transition has been formulated. Close to the transition point the statistics of average magnetic reversal times and fluctuations have been studied and critical exponents numerically given.",1001.1849v2 2010-01-30,Magnetoresistance of a spin MOSFET with ferromagnetic MnAs source and drain contacts,"Spin-dependent transport was investigated in a spin metal-oxide-semiconductor field-effect transistors (spin MOSFET) with ferromagnetic MnAs source and drain (S/D) contacts. The spin MOSFET of bottom-gate type was fabricated by photolithography using an epitaxial MnAs film grown on a silicon-on-insulator (SOI) substrate. In-plane magnetoresistance showed spin-valve-type hysteretic behavior, when the measurements were performed with constant source-drain and source-gate biases. By comparing with the magnetization-related resistance change resulting from the MnAs contacts, we conclude that the spin-polarized electrons are injected from the MnAs source into the Si MOS inversion channel, and detected by the MnAs drain.",1002.0057v1 2010-02-01,Spin relaxation torque in metallic ferromagnets,"Spin relaxation torque ${\cal T}$ in uniform metallic ferromagnets with the spin-orbit interaction arising from random impurities is calculated microscopically. ${\cal T}$ is shown to be written by spatial derivatives of the electric field, but is not always given as a divergence of a torque dipole density. The result is due to the fast dissipation of the orbital angular momentum, and thus a conserving spin current cannot be defined. It is discussed that the $\beta$ term in the spin transfer torque can also be asymmetric.",1002.0207v2 2010-02-10,Phase Diagram of the Low-Density Two-Dimensional Homogeneous Electron Gas,"We have used quantum Monte Carlo methods to calculate the zero-temperature phase diagram of the two-dimensional homogeneous electron gas. We find a transition from a paramagnetic fluid to an antiferromagnetic triangular Wigner crystal at density parameter r_s=31(1) a.u. and a transition to a ferromagnetic crystal at r_s=38(5) a.u. The fully spin-polarized fluid is never stable. We searched for, but did not find, the ferromagnetic hybrid phase proposed by H. Falakshahi and X. Waintal [Phys. Rev. Lett. 94, 046801 (2005)].",1002.2101v1 2010-03-04,"The QCD string tension curve, the ferromagnetic magnetization, and the quark-antiquark confining potential at finite Temperature","We study the string tension as a function of temperature, fitting the SU(3) lattice QCD finite temperature free energy potentials computed by the Bielefeld group. We compare the string tension points with order parameter curves of ferromagnets, superconductors or string models, all related to confinement. We also compare the SU(3) string tension with the one of SU(2) Lattice QCD. With the curve providing the best fit to the finite temperature string tensions, the spontaneous magnetization curve, we then show how to include finite temperature, in the state of the art confining and chiral invariant quark models.",1003.0936v1 2010-03-06,Magnetoresistance studies of $La_{2/3} Sr_{1/3} MnO_3$ - $YBa_2 Cu_3 O_7$ - $La_{2/3} Sr_{1/3} Mn O_3$ trilayers with ferromagnetic coupling along the nodal direction of $YBa_2 Cu_3 O_7$,"I have successfully prepared (110) trilayers of \LSMO-\YBCO-\LSMO. Magnetization measurements on these samples reveal a stronger coupling between the ferromagnetic layers. The coupling is an order of magnitude higher than that seen in the case of (001) trilayers. Magnetoresistance measurements show a first order transition in the data coinciding with the antiferromagnetic regime deduced from the magnetization measurements. I have also measured the anisotropic magnetoresistance (AMR) of these samples revealing an unusually high AMR ($\sim 72000%$). I attribute such a high AMR to the pair breaking effects in these films.",1003.1414v1 2010-03-16,Macroscopic quantum state in a semiconductor device,"We show how nanostructuring of a metallic gate on a field-effect transistor (FET) can lead to a macroscopic, robust and voltage controlled quantum state in the electron channel of a FET. A chain of triple quantum dot molecules created by gate structure realizes a spin-half Heisenberg chain with spin-spin interactions alternating between ferromagnetic and anti-ferromagnetic. The quantum state is a semiconductor implementation of an integer spin-one antiferromagnetic Heisenberg chain with a unique correlated ground state and a finite energy gap, originally conjectured by Haldane.",1003.3162v1 2010-03-16,Interference patterns of multifacet 20x(0-pi-) Josephson junctions with ferromagnetic barrier,"We have realized multifacet Josephson junctions with periodically alternating critical current density (MJJs) using superconductor-insulator-ferromagnet-superconductor heterostructures. We show that anomalous features of critical current vs. applied magnetic field, observed also for other types of MJJs, are caused by a non-uniform flux density (parallel to the barrier) resulting from screening currents in the electrodes in the presence of a (parasitic) off-plane field component.",1003.3216v1 2010-03-19,Dynamics of magnetization on the topological surface,"We investigate theoretically the dynamics of magnetization coupled to the surface Dirac fermions of a three dimensional topological insulator, by deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge current. Both the inverse spin-Galvanic effect and the Gilbert damping coefficient $\alpha$ are related to the two-dimensional diagonal conductivity $\sigma_{xx}$ of the Dirac fermion, while the Berry phase of the ferromagnetic moment to the Hall conductivity $\sigma_{xy}$. The spin transfer torque and the so-called $\beta$-terms are shown to be negligibly small. Anomalous behaviors in various phenomena including the ferromagnetic resonance are predicted in terms of this LLG equation.",1003.3769v1 2010-03-28,Giant magnetic broadening of ferromagnetic resonance in a GMR Co/Ag/Co/Gd quadlayer,"Both magnetic-resonance damping and the giant magnetoresistance effect have been predicted to be strongly affected by the local density of states in thin ferromagnetic films. We employ the antiferromagnetic coupling between Co and Gd to provide a spontaneous change from parallel to antiparallel alignment of two Co films. A sharp increase in magnetic damping accompanies the change from parallel to antiparallel alignment, analogous to resistivity changes in giant magnetoresistance.",1003.5344v1 2010-03-28,Conserved spin and orbital phase along carbon nanotubes connected with multiple ferromagnetic contacts,"We report on spin dependent transport measurements in carbon nanotubes based multi-terminal circuits. We observe a gate-controlled spin signal in non-local voltages and an anomalous conductance spin signal, which reveal that both the spin and the orbital phase can be conserved along carbon nanotubes with multiple ferromagnetic contacts. This paves the way for spintronics devices exploiting both these quantum mechanical degrees of freedom on the same footing.",1003.5387v1 2010-03-29,Quantum magnetism and counterflow supersolidity of up-down bosonic dipoles,"We study a gas of dipolar Bosons confined in a two-dimensional optical lattice. Dipoles are considered to point freely in both up and down directions perpendicular to the lattice plane. This results in a nearest neighbor repulsive (attractive) interaction for aligned (anti-aligned) dipoles. We find regions of parameters where the ground state of the system exhibits insulating phases with ferromagnetic or anti-ferromagnetic ordering, as well as with rational values of the average magnetization. Evidence for the existence of a novel counterflow supersolid quantum phase is also presented.",1003.5591v2 2010-03-30,Colossal anisotropy in diluted magnetic topological insulators,"We consider dilute magnetic doping in the surface of a three dimensional topological insulator where a two dimensional Dirac electron gas resides. We find that exchange coupling between magnetic atoms and the Dirac electrons has a strong and peculiar effect on both. First, the exchange-induced single ion magnetic anisotropy is very large and favors off-plane orientation. In the case of ferromagnetically ordered phase we find a colossal magnetic anisotropy energy, of the order of the critical temperature. Second, a persistent electronic current circulates around the magnetic atom and, in the case of a ferromagnetic phase, around the edges of the surface.",1003.5931v1 2010-04-08,Spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites,"Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites are calculated with realistic parameters in order to provide a quantitative comparison with experimental results for spin stiffness, magnon dispersion, magnon damping, anomalous zone-boundary magnon softening, and Curie temperature. The role of orbital degeneracy, orbital ordering, and orbital correlations on spin dynamics in different doping regimes is highlighted.",1004.1352v2 2010-04-15,Ferromagnetism in repulsive Fermi gases: upper branch of Feshbach resonance versus hard spheres,"We use quantum Monte Carlo, including backflow corrections, to investigate a two-component Fermi gas on the upper branch of a Feshbach resonance and contrast it with the hard sphere gas. We find that, in both cases, the Fermi liquid becomes unstable to ferromagnetism at a $k_F a$ smaller than the mean field result, where $k_F$ is the Fermi wavevector and $a$ the scattering length. Even though the total energies $E(k_F a)$ are similar in the two cases, their pair correlations and kinetic energies are completely different, reflecting the underlying potentials. We discuss the extent to which our calculations shed light on recent experiments.",1004.2680v1 2010-04-26,Majorana fermion chain at the Quantum Spin Hall edge,"We study a realization of a 1d chain of Majorana bound states at the interfaces between alternating ferromagnetic and superconducting regions at a quantum spin Hall insulator edge. In the limit of well separated Majoranas, the system can be mapped to the transverse field Ising model. The disordered critical point can be reached by tuning the relative magnitude or phases of the ferromagnetic and superconducting order parameters. We compute the voltage dependence of the tunneling current from a metallic tip into the Majorana chain as a direct probe of the random critical state.",1004.4585v2 2010-04-26,Ferromagnetism of cold fermions loaded into a decorated square lattice,"We investigate two-component ultracold fermions loaded into a decorated square lattice, which are described by the Hubbard model with repulsive interactions and nearest neighbor hoppings. By combining the real-space dynamical mean-field theory with the numerical renormalization group method, we discuss how a ferromagnetically ordered ground state in the weak coupling regime, which originates from the existence of a dispersionless band, is adiabatically connected to a Heisenberg ferrimagnetic state in the strong coupling limit. The effects of level splitting and hopping imbalance are also addressed.",1004.4650v1 2010-06-04,Scaling Behavior of Response Functions in the Coarsening Dynamics of Disordered Ferromagnets,"We study coarsening dynamics in the ferromagnetic random bond Ising model in d = 1; 2. We focus on the validity of super-universality and the scaling properties of the response functions. In the d = 1 case, we obtain a complete understanding of the evolution, from pre- asymptotic to asymptotic behavior. The corresponding response function shows a clear violation of super-universality. Further, our results for d = 1; 2 settle the controversy regarding the decay exponent which characterizes the response function.",1006.0934v1 2010-06-07,"Entanglement, fidelity, and quantum phase transition in antiferromagnetic-ferromagnetic alternating Heisenberg chain","The fidelity and entanglement entropy in an antiferromagnetic-ferromagnetic alternating Heisenberg chain are investigated by using the method of density-matrix renormalization-group. The effect of anisotropy on fidelity and entanglement entropy are investigated. The relations between fidelity, entanglement entropy and quantum phase transition are analyzed. It is found that the quantum phase transition point can be well characterized by both the ground-state entropy and fidelity for large system.",1006.1155v1 2010-06-09,Impurity effects and ferromagnetism in excitonic insulators,"Both nonmagnetic and magnetic impurity effects in spin singlet and triplet excitonic insulators were investigated. The bound state energies caused by single impurity were given. The different compositions of the bound states can be used to detect the symmetry of the excitonic insulators. In finite concentration problems, nonmagnetic impurities showed same pair-breaking effect in singlet and triplet excitonic insulators while magnetic impurities showed weaker pair-breaking effect in triplet excitonic insulators than in singlet ones. The pair-breaking effects suppressed the ferromagnetic range via doping and gave a natural explaination for experimental results.",1006.1687v1 2010-06-09,Analytical expression of the magneto-optical Kerr effect and Brillouin light scattering intensity arising from dynamic magnetization,"Time-resolved magneto-optical Kerr effect (MOKE) and Brillouin light scattering (BLS) spectroscopy are important techniques for the investigation of magnetization dynamics. Within this article, we calculate analytically the MOKE and BLS signals from prototypical spin-wave modes in the ferromagnetic layer. The reliability of the analytical expressions is confirmed by optically exact numerical calculations. Finally, we discuss the dependence of the MOKE and BLS signals on the ferromagnetic layer thickness.",1006.1906v1 2010-06-17,Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet/Semiconductor Heterostructures,"We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically-injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a MnAs/GaAs interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations exceeding 4 pi radians during transport.",1006.3554v1 2010-06-24,Periodic solutions for the Landau-Lifshitz-Gilbert equation,"Ferromagnetic materials tend to develop very complex magnetization patterns whose time evolution is modeled by the so-called Landau-Lifshitz-Gilbert equation (LLG). In this paper, we construct time-periodic solutions for LLG in the regime of soft and small ferromagnetic particles which satisfy a certain shape condition. Roughly speaking, it is assumed that the length of the particle is greater than its hight and its width. The approach is based on a perturbation argument and the spectral analysis of the corresponding linearized problem as well as the theory of sectorial operators.",1006.4765v1 2010-07-13,Magnetic-field control of the electric polarization in BiMnO3,"We present the microscopic theory of improper multiferroicity in BiMnO3, which can be summarized as follows: (1) the ferroelectric polarization is driven by the hidden antiferromagnetic order in the otherwise centrosymmetric C2/c structure; (2) the relativistic spin-orbit interaction is responsible for the canted spin ferromagnetism. Our analysis is supported by numerical calculations of electronic polarization using Berry's phase formalism, which was applied to the low-energy model of BiMnO3 derived from the first-principles calculations. We explicitly show how the electric polarization can be controlled by the magnetic field and argue that BiMnO3 is a rare and potentially interesting material where ferroelectricity can indeed coexist and interplay with the ferromagnetism.",1007.2039v1 2010-07-13,Tunable phase diagram and vortex pinning in a superconductor-ferromagnet bilayer,"We have observed the evolution of phase diagram and vortex pinning using a single ferromagnet/superconductor bilayer of [Co/Pt]8/Nb through a special demagnetization procedure. It induces a continuous and reversible change of the domain width with equal positive/negative domains enabling the observation of the predicted tunable phase diagram. The tunable domain pattern also systematically affects vortex pinning. We have determined the dependence of the activation energy of vortex pinning on domain width, temperature, and magnetic field.",1007.2179v1 2010-07-14,Frustration-induced insulating chiral spin state in itinerant triangular-lattice magnets,"We study the double-exchange model at half-filling with competing superexchange interactions on a triangular lattice, combining exact diagonalization and Monte-Carlo methods. We find that in between the expected itinerant ferromagnetic and $120^{\circ}$ Yafet-Kittel phases a robust scalar-chiral, insulating spin state emerges. At finite temperatures the ferromagnet - scalar-chiral quantum critical point is characterized by anomalous bad-metal behavior in charge transport as observed in frustrated itinerant magnets R$_2$Mo$_{2}$O$_7$.",1007.2278v2 2010-07-20,Finite-temperature magnetism in bcc Fe under compression,"We investigate the contributions of finite-temperature magnetic fluctuations to the thermodynamic properties of bcc Fe as a function of pressure. First, we apply a tight-binding total-energy model parameterized to first-principles linearized augmented planewave computations to examine various ferromagnetic, anti-ferromagnetic, and noncollinear spin spiral states at zero temperature. The tight-binding data are fit to a generalized Heisenberg Hamiltonian to describe the magnetic energy functional based on local moments. We then use Monte Carlo simulations to compute the magnetic susceptibility, the Curie temperature, heat capacity, and magnetic free energy. Including the finite-temperature magnetism improves the agreement with experiment for the calculated thermal expansion coefficients.",1007.3422v1 2010-07-26,"Spin fluctuations, susceptibility and the dipole oscillation of a nearly ferromagnetic Fermi gas","We discuss the spin fluctuations and the role played by the magnetic susceptibility in an atomic Fermi gas interacting with positive scattering length. Both thermal and quantum fluctuations are considered. Using a sum rule approach and recent {\it ab initio} Monte Carlo results for the magnetic susceptibility of uniform matter we provide explicit predictions for the frequency of the spin dipole oscillation of a gas trapped by a harmonic potential and discuss the deviations from the behaviour of an ideal gas when the system approaches the ferromagnetic transition. The role of the Landau's parameters in the characterization of the magnetic properties is also discussed.",1007.4504v1 2010-07-28,Microwave response of a magnetic single-electron transistor,"We consider a single-electron transistor in the form of a ferromagnetic dot in contact with normal-metal and pinned ferromagnetic leads. Microwave-driven precession by the dot induces a pumped electric current. In open circuits, this pumping produces a measurable reverse bias voltage, which can be enhanced and made highly nonlinear by Coulomb blockade in the dot. The dependence of this bias on the power and spectrum of microwave irradiation may be utilized to develop nanoscale microwave detectors analogous to single-electron transistor-based electrostatic sensors and nanoelectromechanical devices.",1007.4966v2 2010-07-28,Spin-dependent transport through quantum-dot Aharonov-Bohm interferometers,"We study the influence of spin polarization on the degree of coherence of electron transport through interacting quantum dots. To this end, we identify transport regimes in which the degree of coherence can be related to the visibility of the Aharonov-Bohm oscillations in the current through a quantum-dot Aharonov-Bohm interferometer with one normal and one ferromagnetic lead. For these regimes, we calculate the visibility and, thus, the degree of coherence, as a function of the degree of spin polarization of the ferromagnetic lead.",1007.4988v2 2010-07-28,Current-induced torques in the presence of spin-orbit coupling,"In systems with strong spin-orbit coupling, the relationship between spin-transfer torque and the divergence of the spin current is generalized to a relation between spin transfer torques, total angular momentum current, and mechanical torques. In ferromagnetic semiconductors, where the spin-orbit coupling is large, these considerations modify the behavior of the spin transfer torques. One example is a persistent spin transfer torque in a spin valve: the spin transfer torque does not decay away from the interface, but approaches a constant value. A second example is a mechanical torque at single ferromagnetic-nonmagnetic interface.",1007.5037v1 2010-08-02,"Tipping without Flipping: A Novel Metastable ""Tilted"" State in Anisotropic Ferromagnets in External Fields","We show that in suitable anisotropic ferromagnets, both stable and metastable ``tilted'' phases occur, in which the magnetization ${\vec M}$ makes an angle between zero and $180$ degrees with the externally applied ${\vec H}$. Tuning either the magnitude of the external field or the temperature can lead to continuous transitions between these states. A unique feature is that one of these transitions is between two {\it metastable} states. Near the transitions the longitudinal susceptibility becomes anomalous with an exponent which has an {\it exact} scaling relation with the critical exponents.",1008.0215v1 2010-08-02,Tunneling spectroscopy probing magnetic and nonmagnetic electrodes in tunnel junctions,"Tunneling spectroscopy is applied to tunnel junctions with only one or no ferromagnetic electrode to study the excitation of quasi particles in magnetic tunnel junctions. The bias dependence is investigated with high accuracy by inelastic electron tunneling spectroscopy. Both types of junctions show a zero bias anomaly that is different in size and sign compared to magnetic tunnel junctions, i.e. junctions with two ferromagnetic electrodes. A pronounced difference is also found depending on the material that is probed by the tunneling electrons, which might be attributed to the excitation of magnons.",1008.0326v1 2010-08-05,Anisotropic magnetocaloric effect in all-ferromagnetic (La0.7Sr0.3MnO3/SrRuO3) superlattices,"We exploit the magnetic interlayer coupling in La0.7Sr0.3MnO3/SrRuO3 superlattices to realize a crossover between inverse and conventional magnetic entropy changes. Our data reveal a strong anisotropic nature of the magnetocaloric effect due to the magnetic anisotropy of the superlattice. Therefore, artificial superlattices built from ferromagnetic materials that can be used to alter the magnetic structure as well as the magnetic anisotropy, could also be utilized for tuning the magnetocaloric properties, which may open a constructive approach for magnetic refrigeration applications.",1008.1041v1 2010-08-06,Ferromagnetic phases in spin-Fermion systems,"Spin-Fermion systems which obtain their magnetic properties from a system of localized magnetic moments being coupled to conducting electrons are considered. The dynamical degrees of freedom are spin-$s$ operators of localized spins and spin-1/2 Fermi operators of itinerant electrons. Renormalized spin-wave theory, which accounts for the magnon-magnon interaction, and its extension are developed to describe the two ferrimagnetic phases in the system: low temperature phase $0 Re(n) < 0. Here we demonstrate experimentally that in such natural metals as pure Co and FeCo alloy the negative values of the refractive index are achieved close to the frequency of the ferromagnetic resonance. Large values of the negative refraction can be obtained at room temperature and they can easily be tuned in moderate magnetic fields.",1011.3663v1 2010-11-18,Possibility of exchange switching ferromagnet - antiferromagnet junctions,"Current flowing is studied in magnetic junctions consisting of a ferromagnetic metal (FM), antiferromagnetic conductor (AFM) and a nonmagnetic metal closing the electric circuit. The FM layer with high anisotropy and pinned spins of the magnetic atoms in the lattice acts as a spin injector relative to the AFM layer. To obtain resulting magnetization in the AFM layer, magnetic field is applied, which may be varied to control the magnetization. The spin-polarized current from the FM layer creates a torque and makes it possible to switch the magnetization. A possibility is shown to lower the threshold current density by the orders of magnitude by means of the magnetic field.",1011.4303v1 2010-11-22,Wang-Landau study of the triangular Blume-Capel ferromagnet,"We report on numerical simulations of the two-dimensional Blume-Capel ferromagnet embedded in the triangular lattice. The model is studied in both its first- and second-order phase transition regime for several values of the crystal field via a sophisticated two-stage numerical strategy using the Wang-Landau algorithm. Using classical finite-size scaling techniques we estimate with high accuracy phase-transition temperatures, thermal, and magnetic critical exponents and we give an approximation of the phase diagram of the model.",1011.4780v2 2010-11-24,The competition between superconductivity and ferromagnetism in small metallic grains: thermodynamic properties,"We study the thermodynamic properties of a small superconducting metallic grain using a quantum Monte Carlo method. The grain is described by the universal Hamiltonian, containing pairing and ferromagnetic exchange correlations. In particular, we study how the thermodynamic signatures of pairing correlations are affected by the spin exchange interaction. We find the exchange interaction effects to be qualitatively different in the BCS and fluctuation-dominated regimes of pairing correlations.",1011.5421v1 2010-12-06,Local electrical characterization of resonant magnetization motion in a single ferromagnetic sub-micrometer particle in lateral geometry,"In this article a detailed characterization of a magnetization motion in a single sub-micrometer and multi-terminal ferromagnetic structure in lateral geometry is performed in a GHz regime using direct DC characterization technique. We have shown applicability of the Stoner-Wohlfarth model to the magnetic nano-structure with large length to with ratio. Applying the model to experimental data we are able to extract relevant magnetization motion parameters and show a correlation between high frequency inductive currents and local magnetization. Additionally, DC voltage generated over the structure at the resonance, with external magnetic field under an angle to the shape anisotropy axis, is explained by the model.",1012.1266v1 2010-12-10,Active microcantilevers based on piezoresistive ferromagnetic thin films,"We report the piezoresisitivity in magnetic thin films of FeGa and their use for fabricating self transducing microcantilevers. The actuation occurs as a consequence of both the ferromagnetic and magnetostrictive property of FeGa thin films, while the deflection readout is achieved by exploiting the piezoresisitivity of these films. This self-sensing, self-actuating micromechanical system involves a very simple bilayer structure, which eliminates the need for the more complex piezoelectric stack that is commonly used in active cantilevers. Thus, it potentially opens opportunities for remotely actuated, cantilever-based sensors.",1012.2330v1 2010-12-20,Fully electrically read-write device out of a ferromagnetic semiconductor,"We report the realization of a read-write device out of the ferromagnetic semiconductor (Ga,Mn)As as the first step to fundamentally new information processing paradigm. Writing the magnetic state is achieved by current-induced switching and read-out of the state is done by the means of the tunneling anisotropic magneto resistance (TAMR) effect. This one bit demonstrator device can be used to design a electrically programmable memory and logic device.",1012.4252v1 2010-12-21,Spin superconductor in ferromagnetic graphene,"We show a spin superconductor (SSC) in ferromagnetic graphene as the counterpart to the charge superconductor, in which a spin-polarized electron-hole pair plays the role of the spin $2 (\hbar/2)$ `Cooper pair' with a neutral charge. We present a BCS-type theory for the SSC. With the `London-type equations' of the super-spin-current density, we show the existence of an electric `Meissner effect' against a spatial varying electric field. We further study a SSC/normal conductor/SSC junction and predict a spin-current Josephson effect.",1012.4528v2 2010-12-28,Weak ferromagnetism in cobalt oxalate crystals,"Microcrystals of diaquocobalt(II) oxalate have been synthesized by the coprecipitation reaction of aqueous solutions of Cobalt (II) bromide and oxalic acid. Chemical analysis and thermal experiments revealed that there is only one phase present. X-ray powder diffraction studies show that this compound is orthorhombic with space group Cccm. Molar susceptibility versus temperature measurements show the existence of an antiferromagnetic ordering, however, the hysteresis measured in magnetization measurements as a function of magnetic field reveals a weak ferromagnetic behavior.",1012.5824v1 2011-01-07,Novel effects of localization due to `intrinsic disorder' in the `two-fluid' model for manganites,"We discuss the effects of a novel polaronic disorder in the recently proposed two-fluid model for manganites. Using effective field theory as well as direct numerical simulations, we show that this disorder can have dramatic effects in terms of the transition from ferromagnetic insulator to ferromagnetic metal upon hole-doping, including an Anderson localized regime where variable range hopping may be observed.",1101.1370v1 2011-01-08,"Voids and Mn-rich inclusions in a (Ga,Mn)As ferromagnetic semiconductor investigated by transmission electron microscopy","Voids adjacent to both cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in annealed Ga0.995Mn0.005As magnetic semiconductor specimen grown by molecular beam epitaxy. Nano-beam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter from that of bulk MnAs. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals. After annealing at 903 K, the magnetic transition temperature of the specimen likely to be dominated by the presence of cubic ferromagnetic nanocrystals.",1101.1600v1 2011-01-11,Magnon Pumping by a Time-Dependent Transverse Magnetic Field in Ferromagnetic Insulators,"The magnon pumping effect in ferromagnetic insulators under an external time-dependent transverse magnetic field is theoretically studied. Generation of a magnon current is discussed by calculating the magnon source term in the spin continuity equation. This term represents the non-conservation of magnons arising from an applied transverse magnetic field. The magnon source term has a resonance structure as a function of the angular frequency of the transverse field, and this fact is useful to enhance the pumping effect.",1101.2137v2 2011-01-20,Tunneling Hamiltonian description of the atomic-scale 0-pi transition in superconductor/ferromagnetic-insulator junctions,"We show a perturbation theory of the Josephson transport through ferromagnetic insulators (FIs). Recently we have found that the appearance of the atomic scale 0-pi transition in such junctions based on numerical calculations. In order to explore the mechanism of this anomalous transition, we have analytically calculated the Josephson current using the tunneling Hamiltonian theory and found that the spin dependent pi-phase shift in the FI barrier gives the atomic scale 0-pi transition.",1101.3806v1 2011-01-20,Controllable manipulation of superconductivity using magnetic vortices,"The magneto-transport of a superconducting/ferromagnetic hybrid structure consisting of a superconducting thin film in contact with an array of magnetic nanodots in the so-called ""magnetic vortex-state"" exhibits interesting properties. For certain magnetic states, the stray magnetic field from the vortex array is intense enough to drive the superconducting film into the normal state. In this fashion, the normal-to-superconducting phase transition can be controlled by the magnetic history. The strong coupling between superconducting and magnetic subsystems allows characteristically ferromagnetic properties, such as hysteresis and remanence, to be dramatically transferred into the transport properties of the superconductor.",1101.3846v1 2011-01-29,Spin-injection terahertz radiation in magnetic junctions,"Electromagnetic radiation of 1 - 10 THz range has been found at room temperature in a structure with a point contact between a ferromagnetic rod and a thin ferromagnetic film under electric current of high enough density. The radiation is due to nonequilibrium spin injection between the structure components. By estimates, the injection can lead to inverted population of the spin subbands. The radiation power exceeds by orders of magnitude the thermal background (with the Joule heating taking into account) and follows the current without inertia.",1101.5683v1 2011-01-30,Spin-Charge Locking and Tunneling into a Helical Metal,"We derive a kinetic equation for the electrons moving on the surface of a three-dimensional topological insulator. Due to the helical nature of the excitations backward scattering is suppressed in the collision integral, and the spin dynamics is entirely constrained by that of the charge. We further analyze the tunneling between the helical and a conventional metal or ferromagnet. We find that the tunnel resistance strongly depends on the angle between the magnetization in the ferromagnet and the current in the helical metal. A nonmagnetic layer on top of the helical metal amplifies the current-induced spin polarization.",1101.5802v1 2011-02-10,Circularly Polarized Resonant Rayleigh Scattering and Skyrmions in the $ν$ = 1 Quantum Hall Ferromagnet,"We use the circularly polarized resonant Rayleigh scattering (RRS) to study the quantum Hall ferromagnet at $\nu$ = 1. At this filling factor we observe a right handed copolarized RRS which probes the Skyrmion spin texture of the electrons in the photoexcited grounds state. The resonant scattering is not present in the left handed copolarization, and this can be related to the correlation between Skymionic effects, screening and spin wave excitations. These results evidence that RRS is a valid method for the study of the spin texture of the quantum Hall states.",1102.2185v2 2011-02-11,Chiral spin states in polarized kagome spin systems with spin-orbit coupling,"We study quantum spin systems with a proper combination of geometric frustration, spin-orbit coupling and ferromagnetism. We argue that such a system is likely to be in a chiral spin state, a fractional quantum Hall (FQH) state for bosonic spin degrees of freedom. The energy scale of the bosonic FQH state is of the same order as the spin-orbit coupling and ferromagnetism --- overall much higher than the energy scale of FQH states in semiconductors.",1102.2406v1 2011-02-17,Thermodynamics of the frustrated one-dimensional spin-1/2 Heisenberg ferromagnet in a magnetic field,"We calculate the low-temperature thermodynamic quantities (magnetization, correlation functions, transverse and longitudinal correlation lengths, spin susceptibility, and specific heat) of the frustrated one-dimensional spin-half J1-J2 Heisenberg ferromagnet, i.e. for J2< 0.25|J1|, in an external magnetic field using a second-order Green-function formalism and full diagonalization of finite systems. We determine power-law relations for the field dependence of the position and the height of the maximum of the uniform susceptibility. Considering the specific heat at low magnetic fields, two maxima in its temperature dependence are found.",1102.3581v2 2011-03-11,Atomic scale 0-pi transition in a high-Tc superconductor/ferromagnetic-insulator/high-T superconductor Josephson junction,"We study the Josephson transport in a high-Tc superconductor/ferromagnetic-insulator(FI)/high-Tc superconductor numerically. We found the formation of a pi-junction in such systems. More remarkably the ground state of such junction alternates between 0- and pi-states when thickness of FI is increasing by a single atomic layer. We propose an experimental setup for observing the atomic-scale 0-pi transition. Such FI-based pi-junctions can be used to implement highly-coherent quantum bits.",1103.2187v1 2011-03-12,Effective field theories and spin-wave excitations in helical magnets,"We consider two classes of helical magnets. The first one has magnetic ordering close to antiferromagnet and the second one has magnetic ordering close to ferromagnet. The first case is relevant to cuprate superconductors and the second case is realized in FeSrO$_3$ and FeCaO$_3$. We derive the effective field theories for these cases and calculate corresponding excitation spectra. We demonstrate that the ""hourglass"" spin-wave dispersion observed experimentally in cuprates is a fingerprint of the ""antiferromagnetic spin spiral state"". We also show that quantum fluctuations are important for the ""ferromagnetic spin spiral"", they influence qualitative features of the spin-wave dispersion.",1103.2422v1 2011-03-23,Critical scaling to infinite temperature,"Three dimensional Ising model ferromagnets on different lattices with nearest neighbor interactions, and on simple cubic lattices with equivalent interactions out to further neighbors, are studied numerically. The susceptibility data for all these systems are analyzed using the critical Renormalization Group Theory formalism over the entire temperature range above Tc with an appropriate choice of scaling variable and scaling expressions. Representative experimental data on a metallic ferromagnet (Ni) and an elementary fluid (Xe) are interpreted in the same manner so as to estimate effective coordination numbers.",1103.4491v1 2011-03-31,Electromagnetic propulsion and separation by chirality of nanoparticles in liquids,"We introduce a new mechanism for the propulsion and separation by chirality of small ferromagnetic particles suspended in a liquid. Under the action of a uniform d.c. magnetic field H and an a.c. electric field E isomers with opposite chirality move in opposite directions. Such a mechanism could have a significant impact on a wide range of emerging technologies. The component of the chiral velocity that is odd in H is found to be proportional to the intrinsic orbital and spin angular momentum of the magnetized electrons. This effect arises because a ferromagnetic particle responds to the applied torque as a small gyroscope.",1104.0026v1 2011-04-04,Hysteresis in Anti-Ferromagnetic Random-Field Ising Model at Zero Temperature,"We study hysteresis in anti-ferromagnetic random-field Ising model at zero temperature. The external field is cycled adiabatically between -$\infty$ and $\infty$. Two different distributions of the random-field are considered, (i) a uniform distribution of width $2\Delta$ centered at the origin, and (ii) a Gaussian distribution with average value zero and standard deviation $\sigma$. In each case the hysteresis loop is determined exactly in one dimension and compared with numerical simulations of the model.",1104.0586v1 2011-04-28,Temperature dependent anisotropy and elastic effects in ferromagnetic nanowire arrays,"Temperature dependent Ferromagnetic Resonance measurements performed as a function of diameter on Nickel nanowire arrays reveal several interesting features in these systems. With diameter decrease from 100 nm to 15 nm, a transition induced by surface anisotropy increase is observed at 50 nm in easy axis orientation from parallel to perpendicular with respect to individual nanowire geometric axis. Analysis of resonance field $H_{res}$ temperature variation (between liquid Helium and room temperature) reveals underlying strong magneto-elastic effects in small and large diameter nanowire arrays with potential applications in recording and spintronics.",1104.5348v3 2011-05-05,Spontaneous separation of two-component Fermi gases in a double-well trap,"The two-component Fermi gas in a double-well trap is studied using the density functional theory and the density profile of each component is calculated within the Thomas-Fermi approximation. We show that the two components are spatially separate in the two wells once the repulsive interaction exceeds the Stoner point, signaling the occurrence of the ferromagnetic transition. Therefore, the double-well trap helps to explore itinerant ferromagnetism in atomic Fermi gases, since the spontaneous separation can be examined by measuring component populations in one well.",1105.0995v1 2011-05-09,Precessing ball solitons as dissipative structures during a phase transition in a ferromagnet,"Precessing ball solitons (PBS) in a ferromagnet during the first order phase transition induced by a magnetic field directed along the axis of anisotropy, while the action of the periodic field perpendicular to the main magnetic field, have been analyzed. Under this condition, the characteristics of arising equilibrium PBS's are uniquely determined by the frequency of the periodic field, but the solitons with other frequencies are impossible. It is shown that the equilibrium PBS's are essentially the ""dissipative structures"" that can arise in a metastable state.",1105.1807v1 2011-05-16,On the line shape of the electrically detected ferromagnetic resonance,"This work reviews and examines two particular issues related with the new technique of electrical detection of ferromagnetic resonance (FMR). This powerful technique has been broadly applied for studying magnetization and spin dynamics over the past few years. The first issue is the relation and distinction between different mechanisms that give rise to a photovoltage via FMR in composite magnetic structures, and the second is the proper analysis of the FMR line shape, which remains the ""Achilles heel"" in interpreting experimental results, especially for either studying the spin pumping effect or quantifying the spin Hall angles via the electrically detected FMR.",1105.3236v1 2011-05-24,Tunable anisotropic magnetism in trapped two-component Bose gases,"We theoretically address magnetic ordering at zero and finite temperature in both homogeneous and trapped Bose-Bose mixtures in optical lattices. By using Bosonic Dynamical Mean-Field Theory, we obtain the phase diagram of the homogeneous two-component Bose-Hubbard model in a three-dimensional (3D) cubic lattice, which features competing magnetic order of XY-ferromagnetic and anti-ferromagnetic type in addition to the Mott and superfluid states. We show that these magnetic phases persist also in the presence of a harmonic trap.",1105.4886v1 2011-05-25,Non-Gaussian resistance noise in the ferromagnetic insulating state of a hole doped manganite,"We report the observation of a large 1/f noise in the ferromagnetic insulating state (FMI) of a hole doped manganite single crystal of La0.80Ca0.20MnO3 which manifests hopping conductivity in presence of a Coulomb gap. The temperature dependent noise magnitude shows a deep within the FMI state, there is a sharp freeze out of the noise magnitude with temperature on cooling. As the material enters the FMI state, the noise becomes non-Gaussian as seen through probability density function and second spectra. It is proposed to arise from charge fluctuations in a correlated glassy phase of the polaronic carriers which develop in these systems as reported in recent simulation studies.",1105.5079v1 2011-05-30,Dynamical spin-flip susceptibility for a strongly interacting ultracold Fermi gas,"The Stoner model predicts that a two-component Fermi gas at increasing repulsive interactions undergoes a ferromagnetic transition. Using the random-phase approximation we study the dynamical properties of the interacting Fermi gas. For an atomic Fermi gas under harmonic confinement we show that the transverse (spin-flip) dynamical susceptibility displays a clear signature of the ferromagnetic phase in a magnon peak emerging from the Stoner particle-hole continuum. The dynamical spin susceptibilities could be experimentally explored via spin-dependent Bragg spectroscopy.",1105.5997v1 2011-06-01,Ferromagnetic and antiferromagnetic dimer splittings in LaMn0.1Ga0.9O3,"Inelastic neutron scattering was employed to study the magnetic excitations of Mn3+ dimers in LaMn0.1Ga0.9O3. The nearest-neighbor interaction of Mn3+ ions is ferromagnetic in the basal (a,b)-plane, but antiferromagnetic along the c-direction, thus two different types of dimer excitations are simultaneously present in the experiments. From the observed energy spectra we derive Heisenberg-type exchange interactions Jab=0.210(4) meV and Jc=-0.285(5) meV as well as an axial anisotropy parameter D=0.036(6) meV. These parameters considerably differ from those derived for the isostructural parent compound LaMnO3 due to structural effects.",1106.0161v1 2011-06-02,Cubic to hexagonal iron phase transition promoted by interstitial hydrogen,"Using ab-initio density functional theory we study the role of interstitial hydrogen on the energetics of the phase transformation of iron from bcc to hcp along Bain's pathway. The impurity creates an internal stress field that can be released through a tetragonal distortion of the lattice, promoting the bcc (ferromagnetic) $\rightarrow$ fcc (frustrated antiferromagnetic) $\rightarrow$ hcp (ferromagnetic) transition. The transformation between crystal systems is accompanied by a drastic magnetic reorganization and sudden variations of the unit cell volume, that can be one of the reasons for embrittlement and mechanical failure of iron upon hydrogen adsorption.",1106.0386v1 2011-06-06,Rise and fall of defect induced ferromagnetism in SiC single crystals,"6H-SiC (silicon carbide) single crystals containing VSi-VC divacancies are investigated with respect to magnetic and structural properties. We found that an initial increase of structural disorder leads to pronounced ferromagnetic properties at room temperature. Further introduction of disorder lowers the saturation magnetization and is accompanied with the onset of lattice amorphization. Close to the threshold of full amorphization, also divacancy clusters are formed and the saturation magnetization nearly drops to zero.",1106.0966v1 2011-06-07,Goldstone Mode Relaxation in a Quantum Hall Ferromagnet due to Hyperfine Interaction with Nuclei,"Spin relaxation in quantum Hall ferromagnet regimes is studied. As the initial non-equilibrium state, a coherent deviation of the spin system from the ${\vec B}$ direction is considered and the breakdown of this Goldstone-mode state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring non-exponentially with time is studied in terms of annihilation processes in the ""Goldstone condensate"" formed by ""zero spin excitons"". The relaxation rate is calculated analytically even if the initial deviation is not small. This relaxation channel competes with the relaxation mechanisms due to spin-orbit coupling, and at strong magnetic fields it becomes dominating.",1106.1386v1 2011-06-09,Majorana fermions in spin-orbit coupled ferromagnetic Josephson junctions,"We study all possible Majorana modes in two-dimensional spin-orbit coupled ferromagnetic superconductor-normal state-superconductor (SNS) Josephson junctions and propose experiments to detect them. With the S region in a non-trivial topological phase and a superconducting phase difference $\phi = \pi$ across the junction, two delocalized Majorana fermions with no excitation gap appear in the N region. In addition, if S and N belong to different topological phases and have well-separated the Fermi surfaces, localized Majorana fermions with a finite excitation gap also emerge at both SN interfaces for all $\phi$.",1106.1801v2 2011-06-16,Thermomagnonic spin transfer and Peltier effects in insulating magnets,"We study the coupled magnon energy transport and collective magnetization dynamics in ferromagnets with magnetic textures. By constructing a phenomenological theory based on irreversible thermodynamics, we describe motion of domain walls by thermal gradients and generation of heat flows by magnetization dynamics. From microscopic description based on magnon kinetics, we estimate the transport coefficients and analyze the feasibility of energy-related applications in insulating ferromagnets, such as yttrium iron garnet and europium oxide.",1106.3135v2 2011-06-17,Current effect on magnetization oscillations in a ferromagnet - antiferromagnet junction,"Spin-polarized current effect is studied on the static and dynamic magnetization of the antiferromagnet in a ferromagnet - antiferromagnet junction. The macrospin approximation is generalized to antiferromagnets. Canted antiferromagnetic configuration and resulting magnetic moment are induced by an external magnetic field. The resonance frequency and damping are calculated, as well as the threshold current density corresponding to instability appearance. A possibility is shown of generating low-damping magnetization oscillations in terahertz range. The fluctuation effect is discussed on the canted antiferromagnetic configuration.",1106.3519v1 2011-06-17,Gate-dependent spin-torque in a nanoconductor-based spin-valve,"This article discusses the spin-torque effect in a spin-valve made out of two ferromagnetic leads connected through a coherent nanoconductor (NC), in the limit where a single channel of the NC lies near the Fermi energy of the leads. Due to quantum interferences inside the NC, the spin-torque presents clear qualitative differences with respect to the case of a multichannel disordered spin-valve. In particular, it can be modulated with the NC gate voltage. In principle, this modulation can be observed experimentally, assuming that the spin-torque affects a ferromagnetic nano-domain in direct contact with the NC.",1106.3540v1 2011-06-22,Magnetic excitations in iron pnictides,"Spin wave dispersion and damping are investigated in the metallic SDW state of different itinerant electron models including a small interlayer hopping. Magnetic excitations in iron pnictides are shown to be well understood in terms of physical mechanisms characteristic of metallic magnets, such as carrier-induced ferromagnetic spin couplings, intra-band particle-hole excitations, and the spin-charge coupling mechanism, which is also important in ferromagnetic manganites.",1106.4421v1 2011-06-28,Stability of precessing domain walls in ferromagnetic nanowires,"We show that recently reported precessing solution of Landau-Lifshitz-Gilbert equations in ferromagnetic nanowires is stable under small perturbations of initial data, applied field and anisotropy constant. Linear stability is established analytically, while nonlinear stability is verified numerically.",1106.5808v2 2011-07-01,Current induced magnetization reversal on the surface of a topological insulator,"We study dynamics of the magnetization coupled to the surface Dirac fermions of a three di- mensional topological insulator. By solving the Landau-Lifshitz-Gilbert equation in the presence of charge current, we find current induced magnetization dynamics and discuss the possibility of mag- netization reversal. The torque from the current injection depends on the transmission probability through the ferromagnet and shows nontrivial dependence on the exchange coupling. The mag- netization dynamics is a direct manifestation of the inverse spin-galvanic effect and hence another ferromagnet is unnecessary to induce spin transfer torque in contrast to the conventional setup.",1107.0116v1 2011-07-06,Hysteretic magnetic pinning and reversible resistance switching in High-Tc superconductor/ferromagnet multilayers,"We study a high-TC superconducting (YBa2Cu3O7-d) / ferromagnetic (Co/Pt multilayer) hybrid which exhibits resistance switching driven by the magnetic history: depending on the direction of the external field, a pronounced decrease or increase of the mixed-state resistance is observed as magnetization reversal occurs within the Co/Pt multilayer. We demonstrate that stray magnetic fields cause these effects via i) creation of vortices/antivortices and ii) magnetostatic pinning of vortices that are induced by the external field.",1107.1122v1 2011-07-08,Ferromagnetism in lead graphite-pencils and magnetic composite with CoFe2O4 particles,"This work has been initiated with a curiosity to investigate the elemental composition and magnetic response of different grades of lead pencils (6B, 2B, HB, 2H, 5H) that people use in daily life. Interestingly, experimental results landed with a great achievement of observing soft magnetism in lead pencils, indicating a wide scope of magnetic tuning for room temperature applications. A novel magnetic composite has been synthesized by mixing different concentration of CoFe2O4 (CF) nanoparticles in 5H and 6B pencils for studying the magnetic tailoring aspects using pencils. Our results showed different possibilities of controlling disorder induced ferromagnetic parameters and a simple approach of producing sufficiently high coercive magnetic composite using pencils.",1107.1662v1 2011-07-20,Upper critical field of p-wave ferromagnetic superconductors with orthorhombic symmetry,"We extended the Scharnberg-Klemm theory of $H_{c2}(T)$ in p-wave superconductors with broken symmetry to cases of partially broken symmetry in an orthorhombic crystal, as is appropriate for the more exotic ferromagnetic superconductor UCoGe in strong magnetic fields. For some partially broken symmetry cases, $H_{c2}(T)$ can mimic upward curvature in all three crystal axis directions, and reasonably good fits to some of the UCoGe data are obtained.",1107.4071v1 2011-07-26,"Role of interface in ferromagnetism of (Zn,Co)O films","We demonstrate that room temperature ferromagnetic response (RT FR) of ZnCoO films grown at low temperature by the Atomic layer Deposition (ALD) method is due to Co metal accumulations at the ZnCoO/substrate interface region. The accumulated experimental evi evidences allow us to reject several other explanations of this effect in our samples, despite the fact that some of them are likely to be responsible for the low temperature FM in this class of the material.",1107.5188v1 2011-08-09,"Magnetic phases of a highly frustrated magnet, ZnCr2O4, up to an ultra-high magnetic field of 600 T","The Faraday rotation and magneto-optical absorption spectral measurements were conducted to reveal the full-magnetization process and map out a magnetic phase diagram of a typical geometrical frustrated magnet, ZnCr2O4, by using the electromagnetic flux compression method in ultra-magnetic fields up to 600 T. A fully polarized ferromagnetic phase is observed in which the absorption spectra associated with an exciton-magnon-phonon transition disappears. Furthermore, prior to the fully polarized ferromagnetic phase above 410 T, we found a novel magnetic phase above 350 T followed by a canted 3:1 phase.",1108.1932v1 2011-08-16,"All-optical four-state magnetization reversal in (Ga,Mn)As ferromagnetic semiconductors","Using density matrix equations of motion and a tight-binding band calculation, we predict all-optical switching between four metastable magnetic states of (III,Mn)As ferromagnets. This switching is initiated non-thermally within 100fs, during nonlinear coherent photoexcitation. For a single optical pulse, magnetization reversal is completed after $\sim$100 ps and controlled by the coherent femtosecond photoexcitation. Our predicted switching comes from magnetic nonlinearities triggered by a femtosecond magnetization tilt that is sensitive to un--adiabatic light--induced spin interactions.",1108.3193v1 2011-08-19,"Hydrostatic pressure study of paramagnetic-ferromagnetic phase transition in (Ga,Mn)As","The effect of hydrostatic pressure on the paramagnetic - ferromagnetic phase transition has been studied in (Ga,Mn)As. The variation of the Curie temperature (TC) with pressure was monitored by two transport methods: (1) - measurement of zero field resistivity versus temperature {\rho}(T), (2) - dependence on temperature of the Hall voltage hysteresis loop. Two specimens of different resistivity characteristics were examined. The measured pressure-induced changes of TC were relatively small (of the order of 1K/GPa) for both samples, however they were opposite for the two.",1108.3960v1 2011-08-22,Spin Density Wave in Insulating Ferromagnetic Frustrated Chain LiCuVO$_4$,"We study field induced quantum phase in weakly-coupled ferromagnetic frustrated chain LiCuVO$_4$ by neutron diffraction technique. A new incommensurate magnetic peak is observed at $H \ge 8.5$ T. The field dependent propagation vector is identified with the spin density wave correlation in the theoretically predicted magnetic quadrupole order. Quantum fluctuation, geometrical frustration, and interchain interaction induce the exotic spin density wave long-range order in the insulating magnet.",1108.4255v1 2011-09-06,Griffiths phase in solid-solution of ferromagnetic manganite and cobaltite,"We report the existence of Griffiths phase (GP) over a wide range of $x$ in La$_{0.6}$Sr$_{0.4}$Mn$_{1-x}$Co$_x$O$_3$, the solid solution of ferromagnetic (FM) La$_{0.6}$Sr$_{0.4}$MnO$_3$ and La$_{0.6}$Sr$_{0.4}$CoO$_3$, from magnetization measurements. In this compound, GP arises due to the quenching of randomly distributed Co-O-Mn antiferromagnetic bonds in the FM background. In contrary to divalent doped manganites, GP in the present system can exist entirely in the metallic state above $T_C$ (for $x$$<$0.10). Based on the present study, a magnetoelectronic phase diagram is drawn.",1109.1090v1 2011-09-07,Ferromagnetic properties of charged vector bosons condensate in the early universe,"Bose-Einstein condensation in the early universe is considered. The magnetic properties of a condensate of charged vector bosons are studied, showing that a ferromagnetic state is formed. As a consequence, the primeval plasma may be spontaneously magnetized inside macroscopically large domains and primordial magnetic fields can be generated.",1109.1330v1 2011-09-24,Josephson effect between triplet superconductors through a ferromagnetic barrier of finite width,"Charge and spin transport in a junction involving two triplet superconductors and a ferromagnetic barrier are studied. We use Bogoliubov-de Gennes wavefunctions to construct the Green's function, from which we obtain the Josephson currents in terms of the Andreev reflection coefficients. We focus on the consequences of a finite barrier width for the occurrence of 0-\pi transitions and for the spin currents, and examine the appropriateness of the common \delta-function approximation for the tunneling region.",1109.5238v2 2011-09-29,Sources of negative tunneling magneto-resistance in multilevel quantum dots with ferromagnetic contacts,"We analyze distinct sources of spin-dependent energy level shifts and their impact on the tunneling magnetoresistance (TMR) of interacting quantum dots coupled to collinearly polarized ferromagnetic leads. Level shifts due to virtual charge fluctuations can be quantitatively evaluated within a diagrammatic representation of our transport theory. The theory is valid for multilevel quantum dot systems and we exemplarily apply it to carbon nanotube quantum dots, where we show that the presence of many levels can qualitatively influence the TMR effect.",1109.6599v1 2011-09-30,Dephasing in ferromagnetic nanowires: The role of spin waves,"We present a calculation of the dephasing time of electrons in a ferromagnet relevant for the conductance fluctuations. We focus on the contribution from the interaction with spin waves. Explicit results are presented for a quasi-one-dimensional systems. Going beyond previous calculations, we do not restrict ourselves to the limit of a small exchange splitting compared to the electronic elastic scattering time, nor does our calculation rely on the diffusion approximation to describe electronic transport.",1109.6796v1 2011-10-18,Life times and chirality of spin-waves in antiferromagnetic and ferromagnetic FeRh: time depedent density functional theory perspective,"The study of the spin excitations in antiferromagnetic (AFM) and ferromagnetic (FM) phases of FeRh is reported. We demonstrate that although the Fe atomic moments are well defined there is a number of important phenomena absent in the Heisenberg description: Landau damping of spin waves, large Rh moments induced by the AFM magnons, the formation of the optical magnons terminated by Stoner excitations. We relate the properties of the spin-wave damping to the features of the Stoner continuum and compare the chirality of the spin excitations in AFM, FM and paramagnetic (PM) systems.",1110.3913v1 2011-11-30,Non-Fraunhofer Interference Pattern in Inhomogeneous Ferromagnetic Josephson Junctions,"Generic conditions are established for producing a non-Fraunhofer response of the critical supercurrent subject to an external magnetic field in ferromagnetic Josephson junctions. Employing the quasiclassical Keldysh-Usadel method, we demonstrate theoretically that an inhomogeneity in the magnitude of the energy scales in the system, including Thouless energy, exchange field and temperature gradient normal to the transport direction, influences drastically the standard Fraunhofer pattern. The exotic non-Fraunhofer response, similar to that observed in recent experiments, is described in terms of an intricate interplay between multiple '0-pi'-states and is related to the appearance of proximity vortices.",1112.0007v1 2011-12-12,Spin susceptibilities in zigzag graphene nanoribbons,"The simple Hubbard Hamiltonian with the mean field approximation is used to know about the energy bands and spin susceptibilities of zigzag graphene nanoribbons. Depending on the electron doping, antiferromagnetic or ferromagnetic configurations are possible; in the former, an energy gap exits which is proportional to the Hubbard parameter, while in the latter the up and down spin bands intersect the Fermi level. Due to the two dimensional nature of the system, a susceptibility square matrix is necessary to explain the self-correlations and correlations between spins in the ribbon unit cell. The transverse spin susceptibilities are computed and the static case for ferromagnetic solutions is examined, as a function of the electron doping.",1112.2601v1 2011-12-28,Spectral signatures of magnetic Bloch oscillations in one-dimensional easy-axis ferromagnets,"Domain walls in a one-dimensional gapped easy-axis ferromagnet can exhibit Bloch oscillations in an applied magnetic field. We investigate how exchange couplings modify this behavior within an approximation based on noninteracting domain-wall bound states. In particular, we obtain analytical results for the spectrum and the dynamic structure factor, and show where in momentum space to expect equidistant energy levels, the Wannier-Zeeman ladder, which is the spectral signature of magnetic Bloch oscillations. We compare our results to previous calculations employing a single domain-wall approximation, and make predictions relevant for the material $CoCl2\cdot2H2O$.",1112.6195v2 2012-02-14,Magnon qubit on double Bose-Einstein condensate,"We have proposed a magnon qubit based on coupled configuration of Bose-Einstein condensates (BEC) in two ferromagnetic samples placed closely to each other. We have evaluated the magnon BEC qubit realization in the double BEC scheme where we found a quantum synchronism condition providing an effective Hamiltonian of magnon qubit. It has the form of well-known superconducting Josephson qubit. The possibilities for coherent magnon BEC qubit rotation are analyzed. Implementations of the magnon BEC qubit are considered for small samples and thin ferromagnetic films. Advantages of the proposed macroscopic qubit realization are discussed.",1202.3057v1 2012-02-22,Dynamic Critical Behavior in Models of Ferromagnetic Gadolinium,"A numerical technique combining Monte Carlo and molecular dynamics simulations is used for the first time to examine the complex critical dynamics of models of ferromagnetic gadolinium in which both strong exchange interactions and relativistic effects of several different types are taken into account. A finitesize scaling technique is used to calculate the corresponding dynamic critical exponents. The role played by isotropic dipole-dipole interaction in the critical behavior of gadolinium is evaluated. The results obtained provide an explanation for the anomalous dynamic critical behavior of gadolinium.",1202.4878v1 2012-02-29,Valence-band satellite in the ferromagnetic nickel: LDA+DMFT study with exact diagonalization,"The valence-band spectrum of the ferromagnetic nickel is calculated using the LDA+DMFT method. The auxiliary impurity model emerging in the course of the calculations is discretized and solved with the exact diagonalization, or, more precisely, with the Lanczos method. Particular emphasis is given to spin dependence of the valence-band satellite that is observed around 6 eV below the Fermi level. The calculated satellite is strongly spin polarized in accord with experimental findings.",1202.6595v1 2012-02-29,Epitaxial Ferromagnetic Nanoislands of Cubic GdN in Hexagonal GaN,"Periodic structures of GdN particles encapsulated in a single crystalline GaN matrix were prepared by plasma assisted molecular beam epitaxy. High resolution X-ray diffractometery shows that GdN islands, with rock salt structure are epitaxially oriented to the wurtzite GaN matrix. Scanning transmission electron microscopy combined with in-situ reflection high energy electron diffraction allows for the study of island formation dynamics, which occurs after 1.2 monolayers of GdN coverage. Magnetometry reveals two ferromagnetic phases, one due to GdN particles with Curie temperature of 70K and a second, anomalous room temperature phase.",1203.0028v2 2012-03-03,Skyrmion Generation by Current,"Skyrmions, once a hypothesized field-theoretical object believed to describe the nature of elementary particles, became common sightings in recent years among several non-centrosymmetric metallic ferromagnets. For more practical applications of Skyrmionic matter as carriers of information, thus realizing the prospect of ""Skyrmionics"", it is necessary to have the means to create and manipulate Skyrmions individually. We show through extensive simulation of the Landau-Lifshitz-Gilbert equation that a circulating current imparted to the metallic chiral ferromagnetic system can create isolated Skyrmionic spin texture without the aid of external magnetic field.",1203.0638v1 2012-06-06,Magnon-driven quantum-dot heat engine,"We investigate a heat- to charge-current converter consisting of a single-level quantum dot coupled to two ferromagnetic metals and one ferromagnetic insulator held at different temperatures. We demonstrate that this nano engine can act as an optimal heat to spin-polarized charge current converter in an antiparallel geometry, while it acts as a heat to pure spin current converter in the parallel case. We discuss the maximal output power of the device and its efficiency.",1206.1259v2 2012-06-24,Vanishing Meissner effect as a hallmark of in-plane FFLO instability in superconductor - ferromagnet layered systems,"We demonstrate that in a wide class of multilayered superconductor - ferromagnet structures (e.g., S/F, S/F/N and S/F/F') the vanishing Meissner effect signals the appearance of the in-plane Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulated superconducting phase. In contrast to the bulk superconductors the FFLO instability in these systems can emerge at temperatures close to the critical one and is effectively controlled by the S layer thickness and the angle between magnetization vectors in the F/F' bilayers. The predicted FFLO state reveals through the critical temperature oscillations vs the perpendicular magnetic field component.",1206.5488v2 2012-07-05,Evidence for the Presence of Spin Accumulation in Localized States at Ferromagnet-Silicon Interfaces,"We experimentally show evidence for the presence of spin accumulation in localized states at ferromagnet-silicon interfaces, detected by electrical Hanle effect measurements in CoFe/$n^{+}$-Si/$n$-Si lateral devices. By controlling the measurement temperature, we can clearly observe marked changes in the spin-accumulation signals at low temperatures, at which the electron transport across the interface changes from the direct tunneling to the two-step one via the localized states. We discuss in detail the difference in the spin accumulation between in the Si channel and in the localized states.",1207.1154v1 2012-07-08,Non-monotonic behaviour of the superconducting order parameter in Nb/PdNi bilayers observed through point contact spectroscopy,"Point contact spectroscopy measurements have been performed on Nb/PdNi bilayers in which the thickness of the Nb layer, dNb, was kept constant to 40 nm while the thickness of PdNi, dPdNi, was changed from 2 nm to 9 nm. Features related to the superconducting gap induced in the ferromagnet have been observed in the dV/dI versus V curves. These structures show a non-monotonic behaviour as a function of dPdNi as a consequence of the damped oscillatory behaviour of the superconducting order parameter in the ferromagnetic layer.",1207.1879v1 2012-07-13,Universality and itinerant ferromagnetism in rotating strongly interacting Fermi gases,"We analytically determine the properties of three interacting fermions in a harmonic trap subject to an external rotation. Thermodynamic quantities such as the entropy and energy are calculated from the third order quantum virial expansion. By parameterizing the solutions in the rotating frame we find that the energy and entropy are universal for all rotations in the strongly interacting regime. Additionally, we find that rotation suppresses the onset of itinerant ferromagnetism in strongly interacting repulsive three-body systems.",1207.3129v1 2012-07-13,Magnetic relaxation in bilayers of yttrium iron garnet/platinum due to the dynamic coupling at the interface,"We show that in ferromagnetic (FM)/normal metal (NM) bilayers the dynamic coupling at the interface transfers an additional magnetic relaxation from the heavily damped motion of the conduction electron spins in the NM layer to the FM spins. While the FM relaxation rates due to two-magnon scattering and spin pumping decrease rapidly with increasing FM film thickness, the damping due to the dynamic coupling does not depend on the FM film thickness. The proposed mechanism explains the very large broadening of ferromagnetic resonance lines in thick films of yttrium iron garnet after deposition of a Pt layer.",1207.3330v1 2012-07-13,Surface-induced Magnetism Fluctuations in Single Crystal of NiBi3 Superconductor,"We report anistropy in superconducting and normal state of NiBi3 single crystals with Tc = 4.06 K. The magnetoresistance results indicate the absence of scattering usually associated with ferromagnetic metals, suggesting the absence of bulk long range magnetic order below 300 K. However, the electron spin resonance results demonstrate that ferromagnetism fluctuations exist on the surface of the crystal below 150K.",1207.3345v1 2012-07-17,Magnetic properties of Mn-doped Ge46 and Ba8Ge46 clathrates,"We present a detailed study of the magnetic properties of unique cluster assembled solids namely Mn doped Ge46 and Ba8Ge46 clathrates using density functional theory. We find that ferromagnetic (FM) ground states may be realized in both the compounds when doped with Mn. In Mn2Ge44, ferromagnetism is driven by hybridization induced negative exchange splitting, a generic mechanism operating in many diluted magnetic semiconductors. However, for Mn-doped Ba8Ge46 clathrates incorporation of conduction electrons via Ba encapsulation results in RKKY-like magnetic interactions between the Mn ions. We show that our results are consistent with the major experimental observations for this system.",1207.3996v1 2012-08-03,Metal-insulator transition in Sr2-xLaxCoO4 driven by spin-state transition,"We sought the origin of the metal-insulator transition in Sr2-xLaxCoO4, using electron-correlation corrected density functional calculations. Our results show that Sr2CoO4 is in an intermediate-spin (IS) state and a strong Co4+ 3d-O 2p hybridization is responsible for its ferromagnetic metallicity. Upon La doping, however, a spin-state transition occurs in Sr1.5La0.5CoO4: IS-Co4+ x 2 + 1e --> LS-Co4+ + HS-Co3+ (LS: low spin; HS: high spin). Then the spin-state transition suppresses an electron hopping via a spin-blockade and gives rise to the insulating behavior of Sr1.5La0.5CoO4. A corresponding superexchange accounts for its ferromagnetism. Thus, spin state could provide a way to tune materials properties.",1208.0637v1 2012-08-10,Phase Separation in Mixtures of Repulsive Fermi Gases Driven by Mass Difference,"We show that phase separation must occur in a mixture of fermions with repulsive interaction if their mass difference is sufficiently large. This phenomenon is highly dimension-dependent. Consequently, the density profiles of phase separated 3d mixtures are very different from those in 1d. Noting that the ferromagnetic transition of a spin-1/2 repulsive Fermi gas is the equal mass limit of the phase separation in mixtures, we show from the Bethe Ansatz solution that a ferromagnetic transition will take place in the scattering states when the repulsive interaction passes through resonance and becomes attractive.",1208.2211v2 2012-08-15,Inertia and chiral edge modes of a skyrmion magnetic bubble,"The dynamics of a vortex in a thin-film ferromagnet resembles the motion of a charged massless particle in a uniform magnetic field. Similar dynamics is expected for other magnetic textures with a nonzero skyrmion number. However, recent numerical simulations revealed that skyrmion magnetic bubbles show significant deviations from this model. We show that a skyrmion bubble possesses inertia and derive its mass from the standard theory of a thin-film ferromagnet. Besides center-of-mass motion, other low energy modes are waves on the edge of the bubble traveling with different speeds in opposite directions.",1208.3123v2 2012-08-28,Magnon Mediated Electric Current Drag Across a Ferromagnetic Insulator Layer,"In a semiconductor hererostructure, the Coulomb interaction is responsible for the electric current drag between two 2D electron gases across an electron impenetrable insulator. For two metallic layers separated by a ferromagnetic insulator (FI) layer, the electric current drag can be mediated by a nonequilibrium magnon current of the FI. We determine the drag current by using the semiclassical Boltzmann approach with proper boundary conditions of electrons and magnons at the metal-FI interface.",1208.5812v1 2012-08-31,On the interpretation of the angular dependence of the FMR spectrum in heterogeneous ferromagnetic thin films,"We demonstrate that a multi-peak FMR spectrum, with lines corresponding to resonance in different ferromagnetic regions of a heterogeneous thin-film sample, can collapse to a single-peak spectrum if there exists a particular field configuration, or the configuration of the external magnetic field with respect to the film surface, in which $dH_{\text{res}}/dM_{\text{eff}}=0$ within the region magnetically dominating in the sample.",1208.6573v1 2012-09-09,Neutron Brillouin scattering with pulsed spallation neutron source - spin-wave excitations from ferromagnetic powder samples -,"Neutron Brillouin scattering (NBS) method was developed using a pulsed spallation neutron source, and the feasibility of NBS was demonstrated by observing ferromagnetic spin waves in La$_{0.8}$Sr$_{0.2}$MnO$_3$ and SrRuO$_3$ powders. Gapless spin-wave excitations were observed in La$_{0.8}$Sr$_{0.2}$MnO$_3$, which were continuously extended to the lower scattering vector $Q$ from previous results using single crystals. The novel result is a well-defined quadratic $Q$ dependence in the spin-wave dispersion curve with a large energy gap in SrRuO$_3$ indicating robust spin-orbit coupling.",1209.1780v1 2012-09-13,Experimental verification of contact-size estimates in point-contact spectroscopy on superconductor/ferromagnet heterocontacts,"Nanostructured superconductor/ferromagnet heterocontacts are studied in the different transport regimes of point-contact spectroscopy. Direct measurements of the nanocontact size by scanning electron microscopy allow a comparison with theoretical models for contact-size estimates of heterocontacts. Our experimental data give evidence that size estimates yield reasonable values for the point-contact diameter $d$ as long as the samples are carefully characterized with respect to the local electronic parameters.",1209.2821v1 2012-09-21,"Ferromagnetic neutron stars: axial anomaly, dense neutron matter, and pionic wall","We show that a chiral nonlinear sigma model coupled to degenerate neutrons exhibits a ferromagnetic phase at high density. The magnetization is due to the axial anomaly acting on the parallel layers of neutral pion domain walls spontaneously formed at high density. The emergent magnetic field would reach the QCD scale ~ 10^19 [G], which suggests that the quantum anomaly can be a microscopic origin of the magnetars (highly magnetized neutron stars).",1209.4814v1 2012-10-18,Enhanced Inverse Spin-Hall Effect in Ultrathin Ferromagnetic/Normal Metal Bilayers,"We measure electrically detected ferromagnetic resonance in microdevices patterned from ultra-thin Co/Pt bilayers. Spin pumping and rectification voltages are observed and distinguished via their angular dependence. The spin-pumping voltage shows an unexpected increase as the cobalt thickness is reduced below 2 nm. This enhancement allows more efficient conversion of spin to charge current and motivates a theory modelling the dependence of impurity scattering on surface roughness.",1210.5230v1 2012-11-06,Study of planar Ising ferromagnet on the triangular lattice with selective dilution,"In the paper the Curie temperatures of selectively diluted planar Ising ferromagnet on the triangular lattice are calculated vs. concentration of magnetic atoms. Various analytical approaches are compared with the exact numerical calculations for finite clusters, as well as with the exact analytical solutions for the triangular and honeycomb lattices.",1211.1283v2 2012-11-26,Magneto-optical properties of La2/3Sr1/3MnO3 ultrathin films,"Pulse laser deposited La2/3Sr1/3MnO3 ultrathin films on SrTiO3 substrates were characterized by polar and longitudinal Kerr magneto-optical spectroscopy. An agreement between experimental and theoretical spectra was achieved for a 10.7 nm thick film, while a distinction in the Kerr effect amplitudes was obtained for a 5 nm thick film. This points to the slight suppression of ferromagnetism by interface effects. Nevertheless, the room temperature ferromagnetism of the 5 nm thick film was clearly demonstrated. All the studied films exhibit magneto-optical properties similar to single crystals and thick films, which confirms a fully developed perovskite structure even at 5 nm.",1211.6025v1 2012-11-29,Possible Manipulation of Kondo Effect by Transition between Antiferromagnetic and Ferromagnetic s-d Coupling,"Kondo effect originates from antiferromagnetic (AFM) s-d coupling between magnetic impurity and the conduction electron, while it will be totally quenched in ferromagnetic (FM) regime due to malfunction of spin-flip. We investigate the possibility of switching on/off Kondo effect by transition of AFM/FM s-d coupling using 3d-metal phthalocyanine molecule (MPc) on Au(111) as a model system. A Hamiltonian model is constructed based on the feature of MPc molecule to show the condition for AFM/FM s-d coupling. The AFM s-d coupling could transform to FM s-d coupling if the spin state of the lowest unoccupied orbital changes.",1211.6955v1 2012-12-06,Growth and Characterization of Hybrid Insulating Ferromagnet-Topological Insulator Heterostructure Devices,"We report the integration of the insulating ferromagnet GdN with epitaxial films of the topological insulator Bi2Se3 and present detailed structural, magnetic and transport characterization of the heterostructures. Fabrication of multi-channel Hall bars with bare and GdN-capped sections enables direct comparison of magnetotransport properties. We show that the presence of the magnetic overlayer results in suppression of weak anti-localization at the top surface.",1212.1225v2 2012-12-19,Spin-Orbital Entanglement Emerging from Frustration in the Kugel-Khomskii Model,"We investigate the zero-temperature phase diagrams of the bilayer square-lattice Kugel-Khomskii ($d^9$) model involving entangled and singlet phases using mean-field cluster approach. This diagram includes interlayer singlet phase observed in fluoride K$_3$Cu$_2$F$_7$ and exotic entangled spin-orbital phases. For a monolayer case, realized in K$_2$CuF$_4$, we perform similar calculations in finite temperature and show that the alternating-orbital ferromagnet decays first to an entangled uniform ferromagnet and then to a paramagnetic phase. Published in: J. Phys.: Conf. Series 391, 012085 (2012).",1212.4612v1 2013-01-03,Nonequilibrium Rashba field driven domain wall motion in ferromagnetic nanowires,"We study the effects of spin-orbit interaction (SOI) on the current-induced motion of a magnetic (Bloch) domain wall in ultrathin ferromagnetic nanowires. The conspiracy of spin relaxation and SOI is shown to generate a novel strong nonequilibrium Rashba field, which is dominant even for moderate SOI. This field causes intricate spin precession and a transition from translatory to oscillatory wall dynamics with increasing SOI. We show that current pulses of different lengths can efficiently be used to control the domain wall motion.",1301.0389v2 2013-01-17,Imaging the antiferromagnetic to ferromagnetic first order phase transition of FeRh,"The antiferromagnetic (AFM) to ferromagnetic (FM) first order phase transition of an epitaxial FeRh thin-film has been studied with x-ray magnetic circular dichroism using photoemission electron microscopy. The FM phase is magnetized in-plane due to shape anisotropy, but the magnetocrystalline anisotropy is negligible and there is no preferred in-plane magnetization direction. When heating through the AFM to FM phase transition the nucleation of the FM phase occurs at many independent nucleation sites with random domain orientation. The domains subsequently align to form the final FM domain structure. We observe no pinning of the FM domain structure.",1301.4164v1 2013-01-21,Spin-Polarized Electrons in Bilayer Graphene Flakes,"We show that a bilayer graphene flake deposited above a ferromagnetic insulator can behave as a spin-filtering device. The ferromagnetic material induces exchange splitting in the graphene flake, and due to the Fano antiresonances occurring in the transmission of the graphene flake as a function of flake length and energy, it is possible to obtain a net spin current. This happens when an antiresonance for one spin channel coincides with a maximum transmission for the opposite spin. We propose these structures as a means to obtain spin-polarized currents and spin filters in graphene-based systems.",1301.4974v1 2013-01-23,Theoretical investigation of FeTe magnetic ordering under hydrostatic pressure,"We investigate the pressure phase diagram of FeTe, predicting structural and magnetic properties in the normal state at zero temperature within density functional theory (DFT). We carefully examined several possible different crystal structures over a pressure range up to $\approx 30 $ GPa: simple tetragonal (PbO type), simple monoclinic, orthorhombic (MnP type), hexagonal (NiAs and wurzite type) and cubic (CsCl and NaCl type). We predict pressure to drive the system through different magnetic ordering (notably also some ferromagnetic phases) eventually suppressing magnetism at around 17GPa. We speculate the ferromagnetic order to be the reason for the absence of a superconducting phase in FeTe at variance with the case of FeSe.",1301.5517v1 2013-02-06,Penetration of a magnetic wall into thin ferromagnetic electrodes of a nano-contact spin valve,"We theoretically analyzed a magnetic wall confined in a nano-contact spin valve paying special attention to the penetration of the magnetic wall into thin ferromagnetic electrodes. We showed that, compared with the Bloch wall, the penetration of the Neel wall is suppressed by increases of the demagnetization energy. We found the optimal conditions of the radius and height of the nano-contact to maximize the power of the current-induced oscillation of the magnetic wall. We also found that the thermal stability of the Bloch wall increases when the nano-contact's radius increases or height decreases.",1302.1248v1 2013-02-27,Magnetic relaxation in uranium ferromagnetic superconductors,"There is proposed a phenomenological theory of quasi-elastic neutron scattering in the itinerant ferromagnets UGe$_2$ and UCoGe based on their property that magnetization supported by the moments located at uranium atoms is not conserved quantity relaxing to equilibrium by the interaction with itinerant electrons subsystem. As result the line width of quasi elastic neutron scattering at $q\to 0$ acquires nonvanishing value at all temperatures but the Curie temperature.",1302.6755v3 2013-02-27,Ferromagnetic to antiferromagnetic transition of one-dimensional spinor Bose gases with spin-orbit coupling,"We have analytically solved one-dimensional interacting two-component bosonic gases with spin-orbit (SO) coupling by the Bethe-ansatz method. Through a gauge transformation, the effect of SO coupling is incorporated into a spin-dependent twisted boundary condition. Our result shows that the SO coupling can influence the eigenenergy in a periodical pattern. The interplay between interaction and SO coupling may induce the energy level crossing for the ground state, which leads to a transition from the ferromagnetic to antiferromagnetic state.",1302.6901v2 2013-03-05,Phase diagram of the isotropic spin-3/2 model on the z=3 Bethe lattice,"We study an SU(2) symmetric spin-3/2 model on the z=3 Bethe lattice using the infinite Time Evolving Block Decimation (iTEBD) method. This model is shown to exhibit a rich phase diagram. We compute the expectation values of several order parameters which allow us to identify a ferromagnetic, a ferrimagnetic, a anti-ferromagnetic as well as a dimerized phase. We calculate the entanglement spectra from which we conclude the existence of a symmetry protected topological phase that is characterized by S=1/2 edge spins. Details of the iTEBD algorithm used for the simulations are included.",1303.1110v1 2013-03-30,Features of amplification of dipole magnetic field with linear ferromagnetic concentrator,"Amplification of a low frequency magnetic field from a dipole source by means of linear ferromagnetic concentrator of different dimensions (permalloy micro wire) has been investigated experimentally. The results obtained strengthen the validity of the analytical relation for amplification which allows for nonlinear dependence of the magnetic permeability upon the value and distribution of the dipole field over concentrator length.",1304.0111v1 2013-04-05,Magnetic properties of double exchange biased diluted magnetic alloy/ferromagnet/antiferromagnet trilayers,"The magnetic properties of trilayers consisting of a diluted magnetic alloy, CuMn (Cu0.99Mn0.01), a soft ferromagnet, Py(Ni0.8Fe0.2), and an antiferromagnet, alpha-Fe2O3, were investigated. The samples, grown by UHV magnetron sputtering, were magnetically characterized in the temperature range T = 3-100 K. Typical exchange bias features, namely clear hysteresis cycle shifts and coercivity enhancements, were observed. Moreover the presence of an inverse bias, which had been already reported for spin glass-based structures, was also obtained in a well defined range of temperatures and CuMn thicknesses.",1304.1707v1 2013-04-11,Ferromagnetic spin correlations in a few-fermion system,"We study the spin correlations of a few fermions in a quasi one-dimensional trap. Exact diagonalization calculations demonstrate that repulsive interactions between the two species drives ferromagnetic correlations. The ejection probability of an atom provides an experimental probe of the spin correlations. With more than five atoms trapped, the system approaches the itinerant Stoner limit. Losses to Feshbach molecules are suppressed by the discretization of energy levels when fewer than seven atoms are trapped.",1304.3299v3 2013-04-21,Dynamic Magnetoelectric Effect in Ferromagnet-Superconductor Tunnel Junctions,"We study the magnetization dynamics in a ferromagnet-insulator-superconductor tunnel junction and the associated buildup of the electrical polarization. We show that for an open circuit, the induced voltage varies strongly and nonmonotonically with the precessional frequency, and can be enhanced significantly by the superconducting correlations. For frequencies much smaller or much larger than the superconducting gap, the voltage drops to zero, while when these two energy scales are comparable, the voltage is peaked at a value determined by the driving frequency. We comment on the potential utilization of the effect for the low-temperature spatially-resolved spectroscopy of magnetic dynamics.",1304.5750v2 2013-04-25,Self-sustained Clusters and Ergodicity Breaking in Spin Models,"The emergence of self-sustained clusters and their role in ergodicity breaking is investigated in fully connected Ising and Sherrington-Kirkpatick (SK) models. The analysis reveals a clustering behavior at various parameter regimes, as well as yet unobserved phenomena such as the absence of non-trivial clusters in the Ising ferromagnetic and paramagnetic regimes, the formation of restricted spin clusters in SK spin glass and a first order phase transition in cluster sizes in the SK ferromagnet. The method could be adapted to investigate other spin models.",1304.7010v1 2013-05-07,Determination of the Local Symmetry and the Multiferroic-ferromagnetic Crossover in Ni3-xCoxV2O8 by using Raman Scattering Spectroscopy,"Comprehensive vibrational studies on the multiferroic-to-normal-ferromagnetic crossover in isostructural Ni3-xCoxV2O8 (NCVO, x = 0 - 3.0) are performed using Raman scattering spectroscopy. The systematically red-shifted phonon modes are discussed in terms of the mode Gr\""uneisen parameters, and are interpreted as a chemical pressure effect. In addition, we present evidence that the local symmetry is broken in the multiferroic phase.",1305.1377v1 2013-05-11,Magnetic Properties of the Spin-1/2 Deformed Kagome Antiferromagnet Edwardsite,"We prepared a powder sample of edwardsite Cd2Cu3(SO4)2(OH)6 4H2O, which is a new candidate compound for the spin-1/2 kagome antiferromagnet, and studied its magnetic properties by magnetic susceptibility and heat capacity measurements. Edwardsite has a deformed kagome lattice with an average antiferromagnetic interaction of 51 K between nearby spins and shows an antiferromagnetic order accompanied by a small ferromagnetic moment below 4.3 K. The weak ferromagnetism is likely due to spin canting caused by sizable Dzyaloshinsky-Moriya interactions, which may stabilize the long-range magnetic order instead of a spin-liquid state expected for the kagome antiferromagnet.",1305.2477v1 2013-05-12,Effects of Density Conservation and Hydrodynamics on Aging in Nonequilibrium Processes,"Aging in kinetics of three different phase transitions, viz., magnetic, binary solid and single component fluid, have been studied via Monte Carlo and molecular dynamics simulations in three space dimensions with the objective of identifying the effects of order-parameter conservation and hydrodynamics. We observe that the relevant autocorrelations exhibit power-law decay in ferromagnet and binary solid but with different exponents. At early time fluid autocorrelation function nicely follows that of binary solid, the order parameter being conserved for both of them, as opposed to a ferromagnet. At late time the fluid data crosses over to an exponential decay which we identify as a hydrodynamic effect and provide analytical justification for this behavior.",1305.2557v1 2013-05-21,Switching of a single ferromagnetic layer driven by spin Hall effect,"The magnetization switching of a thin ferromagnetic layer placed on top of a heavy metal (such as Pt, Ta or W) driven by an in-plane current has been observed in recent experiments. The magnetization dynamics of these processes is studied in a full micromagnetic framework which takes into account the transfer-torque from spin Hall effect due to the spin-orbit coupling. Simulations indicate that the reversal occurs via nucleation of complex magnetization patterns. In particular, magnetic bubbles appear during the reversal of the magnetization in the perpendicular configuration while for the in-plane configuration, nucleation of vortexes are observed.",1305.4806v1 2013-05-28,Spin-charge disparity of polarons in organic ferromagnets,"Polaron formation in quasi-one-dimensional organic ferromagnets is studied based on an extended Su-Schrieffer-Heeger model combined with a Kondo term. The charge distribution of the polaron is found to be highly asymmetric under spatial reflection, due to the spin radicals. On the contrary, the spin density is nearly symmetric; the spin asymmetry introduced by the extra electron inducing the polaron formation is nearly compensated by the spin polarization of the lower-energy states. We discuss these results on the basis of real-space mean-field calculations and symmetry arguments.",1305.6495v1 2013-06-19,Energetic stability of coreless vortices in spin-1 Bose-Einstein condensates with conserved magnetization,"We show that conservation of longitudinal magnetization in a spinor condensate provides a stabilizing mechanism for a coreless vortex phase-imprinted on a polar condensate. The stable vortex can form a composite topological defect with distinct small- and large-distance topology: the inner ferromagnetic coreless vortex continuously deforms toward an outer singular, singly quantized polar vortex. A similar mechanism can also stabilize a nonsingular nematic texture in the polar phase. A weak magnetization is shown to destabilize a coreless vortex in the ferromagnetic phase.",1306.4700v2 2013-07-01,Spin-controlled superconductivity and tunable triplet correlations in graphene nanostructures,"We study graphene ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures via a microscopic self-consistent Dirac Bogoliubov-de Gennes formalism. We show that as a result of proximity effects, experimentally accessible spin switching phenomena can occur as one tunes the Fermi level $\mu_F$ of the F regions or varies the angle $\theta$ between exchange field orientations. Superconductivity can then be switched on and off by varying either $\theta$ or $\mu_F$ (a spin-controlled superconducting graphene switch). The induced equal-spin triplet correlations in S can be controlled by tuning $\mu_F$, effectively making a graphene based two-dimensional spin-triplet valve.",1307.0546v1 2013-07-02,Size Dependence of Domain Pattern Transfer in Multiferroic Heterostructures,"Magnetoelectric coupling in multiferroic heterostructures can produce large lateral modulations of magnetic anisotropy enabling the imprinting of ferroelectric domains into ferromagnetic films. Exchange and magnetostatic interactions within ferromagnetic films oppose the formation of such domains. Using micromagnetic simulations and a 1-D model, we demonstrate that competing energies lead to the breakdown of domain pattern transfer below a critical domain width. Moreover, rotation of the magnetic field results in abrupt transitions between two scaling regimes with different magnetic anisotropy.",1307.0605v2 2013-07-02,Phase Field Crystal Model for Magneto-Elasticity in Isotropic Ferromagnetic Solids,"A new isotropic magneto-elastic phase field crystal (PFC) model to study the relation between morphological structure and magnetic properties of pure ferromagnetic solids is introduced. Analytic calculations were used to determine the phase diagram and obtain the relationship between elastic strains and magnetization. Time dependent numerical simulations were used to demonstrate the effect of grain boundaries on the formation of magnetic domains. It was shown that the grain boundaries act as nucleating sites for domains of reverse magnetization. Finally, we derive a relation for coercivity versus grain mis-orientation in the isotropic limit.",1307.0758v1 2013-07-03,Large Spin Pumping from Epitaxial Y3Fe5O12 Thin Films to Pt and W Layers,"Epitaxial Y3Fe5O12 thin films have been deposited by off-axis sputtering, which exhibit excellent crystalline quality, enabling observation of large spin pumping signals in Pt/Y3Fe5O12 and W/Y3Fe5O12 bilayers driven by cavity ferromagnetic resonance. The inverse spin Hall voltages reach 2.10 mV and -5.26 mV in 5-mm long Pt/Y3Fe5O12 and W/Y3Fe5O12 bilayers, respectively, excited by a radio-frequency magnetic field of 0.3 Oe. From the ferromagnetic resonance linewidth broadening, the interfacial spin mixing conductance of 4.56E14 {\Omega}-1m-2 and 2.30E14 {\Omega}-1m-2 are obtained for Pt/Y3Fe5O12 and W/Y3Fe5O12 bilayers, respectively.",1307.1172v1 2013-07-16,Analysis of magnetic neutron-scattering data of two-phase ferromagnets,"We have analyzed magnetic-field-dependent small-angle neutron scattering (SANS) data of soft magnetic two-phase nanocomposite ferromagnets in terms of a recent micromagnetic theory for the magnetic SANS cross section [D. Honecker and A. Michels, Phys. Rev. B $\mathbf{87}$, 224426 (2013)]. The approach yields a value for the average exchange-stiffness constant and provides the Fourier coefficients of the magnetic anisotropy field and magnetostatic field, which is related to jumps of the magnetization at internal interfaces.",1307.4206v1 2013-07-17,Huge thermoelectric effects in ferromagnet-superconductor junctions in the presence of a spin-splitting field,"We show that a huge thermoelectric effect can be observed by contacting a superconductor whose density of states is spin-split by a Zeeman field with a ferromagnet with a non-zero polarization. The resulting thermopower exceeds $k_B/e$ by a large factor, and the thermoelectric figure of merit $ZT$ can far exceed unity, leading to heat engine efficiencies close to the Carnot limit. We also show that spin-polarized currents can be generated in the superconductor by applying a temperature bias.",1307.4672v2 2013-10-19,Superconductivity and magnetism on flux grown single crystals of NiBi3,"We present resistivity, magnetization and specific heat measurements on flux grown single crystals of NiBi3. We find typical behavior of a type-II superconductor, with, however, a sizable magnetic signal in the superconducting phase. There is a hysteretic magnetization characteristic of a ferromagnetic compound. By following the magnetization as a function of temperature, we find a drop at temperatures corresponding to the Curie temperature of ferromagnetic amorphous Ni. Thus, we assign the magnetism in NiBi$_3$ crystals to amorphous Ni impurities.",1310.5247v1 2013-10-25,Giant coercivity enhancement and dimensional crossover of superconductivity in Co2FeSi-NbN nanoscale bilayers,"Proximity coupling and magnetic switching dynamics are shown to be correlated in bilayers (B) of fully spin polarized ferromagnet (F) Co2FeSi and superconductor (S) NbN. The upper critical field derived from resistivity measurements shows a dimensional crossover with a reduced effective thickness of the S layer. At temperatures (T) << superconducting Tc, the measured M-H loops show two step switching; one at T-independent value ~ 7 Oe and other at strongly T-dependent value becoming very large ~ 1 kOe at 2 K. These results reveal induced ferromagnetism in S-layer at the S/F interface with vortex pinning influenced dynamics.",1310.6818v1 2013-10-25,Domain wall motion in ferromagnetic nanotubes: Analytic results,"Dynamics of magnetization domain walls (DWs) in thin ferromagnetic nanotubes subject to longitudinal external fields is addressed analytically in the regimes of strong and weak penalization. Explicit functional forms of the DW profiles and formulas for the DW propagation velocity are derived in both regimes. In particular, the DW speed is shown to depend nonlinearly on the nanotube radius.",1310.6883v1 2013-10-30,Spin torque ferromagnetic resonance with magnetic field modulation,"We demonstrate a technique of broadband spin torque ferromagnetic resonance (ST-FMR) with magnetic field modulation for measurements of spin wave properties in magnetic nanostructures. This technique gives great improvement in sensitivity over the conventional ST-FMR measurements, and application of this technique to nanoscale magnetic tunnel junctions (MTJs) reveals a rich spectrum of standing spin wave eigenmodes. Comparison of the ST-FMR measurements with micromagnetic simulations of the spin wave spectrum allows us to explain the character of low-frequency magnetic excitations in nanoscale MTJs.",1310.7996v1 2013-11-19,Anisotropic magneto-thermal transport and Spin-Seebeck effect,"The angular dependence of the thermal transport in insulating or conducting ferromagnets is derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions. The measured thermocouple generated between the extremities of the non-magnetic electrode in thermal contact to the ferromagnet follows this same angular dependence. The sign and amplitude of the magneto-voltaic signal is controlled by the difference of the Seebeck coefficients of the thermocouple.",1311.4868v1 2013-11-24,Role of spin mixing conductance in spin pumping: enhancement of spin pumping efficiency in Ta/Cu/Py structures,"From spin pumping measurements in Ta/Py devices for different thicknesses of Ta, we determine the spin Hall angle to be 0.021 - 0.033 and spin diffusion length to be 8 nm in Ta. We have also studied the effect of changing the properties of non-magnet/ferromagnet interface by adding a Cu interlayer. The experimental results show that the effective spin mixing conductance increases in the presence of Cu interlayer for Ta/Cu/Py devices, whereas it decreases in Pt/Cu/Py devices. Our findings allow the tunability of the spin pumping efficiency by adding a thin interlayer at the non-magnet/ferromagnet interface.",1311.6098v1 2013-12-05,Magnetic properties of ferromagnetic Pu2Pt3Si5,"The structural, magnetic, and thermodynamic properties of a new plutonium based compound, Pu2Pt3Si5, are reported. Single crystals produced by a Sn-flux technique have been analyzed showing a ferromagnetic behavior at 58 K. Pu2Pt3Si5 crystallizes in the U2Co3Si5-type orthorhombic Iabm structure (72) with atomic parameters a = 9.9226(2) \AA, b = 11.4436(2) \AA and c = 6.0148(1) \AA. The effective (\mu_eff ~0.74 \mu_B) and saturated (\mu_sat ~0.32 B/Pu) moments as well as the Sommerfeld coefficient (\gamma_e ~2 mJ.mol-1.K-2/Pu) could point towards 5f localization in this material.",1312.1576v1 2013-12-06,First Principles Study of the Magnetic Properties of LaOMnAs,"Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the $ab$ plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.",1312.1775v1 2013-12-12,Microwave manipulation of electrically injected spin polarized electrons in silicon,"We demonstrate microwave manipulation of the spin states of electrically injected spin-polarized electrons in silicon. Although the silicon channel is bounded by ferromagnetic metal films, we show that moderate microwave power can be applied to the devices without altering the device operation significantly. Resonant microwave irradiation is used to induce spin rotation of spin-polarized electrons as they travel across a silicon channel, and the resultant spin polarization is subsequently detected by a ferromagnetic Schottky barrier spin detector. These results demonstrate the potential for combining advanced electron spin resonance techniques to complement the study of semiconductor spintronic devices beyond standard magnetotransport measurements.",1312.3663v1 2013-12-13,A generalized magnetic refrigeration scheme,"We have investigated the magnetocaloric effects in antiferromagnets and compared them with those in ferromagnets using Monte Carlo simulations. In antiferromagnets, the magnetic entropy reaches a maximum value at a finite magnetic field when the temperature is fixed below the N\'eel temperature. Using the fact, we proposed a protocol for applying magnetic fields to achieve the maximum efficiency for magnetic refrigeration in antiferromagnets. In particular, we found that at low temperatures, antiferromagnets are more useful for magnetic refrigeration than ferromagnets.",1312.3716v2 2013-12-31,Topological response in ferromagnets,"We present a theory of the intrinsic anomalous Hall effect in a model of a doped Weyl semimetal, which serves here as the simplest toy model of a generic three-dimensional metallic ferromagnet with Weyl nodes in the electronic structure. We analytically evaluate the anomalous Hall conductivity as a function of doping, which allows us to explicitly separate the Fermi surface and non Fermi surface contributions to the Hall conductivity by carefully evaluating the zero frequency and zero wavevector limits of the corresponding response function. We show that this separation agrees with the one suggested a long time ago in the context of the quantum Hall effect by Streda.",1401.0066v2 2014-01-02,Giant zero field cooled spontaneous exchange bias effect in phase separated La1.5Sr0.5CoMnO6,"We report a giant zero field cooled exchange bias (ZEB) effect (~0.65 T) in La1.5Sr0.5CoMnO6 sample. Magnetic study has revealed a reentrant spin glass ~90 K, phase separation to spin glass and ferromagnetic phases below 50 K and canted antiferromagnetic transition ~10 K. A small conventional exchange bias (CEB) is established with the advent of spontaneous phase separation down to 10 K. Giant ZEB and enhanced CEB effects are found only below 10 K and are attributed to the large unidirectional anisotropy at the interface of isothermally field induced ferromagnetic phase and canted antiferromagnetic background.",1401.0377v1 2014-01-03,Spin-superflow turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates,"Spin-superflow turbulence (SST) in spin-1 ferromagnetic spinor Bose-Einstein condensates is theoretically and numerically studied by using the spinor Gross-Pitaevskii (GP) equations. SST is turbulence in which the disturbed spin and superfluid velocity fields are coupled. Applying the Kolmogorov-type dimensional scaling analysis to the hydrodynamic equations of spin and velocity fields, we theoretically find that the -5/3 and -7/3 power laws appear in spectra of the superflow kinetic and the spin-dependent interaction energy, respectively. Our numerical calculation of the GP equations confirms SST with the coexistence of disturbed spin and superfluid velocity field with two power laws.",1401.0599v2 2014-01-10,Intrinsic Localized Modes of a Classical Discrete Anisotropic Heisenberg Ferromagnetic Spin Chain,"We report several exact intrinsic localized mode solutions of the classical spin evolution equation of a one-dimensional anisotropic Heisenberg ferromagnetic spin chain in terms of Jacobian elliptic functions. These include one, two and three spin excitations. All these solutions have smooth anticontinuum limits. Their linear stability and semiclassical quantization are also discussed briefly.",1401.2241v2 2014-01-13,Phase transitions in the domain structure of ferromagnetic superconductors,"Starting from the London - type model we study the domain structures in ferromagnetic superconductors taking account of the nucleation of vortices and antivortices coupled to the magnetic texture. We predict that the coupling between domains and vortices results in the formation of two energetically favorable domain configurations: (i) a Meissner - type vortex free configuration with strong domain shrinking and (ii) a more rare domain configuration with a dense vortex -- antivortex lattice. The switching between these configurations is shown to result in the first order phase transitions which could be observable in superconducting uranium based compounds.",1401.2806v1 2014-01-20,An Importance Sampling Scheme on Dual Factor Graphs. I. Models in a Strong External Field,"We propose an importance sampling scheme to estimate the partition function of the two-dimensional ferromagnetic Ising model and the two-dimensional ferromagnetic $q$-state Potts model, both in the presence of an external magnetic field. The proposed scheme operates in the dual Forney factor graph and is capable of efficiently computing an estimate of the partition function under a wide range of model parameters. In particular, we consider models that are in a strong external magnetic field.",1401.4912v5 2014-01-22,A quantum magnetic RC circuit,"We propose a setup that is the spin analog of the charge-based quantum RC circuit. We define and compute the spin capacitance and the spin resistance of the circuit for both ferromagnetic (FM) and antiferromagnetic (AF) systems. We find that the antiferromagnetic setup has universal properties, but the ferromagnetic setup does not. We discuss how to use the proposed setup as a quantum source of spin excitations, and put forward a possible experimental realization using ultracold atoms in optical lattices.",1401.5712v1 2014-01-29,Spin and charge ordering in three-leg ladders in oxyborates,"We study the spin ordering within the 3-leg ladders present in the oxyborate Fe(3)O(2)BO(3) consisting of localized classical spins interacting with conduction electrons (one electron per rung). We also consider the competition with antiferromagnetic superexchange interactions to determine the magnetic phase diagram. Beside a ferromagnetic phase we find (i) a phase with ferromagnetic rungs ordered antiferromagnetically (ii) a zig-zag canted spin ordering along the legs. We also determine the induced charge ordering within the different phases and the interplay with lattice instability. Our model is discussed in connection with the lattice dimerization transition observed in this system, emphasizing on the role of the magnetic structure.",1401.7677v1 2014-02-12,Hydrostatic pressure to trigger and assist magnetic transitions: baromagnetic refrigeration,"The possible application of the barocaloric effect to produce solid state refrigerators is a topic of interest in the field of applied physics. In this work, we present experimental data about the influence of external pressure on the magnetic properties of a manganite with phase separation. Using the Jahn Teller effect associated with the presence of the charge ordering we were able to follow the transition to the ferromagnetic state induced by pressure. We also demonstrated that external pressure can assist the ferromagnetic state, decreasing the magnetic field necessary to generate the magnetic transition.",1402.2824v1 2014-02-13,Giant magnetoresistance for ensembles of ferromagnetic granules in variable range hopping conductivity regime,"We study an effect of moderate magnetic field on variable range hopping conductivity in arrays of ferromagnetic granules separated by tunnel barriers. It is shown that the resulting magnetoresistance can be significantly larger than the standard ""giant"" magnetoresistance in Fe-N-Fe-N... multilayers. The effect is related to a gain in densities of states available for the virtual processes within the intermediate granules due to magnetic-field induced alignment of the granule magnetizations.",1402.3183v1 2014-02-15,Ferromagnetism and transport properties of the Kondo system Ce4Sb1.5Ge1.5,"Ferromagnetic ordering with a small magnetic moment is found below 14 K from SQUID measurements for the compound Ce4Sb1.5Ge1.5. The transport characteristics of a number of Ce4Sb3-xTx (T = Ge, Si, Sn, Pb, Al) systems were measured at room temperature, Ce4Sb1.5Ge1.5 demonstrating the highest Seebeck coefficient. Its transport properties (resistivity, thermal conductivity, thermoelectric power) are experimentally investigated in detail up to low temperatures. The Kondo-lattice behavior of resistivity and its anomaly at the ordering point are found. A comparison with the Wiedemann-Franz law is performed.",1402.3667v2 2014-02-28,"Standard, inverse and triplet spin-valve effects in F1/S/F2 systems","We demonstrate that contrary to the common belief the critical temperature Tc of clean F1/S/F2 spin valves can depend non-monotonically on the angle between the magnetic moments of the ferromagnetic F1 and F2 layers. Depending on the system parameters the minimum of Tc may correspond to parallel, antiparallel or non-collinear mutual orientation of magnetic moments. Such anomalous behavior can reveal itself only provided the ferromagnetic layers differ from each other and it completely disappears in the dirty limit.",1402.7259v1 2014-04-22,Approaching Pomeranchuk Instabilities from Ordered Phase: A Crossing-symmetric Equation Method,"We explore features of a 3D Fermi liquid near generalized Pomeranchuk instabilities using a tractable crossing symmetric equation method. We approach the instabilities from the ordered ferromagnetic phase. We find quantum multi-criticality as approach to the ferromagnetic instability drives instability in other channel(s). It is found that a charge nematic instability precedes and is driven by Pomeranchuk instabilities in both the l = 0 spin and density channels.",1404.5655v1 2014-07-14,Intrinsic magnetoresistance in metal films on ferromagnetic insulators,"We predict a magnetoresistance induced by the interfacial Rashba spin-orbit coupling in normal metal|ferromagnetic insulator bilayer. It depends on the angle between current and magnetization directions identically to the ""spin Hall magnetoresistance"" mechanism caused by a combined action of spin Hall and inverse spin Hall effects. Due to the identical phenomenology it is not obvious whether the magnetoresistance reported by Nakayama et al. is a bulk metal or interface effect. The interfacial Rashba induced magnetoresistance may be distinguished from the bulk metal spin Hall magnetoresistance by its dependence on the metal film thickness.",1407.3571v1 2014-07-18,Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system,"A microwave technique is employed to simultaneously examine the spin pumping and the spin Seebeck effect processes in a YIG|Pt bilayer system. The experimental results show that for these two processes, the spin current flows in opposite directions. The temporal dynamics of the longitudinal spin Seebeck effect exhibits that the effect depends on the diffusion of bulk thermal-magnons in the thermal gradient in the ferromagnetic-insulator|normal-metal system.",1407.4957v2 2014-07-18,Structural and electronic properties of epitaxial YBaCuO/LaCaMnO bilayers grown on SrTiO$_3$ (110) substrates,"Epitaxial bilayers of the high-temperature-superconductor YBa2Cu3O7-d (YBCO) and the ferromagnetic metal La0.67Ca0.33MnO3 (LCMO) were prepared by pulsed laser deposition on (110)-oriented SrTiO3 substrates, such that the CuO2 planes of YBCO are perpendicular to the YBCO-LCMO interface. X-ray diffraction and Raman scattering demonstrate complete (110) orientation of both YBCO and LCMO overlayers. The resistivity and magnetization of the bilayer films are highly anisotropic. The critical temperatures for superconductivity and ferromagnetism as well as the saturation magnetization exhibit modest reductions compared to corresponding bulk values.",1407.5049v1 2014-07-25,Pre-asymptotic critical behavior and effective exponents in disordered metallic quantum ferromagnets,"We determine the pre-asymptotic critical behavior at the quantum ferromagnetic transition in strongly disordered metals. We find that it is given by effective power laws, in contrast to the previously analyzed asymptotic critical behavior, which is valid only in an unobservably small region. The consequences for analyzing experiments are discussed, in particular ways to distinguish between critical behavior and Griffiths-phase effects.",1407.7068v1 2014-08-01,Spin-polarized quasiparticle transport in exchange-split superconducting aluminum on europium sulfide,"We report on nonlocal spin transport in mesoscopic superconducting aluminum wires in contact with the ferromagnetic insulator europium sulfide. We find spin injection and long-range spin transport in the regime of the exchange splitting induced by europium sulfide. Our results demonstrate that spin transport in superconductors can be manipulated by ferromagnetic insulators, and opens a new path to control spin currents in superconductors.",1408.0176v1 2014-08-06,"Ground state, magnetization process and magnetocaloric effect of the exactly tractable spin-electron tetrahedral chain","A hybrid spin-electron system on one-dimensional tetrahedral chain, in which the localized Ising spin regularly alternates with the mobile electron delocalized over three lattice sites, is exactly investigated using the generalized decoration-iteration transformation. The system exhibits either the ferromagnetic or antiferromagnetic ground state depending on whether the ferromagnetic or antiferromagnetic interaction between the Ising spins and mobile electrons is considered. The enhanced magnetocaloric effect during the adiabatic demagnetization suggests a potential use of the investigated system for low-temperature magnetic refrigeration.",1408.1207v1 2014-08-13,Critical field theory for ferromagnetic quantum criticality in the strong coupling regime of Hertz-Moriya-Millis theory,"We develop U(1) slave spin-rotor theory, suggesting a metal-metal transition from Landau's Fermi-liquid state to a bad metal phase, as U(1) slave charge-rotor theory [Phys. Rev. B {\bf 70}, 035114 (2004)] describes a metal-insulator transition from Landau's Fermi-liquid state to a spin-liquid phase. U(1) slave spin-rotor formulation allows us to generalize Hertz-Moriya-Millis theory for ferromagnetic quantum phase transitions, replacing Landau's Fermi-liquid state with an incoherent metallic phase. As a result, we argue that localized magnetic moments emerge to govern quantum critical physics in bad metals.",1408.2993v2 2014-08-14,PT-symmetry in macroscopic magnetic structures,"We introduce the notion of PT-symmetry in magnetic nanostructures and show that they can support a new type of non-Hermitian dynamics. Using the simplest possible set-up consisting of two coupled ferromagnetic films, one with loss and another one with a balanced amount of gain, we demonstrate the existence of a spontaneous PT-symmetry breaking point where both the eigenfrequencies and eigenvectors are degenerate. Below this point the frequency spectrum is real indicating stable dynamics while above this point it is complex signaling unstable dynamics.",1408.3285v1 2014-08-18,Metastable spin textures and Nambu-Goldstone modes of a ferromagnetic spin-1 Bose-Einstein condensate confined in a ring trap,"We investigate the metastability of a ferromagnetic spin-1 Bose-Einstein condensate confined in a quasi-one-dimensional rotating ring trap by solving the spin-1 Gross-Pitaevskii equation. We find analytical solutions that exhibit spin textures. By performing linear stability analysis, it is shown that the solutions can become metastable states. We also find that the number of Nambu-Goldstone modes changes at a certain rotation velocity without changing the continuous symmetry of the order parameter.",1408.4129v4 2014-09-14,Spin-polarized Andreev transport influenced by Coulomb repulsion through two quantum dot system,"Spin-polarized transport through a double quantum dot system attached to a common superconducting lead and two ferromagnetic electrodes (fork geometry) is investigated theoretically. The key objective of the analysis is to describe the influence of electrodes' ferromagnetism on the Andreev tunneling. Both direct and crossed Andreev tunneling processes are considered, in general. The other objective is a detailed analysis of the role of Coulomb interaction and its impact on the Andreev tunneling processes.",1409.4122v1 2014-09-16,"Magnetisms in $p$-type monolayer gallium chalcogenides (GaSe, GaS)","Magnetisms in $p$-type monolayer GaX (X=S,Se) is investigated by performing density-functional calculations. Due to the large density of states near the valence band edge, these monolayer semiconductors are ferromagnetic within a small range of hole doping. The intrinsic Ga vacancies can promote local magnetic moment while Se vacancies cannot. Magnetic coupling between vacancy-induced local moments is ferromagnetic and surprisingly long-range. The results indicate that magnetization can be induced by hole doping and can be tuned by controlled defect generation.",1409.4733v2 2014-09-25,Interfacial Spin and Heat Transfer between Metals and Magnetic Insulators,"We study the role of thermal magnons in the spin and heat transport across a normal-metal/insulating-ferromagnet interface, which is beyond an elastic electronic spin transfer. Using an interfacial exchange Hamiltonian, which couples spins of itinerant and localized orbitals, we calculate spin and energy currents for an arbitrary interfacial temperature difference and misalignment of spin accumulation in the normal metal relative to the ferromagnetic order. The magnonic contribution to spin current leads to a temperature-dependent torque on the magnetic order parameter; reciprocally, the coherent precession of the magnetization pumps spin current into the normal metal, the magnitude of which is affected by the presence of thermal magnons.",1409.7128v1 2014-10-03,Dynamics of a vortex dipole across a magnetic phase boundary in a spinor Bose-Einstein condensate,"Dynamics of a vortex dipole in a spin-1 Bose-Einstein condensate in which magnetic phases are spatially distributed is investigated. When a vortex dipole travels from the ferromagnetic phase to the polar phase, or vice versa, it penetrates the phase boundary and transforms into one of the various spin vortex dipoles, such as a leapfrogging ferromagnetic-core vortex dipole and a half-quantum vortex dipole. Topological connections of spin wave functions across the phase boundary are discussed.",1410.0743v1 2014-10-08,Uniqueness of zero-temperature metastate in disordered Ising ferromagnets,"We study ground states of Ising models with random ferromagnetic couplings, proving the triviality of all zero-temperature metastates. This unexpected result sheds a new light on the properties of these systems, putting strong restrictions on their possible ground state structure. Open problems related to existence of interface-supporting ground states are stated and an interpretation of the main result in terms of first-passage and random surface models in a random environment is presented.",1410.2283v1 2014-10-14,Skyrmion-like states in multilayer exchange coupled ferromagnetic nanostructures with distinct anisotropy directions,"We report the experimental observation of magnetic skyrmion-like states in patterned ferromagnetic nanostructures consisting of perpendicular magnetized Co/Pt multilayer film exchange coupled with Co nanodisks in vortex state. The magnetic force microscopy and micromagnetic simulations show that depending on the magnitude of Co/Pt perpendicular anisotropy in these systems two different modes of skyrmion formation are realized.",1410.4174v1 2014-10-20,Phase diagram of exciton condensate in doped two-band Hubbard model,"Using the dynamical mean-field approximation we investigate formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first-order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a multi-critical point. Rapid but continuous transition between ferromagnetic and non-magnetic excitonic phases allows switching of uniform magnetization by small changes of chemical potential.",1410.5198v1 2014-10-21,Magnetic switching dynamics in a ferrimagnetic two sub-lattice model including ultrafast exchange scattering,"We study the heat-induced magnetization dynamics in a toy model of a ferrimagnetic alloy, which includes localized spins antiferromagnetically coupled to an itinerant carrier system with a Stoner gap. We determine the one-particle spin-density matrix including exchange scattering between localized and itinerant bands as well as scattering with phonons. While a transient ferromagnetic-like state can always be achieved by a sufficiently strong excitation, this transient ferromagnetic-like state only leads to magnetization switching for model parameters that also yield a compensation point in the equilibrium M(T) curve.",1410.5745v2 2014-10-30,Superconducting Gap of UCoGe probed by Thermal Transport,"Thermal conductivity measurements in the superconducting state of the ferromagnet UCoGe were performed at very low temperatures and under magnetic field on samples of different qualities and with the heat current along the three crystallographic axis. This allows to disentangle intrinsic and extrinsic effects, confirm the situation of multigap superconductivity and shed new light on the situation expected or claimed for the gap in these ferromagnetic superconductors, like evidences of absence of ""partially gapped"" Fermi surfaces.",1410.8361v1 2014-11-02,Universal free-energy distribution in the critical point of a random Ising ferromagnet,"We discuss the non-self-averaging phenomena in the critical point of weakly disordered Ising ferromagnet. In terms of the renormalized replica Ginzburg-Landau Hamiltonian in dimensions D <4, we derive an explicit expression for the probability distribution function (PDF) of the critical free energy fluctuations. In particular, using known fixed-point values for the renormalized coupling parameters, we obtain the universal curve for such PDF in the dimension D=3. It is demonstrated that this function is strongly asymmetric: its left tail is much slower than the right one.",1411.0263v2 2014-11-06,Phenomenological description of anisotropy effects in some ferromagnetic superconductors,"We study phenomenologically by using the previously derived Landau free energy, the role of anisotropy in ferromagnetic superconductors UGe2, URhGe, and UCoGe. The three compounds are separately discussed with the special stress on UGe2. The main effect comes from the strong uniaxial anisotropy of magnetization while the anisotropy of Cooper pairs and crystal anisotropy only slightly change the phase diagram in the vicinity of Curie temperature. The limitations of this approach are also discussed.",1411.1748v1 2014-11-24,Flat band ferromagnetism without connectivity conditions in the flat band,"It is known that a system which exhibits a half filled lowest flat band and the localized one-particle Wannier states on the flat band satisfy the connectivity conditions, is always ferromagnetic. Without the connectivity conditions on the flat band, the system is non-magnetic. We show that this is not always true. The reason is connected to a peculiar behavior of the band situated just above the flat band.",1411.6477v1 2014-11-27,Reverse magnetic vortex curling direction of ferromagnetic nanodisk,"We reverse the magnetic vortex curling direction of ferromagnetic nanodisk by applying a circular Oersted field. The nanodisk is fabricated without breaking its symmetry. The Oersted field is induced by passing current through an atomic force microscope tip placed at the center of the disk. Micromagnetic simulation indicates that compared to the uniformly distributed current throughout the cross section of disk, the line current concentrated in the center can reverse the chirality more easily, which is in accordance with our experimental results.",1411.7444v1 2014-12-07,Asymmetric domain walls of small angle in soft ferromagnetic films,"We focus on a special type of domain walls appearing in the Landau-Lifshitz theory for soft ferromagnetic films. These domain walls are divergence-free $S^2$-valued transition layers that connect two directions in $S^2$ (differing by an angle $2\theta$) and minimize the Dirichlet energy. Our main result is the rigorous derivation of the asymptotic structure and energy of such ""asymmetric"" domain walls in the limit $\theta \to 0$. As an application, we deduce that a supercritical bifurcation causes the transition from symmetric to asymmetric walls in the full micromagnetic model.",1412.2382v1 2014-12-16,Charge and spin transport in a metal-semiconductor heterostructure with double Schottky barriers,"Taking into account the available experimental results, we model the electronic properties and current-voltage characteristics of a ferromagnet-semiconductor junction. The Fe/GaAs interface is considered as a Fe/(i-GaAs)/n+-GaAs/n-GaAs multilayer structure with the Schottky barrier. We also calculate numerically the current-voltage characteristics of a double-Schottky-barrier structure Fe/GaAs/Fe, which are in agreement with available experimental data. For this structure, we have estimated the spin current in the GaAs layer, which characterizes spin injection from the ferromagnet to the semiconductor.",1412.4932v1 2014-12-18,Coherent elastic excitation of spin waves,"We model the injection of elastic waves into a ferromagnetic film (F) by a non-magnetic transducer (N). We compare the configurations in which the magnetization is normal and parallel to the wave propagation. The lack of axial symmetry in the former results in the emergence of evanescent interface states. We compute the energy-flux transmission across the N$|$F interface and sound-induced magnetization dynamics in the ferromagnet. We predict efficient acoustically induced pumping of spin current into a metal contact attached to F.",1412.5820v2 2014-12-18,Topological superconductivity and high Chern numbers in 2D ferromagnetic Shiba lattices,"Inspired by the recent experimental observation of topological superconductivity in ferromagnetic chains, we consider a dilute 2D lattice of magnetic atoms deposited on top of a superconducting surface with a Rashba spin-orbit coupling. We show that the studied system supports a generalization of $p_x+ip_y$ superconductivity and that its topological phase diagram contains Chern numbers higher than $\xi/a$ $(\gg1)$, where $\xi$ is the superconducting coherence length and $a$ is the distance between the magnetic atoms. The signatures of nontrivial topology can be observed by STM spectroscopy in finite-size islands.",1412.5834v1 2015-01-06,Critical behavior in lattice models with two symmetric absorbing state,"We analyze nonequilibrium lattice models with up-down symmetry and two absorbing states by mean-field approximations and numerical simulations in two and three dimensions. The phase diagram displays three phases: paramagnetic, ferromagnetic and absorbing. The transition line between the first two phases belongs to the Ising universality class and between the last two, to the direct percolation universality class. The two lines meet at the point describing the voter model and the size $\ell$ of the ferromagnetic phase vanishes with the distance $\varepsilon$ to the voter point as $\ell\sim\varepsilon$, with possible logarithm corrections in two dimensions.",1501.01116v1 2015-01-17,Magnetic and Magnetocaloric Study of the Ferromagnetically Coupled GdF3: The Best Cryogenic Magnetic Coolant Ever,"The magnetic susceptibility and isothermal magnetization for GdF3 were measured, and the isothermal entropy change was evaluated up to 9 T. Combining the large isotropic spin of Gd3+, the dense structure and the weak ferromagnetic interaction, an extremely large -(delta)Sm for GdF3 was observed up to 528 mJ cm-3 K-1 for (delta)H = 9 T, proving itself to be the best cryogenic magnetic coolant ever.",1501.04180v1 2015-02-05,DFT calculations of magnetic anisotropy energy for GeMnTe ferromagnetic semiconductor,"Density functional theory (DFT) calculations of the energy of magnetic anisotropy for diluted ferromagnetic semiconductor GeMnTe were performed using using OpenMX package with fully relativistic pseudopotentials. The influence of hole concentration and magnetic ion neighborhood on magnetic anisotropy energy is presented. Analysis of microscopic mechanism of magnetic anisotropy is provided, in particular the role of spin-orbit coupling, spin polarization and spatial changes of electron density are discussed. The calculations are in accordance with the experimental observation of perpendicular magnetic anisotropy in rhombohedral GeMnTe (111) thin layers.",1502.01715v3 2015-02-06,Microscopic theory of Gilbert damping in metallic ferromagnets,"We present a microscopic theory for magnetization relaxation in metallic ferromagnets of nanoscopic dimensions that is based on the dynamic spin response matrix in the presence of spin-orbit coupling. Our approach allows the calculation of the spin excitation damping rate even for perfectly crystalline systems, where existing microscopic approaches fail. We demonstrate that the relaxation properties are not completely determined by the transverse susceptibility alone, and that the damping rate has a non-negligible frequency dependence in experimentally relevant situations. Our results indicate that the standard Landau-Lifshitz-Gilbert phenomenology is not always appropriate to describe spin dynamics of metallic nanostructure in the presence of strong spin-orbit coupling.",1502.02068v1 2015-02-14,Energy dissipation in single-domain ferromagnetic nanoparticles: Dynamical approach,"We study, both analytically and numerically, the phenomenon of energy dissipation in single-domain ferromagnetic nanoparticles driven by an alternating magnetic field. Our interest is focused on the power loss resulting from the Landau-Lifshitz-Gilbert equation, which describes the precessional motion of the nanoparticle magnetic moment. We determine the power loss as a function of the field amplitude and frequency and analyze its dependence on different regimes of forced precession induced by circularly and linearly polarized magnetic fields. The conditions to maximize the nanoparticle heating are also analyzed.",1502.04222v1 2015-03-05,Single-spin-flip dynamics of the Ising chain,"We consider the most general single-spin-flip dynamics for the ferromagnetic Ising chain with nearest-neighbour influence and spin reversal symmetry. This dynamics is a two-parameter extension of Glauber dynamics corresponding respectively to non-linearity and irreversibility. The associated stationary state measure is given by the usual Boltzmann-Gibbs distribution for the ferromagnetic Hamiltonian of the chain. We study the properties of this dynamics both at infinite and at finite temperature, all over its parameter space, with particular emphasis on special lines and points.",1503.01661v3 2015-03-12,Dyson hierarchical quantum ferromagnetic Ising chain with pure or random transverse fields,"The Dyson hierarchical version of the quantum Ising chain with Long-Ranged power-law ferromagnetic couplings $J(r) \propto r^{-1-\sigma}$ and pure or random transverse fields is studied via real-space renormalization. For the pure case, the critical exponents are explicitly obtained as a function of the parameter $\sigma$, and are compared with previous results of other approaches. For the random case, the RG rules are numerically applied and the critical behaviors are compared with previous Strong Disorder Renormalization results.",1503.03727v2 2015-03-12,Control of current-induced spin-orbit effects in a ferromagnetic heterostructure by electric field,"We study the effects of electrostatic gating on the current-induced phenomena in ultrathin ferromagnet/heavy metal heterostructures. We utilize heterodyne detection and analysis of symmetry with respect to the direction of the magnetic field to separate electric field contributions to the magnetic anisotropy, current-induced field-like torque, and damping torque. Analysis of the electric field effects allows us to estimate the Rashba and the spin Hall contributions to the current-induced phenomena. Electrostatic gating can provide insight into the spin-orbit phenomena, and enable new functionalities in spintronic devices.",1503.03882v1 2015-03-13,Upper bound of one-magnon excitation and lower bound of effective mass for ferromagnetic spinor Bose and Fermi gases,"Using a variational method, we derive an exact upper bound for one-magnon excitation energy in ferromagnetic spinor gases, which limits the quantum corrections to the effective mass of a magnon to be positive. We also derive an upper bound for one-magnon excitation energy in lattice systems. The results hold for both Bose and Fermi systems in $d$ dimensions as long as the interaction is local and invariant under spin rotation.",1503.03951v3 2015-03-16,Impact of Current on Static and Kinetic Depinning Fields of Domain Wall in Ferromagnetic Nanostrip,"The impact of current on static and kinetic depinning fields of a domain wall in an one dimensional ferromagnetic nanostrip is investigated by solving the Landau-Lifshitz-Gilbert equation with adiabatic and non-adiabatic spin-transfer torques analytically and numerically. The results show that in the absence of current, the static depinning field is greater than the kinetic depinning field and both the depinning fields decrease by the increase of current applied in a direction opposite to the direction of the applied field. Both the depinning fields can also be tuned by the current to make them equal.",1503.04553v1 2015-03-18,Long-Range Triplet Supercurrents Induced by Singlet Supercurrents Parallel to Magnetic Interfaces,"Employing a spin-parameterized Kleldysh-Usadel technique for the diffusive regime, we demonstrate that even in the low proximity limit, considerable long-ranged triplet supercurrents can be effectively generated by spin-singlet supercurrents flowing \textit{parallel} to the interfaces of uniform double ferromagnet interlayers with noncollinear exchange fields ""{\it independent}"" of actual junction geometry. The triplet supercurrents are found to be most pronounced when the thicknesses of the ferromagnet strips are unequal. To experimentally verify this generic phenomenon, we propose an accessible and well-controlled structure that can fully isolate the long-range triplet effects.",1503.05584v1 2015-03-29,Cooperative Multiscale Aging in a Ferromagnet/Antiferromagnet Bilayer,"We utilize anisotropic magnetoresistance to study temporal evolution of the magnetization state in epitaxial Ni$_{80}$Fe$_{20}$/Fe$_{50}$Mn$_{50}$ ferromagnet/antiferromagnet bilayers. The resistance exhibits power-law evolution over a wide range of temperatures and magnetic fields, indicating that aging is characterized by a wide range of activation time scales. We show that aging is a cooperative process, i.e. the magnetic system is not a superposition of weakly interacting subsystems characterized by simple Arrhenius activation. The observed effects are reminiscent of avalanches in granular materials, providing a conceptual link to a broad class of critical phenomena in other complex condensed matter systems.",1503.08380v1 2015-04-16,Signatures of odd-frequency correlations in the Josephson current of superconductor/ferromagnet hybrid junctions,"Contacting a bilayer ferromagnet with a singlet even-frequency superconductor allows for the realization of an effective triplet odd-frequency superconductor. In this work, we investigate the Josephson effect between superconductors with different symmetries (e.g. odd- versus even-frequency). In particular, we study the supercurrent flowing between two triplet odd-frequency superconducting leads through a weak singlet even-frequency superconductor. We show that the peculiar temperature dependence of the critical current below the superconducting transition of the weak superconductor is a signature of the competition between odd/odd-frequency and odd/even-frequency Josephson couplings.",1504.04400v1 2015-04-18,Dynamic phase transitions in a ferromagnetic thin film system: A Monte Carlo simulation study,"Dynamic phase transition properties of ferromagnetic thin film system under the influence both bias and time dependent magnetic fields have been elucidated by means of kinetic Monte Carlo simulation with local spin update Metropolis algorithm. The obtained results after a detailed analysis suggest that bias field is the conjugate field to dynamic order parameter, and it also appears to define a phase line between two antiparallel dynamic ordered states depending on the considered system parameters. Moreover, the data presented in this study well qualitatively reproduce the recently published experimental findings where time dependent magnetic behavior of a uniaxial cobalt films is studied in the neighborhood of dynamic phase transition point.",1504.04749v1 2015-04-25,Layered systems at the mean field critical temperature,"We consider the Ising model on $\mathbb Z\times \mathbb Z$ where on each horizontal line $\{(x,i), x\in \mathbb Z\}$, the interaction is given by a ferromagnetic Kac potential with coupling strength $J_\gamma(x,y)\sim \gamma J(\gamma (x-y))$ at the mean field critical temperature. We then add a nearest neighbor ferromagnetic vertical interaction of strength $\epsilon$ and prove that for every $\epsilon >0$ the systems exhibits phase transition provided $\gamma>0$ is small enough.",1504.06767v1 2015-05-01,Anisotropic magneto-capacitance in ferromagnetic-plate capacitors,"The capacitance of a parallel plate capacitor can depend on applied magnetic field. Previous studies have identified capacitance changes induced via classical Lorentz force or spin-dependent Zeeman effects. Here we measure a magnetization direction dependent capacitance in parallel-plate capacitors where one plate is a ferromagnetic semiconductor, gallium manganese arsenide. This anisotropic magneto-capacitance is due to the anisotropy in the density of states dependent on the magnetization through the strong spin-orbit interaction.",1505.00170v1 2015-05-04,Magnetomechanical effects in the elastic polymer composites containing different volume fraction of ferromagnetic powder particles,"In the present work a detailed thermodynamic consideration for the magnetic free energy of the composite material consisting of the ferromagnetic powder particles embedded into a polymer matrix is given. We estimate their magnetostatic interaction energy and its dependence on the microscopic distribution of the magnetization and the magnetic field in the composite material. We also define the hydrostatic component of the mechanical force developed and the volume change effect caused by the magnetostatic interactions in such composites.",1505.00795v1 2015-05-20,Dipolar spin-misalignment correlations in inhomogeneous magnets: comparison between neutron scattering and micromagnetic approaches,"In inhomogeneous bulk ferromagnets, the dominating sources of spin disorder are related to spatial variations of (i) the magnitude of the local saturation magnetization and of (ii) the magnitude and/or direction of the magnetic anisotropy field. For the particular example of a porous ferromagnet, where the magnetization inhomogeneity is at maximum, we demonstrate, by means of experimental neutron scattering data and micromagnetic simulations, the anisotropic character of magnetization fluctuations induced by the dipolar interaction.",1505.05337v1 2015-06-05,On the magnetization process of ferromagnetic materials,"The present article concludes that a ferromagnetic sample could be considered like a paramagnetic system where the role of magnetic moments plays magnetic domains. Based on this conclusion and taking into account presence of an anisotropic field the formula which describes magnetization dependence on the external magnetic field is derived. Expressions for a remanent magnetization and a coercive force are presented. The new parameter to characterize a magnetic stiffness of a material is introduced. A physical expression for a dynamic magnetic susceptibility as a function of material's characteristics, external magnetic field, and a temperature is given.",1506.01805v3 2015-06-18,Dispersive read-out of ferromagnetic resonance for strongly coupled magnons and microwave photons,"We demonstrate the dispersive measurement of ferromagnetic resonance in a yttrium iron garnet sphere embedded within a microwave cavity. The reduction in the longitudinal magnetization at resonance is measured as a frequency shift in the cavity mode coupled to the sphere. This measurement is a result of the intrinsic non-linearity in magnetization dynamics, indicating a promising route towards experiments in magnon cavity quantum electro-dynamics.",1506.05631v1 2015-06-19,Coherent manipulation of a Majorana qubit by a mechanical resonator,"We propose a hybrid system composed of a Majorana qubit and a nanomechanical resonator, implemented by a spin-orbit-coupled superconducting nanowire, using a set of static and oscillating ferromagnetic gates. The ferromagnetic gates induce Majorana bound states in the nanowire, which hybridize and constitute a Majorana qubit. Due to the oscillation of one of these gates, the Majorana qubit can be coherently rotated. By tuning the gate voltage to modulate the local spin-orbit coupling, it is possible to reach the resonance of the qubit-oscillator system for relatively strong couplings.",1506.05879v2 2015-06-23,Effects of magnetic anisotropy on spin dynamics of ferromagnetic frustrated chain,"By exploiting density-matrix renormalization group techniques, we investigate the spin dynamics of a spin-1/2 one-dimensional J1-J2 XXZ model with competing ferromagnetic J1 and antiferromagnetic J2 exchange couplings under applied magnetic fields. Numerical results of spin excitation spectra show that in the field-induced spin quadrupole regime, the longitudinal component has a gapless mode and the transverse component has a gapped mode irrespective of the exchange anisotropy. The excitation gap of the transverse spin excitation increases as the exchange anisotropy increases over the XY-like and Ising-like regions, demonstrating that two-magnon bound states are stabilized due to the easy-axis anisotropy.",1506.06883v1 2015-06-23,Magnetic Excitations of Spin Nematic State in Frustrated Ferromagnetic Chain,"By exploiting density-matrix renormalization group techniques, we investigate the dynamical spin structure factor of a spin-1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions in an applied magnetic field. In a field-induced spin nematic regime, we find gapless longitudinal and gapped transverse spin excitation spectra, in accordance with quasi-long-ranged longitudinal and short-ranged transverse spin correlations, respectively. The gapless point coincides with the dominant longitudinal spin correlation, whereas the gap position exhibits a characteristic field dependence contradicting the dominant transverse spin correlation.",1506.06891v1 2015-07-02,Piezo-Magnetoelectric Effect of Spin Origin in Dysprosium Orthoferrite,"Piezo-magnetoelectric effect, namely simultaneous induction of both the ferromagnetic moment and electric polarization by an application of uniaxial stress, was achieved in the non-ferroelectric and antiferromagnetic ground state of DyFeO$_3$. The induced electric polarization and ferromagnetic moment are coupled with each other, and monotonically increase with increasing uniaxial stress. The present work provides a new way to design spin-driven multiferroic states, that is, magnetic symmetry breaking forced by external uniaxial stress.",1507.00449v1 2015-07-02,Spin-dependent tunneling in semiconductor heterostructures with a magnetic layer,"We present a theory that describes the appearance of circular polarization of the photoluminescence (PL) in ferromagnet-semiconductor hybrid heterostructures due to spin-dependent tunneling of photoexcited carriers from a quantum well into a magnetic layer. The theory succeeds in explaining the experimental data on time-resolved PL for heterostructures consisting of InGaAs-based quantum well (QW) and a spatially separated Mn $\delta$-layer. We show that the circular polarization of the PL originates from dynamic spin polarization of electrons due to spin-dependent leakage from the QW onto Mn donor states split by the exchange field of the ferromagnetic Mn delta-layer.",1507.00585v1 2015-07-13,Effective 90-degree magnetization rotation in Co2FeAl thin film/Piezoelectric system probed by microstripline ferromagnetic,"Microstripline ferromagnetic resonance technique has been used to study the indirect magnetoelectric coupling occurring in an artificial magnetoelectric heterostructure consisting of a magnetostrictive thin film cemented onto a piezoelectric actuator. Two different modes (sweep-field and sweep-frequency modes) of this technique have been employed to quantitatively probe the indirect magnetoelectric coupling and to observe a voltage induced magnetization rotation (of 90 degree). This latter has been validated by the experimental frequency variation of the uniform mode and by the amplitude of the sweep-frequency spectra.",1507.03308v1 2015-08-07,A Holographic Realization of Ferromagnets,"A holographic realization for ferromagnetic systems has been constructed. Owing to the holographic dictionary proposed on the basis of this realization, we obtained relevant thermodynamic quantities such as magnetization, magnetic susceptibility, and free energy. This holographic model reproduces the behavior of the mean field theory near the critical temperature. At low temperatures, the results automatically incorporate the contributions from spin wave excitations and conduction electrons.",1508.01626v1 2015-08-23,Rotational properties of ferromagnetic nanoparticles driven by a precessing magnetic field in a viscous fluid,"We study the deterministic and stochastic rotational dynamics of ferromagnetic nanoparticles in a precessing magnetic field. Our approach is based on the system of effective Langevin equations and on the corresponding Fokker-Planck equation. Two key characteristics of the rotational dynamics, the average angular frequency of precession of nanoparticles and their average magnetization, are of our interest. Using the Langevin and Fokker-Planck equations, we calculate both analytically and numerically these characteristics in the deterministic and stochastic cases, determine their dependence on the model parameters, and analyze in detail the role of thermal fluctuations.",1508.05651v1 2015-08-25,Characterization of Topological Phases of Spin-1/2 Frustrated Ferromagnetic-Antiferromagnetic Alternating Heisenberg Chains by Entanglement Spectrum,"The topological classification of a series of frustration-induced spin-gap phases in the spin-1/2 ferromagnetic--antiferromagnetic alternating Heisenberg chain with next-nearest-neighbour interaction reported in J. Phys. Soc. Jpn. 82, 064703 (2013) is confirmed using two kinds of entanglement spectra defined by different divisions of the whole chain. For the numerical calculation, the iDMRG method is used. The results are consistent with the valence bond solid picture proposed in the previous paper.",1508.06129v2 2015-09-03,Asymptotic Ferromagnetic Ordering of Energy Levels for the Heisenberg Model on Large Boxes,"We prove a result for the spin-$1/2$ quantum Heisenberg ferromagnet on $d$-dimensional boxes $\{1,\dots,L\}^d \subset \mathbb{Z}^d$. For any $n$, if $L$ is large enough, the Hamiltonian satisfies: among all vectors whose total spin is at most $(L^d/2)-n$, the minimum energy is attained by a vector whose total spin is exactly $(L^d/2)-n$.",1509.00907v1 2015-09-15,Magnetic Nernst effect,"The thermodynamics of irreversible processes in continuous media predicts the existence of a Magnetic Nernst effect that results from a magnetic analog to the Seebeck effect in a ferromagnet and magnetophoresis occurring in a paramagnetic electrode in contact with the ferromagnet. Thus, a voltage that has DC and AC components is expected across a Pt electrode as a response to the inhomogeneous magnetic induction field generated by magnetostatic waves of an adjacent YIG slab subject to a temperature gradient. The voltage frequency and dependence on the orientation of the applied magnetic induction field are quite distinct from that of spin pumping.",1509.04440v1 2015-09-18,Observation of thermoelectric currents in high-field superconductor-ferromagnet tunnel junctions,"We report on the experimental observation of thermoelectric currents in superconductor-ferromagnet tunnel junctions in high magnetic fields. The thermoelectric signals are due to a spin-dependent lifting of particle-hole symmetry, and are found to be in excellent agreement with recent theoretical predictions. The maximum Seebeck coefficient inferred from the data is about $-100~\mathrm{\mu V/K}$, much larger than commonly found in metallic structures. Our results directly prove the coupling of spin and heat transport in high-field superconductors.",1509.05568v1 2015-09-24,Nonadiabatic pure spin pumping in zigzag graphene nanoribbons with proximity induced ferromagnetism,"By combining Floquet theory with Green's function formalism, we present non-adiabatic quantum spin and charge pumping through a zigzag ferromagnetic graphene nanoribbon including a double-barriers structure driven weakly by two local $ac$ gate voltages operating with a phase-lag. Over a wide range of Fermi energies, interesting quantum pumping such as i) pure spin pumping with zero net charge pumping, ii) pure charge pumping and iii) fully spin polarized pumping can be achieved by tuning and manipulating driving frequency in the non-adiabatic regime. Spin polarized pumping which is measurable using the current technology depends on the competition between the energy level spacing and driving frequency.",1509.07233v1 2015-09-27,Exchange Bias following Kinetic Arrest,"Exchange bias is often observed when anti-ferromagnetic and ferromagnetic phases coexist. The coexistence of two competing magnetic phases can persist to the lowest temperatures if the disorder-broadened 1st order transition separating them is interrupted, as is proposed in the kinetic arrest phenomenon. The fractions of coexisting phases can, in this phenomenon, be tuned by following different cooling protocols. We discuss predicted behaviours of exchange bias resulting from the kinetic arrest phenomenon. Specifically, for appropriate values of cycling field Hmax and measuring temperature T0 there will be no exchange bias under cooling in zero field, while it will manifest with increasing cooling field, and then saturate.",1509.08052v1 2015-09-28,Superconductivity and Magnetism in Noncentrosymmetric RhGe,"RhGe synthesized at high pressure is crystallized in noncentrosymmetric cubic structure of the B20 type. Measurements of the electrical resistivity and magnetization demonstrate a superconducting state below T$_c$ ~ 4.5 K and a weak ferromagnetism below T$_m$ ~ 140 K. Specific heat data confirm the bulk nature of superconductivity in this ferromagnetic superconductor. The superconducting region forms a dome on the P-T diagram with a maximum of T$_c$ near 4 GPa. Ab initio simulations suggest that the observed weak magnetization emerges from the pronounced spin polarization with magnetic quadrupole and toroidal moments located at Rh and Ge sites.",1509.08249v1 2015-10-22,Molecular Anisotropic Magnetoresistance,"Using density functional theory calculations, we demonstrate that the effect of anisotropic magnetoresistance (AMR) can be enhanced by orders of magnitude with respect to conventional bulk ferromagnets in junctions containing molecules sandwiched between ferromagnetic leads. We study ballistic transport in metal-benzene complexes contacted by $3d$ transition-metal wires. We show that the gigantic AMR can arise from spin-orbit coupling effects in the leads, drastically enhanced by orbital-symmetry filtering properties of the molecules. We further discuss how this molecular anisotropic magnetoresistance (MAMR) can be tuned by proper choice of materials and their electronic properties.",1510.06632v1 2015-11-02,"Uniqueness, Spatial Mixing, and Approximation for Ferromagnetic 2-Spin Systems","We give fully polynomial-time approximation schemes (FPTAS) for the partition function of ferromagnetic 2-spin systems in certain parameter regimes. The threshold we obtain is almost tight up to an integrality gap. Our technique is based on the correlation decay framework. The main technical contribution is a new potential function, with which we establish a new kind of spatial mixing.",1511.00493v3 2015-11-04,Non-Convex Multipartite Ferromagnets,"We investigate a multipartite ferromagnetic model without self-interactions between spins of the same party, so that the Hamiltonian is not a definite quadratic form of the magnetisations. We find the free energy and study the phase transition for all zero external fields. Moreover in the bipartite case we analyse the fluctuations of the rescaled magnetisations, below and at the critical point, and we study the phase transitions with non zero magnetic fields.",1511.01307v2 2015-11-11,Spin Swapping Transport and Torques in Ultrathin Magnetic Bilayers,"Planar spin transport in disordered ultrathin magnetic bilayers comprising a ferromagnet and a normal metal (typically used for spin pumping, spin Seebeck and spin-orbit torque experiments) is investigated theoretically. Using a tight-binding model that treats extrinsic spin Hall effect, spin swapping and spin relaxation on equal footing, we show that the nature of spin-orbit coupled transport dramatically depends on ratio between the layers thickness $d$ and the mean free path $\lambda$. While spin Hall effect dominates in the diffusive limit ($d\gg\lambda$), spin swapping dominates in Knudsen regime ($d\lesssim\lambda$). A remarkable consequence is that the symmetry of the spin-orbit torque exerted on the ferromagnet is entirely different in these two regimes.",1511.03454v1 2016-01-04,Ultrafast transient dynamics in composite multiferroics,"We investigate theoretically the dynamic multiferroic response of coupled ferroelectric/ferromagnetic composites upon excitation by a photo-induced acoustic strain pulse. Two magnetoelectric mechanisms are considered: interface strain- and charge-mediated magnetoelectric couplings. The former results in demagnetization, depolarization and repolarization within tens of picoseconds via respectively magnetostriction and piezoelectricity. Charge magnetoelectric interaction affects the ferroelectric/ferromagnetic feedback response leading to magnetization recovery. Experimental realization based on time-resolved x-ray diffraction is suggested. The findings indicate the potential of composite multiferroics for photo-steered, high-speed, multi-state electronic devices.",1601.00391v1 2016-01-04,Direct and inverse cascades of spin-wave turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates,"We theoretically and numerically study spin-wave turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates, finding direct and inverse cascades with power-law behavior. To derive these power exponents analytically, the conventional weak wave turbulence theory is applied to the spin-1 spinor Gross-Pitaevskii equation. Thus, we obtain the $-7/3$ and $-5/3$ power laws in the transverse spin correlation function for the direct and inverse cascades, respectively. To confirm these power laws, numerical calculations are performed that obtain results consistent with these power laws.",1601.00446v2 2016-01-12,U(1) Emergence versus Chiral Symmetry Restoration in the Ashkin Teller Model,"We show that suppression of vortices in the Ashkin Teller ferromagnet on a square lattice splits the order-disorder transition and opens up an intermediate phase where the macroscopic symmetry enhances to U(1). When we selectively suppress the formation of non-chiral vortices, chiral vortices proliferate and replace the U(1) phase with a new phase where chiral symmetry is restored. This result demonstrates a fascinating phenomenon in which the symmetry information encoded in topological defects manifests itself in the symmetry of the phase where the defects proliferate. We also show that this phenomenon can occur in all $\mathbb{Z}_n$ ferromagnets with even values of $n$.",1601.02755v1 2016-01-24,Multiferroic Two-Dimensional Materials,"The relation between unusual Mexican-hat band dispersion, ferromagnetism and ferroelasticity is investigated using a combination of analytical, first-principles and phenomenological methods. The class of material with Mexican-hat band edge is studied using the $\alpha$-SnO monolayer as a prototype. Such band edge causes a van Hove singularity diverging with $\frac{1}{\sqrt{E}}$, and in p-type material leads to spatial and/or time-reversal spontaneous symmetry breaking. We show that an unexpected multiferroic phase is obtained in a range of hole density for which the material presents ferromagnetism and ferroelasticity simultaneously.",1601.06438v1 2016-01-30,Low-Temperature Spin-Wave Approximation for the Heisenberg Ferromagnet,"We study the low temperature thermodynamics of the quantum Heisenberg ferromagnet in three dimension for any value of the spin $ S \geq 1/2 $. We report on a rigorous proof of the validity of the spin-wave approximation for the excitation spectrum, at the level of the first non-trivial contribution to the free energy as the inverse temperature $ \beta \to \infty $.",1602.00155v1 2016-02-25,Uniaxial pressure induced half-metallic ferromagnetic phase transition in LaMnO$_3$,"We use first-principles theory to predict that the application of uniaxial compressive strain leads to a transition from an antiferromagnetic insulator to a ferromagnetic half-metal phase in LaMnO$_3$. We identify the Q2 Jahn-Teller mode as the primary mechanism that drives the transition, indicating that this mode can be used to tune the lattice, charge, and spin coupling. Applying $\simeq$ 6 GPa of uniaxial pressure along the [010] direction activates the transition to a half-metallic $\textit{pseudo-cubic}$ state. The half-metallicity opens the possibility of producing colossal magnetoresistance in the stoichiometric LaMnO$_3$ compound at significantly lower pressure compared to recently observed investigations using hydrostatic pressure.",1602.07984v1 2016-02-26,Tuning the magnetocrystalline anisotropy in $R$CoPO by means of $R$ substitution: a ferromagnetic resonance study,"We report on broad-band electron spin resonance measurements performed within the itinerant ferromagnetic phase of $R$CoPO ($R$ = La, Pr, Nd and Sm). We reveal that the $R$ substitution is highly effective in gradually introducing a sizeable easy-plane magnetocrystalline anisotropy within the Co sublattice. We explain our results in terms of a subtle interplay of structural effects and of indirect interactions between the $f$ and $d$ orbitals from $R$ and Co, respectively.",1602.08377v1 2016-03-02,Ferromagnetic resonance in submicron permalloy stripes,"We present the results of systematic experimental investigations and micromagnetic simulations for the ferromagnetic resonance in rectangular permalloy microstripes. It is shown that the resonant magnetization oscillations have a complex spatial structure including a quasi-homogeneous precession, lateral spin-wave resonances and localized edge modes, which strongly depend on sample orientation in an external magnetic field.",1603.00694v1 2016-03-10,Resonant excitation of the spin-wave current in hybrid nanostructures,"Using the non-equilibrium statistical operator method (NSO), we have investigated the spin transport through the interface in a semiconductor/ferromagnetic insulator hybrid structure. We have analyzed the approximation of effective parameters, when each of the considered subsystems (conduction electrons, magnons, and phonons) is characterized by its effective temperature. We have constructed the macroscopic equations, describing the spin-wave current caused by both resonantly excited spin system of conduction electrons and by an inhomogeneous thermal field in the ferromagnetic insulator.",1603.03207v1 2016-03-23,Large Magnetoresistance at Room Temperature in Ferromagnet/Topological Insulator Contacts,"We report magnetoresistance for current flow through iron/topological insulator (Fe/TI) and Fe/evaporated-oxide/TI contacts when a magnetic field is used to initially orient the magnetic alignment of the incorporated ferromagnetic Fe bar, at temperatures ranging from 100 K to room temperature. This magnetoresistance is associated with the relative orientation of the Fe bar magnetization and spin-polarization of electrons moving on the surface of the TI with helical spin-momentum locking. The magnitude of the observed magnetoresistance is relatively large compared to that observed in prior work.",1603.07283v1 2016-04-13,Dipolar Rings of Microscopic Ellipsoids: Magnetic Manipulation and Cell Entrapment,"We study the formation and dynamics of dipolar rings composed by microscopic ferromagnetic ellipsoids, which self-assemble in water by switching the direction of the applied field. We show how to manipulate these fragile structures and control their shape via application of external static and oscillating magnetic fields. We introduce a theoretical framework which describes the ring deformation under an applied field, allowing to understand the underlying physical mechanism. Our microscopic rings are finally used to capture, entrap and later release a biological cell via magnetic command, i.e. performing a simple operation which can be implemented in other microfluidic devices which make use of ferromagnetic particles.",1604.03895v2 2016-04-20,Ultrafast and gigantic spin injection in semiconductors,"The injection of spin currents in semiconductors is one of the big challenges of spintronics. Motivated by the ultrafast demagnetisation and spin injection into metals, we propose an alternative femtosecond route based on the laser excitation of superdiffusive spin currents in a ferromagnet such as Ni. Our calculations show that even though only a fraction of the current crosses the Ni-Si interface, the laser-induced creation of strong transient electrical fields at a ferromagnet-semiconductor interface allows for the injection of chargeless spin currents with a record spin polarisations of 80%. Beyond that they are pulsed on the time scale of 100 femtoseconds which opens the door for new experiments and ultrafast spintronics.",1604.05882v1 2016-04-22,Swift thermal steering of domain walls in ferromagnetic MnBi stripes,"We predict a fast domain wall (DW) motion induced by a thermal gradient across a nanoscopic ferromagnetic stripe of MnBi. The driving mechanism is an exchange torque fueled by magnon accumulation at the DWs. Depending on the thickness of the sample, both hot-to-cold and cold-to-hot DW motion directions are possible. The finding unveils an energy efficient way to manipulate DWs as an essential element in magnetic information processing such as racetrack memory.",1604.06575v1 2016-04-28,Ferromagnetism and d+id superconductivity in 1/2 doped correlated systems on triangular lattice,"We investigate the quantum phase diagram of t-J model on triangular lattice at 1/2 doping with various lattice sizes by using a combination of density matrix renormalization group (DMRG), variational Monte Carlo and quantum field theories. To sharply distinguish different phases, we calculated the symmetry quantum numbers of the ground state wave functions, and the results are further confirmed by studying correlation functions. Our results show there is a first order phase transition from ferromagnetism to d+id superconductivity, with the transition taking place at $J/t=0.4\pm0.2$.",1604.08615v1 2016-06-08,Magnetic energy of sc ferromagnetic films with three layers as described by third order perturbed Heisenberg Hamiltonian,"The solution of third order perturbed Heisenberg Hamiltonian of simple cubic ferromagnetic ultra-thin films with three layers were found. All the magnetic energy parameters such as spin exchange interaction, magnetic dipole interaction, second order magnetic anisotropy, fourth order magnetic anisotropy, applied magnetic field, demagnetization factor and stress induced anisotropy were included in the third order perturbed Heisenberg Hamiltonian. 3-D plots of stress induced anisotropy, out of plane magnetic field, demagnetization factor and spin exchange interaction are presented in this manuscript. Magnetic easy and hard directions were determined using these 3-D plots. MATLAB program was employed to solve the equation with seven parameters.",1606.02439v1 2016-06-27,Spin wave power flow and caustics in ultrathin ferromagnets with the Dzyaloshinskii-Moriya interaction,"The Dzyaloshinskii-Moriya interaction in ultrathin ferromagnets can result in nonreciprocal propagation of spin waves. We examine theoretically how spin wave power flow is influenced by this interaction. We show that the combination of the dipole-dipole and Dzyaloshinskii-Moriya interactions can result in unidirectional caustic beams in the Damon-Eshbach geometry. Morever, self-generated interface patterns can also be induced from a point-source excitation.",1606.08159v2 2016-07-05,Fluctuations of the free energy of the spherical Sherrington-Kirkpatrick model with ferromagnetic interaction,"We consider a spherical spin system with pure 2-spin spherical Sherrington-Kirkpatrick Hamiltonian with ferromagnetic Curie-Weiss interaction. The system shows a two-dimensional phase transition with respect to the temperature and the coupling constant. We compute the limiting distributions of the free energy for all parameters away from the critical values. The zero temperature case corresponds to the well-known phase transition of the largest eigenvalue of a rank 1 spiked random symmetric matrix. As an intermediate step, we establish a central limit theorem for the linear statistics of rank 1 spiked random symmetric matrices.",1607.01089v2 2016-07-13,Demagnetization factor dependence of energy of thick ferromagnetic films,"Second order perturbed Heisenberg Hamiltonian was employed to study the variation of energy of ferromagnetic thick films with demagnetization factor. Under the influence of demagnetization factor given by =6.6, the sc(001) film with 10000 layers can be easily oriented in 0.6 radians direction for the values of energy parameters used in this report. Easy direction of thick fcc(001) film with 10000 layers was determined at 0.66 radians, when the demagnetization factor is given by =2.6. The energy of sc(001) thick film is larger than that of fcc(001) thick film. But the energy curve of fcc(001) thick film is smoother than that of sc(001).",1607.03590v1 2016-07-29,Ferromagnetic detection of moduli dark matter,"We propose a scheme to detect light scalar moduli dark matter, based on measuring the change of magnetization induced in a macroscopic hard ferromagnet. Our method can probe moduli dark matter at the natural coupling to the electron mass over several orders of magnitude in the moduli mass. The most attracting feature of the proposed approach, compared to mechanical ones, is that it relies on a nonresonant detection, allowing to probe a much wider region of the parameter space. This is a crucial point, as long as the theory is not able to predict the moduli mass.",1607.08824v1 2016-09-07,Magnetic decoupling of ferromagnetic metals through a graphene spacer,"We study the magnetic coupling between different ferromagnetic metals (FMs) across a graphene (G) layer, and the role of graphene as a thin covalent spacer. Starting with G grown on a FM substrate (Ni or Co), we deposit on top at room temperature different FM metals (Fe, Ni, Co). By measuring the dichroic effect of 3p photoemission lines we detect the magnetization of the substrate and the sign of the exchange coupling in FM overlayer at room temperature. We show that the G layer magnetically decouples the FM metals.",1609.01861v1 2016-09-27,Multiferroic Bi$_2$NiMnO$_6$ Thin Films: A Computational Prediction,"We report first-principles calculations for one of the few materials that is believed to be a ferroelectric ferromagnet, Bi$_2$NiMnO$_6$. Our calculations show that, contrary to what it has been reported so far, bulk Bi$_2$NiMnO$_6$ does not have a polarization. Instead, like BiMnO$_3$, it crystallizes into a centrosymmetric structure with space group $C2/c$. We also predict that Bi$_2$NiMnO$_6$ will indeed be a ferroelectric ferromagnet if it is grown as an epitaxial film on a substrate with in-plane square symmetry and a lattice constant around 4~\AA, such as BaTiO$_3$ or PbZr$_{1-x}$Ti$_{x}$O$_{3}$.",1609.08246v1 2016-10-03,Anomalous Thermal Hall Effect in a Disordered Weyl Ferromagnet,"We investigate the electric and thermal transport properties in a disordered Weyl ferromagnet on an equal footing by using the Keldysh formalism in curved spacetime. In particular, we calculate the anomalous thermal Hall conductivity, which consists of the Kubo formula and the heat magnetization, without relying on the Wiedemann-Franz law. We take nonmagnetic impurities into account within the self-consistent $T$-matrix approximation and reproduce the Wiedemann-Franz law for the extrinsic Fermi-surface and intrinsic Fermi-sea terms, respectively. This is the first step towards a unified theory of the anomalous Hall effect at finite temperature, where we should take into account both disorder and interactions.",1610.00390v2 2016-10-15,Spin excitations in an all-organic double quantum dot molecule,"We realize a strongly coupled double quantum dot in a single all-organic molecule by introducing a non-conjugated bridge in between two identical conjugated moieties. Spin-1/2 Kondo and Kondo enhanced low-energy excitations for respectively the odd and even electron occupation are observed in off-resonant transport. The ground state in the even occupation can be the singlet or the triplet state varying between samples. This observation suggests that both anti-ferromagnetic and ferromagnetic interactions between spins are of the same order of magnitude.",1610.04715v1 2016-10-18,Engineering Curvature Induced Anisotropy in Thin Ferromagnetic Films,"The large curvature effects on micromagnetic energy of a thin ferromagnetic film with nonlocal dipolar energy are considered. We predict that the dipolar interaction and surface curvature can produce perpendicular anisotropy which can be controlled by engineering a special type of periodic surface shape structure. Similar effects can be achieved by a significant surface roughness in the film. We show that in general the anisotropy can point in an arbitrary direction depending on the surface curvature. We provide simple examples of these periodic surface structures to demonstrate how to engineer particular anisotropies in the film.",1610.05574v1 2017-02-08,Annihilation of Domain Walls in a Ferromagnetic Wire,"We study the annihilation of topological solitons in the simplest setting: a one-dimensional ferromagnet with an easy axis. We develop an effective theory of the annihilation process in terms of four collective coordinates: two zero modes of the translational and rotational symmetries $Z$ and $\Phi$, representing the average position and azimuthal angle of the two solitons, and two conserved momenta $\zeta$ and $\varphi$, representing the relative distance and twist. Comparison with micromagnetic simulations shows that our approach captures well the essential physics of the process.",1702.02248v1 2017-02-15,Pressure-induced insulator-metal transition in EuMnO3,"We study the influence of external pressure on the electronic and magnetic structure of EuMnO3 from first-principles calculations. We find a pressure-induced insulator-metal transition at which the magnetic order changes from A-type antiferromagnetic to ferromagnetic with a strong interplay with Jahn-Teller distortions. In addition, we find that the non-centrosymmetric E*-type antiferromagnetic order can become nearly degenerate with the ferromagnetic ground state in the high-pressure metallic state. This situation can be exploited to promote a magnetically-driven realization of a non-centrosymmetric (ferroelectric) metal.",1702.04607v2 2017-02-16,Domain wall nucleation in ferromagnetic nanowire with perpendicular magnetization stimulated by stray field of V-shaped magnetic particle,"We report the results of micromagnetic simulations of domain wall (DW) nucleation and pinning/depinning processes in ferromagnetic planar structure consisting of nanowire (NW) with perpendicular anisotropy and special V-shaped nanoparticle (NP) with in-plane anisotropy located on top of NW. The magnetization reversal features of this system in an external magnetic field are investigated depending on the direction of particle magnetic moment. Possible variants of magnetic logic cells (LCs) based on such system are discussed.",1702.05416v1 2017-02-14,Exact and approximate analytical solutions of Weiss equation of ferromagnetism and their experimental relevance,"The recent progress in the theory of generalized Lambert functions makes possible to solve exactly the Weiss equation of ferromagnetism. However, this solution is quite inconvenient for practical purposes. Precise approximate analytical solutions are obtained, giving the temperature dependence of the spontaneous magnetization, and also the dependence of the magnetization on both temperature and external magnetic field. The experimental relevance of these results, mainly for the determination of the Curie temperature, is discussed.",1702.07225v1 2017-08-05,Magnetization reversal of thin ferromagnetic elements with surface anisotropy,"The magnetization reversal process in thin-film ferromagnetic elements with surface anisotropy of various shapes and sizes is investigated by means of numerical simulation. The dependence of the perpendicular and in-plane hysteresis loops on the element thickness and the value of the surface anisotropy constant is obtained. For sufficiently large values of the surface anisotropy constant the magnetization reversal of thin-film elements is shown to occur due to the nucleation of the buckling mode. For an elongated rectangular element the nucleation field of the buckling mode is proportional to the absolute value of the surface anisotropy constant, and inversely proportional to the element thickness.",1708.01802v1 2017-08-07,Exact description of paramagnetic and ferromagnetic phases of an Ising model on a third-order Cayley tree,"In this paper we analytically study the recurrence equations of an Ising model with three competing interactions on a Cayley tree of order three. We exactly describe paramagnetic and ferromagnetic phases of the Ising model. We obtain some rigorous results: critical temperatures and curves, number of phases, partition function. Ganikhodjaev et al. [J. Concrete and Applicable Mathematics, 9 (1), 26-34 (2011)] have numerically studied the Ising model on a second-order Cayley tree. We compare the numerical results to exact solutions of mentioned model.",1708.02585v1 2017-08-10,Electron transport in magnetic tunnel junctions -- a theoretical study of lattice and continuum models,"Magnetic tunnel junctions comprising of an insulator sandwiched between two ferromagnetic films are the simplest spintronic devices. Theoretically, these can be modeled by a metallic Hamiltonian in both the lattice and the continuum with an addition of Zeeman field. We calculate conductance at arbitrary orientations of the easy axes of the two ferromagnets. When mapped, the lattice and the continuum models show a discrepancy in conductance in the limit of a large Zeeman field. We resolve the discrepancy by modeling the continuum theory in an appropriate way.",1708.03161v2 2017-08-28,Evidence for field induced proximity type behavior in ferromagnetic nanofluid,"We report some unusual magnetic properties observed in CoFe2O4 based ferrofluid (with an average particle size of D = 6 nm). More precisely, in addition to the low-field ferromagnetic (FM) phase transition with an intrinsic Curie temperature T_Cb=350K, a second phase transition with an extrinsic Curie temperature T_Cw = 266K emerges at higher (saturating) magnetic field. The transitions meet at the crossover point T_cr = 210 K. The origin of the second transition is attributed to magnetic field induced proximity type interaction between FM particles through non-FM layers.",1708.08373v1 2018-02-01,Exploring the diluted ferromagnetic p-spin model with a Cavity Master Equation,"We introduce a new solution to Glauber multi-spin dynamics on random graphs. The solution is based on the recently introduced Cavity Master Equation (CME), a time-closure turning the in principle exact Dynamic Cavity Method into a practical method of analysis and of fast simulation. Running CME once is of comparable computational complexity as one Monte Carlo run on the same problem. We show that CME correctly models the ferromagnetic $p$-spin Glauber dynamics from high temperatures down to and below the spinoidal transition. We also show that CME allows a novel exploration of the low-temperature spin-glass phase of the model.",1802.00298v1 2018-02-06,Nonlinear heat transport in ferromagnetic-quantum dot-superconducting systems,"We analyze the heat current traversing a quantum dot sandwiched between a ferromagnetic and a superconducting electrode. The heat flow generated in response to a voltage bias presents rectification as a function of the gate potential applied to the quantum dot. Remarkably, in the thermally driven case the heat shows a strong diode effect with large asymmetry ratios that can be externally tuned with magnetic fields or spin-polarized tunneling. Our results thus demonstrate the importance of hybrid systems as promising candidates for thermal applications.",1802.02024v1 2018-04-12,Peculiar magnetic properties of NC6 and NC12 layered compounds from first principles,"In the context of characterizing nitrogen poor carbo-nitrides for different applications, identification of an unusual onset of spin polarization of N(p) states has been shown. A full saturation up to 3 Bohr magnetons is demonstrated in extended two-dimensional carbon networks of C6N and C12N hexagonal structures refined based on density functional theory calculations. From establishing the energy-volume equations of states in both compounds assuming spin degenerate (non spin polarized) and spin-polarized configurations, the ground state is identified as ferromagnetic. The variation of magnetization with volume points to strongly ferromagnetic behavior",1804.04441v1 2018-11-06,Quantized Josephson phase battery,"A ferromagnetic Josephson junction with a spin-flipper (magnetic impurity) sandwiched in-between acts as a phase battery that can store quantized amounts of superconducting phase difference $\Phi_0$ in the ground state of the junction. Moreover, for such $\Phi_0$-Josephson junction anomalous Josephson current appears at zero phase difference. We study the properties of this quantum spin-flip scattering induced anomalous Josephson current, especially its tun-ability via misorientation angle between two Ferromagnets.",1811.02484v2 2018-11-22,Studying of the interlayer interaction in magnetic multilayers (FM/I/FM) measuring the FMR peak asymmetry,"We experimentally study the interlayer interaction in a magnetic multilayer system ferromagnet/insulator/ferromagnet with different spacer thickness. We demonstrate that the sign and the magnitude of the interaction can be deduced from the FMR peak shape rather than from the FMR peak shift. The proposed technique allows studying the interlayer interaction using a single sample (without a reference sample for comparison).",1811.09228v1 2019-05-23,Devil's staircase structures in $\varphi_{0}$ junction,"The superconductor-ferromagnet-superconductor $\varphi_{0}$ junction provides a direct coupling between Josephson phase and magnetic moment of ferromagnetic barrier. We demonstrate an appearance of additional fractional subharmonic steps in the IV-characteristics of $\varphi_{0}$ junction under external electromagnetic radiation due to spin-orbit coupling. An origin of subharmonic steps is related to the locking of magnetic moment precession to the Josephson oscillations. We prove that the positions of those steps follow a continued fraction algorithm.",1905.09672v1 2020-07-06,Spin-flip excitations and Stoner ferromagnetism in a strongly correlated quantum Hall system,"Spin-flip excitations in a quantum Hall electron system at fixed filling factor nu=2 are modelled and studied under conditions of a strong Coulomb interaction when the `Landau level mixing' is a dominant factor determining the excitation energy. The `one-exciton' approach used for the purely electronic excitations in question allows us to describe the Stoner transition from the unpolarized/paramgnet state to the polarized/ferromagnet one. The theoretical results are compared with the available experimental data.",2007.02827v1 2020-07-27,Electric-Field-Induced Antiferromagnetic Insulating State in a Metallic Ferromagnet,"We show that a static electric field induces the transition from a ferromagnetic metal to an antiferromagnetic insulator owing to the Bloch oscillation of conduction electrons. In the steady state, the electric current is inversely proportional to the applied electric field, implying the nonperturbative insulating nature that is different from the Wannier-Stark localization. Possible experimental realization based on recent terahertz pulse sources is discussed.",2007.13720v1 2010-05-11,A spin quantum bit with ferromagnetic contacts for circuit QED,"We theoretically propose a scheme for a spin quantum bit based on a double quantum dot contacted to ferromagnetic elements. Interface exchange effects enable an all electric manipulation of the spin and a switchable strong coupling to a superconducting coplanar waveguide cavity. Our setup does not rely on any specific band structure and can in principle be realized with many different types of nanoconductors. This allows to envision on-chip single spin manipulation and read-out using cavity QED techniques.",1005.1901v2 2010-05-12,Kinetics of precessing ball solitons in ferromagnet at the first-order transition,"The fundamentals of precessing ball solitons (PBS) arising as a result of the energy fluctuations at the first-order phase transition induced by a magnetic field in ferromagnets with uniaxial anisotropy are presented. When external magnetic field is anti-parallel to the magnetization direction of the crystal, PBS states are possible in a wide range of amplitudes and energies, including the positive and negative energy relative to an initial condition. PBS are born with the greatest probability at near-zero energy, i.e. near the bifurcation point. Evolution of the PBS, at which they transform into macroscopic domains of a new magnetic phase or into quasi-equilibrium solitonic state, is analyzed.",1005.2054v1 2010-05-12,Ferromagnetism without flat bands in thin armchair nanoribbons,"Describing by a Hubbard type of model a thin armchair graphene ribbon in the armchair hexagon chain limit, one shows in exact terms, that even if the system does not have flat bands at all, at low concentration a mesoscopic sample can have ferromagnetic ground state, being metallic in the same time. The mechanism is connected to a common effect of correlations and confinement.",1005.2126v1 2010-05-13,Ferromagnetic spinel CuCr2Se4 studied by Raman spectroscopy and lattice dynamics calculations,"The lattice dynamics of the ferromagnetic spinel CuCr2Se4 (Tc = 430K) was studied experimentally by measuring the Raman spectra and theoretically by calculation of zone center phonon frequencies within a shell model. All Raman allowed modes (A1g + E_g + 3F2g) were identified and assigned to specific atomic motions. The relative intensity of the Raman lines varies strongly with excitation photon energy between 1.58 and 2.71 eV, but no significant phonon anomalies are observed near Tc.",1005.2245v1 2010-05-13,Competition between pairing and ferromagnetic instabilities in ultracold Fermi gases near Feshbach resonances,"We study the quench dynamics of a two-component ultracold Fermi gas from the weak into the strong interaction regime, where the short time dynamics are governed by the exponential growth rate of unstable collective modes. We obtain an effective interaction that takes into account both Pauli blocking and the energy dependence of the scattering amplitude near a Feshbach resonance. Using this interaction we analyze the competing instabilities towards Stoner ferromagnetism and pairing.",1005.2366v1 2010-05-24,The importance of hole concentration in establishing carrier-mediated ferromagnetism in Mn doped Ge,"In the present work, we have prepared Mn-doped Ge using different annealing approaches after Mn ion implantation, and obtained samples with hole concentrations ranging from 10^18 to 2.1x10^20 cm^-3, the latter being the highest reported so far. Based on the magnetotransport properties of Mn doped Ge, we argue that the hole concentration is a decisive parameter in establishing carrier-mediated ferromagnetism in magnetic Ge.",1005.4325v1 2010-05-26,Ferromagnetism in the Highly-Correlated Hubbard Model,"The Hubbard model with strong correlations is treated in the many-electron representation of Hubbard's operators. The regions of stability of saturated and non-saturated ferromagnetism in the n-U plane for the square and simple cubic lattices are calculated. The role of the bare density of states singularities for the magnetic phase diagram is discussed. A comparison with the results of previous works is performed.",1005.4795v1 2013-08-01,Topological States in Ferromagnetic CdO/EuO Superlattices and Quantum Wells,"Based on ab initio calculations, we demonstrate that the ferromagnetic CdO/EuO superlattice is a simple Weyl semimetal with two linear Weyl nodes in the Brillouin zone, and the corresponding CdO/EuO quantum well realizes the stichometric quantum anomalous Hall state without random magnetic doping. In addition, a simple effective model is presented to describe the basic mechanism of spin polarized band inversion in this system.",1308.0349v3 2013-08-06,Broadband ferromagnetic resonance characterization of GaMnAs thin films,"The precessional magnetization dynamics of GaMnAs thin films are characterized by broadband network analyzer ferromagnetic resonance (FMR) in a coplanar geometry at cryogenic temperatures. The FMR frequencies are characterized as function of in-plane field angle and field amplitude. Using an extended Kittel model of the FMR dispersion the magnetic film parameters such as saturation magnetization and anisotropies are derived. The modification of the FMR behavior and of the magnetic parameters of the thin film upon annealing is analyzed.",1308.1232v1 2013-08-14,Ising models on the Regularized Apollonian Network,"We investigate the critical properties of Ising models on a Regularized Apollonian Network (RAN), here defined as a kind of Apollonian Network (AN) in which the connectivity asymmetry associated to its corners is removed. Different choices for the coupling constants between nearest neighbors are considered, and two different order parameters are used to detect the critical behaviour. While ordinary ferromagnetic and anti-ferromagnetic models on RAN do not undergo a phase transition, some anti-ferrimagnetic models show an interesting infinite order transition. All results are obtained by an exact analytical approach based on iterative partial tracing of the Boltzmann factor as intermediate steps for the calculation of the partition function and the order parameters.",1308.3259v1 2013-08-26,Ferromagnetic spin-orbital liquid of dipolar fermions in zigzag lattices,"Two-component dipolar fermions in zigzag optical lattices allow for the engineering of spin-orbital models. We show that dipolar lattice fermions permit the exploration of a regime typically unavailable in solid-state compounds that is characterized by a novel spin-liquid phase with a finite magnetization and spontaneously broken SU(2) symmetry. This peculiar spin liquid may be understood as a Luttinger liquid of composite particles consisting of bound states of spin waves and orbital domain walls moving in an unsaturated ferromagnetic background. In addition, we show that the system exhibits a boundary phase transitions involving non-local entanglement of edge spins.",1308.5519v1 2013-08-29,Optimal interface doping at La_2/3Sr_1/3MnO_3/SrTiO_3(001) heterojunctions for spintronic applications,"We examine, by means of ab initio pseudopotential calculations, La_2/3Sr_1/3MnO_3/SrTiO_3 (LSMO/STO) heterojunctions in which one unit layer of La_(1-x)Sr_xMnO_3 (with 00$, as $|x|\to \infty$. We prove that in dimensions $d\ge 2$ for all $\beta$ large enough if $\alpha>1$ there is a phase transition while if $\alpha<1$ there is a unique DLR state.",1403.7961v3 2016-07-17,Magnetic anisotropy dependence of the energy of oriented thick ferromagnetic films,"Heisenberg Hamiltonian was employed to describe the variation of energy of thick ferromagnetic films with second and fourth order anisotropies. At angle of 1.36 degrees and anisotropies of 1.25, energy is minimum for thick film of sc(001) with 1000 layers. Energy becomes minimum at angle of 1.18 degrees and fourth order anisotropy of 1.15 for thick film of bcc(001) with the same thickness. According to these simulations, these lattices can be easily oriented in some certain directions under the influence of some particular values of anisotropies. Energy varies with second and fourth order anisotropies in similar passion for both types of lattices. The energy gradually decreases with second and fourth order anisotropy for both types of lattices in the range described here.",1611.02227v1 2016-11-11,Incommensurate spiral order from double exchange interactions,"The double exchange model describing interactions of itinerant electrons with localized spins is usually used to explain ferromagnetism in metals. We show that for a variety of crystal lattices of different dimensionalities and for a wide range of model parameters the ferromagnetic state is unstable against a non-collinear spiral magnetic order. We revisit the phase diagram of the double exchange model on a triangular lattice and show in a large part of the diagram the incommensurate spiral state has a lower energy than the previously discussed commensurate states. These results indicate that double exchange systems are inherently frustrated and can host unconventional spin orders.",1611.03689v2 2016-11-14,Low-temperature evolution of the spectral weight of a spin-up carrier moving in a ferromagnetic background,"We derive the lowest-temperature correction to the self-energy of a spin-up particle injected in a ferromagnetic background. The background is modeled with both Heisenberg and Ising Hamiltonians so that differences due to gapless vs. gapped magnons can be understood. Beside the expected thermal broadening of the quasiparticle peak as it becomes a resonance inside a continuum, we also find that spectral weight is transferred to regions lying outside this continuum. We explain the origin of this spectral weight transfer and its low-temperature evolution.",1611.04575v1 2016-11-15,Coarsening and percolation in a disordered ferromagnet,"By studying numerically the phase-ordering kinetics of a two-dimensional ferromagnetic Ising model with quenched disorder -- either random bonds or random fields -- we show that a critical percolation structure forms in an early stage and is then progressively compactified by the ensuing coarsening process. Results are compared with the non-disordered case, where a similar phenomenon is observed, and interpreted within a dynamical scaling framework.",1611.04828v1 2016-11-26,Universal Depinning Transition of Domain Walls in Ultrathin Ferromagnets,"We present a quantitative and comparative study of magnetic field driven domain wall depinning transition in different ferromagnetic ultrathin films over a wide range of temperature. We reveal a universal scaling function accounting for both drive and thermal effects on the depinning transition, including critical exponents. The consistent description we obtain for both the depinning and subthreshold thermally activated creep motion should shed light on the universal glassy dynamics of thermally fluctuating elastic objects pinned by disordered energy landscapes.",1611.08701v2 2017-01-16,Lateral ferromagnetic domain control in Cr2O3/Pt/Co positive exchange bias system,"We investigated the perpendicular exchange bias (PEB) switching from negative- to positive-exchange bias state for Cr2O3/Pt/Co exchange coupling thin film system exhibiting positive exchange bias phenomena. By changing Pt spacer layer thickness or measurements temperature, we demonstrated the control of two kind of intermediate state of the switching; the double hysteresis loop indicating local, non-averaged PEB, and single hysteresis loop indicating averaged PEB. We proposed the way to control the lateral ferromagnetic domain though the control of PEB magnitude.",1701.04252v1 2017-01-16,Asymptotic behaviour of ground states for mixtures of ferromagnetic and antiferromagnetic interactions in a dilute regime,"We consider randomly distributed mixtures of bonds of ferromagnetic and antiferromagnetic type in a two-dimensional square lattice with probability $1-p$ and $p$, respectively, according to an i.i.d. random variable. We study minimizers of the corresponding nearest-neighbour spin energy on large domains in ${\mathbb Z}^2$. We prove that there exists $p_0$ such that for $p\le p_0$ such minimizers are characterized by a majority phase; i.e., they take identically the value $1$ or $-1$ except for small disconnected sets. A deterministic analogue is also proved.",1701.04482v1 2017-04-07,Realization of Quantum Anomalous Hall Effect in Graphene from \textit{n}-\textit{p} Codoping Induced Stable Atomic-Adsorption,"Using first-principles calculation methods, we study the possibility of realizing quantum anomalous Hall effect in graphene from stable 3\textit{d}-atomic adsorption via charge-compensated \textit{n}-\textit{p} codoping scheme. As concrete examples, we show that long-range ferromagnetism can be established by codoping 3\textit{d} transition metal and boron atoms, but only the Ni codopants can open up a global bulk gap to harbour the quantum anomalous Hall effect. Our estimated ferromagnetic Curie transition temperature can reach over 10 Kelvin for various codoping concentrations.",1704.02085v1 2017-04-14,How experimentally to detect a solitary superconductivity in dirty ferromagnet-superconductor trilayers?,"We theoretically study the proximity effect in the thin-film layered ferromagnet (F) - superconductor (S) heterostructures in F$_1$F$_2$S design. We consider the boundary value problem for the Usadel-like equations in the case of so-called ""dirty"" limit. The ""latent"" superconducting pairing interaction in F layers taken into account. The focus is on the recipe of experimental preparation the state with so-called solitary superconductivity. We also propose and discuss the model of the superconducting spin valve based on F$_1$F$_2$S trilayers in solitary superconductivity regime.",1704.04453v1 2017-04-20,Domain percolation in a quenched ferromagnetic spinor condensate,"We show that the easy-axis (EA) magnetic domains formed in a quenched ferromagnetic spinor condensate are described by percolation theory. We introduce a generalized spin rotation to vary the proportion of positive and negative EA domains, allowing us to explore domain percolation. Using simulations we investigate the finite-size scaling behaviour to extract the correlation length critical exponent and the transition point. We analyse the sensitivity of our results to the early-time dynamics of the system, the quadratic Zeeman energy, and the threshold condition used to define the positive (percolating) domains.",1704.06332v2 2017-04-22,Boron Triangular Kagome Lattice with Half-Metallic Ferromagnetism,"Based on the first-principles evolutionary materials design, we report a stable boron Kagome lattice composed of triangles in triangles on a two-dimensional sheet. The Kagome lattice can be synthesized on a silver substrate, with selecting Mg atoms as guest atoms. While the isolated Kagome lattice is slightly twisted without strain, it turns into an ideal triangular Kagome lattice under tensile strain. In the triangular Kagome lattice, we find the exotic electronic properties, such as topologically non-trivial flat band near the Fermi energy and half-metallic ferromagnetism, and predict the quantum anomalous Hall effect in the presence of spin-orbit coupling.",1704.06842v2 2018-05-03,"Exact Intrinsic Localized Excitation of an Anisotropic Ferromagnetic Spin Chain in External Magnetic Field with Gilbert Damping, Spin Current and PT-Symmetry","We obtain the exact one-spin intrinsic localized excitation in an anisotropic Heisenberg ferromagnetic spin chain in a constant/variable external magnetic field with Gilbert damping included. We also point out how an appropriate magnitude spin current term in a spin transfer nano-oscillator (STNO) can stabilize the tendency towards damping. Further, we show how this excitation can be sustained in a recently suggested PT-symmetric magnetic nanostructure. We also briefly consider more general spin excitations.",1805.01230v1 2008-07-11,Breakdown of half-metallic ferromagnetism in zinc-blende II-V compounds,"We investigated the electronic and magnetic properties of a series of zinc-blend II-V compounds by carrying out density-functional-theory calculations including spin-orbit couplings. Contrary to the case of CaN and CaP, the half-metallic characteristics of the II-V compounds such as CaSb and CaBi were found to be destroyed. Our analysis of the valence band structures of CaAs, CaSb, and CaBi revealed a critical role of the spin-orbit coupling interactions on the exchange-split band structure, thereby leading to breakdown of the half-metallic ferromagnetism for the systems with heavier group V elements in the zinc-blend II-V compounds.",0807.1778v1 2008-07-28,Magnetoelectric bistabilities in ferromagnetic resonant tunneling structures,"The conditions for the occurrence of pronounced magnetoelectric bistabilities in the resonant tunneling through a ferromagnetic quantum well are theoretically investigated. The bistability appears due to the mutual feedback of the carriers Coulomb interaction and the carriers exchange coupling with magnetic impurities in the well. It is shown that the well Curie temperature depends strongly on the relative alignment of the quantum well level and the reservoirs chemical potentials, which can be modified electrically. Switching between a ""current-on/magnetism-off"" and a ""current-off/magnetism-on"" mode becomes possible, if the well temperature lies in-between the bistable values of the well Curie temperature.",0807.4445v1 2008-07-29,Thermally activated Peierls dimerization in ferromagnetic spin chains,"We demonstrate that a Peierls dimerization can occur in ferromagnetic spin chains activated by thermal fluctuations. The dimer order parameter and entanglement measures are studied as functions of the modulation of the magnetic exchange interaction and temperature, using a spin-wave theory and the density-matrix renormalization group. We discuss the case where a periodic modulation is caused by spin-phonon coupling and the case where electronic states effectively induce such a modulation. The importance of the latter for a number of transition metal oxides is highlighted.",0807.4693v1 2009-12-07,Thermoelectric spin diffusion in a ferromagnetic metal,"We present a semiclassical theory of spin-diffusion in a ferromagnetic metal subject to a temperature gradient. Spin-flip scattering can generate pure thermal spin currents by short-circuiting spin channels while suppressing spin accumulations. A thermally induced spin density is locally generated when the energy dependence of the density of states is spin polarized.",0912.1213v1 2009-12-09,Microscopic Theory of Current-Spin Interaction in Ferromagnets,"Interplay between magnetization dynamics and electric current in a conducting ferromagnet is theoretically studied based on a microscopic model calculation. First, the effects of the current on magnetization dynamics (spin torques) are studied with special attention to the ""dissipative"" torques arising from spin-relaxation processes of conduction electrons. Next, an analysis is given of the ""spin motive force"", namely, a spin-dependent 'voltage' generation due to magnetization dynamics, which is the reaction to spin torques. Finally, an attempt is presented of a unified description of these effects.",0912.1676v1 2009-12-17,Quantum-level control in a III-V-based ferromagnetic-semiconductor heterostructure with a GaMnAs quantum well and double barriers,"We investigate the spin-dependent tunneling properties in a three-terminal III-V-based ferromagnetic-semiconductor heterostructure with a 2.5-nm-thick GaMnAs quantum well (QW) and double barriers. We successfully control the quantum levels and modulate the spin-dependent current with varying the voltage of the electrode connected to the GaMnAs QW. Our results will open up a new possibility for realizing three-terminal spin resonant-tunneling devices.",0912.3409v1 2009-12-28,Notes on ferromagnetic diluted P-spin model,"In this paper we develop the interpolating cavity field technique for the mean field ferromagnetic p-spin. The model we introduce is a natural extension of the diluted Curie-Weiss model to p>2 spin interactions. Several properties of the free energy are analyzed and, in particular, we show that it recovers the expressions already known for p=2 models and for p>2 fully connected models. Further, as the model lacks criticality, we present extensive numerical simulations to evidence the presence of a first order phase transition and deepen the behavior at the transition line. Overall, a good agreement is obtained among analytical results, numerics and previous works.",0912.5173v1 2009-12-29,"Synthesis, characterization and magnetic properties of room-temperature nanofluid ferromagnetic graphite","We report the chemical synthesis route, structural characterization, and physical properties of nanofluid magnetic graphite (NFMG) obtained from the previously synthesized bulk organic magnetic graphite (MG) by stabilizing the aqueous ferrofluid suspension with an addition of active cationic surfactant. The measured magnetization-field hysteresis curves along with the temperature dependence of magnetization confirmed room-temperature ferromagnetism in both MG and NFMG samples.",0912.5344v1 2011-11-23,Dark solitons in ferromagnetic chains with first- and second-neighbor interactions,"We study the ferromagnetic spin chain with both first- and second-neighbor interactions. We obtained the condition for the appearance and stability of bright and dark solitons for arbitrary wave number inside the Brillouin zone. The influence of the second-neighbor interaction and the anisotropy on the soliton properties is considered. The scattering of dark solitons from point defects in the discrete spin chain is investigated numerically.",1111.5477v1 2012-01-03,A universal scaling law of exchange bias training effect,"Exchange bias training effect in ferromagnetic/antiferromagnetic bilayers is investigated. In some systems the evolution of the exchange bias field $H_E$ with the number of cycle $n$ cannot be fitted by the empirical $1/\sqrt{n}$ function. A unified expression is derived from a discretized Landau-Khalatnikov equation in the framework of the thermodynamics model which is proposed by Ch.\ Binek. This generalized model describes well training effect independent of the magnetization reversal mechanism in the ferromagnetic layers.",1201.0620v1 2012-01-10,Time reversal invariant topological superconductivity in correlated non-centrosymmetric systems,"Using functional renormalization group method, we study the favorable condition for electronic correlation driven time reversal invariant topological superconductivity in symmetry class DIII. For non-centrosymmetric systems we argue that the proximity to ferromagnetic (or small wavevector magnetic) instability can be used as a guideline for the search of this type of superconductivity. This is analogous to the appearance of singlet unconventional superconductivity in the neighborhood of antiferromagnetic instability. We show three concrete examples where ferromagnetic-like fluctuation leads to topological pairing.",1201.2003v1 2012-01-12,Ferromagnet/Insulator/Ferroelectric nanometer multilayer for multiferroictronics,"We have investigated the field-induced-changes in both the magnetization and the polarization in ferromagnet/Insulator/ferroelectric (FM/I/FE) multilayer by following both the Stoner-Wohlfarth (SW) model and the Landau theory. It has been found that with the stresses introduced in the FM/I/FE structure by the fields, both the magnetization and the polarization states can be significantly modified and the combination of their states can be of multiple states. These results demonstrate the feasibility of combining both the spintronics and the ferroelectrics into the multiferroictronics.",1201.2527v1 2012-01-26,Thermal switching rate of a ferromagnetic material with uniaxial anisotropy,"The field dependence of the thermal switching rate of a ferromagnetic material with uniaxial anisotropy was studied by solving the Fokker-Planck equation. We derived the analytical expression of the thermal switching rate using the mean first-passage time approach, and found that Brown's formula [Phys. Rev. 130, 1677 (1963)] is applicable even in the low barrier limit by replacing the attempt frequency with the proper factor which is expressed by the error function.",1201.5460v2 2012-04-09,Graphene nanoring as a tunable source of polarized electrons,"We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.",1204.1828v1 2012-04-10,Flat-Band Ferromagnetism as a Pauli-Correlated Percolation Problem,"We investigate the location and nature of the para-ferro transition of interacting electrons in dispersionless bands using the example of the Hubbard model on the Tasaki lattice. This case can be analyzed as a geometric site-percolation problem where different configurations appear with nontrivial weights. We provide a complete exact solution for the 1D case and develop a numerical algorithm for the 2D case. In two dimensions the paramagnetic phase persists beyond the uncorrelated percolation point, and the grand-canonical transition is via a first-order jump to an unsaturated ferromagnetic phase.",1204.2110v2 2012-04-12,Metamagnetism and Weak Ferromagnetism in Nickel (II) oxalate crystals,"Microcrystals of orthorhombic nickel (II) oxalate dihydrate were synthesized through a precipitation reaction of aqueous solutions of nickel chloride and oxalic acid. Magnetic susceptibility exhibits a sharp peak at 3.3 K and a broadrounded maximum near 43 K. We associated the lower maximum with a metamagnetic transition that occurs when the magnetic field is about \geq 3.5 T. The maximum at 43 K is the typical of 1D antiferromagnets, whereas weak ferromagnetism behavior was observed in the range of 3.3 to 43 K.",1204.2850v1 2012-04-18,Domain wall heat conductance in ferromagnetic wires,"We present a theoretical study of heat transport in electrically insulating ferromagnetic wires containing a domain wall. In the regime of validity of continuum micromagnetism a domain wall is found to have no effect on the heat conductance. However, spin waves are found to be reflected by domain walls with widths of a few lattice spacings, which is associated with emergence of an additional spin wave bound state. The resulting domain wall heat conductance should be significant for thin films of Yttrium Iron Garnet with sharply defined magnetic domains.",1204.4008v2 2012-04-21,Ferromagnetism and Antiferromagnetism of Correlated Topological Insulator with Flat Band,"In this paper, based on the mean field approach and random-phase-approximation, we study the magnetic properties of the spinful Haldane model on honeycomb lattice of topological flat band with on-site repulsive Coulomb interaction. We find that the antiferromagnetic (AF) order is more stable than ferromagnetic (FM) order at (or near) half-filling; while away from half-filling the phase diagram becomes complex: At large doping, FM order is more stable than AF order due to the flatness of band structure. In particular, we find that at quarter filling case, the system becomes a Q=1$ topological insulator which is induced by the FM order.",1204.4766v1 2012-04-21,Collapse of Skyrmions in 2d Ferro- and Antiferromagnets,"Collapse of a skyrmion due to the discreteness of a crystal lattice in isotropic two-dimensional ferro- and antiferromagnets has been studied analytically and by numerical solution of equations of motion for up to 2000 x 2000 classical spins on a square lattice coupled via Heisenberg exchange interaction. Excellent agreement between analytical and numerical results has been achieved. The lifetime of the skyrmion scales with its initial size, lambda_0, as(lambda_0/a)^5 in ferromagnets and as (lambda_0/a)^2.15 in antiferromagnets, with a being the lattice parameter. This makes antiferromagnetic skyrmions significantly shorter lived than ferromagnetic skyrmions.",1204.4848v1 2013-09-10,Spin rectification induced by dynamical Hanle effect,"Dynamic response of spin accumulation to a time-dependent magnetic field has been investigated in a ferromagnetic/nonmagnetic bilayer under ferromagnetic resonance. In this system, magnetization precession driven by a microwave generates direct-current (dc) and alternate-current (ac) spin accumulation in the nonmagnetic layer by the spin pumping. The ac spin accumulation is coupled with the microwave magnetic field through a dynamical Hanle spin precession, giving rise to rectified spin accumulation comparable with the dc spin accumulation directly generated by the spin pumping.",1309.2365v1 2013-09-13,Skyrmion spin texture in ferromagnetic semiconductor-superconductor heterostructures,"We provide a derivation of a spin Skyrmion number classification for two-dimensional topological superconductors constructed from ferromagnetic and Rashba spin-orbit coupled semiconductor-superconductor heterostructures. We show that in the non-trivial topological phase, characterized by a non-zero Chern number, there is always a topological spin texture in the occupied bands represented by a Skyrmion number. The Skyrmion number has the advantage of being both physically intuitive and directly measurable using spin-sensitive band structure imaging techniques. In addition, we show that the Skyrmion classification can be extended to the equivalent one-dimensional topological superconductors.",1309.3411v2 2013-09-13,Tunneling Anisotropic Thermopower and Seebeck Effects in Magnetic Tunnel Junctions,"The Tunneling Anisotropic Magneto-Thermopower (TAMT) and the Tunneling Anisotropic Spin-Seebeck (TASS) effects are studied for a magnetic tunnel junction (MTJ) composed of a ferromagnetic electrode, a zinc-blende semiconductor and a normal metal. We develop a theoretical model for describing the dependence of a thermally induced tunneling current across the MTJ on the in-plane orientation of the magnetization in the ferromagnetic layer. The model accounts for the specific Bychkov-Rashba and Dresselhaus spin-orbit interactions present in these systems, which are responsible for the $C_{2v}$ symmetry we find in the TAMT and the TASS.",1309.3463v1 2013-09-14,Nonconventional Spin Glass Transition in a Chemically Ordered Pyrochlore,"We report on the study of unusual spin glass properties in the geometrically frustrated pyrochlore Tb$_{2}$Mo$_{2}$O$_{7}$, $T$$_{g}$$\simeq$24 K. The analysis of the nonlinear part of dc and complex susceptibilities, near the glass transition regime, suggests the existence of a statistical distribution of relaxation times in short-range ordered ferromagnetic clusters. In addition, the magnetic spins are not sufficiently frozen below the glass transition temperature; apparently, responsible for the non-equilibrium scaling behavior of the static critical exponents of nonlinear susceptibilities. Our report is expected to shed new light in understanding the freezing properties of frustrated pyrochlores with short range ferromagnetic interactions.",1309.3617v1 2013-09-25,Band structure calculations of Ti\raisebox{-.2ex}{\scriptsize 2}FeSn: a new half-metallic compound,"Within the framework of density functional theory, the electronic structure and magnetic properties have been studied for the Ti\raisebox{-.2ex}{\scriptsize 2}FeSn full-Heusler compound. The ferromagnetic state is found to be energetically more favorable than paramagnetic and antiferromagnetic states. The spin-polarized results show that Ti\raisebox{-.2ex}{\scriptsize 2}FeSn compound has half-metallic ferromagnetic character with a total spin moment of $2 \mu_{B}$ and a band gap in the minority spin channel of 0.489 eV, at the equilibrium lattice constant a=6.342 A.",1309.6442v1 2013-09-26,"Tuning the metamagnetic transition in the (Co,Fe)MnP system for magnetocaloric purposes","The inverse magnetocaloric effect taking place at the antiferro-to-ferromagnetic transition of (Co,Fe)MnP phosphides has been characterized by magnetic and direct {\Delta}Tad measurements. In Co0.53Fe0.47MnP, entropy change of 1.5 Jkg-1K-1 and adiabatic temperature change of 0.6 K are found at room temperature for an intermediate field change ({\Delta}B= 1 T). Several methods were used to control the metamagnetic transition properties, in each case a peculiar splitting of the antiferro-to-ferromagnetic transition is observed.",1309.6741v1 2013-09-27,Multiferroicity in V-doped PbTiO$_{3}$,"We report \emph{ab initio} predictions on the proper multiferroic (ferromagnetic, insulating and ferroelectric) character of PbTiO$_{3}$ doped with vanadium. V impurities coupled ferromagnetically carry a magnetization of 1 $\mu_{\rm B}$ each. The coupling is expected to be strong, since the paramagnetic solution is higher by 150 meV/vanadium, and no stable antiferromagnetic solution was found. The electronic gap in the doped system is about 0.2-0.3 eV in GGA, hence the system is properly multiferroic. V doping increases the spontaneous polarization in PbTiO$_{3}$, with an approximate percentual rate of 0.7 $\mu$C/cm$^{2}$.",1309.7205v1 2013-09-28,Superfluid phases of spin-1 bosons in cubic optical lattice,"We analyze theoretically the emergence of different superfluid phases of spin-1 bosons in a three-dimensional cubic optical lattice by generalizing the recently developed Ginzburg-Landau theory for the Bose-Hubbard model to a spinor Bose gas. In particular at zero temperature, our theory distinguishes within its validity range between various superfluid phases for an anti-ferromagnetic interaction with an external magnetic field. In addition, we determine that the superfluid-Mott insulator phase transition is of second order and that the transitions between the respective superfluid phases with anti-ferromagnetic interaction can be both of first and second order.",1309.7449v2 2014-05-05,Multigap Superconductivity in the Ferromagnetic Superconductor UCoGe Revealed by Thermal Conductivity Measurements,"We performed thermal conductivity measurements on a single crystal of the ferromagnetic superconductorUCoGe under magnetic field. Two different temperature dependencies of the thermal conductivity are observed, for H//b linear at low magnetic field and quadratic for magnetic field larger than 1 Tesla. At the same field value, a plateau appears in the field dependency of the residual term of thermal conductivity. Such observations suggest a multigap superconductivity with a line of nodes in the superconducting gap.",1405.0817v2 2014-05-11,Tuning perpendicular magnetic anisotropy in the MgO/CoFeB/Ta thin films,"Understanding the magnetic anisotropy at ferromagnetic metal/oxide interface is a fundamental and intriguing subject. Here we propose an approach to manipulate the strength of perpendicular magnetic anisotropy (PMA) by varying MgO thickness in the MgO/CoFeB/Ta thin films. We identify that the PMA at the MgO/CoFeB interface is tuned by the crystalline structure of bulk MgO layer and decreases dramatically due to the onset of crystalline MgO forming with the increase of MgO thickness. Our work opens an avenue to manipulate the magnetic anisotropy by the modification of the ferromagnetic metal/oxide interface.",1405.2551v2 2014-05-18,Monte Carlo Studies of the Ising Antiferromagnet with a Ferromagnetic Mean-field Term,"The unusual thermodynamic properties of the Ising antiferromagnet supplemented with a ferromagnetic, mean-field term are outlined. This simple model is inspired by more realistic models of spin-crossover materials. The phase diagram is estimated using Metropolis Monte Carlo methods, and differences with preliminary Wang-Landau Monte Carlo results for small systems are noted.",1405.4553v1 2014-06-10,Influence of Ta insertions on the magnetic properties of MgO/CoFeB/MgO films probed by ferromagnetic resonance,"We show by vector network analyzer ferromagnetic resonance measurements that low Gilbert damping {\alpha} down to 0.006 can be achieved in perpendicularly magnetized MgO/CoFeB/MgO thin films with ultra-thin insertions of Ta in the CoFeB layer. While increasing the number of Ta insertions allows thicker CoFeB layers to remain perpendicular, the effective areal magnetic anisotropy does not improve with more insertions, and also comes with an increase in {\alpha}.",1406.2491v2 2014-06-16,In-situ microwave characterization of ferromagnetic microwires-filled polymer composites: a review,"This review describes the emerging research area and relevant physics of polymer-based composites enabled by amorphous ferromagnetic microwires. Fruitful results ranging from their tunable magnetic field and mechanical stress properties and influences of direct current on their microwave behavior are displayed in addition to the brief analysis on the underlying physics. The multifunctionalities exhibited strongly imply a variety of potential applications such as structural health monitoring and high-performance sensors. This article underlines that the future challenge mainly lies in proper microwire tailoring in expectation of a better microwave performance of microwire composites",1406.3903v1 2014-06-24,Interaction created effective flat bands in conducting polymers,"For a general class of conducting polymers with arbitrary large unit cell and different on-site Coulomb repulsion values on different type of sites, I demonstrate in exact terms the emergence possibility of an upper, interaction created ""effective"" flat band. This last appears as a consequence of a kinetic energy quench accompanied by a strong interaction energy decrease, and leads to a non-saturated ferromagnetic state. This ordered state clearly differs from the known flat-band ferromagnetism. This is because it emerges in a system without bare flat bands, requires inhomogeneous on-site Coulomb repulsions values, and possesses non-zero lower interaction limits at the emergence of the ordered phase.",1406.6293v1 2014-06-27,Spin-Ordered States in Multilayer Massless Dirac Fermion Systems,"We investigate the spin-ordered states in multilayer massless Dirac fermion systems under magnetic fields, in which the intralayer interaction is ferromagnetic owing to the exchange interaction, while the interlayer interaction is antiferromagnetic arising from the interlayer hopping and the on-site Coulomb repulsion. The possible spin-ordered states are examined within the mean field theory, and we apply it to alpha-(BEDT-TTF)2I3, which is a multilayer massless Dirac fermion system under pressure. In the weak interlayer coupling regime the system exhibits a ferromagnetically spin-ordered state with the effective Zeeman g-factor less than two contrasting to that observed in the single-layer graphene.",1406.7109v1 2015-12-18,Existence of travelling-wave solutions representing domain wall motion in a thin ferromagnetic nanowire,"We study the dynamics of a domain wall under the influence of applied magnetic fields in a one-dimensional ferromagnetic nanowire, governed by the Landau--Lifshitz--Gilbert equation. Existence of travelling-wave solutions close to two known static solutions is proven using implicit-function-theorem-type arguments.",1512.06016v2 2016-05-02,Majorana fermions at odd junctions in a wire network of ferromagnetic impurities,"We consider a wire network of ferromagnetic impurities on the surface of an $s$-wave superconductor with strong Rashba spin-orbit interaction. Within the topological phase, zero-energy Majorana fermions appear at wire end-points as well as at junctions between an odd number of wire segments, while no low-energy states are present at junctions between an even number of wire segments, providing strong experimentally accessible signatures for Majorana fermions. We also investigate the quasiparticle energy gap with respect to varying the Rashba spin-orbit coupling and magnetic impurity strength.",1605.00696v1 2016-05-05,Theory of magnon motive force in chiral ferromagnets,"We predict that magnon motive force can lead to temperature dependent, nonlinear chiral damping in both conducting and insulating ferromagnets. We estimate that this damping can significantly influence the motion of skyrmions and domain walls at finite temperatures. We also find that in systems with low Gilbert damping moving chiral magnetic textures and resulting magnon motive forces can induce large spin and energy currents in the transverse direction.",1605.01694v2 2016-05-12,Transmission of spin waves in ordered FeRh epitaxial thin films,"We report on B2-ordering dependence of magnetostatic surface spin waves in ferromagnetic FeRh at room temperature. Spin waves transmit over a distance longer than 21 {\mu}m in highly ordered FeRh alloys even with relatively large spin-orbit interaction. The long-range transmission likely arises from the induced Rh moments of the ordered FeRh due to ferromagnetic exchange interaction between Fe and Rh. The results indicate a potential of using FeRh in spintronic and magnonic applications by integrating with other fascinating magnetic characteristics of FeRh such as electric field induced magnetic phase transition.",1605.03798v1 2016-08-02,Ferromagnetic Damping/Anti-damping in a Periodic 2D Helical surface; A Non-Equilibrium Keldysh Green Function Approach,"In this paper, we investigate theoretically the spin-orbit torque as well as the Gilbert damping for a two band model of a 2D helical surface state with a Ferromagnetic (FM) exchange coupling. We decompose the density matrix into the Fermi sea and Fermi surface components and obtain their contributions to the electronic transport as well as the spin-orbit torque (SOT). Furthermore, we obtain the expression for the Gilbert damping due to the surface state of a 3D Topological Insulator (TI) and predicted its dependence on the direction of the magnetization precession axis.",1608.00984v2 2016-08-22,Spin transport in half-metallic ferromagnets,"We theoretically investigate spin transport in half-metallic ferromagnets at finite temperatures. The side-jump and skew-scattering contributions to spin Hall conductivity are derived using the Kubo formula. The electron-magnon interaction causes a finite density of states in the energy gap of the minority-spin band and induces spin Hall conductivity. We show that spin Hall conductivity is proportional to $T^{3/2}$, with $T$ being temperature and is sensitive to $T$. We propose that spin Hall conductivity may be a tool to study the minority-spin state.",1608.06333v2 2016-12-06,Exceptional magneto-electric coupling and spontaneous electric polarization in anti-ferromagnet Co4Nb2O9,"Synthesis and extensive structural, pyroelectric, magnetic, dielectric and magneto-electric characterizations are reported for polycrystalline Co4Nb2O9 towards unraveling the multiferroic state especially in reference to the magnetic spin flop transition. Magnetic measurements confirm the Co4Nb2O9 becomes antiferromagnetic (AFM) at around 28 K but no clear evidence for spin-flop effect was found. Associated with the magnetic phase transition, a sharp peak in pyroelectric current indicates the appearance of the strong magneto-electric coupling below Neel temperature (TN) with a large coupling constant upto 17.8 uC/m^2T. Using temperature oscillation technique, we establish Co4Nb2O9 to be a genuine multiferroic with spontaneous electric polarization in the anti-ferromagnetic state.",1612.01970v1 2016-12-08,"First-principles study on the magnetic properties of ordered Nd$_{6}$(Fe,Ga)$_{14}$ alloys","We studied the stable magnetic structure of ordered Nd$_{6}$Fe$_{14-x}$Ga$_x$ ($x = 0, 1)$ alloys, which appears in the grain-boundary (GB) phase of Nd-Fe-B permanent magnets, using first-principles techniques. Slight Ga doping ($x = 1$) was shown to contribute to the stabilization of an anti-ferromagnetic (AF) state, whereas the non-doped case ($x = 0$) was revealed to favor ferromagnetic state rather than AF state with a slight energy difference.",1612.02633v3 2016-12-12,Breakdown of the Nagaoka phase at finite doping,"The Nagaoka ($U=\infty$) limit of the Hubbard model on a square lattice is mapped onto the itinerant-localized Kondo model at infinitely strong coupling. Such a model is well suited to perform quantum Monte Carlo (QMC) simulations to compute spin correlation functions. This model is shown to exhibit no short-range ferromagnetic (FM) spin correlations at any doping $\delta\ge 0.01$ and finite temperature, $T=0.1t$. Our simulations give no indication that there is a tendency towards ferromagnetic ordering in the ground state.",1612.03646v2 2016-12-19,Thickness dependence study of current-driven ferromagnetic resonance in Y3Fe5O12/heavy metal bilayers,"We use ferromagnetic resonance to study the current-induced torques in YIG/heavy metal bilayers. YIG samples with thickness varying from 14.8 nm to 80 nm, with Pt or Ta thin film on top, are measured by applying a microwave current into the heavy metals and measuring the longitudinal DC voltage generated by both spin rectification and spin pumping. From a symmetry analysis of the FMR lineshape and its dependence on YIG thickness, we deduce that the Oersted field dominates over spin-transfer torque in driving magnetization dynamics.",1612.06111v1 2016-12-19,Electrically driven magnetic antenna based on multiferroic composites,"We suggest and demonstrate via large scale numerical simulations an electrically operated spin- wave inducer based on composite multiferroic junctions. Specifcally, we consider an interfacially coupled ferromagnetic/ferroelectric structure that emits controllably spin waves in the ferromagnets if the ferroelectric polarization is poled by an external electric field. The roles of geometry and material properties are discussed.",1612.06326v1 2016-12-26,Large-scale calculation of ferromagnetic spin systems on the pyrochlore lattice,"We perform the high-performance computation of the ferromagnetic Ising model on the pyrochlore lattice. We determine the critical temperature accurately based on the finite-size scaling of the Binder ratio. Comparing with the data on the simple cubic lattice, we argue the universal finite-size scaling. We also calculate the classical XY model and the classical Heisenberg model on the pyrochlore lattice.",1612.08312v1 2017-06-01,Superconductivity in the Ferromagnet URhGe under uniaxial pressure,"Uniaxial pressure applied in the b crystallographic direction perpendicular to spontaneous magnetization in heavy fermion ferromagnet URhGe strongly stimulates superconductivity in this compound. The phenomenological approach allows point out two mechanisms of superconducting temperature raising. They originates from stimulation by the uniaxial stress both intraband and interband amplitudes of triplet Cooper pairing. The phenomenon of reentrant superconductivity under magnetic field along b-axis is also strongly sensitive to the uniaxial stress in the same direction. The uniaxial stress accelerates suppression the Curie temperature by the transversal magnetic field. The emergence of the first order transition to the paramagnetic state occurs at much lower field than in the absence of uniaxial stress.",1706.00354v1 2017-06-17,The Ferromagnetism in the Vicinity of Lifshitz Topological Transitions,"We show that the critical temperature of a ferromagnetic phase transition in a quasi-two-dimensional hole gas confined in a diluted magnetic semiconductor quantum well strongly depends on the hole chemical potential and hole density. The significant variations of the the Curie temperature occur close to the Lifshitz topological transition points where the hole Fermi surface acquires additional components of topological connectivity due to the filling of excited size-quantization subbands. The model calculations demonstrate that the Curie temperature can be doubled by a small variation of the gate voltage for the CdMnTe/CdMgTe quantum well based device.",1706.05490v1 2017-06-20,Thermally induced antiferromagnetic exchange in magnetic multilayers,"We demonstrate sharp thermally-induced switching between ferromagnetic and antiferromagnetic RKKY exchange in a spin-valve with the spacer incorporating a thin diluted ferromagnetic layer as the core. We illustrate the mechanism behind the effect as due to a change in the effective thickness of the spacer induced by the Curie transition into its paramagnetic state.",1706.06325v1 2017-10-09,Model for Ferromagnetic Quantum Critical Point in a 1D Kondo Lattice,"Motivated by recent experiments, we study a quasi-one dimensional model of a Kondo lattice with Ferromagnetic coupling between the spins. Using bosonization and dynamical large-N techniques we establish the presence of a Fermi liquid and a magnetic phase separated by a local quantum critical point, governed by the Kondo breakdown picture. Thermodynamic properties are studied and a gapless charged mode at the quantum critical point is highlighted.",1710.03345v3 2017-10-26,Fate of Ising ferromagnets and antiferromagnets by zero-temperature Glauber dynamics on the two-dimensional Archimedean and 2-uniform lattices,"The fate of the Ising ferromagnet and antiferromagnet by the zero-temperature Glauber dynamics from random initial spin configuration is investigated in the two-dimensional Archimedean and 2-uniform lattices. Blinker states are found in addition to the ground state and metastable state. We show that an even-coordinated lattice can arrive at a blinker state or a metastable state without stripe structure, in contrast to common expectation. The universal relationship between the critical percolation and the probability of stripe final state is confirmed for six lattices. Results about the fate of the antiferromagnetic Ising model show that the geometric frustration suppresses ordering more and promotes blinker state.",1710.09946v1 2017-12-07,Microscopic mechanism for the unusual antiferromagnetic order and the pressure-induced transition to ferromagnetism in USb$_2$,"Uranium dipnictide USb$_2$ reflects enigmatic properties posing a substantial challenge for a microscopic modeling. Among others, it develops a nonstandard antiferromagnetic order of a $\uparrow\downarrow\downarrow\uparrow$-type along [001] crystallographic direction, and under pressure it undergoes transition to the ferromagnetic phase. Here we propose a minimal low-energy model of USb$_2$ which, as we demonstrate at the mean-field level, accommodates physical mechanism for mentioned observations. Relying on the obtained results we also comment on the features of magnetism observed in other U-based compounds: UAs$_2$, UBi$_2$, UAsSe, URh$_x$Ir$_{1-x}$Ge and UGe$_2$.",1712.02534v1 2017-12-13,Viscous dynamics of vortices in a ferromagnetic film,"We derive viscous forces for vortices in a thin-film ferromagnet. The viscous force acting on vortex $i$ is a linear superposition $\mathbf F_i = - \sum_{j} \hat{D}_{ij} \mathbf V_j$, where $\mathbf V_j$ is the velocity of vortex $j$. Thanks to the long-range nature of vortices, the mutual drag tensor $\hat{D}_{ij}$ is comparable in magnitude to the coefficient of self-drag $D_{ii}$.",1712.05052v1 2017-12-15,Stability of ferromagnetism in many-electron systems,"We construct a model-independent framework describing stabilities of ferromagnetism in strongly correlated electron systems; Our description relies on the operator theoretic correlation inequalities. Within the new framework, we reinterpret the Marshall-Lieb-Mattis theorem and Lieb\rq{}s theorem; in addition, from the new perspective, we prove that Lieb\rq{}s theorem still holds true even if the electron-phonon and electron-photon interactions are taken into account. We also examine the Nagaoka-Thouless theorem and its stabilities. These examples verify the effectiveness of our new viewpoint.",1712.05529v3 2017-12-15,"Wurtzite (Ga,Mn)As nanowire shells with ferromagnetic properties","(Ga,Mn)As in wurtzite crystal structure, is coherently grown by molecular beam epitaxy on the {1100} side facets of wurtizte (Ga,In)As nanowires and further encapsulated by (Ga,Al)As and low temperature GaAs. For the first time a true long-range ferromagnetic magnetic order is observed in non-planar (Ga,Mn)As, which is attributed to a more effective hole confinement in the shell containing Mn by a proper selection/choice of both the core and outer shell materials.",1712.05713v1 2017-12-23,Theory of type-II superconductivity in ferromagnetic metals with triplet pairing,"The superconducting state in uranium compounds UGe2, URhGe and UCoGe is formed at temperatures far below the Curie temperature pointing on nonconventional nature of superconductivity in these materials - namely the superconductivity with triplet pairing. The emergence of superconductivity is accompanied by the slight magnetization expulsion typical for the type-II superconductors. Following classic Abrikosov paper I develop the theory of type-II superconductivity in application to two-band ferromagnetic metal with equal spin triplet pairing.",1712.08750v4 2018-01-10,Bound states induced by the ferromagnetic dimer in a triangular lattice,"The ongoing efforts aiming at control and manipulation of novel topological phases renewed the interest in bound states induced in superconducting substrates by magnetic impurities. First described by Yu, Shiba and Rusinov, those bound states are spin-polarized and exhibit a rather large spatial extent in two dimensional (2D) materials. Here, using the Bogoliubov--de~Gennes formalism, we study the Yu--Shiba--Rusinov bound states induced by a ferromagnetic dimer in 2D triangular lattice. We describe various topographies of the in-gap bound states, depending on the coupling between impurities and superconducting host.",1801.03559v1 2018-09-07,Nuclear spin pumping by pulling effect,"The nuclear-to-electron spin angular momentum conversion via hyperfine coupling in a normal metal (NM)/ferromagnet (FM) bilayer system is theoretically investigated by using the nonequilibrium Green's function method. The spin current generated by the nuclear magnetic resonance (NMR) is found to be enhanced by the pulling effect in the FM when the temperature is lower than NMR resonance frequency. In a Co/Pt bilayer system, we show that the spin current by NMR becomes larger than that of the ferromagnetic resonance (FMR).",1809.02272v1 2018-09-27,Disordered flat bands on the kagome lattice,"We study two models of correlated bond- and site-disorder on the kagome lattice considering both translationally invariant and completely disordered systems. The models are shown to exhibit a perfectly flat ground state band in the presence of disorder for which we provide exact analytic solutions. Whereas in one model the flat band remains gapped and touches the dispersive band, the other model has a finite gap, demonstrating that the band touching is not protected by topology alone. Our model also displays fully saturated ferromagnetic groundstates in the presence of repulsive interactions, an example of disordered flat band ferromagnetism.",1809.10726v2 2018-10-19,Strong coupling phases of partially filled twisted bilayer graphene narrow bands,"We identify states favored by Coulomb interactions projected onto the Wannier basis of the four narrow bands of the ""magic angle"" twisted bilayer graphene. At the filling of two electrons/holes per moire unit cell, such interactions favor an insulating SU(4) ferromagnet. The kinetic terms select the ground state in which the two valleys with opposite spins are equally mixed, with vanishing magnetic moment per particle. We also find extended excited states, the gap to which decreases in magnetic field. An insulating stripe ferromagnetic phase is favored at one electron/hole per unit cell.",1810.08642v1 2019-06-10,High Frequency Domain Wall Oscillations in Ferromagnetic Nanowire with a Nanoscale Dzyaloshinskii Moriya Interaction (DMI) Region,"The Dzyaloshinskii Moriya Interaction (DMI) has laid the foundation for many novel chiral structures such as Skyrmions. In most of the studies so far, the DMI is present in the whole of the magnetic layer. Here, we report our investigations on a ferromagnetic nanowire where DMI is confined to a nanoscale region. We observe that the local modulation of magnetic properties causes oscillation of domain walls under the influence of spin-transfer torque. The oscillation frequency is tunable within a few GHz, making this observation potentially useful for applications in neuromorphic computing.",1906.03844v1 2019-07-11,Magnetization-density distribution in the metallic ferromagnet SrRuO3 determined by polarized neutron diffraction,"The magnetization-density distribution in the metallic ferromagnet SrRuO$_3$ was studied by means of polarized neutron diffraction. The analyzes by multipole refinements and by the maximum entropy method consistently reveal a strong polarization of all oxygen sites carrying 30\% of the total magnetization. The spin-density distribution on the Ru site exhibits a nearly cubic shape in agreement with an almost equal occupation of $t_{2g}$ orbitals and $pd$ hybridization. The experimental analysis is well reproduced by density functional calculations. There is no qualitative change in the magnetization distribution between 2 and 200 K.",1907.05140v1 2019-07-14,Zeros of ferromagnetic 2-spin systems,"We study zeros of the partition functions of ferromagnetic 2-state spin systems in terms of the external field, and obtain new zero-free regions of these systems via a refinement of Asano's and Ruelle's contraction method. The strength of our results is that they do not depend on the maximum degree of the underlying graph. Via Barvinok's method, we also obtain new efficient and deterministic approximate counting algorithms. In certain regimes, our algorithm outperforms all other methods such as Markov chain Monte Carlo and correlation decay.",1907.06156v1 2019-07-15,Exchange biased Anomalous Hall Effect driven by frustration in a magnetic Kagome lattice,"Co3Sn2S2 is a ferromagnetic Weyl semimetal that has been the subject of intense scientific interest due to its large anomalous Hall effect. We show that the coupling of this material's topological properties to its magnetic texture leads to a strongly exchange biased anomalous Hall effect. We argue that this is likely caused by the coexistence of ferromagnetism and spin glass phases, the latter being driven by the geometric frustration intrinsic to the Kagome network of magnetic ions.",1907.06651v1 2019-09-16,Experimental Confirmation of Quantum Hall Ferromagnetic State in an Organic Dirac Fermion System,"We have experimentally confirmed the quantum Hall ferromagnetic state with Chern number \nu=0, characterized by the helical edge state, in a layered organic Dirac fermion system \alpha-(BEDT-TTF)_2I_3. The interlayer resistance saturates at low temperatures and high magnetic fields. It does not scale with the sample cross-sectional area in the saturating region, and resonantly depends on the magnetic field direction. These results strongly suggest that the helical edge state dominates transport. This is the first observation of the topological phase in organic molecular crystals.",1909.07006v1 2019-09-25,Magnetic Quantum Phase Transitions in a Clean Dirac Metal,"We consider clean Dirac metals where the linear band crossing is caused by a strong spin-orbit interaction, and study the quantum phase transitions from the paramagnetic phase to various magnetic phases, including homogeneous ferromagnets, ferrimagnets, canted ferromagnets, and magnetic nematics. We show that in all of these cases the coupling of fermionic soft modes to the order parameter generically renders the quantum phase transition first order, with certain gapless Dirac systems providing a possible exception. These results are surprising since a strong spin-orbit scattering suppresses the mechanism that causes the first order transition in ordinary metals. The important role of chirality in generating a new mechanism for a first-order transition is stressed.",1909.11642v1 2019-10-15,Itinerant-electron magnetism: the importance of many-body correlations,"Do electrons become ferromagnetic just because of their repulisve Coulomb interaction? Our calculations on the three-dimensional electron gas imply that itinerant ferromagnetim of delocalized electrons without lattice and band structure, the most basic model considered by Stoner, is suppressed due to many-body correlations as speculated already by Wigner, and a possible ferromagnetic transition lowering the density is precluded by the formation of the Wigner crystal.",1910.06554v2 2019-10-28,"Geometric Flows of Curves, Two-Component Camassa-Holm Equation and Generalized Heisenberg Ferromagnet Equation","In this paper, we study the generalized Heisenberg ferromagnet equation, namely, the M-CVI equation. This equation is integrable. The integrable motion of the space curves induced by the M-CVI equation is presented. Using this result, the Lakshmanan (geometrical) equivalence between the M-CVI equation and the two-component Camassa-Holm equation is established. Note that these equations are gauge equivalent each to other.",1910.13281v1 2020-01-24,Homogenization of ferromagnetic energies on Poisson random sets in the plane,"We prove that by scaling nearest-neighbour ferromagnetic energies defined on Poisson random sets in the plane we obtain an isotropic perimeter energy with a surface tension characterised by an asymptotic formula. The result relies on proving that cells with `very long' or `very short' edges of the corresponding Voronoi tessellation can be neglected. In this way we may apply Geometry Measure Theory tools to define a compact convergence, and a characterisation of metric properties of clusters of Voronoi cells using limit theorems for subadditive processes.",2001.08919v1 2020-02-02,Topological Valley Transport of Gapped Dirac Magnons in Bilayer Ferromagnetic Insulators,"Bilayer Heisenberg ferromagnetic insulators hold degenerate terahertz Dirac magnon modes associated with two opposite valleys of the hexagonal Brillouin zone. We show that this energy degeneracy can be removed by breaking of the inversion symmetry (I), leading to a topological magnon valley current. We show furthermore that this current leads to valley Seebeck effect for magnons and is thereby detectable. We perform calculations in the specific example of bilayer CrBr3, where I can be broken by electrostatic doping.",2002.00446v2 2020-02-24,Upper critical field in ferromagnetic metals with triplet pairing,"The theory of triplet superconductivity in ferromagnetic metals based on electron-electron interaction by spin fluctuation exchange is developed. The equations for the upper critical field temperature dependence are derived. In contrast to the similar equations for the superconductivity in two band metals they contain the pairing amplitudes and the Fermi velocities depending on magnetic field. The critical field behaviour near the critical temperature and at T = 0 is established analytically.",2002.10156v3 2020-02-21,"Generalized Heisenberg Ferromagnet type Equation and Modified Camassa-Holm Equation: Geometric Formulation, Soliton Solutions and Equivalence","We study the integrability and equivalence of a generalized Heisenberg ferromagnet-type equation (GHFE). The different forms of this equation as well as its reduction are presented. The Lax representation (LR) of the equation is obtained. We observe that the geometrical and gauge equivalent counterpart of the GHFE is the modified Camassa-Holm equation (mCHE) with an arbitrary parameter $\kappa$. Finally, the 1-soliton solution of the GHFE is obtained.",2002.10969v1 2020-04-21,Anisotropic ferromagnet with two spins per site,"Green function diagrammatic technique is used to investigate a model ferromagnet in the case, when the effective spin at a site is compounded of two anisotropically interacting electron spins. The results are: the transverse magnetic excitations and the longitudinal ones are obtained in the random phase approximation, magnetization and critical temperature are calculated to the first order in the reciprocal of the effective number z of ions interacting with a given ion, and exchange interaction at the same site gives the contrary effects in changes of the magnitudes mentioned above in dependence on the exchange-isotropy parameter I and the exchange-anisotropy parameter D.",2004.09884v1 2020-04-25,Torque-induced dispersive readout in a weakly coupled hybrid system,"We propose a quantum state readout mechanism of a weakly coupled qubit in dispersive regime. The hybrid system consists of ferromagnetic insulator and a superconducting qubit in a microwave cavity. The enhancement of the measurement sensitivity is achieved by exerting torque on the ferromagnetic insulator magnetization, which compensates the damping of the system leading to an exceptional point. The proposed machanism allows to measure the qubit state either via the transmission of the cavity or the FMR signal of the magnetic material.",2004.12114v1 2020-04-23,Zakharov-Ito equation and Generalized Heisenberg ferromagnet-type equation: equivalence and related geometric curve flows,"These results continue our studies of integrable generalized Heisenberg ferromagnet-type equations (GHFE) and their equivalent counterparts. We consider the GHFE which is the spin equivalent of the Zakharov-Ito equation (ZIE). We have established that these equations are gauge and geometrical equivalent to each other. The integrable motion of space curves induced by the ZIE is constructed. The 1-soliton solution of the GHFE is obtained from the seed solution of the ZIE.",2004.12816v1 2020-04-28,Magnetic Flux Pumping in Superconducting Loop Containing a Josephson $ψ$ Junction,"We demonstrate that a Josephson junction with a half-metallic weak link integrated into the superconducting loop enables the pumping of magnetic flux piercing the loop. In such junctions, the ground state phase $\psi$ is determined by the mutual orientation of magnetic moments in two ferromagnets surrounding the half-metal. Thus, the precession of magnetic moment in one of two ferromagnets controlled, e.g., by the microwave radiation, results in the accumulation of the phase $\psi$ and subsequent switching between the states with different vorticities. The proposed flux pumping mechanism does not require the application of voltage or external magnetic field which enables the design of electrically decoupled memory cells in superconducting spintronics.",2004.13403v1 2020-12-13,Tunneling magnetoresistance in ensembles of ferromagnetic granules with exchange interaction and random easy axes of magnetic anisotropy,"We study the tunneling magnetoresistance in the ensembles of ferromagnetic granules with random easy axes of magnetic anisotropy taking into account the exchange interaction between granules. It is shown that due to the exchange interaction magnetoresistance is effectively decoupled from magnetization, i.e. the strongest negative magnetoresistance can be observed at the field where magnetization is almost saturated. Under some conditions, the sign of magnetoresistance can be reversed and tunneling magnetoresistance can become positive at certain magnetic fields. Our theory agrees with measurements of magnetoresistance in ensembles of Fe granules in SiCxNy matrix.",2012.07114v1 2021-01-07,Microwave directional dichroism resonant with spin excitations in the polar ferromagnet GaV$_4$S$_8$,"We have investigated the directional dichroism of magnetic resonance spectra in the polar ferromagnet GaV$_4$S$_8$. While four types of structural domains are energetically degenerated under zero field, the magnetic resonance for each domain is well separated by applying magnetic fields due to uniaxial magnetic anisotropy. Consequently, the directional dichroism as large as 20 % is clearly observed without domain cancellation. The present observation therefore demonstrates that not only magnetoelectric mono-domain crystals but also magnetoelectric multi-domain specimens can be used to realize microwave (optical) diodes owing to the lack of inversion domains.",2101.02411v1 2021-01-11,Bose Einstein condensation and ferromagnetism of low density Bose gas of particles with arbitrary spin,"Properties of the ground state and the spectrum of elementary excitations are investigated for the low density ultracold spinor 3D Bose gas of particles with arbitrary nonzero spin. Gross-Pitaevskii equations are derived. Within the framework of the considering interaction Hamiltonian it is shown that the ground state spin structure and spin part of the chemical potential is determined by the renormalized interaction, being defined by the contribution of the virtual large momenta. The ferromagnetic structure of the ground state, and the equation of the phase, density, and spin dynamics are obtained from Gross-Pitaevskii equations.",2101.03744v2 2021-01-14,The magnetic field induced ferromagnetism in EuPd$_2$Sn$_4$ novel compound,"We report on crystal structure, magnetic and thermal physical properties of EuPd$_2$Sn$_4$ stannide. From the magnetic susceptibility measurements a divalent state of Eu rare earth element was determined together with an antiferromagnetic (AFM) order at 11 K. By applying magnetic field, the magnetic behavior variation reveals a complex magnetic structure. Above a critical field Bc a ferromagnetic (FM)-like character starts to prevail. AFM and FM regimes are separated by a metamagnetic region. Heat capacity measurements confirm this behavior.",2101.05854v1 2021-01-19,Direct evidence of ferromagnetism in MnSb2Te4,"We report the magnetic imaging of ferromagnetic domains in the van der Waals single crystal MnSb2Te4 from two different sources using cryogenic magnetic force microscopy. The magnetic field dependence of the domains reveals very weak pinning of domain walls in MnSb2Te4, resulting in a negligibly small magnetic hysteresis loop. The temperature dependence of the domain contrast reveals a mean field like behavior, in good agreement with that of bulk magnetization measurements.",2101.07892v1 2021-01-21,Polarons in a ferromagnetic spinor Bose-Einstein condensates,"We investigate the polarons formed by immersing a spinor impurity in a ferromagnetic state of $F=1$ spinor Bose-Einstein condensate. The ground state energies and effective masses of the polarons are calculated in both weak-coupling regime and strong-coupling regime. In the weakly interacting regime the second order perturbation theory is performed. In the strong coupling regime we use a simple variational treatment. The analytical approximations to the energy and effective mass of the polarons are constructed. Especially, a transition from the mobile state to the self-trapping state of the polaron in the strong coupling regime is discussed. We also estimate the signatures of polaron effects in spinor BEC for the future experiments.",2101.08474v1 2021-01-25,Discovery of an insulating ferromagnetic phase of electrons in two dimensions,"This is a commentary on two papers (PNAS 117 (51) 32244-32250 (2020) and arXiv:2011.06721), which observed a series of ordering transitions in a strongly correlated two-dimensional electron system confined to a AlAs quantum well. We summarize the main discoveries (electron nematicity, insulating ferromagnetism, etc.) of them and speculate on $T-r_s$ phase diagram based on theoretical considerations.",2101.10299v1 2000-06-05,Absence of overscreened Kondo effect in ferromagnetic host,"We study the low temperature behavior of a boundary magnetic impurity S'=1/2 in an open ferromagnetic Takhatajian-Babujian spin-S chain. For antiferromagnetic Kondo coupling, it is show via Bethe ansatz solution that the impurity spin is always locked into the critical behavior the bulk. At low temperature, a local composite of spin S-1/2 forms near the impurity site and its contribution to specific heat is of simple power law T^{1/2}. The absence of overscreened Kondo effect is due to the large correlation length of host spins which is divergent near the quantum critical point.",0006058v1 2000-06-27,Mesoscopic Tunneling Magnetoresistance,"We study spin-dependent transport through ferromagnet/normal-metal/ferromagnet double tunnel junctions in the mesoscopic Coulomb blockade regime. A general transport equation allows us to calculate the conductance in the absence or presence of spin-orbit interaction and for arbitrary orientation of the lead magnetizations. The tunneling magnetoresistance (TMR), defined at the Coulomb blockade conductance peaks, is calculated and its probability distribution presented. We show that mesoscopic fluctuations can lead to the optimal value of the TMR.",0006429v3 2012-05-02,Strain and structure driven complex magnetic ordering of a CoO overlayer on Ir(100),"We have investigated the magnetic ordering in the ultrathin c(10$\times$2) CoO(111) film supported on Ir(100) on the basis of ab-initio calculations. We find a close relationship between the local structural properties of the oxide film and the induced magnetic order, leading to alternating ferromagnetically and anti-ferromagnetically ordered segments. While the local magnetic order is directly related to the geometric position of the Co atoms, the mismatch between the CoO film and the Ir substrate leads to a complex long-range order of the oxide.",1205.0493v1 2012-05-04,Measurement of the gradient of the Casimir force between a nonmagnetic sphere and a magnetic plate,"We measured the gradient of the Casimir force between an Au sphere and a plate made of ferromagnetic metal (Ni). It is demonstrated that the magnetic properties influence the force magnitude. This opens prospective opportunities for the control of the Casimir force in nanotechnology and for obtaining Casimir repulsion by using ferromagnetic dielectrics.",1205.0872v1 2012-05-11,Effective Potential Approach to Quark Ferromagnetization in High Density Quark Matter,"A possibility of spontaneous magnetization in high density symmetric quark matter is investigated using the NJL type effective model of QCD by means of an effective potential with respect to an auxiliary field. It is shown that the quark ferromagnetic condensate has non-vanishing value at high baryon density due to the tensor-type four-point interaction between quarks.",1205.2409v2 2012-05-17,Tunneling anisotropic magnetoresistance in single-molecule magnet junctions,"We theoretically investigate quantum transport through single-molecule magnet (SMM) junctions with ferromagnetic and normal-metal leads in the sequential regime. The current obtained by means of the rate-equation gives rise to the tunneling anisotropic magnetoresistance (TAMR), which varies with the angle between the magnetization direction of ferromagnetic lead and the easy axis of SMM. The angular dependence of TAMR can serve as a probe to determine experimentally the easy axis of SMM. Moreover, it is demonstrated that both the magnitude and sign of TAMR are tunable by the bias voltage, suggesting a promising TAMR based spintronic molecule-device.",1205.3916v2 2017-04-29,Double-Exchange Interaction in Optically Induced Nonequilibrium State: A Conversion from Ferromagnetic to Antiferromagnetic Structure,"The double-exchange (DE) interaction, that is, a ferromagnetic (FM) interaction due to a combination of electron motion and the Hund coupling, is a well known source of a wide class of FM orders. Here, we show that the DE interaction in highly photoexcited states is antiferromagnetic (AFM). Transient dynamics of quantum electrons coupled with classical spins are analyzed. An ac field applied to a metallic FM state results in an almost perfect N\'eel state. A time characterizing the FM-to-AFM conversion is scaled by light amplitude and frequency. This hidden AFM interaction is attributable to the electron-spin coupling under nonequilibrium electron distribution.",1705.00240v1 2017-05-05,Light-induced anisotropic skyrmion and stripe phases in a Rashba ferromagnet,"An external off-resonant pumping is proposed as a tool to control the Dzyaloshinskii-Moriya interaction (DMI) in ferromagnetic layers with strong spin-orbit coupling. Combining theoretical analysis with numerical simulations for an $s$-$d$-like model we demonstrate that linearly polarized off-resonant light may help stabilizing novel noncollinear magnetic phases by inducing a strong anisotropy of the DMI. We also investigate how with the application of electromagnetic pumping one can control the stability, shape and size of individual skyrmions to make them suitable for potential applications.",1705.02261v2 2017-05-11,Bethe Ansatz for two-magnon scattering states in 2D and 3D Heisenberg-Ising ferromagnets,"Various versions of the Bethe ansatz are suggested for evaluation of scattering two-magnon states in 2D and 3D Heisenberg-Ising ferromagnets. It is shown that for 2D square (3D qubic) finite-periodic or infinite lattices about a half (3/4) of states have a correctly 2D- (3D-) generalized Bethe form. The remaining scattering states are treated (on the infinite lattices only) within the degenerative discrete-diffractive modification of the Bethe ansatz previously suggested by the author.",1705.04117v2 2017-05-29,"Part of a collection of reviews on antiferromagnetic spintronics. Antiferromagnetic dynamics, spin-texures, and nanostructures","Antiferromagnets as active elements of spintronics can be faster than their ferromagnetic counterparts and more robust to magnetic noise. Owing to the strongly exchange-coupled magnetic sublattice structure, antiferromagnetic order parameter dynamics are qualitatively different and thus capable of engendering novel device functionalities. In this review, we discuss antiferromagnetic textures -- nanoparticles, domain walls, and skyrmions, -- under the action of different spin torques. We contrast the antiferromagnetic and ferromagnetic dynamics, with a focus on the features that can be relevant for applications.",1705.10377v1 2017-05-30,Synthetic Antiferromagnetic Spintronics: Part of a collection of reviews on antiferromagnetic spintronics,"Spintronic and nanomagnetic devices often derive their functionality from layers of different materials and the interfaces between them. This is especially true for synthetic antiferromagnets - two or more ferromagnetic layers that are separated by metallic spacers or tunnel barriers and which have antiparallel magnetizations. Here, we discuss the new opportunities that arise from synthetic antiferromagnets, as compared to crystal antiferromagnets or ferromagnets.",1705.10526v1 2017-09-05,Gate-controlled magnonic-assisted switching of magnetization in ferroelectric/ferromagnetic junctions,"Interfacing a ferromagnet with a polarized ferroelectric gate generates a non-uniform, interfacial spin density coupled to the ferroelectric polarization allowing so for an electric field control of effective transversal field to magnetization. Here we study the dynamic magnetization switching behavior of such a multilayer system based on the Landau-Lifshitz-Baryakhtar equation, demonstrating that interfacial magnetoelectric coupling is utilizable as a highly localized and efficient tool for manipulating magnetism.",1709.01286v1 2017-09-06,Edge states in a ferromagnetic honeycomb lattice with armchair boundaries,"We investigate the properties of magnon edge states in a ferromagnetic honeycomb lattice with armchair boundaries. In contrast with fermionic graphene, we find novel edge states due to the missing bonds along the boundary sites. After introducing an external on-site potential at the outermost sites we find that the energy spectra of the edge states are tunable. Additionally, when a non-trivial gap is induced, we find that some of the edge states are topologically protected and also tunable. Our results may explain the origin of the novel edge states recently observed in photonic lattices. We also discuss the behavior of these edge states for further experimental confirmations",1709.01855v1 2017-09-13,Ultrafast spin-lattice relaxation in ferromagnets including effective spin-orbit fields,"We investigate ultrafast demagnetization due to electron-phonon interaction in a model band-ferromagnet. We show that the microscopic mechanism behind the spin dynamics due to electron-phonon interaction is the interplay of scattering and the precession around momentum-dependent effective internal spin-orbit magnetic fields. The resulting magnetization dynamics can only be mimicked by spin-flip transitions if the spin precession around the internal fields is sufficiently fast (compared to the scattering time) so that it averages out the transverse spin components.",1709.04253v1 2017-09-17,Magnetic shape-memory effect in SrRuO$_3$,"Like most perovskites, SrRuO$_3$ exhibits structural phase transitions associated with rotations of the RuO$_6$ octahedra. The application of moderate magnetic fields in the ferromagnetically ordered state allows one to fully control these structural distortions, although the ferromagnetic order occurs at six times lower temperature than the structural distortion. Our neutron diffraction and macroscopic measurements unambiguously show that magnetic fields rearrange structural domains, and that for the field along a cubic [110]$_c$ direction a fully detwinned crystal is obtained. Subsequent heating above the Curie temperature causes a magnetic shape-memory effect, where the initial structural domains recover.",1709.05688v1 2017-09-25,Study of the non-linear eddy-current response in a ferromagnetic plate: theoretical analysis for the 2D case,"The non-linear induction problem in an infinite ferromagnetic pate is studied theoretically by means of the truncated region eigenfunction expansion (TREE) for the 2D case. The non-linear formulation is linearised using a fixed-point iterative scheme, and the solution of the resulting linear problem is constructed in the Fourier domain following the TREE formalism. The calculation is carried out for the steady-state response under harmonic excitation and the harmonic distortion is derived from the obtained spectrum. This article is meant to be the theoretical part of a study, which will be complemented by the corresponding experimental work in a future communication.",1709.08380v1 2017-09-26,Three-dimensional Bose-Einstein Condensation in the Spin-1/2 Ferromagnetic-leg Ladder 3-Br-4-F-V,"The critical exponent of the phase boundary has been examined on the three-dimensional incommensurate ordering phase in the spin-1/2 ferromagnetic-leg ladder 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl]. Using the temperature-window fitting technique, we obtained the critical exponents which agreed with the three-dimensional (3D) Bose-Einstein condensation (BEC) universality class at both sides of the lower critical field and the saturation field. 3-Br-4-F-V thus becomes a new member of the quantum magnets which prove the universality of the 3D BEC exponent.",1709.08849v1 2017-09-28,Switching of Co magnetization driven by antiferromagnetic-ferromagnetic phase transition of FeRh alloy in Co/FeRh bilayers,"We show that Co spins in Co/FeRh epitaxial bilayers grown on W(110) switch reversibly between the two orthogonal in-plane directions as the FeRh layer undergoes temperature driven antiferromagnetic-ferromagnetic (AFM-FM) phase transition. Switching of Co magnetization is characterized by a hysteretic behavior owing to a temperature hysteresis of the AFM-FM transition in FeRh. The spin reorientation of Co is driven by the evolution interfacial exchange coupling to FeRh system across the AFM-FM process. Our results provide a new method of writing information purely by a local temperature change.",1709.09928v1 2018-03-13,Spin Seebeck effect in a simple ferromagnet near Tc: A Ginzburg-Landau approach,"A time-dependent Ginzburg-Landau theory is used to examine the longitudinal spin Seebeck effect in a simple ferromagnet in the vicinity of the Curie temperature Tc. It is shown analytically that the spin Seebeck effect is proportional to the magnetization near Tc, whose result is in line with the previous numerical finding. It is argued that the present result can be tested experimentally using a simple magnetic system such as EuO/Pt or EuS/Pt.",1803.04602v1 2018-03-22,Vortex liquid phase in the p-wave ferromagnetic superconductor UCoGe,"The upper critical field for field along the b-axis of the p-wave ferromagnetic superconductor UCoGe has a particular S-shape, akin to the re-entrant superconducting phase of URhGe. To explore the evolution of the superconducting phase under this transverse magnetic field, we report the thermal conductivity and resistivity measurements, revealing a possible field-induced vortex liquid phase, and supporting a field-induced change of the superconducting order parameter.",1803.08468v3 2018-06-04,Dynamical quadrupole structure factor of frustrated ferromagnetic chain,"We investigate the dynamical quadrupole structure factor of a spin-1/2 $J_{1}$-$J_{2}$ Heisenberg chain with competing ferromagnetic $J_{1}$ and antiferromagnetic $J_{2}$ in a magnetic field by exploiting density-matrix renormalization group techniques. In a field-induced spin nematic regime, we observe gapless excitations at $q=\pi$ according to quasi-long-range antiferro-quadrupole correlations. The gapless excitation mode has a quadratic form at the saturation, while it changes into a linear dispersion as the magnetization decreases.",1806.01434v1 2018-06-06,Traveling domain walls in chiral ferromagnets,"We show that chiral symmetry breaking enables traveling domain wall solution for the conservative Landau-Lifshitz equation of a uniaxial ferromagnet with Dzyaloshinskii-Moriya interaction. In contrast to related domain wall models including stray-field based anisotropy, traveling wave solutions are not found in closed form. For the construction we follow a topological approach and provide details of solutions by means of numerical calculations.",1806.02082v1 2018-06-30,Motion of discrete interfaces on the triangular lattice,"We study the motion of discrete interfaces driven by ferromagnetic interactions on the two-dimensional triangular lattice by coupling the Almgren, Taylor and Wang minimizing movements approach and a discrete-to-continuum analysis, as introduced by Braides, Gelli and Novaga in the pioneering case of the square lattice. We examine the motion of origin-symmetric convex ""Wulff-like"" hexagons, i.e. origin-symmetric convex hexagons with sides having the same orientations as those of the hexagonal Wulff shape related to the density of the anisotropic perimeter $\Gamma$-limit of the ferromagnetic energies as the lattice spacing vanishes. We compare the resulting limit motion with the corresponding evolution by crystalline curvature with natural mobility.",1807.00195v2 2018-07-05,Coulomb blockade-tuned indirect exchange in ferromagnetic nanostructures,"We develop a theory of the reversible switching of the magnetic state of the ferromagnet-insulator-normal metalferromagnet (FINF) nanostructure. The switching is controlled by tuning the Coulomb blockade strength via the gate voltage on the normal metal granule. The proposed mechanism allows for realizing the switching without passing a dissipative current through the structure.",1807.02051v1 2018-07-07,Theoretical investigation on the ferromagnetic two-dimensional scandium monochloride sheet that has a high Curie temperature and could be exfoliated from a known material,"A two-dimensional scandium monochloride sheet was investigated by using density functional theory. It could be exfoliated from a known bulk material with a cleavage energy slightly lower than that of graphene. The sheet has a ferromagnetic ground state with a Curie temperature of 100 K. Moreover, the sheet becomes a half-metal under hole doping. The Curie temperature increases to 250 K with the doping amount of 0.4 per primitive cell, which is close to the ice point. The two-dimensional scandium monochloride sheet should be a good candidate for two-dimensional spintronics.",1807.02652v1 2018-07-15,"Anisotropy of Neel ""orange-peel"" coupling in magnetic multilayers","We calculate the energy of the magnetostatic interaction between two ferromagnetic films with uniform magnetization and correlated interfaces (the ""orange-peel"" effect). The ""orange-peel"" coupling is anisotropic: the interaction is ferromagnetic when the films are magnetized in-plane; and it is antiferromagnetic when magnetization is out-of-plane. The interaction anisotropy can be used to distinguish the ""orange-peel"" effect from the interlayer exchange coupling.",1807.05590v1 2018-07-18,Banana and pizza-slice-shaped mesogens give a new constrained ferromagnet universality class,"It has been known that at high density, the local orientation of banana-shaped molecules shows a spontaneously bent state, giving rise to interesting liquid-crystalline phases such as splay-bend and twist-bend. This spontaneous bend can be modelled theoretically by allowing the bend elastic constant in the Frank elastic energy to become negative. Here we extend this idea to polar banana and pizza-slice-shaped molecules which can also splay spontaneously. By allowing both splay and bend elastic constants to be negative we discovered two additional new liquid crystalline phases. In particular, using renormalization group technique, we showed that the phase transition belongs to a new constrained ferromagnet universality class.",1807.07039v1 2018-07-28,Ultrafast spin current generation in ferromagnetic thin films,"Spin currents have been shown to play a key role in the ultrafast laser-driven demagnetization process in ferromagnetic thin films. Here, we show that an oscillating spin current can be generated in the film via the application of a femtosecond laser pulse in the visible range. In particular, we prove that the spin current is due to ballistic electrons traveling back and forth in the film. The dependence of the current intensity on the driving electric field is also investigated.",1807.10946v1 2018-12-06,Ferromagnetic Mott State in Twisted Graphene Bilayers at the Magic Angle,"We address the effective tight-binding Hamiltonian that describes the insulating Mott state of twisted graphene bilayers at a magic angle. In that configuration, twisted bilayers form a honeycomb superlattice of localized states, characterized by the appearance of flat bands with four-fold degeneracy. After calculating the maximally localized superlattice Wannier wavefunctions, we derive the effective spin model that describes the Mott state. We suggest that the system is an exotic ferromagnetic Mott insulator, with well defined experimental signatures.",1812.02550v2 2018-12-08,Magnetic properties of nanowires with ferromagnetic core and antiferromagnetic shell,"We present a Monte Carlo study of the magnetic properties of thin cylindrical nanowires composed of a ferromagnetic core and an antiferromagnetic shell implementing a classical spin Hamiltonian. We address systematically the impact of interface exchange coupling on the loop characteristics and the magnetization reversal mechanism. We study the effect of shell polycrystallinity on the characteristic fields of the isothermal hysteresis loop (coercivity, exchange-bias). We demonstrate that coupling to a polycrystalline antiferromagnetic shell increases the critical core diameter for transition from transverse to vortex domain walls.",1812.03339v1 2018-12-20,Non-local Spin-charge Conversion via Rashba Spin-Orbit Interaction,"We show theoretically that conversion between spin and charge by spin-orbit interaction in metals occurs even in a non-local setup where magnetization and spin-orbit interaction are spatially separated if electron diffusion is taken into account. Calculation is carried out for the Rashba spin-orbit interaction treating the coupling with a ferromagnet perturbatively. The results indicate the validity of the concept of effective spin gauge field (spin motive force) in the non-local configuration. The inverse Rashba-Edelstein effect observed for a trilayer of a ferromagnet, a normal metal and a heavy metal can be explained in terms of the non-local effective spin gauge field.",1812.08884v1 2019-01-05,Coupled Dispersionless and Generalized Heisenberg Ferromagnet Equations with Self-Consistent Sources: Geometry and Equivalence,"We propose a new integrable coupled dispersionless equation with self-consistent sources (CDESCS). We obtain the Lax pair and the equivalent generalized Heisenberg ferromagnet equation (GHFE), demonstrating its integrability. Specifically, we explore the geometry of these equations. Last, we consider the relation between the motion of curves/surfaces and the CDESCS and the GHFE.",1901.01470v3 2019-01-31,"Anisotropic ferromagnetism and structure stability in $4f$-$3d$ intermetallics: ab initio structure optimization and magnetic anisotropy for RCo$_5$ (R=Ce, La, and Y)","Electronic mechanism in the interplay between ferromagnetism and structure stability of $4f$-$3d$ intermetallics in the main phase of rare-earth permanent magnets is investigated from first principles. We present a case study with an archetypical materials family RCo$_5$ (R=Ce, La, Y), which was a part of the earliest rare-earth permanent magnets and from which other representative main-phase compounds can be regarded as a derived type. Comparison with the champion magnet materials family R$_2$T$_{14}$B and recently revisited materials family RT$_{12}$ (T=Co and Fe) points to a direction leading to a mid-class magnet for the possible next generation materials.",1901.11498v1 2019-02-08,Magnetic excitations and transport properties in frustrated ferromagnetic chain,"To clarify the transport property mediated by bound magnons in a spin nematic state, we investigate the spin Drude weight of a spin-1/2 $J_{1}$-$J_{2}$ Heisenberg chain with ferromagnetic $J_{1}$ and antiferromagnetic $J_{2}$ in a magnetic field by numerical diagonalization. At zero magnetic field, numerical results with up to 24 sites suggest that the spin Drude weight would be zero in the thermodynamic limit, indicating diffusive spin transport. With increasing the magnetic field, the spin Drude weight is enhanced at low temperatures, indicating that bound magnons would contribute to the spin transport.",1902.02932v1 2019-02-24,Half-Metallic Ferromagnets and Spin Gapless Semiconductors,"A brief review of experimental and theoretical studies of half-metallic ferromagnets (HMF) and spin gapless semiconductors (SGS) is given. The data on resistivity and magnetoresistivity are presented. An important role of non-quasiparticle states owing to electron-magnon scattering in transport properties is discussed. The problem of low-temperature resistivity in HMF is treated in terms of one-magnon and two-magnon scattering processes.",1902.08972v1 2019-03-05,Electronic structure and magnetism of transition metal dihalides: bulk to monolayer,"Based on first-principles calculations, the evolution of the electronic and magnetic properties of transition metal dihalides MX$_2$ (M= V, Mn, Fe, Co, Ni; X = Cl, Br, I) is analyzed from the bulk to the monolayer limit. A variety of magnetic ground states is obtained as a result of the competition between direct exchange and superexchange. The results predict that FeX$_2$, NiX$_2$, CoCl$_2$ and CoBr$_2$ monolayers are ferromagnetic insulators with sizable magnetocrystalline anisotropies. This makes them ideal candidates for robust ferromagnetism at the single layer level. Our results also highlight the importance of spin-orbit coupling to obtain the correct ground state.",1903.01789v1 2019-03-29,Nonlocal Reductions of a Generalized Heisenberg Ferromagnet Equation,"We study nonlocal reductions of coupled equations in $1+1$ dimensions of the Heisenberg ferromagnet type. The equations under consideration are completely integrable and have a Lax pair related to a linear bundle in pole gauge. We describe the integrable hierarchy of nonlinear equations related to our system in terms of generating operators. We present some special solutions associated with four distinct discrete eigenvalues of the scattering operator. Using Lax pair diagonalization method, we derive recurrence formulas for the conserved densities and find the first two simplest conserved densities.",1904.00797v2 2019-04-26,Density Functional Methods for the Magnetism of Transition Metals: SCAN in Relation to Other Functionals,"We report tests of various density functionals for ferromagnetic, Fe, Co and Ni with a focus on characterizing the behavior of the so-called strongly constrained and appropriately normed (SCAN) functional. It is found that SCAN is closer in behavior to functionals that yield localized behavior, such as hybrid functionals, than other semilocal functionals that are tested. The results are understood in terms of a tendency to differentiate orbitals, favoring integer occupation, which is necessary for a correct description of atomic systems, but inappropriate for the open shell metallic ferromagnetic metals studied here.",1904.11825v1 2019-04-29,Melting of Ferromagnetic Order on a Trellis Ladder,"The ground state properties of a frustrated spin-1/2 system is studied on a trellis ladder which is composed of two zigzag ladders interacting through rung interactions. The presence of rung interaction between the zigzag ladders induces a non-magnetic ground state, although, each of zigzag ladders has ferromagnetic order in weak antiferromagnetic leg interaction limit. The rung interaction also generates rung dimers and opens spin gap which increases rapidly with rung interaction strength. The correlation between spins decreases exponentially with the distance between them.",1904.12521v1 2019-08-22,Anomalous Hall effect at the spontaneously electron-doped polar surface of PdCoO2 ultrathin films,"We revealed the electrical transport through surface ferromagnetic states of a nonmagnetic metal PdCoO2. Electronic reconstruction at the Pd-terminated surface of PdCoO2 induces Stoner-like ferromagnetic states, which could lead to spin-related phenomena among the highly conducting electrons in PdCoO2. Fabricating a series of nanometer-thick PdCoO2 thin films, we detected a surface-magnetization-driven anomalous Hall effect via systematic thickness- and termination-dependent measurements. Besides, we discuss that finite magnetic moments in electron doped CoO2 triangular lattices may have given rise to additional unconventional Hall resistance.",1908.08173v1 2019-12-03,Ferromagnetic spin correlations in the two-dimensional Hubbard model,"We analyze the dynamical nearest-neighbor and next-nearest-neighbor spin correlations in the 4-site and 8-site dynamical cluster approximation to the two-dimensional Hubbard model. Focusing on the robustness of these correlations at long imaginary times, we reveal enhanced ferromagnetic correlations on the lattice diagonal, consistent with the emergence of composite spin-1 moments at a temperature scale that essentially coincides with the pseudo-gap temperature $T^*$. We discuss these results in the context of the spin-freezing theory of unconventional superconductivity.",1912.01260v1 2019-12-05,Spin pumping into a spin glass material,"Spin pumping is a recently established means for generating a pure spin current, whereby spins are pumped from a magnet into the adjacent target material under the ferromagnetic resonance condition. We theoretically investigate the spin pumping from an insulating ferromagnet into spin glass materials. Combining a dynamic theory of spin glasses with the linear-response formulation of the spin pumping, we calculate temperature dependence of the spin pumping near the spin glass transition. The analysis predicts that a characteristic peak appears in the spin pumping signal, reflecting that the spin fluctuations slow down upon the onset of spin freezing.",1912.02337v2 2019-12-11,Dynamical symmetry in quantum dissipative models,"We show that the dynamical symmetry exists in dissipative quantum many-body systems. Under constraints on both Hamiltonian and dissipation parts, the time evolution of particular observables can be symmetric between repulsive and attractive interactions in the Hubbard model, or symmetric between ferromagnetic and anti-ferromagnetic interactions in the Ising model with external fields. We present a theorem to determine the existence of the dynamical symmetry in dissipative systems. This theorem is also responsible for the symmetry of steady states, even without the constraint on the initial state. We demonstrate the applications of our theorem with numerical simulations using tensor network algorithms.",1912.05495v1 2020-03-23,Nernst heat theorem for the Casimir-Polder interaction between a magnetizable atom and ferromagnetic dielectric plate,"We find the low-temperature behavior of the Casimir-Polder free energy for a polarizable and magnetizable atom interacting with a plate made of ferromagnetic dielectric material. It is shown that the corresponding Casimir-Polder entropy goes to zero with vanishing temperature, i.e., the Nernst heat theorem is satisfied, if the dc conductivity of the plate material is disregarded in calculations. If the dc conductivity is taken into account, the Nernst theorem is violated. These results are discussed in light of recent experiments.",2003.10101v1 2020-03-31,Floquet Control of Indirect Exchange Interaction in Periodically Driven Two-Dimensional Electron Systems,"We present a theory for the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction mediated by a two-dimensional (2D) electron system subjected to periodic driving. This is demonstrated for a heterostructure consisting of two ferromagnets laterally sandwiching the 2D metallic spacer. Our calculations reveal new non-analytic features in the spin susceptibility. For weak light-matter coupling, the RKKY interaction shows oscillations with a period tunable by the light amplitude and frequency. For stronger light-matter coupling, the interaction becomes non-oscillatory and remains purely ferromagnetic.",2003.14383v1 2020-05-18,Tuning thermo-magnetic properties of dilute-ferromagnet multilayers using RKKY interaction,"We demonstrate a 20-fold enhancement in the strength of the RKKY interlayer exchange in dilute-ferromagnet/normal-metal multilayers by incorporating ultrathin Fe layers at the interfaces. Additionally, the resulting increase in the interface magnetic polarization profoundly affects the finite-size effects, sharpening the Curie transition of the multilayer, while allowing to separately tune its Curie temperature via intralayer magnetic dilution. These results should be useful for designing functional materials for applications in magneto-caloric micro-refrigeration and thermally-assisted spin-electronics.",2005.08846v1 2020-05-26,Magnetorheological effect in elastomers containing uniaxial ferromagnetic particles,"The description of the collective magnetorheological effect induced by magnetic field in magnetoactive elastomers is proposed. The condition of consistency is used between magnetic and mechanic momenta of forces exerted on magnetically uniaxial ferromagnetic particles in elastomer at their magnetization. The study shows that even in the case of small concentration of particles, the value of magnetically-induced shear can be anomalously large, reaching up to tens of percent. The deformation of magnetoactive elastomer can evolve critically, as a second-order phase transition, if magnetic field is aligned along the easy axis of particles.",2005.12736v1 2020-06-19,Absence of renormalization group pathologies in some critical Dyson-Ising ferromagnets,"The Dyson-Ising ferromagnet is a one-dimensional Ising model with a power law interaction. When the power is between -1 and -2, the model has a phase transition. Van Enter and Le Ny proved that at sufficiently low temperatures the decimation renormalization group transformation is not defined in the sense that the renormalized measure is not a Gibbs measure. We consider a modified model in which the nearest neighbor couplings are much larger than the other couplings. For a family of Hamiltonians which includes critical cases, we prove that the first step of the renormalization group transformation can be rigorously defined for majority rule and decimation.",2006.11429v1 2020-06-22,Macroscale non-local transfer of superconducting signatures to a ferromagnet in a cavity,"Cavity spintronics recently heralded non-local magnonic signal transfer between magnetic samples. Here we show that by including superconductors in the cavity, we can make use of these principles to bring composite superconductor--ferromagnet systems to the macroscale. We analyze how a superconductor's a.c. conductivity influences the spin dynamics of a spatially separated magnet, and we discuss the potential impact on spintronic applications.",2006.12516v2 2020-06-27,Thermal Kosterlitz-Thouless transitions in the $1/r^2$ long-range ferromagnetic quantum Ising chain revisited,"For the inverse square long-range ferromagnetic Ising chain in a transverse field, the thermal phase boundary of the floating Kosterlitz-Thouless phase is obtained for several values of the transverse field down to the quantum critical point. The sharp domain walls in the classical model are increasingly smeared out by the transverse field, which is evidenced by a pronounced broadening of the non-universal bump in the specific heat. The discernability of KT critical scaling in finite-size simulations is discussed.",2006.15402v1 2020-10-16,Zero-magnon sound in quantum Heisenberg ferromagnets,"Using a functional renormalization group approach we show that at low temperatures and in a certain range of magnetic fields the longitudinal dynamic structure factor of quantum Heisenberg ferromagnets in dimensions $D\leq 2$ exhibits a well-defined quasi-particle peak with linear dispersion that we identify as zero-magnon sound. In $D>2$ the larger phase space available for the decay into transverse spin waves leads only to a broad hump centered at zero frequency whose width scales linearly in momentum.",2010.08425v3 2020-10-21,Thermal gating of magnon exchange in magnetic multilayers with antiferromagnetic spacers,"We observe a strong thermally-controlled magnon-mediated interlayer coupling of two ferromagnetic layers via an antiferromagnetic spacer in spin-valve type trilayers. The effect manifests itself as a field-induced coherent switching of the two ferromagnets, which can be controlled by varying temperature and the spacer thickness. We explain the observed behavior as due to a strong hybridization of the ferro- and antiferro-magnetic magnon modes in the trilayer at temperatures just below the N\'eel temperature of the antiferromagnetic spacer.",2010.11134v1 2021-03-10,On the mean-field equations for ferromagnetic spin systems,"We derive mean-field equations for a general class of ferromagnetic spin systems with an explicit error bound in finite volumes. The proof is based on a link between the mean-field equation and the free convolution formalism of random matrix theory, which we exploit in terms of a dynamical method. We present three sample applications of our results to Ka\'{c} interactions, randomly diluted models, and models with an asymptotically vanishing external field.",2103.06207v4 2021-03-25,p+ip-wave pairing symmetry at type-II van Hove singularities,"Based on the random phase approximation calculation in two-orbital honeycomb lattice model, we investigate the pairing symmetry of Ni-based transition-metal trichalcogenides by electron doping access to type-II van Hove singularities (vHs). We find that chiral even-parity d+id-wave (Eg) state is suppressed by odd-parity p+ip-wave (Eu) state when electron doping approaches the type-II vHs. The type-II vHs peak in density of states (DOS) enables to strengthen the ferromagnetic fluctuation, which is responsible for triplet pairing. The competition between antiferromagnetic and ferromagnetic fluctuation results in pairing phase transition from singlet to triplet pairing. The Ni-based transition-metal trichalcogenides provide a promising platform to unconventional superconductor emerging from electronic DOS.",2103.13753v1 2021-03-29,Determination of paramagnetic and ferromagnetic phases of an Ising model on a third-order Cayley tree,"In this present paper, the recurrence equations of an Ising model with three coupling constants on a third-order Cayley tree are obtained. Paramagnetic and ferromagnetic phases associated with the Ising model are characterized. Types of phases and partition functions corresponding to the model are rigorously studied. Exact solutions of the mentioned model are compared with the numerical results given in Ganikhodjaev et al. [J. Concr. Appl. Math., 2011, 9, No. 1, 26-34].",2103.15473v1 2021-04-03,Evidence for spin-polarized bound states in semiconductor-superconductor-ferromagnetic insulator islands,"We report Coulomb blockade transport studies of semiconducting InAs nanowires grown with epitaxial superconducting Al and ferromagnetic insulator EuS on overlapping facets. Comparing experiment to a theoretical model, we associate cotunneling features in even-odd bias spectra with spin-polarized Andreev levels. Results are consistent with zero-field spin splitting exceeding the induced superconducting gap. Energies of subgap states are tunable on either side of zero via electrostatic gates.",2104.01463v2 2021-05-08,Conserved momenta of ferromagnetic solitons through the prism of differential geometry,"The relation between symmetries and conservation laws for solitons in a ferromagnet is complicated by the presence of gyroscopic (precessional) forces, whose description in the Lagrangian framework involves a background gauge field. This makes canonical momenta gauge-dependent and requires a careful application of Noether's theorem. We show that Cartan's theory of differential forms is a natural language for this task. We use it to derive conserved momenta of the Belavin--Polyakov skyrmion, whose symmetries include translation, global spin rotation, and dilation.",2105.03553v2 2021-06-09,Einstein-de Haas Nanorotor,"We propose a nanoscale rotor embedded between two ferromagnetic electrodes that is driven by spin injection. The spin-rotation coupling allows this nanorotor to continuously receive angular momentum from an injected spin under steady current flow between ferromagnetic electrodes in an antiparallel magnetization configuration. We develop a quantum theory of this angular momentum transfer and show that a relaxation process from a precession state into a sleeping top state is crucial for the efficient driving of the nanorotor by solving the master equation. Our work clarifies a general strategy for efficient driving of a nanorotor.",2106.04861v3 2021-07-12,Spin wave driven domain wall motion in easy-plane ferromagnets: a particle perspective,"In easy-plane ferromagnets, we show that the interplay between a domain wall and a spin wave packet can be formulated as the collision of two massive particles with a gravity-like attraction. In the presence of magnetic dissipation, the domain wall mimics a particle subject to viscous friction, while the spin wave packet resembles a particle of variable mass. Due to attractive nature of the interaction, the domain wall acquires a backward displacement as a spin wave packet penetrating the domain wall, even though there is no change in momentum of the wave packet before and after penetration.",2107.05401v2 2021-08-13,Coupling the Higgs mode and ferromagnetic resonance in spin-split superconductors with Rashba spin-orbit coupling,"We consider the Higgs mode at nonzero momentum in superconductors and demonstrate that in the presence of Rashba spin-orbit coupling, it couples linearly with an external exchange field. The Higgs-spin coupling dramatically modifies the spin susceptibility near the superconducting critical temperature and consequently enhances the spin pumping effect in a ferromagnetic insulator/superconductor bilayer system. We show that this effect can be detected by measuring the magnon-induced voltage generated by the inverse spin Hall effect.",2108.06202v2 2021-08-28,Background for the Self Consistent Renormalisation (SCR) Theory,"A detailed review is given on the evolution of the theory of itinerant magnetism. The self-consistent renormalization (SCR) theory is quite successful in addressing the phenomenon of itinerant magnetism in weakly and nearly ferromagnetic and anti-ferromagnetic materials. It goes beyond the Stoner and random phase approximation (RPA) theories in taking the correlation effects into account. The Mathematical machinery of the SCR theory is rather complicated. The aim of these notes is to provide the required background. The problems with Stoner and RPA theory are discussed and the way in which the SCR theory rectifies those problems is also discussed.",2108.12683v1 2021-09-03,Robust spin-transfer torque and magnetoresistance in non-collinear antiferromagnetic junctions,"Ferromagnetic spin-valves and tunneling junctions are crucial for spintronics applications and are one of the most fundamental spintronics devices. Motivated by the potential unique advantages of antiferromagnets for spintronics, we theoretically study here junctions built out of non-collinear antiferromagnets. We demonstrate a large and robust magnetoresistance and spin-transfer torque capable of ultrafast switching between parallel and anti-parallel states of the junction. In addition, we show that the non-collinear order results in a spin-transfer torque that is in several key aspects different from the spin-transfer torque in ferromagnetic junctions.",2109.01399v1 2021-09-17,Fabricacion de un magnetron sputtering para deposito de peliculas nanometricas magneticas,"This paper presents the development of scientific instrumentation for the fabrication of ferromagnetic thin films, by sputtering technique, for the use of 2-inch-diameter targets. Thin films were deposited using Permalloy alloy (Ni80Fe20) as ferromagnetic material at room temperature on Si (001) substrates. The film thicknesses were measured with profilometry and a deposition rate for this alloy of 16.2 nm/min was calculated. Scanning electron microscopy showed a continuous film formation and a chemical composition similar to the target",2109.08649v1 2021-10-06,Sharp Signal Detection Under Ferromagnetic Ising Models,"In this paper we study the effect of dependence on detecting a class of structured signals in Ferromagnetic Ising models. Natural examples of our class include Ising Models on lattices, and Mean-Field type Ising Models such as dense Erd\H{o}s-R\'{e}nyi, and dense random regular graphs. Our results not only provide sharp constants of detection in each of these cases and thereby pinpoint the precise relationship of the detection problem with the underlying dependence, but also demonstrate how to be agnostic over the strength of dependence present in the respective models.",2110.02949v1 2021-11-24,Spin Hall response at finite wave vector in ferromagnets,"Spin Hall effect at finite wave vector in a ferromagnetic conductor is theoretically studied by calculating the spin density as the linear response to an applied electric field. The cases of a spin-orbit interaction due to random impurities and a localized Rashba interaction are considered. It is shown that the spin Hall effect has a broad response for the wave vector $q\lesssim 2\kf$ where $\kf $ is the Fermi wave vector. This fact confirms the local nature of the spin-charge conversion effects.",2111.12247v1 2021-12-21,Hyperscaling analysis of a disorder-induced ferromagnetic quantum critical point in Ni$_{1-x}$Rh$_{x}$ with $x = 0.375$,"Here we report on hyperscaling analysis on thermodynamic measurements as a function of temperature and magnetic field for Ni$_{1-x}$Rh$_{x}$ with $x = 0.375$ where a ferromagnetic quantum critical point has been recently identified [Phys. Rev. Lett. $\textbf{124}$, 117203 (2020)]. The obtained critical exponents agree well with the theory proposed by Belitz, Kirkpatrick, and Vojta for disorder tuned quantum critical point in the preasymptotic region.",2112.10997v1 2021-12-26,Uniqueness result for a weighted pendulum equation modeling domain walls in notched ferromagnetic nanowires,"We prove an existence and uniqueness result for solutions $\varphi$ to a weighted pendulum equation in $\mathbb{R}$ where the weight is non-smooth and coercive. We also establish (in)stability results for $\varphi$ according to the monotonicity of the weight. These results are applied in a reduced model for thin ferromagnetic nanowires with notches to obtain existence, uniqueness and stability of domain walls connecting two opposite directions of the magnetization.",2112.13358v1 2022-01-07,Integrable negative flows of the Heisenberg ferromagnet equation hierarchy,"We study the negative flows of the hierarchy of the integrable Heisenberg Ferromagnet model and their soliton solutions. The first negative flow is related to the so-called short pulse equation. We provide a framework which generates Lax pairs for the other members of the hierarchy. The Inverse scattering, based on the dressing method is illustrated with the derivation of the one-soliton solution.",2201.02655v1 2022-02-05,Gyromagnetic bifurcation in a levitated ferromagnetic particle,"We examine the mechanical rotation of a levitated magnetic particle that is induced by ferromagnetic resonance under microwave irradiation. We show that two stable solutions appear in a certain range of parameters by bifurcation when the rotation frequency is comparable to the microwave frequency. This phenomenon originates from the coexistence of the Barnett and the Einstein-de Haas effects. We also reveal that this measurement is sensitive to the strength of the spin-rotation coupling. Our work provides a platform for accessing a microscopic relaxation process from spin to macroscopic rotation.",2202.02461v2 2022-02-14,Optimizing the topological properties of stacking semiconductor-ferromagnet-superconductor heterostructures,"We study the electronic properties of a planar semiconductor-superconductor heterostructure, in which a thin ferromagnetic insulator layer lies in between and acts as a spin filtering barrier. We find that in such a system one can simultaneously enhance the strengths of all the three important induced physical quantities, i.e., Rashba spin-orbit coupling, exchange coupling, and superconducting pairing potential, for the hybrid mode by external gating. Our results show specific advantage of this stacked device geometry compared to conventional devices. We further discuss how to optimize geometrical parameters for the heterostructure and complement our numerical simulations with analytic calculations.",2202.06604v1 2022-04-08,In situ controllable magnetic phases in doped twisted bilayer transition-metal dichalcogenides,"We study the electronic structure of hole-doped transition metal dichalcogenides for small twist-angels, where the onsite repulsion is extremely strong. Using unbiased diagrammatic Monte Carlo simulations, we find evidence for magnetic correlations which are driven by delocalization and can be controlled in situ via the dielectric environment. For weak spin-orbit coupling, we find that the moderately doped system becomes anti-ferromagnetic, whilst the regime of strong spin-orbit coupling features ferromagnetic correlations. We show that this behavior is accurately predicted by an analytical theory based on moment expansion of the Hamiltonian, and analysis of corresponding particle trajectories.",2204.03917v1 2022-04-13,What Formulation Should One Choose for Modeling a 3D HTS Motor Pole with Ferromagnetic Materials?,"We discuss the relevance of several finite-element formulations for nonlinear systems containing high-temperature superconductors (HTS) and ferromagnetic materials (FM), in the context of a 3D motor pole model. The formulations are evaluated in terms of their numerical robustness and efficiency. We propose a coupled h-phi-a-formulation as an optimal choice, modeling the problem with an a-formulation in the FM and an h-phi-formulation in the remaining domains. While maintaining a low number of degrees of freedom, the h-phi-a-formulation guarantees a robust resolution and strongly reduces the number of iterations required for handling the nonlinearities of HTS and FM compared to standard formulations.",2204.06499v1 2022-04-28,Graphene on a ferromagnetic substrate: instability of the electronic liquid,"We previously show [JETP Letters, {\bf 114}, 763 (2021)] that a graphene sample placed on a ferromagnetic substrate demonstrates a cooperative magnetoelectronic instability. The instability induces a gap in the electronic spectrum and a canting deformation of the magnetization near the graphene-substrate interface. In this paper we prove that the interaction between the electrons in graphene strongly enhances the instability. Our estimates suggest that in the presence of even a moderate interaction the instability can be sufficiently pronounced to be detected experimentally in a realistic setting.",2204.13434v1 2022-04-30,Piezoelectric ferromagnetism in two dimensional FeCl$_2$,"We predict that monolayer FeCl$_2$ is a two-dimensional piezoelectric ferromagnet (PFM) with easy-axis magnetism and a Curie temperature of 260 K. Our ab-initio calculations combined with data mining reveal 2H-FeCl$_2$ as the only easy-axis 2D monolayer PFM, and that its magnetic anisotropy increases many-fold with moderate hole doping. We develop an analysis based on magnetic anisotropy energy densities that explain the magnetic and doping-dependent behavior of FeCl$_2$, as well as VSe$_2$ and CrI$_3$, and can enable the design of future 2D magnetically ordered materials.",2205.00300v1 2022-05-03,On multi-soliton solutions to a generalized inhomogeneous nonlinear Schrodinger equation for the Heisenberg ferromagnetic spin chain,"A generalized inhomogeneous higher-order nonlinear Schrodinger (GIHNLS) equation for the Heisenberg ferromagnetic spin chain system in (1+1)-dimensions under zero boundary condition at infinity is taken into account. The spectral analysis is first performed to generate a related matrix Riemann-Hilbert problem on the real axis. Then, through solving the resulting matrix Riemann-Hilbert problem by taking the jump matrix to be the identity matrix, the general bright multi-soliton solutions to the GIHNLS equation are attained. Furthermore, the one- and two-soliton solutions are written out and analyzed by figures.",2205.01318v2 2022-05-07,Current-driven Dynamics of Skyrmion Bubbles in Achiral Uniaxial Magnets,"We report dynamics of skyrmion bubbles driven by spin-transfer torque in achiral ferromagnetic nanostripes using micromagnetic simulations. In a three-dimensional uniaxial ferromagnet with a quality factor that is smaller than 1, the skyrmion bubble is forced to stay at the central nanostripe by a repulsive force from the geometry border. The coherent motion of skyrmion bubbles in the nanostripe can be realized by increasing the quality factor to ~3.8. Our results should propel the design for future spintronic devices such as artificial neural computing and racetrack memory based on dipole-stabilized skyrmion bubbles.",2205.03641v1 2022-06-02,Itinerant spin polaron and metallic ferromagnetism in semiconductor moiré superlattices,"Itinerant spin polaron and metallic ferromagnetism are theoretically predicted in the Mott insulator in semiconductor moir\'e superlattices doped below and above half filling of the narrow moir\'e band, respectively. The existence of spin polaron can be directly identified from the kink in the dependence of the charge gap on the magnetic field.",2206.01221v3 2022-06-06,Analytical models for the pulse shape of a superconductor-ferromagnet tunnel junction thermoelectric microcalorimeter,"The superconductor-ferromagnet thermoelectric detector (SFTED) is a novel ultrasensitive radiation detector based on the giant thermoelectric effect in superconductorferromagnet tunnel junctions. We demonstrate analytical models and solutions in the time domain for a SFTED operated as a microcalorimeter (pulse excitation), in the linear smallsignal limit. Based on these solutions, the signal current and temperature pulse response were studied for two different electrical circuit models, providing design conditions for stable and non-oscillatory response.",2206.02513v1 2022-06-10,Spin Pumping into Anisotropic Dirac Electrons,"We study spin pumping into an anisotropic Dirac electron system induced by microwave irradiation to an adjacent ferromagnetic insulator theoretically. We formulate the Gilbert damping enhancement due to the spin current flowing into the Dirac electron system using second-order perturbation with respect to the interfacial exchange coupling. As an illustration, we consider the anisotropic Dirac system realized in bismuth to show that the Gilbert damping varies according to the magnetization direction in the ferromagnetic insulator. Our results indicate that this setup can provide helpful information on the anisotropy of the Dirac electron system.",2206.04899v1 2022-07-30,Inertial spin waves in ferromagnets and antiferromagnets,"Inertial effects in spin dynamics are theoretically predicted to emerge at ultrashort time scales, but their experimental signatures are often ambiguous. Here, we calculate the spin-wave spectrum in ferromagnets and two-sublattice antiferromagnets in the presence of inertial effects. It is shown how precession and nutation spin waves hybridize with each other, leading to the renormalization of the frequencies, the group velocities, the effective gyromagnetic ratios and the effective damping parameters. Possible ways of distinguishing between the signatures of inertial dynamics and similar effects explainable within conventional models are discussed.",2208.00131v1 2022-08-01,Calculation of magnon drag force induced by an electric current in ferromagnetic metals,"Magnon drag effect induced by an applied electric field in ferromagnetic metals is theoretically studied by a microscopic calculation of the force on magnons arising from magnon emission/absorption and scattering due to driven electrons. It is shown that magnon scattering contribution dominates over the emission/absorption one in a wide temperature regime in good metals with long elastic lifetime $\taue$, as the latter has a relative suppression factor of $(\Deltasd\taue)^{-2}$ due to the electron spin flip by the magnon, where $\Deltasd$ is the $sd$ exchange interaction energy. Spin-transfer efficiency is discussed including the magnon drag effect.",2208.00622v1 2022-08-13,The Landau-Lifshitz-Bloch equation on the thin film,"We consider the initial boundary value problem of Landau-Lifshitz-Bloch equation on three-dimensional ferromagnetic films, where the effective field contains the stray field controlled by Maxwell equation and the exchange field contains exchange constant. In this paper, we establish the existence of weak solutions of the equation by using the Faedo-Galerkin approximation method. We also derive its two-dimensional limit equation in a mathematically rigorous way when the film thickness tends to zero under appropriate compactness conditions. Moreover, we obtain an equation that can better describe the magnetic dynamic behavior of ferromagnetic films with negligible thickness at high temperature.",2208.06558v1 2022-04-26,Functional nanostructures superconductor-ferromagnet for spintronics,"The work is devoted to the study of the processes of formation and analysis of the parameters of a functional nanostructure -- a superconducting spin valve, which is a multilayer structure consisting of ferromagnetic cobalt nanofilms separated by niobium superconductor films. The studies were carried out using molecular dynamics simulations. The atomic structure of individual nanolayers of the system is considered.",2208.08364v1 2022-08-26,Unified theory of spin and charge in a ferromagnet,"We derive a unified theory of spin and charge degrees of freedom in a ferromagnet. The spin-transfer torque and spin electromotive force are examined from the coarse-grained perspective of collective coordinates. The resulting equations of motion reflect a balance of conservative, gyroscopic (Berry-phase), and dissipative forces. We then expand the space of collective coordinates by adding the electric charge. The adiabatic spin-transfer torque and spin emf turn out to be a gyroscopic force; their nonadiabatic counterparts are a dissipative force.",2208.12800v2 2022-09-20,Possible origin of the interface field in spintronic experiments,"The effective field at the interface between ferromagnetic and metal layers is often observed in spintronic experiments. It is common to ascribe it to specific exchange interactions caused by spin-orbit coupling, namely, Rashba field, Dzyaloshinskiy-Moriya interaction, spin currents, etc. Alternative view involves the global magnetic field (Oersted field) generated by current both inside and outside the conductor. It is shown here that the inhomogeneous stray field surrounding ferromagnetic layers can produce significant contribution into the observed interface field. This mechanism should be taken into account for better understanding the current-induced phenomena in magnetic multilayers.",2209.09738v1 2022-09-20,Field-Induced Spin Nematic Liquid of the $S=1/2$ Bond-Alternating Chain with the Anisotropy,"The $S=1/2$ ferromagnetic-antiferromagnetic bond-alternating spin chain with the anisotropy on the ferromagnetic exchange interaction in magnetic field is investigated using the numerical diagonalization and the density matrix renormalization group analyses. It is found that the nematic-spin-dominant Tomonaga-Luttinger liquid phase is induced by the external magnetic field for sufficiently large anisotropy. The phase diagram with respect to the anisotropy and the magnetization is presented.",2209.09740v1 2022-09-28,Constructing Non-Abelian Quantum Spin Liquids Using Combinatorial Gauge Symmetry,"We construct Hamiltonians with only 1- and 2-body interactions that exhibit an exact non-Abelian gauge symmetry (specifically, combinatiorial gauge symmetry). Our spin Hamiltonian realizes the quantum double associated to the group of quaternions. It contains only ferromagnetic and anti-ferromagnetic $ZZ$ interactions, plus longitudinal and transverse fields, and therefore is an explicit example of a spin Hamiltonian with no sign problem that realizes a non-Abelian topological phase. In addition to the spin model, we propose a superconducting quantum circuit version with the same symmetry.",2209.14333v2 2022-11-09,Giant efficiency of long-range orbital torque in Co/Nb bilayers,"We report unambiguously experimental evidence of a strong orbital current in Nb films with weak spin-orbit coupling via the spin-torque ferromagnetic resonance (ST-FMR) spectrum for Fe/Nb and Co/Nb bilayers. The sign change of the damping-like torque in Co/Nb demonstrates a large spin-orbit correlation and thus great efficiency of orbital torque in Co/Nb. By studying the efficiency as a function of the thickness of Nb sublayer, we reveal a long orbital diffusion length (~3.1 nm) of Nb. Further planar Hall resistance (PHE) measurements at positive and negative applying current confirm the nonlocal orbital transport in ferromagnetic-metal/Nb heterostructures.",2211.04809v1 2022-11-17,Boundary Ferromagnetism in Zigzag Edged Graphene,"The flat band of edge states which occur in the simple tight-binding lattice model of graphene with a zig-zag edge have long been conjectured to take up a ferromagnetic configuration. In this work we demonstrate that, for a large class of interaction Hamiltonians which can be added to the tight-binding model, and at the first order in perturbation theory, the degeneracy of the edge states is resolved in such a way that the ground state is in the maximal, spin j = N/2 representation of the spin symmetry where N is the number of edge states.",2211.09282v1 2022-11-24,Avoided ferromagnetic quantum critical point in CeZn,"Cubic CeZn shows a structural phase transition under pressure, and it modifies the ground state from an antiferromagnetic (AFM) state to a ferromagnetic (FM) state. To investigate how the FM state terminates at a quantum phase transition, we measured the electrical resistivity under pressure for a single crystal CeZn. The transition temperature into the FM state decreases monotonously with increasing pressure, accompanied by the pronounced Kondo effect, but a drastic change in the field response occurs before the ordered phase terminates. This result suggests that the FM quantum critical point is avoided by the appearance of an AFM-like state.",2211.13401v1 2022-12-29,Localized Spin-Wave Modes and Microwave Absorption in Random-Anisotropy Ferromagnets,"The theory of localized spin-wave excitations in random-anisotropy magnets has been developed. Starting with a pure Heisenberg ferromagnet, we study the evolution of standing spin waves in a finite-size sample towards localized modes on increasing the strength of random anisotropy. Profiles of the localized modes and their phases are analyzed and visualized in a 2D sample. Localization length is obtained by several methods and its dependence on random anisotropy is computed. The connection between the localization of spin excitations and the broadband nature of the absorption of microwave power by random-anisotropy magnets is elucidated.",2212.14301v1 2023-02-15,Perfect Sampling of $q$-Spin Systems on $\mathbb Z^2$ via Weak Spatial Mixing,"We present a perfect marginal sampler of the unique Gibbs measure of a spin system on $\mathbb Z^2$. The algorithm is an adaptation of a previous `lazy depth-first' approach by the authors, but relaxes the requirement of strong spatial mixing to weak. It exploits a classical result in statistical physics relating weak spatial mixing on $\mathbb Z^2$ to strong spatial mixing on squares. When the spin system exhibits weak spatial mixing, the run-time of our sampler is linear in the size of sample. Applications of note are the ferromagnetic Potts model at supercritical temperatures, and the ferromagnetic Ising model with consistent non-zero external field at any non-zero temperature.",2302.07821v1 2023-02-21,Efficient asymptotic models for axisymmetric eddy current problems in linear ferromagnetic materials,"The problem under consideration is that of time-harmonic eddy current problems in linear ferromagnetic materials surrounded by a dielectric medium with a smooth common interface. Assuming axisymmetric geometries and orthoradial axisymmetric data, we construct an efficient multiscale expansion for the orthoradial solution that provides reduced computational costs. We investigate numerically the accuracy of the approach using an analytical procedure and infinite cylinders as well. It results that the computation of two asymptotics is sufficient to ensure accurate solutions in the case of low frequencies.",2302.10457v1 2023-02-27,Ferromagnetism and Metal-Insulator transition in F-doped LaMnO3,"We present our studies on polycrystalline samples of fluorine doped LaMnO3 (LaMnO3-yFy). LaMnO2.5F0.5 exhibits remarkable magnetic and electrical properties. It shows ferromagnetic and metallic behavior with a high Curie temperature of ~ 239 K and a high magnetoresistance of -64. This drastic change in magnetic properties in comparison to pure LaMnO3 is ascribed to the presence of mixed-valence Mn ions driven by the F-doping at the O-sites, which enables double exchange (DE) in LMOF. Furthermore, the resistivity data exhibits two resistivity peaks at 239 K and 213 K, respectively. Our results point towards the possibility of multiple double exchange hopping paths of two distinct resistances existing simultaneously in the sample below 213 K.",2302.13845v1 2023-03-02,Spin Pumping into Carbon Nanotubes,"We theoretically study spin pumping from a ferromagnetic insulator (FI) into a carbon nanotube (CNT). By employing the bosonization method, we formulate the Gilbert damping induced by the FI/CNT junction, which can be measured by ferromagnetic resonance. We show that the increase in the Gilbert damping has a temperature dependence characteristic of a Luttinger liquid and is highly sensitive to the Luttinger parameter of the spin sector for a clean interface. We also discuss the experimental relevance of our findings based on numerical estimates, using realistic parameters.",2303.01343v2 2023-03-22,Even-odd effect for spin current through thin antiferromagnetic insulator,"Magnon spin transport in a metal-antiferromagnetic insulator-ferromagnetic insulator heterostructure is considered. The spin current is generated via the spin Seebeck effect and in the limit of clean sample where the effects of interface imperfections and lattice defects are excluded. For NiO as an antiferromagnetic insulator we have a magnetic order of antiferromagnetically combined planes which are internally in ferromagnetic order. We find that the sign of the spin current depends on the magnetization direction of the plane next to the metal resulting in an even-odd effect for the spin current. Moreover, as long as damping is excluded, this even-odd effect is the only remaining dependence on the NiO thickness for high temperatures.",2303.12282v1 2023-03-27,Metastable ferromagnetic clusters in dissipative many-body systems of polar molecules,"We investigate the effect of two-body loss due to chemical reactions on quantum magnetism of fermionic polar molecules in an optical lattice. We show that an interplay between dissipation and strong long-range interactions leads to formation of metastable ferromagnetic clusters. The spin states of clusters are controlled by interaction parameters and reflect the symmetry of interactions. The size of clusters strongly depends on the initial configuration of molecules due to Hilbert-space fragmentation during dissipative many-body dynamics. We construct an effective model to show the emergence of metastable states as quasi-dark states. Application to quantum simulation of the spin-$S$ Heisenberg model is discussed.",2303.14967v2 2023-04-12,Transport properties of very overdoped nonsuperconducting Bi2Sr2CuO6+d thin films,"The transport properties, resistance, Hall effect, and low T magnetoresistance for very oxygen overdoped nonsuperconducting Bi2Sr2CuO6+d (Bi2201) thin films are reported. From 20 to 300K, the temperature dependence of the resistance is well described by a law of the form a+bT4/3, theoretically predicted to occur in the presence of ferromagnetic fluctuations. In addition, this prediction is reinforced by the analysis of the transverse and the longitudinal low T magnetoresistance. Interestingly, the presence of a weak disorder causing low T electronic localization allows us to evidence very short diffusion lengths, as observed in other systems with ferromagnetic fluctuations.",2304.05851v1 2023-06-08,A Macroscopic Theory of Saturated Ferromagnetic Conductors,"A phenomenological theory of rigid and saturated ferromagnetic conductors is constructed from a four-continuum model consisting of a rigid lattice continuum, a bound charge continuum for polarization, a circulating current continuum for magnetization, and a free charge continuum for electrical conduction. The basic laws of physics are applied to the four continua. Thermal couplings and the related dissipative effects are also included. The theory includes the Landau-Lifshitz-Gilbert equation as one of a system of simultaneous equations.",2306.11525v1 2023-06-22,SPT extension of $Z_2$ quantum Ising model's ferromagnetic phase,"This paper focuses on the creation of a model with explicitly defined symmetry-protected topological (SPT) phases on a triangular lattice as an extension of $Z_2$ Ising model's ferromagnetic phase. Unlike in previously known similar works, this model is based on an initially interacting system which is known to describe experimentally realizable physical systems. The Hamiltonian for these edge states contains four-point spin interactions between next-to-next nearest neighbors. As an initially interacting A generic technique for creating SPT models is developed, allowing for the construction of translation-invariant edge models.",2306.13170v2 2023-08-23,Parallel simulation of the magnetic moment reversal within the $φ_0$-Josephson junction model,"Periodic structure of the magnetization reversal (MR) domains is studied within the superconductor-ferromagnetic-superconductor $\phi_0$-junction model. The model is described by the Cauchy problem for the system of nonlinear ordinary equations which is numerically solved by means of the 2-step Gauss-Legendre method. Two versions of parallel implementation on the basis of MPI and OpenMP techniques have been developed. Efficiency of both versions is confirmed by test calculations. An effect of frequency of ferromagnetic resonance on the configuration of MR domains has been investigated.",2308.11996v1 2023-09-08,Nonlinear Stability of Static Néel Walls in Ferromagnetic Thin Films,"In this paper, the nonlinear (orbital) stability of static 180^\circ N\'eel walls in ferromagnetic films, under the reduced wave-type dynamics for the in-plane magnetization proposed by Capella, Melcher and Otto [CMO07], is established. It is proved that the spectrum of the linearized operator around the static N\'eel wall lies in the stable complex half plane with non-positive real part. This information is used to show that small perturbations of the static N\'eel wall converge to a translated orbit belonging to the manifold generated by the static wall.",2309.04432v2 2023-09-11,Reentrant Ferromagnetic Ordering of the Random-Field Heisenberg Model in d>2 Dimensions: Fourier-Legendre Renormalization-Group Theory,"The random-magnetic-field classical Heisenberg model is solved in spatial dimensions d>=2 using the recently developed Fourier-Legendre renormalization-group theory for $4\pi$ steradians continuously orientable spins, with renormalization-group flows of 12,500 variables. The random-magnetic-field Heisenberg model is exactly solved in 10 hierarchical models, for d=2,2.26,2.46,2.58,2.63,2.77,2.89,3. For non-zero random fields, ferromagnetic order is seen for d>2. This ordering shows, at d=3, reentrance as a function of temperature.",2309.05576v1 2023-09-13,Multiscaling in the 3D critical site-diluted Ising ferromagnet,"We have studied numerically the appearance of multiscaling behavior in the three-dimensional ferromagnetic Ising site diluted model, in the form of a multifractal distribution of the decay exponents for the spatial correlation functions at the critical temperature. We have computed the exponents of the long-distance decay of higher moments of the correlation function, up to the 10th power, by studying three different quantities: global susceptibilities, local susceptibilities and correlation functions. We have found very clear evidences for multiscaling behavior.",2309.06752v2 2023-09-13,Symmetry transformation of nonlinear optical current of tilted Weyl nodes and application to ferromagnetic MnBi2Te4,"A Weyl node is characterized by its chirality and tilt. We develop a theory of how $n$th-order nonlinear optical conductivity behaves under transformations of anisotropic tensor and tilt, which clarify how chirality-dependent and -independent parts of optical conductivity transform under the reversal of tilt and chirality. Built on this theory, we propose ferromagnetic $\rm MnBi_{2}Te_{4}$ as a magnetoelectrically regulated, terahertz optical device, by magnetoelectrically switching the chirality-dependent and -independent dc photocurrents. These results are useful for creating nonlinear optical devices based on topological Weyl semimetals.",2309.06760v2 2023-10-06,Near optimal bounds for weak and strong spatial mixing for the anti-ferromagnetic Potts model on trees,"We show that the anti-ferromagnetic Potts model on trees exhibits strong spatial mixing for a near-optimal range of parameters. Our work complements recent results of Chen, Liu, Mani, and Moitra [arXiv.2304.01954] who showed this to be true in the infinite temperature setting, corresponding to uniform proper colorings. We furthermore prove weak spatial mixing results complementing results in [arXiv.2304.01954].",2310.04338v1 2023-10-23,Gyromagnetic effects in dynamics of magnetic microparticles,"We derive equations of motion for paramagnetic and ferromagnetic particles fully accounting for gyromagnetic effects. Considering the Einstein-de Haas effect for an ellipsoidal paramagnetic particle we find that starting from a quiescent non-magnetized state, after the field is switched on a rotation along the short axis is established. This is confirmed by the stability analysis of the fixed points of the corresponding ordinary differential equations. In the case of a ferromagnetic particle we integrate the equations of motion in the dissipationless case by finding the integrals of motion. We also reformulate the equations in a Hamiltonian framework in this case and find a period of small nutation oscillations.",2310.14674v1 2023-10-30,"Magnetic Stability, Fermi Surface Topology, and Spin-Correlated Dielectric Response in Monolayer 1T-CrTe2","We have carried out density-functional theory (DFT) calculations to study the magnetic stability of both ferromagnetic (FM) and anti-ferromagnetic (AFM) states in monolayer 1T-CrTe2. Our results show that the AFM order is lower in energy and thus is the ground state. By tuning the lattice parameters, the AFM order can transition to the FM order, in good agreement with experimental observation. We observe a commensurate SDW alongside the previously predicted CDW, and attribute the AFM order to the SDW. This results in distinct hole and electron Fermi pockets and a pronounced optical anisotropy, suggesting quasi-one-dimensional behavior in this material.",2310.19735v1 2023-11-02,The generation and detection of the spin-valley-polarization in semi-Dirac materials,"We investigated the transport properties in a normal metal/ferromagnet/normal metal/superconductor junction based on semi-Dirac materials with inverted energy gap. With a scattering matrix approach, we show that the electron transport in the junction is spin-valley-polarized due to the ferromagnetic exchange energy. It is also shown that the Andreev reflection is strongly suppressed, which is a clear experimental signal for the spin-valley-polarization in semi-Dirac materials.",2311.00948v1 2023-11-10,Observation of the out-of-plane orbital antidamping-like torque,"The out-of-plane antidamping-like orbital torque fosters great hope for high-efficiency spintronic devices. Here we report experimentally the observation of out-of-plane antidamping-like torque that could be generated by z-polarized orbital current in ferromagnetic-metal/oxidized Cu bilayers, which is presented unambiguously by the magnetic field angle dependence of spin-torque ferromagnetic resonance signal. The oxidized Cu thickness dependence of orbital torque ratios highlights the interfacial effect would be responsible for the generation of orbital current. Besides that, the oxidized Cu thickness dependence of damping parameter further proves the observation of antidamping-like torque. This result contributes to enriching the orbital-related theory of the generation mechanism of the orbital torque.",2311.05868v1 2023-10-13,Electronic structure of CrO$_2$ probed by NMR and DFT,"Electronic structure of ferromagnetic half-metal CrO$_2$ was studied by means of 53Cr nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT). The measured NMR spectrum consists of three distinct spectral lines and is interpreted as a triplet arising due to electric quadrupole interaction. The observed NMR parameters agree well with those obtained from electronic structure calculations, corresponding to the presence of Cr$^{4+}$ with fully occupied localized dxy singlet and partially occupied degenerated dxz and dyz states, as required by ferromagnetic double exchange mechanism. With high accuracy the orbital occupations and valence states of all Cr atoms within the CrO$_2$ structure are found uniform.",2311.12846v1 2023-11-27,Positive magnetoresistance in anapole superconductor junctions,"The article presents a method to detect time-reversal symmetry breaking in non-centrosymmetric superconductors using only transport measurements. Specifically, if time-reversal symmetry is broken via a phase difference between singlet and triplet correlations, as in anapole superconductors, the conductance in SFN junctions is enhanced by increasing the exchange field strength in the ferromagnet. This is in sharp contrast with the negative magnetoresistance when using superconductors in which time reversal symmetry is preserved. Moreover, results show a large quadrupolar component of the magnetoresistance which is qualitatively different from the bipolar giant magnetoresistance in strong ferromagnets.",2311.15708v1 2023-12-07,Oxidation tuning of ferroic transitions in Gd$_2$C monolayer,"Tuning of ferroic phases provides great opportunities for material functionalities, especially in two-dimensional materials. Here, a $4f$ rare-earth carbide Gd$_2$C monolayer is predicted to be ferromagnetic metal with large magnetization, inherited from its bulk property. Based on first-principles calculations, we propose a strategy that the surface passivation can effectively tune its ferroicity, namely switching among ferromagnetic, antiferromagnetic, and ferroelectric phases. Metal-insulator transition also occurs accompanying these ferroic transitions. Our calculation also suggests that the magneto-optic Kerr effect and second harmonic generation are effective methods to monitor these phase transitions.",2312.04011v1 2023-12-17,Symmetry breaking in zero-field two-dimensional electron bilayers,"We theoretically consider bilayers of two dimensional (2D) electron gases as in semiconductor quantum wells, and investigate possible spontaneous symmetry breaking transitions at low carrier densities driven by interlayer Coulomb interactions. We use a self-consistent technique implementing mean field truncations of the interacting four-fermion terms, and find a U(1) layer symmetry breaking transition at low carrier densities where the individual layer identities are lost leading to an effective pseudospin XY ferromagnet in the 2D plane. Our results validate earlier theoretical works using simpler restricted Hartree-Fock techniques, and establish the pseudospin XY ferromagnet as a possible low-density symmetry broken phase of 2D bilayers.",2312.10791v2 2024-01-15,Optimization of thermo-spin voltage in vertical nanostructures by geometrical means,"The thermo-spin conversion provides new concepts for further developing the green energy-harvesting technology because spin can be controlled with minimal energy in nanostructures. Through theoretical analysis of thermo-spin generation, transportation and conversion in ferromagnet/non-ferromagnet/heavy metal (FM/NM/HM) vertical structures, we found that the output transverse thermo-spin voltage is independent of the structure's width, but varies in a linear function with the structure's length. To validate our predictions, we fabricated the thermo-spin devices with a CoFeAl/Cu/Pt structure. Our results indicate that FM/NM/HM structures can be utilized to design flexible thermo-spin conversion devices.",2401.07755v1 2024-02-20,Electron-phonon coupling in ferromagnetic Fe-Co alloys from first principles,"We calculate from first principles the electron-phonon coupling strength in ferromagnetic iron-cobalt Fe$_{1-x}$Co$_x$ alloys for compositions ranging from $x=0$ to $x=0.75$. We find strong, spin-dependent variation of the electron-phonon coupling strength with alloy composition. The minimum of the electron-phonon interaction is found near the composition $x=0.25$. We analyze the variation of the electron-phonon interaction with composition, as a function of electron spin, density of states, electron-phonon matrix elements, and phonon frequencies. Our results are in good qualitative agreement with magnetization dynamics experiments.",2402.12650v1 2024-04-04,Interaction-induced nonlinear magnon transport in noncentrosymmetric ferromagnets,"We study the effect of the magnon-magnon interaction on the nonlinear magnon transport. The magnon-magnon interaction induces nonreciprocal magnon decay when the time-reversal symmetry is broken, and leads to nonlinear thermal responses of magnons. We construct a theoretical framework to study the nonlinear thermal responses due to the nonreciprocal magnon decay by using the imaginary Dyson equation and quantum kinetic theory, which is then applied to a model of honeycomb ferromagnets with Dzyaloshinskii-Moriya interactions. An order estimate shows that the nonlinear thermal response from the present mechanism is feasible for experimental measurement.",2404.03260v1 2024-04-08,Electrical control of superconducting spin valves using ferromagnetic helices,"The geometrical properties of a helical ferromagnet are shown theoretically to control the critical temperature of a proximity-coupled superconductor. Using the Usadel equation for diffusive spin transport, we provide self-consistent analysis of how curvature and torsion modulate the proximity effect. When the helix is attached to a piezoelectric actuator, the pitch of the helix -- and hence the superconducting transition -- can be controlled electrically.",2404.05798v1 1996-10-09,Normal Helium 3: a Mott-Stoner liquid,"A physical picture of normal liquid $^3$He, which accounts for both ``almost localized'' and ``almost ferromagnetic'' aspects, is proposed and confronted to experiments.",9610076v1 1997-11-24,Microscopic conditions favoring itinerant ferromagnetism,"A systematic investigation of the microscopic conditions stabilizing itinerant ferromagnetism of correlated electrons in a single-band model is presented. Quantitative results are obtained by quantum Monte Carlo simulations for a model with Hubbard interaction U and direct Heisenberg exchange interaction F within the dynamical mean-field theory. Special emphasis is placed on the investigation of (i) the distribution of spectral weight in the density of states, (ii) the importance of genuine correlations, and (iii) the significance of the direct exchange, for the stability of itinerant ferromagnetism at finite temperatures. We find that already a moderately strong peak in the density of states near the band edge suffices to stabilize ferromagnetism at intermediate U-values in a broad range of electron densities n. Correlation effects prove to be essential: Slater--Hartree-Fock results for the transition temperature are both qualitatively and quantitatively incorrect. The nearest-neighbor Heisenberg exchange does not, in general, play a decisive role. Detailed results for the magnetic phase diagram as a function of U, F, n, temperature T, and the asymmetry of the density of states are presented and discussed.",9711242v2 1998-08-25,"Comment on ""High-Temperature Series Analysis of the 2D Random-Bond Ising Ferromagnet""","A comment on PRL vol. 80, 4697 (1998) by Roder, Adler and Janke. Minior correction included.",9808280v2 1998-11-19,Magnetism and Electronic States of Systems with Strong Hund Coupling,"This paper is a brief review of our recent studies concerning on magnetism and electronic states of lattice systems with Hund coupling. First we examined the effectiveness of the Hund coupling in realizing ferromagnetism in the doubly degenerate Hubbard model. One- and infinite-dimensional systems were studied and thereby dimensional dependence was discussed. In quarter-filled systems the insulating ferromagnetic state accompanied by alternating orbital order was found stable. In more-than-quarter filling cases metallic ferromagnetism is stabilized by the ``double exchange mechanism''. These results are common to one and infinite dimensions. In less-than-quarter filling cases the ferromagnetic ground state is stable in one dimension but not in infinite dimensions. Secondly we examined the electronic states and the resistivity in the double exchange model by using the one-particle Green function. The splitting and narrowing of the one-particle spectrum due to the Hund coupling were clarified in the framework of a single-site approximation. The resistivity due to the scattering by random localized spins was shown to be too small to explain the experimental results of doped manganites.",9811286v1 1998-11-30,Landau theory of bi-criticality in a random quantum rotor system,"We consider here a generalization of the random quantum rotor model in which each rotor is characterized by an M-component vector spin. We focus entirely on the case not considered previously, namely when the distribution of exchange interactions has non-zero mean. Inclusion of non-zero mean permits ferromagnetic and superconducting phases for M=1 and M=2, respectively. We find that quite generally, the Landau theory for this system can be recast as a zero-mean problem in the presence of a magnetic field. Naturally then, we find that a Gabay-Toulouse line exists for $M>1$ when the distribution of exchange interactions has non-zero mean. The solution to the saddle point equations is presented in the vicinity of the bi-critical point characterized by the intersection of the ferromagnetic (M=1) or superconducting (M=2) phase with the paramagnetic and spin glass phases. All transitions are observed to be second order. At zero temperature, we find that the ferromagnetic order parameter is non-analytic in the parameter that controls the paramagnet/ferromagnet transition in the absence of disorder. Also for M=1, we find that replica symmetry breaking is present but vanishes at low temperatures. In addition, at finite temperature, we find that the qualitative features of the phase diagram, for M=1, are {\it identical} to what is observed experimentally in the random magnetic alloy $LiHo_xY_{1-x}F_4$.",9811418v1 1998-12-09,Ferromagnetism and the temperature-dependent electronic structure in thin Hubbard films,"The magnetic behavior of thin ferromagnetic itinerant-electron films is investigated within the strongly correlated single-band Hubbard model. For its approximate solution we apply a generalization of the modified alloy analogy (MAA) to deal with the modifications due to the reduced translational symmetry. The theory is based on exact results in the limit of strong Coulomb interaction which are important for a reliable description of ferromagnetism. Within the MAA the actual type of the alloy analogy is determined selfconsistently. The MAA allows, in particular, the investigation of quasiparticle lifetime effects in the paramagnetic as well as the ferromagnetic phase. For thin fcc(100) and fcc(111) films the layer magnetizations are discussed as a function of temperature as well as film thickness. The magnetization at the surface-layer is found to be reduced compared to the inner layers. This reduction is stronger in fcc(100) than in fcc(111) films. The magnetic behavior can be microscopically understood by means of the layer-dependent spectral density and the quasiparticle density of states. The quasiparticle lifetime that corresponds to the width of the quasiparticle peaks in the spectral density is found to be strongly spin- and temperature-dependent.",9812151v1 1999-01-21,Resistive Anomalies at Ferromagnetic Transitions Revisited: the case of SrRuO_3,"We show that recent resistivity data on SrRuO_3 for T->T_c are consistent with conventional theory when corrections to scaling are included and a small shift in T_c is allowed.",9901207v2 1999-06-12,Mesoscopic coherence effect in superconductor-metallic ferromagnet structures,The boundary condition in Eq.6 is invalid.,9906177v4 2000-02-24,Skyrmions in spinor Bose-Einstein condensates,"We show that spinor Bose-Einstein condensates not only have line-like vortex excitations, but in general also allow for point-like topological excitations, i.e., skyrmions. We discuss the static and dynamic properties of these skyrmions for spin-1/2 and ferromagnetic spin-1 Bose gases.",0002375v1 2000-02-29,Ferromagnetism in III-V and II-VI semiconductor structures,"The current status of research on the carrier-mediated ferromagnetism in tetrahedrally coordinated semiconductors is briefly reviewed. The experimental results for III-V semiconductors, where Mn atoms introduce both spins and holes, are compared to the case of II-VI compounds, in which the ferromagnetism has been observed for the modulation-doped p-type Cd1-xMnxTe/Cd1-y-zMgyZnzTe:N heterostructures, and more recently, in Zn1-xMnxTe:N epilayers. On the theoretical side, a model is presented, which takes into account: (i) strong spin-orbit and kp couplings in the valence band; (ii) the effect of confinement and strain upon the hole density-of-states and response function, and (iii) the influence of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition. A comparison between experimental and theoretical results demonstrates that the model can describe the magnetic circular dichroism, the values of TC observed in the studied systems as well as explain the directions of the easy axis and the magnitudes of the corresponding anisotropy fields as a function of confinement and biaxial strain. Various suggestions concerning design of novel ferromagnetic semiconductor systems are described.",0002450v2 2001-02-05,"Comment on ``Specific heat of a Fermi system near ferromagnetic quantum phase transition'' by Grosu, Bodea, and Crisan (cond-mat/0101392)","We comment on various incorrect claims made in a recent paper by Grosu et al. (cond-mat/0101392).",0102064v2 2001-05-16,A dynamical mean field theory for the magnetic transition temperature of ferromagnetic semiconductors,"This paper has been withdrawn by the authors due to a numerical error. A revised version will be posted shortly.",0105331v2 2001-11-16,"Ferromagnetism in (III,Mn)V Semiconductors","In this review article we briefly summarize the main experimental properties of (III,Mn)V ferromagnets and outline several different but related approaches that have been explored in an effort to gain insight into these materials. The main body of the paper deals with the development of the semi-phenomenological model in which the low energy degrees of freedom are exchange-coupled valence-band holes and $S=5/2$ Mn local moments. We discuss physical predictions based on a mean-field treatment of the disorder-free model and demonstrate that it successfully describes a number of non-trivial properties of (Ga,Mn)As and (In,Mn)As ferromagnets. Study of collective excitations of these ferromagnets shows that the simple mean-field-theory is reasonably reliable for typical parameters of current samples but must fail at large carrier densities and also in the limit of very strong exchange coupling. Finally we discuss the results of Monte Carlo calculations that describe the effect of collective fluctuations of Mn moment orientations. The method can deal with some of the complications and additional physics, including the possibility of non-collinear ground states, that enters when disorder is added to the theoretical model.",0111314v1 2001-12-13,Kondo effect and STM spectra through ferromagnetic nanoclusters,"Motivated by recent scanning tunneling microscope (STM) experiments on cobalt clusters adsorbed on single wall metallic nanotubes [Odom {\em et al.}, Science {\bf 290}, 1549 (2000)], we study theoretically the size dependence of STM spectra and spin-flip scattering of electrons from finite size ferromagnetic clusters adsorbed on metallic surfaces. We study two models of nanometer size ferromagnets: (i) An itinerant model with delocalized s, p and d electrons and (ii) a local moment model with both localized d-level spins and delocalized cluster electrons. The effective exchange coupling between the spin of the cluster and the conduction electrons of the metallic substrate depends on the specific details of the single particle density of states on the cluster. The calculated Kondo coupling is inversely proportional to the total spin of the ferromagnetic cluster in both models and thus the Kondo temperature is rapidly suppressed as the size of the cluster increases. Mesoscopic fluctuations in the charging energies and magnetization of nanoclusters can lead to large fluctuations in the Kondo temperatures and a very asymmetric voltage dependence of the STM spectra. We compare our results to the experiments.",0112258v2 2002-04-04,Spin dynamics in La_{1-x}Sr_{x}MnO_{3} (x \le 0.175) investigated by high-field ESR spectroscopy,"High-field electron spin resonance (ESR) experiments have been carried out in single crystals of La_{1-x}Sr_xMnO_3 in the concentration range 0 \le x \le 0.175. Different quasioptical arrangements have been utilized in the frequency range 40 < \nu < 700GHz and for magnetic fields B \le 12T. A splitting of the antiferromagnetic resonance (AFMR) mode is observed in the magnetic field for the parent compound LaMnO_3 in agreement with the antiferromagnetic structure of this material. Abrupt changes in the AFMR frequencies have been observed around x = 0.025 and attributed to the possible transition between antiferromagnetic and canted state. For increasing Sr-doping the observed AFMR modes are splitted even in zero field, which can be naturally explained using a concept of a canted magnetic structure for x<0.1. In La_{0.825}Sr_{0.175}MnO_3 the ESR spectra are consistent with the ferromagnetic and metallic state. The lines of ferromagnetic resonance and ferromagnetic antiresonance can be clearly seen in the spectra. For intermediate concentrations 0.1 \le x \le 0.15 a complicated set of ESR spectra has been observed, which can be well explained by a single ferromagnetic resonance mode and taking into account electrodynamic effects.",0204112v1 2002-11-07,NMR evidence for an inhomogeneous transition between the ferromagnetic and antiferromagnetic ground states in Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ manganites,"The low temperature behavior of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ manganites are known to undergo a transition at $x\sim$0.3, from an insulating ferromagnetic state, at low Ca concentration, to an insulating antiferromagnetic state. Above the onset concentration for the charge-ordering effect ($x\sim 0.3$), a metal-insulator transition induced by an external magnetic field is also observed and related to the collapse of the charge-ordered state. In this paper we show that the ferro-antiferromagnetic transition takes place in an inhomogeneous way: around the critical concentration the sample is a mixture of ferromagnetic and antiferromagnetic regions. The zero-field NMR measurements show that a fraction of the ferromagnetic regions is in a fast hopping regime, which suggests that a small fraction of the sample could be metallic, even in zero field. This fraction of fast hopping regions is maximal at $x=$0.3, which is also the concentration for which the insulator-metal transition has been observed in the smallest field.",0211143v1 2002-11-25,Giant frequency dependence of dynamic freezing in nanocrystalline ferromagnetic LaCo0.5Mn0.5O3,"This paper is withdrawn now and soon will be replaced by a complete study on LaCo0.5-MgxMn0.5O3.",0211524v2 2003-02-13,"Electronic phase separation in the rare earth manganates, (La1-xLnx)0.7Ca0.3MnO3 (Ln = Nd, Gd and Y)","All the three series of manganates showsaturation magnetization characteristic of ferromagnetism, with the ferromagnetic Tc decreasing with increasing in x up to a critical value of x, xc (xc = 0.6, 0.3, 0.2 respectively for Nd, Gd, Y). For x > xc, the magnetic moments are considerably smaller showing a small increase around TM, the value of TM decreasing slightly with increase in x or decrease in < rA >. The ferromagnetic compositions (x xc) show insulator-metal (IM) transitions, while the compositions with x > xc are insulating. The magnetic and electrical resistivity behavior of these manganates is consistent with the occurrence of phase separation in the compositions around xc, corresponding to a critical average radius of the A-site cation, < rAc >, of 1.18 A. Both Tc and TIM increase linearly when < rA > > < rAc > or x xc as expected of a homogenous ferromagnetic phase. Both Tc and TM decrease linearly with the A-site cation size disorder at the A-site as measured by the variance s2. Thus, an increase in s2 favors the insulating AFM state. Percolative conduction is observed in the compositions with < rA > > < rAc >. Electron transport properties in the insulating regime for x > xc conforms to the variable range hopping mechanism. More interestingly, when x > xc, the real part of dielectric constant (e') reaches a high value (104-106) at ordinary temperatures dropping to a very small (~500) value below a certain temperature, the value of which decreases with decreasing frequency.",0302268v1 2003-03-21,"Independent Electronic and Magnetic Doping in (Ga,Mn)As Based Digital Ferromagnetic Heterostructures","Ferromagnetic semiconductors promise the extension of metal-based spintronics into a material system that combines widely tunable electronic, optical, and magnetic properties. Here, we take steps towards realizing that promise by achieving independent control of electronic doping in the ferromagnetic semiconductor (Ga,Mn)As. Samples are comprised of superlattices of 0.5 monolayer (ML) MnAs alternating with 20 ML GaAs and are grown by low temperature (230 C) atomic layer epitaxy (ALE). This allows for the reduction of excess As incorporation and hence the number of charge-compensating As-related defects. We grow a series of samples with either Be or Si doping in the GaAs spacers (p- and n-type dopants, respectively), and verify their structural quality by in situ reflection high-energy electron diffraction (RHEED) and ex situ x-ray diffraction. Magnetization measurements reveal ferromagnetic behavior over the entire doping range, and show no sign of MnAs precipitates. Finally, magneto-transport shows the giant planar Hall effect and strong (20%) resistance fluctuations that may be related to domain wall motion.",0303461v1 2003-06-20,Giant lasing effect in magnetic nanoconductors,"We propose a new principle for a compact solid-state laser in the 1-100 THz regime. This is a frequency range where attempts to fabricate small size lasers up till now have met severe technical problems. The proposed laser is based on a new mechanism for creating spin-flip processes in ferromagnetic conductors. The mechanism is due to the interaction of light with conduction electrons; the interaction strength, being proportional to the large exchange energy, exceeds the Zeeman interaction by orders of magnitude. On the basis of this interaction, a giant lasing effect is predicted in a system where a population inversion has been created by tunneling injection of spin-polarized electrons from one ferromagnetic conductor to another -- the magnetization of the two ferromagnets having different orientations. Using experimental data for ferromagnetic manganese perovskites with nearly 100% spin polarization we show the laser frequency to be in the range 1-100 THz. The optical gain is estimated to be of order 10^7 cm^{-1}, which exceeds the gain of conventional semiconductor lasers by 3 or 4 orders of magnitude. A relevant experimental study is proposed and discussed.",0306522v1 2003-08-13,Nonlinear magnetic susceptibility and aging phenomena in reentrant ferromagnet: Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound,"Linear and nonlinear dynamic properties of a reentrant ferromagnet Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound are studied using AC and DC magnetic susceptibility. This compound undergoes successive phase transitions at the transition temperatures $T_{h}$ (= 16 K), $T_{c}$ (= 9.7 K), and $T_{RSG}$ (= 3.5 K). The static and dynamic behaviors of the reentrant spin glass phase below $T_{RSG}$ are characterized by those of normal spin glass phase with critical exponent $\beta$ = 0.57 $\pm$ 0.10, a dynamic critical exponent $x$ = 8.5 $\pm$ 1.8, and an exponent $p$ (= 1.55 $\pm$ 0.13) for the de Almeida -Thouless line. A prominent nonlinear susceptibility is observed between $T_{RSG}$ and $T_{c}$ and around $T_{h}$, suggesting a chaotic nature of the ferromagnetic phase ($T_{RSG} \leq T \leq T_{c}$) and the helical spin ordered phase ($T_{c} \leq T \leq T_{h}$). The aging phenomena are observed both in the RSG and FM phases, with the same qualitative features as in normal spin glasses. The aging of zero-field cooled magnetization indicates a drastic change of relaxation mechanism below and above $T_{RSG}$. The time dependence of the absorption $\chi^{\prime \prime}$ is described by a power law form ($\approx t^{-b^{\prime \prime}}$) in the ferromagnetic phase, where $b^{\prime \prime} \approx 0.074 \pm 0.016$ at $f$ = 0.05 Hz and $T$ = 7 K. No $\omega t$-scaling law for $\chi^{\prime \prime}$ [$\approx (\omega t)^{-b^{\prime \prime}}$] is observed.",0308247v1 2004-01-24,Dielectric and Pyroelectric Properties of Thick Film Ferromagnetic - Piezoelectric Structures,"Layered ferromagnetic-piezoelectric composites show mechanical strain mediated electromagnetic coupling. Here we discuss dielectric and piezoelectric properties of ferrite-lead zirconate titanate (PZT) and lanthanum manganite-PZT samples. Results of our investigations on dielectric and pyroelectric properties of multilayer ferromagnetic-piezoelectric are presented here. Lead zircinate-titanate PbZrxTi1-xO3 (PZT) was used for the piezoelectric phase in all the structures. The following materials were used for the ferromagnetic component: nickel-zinc ferrites Ni0.9Zn 0.1Fe2O4 (NFO1) and Ni0.8Zn0.2Fe2O4 (NFO2), cobalt ferrite (CFO), lithium ferrite (LFO), lanthanum strontium manganite La0.7Sr0.3MnO3 (LSM), and lanthanum-calcium manganite La0.7Ca0.3MnO3 (LCM). The pyroelectric effect was studied by measuring the current J flowing through a closed loop containing the sample and an electrometer as the sample temperature T was slowly varied at the rate 0.1 K/s. Polarized PZT layers generate a pyroelectric current as the temperature changes. The main indicator of pyroelectric nature of the current is the sign reversal when the thermal cycle is switched from heating to cooling. Almost all of the multilayer structures showed a pyroelectric current, but the pyroelectric coefficient varied in a wide range. (i) For NFO1-PZT system the coefficient was in the range 0.01 - 10 nC/(cm2 K), depending on the temperature. (ii) CFO-PZT and LFO-PZT structures showed a large thermal current and a weak pyroelectric effect. (iii) Thermal currents, however, were absent in LCM-PZT within the temperature range from the room temperature to 400 K. (iv) In LSM-PZT, the thermal current exceeded the pyroelectric current. A model is proposed for an understanding of these results.",0401481v1 2004-01-28,Density Functional Calculations near Ferromagnetic Quantum Critical Points,"We discuss the application of the density functional theory in the local density approximation (LDA) near a ferromagnetic quantum critical point. The LDA fails to describe the critical fluctuations in this regime. This provides a fingerprint of a materials near ferromagnetic quantum critical points: overestimation of the tendency to magnetism in the local density approximation. This is in contrast to the typical, but not universal, tendency of the LDA to underestimate the tendency to magnetism in strongly Hubbard correlated materials. We propose a method for correcting the local density calculations by including critical spin fluctuations. This is based on (1) Landau expansion for the free energy, evaluated within the LDA, (2) lowest order expansion of the RPA susceptibility in LDA and (3) extraction of the amplitude of the relevant (critical) fluctuations by applying the fluctuation-dissipation theorem to the difference between a quantum-critical system and a reference system removed from the quantum critical point. We illustrate some of the aspects of this by the cases of Ni3Al and Ni3Ga, which are very similar metals on opposite sides of a ferromagnetic quantum critical point. LDA calculations predict that Ni3Ga is the more magnetic system, but we find that due to differences in the band structure, fluctuation effects are larger in Ni3Ga, explaining the fact that experimentally it is the less magnetic of the two materials.",0401563v1 2004-02-03,Observation of ferromagnetism above 900 K in Cr-GaN and Cr-AlN,"We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN thin films. The saturation magnetization moments in our best films of Cr-GaN and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively, indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated conduction that follows the exponential law expected for variable range hopping between localized states. Hall measurements on a Cr-GaN sample indicate a mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping conduction, and a free carrier density (1.4E20/cm^3), which is similar in magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A large negative magnetoresistance is attributed to scattering from loose spins associated with non-ferromagnetic impurities. The results indicate that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands.",0402103v2 2004-02-18,Interface-Localized Mode In Bilayer Film Ferromagnetic Resonance Spectrum,"Ferromagnetic resonance (FMR) in exchange-coupled bilayer films has been the subject of intensive studies in recent years. From the experimental viewpoint, a characteristic feature of this FMR is that some specimens show single resonance, whereas others show double resonance. Moreover, double resonance can exhibit a regular pattern, in which the high-field (HF) line intensity surpasses that of the low-field (LF) line, or it can exhibit an inverted pattern with the HF line less intense than the LF line. There is a general belief that the inverted FMR pattern occurs when the HF line is an ""optic mode"", i.e. an out-of-phase composition of individual sublayer modes, and the LF line is an ""acoustic mode"", or an in-phase mode composition. The existing theoretical explanations of bilayer ferromagnetic resonance are, as a rule, based on phenomenological equation of motion of the magnetization vector. In this paper, we propose a theory of FMR in ultrathin bilayers based on an entirely microscopic approach, using the Heisenberg model of localized spins and assuming that the two ferromagnetic sublayers are exchange-coupled through their interface. This rigorous microscopic FMR theory does explain the inverted pattern of the bilayer FMR spectrum by assuming the HF line to correspond to an in-phase mode, but of interface-localized nature; this gives a possibility to explore the HF resonance line corresponding to the interface-localized mode as a potential source of information concerning the bilayer interface.",0402475v1 2004-04-12,Double Exchange in a Magnetically Frustrated System,"This work examines the magnetic order and spin dynamics of a double-exchange model with competing ferromagnetic and antiferromagnetic Heisenberg interactions between the local moments. The Heisenberg interactions are periodically arranged in a Villain configuration in two dimensions with nearest-neighbor, ferromagnetic coupling $J$ and antiferromagnetic coupling $-\eta J$. This model is solved at zero temperature by performing a $1/\sqrt{S}$ expansion in the rotated reference frame of each local moment. When $\eta $ exceeds a critical value, the ground state is a magnetically frustrated, canted antiferromagnet. With increasing hopping energy $t$ or magnetic field $B$, the local moments become aligned and the ferromagnetic phase is stabilized above critical values of $t$ or $B$. In the canted phase, a charge-density wave forms because the electrons prefer to sit on lines of sites that are coupled ferromagnetically. Due to a change in the topology of the Fermi surface from closed to open, phase separation occurs in a narrow range of parameters in the canted phase. In zero field, the long-wavelength spin waves are isotropic in the region of phase separation. Whereas the average spin-wave stiffness in the canted phase increases with $t$ or $\eta $, it exhibits a more complicated dependence on field. This work strongly suggests that the jump in the spin-wave stiffness observed in Pr$_{1-x}$Ca$_x$MnO$_3$ with $0.3 \le x \le 0.4$ at a field of 3 T is caused by the delocalization of the electrons rather than by the alignment of the antiferromagnetic regions.",0404268v1 2004-06-17,"Effects of inter-site Coulomb interactions on ferromagnetism: Application to Fe, Co and Ni","We reanalyze the condition for metallic ferromagnetism in the framework of the tight-binding approximation and investigate the consequences of the inter-site Coulomb interactions using the Hartree-Fock approximation. We first consider a non-degenerate $s$ band and we show that the inter-site interactions modify the occurrence of ferromagnetism, and we derive a generalized Stoner criterion. We analyze the main effects due to the renormalization of the hopping integrals by the inter-site Coulomb interactions. These effects are strongly dependent on the relative values of the inter-site electron-electron interactions and on the shape of the density of states as illustrated by a study of cubic crystals from which we establish general trends. We then investigate a realistic $spd$ tight-binding model, including intra (Coulomb and exchange) and inter-site charge-charge Coulomb integrals. This model is used to study the electronic structure (band structure, densities of states, magnetic moment) of bulk ferromagnetic 3d transition metals Fe(bcc), Co(hcp and fcc) and Ni(fcc). An excellent agreement with local spin density functional calculations is obtained for the three metals, in particular concerning the relative widths of the majority and minority spin bands. Thus our tight-binding Hartree-Fock model provides a consistent interpretation of this effect.",0406395v1 2004-09-02,Curie and Neel Temperatures of Quantum Magnets,"We estimate, using high-temperature series expansions, the transition temperatures of the spin 1/2, 1 and 3/2 Heisenberg ferromagnet and antiferromagnet in 3-dimensions. The manner in which the difference between Curie and Neel temperatures vanishes with increasing spin quantum number is investigated.",0409041v1 2004-09-17,Magnetism in Carbon Structures,"We discuss different magnetic phenomena observed in carbon-based structures, in particular the diamagnetism, paramagnetism and ferromagnetism observed in graphite, disordered carbon, fullerenes and irradiated carbon structures.",0409443v1 2004-10-21,A Note on Edwards' Hypothesis for Zero-Temperature Ising Dynamics,"We give a simple criterion for checking the so called Edwards' hypothesis in certain zero-temperature, ferromagnetic spin-flip dynamics and use it to invalidate the hypothesis in various examples in dimension one and higher.",0410543v1 2005-01-11,Ferromagnetic polarons in the one-dimensional ferromagnetic Kondo model with quantum mechanical S=3/2 core spins,"We present an extensive numerical study of the ferromagnetic Kondo lattice model with quantum mechanical S=3/2 core spins. We treat one orbital per site in one dimension using the density matrix renormalization group and include on-site Coulomb repulsion between the electrons. We examine parameters relevant to manganites, treating the range of low to intermediate doping, 00 a large enhancement of the effective mass, ferromagnetism and a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e. ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on NaxCoO2. We propose that `Curie-Weiss metal' phase observed in NaxCoO2 is a consequence of the crossover from ``bad metal'' with incoherent quasiparticles at temperatures T>T* and Fermi liquid behavior with enhanced parameters below T*, where T* is a low energy coherence scale induced by strong local Coulomb electron correlations. We propose a model which contains the charge ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.",0512696v2 2006-03-14,"Theory of ferromagnetic (III,Mn)V semiconductors","The body of research on (III,Mn)V diluted magnetic semiconductors initiated during the 1990's has concentrated on three major fronts: i) the microscopic origins and fundamental physics of the ferromagnetism that occurs in these systems, ii) the materials science of growth and defects and iii) the development of spintronic devices with new functionalities. This article reviews the current status of the field, concentrating on the first two, more mature research directions. From the fundamental point of view, (Ga,Mn)As and several other (III,Mn)V DMSs are now regarded as textbook examples of a rare class of robust ferromagnets with dilute magnetic moments coupled by delocalized charge carriers. Both local moments and itinerant holes are provided by Mn, which makes the systems particularly favorable for realizing this unusual ordered state. Advances in growth and post-growth treatment techniques have played a central role in the field, often pushing the limits of dilute Mn moment densities and the uniformity and purity of materials far beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds, material quality and magnetic properties are intimately connected. In the review we focus on the theoretical understanding of the origins of ferromagnetism and basic structural, magnetic, magneto-transport, and magneto-optical characteristics of simple (III,Mn)V epilayers, with the main emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive literature covering results of complementary ab initio and effective Hamiltonian computational techniques, and on comparisons between theory and experiment.",0603380v2 2006-03-15,"Anomalous Hall Effect in three ferromagnets: EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30","The Hall resistivity (Rho_xy), resistivity (Rho_xx), and magnetization of three metallic ferromagnets are investigated as a function of magnetic field and temperature. The three ferromagnets, EuFe4Sb12 (Tc = 84 K), Yb14MnSb11 (Tc = 53 K), and Eu8Ga16Ge30 (Tc = 36 K) are Zintl compounds with carrier concentrations between 1 x 10^21 cm^-3 and 3.5 x 10^21 cm^-3. The relative decrease in Rho_xx below Tc [Rho_xx(Tc)/Rho_xx(2 K)] is 28, 6.5, and 1.3 for EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively. The low carrier concentrations coupled with low magnetic anisotropies allow a relatively clean separation between the anomalous (Rho_'xy), and normal contributions to the measured Hall resistivity. For each compound the anomalous contribution in the zero field limit is fit to alpha Rho_xx + sigma_xy rho_xx^2 for temperatures T < Tc. The anomalous Hall conductivity, sigma_xy, is -220 +- 5 (Ohm^-1 cm^-1), -14.7 +- 1 (Ohm^-1 cm^-1), and 28 +- 3 (Ohm^-1 cm^-1) for EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively and is independent of temperature for T < Tc if the change in spontaneous magnetization (order parameter) with temperature is taken into account. These data are consistent with recent theories of the anomalous Hall effect that suggest that even for stochiometric ferromagnetic crystals, such as those studied in this article, the intrinsic Hall conductivity is finite at T = 0, and is a ground state property that can be calculated from the electronic structure.",0603410v1 2006-04-11,A single layer of Mn in a GaAs quantum well: a ferromagnet with quantum fluctuations,"Some of the highest transition temperatures achieved for Mn-doped GaAs have been in delta-doped heterostructures with well-separated planes of Mn. But in the absence of magnetic anisotropy, the Mermin-Wagner theorem implies that a single plane of magnetic ions cannot be ferromagnetic. Using a Heisenberg model, we show that the same mechanism that produces magnetic frustration and suppresses the transition temperature in bulk Mn-doped GaAs, due to the difference between the light and heavy band masses, can stabilize ferromagnetic order for a single layer of Mn in a GaAs quantum well. This comes at the price of quantum fluctuations that suppress the ordered moment from that of a fully saturated ferromagnet. By comparing the predictions of Heisenberg and Kohn-Luttinger models, we conclude that the Heisenberg description of a Mn-doped GaAs quantum well breaks down when the Mn concentration becomes large, but works quite well in the weak-coupling limit of small Mn concentrations. This comparison allows us to estimate the size of the quantum fluctuations in the quantum well.",0604288v4 2006-05-11,Quantum critical behavior in itinerant electron systems -- Eliashberg theory and instability of a ferromagnetic quantum-critical point,"We consider the problem of fermions interacting with gapless long-wavelength collective bosonic modes. The theory describes, among other cases, a ferromagnetic quantum-critical point (QCP) and a QCP towards nematic ordering. We construct a controllable expansion at the QCP in two steps: we first create a new, non Fermi-liquid ``zero-order'' Eliashberg-type theory, and then demonstrate that the residual interaction effects are small. We prove that this approach is justified under two conditions: the interaction should be smaller than the fermionic bandwidth, and either the band mass $m_B$ should be much smaller than $m = p_F/v_F$, or the number of fermionic flavors $N$ should be large. For an SU(2) symmetric ferromagnetic QCP, we find that the Eliashberg theory itself includes a set of singular renormalizations which can be understood as a consequence of an effective long-range dynamic interaction between quasi-particles, generated by the Landau damping term. These singular renormalizations give rise to a negative non-analytic $q^{3/2}$ correction to the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate that this effect can be understood in the framework of the $\phi^4$ theory of quantum criticality. We also show that the non-analytic $q^{3/2}$ correction to the bosonic propagator is specific to the SU(2) symmetric case. For systems with a scalar order parameter, the $q^{3/2}$ contributions from individual diagrams cancel out in the full expression of the susceptibility, and the QCP remains stable.",0605306v1 2006-05-19,"Magnetic unipolar features in con- ductivity of point contacts between normal and ferromagnetic d-metals (Co, Ni, Fe)","In nanocontacts between normal and ferromagnetic metals (N--F) abrupt changes of the order of 1% are detected in differential resistance, dV/dI(V), versus bias voltage, V, on achieving of high current densities, ~10^9 A/cm^2. These features in dV/dI(V) are observed when the electron flow is directed from the nonmagnetic metal into the ferromagnet and connected with magnetization excitations in the ferromagnet induced by the current. Applying an external magnetic field leads to a shift of the observed features to higher biasing current, confirming the magnetic nature of the effect. Such effects are observed for the non-ballistic (not spectral) regime of current flow in the nanocontacts. Thus, the current induced magneto-conductance effects in multilayered N--F structures (nanopillars) extensively studied in the recent literature have much more general character and can be stimulated by elastic electron scattering at single N--F interfaces.",0605485v1 2006-06-06,Infinite-range Ising ferromagnet in a time-dependent transverse field: quench and ac dynamics near the quantum critical point,"We study an infinite range ferromagnetic Ising model in the presence of a transverse magnetic field which exhibits a quantum paramagnetic-ferromagnetic phase transition at a critical value of the transverse field. In the thermodynamic limit, the low-temperature properties of this model are dominated by the behavior of a single large classical spin governed by an anisotropic Hamiltonian. Using this property, we study the quench and AC dynamics of the model both numerically and analytically, and develop a correspondence between the classical phase space dynamics of a single spin and the quantum dynamics of the infinite-range ferromagnetic Ising model. In particular, we compare the behavior of the equal-time order parameter correlation function both near to and away from the quantum critical point in the presence of a quench or AC transverse field. We explicitly demonstrate that a clear signature of the quantum critical point can be obtained by studying the AC dynamics of the system even in the classical limit. We discuss possible realizations of our model in experimental systems.",0606137v3 2006-06-15,Competing ferromagnetism in high temperature copper oxide superconductors,"The extreme variability of observables across the phase diagram of the cuprate high temperature superconductors has remained a profound mystery, with no convincing explanation of the superconducting dome. While much attention has been paid to the underdoped regime of the hole-doped cuprates because of its proximity to a complex Mott insulating phase, little attention has been paid to the overdoped regime. Experiments are beginning to reveal that the phenomenology of the overdoped regime is just as puzzling. For example, the electrons appear to form a Landau Fermi liquid, but this interpretation is problematic; any trace of Mott phenomena, as signified by incommensurate antiferromagnetic fluctuations, is absent, and the uniform spin susceptibility shows a ferromagnetic upturn. Here we show and justify that many of these puzzles can be resolved if we assume that competing ferromagnetic fluctuations are simultaneously present with superconductivity, and the termination of the superconducting dome in the overdoped regime marks a quantum critical point beyond which there should be a genuine ferromagnetic phase at zero temperature. We propose new experiments, and make new predictions, to test our theory and suggest that effort must be mounted to elucidate the nature of the overdoped regime, if the problem of high temperature superconductivity is to be solved. Our approach places competing order as the root of the complexity of the cuprate phase diagram.",0606431v3 2006-06-21,Spin Glass and ferromagnetism in disordered Cerium compounds,"The competition between spin glass, ferromagnetism and Kondo effect is analysed here in a Kondo lattice model with an inter-site random coupling $J_{ij}$ between the localized magnetic moments given by a generalization of the Mattis model which represents an interpolation between ferromagnetism and a highly disordered spin glass. Functional integral techniques with Grassmann fields have been used to obtain the partition function. The static approximation and the replica symmetric ansatz have also been used. The solution of the problem is presented as a phase diagram giving $T/{J}$ {\it versus} $J_K/J$ where $T$ is the temperature, $J_{K}$ and ${J}$ are the strengths of the intrasite Kondo and the intersite random couplings, respectively. If $J_K/{J}$ is small, when temperature is decreased, there is a second order transition from a paramagnetic to a spin glass phase. For lower $T/{J}$, a first order transition appears between the spin glass phase and a region where there are Mattis states which are thermodynamically equivalent to the ferromagnetism. For very low ${T/{J}}$, the Mattis states become stable. On the other hand, it is found as solution a Kondo state for large $J_{K}/{J}$ values. These results can improve the theoretical description of the well known experimental phase diagram of $CeNi_{1-x}Cu_{x}$.",0606551v1 2006-06-23,"High temperature ferromagnetism in GdFe2Zn20: large, local moments embedded in the nearly ferromagnetic Fermi liquid compound YFe2Zn20","The RFe$_2$Zn$_{20}$ series manifests strongly correlated electron behavior for the non-magnetic R = Y member and remarkably high temperature, ferromagnetic ordering ($T_C$ = 86 K) for the local moment bearing R = Gd member (a compound that is less than 5% atomic Gd). In contrast, the isostructural RCo$_2$Zn$_{20}$ series manifests a more typical ordering temperature ($T_N$ = 5.7 K for GdCo$_2$Zn$_{20}$) and YCo$_2$Zn$_{20}$ does not show signs of correlated electron behavior. Studies of R(Fe$_x$Co$_{1-x}$)$_2$Zn$_{20}$ (R = Gd, Y), combined with bandstructure calculations for the end members, reveal that YFe$_2$Zn$_{20}$ is a nearly ferromagnetic Fermi liquid and that the remarkably high $T_C$ associated with GdFe$_2$Zn$_{20}$ is the result of submerging a large local moment into such a highly polarizable matrix. These results indicate that the RFe$_2$Zn$_{20}$ series, and more broadly the RT$_2$Zn$_{20}$ (T = Fe, Co, Ni, Mn, Ru, Rh, Os, Ir, Pt) isostructural family of compounds, offer an exceptionally promising phase space for the study of the interaction between local moment and correlated electron effects near the dilute R limit.",0606615v2 2006-07-21,Spin splitting and Kondo effect in quantum dots coupled to noncollinear ferromagnetic leads,"We study the Kondo effect in a quantum dot coupled to two noncollinear ferromagnetic leads. First, we study the spin splitting $\delta\epsilon=\epsilon_{\downarrow}-\epsilon_{\uparrow}$ of an energy level in the quantum dot by tunnel couplings to the ferromagnetic leads, using the Poor man's scaling method. The spin splitting takes place in an intermediate direction between magnetic moments in the two leads. $\delta\epsilon \propto p\sqrt{\cos^2(\theta/2)+v^2\sin^2(\theta/2)}$, where $p$ is the spin polarization in the leads, $\theta$ is the angle between the magnetic moments, and $v$ is an asymmetric factor of tunnel barriers ($-1= 0.5. The lattice parameter versus x (including x = 0) shows a distinct maximum at x ~ 0.4. We propose that these observations can be explained if the oxygen defects are on the O' sublattice of the structure. The magnetic susceptibility versus temperature exhibits Curie-Weiss behavior above 150 K for all x, with a Curie constant C that increases with x as expected in an ionic model. However, the magnetization measurements also show that the (ferromagnetic) Weiss temperature theta and the ferromagnetic ordering temperature T_C both strongly decrease with increasing x instead of increasing as expected from C(x). The T_C decreases from 73 K for x = 0 to 21 K for x = 0.65. Furthermore, the saturation moment at a field of 5.5 T at 5 K is nearly independent of x, with the value expected for a fixed spin 1/2 per V. The latter three observations suggest that Lu2V2O_{7-x} may contain localized spin 1/2 vanadium moments in a metallic background that is induced by oxygen defect doping, instead of being a semiconductor as suggested by the C(x) dependence.",0610680v1 2006-11-09,"Ferromagnetism in the t-t' Hubbard model: interplay of lattice, band dispersion, and interaction effects studied within a Goldstone-mode preserving scheme","Ferromagnetism in the Hubbard model is investigated on sc, bcc, and fcc lattices using a systematic inverse-degeneracy ($1/{\cal N}$) expansion which incorporates self-energy and vertex corrections such that spin-rotation symmetry and the Goldstone mode are explicitly preserved. First-order quantum corrections to magnon energies are evaluated for several cases, providing a comprehensive picture of the interplay of lattice, band dispersion, and interaction effects on the stability of the ferromagnetic state with respect to both long- and short-wavelength fluctuations. Our results support the belief that ferromagnetism is a generic feature of the Hubbard model at intermediate and strong coupling provided the DOS is sufficiently asymmetric and strongly peaked near band edge, as for fcc lattice with finite $t'$. For short-wavelength modes, behavior of a characteristic energy scale $\omega^* \sim T_c$ (magnon-DOS-peak energy) is in excellent agreement with the $T_c$ vs. $n$ behavior within DMFT, both with respect to the stable range of densities ($0.20 < n < 0.85$) as well as the optimal density $n=0.65$. However, our finding of vanishing spin stiffness near optimal density highlights the role of long-wavelength fluctuations in further reducing the stable range of densities.",0611238v1 2006-11-22,Magnetism and structure of magnetic multilayers based on the fully spin polarized Heusler alloys Co2MnGe and Co2MnSn,"Our Introduction starts with a short general review of the magnetic and structural properties of the Heusler compounds which are under discussion in this book. Then, more specifically, we come to the discussion of our experimental results on multilayers composed of the Heusler alloys Co2MnGe and Co2MnSn with V or Au as interlayers. The experimental methods we apply combine magnetization and magneto-resistivity measurements, x-ray diffraction and reflectivity, soft x-ray magnetic circular dichroism and spin polarized neutron reflectivity. We find that below a critical thickness of the Heusler layers at typically dcr = 1.5 nm the ferromagnetic order is lost and spin glass order occurs instead. For very thin ferromagnetic Heusler layers there are peculiarities in the magnetic order which are unusual when compared to conventional ferromagnetic transition metal multilayer systems. In [Co2MnGe/Au] multilayers there is an exchange bias shift at the ferromagnetic hysteresis loops at low temperatures caused by spin glass ordering at the interface. In [Co2MnGe/V] multilayers we observe an antiferromagnetic interlayer long range ordering below a well defined Neel temperature originating from the dipolar stray fields at the magnetically rough Heusler layer interfaces.",0611590v1 2006-12-07,"Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N - relation between structural, electronic, and magnetic properties","We report on the metalorganic chemical vapor deposition (MOCVD) of GaN:Fe and (Ga,Fe)N layers on c-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), spatially-resolved energy dispersive X-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably FexN with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. In layers with iron content x<0.4% the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in- and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.",0612200v2 2006-12-24,Carrier induced ferromagnetism in diluted local-moment systems,"The electronic and magnetic properties of concentrated and diluted ferromagnetic semiconductors are investigated by using the Kondo lattice model, which describes an interband exchange coupling between itinerant conduction electrons and localized magnetic moments. In our calculations, the electronic problem and the local magnetic problem are solved separately. For the electronic part an interpolating self-energy approach together with a coherent potential approximation (CPA) treatment of a dynamical alloy analogy is used to calculate temperature-dependent quasiparticle densities of states and the electronic self-energy of the diluted local-moment system. For constructing the magnetic phase diagram we use a modified RKKY theory by mapping the interband exchange to an effective Heisenberg model. The exchange integrals appear as functionals of the diluted electronic self-energy being therefore temperature- and carrier-concentration-dependent and covering RKKY as well as double exchange behavior. The disorder of the localized moments in the effective Heisenberg model is solved by a generalized locator CPA approach. The main results are: 1) extremely low carrier concentrations are sufficient to induce ferromagnetism; 2) the Curie temperature exhibits a strikingly non-monotonic behavior as a function of carrier concentration with a distinct maximum; 3) $T_C$ curves break down at critical $n/x$ due to antiferromagnetic correlations and 4) the dilution always lowers $T_C$ but broadens the ferromagnetic region with respect to carrier concentration.",0612611v1 2007-01-14,Impact of the Fe Doping on Magnetism in Perovskite Cobaltites,"We systematically studied the magnetic and transport properties for the polycrystalline samples of Fe-doped perovskite cobaltites: Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$ ($y$=0.3, $x$=0-0.15; $y$=0.45, $x$=0-0.3) and Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ ($x$=0-0.3). Fe doping leads to an enhancement of the ferromagnetism in the systems of Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$, while the ferromagnetism is suppressed with further increasing Fe content and spin-glass behavior is observed at high doping level of Fe. In contrast, the ferromagnetism is suppressed in the system Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ as long as Fe is doped, and no spin-glass behavior is observed in the sample with Fe doping up to 0.3. The competition between ferromagnetic interactions through Fe$^{3+}$-O-(LS)Co$^{4+}$ and antiferromagnetic interactions through Fe$^{3+}$-O-Fe$^{3+}$ and Fe$^{3+}$-O-(IS)Co$^{3+}$ is considered to be responsible for the behavior observed above. The average radius of the ions on A sites plays the key role in determining what type of interactions Fe doping mainly introduces.",0701302v1 2004-04-23,Quantum Hall States of Gluons in Quark Matter,"We have recently shown that dense quark matter possesses a color ferromagnetic phase in which a stable color magnetic field arises spontaneously. This ferromagnetic state has been known to be Savvidy vacuum in the vacuum sector. Although the Savvidy vacuum is unstable, the state is stabilized in the quark matter. The stabilization is achieved by the formation of quantum Hall states of gluons, that is, by the condensation of the gluon's color charges transmitted from the quark matter. The phase is realized between the hadronic phase and the color superconducting phase. After a review of quantum Hall states of electrons in semiconductors, we discuss the properties of quantum Hall states of gluons in quark matter in detail. Especially, we evaluate the energy of the states as a function of the coupling constant. We also analyze solutions of vortex excitations in the states and evaluate their energies. We find that the states become unstable as the gauge coupling constant becomes large, or the chemical potential of the quarks becomes small, as expected. On the other hand, with the increase of the chemical potential, the color superconducting state arises instead of the ferromagnetic state. We also show that the quark matter produced by heavy ion collisions generates observable strong magnetic field $\sim 10^{15}$ Gauss when it enters the ferromagnetic phase.",0404201v1 2001-11-02,Spiral-Logarithmic Structure in a Heisenberg Ferromagnet,"Spiral-logarithmic structure is suggested as a stationary solution of a modified equation for the Heisenberg model, and the single- and N-soliton solutions are constructed on this base.",0111004v1 2003-12-03,Magnetostatic Spin Waves,"In general, Maxwell's equations require that a wave of magnetic field be accompanied by a wave of electric field, and vice versa. In magnetic media it is possible to have waves of magnetization with negligible electric field. We discuss an example of this based on ferromagnetic spin waves.",0312026v1 2007-04-09,Excitation Spectrum Gap and Spin-Wave Stiffness of XXZ Heisenberg Chains: Global Renormalization-Group Calculation,"The anisotropic XXZ spin-1/2 Heisenberg chain is studied using renormalization-group theory. The specific heats and nearest-neighbor spin-spin correlations are calculated thoughout the entire temperature and anisotropy ranges in both ferromagnetic and antiferromagnetic regions, obtaining a global description and quantitative results. We obtain, for all anisotropies, the antiferromagnetic spin-liquid spin-wave velocity and the Isinglike ferromagnetic excitation spectrum gap, exhibiting the spin-wave to spinon crossover. A number of characteristics of purely quantum nature are found: The in-plane interaction s_i^x s_j^x + s_i^y s_j^y induces an antiferromagnetic correlation in the out-of-plane s_i^z component, at higher temperatures in the antiferromagnetic XXZ chain, dominantly at low temperatures in the ferromagnetic XXZ chain, and, in-between, at all temperatures in the XY chain. We find that the converse effect also occurs in the antiferromagnetic XXZ chain: an antiferromagnetic s_i^z s_j^z interaction induces a correlation in the s_i^xy component. As another purely quantum effect, (i) in the antiferromagnet, the value of the specific heat peak is insensitive to anisotropy and the temperature of the specific heat peak decreases from the isotropic (Heisenberg) with introduction of either type (Ising or XY) anisotropy; (ii) in complete contrast, in the ferromagnet, the value and temperature of the specific heat peak increase with either type of anisotropy.",0704.1064v2 2007-05-08,Precise measurements of radio-frequency magnetic susceptibility in (anti)ferromagnetic materials,"Dynamic magnetic susceptibility, $\chi$, was studied in several intermetallic materials exhibiting ferromagnetic, antiferromagnetic and metamagnetic transitions. Precise measurements by using a 14 MHz tunnel diode oscillator (TDO) allow detailed insight into the field and temperature dependence of $\chi$. In particular, local moment ferromagnets show a sharp peak in $\chi(T)$ near the Curie temperature, $T_c$. The peak amplitude decreases and shifts to higher temperatures with very small applied dc fields. Anisotropic measurements of CeVSb$_3$ show that this peak is present provided the magnetic easy axis is aligned with the excitation field. In a striking contrast, small moment, itinerant ferromagnets (i.e., ZrZn$_2$) show a broad maximum in $\chi(T)$ that responds differently to applied field. We believe that TDO measurements provide a very sensitive way to distinguish between local and itinerant moment magnetic orders. Local moment antiferromagnets do not show a peak at the N\'eel temperature, $T_N$, but only a sharp decrease of $\chi$ below $T_N$ due to the loss of spin-disorder scattering changing the penetration depth of the ac excitation field. Furthermore, we show that the TDO is capable of detecting changes in spin order as well as metamagnetic transitions. Finally, critical scaling of $\chi(T,H)$ in the vicinity of $T_C$ is discussed in CeVSb$_3$ and CeAgSb$_2$.",0705.1072v2 2007-05-21,Ferromagnetic redshift of the optical gap in GdN,"We report measurements of the optical gap in a GdN film at temperatures from 300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is 1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split bands below the Curie temperature. The paramagnetic gap is larger than was suggested by very early experiments, and has permitted us to refine a (LSDA+U)-computed band structure. The band structure was computed in the full translation symmetry of the ferromagnetic ground state, assigning the paramagnetic-state gap as the average of the majority- and minority-spin gaps in the ferromagnetic state. That procedure has been further tested by a band structure in a 32-atom supercell with randomly-oriented spins. After fitting only the paramagnetic gap the refined band structure then reproduces our measured gaps in both phases by direct transitions at the X point.",0705.2912v7 2007-08-09,Unusual heavy-mass nearly ferromagnetic state with a surprisingly large Wilson ratio in the double layered ruthenates (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ru$_{2}$O$_{7}$,"We report an unusual nearly ferromagnetic, heavy-mass state with a surprisingly large Wilson ratio $R_{\textrm{w}}$ (e.g., $R_{\textrm{w}}\sim$ 700 for $x =$ 0.2) in double layered ruthenates (Sr$_{1-x}$Ca$_{x}$)$_{3}$Ru$_{2}$O$_{7}$ with 0.08 $< x <$ 0.4. This state does not evolve into a long-range ferromagnetically ordered state despite considerably strong ferromagnetic correlations, but freezes into a cluster-spin-glass at low temperatures. In addition, evidence of non-Fermi liquid behavior is observed as the spin freezing temperature of the cluster-spin-glass approaches zero near $x \approx$ 0.1. We discuss the origin of this unique magnetic state from the Fermi surface information probed by Hall effect measurements.",0708.1291v5 2007-10-16,A generalization of Snoek's law to ferromagnetic films and composites,"The present paper establishes characteristics of the relative magnetic permeability spectrum $\mu$(f) of magnetic materials at microwave frequencies. The integral of the imaginary part of $\mu$(f) multiplied with the frequency f gives remarkable properties. A generalisation of Snoek's law consists in this quantity being bounded by the square of the saturation magnetization multiplied with a constant. While previous results have been obtained in the case of non-conductive materials, this work is a generalization to ferromagnetic materials and ferromagnetic-based composites with significant skin effect. The influence of truncating the summation to finite upper frequencies is investigated, and estimates associated to the finite summation are provided. It is established that, in practice, the integral does not depend on the damping model under consideration. Numerical experiments are performed in the exactly solvable case of ferromagnetic thin films with uniform magnetization, and these numerical experiments are found to confirm our theoretical results. Microwave permeability measurements on soft amorphous films are reported. The relation between the integral and the saturation magnetization is verified experimentally, and some practical applications of the theoretical results are introduced. The integral can be used to determine the average magnetization orientation in materials with complex configurations of the magnetization, and furthermore to demonstrate the accuracy of microwave measurement systems. For certain applications, such as electromagnetic compatibility or radar absorbing materials, the relations established herein provide useful indications for the design of efficient materials, and simple figures of merit to compare the properties measured on various materials.",0710.2980v2 2008-01-15,Ferromagnet in a continuously tuneable random field,"The Random-Field Ising Model (RFIM) has been extensively studied as a model system for understanding the effects of disorder in magnets. Since the late 1970s, there has been a particular focus on realizations of the RFIM in site-diluted antiferromagnets. We observe random-field effects in the dilute dipole-coupled ferromagnet $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$. In the presence of a magnetic field transverse to the Ising axis ($H_t$), the behavior of $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$ becomes increasingly dominated by the influence of random-field terms in the effective Hamiltonian. This is seen experimentally in the shape of the ferromagentic-paramagnetic phase boundary and in changes to the critical exponents near the classical critical point. We find that above the classical critical point the magnetic susceptibility diverges as $H_t\to0$, and that the susceptibility both above and below the classical critical point can be collapsed onto a single universal curve using a modified Curie law which explicitly incorporates random-field contributions. The discovery of a ferromagnetic realization of the RFIM opens the door to investigation of the random-field problem with the wide variety of techniques available for probing ferromagnets, including the ability to examine both the statics and dynamics of the random-field problem. It also allows studying the effects of controlled amounts of randomness on the dynamics of domain pinning and the energetics of domain reversal.",0801.2335v1 2008-02-14,"Nonvolatile ferroelectric control of ferromagnetism in (Ga,Mn)As","There is currently much interest in materials and structures that provide coupled ferroelectric and ferromagnetic responses, with a long-term goal of developing new memories and spintronic logic elements. Within the field there is a focus on composites coupled by magnetostrictive and piezoelectric strain transmitted across ferromagnetic-ferroelectric interfaces, but substrate clamping limits the response in the supported multilayer configuration favoured for devices. This constraint is avoided in a ferroelectric-ferromagnetic bilayer in which the magnetic response is modulated by the electric field of the poled ferroelectric. Here, we report the realization of such a device using a diluted magnetic semiconductor (DMS) channel and a polymer ferroelectric gate. Polarization reversal of the gate by a single voltage pulse results in a persistent modulation of the Curie temperature as large as 5%. The device demonstrates direct and quantitatively understood electric-fieldmediated coupling in a multiferroic bilayer and may provide new routes to nanostructured DMS materials and devices via ferroelectric domain nanopatterning. The successful implementation of a polymer-ferroelectric gate fieldeffect transistor (FeFET) with a DMS channel adds a new functionality to semiconductor spintronics and may be of importance for future low-voltage spintronics devices and memory structures.",0802.2074v1 2008-02-22,Thermodynamics of Heisenberg ferromagnets with arbitrary spin in a magnetic field,"The thermodynamic properties (magnetization, magnetic susceptibility, transverse and longitudinal correlation lengths, specific heat) of one- and two-dimensional ferromagnets with arbitrary spin S in a magnetic field are investigated by a second-order Green-function theory. In addition, quantum Monte Carlo simulations for S= 1/2 and S=1 are performed using the stochastic series expansion method. A good agreement between the results of both approaches is found. The field dependence of the position of the maximum in the temperature dependence of the susceptibility fits well to a power law at low fields and to a linear increase at high fields. The maximum height decreases according to a power law in the whole field region. The longitudinal correlation length may show an anomalous temperature dependence: a minimum followed by a maximum with increasing temperature. Considering the specific heat in one dimension and at low magnetic fields, two maxima in its temperature dependence for both the S= 1/2 and S = 1 ferromagnets are found. For S>1 only one maximum occurs, as in the two-dimensional ferromagnets. Relating the theory to experiments on the S= 1/2 quasi-one-dimensional copper salt TMCuC [(CH_3)_4NCuCl_3], a fit to the magnetization as a function of the magnetic field yields the value of the exchange energy which is used to make predictions for the occurrence of two maxima in the temperature dependence of the specific heat.",0802.3395v1 2008-04-10,Near-Zero Moment Ferromagnetism in the Semiconductor SmN,"The magnetic behaviour of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis below T_c. The ferromagnetic state is characterised by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetisation at 2 K, measured after cooling in the maximum field, is 0.035 mu_B per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for Sm3+ in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field . The susceptibility in the paramagnetic phase shows temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the J=7/2 excited state and the 5/2 ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of 0.4 mu_B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm3+ ions.",0804.1595v3 2008-05-03,Intrinsic room temperature ferromagnetism in Co-implanted ZnO,"We report on the structural and magnetic properties of a cobalt-implanted ZnO film grown on a sapphire substrate. X-ray diffraction and transmission electron microscopy reveal the presence of a (10-10)-oriented hexagonal Co phase in the Al2O3 sapphire substrate, but not in the ZnO film. Co clusters, with a diameter of is about 5-6 nm, form a Co rich layer in the substrate close to the ZnO/Al2O3 interface. Magnetization measurements indicate that there exist two different magnetic phases in the implanted region. One originates from the Co clusters in Al2O3, the other one belongs to a homogeneous ferromagnetic phase with a ferromagnetic Curie temperature far above room temperature and can be attributed to Co substitution on Zn sites in the ZnO layer. We have observed magnetic dichroism at the Co L2,3 and O K edges at room temperature as well as the multiplet structure in x-ray absorption spectra around the Co L3 edge, supporting the intrinsic nature of the observed ferromagnetism in Co-implanted ZnO film. The magnetic moment per substituted cobalt is found about 2.81 Bohr magneton which is very close to the theoretical expected value of 3 Bohr magneton per Co atom for Co 2+ in its high spin state.",0805.0361v2 2008-05-26,"Dopant-dependent impact of Mn-site doping on the critical-state manganites: R0.6Sr0.4MnO3 (R=La, Nd, Sm, and Gd)","Versatile features of impurity doping effects on perovskite manganites, $R_{0.6}$Sr$_{0.4}$MnO$_{3}$, have been investigated with varying the doing species as well as the $R$-dependent one-electron bandwidth. In ferromagnetic-metallic manganites ($R$=La, Nd, and Sm), a few percent of Fe substitution dramatically decreases the ferromagnetic transition temperature, leading to a spin glass insulating state with short-range charge-orbital correlation. For each $R$ species, the phase diagram as a function of Fe concentration is closely similar to that for $R_{0.6}$Sr$_{0.4}$MnO$_{3}$ obtained by decreasing the ionic radius of $R$ site, indicating that Fe doping in the phase-competing region weakens the ferromagnetic double-exchange interaction, relatively to the charge-orbital ordering instability. We have also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating manganite ($R$=Gd). There, the impurity-induced ferromagnetic magnetization is observed at low temperatures as a consequence of the collapse of the inherent short-range charge-orbital ordering, while Fe doping plays only a minor role. The observed opposite nature of impurity doping may be attributed to the difference in magnitude of the antiferromagnetic interaction between the doped ions.",0805.3966v1 2008-12-04,One-dimensional Ising ferromagnet frustrated by long-range interactions at finite temperatures,"We consider a one-dimensional lattice of Ising-type variables where the ferromagnetic exchange interaction J between neighboring sites is frustrated by a long-ranged anti-ferromagnetic interaction of strength g between the sites i and j, decaying as |i-j|^-alpha, with alpha>1. For alpha smaller than a certain threshold alpha_0, which is larger than 2 and depends on the ratio J/g, the ground state consists of an ordered sequence of segments with equal length and alternating magnetization. The width of the segments depends on both alpha and the ratio J/g. Our Monte Carlo study shows that the on-site magnetization vanishes at finite temperatures and finds no indication of any phase transition. Yet, the modulation present in the ground state is recovered at finite temperatures in the two-point correlation function, which oscillates in space with a characteristic spatial period: The latter depends on alpha and J/g and decreases smoothly from the ground-state value as the temperature is increased. Such an oscillation of the correlation function is exponentially damped over a characteristic spatial scale, the correlation length, which asymptotically diverges roughly as the inverse of the temperature as T=0 is approached. This suggests that the long-range interaction causes the Ising chain to fall into a universality class consistent with an underlying continuous symmetry. The e^(Delta/T)-temperature dependence of the correlation length and the uniform ferromagnetic ground state, characteristic of the g=0 discrete Ising symmetry, are recovered for alpha > alpha_0.",0812.0907v2 2008-12-16,Evidence for reversible control of magnetization in a ferromagnetic material via spin-orbit magnetic field,"Conventional computer electronics creates a dichotomy between how information is processed and how it is stored. Silicon chips process information by controlling the flow of charge through a network of logic gates. This information is then stored, most commonly, by encoding it in the orientation of magnetic domains of a computer hard disk. The key obstacle to a more intimate integration of magnetic materials into devices and circuit processing information is a lack of efficient means to control their magnetization. This is usually achieved with an external magnetic field or by the injection of spin-polarized currents. The latter can be significantly enhanced in materials whose ferromagnetic properties are mediated by charge carriers. Among these materials, conductors lacking spatial inversion symmetry couple charge currents to spin by intrinsic spin-orbit (SO) interactions, inducing nonequilibrium spin polarization tunable by local electric fields. Here we show that magnetization of a ferromagnet can be reversibly manipulated by the SO-induced polarization of carrier spins generated by unpolarized currents. Specifically, we demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor.",0812.3160v2 2009-03-17,In search for the superconducting spin-switch: Magnetization induced resistance switching effects in La$_{0.67}$Sr$_{0.33}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ bi- and trilayers,"We have studied the influence of the magnetization on the superconducting transition temperature ($T_c$) in bi- and trilayers consisting of the half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) and the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO). We have made use of tilted epitaxial growth in order to achieve contacts between the two materials that are partly in the crystallographic $ab$-plane of the YBCO. As a result of uniaxial magnetic anisotropy in the tilted structures, we observe sharp magnetization switching behavior. At temperatures close to $T_c$, the magnetization switching induces resistance jumps in trilayers, resulting in a magnetization dependence of $T_c$. In bilayers, this switching effect can be observed as well, provided that the interface to the ferromagnetic layer is considerably rough. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor. A simple model describes the observed behavior well. We find no evidence that the switching behavior is caused by a so-called superconducting spin-switch, nor by accumulation of spin-polarized electrons. Observation of magnetic coupling of the ferromagnetic layers, through the superconductor, supports the idea of field induced resistance switching.",0903.2993v1 2009-03-18,Superconductor-ferromagnet nanocomposites created by co-deposition of niobium and dysprosium,"We have created superconductor-ferromagnet composite films in order to test the enhancement of critical current density, Jc, due to magnetic pinning. We co-sputter the type-II superconductor niobium (Nb) and the low-temperature ferromagnet dysprosium (Dy) onto a heated substrate; the immiscibility of the two materials leads to a phase-separated composite of magnetic regions within a superconducting matrix. Over a range of compositions and substrate temperatures, we achieve phase separation on scales from 5 nm to 1 micron. The composite films exhibit simultaneous superconductivity and ferromagnetism. Transport measurements show that while the self-field Jc is reduced in the composites, the in-field Jc is greatly enhanced up to the 3 T saturation field of Dy. In one instance, the phase separation orders into stripes, leading to in-plane anisotropy in Jc.",0903.3142v2 2009-05-19,Magnetism in Nb(1-y)Fe(2+y) - composition and magnetic field dependence,"We present a systematic study of transport and thermodynamic properties of the Laves phase system Nb$_{1-y}$Fe$_{2+y}$. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for $\mid y\mid\geq 0.02$), show bulk ferromagnetism at low temperature. For stoichiometric NbFe$_2$, on the other hand, magnetization, magnetic susceptibility and magnetoresistance results point towards spin-density wave (SDW) order, possibly helical, with a small ordering wavevector $Q \sim 0.05$ \AA$^{-1}$. Our results suggest that on approaching the stoichiometric composition from the iron-rich side, ferromagnetism changes into long-wavelength SDW order. In this scenario, $Q$ changes continuously from 0 to small, finite values at a Lifshitz point in the phase diagram, which is located near $y=+0.02$. Further reducing the Fe content suppresses the SDW transition temperature, which extrapolates to zero at $y\approx -0.015$. Around this Fe content magnetic fluctuations dominate the temperature dependence of the resistivity and of the heat capacity which deviate from their conventional Fermi liquid forms, inferring the presence of a quantum critical point. Because the critical point is located between the SDW phase associated with stoichiometric NbFe$_2$ and the ferromagnetic order which reemerges for very Nb-rich NbFe$_2$, the observed temperature dependences could be attributed both to proximity to SDW order or to ferromagnetism.",0905.3051v1 2009-06-02,Correlation and confinement induced itinerant ferromagnetism in chain structures,"Using a positive semidefinite operator technique one deduces exact ground states for a zig-zag hexagon chain described by a non-integrable Hubbard model with on-site repulsion. Flat bands are not present in the bare band structure, and the operators $\hat B^{\dagger}_{\mu,\sigma}$ introducing the electrons into the ground state, are all extended operators and confined in the quasi 1D chain structure of the system. Consequently, increasing the number of carriers, the $\hat B^{\dagger}_{\mu,\sigma}$ operators become connected i.e. touch each other on several lattice sites. Hence the spin projection of the carriers becomes correlated in order to minimize the ground state energy by reducing as much as possible the double occupancy leading to a ferromagnetic ground state. This result demonstrates in exact terms in a many-body frame that the conjecture made at two-particle level by G. Brocks et al. [Phys.Rev.Lett.93,146405,(2004)] that the Coulomb interaction is expected to stabilize correlated magnetic ground states in acenes is clearly viable, and opens new directions in the search for routes in obtaining organic ferromagnetism. Due to the itinerant nature of the obtained ferromagnetic ground state, the systems under discussion may have also direct application possibilities in spintronics.",0906.0520v1 2009-06-08,SU(2) slave-boson formulation of spin nematic states in S=1/2 frustrated ferromagnets,"An SU(2) slave boson formulation of bond-type spin nematic orders is developed in frustrated ferromagnets, where the spin nematic states are described as the resonating spin-triplet valence bond (RVB) states. The d-vectors of spin-triplet pairing ansatzes play the role of the directors in the bond-type spin quadrupolar states. The low-energy excitations around such spin-triplet RVB ansatzes generally comprise the (potentially massless) gauge bosons, massless Goldstone bosons, and spinon individual excitations. Extending the projective symmetry group argument to the spin-triplet ansatzes, we show how to identify the number of massless gauge bosons efficiently. Applying this formulation, we next (i) enumerate possible mean field solutions for the S=1/2 ferromagnetic J1-J2 Heisenberg model on the square lattice, with ferromagnetic nearest neighbor J1 and competing antiferromagnetic next-nearest neighbor J2, and (ii) argue their stability against small gauge fluctuations. As a result, two stable spin-triplet RVB ansatzes are found in the intermediate coupling regime around J1:J2 \simeq 1:0.4. One is the Z_2 Balian-Werthamer (BW) state stabilized by the Higgs mechanism and the other is the SU(2) chiral p-wave (Anderson-Brinkman-Morel) state stabilized by the Chern-Simon mechanism. The former Z_2 BW state in fact shows the same bond-type spin quadrupolar order as found in the previous exact diagonalization study [N. Shannon et al., Phys. Rev. Lett. 96, 027213 (2006)].",0906.1248v3 2009-06-17,The Kondo effect in ferromagnetic atomic contacts,"Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the $s$ and $p$ electrons, whereas the magnetic moments are mostly in the narrow $d$-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system;this appears as a Fano-Kondo resonance in the conductance characteristics as observed in other artificial nanostructures. The study of hundreds of contacts shows material-dependent lognormal distributions of the resonance width that arise naturally from Kondo theory. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems. Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.",0906.3135v1 2009-07-12,Odd triplet superconductivity in clean and moderately disordered SFFS junctions,"We study the Josephson effect and pairing correlations in SFFS junctions that consist of conventional superconductors (S) connected through two metallic monodomain ferromagnets (F) with transparent and spin inactive interfaces. We solve the Eilenberger equations for arbitrary relative orientation of magnetizations of the two F layers in the clean limit and for moderate disorder in ferromagnets. Spatial variation of pair amplitudes, singlet $f_s$, and odd in frequency triplet $f_{t0}$ and $f_{t1}$, with 0 and $\pm1$ spin projections, as well as the Josephson current-phase relations are calculated for different values of the ferromagnetic layers thickness and angle $\alpha$ between in-plane magnetizations. In contrast to the dirty limit case, we find that for $0<\alpha<\pi$ both spin singlet and triplet pair amplitudes in F layers power-law decay in the same oscillatory manner with distance from the FS interfaces. This decay gets faster as the impurity-scattering rate in ferromagnets is increased. The computed triplet amplitude $f_{t1}$ has opposite signs in the two magnet regions, penetrates into the superconductors and monotonically decays over the same distance, which is the superconducting coherence length, as the singlet amplitude $f_s$ saturates to the bulk value. We point out that influence of misorientation of magnetizations on the Josephson current can not be attributed directly to the appearance of odd triplet correlations.",0907.2042v1 2009-09-13,"Unconventional ferromagnetism and transport properties of (In,Mn)Sb dilute magnetic semiconductor","Narrow-gap higher mobility semiconducting alloys In_{1-x}Mn_{x}Sb were synthesized in polycrystalline form and their magnetic and transport properties have been investigated. Ferromagnetic response in In_{0.98}Mn_{0.02}Sb was detected by the observation of clear hysteresis loops up to room temperature in direct magnetization measurements. An unconventional (reentrant) magnetization versus temperature behavior has been found. We explained the observed peculiarities within the frameworks of recent models which suggest that a strong temperature dependence of the carrier density is a crucial parameter determining carrier-mediated ferromagnetism of (III,Mn)V semiconductors. The correlation between magnetic states and transport properties of the sample has been discussed. The contact spectroscopy method is used to investigate a band structure of (InMn)Sb near the Fermi level. Measurements of the degree of charge current spin polarization have been carried out using the point contact Andreev reflection (AR) spectroscopy. The AR data are analyzed by introducing a quasiparticle spectrum broadening, which is likely to be related to magnetic scattering in the contact. The AR spectroscopy data argued that at low temperature the sample is decomposed on metallic ferromagnetic clusters with relatively high spin polarization of charge carriers (up to 65% at 4.2K) within a cluster.",0909.2407v2 2009-09-23,Phase-controlled proximity-effect in ferromagnetic Josephson junctions: calculation of DOS and electronic specific heat,"We study the thermodynamic properties of a dirty ferromagnetic S$\mid$F$\mid$S Josephson junction with s-wave superconducting leads in the low-temperature regime. We employ a full numerical solution with a set of realistic parameters and boundary conditions, considering both a uniform and non-uniform exchange field in the form of a Bloch domain wall ferromagnetic layer. The influence of spin-active interfaces is incorporated via a microscopic approach. We mainly focus on how the electronic specific heat and density of states (DOS) of such a system is affected by the \textit{proximity effect}, which may be tuned via the superconducting phase difference. Our main result is that it is possible to \textit{strongly modify the electronic specific heat} of the system by changing the phase difference between the two superconducting leads from 0 up to nearly $\pi$ at low temperatures. An enhancement of the specific heat will occur for small values $h\simeq\Delta$ of the exchange field, while for large values of $h$ the specific heat is suppressed by increasing the phase difference between the superconducting leads. These results are all explained in terms of the proximity-altered DOS in the ferromagnetic region, and we discuss possible methods for experimental detection of the predicted effect.",0909.4323v1 2009-10-04,Quasi-Ferromagnet Spintronics in Graphene Nanodisk-Lead System,"A zigzag graphene nanodisk can be interpreted as a quantum dot with an internal degree of freedom. It is well described by the infinite-range Heisenberg model. We have investigated its thermodynamical properties. There exists a quasi-phase transition between the quasi-ferromagnet and quasi-paramagnet states, as signaled by a sharp peak in the specific heat and in the susceptability. We have also analyzed how thermodynamical properties are affected when two leads are attached to the nanodisk. It is shown that lead effects are described by the many-spin Kondo Hamiltonian. There appears a new peak in the specific heat, and the multiplicity of the ground state becomes just one half of the system without leads. Another lead effect is to enhance the ferromagnetic order. Being a ferromagnet, a nanodisk can be used as a spin filter. Furthermore, since the relaxation time is finite, it is possible to control the spin of the nanodisk by an external spin current. We then propose a rich variety of spintronic devices made of nanodisks and leads, such as spin memory, spin amplifier, spin valve, spin-field-effect transistor, spin diode and spin logic gates such as spin-XNOR gate and spin-XOR gate. Graphene nanodisks could well be basic components of future nanoelectronic and spintronic devices.",0910.0593v1 2009-10-15,Spin Transfer from the point of view of the ferromagnetic degrees of freedom,"Spintronics is the generic term that describes magnetic systems coupled to an electric generator, taking into account the spin attached to the charge carriers. For this topical review of {\it Spin Caloritronics}, we focus our attention on the study of {\it irreversible processes} occuring in spintronic devices, that involve both the spins of the conduction electrons and the ferromagnetic degrees of freedom. The aim of this report is to clarify the nature of the different kinds of power dissipated in metallic ferromagnets contacted to an electric generator, and to exploit it in the framework of the theory of mesoscopic non-equilibrium thermodynamics. The expression of the internal power (i.e. the internal entropy production multiplied by the temperature) dissipated by a generic system connected to different reservoirs, allows the corresponding kinetic equations to be derived with the introduction of the relevant phenomenological kinetic coefficients. After derivation of the kinetic equations for the ferromagnetic degrees of freedom (i.e. the Landau-Lifshitz equation) and the derivation of the kinetic equations for the spin-accumulation effects (within a two channel model), the kinetic equations describing spin-transfer are obtained. Both spin-dependent relaxation (usual spin-accumulation) and spin-precession in quasi-ballistic regime (transverse spin-accumulation) are taken into account. The generalization of the Landau-Lifshitz equation to spin-accumulation is then performed with the introduction of two potential energy terms, that are experimentally accessible.",0910.2890v2 2009-10-17,Density functional study of the actinide nitrides,"The full potential all electron linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K, has been used to systematically investigate the structural, electronic, and magnetic properties of the actinide compounds AnN (An = Ac, Th, Pa, U, Np, Pu, Am). The theoretical formalism used is the generalized gradient approximation to density functional theory (GGA-DFT) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Each compound has been studied at six levels of theory: non-magnetic (NM), non-magnetic with spin-orbit coupling (NM+SOC), ferromagnetic (FM), ferromagnetic with spin-orbit coupling (FM+SOC), anti-ferromagnetic (AFM), and anti-ferromagnetic with spin-orbit coupling (AFM+SOC). The structural parameters, bulk moduli, densities of states, and charge distributions have been computed and compared to available experimental data and other theoretical calculations published in the literature. The total energy calculations indicate that the lowest energy structures of AcN, ThN, and PaN are degenerate at the NM+SOC, FM+SOC, and AFM+SOC levels of theory with vanishing total magnetic moments in the FM+SOC and AFM+SOC cases, making the ground states essentially non-magnetic with spin-orbit interaction. The ground states of UN, NpN, PuN, and AmN are found to be FM+SOC at the level of theory used in the present computations. The nature of the interactions between the actinide metals and nitrogen atom, and the implications on 5f electron delocalization and localization are discussed in detail.",0910.3337v1 2009-11-25,Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and πsegments,"Josephson junctions with ferromagnetic barrier can have positive or negative critical current depending on the thickness $d_F$ of the ferromagnetic layer. Accordingly, the Josephson phase in the ground state is equal to 0 (a conventional or 0 junction) or to $\pi$ ($\pi$ junction). When 0 and $\pi$ segments are joined to form a ""0-$\pi$ junction"", spontaneous supercurrents around the 0-$\pi$ boundary can appear. Here we report on the visualization of supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS) junctions by low-temperature scanning electron microscopy (LTSEM). We discuss data for rectangular 0, $\pi$, 0-$\pi$, 0-$\pi$-0 and 20 \times 0-$\pi$ junctions, disk-shaped junctions where the 0-$\pi$ boundary forms a ring, and an annular junction with two 0-$\pi$ boundaries. Within each 0 or $\pi$ segment the critical current density is fairly homogeneous, as indicated both by measurements of the magnetic field dependence of the critical current and by LTSEM. The $\pi$ parts have critical current densities $j_c^\pi$ up to $35\units{A/cm^2}$ at $T = 4.2\units{K}$, which is a record value for SIFS junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology is capable to produce Josephson devices with a unique topology of the 0-$\pi$ boundary.",0911.4831v1 2010-02-01,Spin Torques in Point Contacts to Exchange-Biased Ferromagnetic Films,"Hysteretic magneto-resistance of point contacts formed between non-magnetic tips and single ferromagnetic films exchange-pinned by antiferromagnetic films is investigated. The analysis of the measured current driven and field driven hysteresis agrees with the recently proposed model of the surface spin-valve, where the spin orientation at the interface can be different from that in the bulk of the film. The switching in magneto-resistance at low fields is observed to depend significantly on the direction of the exchange pinning, which allows identifying this transition as a reversal of interior spins of the pinned ferromagnetic films. The switching at higher fields is thus due to a spin reversal in the point contact core, at the top surface of the ferromagnet, and does not exhibit any clear field offset when the exchange-pinning direction or the magnetic field direction is varied. This magnitude of the switching field of the surface spins varies substantially from contact to contact and sometimes from sweep to sweep, which suggests that the surface coercivity can change under very high current densities and/or due to the particular microstructure of the point contact. In contrast, no changes in the effect of the exchange biasing on the interior spins are observed at high currents, possibly due to the rapid drop in the current density away from nanometer sized point contact cores.",1002.0209v1 2010-02-19,Electronic transport through a graphene-based ferromagnetic/normal/ferromagnetic junction,"Electronic transport in a graphene-based ferromagnetic/normal/ferromagnetic junction is investigated by means of Landauer-B\""{u}ttiker formulism and the nonequilibrium Green's function technique. For the zigzag edge case, the results show that the conductance is always larger than $e^{2}/h$ for the parallel configuration of lead magnetizations, but for the antiparallel configuration the conductance becomes zero because of the band-selective rule. So a magnetoresistance (MR) plateau emerges with the value 100% when the Fermi energy is located around the Dirac point. Besides, choosing narrower graphene ribbons can obtain the wider 100% MR plateaus and the length change of the central graphene region does not affect the 100% MR plateaus. Although the disorder will reduce the MR plateau, the plateau value can be still kept about 50% even in a large disorder strength case. In addition, when the magnetizations of the left and right leads have a relative angle, the conductance changes as a cosine function of the angle. What is more, for the armchair edge case, the MR is usually small. So, it is more favorable to fabricate the graphene-based spin valve device by using the zigzag edge graphene ribbon.",1002.3665v1 2010-03-10,"Diffuse Neutron Scattering Study of Magnetic Correlations in half-doped La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) Manganites","The short range ordered magnetic correlations have been studied in half doped La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) compounds by polarized neutron scattering technique. On doping Sr2+ for Ca2+ ion, these compounds with x = 0.1, 0.3, and 0.4 exhibit CE-type, mixture of CE-type and A-type, and A-type antiferromagnetic ordering, respectively. Magnetic diffuse scattering is observed in all the compounds above and below their respective magnetic ordering temperatures and is attributed to magnetic polarons. The correlations are primarily ferromagnetic in nature above T\_N, although a small antiferromagnetic contribution is also evident. Additionally, in samples x = 0.1 and 0.3 with CE-type antiferromagnetic ordering, superlattice diffuse reflections are observed indicating correlations between magnetic polarons. On lowering temperature below T\_N the diffuse scattering corresponding to ferromagnetic correlations is suppressed and the long range ordered antiferromagnetic state is established. However, the short range ordered correlations indicated by enhanced spin flip scattering at low Q coexist with long range ordered state down to 3K. In x = 0.4 sample with A-type antiferromagnetic ordering, superlattice diffuse reflections are absent. Additionally, in comparison to x = 0.1 and 0.3 sample, the enhanced spin flip scattering at low Q is reduced at 310K, and as temperature is reduced below 200K, it becomes negligibly low. The variation of radial correlation function, g(r) with temperature indicates rapid suppression of ferromagnetic correlations at the first nearest neighbor on approaching TN. Sample x = 0.4 exhibits growth of ferromagnetic phase at intermediate temperatures (~ 200K). This has been further explored using SANS and neutron depolarization techniques.",1003.2026v1 2010-03-17,"Kondo behavior, ferromagnetic correlations, and crystal fields in the heavy Fermion compounds Ce3X (X=In, Sn)","We report measurements of inelastic neutron scattering, magnetic susceptibility, magnetization, and the magnetic field dependence of the specific heat for the heavy Fermion compounds Ce$_3$In and Ce$_3$Sn. The neutron scattering results show that the excited crystal field levels have energies $E_1$ = 13.2 meV, $E_2$ = 44.8 meV for Ce$_3$In and $E_1$ = 18.5 meV, $E_2$ = 36.1 meV for Ce$_3$Sn. The Kondo temperature deduced from the quasielastic linewidth is 17 K for Ce$_3$In and 40 K for Ce$_3$Sn. The low temperature behavior of the specific heat, magnetization, and susceptibility can not be well-described by J=1/2 Kondo physics alone, but require calculations that include contributions from the Kondo effect, broadened crystal fields, and ferromagnetic correlations, all of which are known to be important in these compounds. We find that in Ce$_3$In the ferromagnetic fluctuation makes a 10-15 % contribution to the ground state doublet entropy and magnetization. The large specific heat coefficient $\gamma$ in this heavy fermion system thus arises more from the ferromagnetic correlations than from the Kondo behavior.",1003.3462v1 2010-04-09,Epitaxial ferromagnetic semiconductor GdN thin films on Si substrate,This paper has been withdrawn by the author,1004.1545v2 2010-04-22,Magnetic excitations in the metallic single-layer Ruthenates Ca(2-x)Sr(x)RuO(4) studied by inelastic neutron scattering,"By inelastic neutron scattering, we have analyzed the magnetic correlations in the paramagnetic metallic region of the series Ca(2-x)Sr(x)RuO(4), 0.2<=x<=0.62. We find different contributions that correspond to 2D ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors (0.11,0,0), (0.26,0,0) and (0.3,0.3,0). These components constitute the measured response as function of the Sr-concentration x, of the magnetic field and of the temperature. A generic model is applicable to metallic Ca(2-x)Sr(x)RuO(4) close to the Mott transition, in spite of their strongly varying physical properties. The amplitude, characteristic energy and width of the incommensurate components vary only little as function of x, but the ferromagnetic component depends sensitively on concentration, temperature and magnetic field. While ferromagnetic fluctuations are very strong in Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T=1.5 K, they are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of a magnetic field and form a magnon mode above the metamagnetic transition. The inelastic neutron scattering results document how the competition between ferromagnetic and incommensurate antiferromagnetic instabilities governs the physics of this system.",1004.3957v1 2010-06-18,Probing the magnetic state by linear and non linear ac magnetic susceptibility measurements in under doped manganite Nd0.8Sr0.2MnO3,"We have thoroughly investigated the entire magnetic states of under doped ferromagnetic insulating manganite Nd0.8Sr0.2MnO3 through temperature dependent linear and non linear complex ac magnetic susceptibility measurements. This ferromagnetic insulating manganite is found to have frequency independent ferromagnetic to paramagnetic transition temperature at around 140 K. At around 90 K (\approx T_f) the sample shows a second frequency dependent re - entrant magnetic transition as explored through complex ac susceptibility measurements. Non linear ac susceptibility measurements (higher harmonics of ac susceptibility) have also been performed (with and without the superposition of a dc magnetic field) to further investigate the origin of this frequency dependence (dynamic behavior at this re-entrant magnetic transition). Divergence of 3rd order susceptibility in the limit of zero exciting field indicates a spin glass like freezing phenomena. However, large value of spin relaxation time (?0= 10-8 s) and small value of coercivity (~22 Oe) obtained at low temperature (below T_f) from critical slowing down model and dc magnetic measurements, respectively, are in contrast with what generally observed in a canonical spin glass (?0 = 10-12 - 10-14 s and very large value of coercivity below freezing temperature). We have attributed our observation to the formation of finite size ferromagnetic clusters which are formed as consequence of intrinsic separation and undergo cluster glass like freezing below certain temperature in this under doped manganite. The results are supported by the electronic - and magneto - transport data.",1006.3733v1 2010-07-19,Magnetic properties of pseudomorphic epitaxial films of Pr_{0.7}Ca_{0.3}MnO_3 under different biaxial tensile stresses,"In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.",1007.3078v1 2010-08-02,Spiral correlations in frustrated one-dimensional spin-1/2 Heisenberg J1-J2-J3 ferromagnets,"We use the coupled cluster method for infinite chains complemented by exact diagonalization of finite periodic chains to discuss the influence of a third-neighbor exchange J3 on the ground state of the spin-1/2 Heisenberg chain with ferromagnetic nearest-neighbor interaction J1 and frustrating antiferromagnetic next-nearest-neighbor interaction J2. A third-neighbor exchange J3 might be relevant to describe the magnetic properties of the quasi-one-dimensional edge-shared cuprates, such as LiVCuO4 or LiCu2O2. In particular, we calculate the critical point J2^c as a function of J3, where the ferromagnetic ground state gives way for a ground state with incommensurate spiral correlations. For antiferromagnetic J3 the ferro-spiral transition is always continuous and the critical values J2^c of the classical and the quantum model coincide. On the other hand, for ferromagnetic J3 \lesssim -(0.01...0.02)|J1| the critical value J2^c of the quantum model is smaller than that of the classical model. Moreover, the transition becomes discontinuous, i.e. the model exhibits a quantum tricritical point. We also calculate the height of the jump of the spiral pitch angle at the discontinuous ferro-spiral transition.",1008.0317v2 2010-08-30,"Intriguing complex magnetism of Co in RECoAsO (RE=La, Nd and Sm)","We synthesized bulk polycrystalline samples of RECoAsO (RE=La, Nd and Sm) by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Co, in these compounds is in itinerant ferromagnetic state with its paramagnetic moment above 1.5 microB and the same orders ferromagnetically (FM) with small saturation moment of around 0.20 microB below say 80K. This bulk intrinsic magnetism of Co changes dramatically when nonmagnetic La is changed by magnetic Sm and Nd. Although the itinerant ferromagnetism occurs below 80-100K with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 57K and 45K for Sm and at 69K and 14K for Nd. The transition of Co spins from FM to AFM, for magnetic Sm and Nd in RECoAsO is both field and temperature dependent. For applied fields below 100Oe, both TN1 and TN2 are seen, with intermediate fields below 1-2kOe only TN1 and above say 5kOe the AFM transition is not observed. This is evidenced in isothermal magnetization (MH) plots as well. It is clear that Sm/Nd magnetic moments interact with the ordered Co spins in adjacent layer and thus transforms the FM ordering to AFM. All the studied compounds are metallic in nature, and their magneto-transport R(T)H follows the temperature and field dependent FM-AFM transition of ordered Co spins.",1008.4999v3 2010-09-15,Magnetocaloric effect in manganites: metamagnetic transitions for magnetic refrigeration,"We present a study of the magnetocaloric effect in La5/8-yPryCa3/8MnO3 (y=0.3) and Pr0.5Ca0.09Sr0.41MnO3 manganites. The low temperature state of both ystems is the result of a competition between the antiferromagnetic and ferromagnetic phases. The samples display magnetocaloric effect evidenced in an adiabatic temperature change during a metamagnetic transition from an antiferromagnetic to a ferromagnetic phase . As additional features, La5/8-yPryCa3/8MnO3 exhibits phase separation characterized by the coexistence of antiferromagnetic and ferromagnetic phases and Pr0.5Ca0.09Sr0.41MnO3 displays inverse magnetocaloric effect in which temperature decreases while applying an external magnetic field. In both cases, a significant part of the magnetocaloric effect appears from non-reversible processes. As the traditional thermodynamic description of the effect usually deals with reversible transitions, we developed an alternative way to calculate the adiabatic temperature change in terms of the change of the relative ferromagnetic fraction induced by magnetic field. To evaluate our model, we performed direct measurement of the sample's adiabatic temperature change by means of a differential thermal analysis. An excellent agreement has been obtained between experimental and calculated data. These results show that metamagnetic transition in manganites play an important role in the study of magnetic refrigeration.",1009.2914v1 2010-10-04,Defects induced ferromagnetism in Mn doped ZnO,"Single phase Mn doped (2 at %) ZnO samples have been synthesized by solid-state reaction technique. Before the final sintering at 500 C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 hours). The grain sizes of the samples are very close to each other (~ 32 \pm 4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value for saturation magnetization (0.11 {\mu}_B / Mn atom) was achieved for 96 hours milled sample. Electrical resistivity has been found to increase with increasing milling time. The most resistive sample bears the largest saturation magnetization. Variation of average positron lifetime with milling time bears a close similarity with that of the saturation magnetization. This indicates the key role played by open volume vacancy defects, presumably zinc vacancies near grain surfaces, in inducing ferromagnetic order in Mn doped ZnO. To attain optimum defect configuration favorable for ferromagnetism in this kind of samples proper choice of milling period and annealing conditions is required.",1010.0547v1 2010-12-25,Charge inhomogeneities and transport in semiconductor heterostructures with a manganese $δ$-layer,"We study experimentally and theoretically the effects of disorder, nonlinear screening, and magnetism in semiconductor heterostructures containing a $\delta$-layer of Mn, where the charge carriers are confined within a quantum well and hence both ferromagnetism and transport are two-dimensional (2D) and differ qualitatively from their bulk counterparts. Anomalies in the electrical resistance observed in both metallic and insulating structures can be interpreted as a signature of significant ferromagnetic correlations. The insulating samples turn out to be the most interesting as they can give us valuable insights into the mechanisms of ferromagnetism in these heterostructures. At low charge carrier densities, we show how the interplay of disorder and nonlinear screening can result in the organization of the carriers in the 2D transport channel into charge droplets separated by insulating barriers. Based on such a droplet picture and including the effect of magnetic correlations, we analyze the transport properties of this set of droplets, compare it with experimental data, and find a good agreement between the model calculations and experiment. Our analysis shows that the peak or shoulder-like features observed in temperature dependence of resistance of 2D heterostructures $\delta$-doped by Mn lie significantly below the Curie temperature $T_{C}$ unlike the three-dimensional case, where it lies above and close to $T_{C}$. We also discuss the consequences of our description for understanding the mechanisms of ferromagnetism in the heterostructures under study.",1012.5456v1 2011-01-20,Ensemble Inequivalence in the Ferromagnetic p-spin Model in Random Fields,"We study the effect that randomness has on long-range interacting systems by using the ferromagnetic Ising model with $p$-body interactions in random fields. The case with p=2 yields a phase diagram similar to that of previously studied models and shows known features that inequivalence of the canonical and microcanonical ensembles brings with it, for example negative specific heat in a narrow region of the phase diagram. When p>2, however, the canonical phase diagram is completely different from the microcanonical one. The temperature does not necessarily determine the microcanonical phases uniquely, and thus the ferromagnetic and paramagnetic phases are not separated in such a region of a conventional phase diagram drawn with the temperature and field strength as the axes. Below a certain value of the external field strength, part of the ferromagnetic phase has negative specific heat. For large values of the external field strength the ergodicity is broken before the phase transition occurs for p>2. Moreover, for p>2, the Maxwell construction cannot be derived in a consistent manner and therefore, in contrast to previous cases with negative specific heat, the Maxwell construction does not bridge the gap between the ensembles.",1101.3809v2 2011-01-21,Multiple spin state analysis applied to graphite-like carbon-based ferromagnetism,"Recent experiments indicate room-temperature ferromagnetism in graphite like materials. This paper offers an multiple spin state analysis to find out the origine of ferromagnetism in case of nano meter size graphene molecule.First principle density function theory calculation (DFT-GGA with 631-G basis set) is applied to nano meter size asymmetric graphene fifteen molecules. Major results are,(1) Dihydrogenated zigzag edge molecule like C64H27 show that the most stable (lowest molecular energy) spin state is the highest one as Sz=5/2. Examples for spin density map of Sz=1/2,3/2 and 5/2 is shown in Fig.1. In other molecules like C56H24, C64H25, C64H22 and C64H23 also show the highest spin state most stable as shown in Fig.2. Energy difference between most stable spin state and next one overcome temperature difference 1000K,which suggests a stability of room temperature ferromagnetism. (2) Radical carbon zigzag edge molecules are also analysed. As illustrated in Fig.3, in every five molecule, also the highest spin state is most stable. (3) In contrast, nitrogen substituted molecules like C59N5H22, C61N3H22 etc. show opposite result,that is, the lowest spin state is most stable as shown in Fig.4. There are following three key issues to bring those results. (A) Edge specified localized spin arrangement. (B) Up-Up (also Down-Down) complex spin pairs inside of molecule. (C) Optimized atom position rearrangement depend on the spin state. Detailed mechanism will be discussed in the Symposium. Multiple spin state analysis is very useful to design carbon based ferro-magnet and also to design new spintronic devices.",1101.4082v1 2011-03-25,Effect of co-doping of donor and acceptor impurities in the ferromagnetic semiconductor Zn1-xCrxTe studied by soft x-ray magnetic circular dichroism,"We have performed x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the diluted ferromagnetic semiconductor Zn$_{1-\textit{x}}$Cr$_\textit{x}$Te doped with iodine (I) or nitrogen (N), corresponding to electron or hole doping, respectively. From the shape of the Cr $2p$ absorption peak in the XAS spectra, it was concluded that Cr ions in the undoped, I-doped and lightly N-doped samples are divalent (Cr$^{2+}$), while Cr$^{2+}$ and trivalent (Cr$^{3+}$) coexist in the heavily N-doped sample. This result indicates that the doped nitrogen atoms act as acceptors but that doped holes are located on the Cr ions. In the magnetic-field dependence of the XMCD signal at the Cr $2p$ absorption edge, ferromagnetic behaviors were observed in the undoped, I-doped, and lightly N-doped samples, while ferromagnetism was considerably suppressed in heavily N-doped sample, which is consistent with the results of magnetization measurements.",1103.4917v1 2011-04-24,Lambda- and Schottky-anomalies in solid-state phase transitions,"The origin of lambda and Schottky anomalies in solid-state phase transitions are analyzed and illustrated. They are shown to be the latent heat of nucleation-and-growth phase transitions.",1104.4637v1 2011-05-05,"Synthesis, magnetization and magneto transport study of RECoPO (RE = La, Nd & Sm)","We report the electrical, magneto transport and specific heat of the layered polycrystalline RECoPO (RE = La, Nd and Sm) samples. These compounds are iso-structural to recently discovered superconductor LaFeAs(O/F). Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Cobalt in these compounds is in itinerant state with its paramagnetic moment above 1.4\muB and the same orders ferromagnetically (FM) with saturation moment of around 0.20\muB below say 80K. Though, LaCoPO shows single paramagnetic (PM) to ferromagnetic (FM) transition near 35K, the NdCoPO and SmCoPO exhibit successive PM-FM-AFM transitions. Both FM and AFM transition temperatures vary with applied field. Although the itinerant ferromagnetism occurs with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 69K and 14K for Nd and 57K and 45K for Sm. This FM-AFM transition of Co spins in NdCoPO and SmCoPO is both field and temperature dependent. The Magneto-transport of NdCoPO and SmCoPO distinctly follows their successive PM-FM-AFM transitions. It is clear that Sm/Nd (4f) interacts with the Co (3d) in first time synthesized Sm/NdCoPO.",1105.0971v2 2011-08-16,Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface,"LaAlO3 and SrTiO3 are insulating, nonmagnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility,1 superconductivity at low temperatures,2-6 and electric-field-tuned metal-insulator and superconductorinsulator phase transitions.3,6-8 Bulk magnetization and magnetoresistance measurements also suggest some form of magnetism depending on preparation conditions5,9-11 and suggest a tendency towards nanoscale electronic phase separation.10 Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism, and superconductivity. We find submicron patches of ferromagnetism in a uniform background of paramagnetism, with a nonuniform, weak diamagnetic superconducting susceptibility at low temperature. These results demonstrate the existence of nanoscale phase separation as suggested by theoretical predictions based on nearly degenerate interface sub-bands associated with the Ti orbitals.12,13 The magnitude and temperature dependence of the paramagnetic response suggests that the vast majority of the electrons at the interface are localized, and do not contribute to transport measurements.3,6,7 In addition to the implications for magnetism, the existence of a 2D superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background suggests the potential for exotic superconducting phenomena.",1108.3150v1 2011-08-24,Carrier-independent ferromagnetism and giant anomalous Hall effect in magnetic topological insulator,"Breaking the time-reversal symmetry of a topological insulator (TI) by ferromagnetism can induce exotic magnetoelectric phenomena such as quantized anomalous Hall (QAH) effect. Experimental observation of QAH effect in a magnetically doped TI requires ferromagnetism not relying on the charge carriers. We have realized the ferromagnetism independent of both polarity and density of carriers in Cr-doped BixSb2-xTe3 thin films grown by molecular beam epitaxy. Meanwhile, the anomalous Hall effect is found significantly enhanced with decreasing carrier density, with the anomalous Hall angle reaching unusually large value 0.2 and the zero field Hall resistance reaching one quarter of the quantum resistance (h/e2), indicating the approaching of the QAH regime. The work paves the way to ultimately realize QAH effect and other unique magnetoelectric phenomena in TIs.",1108.4754v2 2011-10-11,Superposition of ferromagnetic and antiferromagnetic spin chains in the quantum magnet BaAg2Cu[VO4]2,"Based on density functional theory band structure calculations, quantum Monte-Carlo simulations, and high-field magnetization measurements, we address the microscopic magnetic model of BaAg2Cu[VO4]2 that was recently proposed as a spin-1/2 anisotropic triangular lattice system. We show that the actual physics of this compound is determined by a peculiar superposition of ferromagnetic and antiferromagnetic uniform spin chains with nearest-neighbor exchange couplings of Ja(1) ~ -19 K and Ja(2) ~ 9.5 K, respectively. The two chains featuring different types of the magnetic exchange perfectly mimic the specific heat of a triangular spin lattice, while leaving a clear imprint on the magnetization curve that is incompatible with the triangular-lattice model. Both ferromagnetic and antiferromagnetic spin chains run along the crystallographic 'a' direction, and slightly differ in the mutual arrangement of the magnetic CuO4 plaquettes and non-magnetic VO4 tetrahedra. These subtle structural details are, therefore, crucial for the ferromagnetic or antiferromagnetic nature of the exchange couplings, and put forward the importance of comprehensive microscopic modeling for a proper understanding of quantum spin systems in transition-metal compounds.",1110.2329v2 2012-03-30,Electron cooling in diffusive normal metal - superconductor tunnel junctions with a spin-valve ferromagnetic interlayer,"We investigate heat and charge transport through a diffusive SIF1F2N tunnel junction, where N (S) is a normal (superconducting) electrode, I is an insulator layer and F1,2 are two ferromagnets with arbitrary direction of magnetization. The flow of an electric current in such structures at subgap bias is accompanied by a heat transfer from the normal metal into the superconductor, which enables refrigeration of electrons in the normal metal. We demonstrate that the refrigeration efficiency depends on the strength of the ferromagnetic exchange field h and the angle {\alpha} between the magnetizations of the two F layers. As expected, for values of h much larger than the superconducting order parameter \Delta, the proximity effect is suppressed and the efficiency of refrigeration increases with respect to a NIS junction. However, for h \sim \Delta the cooling power (i.e. the heat flow out of the normal metal reservoir) has a non-monotonic behavior as a function of h showing a minimum at h \approx \Delta. We also determine the dependence of the cooling power on the lengths of the ferromagnetic layers, the bias voltage, the temperature, the transmission of the tunneling barrier and the magnetization misalignment angle {\alpha}.",1203.6735v1 2012-10-18,Magnetocaloric effect and Magnetothermopower in the room temperature ferromagnet Pr0.6Sr0.4MnO3,"We have investigated magnetization(M), magnetocaloric effect(MCE) and magnetothermopower(MTEP) in polycrystalline Pr0.6Sr0.4MnO3, which shows a second-order paramagnetic to ferromagnetic transition near room temperature (TC = 305 K). However, field-cooled M(T) within the long range ferromagnetic state shows an abrupt decrease at TS = 86 K for H < 3 T. The low temperature transition is first-order in nature as suggested by the hysteresis in M(T) and exothermic/endothermic peaks in differential thermal analysis for cooling and warming cycles. The anomaly at TS is attributed to a structural transition from orthorhombic to monoclinic phase. The magnetic entropy change is negative at TC but changes to positive at TS. Thermopower (Q) is negative from 350 K to 20 K, shows a rapid decrease at TC and a small cusp around TS in zero field. The MTEP reaches a maximum value of 25% for deltaH = 3 T around TC which is much higher than 15% dc magnetoresistance for the same field change. A linear relation between MTEP and magnetoresistance, and between delta Sm and Delta Q are found near TC. Further, ac magnetotransport in low dc magnetic fields (H less than or equal to 1 kOe), critical analysis of the paramagnetic to ferromagnetic transition and scaling behavior of the magnetic entropy change versus a reduced temperature under different magnetic fields are also reported.",1210.5216v1 2012-10-22,Large vortex state in ferromagnetic disks,"Magnetic vortices in soft ferromagnetic nano-disks have been extensively studied for at least several decades both for their fundamental (as a ""live"" macroscopic realization of a field theory model of an elementary particle) as well as applied value for high-speed high-density power-independent information storage. Here it is shown that there is another stable vortex state with large thickness-dependent core profile in nano-scale ferromagnetic disks of several exchange lengths in size. Its energy is computed numerically (in the framework of Magnetism@home distributed computing project) and its stability is studied analytically, which allows to plot it on magnetic phase diagram. In cylinders of certain geometries large vortices exist on par with classical ones, while being separated by an energy barrier, controllable by tuning the geometry and material of ferromagnetic disk. They can be an excellent candidate for magnetic information storage not only because the resulting disk sizes are among the smallest, able to support magnetic vortices, but also because it is the closest to the classical vortex state of all other known metastable states of magnetic nano-cylinder. It means that memory, based on switching between these two types of magnetic vortices, may, potentially, achieve the highest possible rate of switching.",1210.5960v3 2012-10-23,Numerical Simulations of Laser Induced Magnetic Bloch Oscillations,"We propose to use a laser to generate magnetic Bloch oscillations in one dimensional easy-axis ferromagnets at low temperatures. This proposal is investigated numerically in details for material parameters relevant for CoCl2*2H2O.",1210.6200v1 2012-12-20,Macroscopic quantum tunneling and coherence of spins,"We present a review of suppression of tunneling rate (energy splitting) for half-odd-integer spins in ferromagnetic and antiferromagnetic particles. We also review the quenching of the tunneling rate in the presence of a magnetic field.",1212.4904v1 2013-01-11,Spin polarization of electrons in quantum wires,"The total energy of a quasi-one-dimensional electron system is calculated using density functional theory. It is shown that spontaneous ferromagnetic state in quantum wire occurs at low one-dimensional electron density. The critical electron density below which electrons are in spin-polarized state is estimated analytically.",1301.2544v1 2013-03-28,New Diluted Ferromagnetic Semiconductor isostructural to 122 type iron pnictide superconductor with TC up to 180 K,"Diluted magnetic semiconductors (DMS) have received much attention due to its potential applications to spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since 1990s. The simultaneous spin and charge doping via hetero-valence (Ga3+,Mn2+) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Previously we synthesized a new diluted ferromagnetic semiconductor of bulk Li(Zn,Mn)As with Tc up to 50K, where isovalent (Zn,Mn) spin doping was separated from charge control via Li concentrations. Here we report the synthesis of a new diluted ferromagnetic semiconductor (Ba1-xKx)(Zn1-yMny)2As2, isostructural to iron 122 system, where holes are doped via (Ba2+, K1+), while spins via (Zn2+,Mn2+) substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit ferromagnetic order with TC up to 180K, comparable to that of record high Tc for Ga(MnAs), significantly enhanced than Li(Zn,Mn)As. Moreover the (Ba,K)(Zn,Mn)2As2 shares the same 122 crystal structure with semiconducting BaZn2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2, which makes them promising to the development of multilayer functional devices.",1303.7157v1 2013-04-08,Dynamical barriers of pure and random ferromagnetic Ising models on fractal lattices,"We consider the stochastic dynamics of the pure and random ferromagnetic Ising model on the hierarchical diamond lattice of branching ratio $K$ with fractal dimension $d_f=(\ln (2K))/\ln 2$. We adapt the Real Space Renormalization procedure introduced in our previous work [C. Monthus and T. Garel, J. Stat. Mech. P02037 (2013)] to study the equilibrium time $t_{eq}(L)$ as a function of the system size $L$ near zero-temperature. For the pure Ising model, we obtain the behavior $t_{eq}(L) \sim L^{\alpha} e^{\beta 2J L^{d_s}} $ where $d_s=d_f-1$ is the interface dimension, and we compute the prefactor exponent $\alpha$. For the random ferromagnetic Ising model, we derive the renormalization rules for dynamical barriers $B_{eq}(L) \equiv (\ln t_{eq}/\beta)$ near zero temperature. For the fractal dimension $d_f=2$, we obtain that the dynamical barrier scales as $ B_{eq}(L)= c L+L^{1/2} u$ where $u$ is a Gaussian random variable of non-zero-mean. While the non-random term scaling as $L$ corresponds to the energy-cost of the creation of a system-size domain-wall, the fluctuation part scaling as $L^{1/2}$ characterizes the barriers for the motion of the system-size domain-wall after its creation. This scaling corresponds to the dynamical exponent $\psi=1/2$, in agreement with the conjecture $\psi=d_s/2$ proposed in [C. Monthus and T. Garel, J. Phys. A 41, 115002 (2008)]. In particular, it is clearly different from the droplet exponent $\theta \simeq 0.299$ involved in the statics of the random ferromagnet on the same lattice.",1304.2134v1 2013-06-08,Observation of a Berry phase anti-damping spin-orbit torque,"Recent observations of current-induced magnetization switching at ferromagnet/normal-conductor interfaces have important consequences for future magnetic memory technology. In one interpretation, the switching originates from carriers with spin-dependent scattering giving rise to a relativistic anti-damping spin-orbit torque (SOT) in structures with broken space-inversion symmetry. The alternative interpretation combines the relativistic spin Hall effect (SHE), making the normal-conductor an injector of a spin-current, with the non-relativistic spin-transfer torque (STT) in the ferromagnet. Remarkably, the SHE in these experiments originates from the Berry phase effect in the band structure of a clean crystal and the anti-damping STT is also based on a disorder-independent transfer of spin from carriers to magnetization. Here we report the observation of an anti-damping SOT stemming from an analogous Berry phase effect to the SHE. The SOT alone can therefore induce magnetization dynamics based on a scattering-independent principle. The ferromagnetic semiconductor (Ga,Mn)As we use has a broken space-inversion symmetry in the crystal. This allows us to consider a bare ferromagnetic element which eliminates by design any SHE related contribution to the spin torque. We provide an intuitive picture of the Berry phase origin of the anti-damping SOT and a microscopic modeling of measured data.",1306.1893v1 2013-06-14,Magnetic structures of Mn3-xFexSn2: an experimental and theoretical study,"We investigate the magnetic structure of Mn3-xFexSn2 using neutron powder diffraction experiments and electronic structure calculations. These alloys crystallize in the orthorhombic Ni3Sn2 type of structure (Pnma) and comprise two inequivalent sites for the transition metal atoms (4c and 8d) and two Sn sites (4c and 4c). The neutron data show that the substituting Fe atoms predominantly occupy the 4c transition metal site and carry a lower magnetic moment than Mn atoms. Four kinds of magnetic structures are encountered as a function of temperature and composition: two simple ferromagnetic structures (with the magnetic moments pointing along the b or c axis) and two canted ferromagnetic arrangements (with the ferromagnetic component pointing along the b or c axis). Electronic structure calculations results agree well with the low-temperature experimental magnetic moments and canting angles throughout the series. Comparisons between collinear and non-collinear computations show that the canted state is stabilized by a band mechanism through the opening of a hybridization gap. Synchrotron powder diffraction experiments on Mn3Sn2 reveal a weak monoclinic distortion at low temperature (90.08 deg at 175 K). This lowering of symmetry could explain the stabilization of the c-axis canted ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a circumstance that would otherwise require unusually large high-order terms in the spin Hamiltonian.",1306.3339v1 2013-06-16,Zero-Field Fiske Resonance Coupled with Spin-waves in Ferromagnetic Josephson Junctions,"AC Josephson current density in a Josephson junction with DC bias is spatially modulated by an external magnetic field, and induces an electromagnetic (EM) field inside the junction. The current-voltage ($I$-$V$) curve exhibits peaks due to the resonance between the EM field and the spatially modulated AC Josephson current density. This is called {\it Fiske resonance}. Such a spatially modulated Josephson current density can be also induced by a non-uniform insulating barrier and the Fiske resonance appears without external magnetic field. This is called zero-field Fiske resonance (ZFFR). In this paper, we theoretically study the ZFFR coupled with spin-waves in a superconductor/ferromagnetic insulator/superconductor junction (ferromagnetic Josephson junction) with a non-uniform ferromagnetic insulating barrier. The resonant mode coupled with spin-waves can be induced without external magnetic field. We find that the $I$-$V$ curve shows resonant peaks associated with composite excitations of spin-waves and the EM field in the junction. The voltage at the resonance is obtained as a function of the normal modes of EM field. The ZFFRs coupled with spin-waves are found as peak structures in the DC Josephson current density as a function of bias voltage.",1306.3652v2 2013-06-19,"Asymmetric Ferromagnetic Resonance, Universal Walker Breakdown, and Counterflow Domain Wall Motion in the Presence of Multiple Spin-Orbit Torques","We study the motion of several types of domain wall profiles in spin-orbit coupled magnetic nanowires and also the influence of spin-orbit interaction on the ferromagnetic resonance of uniform magnetic films. We extend previous studies by fully considering not only the field-like contribution from the spin-orbit torque, but also the recently derived Slonczewski-like spin-orbit torque. We show that the latter interaction affects both the domain wall velocity and the Walker breakdown threshold non-trivially, which suggests that it should be accounted in experimental data analysis. We find that the presence of multiple spin-orbit torques may render the Walker breakdown to be universal in the sense that the threshold is completely independent on the material-dependent Gilbert damping, non-adiabaticity, and the chirality of the domain wall. We also find that domain wall motion against the current injection is sustained in the presence of multiple spin-orbit torques and that the wall profile will determine the qualitative influence of these different types of torques (e.g. field-like and Slonczewski-like). In addition, we consider a uniform ferromagnetic layer under a current bias, and find that the resonance frequency becomes asymmetric against the current direction in the presence of Slonczewski-like spin-orbit coupling. This is in contrast with those cases where such an interaction is absent, where the frequency is found to be symmetric with respect to the current direction. This finding shows that spin-orbit interactions may offer additional control over pumped and absorbed energy in a ferromagnetic resonance setup by manipulating the injected current direction.",1306.4680v1 2013-07-12,Dilute ferromagnetic semiconductors: Physics and spintronic structures,"This review compiles results of experimental and theoretical studies on thin films and quantum structures of semiconductors with randomly distributed Mn ions, which exhibit spintronic functionalities associated with collective ferromagnetic spin ordering. Properties of p-type Mn-containing III-V as well as II-VI, V_2-VI_3, IV-VI, I-II-V, and elemental group IV semiconductors are described paying particular attention to the most thoroughly investigated system (Ga,Mn)As that supports the hole-mediated ferromagnetic order up to 190 K for the net concentration of Mn spins below 10%. Multilayer structures showing efficient spin injection and spin-related magnetotransport properties as well as enabling magnetization manipulation by strain, light, electric fields, and spin currents are presented together with their impact on metal spintronics. The challenging interplay between magnetic and electronic properties in topologically trivial and non-trivial systems is described, emphasizing the entangled roles of disorder and correlation at the carrier localization boundary. Finally, the case of dilute magnetic insulators is considered, such as (Ga,Mn)N, where low temperature spin ordering is driven by short-ranged superexchange that is ferromagnetic for certain charge states of magnetic impurities.",1307.3429v3 2013-11-03,Thickness-dependent ferromagnetic metal to paramagnetic insulator transition in La$_{0.6}$Sr$_{0.4}$MnO$_3$ thin films studied by x-ray magnetic circular dichroism,"Metallic transition-metal oxides undergo a metal-to-insulator transition (MIT) as the film thickness decreases across a ritical thickness of several monolayers (MLs), but its driving mechanism remains controversial. We have studied the thickness-dependent MIT of the ferromagnetic metal La$_{0.6}$Sr$_{0.4}$MnO$_3$ by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. As the film thickness was decreased across the critical thickness of the MIT (6-8 ML), a gradual decrease of the ferromagnetic signals and a concomitant increase of paramagnetic signals were observed, while the Mn valence abruptly decreased towards Mn$^{3+}$. These observations suggest that the ferromagnetic phase gradually and most likely inhomogeneously turns into the paramagnetic phase and both phases abruptly become insulating at the critical thickness.",1311.0520v4 2013-11-26,"Recent Progress in III-V based ferromagnetic semiconductors: Band structure, Fermi level, and tunneling transport","Spin-based electronics or spintronics is an emerging field, in which we try to utilize spin degrees of freedom as well as charge transport in materials and devices. While metal-based spin-devices, such as magnetic-field sensors and magnetoresistive random access memory using giant magnetoresistance and tunneling magnetoresistance, are already put to practical use, semiconductor-based spintronics has greater potential for expansion because of good compatibility with existing semiconductor technology. Many semiconductor-based spintronics devices with useful functionalities have been proposed and explored so far. To realize those devices and functionalities, we definitely need appropriate materials which have both the properties of semiconductors and ferromagnets. Ferromagnetic semiconductors (FMS), which are alloy semiconductors containing magnetic atoms such as Mn and Fe, are one of the most promising classes of materials for this purpose, and thus have been intensively studied for the past two decades. Here, we review the recent progress in the studies of the most prototypical III-V based FMS, p-type (GaMn)As, and its heterostructures with focus on tunneling transport, Fermi level, and bandstructure. Furthermore, we cover the properties of a new n-type FMS, (InFe)As, which shows electron-induced ferromagnetism. These FMS materials having zinc-blende crystal structure show excellent compatibility with well-developed III-V heterostructures and devices.",1311.6616v1 2013-12-13,On Vortex Solutions of Landau-Lifshitz Equations,"We study the Landau-Lifshitz equation of ferromagnetism on $\mathbb{R}^2$ with an easy-axis anisotropy. We give the necessary condition for the existence of the finite energy vortex solutions and show the behaviors of the solutions.",1312.3816v1 2014-01-15,Characterization of the Spin-1/2 Linear-Chain Ferromagnet CuAs$_2$O$_4$,"The magnetic and lattice properties of the $S$=1/2 quantum-spin-chain ferromagnet, CuAs$_2$O$_4$, mineral name trippkeite, were investigated. The crystal structure of CuAs$_2$O$_4$ is characterized by the presence of corrugated CuO$_2$ ribbon chains. Measurements of the magnetic susceptibility, heat capacity, electron paramagnetic resonance and Raman spectroscopy were performed. Our experiments conclusively show that a ferromagnetic transition occurs at $\sim$7.4 K. $\textit{Ab initio}$ DFT calculations reveal dominant ferromagnetic nearest-neighbor and weaker antiferromagnetic next- nearest-neighbor spin exchange interactions along the ribbon chains. The ratio of $J_{\rm nn}$/$J_{\rm nnn}$ is near -4, placing CuAs$_2$O$_4$ in close proximity to a quantum critical point in the $J_{\rm nn}$ - $J_{\rm nnn}$ phase diagram. TMRG simulations used to analyze the magnetic susceptibility confirm this ratio. Single-crystal magnetization measurements indicate that a magnetic anisotropy forces the Cu$^{2+}$ spins to lie in an easy plane perpendicular to the $c$-axis. An analysis of the field and temperature dependent magnetization by modified Arrott plots reveals a 3d-XY critical behavior. Lattice perturbations induced by quasi-hydrostatic pressure and temperature were mapped via magnetization and Raman spectroscopy.",1401.3400v1 2014-02-26,Ferromagnetism of magnetic impurities coupled indirectly via conduction electrons: Insights from various theoretical approaches,"The magnetic ground-state properties of the periodic Anderson model with a regular depletion of the correlated sites are analyzed within different theoretical approaches. We consider the model on the one-dimensional chain and on the two-dimensional square lattice with hopping between nearest neighbors. At half-filling and with correlated impurities present at every second site, the depleted Anderson lattice is the most simple system where the indirect magnetic coupling mediated by the conduction electrons is ferromagnetic. We discuss the underlying electronic structure and the possible mechanisms that result in ferromagnetic long-range order. To this end, different numerical and analytical concepts are applied to the depleted Anderson and also to the related depleted Kondo lattice and are contrasted with each other. This includes numerical approaches, i.e. Hartree-Fock theory, density-matrix renormalization and dynamical mean-field theory, as well as analytical concepts, namely a variant of the Lieb-Mattis theorem and the concept of flat-band ferromagnetism, and finally perturbative approaches, i.e. the effective RKKY exchange in the limit of weak and the ""inverse indirect magnetic exchange"" in the limit of strong coupling between the conduction band and the impurities.",1402.6657v2 2014-07-20,A walk in the statistical mechanical formulation of neural networks,"Neural networks are nowadays both powerful operational tools (e.g., for pattern recognition, data mining, error correction codes) and complex theoretical models on the focus of scientific investigation. As for the research branch, neural networks are handled and studied by psychologists, neurobiologists, engineers, mathematicians and theoretical physicists. In particular, in theoretical physics, the key instrument for the quantitative analysis of neural networks is statistical mechanics. From this perspective, here, we first review attractor networks: starting from ferromagnets and spin-glass models, we discuss the underlying philosophy and we recover the strand paved by Hopfield, Amit-Gutfreund-Sompolinky. One step forward, we highlight the structural equivalence between Hopfield networks (modeling retrieval) and Boltzmann machines (modeling learning), hence realizing a deep bridge linking two inseparable aspects of biological and robotic spontaneous cognition. As a sideline, in this walk we derive two alternative (with respect to the original Hebb proposal) ways to recover the Hebbian paradigm, stemming from ferromagnets and from spin-glasses, respectively. Further, as these notes are thought of for an Engineering audience, we highlight also the mappings between ferromagnets and operational amplifiers and between antiferromagnets and flip-flops (as neural networks -built by op-amp and flip-flops- are particular spin-glasses and the latter are indeed combinations of ferromagnets and antiferromagnets), hoping that such a bridge plays as a concrete prescription to capture the beauty of robotics from the statistical mechanical perspective.",1407.5300v1 2014-10-01,Evidence for magnetic clusters in Ni$_{1-x}$V$_{x}$ close to the quantum critical concentration,"The d-metal alloy Ni$_{1-x}$V$_{x}$ undergoes a quantum phase transition from a ferromagnetic ground state to a paramagnetic ground state as the vanadium concentration $x$ is increased. We present magnetization, ac-susceptibility and muon-spin relaxation data at several vanadium concentrations near the critical concentration $x_c \approx11.6%$ at which the onset of ferromagnetic order is suppressed to zero temperature. Below $x_c$, the muon data reveal a broad magnetic field distribution indicative of long-range ordered ferromagnetic state with spatial disorder. We show evidence of magnetic clusters in the ferromagnetic phase and close to the phase boundary in this disordered itinerant system as an important generic ingredient of a disordered quantum phase transition. In contrast, the temperature dependence of the magnetic susceptibility above $x_c$ is best described in terms of a magnetic quantum Griffiths phase with a power-law distribution of fluctuation rates of dynamic magnetic clusters. At the lowest temperatures, the onset of a short-range ordered cluster-glass phase is recognized by an increase in the muon depolarization in transverse fields and maxima in ac-susceptibility.",1410.0094v1 2014-10-29,"(Ca,Na)(Zn,Mn)2As2: a new spin & charge doping decoupled diluted ferromagnetic semiconductor with hexagonal CaAl2Si2 structure","Here we report the successful synthesis of a spin- & charge-decoupled diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2As2, crystallizing into the hexagonal CaAl2Si2 structure. The compound shows a ferromagnetic transition with a Curie temperature up to 33 K with 10% Na doping, which gives rise to carrier density of np~10^20 cm^-3. The new DMS is a soft magnetic material with HC<400 Oe. The anomalous Hall effect is observed below the ferromagnetic ordering temperature. With increasing Mn doping, ferromagnetic order is accompanied by an interaction between the local spin and mobile charge, giving rise to a minimum in resistivity at low temperatures and localizing the conduction electrons. The system provides an ideal platform for studying the interaction of the local spins and conduction electrons.",1410.8015v1 2014-12-02,Interplay of spin-orbit torque and thermoelectric effects in ferromagnet/normal metal bilayers,"We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field-dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6nm)/Co(2.5nm) and negligible in Pt(6nm)/Co(2.5nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the field-like SOTs are about an order of magnitude smaller.",1412.0865v1 2015-02-13,Magnon transport through microwave pumping,"We present a microscopic theory of magnon transport in ferromagnetic insulators (FIs). Using magnon injection through microwave pumping, we propose a way to generate magnon dc currents and show how to enhance their amplitudes in hybrid ferromagnetic insulating junctions. To this end focusing on a single FI, we first revisit microwave pumping at finite (room) temperature from the microscopic viewpoint of magnon injection. Next, we apply it to two kinds of hybrid ferromagnetic insulating junctions. The first is the junction between a quasi-equilibrium magnon condensate and magnons being pumped by microwave, while the second is the junction between such pumped magnons and noncondensed magnons. We show that quasi-equilibrium magnon condensates generate ac and dc magnon currents, while noncondensed magnons produce essentially a dc magnon current. The ferromagnetic resonance (FMR) drastically increases the density of the pumped magnons and enhances such magnon currents. Lastly, using microwave pumping in a single FI, we discuss the possibility that a magnon current through an Aharonov-Casher phase flows persistently even at finite temperature. We show that such a magnon current arises even at finite temperature in the presence of magnon-magnon interactions. Due to FMR, its amplitude becomes much larger than the condensed magnon current.",1502.03865v2 2015-02-26,Chiral anomaly and transport in Weyl metals,"We present an overview of our recent work on transport phenomena in Weyl metals, which may be connected to their nontrivial topological properties, particularly to chiral anomaly. We argue that there are two basic phenomena, which are related to chiral anomaly in Weyl metals: Anomalous Hall Effect (AHE) and Chiral Magnetic Effect (CME). While AHE is in principle present in any ferromagnetic metal, we demonstrate that a magnetic Weyl metal is distinguished from an ordinary ferromagnetic metal by the absence of the extrinsic and the Fermi surface part of the intrinsic contributions to the AHE, as long as the Fermi energy is sufficiently close to the Weyl nodes. The AHE in a Weyl metal is thus shown to be a purely intrinsic, universal property, fully determined by the location of the Weyl nodes in the first Brillouin zone. In other words, a ferromagnetic Weyl metal may be thought of as the only example of a ferromagnetic metal with a purely intrinsic AHE. We further develop a fully microscopic theory of diffusive magnetotransport in Weyl metals. We derive coupled diffusion equations for the total and axial (i.e. node-antisymmetric) charge densities and show that chiral anomaly manifests as a magnetic-field-induced coupling between them. We demonstrate that an experimentally-observable consequence of CME in magnetotransport in Weyl metals is a quadratic negative magnetoresistance, which will dominate all other contributions to magnetoresistance under certain conditions and may be regarded as a smoking-gun transport characteristic, unique to Weyl metals.",1502.07609v1 2015-03-24,Itinerant ferromagnetism in the As 4$p$ conduction band of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ identified by x-ray magnetic circular dichroism,"X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ show that the ferromagnetism below $T_{\textrm{C}}\approx$ 100 K arises in the As $4p$ conduction band. No XMCD signal is observed at the Mn x-ray absorption edges. Below $T_{\textrm{C}}$, however, a clear XMCD signal is found at the As $K$ edge which increases with decreasing temperature. The XMCD signal is absent in data taken with the beam directed parallel to the crystallographic $\textrm{c}$ axis indicating that the orbital magnetic moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant ferromagnetism is associated with the As $4p$ conduction band and that distinct local-moment antiferromagnetism and itinerant ferromagnetism with perpendicular easy axes coexist in this compound at low temperature.",1503.07197v2 2015-04-29,Role of transparency of platinum-ferromagnet interface in determining intrinsic magnitude of spin Hall effect,"The spin Hall effect (SHE) converts charge current to pure spin currents in orthogonal directions in materials that have significant spin-orbit coupling.The efficiency of the conversion is described by the spin Hall Angle (SHA). The SHA can most readily be inferred by using the generated spin currents to excite or rotate the magnetization of ferromagnetic films or nano-elements via spin-transfer torques.Some of the largest spin torque derived spin Hall angles (ST-SHA) have been reported in platinum. Here we show, using spin torque ferromagnetic resonance (ST-FMR) measurements, that the transparency of the Pt-ferromagnet interface to the spin current plays a central role in determining the magnitude of the ST-SHA. We measure a much larger ST-SHA in Pt/cobalt (~0.11) compared to Pt/permalloy (~0.05) bilayers when the interfaces are assumed to be completely transparent. Taking into account the transparency of these interfaces, as derived from spin-mixing conductances, we find that the intrinsic SHA in platinum has a much higher value of 0.19 +- 0.04 as compared to the ST-SHA. The importance of the interface transparency is further exemplified by the insertion of atomically thin magnetic layers at the Pt/permalloy interface that we show strongly modulates the magnitude of the ST-SHA.",1504.07929v1 2015-08-19,Independent tuning of electronic properties and induced ferromagnetism in topological insulators with heterostructure approach,"The quantum anomalous Hall effect (QAHE) has been recently demonstrated in Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures below 100 mK. In those materials, the spins of unfilled d-electrons in the transition metal dopants are exchange coupled to develop a long-range ferromagnetic order, which is essential for realizing QAHE. However, the addition of random dopants does not only introduce excess charge carriers that require readjusting the Bi/Sb ratio, but also unavoidably introduces paramagnetic spins that can adversely affect the chiral edge transport in QAHE. In this work, we show a heterostructure approach to independently tune the electronic and magnetic properties of the topological surface states in (BixSb1-x)2Te3 without resorting to random doping of transition metal elements. In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium iron garnet (YIG), a high Curie temperature (~ 550 K) magnetic insulator, we find that the TI surface in contact with YIG becomes ferromagnetic via proximity coupling which is revealed by the anomalous Hall effect (AHE). The Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is varied, the AHE resistivity scales with the longitudinal resistivity. In this approach, we decouple the electronic properties from the induced ferromagnetism in TI. The independent optimization provides a pathway for realizing QAHE at higher temperatures, which is important for novel spintronic device applications.",1508.04719v1 2015-09-07,Josephson current in a normal-metal nanowire coupled to superconductor/ferromagnet/superconductor junction,"We consider superconducting nanowire proximity coupled to superconductor / ferromagnet / superconductor junction, where the magnetization penetrates into superconducting segment in nanowire decaying as $\sim\exp[-\frac{\mid n \mid}{\xi}]$ with site index $n$ and the decay length $\xi$. We tune chemical potential and spin-orbit coupling so that topological superconducting regime hosting Majorana fermion is realized for long $\xi$. We find that when $\xi$ becomes shorter, zero energy state at the interface between superconductor and ferromagnet splits into two away from zero energy. Accordingly, the behavior of Josephson current is drastically changed due to this ""zero mode-non-zero mode crossover"". By tuning the model parameters, we find an almost second-harmonic current-phase relation, $\sin2\varphi$, with phase difference $\varphi$. Based on the analysis of Andreev bound state (ABS), we clarify that current-phase relation is determined by coupling of the states within the energy gap. We find that the emergence of crossing points of ABS is a key ingredient to generate $\sin2\varphi$ dependence in current-phase relation. We further study both the energy and $\varphi$ dependence of pair amplitudes in the ferromagnetic region. For long $\xi$, odd-frequency spin-triplet $s$-wave component is dominant. The magnitude of the odd-frequency pair amplitude is enhanced at the energy level of ABS.",1509.01914v2 2016-01-21,Real-space renormalization for the finite temperature statics and dynamics of the Dyson Long-Ranged Ferromagnetic and Spin-Glass models,"The finite temperature dynamics of the Dyson hierarchical classical spins models is studied via real-space renormalization rules concerning the couplings and the relaxation times. For the ferromagnetic model involving Long-Ranged coupling $J(r) \propto r^{-1-\sigma}$ in the region $1/2<\sigma<1$ where there exists a non-mean-field-like thermal Ferromagnetic-Paramagnetic transition, the RG flows are explicitly solved: the characteristic relaxation time $\tau(L)$ follows the critical power-law $\tau(L)\propto L^{z_c(\sigma)} $ at the phase transition and the activated law $\ln \tau(L)\propto L^{\psi} $ with $\psi=1-\sigma$ in the ferromagnetic phase. For the Spin-Glass model involving random Long-Ranged couplings of variance $\overline{J^2(r)} \propto r^{-2\sigma}$ in the region $2/3<\sigma<1$ where there exists a non-mean-field-like thermal SpinGlass-Paramagnetic transition, the coupled RG flows of the couplings and of the relaxation times are studied numerically : the relaxation time $\tau(L)$ follows some power-law $\tau(L)\propto L^{z_c(\sigma)} $ at criticality and the activated law $\ln \tau(L)\propto L^{\psi} $ in the Spin-Glass phase with the dynamical exponent $\psi=1-\sigma=\theta$ coinciding with the droplet exponent governing the flow of the couplings $J(L) \propto L^{\theta} $.",1601.05643v2 2016-02-01,Facet-dependent giant spin orbit torque in single crystalline antiferromagnetic Ir-Mn / ferromagnetic permalloy bilayers,"There has been considerable interest in spin-orbit torques for the purpose of manipulating the magnetization of ferromagnetic (FM) films or nano-elements for spintronic technologies. Spin-orbit torques are derived from spin currents created from charge currents in materials with significant spin-orbit coupling that diffuse into an adjacent FM material. There have been intensive efforts to search for candidate materials that exhibit large spin Hall angles, i.e. efficient charge to spin current conversion. Here we report, using spin torque ferromagnetic resonance, the observation of a giant spin Hall angle of up to ~0.35 in (100) oriented single crystalline antiferromagnetic (AF) IrMn3 thin films, coupled to ferromagnetic permalloy layers, and a spin Hall angle that is about three times smaller in (111) oriented films. For the (100) oriented samples we show that the magnitude of the spin Hall angle can be significantly changed by manipulating the populations of the various AF domains through field annealing. Using ab-initio calculations we show that the triangular AF structure of IrMn3 gives rise to a substantial intrinsic spin Hall conductivity that is three times larger for the (100) than for the (111) orientations, consistent with our experimental findings.",1602.00670v1 2016-02-08,A Precessing Ferromagnetic Needle Magnetometer,"A ferromagnetic needle is predicted to precess about the magnetic field axis at a Larmor frequency $\Omega$ under conditions where its intrinsic spin dominates over its rotational angular momentum, $N\hbar \gg I\Omega$ ($I$ is the moment of inertia of the needle about the precession axis and $N$ is the number of polarized spins in the needle). In this regime the needle behaves as a gyroscope with spin $N\hbar$ maintained along the easy axis of the needle by the crystalline and shape anisotropy. A precessing ferromagnetic needle is a correlated system of $N$ spins which can be used to measure magnetic fields for long times. In principle, by taking advantage of rapid averaging of quantum uncertainty, the sensitivity of a precessing needle magnetometer can far surpass that of magnetometers based on spin precession of atoms in the gas phase. Under conditions where noise from coupling to the environment is subdominant, the scaling with measurement time $t$ of the quantum- and detection-limited magnetometric sensitivity is $t^{-3/2}$. The phenomenon of ferromagnetic needle precession may be of particular interest for precision measurements testing fundamental physics.",1602.02818v2 2016-03-17,Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations,"Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with \textsf{OOMMF} and \textsf{Nmag} simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.",1603.05419v1 2016-03-31,"Energetic, electronic and magnetic properties of Mn-dimers on reconstructed (001) GaAs surfaces","We study energetic, magnetic, and electronic properties of diluted substitutional Mn-pairs on the reconstructed $(001)$ GaAs surfaces. The studies are based on first-principles calculations in the framework of the density functional theory. We demonstrate that the stability of the systems strongly depends on the position, orientation, and the distance between the Mn-atoms constituting the pair. Independently of the considered surface reconstruction pattern, the Mn-pairs with Mn-atoms being the nearest neighbors (NN) on cationic sublattice turn out to be energetically more favorable than the pairs with the larger distance between the Mn-atoms. However, the preferential build-up orientation of the Mn-NN-pair depends on the surface reconstruction and is parallel either to $[110]$ or $[1\bar{1}0]$ crystallographic direction. We reveal also the mechanisms of the magnetic ordering of Mn-NN-pairs. The Mn-NN-pairs along the $[110]$ crystallographic direction exhibit always ferromagnetic alignment of Mn spins, whereas the spins in the Mn-NN-pairs along $[1\bar{1}0]$ direction are mostly anti-ferromagnetically aligned. In the electronic structure of the systems containing Mn-pairs with ferromagnetically aligned spins, we observe the valence band hole states in the neighborhood of Fermi energy. This indicates that the surface ferromagnetism in this prototype of dilute magnetic semiconductors can be explained in terms of the $p$-$d$ Zener model.",1603.09606v4 2016-04-12,An invisible non-volatile solid-state memory,"Information technologies require entangling data stability with encryption for a next generation of secure data storage. Current magnetic memories, ranging from low-density stripes up to high-density hard drives, can ultimately be detected using routinely available probes or manipulated by external magnetic perturbations. Antiferromagnetic resistors feature unrivalled robustness but the stable resistive states reported scarcely differ by more than a fraction of a percent at room temperature. Here we show that the metamagnetic (ferromagnetic to antiferromagnetic) transition in intermetallic Fe0.50Rh0.50 can be electrically controlled in a magnetoelectric heterostructure to reveal or cloak a given ferromagnetic state. From an aligned ferromagnetic phase, magnetic states are frozen into the antiferromagnetic phase by the application of an electric field, thus eliminating the stray field and likewise making it insensitive to external magnetic field. Application of a reverse electric field reverts the antiferromagnetic state to the original ferromagnetic state. Our work demonstrates the building blocks of a feasible, extremely stable, non-volatile, electrically addressable, low-energy dissipation, magnetoelectric multiferroic memory.",1604.03383v2 2016-06-28,Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra,"As charge carriers traverse a single superconductor ferromagnet interface they experience an additional spin-dependent phase angle which results in spin mixing and the formation of a bound state called the Andreev Bound State. This state is an essential component in the generation of long range spin triplet proximity induced superconductivity and yet the factors controlling the degree of spin mixing and the formation of the bound state remain elusive. Here we demonstrate that point contact Andreev reflection can be used to detect the bound state and extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7 single crystal - Pb junctions, together with a compilation of literature data on highly spin polarised systems, we show that the existence of the Andreev Bound State both resolves a number of long standing controversies in the Andreev literature as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly we find that for these high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.",1606.08619v1 2016-07-06,"Impurity states in the magnetic topological insulator V:(Bi,Sb)$_2$Te$_3$","The ferromagnetic topological insulator V:(Bi,Sb)$_2$Te$_3$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of V:(Bi,Sb)$_2$Te$_3$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted, and exhibits a high density of states near Fermi level. First-principles calculations support the experimental results and indicate the coexistence of ferromagnetic superexchange and double exchange interactions. The observed impurity band is thus expected to contribute to the ferromagnetism via the interplay of different mechanisms.",1607.01662v2 2016-07-13,Half-Metallic Superconducting Triplet Spin Valve,"We theoretically study a finite size $SF_1NF_2$ spin valve, where a normal metal ($N$) insert separates a thin standard ferromagnet ($F_1$) and a thick half-metallic ferromagnet ($F_2$). For sufficiently thin superconductor ($S$) widths close to the coherence length $\xi_0$, we find that changes to the relative magnetization orientations in the ferromagnets can result in substantial variations in the transition temperature $T_c$, consistent with experiment [Singh et al., Phys. Rev. X 5, 021019 (2015)]. Our results demonstrate that, in good agreement with the experiment, the variations are largest in the case where $F_2$ is in a half-metallic phase and thus supports only one spin direction. To pinpoint the origins of this strong spin-valve effect, both the equal-spin $f_1$ and opposite-spin $f_0$ triplet correlations are calculated using a self-consistent microscopic technique. We find that when the magnetization in $F_1$ is tilted slightly out-of-plane, the $f_1$ component can be the dominant triplet component in the superconductor. The coupling between the two ferromagnets is discussed in terms of the underlying spin currents present in the system. We go further and show that the zero energy peaks of the local density of states probed on the $S$ side of the valve can be another signature of the presence of superconducting triplet correlations. Our findings reveal that for sufficiently thin $S$ layers, the zero energy peak at the $S$ side can be larger than its counterpart in the $F_2$ side.",1607.03899v1 2016-07-25,On the Explanation of the Paramagnetic Meissner Effect in Superconductor/Ferromagnet Heterostructures,"An increase of the magnetic moment in superconductor/ferromagnet (S/F) bilayers V(40nm)/F [F$=$Fe(1,3nm), Co(3nm), Ni(3nm)] was observed using SQUID magnetometry upon cooling below the superconducting transition temperature Tc in magnetic fields of 10 Oe to 50 Oe applied parallel to the sample surface. A similar increase, often called the paramagnetic Meissner effect (PME), was observed before in various superconductors and superconductor/ferromagnet systems. To explain the PME effect in the presented S/F bilayers a model based on a row of vortices located at the S/F interface is proposed. According to the model the magnetic moment induced below Tc consists of the paramagnetic contribution of the vortex cores and the diamagnetic contribution of the vortex-free region of the S layer. Since the thickness of the S layer is found to be 3-4 times less than the magnetic field penetration depth, this latter diamagnetic contribution is negligible. The model correctly accounts for the sign, the approximate magnitude and the field dependence of the paramagnetic and the Meissner contributions of the induced magnetic moment upon passing the superconducting transition of a ferromagnet/superconductor bilayer.",1607.07341v2 2016-09-15,Ferromagnetism and the Effect of Free Charge Carriers on Electric Polarization in Y_2NiMnO_6 Double Perovskite,"The double perovskite Y_2NiMnO_6 displays ferromagnetic transition at Tc = 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (M_s) and also, validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P 21/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of frequency dependent dielectric constant and equivalent circuit analysis of impedance data takes into account the bulk contribution to total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T_c). Pyrocurrent measurements register a current flow with onset near Tc and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to re-orientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material",1609.04511v1 2016-09-18,Correlation and transport phenomena in topological nodal-loop semimetals,"We study the unique physical properties of topological nodal-loop semimetals protected by the coexistence of time-reversal and inversion symmetries with negligible spin-orbit coupling. We argue that strong correlation effects occur at the surface of such systems for relatively small Hubbard interaction $U$, due to the narrow bandwidth of the ""drumhead"" surface states. In the Hartree-Fock approximation, at small $U$ we obtain a surface ferromagnetic phase through a continuous quantum phase transition characterized by the surface-mode divergence of the spin susceptibility, while the bulk states remain very robust against local interactions and remain non-ordered. At slightly increased interaction strength, the system quickly changes from a surface ferromagnetic phase to a surface charge-ordered phase through a first-order transition. When Rashba-type spin-orbit coupling is applied to the surface states, a canted ferromagnetic phase occurs at the surface for intermediate values of $U$. The quantum critical behavior of the surface ferromagnetic transition is nontrivial in the sense that the surface spin order parameter couple to Fermi-surface excitations from both surface and bulk states. This leads to unconventional Landau damping and consequently a na\""ive dynamical critical exponent $z\!\approx\!1$ when the Fermi level is close to the bulk nodal energy. We also show that, already without interactions, quantum oscillations arise due to bulk states, despite the absence of a Fermi surface when the chemical potential is tuned to the energy of the nodal loop. The bulk magnetic susceptibility diverges logarithmically whenever the nodal loop exactly overlaps with a quantized magnetic orbit in the bulk Brillouin zone. These correlation and transport phenomena are unique signatures of nodal loop states.",1609.05529v1 2016-10-13,Role of direct exchange and Dzyaloshinskii-Moriya interactions in magnetic properties of graphene derivatives: C$_2$F and C$_2$H,"According to the Lieb's theorem the ferromagnetic interaction in graphene-based materials with bipartite lattice is a result of disbalance between the number of sites available for $p_z$ electrons in different sublattices. Here, we report on another mechanism of the ferromagnetism in functionalized graphene that is the direct exchange interaction between spin orbitals. By the example of the single-side semihydrogenated (C$_2$H) and semifluorinated (C$_2$F) graphene we show that such a coupling can partially or even fully compensate antiferromagnetic character of indirect exchange interactions reported earlier [Phys. Rev. B {\bf 88}, 081405(R) (2013)]. As a result, C$_2$H is found to be a two-dimensional material with the isotropic ferromagnetic interaction and negligibly small magnetic anisotropy, which prevents the formation of the long-range magnetic order at finite temperature in accordance with the Mermin-Wagner theorem. This gives a rare example of a system where direct exchange interactions play a crucial role in determining a magnetic structure. In turn, C$_2$F is found to be at the threshold of the antiferromagnetic-ferromagnetic instability, which in combination with the Dzyaloshinskii-Moriya interaction can lead to a skyrmion state.",1610.04059v1 2017-03-22,Fast domain wall motion induced by antiferromagnetic spin dynamics at the angular momentum compensation temperature of ferrimagnets,"Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices. A central motivation toward this direction is that antiferromagnetic spin dynamics is expected to be much faster than ferromagnetic counterpart because antiferromagnets have higher resonance frequencies than ferromagnets. Recent theories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs. However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored mainly because of the immunity of antiferromagnets to magnetic fields. Furthermore, this immunity makes field-driven antiferromagnetic DW motion impossible despite rich physics of field-driven DW dynamics as proven in ferromagnetic DW studies. Here we show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare-earth 3d-transition metal ferrimagnetic compounds where net magnetic moment is nonzero at TA, the field-driven DW mobility remarkably enhances up to 20 km/sT. The collective coordinate approach generalized for ferrimagnets and atomistic spin model simulations show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics.",1703.07515v1 2017-07-31,Theoretical study of magnetism induced by proximity effect in a ferromagnetic Josephson junction with a normal metal,"We theoretically study the magnetism induced by the proximity effect in the normal metal of ferromagnetic Josephson junction composed of two $s$-wave superconductors separated by ferromagnetic metal/normal metal/ferromagnetic metal junction (${S}/{F}/{N}/{F}/{S}$ junction). We calculate the magnetization in the $N$ by solving the Eilenberger equation. We show that the magnetization arises in the ${N}$ when the product of anomalous Green's functions of the spin-triplet even-frequency odd-parity Cooper pair and spin-singlet odd-frequency odd-parity Cooper pair in the ${N}$ has a finite value. The induced magnetization $M(d,\theta)$ can be decomposed into two parts, $M(d,\theta)=M^{\rm I}(d)+M^{\rm II}(d,\theta)$, where $d$ is the thickness of $N$ and $\theta$ is superconducting phase difference between two ${S}$s. Therefore, $\theta$ dependence of $M(d,\theta)$ allows us to control the amplitude of magnetization by changing $\theta$. The variation of $M(d,\theta)$ with $\theta$ is indeed the good evidence of the magnetization induced by the proximity effect, since some methods of magnetization measurement pick up total magnetization in the ${S}/{F}/{N}/{F}/{S}$ junction.",1708.00782v1 2017-08-06,Electric-field-induced extremely large change in resistance in graphene ferromagnets,"A colossal magnetoresistance ($\sim 100\times10^3\%$) and an extremely large magnetoresistance ($\sim 1\times10^6\%$) have been previously explored in manganite perovskites and Dirac materials, respectively. However, the requirement of an extremely strong magnetic field (and an extremely low temperature) makes them not applicable for realistic devices. In this work, we propose a device that can generate even larger changes in resistance in a zero-magnetic field and at a high temperature. The device is composed of a graphene under two strips of yttrium iron garnet (YIG), where two gate voltages are applied to cancel the heavy charge doping in the YIG-induced half-metallic ferromagnets. By calculations using the Landauer-B\""{u}ttiker formalism, we demonstrate that, when a proper gate voltage is applied on the free ferromagnet, changes in resistance up to $305\times10^6\%$ ($16\times10^3\%$) can be achieved at the liquid helium (nitrogen) temperature and in a zero magnetic field. We attribute such a remarkable effect to a gate-induced full-polarization reversal in the free ferromagnet, which results in a metal-state to insulator-state transition in the device. We also find that, the proposed effect can be realized in devices using other magnetic insulators such as EuO and EuS. Our work should be helpful for developing a realistic switching device that is energy saving and CMOS-technology compatible.",1708.01858v2 2017-08-08,Spin-orbit-torque driven magnetoimpedance in Pt-layer/magnetic-ribbon heterostructures,"When a flow of electron passes through a paramagnetic layer with strong spin-orbit-coupling such as platinum (Pt), a net spin current is produced via spin Hall effect (SHE). This spin current can exert a torque on the magnetization of an adjacent ferromagnetic layer which can be probed via magnetization dynamic response, e.g. spin-torque ferromagnetic resonance (ST-FMR). Nevertheless, that effect in lower frequency magnetization dynamic regime (MHz) where skin effect occurs in high permeability ferromagnetic conductors namely the magneto-impedance (MI) effect can be fundamentally important which has not been studied so far. Here, by utilizing the MI effect in magnetic-ribbon/Pt heterostructure with high magnetic permeability that allows the ac current effectively confined at the skin depth of ~100 nm thickness, the effect of spin-orbit-torque (SOT) induced by the SHE probed via MI measurement is investigated. We observed a systematic MI frequency shift that increases by increasing the applied current amplitude and thickness of the Pt layer (varying from 0 nm to 20 nm). In addition, the role of Pt layer in ribbon/Pt heterostructure is evaluated with ferromagnetic resonance (FMR) effect representing standard Gilbert damping increase as the result of presence of the SHE. Our results unveil the role of SOT in dynamic control of the transverse magnetic permeability probed with impedance spectroscopy as useful and valuable technique for detection of future SHE devices.",1708.02402v2 2017-08-08,Nonlinear electric and thermoelectric Andreev transport through a hybrid quantum dot coupled to ferromagnetic and superconducting leads,"We discuss the nonlinear Andreev current of an interacting quantum dot coupled to spin-polarized and superconducting reservoirs when voltage and temperature biases are applied across the nanostructure. Due to the particle-hole symmetry introduced by the superconducting (S) lead, the subgap spin current vanishes identically. Nevertheless, the Andreev charge current depends on the degree of polarization in the ferromagnetic (F) contact since the shift of electrostatic internal potential of the conductor depends on spin orientation of the charge carrier. This spin-dependent potential shift characterizes nonlinear responses in our device. We show how the subgap current versus the bias voltage or temperature difference depends on the lead polarization in two cases, namely (i) S-dominant case, when the dot-superconductor tunneling rate ($\Gamma_R$) is much higher than the ferromagnet-dot tunnel coupling ($\Gamma_L$), and (ii) F-dominant case, when $\Gamma_L\gg \Gamma_R$. For the ferromagnetic dominant case the spin-dependent potential shows a nonmonotonic behavior as the dot level is detuned. Thus the subgap current can also exhibit interesting behaviors such as current rectification and the maximization of thermocurrents with smaller thermal biases when the lead polarization and the quantum dot level are adjusted.",1708.02451v1 2018-02-11,Strongly suppressed superconducting proximity effect and ferromagnetism in trilayers of $\rm Bi_2Se_3$ / $\rm SrRuO_3$ / underdoped $\rm YBa_2Cu_3O_x$: A possible new platform for Majorana nano-electronics,"We report properties of topological insulator - ferromagnet - superconductor trilayers comprised of thin films of 20 nm thick $\rm Bi_2Se_3$ on 10 nm $\rm SrRuO_3$ on 30 nm $\rm YBa_2Cu_3O_x$. As deposited trilayers are underdoped and have a superconductive transition with $\rm T_c$ onset at 75 K, zero resistance at 65 K, $\rm T_{Cueri}$ at 150 K and $\rm T^*$ of about 200 K. Further reannealing under vacuum yields the 60 K phase of $\rm YBa_2Cu_3O_x$ which still has zero resistance below about 40 K. Only when $10\times 100$ micro-bridges were patterned in the trilayer, some of the bridges showed resistive behavior all the way down to low temperatures. Magnetoresistance versus temperature of the superconductive ones showed the typical peak due to flux flow against pinning below $\rm T_c$, while the resistive ones showed only the broad leading edge of such a peak. All this indicates clearly weak-link superconductivity in the resistive bridges between superconductive $\rm YBa_2Cu_3O_x$ grains via the topological and ferromagnetic cap layers. Comparing our results to those of a reference trilayer with the topological $\rm Bi_2Se_3$ layer substituted by a non-superconducting highly overdoped $\rm La_{1.65}Sr_{0.35}CuO_4$, indicates that the superconductive proximity effect as well as ferromagnetism in the topological trilayer are actually strongly suppressed compared to the non-topological reference trilayer. This strong suppression is likely to originate in strong proximity induced edge currents in the SRO/YBCO layer that can lead to Majorana bound states, a possible signature of which is observed in the present study as zero bias conductance peaks.",1802.03728v2 2018-02-19,Spin current and spin transfer torque in ferromagnet/superconductor spin valves,"Using fully self consistent methods, we study spin transport in realistic, fabricable experimental spin valve systems consisting of two magnetic layers, a superconducting layer, and a spacer normal layer between the ferromagnets. Our methods ensure that the proper relations between spin current gradients and spin transfer torques are satisfied. We present results as a function of geometrical parameters, interfacial barrier values, misalignment angle between the ferromagnets, and bias voltage. Our main results are for the spin current and spin accumulation as functions of position within the spin valve structure. We see precession of the spin current about the exchange fields within the ferromagnets, and penetration of the spin current into the superconductor for biases greater than the critical bias, defined in the text. The spin accumulation exhibits oscillating behavior in the normal metal, with a strong dependence on the physical parameters both as to the structure and formation of the peaks. We also study the bias dependence of the spatially averaged spin transfer torque and spin accumulation. We examine the critical bias effect of these quantities, and their dependence on the physical parameters. Our results are predictive of the outcome of future experiments, as they take into account imperfect interfaces and a realistic geometry.",1802.06834v1 2018-04-02,"Anisotropic bulk and planar Heisenberg ferromagnets in uniform, arbitrarily oriented magnetic fields","Today, further downscaling of mobile electronic devices poses serious problems, such as energy consumption and local heat dissipation. In this context, spin wave majority gates made of very thin ferromagnetic films may offer a viable alternative. However, similar downscaling of magnetic thin films eventually enforces the latter to operate as quasi-two dimensional magnets, the magnetic properties of which are not yet fully understood, especially those related to anisotropies and external magnetic fields in arbitrary directions. To this end, we have investigated the behaviour of an easy-plane and easy-axis anisotropic ferromagnet -- both in two and three dimensions -- subjected to a uniform magnetic field, applied along an arbitrary direction. In this paper, a spin-1/2 Heisenberg Hamiltonian with anisotropic exchange interactions is solved using double-time temperature-dependent Green's functions and the Tyablikov decoupling approximation. We determine various magnetic properties such as the Curie temperature and the magnetization as a function of temperature and the applied magnetic field, discussing the impact of the system's dimensionality and the type of anisotropy. The magnetic reorientation transition taking place in anisotropic Heisenberg ferromagnets is studied in detail. Importantly, spontaneous magnetization is found to be absent for easy-plane two-dimensional spin systems with short range interactions.",1804.00436v1 2018-04-19,Chiral surface and edge plasmons in ferromagnetic conductors,"The recently introduced concept of ""surface Berry plasmons"" is studied in the concrete instance of a ferromagnetic conductor in which the Berry curvature, generated by spin-orbit (SO) interaction, has opposite signs for carriers parallel or antiparallel to the magnetization. By using collisionless hy- drodynamic equations with appropriate boundary conditions, we study both the surface plasmons of a three-dimensional ferromagnetic conductor and the edge plasmons of a two-dimensional one. The anomalous velocity and the broken inversion symmetry at the surface or the edge of the conductor create a ""handedness"", whereby the plasmon frequency depends not only on the angle between the wave vector and the magnetization, but also on the direction of propagation along a given line. In particular, we find that the frequency of the edge plasmon depends on the direction of propagation along the edge. These Berry curvature effects are compared and contrasted with similar effects induced in the plasmon dispersion by an external magnetic field in the absence of Berry curvature. We argue that Berry curvature effects may be used to control the direction of propagation of the surface plasmons via coupling with the magnetization of ferromagnetic conductors, and thus create a link between plasmonics and spintronics.",1804.06967v2 2018-11-02,Evidence for exchange Dirac gap in magneto-transport of topological insulator-magnetic insulator heterostructures,"Transport signatures of exchange gap opening because of magnetic proximity effect (MPE) are reported for bilayer structures of Bi2Se3 thin films on yttrium iron garnet (YIG) and thulium iron garnet (TmIG) of perpendicular magnetic anisotropy (PMA). Pronounced negative magnetoresistance (MR) was detected, and attributed to an emergent weak localization (WL) effect superimposing on a weak antilocalization (WAL). Thickness-dependent study shows that the WL originates from the time-reversal-symmetry breaking of topological surface states by interfacial exchange coupling. The weight of WL declined when the interfacial magnetization was aligned toward the in-plane direction, which is understood as the effect of tuning the exchange gap size by varying the perpendicular magnetization component. Importantly, magnetotransport study revealed anomalous Hall effect (AHE) of square loops and anisotropic magnetoresistance (AMR) characteristic, typifying a ferromagnetic conductor in Bi2Se3/TmIG, and the presence of an interfacial ferromagnetism driven by MPE. Coexistence of MPE-induced ferromagnetism and the finite exchange gap provides an opportunity of realizing zero magnetic-field dissipation-less transport in topological insulator/ferromagnetic insulator heterostructures.",1811.00689v1 2018-11-05,Evidence of long-range ferromagnetic order and spin frustration effects in the double perovskite $La_2CoMnO_6$,"We present a comprehensive study on the magnetic structure, dynamics, and phase evolution in the single-phase double perovskite $La_2CoMnO_6$. The mixed valence state due to oxygen deficiency is verified by X-ray photoelectron spectroscopy, and confirms a double ferromagnetic transition observed in DC magnetization. Neutron diffraction reveals that the magnetic structure is dominated by long-range ferromagnetic ordering, which is further corroborated by a critical exponents analysis of the paramagnetic to ferromagnetic phase transition. An analysis of the magnetization dynamics by means of linear and nonlinear ac magnetic susceptibilities marks the presence of two distinct cluster glass-like states that emerge at low temperatures. The isothermal entropy change as a function of temperature and magnetic field (H) is exploited to investigate the mechanism of stabilization of the magnetic phases across the H-T phase diagram. In the regime of the phase diagram where thermal energy is sufficiently low, regions of competing interactions due to local disorder become stabilized and display glass-like dynamics. The freezing mechanism of clusters is illustrated using a unique probe of transverse susceptibility that isolates the effects of the local anisotropy of the spin clusters. The results are summarized in a new H-T phase diagram of $La_2CoMnO_6$ revealed for the first time from these data.",1811.01835v1 2018-11-13,Ferromagnetic resonance and magnetic precessions in $\varphi_0$ junction,"The Josephson $\varphi_0$ junctions with the current-phase relation $I = I_c \sin (\varphi-\varphi_0)$, where the phase shift $\varphi_0$ is proportional to the magnetic moment perpendicular to the gradient of the asymmetric spin-orbit potential, demonstrate a number of unique features important for superconducting spintronics and modern informational technologies. Here we show that a current sweep along IV-characteristic of the $\varphi_0$ junction may lead to regular magnetization dynamics with a series of specific phase trajectories. The origin of these trajectories is related to a direct coupling between the magnetic moment and the Josephson oscillations in these junctions, and ferromagnetic resonance when Josephson frequency coincides with the ferromagnetic one. We demonstrate that an external electromagnetic field can control the dynamics of magnetic moment within a current interval corresponding to a Shapiro step and produce topological transformation of specific precession trajectories. We demonstrate the appearance of the DC component of superconducting current and clarify its role in the transformation of IV-characteristics in the resonance region. Good agreement between numerical and analytical results has been found in the ferromagnetic resonance region. The presented results might be used for developing novel resonance methods of determination of the spin-orbit coupling parameter in the non-centrosymmetric materials. We discuss experiments which can test our results.",1811.05282v2 2018-11-14,Interface-induced enhancement of the anomalous Nernst effect in ferromagnetic Mn$_5$Ge$_3$C$_{0.8}$ films,"The anomalous Nernst effect of thin bilayers comprising a thin normal metal film and a ferromagnetic Mn$_5$Ge$_3$C$_{0.8}$ film has been investigated. Epitaxial c-axis grown Mn$_5$Ge$_3$C$_{0.8}$ films have been obtained by e-beam evaporation as well as by magnetron sputtering on Ge(111) substrates. The size of the anomalous Nernst coefficient is independent of the growth method. Pt, Ta, and Pt/Cu bilayers have been used as voltage leads on top of the ferromagnetic film. We show that the interface between the normal metal electrode and the ferromagnetic film contributes significantly to the anomalous Nernst voltage which is suprisingly independent of the choice of the metal. The interface-induced enhancement of the anomalous Nernst effect is verified for samples with varying Mn content or undergoing an additional annealing step. The results suggest that the interface quality has a strong impact on the size of the anomalous Nernst effect.",1811.05857v2 2018-11-19,"Electronic structure of the novel high-$T_{\rm C}$ ferromagnetic semiconductor (Ga,Fe)Sb: x-ray magnetic circular dichroism and resonance photoemission spectroscopy studies","The electronic structure and the magnetism of the novel ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $T_{\rm C}$ can exceed room temperature, were investigated by means of x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable amount of 3$d^{6}$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and non-dispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe 3$d$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3$d$ states have finite contribution to the DOS at the Fermi energy. These states presumably consisting of majority-spin $p$-$d$ hybridized states or minority-spin $e$ states would be responsible for the ferromagnetic order in this material.",1811.07467v1 2019-05-03,Effective spin-mixing conductance of topological-insulator/ferromagnet and heavy-metal/ferromagnet spin-orbit-coupled interfaces: A first-principles Floquet-nonequilibrium-Green-function approach,"The spin mixing conductance (SMC) is a key quantity determining efficiency of spin transport across interfaces. Thus, knowledge of its precise value is required for accurate measurement of parameters quantifying numerous effects in spintronics, such as spin-orbit torque, spin Hall magnetoresistance, spin Hall effect and spin pumping. However, the standard expression for SMC, provided by the scattering theory in terms of the reflection probability amplitudes, is inapplicable when strong spin-orbit coupling (SOC) is present directly at the interface. This is the precisely the case of topological-insulator/ferromagnet and heavy-metal/ferromagnet interfaces of great contemporary interest. We introduce an approach where first-principles Hamiltonian of these interfaces, obtained from noncollinear density functional theory (ncDFT) calculations, is combined with charge conserving Floquet-nonequilibrium-Green-function formalism to compute {\em directly} the pumped spin current $I^{S_z}$ into semi-infinite left lead of two-terminal heterostructures Cu/X/Co/Cu or Y/Co/Cu---where X=Bi$_2$Se$_3$ and Y=Pt or W---due to microwave-driven steadily precessing magnetization of the Co layer. This allows us extract an effective SMC as a prefactor in $I^{S_z}$ vs. precession cone angle $\theta$ dependence, as long as it remains the same, $I^{S_z} \propto \sin^2 \theta$, as in the case where SOC is absent. By comparing calculations where SOC in switched off vs. switched on in ncDFT calculations, we find that SOC consistently reduces the pumped spin current and, therefore, the effective SMC.",1905.01299v2 2019-05-03,Ferromagnetic Anomalous Hall Effect in Cr-doped Bi$_2$Se$_3$ Thin Films via Surface-State Engineering,"The anomalous Hall effect (AHE) is a non-linear Hall effect appearing in magnetic conductors, boosted by internal magnetism beyond what is expected from the ordinary Hall effect. With the recent discovery of the quantized version of the AHE, the quantum anomalous Hall effect (QAHE), in Cr- or V-doped topological insulator (TI) (Sb,Bi)$_2$Te$_3$ thin films, the AHE in magnetic TIs has been attracting significant interest. However, one of the puzzles in this system has been that while Cr- or V-doped (Sb,Bi)$_2$Te$_3$ and V-doped Bi$_2$Se$_3$ exhibit AHE, Cr-doped Bi$_2$Se$_3$ has failed to exhibit even ferromagnetic AHE, the expected predecessor to the QAHE, though it is the first material predicted to exhibit the QAHE. Here, we have successfully implemented ferromagnetic AHE in Cr-doped Bi$_2$Se$_3$ thin films by utilizing a surface state engineering scheme. Surprisingly, the observed ferromagnetic AHE in the Cr-doped Bi$_2$Se$_3$ thin films exhibited only positive slope regardless of the carrier type. We show that this sign problem can be explained by the intrinsic Berry curvature of the system as calculated from a tight-binding model combined with a first-principles method.",1905.01300v1 2020-07-03,Light-controlled room temperature ferromagnetism in vanadium-doped tungsten diselenide semiconducting monolayers,"Atomically thin transition metal dichalcogenide (TMD) semiconductors hold enormous potential for modern optoelectronic devices and quantum computing applications. By inducing long-range ferromagnetism (FM) in these semiconductors through the introduction of small amounts of a magnetic dopant, it is possible to extend their potential in emerging spintronic applications. Here, we demonstrate light-mediated, room temperature (RT) FM, in V-doped WS2 (V-WS2) monolayers. We probe this effect using the principle of magnetic LC resonance, which employs a soft ferromagnetic Co-based microwire coil driven near its resonance in the radio frequency (RF) regime. The combination of LC resonance with an extraordinary giant magneto-impedance effect, renders the coil highly sensitive to changes in the magnetic flux through its core. We then place the V-WS2 monolayer at the core of the coil where it is excited with a laser while its change in magnetic permeability is measured. Notably, the magnetic permeability of the monolayer is found to depend on the laser intensity, thus confirming light control of RT magnetism in this two-dimensional (2D) material. Guided by density functional calculations, we attribute this phenomenon to the presence of excess holes in the conduction and valence bands, as well as carriers trapped in the magnetic doping states, which in turn mediates the magnetization of the V-WS2 monolayer. These findings provide a unique route to exploit light-controlled ferromagnetism in low powered 2D spintronic devices capable of operating at RT.",2007.01505v1 2020-07-03,Transition between half-metal and ferromagnetic semiconductor induced by silicon vacancy in epitaxial silicene,"Since the inevitability in experimental synthesis, defects show great importance to many materials. They will deeply regulate the properties of the materials, and then affect the further applications. Thus, exploring the effects of defects on the properties of materials is desired. Here, by using first-principles calculations, we systematically studied the effect of silicon vacancy defects on the properties of silicene generated on Nterminated cubic boron nitride (111) surface. It is found that the introduction of silicon vacancy would trigger transition between half-metal and ferromagnetic semiconductor. With small vacancy ratios of 1/36 and 1/24, the ground-state of the samples would behave as ferromagnetic semiconductors, and the band gaps are about 1.25 and 0.95 eV, respectively. When the vacancy ratio is increased up to 1/6, the sample would turn into a ferromagnetic half-metal with a half-metallic gap of around 0.15 eV. The change of the electronic structure of the samples is driven by the different electron transfer between silicon layer and substrate, i.e., there will be different amount of electrons transferred from the silicon layer to the substrate when the vacancy ratio is altered. This work would open a new way to regulate the properties of materials and extend applications in nanoelectronic field.",2007.01646v1 2020-07-11,Magnon-bipolar carrier drag thermopower in antiferromagnetic/ferromagnetic semiconductors: theoretical formulation and experimental evidence,"Quantized spin-wave known as magnon, a bosonic quasiparticle, can drag electrons or holes via sd exchange interaction and boost the thermopower over the conventional diffusive thermopower. P-type magnon-drag thermopower has been observed in both ferromagnetic and antiferromagnetic metallic and degenerate semiconductors. However, it has been less reported for intrinsic or n-type magnetic semiconductors; therefore, the impact of magnon-bipolar carrier drag on thermopower has remained unexplored. Here, a theoretical model for magnon-bipolar carrier drag thermopower is derived based on the magnon-carrier interaction lifetimes. The model predicts that the bipolar carrier drag thermopower becomes independent of both the carrier and magnon relaxation times. A proof of concept experiment is presented that confirms this prediction. We also report the observation of magnon-carrier drag thermopower in n-type and intrinsic ferromagnetic semiconductors experimentally. The p-type antiferromagnetic MnTe is doped with different amounts of Cr to produce non-degenerate and n-type semiconductors of various carrier concentrations. Cr dopants have a donor nature and create ferromagnetic-antiferromagnetic clusters due to the Cr3+ oxidation state. Heat capacity measurements confirm the presence of magnons in Cr-doped MnTe. It is shown that the magnon-drag thermopower is significantly reduced for 3%-5% Cr-doped samples due to bipolar drag effects and becomes negative for 14% and 20% Cr-doped MnTe due to dominant magnon-electron drag thermopower.",2007.05682v1 2020-07-13,Molecule Oxygen Induced Ferromagnetism and Half-metallicity in α-BaNaO4: A First Principles Study,"Molecule oxygen resembles 3d and 4f metals in exhibiting long-range spin ordering and electron strong correlated behaviors in compounds. The ferromagnetic spin ordering and half-metallicity, however, are quite elusive and have not been well acknowl-edged. In this article, we address this issue to study how spins will interact each other if the oxygen dimers are arranged in a dif-ferent way from that in the known super- and per-oxides by first principles calculations. Based on the results of structure search, thermodynamic study and lattice dynamics, we show that tetragonal {\alpha}-BaNaO4 is a stable half-metal with a Curie temperature at 120 K, a first example in this class of compounds. Like 3d and 4f metals, the O2 dimer carries a local magnetic moment 0.5 {\mu}B due to the unpaired electrons in its {\pi}* orbitals. This compound can be regarded as forming from the O2 dimer layers stacking in a head to head way. Different from that in AO2 (A=K, Rb, Cs), the spins are both ferromagnetically coupled within and between the layers. Spin polarization occurs in {\pi}* orbitals with spin-up electrons fully occupying the valence band and spin-down electrons partially the conduction band, forming the semiconducting and metallic channels, respectively. Our results highlight the im-portance of geometric arrangement of O2 dimers in inducing ferromagnetism and other novel properties in O2 dimer containing compounds.",2007.06433v1 2020-07-29,Impact of magnetic dopants on magnetic and topological phases in magnetic topological insulators,"A topological insulator doped with random magnetic impurities is studied. The system is modelled by the Kane-Mele model with a random spin exchange between conduction electrons and magnetic dopants. The dynamical mean field theory for disordered systems is used to investigate the electron dynamics. The magnetic long-range order and the topological invariant are calculated within the mean field theory. They reveal a rich phase diagram, where different magnetic long-range orders such as antiferromagnetic or ferromagnetic one can exist in the metallic or insulating phases, depending on electron and magnetic impurity fillings. It is found that insulator only occurs at electron half filling, quarter filling and when electron filling is equal to magnetic impurity filling. However, non-trivial topology is observed only in half-filling antiferromagnetic insulator and quarter-filling ferromagnetic insulator. At electron half filling, the spin Hall conductance is quantized and it is robust against magnetic doping, while at electron quarter filling, magnetic dopants drive the ferromagnetic topological insulator to ferromagnetic metal. The quantum anomalous Hall effect is observed only at electron quarter filling and dense magnetic doping.",2007.14687v1 2016-11-10,Pressure-induced quantum phase transition in the itinerant ferromagnet UCoGa,"In this paper, we report the results of a high pressure study of the itinerant 5f-electron ferromagnet UCoGa. The work is focused on probing the expected ferromagnet-to-paramagnet quantum phase transition induced by high pressure and on the general features of the P-T(-H) phase diagram. Diamond anvil cells were employed to measure the magnetization and electrical resistivity under pressures up to ~ 10 GPa.At ambient pressure, UCoGa exhibits collinear ferromagnetic ordering of uranium magnetic moments {\mu}U ~ 0.74 {\mu}B (at 2 K) aligned along the c-axis of the hexagonal crystal structure below Curie temperature TC = 48K. With the application of pressure, gradual decrease of both, TC and the saturated magnetic moment, has been observed up to pressures ~ 6 GPa. This is followed by a sharp drop of magnetic moment and a sudden disappearance of the magnetic order at the pressure of 6.5 GPa, suggesting a first-order phase transition, as expected for a clean system. The low temperature power law dependence of the electrical resistivity shows distinct anomalies around the ~ 6 GPa, consistent with the pressure evolution of the magnetic moment and the ordering temperature. The tricritical point of the UCoGa phase diagram is located at approximately ~ 30 K and ~ 6 GPa.",1611.03276v1 2016-11-17,Inductive detection of field-like and damping-like AC inverse spin-orbit torques in ferromagnet/normal metal bilayers,"Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both damping-like and field-like spin-orbit torques have been observed in the forward process of current-driven spin torque and damping-like inverse spin-orbit torque has been well-studied via spin pumping into heavy metal layers. Here we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the AC charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly-extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all AC charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Ni$_{80}$Fe$_{20}$/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques. While the magnitudes of both the damping-like and field-like inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the damping-like magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the damping-like spin-to-charge conversion.",1611.05798v2 2016-11-22,Uranium ferromagnet with negligible magnetocrystalline anisotropy - $\mathrm{U_{4}Ru_{7}Ge_{6}}$,"Strong magnetocrystalline anisotropy is a well-known property of uranium compounds. The almost isotropic ferromagnetism in $\mathrm{U_{4}Ru_{7}Ge_{6}}$ reported in this paper represents a striking exception. We present results of magnetization, AC susceptibility, thermal expansion, specific heat and electrical resistivity measurements performed on a $\mathrm{U_{4}Ru_{7}Ge_{6}}$ single crystal at various temperatures and magnetic fields and discuss them in conjunction with results of first-principles electronic-structure calculations. $\mathrm{U_{4}Ru_{7}Ge_{6}}$ behaves as an itinerant 5$f$-electron ferromagnet ($T_{\mathrm{C}}= 10.7 K$, $\mu_{\mathrm{S}}= 0.85 \mu_{\mathrm{B}}/f.u.$ at 1.9 K. The ground-state easy-magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field $\mu_{0}H_{\mathrm{a}}$ along the [001] direction is only of $\sim0.3 T$, which is at least 3 orders of magnitude smaller value than in other U ferromagnets. At $T_{r}=5.9 K$ the easy magnetization direction changes for [001] which holds at temperatures up to $T_{\mathrm{C}}$. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in case of the [111]([001]) easy-magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. The ab initio calculated ground-state magnetic moment of $1.01 \mu_{\mathrm{B}}/f.u.$ is oriented along [111]. The two crystallographically inequivalent U sites are a consequence of spin-orbit coupling of the U 5$f$-electrons. In the excited state which is only 0.9 meV above the ground state the moment points to the [001] direction in agreement with experiment.",1611.07335v1 2017-01-09,The Coprime Quantum Chain,"In this paper we introduce and study the coprime quantum chain, i.e. a strongly correlated quantum system defined in terms of the integer eigenvalues $n_i$ of the occupation number operators at each site of a chain of length $M$. The $n_i$'s take value in the interval $[2,q]$ and may be regarded as $S_z$ eigenvalues in the spin representation $j = (q-2)/2$. The distinctive interaction of the model is based on the coprimality matrix $\bf \Phi$: for the ferromagnetic case, this matrix assigns lower energy to configurations where occupation numbers $n_i$ and $n_{i+1}$ of neighbouring sites share a common divisor, while for the anti-ferromagnetic case it assigns lower energy to configurations where $n_i$ and $n_{i+1}$ are coprime. The coprime chain, both in the ferro and anti-ferromagnetic cases, may present an exponential number of ground states whose values can be exactly computed by means of graph theoretical tools. In the ferromagnetic case there are generally also frustration phenomena. A fine tuning of local operators may lift the exponential ground state degeneracy and, according to which operators are switched on, the system may be driven into different classes of universality, among which the Ising or Potts universality class. The paper also contains an appendix by Don Zagier on the exact eigenvalues and eigenvectors of the coprimality matrix in the limit $q \rightarrow \infty$.",1701.02317v1 2017-01-11,Carrier driven coupling in ferromagnetic oxide heterostructures,"Transition metal oxides are well known for their complex magnetic and electrical properties. When brought together in heterostructure geometries, they show particular promise for spintronics and colossal magnetoresistance applications. In this letter, we propose a new mechanism for the coupling between layers of itinerant ferromagnetic materials in heterostructures. The coupling is mediated by charge carriers that strive to maximally delocalize through the heterostructure to gain kinetic energy. In doing so, they force a ferromagnetic or antiferromagnetic coupling between the constituent layers. To illustrate this, we focus on heterostructures composed of SrRuO$_3$ and La$_{1-x}$A$_{x}$MnO$_3$ (A=Ca/Sr). Our mechanism is consistent with antiferromagnetic alignment that is known to occur in multilayers of SrRuO$_3$-La$_{1-x}$A$_{x}$MnO$_3$. To support our assertion, we present a minimal Kondo-lattice model which reproduces the known magnetization properties of such multilayers. In addition, we discuss a quantum well model for heterostructures and argue that the spin-dependent density of states determines the nature of the coupling. As a smoking gun signature, we propose that bilayers with the same constituents will oscillate between ferromagnetic and antiferromagnetic coupling upon tuning the relative thicknesses of the layers.",1701.03117v1 2017-01-18,Step Stone Effect: A sp anti-bonding Mediated Long-Range Ferromagnetism in Cr-doped Carrier-Free Bi2Te3,"Despite the recent success in the realization of the quantum anomalous Hall effect, the underlying physical mechanism of the long range Ferromagnetism is still unclear. Based on our density functional theory calculations, we discovered an intriguing long-range ferromagnetic order in Cr-doped, carrier-free Bi2Te3, with the separation between dopants more than 8 {\AA}. We found that this magnetic coupling is facilitated by an anti-bonding state originated from the lone pair of a Te 5p state and a Bi 6s state, despite this anti-bonding state lies below the valence band maximum. Such a state serves as a hidden step stone merged in the electron sea and mediates the long-range magnetic order. An effective electron hopping model is proposed to explain this mechanism. This novel mechanism sheds light on the understanding of long-range ferromagnetism in insulators and may lead to the realization of the long-range magnetic order in dilute magnetic semiconductors.",1701.04943v5 2017-01-02,Ferromagnetic films with three to twenty spin layers as described using second order perturbed Heisenberg Hamiltonian,"Modified second order perturbed Heisenberg Hamiltonian was employed to describe the magnetic properties of ferromagnetic films with three to twenty spin layers for the first time. Previously, the solution of second order perturbed Heisenberg Hamiltonian was found for ferromagnetic films with two to five layers under some assumptions by us. In this report, the exact solution is presented without any assumptions by calculating the pseudo inverse of matrix C. All eight magnetic parameters such as spin exchange interaction, second order magnetic anisotropy, fourth order magnetic anisotropy, stress induced magnetic anisotropy, demagnetization factor, in plane magnetic field, out of plane magnetic field and magnetic dipole interaction were taken into account. The easy and hard directions were found using 3-D and 2-D plots of total magnetic energy versus these magnetic parameters. Angle between easy and hard directions was nearly 107 degrees in many cases. The magnetic easy axis gradually rotates from out of plane to in plane direction by indicating a preferred in plane orientation of magnetic easy axis for films with higher number of spin layers. These theoretical results agree with some experimental research data of ferromagnetic thin films.",1701.07780v1 2017-04-28,Ferromagnetism and spin-dependent transport at a complex oxide interface,"Complex oxide interfaces are a promising platform for studying a wide array of correlated electron phenomena in low-dimensions, including magnetism and superconductivity. The microscopic origin of these phenomena in complex oxide interfaces remains an open question. Here we investigate for the first time the magnetic properties of semi-insulating NdTiO$_3$/SrTiO$_3$ (NTO/STO) interfaces and present the first milli-Kelvin study of NTO/STO. The magnetoresistance (MR) reveals signatures of local ferromagnetic order and of spin-dependent thermally-activated transport, which are described quantitatively by a simple phenomenological model. We discuss possible origins of the interfacial ferromagnetism. In addition, the MR also shows transient hysteretic features on a timescale of ~10-100 seconds. We demonstrate that these are consistent with an extrinsic magneto-thermal origin, which may have been misinterpreted in previous reports of magnetism in STO-based oxide interfaces. The existence of these two MR regimes (steady-state and transient) highlights the importance of time-dependent measurements for distinguishing signatures of ferromagnetism from other effects that can produce hysteresis at low temperatures.",1704.08828v3 2018-05-07,Appearance of ferromagnetism property for Si nano-polycrystalline body and vanishing of electrical resistances at local high frequencies,"Reduction in the skin effect for the sintered Si nanopolycrystalline body as an electricity conductor at a high frequency due to its nano-structure was studied. Singular vanishing of electrical resistances near a local high magnetic harmonic frequency of a few MHz was observed. This phenomenon has not been observed for conventional ferromagnetic metals. The measured electrical resistances changed to almost 0 m{\Omega} at room temperature. At the same time, negative resistance of the sintered Si nano-polycrystalline body was observed. It will be applicable to electronic transmittance lines or semiconductors. Numerical calculation was also performed on the electrical resistance with frequency dependency while considering the electric field and magnetic field in the sintered Si nanopolycrystalline body. The calculation could explain the variation of the relative permittivity of the Si nanopolycrystalline and the phenomenon for vanishing the resistivity at frequency of MHz theoretically. Reduced Si nanoparticles from SiO2 powder were synthesized by laser ablation in liquid. A Si nano-polycrystalline body made of the reduced Si nanoparticles was fabricated. It was found by measuring the magnetization property of the body that the sintered Si nano-polycrystalline body has ferromagnetism. High-density dangling bonds cause the sintered Si nanopolycrystalline to have ferromagnetism. In this study, the density of the unpaired electrons in the sintered Si nanopolycrystalline was observed using ESR. It has been clarified that the Si nanopowder and the sintered Si nanopolycrystalline have numerous dangling bonds. Both densities of the dangling bonds were evaluated.",1805.02312v1 2018-05-07,Universality class of the nonequilibrium phase transition in two dimensional Ising ferromagnet driven by propagating magnetic field wave,"The finite size analysis of the nonequilibrium phase transition, in two dimensional Ising ferromagnet driven by plane propagating magneticwave, is studied by Monte Carlo simulation. It is observed that the system undergoes a nonequilibrium dynamicphase transition from a high temperature dynamically symmetric ({\it propagating}) phase to a low temperature dynamically symmetry-broken ({\it pinned}) phase as the system is cooled below the transition temperature. This transition temperature is determined precisely by studying the fourth order Binder Cumulant of the dynamic order parameter as a function of temperature for different system sizes ($L$). From the finite size analysis of dynamic order parameter ($Q_L \sim L^{-{{\beta} \over {\nu}}}$) and the dynamic susceptibility ($\chi^{Q}_{L} \sim L^{{\gamma} \over {\nu}}$), we have estimated the critical exponents $\beta/\nu=0.146\pm0.025$ and $\gamma/\nu=1.869\pm0.135$ (measured from the data read at the critical temperature obtained from Binder cumulant), and $\gamma/\nu=1.746\pm0.017$ (measured from the peak positions of dynamic susceptibility). Our results indicate that such driven Ising ferromagnet belongs to the same universality class of the two-dimensional equilibrium Ising ferromagnet (where $\beta/\nu=1/8$ and $\gamma/\nu=7/4$), within the limits of statistical errors.",1805.02389v2 2018-05-13,Pressure-induced spin reorientation transition in layered ferromagnetic insulator Cr2Ge2Te6,"Anisotropic magnetoresistance (AMR) of Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. The easy axis direction of the magnetization is inferred from the AMR saturation feature in the presence and absence of the applied pressure. At zero applied pressure, the easy axis is along the c-direction or perpendicular to the layer. Upon application of a hydrostatic pressure>1 GPa, the uniaxial anisotropy switches to easy-plane anisotropy which drives the equilibrium magnetization from the c-axis to the ab-plane at zero magnetic field, which amounts to a giant magnetic anisotropy energy change (>100%). As the temperature is increased across the Curie temperature, the characteristic AMR effect gradually decreases and disappears. Our first-principles calculations confirm the giant magnetic anisotropy energy change with moderate pressure and assign its origin to the increased off-site spin-orbit interaction of Te atoms due to a shorter Cr-Te distance. Such a pressure-induced spin reorientation transition is very rare in three-dimensional ferromagnets, but it may be common to other layered ferromagnets with similar crystal structures to CGT, and therefore offers a unique way to control magnetic anisotropy.",1805.04937v1 2018-05-22,FMR-related phenomena in spintronic devices,"Spintronic devices, such as non-volatile magnetic random access memories and logic devices, have attracted considerable attention as potential candidates for future high efficient data storage and computing technology. In a heavy metal or other emerging material with strong spin-orbit coupling (SOC), the charge currents induce spin currents or spin accumulations via SOC. The generated spin currents can exert spin-orbit torques (SOTs) on an adjacent ferromagnet, which opens up a new way to realize magnetization dynamics and switching of the ferromagnetic layer for spintronic devices. In the SOT scheme, the charge-to-spin interconversion efficiency (SOT efficiency) is an important figure of merit for applications. For the effective characterization of this efficiency, the ferromagnetic resonance (FMR) based methods, such as the spin transfer torque ferromagnetic resonance (ST-FMR) and the spin pumping, are common utilized in addition to low frequency harmonic or dc measurements. In this review, we focus on the ST-FMR measurements for the evaluation of the SOT efficiency. We provide a brief summary of the different ST-FMR setups and data analysis methods. We then discuss ST-FMR and SOT studies in various materials, including heavy metals and alloys, topological insulators, two dimensional (2D) materials, interfaces with strong Rashba effect, antiferromagnetic materials, two dimensional electron gas (2DEG) in oxide materials and oxidized nonmagnetic materials.",1805.08496v1 2018-05-25,Learning Restricted Boltzmann Machines via Influence Maximization,"Graphical models are a rich language for describing high-dimensional distributions in terms of their dependence structure. While there are algorithms with provable guarantees for learning undirected graphical models in a variety of settings, there has been much less progress in the important scenario when there are latent variables. Here we study Restricted Boltzmann Machines (or RBMs), which are a popular model with wide-ranging applications in dimensionality reduction, collaborative filtering, topic modeling, feature extraction and deep learning. The main message of our paper is a strong dichotomy in the feasibility of learning RBMs, depending on the nature of the interactions between variables: ferromagnetic models can be learned efficiently, while general models cannot. In particular, we give a simple greedy algorithm based on influence maximization to learn ferromagnetic RBMs with bounded degree. In fact, we learn a description of the distribution on the observed variables as a Markov Random Field. Our analysis is based on tools from mathematical physics that were developed to show the concavity of magnetization. Our algorithm extends straighforwardly to general ferromagnetic Ising models with latent variables. Conversely, we show that even for a contant number of latent variables with constant degree, without ferromagneticity the problem is as hard as sparse parity with noise. This hardness result is based on a sharp and surprising characterization of the representational power of bounded degree RBMs: the distribution on their observed variables can simulate any bounded order MRF. This result is of independent interest since RBMs are the building blocks of deep belief networks.",1805.10262v2 2008-07-06,Low voltage control of ferromagnetism in a semiconductor p-n junction,"The concept of low-voltage depletion and accumulation of electron charge in semiconductors, utilized in field-effect transistors (FETs), is one of the cornerstones of current information processing technologies. Spintronics which is based on manipulating the collective state of electron spins in a ferromagnet provides complementary technologies for reading magnetic bits or for the solid-state memories. The integration of these two distinct areas of microelectronics in one physical element, with a potentially major impact on the power consumption and scalability of future devices, requires to find efficient means for controlling magnetization electrically. Current induced magnetization switching phenomena represent a promising step towards this goal, however, they relay on relatively large current densities. The direct approach of controlling the magnetization by low-voltage charge depletion effects is seemingly unfeasible as the two worlds of semiconductors and metal ferromagnets are separated by many orders of magnitude in their typical carrier concentrations. Here we demonstrate that this concept is viable by reporting persistent magnetization switchings induced by short electrical pulses of a few volts in an all-semiconductor, ferromagnetic p-n junction.",0807.0906v1 2011-11-18,Nanoscale inhomogeneities: A new path toward high Curie temperature ferromagnetism in diluted materials,"Room temperature ferromagnetism has been one of the most sought after topics in today's emerging field of spintronics. It is strongly believed that defect- and inhomogeneity- free sample growth should be the optimal route for achieving room-temperature ferromagnetism and huge efforts are made in order to grow samples as ""clean"" as possible. However, until now, in the dilute regime it has been difficult to obtain Curie temperatures larger than that measured in well annealed samples of (Ga,Mn)As ($\sim$190 K for 12% doping). In the present work, we propose an innovative path to room-temperature ferromagnetism in diluted magnetic semiconductors. We theoretically show that even a very small concentration of nanoscale inhomogeneities can lead to a tremendous boost of the critical temperatures: up to a 1600% increase compared to the homogeneous case. In addition to a very detailed analysis, we also give a plausible explanation for the wide variation of the critical temperatures observed in (Ga,Mn)N and provide a better understanding of the likely origin of very high Curie temperatures measured occasionally in some cases. The colossal increase of the ordering temperatures by nanoscale cluster inclusions should open up a new direction toward the synthesis of materials relevant for spintronic functionalities.",1111.4355v2 2011-11-23,Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb_2Ti_2O_7,"In a class of frustrated magnets known as spin ice, magnetic monopoles emerge as classical defects and interact via the magnetic Coulomb law. With quantum-mechanical interactions, these magnetic charges are carried by fractionalised bosonic quasi-particles, spinons, which can undergo Bose-Einstein condensation through a first-order transition via the Higgs mechanism. Here, we report evidence of a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in single-crystal Yb2Ti2O7. Polarised neutron-scattering experiments show that the diffuse [111]-rod scattering and pinch-point features which develop on cooling are suddenly suppressed below T_C ~ 0.21 K, where magnetic Bragg peaks and a full depolarisation of the neutron spins are observed with thermal hysteresis, indicating a first-order ferromagnetic transition. Our results are explained on the basis of a quantum spin-ice model, whose high-temperature phase is effectively described as a magnetic Coulomb liquid, while the ground state shows a nearly collinear ferromagnetism with gapped spin excitations.",1111.5406v2 2012-04-19,Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons,"Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While the low temperature ferromagnet-like feature is observed in both types of ribbons, such room temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a 'positive exchange bias'. Electron spin resonance measurements infer that the carbon related defects may responsible for the observed magnetic behaviour in both types of ribbons. Furthermore, proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance provides no indications for the presence of potassium (cluster) related signals, emphasizing the intrinsic magnetic nature of the ribbons. Our combined experimental results may infer the coexistence of ferromagnetic clusters with anti-ferromagnetic regions leading to disordered magnetic phase. We discuss the origin of the observed contrast in the magnetic behaviours of these two types of ribbons.",1204.4401v1 2012-04-27,Energetically stable singular vortex cores in an atomic spin-1 Bose-Einstein condensate,"We analyze the structure and stability of singular singly quantized vortices in a rotating spin-1 Bose-Einstein condensate. We show that the singular vortex can be energetically stable in both the ferromagnetic and polar phases despite the existence of a lower-energy nonsingular coreless vortex in the ferromagnetic phase. The spin-1 system exhibits an energetic hierarchy of length scales resulting from different interaction strengths and we find that the vortex cores deform to a larger size determined by the characteristic length scale of the spin-dependent interaction. We show that in the ferromagnetic phase the resulting stable core structure, despite apparent complexity, can be identified as a single polar core with axially symmetric density profile which is nonvanishing everywhere. In the polar phase, the energetically favored core deformation leads to a splitting of a singly quantized vortex into a pair of half-quantum vortices that preserves the topology of the vortex outside the extended core region, but breaks the axial symmetry of the core. The resulting half-quantum vortices exhibit nonvanishing ferromagnetic cores.",1204.6347v4 2013-09-19,Anomalous Hall effect in ferromagnets with Gaussian disorder,"Using the Kubo formalism we derived expressions and implemented the method for calculating the anomalous Hall conductivity (AHC) in ferromagnets with short-range Gaussian disorder directly from first-principles electronic structure of the perfect crystal. We used this method to calculate the AHC in bcc Fe, fcc Co, L1o-FePd, L1o-FePt as well as thin bcc Fe(001) films. Within our approach we can transparently decompose the conductivity into intrinsic (IC), side jump (SJ) and intrinsic skew-scattering (ISK) contributions. The existence of ISK, which originates from asymmetric Mott scattering but is clearly distinguishable from conventional skew-scattering in that it converges to a finite value in clean limit, was pointed out by Sinitsyn et al. [Phys. Rev. B 75, 045315 (2007)]. Here, we collect all contributions to the AHC in ferromagnets which result in ""scattering-independent"" AHE in clean limit, and analyze their relative magnitude from first principles calculations. By comparing our results to existing experiments we show that the Gaussian disorder is well suited to model various types of disorder present in real materials, to some extent including the effect of temperature. In particular, we show that in addition to intrinsic and side-jump AHE, the intrinsic skew-scattering can be a major player in determining the magnitude of the AHE in ferromagnets.",1309.4979v1 2014-05-19,Infrared electrodynamics and ferromagnetism in the topological semiconductors Bi$_2$Te$_3$ and Mn-doped Bi$_2$Te$_3$,"We report on infrared (IR) optical experiments on Bi$_2$Te$_3$ and Mn-doped Bi$_2$Te$_3$ epitaxial thin films. In the latter film, dilute Mn doping (4.5\%) of the topologically nontrivial semiconductor host results in time-reversal-symmetry-breaking ferromagnetic order below $T_C$=15 K. Our spectroscopic study shows both materials share the Bi$_2$Te$_3$ crystal structure, as well as classification as bulk degenerate semiconductors. Hence the Fermi energy is located in the Bi$_2$Te$_3$ conduction band in both materials, and furthermore, there is no need to invoke topological surface states to describe the conductivity spectra. We also demonstrate that the Drude oscillator strength gives a simple metric with which to distinguish the possibility of topological surface state origins of the low frequency conductance, and conclude that in both the pristine and Mn-doped Bi$_2$Te$_3$ samples the electromagnetic response is indeed dominated by the bulk material properties, rather than those of the surface. An encouraging aspect for taking advantage of the interplay between nontrivial topology and magnetism, however, is that the temperature dependence of the Mn-doped Bi$_2$Te$_3$ film suggests bulk charge carriers do not play a significant role in mediating ferromagnetism. Thus, a truly insulating bulk may still be suitable for the formation of a ferromagnetic ground state in this dilute magnetic topological semiconductor.",1405.4916v1 2014-05-26,Amorphous ferromagnetism and re-entrant magnetic glassiness in Sm$_{2}$Mo$_{2}$O$_{7}$: new insights into the electronic phase diagram of pyrochlore molybdates,"We discuss the magnetic properties of a Sm$_{2}$Mo$_{2}$O$_{7}$ single crystal as investigated by means of different experimental techniques. In the literature, a conventional itinerant ferromagnetic state is reported for the Mo$^{4+}$ sublattice below $\sim 78$ K. However, our results of dc magnetometry, muon spin spectroscopy ($\mu^{+}$SR) and high-harmonics magnetic ac susceptibility unambiguously evidence highly disordered conditions in this phase, in spite of the crystalline and chemical order. This disordered magnetic state shares several common features with amorphous ferromagnetic alloys. This scenario for Sm$_{2}$Mo$_{2}$O$_{7}$ is supported by the anomalously high values of the critical exponents, as mainly deduced by a scaling analysis of our dc magnetization data and confirmed by the other techniques. Moreover, $\mu^{+}$SR detects a significant static magnetic disorder at the microscopic scale. At the same time, the critical divergence of the third-harmonic component of the ac magnetic susceptibility around $\sim 78$ K leads to additional evidence towards the glassy nature of this magnetic phase. Finally, the longitudinal relaxation of $\mu^{+}$ spin polarization (also supported by results of ac susceptibility) evidences re-entrant glassy features similar to amorphous ferromagnets.",1405.6608v1 2014-06-13,"{Effect of on-site Coulomb interaction (\textit{U}) on the electronic and magnetic properties of Fe$_{2}$MnSi, Fe$_{2}$MnAl and Co$_{2}$MnGe","The electronic band structures, density of states plots and magnetic moments of Fe$_{2 }$MnSi, Fe$_{2 }$MnAl, and Co$_{2 }$MnGe are studied by using the first principles calculations. The FM solutions using LSDA without \textit{U} show the presence of half-metallic ferromagnetic (HFM) ground state in Fe$_{2 }$MnSi, whereas the ground state of Fe$_{2 }$MnAl is found to be metallic. In both compounds the maximum contribution to the total magnetic moment is from the Mn atom, while the Fe atom contributes very less. The electronic structures and magnetic moments of Fe-based compounds affected significantly by \textit{U}, whereas its effect is very less on Co$_{2}$MnGe. The magnetic moment of Fe atom in Fe$_{2 }$MnSi (Fe$_{2 }$MnAl), increased by $\sim$ 70 % ($\sim$ 75 %) and in Mn atom it decreases by $\sim$ 50 % ($\sim$ 70 %) when the value of \textit{U} is increased from 1 to 5 eV. The Hund's like exchange interactions are increasing in Fe atom while decreasing in Mn atom with increase in \textit{U}. The Fe and Mn moments are ferromagnetically coupled in Fe$_{2 }$MnSi for all values of \textit{U}, whereas in Fe$_{2 }$MnAl they coupled antiferromagnetically below \textit{U} = 2 eV and ferromagnetically above it. Above \textit{U} = 2 eV the metallic ground state of Fe$_{2 }$MnAl changes to semiconducting ground state and the ferromagnetic coupling between Fe and Mn atoms appears to be responsible for this.",1406.3505v1 2014-06-27,Optimal bounds for periodic mixtures of ferromagnetic interactions,"In this paper we give optimal bounds for the homogenization of periodic Ising systems when the coefficients may take two given values in given proportions.",1406.7152v1 2014-06-29,Trajectory of Anomalous Hall Effect toward the Quantized State in a Ferromagnetic Topological Insulator,"Topological insulators are bulk electronic insulators which possess symmetry protected gapless modes on their surfaces. Breaking the symmetries that underlie the gapless nature of the surface modes is predicted to give rise to exotic new states of matter. In particular, it has recently been predicted and shown that breaking of time reversal symmetry in the form of ferromagnetism can give rise to a gapped state characterized by a zero magnetic field quantized Hall response and dissipationless longitudinal transport known as the Quantum Anomalous Hall (QAH) state. A key question that has thus far remained experimentally unexplored is the relationship of this new type of quantum Hall state with the previously known orbitally driven quantum Hall states. Here, we show experimentally that a ferromagnetic topological insulator exhibiting the QAH state is well described by the global phase diagram of the quantum Hall effect. By mapping the behavior of the conductivity tensor in the parameter space of temperature, magnetic field, and chemical potential in the vicinity of the QAH phase, we find evidence for quantum criticality and delocalization behavior that can quantitatively be described by the renormalization group properties of the quantum Hall ground state. This result demonstrates that the QAH state observed in ferromagnetic topological insulators can be understood within the context of the law of corresponding states which governs the quantum Hall state. This suggests a roadmap for studying the QAH effect including transitions to possible adjacent topologically non-trivial states and a possible universality class for the QAH transition.",1406.7450v1 2015-12-08,Density-matrix renormalization group study of Kitaev-Heisenberg model on the triangular lattice,"We study the Kitaev-Heisenberg model on the triangular lattice by using the two-dimensional density-matrix renormalization group method. Calculating the ground-state energy and spin structure factors, we obtain the ground state phase diagram of the Kitaev-Heisenberg model. As suggested by previous studies, we find a 120$^\circ$ antiferromagnetic (AFM) phase, a $\mathbb{Z}_2$-vortex crystal phase, a nematic phase, a dual $\mathbb{Z}_2$-vortex crystal phase (dual counterpart of $\mathbb{Z}_2$-vortex crystal phase), a $\mathbb{Z}_6$ ferromagnetic phase, and a dual ferromagnetic phase (dual counterpart of $\mathbb{Z}_6 $ ferromagnetic phase). Spin correlations discontinuously change at the phase boundaries because of first-order phase transitions. In the (dual) $\mathbb{Z}_6$ ferromagnetic phase, we find that the dominant spin component of the spin structure factors is different on either side of the SU(2) symmetric point, although there is no phase transition. Furthermore, we study the relation among von Neumann entanglement entropy, entanglement spectrum, and phase transitions of the model. We find that the Schmidt gap closes at the phase boundaries and thus the entanglement entropy clearly changes as well. This is different from the Kitaev-Heisenberg model on the honeycomb lattice, where the Schmidt gap and the entanglement entropy are not necessary a good measure of phase transitions.",1512.02334v3 2016-05-03,Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2,"We report on the magnetic properties of CsCo$_2$Se$_2$ with ThCr$_2$Si$_2$ structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo$_2$Se2$_2$ undergoes a phase transition to an antiferromagnetically ordered state with a N\'eel temperature of $T_{\rm N} \approx$ 66 K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of $\Theta \approx$ 51.0 K. We find that the magnetic structure of CsCo$_2$Se$_2$ consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at $T$ = 1.5 K is found to be only 0.20(1)$\mu_{\rm Bohr}$/Co. Already in comparably small magnetic fields of $\mu_0 H_{MM}$(5K) $\approx$ 0.3 T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo$_2$Se$_2$, with the moments fully ferromagnetically saturated in a magnetic field of $\mu_0 H_{\rm FM}$(5K) $\approx$ 6.4 T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo$_2$Se$_2$ with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo$_2$Se$_2$, which is chemically and electronically posed closely to the $A_xFe_{2-y}Se_2$ superconductors, as a host of versatile magnetic interactions.",1605.01113v1 2016-05-13,Coarsening and thermalisation properties of a quenched ferromagnetic spin-1 condensate,"We examine the dynamics of a quasi-two-dimensional spin-1 condensate in which the quadratic Zeeman energy q is suddenly quenched to a value where the system has a ferromagnetic ground state. There are two distinct types of ferromagnetic phases, i.e. a range of q values where the magnetization prefers to be in the direction of the external field (easy-axis), and a range of q values where it prefers to be transverse to the field (easy-plane). We study the quench dynamics for a variety of q values and show that there is a single dynamical critical exponent to characterize the scale invariant domain growth for each ferromagnetic phase. For both quenches we give simple analytic models that capture the essential scale invariant dynamics, and correctly predict the exponents. Because the order parameter for each phase is different, the natures of the domains and the relevant topological defects in each type of coarsening is also different. To explore these differences we characterize the fractal dimension of the domain walls, and the relationship of polar-core spin vortices to the domains in the easy-plane phase. Finally, we consider how the energy liberated from the quench thermalizes in the easy-axis quench. We show that local equilibrium is established in the spin waves on moderate time scales, but continues to evolve as the domains anneal.",1605.04016v2 2016-05-17,Electronic Structure of the Ferromagnetic Semiconductor Fe-doped Ge Revealed by Soft X-ray Angle-Resolved Photoemission Spectroscopy,"Ge$_{1-x}$Fe$_{x}$ (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K, and hence is a promising material for spintronic applications compatible with Si technology. We have studied its electronic structure by soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurements in order to elucidate the mechanism of the ferromagnetism. We observed finite Fe 3$d$ components in the states at the Fermi level ($E_{F}$) in a wide region in momentum space and $E_{F}$ was located above the valence-band maximum (VBM). First-principles supercell calculation also suggested that the $E_{F}$ is located above the VBM, within the narrow spin-down $d$($e$) band and within the spin-up impurity band of the deep acceptor-level origin derived from the strong $p$-$d$($t_{2}$) hybridization. We conclude that the narrow $d$($e$) band is responsible for the ferromagnetic coupling between Fe atoms while the acceptor-level-originated band is responsible for the transport properties of Ge:Fe.",1605.05275v1 2016-08-09,Griffiths phase and critical behavior in layered Sr$_2$IrO$_4$ ferromagnet,"We report the existence of Griffiths phase (GP) and its influence on critical phenomena in layered Sr$_2$IrO$_4$ ferromagnet (T$_C$ = 221.5 K). The power law behavior of inverse magentic susceptibility, 1/$\chi$(T) with exponent $\lambda = 0.18(2)$ confirm the GP in the regime T$_C$ $<$ T $\leq$ T$_G$ = 279.0(5) K. Moreover, the detailed critical analysis via modified Arrott plot method exhibits unrealistic critical exponents $\beta$ = 0.77(1), $\gamma$ = 1.59(2) and $\delta = 3.06(4)$, in corroboration with magneto-caloric study. The abnormal exponent values have been viewed in context of ferromagnetic-Griffiths phase transition. The GP has been further analyzed using Bray model, which yields a reliable value of $\beta$ = 0.19(2), belonging to the two-dimensional (2D) XYh$_4$ universality class with strong anisotropy present in Sr$_2$IrO$_4$. The present study proposes Bray model as a possible tool to investigate the critical behavior for Griffiths ferromagnets in place of conventional Arrott plot analysis. The possible origins of GP and its correlation with insulating nature of Sr$_2$IrO$_4$ have been discussed.",1608.02726v1 2016-08-18,Phase diagram and spin correlations of the Kitaev-Heisenberg model: Importance of quantum effects,"We explore the phase diagram of the Kitaev-Heisenberg model with nearest neighbor interactions on the honeycomb lattice using the exact diagonalization of finite systems combined with the cluster mean field approximation, and supplemented by the insights from the linear spin-wave and second--order perturbation theories. This study confirms that by varying the balance between the Heisenberg and Kitaev term, frustrated exchange interactions stabilize in this model four phases with magnetic long range order: N\'eel phase, ferromagnetic phase, and two other phases with coexisting antiferromagnetic and ferromagnetic bonds, zigzag and stripy phases. They are separated by two disordered quantum spin-liquid phases, and the one with ferromagnetic Kitaev interactions has a substantially broader range of stability as the neighboring competing ordered phases, ferromagnetic and stripy, have very weak quantum fluctuations. Focusing on the quantum spin-liquid phases, we study spatial spin correlations and dynamic spin structure factor of the model by the exact diagonalization technique, and discuss the evolution of gapped low-energy spin response across the quantum phase transitions between the disordered spin liquid and magnetic phases with long range order.",1608.05333v1 2016-12-08,Two-dimensional ReN$_2$ materials from first principles,"The discovery of graphene makes it highly desirable to seek new two-dimensional materials. Through first-principles investigation, we predict two-dimensional materials of ReN$_{2}$: honeycomb and tetragonal structures. The phonon spectra establish the dynamical stability for both of the two structures, and the calculated in-plane stiffness constants proves their mechanical stability. The energy bands near the Fermi level consist of N-p and Re-d orbitals for the honeycomb structure, and are mainly from Re d orbitals for the tetragonal structure. While the tetragonal structure is non-magnetic, the honeycomb structure has N-based ferromagnetism, which will transit to anti-ferromagnetism under 14$\%$ biaxial strain. The calculated electron localization function and spin density indicate that direct N-N bond can occur only in the honeycomb structure. The ferromagnetism allows us to distinguish the two 2D phases easily. The tetragonal phase has lower energy than the honeycomb one, which means that the tetragonal phase is more stable, but the hexagonal phase has much larger bulk, shear, and Young's muduli than the tetragonal phase. The tetragonal phase is a three-bands metal, and the hexagonal phase is a ferromagnetic semi-metal. The special structural, electronic, magnetic, and optical properties in the honeycomb and tetragonal structures make them promising for novel applications.",1612.02748v2 2016-12-28,Robust Ferromagnetism in Silicene Nanoflakes through Patterned Hydrogenation,"Considerably different properties emerge in nanomaterials as a result of quantum confinement and edge effects. In this study, the electronic and magnetic properties of quasi zero dimensional silicene nanoflakes (SiNFs) are investigated using first principles calculations. Whilst the zigzag edged hexagonal SiNFs exhibit nonmagnetic semiconducting character, the zigzag edged triangular SiNFs are magnetic semiconductors. One effective method of harnessing the properties of silicene is hydrogenation owing to its reversibility and controllability. From bare SiNFs to half hydrogenated and then to fully hydrogenated, a triangular SiNF experiences a change from ferrimagnetic to very strong ferromagnetic, and then to non-magnetic. Nonetheless, a hexagonal SiNF undergoes a transfer from nonmagnetic to very strong ferromagnetic, then to nonmagnetic. The half hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flakes size. It has been revealed that the strong induced spin magnetizations align parallel and demonstrates a collective character by large range ferromagnetic exchange coupling, giving rise to its potential use in spintronic circuit devices. Spin switch models are offered as an example of one of the potential applications of SiNFs in tuning the transport properties by controlling the hydrogen coverage.",1612.09005v4 2017-06-02,A new class of ferromagnetic semiconductors with high Curie temperatures,"Ferromagnetic semiconductors (FMSs), which have the properties and functionalities of both semiconductors and ferromagnets, provide fascinating opportunities for basic research in condensed matter physics and device applications. Over the past two decades, however, intensive studies on various FMS materials, inspired by the influential mean-field Zener (MFZ) model have failed to realise reliable FMSs that have a high Curie temperature (Tc > 300 K), good compatibility with semiconductor electronics, and characteristics superior to those of their non-magnetic host semiconductors. Here, we demonstrate a new n type Fe-doped narrow-gap III-V FMS, (In,Fe)Sb, in which ferromagnetic order is induced by electron carriers, and its Tc is unexpectedly high, reaching ~335 K at a modest Fe concentration of 16%. Furthermore, we show that by utilizing the large anomalous Hall effect of (In,Fe)Sb at room temperature, it is possible to obtain a Hall sensor with a very high sensitivity that surpasses that of the best commercially available InSb Hall sensor devices. Our results reveal a new design rule of FMSs that is not expected from the conventional MFZ model. (This work was presented at the JSAP Spring meeting, presentation No. E15a-501-2: https://confit.atlas.jp/guide/event/jsap2017s/subject/15a-501-2/advanced)",1706.00735v1 2017-06-10,Tricritical behavior of two-dimensional intrinsic ferromagnetic semiconducting CrGeTe3,"CrGeTe3 recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike CrSiTe3 whose magnetism can be understood using the 2D-Ising model, CrGeTe3 exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior CrGeTe3 around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis, which show that the magnetism of CrGeTe3 follows the tricritical mean-field model with the critical exponents \b{eta}, {\gamma}, and {\delta} of 0.240, 1.000, and 5.070, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for CrGeTe3 should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor.",1706.03239v1 2017-06-11,Dirac magnons in honeycomb ferromagnets,"The discovery of the Dirac electron dispersion in graphene led to the question of the Dirac cone stability with respect to interactions. Coulomb interactions between electrons were shown to induce a logarithmic renormalization of the Dirac dispersion. With a rapid expansion of the list of compounds and quasiparticle bands with linear band touching, the concept of bosonic Dirac materials has emerged. We consider a specific case of ferromagnets consisting of the Van der Waals-bonded stacks of honeycomb layers, e.g chromium trihalides CrX3 (X = F, Cl, Br and I), that display two spin wave modes with energy dispersion similar to that for the electrons in graphene. At the single particle level, these materials resemble their fermionic counterparts. However, how different particle statistics and interactions affect the stability of Dirac cones has yet to be determined. To address the role of interacting Dirac magnons, we expand the theory of ferromagnets beyond the standard Dyson theory to a case of non-Bravais honeycomb layers. We demonstrate that magnon-magnon interactions lead to a significant momentum-dependent renormalization of the bare band structure in addition to strongly momentum-dependent magnon lifetimes. We show that our theory qualitatively accounts for hitherto unexplained anomalies in a nearly half century old magnetic neutron scattering data for CrBr3. We also show that honeycomb ferromagnets display dispersive surface and edge states, unlike their electronic analogs.",1706.03384v1 2017-06-14,Engineering the breaking of time-reversal symmetry in gate-tunable hybrid ferromagnet/topological insulator heterostructures,"Studying the influence of breaking time-reversal symmetry on topological insulator surface states is an important problem of current interest in condensed matter physics and could provide a route toward proof-of-concept spintronic devices that exploit spin-textured surface states. Here, we develop a new model system for studying the effect of breaking time-reversal symmetry: a hybrid heterostructure wherein a ferromagnetic semiconductor Ga1-xMnxAs, with an out-of-plane component of magnetization, is cleanly interfaced with a three-dimensional topological insulator (Bi,Sb)2(Te,Se)3 by molecular beam epitaxy. Lateral electrical transport in this bilayer is dominated by conduction through the topological insulator whose conductivity is a few orders of magnitude higher than that of the highly resistive ferromagnetic semiconductor with a low Mn concentration. Electrical transport measurements of a top-gated heterostructure device reveal a crossover from weak anti-localization (negative magneto-conductance) to weak localization (positive magneto-conductance) as the temperature is lowered or as the chemical potential approaches the Dirac point. This is accompanied by a systematic emergence of an anomalous Hall effect. These results are interpreted in terms of the opening of a gap at the Dirac point as a result of the exchange coupling between the topological insulator surface state and the ferromagnetic ordering in the Ga1-xMnxAs layer. Our study shows that this hybrid system is well suited to explore topological quantum phenomena and to realize proof-of-concept demonstrations of topological spintronic devices at cryogenic temperatures.",1706.04661v1 2017-06-18,"Walls, Anomalies, and (De)Confinement in Quantum Anti-Ferromagnets","We consider the Abelian-Higgs model in 2+1 dimensions with instanton-monopole defects. This model is closely related to the phases of quantum anti-ferromagnets. In the presence of $\mathbb{Z}_2$ preserving monopole operators, there are two confining ground states in the monopole phase, corresponding to the Valence Bond Solid (VBS) phase of quantum magnets. We show that the domain-wall carries a 't Hooft anomaly in this case. The anomaly can be saturated by, e.g., charge-conjugation breaking on the wall or by the domain wall theory becoming gapless (a gapless model that saturates the anomaly is $SU(2)_1$ WZW). Either way the fundamental scalar particles (i.e. spinons) which are confined in the bulk are deconfined on the domain-wall. This $\mathbb{Z}_2$ phase can be realized either with spin-1/2 on a rectangular lattice, or spin-1 on a square lattice. In both cases the domain wall contains spin-1/2 particles (which are absent in the bulk). We discuss the possible relation to recent lattice simulations of domain walls in VBS. We further generalize the discussion to Abrikosov-Nielsen-Olsen (ANO) vortices in a dual superconductor of the Abelian-Higgs model in 3+1 dimensions, and to the easy-plane limit of anti-ferromagnets. In the latter case the wall can undergo a variant of the BKT transition (consistent with the anomalies) while the bulk is still gapped. The same is true for the easy-axis limit of anti-ferromagnets. We also touch upon some analogies to Yang-Mills theory.",1706.05731v2 2017-06-29,Origin of Contact Resistance at Ferromagnetic Metal-Graphene Interfaces,"Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfaces is much lower than that of surface-contacted ones, for all devices studied and especially for the single-layer graphene systems. We show that this difference in resistance is not due to differences in the metal-graphene coupling strength, which we quantify using Hamiltonian matrix elements. Instead, the larger contact resistance in surface contacts results from spin filtering at the interface, in contrast to the edge-contacted case where both spins are transmitted. Temperature-dependent resistance measurements beyond the Curie temperature TC show that the spin degree of freedom is indeed important for the experimentally measured contact resistance. These results show that it is possible to induce a large change in contact resistance by changing the temperature in the vicinity of TC, thus paving the way for temperature-controlled switches based on spin.",1706.09591v1 2017-07-14,Symmetry-broken dissipative exchange flows in thin-film ferromagnets with in-plane anisotropy,"Planar ferromagnetic channels have been shown to theoretically support a long-range ordered and coherently precessing state where the balance between local spin injection at one edge and damping along the channel establishes a dissipative exchange flow, sometimes referred to as a spin superfluid. However, realistic materials exhibit in-plane anisotropy, which breaks the axial symmetry assumed in current theoretical models. Here, we study dissipative exchange flows in a ferromagnet with in-plane anisotropy from a dispersive hydrodynamic perspective. Through the analysis of a boundary value problem for a damped sine-Gordon equation, dissipative exchange flows in a ferromagnetic channel can be excited above a spin current threshold that depends on material parameters and the length of the channel. Symmetry-broken dissipative exchange flows display harmonic overtones that redshift the fundamental precessional frequency and lead to a reduced spin pumping efficiency when compared to their symmetric counterpart. Micromagnetic simulations are used to verify that the analytical results are qualitatively accurate, even in the presence of nonlocal dipole fields. Simulations also confirm that dissipative exchange flows can be driven by spin transfer torque in a finite-sized region. These results delineate the important material parameters that must be optimized for the excitation of dissipative exchange flows in realistic systems.",1707.04664v3 2017-10-03,Temperature- dependence of anomalous Hall conductivity in Rashba-type ferromagnets,"The applicability and usefulness of Rashba model have been extended by recent observations in the field of spintronics, such as the spin-orbit torque at the junction interfaces between ferromagnetic (FM) metals and non-magnetic (NM) metals and the perpendicular anomalous magnetoresistance (AMR) in heterostructures such as FI/NM or FM/NI (I denotes an insulator). In particular, the observations of the perpendicular AMR effect stimulate further interest in the Rashba-type spin-orbit interaction (SOI) at interfaces. Thus, the Rashba model with exchange splitting (EXS) is considered not only to play as an effective model for the physical understanding but also to reflect actual bi-layer systems in current spintronics devices. In the present work, we have first investigated the temperature dependence of anomalous Hall conductivity (AHC) of Rashba-type ferromagnets considered effects of spin fluctuations within the disordered local moment (DLM) scheme. The most distinctive feature that we observed is that intrinsic AHC increases with increasing temperature. This can be understood from the aspect of spin chirality, which indicates that the AHC increases with decreasing EXS when the SOI is much smaller than the EXS. The extrinsic part of the Fermi surface term also increases with increasing temperature and has a large contribution, comparable to that of the intrinsic part. Although, such a behaviour has not yet been observed experimentally, we suggest that the physical picture found in this work might lurk in an anomalous Hall effects in Rashbe-type ferromagnets.",1710.00982v1 2017-10-31,Kitaev-Heisenberg Hamiltonian for High-Spin $d^7$ Mott Insulators,"In the search for quantum spin liquids, candidate materials for the Kitaev model and its extensions have been intensively explored during the past decade, as the models realize the exact quantum spin liquids in the ground state. Thus far, insulating magnets in the low-spin $d^5$ electron configuration under the strong spin-orbit coupling have been studied for realizing the Kitaev-type bond-dependent anisotropic interactions between the spin-orbital entangled Kramers doublets. To extend the candidates, here we investigate the systems in a high-spin $d^7$ electron configuration, whose ground state is described by the spin-orbital entangled Kramers doublet. By the second- order perturbation in terms of the $t_{2g}$-$t_{2g}$ and $t_{2g}$-$e_g$ hoppings, we show that the effective spin model possesses the anisotropic Kitaev interactions as well as the isotropic Heisenberg ones. While the Kitaev interaction is always ferromagnetic, the Heisenberg interaction can become either ferromagnetic or antiferromagnetic depending on the Coulomb interactions and the crystalline electric fields. We also derive the effective model for the low-spin $d^5$ electron configuration within the same perturbation scheme, in which the Kitaev interaction becomes both ferromagnetic and antiferromagnetic, while the Heisenberg one always ferromagnetic. Referring to the previous study for the Kitaev-Heisenberg model, we find that the quantum spin liquid phase exists in the reasonable parameter region in both $d^7$ and $d^5$ cases, while the former has a richer structure of the phase diagram. We discuss the advantages of the $d^7$ case in comparison with the $d^5$ case. Our results indicate that the high-spin $d^7$ state provides another platform for the Kitaev-type quantum spin liquid.",1710.11357v2 2017-12-17,"Low temperature ferromagnetic properties, magnetic field induced spin order and random spin freezing effect in Ni1.5Fe1.5O4 ferrite; prepared at different pH values and annealing temperatures","We present the low temperature magnetic properties in Ni1.5Fe1.5O4 ferrite as the function of pH at which the material was prepared by chemical route and post annealing temperature. The material is a ferri or ferromagnet, but showed magnetic blocking and random spin freezing process on lowering the measurement temperature down to 5 K. The sample prepared at pH =12 and annealed at 800 ^C showed a sharp magnetization peak at 105 K, the superparamagnetic blocking temperature of the particles. The magnetization peak remained incomplete within measurement temperature up to 350 K for rest of the samples, although peak temperature was brought down by increasing applied dc field. The fitting of temperature dependence of coercivity data according to Kneller law suggested random orientation of ferromagnetic particles. The fitting of saturation magnetization according to Bloch law provided the exponent that largely deviated from 1.5, a typical value for long ranged ferromagnet. An abrupt increase of saturation magnetization below 50 K suggested the active role of frozen surface spins in low temperature magnetic properties. AC susceptibility data elucidated the low temperature spin freezing dynamics and exhibited the characters of cluster spin glass in the samples depending on pH value and annealing temperature.",1712.06114v1 2017-12-20,Optical Manipulation of Magnetic Vortex Visualized in situ by 4D Electron Microscopy,"Understanding the fundamental dynamics of topological vortex and antivortex naturally formed in micro/nanoscale ferromagnetic building blocks under external perturbations is crucial to magnetic vortex based information processing and spintronic devices. All previous studies have focused on magnetic vortex-core switching via external magnetic fields, spin-polarized currents, or spin waves, which have largely prohibited the investigation of novel spin configurations that could emerge from the ground states in ferromagnetic disks and their underlying dynamics. Here, we report in situ visualization of femtosecond laser quenching induced magnetic vortex change in various symmetric ferromagnetic Permalloy disks by Lorentz phase imaging using 4D electron microscopy. Besides the switching of magnetic vortex chirality and polarity, we observed with distinct occurrence frequencies a plenitude of complex magnetic structures that have never been observed by magnetic field or current assisted switching. These complex magnetic structures consist of a number of newly created topological magnetic defects (vortex and antivortex) strictly conserving the topological winding number, demonstrating the direct impact of topological invariant on the magnetization dynamics in ferromagnetic disks. Their spin configurations show mirror or rotation symmetry due to the geometrical confinement of the disks. Combined micromagnetic simulations with the experimental observations reveal the underlying magnetization dynamics and formation mechanism of the optical quenching induced complex magnetic structures. Their distinct occurrence rates are pertinent to their formation-growth energetics and pinning effects at the disk edge. Based on these findings, we propose a paradigm of optical-quenching-assisted fast switching of vortex cores for the control of magnetic vortex based information recording and spintronic devices.",1712.07280v1 2018-01-08,Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure,"Vortices are among the simplest topological structures, and occur whenever a flow field `whirls' around a one-dimensional core. They are ubiquitous to many branches of physics, from fluid dynamics to superconductivity and superfluidity, and are even predicted by some unified theories of particle interactions, where they might explain some of the largest-scale structures seen in today's Universe. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are only observed when the effects of the dipole-dipole interaction is modified by confinement at the nanoscale, or when the parameter associated with the vorticity does not couple directly with strain. Here, we present the discovery of a novel form of vortices in antiferromagnetic (AFM) hematite ($\alpha$-Fe$_2$O$_3$) epitaxial films, in which the primary whirling parameter is the staggered magnetisation. Remarkably, ferromagnetic (FM) topological objects with the same vorticity and winding number of the $\alpha$-Fe$_2$O$_3$ vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of-plane core magnetisation), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, H$_{\parallel}$, giving rise to large-scale vortex-antivortex annihilation.",1801.02560v2 2018-01-30,Playing with universality classes of Barkhausen avalanches,"Many systems crackle, from earthquakes and financial market to Barkhausen effect in ferromagnetic materials. Despite the diversity in essence, the noise emitted in these dynamical systems consists of avalanche-like events with broad range of sizes and durations, characterized by power-law avalanche distributions and typical average avalanche shape that are signatures dependent on the universality class of the underlying dynamics. Here we focus on the crackling noise in ferromagnets and scrutinize the traditional statistics of Barkhausen avalanches in polycrystalline and amorphous ferromagnetic films having different thicknesses. We show how scaling exponents and average shape of the avalanches evolve with the structural character of the materials and film thickness. We find quantitative agreement between experiment and theoretical predictions of models for the magnetic domain wall dynamics, and then elucidate the universality classes of Barkhausen avalanches in ferromagnetic films. Thereby, we observe for the first time the dimensional crossover in the domain wall dynamics, and the outcomes of the interplay between system dimensionality and range of interactions governing the domain wall dynamics on Barkhausen avalanches.",1801.09948v3 2018-10-03,Ferromagnetism-induced Phase Separation in a Two-dimensional Spin Fluid,"We study the liquid-gas phase separation observed in a system of repulsive particles dressed with ferromagnetically aligning spins, a so-called `spin fluid'. Microcanonical ensemble numerical simulations of finite-size systems reveal that magnetization sets in and induces a liquid-gas phase separation between a disordered gas and a ferromagnetic dense phase at low enough energies and large enough densities. The dynamics after a quench into the coexistence region show that the order parameter associated to the liquid-vapour phase separation follows an algebraic law with an unusual exponent, as it is forced to synchronize with the growth of the magnetization: this suggests that for finite size systems the magnetization sets in along a Curie line, which is also the gas-side spinodal line, and that the coexistence region ends at a tricritical point. This picture is confirmed at the mean-field level with different approximation schemes, namely a Bethe lattice resolution and a virial expansion complemented by the introduction of a self-consistent Weiss-like molecular field. However, a detailed finite-size scaling analysis shows that in two dimensions the ferromagnetic phase escapes the Berezinskii-Kosterlitz-Thouless scenario, and that the long-range order is not destroyed by the unbinding of topological defects. The Curie line becomes thus a magnetic crossover in the thermodynamic limit. Finally, the effects of the magnetic interaction range and those of the interaction softness are characterized within a mean-field semi-analytic low-density approach.",1810.01690v2 2018-10-23,Dynamics of Order Parameters of Non-stoquastic Hamiltonians in the Adaptive Quantum Monte Carlo Method,"We derive macroscopically deterministic flow equations with regard to the order parameters of the ferromagnetic $p$-spin model with infinite-range interactions. The $p$-spin model has a first-order phase transition for $p>2$. In the case of $p\geq5$ ,the $p$-spin model with anti-ferromagnetic XX interaction has a second-order phase transition in a certain region. In this case, however, the model becomes a non-stoqustic Hamiltonian, resulting in a negative sign problem. To simulate the $p$-spin model with anti-ferromagnetic XX interaction, we utilize the adaptive quantum Monte Carlo method. By using this method, we can regard the effect of the anti-ferromagnetic XX interaction as fluctuations of the transverse magnetic field. A previous study derived deterministic flow equations of the order parameters in the quantum Monte Carlo method. In this study, we derive macroscopically deterministic flow equations for the magnetization and transverse magnetization from the master equation in the adaptive quantum Monte Carlo method. Under the Suzuki-Trotter decomposition, we consider the Glauber-type stochastic process. We solve these differential equations by using the Runge-Kutta method and verify that these results are consistent with the saddle-point solution of mean-field theory. Finally, we analyze the stability of the equilibrium solutions obtained by the differential equations.",1810.09943v3 2019-06-06,Higher-Order Topological Superconductivity of Spin-Polarized Fermions,"We study the superconductivity of spin-polarized electrons in centrosymmetric ferromagnetic metals. Due to the spin-polarization and the Fermi statistics of electrons, the superconducting pairing function naturally has odd parity. According to the parity formula proposed by Fu, Berg, and Sato, odd-parity pairing leads to conventional first-order topological superconductivity when a normal metal has an odd number of Fermi surfaces. Here, we derive generalized parity formulae for the topological invariants characterizing higher-order topology of centrosymmetric superconductors. Based on the formulae, we systematically classify all possible band structures of ferromagnetic metals that can induce inversion-protected higher-order topological superconductivity. Among them, doped ferromagnetic nodal semimetals are identified as the most promising normal state platform for higher-order topological superconductivity. In two dimensions, we show that odd-parity pairing of doped Dirac semimetals induces a second-order topological superconductor. In three dimensions, odd-parity pairing of doped nodal line semimetals generates a nodal line topological superconductor with monopole charges. On the other hand, odd-parity pairing of doped monopole nodal line semimetals induces a three-dimensional third-order topological superconductor. Our theory shows that the combination of superconductivity and ferromagnetic nodal semimetals opens up a new avenue for future topological quantum computations using Majorana zero modes.",1906.02709v3 2019-06-12,Spin transport and Spin Tunnelling Magneto-Resistance (STMR) of F$|$NCSC$|$F spin valve,"In this work, we study the spin transport at the Ferromagnet$|$Noncentrosymmetric Superconductor (F$|$NCSC) junction of a Ferromagnet$|$Noncentrosymmetric Superconductor$|$Ferromagnet (F$|$NCSC$|$F) spin valve. We investigate the Tunnelling Spin-Conductance (TSC), spin current and Spin Tunnelling Magneto-Resistance (STMR), and their dependence on various important parameters like Rashba Spin-Orbit Coupling (RSOC), strength and orientation of magnetization, an external in-plane magnetic field, barrier strength and a significant Fermi Wavevector Mismatch (FWM) at the ferromagnetic and superconducting regions. The study has been carried out for different singlet-triplet mixing of the NCSC gap parameter. We develop Bogoliubov-de Gennes (BdG) Hamiltonian and use the extended Blonder - Tinkham - Klapwijk (BTK) approach along with the scattering matrix formalism to calculate the scattering coefficients. Our results strongly suggest that the TSC is highly dependent on RSOC, magnetization strength and its orientation, and singlet-triplet mixing of the gap parameter. It is observed that NCSC with moderate RSOC shows maximum conductance for a partially opaque barrier in presence of low external magnetic field. For a strongly opaque barrier and a nearly transparent barrier a moderate value and a low value of field respectively are found to be suitable. Moreover, NCSC with large singlet component is appeared to be useful. In addition, for NCSC with large RSOC and low magnetization strength, a giant STMR ($\%$) is observed. We have also seen that the spin current is strongly magnetization orientation dependent. With the increase in bias voltage spin current increases in transverse direction, but the component along the direction of flow is almost independent.",1906.05081v1 2019-07-11,Anomalous anisotropic behaviour of spin-triplet proximity effect in Au/SrRuO$_3$/Sr$_2$RuO$_4$ junctions,"Spin-polarized supercurrents can be generated with magnetic inhomogeneity at a ferromagnet/spin-singlet-superconductor interface. In such systems, complex magnetic inhomogeneity makes it difficult to functionalise the spin-polarized supercurrents. However, spin-polarized supercurrents in ferromagnet/spin-triplet-superconductor junctions can be controlled by angle between magnetization and spin of Copper pairs ($d$-vector), that can effectively be utilized in developing of a field of research known as superconducting spintronics. Recently, we found induction of spin-triplet correlation into a ferromagnet SrRuO$_3$ epitaxially deposited on a spin-triplet superconductor Sr$_2$RuO$_4$, without any electronic spin-flip scattering. Here, we present systematic magnetic field dependence of the proximity effect in Au/SrRuO$_3$/ Sr$_2$RuO$_4$ junctions. It is found that induced triplet correlations exhibit strong anisotropic field response. Such behaviour attributes to the rotation of the $d$-vector of Sr$_2$RuO$_4$. This anisotropic behaviour is in contrast with the vortex dynamic. Our results will stimulate study of interaction between ferromagnetism and unconventional superconductivity.",1907.05175v2 2019-07-07,"Spin-1 Dirac half-metal, spin-gapless semiconductor, and spin-polarized massive Dirac dispersion in transition metal dihalide monolayers","Spin-1 condensed matter systems characterized by the combination of a Dirac-like dispersion and flat bands are ideal for realizing high-temperature electronics and spintronic technologies in the absence of external magnetic field. In this study, we propose a three-band tight binding model, with spin-polarized Haldane-like next-nearest-neighbour tunnelling, on dice lattice and show that spin-1 Dirac half-metal, spin-1 Dirac spin-gapless semiconductor, and spin-polarized spin-1 massive Dirac dispersion with nontrivial topology can exist in two-dimensional ferromagnetic condensed matter systems with electron spin polarization P = 1. The proposed spin-polarized spin-1 phases can be realized in ferromagnetic transition metal dihalides MX2 monolayers effectively. By using first principle calculations, we show that a small compressive strain leads MX2 monolayers to be spin-one Dirac half-metal for M = Fe and X = Br, Cl while spin-one Dirac spin-gapless semiconductor for M = Co and X = Br, Cl. Spin-one Dirac spin-gapless semiconductors CoBr2 and CoCl2 embeds flat band ferromagnetism where spin-orbit coupling opens a topologically non-trivial Dirac gap between dispersing valance and conduction band while leaving flat band unaffected. The intrinsic flat-band ferromagnetism in spin-polarized spin-1 massive Dirac dispersion plays key role in materializing quantum anomalous Hall state with Chern number C = -2.",1907.07756v1 2019-07-19,Multiple ferromagnetic transitions and structural distortion in the van-der-Waals ferromagnet VI$_3$ at ambient and finite pressures,"We present a combined study of zero-field $^{51}$V and $^{127}$I NMR at ambient pressure and specific heat and magnetization measurements under pressure up to 2.08 GPa on bulk single crystals of the van-der-Waals ferromagnet VI$_3$. At ambient pressure, our results consistently demonstrate that VI$_3$ undergoes a structural transition at $T_s \approx $78 K, followed by two subsequent ferromagnetic transitions at $T_{FM1} \approx $50 K and $T_{FM2} \approx $36 K upon cooling. At lowest temperature ($T < T_{FM2}$), two magnetically-ordered V sites exist, whereas only one magnetically-ordered V site is observed for $T_{FM1} < T\,< T_{FM2}$. Whereas $T_{FM1}$ is almost unaffected by external pressure, $T_{FM2}$ is highly responsive to pressure and merges with the $T_{FM1}$ line at $p \approx 0.6 $GPa. At even higher pressures ($p \approx $1.25\,GPa), the $T_{FM2}$ line merges with the structural transition at $T_s$ which becomes moderately suppressed with $p$ for $p < 1.25$ GPa. Taken together, our data point towards a complex magnetic structure and an interesting interplay of magnetic and structural degrees of freedom in VI$_3$.",1907.08550v2 2019-09-23,Skyrmion Brownian circuit implemented in a continuous ferromagnetic thin film,"The fabrication of a skyrmion circuit which stabilizes skyrmions is important to realize micro- to nano-sized skyrmion devices. One example of promising skyrmion-based device is Brownian computers, which have been theoretically proposed, but not realized. It would require a skyrmion circuit in which the skyrmion is stabilized and easily movable. However, the usual skyrmion circuits fabricated by etching of the ferromagnetic film decrease the demagnetization field stabilizing the skyrmions, and thus prevent their formation. In this study, a skyrmion Brownian circuit implemented in a continuous ferromagnetic film with patterned SiO$_2$ capping to stabilize the skyrmion formation. The patterned SiO$_2$ capping controls the saturation field of the ferromagnetic layer and forms a wire-shaped skyrmion potential well, which stabilizes skyrmion formation in the circuit. Moreover, we implement a hub (Y-junction) circuit without pinning sites at the junction by patterned SiO$_2$ capping. This technique enables the efficient control of skyrmion-based memory and logic devices, as well as Brownian computers.",1909.10130v2 2019-09-24,Unconventional Hund Metal in a Weak Itinerant Ferromagnet,"The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi liquid theory. While magnetic fluctuations play an important role in the materials' unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund's exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets.",1909.11195v2 2019-10-15,Superconducting Correlations in the One-Dimensional Kondo Lattice Models under Magnetic Fields,"We analyze superconducting correlations in the one-dimensional Kondo lattice models with Ising anisotropy under transverse magnetic fields, using the density matrix renormalization group. For the spin-1/2 local spin model, the Ising anisotropy is introduced by the ferromagnetic Ising interaction between the local spins, while for the spin-1 model, it is taken by the single-ion anisotropy. The magnetic properties under the transverse fields for the spin-1/2 model are very similar to those for the spin-1 model [K. Suzuki and K. Hattori, J. Phys. Soc. Jpn. 88, 024707 (2019).]. For the superconducting correlations, we analyze various Cooper pairs within nearest-neighbor pairs including composite ones between the local spins and the electrons. We find that, for the spin-1/2 model, the superconducting correlations are highly enhanced in the Tomonaga-Luttinger liquid state near the Kondo-plateau phase, where the conduction electrons and the local spins are strongly coupled with a finite spin gap for the Ising axis. This is a clear contrast to the model under the longitudinal magnetic fields, where there are no noticeable superconducting correlations. Competitions between the transverse magnetic field and the Kondo singlet formation lead to this enhanced superconducting correlations. For the spin-1 model, the single-ion anisotropy suppresses the superconducting correlation and there is no noticeable enhancement. We also examine the large Kondo exchange coupling limit. For the moderate ferromagnetic Ising interaction between the local spins, we find that another type of superconducting correlation is enhanced inside the ferromagnetic phase. We discuss a possible relation between our results and reentrant superconductivity in U-based ferromagnetic superconductors under transverse magnetic fields.",1910.06545v2 2019-10-22,Magnetic topological insulator MnBi6Te10 with zero-field ferromagnetic state and gapped Dirac surface states,"Magnetic topological insulators (TIs) with nontrivial topological electronic structure and broken time-reversal symmetry exhibit various exotic topological quantum phenomena. The realization of such exotic phenomena at high temperature is one of central topics in this area. We reveal that MnBi6Te10 is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topology is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with Z4 = 2 emerges once spins polarized at field as low as 0.1 T, accompanied with saturated anomalous Hall resistivity up to 10 K. Such a ferromagnetic state is preserved even external field down to zero at 2 K. Theoretical calculations indicate that the few-layer ferromagnetic MnBi6Te10 is also topologically nontrivial with a non-zero Chern number. Angle-resolved photoemission spectroscopy experiments further reveal three types of Dirac surface states arising from different terminations on the cleavage surfaces, one of which has insulating behavior with an energy gap of ~ 28 meV at the Dirac point. These outstanding features suggest that MnBi6Te10 is a promising system to realize various topological quantum effects at zero field and high temperature.",1910.10101v2 2019-10-24,Experimental evidence of inertial dynamics in ferromagnets,"The understanding of how spins move at pico- and femtosecond time scales is the goal of much of modern research in condensed matter physics, with implications for ultrafast and more energy-efficient data storage. However, the limited comprehension of the physics behind this phenomenon has hampered the possibility of realising a commercial technology based on it. Recently, it has been suggested that inertial effects should be considered in the full description of the spin dynamics at these ultrafast time scales, but a clear observation of such effects in ferromagnets is still lacking. Here, we report the first direct experimental evidence of inertial spin dynamics in ferromagnetic thin films in the form of a nutation of the magnetisation at a frequency of approximately 0.6 THz. This allows us to evince that the angular momentum relaxation time in ferromagnets is on the order of 10 ps.",1910.11284v1 2020-01-19,Ultrafast kinetics of the antiferromagnetic-ferromagnetic phase transition in FeRh,"Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is a test specimen for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation can induce ferromagnetism in antiferromagnetic FeRh, but the mechanism and dynamics of this transition are topics of intense debates. Employing double-pump THz emission spectroscopy has enabled us to dramatically increase the temporal detection window of THz emission probes of transient states without sacrificing any loss of resolution or sensitivity. It allows us to study the kinetics of emergent ferromagnetism from the femtosecond up to the nanosecond timescales in FeRh/Pt bilayers. Our results strongly suggest a latency period between the initial pump-excitation and the emission of THz radiation by ferromagnetic nuclei.",2001.06799v4 2020-02-23,Large anomalous Hall effect in a hexagonal ferromagnetic Fe5Sn3 single crystal,"In this paper, we report an experimental observation of the large anomalous Hall effect (AHE) in a hexagonal ferromagnetic Fe5Sn3 single crystal with current along the b axis and a magnetic field normal to the bc plane. The intrinsic contribution of the anomalous Hall conductance sigma_AH^int was approximately 613 {\Omega}-1 cm-1, which was more than 3 times the maximum value in the frustrated kagome magnet Fe3Sn2 and nearly independent of the temperature over a wide range between 5 and 350 K. The analysis results revealed that the large AHE was dominated by a common, intrinsic term, while the extrinsic contribution, i.e., the skew scattering and side jump, turned out to be small. In addition to the large AHE, it was found the types of majority carriers changed at approximately 275 and 30 K, consistent with the critical temperatures of the spin reorientation. These findings suggest that the hexagonal ferromagnetic Fe5Sn3 single crystal is an excellent candidate to use for the study of the topological features in ferromagnets.",2002.09872v2 2020-04-22,Spectroscopic Signature for Local-moment Magnetism in van der Waals Ferromagnet Fe$_3$GeTe$_2$,"The van der Waals ferromagnet Fe$_3$GeTe$_2$ has recently attracted extensive research attention due to its intertwined magnetic, electronic and topological properties. Here, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the temperature evolution of the electronic structure of bulk Fe$_3$GeTe$_2$. We observe largely dispersive energy bands that are narrowed by a factor of 1.6 compared with ab-initio calculation. Upon heating towards the ferromagnetic transition near 225 K, we observe a massive reduction of quasiparticle coherence in a large energy range, which is attributed to the enhanced magnetic fluctuation in the system. Remarkably, the electron bands barely shift with increasing temperature, which deviates from the exchange splitting picture within the itinerant Stoner model. We argue that the local magnetic moments play a crucial role in the ferromagnetism of Fe$_3$GeTe$_2$, despite its strongly itinerant nature. Our results provide important insights into the electronic and magnetic properties of Fe$_3$GeTe$_2$ and shed light on the generic understanding of itinerant magnetism in correlated materials.",2004.10379v1 2020-12-06,Ultrafast ferromagnetic fluctuations preceding magnetoelastic first-order transitions,"First-order magnetic transitions are of both fundamental and technological interest given that a number of emergent phases and functionalities are thereby created. Of particular interest are giant magnetocaloric effects, which are attributed to first-order magnetic transitions and have attracted broad attention for solid-state refrigeration applications. While the conventional wisdom is that atomic lattices play an important role in first-order magnetic transitions, a coherent microscopic description of the lattice and spin degrees of freedom is still lacking. Here, we study the magnetic phase transition dynamics on the intermetallic LaFe13-xSix, which is one of the most classical giant magnetocaloric systems, in both frequency and time domains utilizing neutron scattering and ultrafast X-ray diffraction. We have observed a strong magnetic diffuse scattering in the paramagnetic state preceding the first-order magnetic transition, corresponding to picosecond ferromagnetic fluctuations. Upon photon-excitation, the ferromagnetic state is completely suppressed in 0.9 ps and recovered in 20 ps. The ultrafast dynamics suggest that the magnetic degree of freedom dominates this magnetoelastic transition and ferromagnetic fluctuations might be universally relevant for this kind of compounds.",2012.03186v1 2020-12-22,Quantum Anomalous Hall Effect in Magnetic Doped Topological Insulators and Ferromagnetic Spin-Gapless Semiconductors -- A Perspective Review,"Quantum anomalous Hall effect, with a trademark of dissipationless chiral edge states for electronics/spintronics transport applications, can be realized in materials with large spin-orbit coupling and strong intrinsic magnetization. After Haldane seminal proposal, several models have been presented to control/enhance the spin-orbit coupling and intrinsic magnetic exchange interaction. After brief introduction of Haldane model for spineless fermions, following three fundamental quantum anomalous Hall models are discussed in this perspective review: (i) low-energy effective four band model for magnetic-doped topological insulator (Bi,Sb)2Te3 thin films, (ii) four band tight-binding model for graphene with magnetic adatoms, and (iii) two (three) band spinfull tight-binding model for ferromagnetic spin-gapless semiconductors with honeycomb (kagome) lattice where ground state is intrinsically ferromagnetic. These models cover two-dimensional Dirac materials hosting spinless, spinful and spin-degenerate Dirac points where various mass terms open a band gap and lead to quantum anomalous Hall effect. With emphasize on the topological phase transition driven by ferromagnetic exchange interaction and its interplay with spin-orbit-coupling, we discuss various symmetry constraints on the nature of mass term and the materialization of these models. We hope this study will shed light on the fundamental theoretical perspectives of quantum anomalous Hall materials.",2101.01074v1 2021-01-05,Itinerant ferromagnetism mediated by giant spin polarization of metallic ligand band in van der Waals magnet Fe5GeTe2,"We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand p bands near EF, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for none-magnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in Fe5GeTe2. Our finding provides a fundamental elemental selective view-point for understanding mechanism of itinerant ferromagnetism in low dimensional compounds, which also leads insight for designing exotic magnetic states by interfacial band engineering in heterostructures.",2101.01324v1 2021-01-13,Spin pumping between noncollinear ferromagnetic insulators through thin superconductors,"Dynamical magnets can pump spin currents into superconductors. To understand such a phenomenon, we develop a method utilizing the generalized Usadel equation to describe time-dependent situations in superconductors in contact with dynamical ferromagnets. Our proof-of-concept theory is valid when there is sufficient dephasing at finite temperatures, and when the ferromagnetic insulators are weakly polarized. We derive the effective equation of motion for the Keldysh Green's function focusing on a thin film superconductor sandwiched between two noncollinear ferromagnetic insulators of which one is dynamical. In turn, we compute the spin currents in the system as a function of the temperature and the magnetizations' relative orientations. When the induced Zeeman splitting is weak, we find that the spin accumulation in the superconducting state is smaller than in the normal states due to the lack of quasiparticle states inside the gap. This feature gives a lower backflow spin current from the superconductor as compared to a normal metal. Furthermore, in superconductors, we find that the ratio between the backflow spin current in the parallel and anti-parallel magnetization configuration depends strongly on temperature, in contrast to the constant ratio in normal metals.",2101.05128v1 2017-09-06,"Tuning ferromagnetism and spin state in La$_{(1-x)}$$A_x$CoO$_3$ ($A=$ Sr, Ca) nanoparticles","We use the x-ray diffraction, magnetic susceptibility, isothermal magnetization, and photoelectron spectroscopy to study the structural, magnetic and electronic properties of La$_{(1-x)}$$A_x$CoO$_3$ ($A=$ Sr, Ca; $x=$ 0 -- 0.2) nanoparticles. The Rietveld refinements of room temperature powder x-ray diffraction data confirm the single phase and the rhombohedral crystal structure with R$\bar{3}$C space group. We find drastic changes in the magnetic properties and spin-states with Sr/Ca substitution (hole doping). For $x=$ 0 sample, the magnetic measurements show a ferromagnetic transition at T$_{\rm C}$$\approx$85 K, which shifted significantly to higher temperatures with hole doping; simultaneously a significant increase in the spontaneous magnetic moment has been observed. Whereas, the coercive field H$_{\rm C}$ values are 7, 4.4 and 13.2~kOe for $x=$ 0, 0.2 (Sr), and 0.2(Ca) samples. Furthermore, the FC magnetization shows a ferromagnetic Brillouin function like behavior at low temperatures for Ca samples. We demonstrate that the Sr/Ca substitution increases the population of IS (Co$^{3+}$) and LS (Co$^{4+}$) states and tune the ferromagnetism in nanoparticles via double-exchange interactions between Co$^{3+}$-- Co$^{4+}$. Our results suggest an important role of hole carriers and nano-size effect in controlling the spin-state and magnetism in La$_{(1-x)}$$A_x$CoO$_3$ nanoparticles.",1709.01745v1 2017-09-20,Property Testing in High Dimensional Ising models,"This paper explores the information-theoretic limitations of graph property testing in zero-field Ising models. Instead of learning the entire graph structure, sometimes testing a basic graph property such as connectivity, cycle presence or maximum clique size is a more relevant and attainable objective. Since property testing is more fundamental than graph recovery, any necessary conditions for property testing imply corresponding conditions for graph recovery, while custom property tests can be statistically and/or computationally more efficient than graph recovery based algorithms. Understanding the statistical complexity of property testing requires the distinction of ferromagnetic (i.e., positive interactions only) and general Ising models. Using combinatorial constructs such as graph packing and strong monotonicity, we characterize how target properties affect the corresponding minimax upper and lower bounds within the realm of ferromagnets. On the other hand, by studying the detection of an antiferromagnetic (i.e., negative interactions only) Curie-Weiss model buried in Rademacher noise, we show that property testing is strictly more challenging over general Ising models. In terms of methodological development, we propose two types of correlation based tests: computationally efficient screening for ferromagnets, and score type tests for general models, including a fast cycle presence test. Our correlation screening tests match the information-theoretic bounds for property testing in ferromagnets.",1709.06688v2 2017-09-20,Efficient and controlled domain wall nucleation for magnetic shift registers,"Ultrathin ferromagnetic strips with high perpendicular anisotropy have been proposed for the development of memory devices where the information is coded in tiny domains separated by domain walls. The design of practical devices requires creating, manipulating and detecting domain walls in ferromagnetic strips. Recent observations have shown highly efficient current-driven domain wall dynamics in multilayers lacking structural symmetry, where the walls adopt a chiral structure and can be driven at high velocities. However, putting such a device into practice requires the continuous and synchronous injection of domain walls as the first step. Here, we propose and demonstrate an efficient and simple scheme for nucleating domain walls using the symmetry of the spin orbit torques. Trains of short sub-nanosecond current pulses are injected in a double bit line to generate a localized longitudinal Oersted field in the ferromagnetic strip. Simultaneously, other current pulses are injected through the heavy metal under the ferromagnetic strip. Notably, the Slonczewski-like spin orbit torque assisted by the Oersted field allows the controlled injection of a series of domain walls, giving rise to a controlled manner for writing binary information and, consequently, to the design of a simple and efficient domain wall shift register.",1709.06753v1 2017-09-28,Direct Evidence of Spontaneous Abrikosov Vortex State in Ferromagnetic Superconductor EuFe$_2$(As$_{1-x}$P$_x$)$_2$ with $x=0.21$,"Using low-temperature Magnetic Force Microscopy (MFM) we provide direct experimental evidence for spontaneous vortex phase (SVP) formation in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$ single crystal with the superconducting $T^{\rm 0}_{\rm SC}=23.6$~K and ferromagnetic $T_{\rm FM}\sim17.7$~K transition temperatures. Spontaneous vortex-antivortex (V-AV) pairs are imaged in the vicinity of $T_{\rm FM}$. Also, upon cooling cycle near $T_{\rm FM}$ we observe the first-order transition from the short period domain structure, which appears in the Meissner state, into the long period domain structure with spontaneous vortices. It is the first experimental observation of this scenario in the ferromagnetic superconductors. Low-temperature phase is characterized by much larger domains in V-AV state and peculiar branched striped structures at the surface, which are typical for uniaxial ferromagnets with perpendicular magnetic anisotropy (PMA). The domain wall parameters at various temperatures are estimated.",1709.09802v2 2017-09-28,Massive Dirac fermions in a ferromagnetic kagome metal,"The kagome lattice is a two-dimensional network of corner-sharing triangles known as a platform for exotic quantum magnetic states. Theoretical work has predicted that the kagome lattice may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these have evaded experimental detection to date. Here we study the d-electron kagome metal Fe$_3$Sn$_2$ designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature independent intrinsic anomalous Hall conductivity persisting above room temperature suggestive of prominent Berry curvature from the time-reversal breaking electronic bands of the kagome plane. Using angle-resolved photoemission, we discover a pair of quasi-2D Dirac cones near the Fermi level with a 30 meV mass gap that accounts for the Berry curvature-induced Hall conductivity. We show this behavior is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state with atomic spin-orbit coupling. This report provides the first evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. This offers insight into recent discoveries of exotic electronic behavior in kagome lattice antiferromagnets and may provide a stepping stone toward lattice model realizations of fractional topological quantum states.",1709.10007v1 2017-11-02,A Comparative Study on Spin-Orbit Torque Efficiencies from W/ferromagnetic and W/ferrimagnetic Heterostructures,"It has been shown that W in its resistive form possesses the largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient dampinglike spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ or ferrimagnetic Co$_{63}$Tb$_{37}$ with perpendicular magnetic anisotropy. The DL-SOT efficiency $|\xi_{DL}|$, which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ systems. Maximum values of $|\xi_{DL}|\approx 0.144$ and $|\xi_{DL}|\approx 0.116$ are achieved when the W layer is partially amorphous in the W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ heterostructures, respectively. Our results suggest that the spin Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism.",1711.00630v2 2017-11-12,Magnetic proximity effect of a topological insulator and a ferromagnet in thin film bilayers of $BiSbTe_3$ and $SrRuO_3$,"Magnetic proximity effect of a topological insulator in contact with a ferromagnet is reported in thin film bilayers of 15 nm thick $BiSbTe_3$ on either 15 or 40 nm thick $SrRuO_3$ on (100) $SrTiO_3$ wafers. Magneto transport results of the bilayers were compared with those of reference films of 15 nm $BiSbTe_3$ and 15 or 40 nm $SrRuO_3$. Comparison of the temperature coefficient of resistance [(1/R)$\times$dR/dT which is qualitatively proportional to the magnetization] of the bilayer and reference ferromagnetic film normalized above $T_c$, shows a clear suppression in the bilayer by about 50% just below $T_c$, indicating a weaker proximity magnetization in the bilayer. Resistance hysteresis loops versus field at 1.85$\pm$0.05 K in the bilayer and reference films show a clear magnetic proximity effect, where the peak resistance of the bilayer at the coercive field shifts to lower fields by $\sim$30% compared to a hypothetical bilayer of two resistors connected in parallel with no interaction between the layers. Narrowing of the coercive peaks of the bilayers as compared to those of the reference ferromagnetic films by 25-35% was also observed, which represents another signature of the magnetic proximity effect.",1711.04275v2 2017-11-16,Spin-orbit torques and tunable Dzyaloshinskii-Moriya interaction in Co/Cu/Co trilayers,"We study the spin-orbit torques (SOTs) in Co/Cu/Co magnetic trilayers based on first-principles density-functional theory calculations in the case where the applied electric field lies in-plane, i.e., parallel to the interfaces. We assume that the bottom Co layer has a fixed in-plane magnetization, while the top Co layer can be switched. We find that the SOT on the top ferromagnet can be controlled by the bottom ferromagnet because of the nonlocal character of the SOT in this system. As a consequence the SOT is anisotropic, i.e., its magnitude varies with the direction of the applied electric field. We show that the Dzyaloshinskii-Moriya interaction (DMI) in the top layer is anisotropic as well, i.e., the spin-spiral wavelength of spin-spirals in the top layer depends on their in-plane propagation direction. This effect suggests that DMI can be tuned easily in magnetic trilayers via the magnetization direction of the bottom layer. In order to understand the influence of the bottom ferromagnet on the SOTs and the DMI of the top ferromagnet we study these effects in Co/Cu magnetic bilayers for comparison. We find the SOTs and the DMI to be surprisingly large despite the small spin-orbit interaction of Cu.",1711.06102v2 2018-03-02,Broadband spectroscopy of thermodynamic magnetization fluctuations through a ferromagnetic spin-reorientation transition,"We use scanning optical magnetometry to study the broadband frequency spectra of spontaneous magnetization fluctuations, or ""magnetization noise"", in an archetypal ferromagnetic film that can be smoothly tuned through a spin reorientation transition (SRT). The SRT is achieved by laterally varying the magnetic anisotropy across an ultrathin Pt/Co/Pt trilayer, from the perpendicular to in-plane direction, via graded Ar$^+$ irradiation. In regions exhibiting perpendicular anisotropy, the power spectrum of the magnetization noise, $S(\nu)$, exhibits a remarkably robust $\nu^{-3/2}$ power law over frequencies $\nu$ from 1~kHz to 1~MHz. As the SRT region is traversed, however, $S(\nu)$ spectra develop a steadily-increasing critical frequency, $\nu_0$, below which the noise power is spectrally flat, indicating an evolving low-frequency cutoff for magnetization fluctuations. The magnetization noise depends strongly on applied in- and out-of-plane magnetic fields, revealing local anisotropies and also a field-induced emergence of fluctuations in otherwise stable ferromagnetic films. Finally, we demonstrate that higher-order correlators can be computed from the noise. These results highlight broadband spectroscopy of thermodynamic fluctuations as a powerful tool to characterize the interplay between thermal and magnetic energy scales, and as a means of characterizing phase transitions in ferromagnets.",1803.00962v1 2018-03-02,Response and phase transition of a Kitaev spin liquid in a local magnetic field,"We study the response of the Kitaev spin liquid (KSL) to a local magnetic field perpendicular to the Kitaev honeycomb lattice. The local magnetic field induces a dynamical excitation of a flux pair in the spin liquid and the system can be described by a generally particle-hole asymmetric interacting resonant level model. The dynamical excitation of the flux pair closes the flux gap in the spectrum of the spin correlation function locally for the gapless KSL even from the perturbative response to a weak magnetic field. Beyond the perturbative regime, the p-h asymmetry competes with the magnetic field and results in a rich phase diagram. Moreover, the magnetic field breaks the gauge equivalence of the ferromagnetic and anti-ferromagnetic Kitaev couplings of the ground state and leads to very different behaviors for the two cases. The anti-ferromagnetic case experiences a first order phase transition to the polarized state during magnetization whereas the ferromagnetic case does not. This study can be generalized to the Kitaev model in a uniform magnetic field and may help understand issues in recent experiments on KSL candidates.",1803.01011v4 2018-03-05,Electric-field modification of interfacial spin-orbit field-vector,"Current induced spin-orbit magnetic fields (iSOFs), arising either in single-crystalline ferromagnets with broken inversion symmetry1,2 or in non-magnetic metal/ferromagnetic metal bilayers3,4, can produce spin-orbit torques which act on a ferromagnet's magnetization,thus offering an efficient way for its manipulation.To further reduce power consumption in spin-orbit torque devices, it is highly desirable to control iSOFs by the field-effect, where power consumption is determined by charging/discharging a capacitor5,6. In particular, efficient electric-field control of iSOFs acting on ferromagnetic metals is of vital importance for practical applications. It is known that in single crystalline Fe/GaAs (001) heterostructures with C2v symmetry, interfacial SOFs emerge at the Fe/GaAs (001) interface due to the lack of inversion symmetry7,8. Here, we show that by applying a gate-voltage across the Fe/GaAs interface, interfacial SOFs acting on Fe can be robustly modulated via the change of the magnitude of the interfacial spin-orbit interaction. Our results show that, for the first time, the electric-field in a Schottky barrier is capable of modifying SOFs, which can be exploited for the development of low-power-consumption spin-orbit torque devices.",1803.01656v2 2018-03-20,Direct demonstration of the emergent magnetism resulting from the multivalence Mn in a LaMnO3 epitaxial thin film system,"Atomically engineered oxide heterostructures provide a fertile ground for creating novel states. For example, a two-dimensional electron gas at the interface between two oxide insulators, giant thermoelectric Seebeck coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and colossal ionic conductivity. Extensive research efforts reveal that oxygen deficiency or lattice strain play an important role in determining these unexpected properties. Herein, by studying the abrupt presence of robust ferromagnetism (up to 1.5 uB/Mn) in LaMnO3-based heterostructures, we find the multivalence states of Mn that play a decisive role in the emergence of ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films. Combining spatially resolved electron energy-loss spectroscopy, X-ray absorption spectroscopy and X-ray magnetic circular dichroism techniques, we determine unambiguously that the ferromagnetism results from a conventional Mn3+-O-Mn4+ double-exchange mechanism rather than an interfacial effect. In contrast, the magnetic dead layer of 5 unit cell in proximity to the interface is found to be accompanied with the accumulation of Mn2+ induced by electronic reconstruction. These findings provide a hitherto-unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3 and BiMnO3, and shed new light on all-oxide spintronic devices.",1803.07402v1 2018-07-29,"A family of high-temperature ferromagnetic monolayers with locked spin-dichroism-mobility anisotropy: MnNX and CrCX (X=Cl, Br, I; C=S, Se, Te)","Two-dimensional magnets have received increasing attention since Cr2Ge2Te6 and CrI3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature (Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tc ferromagnetic monolayers, namely MnNX and CrCX (X=Cl, Br and I; C=S, Se and Te). Their Tc values were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ~101 for effective masses and ~102 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials. Together with the mobility anisotropy, it offers a spin-, dichroism- and mobility-anisotropy locking. These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.",1807.11000v3 2018-08-20,Unconventional exchange bias coupling at perovskite/brownmillerite interface in spontaneously stabilized SrCoO3-δ/SrCoO2.5 bi-layer,"Interface effect in complex oxide thin film heterostructures lies at the vanguard of current research to design technologically relevant functionality and explore emergent physical phenomena. While most of the previous works focus on the perovskite/perovskite heterostructures, the study on perovskite/brownmillerite interfaces remain at its infancy. Here, we investigate spontaneously stabilized perovskite-ferromagnet (SrCoO3-{\delta})/brownmillertite-antiferromagnet (SrCoO2.5) bi-layer with TN > TC and discover an unconventional interfacial magnetic exchange bias effect. From magnetometry investigations, it is rationalized that the observed effect stems from the interfacial ferromagnet/antiferromagnet coupling. The possibility for coupled ferromagnet/spinglass interface engendering such effect is ruled out. Strikingly, a finite coercive field persists in the paramagnetic state of SrCoO3-{\delta} whereas the exchange bias field vanishes at TC. We conjecture the observed effect to be due to the effective external quenched staggered field provided by the antiferromagtic layer for the ferromagnetic spins at the interface. Our results not only unveil a new paradigm to tailor the interfacial magnetic properties in oxide heterostructures without altering the cations at the interface, but also provide a purview to delve into the fundamental aspects of exchange bias in such unusual systems paving a big step forward in thin film magnetism.",1808.06321v1 2018-12-02,Direct detection of induced magnetic moment and efficient spin-to-charge conversion in graphene/ferromagnetic structures,"This article shows that the spin-to-charge current conversion in single-layer graphene (SLG) by means of the inverse Rashba-Edelstein effect (IREE) is made possible with the integration of this remarkable 2D-material with the unique ferrimagnetic insulator yttrium iron garnet (YIG = $Y_{3}Fe_{5}O_{12}$) as well as with the ferromagnetic metal permalloy (Py = $Ni_{81}Sb_{19}$). By means of X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) techniques, we show that the carbon atoms of the SLG acquires an induced magnetic moment due to the proximity effect with the magnetic layer. The spin currents are generated in the magnetic layer by spin pumping from microwave driven ferromagnetic resonance and are detected by a dc voltage along the graphene layer, at room temperature. The spin-to-charge current conversion, occurring at the graphene layer, is explained by the extrinsic spin-orbit interaction (SOI) induced by the proximity effect with the ferromagnetic layer. The results obtained for the SLG/YIG and SLG/Py systems confirm very similar values for the IREE parameter, which are larger than the values reported in previous studies for SLG. We also report systematic investigations of the electronic and magnetic properties of the SLG/YIG by means of scanning tunneling microscopy (STM).",1812.00455v1 2018-12-03,High-resolution neutron depolarization microscopy of the ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ under pressure,"We performed neutron imaging of ferromagnetic transitions in Ni$_3$Al and HgCr$_2$Se$_4$ crystals. These neutron depolarization measurements revealed bulk magnetic inhomogeneities in the ferromagnetic transition temperature with spatial resolution of about 100~$\mu$m. To obtain such spatial resolution, we employed a novel neutron microscope equipped with Wolter mirrors as a neutron image-forming lens and a focusing neutron guide as a neutron condenser lens. The images of Ni$_3$Al show that the sample does not homogeneously go through the ferromagnetic transition; the improved resolution allowed us to identify a distribution of small grains with slightly off-stoichiometric composition. Additionally, neutron depolarization imaging experiments on the chrome spinel, HgCr$_2$Se$_4$, under pressures up to 15~kbar highlight the advantages of the new technique especially for small samples or sample environments with restricted sample space. The improved spatial resolution enables one to observe domain formation in the sample while decreasing the acquisition time despite having a bulky pressure cell in the beam.",1812.00864v1 2019-01-04,Cosmic Ferromagnetism of Magninos,"We study the physical conditions for the occurrence of ferromagnetic instability in a neutral plasma of fermions. We consider a system of two species $M$ and $Y$ which are oppositely charged under a local $U(1)_{X}$, with $M$ much lighter than $Y$. The leading correction to free quasiparticle behaviour for the lighter species arises from the exchange interaction, while the heavier species remain spectators. This plasma, which is abelian, asymmetric and idealised, is shown to be naturally susceptible to the formation of a completely spin-imbalanced ferromagnetic state for the lighter species (dubbed a magnino) in large parts of parameter space. It is shown that the domain structure formed by this ferromagnetic state can mimic Dark Energy, determining the masses of the two fermion species involved, depending on their abundance relative to the standard photons. Incomplete cancellation of the X-magnetic fields among the domains can give rise to residual long range $X$-magnetic fields. Under the assumption that this $U(1)_{X}$ mixes with Maxwell electromagnetism, this provides a mechanism for the seed for cosmic-scale magnetic fields. An extended model with several flavours $M_a$ and $Y_a$ of the species can incorporate Dark Matter. Thus the scenario shows the potential for explaining the large scale magnetic fields, and what are arguably the two most important outstanding puzzles of cosmology: Dark Matter and Dark Energy.",1901.00995v2 2019-02-09,Antiferromagnetic phase transition in $Cr_{2}As$ via anisotropy of exchange interactions,"The electronic structure of anti-ferromagnetic $Cr_{2}As$ is investigated. Anisotropy of exchange interactions between chrome sub-lattices is determined ($J^{X}(Cr_{I}-Cr_{II}) =4.77 meV,J^{Y}(Cr_{I}-Cr_{II}) =-6.36 meV$). The behavior of exchange integrals from magnetic structure is analyzed.",1902.03337v1 2019-02-15,Interlayer RKKY Coupling in Bulk Rashba Semiconductors under Topological Phase Transition,"The bulk Rashba semiconductors BiTeX (X=I, Cl and Br) with intrinsically enhanced Rashba spin-orbit coupling provide a new platform for investigation of spintronic and magnetic phenomena in materials. We theoretically investigate the interlayer exchange interaction between two ferromagnets deposited on opposite surfaces of a bulk Rashba semiconductor BiTeI in its trivial and topological insulator phases. In the trivial phase BiTeI, we find that for ferromagnets with a magnetization orthogonal to the interface, the exchange coupling is reminiscent of that of a conventional three-dimensional metal. Remarkably, ferromagnets with a magnetization parallel to the interface display a magnetic exchange qualitatively different from that of conventional three-dimensional metal due to the spin-orbit coupling. In this case, the interlayer exchange interaction acquires two periods of oscillations and decays as the inverse of the thickness of the BiTeI layer. For topological BiTeI, the magnetic exchange interaction becomes mediated only by the helical surface states and acts between the one-dimensional spin chains at the edges of the sample. The surface state-mediated interlayer exchange interaction allows for the coupling of ferromagnets with non-collinear magnetization and displays a decay power different from that of trivial BiTeI, allowing the detection of the topological phase transition in this material. Our work provides insights into the magnetic properties of these newly discovered materials and their possible functionalization.",1902.06001v2 2019-03-27,High throughput computational screening for 2D ferromagnetic materials: the critical role of anisotropy and local correlations,"The recent observation of ferromagnetic order in two-dimensional (2D) materials has initiated a booming interest in the subject of 2D magnetism. In contrast to bulk materials, 2D materials can only exhibit magnetic order in the presence of magnetic anisotropy. In the present work we have used the Computational 2D Materials Database (C2DB) to search for new ferromagnetic 2D materials using the spinwave gap as a simple descriptor that accounts for the role of magnetic anisotropy. In addition to known compounds we find 12 novel insulating materials that exhibit magnetic order at finite temperatures. For these we evaluate the critical temperatures from classical Monte Carlo simulations of a Heisenberg model with exchange and anisotropy parameters obtained from first principles. Starting from 150 stable ferromagnetic 2D materials we find five candidates that are predicted to have critical temperatures exceeding that of CrI3. We also study the effect of Hubbard corrections in the framework of DFT+U and find that the value of U can have a crucial influence on the prediction of magnetic properties. Our work provides new insight into 2D magnetism and identifies a new set of promising monolayers for experimental investigation.",1903.11466v2 2019-04-16,Universal critical behavior in the ferromagnetic superconductor Eu(Fe$_{0.75}$Ru$_{0.25}$)$_{2}$As$_{2}$,"The study of universal critical behavior is a crucial issue in a continuous phase transition, which groups various critical phenomena into universality classes for revealing microscopic electronic behaviors. The understanding of the nature of magnetism in Eu-based ferromagnetic superconductors is largely impeded by the infeasibility of performing inelastic neutron scattering measurements to deduce the microscopic magnetic behaviors and the effects on the superconductivity, due to the significant neutron absorption effect of natural $^{152}$Eu and unavailability of large single crystals. However, by systematically combining the neutron diffraction experiment, the first-principles calculations, and the quantum Monte Carlo simulations, we have obtained a perfectly consistent universal critical exponent value of $\beta=0.385(13)$ experimentally and theoretically for Eu(Fe$_{0.75}$Ru$_{0.25}$)$_{2}$As$_{2}$, from which the magnetism in the Eu-based ferromagnetic superconductors is identified as the universal class of a three-dimensional anisotropic quantum Heisenberg model with long-range magnetic exchange coupling. This study not only clarifies the nature of microscopic magnetic behaviors in the Eu-based ferromagnetic superconductors, but also opens a new avenue of systemic methodology for studying the universal critical behaviors associated with magnetic phase transitions in the area of magnetism and the spin fluctuations effects on the unconventional superconductivity.",1904.07417v3 2019-04-18,Insight into the magnetic behavior of Sr$_2$IrO$_4$:A spontaneous magnetization study,"Sr$_2$IrO$_4$ is a weak ferromagnet where the spin arrangement is canted anti-ferromagnetic (AF). Moreover, the spin-structure coupling plays and important role in magnetic behavior of Sr$_2$IrO$_4$. In this concern the magnetization under zero applied field i.e. spontaneous magnetization would be interesting to study and would give insight into the novel magnetic behavior of Sr$_2$IrO$_4$. Sophisticated techniques like neutron diffraction, $\mu$ \textit{SR} etc has been used to understand the magnetic behavior of Sr$_2$IrO$_4$ under zero applied field. To understand the magnetic behavior we have performed detail field and temperature dependent magnetization study, the measured field and temperature dependent magnetic data is analyzed rigorously. We have attempted the understand the temperature dependance of spontaneous magnetization, remanent magnetization and coercive force. We observe that the spontaneous magnetization extracted from Arrott plot shows that the Sr$_2$IrO$_4$ is not an ideal ferromagnet. The temperature dependent coercive field is found to follows Guant's model of strong domain wall pinning. Our investigation explicit the temperature dependence of various magnetic properties shows the magnetic transitions from paramagnetic to ferromagnetic phase with $T_c$ around 225 K and a low temperature evolution of magnetic magnetic moment around $T_M$ $\sim$90 K.",1904.08792v1 2019-12-13,Frustration induced highly anisotropic magnetic patterns in classical $XY$ model on kagome lattice,"We predict and observed novel highly anisotropic magnetic patterns obtained in the model of frustrated planar interacting magnetic moments (the classical $X-Y$ model) on the regular kagome lattice. The frustration is provided by the presence of both ferromagnetic and anti-ferromagnetic interactions between adjacent magnetic moments defined on the lattice nodes. At the critical value of the frustration $f=f_{cr}=3/4$ such a systems displays the phase transition from the ordered ferromagnetic state to the disordered frustration regime characterized by the highly-degenerated ground state. In the frustrated regime, $f_{cr}< f \leq 1$, unexpected scaling of spatially averaged magnetization $\langle \vec{M} \rangle $ on the total number of nodes,$N$, i.e. $\langle \vec{M} \rangle \simeq N^{-1/4}$, has been obtained. Such scaling is provided by highly anisotropic magnetic patterns displaying the ferromagnetic ordering along the $y$-direction, and short-range correlations of magnetic moments along the $x$-direction. We conjecture that all these intriguing features are explained by the presence of the double-degenerated ground state in the basic cell, i.e. single triangle, of the kagome lattice accompanying a large amount of intrinsic constraints. We anticipate the implementation of the phase transition and anisotropic magnetic patterns in various systems, e.g. natural magnetic molecular clusters, artificially prepared Josephson junctions networks, trapped-ions and/or photonic crystals.",1912.06397v1 2019-12-14,Spin-mediated charge-to-heat current conversion phenomena in ferromagnetic binary alloys,"Spin-mediated charge-to-heat current conversion phenomena, i.e., the anomalous Ettingshausen effect (AEE) and the anisotropic magneto-Peltier effect (AMPE), have been investigated in various ferromagnetic Ni-Fe, Ni-Pt, Ni-Pd, and Fe-Pt binary alloys at room temperature. When a charge current is applied to a ferromagnetic conductor, the AMPE modulates the Peltier coefficient depending on the angle between the directions of the charge current and magnetization, while the AEE generates a heat current in the direction perpendicular to both the charge current and magnetization. We observed the strong material dependence of the thermoelectric conversion coefficients and figures of merit of these phenomena. Among the ferromagnetic alloys used in this study, Ni$_{95}$Pt$_{5}$ exhibits the largest AMPE of which the anisotropy of the Peltier coefficient is $\sim 12\%$. In contrast, the magnitude of the AEE signals is moderate in Ni$_{95}$Pt$_{5}$ but largest in Ni$_{75}$Pt$_{25}$ and Ni$_{50}$Fe$_{50}$. We discuss these behaviors by exploring the relations between these charge-to-heat current conversion phenomena and other transport as well as magnetic properties. This systematic study will provide a clue for clarifying the mechanisms of the AMPE and AEE and for enhancing the thermoelectric conversion efficiency of these phenomena.",1912.06799v2 2019-12-16,High-Temperature Quantum Anomalous Hall Insulators in Lithium-Decorated Iron-Based Superconductor Materials,"Quantum anomalous Hall (QAH) insulator is the key material to study emergent topological quantum effects, but its ultralow working temperature limits experiments. Here, by first-principles calculations, we find a family of stable two-dimensional (2D) structures generated by lithium decoration of layered iron-based superconductor materials FeX (X = S, Se, Te), and predict room-temperature ferromagnetic semiconductors together with large-gap high-Chern-number QAH insulators in the 2D materials. The extremely robust ferromagnetic order is induced by the electron injection from Li to Fe and stabilized by strong ferromagnetic kinetic exchange in the 2D Fe layer. While in the absence of spin-orbit coupling (SOC), the ferromagnetism polarizes the system into a half Dirac semimetal state protected by mirror symmetry, the SOC effect results in a spontaneous breaking of mirror symmetry and introduces a Dirac mass term, which creates QAH states with sizable gaps (several tens of meV) and multiple chiral edge modes. We also find a 3D QAH insulator phase featured by macroscopic number of chiral conduction channels in bulk LiOH-LiFeX. The findings open new opportunities to realize novel QAH physics and applications at high temperatures.",1912.07461v4 2019-12-24,Direct observation of the energy gain underpinning ferromagnetic superexchange in the electronic structure of CrGeTe$_3$,"We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe$_3$. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through ${T_\mathrm{C}}$. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te ${5p}$ and the Cr ${e_g}$ orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr ${t_{2g}}$ states that carry the majority of the spin moment. The ${t_{2g}}$ states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.",1912.11314v1 2019-12-26,Orbitally-resolved ferromagnetism of monolayer CrI$_3$,"Few-layer CrI$_3$ is the most known example among two-dimensional (2D) ferromagnets, which have attracted growing interest in recent years. Despite considerable efforts and progress in understanding the properties of 2D magnets both from theory and experiment, the mechanism behind the formation of in-plane magnetic ordering in chromium halides is still under debate. Here, we propose a microscopic orbitally-resolved description of ferromagnetism in monolayer CrI$_3$. Starting from first-principles calculations, we construct a low-energy model for the isotropic Heisenberg exchange interactions. We find that there are two competing contributions to the long-range magnetic ordering in CrI$_3$: (i) Antiferromagnetic Anderson's superexchange between half-filled $t_{2g}$ orbitals of Cr atoms; and (ii) Ferromagnetic exchange governed by the Kugel-Khomskii mechanism, involving the transitions between half-filled $t_{2g}$ and empty $e_g$ orbitals. Using numerical calculations, we estimate the exchange interactions in momentum-space, which allows us to restore the spin-wave spectrum, as well as estimate the Curie temperature. Contrary to the nearest-neighbor effective models, our calculations suggest the presence of sharp resonances in the spin-wave spectrum at 5--7 meV, depending on the vertical bias voltage. Our estimation of the Curie temperature in monolayer CrI$_3$ yields 55--65 K, which is in good agreement with experimental data.",1912.11828v1 2020-03-01,Roles of easy-plane and easy-axis XXZ anisotropy and bond alternation on a frustrated ferromagnetic spin-$1/2$ chain,"The spin-$1/2$ Heisenberg chain with a ferromagnetic first-neighbor exchange coupling $J_1$ and an antiferromagnetic second-neighbor $J_2$ has a Haldane dimer ground state with an extremely small spin gap. Thus, the ground state is readily altered by perturbations. Here, we investigate the effects of XXZ exchange magnetic anisotropy of both the easy-axis and easy-plane types and an alternation in $J_1$ on the ground state, the spin gap, and magnetic properties of the frustrated ferromagnetic spin-$1/2$ chain. It is found that there are two distinct dimerized spin-gap phases, in one of which the spin gap and the magnetic susceptibility are extremely small around the SU(2) symmetric case and in the other they are moderately large far away from the SU(2) symmetric case. A small alternation in the amplitude of $J_1$ rapidly shortens the pitch of spin correlations towards the four-spin periodicity, as in the limit of $J_1/J_2\to0$. These effects are not sufficient to quantitatively explain overall experimentally observed magnetic properties in the quasi-one-dimensional spin-gapped magnetoelectric cuprate Rb$_2$Cu$_2$Mo$_3$O$_{12}$ that exhibits ferroelectricity stabilized by a magnetic field. Our results are also relevant to Cs$_2$Cu$_2$Mo$_3$O$_{12}$, where the ferromagnetic intrachain and antiferromagnetic interchain order has recently been found, in a single chain level. We also reveal the nature of symmetry-protected topological phase transitions in the model by mapping onto effective spin-1 chain models.",2003.00554v2 2020-03-20,Current cross-correlations and waiting time distributions in Andreev transport through Cooper pair splitters based on triple quantum dots,"We study the spin-resolved subgap transport in a triple quantum-dot system coupled to one superconducting and two ferromagnetic leads. We examine the Andreev processes in the parallel and antiparallel alignments of ferromagnets magnetic moments in both the linear and nonlinear response regimes. The emphasis is put on the analysis of the current cross-correlations between the currents flowing through the left and right arms of the device and relevant electron waiting time distributions. We show that both quantities can give an important insight into the subgap transport processes and their analysis can help optimizing the system parameters for achieving the considerable Andreev current and efficient Cooper pair splitting. Strong positive values of cross-correlations are associated with the presence of tunneling processes enhancing the Cooper pair splitting efficiency, while short waiting times for electrons tunneling through distinct ferromagnetic contacts indicate fast splitting of emitted Cooper pairs. In particular, we study two detuning schemes and show that antisymmetric shift of side quantum dots energy levels is favorable for efficient Cooper pair splitting. The analysis of spin-resolved waiting time distributions supports the performance enhancement due to the presence of ferromagnetic contacts, which is in particular revealed for short times. Finally, we consider the effect of changing the inter-dot hopping amplitude and predict that strong inter-dot correlations lead to a reduction of Andreev transport properties in low-bias limit.",2003.09165v1 2020-03-23,Evolution of Griffiths Phase and Critical Behaviour of La1-xPbxMnO3+-y Solid Solutions,"Polycrystalline La1-xPbxMnO3+-y (x = 0.3, 0.35, 0.4) solid solutions were prepared by solid state reaction method and their magnetic properties have been investigated. Rietveld refinement of X-ray powder diffraction patterns showed that all samples are single phase and crystallized with the rhombohedral structure in the R-3c space group. A second order paramagnetic to ferromagnetic phase transition was observed for all materials. The Griffiths phase (GP), identified from the temperature dependence of the inverse susceptibility, was suppressed by increasing magnetic field and showed a significant dependence on A-site chemical substitution. The critical behaviour of the compounds was investigated near to their Curie temperatures, using intrinsic magnetic field data. The critical exponents (\b{eta}, {\gamma} and {\delta}) are close to the mean-field approximation values for all three compounds. The observed mean-field like behaviour is a consequence of the GP and the formation of ferromagnetic clusters. Long-range ferromagnetic order is established as the result of long-range interactions between ferromagnetic clusters. The magnetocaloric effect was studied in terms of the isothermal entropy change. Our study shows that the material with the lowest chemical substitution (x = 0.3) has the highest potential (among the three compounds) as magnetic refrigerant, owing to its higher relative cooling power (258 J/kg at 5 T field) and a magnetic phase transition near room temperature.",2003.10207v2 2020-05-01,Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-doped Tungsten Diselenide Monolayers at Room Temperature,"The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long-range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step towards modern TMD-based spintronics, but also enables exploration of new and exciting dimensionality-driven magnetic phenomena. To this end, we show tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V-doped WSe2. As vanadium concentrations increase within the WSe2 monolayers the saturation magnetization increases, and it is optimal at ~4at.% vanadium; the highest doping/alloying level ever achieved for V-doped WSe2 monolayers. The TISF occurs at ~175 K and becomes more pronounced upon increasing the temperature towards room temperature. We demonstrate that TISF can be manipulated by changing the vanadium concentration within the WSe2 monolayers. We attribute TISF to the magnetic field and temperature dependent flipping of the nearest W-site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin-polarized density functional theory calculation. Our findings pave the way for the development of novel spintronic and valleytronic nanodevices based on atomically thin magnetic semiconductors and stimulate further studies in this rapidly expanding research field of 2D magnetism.",2005.00493v1 2020-05-29,Electron spin resonance and ferromagnetic resonance spectroscopy in the high-field phase of the van der Waals magnet CrCl$_3$,"We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet CrCl$_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in CrCl$_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.",2005.14559v1 2020-06-01,Vortex-lattice formation in a spin-orbit coupled rotating spin-1 condensate,"We study the vortex-lattice formation in a rotating {Rashba} spin-orbit (SO) coupled quasi-two-dimensional (quasi-2D) hyper-fine spin-1 spinor Bose-Einstein condensate (BEC) in the $x-y$ plane using a numerical solution of the underlying mean-field Gross-Pitaevskii equation. % The wave function for this system %has three components corresponding to the three projections of hyper-fine spin $F_z= +1,0,-1$. In this case, the non-rotating {Rashba} SO-coupled spinor BEC can have topological excitation in the form of vortices of different angular momenta in the three components, e.g. the $(0,+1,+2)$- and $(-1,0,+1)$-type states in ferromagnetic and anti-ferromagnetic spinor BEC: the numbers in the parenthesis denote the intrinsic angular momentum of the vortex states of the three components with the negative sign denoting an anti-vortex. The presence of these states with intrinsic vorticity breaks the symmetry between rotation with vorticity along the $z$ and $-z$ axes and thus generates a rich variety of vortex-lattice and anti-vortex-lattice states in a rotating quasi-2D spin-1 spinor ferromagnetic and anti-ferromagnetic BEC, not possible in a scalar BEC. {For weak SO coupling, } we find two types of symmetries of these states $-$ hexagonal and ""square"". The hexagonal (square) symmetry state has vortices arranged in closed concentric orbits with a maximum of $6, 12, 18...$ ($8,12,16...$) vortices in successive orbits. Of these two symmetries, the square vortex-lattice state is found to have the smaller energy.",2006.00646v3 2020-06-04,Yu-Shiba-Rusinov bands in ferromagnetic superconducting diamond,"The combination of different exotic properties in materials paves the way for the emergence of their new potential applications. An example is the recently found coexistence of the mutually antagonistic ferromagnetism and superconductivity in hydrogenated boron-doped diamond, which promises to be an attractive system with which to explore unconventional physics. Here, we show the emergence of Yu-Shiba-Rusinov (YSR) bands with a spatial extent of tens of nanometers in ferromagnetic superconducting diamond using scanning tunneling spectroscopy. We demonstrate theoretically how a two-dimensional (2D) spin lattice at the surface of a three-dimensional (3D) superconductor gives rise to the YSR bands, and how their density-of-states profile correlates with the spin lattice structure. The established strategy to realize new forms of the coexistence of ferromagnetism and superconductivity opens a way to engineer the unusual electronic states and also to design better performing superconducting devices.",2006.02853v1 2020-06-21,"Crystal Structure, Magnetism, and Electronic Properties of New Rare-Earth-Free Ferromagnetic MnPt5As","The design and synthesis of targeted functional materials have been a long-term goal for material scientists. Although a universal design strategy is difficult to generate for all types of materials, however, it is still helpful for a typical family of materials to have such design rules. Herein, we incorporated several significant chemical and physical factors regarding magnetism, such as structure type, atom distance, spin-orbit coupling, and successfully synthesized a new rare-earth-free ferromagnet, MnPt5As, for the first time. MnPt5As can be prepared by using high-temperature pellet methods. According to X-ray diffraction results, MnPt5As crystallizes in a tetragonal unit cell with the space group P4/mmm (Pearson symbol tP7). Magnetic measurements on MnPt5As confirm ferromagnetism in this phase with a Curie temperature of ~301 K and a saturated moment of 3.5 uB per formula. Evaluation by applying the Stoner Criterion also indicates that MnPt5As is susceptible to ferromagnetism. Electronic structure calculations using the WIEN2k program with local spin density approximation imply that the spontaneous magnetization of this phase arises primarily from the hybridization of d orbitals on both Mn and Pt atoms. The theoretical assessments are consistent with the experimental results. Moreover, the spin-orbit coupling effects heavily influence on magnetic moments in MnPt5As. MnPt5As is the first high-performance magnetic material in this structure type. The discovery of MnPt5As offers a platform to study the interplay between magnetism and structure.",2006.11897v1 2020-08-05,Proximate ferromagnetic state in the Kitaev model material $α$-RuCl$_{3}$,"$\alpha$-RuCl$_{3}$ is a major candidate for the realization of the Kitaev quantum spin liquid, but its zigzag antiferromagnetic order at low temperatures indicates deviations from the Kitaev model. We have quantified the spin Hamiltonian of $\alpha$-RuCl$_{3}$ by a resonant inelastic x-ray scattering study at the Ru $L_{3}$ absorption edge. In the paramagnetic state, the quasi-elastic intensity of magnetic excitations has a broad maximum around the zone center without any local maxima at the zigzag magnetic Bragg wavevectors. This finding implies that the zigzag order is fragile and readily destabilized by competing ferromagnetic correlations. The classical ground state of the experimentally determined Hamiltonian is actually ferromagnetic. The zigzag state is stabilized via a quantum ""order by disorder"" mechanism, leaving ferromagnetism -- along with the Kitaev spin liquid -- as energetically proximate metastable states. The three closely competing states and their collective excitations hold the key to the theoretical understanding of the unusual properties of $\alpha$-RuCl$_{3}$ in magnetic fields.",2008.02037v2 2020-08-27,"Different universality classes of isostructural U$TX$ compounds ($T$ = Rh, Co, Co$_{0.98}$Ru$_{0.02}$; $X$ = Ga, Al)","Magnetization isotherms of the 5f-electron ferromagnets URhGa, UCoGa and UCo0.98Ru0.02Al were measured at temperatures in the vicinity of their Curie temperature in order to investigate the critical behavior near the ferromagnetic phase transition. These compounds adopt the layered hexagonal ZrNiAl-type structure and exhibit huge uniaxial magnetocrystalline anisotropy. The critical \b{eta}, {\gamma} and {\delta} exponents were determined by analyzing Arrott-Noakes plots, Kouvel-Fisher plots, critical isotherms, scaling theory and Widom scaling relations. The values obtained for URhGa and UCoGa can be explained by the results of the renormalization group theory for a 2D Ising system with long-range interactions similar to URhAl reported by other investigators. On the other hand, the critical exponents determined for UCo0.98Ru0.02Al are characteristic of a 3D Ising ferromagnet with short-range interactions suggested in previous studies also for the itinerant 5f-electron paramagnet UCoAl situated near a ferromagnetic transition. The change from the 2D to the 3D Ising system is related to the gradual delocalization of 5f electrons in the series of the URhGa, URhAl, UCoGa to UCo0.98Ru0.02Al and UCoAl compounds and appears close to the strongly itinerant nonmagnetic limit. This indicates possible new phenomena that may be induced by the change of dimensionality in the vicinity of the quantum critical point.",2008.12061v1 2020-10-20,Pressure-induced huge increase of Curie temperature of the van der Waals ferromagnet VI3,"Evolution of magnetism in single crystals of the van der Waals compound VI3 in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at T1 = 54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature TC and the transition between two different ferromagnetic phases TFM, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. TC was found to be almost unaffected by pressures up to 0.6 GPa whereas TFM increases rapidly with increasing pressure and reaches TC at a triple point at ~ 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is dramatically reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. We discussed these results in light of recent theoretical studies to analyze exchange interactions and provide how to increase the Curie temperature of VI3.",2010.10319v1 2020-10-26,Ultra-strong photon-to-magnon coupling in multilayered heterostructures involving superconducting coherence via ferromagnetic layers,"The critical step for future quantum industry demands realization of efficient information exchange between different-platform hybrid systems, including photonic and magnonic systems, that can harvest advantages of distinct platforms. The major restraining factor for the progress in certain hybrid systems is the fundamentally weak coupling parameter between the elemental particles. This restriction impedes the entire field of hybrid magnonics by making realization of scalable on-chip hybrid magnonic systems unattainable. In this work, we propose a general flexible approach for realization of on-chip hybrid magnonic systems with unprecedentedly strong coupling parameters. The approach is based on multilayered micro-structures containing superconducting, insulating and ferromagnetic layers with modified both photon phase velocities and magnon eigen-frequencies. Phenomenologically, the enhanced coupling strength is provided by the radically reduced photon mode volume. The microscopic mechanism of the phonon-to-magnon coupling in studied systems evidences formation of the long-range superconducting coherence via thick strong ferromagnetic layers. This coherence is manifested by coherent superconducting screening of microwave fields by the superconductor/ferromagnet/superconductor three-layers in presence of magnetization precession. This discovery offers new opportunities in microwave superconducting spintronics for quantum technologies.",2010.13553v1 2020-11-01,"Tunable magneto-optical effect, anomalous Hall effect and anomalous Nernst effect in two-dimensional room-temperature ferromagnet $1T$-CrTe$_2$","Utilizing the first-principles density functional theory calculations together with group theory analyses, we systematically investigate the spin order-dependent magneto-optical effect (MOE), anomalous Hall effect (AHE), and anomalous Nernst effect (ANE) in a recently discovered two-dimensional room-temperature ferromagnet $1T$-CrTe$_2$. We find that the spin prefers an in-plane direction by the magnetocrystalline anisotropy energy calculations. The MOE, AHE, and ANE display a period of $2\pi/3$ when the spin rotates within the atomic plane, and they are forbidden if there exists a mirror plane perpendicular to the spin direction. By reorienting the spin from in-plane to out-of-plane direction, the MOE, AHE, and ANE are enhanced by around one order of magnitude. Moreover, we establish the layer-dependent magnetic properties for multilayer $1T$-CrTe$_2$ and predict antiferromagnetism and ferromagnetism for bilayer and trilayer $1T$-CrTe$_2$, respectively. The MOE, AHE, and ANE are prohibited in antiferromagnetic bilayer $1T$-CrTe$_2$ due to the existence of the spacetime inversion symmetry, whereas all of them are activated in ferromagnetic trilayer $1T$-CrTe$_2$ and the MOE is significantly enhanced compared to monolayer $1T$-CrTe$_2$. Our results show that the magneto-optical and anomalous transports proprieties of $1T$-CrTe$_2$ can be effectively modulated by altering spin direction and layer number.",2011.00394v1 2020-11-20,Phase diagram of vortices in the polar phase of spin-1 Bose-Einstein condensates,"The phase diagram of lowest-energy vortices in the polar phase of spin-1 Bose--Einstein condensates is investigated theoretically. Singly quantized vortices are categorized by the local ordered state in the vortex core and three types of vortices are found as lowest-energy vortices, which are elliptic AF-core vortices, axisymmetric F-core vortices, and N-core vortices. These vortices are named after the local ordered state, ferromagnetic (F), antiferromagnetic (AF), broken-axisymmetry (BA), and normal (N) states apart from the bulk polar (P) state. The N-core vortex is a conventional vortex, in the core of which the superfluid order parameter vanishes. The other two types of vortices are stabilized when the quadratic Zeeman energy is smaller than a critical value. The axisymmetric F-core vortex is the lowest-energy vortex for ferromagnetic interaction, and it has an F core surrounded by a BA skin that forms a ferromagnetic-spin texture, as exemplified by the localized Mermin--Ho texture. The elliptic AF-core vortex is stabilized for antiferromagnetic interaction; the vortex core has both nematic-spin and ferromagnetic orders locally and is composed of the AF-core soliton spanned between two BA edges. The phase transition from the N-core vortex to the other two vortices is continuous, whereas that between the AF-core and F-core vortices is discontinuous. The critical point of the continuous vortex-core transition is computed by the perturbation analysis of the Bogoliubov theory and the Ginzburg--Landau formalism describes the critical behavior. The influence of trapping potential on the core structure is also investigated.",2011.10342v3 2021-02-03,Low temperature ferromagnetism in perovskite SrIrO$_3$ films,"The 5$d$ based SrIrO$_3$ represents prototype example of nonmagnetic correlated metal which mainly originates from a combined effect of spin-orbit coupling, lattice dimensionality and crystal structure. Therefore, tuning of these parameters results in diverse physical properties in this material. Here, we study the structural, magnetic and electrical transport behavior in epitaxial SrIrO$_3$ film ($\sim$ 40 nm) grown on SrTiO$_3$ substrate. Opposed to bulk material, the SrIrO$_3$ film exhibits a ferromagnetic ordering at low temperature below $\sim$ 20 K. The electrical transport data indicate an insulating behavior where the nature of charge transport follows Mott's variable-range-hopping model. A positive magnetoresistance is recorded at 2 K which has correlation with magnetic moment. We further observe a nonlinear Hall effect at low temperature ($<$ 20 K) which arises due to an anomalous component of Hall effect. An anisotropic behavior of both magnetoresistance and Hall effect has been evidenced at low temperature which coupled with anomalous Hall effect indicate the development of ferromagnetic ordering. We believe that an enhanced (local) structural distortion caused by lattice strain at low temperatures induces ferromagnetic ordering, thus showing structural instability plays vital role to tune the physical properties in SrIrO$_3$.",2102.02150v1 2021-02-22,High T$_C$ ferromagnetic inverse Heusler alloys: A comparative study of Fe$_2$RhSi and Fe$_2$RhGe,"We report the results of experimental investigations on structural, magnetic, resistivity, caloric properties of Fe$_2$RhZ (Z=Si,Ge) along with \textit{ab-initio} band structure calculations using first principle simulations. Both these alloys are found to crystallize in inverse Heusler structure but with disorder in tetrahedral sites between Fe and Rh. Fe$_2$RhSi has saturation moment of 5.00 $\mu_B$ and while its counterpart has 5.19 $\mu_B$. Resistivity measurement reveals metallic nature in both of them. Theoretical simulations using generalized gradient approximation(GGA) predict inverse Heusler structure with ferromagnetic ordering as ground state for both the alloys. However it underestimates the experimentally observed moments. GGA+$U$ approach, with Hubbard $U$ values estimated from density functional perturbation theory helps to improve the comparison of the experimental results. Fe$_2$RhSi is found to be half metallic ferromagnet while Fe$_2$RhGe is not. Varying $U$ values on Fe and Rh sites does not change the net moment much in Fe$_2$RhSi, unlike in Fe$_2$RhGe. Relatively small exchange splitting of orbitals in Fe$_2$RhGe compared to that of Fe$_2$RhSi is the reason for not opening the band gap in the minority spin channel in the former. High ordering temperature and moment make Fe$_2$RhSi useful for spintronics applications.",2102.10967v1 2021-02-27,Room temperature ferromagnetism of monolayer chromium telluride with perpendicular magnetic anisotropy,"The realization of long-range magnetic ordering in two-dimensional (2D) systems can potentially revolutionize next-generation information technology. Here, we report the successful fabrication of crystalline Cr3Te4 monolayers with room temperature ferromagnetism. Using molecular beam epitaxy, the growth of 2D Cr3Te4 films with monolayer thickness is demonstrated at low substrate temperatures (~100C), compatible with Si CMOS technology. X-ray magnetic circular dichroism measurements reveal a Curie temperature (Tc) of ~344 K for the Cr3Te4 monolayer with an out-of-plane magnetic easy axis, which decreases to ~240 K for the thicker film (~ 7 nm) with an in-plane easy axis. The enhancement of ferromagnetic coupling and the magnetic anisotropy transition is ascribed to interfacial effects, in particular the orbital overlap at the monolayer Cr3Te4/graphite interface, supported by density-functional theory calculations. This work sheds light on the low-temperature scalable growth of 2D nonlayered materials with room temperature ferromagnetism for new magnetic and spintronic devices.",2103.00251v1 2021-02-28,Spin and charge interconversion in Dirac semimetal thin films,"We report spin-to-charge and charge-to-spin conversion at room temperature in heterostructure devices that interface an archetypal Dirac semimetal, Cd3As2, with a metallic ferromagnet, Ni0.80Fe0.20 (permalloy). The spin-charge interconversion is detected by both spin torque ferromagnetic resonance and ferromagnetic resonance driven spin pumping. Analysis of the symmetric and anti-symmetric components of the mixing voltage in spin torque ferromagnetic resonance and the frequency and power dependence of the spin pumping signal show that the behavior of these processes is consistent with previously reported spin-charge interconversion mechanisms in heavy metals, topological insulators, and Weyl semimetals. We find that the efficiency of spin-charge interconversion in Cd3As2/permalloy bilayers can be comparable to that in heavy metals. We discuss the underlying mechanisms by comparing our results with first principles calculations.",2103.00653v1 2021-03-16,Emergent Magnetic Phenomenon with Unconventional Structure in Epitaxial Manganate Thin Films,"A variety of emergent phenomena has been enabled by interface engineering in the complex oxides heterostructures. While extensive attention has been attracted to LaMnO3 (LMO) thin films for observing the control of functionalities at its interface with substrate, the nature of the magnetic phases in the thin film is, however,controversial. Here, it is reported that the ferromagnetism in 2 and 5 unit cells thick LMO films epitaxially deposited (001)-SrTiO3 substrates ferromagnetic/ferromagnetic coupling in 8 and 10-unit-cell ones, and a striking ferromagnetic/antiferromagnetic pinning effect with apparent positive exchange bias in 15 and 20-unit-cell ones are observed. This novel phenomenon in both 15 and 20-unit-cell films indicates a coexistence of three magnetic orderings in a single LMO film.The high-resolution scanning transmission electron microscopy suggests a P21/n to Pbnm symmetry transition from interface to surface, with the spatial stratification of MnO6 octahedral morphology, corresponding to different magnetic orderings. These results should shed some new lights on manipulating the functionality of oxides by interface engineering.",2103.09133v1 2021-03-30,Z2 topological order and first-order quantum phase transitions in systems with combinatorial gauge symmetry,"We study a generalization of the two-dimensional transverse-field Ising model, combining both ferromagnetic and antiferromagnetic two-body interactions, that hosts exact global and local Z2 gauge symmetries. Using exact diagonalization and stochastic series expansion quantum Monte Carlo methods, we confirm the existence of the topological phase in line with previous theoretical predictions. Our simulation results show that the transition between the confined topological phase and the deconfined paramagnetic phase is of first-order, in contrast to the conventional Z2 lattice gauge model in which the transition maps onto that of the standard Ising model and is continuous. We further generalize the model by replacing the transverse field on the gauge spins with a ferromagnetic XX interaction while keeping the local gauge symmetry intact. We find that the Z2 topological phase remains stable, while the paramagnetic phase is replaced by a ferromagnetic phase. The topological-ferromagnetic quantum phase transition is also of first-order. For both models, we discuss the low-energy spinon and vison excitations of the topological phase and their avoided level crossings associated with the first-order quantum phase transitions.",2103.16625v1 2021-04-27,Observation of a phase transition within the domain walls of ferromagnetic Co3Sn2S2,"The ferromagnetic phase of Co$_3$Sn$_2$S$_2$ is widely considered to be a topological Weyl semimetal, with evidence for momentum-space monopoles of Berry curvature from transport and spectroscopic probes. As the bandstructure is highly sensitive to the magnetic order, attention has focused on anomalies in magnetization, susceptibility and transport measurements that are seen well below the Curie temperature, leading to speculation that a ""hidden"" phase coexists with ferromagnetism. Here we report spatially-resolved measurements by Kerr effect microscopy that identify this phase. We find that the anomalies coincide with a deep minimum in domain wall (DW) mobility, indicating a crossover between two regimes of DW propagation. We demonstrate that this crossover is a manifestation of a 2D phase transition that occurs within the DW, in which the magnetization texture changes from continuous rotation to unidirectional variation. We propose that the existence of this 2D transition deep within the ferromagnetic state of the bulk is a consequence of a giant quality factor for magnetocrystalline anisotropy unique to this compound. This work broadens the horizon of the conventional binary classification of DWs into Bloch and N\'eel walls, and suggests new strategies for manipulation of domain walls and their role in electron and spin transport.",2104.13381v2 2021-05-19,"Determination of the spin Hall angle by the inverse spin Hall effect, device level ferromagnetic resonance, and spin torque ferromagnetic resonance: a comparison of methods","The spin torque ferromagnetic resonance (STFMR) is one of the popular methods for measurement of the spin Hall angle (SHA). However, in order to accurately determine SHA from STFMR measurements, the acquired data must be carefully analyzed: The resonance linewidth should be determined to an accuracy of a fraction of an Oe, while the dynamical interaction leading to the measured response consists of the conventional field-induced ferromagnetic resonance (FMR), spin-torque induced FMR, and of the inverse spin Hall effect (ISHE). Additionally, the signal often deteriorates when DC current is passed through the device. In this work we compare the STFMR method with two other FMR-based methods that are used to extract SHA. The first is a device-level FMR and the second is based on the ISHE. We identify artefacts that are caused by the noise floor of the instrumentation that make the measurement of SHA illusive even when the signal to noise ratio seems to be reasonable. Additionally, we estimate a 10% error in SHA that results from neglecting the magnetic anisotropies as in conventional measurements. Overall, we find the STFMR to be the most robust of the three methods despite the complexity of the interaction taking place therein. The conclusions of our work lead to a more accurate determination of SHA and will assist in the search of novel materials for energy efficient spin-based applications.",2105.09023v1 2021-06-03,Reentrant Bloch ferromagnetism,"An interacting electron liquid in two (2D) and three (3D) dimensions may undergo a paramagnetic-to-ferromagnetic quantum spin polarization transition at zero applied magnetic field, driven entirely by exchange interactions, as the system density ($n$) is decreased. This is known as Bloch ferromagnetism. We show theoretically that the application of an external magnetic field ($B$), which directly spin polarizes the system through Zeeman spin splitting, has an interesting effect on Bloch ferromagnetism if the applied field and carrier density are both decreased (from some initial applied high magnetic field at a high carrier density) in a power-law manner, $B\sim n^p$. For $pp_c$, the system may undergo two transitions if starting from the fully spin-polarized state: first, a weak second order transition at high density and field from the field-induced fully polarized phase to the partially polarized phase; and then, at a lower field and density, a reentrant first order transition back to the fully spin-polarized phase again with a single Fermi surface.",2106.02038v2 2021-06-04,Readout of a antiferromagnetic spintronics systems by strong exchange coupling of Mn2Au and Permalloy,"In antiferromagnetic spintronics, the read-out of the staggered magnetization or Neel vector is the key obstacle to harnessing the ultra-fast dynamics and stability of antiferromagnets for novel devices. Here, we demonstrate strong exchange coupling of Mn2Au, a unique metallic antiferromagnet that exhibits Neel spin-orbit torques, with thin ferromagnetic Permalloy layers. This allows us to benefit from the well-estabished read-out methods of ferromagnets, while the essential advantages of antiferromagnetic spintronics are retained. We show one-to-one imprinting of the antiferromagnetic on the ferromagnetic domain pattern. Conversely, alignment of the Permalloy magnetization reorients the Mn2Au Neel vector, an effect, which can be restricted to large magnetic fields by tuning the ferromagnetic layer thickness. To understand the origin of the strong coupling, we carry out high resolution electron microscopy imaging and we find that our growth yields an interface with a well-defined morphology that leads to the strong exchange coupling.",2106.02333v1 2021-06-15,Phase separation of self-propelled disks with ferromagnetic and nematic alignment,"We present a comprehensive study of a model system of repulsive self-propelled disks in two dimensions with ferromagnetic and nematic velocity alignment interactions. We characterize the phase behavior of the system as a function of the alignment and self-propulsion strength, featuring orientational order for strong alignment and Motility-Induced Phase Separation (MIPS) at moderate alignment but high enough self-propulsion. We derive a microscopic theory for these systems yielding a close set of hydrodynamic equations from which we perform a linear stability analysis of the homogenous disordered state. This analysis predicts MIPS in the presence of aligning torques. The nature of the continuum theory allows for an explicit quantitative comparison with particle-based simulations, which consistently shows that ferromagnetic alignment fosters phase separation, while nematic alignment does not alter either the nature or the location of the instability responsible for it. In the ferromagnetic case, such behavior is due to an increase of the imbalance of the number of particle collisions along different orientations, giving rise to the self-trapping of particles along their self-propulsion direction. On the contrary, the anisotropy of the pair correlation function, which encodes this self-trapping effect, is not significantly affected by nematic torques. Our work shows the predictive power of such microscopic theories to describe complex active matter systems with different interaction symmetries and sheds light on the impact of velocity-alignment interactions in Motility-Induced Phase Separation.",2106.08228v2 2021-06-29,Spin-orbit torques and magnetotransport properties of $α$-Sn and $β$-Sn heterostructures,"Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\alpha$-Sn. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\alpha$-Sn and $\beta$-Sn-based ferromagnetic heterostructures. We show that $\alpha$-Sn grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\alpha$-Sn grown without Bi surfactant and the non-topological phase, $\beta$-Sn, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\alpha$-Sn with Bi are 0.12 and 0.18, respectively. Further, we show that $\alpha$-Sn grown with and without Bi presents a spin Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\alpha$-Sn ferromagnetic heterostructures, showing that $\alpha$-Sn is a promising material for current-induced magnetization control in spintronic devices.",2106.15386v1 2021-07-07,"Coexistence of ferromagnetism, antiferromagnetism, and superconductivity in magnetically anisotropic (Eu,La)FeAs2","Materials with exceptional magnetism and superconductivity usually conceive emergent physical phenomena. Here, we investigate the physical properties of the (Eu,La)FeAs2 system with double magnetic sublattices. The parent EuFeAs2 shows anisotropy-associated magnetic behaviors, such as Eu-related moment canting and exchange bias. Through La doping, the magnetic anisotropy is enhanced with ferromagnetism of Eu2+ realized in the overdoped region, and a special exchange bias of the superposed ferromagnetic/superconducting loop revealed in Eu0.8La0.2FeAs2. Meanwhile, the Fe-related antiferromagnetism shows unusual robustness against La doping. Theoretical calculation and 57Fe M\""ossbauer spectroscopy investigation reveal a doping-tunable dual itinerant/localized nature of the Fe-related antiferromagnetism. Coexistence of the Eu-related ferromagnetism, Fe-related robust antiferromagnetism, and superconductivity is further revealed in Eu0.8La0.2FeAs2, providing a platform for further exploration of potential applications and emergent physics. Finally, an electronic phase diagram is established for (Eu,La)FeAs2 with the whole superconducting dome adjacent to the Fe-related antiferromagnetic phase, which is of benefit for seeking underlying clues to high-temperature superconductivity.",2107.03526v1 2021-09-02,Supercurrent-induced long-range triplet correlations and controllable Josephson effect in superconductor/ferromagnet hybrids with extrinsic SOC,"We predict that long-range triplet correlations (LRTC) in superconductor/ferromagnet (S/F) hybrids with extrinsic impurity spin-orbit coupling (SOC) can be generated and manipulated by supercurrent flowing in the superconducting leads along the S/F interfaces. The LRTC appear via two basic mechanisms. The essence of the first one is the generation of triplets by the superconducting spin Hall effect. These pairs are long-range in the ferromagnet under the appropriate mutual orientation of the condensate momentum and the ferromagnet magnetization. The second mechanism is based on the singlet-triplet conversion at the S/F interface followed by the rotation of the spin of the obtained short-range opposite-spin pairs via the spin current swapping mechanism. The structure of the supercurrent-induced LRTC is studied both for S/F bilayers and S/F/S Josephson junctions. We demonstrate that in S/F/S junctions, where the Josephson coupling is realized via the supercurrent-induced LRTC, the ground state phase can be switched between $0$ and $\pi$ in a controllable manner. The switching is performed by reversing the supercurrent in one of superconducting leads, thus realizing a new physical principle of the $0-\pi$ shifter.",2109.00966v2 2021-09-08,Signature of half-metallicity in $\text{BiFeO}_\text{3}$,"$\text{BiFeO}_\text{3}$ has drawn a great attention over last several decades due to its promising multiferroic character. In the ground state the bulk $\text{BiFeO}_\text{3}$ is found to be in the rhombohedral phase. However, it has been possible to stabilize $\text{BiFeO}_\text{3}$ with tetragonal structure. The importance of tetragonal phase is due to its much larger value of the electric polarization and the possible stabilization of ferromagnetism as in the rhombohedral phase. Furthermore, the tetragonal structure of $\text{BiFeO}_\text{3}$ has been reported with different $c/a$ ratio, opening up the possibility of a much richer set of electronic phases. In this work, we have used density functional theory based first-principle method to study the ferromagnetic phase of the tetragonal $\text{BiFeO}_\text{3}$ structure as a function of the $c/a$ ratio. We have found that as the $c/a$ ratio decreases from $1.264$ to $1.016$, the tetragonal $\text{BiFeO}_\text{3}$ evolve from a ferromagnetic semiconductor to a ferromagnetic metal, while passing through a \emph{half-metallic} phase. This evolution of the electronic properties becomes even more interesting when viewed with respect to the volume of each structure. The most stable half-metallic phase initially counter-intuitively evolve to the magnetic-semiconducting phase with a reduction in the volume, and after further reduction in the volume it finally becomes a metal. So far, this type of metal to insulator transition on compression was known to exist only in alkali metals, especially in Lithium, in heavy alkaline earth metals, and in some binary compound.",2109.03886v1 2021-09-17,Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet,"Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.",2109.08538v1 2021-10-11,Epitaxial growth of bilayer Bi(110) on two-dimensional ferromagnetic Fe3GeTe2,"Heterostructures of two-dimensional (2D) layered materials with selective compositions play an important role in creating novel functionalities. Effective interface coupling between 2D ferromagnet and electronic materials would enable the generation of exotic physical phenomena caused by intrinsic symmetry breaking and proximity effect at interfaces. Here, epitaxial growth of bilayer Bi(110) on 2D ferromagnetic Fe3GeTe2 (FGT) with large magnetic anisotropy has been reported. Bilayer Bi(110) islands are found to extend along fixed lattice directions of FGT. The six preferred orientations could be divided into two groups of three-fold symmetry axes with the difference approximately to 26{\deg}. Moreover, dI/dV measurements confirm the existence of interface coupling between bilayer Bi(110) and FGT. A variation of the energy gap at the edges of bilayer Bi(110) is also observed which is modulated by the interface coupling strengths associated with its buckled atomic structure. This system provides a good platform for further study of the exotic electronic properties of epitaxial Bi(110) on 2D ferromagnetic substrate and promotes potential applications in the field of spin devices.",2110.05066v3 2021-11-01,Interface-enhanced ferromagnetism with long-distance effect in van der Waals semiconductor,"Ferromagnetic semiconductors discovered in two-dimensional (2D) materials open an avenue for highly integrated and multifunctional spintronics. The Curie temperature (TC) of existed 2D ferromagnetic semiconductors is extremely low and the modulation effect of their magnetism is limited compared with their 2D metallic counterparts. The interfacial effect was found to effectively manipulate the three-dimensional magnetism, providing a unique opportunity for tailoring the 2D magnetism. Here we demonstrate that the TC of 2D ferromagnetic semiconductor Cr2Ge2Te6 can be enhanced by 130% (from ~65 K to above 150 K) when adjacent to a tungsten layer. The interfacial W-Te bonding contributes to the TC enhancement with a strong perpendicular magnetic anisotropy (PMA), guaranteeing an efficient magnetization switching by the spin-orbit torque with a low current density at 150 K. Distinct from the rapid attenuation in conventional magnets, the interfacial effect exhibits a weak dependence on Cr2Ge2Te6 thickness and a long-distance effect (more than 10 nanometers) due to the weak interlayer coupling inherent to 2D magnets. Our work not only reveals a unique interfacial behavior in 2D materials, but also advances the process towards practical 2D spintronics.",2111.00645v1 2021-11-11,Tuning the Room Temperature Ferromagnetism in Fe5GeTe2 by Arsenic Substitution,"In order to tune the magnetic properties of the cleavable high-Curie temperature ferromagnet Fe$_{5-x}$GeTe$_2$, the effect of increasing the electron count through arsenic substitution has been investigated. Small additions of arsenic (2.5 and 5%) seemingly enhance ferromagnetic order in polycrystalline samples by quenching fluctuations on one of the three magnetic sublattices, whereas larger As concentrations decrease the ferromagnetic Curie temperature ($T_{\rm C}$) and saturation magnetization. This work also describes the growth and characterization of Fe$_{4.8}$AsTe$_2$ single crystals that are structurally analogous to Fe$_{5-x}$GeTe$_2$ but with some phase stability complications. Magnetization measurements reveal dominant antiferromagnetic behavior in Fe$_{4.8}$AsTe$_2$ with a N\'{e}el temperature of $T_{\rm N}$ $\approx$42K. A field-induced spin-flop below $T_{\rm N}$ results in a switch from negative to positive magnetoresistance, with significant hysteresis causing butterfly-shaped resistance loops. In addition to reporting the properties of Fe$_{4.8}$AsTe$_2$, this work shows the importance of manipulating the individual magnetic sublattices in Fe$_{5-x}$GeTe$_2$ and motivates further efforts to control the magnetic properties in related materials by fine tuning of the Fermi energy or crystal chemistry.",2111.06439v1 2021-11-18,Spin transport at finite temperatures: A first-principles study for ferromagnetic$|$nonmagnetic interfaces,"Symmetry lowering at an interface leads to an enhancement of the effect of spin-orbit coupling and to a discontinuity of spin currents passing through the interface. This discontinuity is characterized by a ""spin-memory loss"" (SML) parameter $\delta$ that has only been determined directly at low temperatures. Although $\delta$ is believed to be significant in experiments involving interfaces between ferromagnetic and nonmagnetic metals, especially heavy metals like Pt, it is more often than not neglected to avoid introducing too many unknown interface parameters in addition to often poorly known bulk parameters like the spin-flip diffusion length $l_{\rm sf}$. In this work, we calculate $\delta$ along with the interface resistance $AR_{\rm I}$ and the spin-asymmetry parameter $\gamma$ as a function of temperature for Co$|$Pt and Py$|$Pt interfaces where Py is the ferromagnetic Ni$_{80}$Fe$_{20}$ alloy, permalloy. We use first-principles scattering theory to calculate the conductance as well as local charge and spin currents, modeling temperature-induced disorder with frozen thermal lattice and, for ferromagnetic materials, spin disorder within the adiabatic approximation. The bulk and interface parameters are extracted from the spin currents using a Valet-Fert model generalized to include SML.",2111.09731v1 2021-11-26,Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO$_3$,"In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO$_3$, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, $T_c$ = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to $T_{neq} >$ 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti $t_{2g}$ levels. Notably, light-induced high temperature ferromagnetism in YTiO$_3$ is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.",2111.13622v1 2021-12-01,Inter-generational comparison of quantum annealers in solving hard scheduling problems,"We compare the performance of four quantum annealers, the D-Wave Two, 2X, 2000Q, and Advantage in solving an identical ensemble of a parametrized family of scheduling problems. These problems are NP-complete and, in fact, equivalent to vertex coloring problems. They are also practically motivated and closely connected to planning problems from artificial intelligence. We examine factors contributing to the performance differences while separating the contributions from hardware upgrades, support for shorter anneal times, and possible optimization of ferromagnetic couplings. While shorter anneal times can improve the time to solution (TTS) at any given problem size, the scaling of TTS with respect to the problem size worsens for shorter anneal times. In contrast, optimizing the ferromagnetic coupling improves both the absolute TTS and the scaling. There is a statistically significant improvement in performance between D-Wave Two and 2X and from all older generation annealers to Advantage, even when operated under identical anneal time and ferromagnetic couplings. However, the performance improvement from 2X to 2000Q requires the anneal time and ferromagnetic couplings to be optimized. Overall, owing to these inter-generational hardware improvements and optimizations, the scaling exponent reduces from $1.01 \pm 0.01$ on Two to $0.173 \pm 0.009$ on Advantage.",2112.00727v1 2021-12-06,First principles theory for the magnetic and charge instabilities in AV$_3$Sb$_5$ systems,"Vanadium-based materials AV$_3$Sb$_5$ (A=K, Rb, Cs) with layered kagome lattice structures have drawn great attention recently due to the discoveries of topologically nontrivial band structures, charge density wave states, giant anomalous Hall effect, as well as unusual superconducting phase at low temperatures. In this work, we theoretically study the magnetic and charge instabilities for this class of materials based on first principles calculations. We develop a method to calculate the generalized susceptibility tensor defined in the sublattice-orbital-spin space, with the effects of Coulomb interactions treated by generalized random phase approximation (RPA). The RPA susceptibility calculations indicate that there are three leading ferromagnetic instability modes at $\Gamma$ point, which are further verified by unrestricted self-consistent Hartree-Fock calculations including both the on-site and inter-site Coulomb interactions. The inclusion of inter-site interactions tend to suppress the spin ferromagnetism due to charge transfer from the V to Sb sites, leading to weak spin magnetic moments $\sim 0.1\mu_{\textrm{B}}$ per V atom with small intrinsic anomalous Hall conductivity. Current loops can be generated in such weak spin ferromagnetic states as a result of spin-orbit coupling effects. The electronic structures in the ferromagnetic states are significantly reconstructed which have nearly compensated electron and hole carriers from two bands. On the other hand, we do not find any diverging instability mode at $M$ point driven by electron-electron Coulomb interactions. First principles phonon calculations indicate that there are unstable phonon modes which tend to drive the system into an inverse star-of-David structure. Our results indicate that there may be separate phase transitions in the magnetic and charge channels in the system.",2112.02808v1 2022-01-11,Resonant Precession of Magnetization and Precession -- Induced DC voltages in FeGaB Thin Films,"Measurements of frequency dependent ferromagnetic resonance (FMR) and spin pumping driven dc voltage (V_{dc}) are reported for amorphous films of Fe_{78}Ga_{13}B_{9} (FeGaB) alloy to address the phenomenon of self-induced inverse spin Hall effect (ISHE) in plain films of metallic ferromagnets. The V_{dc} signal, which is antisymmetric on field reversal, comprises of symmetric and asymmetric Lorentzians centered around the resonance field. Dominant role of thin film size effects is seen in setting the magnitude of static magnetization, V_{dc} and dynamics of magnetization precession in thinner films (\leq 8 nm). The film thickness dependence of magnetization parameters indicates the presence of a magnetically disordered region at the film-substrate interface, which may promote preferential flow of spins generated by the precessing magnetization towards the substrate. However, the V_{dc} signal also draws contributions from rectification effects of a \approx 0.4 \% anisotropic magnetoresistance and a large (\approx 54 n\Omega.m) anomalous Hall resistivity (AHR) of these films which ride over the effect of spin-orbit coupling driven spin-to-charge conversion near the film-substrate interface. We have addressed these data in the framework of the existing theories of electrodynamics of a ferromagnetic film subjected to radio-frequency field in a coplanar waveguide geometry. Our estimation of the self-induced ISHE for the sample with 54 n\Omega.m AHR shows that it may contribute significantly (\approx 90\%) to the measured symmetric voltage. This study is expected to be very useful for fully understanding the spin pumping induced dc voltages in metallic ferromagnets with disordered interfaces and large anomalous Hall effect.",2201.03739v1 2022-01-23,Van der Waals pi Josephson junctions,"Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs with a finite center-of-mass momentum. The resultant spatially modulated superconducting order parameter is able to stabilize Josephson junctions (JJs) with pi phase difference in superconductor-ferromagnet heterostructures and realize 'quiet' phase qubits. The emergence of two-dimensional (2D) layered superconducting and magnetic materials promises a new platform for realizing pi JJs with atomically sharp interfaces by van der Waals stacking. Here we demonstrate a thickness-driven 0-pi transition in JJs made of NbSe2 (an Ising superconductor) with a Cr2Ge2Te6 (a ferromagnetic semiconductor) weak link. By systematically varying the Cr2Ge2Te6 thickness, we observe a vanishing supercurrent at a critical thickness around 8 nm, followed by a re-entrant supercurrent upon further increase in thickness. Near the critical thickness, we further observe unusual supercurrent interference patterns with vanishing critical current around zero in-plane magnetic field. They signify the formation of 0-pi JJs (with both 0 and pi regions) likely induced by the nanoscale magnetic domains in Cr2Ge2Te6. Our work highlights the potential of van der Waals superconductor-ferromagnet heterostructures for the explorations of unconventional superconductivity and superconducting electronics.",2201.09185v3 2022-01-24,Direct observation of strong anomalous Hall effect and proximity-induced ferromagnetic state in SrIrO3,"The 5d iridium-based transition metal oxides have gained broad interest because of their strong spin-orbit coupling which favors new or exotic quantum electronic states. On the other hand, they rarely exhibit more mainstream orders like ferromagnetism due to generally weak electron-electron correlation strength. Here, we show a proximity-induced ferromagnetic (FM) state with TC = 100 K and strong magnetocrystalline anisotropy in a SrIrO3 (SIO) heterostructure via interfacial charge transfer by using a ferromagnetic insulator in contact with SIO. Electrical transport allows to selectively probe the FM state of the SIO layer and the direct observation of a strong, intrinsic and positive anomalous Hall effect (AHE). For T < 20 K, the AHE displays unusually large coercive and saturation field, a fingerprint of a strong pseudospin-lattice coupling. A Hall angle, sxyAHE/sxx, larger by an order of magnitude than in typical 3d metals and a FM net moment of about 0.1 mB/Ir, is reported. This emphasizes how efficiently the nontrivial topological band properties of SIO can be manipulated by structural modifications and the exchange interaction with 3d TMOs.",2201.09570v2 2022-02-28,Pervasive beyond room-temperature ferromagnetism in a doped van der Waals magnet: Ni doped Fe$_5$GeTe$_2$ with $T_{\text{C}}$ up to 478 K,"The existence of long range magnetic order in low dimensional magnetic systems, such as the quasi-two-dimensional (2D) van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe$_N$GeTe$_2$ ($N$ = 3, 4, 5; FGT) family is exceptional owing to its vast tunability of magnetic properties. Particularly, a ferromagnetic ordering temperature ($T_{\text{C}}$) above room temperature at $N$ = 5 (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron (Fe) in F5GT, a record high $T_{\text{C}}$ = 478(6) K is achieved. Importantly, pervasive, beyond-room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, in which the structural alteration enhanced 3D magnetic couplings might be critical for enhancing the ferromagnetic order.",2203.00125v2 2022-03-10,Prediction of single-atom-thick transition metal nitride CrN$_4$ with a square-planar network and high-temperature ferromagnetism,"Single-atom-thick two-dimensional materials such as graphene usually have a hexagonal lattice while the square-planar lattice is uncommon in the family of two-dimensional materials. Here, we demonstrate that single-atom-thick transition metal nitride CrN$_4$ monolayer is a stable free-standing layer with a square-planar network. The stability of square-planar geometry is ascribed to the combination of N=N double bond, Cr-N coordination bond, and $\pi$-d conjugation, in which the double $\pi$-d conjugation is rarely reported in previous studies. This mechanism is entirely different from that of the reported two-dimensional materials, leading to lower formation energy and more robust stability compared to the synthesized g-C$_3$N$_4$ monolayer. On the other hand, CrN$_4$ layer has a ferromagnetic ground state, in which the ferromagnetic coupling between two Cr atoms is mediated by electrons of the half-filled large $\pi$ orbitals from $\pi$-d conjugation. The high-temperature ferromagnetism in CrN$_4$ monolayer is confirmed by solving the Heisenberg model with Monte Carlo method.",2203.05361v2 2022-03-16,Effect of hydrogen gas on magnetic properties of alloys of ferromagnetic metals with Pd and its application in hydrogen gas sensing,"The mass-production of fuel-cell vehicles and the eventual transition to the hydrogen economy will require safe, inexpensive and reliable sensors capable of simultaneously detecting low concentrations of leaking hydrogen and measuring broad ranges of hydrogen concentration in storage and energy generating systems. Although several competing sensor technologies can potentially be used in this role, just a few of them have thus far demonstrated a combination of all desirable characteristics. This group of devices also includes magneto-electronic sensors that can detect the presence of hydrogen gas in a range of hydrogen concentrations from zero to 100% at atmospheric pressure with the response time approaching the industry standard of one second. The hydrogen gas sensing mechanism underpinning the operation of magneto-electronic sensors is based on the physical processes of ferromagnetic resonance, magneto-optical Kerr effect and anomalous Hall effect that enable one to measure hydrogen-induced changes in the magnetic properties of structures combining Pd with one or several ferromagnetic metals such as Co, Fe or Ni. In this chapter, we overview the physical foundations of emergent ferromagnetic Pd-alloy-based magneto-electronic hydrogen sensors and compare their characteristics with those of high-performing multilayer thin film-based counterparts that have already demonstrated a potential to find commercial applications.",2203.08575v1 2022-03-24,Critical behavior in the Mn$_{5}$Ge$_{3}$ ferromagnet,"High-Curie-temperature ferromagnets are promising candidates for designing new spintronic devices. Here we have successfully synthesized a single-crystal sample of the itinerant ferromagnet Mn$ _{5}$Ge$_{3}$ used flux method and its critical properties were investigated by means of bulk dc-magnetization at the boundary between the ferromagnetic (FM) and paramagnetic (PM) phase. Critical exponents $ \beta=0.336 \pm 0.001 $ with a critical temperature $ T_{c}=300.29 \pm 0.01 $ K and $ \gamma=1.193 \pm 0.003 $ with $ T_{c} = 300.15 \pm 0.05 $ K are obtained by the modified Arrott plot, whereas $ \delta = 4.61 \pm 0.03 $ is deduced by a critical isotherm analysis at $ T_{c} = 300 $ K. The self-consistency and reliability of these critical exponents are verified by the Widom scaling law and the scaling equations. Further analysis reveals that the spin coupling in Mn$ _{5}$Ge$_{3}$ exhibits three-dimensional Ising-like behavior. The magnetic exchange is found to decay as $ J(r)\approx r^{-4.855} $ and the spin interactions are extended beyond the nearest neighbors, which may be related to different set of Mn--Mn interactions with unequal magnitude of exchange strengths. Additionally, the existence of noncollinear spin configurations in Mn$ _{5} $Ge$ _{3} $ results in a small deviation of obtained critical exponents from those for standard 3D-Ising model.",2203.12778v1 2022-03-28,Theory-guided investigation on magnetic evolution of MnPt$_{5-x}$Pd$_x$P and discovery of anti-CeCoIn$_5$-type ferromagnetic MnPd$_5$P,"We report the magnetic changes from canted antiferromagnetic to ferromagnetic orderings in anti-115-type MnPt$_{5-x}$Pd$_x$P ($x$ = 1, 2, 2.5, 3, 4, and 5) and the discovery of a new rare-earth-free ferromagnet, MnPd$_5$P by both theoretical prediction and experimental investigation. The family compounds were synthesized using high temperature solid state method and characterized to crystalize in the anti-CeCoIn$_5$ type with the space group P4/mmm exhibiting a two-dimensional layered structural feature. The magnetic property measurements indicate that the compounds ordered from canted A-type antiferromagnet in MnPt$_5$P to ferromagnet above the room temperature with varying degrees of coercivity and magnetic moments in MnPd$_5$P by reducing the spin orbital coupling. The results of the MnPt$_{5-x}$Pd$_x$P have been analyzed in comparison to the other candidates of the 151 family of Mn(Pt/Pd)$_5$(P/As) to understand the complex structure-magnetism relationships.",2203.15016v1 2022-04-28,Multi-wavelength magnetic coding of helical luminescence in ferromagnetic 2D layered CrI3,"Two-dimensional (2D) van der Waals (vdW) ferromagnets have opened new avenues for manipulating spin at the limits of single or few atomic layers, and for creating unique magneto-exciton devices through the coupling of long-range ferromagnetic (FM) orders and excitons. However, 2D vdW ferromagnets explored so far have rarely possessed exciton behaviors; to date, FM CrI3 have been recently revealed to show ligand-field photoluminescence correlated with FM ordering, but typically with a broad emission peak. Alternatively, many-body excitons have been observed in antiferromagnetic (AFM) NiPS3, but the coupling of excitons with AFM orders is exponentially more difficult, owing to extremely high coercivity. Here, we report a straightforward approach to realize strong coupling of narrow helical emission and FM orders at a low magnetic field in CrI3 through a relatively simple microsphere cavity. We show that the resonant whispering-gallery-modes (WGM) of SiO2 microspheres give rising to a series of strong oscillation helical emissions with a full width at half-maximum (FWHM) of ~5 nm under continuous wave excitation. Reversible magnetic control and coding of helical luminescence with multiwavelength is realized in the range of 950-1100 nm. This work enables plenty of opportunities for creating magnetic encoding lasing for photonic integrated chips.",2204.13318v1 2022-06-08,Macroscopic Quantum Tunneling of a Topological Ferromagnet,"The recent advent of topological states of matter spawned many significant discoveries. The quantum anomalous Hall effect[1-3] is a prime example due to its potential for applications in quantum metrology[4, 5] as well as its influence on fundamental research into the underlying topological and magnetic states[6-11] and axion electrodynamics[2, 12-14]. Here, we perform electronic transport studies on a (V,Bi,Sb)2Te3 ferromagnetic topological insulator nanostructure in the quantum anomalous Hall regime. This allows us access to the dynamics of an individual ferromagnetic domain. The volume of the domain is estimated to be about 85 000 nm3, containing some 50 000 vanadium atoms, spread over a macroscopic distance of 115 nm. Telegraph noise resulting from the magnetization fluctuations of this domain is observed in the Hall signal. Careful analysis of the influence of temperature and external magnetic field on the domain switching statistics provides evidence for quantum tunneling of magnetization[15-22] in a macrospin state. This ferromagnetic macrospin is not only the largest magnetic object in which quantum tunneling has been observed, but also the first observation of the effect in a topological state of matter.",2206.03972v1 2022-06-27,Gate-tunable anomalous Hall effect in stacked van der Waals ferromagnetic insulator - topological insulator heterostructures,"The search of novel topological phases, such as the quantum anomalous Hall insulator (QAHI) or the axion insulator, has motivated different schemes to introduce magnetism into topological insulators. One scheme is to introduce ferromagnetic dopants in topological insulators. However, it is generally challenging and requires carefully engineered growth/heterostructures or relatively low temperatures to observe the QAHI due to issues such as the added disorder with ferromagnetic dopants. Another promising scheme is using the magnetic proximity effect with a magnetic insulator to magnetize the topological insulator. Most of these heterostructures are synthesized so far by growth techniques such as molecular beam epitaxy and metallic organic chemical vapor deposition. These are not readily applicable to allow mixing and matching many of the available ferromagnetic and topological insulators due to difference in growth conditions and lattice mismatch. Here, we demonstrate that the magnetic proximity effect can still be obtained in stacked heterostructures assembled via the dry transfer of exfoliated micrometer-sized thin flakes of van der Waals topological insulator and magnetic insulator materials (BiSbTeSe2/Cr2Ge2Te6), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures can allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by magnetic proximity effect, a key step towards further realization of novel topological phases such as QAHI and axion insulators.",2206.13045v1 2022-07-01,Light-induced giant and persistent changes in the converse magnetoelastic effects in Ni/BaTiO3 multiferroic heterostructure,"Magnetoelastic and magnetoelectric coupling in the artificial multiferroic heterostructures facilitate valuable features for device applications such as magnetic field sensors and electric write magnetic-read memory devices. In a ferromagnetic/ferroelectric heterostructures, the strain mediated coupling exploits piezoelectricity/electrostriction in ferroelectric phase and magnetostriction/piezomagnetism in ferromagnetic phase. Such verity of these combined effect can be manipulated by an external perturbation, such as electric field, temperature or magnetic field. Here, we demonstrate the remote-controlled tunability of these effects under the visible, coherent and polarized light. The combined surface and bulk magnetic study of domain-correlated Ni/BaTiO3 heterostructure reveals that the system is strong sensitive about the light illumination via the combined effect of converse piezoelectric, magnetoelastic coupling and converse magnetostriction. Well-defined ferroelastic domain structure is fully transferred from a tetragonal ferroelectric to magnetostrictive layer via interface strain transfer during the film growth. The visible light illumination is used to manipulate the original ferromagnetic microstructure by the light-induced domain wall motion in ferroelectric, consequently the domain wall motion in the ferromagnetic layer. Our findings mimic the attractive remote-controlled ferroelectric random-access memory write and magnetic random-access memory read application scenarios, hence, can be proven as a novel perspective for room temperature device applications.",2207.00439v3 2022-07-29,Two-dimensional ferromagnetic extension of a topological insulator,"Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a 3D topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface and not in the bulk, e.g. through proximity of a ferromagnetic (FM) layer. However, such a proximity-induced Dirac mass gap has not been observed, likely due to insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely FM extension, using a thin film of the 3D TI Bi$_2$Te$_3$, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi$_2$Te$_4$. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of $\text{T}_\text{c}\approx$~15 K is demonstrated by X-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above T$_c$. This sizable gap indicates a relocation of the TSS to the FM ordered Mn moments near the surface, which leads to a large mutual overlap.",2207.14421v1 2022-08-07,Strain tunability of perpendicular magnetic anisotropy in van der Waals ferromagnets VI3,"Layered ferromagnets with high coercivity have special applications in nanoscale memory elements in electronic circuits, such as data storage. Therefore, searching for new hard ferromagnets and effectively tuning or enhancing the coercivity are the hottest topics in layered magnets today. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements. For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis loop with perpendicular magnetic anisotropy and a large coercivity (up to 1.775 T at 10 K). Furthermore, the coercivity can be enhanced to a maximum of 2.6 T at 10 K under a 2.9% in-plane tensile strain. Our DFT calculations show that the magnetic anisotropy energy (MAE) can be dramatically increased after applying an in-plain tensile strain, which contributes to the enhancement of coercivity in the VI3 flake. Meanwhile, the strain tunability on the coercivity of CrI3, with a similar crystal structure, is limited. The main reason is the strong spin-orbital coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes highlights its potential for integration into vdW heterostructures, paving the way toward nanoscale spintronic devices and applications in the future.",2208.03678v1 2022-08-11,Flat-band based ferromagnetic semiconducting state in the graphitic C$_4$N$_3$ monolayer,"A new set of lattice-models based on the hexagonal $\sqrt{N}\times\sqrt{N}$ super-cells of the well-known honeycomb lattice with single-hole defect (HL-D-1/2N) are proposed to realize the nontrivial isolated flat-bands. Through performing both tight-binding and density functional theory calculations, we demonstrate that the experimentally realized graphitic carbon nitride (Adv. Mater., 22, 1004, 2010; Nat. Commun., 9, 3366, 2018), the HL-D-1/8 based C$_4$N$_3$, is a perfect system to host such flat bands. For the flat high-energy P-6m2 C$_4$N$_3$ structure, it displays the ferromagnetic half-metallicity which is not related to the isolated flat bands. However, the P-6m2 C$_4$N$_3$ structure is dynamically unstable. Using a structure searching method based on group and graph theory, we find that a new corrugated Pca21 C4N3 structure has the lowest energy among all known C$_4$N$_3$ structures. This Pca21 C$_4$N$_3$ structure is an intrinsic ferromagnetic half-semiconductor (Tc$\approx$241 K) with one semiconducting spin-channel (1.75 eV) and one insulating spin-channel (3.64 eV), which is quite rare in the two-dimensional (2D) systems. Its ferromagnetic semiconducting property originates from the isolated p$_z$-state flat-band as the corrugation shift the flat band upward to the Fermi level. Interestingly, this Pca21 C$_4$N$_3$ structure is found to be piezoelectric and ferroelectric, which makes C$_4$N$_3$ an unusual transition-metal-free 2D multiferroic.",2208.06065v1 2022-08-26,Quasiparticle characteristics of the weakly ferromagnetic Hund's metal MnSi,"Hund's metals are multi-orbital systems with $3d$ or $4d$ electrons exhibiting both itinerant character and local moments, and they feature Kondo-like screenings of local orbital and spin moments, with suppressed coherence temperature driven by Hund's coupling $J_H$. They often exhibit magnetic order at low temperature, but how the interaction between the Kondo-like screening and long-range magnetic order is manifested in the quasiparticle spectrum remains an open question. Here we present spectroscopic signature of such interaction in a Hund's metal candidate MnSi exhibiting weak ferromagnetism. Our photoemission measurements reveal renormalized quasiparticle bands near the Fermi level with strong momentum dependence: the ferromagnetism manifests through possibly exchange-split bands (Q1) below $T_C$ , while the spin/orbital screenings lead to gradual development of quasiparticles (Q2) upon cooling. Our results demonstrate how the characteristic spin/orbital coherence in a Hund's metal could coexist and compete with the magnetic order to form a weak itinerant ferromagnet, via quasiparticle bands that are well separated in momentum space and exhibit distinct temperature dependence. Our results imply that the competition between the spin/orbital screening and the magnetic order in a Hund's metal bears intriguing similarity to the Kondo lattice systems.",2208.12456v2 2022-09-20,Tuning the hysteresis loop for the anomalous Hall effect in Pt ultrathin films on $\rm{CoFe_2O_4}$ by electrolyte gating,"Pt ultrathin films on ferromagnetic insulators have been widely studied for spintronics applications, and magnetic moments of interface Pt atoms were considered to be ferromagnetically ordered due to a magnetic proximity effect (MPE). An anomalous Hall effect (AHE) is usually used to examine an out-of-plane magnetic moments of the Pt layer. To tune ferromagnetic properties of an Pt ultrathin film, we fabricated electric double layer transistors on Pt thin films with thicknesses of 5.9 nm and 7.0 nm on a $\rm{CoFe_2O_4}$ (CFO) ferrimagnetic insulator. For the Pt (7.0 nm)/CFO sample, a hysteresis loop was observed in the anomalous Hall resistivity without the gate bias, and the coercive field was tuned by applying the gate bias. For the Pt (5.9 nm)/CFO sample, a hysteresis loop was not observed without a gate bias, but was opened by applying a gate bias ($V\rm{_G} =$ $\pm$3 V). This indicated that the long-range ferromagnetic ordering of magnetic moments in the Pt film was switched on and off by the electric field effect. The hysteresis loop was observed up to 19.5 K for a $V\rm{_G}$ of +3 V, while the AHE was observed up to approximately room temperature.",2209.09797v2 2022-09-27,Ferromagnetism in an extended coherently-coupled atomic superfluid,"Ferromagnetism is an iconic example of a first-order phase transition taking place in spatially extended systems and is characterized by hysteresis and the formation of domain walls. In this paper we demonstrate that an extended atomic superfluid in the presence of a coherent coupling between two internal states exhibits a quantum phase transition from a para- to a ferromagnetic state. The nature of the transition is experimentally assessed by looking at the phase diagram as a function of the control parameters, at hysteresis phenomena, at the magnetic susceptibility and the magnetization fluctuations around the critical point. We show that the observed features are in good agreement with mean-field calculations. Additionally, we develop experimental protocols to deterministically generate domain walls that separate spatial regions of opposite magnetization in the ferromagnetic state. Thanks to the enhanced coherence properties of our atomic superfluid system compared to standard condensed matter systems, our results open the way towards the study of different aspects of the relaxation dynamics in isolated coherent many-body quantum systems.",2209.13235v3 2022-09-30,Absence of magnetic interactions in Ni-Nb ferromagnet-superconductor bilayers,"Studies of ferromagnet-superconductor hybrid systems have uncovered magnetic interactions between the competing electronic orderings. The electromagnetic (EM) proximity effect predicts the formation of a spontaneous vector potential inside a superconductor placed in proximity to a ferromagnet. In this work, we use a Nb superconducting layer and Ni ferromagnetic layer to test for such magnetic interactions. We use the complementary, but independent, techniques of polarised neutron reflectometry and detection Josephson junctions to probe the magnetic response inside the superconducting layer at close to zero applied field. In this condition, Meissner screening is negligible, so our measurements examine only additional magnetic and screening contributions from proximity effects. We report the absence of any signals originating from EM proximity effect in zero applied field. Our observations indicate that either EM proximity effect is below the detection resolution of both of our experiments or may indicate a new phenomenon that requires extension of current theory. From our measurements, we estimate a limit of the size of the zero field EM proximity effect in our Ni-Nb samples to be $\pm0.27$ mT.",2209.15366v2 2022-10-13,The Random-Bond Ising Model and its dual in Hyperbolic Spaces,"We analyze the thermodynamic properties of the random-bond Ising model (RBIM) on closed hyperbolic surfaces using Monte Carlo and high-temperature series expansion techniques. We also analyze the dual-RBIM, that is the model that in the absence of disorder is related to the RBIM via the Kramers-Wannier duality. Even on self-dual lattices this model is different from the RBIM, unlike in the euclidean case. We explain this anomaly by a careful re-derivation of the Kramers--Wannier duality. For the (dual-)RBIM, we compute the paramagnet-to-ferromagnet phase transition as a function of both temperature $T$ and the fraction of antiferromagnetic bonds $p$. We find that as temperature is decreased in the RBIM, the paramagnet gives way to either a ferromagnet or a spin-glass phase via a second-order transition compatible with mean-field behavior. In contrast, the dual-RBIM undergoes a strongly first order transition from the paramagnet to the ferromagnet both in the absence of disorder and along the Nishimori line. We study both transitions for a variety of hyperbolic tessellations and comment on the role of coordination number and curvature. The extent of the ferromagnetic phase in the dual-RBIM corresponds to the correctable phase of hyperbolic surface codes under independent bit- and phase-flip noise.",2210.07227v2 2022-11-02,2D-Double transition metal MXenes for spintronics applications: surface functionalization induced ferromagnetic half-metallic complexes,"MXenes are rapidly emerging two-dimensional (2D) materials with thickness, composition, and functionalization-dependent outstanding properties having applications in diverse fields. To disclose nano-spintronic applications of 2D-double transition metal (DTM) carbide and nitride-based pristine and surface-functionalized MXenes (M'2M""X2Tx, M' and M"" = Cr, Mo, W; X = C/N; T = -F/-OH/=O), a systematic investigation has been performed on structural stability, magnetic properties and electronic structure using spin-polarized first-principles calculations. 36 stables functionalized MXenes were screened from 144 explored DTM based MXenes. The explored materials exhibit striking properties, having wide range of magnetic ground states, from non-magnetic to ferromagnetic, and then to antiferromagnetic, accompanied by metallic to half-metallic or gapless half-metallic properties, depending on transition metal(s) and terminating group. Mo and W-based MXenes are found to be nonmagnetic and metallic, whereas Cr-Mo and Cr-W-based MXenes are magnetic with varying metallic behavior. W2CrN2O2 and Mo2CrN2O2 systems are found to be ferromagnetic half-metallic 2D materials with a direct band gap of 1.35 eV and 0.77 eV respectively, in the minority spin channel. The comprehensive study on DTM MXenes, provide intrinsic half-metallic properties along with robust ferromagnetism, opens up a class of promising new 2D materials with tunable magnetic and electronic properties for potential device applications in nano-spintronics and electronics.",2211.00846v1 2022-11-02,Hydroxide-based magneto-ionics: electric-field control of reversible paramagnetic-to-ferromagnetic switch in $α$-Co(OH)$_{2}$ films,"Magneto-ionics has emerged as a promising approach to manipulate magnetic properties, not only by drastically reducing power consumption associated with electric current based devices but also by enabling novel functionalities. To date, magneto-ionics have been mostly explored in oxygen-based systems, while there is a surge of interests in alternative ionic systems. Here we demonstrate highly effective hydroxide-based magneto-ionics in electrodeposited ${\alpha}$-Co(OH)$_{2}$ films. The ${\alpha}$-Co(OH)$_{2}$, which is a room temperature paramagnet, is switched to ferromagnetic after electrolyte gating with a negative voltage. The system is fully, magnetically reversible upon positive voltage application. The origin of the reversible paramagnetic-to-ferromagnetic transition is attributed to the ionic diffusion of hydroxyl groups, promoting the formation of metallic cobalt ferromagnetic regions. Our findings demonstrate one of the lowest turn-on voltages reported for propylene carbonate gated experiments. By tuning the voltage magnitude and sample area we demonstrate that the speed of the induced ionic effect can be drastically enhanced.",2211.01191v1 2022-11-16,Continuous Electrical Manipulation of Magnetic Anisotropy and Spin Flopping in van der Waals Ferromagnetic Devices,"Controlling the magnetic anisotropy of ferromagnetic materials plays a key role in magnetic switching devices and spintronic applications. Examples of spin-orbit torque devices with different magnetic anisotropy geometries (in-plane or out-of-plane directions) have been demonstrated with novel magnetization switching mechanisms for extended device functionalities. Normally, the intrinsic magnetic anisotropy in ferromagnetic materials is unchanged within a fixed direction, and thus, it is difficult to realize multifunctionality devices. Therefore, continuous modulation of magnetic anisotropy in ferromagnetic materials is highly desired but remains challenging. Here, we demonstrate a gate-tunable magnetic anisotropy transition from out-of-plane to canted and finally to in-plane in layered Fe$_5$GeTe$_2$ by combining the measurements of the angle-dependent anomalous Hall effect and magneto-optical Kerr effect with quantitative Stoner-Wohlfarth analysis. The magnetic easy axis continuously rotates in a spin-flop pathway by gating or temperature modulation. Such observations offer a new avenue for exploring magnetization switching mechanisms and realizing new spintronic functionalities.",2211.08909v1 2022-12-16,Transverse currents in spin transistors,"In many systems, planar Hall effect wherein transverse signal appears in response to longitudinal stimulus is rooted in spin-orbit coupling. A spin transistor put forward by Datta and Das on the other hand consists of ferromagnetic leads connected to spin-orbit coupled central region and its conductance can be controlled by tuning the strength of spin-orbit coupling. We find that transverse currents also appear in Datta-Das transistors made by connecting two two-dimensional ferromagnetic reservoirs to a central spin-orbit coupled two-dimensional electron gas. We find that the spin transistor exhibits a nonzero transverse conductivity which depends on the direction of polarization in ferromagnets and the location where it is measured. We study the conductivities for the system with finite and infinite widths. The conductivities exhibit Fabry-P\'erot type oscillations as the length of the spin-orbit coupled regions is varied. Interestingly, even in the limit when longitudinal conductivity is made zero by cutting off the junction between the central spin-orbit coupled region and the ferromagnetic lead on one side (right), the transverse conductivities remain nonzero in the regions that are on the left side of the cut-off junction.",2212.08301v4 2023-01-05,Two-dimensional Heisenberg models with materials-dependent superexchange interactions,"The two-dimensional (2D) van der Waals ferromagnetic semiconductors, such as CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, and the 2D ferromagnetic metals, such as Fe$_3$GeTe$_2$ and MnSe$_2$, have been obtained in recent experiments and attracted a lot of attentions. The superexchange interaction has been suggested to dominate the magnetic interactions in these 2D magnetic systems. In the usual theoretical studies, the expression of the 2D Heisenberg models were fixed by hand due to experiences. Here, we propose a method to determine the expression of the 2D Heisenberg models by counting the possible superexchange paths with the density functional theory (DFT) and Wannier function calculations. With this method, we obtain a 2D Heisenberg model with six different nearest-neighbor exchange coupling constants for the 2D ferromagnetic metal Cr$_3$Te$_6$, which is very different for the crystal structure of Cr atoms in Cr$_3$Te$_6$. The calculated Curie temperature Tc = 328 K is close to the Tc = 344 K of 2D Cr$_3$Te$_6$ reported in recent experiment. In addition, we predict two stable 2D ferromagnetic semiconductors Cr$_3$O$_6$ and Mn$_3$O$_6$ sharing the same crystal structure of Cr$_3$Te$_6$. The similar Heisenberg models are obtained for 2D Cr$_3$O$_6$ and Mn$_3$O$_6$, where the calculated Tc is 218 K and 208 K, respectively. Our method offers a general approach to determine the expression of Heisenberg models for these 2D magnetic semiconductors and metals, and builds up a solid basis for further studies.",2301.01923v1 2023-01-08,AdS/BCFT correspondence and Horndeski gravity in the presence of gauge fields: holographic paramagnetism/ferromagnetism phase transition,"This paper presents a dual gravity model for a (2+1)-dimensional system with a limit on finite charge density and temperature, which will be used to study the properties of the holographic phase transition to paramagnetism-ferromagnetism in the presence of Horndeski gravity terms. In our model, the non-zero charge density is supported by a magnetic field. As a result, the radius $\rho/B$ indicates a localized condensate, as we increase the Horndeski gravity parameter, that is represented by $\gamma$. Furthermore, such condensate shows quantum Hall-type behavior. This radius is also inversely related to the total action coefficients of our model. It was observed that increasing the Horndeski parameter decreases the critical temperature of the holographic model and leads to the harder formation of the magnetic moment at the bottom of the black hole. However, when removing the magnetic field, the ferromagnetic material presents a disorder of its magnetic moments, which is observed through the entropy of the system. We also found that at low temperatures, spontaneous magnetization and ferromagnetic phase transition.",2301.03121v3 2023-02-03,Signature of the coexistence of ferromagnetism and superconductivity at KTaO$_3$ heterointerfaces,"The coexistence of superconductivity and ferromagnetism is a long-standing issue in the realm of unconventional superconductivity due to the antagonistic nature of these two ordered states. Experimentally identifying and characterizing novel heterointerface superconductors that coexist with magnetism is challenging. Here, we report the experimental observation of long-range ferromagnetic order at the verge of two-dimensional superconductivity at KTaO$_3$ heterointerfaces. Remarkably, we observe in-plane magnetization hysteresis loop persisting to above room temperature with direct current superconducting quantum interference device measurements. Furthermore, both the first-principles calculations and the x-ray magnetic circular dichroism measurements reveal that the observed robust ferromagnetism is attributed to the presence of oxygen vacancies that localize electrons in nearby Ta 5$d$ states. Our findings not only indicate KTaO$_3$ heterointerfaces as unconventional superconductors with time-reversal symmetry breaking, but also inject a new momentum to the study of the delicate interplay between superconductivity and magnetism boosted by strong spin-orbit coupling inherent to the heavy Ta in 5$d$ orbitals of KTaO$_3$ heterointerfaces.",2302.01549v4 2023-02-25,Goldstone modes and the golden spiral in the ferromagnetic spin-1 biquadratic model,"Ferromagnetic ground states have often been overlooked in comparison to seemingly more interesting antiferromagnetic ground states. However, both the physical and mathematical structure of ferromagnetic ground states are particularly rich. We show that the highly degenerate and highly entangled ground states of the ferromagnetic spin-1 biquadratic model are scale invariant, originating from spontaneous symmetry breaking from ${\rm SU}(3)$ to ${\rm U}(1)\times {\rm U}(1)$ with two type-B Goldstone modes. The ground state degeneracies are characterized as the Fibonacci-Lucas sequences -- an ancient mathematical gem, under open and periodic boundary conditions, with the residual entropy being non-zero. This implies that the ground state degeneracies for this model are asymptotically the golden spiral. In addition, sequences of degenerate ground states generated from highest and generalized highest weight states are constructed to establish that the entanglement entropy scales logarithmically with the block size in the thermodynamic limit, with the prefactor being half the number of type-B Goldstone modes. The latter in turn is identified to be the fractal dimension.",2302.13126v3 2023-03-06,Magnetic domain depinning as possible evidence for two ferromagnetic phases in LaCrGe$_3$,"Two ferromagnetic phases, FM1 and FM2, were first proposed to exist in LaCrGe$_3$ based on a broad maximum in the temperature derivative of resistivity resembling that of the superconducting ferromagnet UGe$_2$ where FM1 and FM2 are well-established. While evidence for two FM phases can be found in certain additional probes, corresponding anomalies in magnetization have not been recognized until now. Our spatially-resolved images of the magnetic domains show a substantial change in the domain structure between the higher temperature FM1 phase and the lower temperature FM2 phase. Furthermore, our measurements of the coercive field and virgin magnetization curves reveal an unconventional magnetic domain pinning region in the FM1 phase, followed by a depinning region at lower temperatures where the system is reported to crossover into the FM2 phase. We incorporate this discovery into a simple domain magnetization model that demystifies the magnetization curve seen in all previous studies. Finally, we find that the unusual domain behavior can be explained by a change in the ferromagnetic exchange interaction and magnetic moment, both of which are consistent with the existence of two FM phases. This revelation may help explain a range of anomalous behaviors observed in LaCrGe$_3$ and rekindles the discussion about the prevalence of multiple FM phases in fragile FM systems.",2303.03447v2 2023-03-16,Antiferromagnet-mediated interlayer exchange: hybridization versus proximity effect,"We investigate the interlayer coupling between two thin ferromagnetic (F) films mediated by an antiferromagnetic (AF) spacer in F*/AF/F trilayers and show how it transitions between different regimes on changing the AF thickness. Employing layer-selective Kerr magnetometry and ferromagnetic-resonance techniques in a complementary manner enables us to distinguish between three functionally distinct regimes of such ferromagnetic interlayer coupling. The F layers are found to be individually and independently exchange-biased for thick FeMn spacers - the first regime of no interlayer F-F* coupling. F-F* coupling appears on decreasing the FeMn thickness below 9 nm. In this second regime found in structures with 6.0-9.0 nm thick FeMn spacers, the interlayer coupling exists only in a finite temperature interval just below the effective N\'eel temperature of the spacer, which is due to magnon-mediated exchange through the thermally softened antiferromagnetic spacer, vanishing at lower temperatures. The third regime, with FeMn thinner than 4 nm, is characterized by a much stronger interlayer coupling in the entire temperature interval, which is attributed to a magnetic-proximity induced ferromagnetic exchange. These experimental results, spanning the key geometrical parameters and thermal regimes of the F*/AF/F nanostructure, complemented by a comprehensive theoretical analysis, should broaden the understanding of the interlayer exchange in magnetic multilayers and potentially be useful for applications in spin-thermionics.",2303.09301v2 2023-03-17,Chemically induced ferromagnetism near room temperature in single crystal (Zn$_{1-x}$Cr$_{x}$)Te half-metal,"Magnetic semiconductors are at the core of recent spintronics research endeavors. Chemically doped II-VI diluted magnetic semiconductors, such as (Zn$_{1-x}$Cr$_{x}$)Te, provide promising platform in this quest. However, a detailed knowledge of the microscopic nature of magnetic ground state is necessary for any practical application. Here, we report on the synergistic study of (Zn$_{1-x}$Cr$_{x}$)Te single crystals using elastic neutron scattering measurements and density functional calculations. For the first time, our research unveils the intrinsic properties of ferromagnetic state in macroscopic specimen of (Zn$_{0.8}$Cr$_{0.2}$)Te. The ferromagnetism is onset at $T_c \sim$ 290 K and remains somewhat independent to modest change in the substitution coefficient x. We show that magnetic moments on Zn/Cr site develop ferromagnetic correlation in a-c plane with large ordered moment of $\mu$ = 3.08 $\mu_B$. Magnetic moment across the lattice is induced via the mediation of Te site, uncoupled to the number of dopant carriers as inferred from the density functional calculation. Additionally, the ab-initio calculations also reveal half-metallicity in x = 0.2 composition. These properties are highly desirable for future spintronic applications.",2303.09740v1 2023-03-17,Intrinsic and extrinsic anomalous transport properties of Heusler ferromagnets Fe$_2$CoAl and Fe$_2$NiAl from first principles,"Recently, Heusler ferromagnets have been found to exhibit unconventional anomalous electric, thermal, and thermoelectric transport properties. In this study, we employed first-principles density functional theory calculations to systematically investigate both intrinsic and extrinsic contributions to the anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and anomalous thermal Hall effect (ATHE) in two Heusler ferromagnets: Fe$_2$CoAl and Fe$_2$NiAl. Our analysis reveals that the extrinsic mechanism originating from disorder dominates the AHE and ATHE in Fe$_2$CoAl , primarily due to the steep band dispersions across the Fermi energy and corresponding high longitudinal electronic conductivity. Conversely, the intrinsic Berry phase mechanism, physically linked to nearly flat bands around the Fermi energy and gapped by spin-orbit interaction band crossings, governs the AHE and ATHE in Fe$_2$NiAl. With respect to ANE, both intrinsic and extrinsic mechanisms are competing in Fe$_2$CoAl as well as in Fe$_2$NiAl. Furthermore, Fe$_2$CoAl and Fe$_2$NiAl exhibit tunable and remarkably pronounced anomalous transport properties. For instance, the anomalous Nernst and anomalous thermal Hall conductivities in Fe$_2$NiAl attain giant values of 8.29 A/Km and 1.19 W/Km, respectively, at room temperature. To provide a useful comparison, we also thoroughly investigated the anomalous transport properties of Co$_2$MnGa. Our findings suggest that Heusler ferromagnets Fe$_2$CoAl and Fe$_2$NiAl are promising candidates for spintronics and spin-caloritronics applications.",2303.09812v2 2023-04-01,High-quality NiFe thin films on oxide/non-oxide platforms via pulsed laser deposition at room temperature,"Soft ferromagnetic NiFe thin films are promising for applications in spintronic devices because of their constituent electrical and magnetic properties. Electron beam evaporation and sputtering techniques have been used to deposit NiFe thin films. For in-situ stacking of NiFe with functional complex oxides, the pulsed laser deposition (PLD) method is highly desirable. However, the growth of high-quality NiFe (and non-oxide thin films in general) by PLD remains a formidable task. Here, we report high-quality NiFe thin films of various thicknesses on oxide/non-oxide substrates with desirable magnetic properties by PLD at room temperature. The magnetic properties are found to be strongly dependent on the laser fluence of the deposition process. The laser fluence of 4 Joule/cm$^2$ produces the highest magnetization of ~547 emu/cc. The small coercivity (few Oersted) and sharp ferromagnetic switching behaviour indicate uniaxial anisotropy with an easy axis along the in-plane direction. In addition, thickness-dependent magnetodynamics characterizations are studied via ferromagnetic resonance. Our findings indicate the ferromagnetic characteristics are sensitive to the quality of the oxide/non-oxide substrate surface. These results offer significant insight into the PLD-based development of thin metal magnetic films.",2304.00357v1 2023-04-06,All-electrical operation of a Curie-switch at room temperature,"We present all-electrical operation of a Fe$_x$Cr$_{1-x}$-based Curie switch at room temperature. More specifically, we study the current-induced thermally-driven transition from ferromagnetic to antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect coupling in a Fe/Cr/Fe$_{17.5}$Cr$_{82.5}$/Cr/Fe multilayer. Magnetometry measurements at different temperatures show that the transition from the ferromagnetic to the antiferromagnetic coupling at zero field is observed at $\sim$325K. Analytical modelling confirms that the observed temperature-dependent transition from indirect ferromagnetic to indirect antiferromangetic interlayer exchange coupling originates from the modification of the effective interlayer exchange constant through the ferromagnetic-to-paramagnetic transition in the Fe$_{17.5}$Cr$_{82.5}$ spacer with minor contributions from the thermally-driven variations of the magnetization and magnetic anisotropy of the Fe layers. Room-temperature current-in-plane magnetotransport measurements on the patterned Fe/Cr/Fe$_{17.5}$Cr$_{82.5}$/Cr/Fe strips show the transition from the 'low-resistance' parallel to the 'high-resistance' antiparallel remanent magnetization configuration, upon increased probing current density. Quantitative comparison of the switching fields, obtained by magnetometry and magnetotransport, confirms that the Joule heating is the main mechanism responsible for the observed current-induced resistive switching.",2304.03040v2 2023-04-11,Syntropic spin alignment at the interface between ferromagnetic and superconducting nitrides,"The magnetic correlations at the superconductor/ferromagnet (S/F) interfaces play a crucial role in realizing dissipation-less spin-based logic and memory technologies, such as triplet-supercurrent spin-valves and ""{\pi}"" Josephson junctions. Here we report the coexistence of an induced large magnetic moment and a crypto ferromagnetic state at high-quality nitride S/F interfaces. Using polarized neutron reflectometry and d. c. SQUID measurements, we quantitatively determined the magnetization profile of S/F bilayer and confirmed the induced magnetic moment in the adjacent superconductor only exists below TC. Interestingly, the direction of the induced moment in the superconductors was unexpectedly parallel to that in the ferromagnet, which contrasts with earlier findings in S/F heterostructures based on metals or oxides. The first-principles calculations verify the observed unusual interfacial spin texture is caused by the Heisenberg direct exchange coupling through d orbital overlapping and severe charge transfer across the interfaces. Our work establishes an incisive experimental probe for understanding the magnetic proximity behavior at S/F interfaces and provides a prototype epitaxial building block for superconducting spintronics.",2304.05234v1 2023-04-20,Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2,"The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.",2304.10222v1 2023-04-24,Observation of colossal topological Hall effect in noncoplanar ferromagnet Cr5Te6 thin films,"The topological Hall effect (THE) is critical to the exploration of the spin chirality generated by the real-space Berry curvature, which has attracted worldwide attention for its prospective applications in spintronic devices. However, the prominent THE remains elusive at room temperature, which severely restricts the practical integration of chiral spin textures. Here, we show a colossal intrinsic THE in large-area ferromagnet Cr5Te6 thin films epitaxially grown by pulsed laser deposition. Such a THE can be maintained until 270 K, which is attributed to the field-stimulated noncoplanar spin textures induced by the interaction of the in-plane ferromagnet and antiferromagnet infrastructures. Our first-principles calculations further verify the considerable Dzyaloshinskii-Moriya interaction in Cr5Te6. This work not only paves the way for robust chiral spin textures near room temperature in large-area low-dimensional ferromagnetic films for practical applications, but also facilitates the development of high-density and dissipationless spintronic devices.",2304.11778v1 2023-05-07,Effect of random antiferromagnetic exchange on the spin waves in a three-dimensional Heisenberg ferromagnet,"Neutron scattering is used to study spin waves in the three-dimensional Heisenberg ferromagnet YTiO$_3$, with spin-spin exchange disorder introduced $via$ La-substitution at the Y site. No significant changes are observed in the spin-wave dispersion up to a La concentration of 20%. However, a strong broadening of the spectrum is found, indicative of shortened spin-wave lifetimes. Density-functional theory calculations predict minimal changes in exchange constants as a result of average structural changes due to La substitution, in agreement with the data. The absence of significant changes in the spin-wave dispersion, the considerable lifetime effect, and the reduced ordered magnetic moment previously observed in the La-substituted system are qualitatively captured by an isotropic, nearest-neighbor, three-dimensional Heisenberg ferromagnet model with random antiferromagnetic exchange. We therefore establish Y$_{1-x}$La$_x$TiO$_3$ as a model system to study the effect of antiferromagnetic spin-exchange disorder in a three-dimensional Heisenberg ferromagnet.",2305.04280v1 2023-05-17,Potential-tuned magnetic switches and half-metallicity transition in zigzag graphene nanoribbons,"Realizing controllable room-temperature ferromagnetism in carbon-based materials is one of recent prospects. The magnetism in graphene nanostructures reported previously is mostly formed near the vacancies, zigzag edges, or impurities by breaking the local sublattice imbalance, though a bulk chiral spin-density-wave ground state is also reported at van Hove filling due to its perfectly nested Fermi surface. Here, combining of the first-principles and tight-binding model simulations, we predict a robust ferromagnetic domain lies between the inter-chain carbon atoms inside the zigzag graphene nanoribbons by applying a potential drop. We show that the effective zigzag edges provide the strong correlation background through narrowing the band width, while the internal Van Hove filling provides the strong ferromagnetic background inherited from the bulk. The induced ferromagnetism exhibit interesting switching effect when the nominal Van Hove filling crosses the intra- and inter-chain region by tuning the potential drops. We further observe a robust half-metallicity transition from one spin channel to another within the same magnetic phase. These novel properties provide promising ways to manipulate the spin degree of freedom in graphene nanostructures.",2305.10325v1 2023-05-25,Nonlinear spin dynamics of ferromagnetic ring in the vortex state and its application for spin-transfer nano-oscillator,"We study a nonlinear spin dynamics of a ferromagnetic ring in a vortex state induced by the spin-polarized current. We also suggest to use the ferromagnetic ring as a free layer of a coreless vortex spin-transfer nano-oscillator. The calculated working frequency is about several GHz, that is much higher than the gyromode frequency of the disk-based vortex oscillator. The response of the vortex-state ring to the spin-polarized current has hysteretic behavior with the reasonable values of the thresholds current densities: ignition threshold is about $10^{8} \text{A}\text{cm}^{-2}$, and elimination current to maintain the oscillations has much lower values about $10^{6} \text{A} \text{cm}^{-2}$. The output signal can be extracted by the help of the inverse spin Hall effect or by the giant magnetoresistance. The output electromotive force averaged over all sample vanishes, and we suggest to use a ferromagnetic ring or disk in a vortex state as a GMR analyzer. For an inverse spin Hall analyser we advise to use two heavy metals with different signs of Spin-Hall angle. The ring-based STNO is supposed to increase the areas of practical application of the STNOs.",2305.16019v2 2023-05-25,Observation of c-axis Magnetization at Low Temperatures in Weak Ferromagnet FeBO$_3$ Reveals a Spin-Reorientation Transition,"The weak ferromagnet FeBO$_3$ is well known for being a unique system for modelling and testing magnetic dynamics primarily due to relatively simple and localized magnetic structure and its interesting spin wave dynamics. At room temperature, it has slightly canted iron moments lying in the a-b plane that result in a strong antiferromagnetic moment and a weak ferromagnetic moment, which results in pronounced ferromagnetic and antiferromagnetic spin modes. However, some previous studies have shown unusual low-temperature behavior that suggests a phase transition. By performing low-temperature magnetization measurements, both in bulk and on the mesoscale, we have observed a low temperature magnetic texture in this material in which a large c-axis magnetization occurs. Magnetic fields along the c-axis as high as 1300 Oe were observed close to the sample surface. This presents evidence for the onset of a Morin transition or another type of spin-reorientation phase transition wherein the Fe3+ moments would acquire a c-axis component to their canting below a critical temperature. The observation of this c-axis magnetization suggests that there is a different ground state in this material than has been previously expected and could be due to as yet unexplored intricacies of the Dzyaloshinskii-Moriya interaction.",2305.16082v1 2023-06-01,Constraints on proximity-induced ferromagnetism in a Dirac semimetal (Cd$_3$As$_2$)/ferromagnetic semiconductor (Ga$_{1-x}$Mn$_x$Sb) heterostructure,"Breaking time-reversal symmetry in a Dirac semimetal Cd$_3$As$_2$ through doping with magnetic ions or by the magnetic proximity effect is expected to cause a transition to other topological phases (such as a Weyl semimetal). To this end, we investigate the possibility of proximity-induced ferromagnetic ordering in epitaxial Dirac semimetal (Cd$_3$As$_2$)/ferromagnetic semiconductor (Ga$_{1-x}$Mn$_x$Sb) heterostructures grown by molecular beam epitaxy. We report the comprehensive characterization of these heterostructures using structural probes (atomic force microscopy, x-ray diffraction, scanning transmission electron microscopy), angle-resolved photoemission spectroscopy, electrical magneto-transport, magnetometry, and polarized neutron reflectometry. Measurements of the magnetoresistance and Hall effect in the temperature range 2 K - 20 K show signatures that could be consistent with either a proximity effect or spin-dependent scattering of charge carriers in the Cd$_3$As$_2$ channel. Polarized neutron reflectometry sets constraints on the interpretation of the magnetotransport studies by showing that (at least for temperatures above 6 K) any induced magnetization in the Cd$_3$As$_2$ itself must be relatively small ($<$ 14 emu/cm$^3$).",2306.01167v1 2023-06-27,Interfacial-antiferromagnetic-coupling driven magneto-transport properties in ferromagnetic superlattices,"We explore the role of interfacial antiferromagnetic interaction in coupled soft and hard ferromagnetic layers to ascribe the complex variety of magneto-transport phenomena observed in $La_{0.7}Sr_{0.3}MnO_3/SrRuO_3$ (LSMO/SRO) superlattices (SLs) within a one-band double exchange model using Monte-Carlo simulations. Our calculations incorporate the magneto-crystalline anisotropy interactions and super-exchange interactions of the constituent materials, and two types of antiferromagnetic interactions between Mn and Ru ions at the interface: (i) carrier-driven and (ii) Mn-O-Ru bond super-exchange in the model Hamiltonian to investigate the properties along the hysteresis loop. We find that the antiferromagnetic coupling at the interface induces the LSMO and SRO layers to align in anti-parallel orientation at low temperatures. Our results reproduce the positive exchange bias of the minor loop and inverted hysteresis loop of LSMO/SRO SL at low temperatures as reported in experiments. In addition, conductivity calculations show that the carrier-driven antiferromagnetic coupling between the two ferromagnetic layers steers the SL towards a metallic (insulating) state when LSMO and SRO are aligned in anti-parallel (parallel) configuration, in good agreement with the experimental data. This demonstrate the necessity of carrier-driven antiferromagnetic interactions at the interface to understand the one-to-one correlation between the magnetic and transport properties observed in experiments. For high temperature, just below the ferromagnetic $T_C$ of SRO, we unveiled the unconventional three-step flipping process along the magnetic hysteresis loop. We emphasize the key role of interfacial antiferromagnetic coupling between LSMO and SRO to understand these multiple-step flipping processes along the hysteresis loop.",2306.15143v1 2023-08-07,"Buzdin, Shapiro and Chimera Steps in $\varphi_0$ Josephson Junctions","The unique resonance and locking phenomena in the superconductor-ferromagnet-superconductor $\varphi_0$ Josephson junction under external electromagnetic radiation are demonstrated when not just the electric but also the magnetic component of external radiation is taken into account. Due to the coupling of superconductivity and magnetism in this system, the magnetic moment precession of the ferromagnetic layer caused by the magnetic component of external radiation can lock the Josephson oscillations, which results in the appearance of a particular type of steps in the current-voltage characteristics, completely different from the well-known Shapiro steps. We call these steps the Buzdin steps in the case when the system is driven only by the magnetic component and the Chimera steps in the case when both magnetic and electric components are present. Unlike the Shapiro steps where the magnetization remains constant along the step, here it changes though the system is locked. The spin-orbit coupling substantially contributes to the amplitude, i.e., the size of these steps. Dramatic changes in their amplitudes are also observed at frequencies near the ferromagnetic resonance. Combinations of the Josephson and Kittel ferromagnetic resonances together with different types of locking pronounced in dynamics and current-voltage characteristics make the physics of this system very interesting and open up a series of new applications.",2308.03701v2 2023-08-23,Enhancing the Capture of Magnetic Nanoparticles Inside of Ferromagnetic Nanostructures Using External Magnetic Fields,"The influence of an external magnetic field upon the capture of 130 nm magnetic nanoparticles (MNPs) by ferromagnetic nanostructures was investigated. The magnetophoretic forces acting upon a nanoparticle were simulated in external magnetic fields parallel and perpendicular to ferromagnetic nanostructures consisting of arrays of antidots and dots. Changing the direction of the external field was found to dramatically alter the magnetophoretic forces acting on the particle and the trajectories of the MNPs. A field parallel to the nanostructures' surfaces generated magnetophoretic forces that directed the nanoparticle into the nanostructures. A perpendicular field produced forces directing particles onto the structures' surfaces. Nanostructures were etched into the surfaces of Permalloy films using ion beam lithography. MNPs were then deposited onto the films' surfaces under a parallel or perpendicular magnetic field. The distributions of particles in the nanostructures were analysed to obtain the capture efficiencies of each structure which indicate the proportion of particles trapped inside. Without an external field, the highest efficiency was displayed by arrays of circular antidots with circular dot arrays displaying the lowest. Antidot arrays displayed higher capture efficiencies than dot arrays. Addition of a field parallel to the surface significantly increased the capture efficiencies and addition of a field perpendicular to the surface decreased the efficiencies. Under the perpendicular field, the particles were instead caught on the outer edges of the nanostructures. These results suggest that application of a parallel external magnetic field promotes the capture of MNPs within ferromagnetic nanostructures and a perpendicular field increases the capture of MNPs onto the outer surface and edges of nanostructures.",2308.12321v1 2023-10-23,Nonlinear Optical Effects Due to Magnetization Dynamics In a Ferromagnet,"We theoretically consider magnetization dynamics in a ferromagnetic slab induced by the magnetic field of a strong femtosecond laser pulse. The longitudinal geometry, in which the initial magnetization lies in both the plane of incidence and the sample plane, is studied. The magnetization oscillations at the optical wave frequency are calculated with the use of the Kapitza pendulum approach taking into account that the optical frequency is much greater than the magnetization oscillation eigenfrequency. We study the reflection of the electromagnetic wave from a ferromagnet %with such a dynamics and show that this laser-induced low-frequency magnetization dynamics leads to the appearance of the second-order nonlinearity in the Maxwell's equations, which in turn gives rise to both the second harmonic generation (SHG) and rectification effect. Although the amplitude of the magnetization oscillations is small, the considered effect may be responsible for the SHG with the efficiency comparable to that of nonmagnetic SHG from metal surfaces. Our estimations show that the suggested mechanism may explain the recent experiments on magnetization induced modulation of the SHG intensity in a ``forbidden'' $P_{in}P_{out}$ combination of incident and reflected waves in cobalt/heavy metal systems, where it can be even more pronounced due to the spin current flow through the ferromagnet/ heavy metal interface.",2310.14967v1 2023-11-03,Robust room temperature ferromagnetism in an itinerant van der Waals antiferromagnet,"The coexistence of antiferromagnetic and ferromagnetic order at room temperature in single-phase van der Waals materials, particularly within the two-dimensional limit, has attracted significant research interest. Nonetheless, such materials are rare. In this work, we introduce an itinerant van der Waals antiferromagnet (Fe0.56Co0.44)5GeTe2, where the ferromagnetic order of its exfoliated flakes remains discernible up to room temperature, extending down to the monolayer limit. A notable phenomenon observed is the evident odd-even layer-number effect at high temperature (e.g., T = 150 K). Such behaviour can be expounded by a linear-chain model. Of particular interest is the robust ferromagnetic order observed in even-layer flakes at low temperature (e.g., T = 2 K), which could potentially be attributed to spin-polarized defects. The intricate interplay among magnetic field strength, layer number, and temperature gives rise to a diverse array of phenomena, holding promise not only for new physics but also for practical applications.",2311.01735v1 2023-11-07,Revealing the ultra-fast domain wall motion in Manganese Gold through permalloy capping,"Antiferromagnets offer much faster dynamics compared to their ferromagnetic counterparts but their order parameter is extremely difficult to detect and control. So far, controlling the N\'eel order parameter electrically is limited to only very few materials where N\'eel spin-orbit torques are allowed by symmetry. In this work, we show that coupling a thin ferromagnet (permalloy) layer on top of an antiferromagnet (Mn$_2$Au) solves a major roadblock -- the controlled reading, writing, and manipulation of antiferromagnetic domains. We confirm by atomistic spin dynamics simulations that the domain wall patterns in the Mn$_2$Au are imprinted on the permalloy, therefore allowing for indirect imaging of the N\'eel order parameter. Our simulations show that the coupled domain wall structures in Mn$_2$Au-Py bilayers can be manipulated by either acting on the N\'eel order parameter via N\'eel spin-orbit torques or by acting on the magnetisation (the ferromagnetic order parameter) via magnetic fields. In both cases, we predict ultra-high domain wall speeds on the order of 8.5 km/s. Thus, employing a thin ferromagnetic layer has the potential to easily control the N\'eel order parameter in antiferromagnets even where N\'eel spin-orbit torques are forbidden by symmetry. The controlled manipulation of the antiferromagnetic order parameter provides a promising basis for the development of high-density storage and efficient computing technologies working in the THz regime.",2311.04305v1 2023-11-17,Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure,"We report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure ($BiSbTe_{1.5}Se_{1.5}/Cu/Ni_{80}Fe_{20}$) by using spin-pumping technique where $BiSbTe_{1.5}Se_{1.5}$ is the TI and $Ni_{80}Fe_{20}$ is the ferromagnetic layer. The SCC, characterized by inverse Edelstein effect length ($\lambda_{IEE}$) in the TI material gets altered with an intervening Copper (Cu) layer and it depends on the interlayer thickness. The introduction of Cu layer at the interface of TI and ferromagnetic metal (FM) provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the linewidth of ferromagnetic resonance (FMR) absorption spectra due to the insertion of Cu layer at the interface indicates a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of ferromagnetic metal. The temperature dependence (from 8K to 300K) of the evaluated $\lambda_{IEE}$ data for all the trilayer systems, TI/Cu/FM with different Cu thickness confirms the effect of exchange coupling between the TI and FM layer on the spin to charge conversion efficiency of the topological surface state.",2311.10460v1 2023-12-04,Survival of magnetic correlations above ordering temperature in a ferromagnetically ordered classical kagomé magnet: Li9Cr3(P2O7)3(PO4)2,"Motivated by the recent discovery of a semiclassical nematic spin liquid state in a Heisenberg kagom\'e antiferromagnet Li9Fe3(P2O7)3(PO4)2 (LFPO) with S=5/2 [Kermarrec et al. Phys. Rev. Lett. 127, 157202 (2021)], we now investigate the impact of spin quantum number S on the ground state properties by studying the isostructural kagom\'e magnet Li9Cr3(P2O7)3(PO4)2 (LCPO) with active t2g orbitals and S = 3/2. Thermodynamic measurements reveal that the ground state properties of LCPO is dominated by the ferromagnetic interactions with a mean-field temperature $\theta \sim$ 3 K (J < 1 K) and the ordering temperature, Tc ~ 2.7 K, and the size of the ordered moment $\sim 1.05 \pm 0.25 $ ~${\mu}_B$ is significantly reduced from that of a fully ordered moment. The ab initio electronic structure calculations nicely corroborate the thermodynamic results and suggest the presence of additional in and out-of-plane further neighbor antiferromagnetic couplings, though significantly weaker in comparison to the dominant first-nearest neighbor ferromagnetic coupling. The spin-lattice relaxation rate measured with fields larger than the saturation field shows a magnetic field induced gap ($\Delta \propto B$) in the excitation spectrum, and in $B \rightarrow 0$ limit the gap has a finite intercept ~ 3 K, equivalent to the mean-field scale. We interpret the origin of this gap is associated with the magnetic interactions inherent to the material. With our experimental results, we establish the stabilization of a ferromagnetic like ground state and the persistence of magnetic-correlations above the ordering temperature in LCPO.",2312.01731v1 2023-12-31,Magnetic properties at various fillings of the quasiflat band in a fermionic two-leg ladder model,"A recent study has demonstrated that a fermionic two-leg ladder model, threaded by a flux and characterized by a spatially varying interleg hopping term, gives rise to a quasiflat low-energy band. This band exhibits an unusual ground state at half filling in the presence of interaction -- a ferromagnetic Mott insulator. In this paper, we extend the study of this model to other fillings of the quasiflat band and explore the magnetic properties of the ground state at these fillings. In particular, we study four fillings: one-quarter, three-quarters, slightly above half filling (half filling plus two electrons), and slightly below half-filling (half filling minus two electrons). Incorporating interaction within the Hubbard model and using the Density Matrix Renormalization Group method to find the ground states, we find that the spin-spin correlation is ferromagnetic at fillings less than half, similar to that observed at half filling, but is antiferromagnetic beyond half filling. Interestingly, these results hold only when mixing between the lowest quasiflat band and the next-to-lowest dispersive band is negligible; once mixing between the two bands is facilitated by increasing the interaction strength, the correlation becomes ferromagnetic above half filling as well. Additionally, by reducing the strength of the interaction in comparison to the bandwidth, a transition from the ferromagnetic to the antiferromagnetic state is observed in all the cases.",2401.00483v1 2024-01-24,Two-dimensional ferromagnetic semiconductor Cr2XP: First-principles calculations and Monte Carlo simulations,"According to the Mermin Wagner theorem, two-dimensional material is difficult to have the Curie temperature above room temperature. By using the method of band engineering, we design a promising two-dimensional ferromagnetic semiconductor Cr2XP (X=P, As, Sb) with large magnetization, high Curie temperature and sizable band gap. The formation of gap is discussed in terms of the hybridizations, occupation and distribution of electronic states and charge transfer. Large magnetic moments about 6.16~6.37uB origin from the occupation of Cr-d electrons in crystal field.Competition and cooperation between d-d (Cr-d~Cr-d) and d-p-d (Cr-d~X-p~Cr-d) exchange interactions lead to the emergence of ferromagnetic ordering phase. Furthermore, Curie temperatures, approaching to 269 K, 332 K and 400 K for Cr2P2, Cr2AsP and Cr2SbP, are estimated by employing Monte Carlo simulation based on the Heisenberg model. Magnetic anisotropy energy of Cr2XP is determined by calculating the total energy dependence on the angle along different directions, and the origin is also discussed by the second-order perturbation theory. In addition, the Cr2XP possesses excellent thermodynamical, dynamical and mechanical stabilities, and can overcome their own gravity to keep their planar structure without the support of substrate. These above-mentioned advantages will offer some valuable hints for two-dimensional ferromagnetic semiconductor Cr2XP in spintronic devices.",2401.13425v1 2024-01-25,A promising candidate for ising ferromagnetism of two-dimensional kagome V$_2$O$_3$ honeycomb monolayer,"Due to the low dimensionality in the quantization of the electronic states and degree of freedom for device modulation, two-dimensional (2D) ferromagnetism plays a critical role in lots of fields. In this study, we perform first-principles calculation to investigate the ising ferromagnetism and half-metallicity of kagome V$_2$O$_3$ monolayer. Based on the calculations using different functional, it is found that GGA-PBE gives a half-metallic band gap while the GGA+U gives a semiconductor narrow band gap (~1.1 meV), which shows quasi-half metallic nature. By studying the magnetic properties with LDA, GGA-PBE, and GGA+U, we get a robust ferromagnetic ground state, where the giant perpendicular magnetic anisotropy energy of ~0.544 meV is achieved by applying the spin-orbit coupling (SOC) with GGA+U. Furthermore, by exploring the orbital contribution to the electronic bands and the magnetic crystalline anisotropy, it is uncovered that the 3d (V) orbitals contribute to the out-of-plane. The electronic band structure shows two flat bands (F1 and F2) and Dirac points (D1 and D2) which further confirm that kagome V$_2$O$_3$ ML can also be used for topological properties. Besides, the Curie temperature of the V$_2$O$_3$ ML is calculated to be 640 K by Metropolis Monte Carlo (MC) simulations.",2401.14035v1 2024-02-06,"Ab initio calculation of the effective Coulomb interactions in MX2 (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=S, Se, Te): intrinsic magnetic ordering and Mott insulating phase","Correlated phenomena such as magnetism and Mott phase are a very controversial issue in two-dimensional transition metal dichalcogenides (TMDCs). With the aim of finding the value of correlation strength and understanding the origin of ferromagnetic order in TMDCs, we first identify relevant low-energy degrees of freedom on both octahedral T and trigonal prismatic H lattices in MX2 (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=S, Se, Te) and then determine the strength of the effective Coulomb interactions between localized d electrons from the first principles using the constrained random-phase approximation. The on-site Coulomb interaction (Hubbard U) values lie in the range 1.4-3.7 eV (1.1-3.6 eV) and depend on the ground-state electronic structure, d-electron number, and correlated subspace. For most of the TMDCs we obtain 1 < U/W_b < 2 (the bandwidth W_b), which turn out to be larger than the corresponding values in elementary transition metals. On the basis of the calculated U and exchange J interaction, we have checked the condition to be fulfilled for the formation of the ferromagnetic order by Stoner criterion. The results indicate that experimentally observed MnX2 (X=S, Se) and VX2 (X=S, Se) have an intrinsic ferromagnetic behavior in pristine form, although V-based materials are close vicinity to the critical point separating ferromagnetic from paramagnetic phase.",2402.04199v1 2024-02-06,Ultrafast terahertz field control of the emergent magnetic and electronic interactions at oxide interfaces,"Ultrafast electric-field control of emergent electronic and magnetic states at oxide interfaces offers exciting prospects for the development of new generations of energy-efficient devices. Here, we demonstrate that the electronic structure and emergent ferromagnetic interfacial state in epitaxial LaNiO3/CaMnO3 superlattices can be effectively controlled using intense single-cycle THz electric-field pulses. We employ a combination of polarization-dependent X-ray absorption spectroscopy with magnetic circular dichroism and X-ray resonant magnetic reflectivity to measure a detailed magneto-optical profile and thickness of the ferromagnetic interfacial layer. Then, we use time-resolved and temperature-dependent magneto-optical Kerr effect, along with transient optical reflectivity and transmissivity measurements, to disentangle multiple correlated electronic and magnetic processes driven by ultrafast high-field (~1 MV/cm) THz pulses. These processes include an initial sub-picosecond electronic response, consistent with non-equilibrium Joule heating; a rapid (~270 fs) demagnetization of the ferromagnetic interfacial layer, driven by THz-field-induced nonequilibrium spin-polarized currents; and subsequent multi-picosecond dynamics, possibly indicative of a change in the magnetic state of the superlattice due to the transfer of spin angular momentum to the lattice. Our findings shed light on the intricate interplay of electronic and magnetic phenomena in this strongly correlated material system, suggesting a promising avenue for efficient control of two-dimensional ferromagnetic states at oxide interfaces using ultrafast electric-field pulses.",2402.04302v1 2024-02-10,Space-time shape optimization of rotating electric machines,"This article is devoted to the shape optimization of the internal structure of an electric motor, and more precisely of the arrangement of air and ferromagnetic material inside the rotor part with the aim to increase the torque of the machine. The governing physical problem is the time-dependent, non linear magneto-quasi-static version of Maxwell's equations. This multiphase problem can be reformulated on a 2d section of the real cylindrical 3d configuration; however, due to the rotation of the machine, the geometry of the various material phases at play (the ferromagnetic material, the permanent magnets, air, etc.) undergoes a prescribed motion over the considered time period. This original setting raises a number of issues. From the theoretical viewpoint, we prove the well-posedness of this unusual non linear evolution problem featuring a moving geometry. We then calculate the shape derivative of a performance criterion depending on the shape of the ferromagnetic phase via the corresponding magneto-quasi-static potential. Our numerical framework to address this problem is based on a shape gradient algorithm. The non linear time periodic evolution problems for the magneto-quasi-static potential is solved in the time domain, with a Newton-Raphson method. The discretization features a space-time finite element method, applied on a precise, meshed representation of the space-time region of interest, which encloses a body-fitted representation of the various material phases of the motor at all the considered stages of the time period. After appraising the efficiency of our numerical framework on an academic problem, we present a quite realistic example of optimal design of the ferromagnetic phase of the rotor of an electric machine.",2402.07017v1 2024-02-22,Unexpected versatile electrical transport behaviors of ferromagnetic nickel films,"Perpendicular magnetic anisotropy (PMA) of magnets is paramount for electrically controlled spintronics due to their intrinsic potentials for higher memory density, scalability, thermal stability and endurance, surpassing an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental element ferromagnet, yet its electrical transport behavior associated with magnetism has not been comprehensively studied, hindering corresponding spintronic applications exploiting nickel-based compounds. Here, we systematically investigate the highly versatile magnetism and corresponding transport behavior of nickel films. As the thickness reduces within the general thickness regime of a magnet layer for a memory device, the hardness of nickel films' ferromagnetic loop of anomalous Hall effect increases and then decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA. Additionally, the square ferromagnetic loop changes from a hard to a soft one at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we observe a butterfly magnetoresistance resulting from the anisotropic magnetoresistance effect, which evolves in conjunction with the thickness and temperature-dependent magnetic transformations as a complementary support. Our findings unveil the rich magnetic dynamics and most importantly settle down the most useful guiding information for current-driven spintronic applications based on nickel film: The hysteresis loop is squarest for the ~8 nm-thick nickel film, of highest hardness with Rxyr/Rxys~1 and minimum Hs-Hc, up to 125 K; otherwise, extra care should be taken for a different thickness or at a higher temperature.",2402.14275v1 2024-02-22,On the origin of the above-room-temperature magnetism in the 2D van der Waals ferromagnet Fe$_3$GaTe$_2$,"Recent advancements in 2D magnetic materials have attracted a growing interest driven by their unique properties and potential applications in spintronic devices. However, the scarcity of systems that exhibit magnetism at room-temperature has limited their practical implementation into functional devices. In this work we focus on the recently synthetised van der Waals (vdW) ferromagnet Fe$_3$GaTe$_2$, which exhibits above-room-temperature magnetism (T$_{\mathrm{c}}$ = 350-380 K) and strong perpendicular magnetic anisotropy. Through first-principles calculations, we examine the magnetic properties of Fe$_3$GaTe$_2$ and compare them with the widely known Fe$_3$GeTe$_2$ ferromagnet. Our calculations unveil the complex microscopic mechanisms governing their magnetic behaviour, emphasizing the pivotal role of the ferromagnetic in-plane exchange interactions in the stabilization of the elevated T$_{\mathrm{c}}$ in Fe$_3$GaTe$_2$. Additionally, we predict the stability, strong perpendicular anisotropy and high T$_{\mathrm{c}}$ of single-layer Fe$_3$GaTe$_2$. We also demonstrate the potential of strain engineering and electrostatic doping to modulate its magnetic exchange interactions and anisotropy. Our results incentivise the isolation of the monolayer and pave the way for the future optimization of Fe$_3$GaTe$_2$ in magnetic and spintronic nanodevices.",2402.14618v1 2024-03-05,Unraveling spin texture and spin-orbit coupling contributions in spin triplet superconductivity,"Over the past decade, it has been proposed theoretically and confirmed experimentally that long-range spin triplet (LRT) superconductivity can be generated in ferromagnet-superconductor hybrids either by the presence of spin textures (ST-LRT) or thanks to spin-orbit coupling (SOC-LRT). Nevertheless, there has been no theoretical or experimental investigation to date suggesting that both contributions could simultaneously exist within an experimental system. To disentangle these contributions, we present a comprehensive study of superconducting quasiparticle interference effects taking place inside a ferromagnetic layer interfacing a superconductor, through the investigation of above-gap conductance anomalies (CAs) related to MacMillan-Rowell resonances. The bias dependence of the CAs has been studied under a wide range of in-plane (IP) and out-of-plane (OOP) magnetic fields in two types of epitaxial, V/MgO/Fe-based ferromagnet-superconductor junctions with interfacial spin-orbit coupling. We observe an anisotropy of the CAs amplitude under small IP and OOP magnetic fields while remaining weakly affected by high fields, and implement micromagnetic simulations to help us distinguish between the ST-LRT and SOC-LRT contributions. Our findings suggest that further exploration of Fabry-P\'erot type interference effects in electron transport could yield valuable insights into the hybridization between superconductors and ferromagnets induced by spin-orbit coupling and spin textures.",2403.02819v2 2024-03-06,Tracing Dirac points of topological surface states by ferromagnetic resonance,"Ferromagnetic resonance is used to reveal features of the buried electronic band structure at interfaces between ferromagnetic metals and topological insulators. By monitoring the evolution of magnetic damping, the application of this method to a hybrid structure consisting of a ferromagnetic layer and a 3D topological insulator reveals a clear fingerprint of the Dirac point and exhibits additional features of the interfacial band structure not otherwise observable. The underlying spin-pumping mechanism is discussed in the framework of dissipation of angular momentum by topological surface states (TSSs). Tuning of the Fermi level within the TSS was verified both by varying the stoichiometry of the topological insulator layer and by electrostatic backgating and the damping values obtained in both cases show a remarkable agreement. The high energy resolution of this method additionally allows us to resolve the energetic shift of the local Dirac points generated by local variations of the electrostatic potential. Calculations based on the chiral tunneling process naturally occurring in TSS agree well with the experimental results.",2403.03518v2 2024-03-06,"Crystal, ferromagnetism, and magnetoresistance with sign reversal in a EuAgP semiconductor","We synthesized the ferromagnetic EuAgP semiconductor and conducted a comprehensive study of its crystalline, magnetic, heat capacity, band gap, and magnetoresistance properties. Our investigation utilized a combination of X-ray diffraction, optical, and PPMS DynaCool measurements. EuAgP adopts a hexagonal structure with the $P6_3/mmc$ space group. As the temperature decreases, it undergoes a magnetic phase transition from high-temperature paramagnetism to low-temperature ferromagnetism. We determined the ferromagnetic transition temperature to be $T_{\textrm{C}} =$ 16.45(1) K by fitting the measured magnetic susceptibility using a Curie-Weiss law. Heat capacity analysis of EuAgP considered contributions from electrons, phonons, and magnons, revealing $\eta$ = 0.03 J/mol/$\textrm{K}^\textrm{2}$, indicative of semiconducting behavior. Additionally, we calculated a band gap of $\sim$ 1.324(4) eV based on absorption spectrum measurements. The resistivity versus temperature of EuAgP measured in the absence of an applied magnetic field shows a pronounced peak around $T_{\textrm{C}}$, which diminishes rapidly with increasing applied magnetic fields, ranging from 1 to 14 T. An intriguing phenomenon emerges in the form of a distinct magnetoresistance transition, shifting from positive (e.g., 1.95\% at 300 K and 14 T) to negative (e.g., -30.73\% at 14.25 K and 14 T) as the temperature decreases. This behavior could be attributed to spin-disordered scattering.",2403.03650v1 2024-03-21,Spin injection and detection in a Si-based ferromagnetic tunnel junction: A theoretical model based on the band diagram and experimental demonstration,"We have experimentally and theoretically investigated the spin injection/detection polarization in a Si-based ferromagnetic tunnel junction with an amorphous MgO layer, and demonstrated that the experimental features of the spin polarization in a wide bias range can be well explained using our theoretical model based on the band diagram of the junction and the direct tunneling mechanism. It is shown that the spin polarization originates from the band diagrams of the ferromagnetic Fe layer and n+-Si channel in the junction, while the spin selectivity of the MgO tunnel barrier is not necessary. Besides, we clarified the mechanism of the reduction in spin polarization when the bias is high and nonlinear properties are prominent, where the widely-used spin injection/detection model proposed by Valet and Fert is not applicable. The dominant mechanism of such reduction is found to be spin accumulation saturation (SAS) at the n+-Si interface in contact with the MgO layer as the bias is increased in the spin extraction geometry, which is inevitable in semiconductor-based ferromagnetic tunnel junctions. We performed numerical calculations on a two-terminal spin transport device with a n+-Si channel using the junction properties extracted from the experiments, and revealed that the magnetoresistance (MR) ratio is suppressed mainly by SAS in a higher bias range. Furthermore, we proposed methods for improving the MR ratio in two-terminal spin transport devices. Our experiments and theoretical model provide a deep understanding of the spin injection/detection phenomena in semiconductor-based spin transport devices, toward the realization of high performance under reasonably high bias conditions for practical use.",2403.14127v1 2005-01-17,Magnetic domain walls of relic fermions as Dark Energy,"We show that relic fermions of the Big Bang can enter a ferromagnetic state if they possess a magnetic moment and satisfy the requirements of Stoner theory of itinerant ferromagnetism. The domain walls of this ferromagnetism can successfully simulate Dark Energy over the observable epoch spanning $\sim 10$ billion years. We obtain conditions on the anomalous magnetic moment of such fermions and their masses. Known neutrinos fail to satisfy the requirements thus pointing to the possibility of a new ultralight sector in Particle Physics.",0501348v3 1994-01-31,The Free Energy and the Scaling Function of the Ferromagnetic Heisenberg Chain in a Magnetic Field,"A nonlinear susceptibilities (the third derivative of a magnetization $m_S$ by a magnetic field $h$ ) of the $S$=1/2 ferromagnetic Heisenberg chain and the classical Heisenberg chain are calculated at low temperatures $T.$ In both chains the nonlinear susceptibilities diverge as $T^{-6}$ and a linear susceptibilities diverge as $T^{-2}.$ The arbitrary spin $S$ Heisenberg ferromagnet $[$ ${\cal H} = \sum_{i=1}^{N} \{ - J{\bf S}_{i} {\bf S}_{i+1} - (h/S) S_{i}^{z} \}$ $(J>0),$ $]$ has a scaling relation between $m_S,$ $h$ and $T:$ $m_S(T,h) = F( S^2 Jh/T^2).$ The scaling function $F(x)$=(2$x$/3)-(44$x^{3}$/135) + O($x^{5}$) is common to all values of spin $S.$",9401074v1 1994-10-10,Monte Carlo Simulation of Ising Models with Dipole Interaction,"Recently, a new memory effect was found in the metamagnetic domain structure of the diluted Ising antiferromagnet $Fe_x Mg_{1-x} Cl_2$ by domain imaging with Faraday contrast. Essential for this effect is the dipole interaction. We simulate the low temperature behavior of diluted Ising-antiferromagnets by a Monte Carlo simulation considering long range interaction. The metamagnetic domain structure occurring due to the dipole interaction is investigated by graphical representation. In the model considered the antiferromagnetic state is stable for an external magnetic field smaller than a lower boundary $B_{c1}$ while for fields larger than an upper boundary $B_{c2}$ the system is in the saturated paramagnetic phase, where the spins are ferromagnetically polarized. For magnetic fields in between these two boundaries a mixed phase occurs consisting of ferromagnetic domains in an antiferromagnetic background. The position of these ferromagnetic domains is stored in the system: after a cycle in which the field is first removed and afterwards applied again the domains reappear at their original positions. The reason for this effect can be found in the frozen antiferromagnetic domain state which occurs after removing the field.",9410028v1 1994-12-29,Magnetization switching in nanoscale ferromagnetic grains: description by a kinetic Ising model,"The magnetic relaxation of ferromagnetic powders has been studied for many years, largely due to its importance to recording technologies. However, only recently have experiments been performed that resolve the magnetic state of individual sub-micron particles. Motivated by these experimental developments, we use droplet theory and Monte Carlo simulations to study the time and field dependence of some quantities that can be observed by magnetic force microscopy. Particular emphasis is placed on the effects of finite particle size. The qualitative agreement between experiments on switching and our simulations in individual single-domain ferromagnets suggests that the switching mechanism in such particles may involve local nucleation and subsequent growth of droplets of the stable phase.",9412120v1 1995-01-26,Ground states of the infinite q-deformed Heisenberg ferromagnet,"We set up a general structure for the analysis of ``frustration-free ground states'', or ``zero-energy states'', i.e., states minimizing each term in a lattice interaction individually. The nesting of the finite volume ground state spaces is described by a generalized inductive limit of observable algebras. The limit space of this inductive system has a state space which is canonically isomorphic (as a compact convex set) to the set of zero-energy states. We show that for Heisenberg ferromagnets, and for generalized valence bond solid states, the limit space is an abelian C*-algebra, and all zero-energy states are translationally invariant or periodic. For the $q$-deformed spin-$1/2$ Heisenberg ferromagnet in one dimension (i.e., the XXZ-chain with S$_q$U(2)-invariant boundary conditions) the limit space is an extension of the non-commutative algebra of compact operators by two points, corresponding to the ``all spins up'' and the ``all spins down'' states, respectively. These are the only translationally invariant zero-energy states. The remaining ones are parametrized by the density matrices on a Hilbert space, and converge weakly to the ``all up'' (resp.\ ``all down'') state for shifts to $-\infty$ (resp.\ $+\infty$).",9501123v1 1995-05-03,Magnetic and Transport Properties of the Kondo Lattice Model with Ferromagnetic Exchange Coupling,"The Kondo lattice model with Hund's ferromagnetic spin coupling is investigated as a microscopic model of the perovskite-type $3d$ manganese oxide (R,A)MnO$_3$ where $R$ and $A$ are rare earth element and alkaline earth element, respectively. We take the classical spin limit $S=\infty$ for the simplicity of the calculation, since the quantum exchange process seems to be irrelevant in the high temperature paramagnetic phase. Magnetic and transport properties of the system are calculated. In the hole doped systems, ferromagnetic instabilities are observed as the temperature is lowered. The giant magnetoresistance of this model is in excellent agreement with the experimental data of (La,Sr)MnO$_3$.",9505011v3 1995-08-09,Kinetic Ising Systems as Models of Magnetization Switching in Submicron Ferromagnets,"Recently experimental techniques, such as magnetic force microscopy (MFM), have enabled the magnetic state of individual sub-micron particles to be resolved. Motivated by these experimental developments, we use Monte Carlo simulations of two-dimensional kinetic Ising ferromagnets to study the magnetic relaxation in a negative applied field of a grain with an initial magnetization $m_0 = +1$. The magnetostatic dipole-dipole interactions are treated to lowest order by adding to the Hamiltonian a term proportional to the square of the magnetization. We use droplet theory to predict the functional forms for some quantities observed by MFM, such as the probability that the magnetization is positive. Our simulations are in excellent agreement with droplet-theoretical predictions. The qualitative agreement between experiments and our simulations of switching in individual single-domain ferromagnets indicates that the switching mechanism in such particles may involve local nucleation and subsequent growth of droplets of the stable phase.",9508031v1 1995-09-22,Critical Dynamics of Magnets,"We review our current understanding of the critical dynamics of magnets above and below the transition temperature with focus on the effects due to the dipole--dipole interaction present in all real magnets. Significant progress in our understanding of real ferromagnets in the vicinity of the critical point has been made in the last decade through improved experimental techniques and theoretical advances in taking into account realistic spin-spin interactions. We start our review with a discussion of the theoretical results for the critical dynamics based on recent renormalization group, mode coupling and spin wave theories. A detailed comparison is made of the theory with experimental results obtained by different measuring techniques, such as neutron scattering, hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and magnetic relaxation, in various materials. Furthermore we discuss the effects of dipolar interaction on the critical dynamics of three--dimensional isotropic antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a discussion of the consequences of dipolar anisotropies on the existence of magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and antiferromagnets. We close our review with a formulation of critical dynamics in terms of nonlinear Langevin equations.",9509141v1 1995-10-13,Plateau of the Magnetization Curve of the S=1/2 Ferromagnetic-Ferromagnetic-Antiferromagnetic Spin Chain,"I analytically study the plateau of the magnetization curve at $M/M_{\rm S} = 1/3$ (where $M_{\rm S}$ is the saturation magnetization) of the one-dimensional $S=1/2$ trimerized Heisenberg spin system with ferromagnetic ($J_{\rm F}$)-ferromagnetic ($J_{\rm F}$)-antiferromagnetic ($J_{\rm A}$) interactions at $T=0$. I use the bosonization technique for the fermion representation of the spin Hamiltonian through the Jordan-Wigner transformation. The plateau appears when $\gamma \equiv J_{\rm F}/J_{\rm A} \allowbreak < \gamma_{\rm C}$, and vanishes when $\gamma > \gamma_{\rm C}$, where the critical value $\gamma_{\rm C}$ is estimated as $\gamma_{\rm C} = 5 \sim 6$. The behavior of the width of the plateau near $\gamma_{\rm C}$ is of the Kosterlitz-Thouless type. The present theory well explains the numerical result by Hida.",9510070v1 1995-11-27,Topology and dynamics in ferromagnetic media,"A direct link between the topological complexity of ferromagnetic media and their dynamics has recently been established through the construction of unambiguous conservation laws as moments of a topological vorticity. In the present paper we carry out this program under completely realistic conditions, with due account of the long-range magnetostatic field and related boundary effects. In particular, we derive unambiguous expressions for the linear and angular momentum in a ferromagnetic film which are then used to study the dynamics of magnetic bubbles under the influence of an applied magnetic-field gradient. The semi-empirical golden rule of bubble dynamics is verified in its gross features but not in its finer details. A byproduct of our analysis is a set of virial theorems generalizing Derrick's scaling relation as well as a detailed recalculation of the fundamental magnetic bubble.",9511126v2 1995-12-14,Polymerization in a Ferromagnetic Spin Model with Threshold,"We propose a spin model with a new kind of ferromagnetic interaction, which may be called {\it ferromagnetic coupling with threshold}. In this model the contribute of a given spin to the total energy has only two possible values and depends on the number of parallel spins among its nearest and next to the nearest neighbors. By mapping the model onto the Ising version of the isotropic eight vertex model, we obtain some evidence of a low temperature phase made of alternate parallel pluses and minuses polymers.",9512116v3 1996-03-06,Ferromagnetism in multi--band Hubbard models: From weak to strong Coulomb repulsion,"We propose a new mechanism which can lead to ferromagnetism in Hubbard models containing triangles with different on-site energies. It is based on an effective Hamiltonian that we derive in the strong coupling limit. Considering a one-dimensional realization of the model, we show that in the quarter-filled, insulating case the ground-state is actually ferromagnetic in a very large parameter range going from Tasaki's flat-band limit to the strong coupling limit of the effective Hamiltonian. This result has been obtained using a variety of analytical and numerical techniques. Finally, the same results are shown to apply away from quarter-filling, in the metallic case.",9603042v1 1996-06-05,Transport and magnetic properties in ferromagnetic manganese-oxide thin films,"The transport and magnetic properties in ferromagnetic manganese-oxide thin films are studied based on the model of the coupling between the mobile d-electrons and the core spins in Mn ions. The spontaneous magnetization and the resistivity are obtained for various magnetic fields and temperature. The resistivity in absence of magnetic field and the magnetoresistance exhibit maxima near the Curie temperature, the applied magnetic field moves the position of the resistivity peak to high temperature and suppresses the peak value, which agree with the experimental results. The Hall resistivity is predicted to exhibit maximum near the Curie point. The pressure effect of the magnetoresistance can also be explained qualitatively in this mechanism. The colossal magnetoresistance in ferromagnetic manganese-oxide thin films is attributed to the spin-correlation fluctuation scattering and the low dimensional effect.",9606029v1 1996-06-17,A note on ferromagnetism in the Hubbard model on the complete graph,"Recently there have appeared some papers which discuss the existence of ferromagnetism in the Hubbard model defined on the complete graph. At least for the special electron number N_e=N-1, where N denotes the number of sites in the lattice, the existence of ferromagnetism in this model was established rigorously some time ago, as special (and indeed the simplest) cases of more general classes of models. Here we explain these implications to clarify the situation, although we believe the implications are straightforward. We are posting this note to the preprint archive to make it public, but we are not planning to publish it in other forms. This is because we do not think the problem warrants any extra publications, and we believe that the validity of our explanation is evident to the readers.",9606115v1 1996-08-07,Power-law correlated phase in random-field XY models and randomly pinned charge-density waves,"Monte Carlo simulations have been used to study the Z6 ferromagnet in a random field on simple cubic lattices, which is a simple model for randomly pinned charge-density waves. The random field is chosen to have infinite strength on a fraction x of the sites of the lattice, and to be zero on the remaining sites. For x= 1/16 there are two phase transitions. At low temperature there is a ferromagnetic phase, which is stabilized by the six-fold nonrandom anisotropy. The intermediate temperature phase is characterized by a |k|^(-3) decay of two-spin correlations, but no true ferromagnetic order. At the transition between the power-law correlated phase and the paramagnetic phase the magnetic susceptibility diverges, and the two-spin correlations decay approximately as |k|^(-2.87).",9608036v1 1996-08-09,Chiral Metal as a Ferromagnetic Super Spin Chain,"The electrons on the surface of a disordered multi-layer integer quantum Hall system constitute an unusual chiral metal with ballistic motion transverse to the field, and diffusive motion parallel to it. We present a non-perturbative analytic treatment of an appropriate model, consisting of disordered chiral Fermions in two dimensions. A supersymmetric generating functional is set up for the correlation functions of this system. The strong disorder limit is mapped into a supersymmetric spin chain, with ferromagnetic exchange coupling, reflecting the electron's chiral motion. The ferromagnetic ground state and the spin wave excitations, corresponding to the diffusion modes of the chiral metal, are found exactly. The parametric density of states correlator in the ergodic limit is computed from a Boltzmann-weighted sum over low energy spin states. The result is of a universal form and coincides with that for a Hermitian random matrix.",9608049v2 1996-09-06,Quantum critical behavior of itinerant ferromagnets,"The quantum ferromagnetic transition of itinerant electrons is considered. We give a pedagogical review of recent results which show that zero-temperature soft modes that are commonly neglected, invalidate the standard Landau-Ginzburg-Wilson description of this transition. If these modes are taken into account, then the resulting order parameter field theory is nonlocal in space and time. Nevertheless, for both disordered and clean systems the critical behavior has been exactly determined for spatial dimensions d>2 and d>1, respectively. The critical exponents characterizing the paramagnetic-to-ferromagnetic transition are dimensionality dependent, and substantially different from both mean-field critical exponents, and from the classical Heisenberg exponents that characterize the transition at finite temperatures. Our results should be easily observable, particularly those for the disordered case, and experiments to check our predictions are proposed.",9609070v2 1996-12-24,Quantum critical behavior of clean itinerant ferromagnets,"We consider the quantum ferromagnetic transition at zero temperature in clean itinerant electron systems. We find that the Landau-Ginzburg-Wilson order parameter field theory breaks down since the electron-electron interaction leads to singular coupling constants in the Landau-Ginzburg-Wilson functional. These couplings generate an effective long-range interaction between the spin or order parameter fluctuations of the form 1/r^{2d-1}, with d the spatial dimension. This leads to unusual scaling behavior at the quantum critical point in 1 < d\leq 3, which we determine exactly. We also discuss the quantum-to-classical crossover at small but finite temperatures, which is characterized by the appearance of multiple temperature scales. A comparison with recent results on disordered itinerant ferromagnets is given.",9612224v2 1997-05-22,The field theory of Skyrme lattices in quantum Hall ferromagnets,"We report the application of the nonlinear $\sigma$ model to study the multi-skyrmion problem in the quantum Hall ferromagnet system. We show that the ground state of the system can be described by a ferromagnet triangular Skyrme lattice near $\nu=1$ where skyrmions are extremely dilute. We find a transition into antiferromagnet square lattice by increasing the skyrmion density and therefore $|\nu-1|$. We investigate the possibility that the square Skyrme lattice deforms to a single skyrmion with the same topological charge when the Zeeman energy is extremely smaller than the Coulomb energy. We explicitly show that the energy of a skyrmion with charge two is less than the energy of two skyrmions each with charge one when $g \leq g_c$. By taking the quantum fluctuations into account, we also argue the possibility of the existence of a non-zero temperature Kosterlitz-Thouless and a superconductor-insulator phase transition.",9705224v1 1997-05-27,Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction,"We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies kappa=J'a^2c/(g_L\mu_B)^2. For increasing values of \kappa, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120^o structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF_3, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are separated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continuously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.",9705280v1 1997-05-30,Ferromagnetism in the Hubbard model: Influence of the lattice structure,"By use of the spectral density approach the influence of the lattice structure on the possibility of ferromagnetism in the single band Hubbard model is investigated. The d=\infty hypercubic lattice does not show magnetic phase transitions of second order irrespective of the strength of the Coulomb coupling. However, first order transitions to finite magnetic moments, not visible as singularities of the paramagnetic susceptibility, may appear in the very strong coupling regime. In d=3 second order transitions are found but only for very strong couplings, where the non-locality of the electronic self-energy acts in favour of the spontaneous magnetic moment. The influence of the non-local part of the self-energy is particularly strong for lattices with small coordination number. The non-bipartite fcc lattice exhibits saturated ferromagnetism for all band occupations $1\le n\le 2$ while for less than half filled bands ($0\le n\le 1$) the system remains in any case paramagnetic, and that for d=3 as well as d=\infty. The Curie temperature runs through a maximum at about $n=1.4$ and vanishes for $n\to 1$ and $n\to 2$.",9705305v1 1997-07-28,Lattice dependence of saturated ferromagnetism in the Hubbard model,"We investigate the instability of the saturated ferromagnetic ground state (Nagaoka state) in the Hubbard model on various lattices in dimensions d=2 and d=3. A variational resolvent approach is developed for the Nagaoka instability both for U = infinity and for U < infinity which can easily be evaluated in the thermodynamic limit on all common lattices. Our results significantly improve former variational bounds for a possible Nagaoka regime in the ground state phase diagram of the Hubbard model. We show that a pronounced particle-hole asymmetry in the density of states and a diverging density of states at the lower band edge are the most important features in order to stabilize Nagaoka ferromagnetism, particularly in the low density limit.",9707286v2 1997-09-17,Theory of Hysteresis Loop in Ferromagnet,"A theory of the hysteresis loop in ferromagnets controlled by the domain wall motion is presented. Domain walls are considered as plane or linear interfaces moving in a random medium under the action of the external ac magnetic field $H=H_0\sin\omega t$. We introduce important characteristics of the hysteresis loop, such as dynamic threshold fields, reversal field etc. together with well known characteristics as coercive field and hysteresis loop area (HLA) $\cal A$. We show that all these characteristics are regulated by two dimensionless combinations of the $H_0$ and $\omega$and intrinsic characteristics of the ferromagnet. The moving domain wall can create magnetic bubbles playing the role of pre-existing nuclei of the reversed magnetization. We discuss a simple model of this process. For magnetization reversal determined by domain inflation we predict that HLA scales as ${\cal A}\propto \omega^{\beta}H_0^{\alpha}$ with $\alpha =1/2$ and $\beta =1/2$. Numerical simulations confirmthis result.",9709194v2 1997-10-09,Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles,"A formula suitable for a quantitative evaluation of the tunneling effect in a ferromagnetic particle is derived with the help of the instanton method. The tunneling between n-th degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration. The low-lying level-splitting previously obtained with the LSZ method in field theory in which the tunneling is viewed as the transition of n bosons induced by the usual (vacuum) instanton is recovered. The observation made with our new result is that the tunneling effect increases at excited states. The results should be useful in analyzing results of experimental tests of macroscopic quantum coherence in ferromagnetic particles.",9710090v1 1997-10-16,"Hole doping and disorder effects on the one-dimensional Kondo lattice, for ferromagnetic Kondo couplings","We investigate the one-dimensional Kondo lattice model (1D KLM) for ferromagnetic Kondo couplings. The so-called ferromagnetic 2-leg spin ladder and the S=1 antiferromagnet occur as new one-dimensional Kondo insulators. Both exhibit a spin gap. But, in contrast to the strong coupling limit, the Haldane state which characterizes the 2-leg spin ladder Kondo insulator cannot fight against very weak exterior perturbations. First, by using standard bosonization techniques, we prove that an antiferromagnetic ground state occurs by doping with few holes; it is characterized by a form factor of the spin-spin correlation functions which exhibits two structures respectively at $q=\pi$ and $q=2k_F$. Second, we prove precisely by using renormalization group methods that the Anderson-localization inevitably takes place in that weak-coupling Haldane system, by the introduction of quenched randomness; the spin-fixed point rather corresponds to a ``glass'' state. Finally, a weak-coupling ``analogue'' of the S=1 antiferromagnet Kondo insulator is proposed; we show that the transition into the Anderson-localization state may be avoided in that unusual weak-coupling Haldane system.",9710166v1 1997-10-24,Magnetization Switching in Single-Domain Ferromagnets,"A model for single-domain uniaxial ferromagnetic particles with high anisotropy, the Ising model, is studied. Recent experimental observations have been made of the probability that the magnetization has not switched. Here an approach is described in which it is emphasized that a ferromagnetic particle in an unfavorable field is in fact a metastable system, and the switching is accomplished through the nucleation and subsequent growth of localized droplets. Nucleation theory is applied to finite systems to determine the coercivity as a function of particle size and to calculate the probability of not switching. Both of these quantities are modified by different boundary conditions, magnetostatic interactions, and quenched disorder.",9710267v1 1997-12-29,Ferromagnetic transition and phase diagram of the one-dimensional Hubbard model with next-nearest-neighbor hopping,"We study the phase diagram of the one-dimensional Hubbard model with next-nearest-neighbor hopping using exact diagonalization, the density-matrix renormalization group, the Edwards variational ansatz, and an adaptation of weak-coupling calculations on the two-chain Hubbard model. We find that a substantial region of the strong-coupling phase diagram is ferromagnetic, and that three physically different limiting cases are connected in one ferromagnetic phase. At a point in the phase diagram at which there are two Fermi points at weak coupling, we study carefully the phase transition from the paramagnetic state to the fully polarized one as a function of the on-site Coulomb repulsion. We present evidence that the transition is second order and determine the critical exponents numerically. In this parameter regime, the system can be described as a Luttinger liquid at weak coupling. We extract the Luttinger-liquid parameters and show how their behavior differs from that of the nearest-neighbor Hubbard model. The general weak-coupling phase diagram can be mapped onto that of the two-chain Hubbard model. We exhibit explicitly the adapted phase diagram and determine its validity by numerically calculating spin and charge gaps using the density-matrix renormalization group.",9712310v1 1998-02-10,Ferromagnetism in a Hubbard model for an atomic quantum wire: a realization of flat-band magnetism from even-membered rings,"We have examined a Hubbard model on a chain of squares, which was proposed by Yajima et al as a model of an atomic quantum wire As/Si(100), to show that the flat-band ferromagnetism according to a kind of Mielke-Tasaki mechanism should be realized for an appropriate band filling in such a non-frustrated lattice. Reflecting the fact that the flat band is not a bottom one, the ferromagnetism vanishes, rather than intensified, as the Hubbard U is increased. The exact diagonalization method is used to show that the critical value of U is in a realistic range. We also discussed the robustness of the magnetism against the degradation of the flatness of the band.",9802101v2 1998-02-16,Ordering of localized moments in Kondo lattice models,"We describe the transition from a ferromagnetic phase, to a disordered para- magnetic phase, which occurs in one-dimensional Kondo lattice models with partial conduction band filling. The transition is the quantum order-disorder transition of the transverse-field Ising chain, and reflects double-exchange ordered regions of localized spins being gradually destroyed as the coupling to the conduction electrons is reduced. For incommensurate conduction band filling, the low-energy properties of the localized spins near the transition are dominated by anomalous ordered (disordered) regions of localized spins which survive into the paramagnetic (ferromagnetic) phase. Many interesting properties follow, including a diverging susceptibility for a finite range of couplings into the paramagnetic phase. Our critical line equation, together with numerically determined transition points, are used to determine the range of the double-exchange interaction. Models we consider are the spin 1/2 Kondo lattices with antiferromagnetic (Kondo) coupling, with ferromagnetic (Hund's rule) coupling, and the Kondo lattice with repulsive interactions between the conduction electrons.",9802164v1 1998-04-20,Double-exchange model: phase separation versus canted spins,"We study the competition between different possible ground states of the double-exchange model with strong ferromagnetic exchange interaction between itinerant electrons and local spins. Both for classical and quantum treatment of the local spins the homogeneous canted state is shown to be unstable against a phase separation. The conditions for the phase separation into the mixture of the antiferromagnetic and ferromagnetic/canted states are given. We also discuss another possible realization of the phase-separated state: ferromagnetic polarons embedded into an antiferromagnetic surrounding. The general picture of a percolated state, which emerges from these considerations, is discussed and compared with results of recent experiments on doped manganaties.",9804213v3 1998-05-05,Fokker-Planck and Landau-Lifshitz-Bloch Equations for Classical Ferromagnets,"A macroscopic equation of motion for the magnetization of a ferromagnet at elevated temperatures should contain both transverse and longitudinal relaxation terms and interpolate between Landau-Lifshitz equation at low temperatures and the Bloch equation at high temperatures. It is shown that for the classical model where spin-bath interactions are described by stochastic Langevin fields and spin-spin interactions are treated within the mean-field approximation (MFA), such a ``Landau-Lifshitz-Bloch'' (LLB) equation can be derived exactly from the Fokker-Planck equation, if the external conditions change slowly enough. For weakly anisotropic ferromagnets within the MFA the LLB equation can be written in a macroscopic form based on the free-energy functional interpolating between the Landau free energy near T_C and the ``micromagnetic'' free energy, which neglects changes of the magnetization magnitude |{\bf M}|, at low temperatures.",9805054v2 1998-06-01,Composite Polarons in Ferromagnetic Narrow-band Metallic Manganese Oxides,"A new mechanism is proposed to explain the colossal magnetoresistance and related phenomena. Moving electrons accompanied by Jahn-Teller phonon and spin-wave clouds may form composite polarons in ferromagnetic narrow-band manganites. The ground-state and finite-temperature properties of such composite polarons are studied in the present paper. By using a variational method, it is shown that the energy of the system at zero temperature decreases with the formation of composite polaron; the energy spectrum and effective mass of the composite polaron at finite temperature is found to be strongly renormalized by the temperature and the magnetic field. It is suggested that the composite polaron contribute significantly to the transport and the thermodynamic properties in ferromagnetic narrow-band metallic manganese oxides.",9806017v1 1998-06-30,Effects of doping on spin correlations in the periodic Anderson model,"We studied the effects of hole doping on spin correlations in the periodic Anderson model, mainly at the full and three-quarters-full lower bands cases. In the full lower band case, strong anti-ferromagnetic correlations develop when the on-site repulsive interaction strength $U$ becomes comparable to the quasi-particle band width. In the three-quarters full case, a novel kind of spin correlation develops that is consistent with the resonance between a $(\pi,0)$ and a $(0,\pi)$ spin-density wave. In this state the spins on different sublattices appear uncorrelated. Hole doping away from the completely full case rapidly destroys the long-range anti-ferromagnetic correlations, in a manner reminiscent of the destruction of anti-ferromagnetism in the Hubbard model. In contrast to the Hubbard model, the doping does not shift the peak in the magnetic structure factor from the $(\pi,\pi)$ position. At dopings intermediate to the full and three-quarters full cases, only weak spin correlations exist.",9806374v1 1998-07-02,"Diamagnetic Effects, Spin Dependent Fermi Surfaces, and the Giant Magnetoresistance in Metallic Multilayers","We study the role of diamagnetic effects on the transport properties of metallic magnetic multilayers to elucidate whether they can explain the Giant Magnetoresistance (GMR) effect observed in those systems. Realistic Fermi surface topologies in layered ferromagnets are taken into account, with the possibilities of different types of orbits depending on the electron spin. Both configurations, with ferromagnetic and anti-ferromagnetic couplings between magnetic layers, are considered and the transmission coefficient for scattering at the interface boundary is modelled to include magnetic and roughness contributions. We assume that scattering processes conserve the electron spin, due to large spin diffusion lengths in multilayer samples. Scattering from the spacer mixes different orbit topologies in a way similar to magnetic `breakdown' phenomena. For antiferromagnetic coupling, majority and minority spins are interchanged from one magnetic layer to the next. Cyclotron orbits are also traveled in opposite directions, producing a compensation-like effect that yields a huge GMR, particularly for closed orbits. For open orbits, one may get the `inverse' magnetoresistance effect along particular directions.",9807026v1 1998-07-16,Field Theory of Skyrme Lattices in Quantum Hall Ferromagnets,"We report the application of the nonlinear $\sigma$ model to study the multi-skyrmion problem in the quantum Hall ferromagnet system. We make use of a first-principle calculation to derive an analytical form for the inter-skyrmionic interaction to show that the ground state of the system can be described by a ferromagnet triangular Skyrme lattice near $\nu=1$ where skyrmions are extremely dilute and a continuous transition into antiferromagnet square lattice occurs by increasing the skyrmion density and therefore $|\nu-1|$. Using these results we demonstrate that the transition for a triangular to a square lattice which was previously derived, using the Hartree-Fock method, can also be seen in the field theory picture. We investigate the possibility that the skyrmions bound in pair to make a bi-skyrmion triangular lattice when the Zeeman energy is extremely small. We show that the energy of a skyrmion with charge $Q$ is less than the energy of $Q$ skyrmions each with charge one when the short range interaction among them is considered. By taking the quantum fluctuations into account, we also argue the possibility of the existence of a %SMG superconductor-insulator and the non-zero temperature phase transitions.",9807236v1 1998-07-17,Optical conductivity of colossal magnetorestistance compounds: Role of orbital degeneracy in the ferromagnetic phase,"Recent optical conductivity $\sigma(\omega)$ experiments have revealed an anomalous spectral distribution in the ferromagnetic phase of the perovskite system $La_{1-x}Sr_xMnO_3$. Using finite temperature diagonalization techniques we investigate $\sigma(\omega)$ for a model that contains only the $e_g$-orbital degrees of freedom. Due to strong correlations the orbital model appears as a generalized t-J model with anisotopic interactions and 3-site hopping. In the orbital t-J model $\sigma(\omega)$ is characterized by a broad incoherent spectrum with increasing intensity as temperature is lowered, and a Drude peak with small weight, consistent with experiment. Our calculations for two-dimensional systems, which may have some particular relevance for the double-layer manganites, show that the scattering from orbital fluctuations can explain the order of magnitude of the incoherent part of $\sigma(\omega)$ in the low temperature ferromagnetic phase. Moreover orbital correlation functions are studied and it is shown that $x^2$-$y^2$ orbital order is prefered in the doped planar model at low temperature.",9807255v1 1998-08-07,Electromagnetic Interactions in the Quantum Hall Ferromagnet,"The $\nu =1$ quantum Hall ground state in samples like GaAs is well known to be ferromagnetic. The global SU(2) spin symmetry of the microscopic action is broken down to a U(1) symmetry by the ground state. The Goldstone bosons corresponding to this spontaneous breaking of symmetry are the ferromagnetic magnons which are neutral spin waves. In addition, there are topologically nontrivial, electrically charged spin excitations known as spin skyrmions, which in these samples are the favoured charge carriers. In this letter, we look into the electromagnetic coupling of these spin excitations. The discrete symmetries P and T are also broken by the ground state and we find that to the leading order, the electromagnetic interaction of the spin waves occurs through a nonminimal coupling given by a Chern-Simons like term containing both the electromagnetic potentials and the two Euler angles that specify the coset SU(2)/U(1) and thereby, the Goldstone bosons.",9808081v1 1998-09-05,Griffith Singularities and the Replica Instantons in the Random Ferromagnet,"The problem of existence of non-analytic (Griffith-like) contributions to the free energy of weakly disordered Ising ferromagnet is studied from the point of view of the replica theory. The consideration is done in terms of the usual random temperature Ginzburg-Landau Hamiltonian in space dimensions $D < 4$ in the zero external magnetic field. It is shown that in the paramagnetic phase, at temperatures not too close to $T_{c}$ (where the behaviour of the pure system is correctly described by the Gaussian approximation), the free energy of the system has additional non-perturbative contribution of the form $\exp{-(const) \frac{\tau^{(4-D)/2}}{u}}$ (where $\tau = (T-T_{c})/T_{c}$), which has essential singularity in the parameter $u \to 0$ which describes the strength of the disorder. It is demonstrated that this contribution appears due to non-linear localized (instanton-like) solutions of the mean-field stationary equations which are characterized by the special type of the replica symmetry breaking . It is argued that physically these replica instantons describe the contribution from rare spatial ""ferromagnetic islands"" in which local (random) temperature is below $T_{c}$",9809097v1 1998-10-06,Magnetoresistance of Granular Ferromagnets - Observation of a Magnetic Proximity Effect?,"We have observed a superparamagnetic to ferromagnetic transition in films of isolated Ni grains covered by non-magnetic overlayers. The magnetoresistance (MR) of the films was measured as a function of the overlayer thickness. Initially, the granular Ni films exhibited negative MR curves peaked at H=0. As different materials were deposited onto the grains hysteresis developed in the MR. This behavior is ascribed to an increase of the typical domain size due to magnetic coupling between grains. The strength of the inter-grain coupling is found to correlate with the magnetic susceptibility of the overlayer material. We discuss possible mechanisms for this coupling and suggest that the data may reflect the existence of a magnetic proximity effect (analogous to the well-known effect in superconductivity) in which a ferromagnetic moment is induced in the metallic non-magnetic medium.",9810061v1 1998-11-17,The Anti-Jahn-Teller Polaron in LaMnO$_3$,"Distortions of the oxygen sublattice couple to $e_g$ orbitals of Mn$^{3+}$ and drive a cooperative Jahn-Teller (orbital ordering) transition in LaMnO$_3$. A simple model for this transition is studied. Without further adjustment, the model predicts the shape and stability of small (anti-Jahn-Teller) polarons which form when holes are doped into the material. This leads to a new description of the lightly doped insulator, the anti-ferromagnetic to ferromagnetic transition, and the metal-insulator transition.",9811250v3 1998-12-27,Two Ferromagnetic States in Magnetoresistive Manganites - First Order Transition Driven by Orbitals -,"A systematic study of the electronic structure in perovskite manganites is presented. The effective Hamiltonian is derived by taking into account the degeneracy of $e_g$ orbitals and strong electron correlation in Mn ions. The spin and orbital orderings are examined as functions of carrier concentration in the mean-field approximation applied to the effective Hamiltonian. We obtain the first order phase transition between ferromagnetic metallic and ferromagnetic insulating states in the lightly doped region. The transition is accompanied with the orbital order-disorder one which is directly observed in the anomalous X-ray scattering experiments. The present investigation shows a novel role of the orbital degree of freedom on metal-insulator transition in manganites.",9812405v1 1999-01-06,``Smoke Rings'' in Ferromagnets,"It is shown that bulk ferromagnets support propagating non-linear modes that are analogous to the vortex rings, or ``smoke rings'', of fluid dynamics. These are circular loops of {\it magnetic} vorticity which travel at constant velocity parallel to their axis of symmetry. The topological structure of the continuum theory has important consequences for the properties of these magnetic vortex rings. One finds that there exists a sequence of magnetic vortex rings that are distinguished by a topological invariant (the Hopf invariant). We present analytical and numerical results for the energies, velocities and structures of propagating magnetic vortex rings in ferromagnetic materials.",9901037v2 1999-01-07,Interplay between spin-relaxation and Andreev reflection in ferromagnetic wires with superconducting contacts,"We analyze the change in the resistance of a junction between a diffusive ferromagnetic (F) wire and normal metal electrode, due to the onset of superconductivity (S) in the latter and a double Andreev scattering process leading to a complete internal reflection of a large fraction of the spin-polarized electrons back into the ferromagnet. The superconducting transition results in an additional contact resistance arising from the necessity to match spin-polarized current in F-wire to spin-less current in S-reservoir, which is comparable to the resistance of a piece of a F-wire with the length equal to the spin-relaxation length.",9901051v1 1999-01-08,Quantum Ferrimagnets,"We study quantum ferrimagnets in one, two, and three dimensions by using a variety of methods and approximations. These include: (i) a treatment based on the spin coherent state path-integral formulation of quantum ferrimagnets by taking into account the leading order quantum and thermal fluctuations (ii) a field-theoretical (non-linear $\sigma$-model type) formulation of the special case of one-dimensional quantum ferrimagnets at zero temperature (iii) an effective description in terms of dimers and quantum rotors, and (iv) a quantum renormalization group study of ferrimagnetic Heisenberg chains. Some of the formalism discussed here can be used for a unified treatment of both ferromagnets and antiferromagnets in the semiclassical limit. We show that the low (high) energy effective Hamiltonian of a (S_1, S_2) Heisenberg ferrimagnet is a ferromagnetic (antiferromagnetic) Heisenberg model. We also study the phase diagram of quantum ferrimagnets in the presence of an external magnetic field h ($h_{c1} < h < h_{c2}$) and show that the low- and the high-field phases correspond respectively to the classical N\'eel and the fully polarized ferromagnetic states. We also calculate the transition temperature for the Berezinskii-Kosterlitz-Thouless phase transition in the special case of two-dimensional quantum ferrimagnets.",9901063v2 1999-01-11,Temperature dependence of the resistivity in the double-exchange model,"The resistivity around the ferromagnetic transition temperature in the double exchange model is studied by the Schwinger boson approach. The spatial spin correlation responsible for scattering of conduction electrons are taken into account by adopting the memory function formalism. Although the correlation shows a peak lower than the transition temperature, the resistivity in the ferromagnetic state monotonically increases with increasing temperature due to a variation of the electronic state of the conduction electron. In the paramagnetic state, the resistivity is dominated by the short range correlation of scattering and is almost independent of the temperature. It is attributed to a cancellation between the nearest-neighbor spin correlation, the fermion bandwidth, and the fermion kinetic energy. This result implies the importance of the temperature dependence of the electronic states of the conduction electron as well as the localized spin states in both ferromagnetic and paramagnetic phases.",9901078v1 1999-01-11,Coexistence of ferromagnetism and superconductivity in the hybrid ruthenate-cuprate compound RuSr_2GdCu_2O_8 studied by muon spin rotation (μ SR) and DC-magnetization,"We have investigated the magnetic and the superconducting properties of the hybrid ruthenate-cuprate compound RuSr_{2}GdCu_{2}O_{8} by means of zero-field muon spin rotation- (ZF-$\mu $SR) and DC magnetization measurements. The DC-magnetisation data establish that this material exhibits ferromagnetic order of the Ru-moments ($\mu (Ru) \approx 1 \mu_{B}$) below T_{Curie} = 133 K and becomes superconducting at a much lower temperature T_c = 16 K. The ZF-$\mu $SR experiments indicate that the ferromagnetic phase is homogeneous on a microscopic scale and accounts for most of the sample volume. They also suggest that the magnetic order is not significantly modified at the onset of superconductivity.",9901084v2 1999-01-16,Transition between two ferromagnetic states driven by orbital ordering in La_{0.88}Sr_{0.12}MnO_3,"A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a novel ferromagnetic insulator.We identify that the key parameter in the transition is the orbital degree of freedom in e_g electrons. By utilizing the resonant x-ray scattering technique, orbital ordering is directly detected below T_{OO}, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites.",9901148v1 1999-03-07,Correlation between Spin Polarization and Magnetic Moment in Ferromagnetic Alloys,"The correlation between the magnetic moment in ferromagnetic alloys and the tunneling spin polarization in ferromagnet-insulator-superconductor tunneling experiments has been a mystery. The measured spin polarization for Fe, Co, Ni, and various Ni alloys is positive and roughly proportional to their magnetic moments, which can not be explained by considering the net density of states. Using a tight-binding coherent potential approximation (CPA) model, we show that while the polarization of the net density of states is not correlated with the magnetic moment, the polarization of the density of states of {\it s} electrons is correlated with the magnetic moment in the same manner as observed by the tunneling experiments. We also discuss the spin polarization measurements by Andreev reflection experiments, some of which obtained different results from the tunneling experiments and our calculations.",9903118v2 1999-03-09,Monte Carlo study of a two-dimensional quantum ferromagnet,"We present quantum Monte Carlo results for the field and temperature dependence of the magnetization and the spin-lattice relaxation rate $1/T_1$ of a two-dimensional $S=1/2$ quantum Heisenberg ferromagnet. The Monte Carlo method, which yields results free of systematic errors, is described in detail. The high accuracy of the calculated magnetization allows for stringent tests of recent approximate analytical calculations. We also compare our results with recent experimental data for a $\nu=1$ quantum Hall ferromagnet, which is expected to be well described by the Heisenberg model. The dynamic response function needed to extract $1/T_1$ is obtained using maximum-entropy analytic continuation of the corresponding imaginary-time dependent correlation function. We discuss the reliability of this approach.",9903148v1 1999-03-12,On the Coexistence in RuSr2GdCu2O8 of Superconductivity and Ferromagnetism,"We review the reasons that make superconductivity unlikely to arise in a ferromagnet. Then, in light of the report by Tallon and collaborators that RuSr2GdCu2O8 becomes superconducting at approximately 35 K which is well below the Curie temperature of 132 K, we consider whether the objections really apply to this compound. Our considerations are supported by local spin density calculations for this compound, which indeed indicate a ferromagnetic RuO2 layer. The Ru moment resides in t_2g orbitals but is characteristic of itinerant magnetism (and is sensitive to choice of exchange-correlation potential and to the atomic positions). Based on the small exchange splitting that is induced in the Cu-O layers, the system seems capable of supporting singlet superconductivity an FFLO-type order parameter and possibly a pi-phase alternation between layers. If instead the pairing is triplet in the RuO2 layers, it can be distinguished by a spin-polarized supercurrent. Either type of superconductivity seems to imply a spontaneous vortex phase if the magnetization is rotated out of the plane.",9903210v2 1999-04-29,Spin glass behavior upon diluting frustrated magnets and spin liquids: a Bethe-Peierls treatment,"A Bethe-Peierls treatment to dilution in frustrated magnets and spin liquids is given. A spin glass phase is present at low temperatures and close to the percolation point as soon as frustration takes a finite value in the dilute magnet model; the spin glass phase is reentrant inside the ferromagnetic phase. An extension of the model is given, in which the spin glass / ferromagnet phase boundary is shown not to reenter inside the ferromagnetic phase asymptotically close to the tricritical point whereas it has a turning point at lower temperatures. We conjecture similar phase diagrams to exist in finite dimensional models not constraint by a Nishimori's line. We increase frustration to study the effect of dilution in a spin liquid state. This provides a ``minimal'' ordering by disorder from an Ising paramagnet to an Ising spin glass.",9904432v2 1999-05-18,From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model,"The magnetic properties of the two-channel periodic Anderson model for uranium ions, comprised of a quadrupolar and a magnetic doublet are investigated through the crossover from the mixed-valent to the stable moment regime using dynamical mean field theory. In the mixed-valent regime ferromagnetism is found for low carrier concentration on a hyper-cubic lattice. The Kondo regime is governed by band magnetism with small effective moments and an ordering vector $\q$ close to the perfect nesting vector. In the stable moment regime nearest neighbour anti-ferromagnetism dominates for less than half band filling and a spin density wave transition for larger than half filling. $T_m$ is governed by the renormalized RKKY energy scale $\mu_{eff}^2 ^2 J^2\rho_0(\mu)$.",9905260v1 1999-06-23,Spin-orbit coupling effect on quantum Hall ferromagnets with vanishing Zeeman energy,"We present the phase diagram of a ferromagnetic quantum Hall effect liquid in a narrow quantum well with vanishing single-particle Zeeman splitting, $\epsilon_{{\rm Z}}$ and pronounced spin-orbit coupling. Upon decreasing $\epsilon_{{\rm Z}}$, the spin-polarization field of a liquid takes, first, the easy-axis configuration, followed by the formation of a helical state, which affects the transport and NMR properties of a liquid and the form of topological defects in it. The analysis is extended over high odd integer filling factors.",9906354v2 1999-08-13,Theory of spin effect on Coulomb blockade of single-electron tunneling in ferromagnetic system,"Spin dependent single electron tunneling in a ferromagnetic double junction is investigated theoretically in the limit of incoherent sequential tunneling. The junction consists of a small nonmagnetic metallic grain with discrete energy levels, which is connected to two ferromagnetic electrodes. We have developed a new theoretical model taking into account charge as well as spin degrees of freedom. The model allows us to investigate new phenomena such as spin accumulation and spin fluctuations.",9908196v1 1999-08-13,On the magnetic stability at the surface in strongly correlated electron systems,"The stability of ferromagnetism at the surface at finite temperatures is investigated within the strongly correlated Hubbard model on a semi-infinite lattice. Due to the reduced surface coordination number the effective Coulomb correlation is enhanced at the surface compared to the bulk. Therefore, within the well-known Stoner-picture of band ferromagnetism one would expect the magnetic stability at the surface to be enhanced as well. However, by taking electron correlations into account well beyond the Hartree-Fock (Stoner) level we find the opposite behavior: As a function of temperature the magnetization of the surface layer decreases faster than in the bulk. By varying the hopping integral within the surface layer this behavior becomes even more pronounced. A reduced hopping integral at the surface tends to destabilize surface ferromagnetism whereas the magnetic stability gets enhanced by an increased hopping integral. This behavior represents a pure correlation effect and can be understood in terms of general arguments which are based on exact results in the limit of strong Coulomb interaction.",9908198v1 1999-08-13,Spin Wave Theory of Double Exchange Ferromagnets,"We construct the 1/S spin-wave expansion for double exchange ferromagnets at T=0. It is assumed that the value of Hund's rule coupling, J_H, is sufficiently large, resulting in a fully saturated, ferromagnetic half-metallic ground state. We evaluate corrections to the magnon dispersion law, and we also find that, in contrast to earlier statements in the literature, magnon-electron scattering does give rise to spin wave damping. We analyse the momentum dependence of these quantities and discuss the experimental implications for colossal magnetoresistance compounds.",9908202v3 1999-08-13,Influence of uncorrelated overlayers on the magnetism in thin itinerant-electron films,"The influence of uncorrelated (nonmagnetic) overlayers on the magnetic properties of thin itinerant-electron films is investigated within the single-band Hubbard model. The Coulomb correlation between the electrons in the ferromagnetic layers is treated by using the spectral density approach (SDA). It is found that the presence of nonmagnetic layers has a strong effect on the magnetic properties of thin films. The Curie temperatures of very thin films are modified by the uncorrelated overlayers. The quasiparticle density of states is used to analyze the results. In addition, the coupling between the ferromagnetic layers and the nonmagnetic layers is discussed in detail. The coupling depends on the band occupation of the nonmagnetic layers, while it is almost independent of the number of the nonmagnetic layers. The induced polarization in the nonmagnetic layers shows a long-range decreasing oscillatory behavior and it depends on the coupling between ferromagnetic and nonmagnetic layers.",9908203v2 1999-08-30,Spin-polarized tunneling of La0.67Sr0.33MnO3/YBa2Cu3O7-d junctions,"The transport properties between ferromagnets and high-Tc superconductors are investigated in La0.67Sr0.33MnO3/YBa2Cu3O7-d (LSMO/YBCO)junctions in the geometry of cross-strip lines. The conductance spectra show zero-bias conductance peaks (ZBCP), reflecting the charge transport in the ab-plane. When an external magnetic field is applied to the junctions, the conductance spectra show two notable features, i.e., an increase of background conductance and an asymmetric ZBCP splitting whose amplitude responds nonlinearly to the applied field. It is shown that the magnetic field response are consistent with a theoretical prediction of tunneling spectroscopy when the presence of a ferromagnetic barrier between a spin-polarized ferromagnet and a d-wave superconductor is assumed.",9908431v1 1999-10-21,Dynamics of Excited Electrons in Copper and Ferromagnetic Transition Metals: Theory and Experiment,"Both theoretical and experimental results for the dynamics of photoexcited electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and Ni are presented. A model for the dynamics of excited electrons is developed, which is based on the Boltzmann equation and includes effects of photoexcitation, electron-electron scattering, secondary electrons (cascade and Auger electrons), and transport of excited carriers out of the detection region. From this we determine the time-resolved two-photon photoemission (TR-2PPE). Thus a direct comparison of calculated relaxation times with experimental results by means of TR-2PPE becomes possible. The comparison indicates that the magnitudes of the spin-averaged relaxation time \tau and of the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation times for the different ferromagnetic transition metals result not only from density-of-states effects, but also from different Coulomb matrix elements M. Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.",9910326v2 1999-10-26,Theory of Spin-Resolved Auger-Electron Spectroscopy from Ferromagnetic 3d-Transition Metals,"CVV Auger electron spectra are calculated for a multi-band Hubbard model including correlations among the valence electrons as well as correlations between core and valence electrons. The interest is focused on the ferromagnetic 3d-transition metals. The Auger line shape is calculated from a three-particle Green function. A realistic one-particle input is taken from tight-binding band-structure calculations. Within a diagrammatic approach we can distinguish between the \textit{direct} correlations among those electrons participating in the Auger process and the \textit{indirect} correlations in the rest system. The indirect correlations are treated within second-order perturbation theory for the self-energy. The direct correlations are treated using the valence-valence ladder approximation and the first-order perturbation theory with respect to valence-valence and core-valence interactions. The theory is evaluated numerically for ferromagnetic Ni. We discuss the spin-resolved quasi-particle band structure and the Auger spectra and investigate the influence of the core hole.",9910421v1 1999-11-05,Magnetic Order in the Double Exchange Model in Infinite Dimensions,"We studied magnetic properties of the double exchange (DE) model with S=1/2 localized spins at T=0, using exact diagonalization in the framework of the dynamical mean field theory. Obtained phase diagram contains ferromagnetic, antiferromagnetic and paramagnetic phases. Comparing the phase diagram with that of the DE model with classical localized spins, we found that the quantum fluctuations of localized spins partly destabilize the ferromagnetism and expand the paramagnetic phase region. We found that phase separations occur between the antiferromagnetic and paramagnetic phases as well as the paramagnetic and ferromagnetic ones.",9911091v2 1999-11-12,Ferromagnetic Liquid Thin Films Under Applied Field,"Theoretical calculations, computer simulations and experiments indicate the possible existence of a ferromagnetic liquid state, although definitive experimental evidence is lacking. Should such a state exist, demagnetization effects would force a nontrivial magnetization texture. Since liquid droplets are deformable, the droplet shape is coupled with the magnetization texture. In a thin-film geometry in zero applied field, the droplet has a circular shape and a rotating magnetization texture with a point vortex at the center. We calculate the elongation and magnetization texture of such ferromagnetic thin film liquid droplet confined between two parallel plates under a weak applied magnetic field. The vortex stretches into a domain wall and exchange forces break the reflection symmetry. This behavior contrasts qualitatively and quantitatively with the elongation of paramagnetic thin films.",9911202v1 1999-11-30,Two-dimensional frustrated Heisenberg model: Variational study,"The stability of the ferromagnetic phase of the 2D quantum spin-1/2 model with nearest-neighbor ferro- and next-nearest neighbor antiferromagnetic interactions is studied. It turns out that values of exchange integrals at which the ferromagnetic state becomes unstable with respect to a creation of one and two magnon are different. This difference shows that the classical approximation is inapplicable to the study of the transition from the ferromagnetic to the singlet state in contrast with 1D case. This problem is investigated using a variational function of new type. It is based on the boson representation of spin operators which is different from the Holstein-Primakoff approximation. This allows us to obtain the accurate estimate of the transition point and to study the character of the phase transition.",9911491v1 1999-12-02,Electrical transport in the ferromagnetic state of manganites: Small-polaron metallic conduction at low temperatures,"We report measurements of the resistivity in the ferromagnetic state of epitaxial thin films of La_{1-x}Ca_{x}MnO_{3} and the low temperature specific heat of a polycrystalline La_{0.8}Ca_{0.2}MnO_{3}. The resistivity below 100 K can be well fitted by \rho - \rho_{o} = E \omega_{s}/sinh^{2}(\hbar \omega_{s}/2k_{B}T) with \hbar \omega_{s}/k_{B} \simeq 100 K and E being a constant. Such behavior is consistent with small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode. The specific heat data also suggest the existence of such a phonon mode. The present results thus provide evidence for small-polaron metallic conduction in the ferromagnetic state of manganites.",9912037v1 1999-12-17,Ferromagnetic Semiconductor - Singlet (or Triplet) Superconductor - Ferromagnetic Semiconductor Systems as Possible Logic Circuits and Switches,"We consider thin superconducting (S) films of thickness d $\ll \xi_{0}$, sandwiched between two ferromagnetic semiconducting insulators (FI) with differently orientated magnetizations - the FI-S-FI system. We calculate the dependence of the superconducting critical temperature on the orientation of the magnetization in the insulators and on the thickness of the superconducting film. The calculations are done for singlet as well as triplet superconductors. In the singlet case T_{c} depends on the relative orientation of the left and right magnetization only, while in the triplet case T_{c} depends on the absolute orientation of magnetization. The latter property can serve as a kind of spin-spectroscopy of triplet and unconventional superconductors, for instance in resolving the structure of the triplet order parameter in the recently discovered layered superconductor Sr_{2}RuO_{4}. The possibility of logic circuits and switches, which are based on the FI-S-FI systems with arbitrary orientation of magnetizations in FI films, is analyzed too.",9912315v1 2000-01-04,Itinerant ferromagnetism in half-metallic CoS_2,"We have investigated electronic and magnetic properties of the pyrite-type CoS_2 using the linearized muffin-tin orbital (LMTO) band method. We have obtained the ferromagnetic ground state with nearly half-metallic nature. The half-metallic stability is studied by using the fixed spin moment method. The non-negligible orbital magnetic moment of Co 3d electrons is obtained as $\mu_L = 0.06 \mu_B$ in the local spin density approximation (LSDA). The calculated ratio of the orbital to spin angular momenta /< S_z > = 0.15 is consistent with experiment. The effect of the Coulomb correlation between Co 3d electrons is also explored with the LSDA + U method. The Coulomb correlation at Co sites is not so large, $U \lesssim 1$ eV, and so CoS_2 is possibly categorized as an itinerant ferromagnet. It is found that the observed electronic and magnetic behaviors of CoS_2 can be described better by the LSDA than by the LSDA + U.",0001028v1 2000-01-21,Superconductivity in a Ferromagnetic Layered Compound,"We examine superconductivity in layered systems with large Fermi-surface splitting due to coexisting ferromagnetic layers. In particular, the hybrid ruthenate-cuprate compound RuSr_2GdCu_2O_8 is examined on the coexistence of the superconductivity and the ferromagnetism, which has been observed recently. We calculate critical fields of the superconductivity taking into account the Fulde-Ferrell-Larkin-Ovchinnikov state in a model with Fermi-surfaces which shapes are similar to those obtained by a band calculation. It is shown that the critical field is enhanced remarkably due to a Fermi-surface effect, and can be high enough to make the coexistence possible in a microscopic scale. We also clarify the direction of the spatial oscillation of the order parameter, which may be observed by scanning tunneling microscope experiments.",0001318v2 2000-02-17,"Excitonic order at strong-coupling: pseudo-spins, doping, and ferromagnetism","A tight binding model is introduced to describe the strong interaction limit of excitonic ordering. At stoichiometry, the model reduces in the strong coupling limit to a pseudo-spin model with approximate U(4) symmetry. Excitonic order appears in the pseudo-spin model as in-plane pseudo-magnetism. The U(4) symmetry unifies all possible singlet and triplet order parameters describing such states. Super-exchange, Hunds-rule coupling, and other perturbations act as anisotropies splitting the U(4) manifold, ultimately stabilizing a paramagnetic triplet state. The tendency to ferromagnetism with doping (observed experimentally in the hexaborides) is explained as a spin-flop transition to a different orientation of the U(4) order parameter. The physical mechanism favoring such a reorientation is the enhanced coherence (and hence lower kinetic energy) of the doped electrons in a ferromagnetic background relative to the paramagnet. A discussion of the physical meaning of various excitonic states and their experimental consequences is also provided.",0002259v1 2000-02-29,Magnetic properties of the Hubbard model on three-dimensional lattices: fluctuation-exchange and two-particle self-consistent studies,"The relation between three-dimensional lattice structure and magnetism in correlated electron systems is explored for face centered cubic (FCC), body centered cubic (BCC), and simple cubic (SC) lattices. In particular, we question which lattice structure has the strongest tendency toward the ferromagnetism or antiferromagnetism. We employ the Hubbard model to calculate the spin susceptibility and the single-particle spectrum with the fluctuation-exchange (FLEX) and the two-particle self-consistent (TPSC) approximations in the weak coupling regime. We have shown that (i) ferromagnetic spin fluctuations become dominant when the Fermi level lies around a sharp peak (divergence) in the density of states (D(E)) near the bottom of the band, which occurs for FCC with/without next nearest neighbor hoppings (t') or BCC with an appropriate value of t'. Among the cases studied, the ferromagnetic fluctuation is found to be the strongest for FCC with a finite t'. (ii) When the peak in D(E) resides around the band center as in bipartite SC or BCC, antiferromagnetic fluctuations become dominant when the band is close to the half-filling, with the fluctuation being much stronger in BCC.",0002441v1 2000-03-13,Electron - hole asymmetry and activation energy in quantum Hall ferromagnets,"We argue that the dissipative transport in ferromagnetic quantum Hall effect liquids at $\nu=2N+1$ is dominated by the thermal activation of pairs consisting of an electron and an antiskyrmion (topological texture which represents a hole with 'screened' exchange interaction), thus manifesting the lack of electron-hole symmetry in quantum Hall ferromagnets. We find that the activation energy of such a pair is not the exchange energy, but is determined by the interplay between the excess Zeeman energy of a skyrmion and the charging energy of its topological texture: $${\cal E}=a\epsilon_{{\rm Z}}^{1/3}E_{{\rm C}}^{2/3}\ln ^{1/3}(\frac{\Im_{i}}{E_{{\rm C}}^{2/3}\epsilon_{{\rm Z}}^{1/3}}), E_{{\rm C}}=\frac{e^{2}}{\chi \lambda}, $$ with $ a\approx 1.75$.",0003224v2 2000-04-10,Weak localization correction to the FS interface resistance,"The classical resistance of a contact between a ferromagnet (F) and a superconductor (S) acquires an additional contact term as compared to the contact between a ferromagnet and a normal metal. The necessity to match spin-polarized current in a ferromagnet to spin-less current in the superconductor results in the accumulation of non-equilibrium polarization near the F/S interface. In the present work, we show that the weak localization correction to the classical diffusion coefficient, $\delta D$, is dependent on the degree of polarization, with majority spins more likely to be reflected from the interface than minority spins. Taking into account the change in the spin polarized particle distribution in the F wire arising from $\delta D$, we calculate the weak localization correction to the F/S contact resistance.",0004140v1 2000-04-10,T=0 Phase Diagram of the Double-Exchange Model,"We present the T=0 phase diagram of the double-exchange model (ferromagnetic Kondo lattice model) for all values of the carrier concentration $n$ and Hund's couplng $J$, within dynamical mean field theory. We find that depending on the values of $n$ and $J$, the ground state is either a ferromagnet, a commensurate antiferromagnet or some other incommensurate phase with intermediate wave vectors . The antiferromagnetic phase is separated by first order phase boundaries and wide regimes of phase separation. The transition from the ferromagnetic phase to an incommensurate phase is second order.",0004151v2 2000-05-23,Spin tunneling of trigonal and hexagonal ferromagnets in an arbitrarily directed magnetic field,"The quantum tunneling of the magnetization vector are studied theoretically in single-domain ferromagnetic nanoparticles placed in an external magnetic field at an arbitrarily directed angle in the $ZX$ plane. We consider the magnetocrystalline anisotropy with trigonal and hexagonal crystal symmetry, respectively. By applying the instanton technique in the spin-coherent-state path-integral representation, we calculate the tunnel splittings, the tunneling rates and the crossover temperatures in the low barrier limit for different angle ranges of the external magnetic field ($\theta_{H}=\pi/2$, $\pi/2\ll\theta_{H}\ll\pi$, and $\theta_{H}=\pi$). Our results show that the tunnel splittings, the tunneling rates and the crossover temperatures depend on the orientation of the external magnetic field distinctly, which provides a possible experimental test for magnetic quantum tunneling in nanometer-scale single-domain ferromagnets.",0005373v1 2000-05-29,Quantum nucleation in ferromagnets with tetragonal and hexagonal symmetries,"The phenomenon of quantum nucleation is studied in a ferromagnet in the presence of a magnetic field at an arbitrary angle. We consider the magnetocrystalline anisotropy with tetragonal symmetry and that with hexagonal symmetry, respectively. By applying the instanton method in the spin-coherent-state path-integral representation, we calculate the dependence of the rate of quantum nucleation and the crossover temperature on the orientation and strength of the field for a thin film and for a bulk solid. Our results show that the rate of quantum nucleation and the crossover temperature depend on the orientation of the external magnetic field distinctly, which provides a possible experimental test for quantum nucleation in nanometer-scale ferromagnets.",0005491v2 2000-05-31,Magnetization plateaus in antiferromagnetic-(ferromagnetic)_{n} polymerized S=1/2 XXZ chains,"The plateau-non-plateau transition in the antiferromagnetic-(ferromagnetic)$_{n}$ polymerized $S=1/2$ XXZ chains under the magnetic field is investigated. The universality class of this transition belongs to the Brezinskii-Kosterlitz-Thouless (BKT) type. The critical points are determined by level spectroscopy analysis of the numerical diagonalization data for $4 \leq p \leq 13$ where $p(\equiv n+1)$ is the size of a unit cell. It is found that the critical strength of ferromagnetic coupling decreases with $p$ for small $p$ but increases for larger enough $p$. It is also found that the plateau for large $p$ is wide enough for moderate values of exchange coupling so that it should be easily observed experimentally. This is in contrast to the plateaus for $p = 3$ chains which are narrow for a wide range of exchange coupling even away from the critical point.",0005544v1 2000-07-04,Correlation Effects on the Double Exchange Model in a Ferromagnetic Metallic Phase,"The effect of the Hubbard interaction among conduction electrons on the double exchange model is investigated in a ferromagnetic metallic phase. Applying iterative perturbation theory to the Hubbard interaction within dynamical mean field theory, we calculate the one-particle spectral function and the optical conductivity, in which coherent-potential approximation is further used to treat the ferromagnetic Hund coupling between conduction electrons and localized spins. Identifying the decrease of the magnetization for the localized spin with the increase of the temperature, we discuss the temperature dependence of the one-particle spectrum and the optical conductivity. It is found that the interplay between the Hund coupling and the Hubbard interaction dramatically changes the spectral function, while it is somehow obscured in the optical conductivity.",0007041v1 2000-07-07,"Origin and pressure dependence of ferromagnetism in A2Mn2O7 pyrochlores (A=Y, In, Lu, and Tl)","Non-conventional mechanisms have been recently invoked in order to explain the ferromagnetic ground state of A2Mn2O7 pyrochlores (A=Y, In, Lu and Tl) and the puzzling decrease of their Curie temperatures with applied pressure. Here we show, using a perturbation expansion in the Mn-O hopping term, that both features can be understood within the superexhange model, provided that the intra-atomic oxygen interactions are properly taken into account. An additional coupling between the Mn ions mediated by the In(5s)/Tl(6s) bands yields the higher Tc's of these two compounds, this mechamism enhancing their ferromagnetism for higher pressures.",0007124v3 2000-07-11,Hole-mediated ferromagnetism in tetrahedrally coordinated semiconductors,"A mean field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account: (i) strong spin-orbit and kp couplings in the valence band; (ii) the effect of strain upon the hole density-of-states, and (iii) the influence of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that theory describes the values of the Curie temperatures observed in the studied systems as well as explain the directions of the easy axis and the magnitudes of the corresponding anisotropy fields as a function of biaxial strain. Furthermore, the model reproduces unusual sign, magnitude, and temperature dependence of magnetic circular dichroism in the spectral region of the fundamental absorption edge. Chemical trends and various suggestions concerning design of novel ferromagnetic semiconductor systems are described.",0007190v2 2000-07-26,Critical Temperature of Ferromagnetic Transition in Three-Dimensional Double-Exchange Models,"Ferromagnetic transition in three-dimensional double-exchange models is studied by the Monte Carlo method. Critical temperature $T_{\rm c}$ is precisely determined by finite-size scaling analysis. Strong spin fluctuations in this itinerant system significantly reduce $T_{\rm c}$ from mean-field estimates. By choosing appropriate parameters, obtained values of $T_{\rm c}$ quantitatively agree with experiments for the ferromagnetic metal regime of (La,Sr)MnO$_{3}$, which is a typical perovskite manganite showing colossal magnetoresistance. This indicates that the double-exchange mechanism alone is sufficient to explain $T_{\rm c}$ in this material. Critical exponents are also discussed.",0007408v2 2000-07-31,Carrier-induced ferromagnetism in p-Zn1-xMnxTe,"We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped Zn1-xMnxTe epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY) model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.",0007502v1 2000-08-10,Thermal properties of ferrimagnetic systems,"The heat capacity of some ferrimagnets has additional structures like a shoulder in the Schottky-like peak, or emergence of a second peak when an external magnetic field is applied. It is shown here that as long as spin wave-spin wave interactions are ignored in a ferrimagnet, the ferromagnetic and antiferromagnetic elementary excitation spectra give rise to two independent heat capacity peaks, one enveloped by the other, which add up to give the peak for the total system. Taking this into account helps understand the additional structures in the peaks. Moreover, the classification of ferrimagnets into predominantly antiferromagnetic, ferromagnetic, or a mixture of the two is shown to be validated by studying them under additional influences like dimerization and frustration. Because these two are shown to influence the ferromagnetic and antiferromagnetic dispersion relations - and hence the quantities like heat capacity and magnetic susceptibility - by different amounts, the characterisation of ferrimagnetic systems ($1,1/2$), ($3/2,1$) and ($3/2,1/2$) is brought out more clearly. Both these influences enhance antiferromagnetic character.",0008167v1 2000-08-24,Coupling of two superconductors through a ferromagnet : evidence for a pi-junction,"We report measurements of the temperature dependence of the critical current in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a Cu$_x$Ni$_{1-x}$ alloy, with $x$ around 0.5. With decreasing temperature $I_c$ generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which $I_c$ rises again. Such a sharp cusp can only be explained by assuming that the junction changes from a 0-phase state at high temperatures to a $\pi$-phase state at low temperatures.",0008364v1 2000-08-24,Ferromagnetic Polarons in Manganites,"Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J_h > t) and no binding in the weak limit (J_h \simeq t).",0008367v1 2000-08-30,Ferromagnetism in the Periodic Anderson Model - a Modified Alloy Analogy,"We introduce a new aproximation scheme for the periodic Anderson model (PAM). The modified alloy approximation represents an optimum alloy approximation for the strong coupling limit, which can be solved within the CPA-formalism. Zero-temperature and finite-temperature phase diagrams are presented for the PAM in the intermediate-valence regime. The diversity of magnetic properties accessible by variation of the system parameters can be studied by means of quasiparticle densities of states: The conduction band couples either ferro- or antiferromagneticaly to the f-levels. A finite hybridization is a necessary precondition for ferromagnetism. However, too strong hybridization generally suppresses ferromagnetism, but can for certain system parameters also lead to a semi-metallic state with unusual magnetic properties. By comparing with the spectral density approximation, the influence of quasiparticle damping can be examined.",0008441v1 2000-09-26,Soft tetragonal distortions in ferromagnetic Ni_2MnGa and related materials from first principles,"A detailed examination of the energy landscape, density of states and magnetic moment of tetragonally distorted ferromagnetic Ni_2MnGa was performed using first-principles local-spin-density (LSD) pseudopotential calculations, varying V as well as c/a. The energy of tetragonal Ni_2MnGa is found to be nearly constant for values of c/a over a wide range, with shallow minima near c/a = 1 and 1.08 in addition to that near 1.2. This flat energy surface is consistent with the wide range of observed values of c/a. It also explains the observation of pseudomorphic growth of Ni_2MnGa on GaAs, despite a nominal 3% lattice mismatch. The related materials Ni_2MnAl, Ni_2MnIn and ferromagnetic NiMn were examined for similar behavior, but all are seen to have a single well-defined minimum at c/a near 1, consistent with available experimental information. For NiMn, the ground state antiferromagnetic ordering and structural parameters are correctly predicted within the LSD approach.",0009406v1 2000-10-01,Critical exponents of the random-field O(N) model,"The critical behavior of the random-field Ising model has been a puzzle for a long time. Different theoretical methods predict that the critical exponents of the random-field ferromagnet in D dimensions are the same as in the pure (D-2)-dimensional ferromagnet with the same number of the magnetization components. This result contradicts the experiments and simulations. We calculate the critical exponents of the random-field O(N) model with the (4+\epsilon)-expansion and obtain values different from the critical exponents of the pure ferromagnet in 2+\epsilon dimensions. In contrast to the previous approaches we take into account an infinite set of relevant operators emerging in the problem. We demonstrate how these previously missed relevant operators lead to the breakdown of the (6-\epsilon)-expansion for the random-field Ising model.",0010012v1 2000-10-02,"Limits on the Curie temperature of (III,Mn)V ferromagnetic semiconductors","Mean-field-theory predicts that the Curie temperature T_c of a (III,Mn)V ferromagnet will be proportional to the valence band density-of-states of its host (III,V) semiconductor, suggesting a route toward room-temperature ferromagnetism in this materials class. In this Letter, we use theoretical estimates of spin-wave energies and Monte-Carlo simulations to demonstrate that long-wavelength collective fluctuations, neglected by mean-field theory, will limit the critical temperature in large density-of-states materials. We discuss implications for high T_c searches.",0010036v2 2000-10-04,Spin Injection from Ferromagnetic Metals into Gallium Nitride,"The injection of spin polarized electrons from ferromagnetic metals (Fe and Co) into gallium nitride (GaN) via scanning tunneling microscopy (STM) is demonstrated. Electrons from STM tips are injected into the semiconductor. Net circular polarization of the emitted light is observed, which changes sign on reversal of the magnetization of the tip. The polarization is found to be in qualitative agreement with that expected from considerations based on the splitting of the valence bands due to spin-orbit coupling and the crystal field splitting corresponding to the wurtzite structure, and the magnitude of the spin polarization from the ferromagnetic metal. We find a lower bound for the spin injection efficiency of 25%, corresponding to a net spin polarization in the semiconductor of 10%. This is the largest reported value for a room temperature measurement of spin injection into semiconductors in air.",0010058v1 2000-10-11,Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors,"It is predicted that certain atomically ordered interfaces between some ferromagnetic metals (F) and semiconductors (S) should act as ideal spin filters that transmit electrons only from the majority spin bands or only from the minority spin bands of the F to the S at the Fermi energy, even for F with both majority and minority bands at the Fermi level. Criteria for determining which combinations of F, S and interface should be ideal spin filters are formulated. The criteria depend only on the bulk band structures of the S and F and on the translational symmetries of the S, F and interface. Several examples of systems that meet these criteria to a high degree of precision are identified. Disordered interfaces between F and S are also studied and it is found that intermixing between the S and F can result in interfaces with spin anti-filtering properties, the transmitted electrons being much less spin polarized than those in the ferromagnetic metal at the Fermi energy. A patent application based on this work has been commenced by Simon Fraser University.",0010153v1 2000-10-11,"The frequency, temperature, and magnetic field dependence of ferromagnetic resonance and anti-resonance in La$_{0.8}$Sr$_{0.2}$MnO$_3$","Employing a broadband microwave reflection configuration, we have measured the complex surface impedance, $Z_S(\omega,T,H)$, of single crystal La$_{0.8}$Sr$_{0.2}$MnO$_3$, as a function of frequency (0.045-45 GHz), temperature (250-325 K), and magnetic field (0-1.9 kOe). The microwave surface impedance depends not only on the resistivity of the material, but also on the magnetic permeability, $\hat\mu(\omega,T,H)$, which gives rise to ferromagnetic resonance (FMR) and ferromagnetic anti-resonance (FMAR). The broadband nature of this experiment allows us to follow the FMR to low frequency and to deduce the behavior of both the local internal fields and the local magnetization in the sample.",0010172v1 2000-10-18,Current induced switching of magnetic domains to a perpendicular configuration,"In a ferromagnet--normal-metal--ferromagnet trilayer, a current flowing perpendicularly to the layers creates a torque on the magnetic moments of the ferromagnets. When one of the contacts is superconducting, the torque not only favors parallel or antiparallel alignment of the magnetic moments, as is the case for two normal contacts, but can also favor a configuration where the two moments are perpendicular. In addition, whereas the conductance for parallel and antiparallel magnetic moments is the same, signalling the absence of giant magnetoresistance in the usual sense, the conductance is greater in the perpendicular configuration. Thus, a negative magnetoconductance is predicted, in contrast with the usual giant magnetoresistance.",0010255v2 2000-10-23,Vertical Boundary at x ~ 0.11 in the Structural Phase Diagram of the La1-xSrxMnO3 System (0.08 <= x <= 0.125),"The structural phase diagram of the La1-xSrxMnO3 system in the compositional range 0.08 <= x <= 0.125 has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between 20-600 K. Recent studies have reported that there is an unusual rentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic Pbnm. The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with x =0.11-0.125 there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic Pbnm region with a near-vertical phase boundary just below x = 0.11. The new phases indicate the presence of some novel type of orbital ordering unlike that found in LaMnO3.",0010339v1 2000-10-29,Theory of Ferromagnetism in Diluted Magnetic Semiconductors,"Carrier-induced ferromagnetism has been observed in several (III,Mn)V semiconductors. We review the theoretical picture of these ferromagnetic semiconductors that emerges from a model with kinetic-exchange coupling between localized Mn spins and valence-band carriers. We discuss the applicability of this model, the validity of a mean-field approximation for its interaction term widely used in the literature, and validity limits for the simpler RKKY model in which only Mn spins appear explicitly. Our conclusions are based in part on our analysis of the dependence of the system's elementary spin excitations on carrier density and exchange-coupling strength. The analogy between this system and spin-model ferrimagnets is explored. Finally, we list several extensions of this model that can be important in realistic modeling of specific materials.",0010471v1 2000-11-06,Quantum Hall Ferromagnetism in a Two-Dimensional Electron System,"Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction $\nu = 1/3$) and spin-unpolarized ($\nu = 2/5$) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.",0011081v1 2000-11-10,Triplet superconductivity induced by screened phonon interactions in ferromagnetic compounds,"We propose that screened pairing interactions mediated by phonons could give rise to a spin triplet superconductivity in ferromagnetic compounds such as UGe_2. It is pointed out that the pairing interactions include anisotropic components such as those of p, d, f-waves in addition to dominant s-wave component due to the momentum dependence. Since the ferromagnetic long-range order coexists, there is a large splitting of the Fermi surfaces of up and down spin electrons, which suppresses singlet pairing. Therefore, triplet pairing occurs at last due to the sub-dominant anisotropic interactions, even in the absence of magnetic contribution to the pairing interactions.",0011188v3 2000-12-19,Model study for the nonequlibrium magnetic domain structure during the growth of nanostructured ultrathin films,"The nonequilibrium magnetic domain structure of growing ultrathin ferromagnetic films with a realistic atomic structure is studied as a function of coverage and temperature. We apply a kinetic Monte Carlo method to a micromagnetic model describing the transition from superparamagnetic islands at low coverages to a closed ferromagnetic film. The magnetic relaxation and the island growth happen simultaneously. Near the percolation threshold a metastable magnetic domain structure is obtained with an average domain area ranging between the area of individual magnetic islands and the area of the large domains observed for thicker ferromagnetic films. We conclude that this micro-domain structure is controlled and stabilized by the nonuniform atomic nanostructure of the ultrathin film, causing a random interaction between magnetic islands with varying sizes and shapes. The average domain area and domain roughness are determined. A maximum of the domain area and a minimum of the domain roughness are obtained as a function of the temperature.",0012355v3 2000-12-19,Resistance Spikes at Transitions between Quantum Hall Ferromagnets,"We report a new manifestation of first-order magnetic transitions in two-dimensional electron systems. This phenomenon occurs in aluminum arsenide quantum wells with sufficiently low carrier densities and appears as a set of hysteretic spikes in the resistance of a sample placed in crossed parallel and perpendicular magnetic fields, each spike occurring at the transition between states with different partial magnetizations. Our experiments thus indicate that the presence of magnetic domains at the transition starkly increases dissipation, an effect also suspected in other ferromagnetic materials. Analysis of the positions of the transition spikes allows us to deduce the change in exchange-correlation energy across the magnetic transition, which in turn will help improve our understanding of metallic ferromagnetism.",0012367v1 2001-01-10,Depairing currents in the superconductor/ferromagnet proximity system Nb/Fe,"We have investigated the behaviour of the depairing current J_{dp} in ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the thickness d_s of the superconducting layers. Theoretically, J_{dp} depends on the superconducting order parameter or the pair density function, which is not homogeneous across the film due to the proximity effect. We use a proximity effect model with two parameters (proximity strength and interface transparency), which can also describe the dependence of the superconducting transition temperature T_c on d_s. We compare the computations with the experimentally determined zero-field critical current J_{c0} of small strips (typically 5~ \mu m wide) of Fe/Nb/Fe trilayers with varying thickness d_{Nb} of the Nb layer. Near T_c the temperature dependence J_{c0}(T) is in good agreement with the expected behaviour, which allows extrapolation to T = 0. Both the absolute values of J_{c0}(0) and the dependence on d_{Nb} agree with the expectations for the depairing current. We conclude that J_{dp} is correctly determined, notwithstanding the fact that the strip width is larger than both the superconducting penetration depth and the superconducting coherence length, and that J_{dp}(d_s) is correctly described by the model.",0101138v1 2001-01-11,Internal Modes and Magnon Scattering on Topological Solitons in 2d Easy-Axis Ferromagnets,"We study the magnon modes in the presence of a topological soliton in a 2d Heisenberg easy-axis ferromagnet. The problem of magnon scattering on the soliton with arbitrary relation between the soliton radius R and the ""magnetic length"" Delta_0 is investigated for partial modes with different values of the azimuthal quantum numbers m. Truly local modes are shown to be present for all values of m, when the soliton radius is enough large. The eigenfrequencies of such internal modes are calculated analytically on limiting case of a large soliton radius and numerically for arbitrary soliton radius. It is demonstrated that the model of an isotropic magnet, which admits an exact analytical investigation, is not adequate even for the limit of small radius solitons, R<40$ K) temperature data reveals that doping decreases $T_{N}$ from 122 K for the undoped sample to 103 K for $x=0.10$. The low temperature ($T<20$ K) heat capacity data is consistent with phase separation. The undoped sample displays a finite density of states and typical antiferromagnetic behavior. The addition of electrons in the $x\leq 0.03$ samples creates local ferromagnetism as evidenced by a decreased intermanl field and the need to add a ferromagnetic component to the heat capacity data for $x=0.03$. Further substitution enhances the ferromagnetism as evidenced by the formation of a long range spin density wave.",0108239v3 2001-08-23,Theory of High \tc Ferromagnetism in $SrB_6$ family: A case of Doped Spin-1 Mott insulator in a Valence Bond Solid Phase,"Doped divalent hexaborides such as $Sr_{1-x}La_xB_6$ exhibit high \tc ferromagnetism. We isolate a degenerate pair of $2p$-orbitals of boron with two valence electrons, invoke electron correlation and Hund coupling, to suggest that the undoped state is better viewed as a spin-1 Mott insulator; it is predicted to be a type of 3d Haldane gap phase with a spin gap $\sim 0.1 eV$, much smaller than the charge gap of $ > 1.0 eV$ seen in ARPES. The experimentally seen high \tc `ferromagnetism' is argued to be a complex magnetic order in disguise - either a canted 6-sublattice AFM ($\approx 120^0$) order or its quantum melted version, a chiral spin liquid state, arising from a type of double exchange mechanism.",0108384v3 2001-08-27,Superconductivity and Quantum Phase Transitions in Weak Itinerant Ferromagnets,"It is argued that the phase transition in low-T_c clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical endpoint. A semi-quantitative discussion is given in terms of recent experiments on MnSi and UGe_2. It is then shown that the critical temperature for spin-triplet, p-wave superconductivity mediated by spin fluctuations is generically much higher in a Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling of magnons to the longitudinal magnetic susceptibility. This qualitatively explains the phase diagram recently observed in UGe_2 and ZrZn_2.",0108443v1 2001-09-18,"Spin textures, screening and excitations in dirty quantum Hall ferromagnets","We study quantum Hall ferromagnets in the presence of a random electrostatic impurity potential. Describing these systems with a classical non-linear sigma model and using analytical estimates supported by results from numerical simulations, we examine the nature of the ground state as a function of disorder strength, $\Delta$, and deviation, $\delta \nu$, of the average Landau level filling factor from unity. Screening of an impurity potential requires distortions of the spin configuration, and in the absence of Zeeman coupling there is a disorder-driven, zero-temperature phase transition from a ferromagnet at small $\Delta$ and $|\delta \nu|$ to a spin glass at larger $\Delta$ or $|\delta \nu|$. We examine ground-state response functions and excitations.",0109326v1 2001-10-17,Nonequilibrium excitations in Ferromagnetic Nanoparticles,"In recent measurements of tunneling transport through individual ferromagnetic Co nanograins, Deshmukh, Gu\'eron, Ralph et al. \cite{mandar,gueron} (DGR) observed a tunneling spectrum with discrete resonances, whose spacing was much smaller than what one would expect from naive independent-electron estimates. In a previous publication, \cite{prl_kleff} we had suggested that this was a consequence of nonequilibrium excitations, and had proposed a ``minimal model'' for ferromagnetism in nanograins with a discrete excitation spectrum as a framework for analyzing the experimental data. In the present paper, we provide a detailed analysis of the properties of this model: We delineate which many-body electron states must be considered when constructing the tunneling spectrum, discuss various nonequilibrium scenarios and compare their results with the experimental data of Refs. \cite{mandar,gueron}. We show that a combination of nonequilibrium spin- and single-particle excitations can account for most of the observed features, in particular the abundance of resonances, the resonance spacing and the absence of Zeeman splitting.",0110348v1 2001-10-22,Charge current in ferromagnet-superconductor junction with pairing state of broken time-reversal symmetry,"We calculate the tunneling conductance spectra of a ferromagnetic metal/insulator/superconductor using the Blonder-Tinkham-Klapwijk (BTK) formulation. Two possible states for the superconductor are considered with the time reversal symmetry ($\cal{T}$) broken, i.e., $d_{x^2-y^2}+is$, or $d_{x^2-y^2}+id_{xy}$. In both cases the tunneling conductance within the gap is suppressed with the increase of the exchange interaction due to the suppression of the Andreev reflection. In the $(d_{x^2-y^2}+is)$-wave case the peaks that exist when the ferromagnet is a normal metal in the amplitude of the s-wave component due to the bound state formation are reduced symmetrically, with the increase of the exchange field, while in the $(d_{x^2-y^2}+id_{xy})$-wave case the residual density of states within the gap develops a dip around E=0 with the increase of the exchange field. These results would be useful to discriminate between $\cal{T}$-broken pairing states near the surface in high-$T_c$ superconductors",0110483v1 2001-10-26,Charge current in ferromagnet - triplet superconductor junctions,"We calculate the tunneling conductance spectra of a ferromagnetic metal / insulator / triplet superconductor from the reflection amplitudes using the Blonder-Tinkham-Klapwijk (BTK) formula. For the triplet superconductor, we assume one special $p$-wave order parameter, having line nodes, and two two dimensional $f$-wave order parameters with line nodes, breaking the time reversal symmetry. Also we examine nodeless pairing potentials. The evolution of the spectra with the exchange potential depends solely on the topology of the gap. The weak Andreev reflection within the ferromagnet results in the suppression of the tunneling conductance and eliminates the resonances due to the anisotropy of the pairing potential. The tunneling spectra splits asymmetrically with respect to $E=0$ under the influence of an external magnetic field. The results can be used to distinguish between the possible candidate pairing states of the superconductor Sr$_2$RuO$_4$.",0110544v1 2001-11-09,Inhomogeneous superconductivity induced in a weak ferromagnet,"Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g. in the so-called pi-coupling in S/F/S junctions. In principle, the inhomogeneous state can be induced at T_c as function of the F-layer thickness d_F in S/F bilayers and multilayers, which should result in a dip-like characteristic of T_c(d_F). We show the results of measurements on the S/F system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where such effects might be expected. We find that the critical thickness for the occurrence of superconductivity is still relatively high, even for these weak ferromagnets. The resulting dip then is intrinsically shallow and difficult to observe, which explains the lack of a clear signature in the T_c(d_F) data.",0111178v1 2001-11-14,Enhancement of magnetic ordering by the stress fields of grain boundaries in ferromagnets,"In the paper we predict a distinctive change of magnetic properties and considerable increase of the Curie temperature caused by the strain fields of grain boundaries in ferromagnetic films. It is shown that a sheet of spontaneous magnetization may arise along a grain boundary at temperatures greater than the bulk Curie temperature. The temperature dependence and space distribution of magnetization in a ferromagnetic film with grain boundaries are calculated. We found that $45^\circ$ grain boundaries can produce long-range strain fields that results in the width of the magnetic sheet along the boundary of the order of $ 0.5 \div 1 \mu m$ at temperatures grater than the bulk Curie temperature by about $10^2$ K.",0111252v1 2001-11-24,"Influence of quantum confinement on the ferromagnetism of (Ga,Mn)As diluted magnetic semiconductor","We investigate the effect of quantum confinement on the ferromagnetism of diluted magnetic semiconductor Ga$_{1-x}$Mn$_x$As using a combination of tight-binding and density functional methods. We observe strong majority-spin Mn $d$-As $p$ hybridization, as well as half metallic behavior, down to sizes as small as 20 \AA in diameter. Below this critical size, the doped holes are self-trapped by the Mn-sites, signalling both valence and electronic transitions. Our results imply that magnetically doped III-V nanoparticles will provide a medium for manipulating the electronic structure of dilute magnetic semiconductors while conserving the ferromagnetic properties and even enhancing it in certain size regime.",0111475v1 2001-11-26,Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors,"The magnetic behavior of insulating doped diluted magnetic semiconductors (DMS) is characterized by the interaction of large collective spins known as bound magnetic polarons. Experimental measurements of the susceptibility of these materials have suggested that the polaron-polaron interaction is ferromagnetic, in contrast to the antiferromagnetic carrier-carrier interactions that are characteristic of nonmagnetic semiconductors. To explain this behavior, a model has been developed in which polarons interact via both the standard direct carrier-carrier exchange interaction (due to virtual carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due to the interactions of polarons with magnetic ions in an interstitial region). Using a variational procedure, the optimal values of the model parameters were determined as a function of temperature. At temperatures of interest, the parameters describing polaron-polaron interactions were found to be nearly temperature-independent. For reasonable values of these constant parameters, we find that indirect ferromagnetic interactions can dominate the direct antiferromagnetic interactions and cause the polarons to align. This result supports the experimental evidence for ferromagnetism in insulating doped DMS.",0111497v1 2001-11-27,Light and electric field control of ferromagnetism in magnetic quantum structures,"A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible cross-over between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.",0111511v2 2001-12-07,Induced Ferromagnetism and Colossal Magnetoresistance by Ir-Doping in Pr1-xCaxMnO3,"The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_{1-x}Ca_xMnO_3 (0.4 < x < 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180K and CMR ratio in 7T as large as 10^4. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr, ...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state.",0112118v1 2001-12-10,Unusual magneto-elastic forces stabilize the insulating ferromagnetic state of manganites: the La(0.8)Ca(0.2)$MnO(3) case,"The ferromagnetic and insulating state observed in La$_{1-x}$Ca$_{x}$MnO$_3$, 0.125$<$x$<$0.2, is characterized by structural and magnetic anomalies below T$_C$, similar to those observed in the x$_{Sr}$$\approx$1/8. A neutron scattering study of the superlattice {\bf Q$_0$}= (0,0,1+/4)$_{cub}$ peak, and of the magnetic excitations are reported in the x$_{Ca}$=0.2 sample. The occurrence of this superstructure is associated with the observation of a gap in the spin dynamics, at a {\bf q$_0$} wave-vector ({\bf q$_0$}={\bf Q$_0$}-$\tau$) with the same modulus $|${\bf q$_0$}$|$ in all directions, which divides the dispersion into two regimes. For $|${\bf q}$|$$<$$|${\bf q$_0$}$|$ the dispersion is splitted into two or three curves. For $|${\bf q}$|$$>$$|${\bf q$_0$}$|$, magnetic excitations lock on acoustic and optic phonon energies, revealing a new kind of magneto-vibrational coupling. We suggest an analysis in terms of two distinct magnetic couplings, associated with two ferromagnetic media involved into a collective state.",0112159v1 2001-12-13,Ferromagnetism of $^3$He Films in the Low Field Limit,"We provide evidence for a finite temperature ferromagnetic transition in 2-dimensions as $H \to 0$ in thin films of $^3$He on graphite, a model system for the study of two-dimensional magnetism. We perform pulsed and CW NMR experiments at fields of 0.03 - 0.48 mT on $^3$He at areal densities of 20.5 - 24.2 atoms/nm$^2$. At these densities, the second layer of $^3$He has a strongly ferromagnetic tendency. With decreasing temperature, we find a rapid onset of magnetization that becomes independent of the applied field at temperatures in the vicinity of 1 mK. Both the dipolar field and the NMR linewidth grow rapidly as well, which is consistent with a large (order unity) polarization of the $^3$He spins.",0112237v1 2001-12-14,Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties,"We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML) (23-45 \AA), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 \AA). While previous reports state that GaMnAs layers thinner than 50 \AA are paramagnetic in the whole Mn composition range achievable using MBE growth (up to 8% Mn), we have found that short period superlattices exhibit a paramagnetic-to-ferromagnetic phase transition with a transition temperature which depends on both the thickness of the magnetic GaMnAs layer and the nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the magnetic layers in superlattices are ferromagnetically coupled for both thin (below 50 \AA) and thick (above 50 \AA) GaMnAs layers.",0112264v1 2002-01-14,Temperature dependence of the spin and orbital magnetization density in $Sm_{1-x}Gd_{x} Al_{2}$ around the spin-orbital compensation point,"Non-resonant ferromagnetic x-ray diffraction has been used to separate the spin and orbital contribution to the magnetization density of the proposed zero-moment ferromagnet $Sm_{0.982}Gd_{0.018} Al_{2}$. The alignment of the spin and orbital moments relative to the net magnetization shows a sign reversal at 84K, the compensation temperature. Below this temperature the orbital moment is larger than the spin moment, and vice versa above it. This result implies that the compensation mechanism is driven by the different temperature dependencies of the $4f$ spin and orbital moments. Specific heat data indicate that the system remains ferromagnetically ordered throughout.",0201211v2 2002-01-16,Ferromagnetic semiconductors,"The current status and prospects of research on ferromagnetism in semiconductors are reviewed. The question of the origin of ferromagnetism in europium chalcogenides, chromium spinels and, particularly, in diluted magnetic semiconductors is addressed. The nature of electronic states derived from 3d of magnetic impurities is discussed in some details. Results of a quantitative comparison between experimental and theoretical results, notably for Mn-based III-V and II-VI compounds, are presented. This comparison demonstrates that the current theory of the exchange interactions mediated by holes in the valence band describes correctly the values of Curie temperatures T_C magnetic anisotropy, domain structure, and magnetic circular dichroism. On this basis, chemical trends are examined and show to lead to the prediction of semiconductor systems with T_C that may exceed room temperature, an expectation that are being confirmed by recent findings. Results for materials containing magnetic ions other than Mn are also presented emphasizing that the double exchange involving hoping through d states may operate in those systems.",0201282v1 2002-01-26,"Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C","Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic behavior above room temperature for optimized Mn concentrations near 3 at.%. The magnetism is suppressed when the Mn dose is increased or decreased away from the 3 at.% value, or when n-type GaP substrates are used. At low temperatures the saturated moment is on the order of one Bohr magneton, and the spin wave stiffness inferred from the Bloch-law T^3/2 dependence of the magnetization provides an estimate Tc = 385K of the Curie temperature that exceeds the experimental value, Tc = 270K. The presence of ferromagnetic clusters and hysteresis to temperatures of at least 330K is attributed to disorder and proximity to a metal-insulating transition.",0201492v1 2002-02-05,Ferromagnetism and giant magnetoresistance in the rare earth fullerides Eu6-xSrxC60,"We have studied crystal structure, magnetism and electric transport properties of a europium fulleride Eu6C60 and its Sr-substituted compounds, Eu6-xSrxC60. They have a bcc structure, which is an isostructure of other M6C60 (M represents an alkali atom or an alkaline earth atom). Magnetic measurements revealed that magnetic moment is ascribed to the divalent europium atom with S = 7/2 spin, and a ferromagnetic transition was observed at TC = 10 - 14 K. In Eu6C60, we also confirm the ferromagnetic transition by heat capacity measurement. The striking feature in Eu6-xSrxC60} is very large negative magnetoresistance at low temperature; the resistivity ratio \rho(H = 9 T)/\rho(H = 0 T) reaches almost 10^{-3} at 1 K in Eu6C60. Such large magnetoresistance is the manifestation of a strong pi-f interaction between conduction carriers on C60 and 4f electrons of Eu.",0202066v1 2002-03-06,"Aging, rejuvenation and memory effects in re-entrant ferromagnets","We have studied the slow dynamics of the ferromagnetic phases of the re-entrant CdCr_{2x}In_{2-2x}S_4 system for 0.85475 C. For lower growth temperatures and higher Mn concentrations, nanometer scale MnAs precipitates were detected within the In1-xMnxAs matrix. Magnetic properties of the films were investigated using a superconducting quantum interference device (SQUID) magnetometer. Room-temperature ferromagnetic order was observed in a sample with x=0.1. Magnetization measurements indicated a Curie temperature of 333 K and a room-temperature saturation magnetization of 49 emu/cm^3. The remnant magnetization and the coercive field were small, with values of 10 emu/cm^3 and 400 Oe, respectively. A mechanism for this high-temperature ferromagnetism is discussed in light of the recent theory based on the formation of small clusters of a few magnetic atoms.",0205602v1 2002-06-04,Magnetization plateaus in the ferromagnetic-ferromagnetic-antiferromagnetic Ising chain,"The Ising chain consisting of the antiferromagneticaly coupled ferromagnetic trimer is considered in the external magnetic field. In the framework of the transfer-matrix formalism the thermodynamics of the system is described. The magnetization per site (m) is obtained as the explicit function of the external magnetic field (H). The corresponding plots of m(H) are drawn. Two qualitatively different regions of the values of coupling constants are established: weak antiferromagnetic coupling (J_A<3J_F) and the strong antiferromagnetic coupling $(J_A\geq3J_F)$. For the latter case the magnetization curve with plateau at $m/m_{sat}=1/3$ is obtained. It is proven that the plateau is caused by the stability of spatially modulated spin structure <3111>. The values of magnetic field determining the width of the plateau are obtained in the limit of zero temperature.",0206036v1 2002-06-19,"Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices","Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The structures contain magnetic (Ga,Mn)As layers, separated by thin layers of non-magnetic GaAs spacer. The short period Ga$_{0.93}$Mn$_{0.07}$As/GaAs superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to 60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For Ga$_{0.96}$Mn$_{0.04}$As/GaAs superlattices of similar dimensions, the Curie temperature associated with the ferromagnetic transition is found to oscillate with the thickness of non magnetic spacer. The observed oscillations are related to an interlayer exchange interaction mediated by the polarized holes of the (Ga,Mn)As layers.",0206380v1 2002-07-04,Spin Polarized Current in the Ground State of Superconductor - Ferromagnet - Insulator Trilayers,"We study the ground state properties of a superconductor - ferromagnet - insulator trilayer on the basis of a Hubbard Model featuring exchange splitting in the ferromagnet and electron - electron attraction in the superconductor. We solve the spin - polarized Hartree - Fock - Gorkov equations together with the Maxwell's equation (Ampere's law) fully self-consistently. For certain values of the exchange splitting we find that a spontaneous spin polarized current is generated in the ground state and is intimately related to Andreev bound states at the Fermi level. Moreover, the polarization of the current strongly depends on the band filling.",0207135v1 2002-07-10,p-Type doping of II-VI heterostructures from surface states: application to ferromagnetic Cd$_{1-x}$Mn$_x$Te quantum wells,"We present a study of p-type doping of CdTe and Cd$_{1-x}$Mn$_x$Te quantum wells from surface states. We show that this method is as efficient as usual modulation doping with nitrogen acceptors, and leads to hole densities exceeding $2 \times 10^{11}$ cm$^{-2}$. Surface doping was successfully applied to obtain carrier-induced ferromagnetism in a Cd$_{1-x}$Mn$_x$Te quantum well. The observed temperature dependence of photoluminescence spectra, and the critical temperature, correspond well to those previously reported for ferromagnetic quantum wells doped with nitrogen.",0207259v2 2002-07-16,Spin-polarized currents in superconducting films,"We present a microscopic theory of coherent quantum transport through a superconducting film between two ferromagnetic electrodes. The scattering problem is solved for the general case of ferromagnet/superconductor/ferromagnet (FSF) double-barrier junction, including the interface transparency from metallic to tunnel limit, and the Fermi velocity mismatch. Charge and spin conductance spectra of FSF junctions are calculated for parallel (P) and antiparallel (AP) alignment of the electrode magnetization. Limiting cases of nonmagnetic normal-metal electrodes (NSN) and of incoherent transport are also presented. We focus on two characteristic features of finite size and coherency: subgap tunneling of electrons, and oscillations of the differential conductance. Periodic vanishing of the Andreev reflection at the energies of geometrical resonances above the superconducting gap is a striking consequence of the quasiparticle interference. Also, the non-trivial spin-polarization of the current is found for FSF junctions in AP alignment. This is in contrast with the incoherent transport, where the unpolarized current is accompanied by excess spin accumulation and destruction of superconductivity. Application to spectroscopic measurements of the superconducting gap and the Fermi velocity is also discussed.",0207375v1 2002-07-19,"Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4-xDyxCe0.6Sr2RuCu2O10 (x=0 - 0.6)","The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd1.4-xDyxCe0.6Sr2RuCu2O10 (x=0-0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterised by high temperature superconductivity (Tc ranging from 20-40 K depending upon the Dy content) coexisting with weak ferromagnetism with two magnetic transitions (TM2 ranging from 95-106 K and TM1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c/a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.",0207478v1 2002-08-05,Coreless vortex ground state of the rotating spinor condensate,"We study the ground state of the rotating spinor condensate and show that for slow rotation the ground state of the ferromagnetic spinor condensate is a coreless vortex. While coreless vortex is not topologically stable, we show that there is an energetic threshold for the creation of a coreless vortex. This threshold corresponds to a critical rotation frequency that vanishes as the system size increases. Also, we demonstrate the dramatically different behavior of the spinor condensate with anti-ferromagnetic interactions. For anti-ferromagnetic spinor condensate the angular momentum as a function of rotation frequency exhibits the familiar staircase behavior, but in contrast to an ordinary condensate the first step is to the state with angular momentum 1/2 per particle.",0208078v2 2002-08-06,Dipolar interaction between two-dimensional magnetic particles,"We determine the effective dipolar interaction between single domain two-dimensional ferromagnetic particles (islands or dots), taking into account their finite size. The first correction term decays as 1/D^5, where D is the distance between particles. If the particles are arranged in a regular two-dimensional array and are magnetized in plane, we show that the correction term reinforces the antiferromagnetic character of the ground state in a square lattice, and the ferromagnetic one in a triangular lattice. We also determine the dipolar spin-wave spectrum and evaluate how the Curie temperature of an ensemble of magnetic particles scales with the parameters defining the particle array: height and size of each particle, and interparticle distance. Our results show that dipolar coupling between particles might induce ferromagnetic long range order at experimentally relevant temperatures. However, depending on the size of the particles, such a collective phenomenon may be disguised by superparamagnetism.",0208097v1 2002-08-06,Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2,"The recent discovery that superconductivity occurs in several clean itinerant ferromagnets close to low temperature magnetic instabilities naturally invites an interpretation based on a proximity to quantum criticality. Here we report measurements of the pressure dependence of the low temperature magnetisation in one of these materials, UGe_2. Our results show that both of the magnetic transitions observed in this material as a function of pressure are first order transitions and do not therefore correspond to quantum critical points. Further we find that the known pressure dependence of the superconducting transition is not reflected in the pressure dependence of the static susceptibility. This demonstrates that the spectrum of excitations giving superconductivity is not that normally associated with a proximity to quantum criticality in weak itinerant ferromagnets. In contrast our data suggest that instead the pairing spectrum might be related to a sharp spike in the electronic density of states that also drives one of the magnetic transitions.",0208115v1 2002-08-16,In-plane dipole coupling anisotropy of a square ferromagnetic Heisenberg monolayer,"In this study we calculate the dipole-coupling-induced quartic in-plane anisotropy of a square ferromagnetic Heisenberg monolayer. This anisotropy increases with an increasing temperature, reaching its maximum value close to the Curie temperature of the system. At T=0 the system is isotropic, besides a small remaining anisotropy due to the zero-point motion of quantum mechanical spins. The reason for the dipole-coupling-induced anisotropy is the disturbance of the square spin lattice due to thermal fluctuations ('order-by-disorder' effect). For usual ferromagnets its strength is small as compared to other anisotropic contributions, and decreases by application of an external magnetic field. The results are obtained from a Heisenberg Hamiltonian by application of a mean field approach for a spin cluster, as well as from a many-body Green's function theory within the Tyablikov-decoupling (RPA).",0208324v1 2002-08-30,Itinerant Ferromagnetism in the Periodic Anderson Model,"We introduce a novel mechanism for itinerant ferromagnetism, based on a simple two-band model. The model includes an uncorrelated and dispersive band hybridized with a second band which is narrow and correlated. The simplest Hamiltonian containing these ingredients is the Periodic Anderson Model (PAM). Using quantum Monte Carlo and analytical methods, we show that the PAM and an extension of it contain the new mechanism and exhibit a non-saturated ferromagnetic ground state in the intermediate valence regime. We propose that the mechanism, which does not assume an intra atomic Hund's coupling, is present in both the iron group and in some f electron compounds like Ce(Rh_{1-x} Ru_x)_3 B_2, La_x Ce_{1-x} Rh_3 B_2 and the uranium monochalcogenides US, USe, and UTe.",0208604v1 2002-09-08,"Nature of the Quantum Phase Transition in Clean, Itinerant Heisenberg Ferromagnets","A comprehensive theory of the quantum phase transition in clean, itinerant Heisenberg ferromagnets is presented. It is shown that the standard mean-field description of the transition is invalid in spatial dimensions $d\leq 3$ due to the existence of soft particle-hole excitations that couple to the order parameter fluctuations and lead to an upper critical dimension $d_c^+ = 3$. A generalized mean-field theory that takes these additional modes into account predicts a fluctuation-induced first-order transition. In a certain parameter regime, this first-order transition in turn is unstable with respect to a fluctuation-induced second-order transition. The quantum ferromagnetic transition may thus be either of first or of second-order, in agreement with experimental observations. A detailed discussion is given of the stability of the first-order transition, and of the critical behavior at the fluctuation-induced second-order transition. In $d=3$, the latter is mean field-like with logarithmic corrections to scaling, and in $d<3$ it can be controlled by means of a $3-\epsilon$ expansion.",0209190v3 2002-09-16,Phase Competition in Ln0.5a0.5mno3 Perovskites,"Single crystals of the systems Pr0.5(Ca1-xSrx)0.5MnO3, (Pr1-yYy)0.5(Ca1-xSrx)0.5MnO3, and Sm0.5Sr0.5MnO3 were grown to provide a series of samples with fixed ratio Mn(III)/Mn(IV)=1 having geometric tolerance factors that span the transition from localized to itinerant electronic behavior of the MnO3 array. A unique ferromagnetic phase appears at the critical tolerance factor tc= 0.975 that separates charge ordering and localized-electron behavior for ttc. This ferromagnetic phase, which has to be distinguished from the ferromagnetic metallic phase stabilized at tolerance factors t>tc, separates two distinguishable Type-CE antiferromagnetic phases that are metamagnetic. Measurements of the transport properties under hydrostatic pressure were carried out on a compositions t a little below tc in order to compare the effects of chemical vs. hydrostatic pressure on the phases that compete with one another near t=tc.",0209370v1 2002-09-23,Spin injection into a ballistic semiconductor microstructure,"A theory of spin injection across a ballistic ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann regime. Spin injection coefficient $\gamma$ is suppressed by the Sharvin resistance of the semiconductor $r_N^*=(h/e^2)(\pi^2/S_N)$, where $S_N$ is the Fermi-surface cross-section. It competes with the diffusion resistances of the ferromagnets $r_F$, and $\gamma\sim r_F/r_N^*\ll 1$ in the absence of contact barriers. Efficient spin injection can be ensured by contact barriers. Explicit formulae for the junction resistance and the spin-valve effect are presented.",0209539v2 2002-10-10,Spontaneous spin current near the interface between ferromagnets and unconventional superconductors,"Proximity effects between ferromagnets (F) and superconductors (S) with broken time-reversal symmetry (T) are studied theoretically. For the S side we consider a chiral (p_x \pm ip_y)-wave, and a d_{x^2-y^2}-wave superconductor, the latter of which can form T-breaking surface state, i.e., (d_{x^2-y^2} \pm is)-state. The Bogoliubov de Gennes equation which describes the spatial variations of the superconducting order parameter and the magnetization is derived and solved numerically. It is found that a spontaneous spin current flows along the interface between the (p_x \pm ip_y)-wave superconductor and the ferromagnet. On the contrary, in the case of a [110] interface of the d_{x^2-y^2}-wave SC, the surface state has a (d \pm p_x \pm p_y)-wave (or (d_{x^2-y^2} \pm is)-wave) symmetry, and thus no (only charge) spontaneous current arises.",0210232v1 2002-10-10,"High temperature specific heat and magnetic measurements in Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd, Sm, Dy and Ho) samples","We have made a magnetic characterization of Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3, Sm0.5Ca0.5MnO3, Dy0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3 polycrystalline samples. Ferromagnetic, antiferromagnetic and charge ordering transitions in our samples agree with previous reports. We also report specific heat measurements with applied magnetic fields between 0 and 9 T and temperatures between 2 and 300 K in all cases. Each curve was successfully fitted at high temperatures by an Einstein model with three optical phonon modes. Close to the charge ordering and ferromagnetic transition temperatures the specific heat curves showed peaks superposed to the characteristic response of the lattice oscillations. The entropy variation corresponding to the charge ordering transition was higher than the one corresponding to the ferromagnetic transition. The external magnetic field seems to have no effect in specific heat of the CO phase transition.",0210243v3 2002-10-15,Theoretical models of ferromagnetic III-V semiconductors,"Recent materials research has advanced the maximum ferromagnetic transition temperature in semiconductors containing magnetic elements toward room temperature. Reaching this goal would make information technology applications of these materials likely. In this article we briefly review the status of work over the past five years which has attempted to achieve a theoretical understanding of these complex magnetic systems. The basic microscopic origins of ferromagnetism in the (III,Mn)V compounds that have the highest transition temperatures appear to be well understood, and efficient computation methods have been developed which are able to model their magnetic, transport, and optical properties. However many questions remain.",0210318v1 2002-10-17,Glass component induced hysteresis/memory effect in magnetoresistance of ferromagnetic Pr0.9Sr0.1CoO2.99,"Pr0.9Sr0.1CoO2.99 sample exhibits magnetoresistivity (MR) of up to 40 % at 5 K with a strong hysteresis/memory effect. Magnetisation measurements on Pr0.9Sr0.1CoO2.99 in an applied field of 100 Oe show that, as temperature decreases, the zero-field-cooled (ZFC) and field-cooled (FC) magnetisation curves branch clearly at 50 K, and a cusp appears in the ZFC branch at Tcusp 20 K. Magnetisation measurements in various fields between 100 and 10,000 Oe show that both the ZFC-FC branching temperature and, Tcusp, decrease with increasing field. The magnetization-field isotherms at 5 and 10 K show hysteresis loops typical of ferromagnets. No appreciable MR is seen in this compound at 50 K, i.e. at a temperature close to ZFC-FC branching temperature. At 20 K, negative MR of above 16% is observed without any hysteresis effect. We believe that the appearance of a ferromagnetic component at 5 K and 10 K (i.e. at temperatures below Tcusp) within the spin glass state of Co spins is responsible for both large MR and the prominent hysteresis/memory effect in MR.",0210360v1 2002-10-19,Dirty quantum Hall ferromagnets and quantum Hall spin glasses,"We study quantum Hall ferromagnets in the presence of a random electrostatic impurity potential, within the framework of a classical non-linear sigma model. We discuss the behaviour of the system using a heuristic picture for the competition between exchange and screening, and test our conclusions with extensive numerical simulations. We obtain a phase diagram for the system as a function of disorder strength and deviation of the average Landau level filling factor from unity. Screening of an impurity potential requires distortions of the spin configuration. In the absence of Zeeman coupling there is a disorder-driven, zero-temperature phase transition from a ferromagnet at weak disorder and small deviation from integer filling to a spin glass at stronger disorder or large charge deviation. We characterise the spin glass phase in terms of its magnetic and charge response, as well as its ac conductivity.",0210424v1 2002-10-24,Ferromagnetic superconductivity driven by changing Fermi surface topology,"We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase diagram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetisation. The twin peak DOS may be derived from a tight-binding, quasi-one-dimensional bandstructure, inspired by previous bandstructure calculations.",0210552v2 2002-10-29,Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors,"We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the DMS quantum dots and the itinerant holes in the semiconductor valence band makes hole transfer between quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the DMS quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.",0210643v1 2002-11-05,Persistence in the Zero-Temperature Dynamics of the Random Ising Ferromagnet on a Voronoi-Delaunay lattice,"The zero-temperature Glauber dynamic is used to investigate the persistence probability $P(t)$ in the randomic two-dimensional ferromagnetic Ising model on a Voronoi-Delaunay tessellation. We consider the coupling factor $J$ varying with the distance $r$ between the first neighbors to be $J(r) \propto e^{-\alpha r}$, with $\alpha \ge 0$. The persistence probability of spins flip, that does not depends on time $t$, is found to decay to a non-zero value $P(\infty)$ depending on the parameter $\alpha$. Nevertheless, the quantity $p(t)=P(t)-P(\infty)$ decays exponentially to zero over long times. Furthermore, the fraction of spins that do not change at a time $t$ is a monotonically increasing function of the parameter $\alpha$. Our results are consistent with the ones obtained for the diluted ferromagnetic Ising model on a square lattice.",0211097v1 2002-11-16,"Magnetic Transitions and Ferromagnetic Clusters in RuSr2(Eu,Ce)2Cu2O10+d","The macroscopic magnetizations of a RuSr2(Eu0.7Ce0.3)2Cu2O10+d sample were investigated. A ferromagnet-like transition occurs around T_M in the low-field magnetization. Highly nonlinear M(H), non-Curie-Weiss susceptibility, and slow spin-dynamics, however, were observed up to T_1 approx 2-3 T_M. In addition, an antiferromagnet-like differential-susceptibility maximum of Ru appears around a separate temperature T_AM between T_1 and T_M. The data are therefore consistent with a phase-separation model: superparamagnetic clusters (or short-range spin-orders) are first precipitated from the paramagnetic matrix below T_1, followed by an antiferromagnetic transition of the matrix at T_AM and an apparent ferromagnetic (FM) transition around T_M, where the long-range spin-order is established in the FM species imbedded in the matrix.",0211342v1 2002-11-20,Sensitivity of the Superconducting Transition Temperature to Changes in the Spin-Fluctuation Spectral Weight,"In the simplest model of magnetic pairing, the transition temperature to the superconducting state depends on the dynamical susceptibility $\chi({\bf q},\omega)$. We discuss how $T_c$ is affected by different momentum and frequency parts of $\chi({\bf q},\omega)$ for nearly antiferromagnetic and nearly ferromagnetic metals in two dimensions. While in the case of phonon-mediated superconductivity any addition of spectral weight to $\alpha^2F(\omega)$ at $\omega >0$ leads to an increase in $T_c$, we find that adding magnetic spectral weight at any momentum ${\bf q}$ and low frequencies ($[0:3T_c]$ and $[0:(5-9)T_c]$ for nearly antiferromagnetic and ferromagnetic metals respectively) leads to a suppression of $T_c$. The most effective frequency and momentum range consists of large momenta ${\bf q} \sim (\pi,\pi)$ and frequencies around $10T_c$ for nearly antiferromagnetic metals and small momenta ${\bf q} \sim 0$ and frequencies of approximately $(13-22)T_c$ for nearly ferromagnetic metals.",0211436v1 2002-12-04,Spin interactions of interstitial Mn ions in ferromagnetic GaMnAs,"The recently reported Rutherford backscattering and particle-induced X-ray emission experiments have revealed that in low-temperature MBE grown GaMnAs a significant part of the incorporated Mn atoms occupies tetrahedral interstitial sites in the lattice. Here we study the magnetic properties of these interstitial ions. We show that they do not participate in the hole-induced ferromagnetism. Moreover, Mn interstitial double donors may form pairs with the nearest substitutional Mn acceptors - our calculations evidence that the spins in such pairs are antiferromagnetically coupled by the superexchange. We also show that for the Mn ion in the other, hexagonal, interstitial position (which seems to be the case in the GaMnBeAs samples) the p-d interactions with the holes, responsible for the ferromagnetism, are very much suppressed.",0212093v1 2002-12-11,Ferromagnetic resonant tunneling diodes as spin polarimeters and polarizers,"A method for measuring the degree of spin polarization of magnetic materials based on spin-dependent resonant tunneling is proposed. The device we consider is a ballistic double-barrier resonant structure consisting of a ferromagnetic layer embedded between two insulating barriers. A simple procedure, based on a detailed analysis of the differential conductance, allows to accurately determine the polarization of the ferromagnet. The spin-filtering character of such a system is furthermore addressed. We show that a 100% spin selectivity can be achieved under appropriate conditions. This approach is believed to be well suited for the investigation of diluted magnetic semiconductor heterostructures.",0212242v1 2002-12-24,The specific heat of the rwo-dimensional +/-J Ising model,"The specific heat of the two-dimensional $\pm J$ Ising model has been investigated by the numerical transfer matrix method and Monte Carlo simulations from a new point of view. The region where a part of the specific heat takes the negative value has been investigated, which is characteristic of frustrated systems and reflects the non-trivial degeneracy of the ground state. The region mentioned above is found to be fairly large in the $p-T$ plane ($p$ is the concentration of the ferromagnetic bond and $T$ is the temperature). Moreover, it includes the Nishimori line. Namely, it includes a part of the paramagnetic-ferromagnetic phase boundary, on which the specific heat cannot diverge. The present result indicates that the specific heat does not diverge at least on a part of the paramagnetic-ferromagnetic phase boundary above the multicritical point, which is in conflict with previous results.",0212579v1 2002-12-24,Density-of-states picture and stability of ferromagnetism in the highly-correlated Hubbard model,"The problem of stability of saturated and non-saturated ferromagnetism in the Hubbard model is considered in terms of the one-particle Green's functions. Approximations by Edwards and Hertz and some versions of the self-consistent approximations based on the 1/z-expansion are considered. The account of longitudinal fluctuations turns out to be essential for description of the non-saturated state. The corresponding pictures of density of states are obtained. ""Kondo"" density-of-states singularities owing to spin-flip processes are analyzed. The critical electron concentrations for instabilities of saturated ferromagnetism and paramagnetic state are calculated for various lattices. Drawbacks of various approximations are discussed. A comparison with the results of previous works is performed.",0212586v2 2003-01-14,Writing Spin in a Quantum Dot with Ferromagnetic and Superconducting Electrodes,"We propose an efficient mechanism for the operation of writing spin in a quantum dot, which is an ideal candidate for qubit. The idea is based on the Andreev reflection induced spin polarization (ARISP) in a ferromagnetic / quantum-dot / superconductor system. We find that on the resonance of Andreev reflection, the spin polarization of quantum dot strongly denpends on the magnetization of ferromagnetic electrode, and the sign of the spin polarization is controllable by bias voltage. In the presence of intradot Coulomb interaction, we show that ARISP effect can still survive as long as the charging energy is comparable to the superconducting gap. Detailed conditions and properties of ARISP are also discussed.",0301213v1 2003-01-23,High Temperature Ferromagnetism with Giant Magnetic Moment in Transparent Co-doped SnO2-d,"Occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn1-xCoxO2-d (x<0.3). Interestingly, films of Sn0.95Co0.05O2-d grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of about 7 Bohr-Magneton/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.",0301456v1 2003-02-04,Theory of Spin Orientation of Semiconductor Carriers at a Ferromagnetic Interface,"A quantum theory of the spin-dependent scattering of semiconductor electrons by a Schottky barrier at an interface with a ferromagnet is presented. The reflection of unpolarized non-equilibrium carriers produces spontaneous spin-polarization in the semiconductor. If a net spin-polarization pre-exists in the semiconductor, the combination of the ferromagnet magnetization and the incident carrier polarization combine to tilt the reflected polarization in the semiconductor. The spin reflection properties are investigated as functions of the system characteristics: the Schottky barrier height, semiconductor doping and applied bias. The effect on reflection due to the variation of the barrier width with electron energy is contrasted for two means of excitation: optical or electrical. Optically excited electrons have a wider energy spread than the near-equilibrium excitation from non-magnetic ohmic contacts.",0302088v1 2003-02-06,Weak ferromagnetism with very large canting in a chiral lattice: (pyrimidine)2FeCl2,"The transition metal coordination compound (pyrimidine)2FeCl2 crystallizes in a chiral lattice, space group I 4_1 2 2 (or I4_3 2 2). Combined magnetization, Mossbauer spectroscopy and powder neutron diffraction studies reveal that it is a canted antiferromagnet below T_N = 6.4 K with an unusually large canting of the magnetic moments of 14 deg. from their general antiferromagnetic alignment, one of the largest reported to date. This results in weak ferromagnetism with a ferromagnetic component of 1 mu_B. The large canting is due to the interplay between the antiferromagnetic exchange interaction and the local single-ion anisotropy in the chiral lattice. The magnetically ordered structure of (pyrimidine)2FeCl2, however, is not chiral. The implications of these findings for the search of molecule based materials exhibiting chiral magnetic ordering is discussed.",0302121v1 2003-02-12,Crossed Andreev reflection at ferromagnetic domain walls,"We investigate several factors controlling the physics of hybrid structures involving ferromagnetic domain walls (DWs) and superconducting (S) metals. We discuss the role of non collinear magnetizations in S/DW junctions in a spin $\otimes$ Nambu $\otimes$ Keldysh formalism. We discuss transport in S/DW/N and S/DW/S junctions in the presence of inelastic scattering in the domain wall. In this case transport properties are similar for the S/DW/S and S/DW/N junctions and are controlled by sequential tunneling of spatially separated Cooper pairs across the domain wall. In the absence of inelastic scattering we find that a Josephson current circulates only if the size of the ferromagnetic region is smaller than the elastic mean free path meaning that the Josephson effect associated to crossed Andreev reflection cannot be observed under usual experimental conditions. Nevertheless a finite dc current can circulate across the S/DW/S junction due to crossed Andreev reflection associated to sequential tunneling.",0302236v4 2003-02-12,Temperature Dependence of the Dielectric Constant and Resistivity of Diluted Magnetic Semiconductors,"We study the effect that the ferromagnetic order has on the electrical properties of Diluted Magnetic Semiconductors. We analyze the temperature dependence of the dielectric constant and of the resistivity of Ga$_{1-x}$Mn$_x$As. In our treatment the electronic structure of the semiconductor is described by a six band Kohn-Luttinger Hamiltonian, the thermal fluctuations of the Mn magnetic moments are treated in the mean field approximation, the carrier-carrier interaction within the random phase approximation, and the transport properties using the relaxation time approximation. We find that the Thomas-Fermi length changes near 8% when going from the ferromagnetic to the paramagnetic phase. We also find, in good agreement with the experiments, that the resistivity changes near 20% when going from zero to the Curie temperature. We explain this change in the resistivity in terms of the variation of the Fermi surface and the transport scattering time when going from the ferromagnetic phase to the paramagnetic phase.",0302237v1 2003-02-12,Metastability and Avalanches in a Nonequilibrium Ferromagnetic System,"We present preliminary results on the metastable behavior of a nonequilibrium ferromagnetic system. The metastable state mean lifetime is a non-monotonous function of temperature; it shows a maximum at certain non-zero temperature which depends on the strengh of the nonequilibrium perturbation. This is in contrast with the equilibrium case in which lifetime increases monotonously as the temperature is decreasesed. We also report on avalanches during the decay from the metastable state. Assuming both free boundaries and nonequilibrium impurities, the avalanches exhibit power-law size and lifetime distributions. Such scale free behavior is very sensible. The chances are that our observations may be observable in real (i.e. impure) ferromagnetic nanoparticles.",0302241v1 2003-02-24,Magnetization study of RuSr2Y1.5Ce 0.5Cu2O10 (Ru-1222),"We have studied the magnetic properties of the non-superconducting RuSr2Ln1.5Ce0.5Cu2O10 (Ln=Y, Dy and Ho, Ru-1222) compounds synthesized under high pressure (6 Gpa) at elevated temperature. The materials become magnetically ordered at TM =152(2) K regardless of Ln. The wide ferromagnetic-like hysteresis loops which open at 5 K, close themselves around Tirr= 90-100 K and the remanent magnetizations (Mrem) and the coercive fields (HC) become zero. Surprisingly, at Tirr\sim e^{i\theta}$. The crucial step in our analysis is the fact that the $z$-component of the spin is canonically conjugate to the phase of the planar contribution: $[\theta,S^z]=i$. This is analogous to the commutation relation $[\phi,N]=i$ in superconductors, where $\phi$ is the phase associated to the superconducting order parameter and $N$ is the Cooper pair number operator. We briefly discuss the experimental consequences of our theoretical analysis.",0302528v3 2003-02-28,Ferromagnetic Polarons in La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3,"Unrestricted Hartree-Fock calculations on La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3 in the full magnetic unit cell show that the magnetic ground states of these compounds consist of 'ferromagnetic molecules' or polarons ordered in herring-bone patterns. Each polaron consists of either three or five Mn ions separated by O- ions with a magnetic moment opposed to those of the Mn ions. Ferromagnetic coupling within the polarons is strong while coupling between them is relatively weak. Magnetic moments on the Mn ions range between 3.8 and 3.9 Bohr magnetons in La0.5Ca0.5MnO3 and moments on the O- ions are -0.7 Bohr magnetons. Each polaron has a net magnetic moment of 7.0 Bohr magnetons, in good agreement with recently reported magnetisation measurements from electron microscopy. The polaronic nature of the electronic structure reported here is obviously related to the Zener polaron model recently proposed for Pr0.6Ca0.4MnO3 on the basis of neutron scattering data.",0302622v2 2003-03-15,Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors,"Applying the dynamical coherent potential approximation to a simple model, we have systematically studied the carrier states in $A_{1-x}$Mn$_xB$-type diluted magnetic semiconductors (DMS's). The model calculation was performed for three typical cases of DMS's: The cases with strong and moderate exchange interactions in the absence of nonmagnetic potentials, and the case with strong attractive nonmagnetic potentials in addition to moderate exchange interaction. When the exchange interaction is sufficiently strong, magnetic impurity bands split from the host band. Carriers in the magnetic impurity band mainly stay at magnetic sites, and coupling between the carrier spin and the localized spin is very strong. The hopping of the carriers among the magnetic sites causes ferromagnetism through a {\it double-exchange (DE)-like} mechanism. We have investigated the condition for the DE-like mechanism to operate in DMS's. The result reveals that the nonmagnetic attractive potential at the magnetic site assists the formation of the magnetic impurity band and makes the DE-like mechanism operative by substantially enhancing the effect of the exchange interaction. Using conventional parameters we have studied the carrier states in Ga$_{1-x}$Mn$_x$As. The result shows that the ferromagnetism is caused through the DE-like mechanism by the carriers in the bandtail originating from the impurity states.",0303285v1 2003-04-15,Ferromagnetic Kondo-Lattice Model,"We present a many-body approach to the electronic and magnetic properties of the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The coupling between itinerant conduction electrons and localized magnetic moments leads, on the one hand, to a distinct temperature-dependence of the electronic quasiparticle spectrum and, on the other hand, to magnetic properties, as e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly influenced by the band electron selfenergy and therewith in particular by the carrier density. We present results for the single-band Kondo-lattice model in terms of quasiparticle densities of states and quasiparticle band structures and demonstrate the density-dependence of the self-consistently derived Curie temperature. The transition from weak-coupling (RKKY) to strong-coupling (double exchange) behaviour is worked out. The multiband model is combined with a tight-binding-LMTO bandstructure calculation to describe real magnetic materials. As an example we present results for the archetypal ferromagnetic local-moment systems EuO and EuS. The proposed method avoids the double counting of relevant interactions and takes into account the correct symmetry of atomic orbitals.",0304338v1 2003-04-16,NRG study of the Kondo effect in the presence of itinerant-electron ferromagnetism,"The Kondo effect in quantum dots (QDs) - artificial magnetic impurities - attached to ferromagnetic leads is studied with the numerical renormalization group (NRG) method. It is shown that the QD level is spin-split due to presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally-screened state, describable by Fermi liquid theory and a generalized Friedel sum rule, and transport in the unitary limit is not spin dependent.",0304385v2 2003-04-25,Ising-like Spin Anisotropy and Competing Antiferromagnetic - Ferromagnetic Orders in GdBaCo_{2}O_{5.5} Single Crystals,"In RBaCo_{2}O_{5+x} compounds (R is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned GdBaCo_{2}O_{5.5} single crystals, and find a remarkable uniaxial anisotropy of Co^{3+} spins which is tightly linked with the chain oxygen ordering in GdO_{0.5} planes. Reflecting the underlying oxygen order, CoO_2 planes also develop a spin-state order consisting of Co^{3+} ions in alternating rows of S=1 and S=0 states. The magnetic structure appears to be composed of weakly coupled ferromagnetic ladders with Ising-like moments, which gives a simple picture for magnetotransport phenomena.",0304578v1 2003-04-28,Magnetism and Superconductivity in a Two-band Hubbard Model in Infinite Dimensions,"We study a two-band Hubbard model using the dynamical mean-field theory combined with the exact diagonalization method. At the electron density $n=2$, a transition from a band-insulator to a correlated semimetal occurs when the on-site Coulomb interaction $U$ is varied for a fixed value of the charge-transfer energy $\Delta$. At low temperature, the correlated semimetal shows ferromagnetism or superconductivity. With increasing doping $|n-2|$, the ferromagnetic transition temperature rapidly decreases and finally becomes zero at a critical value of $n$. The second-order phase transition occurs at high temperature, while a phase separation of ferromagnetic and paramagnetic states takes place at low temperature. The superconducting transition temperature gradually decreases and finally becomes zero near $n=1$ ($n=3$) where the system is Mott insulator which shows antiferromagnetism at low temperature.",0304624v1 2003-05-02,Enhancement of Curie temperature in Ga(1-x)Mn(x)As/Ga(1-y)Al(y)As ferromagnetic heterostructures by Be modulation doping,"The effect of modulation doping by Be on the ferromagnetic properties of Ga(1-x)Mn(x)As is investigated in Ga(1-x)Mn(x)As/Ga(1-y)Al(y)As heterojunctions and quantum wells. Introducing Be acceptors into the Ga(1-y)Al(y)As barriers leads to an increase of the Curie temperature T_C of Ga(1-x)Mn(x)As, from 70 K in undoped structures to over 100 K with the modulation doping. This increase is qualitatively consistent with a multi-band mean field theory simulation of carrier-mediated ferromagnetism. An important feature is that the increase of T_C occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, but not when the Be-doped layer is grown first. This behavior is expected from the strong sensitivity of Mn interstitial formation to the value of the Fermi energy during growth.",0305047v1 2003-05-06,Pressure induced transition from a spin glass to an itinerant ferromagnet in half doped manganite Ln0.5Ba0.5MnO3 (Ln=Sm and Nd) with quenched disorder,"The effect of quenched disorder on the multiphase competition has been investigated by examining the pressure phase diagram of half doped manganite Ln0.5B0.5MnO3 (Ln = Sm and Nd) with A-site disorders. Sm0.5Ba0.5MnO3, a spin glass insulator at ambient pressure, switches to a ferromagnetic metal with increasing pressure, followed by a rapid increase of the ferromagnetic transition temperature Tc. The rapid increase of Tc was confirmed also for Nd0.5Ba0.5MnO3. These observations indicate that the unusual suppression of the multicritical phase boundary in the A-site disordered system, previously observed as a function of the averaged A-site ionic radius, is essentially controlled by the pressure and hence the band width. The effect of quenched disorder is therefore much enhanced with approaching the multicritical region.",0305114v1 2003-05-19,Effects of local oxygen distortions on electronic structures of Na$_{x}$CoO$_{2}$,"By using pseudopotential method with local spin density functional approximation, the electronic band structures of Na$_{x}$CoO$_{2}$ are calculated for $x=0.25$, 0.5, 0.75, and $x=1$ in the presence of the structure relaxations. As increasing Na content, the hybridization between cobalt and oxygen orbitals is decreased, and a phase transition is predicted from a wide-band ferromagnetic to a narrow band paramagnetic metals. The itinerant ferromagnetism is strongly suppressed by the local distortions of the oxygens around the cobalts. Moreover, straining the CoO$_{2}$ layers corresponding to the hydrated superconductor Na$_{0.35}$CoO$_{2}\cdot $1.3H$%_{2}$O strongly enhances both the hybridization and ferromagnetism.",0305423v2 2003-05-19,Josephson effect in double-barrier superconductor-ferromagnet junctions,"We study the Josephson effect in ballistic double-barrier SIFIS planar junctions, consisting of bulk superconductors (S), a clean metallic ferromagnet (F), and insulating interfaces (I). We solve the scattering problem based on the Bogoliubov--de Gennes equations and derive a general expression for the dc Josephson current, valid for arbitrary interfacial transparency and Fermi wave vectors mismatch (FWVM). We consider the coherent regime in which quasiparticle transmission resonances contribute significantly to the Andreev process. The Josephson current is calculated for various parameters of the junction, and the influence of both interfacial transparency and FWVM is analyzed. For thin layers of strong ferromagnet and finite interfacial transparency, we find that coherent (geometrical) oscillations of the maximum Josephson current are superimposed on the oscillations related to the crossover between 0 and $\pi$ states. For the same case we find that the temperature-induced $0-\pi$ transition occurs if the junction is very close to the crossovers at zero temperature.",0305437v2 2003-05-22,Domain wall superconductivity in hybrid superconductor -- ferromagnetic structures,"On the basis of phenomenological Ginzburg-Landau approach we investigate the problem of order parameter nucleation in hybrid superconductor/ferromagnetic systems with a domain structure in applied external magnetic field. Both the isolated domain boundaries and periodic domain structures in ferromagnetic layers are considered. We study the interplay between the superconductivity localized at the domain walls and far from the walls and show that such interplay determines a peculiar field dependence of the critical temperature Tc. For a periodic domain structure the behavior of the upper critical field of superconductivity nucleation near Tc is strongly influenced by the overlapping of the superconducting nuclei localized over different domains.",0305520v1 2003-05-28,Giant frequency dependence of dynamic freezing in nanocrystalline ferromagnetic LaCo0.5Mn0.5O3,"We have investigated the magnetic properties of nanocrystalline LaCo0.5Mn0.5O3. The temperature dependence of the imaginary part of the a.c. susceptibility shows a strongly frequency dependent maximum at a temperature, Tf, which is well below the ferromagnetic transition temperature (TC ~ 230 K). The frequency dependence of Tf obeys the Arrhenius relation, f = foexp(-Ea/kBT), with physically reasonable values of fo = 109 Hz and Ea/kB = 1518 K. The frequency shift of Tf per decade of frequency is one of the highest values observed in any magnetic system, and a similarly large value is also found in LaCo0.4Mg0.1Mn0.5O3, suggesting that such behavior is intrinsic despite the apparent presence of long range ferromagnetic order.",0305653v1 2003-06-04,Magnetization reversal and two level fluctuations by spin-injection in a ferromagnetic metallic layer,"Slow magnetic relaxation and two level fluctuations measurements under high current injection is performed in single-contacted ferromagnetic nanostructures. The magnetic configurations of the samples are described by two metastable states of the uniform magnetization. The current-dependent effective energy barrier due to spin-transfer from the current to the magnetic layer is measured. The comparison between the results obtained with Ni nanowires of 6 $\mu $m length and 60 nm diameter, and Co (10 nm)/Cu (10 nm)/Co(30 nm) nanometric pillars of about 40 nm in diameter refined the characterization of this effect. It is shown that all observed features cannot be reduced to the action of a current dependent effective field. Instead, all measurements can be described in terms of an effective temperature, which depends on the current amplitude and direction. The system is then analogous to an unstable open system. The effect of current induced magnetization reversal is interpreted as the balance of spin injection between both interfaces of the ferromagnetic layer.",0306103v1 2003-06-10,Magnetic Field Melting of the Charge-Ordered State of La0.5Ca0.5MnO3: A Local Structure Perspective,"The local structure about the Mn site in the half doped system La0.5Ca0.5MnO3 was measured in magnetic fields up 10 T to probe the melting of the charge ordered state. Examination of the Mn-O and Mn-Mn correlations reveal three distinct regions in the structure-field diagram. A broad region with weak field dependence (mainly antiferromatnetic phase below 7.5 T), a narrow-mixed phase region near ~ 8.5 T followed by a ferromagnetic phase region with strong field-structure coupling. At high field the Mn-O radial distribution becomes Gaussian and the Mn-Mn correlations are enhanced - consistent with the dominance of a ferromagnetic phase. The exponential change in resistivity in the first region (observed in transport measurements) is dominated by the reordering of the moments on the Mn sites from CE type antiferromagnetic to ferromagnetic order with only a weak change in the local distortions of the MnO6 octahedra.",0306229v1 2003-06-11,Weak ferromagnetism and other instabilities of the two-dimensional t-t' Hubbard model at Van Hove fillings,"We investigate magnetic and superconducting instabilities of the two-dimensional t-t' Hubbard model on a square lattice at Van Hove densities from weak to intermediate coupling by means of the two-particle self-consistent approach. We find that as the next-nearest-neighbor hopping |t'| increases from zero, the leading instability is towards an incommensurate spin-density wave whose wave vector moves slowly away from (pi, pi). For intermediate values of |t'|, the leading instability is towards d_{x2-y2}-wave superconductivity. For larger |t'|>0.33t, there are signs of a crossover to ferromagnetism at extremely low temperatures. The suppression of the crossover temperature is driven by Kanamori screening that strongly renormalizes the effective interaction and also causes the crossover temperature to depend only weakly on t'. Electronic self-energy effects for large |t'| lead to considerable reduction of single-particle spectral weight at zero energy at temperatures as high as T<0.1t, an effect that may be detrimental to the existence of a ferromagnetic ground state at weak coupling.",0306296v2 2003-06-12,On Aharonov-Bohm oscillation in a ferromagnetic ring,"Aharonov-Bohm effect in a ferromagnetic thin ring in diffusive regime is theoretically studied by calculating the Cooperon and Diffuson. In addition to the spin-orbit interaction, we include the spin-wave excitation and the spin splitting, which are expected to be dominant sources of dephasing in ferromagnets at low temperatures. The spin splitting turns out to kill the spin-flip channel of Cooperon but leaves the spin-conserving channel untouched. For the experimental confirmation of interference effect (described by Cooperons) such as weak localization and Aharonov-Bohm oscillation with period $h/2e$, we need to suppress the dominant dephasing by orbital motion. To do this we propose experiments on a thin film or thin ring with magnetization and external field perpendicular to the film, in which case the effective field inside the sample is equal to the external field (magnetization does not add up). The field is first applied strong enough to saturate the magnetization and then carrying out the measurement down to zero field keeping the magnetization nearly saturated, in order to avoid domain formations (negative fields may also be investigated if the coercive field is large enough).",0306309v1 2003-06-18,On the theory of superconductivity in ferromagnetic superconductors with triplet pairing,"We point out that ferromagnetic superconductors with triplet pairing and strong spin-orbit coupling are even in the simplest case at least two-band superconductors. The Gor'kov type formalism for such superconductors is developed and the Ginzburg-Landau equations are derived. The dependence of the critical temperature on the concentration of ordinary point-like impurities is found. Its nonuniversality could serve as a qualitative measure of the two-band character of ferromagnetic superconductors. The problem of the upper critical field determination is also discussed.",0306471v3 2003-06-23,"Ferromagnetic transition temperature enhancement in (Ga,Mn)As semiconductor by carbon co-doping","We present a theoretical study of (Ga,Mn)(As,C) diluted magnetic semiconductors with high C acceptor density that combines insights from phenomenological model and microscopic approaches. A tight-binding coherent potential approximation is used to describe the electronic structure in the presence of Mn$_{\rm Ga}$ and C$_{\rm As}$ impurities. We find only a small effect of C on the distribution and coherence of electronic states close to the top of the valence band and on the coupling between Mn moments, even at doping levels of several per cent. These results justify applying the model of ferromagnetic Mn-Mn coupling mediated by itinerant holes in the valence band also to C doped samples. The increase of ferromagnetic transition temperature due to the presence of C acceptors is illustrated by calculations that use the k.p Kohn-Luttinger description of the GaAs valence band and assume systems where Mn local moment and itinerant hole densities can be varied independently.",0306557v1 2003-07-01,Growth and properties of ferromagnetic In(1-x)Mn(x)Sb alloys,"We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In(1-x)Mn(x)Sb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In(1-x)Mn(x)Sb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K, which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method.",0307032v1 2003-07-03,"Cyclotron Resonance in Ferromagnetic InMnAs/(Al,Ga)Sb Heterostructures","We report the observation of hole cyclotron resonance (CR) in InMnAs/(Al,Ga)Sb heterostructures in a wide temperature range covering both the paramagnetic and ferromagnetic phases. We observed two pronounced resonances that exhibit drastic changes in position, linewidth, and intensity at a temperature higher than the Curie temperature, indicating possible local magnetic ordering or clustering. We attribute the two resonances to the fundamental CR transitions expected for delocalized valence-band holes in the quantum limt. Using an 8-band {\bf k$\cdot$p} model, which incorporates ferromagnetism within a mean-field approximation, we show that the temperature-dependent CR peak shift is a direct measure of the carrier-Mn exchange interaction. Significant line narrowing was observed at low temperatures, which we interpret as the suppression of localized spin fluctuations.",0307087v1 2003-07-07,"Mn Interstitial Diffusion in (Ga,Mn)As","We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, and resistivity measurements during annealing, show that the observed changes are due to out-diffusion of Mn interstitials towards the surface, governed by an energy barrier of about 0.7-0.8 eV. The Mn interstitial is a double donor resulting in compensation of charge carriers and suppression of ferromagnetism. Electric fields induced by high concentrations of substitutional Mn acceptors have a significant effect on the diffusion.",0307140v1 2003-07-11,"Photo-induced spin dynamics in ferromagnetic semiconductor $p$-(Ga,Mn)As","Spin dynamics in ferromagnetic $p$-(Ga,Mn)As ($x$ = 0.011, $T_{C}$ = 30 K) has been studied by carefully comparing the decay time of the photo-induced reflectivity change with the transient behavior of polar Kerr rotation induced by photo-generated carrier spins with a femtosecond light pulse of various polarizations. As to the rising process, the rate of Kerr rotation is found comparable to the generation rate of spin-polarized carriers. For the decay process, the Kerr rotation and reflectivity signal both show the same decay rate at above the $T_{C}$, whereas, below the $T_{C}$, the former becomes slower than the latter. The magnitude of Kerr rotation suggests that 10$^{2}$ Mn spins are revolved by injecting one hole spin. On the basis of these observations, collective rotation of ferromagnetically coupled Mn spins is discussed in terms of $p$-$d$ exchange interaction and successive transverse spin relaxation. Development of another long-lived behavior under external perpendicular magnetic fields is also disclosed.",0307268v1 2003-07-28,Magnetic properties of Yb2Mo2O7 and Gd2Mo2O7 from rare earth Mossbauer measurements,"Using 170-Yb and 155-Gd Mossbauer measurements down to 0.03K, we have examined the semiconducting pyrochlore Yb2Mo2O7 where the Mo intra-sublattice interaction is anti-ferromagnetic and the metallic pyrochlore Gd2Mo2O7 where this interaction is ferromagnetic. Additional information was obtained from susceptibility, magnetisation and 172-Yb perturbed angular correlation measurements. The microscopic measurements evidence lattice disorder which is important in Yb2Mo2O7 and modest in Gd2Mo2O7. Magnetic irreversibilities occur at 17K in Yb2Mo2O7 and at 75K in Gd2Mo2O7 and below these temperatures the rare earths carry magnetic moments which are induced through couplings with the Mo sublattice. In Gd2Mo2O7, we observe the steady state Gd hyperfine populations at 0.027K are out of thermal equilibrium, indicating that Gd and Mo spin fluctuations persist at very low temperatures. Frustration is thus operative in this essentially isotropic pyrochlore where the dominant Mo intra-sublattice interaction is ferromagnetic.",0307677v1 2003-07-31,Thermodynamic properties of ferromagnetic mixed-spin chain systems,"Using a combination of high-temperature series expansion, exact diagonalization and quantum Monte Carlo, we perform a complementary analysis of the thermodynamic properties of quasi-one-dimensional mixed-spin systems with alternating magnetic moments. In addition to explicit series expansions for small spin quantum numbers, we present an expansion that allows a direct evaluation of the series coefficients as a function of spin quantum numbers. Due to the presence of excitations of both acoustic and optical nature, the specific heat of a mixed-spin chain displays a double-peak-like structure, which is more pronounced for ferromagnetic than for antiferromagnetic intra-chain exchange. We link these results to an analytically solvable half-classical limit. Finally, we extend our series expansion to incorporate the single-ion anisotropies relevant for the molecular mixed-spin ferromagnetic chain material MnNi(NO$_{2}$)$_{4}$(ethylenediamine)$_{2}$, with alternating spins of magnitude 5/2 and 1. Including a weak inter-chain coupling, we show that the observed susceptibility allows for an excellent fit, and the extraction of microscopic exchange parameters.",0307761v2 2003-08-01,What is the value of the superconducting gap of a F/S/F trilayer ?,"Based on the model of F/S/F trilayer with atomic thickness [A. Buzdin and M. Daumens, cond-mat/0305320] we discuss the relative roles of pair-breaking and proximity effects, as a function of the exchange field, of disorder and of a finite thickness in the superconducting layer. The exchange field can be small (weak ferromagnets) or large (strong ferromagnets) compared to the superconducting gap. With weak ferromagnets we show the existence of a reentrant superconducting gap for the F/S/F trilayer with atomic thickness in the parallel alignment (equivalent to the F/S bilayer). Qualitatively small disorder is equivalent to reducing the value of the hopping parameters. In the presence of a finite thickness in the superconducting layer the superconducting gap in the antiparallel alignment is larger than in the parallel alignment, meaning that pair breaking dominates over the proximity effect.",0308007v2 2003-08-19,"Magnetization relaxation in (Ga,Mn)As ferromagnetic semiconductors","We describe a theory of Mn local-moment magnetization relaxation due to p-d kinetic-exchange coupling with the itinerant-spin subsystem in the ferromagnetic semiconductor (Ga,Mn)As alloy. The theoretical Gilbert damping coefficient implied by this mechanism is calculated as a function of Mn moment density, hole concentration, and quasiparticle lifetime. Comparison with experimental ferromagnetic resonance data suggests that in annealed strongly metallic samples, p-d coupling contributes significantly to the damping rate of the magnetization precession at low temperatures. By combining the theoretical Gilbert coefficient with the values of the magnetic anisotropy energy, we estimate that the typical critical current for spin-transfer magnetization switching in all-semiconductor trilayer devices can be as low as $\sim 10^{5} {\rm A cm}^{-2}$.",0308386v3 2003-08-25,Co distribution in ferromagnetic rutile Co-doped TiO$_2$ thin films grown by laser ablation on silicon substrates,"Pure rutile Co-doped TiO$_2$ films were fabricated successfully by the conventional pulsed laser deposition technique on silicon substrates from a ceramic target. Under the right fabrication conditions, Co concentration in the films could be almost the same as in the synthesized target, and films under various conditions all are ferromagnetic well above room temperature. Even though Rutherford backscattering spectroscopy measurements show that Co atoms seem to be mostly localized near the surface of the films and less exist in deeper levels, other experimental evidences show that the ferromagnetism does not come from Co segregations but from the Co-doped TiO$_2$ matrix. Rutile Ti$_1-x$Co$_x$O$_2$ thin films grown by a very simple technique on low-price silicon substrates showing Curie temperature (TC) above 400 K appear to be very attractive to applications.",0308494v1 2003-08-28,External control of the direction of magnetization in ferromagnetic InMnAs/GaSb heterostructures,"In this paper, we demonstrate external control over the magnetization direction in ferromagnetic (FM) In_{1-x}Mn_{x}As/GaSb heterostructures. FM ordering with T_C as high as 50 K is confirmed by SQUID magnetization, anomalous Hall effect (AHE), and magneto-optical Kerr effect (MOKE) measurements. Even though tensile strain is known to favor an easy axis normal to the layer plane, at low temperatures we observe that the magnetization direction in several samples is intermediate between the normal and in-plane axes. As the temperature increases, however, the easy axis rotates to the direction normal to the plane. We further demonstrate that the easy magnetization axis can be controlled by incident light through a bolometric effect, which induces a pronounced increase in the amplitude of the AHE. A mean-field-theory model for the carrier-mediated ferromagnetism reproduces the tendency for dramatic reorientations of the magnetization axis, but not the specific sensitivity to small temperature variations.",0308605v1 2003-09-02,The enhancement of phase separation aspect in electron doped manganite Ca0.8Sm0.16Nd0.04MnO3,"The complex lanthanide doping of electron manganites results in enhancement of various phase separation effects in physical properties of these compounds. Selecting Ca0.8Sm0.16Nd0.04MnO3 as a model case we show that the first order structural phase transition from paramagnetic semi-metallic phase into anti-ferromagnetic semi-metallic phase at TS ~ 158 +- 4 K is marked by an abrupt decrease in magnetization, a step like anomaly DL/L = 10-4 in thermal expansion and large latent heat DQ = 610 J/mol. In a certain temperature range below TS, the high field magnetization exhibits hysteretic metamagnetic behavior due to field-induced first order transformation. ac-susceptibility, magnetization and resistivity data suggest rather a non-uniform state in Ca0.8Sm0.16Nd0.04MnO3 at low temperatures. The metal - insulator transition occurs at TMI ~112 +- 3 K, accompanied by a step-like increase in magnetization. These features could be ascribed to ""sponging"" of electrons from neighboring anti-ferromagnetic matrix by clusters undergoing the ferromagnetic ordering.",0309065v1 2003-09-08,Local Ferromagnetism in Microporous Carbon with the Structural Regularity of Zeolite Y,"Magnetization M(H,T) measurements have been performed on microporous carbon (MC) with a three-dimensional nano-array structure corresponding to that of a zeolite Y supercage. The obtained results unambiguously demonstrate the occurrence of high-temperature ferromagnetism in MC, probably originating from a topological disorder associated with curved graphene sheets. The results provide evidence that the ferromagnetic behavior of MC is governed by isolated clusters in a broad temperature range, and suggest the occurrence of percolative-type transition with the temperature lowering. A comparative analysis of the results obtained on MC and related materials is given.",0309201v1 2003-09-19,Short-range spin- and pair-correlations: a variational wave-function,"A many-body wavefuction is postulated, which is sufficiently general to describe superconducting pair-correlations, and/or spin-correlations, which can occur either as long-range order or as finite-range correlations. The proposed wave-function appears to summarize some of the more relevant aspects of the rich phase-diagram of the high-Tc cuprates. Some of the states represented by this wavefunction are reviewed: For superconductivity in the background of robust anti-ferromagnetism, the Cooper-pairs are shown to be a superposition of spinquantum numbers S=0 and S=1. If the anti-ferromagnetism is weak, a continuous super-symmetric rotation is identified connecting s-wave superconductivity to anti-ferromagnetism.",0309453v2 2003-09-19,High-frequency spin valve effect in ferromagnet-semiconductor-ferromagnet structure based on precession of injected spins,"New mechanism of magnetoresistance, based on tunneling-emission of spin polarized electrons from ferromagnets (FM) into semiconductors (S) and precession of electron spin in the semiconductor layer under external magnetic field, is described. The FM-S-FM structure is considered, which includes very thin heavily doped (delta-doped) layers at FM-S interfaces. At certain parameters the structure is highly sensitive at room-temperature to variations of the field with frequencies up to 100 GHz. The current oscillates with the field, and its relative amplitude is determined only by the spin polarizations of FM-S junctions at relatively large bias voltage.",0309473v3 2003-09-29,Magnons in CMR pyrochlore Tl2Mn2O7,"Well defined spin waves were observed when the spin dynamics of Tl2Mn2O7, the first pyrochlore compound found to exhibit colossal magnetoresistance, was measured [J.W.Lynn et al., Phys.Rev.Lett. 80,4582(1998)], in stark contrast with the experimental results on the larger family of magnetoresistive manganites with perovskite structure. In this work, we present our calculation for the spin waves in Tl2Mn2O7, which we described using the microscopic generic model proposed recently for this compound [C.I.Ventura and M.A.Gusmao, Phys.Rev.B 65, 14422(2002)]. We have employed a canonical transformation to determine perturbatively the effective spin-wave Hamiltonian, obtaining therefrom the renormalization of the ferromagnetic spin waves related to the localized Mn$^{4+}$ spins, due to their coupling with the conduction electrons present. We have calculated the magnon dispersion relations along different paths in the first Brillouin zone, comparing them with those which are obtained for an ideal isotropic ferromagnet. This comparison evidences an agreement between the ferromagnetic magnons obtained from the generic model and the bare spin waves, such as had been found in neutron scattering experiments.",0309684v1 2003-10-07,Phase diagram of a superconductor / ferromagnet bilayer,"The magnetic field (H) - temperature (T) phase diagram of a superconductor is significantly altered when domains are present in an underlying ferromagnet with perpendicular magnetic anisotropy. When the domains have a band-like shape, the critical temperature Tc of the superconductor in zero field is strongly reduced, and the slope of the upper critical field as a function of T is increased by a factor of 2.4 due to the inhomogeneous stray fields of the domains. Field compensation effects can cause an asymmetric phase boundary with respect to H when the ferromagnet contains bubble domains. For a very inhomogeneous domain structure, Tc~H^2 for low H and Tc~H for higher fields, indicating a dimensional crossover from a one-dimensional network-like to a two-dimensional behavior in the nucleation of superconductivity.",0310132v2 2003-10-27,Why Ni$_3$Al is an itinerant ferromagnet but Ni$_3$Ga is not,"Ni$_3$Al and Ni$_3$Ga are closely related materials on opposite sides of a ferromagnetic quantum critical point. The Stoner factor of Ni is virtually the same in both compounds and the density of states is larger in Ni$_3$Ga. So, according to the Stoner theory, it should be more magnetic, and, in LDA calculations, it is. However, experimentally, it is a paramagnet, while Ni$_3$Al is an itinerant ferromagnet. We show that the critical spin fluctuations are stronger than in Ni$_3$Ga, due to a weaker q-dependence of the susceptibility, and this effect is strong enough to reverse the trend. The approach combines LDA calculations with the Landau theory and the fluctuation-dissipation theorem using the same momentum cut-off for both materials. The calculations provide evidence for strong, beyond LDA, spin fluctuations associated with the critical point in both materials, but stronger in Ni$_3$Ga than in Ni$_3$Al.",0310629v2 2003-11-11,Investigation of the ferromagnetic transition in the correlated 4d perovskites SrRu$_{1-x}$Rh$_x$O$_3$,"The solid-solution SrRu$_{1-x}$Rh$_x$O$_3$ ($0\le x \le1$) is a variable-electron-configuration system forming in the nearly-cubic-perovskite basis, ranging from the ferromagnetic 4$d^4$ to the enhanced paramagnetic 4$d^5$. Polycrystalline single-phase samples were obtained over the whole composition range by a high-pressure-heating technique, followed by measurements of magnetic susceptibility, magnetization, specific heat, thermopower, and electrical resistivity. The ferromagnetic order in long range is gradually suppressed by the Rh substitution and vanishes at $x \sim 0.6$. The electronic term of specific-heat shows unusual behavior near the critical Rh concentration; the feature does not match even qualitatively with what was reported for the related perovskites (Sr,Ca)RuO$_3$. Furthermore, another anomaly in the specific heat was observed at $x \sim 0.9$.",0311228v1 2003-11-22,Influence of a Uniform Current on Collective Magnetization Dynamics in a Ferromagnetic Metal,"We discuss the influence of a uniform current, $\vec{j} $, on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy $\epsilon(\vec{q})$ has a current-induced contribution proportional to $\vec{q}\cdot \vec{\cal J}$, where $\vec{\cal J}$ is the spin-current, and predict that collective dynamics will be more strongly damped at finite ${\vec j}$. We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for $j \gtrsim 10^{9} {\rm A} {\rm cm}^{-2}$. We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems.",0311522v1 2003-12-10,Tunable spin polarization in III-V quantum wells with a ferromagnetic barrier,"We demonstrate the epitaxial growth of optical-quality electrically-gated III-V ferromagnetic quantum structures. Photoluminescence spectroscopy reveals that initially unpolarized photoexcited holes in a GaAs quantum well become spin-polarized opposite to the magnetization of an adjacent digital ferromagnetic layer in the AlGaAs barrier. A vertical bias is used to tune the spin polarization from -0.4 to 6.3 percent at T = 5 K and B = 1 kG during which the luminescence becomes quenched, indicating that the polarization is mediated by wave function overlap between heavy holes in the quantum well and Mn-ions in the barrier. Polarization is observed under negligible current flow and is insensitive to the initial spin orientation of the carriers, differentiating the effect from both electrical and optical spin injection.",0312240v1 2003-12-10,Exchange Biasing of the Ferromagnetic Semiconductor Ga1-xMnxAs,"We demonstrate the exchange coupling of a ferromagnetic semiconductor (Ga1-xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet (T_B = 48 +- 2 K) and the Curie temperature of the ferromagnet (T_C = 55.1 +- 0.2 K) are comparable. The resulting exchange bias manifests itself as a clear shift in the magnetization hysteresis loop when the bilayer is cooled in the presence of an applied magnetic field and an enhancement of the coercive field.",0312259v2 2003-12-18,Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge,"A first-principles investigation of the anomalous ferromagnetism of a quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our calculations show that the symmetry breaking at the step leads to an easy magnetization axis at an odd angle of $\sim20^{\circ}$ {\em towards} the Pt step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature} {\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A significant enhancement of the Co orbital magnetic moment is achieved when modest electron correlations are treated within LSDA+$U$ calculations.",0312467v1 2003-12-25,Influence of Fe and Co on Phase Transitions in Ni-Mn-Ga Alloys,"Differential scanning calorimetry (DSC) and magnetic measurements were performed to study the influence of ferromagnetic 3-d transition elements Fe and Co on structural and magnetic properties of ferromagnetic shape memory alloys Ni2MnGa. Addition of Fe or Co on the Ni sites decreases the temperature of martensitic phase transition Tm, whereas addition of Co on the Mn sites results in a considerable increase of Tm. Magnetic measurement revealed that Curie temperature TC increases upon substitution of Fe or Co for Ni. This observation is of importance for design of high temperature ferromagnetic shape memory alloys.",0312639v1 2004-01-23,Nonmonotonic inelastic tunneling spectra due to surface spin excitations in ferromagnetic junctions,"The paper addresses inelastic spin-flip tunneling accompanied by surface spin excitations (magnons) in ferromagnetic junctions. The inelastic tunneling current is proportional to the magnon density of states which is energy-independent for the surface waves and, for this reason, cannot account for the bias-voltage dependence of the observed inelastic tunneling spectra. This paper shows that the bias-voltage dependence of the tunneling spectra can arise from the tunneling matrix elements of the electron-magnon interaction. These matrix elements are derived from the Coulomb exchange interaction using the itinerant-electron model of magnon-assisted tunneling. The results for the inelastic tunneling spectra, based on the nonequilibrium Green's function calculations, are presented for both parallel and antiparallel magnetizations in the ferromagnetic leads.",0401460v6 2004-01-30,Magnetoelectric Effects in Ferromagnetic Metal-Piezoelectric Oxide Layered Structures,"Frequency dependence of magnetoelectric (ME) coupling is investigated in trilayers of ferromagnetic alloy and piezoelectric lead zirconate titanate (PZT). The ferromagnetic phases studied include permendur, a soft magnet with high magnetostriction, iron, nickel, and cobalt. Low frequency data on ME voltage coefficient versus bias magnetic field indicate strong coupling only for trilayers with permendure or Ni. Measurements of frequency dependence of ME voltage reveal a giant ME coupling at electromechanical resonance. The ME interactions for transverse fields is an order of magnitude stronger than for longitudinal fields. The maximum voltage coefficient of 90 V/cm Oe at resonance is measured for samples with nickel or permendure and is three orders of magnitude higher than low-frequency values.",0401648v1 2004-02-04,Magnetoresistance Effect in Spin-Polarized Junctions of Ferromagnetically Contacting Multiple Conductive Paths: Applications to Atomic Wires and Carbon Nanotubes,"For spin-polarized junctions of ferromagnetically contacting multiple conductive paths, such as ferromagnet (FM)/atomic wires/FM and FM/carbon nanotubes/FM junctions, we theoretically investigate spin-dependent transport to elucidate the intrinsic relation between the number of paths and conduction, and to enhance the magnetoresistance (MR) ratio. When many paths are randomly located between the two FMs, electronic wave interference between the FMs appears, and then the MR ratio increases with increasing number of paths. Furthermore, at each number of paths, the MR ratio for carbon nanotubes becomes larger than that for atomic wires, reflecting the characteristic shape of points in contact with the FM.",0402119v1 2004-02-10,Magnetism in semiconductors: A dynamical mean field study of ferromagnetism in Ga_{1-x}Mn_xAs,"We employ the dynamical mean field approximation to perform a systematic study of magnetism in Ga_{1-x}Mn_xAs. Our model incorporates the effects of the strong spin-orbit coupling on the J=3/2 GaAs valence bands and of the exchange interaction between the randomly distributed magnetic ions and the itinerant holes. The ferromagnetic phase transition temperature T_c is obtained for different values of the impurity-hole coupling J_c and of the hole concentration n_h at the Mn doping of x=0.05. We also investigate the temperature dependence of the local magnetization and spin polarization of the holes. By comparing our results with those for a single band Hamiltonian in which the spin-orbit coupling is switched off, we conclude that the spin-orbit coupling in Ga_{1-x}Mn_xAs gives rise to frustration in the ferromagnetic order, strengthening recent findings by Zarand and Janko (Phys. Rev. Lett. 89, 047201 (2002)).",0402289v1 2004-02-19,Fulde-Ferrell-Larkin-Ovchinnikov-like state in Ferromagnet-Superconductor Proximity System,"We discuss some properties of the ferromagnet-superconductor proximity system. In particular, the emphasis is put on the physics of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state. In addition to Andreev reflections it features a number of unusual thermodynamic and transport properties, like: oscillatory behavior of the pairing amplitude, density of states and superconducting transition temperature as a function of the ferromagnet thickness. Surprisingly, under certain conditions spontaneous spin polarized current is generated in the ground state of such a system. We provide some informations regarding experimental observations of this exotic state.",0402492v1 2004-02-24,"Magneto-transport and magneto-optical properties of ferromagnetic (III,Mn)V semicondcutors: a review","Rapid developments in material research of metallic ferromagnetic (III,Mn)V semiconductors over the past few years have brought a much better understanding of these complex materials. We review here some of the main developments and current understanding of the bulk properties of these systems within the metallic regime, focusing principally on the magneto-transport and magneto-optical properties. Although several theoretical approaches are reviewed, the bulk of the review uses the effective Hamiltonian approach, which has proven useful in describing many of these properties namely in (Ga,Mn)As and (In,Mn)As. The model assumes a ferromagnetic coupling between Mn d-shell local moments mediated by holes in the semiconductor valence band.",0402568v1 2004-02-26,"Optical properties of metallic (III,Mn)V ferromagnetic semiconductors in the infrared to visible range","We report on a study of the ac conductivity and magneto-optical properties of metallic ferromagnetic (III,Mn)V semiconductors in the infrared to visible spectrum. Our analysis is based on the successful kinetic exchange model for (III,Mn)V ferromagnetic semiconductors. We perform the calculations within the Kubo formalism and treat the disorder effects pertubatively within the Born approximation, valid for the metallic regime. We consider an eight-band Kohn-Luttinger model (six valence bands plus two conduction bands) as well as a ten-band model with additional dispersionless bands simulating phenomenologically the upper-mid-gap states induced by antisite and interstitial impurities. These models qualitatively account for optical-absorption experiments and predict new features in the mid-infrared Kerr angle and magnetic-circular-dichroism properties as a function of Mn concentration and free carrier density.",0402661v2 2004-03-01,Spin glass models with ferromagnetically biased couplings on the Bethe lattice: analytic solutions and numerical simulations,"We derive the zero-temperature phase diagram of spin glass models with a generic fraction of ferromagnetic interactions on the Bethe lattice. We use the cavity method at the level of one-step replica symmetry breaking (1RSB) and we find three phases: A replica-symmetric (RS) ferromagnetic one, a magnetized spin glass one (the so-called mixed phase), and an unmagnetized spin glass one. We are able to give analytic expressions for the critical point where the RS phase becomes unstable with respect to 1RSB solutions: we also clarify the mechanism inducing such a phase transition. Finally we compare our analytical results with the outcomes of a numerical algorithm especially designed for finding ground states in an efficient way, stressing weak points in the use of such numerical tools for discovering RSB effects. Some of the analytical results are given for generic connectivity.",0403053v3 2004-03-01,Enhancing $T_c$ in ferromagnetic semiconductors,"We theoretically investigate disorder effects on the ferromagnetic transition ('Curie') temperature $T_c$ in dilute III$_{1-x}$Mn$_x$V magnetic semiconductors (e.g. Ga$_{1-x}$Mn$_x$As) where a small fraction ($x \approx 0.01-0.1$) of the cation atoms (e.g. Ga) are randomly replaced by the magnetic dopants (e.g. Mn), leading to long-range ferromagnetic ordering for $TT_B$.",0403446v1 2004-03-18,Measurement of Spin Polarization by Andreev Reflection in Ferromagnetic In1-xMnxSb Epilayers,"We carried out Point Contact Andreev Reflection (PCAR) spin spectroscopy measurements on epitaxially-grown ferromagnetic In1-xMnxSb epilayers with a Curie temperature of ~9K. The spin sensitivity of PCAR in this material was demonstrated by parallel control studies on its non-magnetic analog, In1-yBeySb. We found the conductance curves of the Sn point contacts with In1-yBeySb to be fairly conventional, with the possible presence of proximity-induced superconductivity effects at the lowest temperatures. The experimental Z-values of interfacial scattering agreed well with the estimates based on the Fermi velocity mismatch between the semiconductor and the superconductor. These measurements provided control data for subsequent PCAR measurements on ferromagnetic In1-xMnxSb, which indicated spin polarization in In1-xMnxSb to be 52 +- 3%.",0403451v1 2004-03-21,Phase diagram and magnetic properties of La$_{1-x}$Ca$_x$MnO$_3$ compound for $0\leq x \leq 0.23$,"In this article a detailed study of La$_{1-x}$Ca$_x$MnO$_3$ ($0\leq x \leq 0.23$) phase diagram using powder x-ray diffraction and magnetization measurements is presented. Unfortunately, in the related literature no properly characterized samples have been used, with consequence the smearing of the real physics in this complicated system. As the present results reveal, there are two families of samples. The first family concerns samples prepared in atmosphere ($P({\rm O}_2)=0.2$ Atm) which are all ferromagnetic with Curie temperature rising with $x$. The second family concerns samples, where a post annealing in nearly zero oxygen partial pressure is applied. These samples show a canted antiferromagnetic structure for $0\leq x \leq 0.1$ below $T_N$, while for $0.125\leq x <0.23$ an unconventional ferromagnetic insulated phase is present below $T_c$. The most important difference between nonstoichiometric and stoichiometric samples concerning the magnetic behavior, is the anisotropy in the exchange interactions, in the stoichiometric samples putting forward the idea that a new orbital ordered phase is responsible for the ferromagnetic insulating regime in the La$_{1-x}$Ca$_x$MnO$_3$ compound.",0403543v1 2004-03-24,Phase diagram and magnetic collective excitations of the Hubbard model in graphene sheets and layers,"We discuss the magnetic phases of the Hubbard model for the honeycomb lattice both in two and three spatial dimensions. A ground state phase diagram is obtained depending on the interaction strength U and electronic density n. We find a first order phase transition between ferromagnetic regions where the spin is maximally polarized (Nagaoka ferromagnetism) and regions with smaller magnetization (weak ferromagnetism). When taking into account the possibility of spiral states, we find that the lowest critical U is obtained for an ordering momentum different from zero. The evolution of the ordering momentum with doping is discussed. The magnetic excitations (spin waves) in the antiferromagnetic insulating phase are calculated from the random-phase-approximation for the spin susceptibility. We also compute the spin fluctuation correction to the mean field magnetization by virtual emission/absorpion of spin waves. In the large $U$ limit, the renormalized magnetization agrees qualitatively with the Holstein-Primakoff theory of the Heisenberg antiferromagnet, although the latter approach produces a larger renormalization.",0403605v2 2004-04-01,Cooperative dynamics in doped manganite films: phonon anomalies in the ferromagnetic state,"We present optical measurements of phononic excitations in La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) and La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) thin films covering the full temperature range from the metallic ferromagnetic to the insulating paramagnetic phase. All eight phonons expected for the R$\bar{3}$c symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma symmetry in LCMO have been determined. Close to the ferromagnetic-to-paramagnetic transition both compounds reveal an anomalous behavior but with different characteristics. Anomalies in the phononic spectra are a manifestation of the coupling of lattice degrees of freedom (DOF) to electronic DOF. Specifically, the low-frequency external group proves to be an indicator for lattice modifications induced by electronic correlations. The enhanced electron-phonon coupling in LCMO is responsible for Fano-like interference effects of distinct phonon modes with electronic continuum excitations: we observe asymmetric phonon line shapes, mode splitting and spectral weight transfer between modes.",0404024v1 2004-04-08,Exchange induced ordinary reflection in a single-channel superconductor-ferromagnet-superconductor junction,"The stationnary Josephson effect in a clean Superconductor-Ferromagnet-Superconductor junction is revisited for arbitrarily large spin polarizations. The quasiclassical calculation of the supercurrent assumes that the Andreev reflection is complete for all channels. However, De Jong and Beenakker have shown that the Andreev reflection at a clean FS interface is incomplete, due to the exchange interaction in the ferromagnet. Taking into account this incomplete Andreev reflection, we investigate the quasiparticle spectrum, the Josephson current and the $0-\pi$ transition in a ballistic single channel SFS junction. We find that energy gaps open in the phase dependent spectrum. Although the spectrum is strongly modified when the exchange energy increases, the Josephson current and the $0-\pi$ transition are only weakly affected by the incomplete Andreev reflection, except when the exchange energy is close to the Fermi energy.",0404190v3 2004-04-15,Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures,"We have measured the non-local resistance of aluminum-iron spin-valve structures fabricated by e-beam lithography and shadow evaporation. The sample geometry consists of an aluminum bar with two or more ferromagnetic wires forming point contacts to the aluminum at varying distances from each other. In the normal state of aluminum, we observe a spin-valve signal which allows us to control the relative orientation of the magnetizations of the ferromagnetic contacts. In the superconducting state, at low temperatures and excitation voltages well below the gap, we observe a spin-dependent non-local resistance which decays on a smaller length scale than the normal-state spin-valve signal. The sign, magnitude and decay length of this signal is consistent with predictions made for crossed Andreev reflection (CAR).",0404360v2 2004-04-20,Confirmation of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor and antiferromagnetic second-nearest-neighbor interactions in Rb${}_{2}$Cu${}_{2}$Mo${}_{3}$O${}_{12}$,"We have investigated magnetic properties of Rb$_2$Cu$_2$Mo$_3$O$_{12}$ powder. Temperature dependence of magnetic susceptibility and magnetic-field dependence of magnetization have shown that this cuprate is a model compound of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor (1NN) and antiferromagnetic second-nearest-neighbor (2NN) competing interactions (competing system). Values of the 1NN and 2NN interactions are estimated as $J_1 = -138$ K and $J_2 = 51$ K ($\alpha \equiv J_2 / J_1 = -0.37$). This value of $\alpha$ suggests that the ground state is a spin-singlet incommensurate state. In spite of relatively large $J_1$ and $J_2$, no magnetic phase transition appears down to 2 K, while an antiferromagnetic transition occurs in other model compounds of the competing system with ferromagnetic 1NN interaction. For that reason, Rb$_2$Cu$_2$Mo$_3$O$_{12}$ is an ideal model compound to study properties of the incommensurate ground state that are unconfirmed experimentally.",0404463v1 2004-04-20,Ferromagnetism and temperature-dependent Electronic Structure of hcp Gadolinium,"We use a combination of a many-body model analysis with an ab initio band structure calculation to derive the temperature dependent electronic quasiparticle structure of the rare-earth metal Gadolinium. As a local-moment system Gd is properly represented by the ferromagnetic (multiband) Kondo-lattice model (s-f (d-f) model). The single-particle part of the model-Hamiltonian is taken from an augmented spherical wave (ASW) band calculation. The proposed method avoids the double counting of relevant interactions by exploiting an exact limiting case of the model and takes into account the correct symmetry of atomic orbitals. The a priori only weakly correlated 5d conduction bands get via interband exchange coupling to the localized 4f levels a distinct temperature dependence which explains by a Rudermann-Kittel-Kasuya-Yosida (RKKY) -type mechanism the ferromagnetism of Gd. We get a self-consistently derived Curie temperature of 294.1 K and a T=0-moment of 7.71 $\mu_{\rm B}$, surprisingly close to the experimental values. The striking induced temperature-dependence of the 5d conduction bands explains respective photoemission data. The only parameter of the theory (interband exchange coupling J) is uniquely fixed by the band calculation.",0404472v1 2004-05-25,Dissipationless Anomalous Hall Current in the Ferromagnetic Spinel CuCr$_2$Se$_{4-x}$Br$_x$,"In a ferromagnet, an applied electric field $\bf E$ invariably produces an anomalous Hall current ${\bf J}_H$ that flows perpendicular to the plane defined by $\bf E$ and $\bf M$ (the magnetization). For decades, the question whether ${\bf J}_H$ is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr$_2$Se$_{4-x}$Br$_x$, the resistivity $\rho$ (at low temperature) may be increased 1000 fold by varying $x$(Br), without degrading the $\bf M$. We show that ${\bf J}_H/E$ (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples.",0405584v1 2004-05-27,"Microphotoluminescence study of disorder in ferromagnetic (Cd,Mn)Te quantum well","Microphotoluminescence mapping experiments were performed on a modulation doped (Cd,Mn)Te quantum well exhibiting carrier induced ferromagnetism. The zero field splitting that reveals the presence of a spontaneous magnetization in the low-temperature phase, is measured locally; its fluctuations are compared to those of the spin content and of the carrier density, also measured spectroscopically in the same run. We show that the fluctuations of the carrier density are the main mechanism responsible for the fluctuations of the spontaneous magnetization in the ferromagnetic phase, while those of the Mn spin density have no detectable effect at this scale of observation.",0405657v2 2004-06-07,"Electronic self-doping of Mo-states in A2FeMoO6 (A=Ca, Sr and Ba) half-metallic ferromagnets - a Nuclear Magnetic Resonance study","A systematic study of (A,A')2FeMoO6 (A,A'=Ca, Sr, Ba) ferromagnetic oxides with double perovskite structure has been performed using 95,97Mo and 57Fe NMR spectroscopy. These oxides are isoelectronic but have substantially different Curie temperatures. The NMR analysis provides clear evidence that the magnetic moment at Mo sites is not constant but varies sensitively with the ionic size of the alkaline ions. The 95,97Mo frequency, and thus the electronic charge at Mo ions, is found to be smaller in Ba and Ca than in Sr-based oxides. The charge release from Mo sites is accompanied by an uptake at Fe sites, and thus a self-doping Fe-Mo process is observed. This process is controlled by relevant structural parameters: the Fe-O-Mo bond length and bending. A clear relationship between the Curie temperature and the magnetic moment and thus electron density at Mo sites has been disclosed. The relevance of these findings for the understanding of ferromagnetic coupling in double perovskites is discussed.",0406161v1 2004-06-11,sin(2 phi) current-phase relation in SFS junctions with decoherence in the ferromagnet,"We propose a theoretical description of the sin(2 phi) current-phase relation in SFS junctions at the 0-$\pi$ cross-over obtained in recent experiments by Sellier et al. [Phys. Rev. Lett. 92, 257005 (2004)] where it was suggested that a strong decoherence in the magnetic alloy can explain the magnitude of the residual supercurrent at the 0-pi cross-over. To describe the interplay between decoherence and elastic scattering in the ferromagnet we use an analogy with crossed Andreev reflection in the presence of disorder. The supercurrent as a function of the length R of the ferromagnet decays exponentially over a length xi, larger than the elastic scattering length $l_d$ in the absence of decoherence, and smaller than the coherence length $l_\phi$ in the absence of elastic scattering on impurities. The best fit leads to $\xi \simeq \xi_h^{({\rm diff})}/3$, where $\xi_h^{({\rm diff})}$ is exchange length of the diffusive system without decoherence (also equal to $\xi$ in the absence of decoherence). The fit of experiments works well for the amplitude of both the sin(phi) and sin(2 phi) harmonics.",0406275v2 2004-06-21,Nonquasiparticle states in half-metallic ferromagnets,"Anomalous magnetic and electronic properties of the half-metallic ferromagnets (HMF) have been discussed. The general conception of the HMF electronic structure which take into account the most important correlation effects from electron-magnon interactions, in particular, the spin-polaron effects, is presented. Special attention is paid to the so called non-quasiparticle (NQP) or incoherent states which are present in the gap near the Fermi level and can give considerable contributions to thermodynamic and transport properties. Prospects of experimental observation of the NQP states in core-level spectroscopy is discussed. Special features of transport properties of the HMF which are connected with the absence of one-magnon spin-flip scattering processes are investigated. The temperature and magnetic field dependences of resistivity in various regimes are calculated. It is shown that the NQP states can give a dominate contribution to the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity. First principle calculations of the NQP-states for the prototype half-metallic material NiMnSb within the local-density approximation plus dynamical mean field theory (LDA+DMFT) are presented.",0406487v1 2004-06-23,Weak itinerant ferromagnetism in YCo9Si4,"Weak itinerant ferromagnetism in YCo9Si4 below about 25 K is studied by means of magnetisation, specific heat, and resistivity measurements. Single crystal X-ray Rietveld refinements at room temperature reveal a fully ordered distribution of Y, Co and Si atoms within the tetragonal space group I4/mcm isostructural with LaCo9Si4. The latter exhibits itinerant electron metamagnetism with an induced moment of about 1 mu_B/f.u. above 6 T, whereas YCo9Si4 exhibits a spontaneous magnetisation M0~12 Am^2/kg at 2 K which corresponds to an ordered moment of about 1.6 mu_B/f.u. indicating weak itinerant ferromagnetism.",0406548v1 2004-06-23,Intermediate valence behavior in CeCo9Si4,"The novel ternary compound CeCo$_9$Si$_4$ has been studied by means of specific heat, magnetisation, and transport measurements. Single crystal X-ray Rietveld refinements reveal a fully ordered distribution of Ce, Co and Si atoms with the tetragonal space group I4/mcm isostructural with other RCo9Si4. The smaller lattice constants of CeCo9Si4 in comparison with the trend established by other RCo9Si4 is indicative for intermediate valence of cerium. While RCo9Si4 with R= Pr, .. Tb, and Y show ferromagnetism and LaCo9Si4 is nearly ferromagnetic, CeCo9Si4 remains paramagnetic even in external fields as large as 40 T, though its electronic specific heat coefficient (g~190 mJ/molK^2) is of similar magnitude as that of metamagnetic LaCo9Si4 and weakly ferromagnetic YCo9Si4.",0406551v1 2004-07-12,Sign of the crossed conductances at a FSF double interface,"Crossed conductance in hybrid Ferromagnet / Superconductor / Ferromagnet (FSF) structures results from the competition between normal transmission and Andreev reflection channels. Crossed Andreev reflection (CAR) and elastic cotunneling (EC) between the ferromagnets are dressed by local Andreev reflections, which play an important role for transparent enough interfaces and intermediate spin polarizations. This modifies the simple result previously obtained at lowest order, and can explain the sign of the crossed resistances in a recent experiment [D. Beckmann {\sl et al.}, cond-mat/0404360]. This holds both in the multiterminal hybrid structure model (where phase averaging over the Fermi oscillations is introduced ``by hand'' within the approximation of a single non local process) and for infinite planar interfaces (where phase averaging naturally results in the microscopic solution with multiple non local processes).",0407283v2 2004-07-19,Anomalous self-energy and Fermi surface quasi-splitting in the vicinity of a ferromagnetic instability,"We discuss the low-temperature behavior of the electronic self-energy in the vicinity of a ferromagnetic instability in two dimensions within the two-particle self-consistent approximation, functional renormalization group and Ward-identity approaches. Although the long-range magnetic order is absent at T>0, the self-energy has a non-Fermi liquid form at low energies w<\Delta_0 near the Fermi level, where Delta_0 is the ground-state spin splitting. The spectral function at temperatures T400 K as well as spin polarization of the charge carriers have been observed in ZnMnO thin films grown on Al2O3 and MgO substrates. The magnetic properties depended on the exact Mn concentration and the growth parameters. In well-characterized single-phase films, the magnetic moment is 4.8?B/Mn at 350 K, the highest moment yet reported for any Mn doped magnetic semiconductor. Anomalous Hall effect shows that the charge carriers (electrons) are spin polarized and participate in the observed ferromagnetic behavior.",0408294v1 2004-08-27,"Confined spin waves reveal an assembly of nanosize domains in ferromagnetic La(1-x)CaxMnO3 (x=0.17,0.2)","We report a study of spin-waves in ferromagnetic La$_{1-x}$Ca$_{x}$MnO$_3$, at concentrations x=0.17 and x=0.2 very close to the metallic transition (x=0.225). Below T$_C$, in the quasi-metallic state (T=150K), nearly q-independent energy levels are observed. They are characteristic of standing spin waves confined into finite-size ferromagnetic domains, defined in {\bf a, b) plane for x=0.17 and in all q-directions for x=0.2. They allow an estimation of the domain size, a few lattice spacings, and of the magnetic coupling constants inside the domains. These constants, anisotropic, are typical of an orbital-ordered state, allowing to characterize the domains as ""hole-poor"". The precursor state of the CMR metallic phase appears, therefore, as an assembly of small orbital-ordered domains.",0408611v2 2004-09-06,Bubble domains in disc-shaped ferromagnetic particles,"We study the fundamental magnetic states of disc-shaped ferromagnetic particles with a uniaxial anisotropy along the symmetry axis. Besides the monodomain, a bidomain state is also identified and studied both numerically and theoretically. This bidomain state consists of two coaxial oppositely magnetized cylindrically symmetric domains and remains stable even at zero bias field, unlike magnetic bubbles in ferromagnetic films. For a given disc thickness we find the critical radius above which the magnetization configuration falls into the bidomain bubble state. The critical radius depends strongly on the film thickness especially for ultrathin films. The effect of an external field is also studied and the bidomain state is found to remain stable over a range of field strengths.",0409129v1 2004-09-07,From itinerant ferromagnetism to insulating antiferromagnetism: A magnetic and transport study of single crystal SrRu1-xMnxO3,"We report results of a magnetic and transport study of SrRu1-xMnxO3, i.e., Mn doped SrRuO3. The Mn doping drives the system from the itinerant ferromagnetic state (TC=165 K for x=0) through a quantum critical point at xc=0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Neel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59. Accompanying this quantum phase transition is a drastic change in resistivity by as much as 8 orders of magnitude as a function of x at low temperatures. The critical composition xc=0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.",0409157v1 2004-09-12,Unusual direction dependence of exchange energies in GaAs:Mn - Is the RKKY description relevant,"Ferromagnetism in Mn-doped GaAs, the prototypical dilute magnetic semiconductor, has so far been attributed to hole mediated RKKY-type interactions. First-principles calculations reveal a strong direction dependence of the ferromagnetic (FM) stabilization energy of two magnetic ions, a dependence that cannot be explained within RKKY. In the limit of host-like hole (engineered here by an GGA+U approach with large U) where the RKKY model is applicable, we find that the exchange energies are strongly reduced, suggesting that this limit cannot explain the observed ferromagnetism. The dominant contribution stabilizing the FM state is found to be maximal for <110>-oriented pairs and minimal for <100> oriented pairs, providing an alternate explanation for magnetism in such materials in terms of energy lowering due to p-d hopping interactions, and offering a new design degree of freedom to enhance FM.",0409295v1 2004-09-25,Bose-Einstein condensation of trapped interacting spin-1 atoms,"We investigate Bose-Einstein condensation of trapped spin-1 atoms with ferromagnetic or antiferromagnetic two-body contact interactions. We adopt the mean field theory and develop a Hartree-Fock-Popov type approximation in terms of a semiclassical two-fluid model. For antiferromagnetic interactions, our study reveals double condensations as atoms in the $|m_F=0>$ state never seem to condense under the constraints of both the conservation of total atom number $N$ and magnetization $M$. For ferromagnetic interactions, however, triple condensations can occur. Our results can be conveniently understood in terms of the interplay of three factors: (anti) ferromagnetic atom-atom interactions, $M$ conservation, and the miscibilities between and among different condensed components.",0409680v1 2004-10-04,Magnetotransport properties of ferromagnetic Pr0.85Ca0.15MnO3/ferroelectric Ba0.6Sr0.4TiO3 superlattice films,"Artificial superlattices designed with ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were grown on (100) SrTiO3 substrates. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low field effect, was observed as the number of ferroelectric Ba0.6Sr0.4TiO3 layer thickness increases even up to 9 unit cells. For example, the superlattice [(Pr0.85Ca0.15MnO3)10(Ba0.6Sr0.4TiO3)9]25 shows 35 % MR at 100 K, though the Pr0.85Ca0.15MnO3 film was a robust insulator with negligible MR even at high applied magnetic field. This observed large MR cannot be explained by simple interfacial ferromagnetism or by the tunneling magnetoresistance. One possible explanation could be the effect due to the ferroelectric spacer layer and the associated magnetoelectric coupling.",0410081v1 2004-10-09,Electronic Structure of Strained Manganite Thin Films with Room Temperature Ferromagnetism Investigated by Hard X-ray Photoemission Spectroscopy,"We report the bulk sensitive Hard X-ray (hv=5.95keV) core level photoemission spectroscopy to investigate the intrinsic electronic structure of strained (La0.85Ba0.15)MnO3 thin films. In a 20nm thick well-strained film with strongly enhanced ferromagnetism, a new sharp satellite peak appeared at the low energy site of the Mn 2p3/2 main peak, whereas a broader signal was observed for the unstrained film with 300nm thickness. Cluster calculations revealed that the intensity corresponded to the density of the state at the Fermi level relating to the magnitude of the ferromagnetic order. The satellite intensity also agreed quantitatively with the square of the magnetization.",0410223v1 2004-10-29,Weak ferromagnetism in antiferromagnets: Fe$_{2}$O$_{3}$ and La$_{2}$CuO$_{4}$,"The problem of weak ferromagnetism in antiferromagnets due to canting of magnetic moments was treated using Green's function technique. At first the eigenvalues and eigenfunctions of the electronic Hamiltonian corresponding to collinear magnetic configuration are calculated which are then used to determine first and second variations of the total energy as a function of the magnetic moments canting angle. Spin-orbit coupling is taken into account via perturbation theory. The results of calculations are used to determine an effective spin Hamiltonian. This Hamiltonian can be mapped on conventional spin Hamiltonian that allows to determine parameters of isotropic and anisotropic (Dzyaloshinskii-Moriya) exchange interactions. The method was applied to the typical antiferromagnets with weak ferromagnetism Fe$_{2}$O$_{3}$ and La$_{2}$CuO$_{4}$. The obtained directions and values of the magnetic moments canting angles are in a reasonable agreement with experimental data.",0410767v1 2004-10-30,Dynamics of Domain Wall in a Biaxial Ferromagnet With Spin-torque,"The dynamics of the domain wall (DW) in a biaxial ferromagnet interacting with a spin-polarized current are described by sine-gordon (SG) equation coupled with Gilbert damping term in this paper. Within our frame-work of this model, we obtain a threshold of the current in the motion of a single DW with the perturbation theory on kink soliton solution to the corresponding ferromagnetic system, and the threshold is shown to be dependent on the Gilbert damping term. Also, the motion properties of the DW are discussed for the zero- and nonzero-damping cases, which shows that our theory to describe the dynamics of the DW are self-consistent.",0411005v3 2004-11-08,Local Density of States and Order Parameter Configurations in Layered Ferromagnet-Superconductor Structures,"We analyze the local density of states (LDOS) of heterostructures consisting of alternating ferromagnet, $F$, and superconductor, $S$, layers. We consider structures of the $SFS$ and $SFSFSFS$ type, with thin nanometer scale $F$ and $S$ layers, within the ballistic regime. The spin-splitting effects of the ferromagnet and the mutual coupling between the $S$ regions, yield several nontrivial stable and metastable pair amplitude configurations, and we find that the details of the spatial behavior of the pair amplitude govern the calculated electronic spectra. These are reflected in discernible signatures of the LDOS. The roles that the magnetic exchange energy, interface scattering strength, and the Fermi wavevector mismatch each have on the LDOS for the different allowed junction configurations, are systematically investigated.",0411203v1 2004-11-09,Spin Glass and ferromagnetism in disordered Kondo lattice,"The competition among spin glass (SG), ferromagnetism and Kondo effect has been analysed in a Kondo lattice model where the inter-site coupling $J_{ij}$ between the localized magnetic moments is given by a generalized Mattis model \cite{Mattis} which represents an interpolation between ferromagnetism and a highly disordered spin glass. Functional integral techniques with of Grassmann fields has been used to obtain the partition function. The static approximation and the replica symmetric ansatz has also been used. The solution of the problem is presented as a phase diagram temperature $T$ {\it versus} $J_K$ (the strength of the intra-site interaction). If $J_K$ is small, for decreasing temperature there is a second order transition from a paramagnetic to a spin glass phase For lower temperatures, a first order transition appears where solutions for the spin glass order parameter and the local magnetizations are simultaneously non zero. For very low temperatures, the local magnetizations becomes thermodinamically stables. For high $J_K$, the Kondo state is dominating. These results could be helpful to clarify the experimental situation of $CeNi_{1-x}Cu_{x}$.",0411229v1 2004-11-14,"Transitions between ""$π$"" and ""0"" states in superconductor -- ferromagnet -- superconductor junctions","Experimental and theoretical study of superconductor (S) -- ferromagnet (F) -- superconductor junctions showed that in certain range of parameters (e.g., the length of the ferromagnet $d_F$, the exchange field, $E_{\rm ex}$) the ground state of a SFS junction corresponds to superconducting phase difference $\pi$ or 0. The phase diagram of a SFS junction with the respect to $\pi$ and 0 states is investigated in this letter in $E_{\rm ex}, d_{F}, T$ space. It is shown that the phase diagram is very sensitive to the geometry of the system, in particular, to the amount of disorder.",0411367v2 2004-11-17,Spin-fluctuation dominated electrical transport of Ni3Al at high pressure,"We present the first study of a magnetic quantum phase transition in the itinerant-electron ferromagnet Ni3Al at high pressures. Electrical resistivity measurements in a diamond anvil cell at hydrostatic pressures up to 100 kbar and temperatures as low as 50 mK indicate that the Curie temperature collapses towards absolute zero at a critical pressure pc=82(2) kbar. Over wide ranges in pressure and temperature, both in the ferromagnetic and paramagnetic states, the temperature variation of the resistivity is found to deviate from the conventional Fermi-liquid form. We consider the extent to which this deviation can be understood in terms of a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions.",0411451v1 2004-11-18,Ferromagnetic/DMS hybrid structures: one- and zero-dimensional magnetic traps for quasiparticles,"We investigated possibility of using local magnetic field originating from ferromagnetic island deposited on the top of semiconductor quantum well to produce zero- and one-dimensional traps for quasi-particles with spin. In particular we considered two shapes of experimentally made magnets - cylindrical and rectangular. In the case of ferromagnetic micro-disk the trap can localize spin in three dimensions, contrary to the rectangular micro-magnet which creates a trap that allows free propagation in one direction. We present in detail prediction for absorption spectrum around the main absorption edge in both type of micro-magnets.",0411482v1 2004-11-19,Phenomenological study of spin-triplet ferromagnetic superconductors,"Unconventional superconductivity with spin-triplet Cooper pairing is reviewed on the basis of the quasi-phenomenological Ginzburg-Landau theory. The superconductivity, in particular, the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the spontaneous magnetization. The mixed phase is stable whereas the other superconducting phases that usually exist in unconventional superconductors are either unstable, or, for particular values of the parameters of the theory, some of these phases are metastable at relatively low temperatures in a quite narrow domain of the phase diagram. The phase transitions from the normal phase to the phase of coexistence is of first order while the phase transition from the ferromagnetic phase to the coexistence phase can be either of first or second order depending on the concrete substance. The Cooper pair and crystal anisotropy are relevant to a more precise outline of the phase diagram shape and reduce the degeneration of the ground states of the system but they do not drastically influence the phase stability domains and the thermodynamic properties of the respective phases.",0411503v1 2004-11-24,Electrical Spin Injection in Multi-Wall carbon NanoTubes with transparent ferromagnetic contacts,"We report on electrical spin injection measurements on MWNTs . We use a ferromagnetic alloy Pd$_{1-x}$Ni$_{x}$ with x $\approx$ 0.7 which allows to obtain devices with resistances as low as 5.6 $k\Omega$ at 300 $K$. The yield of device resistances below 100 $k\Omega$, at 300 $K$, is around 50%. We measure at 2 $K$ a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel (AP) orientation and the parallel (P) orientation is about 2%.",0411623v2 2004-12-03,Metastability in zero-temperature dynamics: Statistics of attractors,"The zero-temperature dynamics of simple models such as Ising ferromagnets provides, as an alternative to the mean-field situation, interesting examples of dynamical systems with many attractors (absorbing configurations, blocked configurations, zero-temperature metastable states). After a brief review of metastability in the mean-field ferromagnet and of the droplet picture, we focus our attention onto zero-temperature single-spin-flip dynamics of ferromagnetic Ising models. The situations leading to metastability are characterized. The statistics and the spatial structure of the attractors thus obtained are investigated, and put in perspective with uniform a priori ensembles. We review the vast amount of exact results available in one dimension, and present original results on the square and honeycomb lattices.",0412077v1 2004-12-10,Thermodynamic properties of tetrameric bond-alternating spin chains,"Thermodynamic properties of a tetrameric bond-alternating Heisenberg spin chain with ferromagnetic-ferromagnetic-antiferromagnetic-antiferromagnetic exchange interactions are studied using the transfer-matrix renormalization group and compared to experimental measurements. The temperature dependence of the uniform susceptibility exhibits typical ferrimagnetic features. Both the uniform and staggered magnetic susceptibilities diverge in the limit $T\to 0$, indicating that the ground state has both ferromagnetic and antiferromagnetic long-range orders. A double-peak structure appears in the temperature dependence of the specific heat. Our numerical calculation gives a good account for the temperature and field dependence of the susceptibility, the magnetization, and the specific heat for Cu(3-Clpy)$_{2}$(N$_{3}$)$_{2}$ (3-Clpy=3-Chloroyridine).",0412275v2 2004-12-13,"Quantitative Study of Magnetotransport through a (Ga,Mn)As Single Ferromagnetic Domain","We have performed a systematic investigation of the longitudinal and transverse magnetoresistance of a single ferromagnetic domain in (Ga,Mn)As. We find that, by taking into account the intrinsic dependence of the resistivity on the magnetic induction, an excellent agreement between experimental results and theoretical expectations is obtained. Our findings provide a detailed and fully quantitative validation of the theoretical description of magnetotransport through a single ferromagnetic domain. Our analysis furthermore indicates the relevance of magneto-impurity scattering as a mechanism for magnetoresistance in (Ga,Mn)As.",0412290v2 2004-12-14,Ising model in scale-free networks: A Monte Carlo simulation,"The Ising model in uncorrelated scale-free networks has been studied by means of Monte Carlo simulations. These networks are characterized by a degree (or connectivity) distribution $P(k) \sim k^{-\gamma}$. The ferromagnetic-paramagnetic transition temperature has been studied as a function of the parameter $\gamma$. For $\gamma > 3$ our results agree with earlier analytical calculations, which found a phase transition at a temperature $T_c(\gamma)$ in the thermodynamic limit. For $\gamma \leq 3$, a ferromagnetic-paramagnetic crossover occurs at a size-dependent temperature $T_{co}$, and the system is in the ordered ferromagnetic state at any temperature for a system size $N \to \infty$. For $\gamma = 3$ and large enough $N$, the crossover temperature is found to be $T_{co} \approx A \ln N$, with a prefactor $A$ proportional to the mean degree. For $2 < \gamma < 3$, we obtain $T_{co} \sim < k > N^z$, with an exponent $z$ that decreases as $\gamma$ increases. This exponent is found to be lower than predicted by earlier calculations.",0412355v1 2004-12-21,GaN:Gd: A superdilute ferromagnetic semiconductor with a Curie temperature above 300 K,"We investigate the magnetic and magneto-optic properties of epitaxial GaN:Gd layers as a function of the external magnetic field and temperature. An unprecedented magnetic moment is observed in this diluted magnetic semiconductor. The average value of the moment per Gd atom is found to be as high as 4000 \mub as compared to its atomic moment of 8 \mub. The long-range spin-polarization of the GaN matrix by Gd is also reflected in the circular polarization of magneto-photoluminescence measurements. Moreover, the materials system is found to be ferromagnetic above room temperature in the entire concentration range under investigation (7$\times10^{15}$ to 2$\times10^{19}$ cm$^{-3}$). We propose a phenomenological model to understand the macroscopic magnetic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd.",0412564v2 2004-12-27,Ferromagnetism and possible heavy fermion behavior in single crystals of NdOs$_4$Sb$_{12}$,"Single crystals of the filled-skutterudite compound NdOs$_4$Sb$_{12}$ have been investigated by means of electrical resistivity, magnetization, and specific heat measurements. The NdOs$_4$Sb$_{12}$ crystals have the LaFe$_4$P$_{12}$-type cubic structure with a lattice parameter of 9.3 \AA. Possible heavy-fermion behavior is inferred from specific heat measurements, which reveal a large electronic specific heat coefficient $\gamma \approx 520$ mJ/mol-K$^2$, corresponding to an effective mass $m^* \sim$ 98 $m_e$. Features related to a ferromagnetic transition at {$\sim$ 0.9 K} can be observed in electrical resistivity, magnetization and specific heat. Conventional Arrott-plot analysis indicates that NdOs$_4$Sb$_{12}$ conforms to mean-field ferromagnetism.",0412713v3 2005-01-12,Ferromagnetism in Ga1-xMnxP: evidence for inter-Mn exchange mediated by localized holes within a detached impurity band,"We demonstrate that in ferromagnetic Ga1-xMnxP exchange is mediated by holes localized in a Mn-derived band. For x<0.06, infrared absorption and photoconductivity spectra indicate the presence of a Mn impurity band which is not merged with the valence band. At temperatures above TC (<65 K) electrical transport is dominated by excitation across this energy gap while nearest neighbor hopping dominates below TC. Magnetization measurements reveal a moment of 3.5 Bohr magnetons per substitutional Mn, while the large anomalous Hall signal unambiguously demonstrates that the ferromagnetism is carrier-mediated.",0501275v3 2005-01-14,Electronic self-energy and triplet pairing fluctuations in the vicinity of a ferromagnetic instability in 2D systems: the quasistatic approach,"The self-energy, spectral functions and susceptibilities of 2D systems with strong ferromagnetic fluctuations are considered within the quasistatic approach. The self-energy at low temperatures T has a non-Fermi liquid form in the energy window w>(T/vF)^(-1/3) are obtained.",0501331v2 2005-02-01,Orbital Ordering in ferromagnetic Lu2V2O7,"We have observed the orbital ordering in the ferromagnetic Mott-insulator Lu2V2O7 by the polarized neutron diffraction technique. The orbital ordering pattern determined from the observed magnetic form factors can be explained in terms of a linear combination of wave functions |yz>, |zx> and |xy>; |0> = (1/3)^(1/2) |xy> + (1/3)^(1/2)|yz> + (1/3)^(1/2) |zx> which is proportional to |(x + y + z)^2 - r^2>; where each orbital is extended toward the center-of-mass of the V tetrahedron. We discuss the stability of the ferromagnetic Lu2V2O7, using a Hubbard Hamiltonian with these three orbitals.",0502020v1 2005-02-09,Weak ferromagnetism and spiral spin structures in honeycomb Hubbard planes,"Within the Hartree Fock- RPA analysis, we derive the spin wave spectrum for the weak ferromagnetic phase of the Hubbard model on the honeycomb lattice. Assuming a uniform magnetization, the polar (optical) and acoustic branches of the spin wave excitations are determined. The bipartite lattice geometry produces a q-dependent phase difference between the spin wave amplitudes on the two sub-lattices. We also find an instability of the uniform weakly magnetized configuration to a weak antiferromagnetic spiraling spin structure, in the lattice plane, with wave vector Q along the Gamma-K direction, for electron densities n>0.6. We discuss the effect of diagonal disorder on both the creation of electron bound states, enhancement of the density of states, and the possible relevance of these effects to disorder induced ferromagnetism, as observed in proton irradiated graphite.",0502249v2 2005-02-11,Pt steps decorated by 3d nanowires calculated via an order-N method,"We present the magnetization energy and magnetic moments of 3d monowires aligned along the step edges of Pt (533) and (322) vicinal substrates. We employ an electronic structure method which scales linearly with the size of the system. This allows us to treat with ease both ferromagnetic and antiferromagnetic solutions, and in principle the method should allow for the evaluation of more complex systems, such as wires deposited on rough step edges. We find that Co, Fe, and Mn are almost perfect Heisenberg systems, with almost no change in the moment between antiferromagnetic and ferromagnetic solutions. A large polarisation of the Pt substrate is also observed. Our results are in close agreement w ith recent experimental results obtained for the case of a Co monowire. As expected the trend is for the late d series to be stabalised by ferromagnetic long range order and the middle of the d series by antiferromagnetic order. Since our method is only \emph{locally} self consistent we discuss as some length the convergence of the results with respect to the size of the region which is treated self-consistently.",0502295v1 2005-03-03,Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory,"The transport properties of diluted magnetic semiconductors (DMS) are calculated using dynamical mean field theory (DMFT) and Boltzmann transport theory. Within DMFT we study the density of states and the dc-resistivity, which are strongly parameter dependent such as temperature, doping, density of the carriers, and the strength of the carrier-local impurity spin exchange coupling. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism. We find that spin-disorder scattering, formation of bound state, and the population of the minority spin band are all operational in DMFT in different parameter range. We also develop a complementary Boltzmann transport theory for scattering by screened ionized impurities. The difference in the screening properties between paramagnetic ($T>T_c$) and ferromagnetic ($T10$ mJ/cm$^2$); the saturation value indicates a {\em complete quenching} of ferromagnetism on a sub-picosecond time scale. We attribute this fast dynamics to spin heating through $p$-$d$ exchange interaction between photo-carriers and Mn ions while the $\sim$100 ps component is interpreted as spin-lattice relaxation.",0504611v2 2005-04-27,Perpendicular spin valves with ultra-thin ferromagnetic layers,"We address two finite size effects in perpendicular transport through magnetic multilayers. When the magnetic layer thickness in spin valves becomes of the order or smaller than the spin-flip diffusion length, structural asymmetries affect the transport properties. A magnetic layer with thickness approaching the magnetic coherence length becomes transparent for spin currents polarized perpendicular to the magnetization. We use the generalized magnetoelectronic circuit theory to investigate both effects on the angular magnetoresistance (aMR) and spin transfer torque. We analyze recent aMR experiments to determine the spin-flip diffusion length in the ferromagnet as well as the interface spin-mixing conductance and propose a method to measure the ferromagnetic coherence length.",0504705v2 2005-04-27,Tunneling density of states as a function of thickness in superconductor/ strong ferromagnet bilayers,"We have made an experimental study of the tunneling density of states (DOS) in strong ferromagnetic thin films (CoFe) in proximity with a thick superconducting film (Nb) as a function of $d_F$, the ferromagnetic thickness. Remarkably, we find that as $d_F$ increases, the superconducting DOS exhibits a scaling behavior in which the deviations from the normal-state conductance have a universal shape that decreases exponentially in amplitude with characteristic length $d^*\approx 0.4$ nm. We do not see oscillations in the DOS as a function of $d_F$, as expected from predictions based on the Usadel equations, although an oscillation in $T_c(d_F)$ has been seen in the same materials.",0504739v2 2005-04-28,Effect of electron interactions on the conductivity and exchange coupling energy of disordered metallic magnetic multilayer,"We consider the effect of electron-electron interactions on the current-in-plane (CIP) conductivity and exchange coupling energy of a disordered metallic magnetic multilayer. We analyze its dependence on the value of ferromagnetic splitting of conducting electrons and ferromagnetic layers relative magnetizations orientation. We show that contribution to the CIP conductivity and exchange coupling energy as a periodic function of the angle of magnetizations relative orientation experience $ 2\pi \to \pi $ transition depending on the characteristic energies: ferromagnetic splitting of the conducting electrons and the Thouless energy of paramagnetic layer.",0504754v3 2005-05-08,Unusual Exchange Bias Associated with Phase Separation in Perovskite Cobaltites,"We report the observation of unusual exchange bias phenomena in the doped perovskite cobaltites La_{1-x}Sr_{x}CoO_{3} (x = 0.15, 0.18, and 0.30) in which a spontaneous phase separation into ferromagnetic clusters embedded in a non-ferromagnetic matrix occurs. When the La_{1-x}Sr_{x}CoO_{3} samples are cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops exhibit exchange bias, i.e., the loops shift to the negative field and the magnetization becomes asymmetric. Moreover, exchange bias disappears when the measuring magnetic field is high enough. These results suggest that the intrinsic phase inhomogeniety in a spontaneously phase-separated system may induce an interfacial exchange anisotropy after a field cooling. The diminution of exchange bias in high magnetic fields is ascribed to the propagation of ferromagnetic regions with increasing magnetic field.",0505189v1 2005-05-09,Long range statistical fluctuations of the crossed Josephson current,"We investigate the crossed Josephson effect in a geometry consisting of a double ferromagnetic bridge between two superconductors, with tunnel interfaces. The crossed Josephson current vanishes on average because the Andreev reflected hole does not follow the same sequence of impurities as the incoming electron. We show that i) the root mean square of the crossed Josephson current distribution is proportional to the square root of the junction area; and ii) the coherent coupling mediated by fluctuations is ``long range'' since it decays over the ferromagnet phase coherence length $l_\phi$, larger than the exchange length. We predict a crossed Josephson current due to fluctuations if the length of the ferromagnets is smaller than $l_\phi$ and larger than the exchange length $\xi_h$.",0505211v2 2005-05-10,Percolative Ferromagnetism in Anatase Co:TiO2,"We revisit the most widely investigated and controversial oxide diluted magnetic semiconductor (DMS), Co:TiO2, with a new high temperature film growth, and show that the corresponding material is not only an intrinsic DMS ferromagnet, but also supports a percolative mechanism of ferromagnetism. We establish the uniformity of dopant distribution across the film cross section by Z-contrast imaging via scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at spatial resolution of 0.4 nm and the oxidized 2+ valence state of cobalt by x-ray absorption spectroscopy (XAS). The dependence of magnetic properties on cobalt concentration is consistent with the defect polaron percolation model. The peculiar increase in the transport activation energy above a specific cobalt concentration further emphasizes the polaron contribution to magnetic order.",0505265v1 2005-05-16,Characterization of Magnetic Components in the Diluted Magnetic Semiconductor Zn$_{1-x}$Co$_x$O by X-ray Magnetic Circular Dichroism,"We report on the results of x-ray absorption (XAS), x-ray magnetic circular dichroism (XMCD), and photoemission experiments on {\it n}-type Zn$_{1-x}$Co$_x$O ($x=0.05$) thin film, which shows ferromagnetism at room temperature. The XMCD spectra show a multiplet structure, characteristic of the Co$^{2+}$ ion tetrahedrally coordinated by oxygen, suggesting that the ferromagnetism comes from Co ions substituting the Zn site in ZnO. The magnetic field and temperature dependences of the XMCD spectra imply that the non-ferromagnetic Co ions are strongly coupled antiferromagnetically with each other.",0505387v1 2005-05-17,Superconducting proximity effect in a diffusive ferromagnet with spin-active interfaces,"We reconsider the problem of the superconducting proximity effect in a diffusive ferromagnet bounded by tunneling interfaces, using spin-dependent boundary conditions. This introduces for each interface a phase-shifting conductance Gphi which results from the spin dependence of the phase shifts acquired by the electrons upon scattering on the interface. We show that Gphi strongly affects the density of states and supercurrents predicted for superconducting/ferromagnetic hybrid circuits. We show the relevance of this effect by identifying clear signatures of Gphi in the data of T. Kontos et al [Phys. Rev. Lett. 86, 304 (2001), ibid. 89, 137007 (2002)].",0505403v2 2005-05-19,Soft X-ray Resonant Magnetic Scattering Studies on Fe/CoO Exchange Bias System,"We have used soft X-ray Resonant Magnetic Scattering (XRMS) to search for the presence of an effective ferromagnetic moment belonging to the antiferromagnetic (AF) layer which is in close contact with a ferromagnetic (F) layer. Taking advantage of the element specificity of the XRMS technique, we have measured hysteresis loops of both Fe and CoO layers of a CoO(40 \AA)/Fe(150 \AA) exchange bias bilayer. From these measurements we have concluded that the proximity of the F layer induces a magnetic moment in the AF layer. The F moment of the AF layer has two components: one is frozen and does not follow the applied magnetic field and the other one follows in phase the ferromagnetic magnetization of the F layer. The temperature dependence of the F components belonging to the AF layer is shown and discussed.",0505482v1 2005-05-25,Polar magneto-optical Kerr effect for low-symmetric ferromagnets,"The polar magneto-optical Kerr effect (MOKE) for low-symmetric ferromagnetic crystals is investigated theoretically based on first-principle calculations of optical conductivities and a transfer matrix approach for the electrodynamics part of the problem. Exact average magneto-optical properties of polycrystals are described, taking into account realistic models for the distribution of domain orientations. It is shown that for low-symmetric ferromagnetic single crystals the MOKE is determined by an interplay of crystallographic birefringence and magnetic effects. Calculations for single and bi-crystal of hcp 11-20 Co and for a polycrystal of CrO_2 are performed, with results being in good agreement with experimental data.",0505614v1 2005-06-01,Ferromagnetism in one dimension: Critical Temperature,"Ferromagnetism in one dimension is a novel observation which has been reported in a recent work (P. Gambardella et.al., Nature {\bf 416}, 301 (2002)), anisotropies are responsibles in that relevant effect. In the present work, another approach is used to obtain transition between two different magnetic ordering phases. Critical temperature has been estimated by Binder method. Ferromagnetic long range interactions have been included in a special Hamiltonian through a power law that decays at large inter-particle distance $r$ as $r^{-\alpha}$, where $\alpha\geq0$. For the present model, we have found that the trend of the critical temperature vanishes when the range of interactions decreases ($\alpha\to\infty$) and close to mean field approximation when the range of interactions increases ($\alpha\to0$). The crossover between two these limit situations is discussed",0506033v2 2005-06-17,Theory of spin-polarized transport in ferromagnet-semiconductor structures: Unified description of ballistic and diffusive transport,"A theory of spin-polarized electron transport in ferromagnet-semiconductor heterostructures, based on a unified semiclassical description of ballistic and diffusive transport in semiconductors, is outlined. The aim is to provide a framework for studying the interplay of spin relaxation and transport mechanism in spintronic devices. Transport inside the (nondegenerate) semiconductor is described in terms of a thermoballistic current, in which electrons move ballistically in the electric field arising from internal and external electrostatic potentials, and are thermalized at randomly distributed equilibration points. Spin relaxation is allowed to take place during the ballistic motion. For arbitrary potential profile and arbitrary values of the momentum and spin relaxation lengths, an integral equation for a spin transport function determining the spin polarization in the semiconductor is derived. For field-driven transport in a homogeneous semiconductor, the integral equation can be converted into a second-order differential equation that generalizes the spin drift-diffusion equation. The spin-polarization in ferromagnet semiconductor structures is obtained by matching the spin-resolved chemical potentials at the interfaces, with allowance for spin-selective interface resistances. Illustrative examples are considered.",0506441v1 2005-06-27,Ferromagnetism of the Hubbard Model at Strong Coupling in the Hartree-Fock Approximation,"As a contribution to the study of Hartree-Fock theory we prove rigorously that the Hartree-Fock approximation to the ground state of the d-dimensional Hubbard model leads to saturated ferromagnetism when the particle density (more precisely, the chemical potential mu) is small and the coupling constant U is large, but finite. This ferromagnetism contradicts the known fact that there is no magnetization at low density, for any U, and thus shows that HF theory is wrong in this case. As in the usual Hartree-Fock theory we restrict attention to Slater determinants that are eigenvectors of the z-component of the total spin, {S}_z = sum_x n_{x,\uparrow} - n_{x,\downarrow}, and we find that the choice 2{S}_z = N = particle number gives the lowest energy at fixed 0 < mu < 4d.",0506695v2 2005-07-04,Suppression of ferromagnetism in URhGe doped with Ru,"In the correlated metal URhGe ferromagnetic order (T_C = 9.5 K) and superconductivity (T_s= 0.25 K) coexist at ambient pressure. Here we report on alloying URhGe by Ru, which enables one to tune the Curie temperature to 0 K. URuGe has a paramagnetic ground state and is isostructural to URhGe. We have prepared a series of polycrystalline URh_{1-x}Ru_{x}Ge samples over a wide range of x values. Magnetization and electrical resistivity data (T > 2 K) show, after an initial increase, a linear suppression of T_C with increasing x. The critical Ru concentration for the suppression of ferromagnetic order is x_cr ~ 0.38.",0507068v1 2005-07-11,Quantum Criticality in Ferromagnetic Single-Electron Transistors,"Considerable evidence exists for the failure of the traditional theory of quantum critical points (QCPs), pointing to the need to incorporate novel excitations. The destruction of Kondo entanglement and the concomitant critical Kondo effect may underlie these emergent excitations in heavy fermion metals -- a prototype system for quantum criticality -- but the effect remains poorly understood. Here, we show how ferromagnetic single-electron transistors can be used to study this effect. We theoretically demonstrate a gate-voltage induced quantum phase transition. The critical Kondo effect is manifested in a fractional-power-law dependence of the conductance on temperature ($T$). The AC conductance and thermal noise spectrum have related power-law dependences on frequency ($\omega$) and, in addition, show an $\omega/T$ scaling. Our results imply that the ferromagnetic nanostructure constitutes a realistic model system to elucidate magnetic quantum criticality that is central to the heavy fermions and other bulk materials with non-Fermi liquid behavior.",0507215v3 2005-07-13,Frustrated ferromagnetic spin-1/2 chain in the magnetic field,"We study the ground state properties of the Heisenberg spin-1/2 chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions using two approximate methods. One of them is the Jordan-Wigner mean-field theory and another approach based on the transformation of spin operators to bose-ones and on the variational treatment of bosonic Hamiltonian. Both approaches give close results for the ground state energy and the T=0 magnetization curve. It is shown that quantum fluctuations change the classical critical exponents in the vicinity of the transition point from the ferromagnetic to the singlet ground state. The magnetization process displays the different behavior in the regions near and far from the transition point. The relation of the obtained results to experimental magnetization curve in $Rb_{2}Cu_{2}Mo_{3}O_{12}$ is discussed.",0507301v2 2005-07-20,Superconducting Proximity Effect through a Magnetic Domain Wall,"We study the superconducting proximity effect in a superconductor-ferromagnet-superconductor (SFS) heterostructure, containing a domain wall in the ferromagnetic region. For the ferromagnet we assume an alloy with an exchange splitting of the conduction bands comparable to the superconducting gaps. We calculate the modification of the density of states in the center of the domain wall as a result of the proximity effect. We show that the density of states is sensitive to domain wall parameters due to triplet-pairing correlations created in vicinity of the domain wall. We present a theoretical tool which in a very effective way enables retaining the full spatially dependent spin-space structure of the problem.",0507482v1 2005-07-21,Ferromagnetic semiconductor single wall carbon nanotube,"Possibility of a ferromagnetic semiconductor single wall carbon nanotube (SWCNT), where ferromagnetism is due to coupling between doped magnetic impurity on a zigzag SWCNT and electrons spin, is investigate. We found, in the weak impurity-spin couplings, at low impurity concentrations the spin up electrons density of states remain semiconductor while the spin down electrons density of states shows a metallic behavior. By increasing impurity concentrations the semiconducting gap of spin up electrons in the density of states is closed, hence a semiconductor to metallic phase transition is take place. In contrast, for the case of strong coupling, spin up electrons density of states remain semiconductor and spin down electron has metallic behavior. Also by increasing impurity spin magnitude, the semiconducting gap of spin up electrons is increased.",0507503v1 2005-07-25,Interplay between magnetism and superconductivity in URhGe,"The magnetization rotation transition occurs in the itinerant ferromagnet URhGe when the field about 12T is applied in direction perpendicular to spontaneous magnetization in the plane of the smallest magnetic anisotropy energy.The transition is accompanied by the maximum of resistivity in the normal state and by reentrance of superconductivity at lower temperatures in the field interval between 8 and 13 Tesla [F.Levy et al, Science, to be published (2005)]. We discuss the magnetization orientation transition and the modification of triplet pairing superconducting state coexisting with ferromagnetism up to the fields about 2 Tesla and then reappearing in the broad vicinity of the transition. The nonsymmorphic space group crystal symmetry of ferromagnetic URhGe allows existance of antiferromagnetic ordering of magnetic moments of pairs of uranium atoms along a-axis. We show that the amplitude of this weak antiferromagnetic ordering increases below the phase transition into superconducting state due to Cooper pairs spontaneous magnetism.",0507572v1 2005-07-26,Transverse instability at a compensated interface of ferromagnetic antiferromagnetic bilayer,"The analytical theory for a ferromagnetic antiferromagnetic bilayer with a compensated interface has been developed using an explicit expression for the interfacial interaction energy density. A stability of a solution describing domain walls within ferromagnetic and antiferromagnetic films has been investigated. It is shown that even for high values of surface interaction strength a transverse instability originates within the antiferromagnetic film, because the deviation of the unit antiferromagnetic vector out of the interface plane becomes energetically favorable in large enough external magnetic field. However, it is possible to stabilize the spin distribution near the interface assuming additional in-plane anisotropy within the antiferromagnetic layer. In principle, this opens a way to control exchange bias just avoiding a complicated problem of domain rearrangement within the antiferromagnetic film.",0507613v1 2005-07-28,Theory of transverse spin dynamics in a polarized Fermi liquid and an itinerant ferromagnet,"The linear equations for transverse spin dynamics in a weakly polarized degenerate Fermi liquid with arbitrary relationship between temperature and polarization are derived from Landau-Silin phenomenological kinetic equation with general form of two-particle collision integral. Unlike the previous treatment where Fermi velocity and density of states have been taken as constants independent of polarization here we made derivation free from this assumption. The obtained equations are applicable for description of spin dynamics in paramagnetic Fermi liquid with finite polarization as well in an itinerant ferromagnet. In both cases transverse spin wave frequency is found to be proportional to the square of the wave vector with complex constant of proportionality (diffusion coefficient) such that the damping has a finite value at T=0. The polarization dependence of the diffusion coefficient is found to be different for a polarized Fermi liquid and for an itinerant ferromagnet. These conclusions are confirmed by derivation of transverse spin wave dispersion law in frame of field theoretical methods from the integral equation for the vortex function. It is shown that similar derivation taking into consideration the divergency of static transverse susceptibility also leads to the same attenuating spin wave spectrum.",0507675v1 2005-09-05,Magnetic behavior of EuCu2As2: Delicate balance between antiferromagnetic and ferromagnetic order,"The Eu-based compound, EuCu2As2, crystallizing in the ThCr2Si2-type tetragonal structure, has been synthesized and its magnetic behavior has been investigated by magnetization (M), heat-capacity (C) and electrical resistivity (rho) measurements as a function of temperature (T) and magnetic field (H) as well as by 151Eu Moessbauer measurements. The results reveal that Eu is divalent ordering antiferromagnetically below 15 K in the absence of magnetic field, apparently with the formation of magnetic Brillouin-zone boundary gaps. A fascinating observation is made in a narrow temperature range before antiferromagnetism sets in: That is, there is a remarkable upturn just below 20 K in the plot of magnetic susceptibility versus T even at low fields, as though the compound actually tends to order ferromagnetically. There are corresponding anomalies in the magnetocaloric effect data as well. In addition, a small application of magnetic field (around 1 kOe at 1.8 K) in the antiferromagnetic state causes spin-reorientation effect. These results suggest that there is a close balance between antiferromagnetism and ferromagnetism in this compound",0509108v1 2005-09-06,"First- and Second-Order Phase Transitions, Fulde-Ferrel Inhomogeneous State and Quantum Criticality in Ferromagnet/Superconductor Double Tunnel Junctions","First- and second-order phase transitions, Fulde-Ferrel (FF) inhomogeneous superconducting (SC) state and quantum criticality in ferromagnet/superconductor/ferromagnet double tunnel junctions are investigated. For the antiparallel alignment of magnetizations, it is shown that a first-order phase transition from the homogeneous BCS state to the inhomogeneous FF state occurs at a certain bias voltage $V^{\ast}$; while the transitions from the BCS state and the FF state to the normal state at $% V_{c}$ are of the second-order. A phase diagram for the central superconductor is presented. In addition, a quantum critical point (QCP), $% V_{QCP}$, is identified. It is uncovered that near the QCP, the SC gap, the chemical potential shift induced by the spin accumulation, and the difference of free energies between the SC and normal states vanish as $% |V-V_{QCP}|^{z\nu}$ with the quantum critical exponents $z\nu =1/2$, 1 and 2, respectively. The tunnel conductance and magnetoresistance are also discussed.",0509128v1 2005-09-19,Effect of random disorder and spin frustration on the reentrant spin glass phase and ferromagnetic phase in stage-2 Cu_{0.93}Co_{0.07}Cl_{2} graphite intercalation compound near the multicritical point,"Stage-2 Cu$_{0.93}$Co$_{0.07}$Cl$_{2}$ graphite intercalation compound magnetically behaves like a reentrant ferromagnet near the multicritical point ($c_{MCP} \approx 0.96$). It undergoes two magnetic phase transitions at $T_{RSG}$ ($= 6.64 \pm 0.05$ K) and $T_{c}$ ($= 8.62 \pm 0.05$ K). The static and dynamic nature of the ferromagnetic and reentrant spin glass phase has been studied using DC and AC magnetic susceptibility. Characteristic memory phenomena of the DC susceptibility are observed at $T_{RSG}$ and $T_{c}$. The nonlinear AC susceptibility $\chi_{3}^{\prime}$ has a positive local maximum at $T_{RSG}$, and a negative local minimum at $T_{c}$. The relaxation time $\tau$ between $T_{RSG}$ and $T_{c}$ shows a critical slowing down: $\tau$ with $x = 13.1 \pm 0.4$ and $\tau_{0}^{*} = (2.5 \pm 0.5) \times 10^{-13}$ sec. The influence of the random disorder on the critical behavior above $T_{c}$ is clearly observed: $\alpha = -0.66$, $\beta = 0.63$, and $\gamma = 1.40$. The exponent of $\alpha$ is far from that of 3D Heisenberg model.",0509494v1 2005-09-29,Vortex Lattices in Stripe Domains of Ferromagnet/Superconductor Bilayer,"The continuum theory of domain structures in ferromagnetic/superconducting bilayers fails when the equilibrium domain size becomes comparable with effective penetration depth $\Lambda$. Instead, a lattice of discrete vortices must be considered. Here, we report our results on the discrete vortex lattices in stripe domain structures of ferromagnetic/superconducting bilayers. The vortices are assumed to be situated periodically on chains in stripe domains. We study the configurations containing up to three chains per domain, and calculate their equilibrium energies, equilibrium domain size and vortex positions through a method based on London-Maxwell equations. In equilibrium, the vortices in the neighbor domains are half-way shifted while they are next to each other in the same domain. Additionally, more vortex chains per domain appear spontaneously depending on magnetization and domain wall energy.",0509782v1 2005-10-06,Organometallic Benzene-Vanadium Wire: One-Dimensional Half-Metallic Ferromagnet,"Using density functional theory we have performed theoretical investigations of the electronic properties of a free-standing one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multi-decker V_n(C6H6)_{n+1} clusters which can be synthesized. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We found that the half-metallic behavior is conserved up to 12%, longitudinal elongation of the wire. However, under further stretching, the system exhibits a transition to a high-spin ferromagnetic state that is accompanied by an abrupt jump of the magnetic moment and a gain of exchange energy.",0510144v1 2005-10-06,Magneto-optical evidence of double exchange in a percolating lattice,"Substituting $Eu$ by $Ca$ in ferromagnetic $EuB_6$ leads to a percolation limited magnetic ordering. We present and discuss magneto-optical data of the $Eu_{1-x}Ca_{x}B_6$ series, based on measurements of the reflectivity $R(\omega)$ from the far infrared up to the ultraviolet, as a function of temperature and magnetic field. Via the Kramers-Kronig transformation of $R(\omega)$ we extract the complete absorption spectra of samples with different values of $x$. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing $Ca$-content at low temperatures.",0510155v1 2005-10-07,Theory of orbital state and spin interactions in ferromagnetic titanates,"A spin-orbital superexchange Hamiltonian in a Mott insulator with $t_{2g}$ orbital degeneracy is investigated. More specifically, we focus on a spin ferromagnetic state of the model and study a collective behavior of orbital angular momentum. Orbital order in the model occurs in a nontrivial way -- it is stabilized exclusively by quantum effects through the order-from-disorder mechanism. Several energetically equivalent orbital orderings are identified. Some of them are specified by a quadrupole ordering and have no unquenched angular momentum at low energy. Other states correspond to a noncollinear ordering of the orbital angular momentum and show the magnetic Bragg peaks at specific positions. Order parameters are unusually small because of strong quantum fluctuations. Orbital contribution to the resonant x-ray scattering is discussed. The dynamical magnetic structure factor in different ordered states is calculated. Predictions made should help to observe elementary excitations of orbitals and also to identify the type of the orbital order in ferromagnetic titanates. Including further a relativistic spin-orbital coupling, we derive an effective low-energy spin Hamiltonian and calculate a spin-wave spectrum, which is in good agreement with recent experimental observations in YTiO$_3$.",0510175v1 2005-10-12,Orbital ordering in the two-dimensional ferromagnetic semiconductor Rb_2CrCl_4,"We present the results of electronic structure calculations for the two-dimensional ferromagnet Rb_2CrCl_4. They are obtained by the augmented spherical wave method as based on density functional theory and the local density approximation. In agreement with experimental data Rb_2CrCl_4 is found to be semiconducting and displays long-range ferromagnetic order of the localized Cr 3d moments. The magnetic properties are almost independent of the structural modifications arising from the Jahn-Teller instability, which leads from the parent body-centered tetragonal K_2NiF_4 structure to a side-centered orthorhombic lattice. In contrast, our calculations give evidence for a strong response of the optical band gap to the corresponding structural changes.",0510307v1 2005-10-27,High spin polarization in the ferromagnetic filled skutterudites KFe4Sb12 and NaFe4Sb12,"The spin polarization of ferromagnetic alkali-metal iron antimonides KFe4Sb12 and NaFe4Sb12 is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization Pt of 67% and 60% for the K and Na compound, respectively, is inferred which establishes these materials as a new class of highly spin polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation (LSDA) that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on Pt.",0510736v1 2005-11-01,Spin-transfer mechanism of ferromagnetism in polymerized fullerenes: $Ab initio$ calculations,"A mechanism of the high temperature ferromagnetism in polymerized fullerenes is suggested. It is assumed that some of the C$_{60}$ molecules in the crystal become magnetically active due to spin and charge transfer from the paramagnetic impurities (atoms or groups), such as hydrogen, fluorine, hydroxyl group OH, amino group NH$_2$, or methyl group CH$_3$, dispersed in the fullerene matrix. The exchange interaction between the spins localized on the magnetically active fullerenes is evaluated using \textit{ab initio} calculations. The nearest neighbour and next nearest neighbour exchange interaction is found to be in the range $0.1\div 0.3 $ eV, that is, high enough to account for the room temperature ferromagnetism.",0511025v2 2005-11-04,Weak ferromagnetic spin and charge stripe order in La[sub 5/3]Sr[sub 1/3]NiO[sub 4],"We present magnetization and specific heat data of a La5/3Sr1/3NiO4 single crystal in high magnetic fields. From the charge and spin stripe ordering temperatures, as well as a magnetic low temperature transition, we have constructed the electronic phase diagram for fields up to 14 Tesla. While the charge stripe ordering temperature TCO is independent of the magnetic field, there is a significant shift of the spin stripe ordering temperature TSO of about 1.5 K/ Tesla, if the magnetic fields are applied parallel to the NiO2-planes. The specific heat measurements indicate a large anomalous entropy change at TCO. In contrast, no significant entropy change is observed at the spin stripe transition. The high field magnetization experiments reveal the presence of in-plane weak ferromagnetic moments in the charge stripe ordered phase. From a phenomenological analysis, the magnetic correlation length of these moments is determined. We suggest that the weak ferromagnetism is due either to the presence of bond-centered charge stripes or to double exchange interactions across site-centered charge stripes.",0511105v2 2005-11-17,Effective ferromagnetic coupling between a superconductor and a ferromagnet in LaCaMnO/Nb hybrids,"In this work we present magnetization data on hybrids consisting of multilayers (MLs) of man- ganites [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3]15 in contact with a low-Tc Nb superconductor (SC). Although a pure SC should behave diamagnetically in respect to the external magnetic field in our ML-SC hybrids we observed that the magnetization of the SC follows that of the ML. Our intriguing experimental results show that the SC below its TSC c becomes ferromagnetically coupled to the ML. As a result in the regime where diamagnetic behaviour of the SC was expected its bulk magne- tization switches only whenever the coercive field of the ML is exceeded. By employing specific experiments where the ML was selectively biased or not we demonstrate that the ML inflicts its magnetic properties on the whole hybrid. Possible explanations are discussed in connection to recent theoretical proposals and experimental findings that were obtained in relative hybrids.",0511426v1 2005-11-24,Two-dimensional quantum spin-1/2 Heisenberg model with competing interactions,"We study the quantum spin-1/2 Heisenberg model in two dimensions, interacting through a nearest-neighbor antiferromagnetic exchange ($J$) and a ferromagnetic dipolar-like interaction ($J_d$), using double-time Green's function, decoupled within the random phase approximation (RPA). We obtain the dependence of $k_B T_c/J_d$ as a function of frustration parameter $\delta$, where $T_c$ is the ferromagnetic (F) transition temperature and $\delta$ is the ratio between the strengths of the exchange and dipolar interaction (i.e., $\delta = J/J_d$). The transition temperature between the F and paramagnetic phases decreases with $\delta$, as expected, but goes to zero at a finite value of this parameter, namely $\delta = \delta_c = \pi /8$. At T=0 (quantum phase transition), we analyze the critical parameter $\delta_c(p)$ for the general case of an exchange interaction in the form $J_{ij}=J_d/r_{ij}^{p}$, where ferromagnetic and antiferromagnetic phases are present.",0511606v1 2005-12-13,Fe implanted ferromagnetic ZnO,"Room-temperature ferromagnetism has been induced within ZnO single crystals by implant-doping with Fe ions. For an implantation temperature of 620 K and an ion fluence of 4x10^16 cm^-2, very tiny Fe particles, formed inside the host matrix, are responsible for the ferromagnetic properties. They were identified using synchrotron X-ray diffraction and Moessbauer spectroscopy. On the other hand, Fe ions implanted at a temperature of 253 K and an ion fluence of 4x10^15 cm^-2 are incorporated into the host matrix and develop a room temperature diluted magnetic semiconductor (DMS).",0512267v1 2005-12-20,Spin-dependent Quantum Interference in Single-Wall Carbon Nanotubes with Ferromagnetic Contacts,"We report the experimental observation of spin-induced magnetoresistance in single-wall carbon nanotubes contacted with high-transparency ferromagnetic electrodes. In the linear regime the spin-induced magnetoresistance oscillates with gate voltage in quantitative agreement with calculations based on a Landauer-Buttiker model for independent electrons. Consistent with this interpretation, we find evidence for bias-induced oscillation in the spin-induced magnetoresistance signal on the scale of the level spacing in the nanotube. At higher bias, the spin-induced magnetoresistance disappears because of a sharp decrease in the effective spin-polarization injected from the ferromagnetic electrodes.",0512505v2 2005-12-22,Observation of Periodic pi-Phase Shifts in Ferromagnet-Superconductor Multilayers,"We report complementary studies of the critical temperature and the critical current in ferromagnet (Ni) - superconductor (Nb) multilayers. The observed oscillatory behavior of both quantities upon variation of the thickness of the ferromagnetic layer is found to be in good agreement with theory. The length scale of oscillations is identical for both quantities and is set by the magnetic length corresponding to an exchange field of 200 meV in Ni. The consistency between the behavior of the two quantities provides strong evidence for periodic pi- phase shifts in these devices.",0512593v1 2005-12-23,Magnetization-dependent $T_c$ shift in F/S/F trilayers with a strong ferromagnet,"We have measured the superconducting transition temperature $T_c$ of Ni/Nb/Ni trilayers when the magnetizations of the two outer Ni layers are parallel (P) and antiparallel (AP). The largest difference in $T_c$ occurs when the Nb thickness is just above the critical thickness at which superconductivity disappears completely. We have observed a difference in $T_c$ between the P and AP states as large as 41 mK - a significant increase over earlier results in samples with higher $T_c$ and with a CuNi alloy in place of the Ni. Our result also demonstrates that strong elemental ferromagnets are promising candidates for future investigations of ferromagnet/superconductor heterostructures.",0512623v1 2006-01-23,Consequences of niobium doping for the ferromagnetism and microstructure of anatase Co: TiO2 films,"It is shown that dilute niobium doping has significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO2 films. Epitaxial films of anatase TiO2 with 3% Co, without and with 1% niobium doping were grown by pulsed-laser deposition at 875 C at different oxygen pressures. For growth at 10^{-5} Torr niobium doping suppresses the ferromagnetism, while it enhances the same in films grown at 10^{-4} Torr. High-resolution Z-contrast Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy show uniform surface segregation of cobalt-rich Ti_{1-x-y}Co_{x}Nb_{y}O_{2-d} phase, but without cobalt metal clusters.",0601528v1 2006-01-25,Origin of Ferromagnetism and its pressure and doping dependence in Tl$_{2}$Mn$_{2}$O$_{7}$,"Using NMTO-{\it downfolding} technique, we explore and establish the origin of ferromagnetism in the pyrochlore system, Tl$_{2}$Mn$_{2}$O$_{7}$. It is found to be driven by hybridization induced spin-polarization of the delocalized charge carriers derived from Tl-$s$ and O-$p$ states. The mean-field estimate of the ferromagnetic transition temperature, T$_c$, estimated using computed exchange integrals are found to be in good agreement with the measurements. We find an enhancement of T$_{c}$ for moderate doping with nonmagnetic Sb and a suppression of T$_{c}$ upon application of pressure, both in agreement with experimental findings.",0601563v1 2006-01-30,"Magnetism of two-dimensional defects in Pd: stacking faults, twin boundaries and surfaces","Careful first-principles density functional calculations reveal the importance of hexagonal versus cubic stacking of closed packed planes of Pd as far as local magnetic properties are concerned. We find that, contrary to the stable face centered cubic phase, which is paramagnetic, the hexagonal close-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35 $\mu_{B}$/atom. Our results show that two-dimensional defects with local hcp stacking, like twin boundaries and stacking faults, in the otherwise fcc Pd structure, increase the magnetic susceptibility. The (111) surface also increases the magnetic susceptibility and it becomes ferromagnetic in combination with an individual stacking fault or twin boundary close to it. On the contrary, we find that the (100) surface decreases the tendency to ferromagnetism. The results are consistent with the magnetic moment recently observed in small Pd nanoparticles, with a large surface area and a high concentration of two-dimensional stacking defects.",0601658v1 2006-01-31,Striking properties of Superconductor/Ferromagnet structures with spin-dependent scattering,"We investigate Superconductor/Ferromagnet (S/F) hybrid structures in the dirty limit, described by the Usadel equations. More precisely, the oscillations of the critical temperature and critical current with the thickness of the ferromagnetic layers are studied. We show that spin-flip and spin-orbit scattering lead to the decrease of the decay length and the increase of the oscillations period. The critical current decay is more sensitive to these pair-breaking mechanisms than that of the critical temperature. These two scattering mechanisms should be taken into account to get a better agreement between experimental results and theoretical description. We also study the influence of the interface transparency on the properties of S/F structures.",0601707v2 2006-02-01,The metal-insulator transition of the ferromagnetic La7/8Sr1/8MnO3 probed by spin waves: a 2D stripe superstructure,"We report a study of spin waves in hole-doped ferromagnetic La7/8Sr1/8MnO3, in the metallic state (165K) below TC (181K) and through the puzzling metal-insulator transition which occurs at TO'O''=159K. They reveal very unusual excitations. Propagating spin waves are observed in the small q-range up to q=0.25 (lambda=4a), and, beyond, four dispersionless levels. Both types of excitations have a quasi two-dimensional (2D) character. The transition is revealed by a folding of the dispersed magnon branch at q=1/8. In the metallic state, the dispersionless levels reveal ferromagnetic domains with 4 lattice spacings for their size along a and b. They lead to a picture of charge segregation with hole-poor domains surrounded with hole-rich paths. Within this description, the transition appears as the ordering of domains, which can be interpreted in terms of a 2D superstructure of orthogonal stripes.",0602026v1 2006-02-07,Field-Induced Ferromagnetic Order and Colossal Magnetoresistance in La_{1.2}Sr_{1.8}Mn_2O_7: a ^{139}La NMR study,"In order to gain insights into the origin of colossal magneto-resistance (CMR) in manganese oxides, we performed a ^{139}La NMR study in the double-layered compound La_{1.2}Sr_{1.8}Mn_2O_7. We find that above the Curie temperature T_C=126 K, applying a magnetic field induces a long-range ferromagnetic order that persists up to T=330 K. The critical field at which the induced magnetic moment is saturated coincides with the field at which the CMR effect reaches to a maximum. Our results therefore indicate that the CMR observed above T_C in this compound is due to the field-induced ferromagnetism that produces a metallic state via the double exchange interaction.",0602161v1 2006-02-13,Magnetic properties of polymerized C$_{60}$ with Fe,"We provide evidence that high-pressure high-temperature (2.5 GPa and 1040 K) treatment of mixtures of iron with fullerene powders leads to the complete transformation of iron into iron carbide Fe$_3$C. The comparison of the magnetic properties (Curie temperature and magnetic moment) of the here studied samples and those for the ferromagnetic polymer Rh-C$_{60}$ indicates that the main ferromagnetic signal reported in those samples is due to Fe$_3$C and not related to the ferromagnetism of carbon as originally interpreted. Taking into account the results obtained in this study the original paper on ``Magnetic carbon"" (Nature {\bf 413}, 716 (2001)) was recently retracted.",0602306v3 2006-03-03,Dark solitons in F=1 spinor Bose--Einstein condensate,"We study dark soliton solutions of a multi-component Gross--Pitaevskii equation for hyperfine spin F=1 spinor Bose--Einstein condensate. The interactions are supposed to be inter-atomic repulsive and anti-ferromagnetic ones of equal magnitude. The solutions are obtained from those of an integrable $2\times 2$ matrix nonlinear Schr\""{o}dinger equation with nonvanishing boundary conditions. We investigate the one-soliton and two-soliton solutions in detail. One-soliton is classified into two kinds. The ferromagnetic state has wavefunctions of domain-wall shape and its total spin is nonzero. The polar state provides a hole soliton and its total spin is zero. These two states are selected by choosing the type of the boundary conditions. In two-soliton collisions, we observe the spin-mixing or spin-transfer. It is found that, as ""magnetic"" carriers, solitons in the ferromagnetic state are operative for the spin-mixing while those in the polar are passive.",0603070v1 2006-03-07,Indication of intrinsic room-temperature ferromagnetism in Ti1-xCoxO2-d thin film: An x-ray magnetic circular dichroism study,"Soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3 edges of Co doped rutile TiO2 at room temperature have revealed clear multiplet features characteristic of ferromagnetic Co2+ ions coordinated by O2- ions, being in sharp contrast to the featureless XMCD spectrum of Co metal or metallic clusters. The absorption and XMCD spectra agree well with a full atomic-multiplet calculation for the Co2+ high-spin state in the D2h-symmetry crystal field at the Ti site in rutile TiO2. The results indicate that the ferromagnetism arises from the Co2+ ions substituting the Ti4+ ions.",0603149v1 2006-03-17,Spin in fractional quantum Hall systems,"A system at filling factor 2/3 could be a candidate for a quantum Hall ferromagnet at integer filling factor of composite fermions. Using exact diagonalization with electrons on a torus we study the transition from the singlet ground state to the polarized ground state at this filling and look for signatures of quantum Hall ferromagnetism. Differences between the fractional and corresponding integer systems are emphasised. Most interestingly, we find around the transition a low excited half-polarized state which might become the ground state in the thermodynamical limit. We study its structure and compare it to the singlet and polarized ground states. A new interpretation of the singlet state is suggested and comparison of the filling factors 2/3 and 2/5 is presented. Adding magnetic inhomogeneities into the system we investigate the stability of all the three involved states and the tendency to build up domains like in conventional ferromagnets.",0603451v1 2006-03-17,An Extended Scaling Scheme for Critically Divergent Quantities in Ferromagnets and Spin Glasses,"From a consideration of high temperature series expansions in ferromagnets and in spin glasses, we propose an extended scaling scaling scheme involving a set of scaling formulae which express to leading order the temperature (T) and the system size (L) dependences of thermodynamic observables over a much wider range of T than the corresponding one in the conventional scaling scheme. The extended scaling, illustrated by data on the canonical 2d ferromagnet and on the 3d binomial Ising spin glass, leads to consistency for the estimates of critical parameters obtained from scaling analyses for different observables.",0603453v3 2006-03-28,Ferromagnetic one dimensional Ti atomic chain,"Using the full potential linearized augmented plane wave (FLAPW) method, we have explored the magnetic properties of one dimensional (1D) Ti atomic chain. Astonishingly, we for the first time observed that the 1D Ti atomic chain has ferromagnetic ground state even on NiAl(110) surface although the Ti has no magnetic moment in bulk or macroscopic state. It was found that the physical property of direct exchange interaction among Ti atoms occurred in free standing state is well preserved on NiAl(110) surface and this feature has an essential role in ferromagnetism of 1D Ti atomic chain. It was shown that the m=$|2|$ state has the largest contribution to the magnetic moment of Ti atom grown on NiAl(110) surface. In addition, we found that the magnetic dipole interaction is a key factor in the study of magnetic anisotropy, not the magnetocrystalline anisotropy arising from spin-orbit interaction.",0603740v1 2006-03-29,Electronic Structure and Optical Properties of the Co-doped Anatase TiO$_{2}$ Studied from First Principles,"The Co-doped anatase TiO$_{2}$, a recently discovered room-temperature ferromagnetic insulator, has been studied by the first-principles calculations in the pseudo-potential plane-wave formalism within the local-spin-density approximation (LSDA), supplemented by the full-potential linear augmented plane wave (FP-LAPW) method. Emphasis is placed on the dependence of its electronic structures and linear optical properties on the Co-doping concentration and oxygen vacancy in the system in order to pursue the origin of its ferromagnetism. In the case of substitutional doping of Co for Ti, our calculated results are well consistent with the experimental data, showing that Co is in its low spin state. Also, it is shown that the oxygen vacancy enhances the ferromagnetism and has larger effect on both the electronic structure and optical properties than the Co-doping concentration only.",0603779v1 2006-04-02,The semiconductor-to-ferromagnetic-metal transition in FeSb2,"We propose FeSb2 to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb2 and FeSi (FeSi1-xGex alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment 1 muB per Fe ion with external magnetic field.",0604020v2 2006-04-03,Interplay between superconductivity and ferromagnetism in epitaxial Nb(110)/Au(111)/Fe(110) trilayers,"In order to clarify the influence of ferromagnetism on superconductivity through a normal metal layer, the superconducting properties of epitaxial Nb(110)/Au(111)/Fe(110) trilayers were studied as a function of the thickness tAu of the intervening Au layer. Structural characterization of the samples revealed sharp interfaces, almost free from roughness. A strong suppression of the superconducting transition temperature Tc was observed for tAu<10 A, implying a strong spin-polarization of the Au layer in the vicinity of the Au/Fe interface. A periodic change of Tc with a period of ~21 A (~9 atomic monolayer of Au) was observed for 20 A0 is argued to be described exactly by the sine-Gordon field theory at a specific value of the coupling. The solution of the scaling ferromagnetic case is recalled for comparison. The field theory describing the crossover from antiferromagnetic to ferromagnetic behaviour is also introduced.",0110181v1 2001-11-14,Approach to Fixation for Zero-Temperature Stochastic Ising Models on the Hexagonal Lattice,"We investigate zero-temperature dynamics on the hexagonal lattice H for the homogeneous ferromagnetic Ising model with zero external magnetic field and a disordered ferromagnetic Ising model with a positive external magnetic field h. We consider both continuous time (asynchronous) processes and, in the homogeneous case, also discrete time synchronous dynamics (i.e., a deterministic cellular automaton), alternating between two sublattices of H. The state space consists of assignments of -1 or +1 to each site of H, and the processes are zero-temperature limits of stochastic Ising ferromagnets with Glauber dynamics and a random (i.i.d. Bernoulli) spin configuration at time 0. We study the speed of convergence of the configuration $\sigma^t$ at time t to its limit $\sigma^{\infty}$ and related issues.",0111170v1 2006-08-03,On the absence of ferromagnetism in typical 2D ferromagnets,"We consider the Ising systems in $d$ dimensions with nearest-neighbor ferromagnetic interactions and long-range repulsive (antiferromagnetic) interactions which decay with a power, $s$, of the distance. The physical context of such models is discussed; primarily this is $d=2$ and $s=3$ where, at long distances, genuine magnetic interactions between genuine magnetic dipoles are of this form. We prove that when the power of decay lies above $d$ and does not exceed $d+1$, then for all temperatures, the spontaneous magnetization is zero. In contrast, we also show that for powers exceeding $d+1$ (with $d\ge2$) magnetic order can occur.",0608009v3 2004-03-26,Numerical analysis of the existence and stability of nonlinear excitations in a parametric model of ferromagnetic chain,"A parametrized spin model was recently introduced and intended for one-dimensional ferromagnets with a deformable Zeeman energy. This model is revisited and given more realistic interpretation in terms of a model for ferromagnetic systems with nonconvex anisotropies. A main virtue of the improved form is its exact reduction to the discrete Remoissenet-Peyrard model, i.e. a parametrized version of the Takeno-Homma's discrete sine-Gordon model. The spin-wave phase of the improved parametrized spin model is investigated assuming both harmonic and anharmonic excitations. Intrinsic-self-localized modes, regarded as zone-boundary breather spin waves, are pointed out by simulating the nonlinear difference equations describing the spin equilibrium positions in the chain, and are shown to exist irrespective of values of the model parameter. Domain-wall textures of the model are also numerically examined in terms of kink solitons and with regard to the parametrization.",0403048v1 2005-01-01,Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities,"We investigate the nonlinear dynamics underlying the evolution of a 2-D nanoscale ferromagnetic film with uniaxial anisotropy in the presence of perpendicular pumping. Considering the associated Landau-Lifshitz spin evolution equation with Gilbert damping together with Maxwell equation for the demagnetization field, we study the dynamics in terms of the stereographic variable. We identify several new fixed points for suitable choice of external field in a rotating frame of reference. In particular, we identify explicit equatorial and related fixed points of the spin vector in the plane transverse to the anisotropy axis when the pumping frequency coincides with the amplitude of the static parallel field. We then study the linear stability of these novel fixed points under homogeneous and spin wave perturbations and obtain a generalized Suhl's instability criterion, giving the condition for exponential growth of P-modes under spin wave perturbations. Two parameter phase diagrams (in terms of amplitudes of static parallel and oscillatory perpendicular magnetic fields) for stability are obtained, which differ qualitatively from those for the conventional ferromagnetic resonance near thermal equilibrium and are amenable to experimental tests.",0501002v2 2006-06-27,Periodic structure of spin-transfer current in ferromagnetic multilayers,"We show that the drift-diffusion mechanism in a normal-metal layer in combination with the resonance electron-magnon interactions at ferromagnet-normal interface of F-N-F heterostucture creates spatial instability modes and, out of these modes, a quasi-stable periodic structure of spin-transfer d.c. current can arise with certain channel step and step-to-radius ratio. The ferromagnetic resonance conditions determine spin-transfer current density. Independent nano-oscillators creating intersecting arrays of channels can phase-lock on sub-micrometer distance, which depends on multilayer geometry and applied fields. By decreasing the layer thickness, the number of channels affected by each independent d.c.-current source and their radius may be diminished. Phase-locking of multiple independent nano-oscillators can be used for enhancement of output power.",0606064v1 2004-03-20,Spin polarized states in nuclear matter with Skyrme effective interaction,"The possibility of appearance of spin polarized states in symmetric and strongly asymmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with the Skyrme effective interaction. The zero temperature dependence of the neutron and proton spin polarization parameters as functions of density is found for SkM$^*$, SGII (symmetric case) and SLy4, SLy5 (strongly asymmetric case) effective forces. By comparing free energy densities, it is shown that in symmetric nuclear matter ferromagnetic spin state (parallel orientation of neutron and proton spins) is more preferable than antiferromagnetic one (antiparallel orientation of spins). Strongly asymmetric nuclear matter undergoes at some critical density a phase transition to the state with the oppositely directed spins of neutrons and protons while the state with the same direction of spins does not appear. In comparison with neutron matter, even small admixture of protons strongly decreases the threshold density of spin instability. It is clarified that protons become totally polarized within a very narrow density domain while the density profile of the neutron spin polarization parameter is characterized by the appearance of long tails near the transition density.",0403059v1 2004-08-26,Antiferromagnetic spin phase transition in nuclear matter with effective Gogny interaction,"The possibility of ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with the effective Gogny interaction. It is shown that at some critical density nuclear matter with D1S effective force undergoes a phase transition to the antiferromagnetic spin state (the opposite direction of neutron and proton spins). The self--consistent equations of spin polarized nuclear matter with D1S force have no solutions, corresponding to the ferromagnetic spin ordering (the same direction of neutron and proton spins) and, hence, the ferromagnetic transition does not appear. The dependence of antiferromagnetic spin polarization parameter as a function of density is found at zero temperature.",0408064v2 2002-04-22,Giant Magnetic Moments of Nitrogen Stabilized Mn Clusters and Their Relevance to Ferromagnetism in Mn Doped GaN,"Using first principles calculations based on density functional theory, we show that the stability and magnetic properties of small Mn clusters can be fundamentally altered by the presence of nitrogen. Not only are their binding energies substantially enhanced, but also the coupling between the magnetic moments at Mn sites remains ferromagnetic irrespective of their size or shape. In addition, these nitrogen stabilized Mn clusters carry giant magnetic moments ranging from 4 Bohr magnetons in MnN to 22 Bohr magnetons in Mn_5N. It is suggested that the giant magnetic moments of Mn_xN clusters may play a key role in the ferromagnetism of Mn doped GaN which exhibit a wide range (10K - 940K) of Curie temperatures.",0204064v1 2004-08-23,Ferromagnetic/superconducting bilayer structure: A model system for spin diffusion length estimation,"We report detailed studies on ferromagnet--superconductor bilayer structures. Epitaxial bilayer structures of half metal--colossal magnetoresistive La$_{\mathrm{2/3}}$Ca$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ (HM--CMR) and high--$T_{\mathrm{c}}$ superconducting YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-\delta}}$(HTSC) are grown on SrTiO$_3$ (100) single--crystalline substrates using pulsed laser deposition. Magnetization $M$(T) measurements show the coexistence of ferromagnetism and superconductivity in these structures at low temperatures. Using the HM--CMR layer as an electrode for spin polarized electrons, we discuss the role of spin polarized self injection into the HTSC layer. The experimental results are in good agreement with a presented theoretical estimation, where the spin diffusion length $\xi_{\mathrm {FM}}$ is found to be in the range of $\xi_{\mathrm{FM}} \approx$ 10 nm.",0408102v1 2006-03-05,Stray Field Magnetic Resonance Tomography using Ferromagnetic Spheres,"The methodology for obtaining two- and three-dimensional magnetic resonance images by using azimuthally symmetric dipolar magnetic fields from ferromagnetic spheres is described. We utilize the symmetric property of a geometric sphere in the presence of a large externally applied magnetic field to demonstrate that a complete two- or three-dimensional structured rendering of a sample can be obtained without the motion of the sample relative to the sphere. Sequential positioning of the integrated sample-sphere system in an external magnetic field at various angular orientations provides all the required imaging slices for successful computerized tomographic image reconstruction. The elimination of the requirement to scan the sample relative to the ferromagnetic tip in this imaging protocol is a potentially valuable simplification compared to previous scanning probe magnetic resonance imaging proposals.",0603034v1 2006-05-09,Ising Ferromagnet: Zero-Temperature Dynamic Evolution,"The dynamic evolution at zero temperature of a uniform Ising ferromagnet on a square lattice is followed by Monte Carlo computer simulations. The system always eventually reaches a final, absorbing state, which sometimes coincides with a ground state (all spins parallel), and sometimes does not (parallel stripes of spins up and down). We initiate here the numerical study of ``Chaotic Time Dependence'' (CTD) by seeing how much information about the final state is predictable from the randomly generated quenched initial state. CTD was originally proposed to explain how nonequilibrium spin glasses could manifest equilibrium pure state structure, but in simpler systems such as homogeneous ferromagnets it is closely related to long-term predictability and our results suggest that CTD might indeed occur in the infinite volume limit.",0605077v1 2003-12-03,Entanglement and Quantum Phases in the Anisotropic Ferromagnetic Heisenberg Chain in the Presence of Domain Walls,"We discuss entanglement in the spin-1/2 anisotropic ferromagnetic Heisenberg chain in the presence of a boundary magnetic field generating domain walls. By increasing the magnetic field, the model undergoes a first-order quantum phase transition from a ferromagnetic to a kink-type phase, which is associated to a jump in the content of entanglement available in the system. Above the critical point, pairwise entanglement is shown to be non-vanishing and independent of the boundary magnetic field for large chains. Based on this result, we provide an analytical expression for the entanglement between arbitrary spins. Moreover the effects of the quantum domains on the gapless region and for antiferromagnetic anisotropy are numerically analysed. Finally multiparticle entanglement properties are considered, from which we establish a characterization of the critical anisotropy separating the gapless regime from the kink-type phase.",0312039v3 2007-04-03,The 3D +-J Ising model at the ferromagnetic transition line,"We study the critical behavior of the three-dimensional $\pm J$ Ising model [with a random-exchange probability $P(J_{xy}) = p \delta(J_{xy} - J) + (1-p) \delta(J_{xy} + J)$] at the transition line between the paramagnetic and ferromagnetic phase, which extends from $p=1$ to a multicritical (Nishimori) point at $p=p_N\approx 0.767$. By a finite-size scaling analysis of Monte Carlo simulations at various values of $p$ in the region $p_N$ axis below the first order transition to the non-chiral ferromagnetic state. This unexpected phenomenon is explained by the analysis of the spin-wave spectrum. We demonstrate that the square of the spin-wave gap becomes negative under magnetic field applied along $<111>$ and $<110>$ but not along the $<100>$ direction. It is a result of competition between the spin-wave interaction and cubic anisotropy. This negative sign means an instability of the spin wave spectrum for the helix and leads to a destruction of the helical order, giving rise to the partially disordered state below the first order ferromagnetic transition.",0704.0683v1 2007-04-17,Ferromagnetic quantum critical point in URhGe doped with Ru,"We have investigated the thermal, transport and magnetic properties of URh$_{1-x}$Ru$_x$Ge alloys near the critical concentration $x_{cr} = 0.38$ for the suppression of ferromagnetic order. The Curie temperature vanishes linearly with $x$ and the ordered moment $m_0$ is suppressed in a continuous way. At $x_{cr}$ the specific heat varies as $c \sim TlnT$, the $\gamma$-value $c/T|_{0.5K}$ is maximum and the temperature exponent of the resistivity $\rho \sim T^n$ attains a minimum value $n=1.2$. These observations provide evidence for a ferromagnetic quantum phase transition. Interestingly, the coefficient of thermal expansion and the Gr\""uneisen parameter $\Gamma$ remain finite at $x_{cr}$ (down to $T = 1$ K), which is at odds with recent scaling results for a metallic quantum critical point.",0704.2116v1 2007-04-18,Effect of a Zeeman field on the superconductor-ferromagnet transition in metallic grains,"We investigate the competition between pairing correlations and ferromagnetism in small metallic grains in the presence of a Zeeman field. Our analysis is based on the universal Hamiltonian, valid in the limit of large Thouless conductance. We show that the coexistence regime of superconducting and ferromagnetic correlations can be made experimentally accessible by tuning an external Zeeman field. We compare the exact solution of the model with a mean-field theory and find that the latter cannot describe pairing correlations in the intermediate regime. We also study the occurrence of spin jumps across the phase boundary separating the superconducting and coexistence regimes.",0704.2257v1 2007-04-19,Ferromagnetic resonance force microscopy on a thin permalloy film,"Ferromagnetic Resonance Force Microscopy (FMRFM) offers a means of performing local ferromagnetic resonance. We have studied the evolution of the FMRFM force spectra in a continuous 50 nm thick permalloy film as a function of probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-film interaction force, accounting for the presence of the localized strongly nonuniform magnetic field of the FMRFM probe magnet. Excellent agreement between the experimental data and the simulation results provides insight into the mechanism of FMR mode excitation in an FMRFM experiment.",0704.2442v1 2007-04-19,Experimental studies on vacancy induced ferromagnetism in undoped TiO2,"Room temperature ferromagnetism is observed in undoped TiO2 films deposited on Si substrates using pulsed laser deposition (PLD). The ferromagnetic properties of the samples depend on the oxygen partial pressure during the PLD synthesis. The appearance of higher binding energy component (HBEC) in the oxygen 1s core peak from x-ray photoelectron spectroscopy (XPS) suggests the presence of oxygen vacancies in these samples. The amount of oxygen during the synthesis determines the vacancy concentration in the samples which is directly related to the magnetic behavior of the samples. The magnetic moment decreases with oxygen vacancy concentration in the samples. Valence band measurements were performed to study the electronic structure of both stoichometric and reduced TiO2. The analyses show the presence of Ti 3d band near the Fermi level in reduced TiO2 samples. These bands are otherwise empty in stoichiometric TiO2 and reside in the conduction band which makes them unobservable by XPS. The existence of this Ti 3d band near the Fermi level can possibly lead to Stoner splitting of the band.",0704.2621v1 2007-05-03,"On the Dynamical Ferromagnetic, Quantum Hall, and Relativistic Effects on the Carbon Nanotubes Nucleation and Growth Mechanism","The mechanism of carbon nanotube (CNT) nucleation and growth has been a mystery for over 15 years. Prior models have attempted the extension of older classical transport mechanisms. In July 2000, a more detailed and accurate nonclassical, relativistic mechanism was formulated considering the detailed dynamics of the electronics of spin and orbital rehybridization between the carbon and catalyst via novel mesoscopic phenomena and quantum dynamics. Ferromagnetic carbon was demonstrated. Here, quantum (Hall) effects and relativistic effects of intense many body spin-orbital interactions for novel orbital rehybridization dynamics (Little Effect) are proposed in this new dynamical magnetic mechanism. This dynamic ferromagnetic mechanism is proven by imposing dynamic and static magnetic fields during CNT syntheses and observing the different influence of these external magnetic environments on the catalyzing spin currents and spin waves and the resulting CNT formation.",0705.0407v1 2007-05-23,Revelation of the role of impurities and conduction electron density in the high resolution photoemission study of ferromagnetic hexaborides,"We investigate the temperature evolution of the electronic structure of ferromagnetic CaB$_6$ using ultra-high resolution photoemission spectroscopy; electronic structure of paramagnetic LaB$_6$ is used as a reference. High resolution spectra of CaB$_6$ reveal finite density of states at the Fermi level, $\epsilon_F$ at all the temperatures and evidence of impurity induced localized features in the vicinity of $\epsilon_F$, which are absent in the spectra of LaB$_6$. Analysis of the high resolution spectra suggests that disorder in B-sublattice inducing partial localization in the mobile electrons and low electron density at $\epsilon_F$ is important to achieve ferromagnetism in these systems.",0705.3327v1 2007-05-30,Spontaneous ferromagnetic spin ordering at the surface of La$_2$CuO$_4$,"Magnetic properties of high purity stoichiometric La$_2$CuO$_4$ nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range $5 - 350$ K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed.",0705.4391v1 2007-06-05,Universal low-temperature behavior of the CePd_{1-x}Rh_x ferromagnet,"The heavy-fermion metal CePd_{1-x}Rh_x evolves from ferromagnetism at x=0 to a non-magnetic state at some critical concentration x_c. Utilizing the quasiparticle picture and the concept of fermion condensation quantum phase transition (FCQPT), we address the question about non-Fermi liquid (NFL) behavior of ferromagnet CePd_{1-x}Rh_x and show that it coincides with that of both antiferromagnet YbRh_2(Si_{0.95}Ge_{0.05})_2 and paramagnet CeRu_2Si_2 and CeNi_2Ge_2. We conclude that the NFL behavior being independent of the peculiarities of specific alloy, is universal, while numerous quantum critical points assumed to be responsible for the NFL behavior of different HF metals can be well reduced to the only quantum critical point related to FCQPT.",0706.0742v2 2007-06-19,Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder: a quantum Monte Carlo study,"We consider a spin-1/2 ladder with a ferromagnetic rung coupling J_\perp and inequivalent chains. This model is obtained by a twist (\theta) deformation of the ladder and interpolates between the isotropic ladder (\theta=0) and the SU(2) ferromagnetic Kondo necklace model (\theta=\pi). We show that the ground state in the (\theta,J_\perp) plane has a finite string order parameter characterising the Haldane phase. Twisting the chain introduces a new energy scale, which we interpret in terms of a Suhl-Nakamura interaction. As a consequence we observe a crossover in the scaling of the spin gap at weak coupling from \Delta/J_\| \propto J_\perp/J_\| for \theta < \theta_c \simeq 8\pi/9 to \Delta/J_\| \propto (J_\perp/J_\|)^2 for \theta > \theta_c. Those results are obtained on the basis of large scale Quantum Monte Carlo calculations.",0706.2769v2 2007-06-20,A scaling relation of anomalous Hall effect in ferromagnetic semiconductors and metals,"A scaling relation of the anomalous Hall effect recently found in a ferromagnetic semiconductor (Ti,Co)O_2_ is compared with those of various ferromagnetic semiconductors and metals. Many of these compounds with relatively low conductivity sigma_xx_ < 10^4 ohm^-1 cm^-1 are also found to exhibit similar relation: anomalous Hall conductivity sigma_AH_ approximately scales as sigma_AH_ proportional to sigma_xx_^1.6, that is coincident with a recent theory. This relation is valid over five decades of sigma_xx_ irrespective of metallic or hopping conduction.",0706.2924v1 2007-06-22,Orbital ordering in the ferromagnetic insulator Cs$_2$AgF$_4$ from first principles,"We found, using density-functional theory calculations within the generalized gradient approximation, that Cs$_2$AgF$_4$ is stabilized in the insulating orthorhombic phase rather than in the metallic tetragonal phase. The lattice distortion present in the orthorhombic phase corresponds to the $x^2-z^2$/$y^2-z^2$ hole-orbital ordering of the Ag$^{2+}$ $4d^9$ ions, and this orbital ordering leads to the observed ferromagnetism, as confirmed by the present total-energy calculations. This picture holds in the presence of moderate 4d-electron correlation. The results are compared with the picture of ferromagnetism based on the metallic tetragonal phase.",0706.3413v2 2007-07-11,Stripe width and non-local domain walls in the two-dimensional Dipolar Frustrated Ising Ferromagnet,"We describe a novel type of magnetic domain wall which, in contrast to Bloch or Neel walls, is non-localized and, in a certain temperature range, non-monotonic. The wall appears as the mean-field solution of the two-dimensional ferromagnetic Ising model frustrated by the long-ranged dipolar interaction. We provide experimental evidence of this wall delocalization in the stripe-domain phase of perpendicularly magnetized ultrathin magnetic films. In agreement with experimental results, we find that the stripe width decreases with increasing temperature and approaches a finite value at the Curie-temperature following a power law. The same kind of wall and a similar temperature dependence of the stripe width is expected in the mean-field approximation of the two-dimensional Coulomb frustrated Ising ferromagnet.",0707.1582v2 2007-07-18,Suppression of hole-mediated ferromagnetism in GaMnP by hydrogen,"We report the successful passivation of the Mn acceptors in GaMnP upon exposure to a remote dc hydrogen plasma. The as-grown films are non-metallic and ferromagnetic with a Curie temperature of T_C=55K. After hydrogenation the sample resistivity increases by approximately three orders of magnitude at room temperature and six orders of magnitude at 25 K. Furthermore, the hydrogenated samples are paramagnetic, which is evidenced by a magnetization curve at 5 K that is best described by a Brillouin function with g=2 and J=5/2 expected for Mn atoms in the 3d^5 configuration. These observations unambiguously proof that the ferromagnetism is carrier-mediated also in GaMnP.",0707.2777v1 2007-07-19,Magnetic-glassy multicritical behavior of the three-dimensional +- J Ising model,"We consider the three-dimensional $\pm J$ model defined on a simple cubic lattice and study its behavior close to the multicritical Nishimori point where the paramagnetic-ferromagnetic, the paramagnetic-glassy, and the ferromagnetic-glassy transition lines meet in the T-p phase diagram (p characterizes the disorder distribution and gives the fraction of ferromagnetic bonds). For this purpose we perform Monte Carlo simulations on cubic lattices of size $L\le 32$ and a finite-size scaling analysis of the numerical results. The magnetic-glassy multicritical point is found at $p^*=0.76820(4)$, along the Nishimori line given by $2p-1={\rm Tanh}(J/T)$. We determine the renormalization-group dimensions of the operators that control the renormalization-group flow close to the multicritical point, $y_1 = 1.02(5)$, $y_2 = 0.61(2)$, and the susceptibility exponent $\eta = -0.114(3)$. The temperature and crossover exponents are $\nu=1/y_2=1.64(5)$ and $\phi=y_1/y_2 = 1.67(10)$, respectively. We also investigate the model-A dynamics, obtaining the dynamic critical exponent $z = 5.0(5)$.",0707.2866v1 2007-07-26,Spin transport in proximity induced ferromagnetic graphene,"Magnetic gates in close proximity to graphene can induce ferromagnetic correlations. We study the effect of such induced magnetization dependent Zeeman splittings on the graphene transport properties. We estimate that induced spin splittings of the order of \Delta ~ 5 meV could be achieved with the use of magnetic insulator gates, e.g. EuO-gates, deposited on top of graphene. We demonstrate that such splittings in proximity induced ferromagnetic graphene could be determined directly from the tunneling resonances in the linear response conductance, as the top gate creates also a tunable barrier in the graphene layer. We show how such splittings could also be determined independently by magnetoresistance measurements in a spin-valve geometry. Because the spin polarization of the current near the Dirac point increases with the length of the barrier, long magnetic gates are desirable for determining \Delta experimentally.",0707.3976v2 2007-07-30,Tuning of ferromagnetism through anion substitution in Ga-Mn-pnictide ferromagnetic semiconductors,"We have synthesized Ga1-xMnxAs1-yPy and Ga1-xMnxP1-yNy by the combination of ion implantation and pulsed-laser melting. We find that the incorporation of isovalent impurities with smaller atomic radii leads to a realignment of the magnetic easy axis in Ga1-xMnxP1-yNy/GaP and Ga1-xMnxAs1-yPy/GaAs thin films from in-plane to out-of-plane. This tensile-strain-induced magnetic anisotropy is reminiscent of that observed in Ga1-xMnxAs grown on larger lattice constant (In,Ga)As buffer layers indicating that the role of strain in determining magnetic anisotropy is fundamental to III-Mn-V materials. In addition, we observe a decrease in the ferromagnetic Curie temperature in Ga1-xMnxAs1-yPy with increasing y from 0 to 0.028. Such a decrease may result from localization of holes as the P/As ratio on the Group V sublattice increases.",0707.4490v1 2007-08-02,Hydrogen patterning of Ga1-xMnxAs for planar spintronics,"We demonstrate two patterning techniques based on hydrogen passivation of Ga1-xMnxAs to produce isolated ferromagnetically active regions embedded uniformly in a paramagnetic, insulating host. The first method consists of selective hydrogenation of Ga1-xMnxAs by lithographic masking. Magnetotransport measurements of Hall-bars made in this manner display the characteristic properties of the hole-mediated ferromagnetic phase, which result from good pattern isolation. Arrays of Ga1-xMnxAs dots as small as 250 nm across have been realized by this process. The second process consists of blanket hydrogenation of Ga1-xMnxAs followed by local reactivation using confined low-power pulsed-laser annealing. Conductance imaging reveals local electrical reactivation of micrometer-sized regions that accompanies the restoration of ferromagnetism. The spatial resolution achievable with this method can potentially reach <100 nm by employing near-field laser processing. The high spatial resolution attainable by hydrogenation patterning enables the development of systems with novel functionalities such as lateral spin-injection as well as the exploration of magnetization dynamics in individual and coupled structures made from this novel class of semiconductors.",0708.0389v1 2007-08-08,Onset of ferromagnetism in low-doped GaMnAs,"We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of GaMnAs and compare with experimental measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originates from disorder in the Mn distribution as well as the presence of As antisite defects. The experimentally-determined hopping energy at the Curie temperature is roughly constant over a series of samples whose conductivities vary more than 10^4 and whose hole concentrations vary more than 10^2. Thus in this regime the hopping energy is an excellent predictor of the Curie temperature for a sample, in agreement with the theory.",0708.1063v1 2007-08-08,The anomalous Hall Effect and magnetoresistance in the layered ferromagnet Fe_{1/4}TaS_2: the inelastic regime,"The large magnetic anisotropy in the layered ferromagnet Fe_{1/4}TaS_2 leads to very sharp reversals of the magnetization $\bf M$ at the coercive field. We have exploited this feature to measure the anomalous Hall effect (AHE), focussing on the AHE conductivity $\sigma^A_{xy}$ in the inelastic regime. At low temperature T (5-50 K), $\sigma^A_{xy}$ is T-independent, consistent with the Berry-phase/Karplus-Luttinger theory. Above 50 K, we extract an inelastic AHE conductivity $\sigma^{in}_{xy}$ that scales as the square of $\Delta\rho$ (the T dependent part of the resistivity $\rho$). The term $\sigma^{in}_{xy}$ clarifies the T dependence and sign-reversal of the AHE coefficient R_s(T). We discuss the possible ubiquity of $\sigma^{in}_{xy}$ in ferromagnets, and ideas for interpreting its scaling with $(\Delta\rho)^2$. Measurements of the magnetoresistance (MR) reveal a rich pattern of behavior vs. T and field tilt-angle. We show that the 2 mechanisms, the anisotropic MR effect and field-suppression of magnons, account for the intricate MR behavior, including the bow-tie features caused by the sharp reversals in $\bf M$.",0708.1143v1 2007-08-17,Current-Induced Motion of Narrow Domain Walls and Dissipation in Ferromagnetic Metals,"Spin transport equations in a non-homogeneous ferromagnet are derived in the limit where the sd exchange coupling between the electrons in the conduction band and those in the d band is dominant. It is shown that spin diffusion in ferromagnets assumes a tensor form. The diagonal terms are renormalized with respect to that in normal metals and enhances the dissipation in the magnetic system while the off-diagonal terms renormalize the precessional frequency of the conduction electrons and enhances the non-adiabatic spin torque. To demonstrate the new physics in our theory, we show that self-consistent solutions of the spin diffusion equations and the Landau-Lifshitz equations in the presence of a current lead to a an increase in the terminal velocity of a domain wall which becomes strongly dependent on its width. We also provide a simplified equation that predicts damping due to the conduction electrons.",0708.2412v1 2007-08-29,Separation of the strain and finite size effect on the ferromagnetic properties of La_{0.5}Sr_{0.5}CoO_3 thin films,"The ferromagnetic properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films have been studied. The magnetic transition is affected by both strain and finite thickness. We have used a series of films of different thickness and on different substrates in order to quantitatively determine the change in Curie temperature contributed by each effect. The phase diagram of T$_C$ versus in-plane strain suggests that the ferromagnetic transition temperature is suppressed by tensile strain and enhanced by compressive strain. The general method of separating strain and finite thickness effects should be applicable to any ordering phase transition in thin films.",0708.3964v1 2007-08-29,Spin-charge separation in a strongly correlated spin-polarized chain,"We combine the first-quantized path-integral formalism and bosonization to develop a phenomenological theory for spin-charge coupled dynamics in one-dimensional (1D) ferromagnetic systems with strong interparticle repulsion, at low temperatures. We assume an effective spin-charge separation and retain the standard Luttinger-liquid plasmon branch, which is explicitly coupled to a ferromagnetic spin-wave texture with a quadratic dispersion. The dynamic spin structure severely suppresses the plasmon peak in the single-particle propagator, in both fermionic and bosonic systems. Our analysis provides an effective theory for the new universality class of 1D ferromagnetic systems, capturing both the trapped spin and propagating spin-wave regimes of the long-time behavior.",0708.4012v4 2007-08-30,Origin of Ferromagnetism in nitrogen embedded ZnO:N thin films,"Nitrogen embedded ZnO:N films prepared by pulsed laser deposition exhibit significant ferromagnetism. The nitrogen ions contained in ZnO confirmed by Secondary Ion Microscopic Spectrum and Raman experiments and the embedded nitrogen ions can be regarded as defects. According to the experiment results, a mechanism is proposed based on one of the electrons in the completely filled d-orbits of Zn that compensates the dangling bonds of nitrogen ions and leads to a net spin of one half in the Zn orbits. These one half spins strongly correlate with localized electrons that are captured by defects to form ferromagnetism. Eventually, the magnetism of nitrogen embedded ZnO:N films could be described by a bound magnetic polaron model.",0708.4053v1 2007-09-11,Magnetically asymmetric interfaces in a (LaMnO$_3$)/(SrMnO$_3$) superlattice due to structural asymmetries,"Polarized neutron reflectivity measurements of a ferromagnetic [(LaMnO$_3$)$_{11.8}$/(SrMnO$_3$)$_{4.4}$]$_6$ superlattice reveal a modulated magnetic structure with an enhanced magnetization at the interfaces where LaMnO$_3$ was deposited on SrMnO$_3$ (LMO/SMO). However, the opposite interfaces (SMO/LMO) are found to have a reduced ferromagnetic moment. The magnetic asymmetry arises from the difference in lateral structural roughness of the two interfaces observed via electron microscopy, with strong ferromagnetism present at the interfaces that are atomically smooth over tens of nanometers. This result demonstrates that atomic-scale roughness can destabilize interfacial phases in complex oxide heterostructures.",0709.1715v2 2007-09-19,Interplay of ferromagnetism and triplet superconductivity in a Josephson junction,"In this paper we extend our earlier analysis of the novel Josephson effect in triplet superconductor--ferromagnet--triplet superconductor (TFT) junctions [B. Kastening \emph{et al.}, Phys. Rev. Lett. {\bf{96}}, 047009 (2006)]. In our more general formulation of the TFT junction we allow for potential scattering at the barrier and an arbitrary orientation of the ferromagnetic moment. Several new effects are found upon the inclusion of these extra terms: for example, we find that a Josephson current can flow even when there is vanishing phase difference between the superconducting condensates on either side of the barrier. The critical current for a barrier with magnetization parallel to the interface is calculated as a function of the junction parameters, and is found to display strong non-analyticities. Furthermore, the Josephson current switches first identified in our previous work are found to be robust features of the junction, while the unconventional temperature-dependence of the current is very sensitive to the extra terms in the barrier Hamiltonian.",0709.2918v1 2007-09-18,Theory of current-driven magnetization dynamics in inhomogeneous ferromagnets,"We give a brief account of recent developments in the theoretical understanding of the interaction between electric currents and inhomogeneous ferromagnetic order parameters. We start by discussing the physical origin of the spin torques responsible for this interaction and construct a phenomenological description. We then consider the electric current-induced ferromagnetic instability and domain-wall motion. Finally, we present a microscopic justification of the phenomenological description of current-driven magnetization dynamics, with particular emphasis on the dissipative terms, the so-called Gilbert damping $\alpha$ and the $\beta$ component of the adiabatic current-driven torque.",0709.2937v2 2007-09-20,Magnetic fluctuations and itinerant ferromagnetism in two-dimensional systems with van Hove singularities,"We use the quasistatic approach to analyze the criterion of ferromagnetism for two-dimensional (2D) systems with the Fermi level near Van Hove singularities (VHS) of the electronic spectrum. It is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction I between electrons and paramagnons, which corresponds to the Hubbard on-site repulsion U, is sufficiently large. The critical interactions I_c and U_c remain finite at Van Hove filling and exceed considerably their values obtained from the Stoner criterion due to incommensurate spin fluctuations which are important near the ferromagnetic quantum phase transition. Combining the quasistatic approximation and the equation-of -motion method for the Green functions we obtain the results for the electronic self-energy to first order in the inverse number of spin components.",0709.3219v2 2007-10-11,X-ray magnetic circular dichroism and photoemission study of the diluted ferromagnetic semiconductor Zn$_{1-x}$Cr$_x$Te,"We have performed x-ray magnetic circular dichroism (XMCD) and valence-band photoemission studies of the diluted ferromagnetic semiconductor Zn$_{1-x}$Cr$_x$Te. XMCD signals due to ferromagnetism were observed at the Cr 2p absorption edge. Comparison with atomic multiplet calculations suggests that the magnetically active component of the Cr ion was divalent under the tetrahedral crystal field with tetragonal distortion along the crystalline a-, b-, and c-axes. In the valence-band spectra, spectral weight near the Fermi level was strongly suppressed, suggesting the importance of Jahn-Teller effect and the strong Coulomb interaction between the Cr 3d electrons.",0710.2170v1 2007-10-15,Fermionic representation for the ferromagnetic Kondo lattice model -- diagrammatic study of spin-charge coupling effects on magnon excitations,"A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zone-boundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.",0710.2828v1 2007-10-17,Superfluid density of superconductor-ferromagnet bilayers,"We report the first measurements of the effective superfluid density n_S(T) \propto \lambda^{-2}(T) of Superconductor-Ferromagnet (SC/FM) bilayers, where \lambda is the effective magnetic field penetration depth. Thin Nb/Ni bilayers were sputtered in ultrahigh vacuum in quick succession onto oxidized Si substrates. Nb layers are 102 A thick for all samples, while Ni thicknesses vary from 0 to 100 A. T_C determined from \lambda^{-2}(T) decreases rapidly as Ni thickness d_Ni increases from zero to 15 A, then it has a shallow minimum at d_Ni \approx 25 A. \lambda^{-2}(0) behaves similarly, but has a minimum several times deeper. In fact, \lambda^{-2}(0) continues to increase with increasing Ni thickness long after T_C has stopped changing. We argue that this indicates a substantial superfluid density inside the ferromagnetic Ni films.",0710.3376v1 2007-10-29,Disorder and temperature dependence of the Anomalous Hall Effect in thin ferromagnetic films: Microscopic model,"We consider the Anomalous Hall Effect (AHE) in thin disordered ferromagnetic films. Using a microscopic model of electrons in a random potential of identical impurities including spin-orbit coupling, we develop a general formulation for strong, finite range impurity scattering. Explicit calculations are done within a short range but strong impurity scattering to obtain AH conductivities for both the skew scattering and side jump mechanisms. We also evaluate quantum corrections due to interactions and weak localization effects. We show that for arbitrary strength of the impurity scattering, the electron-electron interaction correction to the AH conductivity vanishes exactly due to general symmetry reasons. On the other hand, we find that our explicit evaluation of the weak localization corrections within the strong, short range impurity scattering model can explain the experimentally observed logarithmic temperature dependences in disordered ferromagnetic Fe films.",0710.5416v1 2007-11-01,Two magnetic regimes in doped ZnO corresponding to a dilute magnetic semiconductor and a dilute magnetic insulator,"Films of ZnO doped with magnetic ions, Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behaviour is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localised spins with static localised states. The magnetism is quenched when carriers in the localised states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarisation of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.",0711.0172v2 2007-11-02,Phase space gaps and ergodicity breaking in systems with long range interactions,"We study a generalized isotropic XY-model which includes both two-spin and four-spin mean-field interactions. This model can be solved in the microcanonical ensemble. It is shown that in certain parameter regions the model exhibits gaps in the magnetization at fixed energy, resulting in ergodicity breaking. This phenomenon has previously been reported in anisotropic and discrete spin models. The entropy of the model is calculated and the microcanonical phase diagram is derived, showing the existence of first order phase transitions from the ferromagnetic to a paramagnetic disordered phase. It is found that ergodicity breaking takes place both in the ferromagnetic and the paramagnetic phases. As a consequence, the system can exhibit a stable ferromagnetic phase within the paramagnetic region, and conversely a disordered phase within the magnetically ordered region.",0711.0268v1 2007-11-02,Effects of Ferromagnetic Magnetic Ordering and Phase Transition on the Resistivity of Spin Current,"It has been shown experimentally a long time ago that the magnetic ordering causes an anomalous behavior of the electron resistivity in ferromagnetic crystals. Phenomenological explanations based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. We show by extensive Monte Carlo simulation that this behavior is also observed for the resistivity of the spin current calculated as a function of temperature ($T$) from low-$T$ ordered phase to high-$T$ paramagnetic phase in a ferromagnet. We show in particular that across the critical region, the spin resistivity undergoes a huge peak. The origin of this peak is shown to stem from the formation of magnetic domains near the phase transition. The behavior of the resistivity obtained here is compared to experiments and theories. A good agreement is observed.",0711.0298v1 2007-11-14,Kondo effect in a semiconductor quantum dot coupled to ferromagnetic electrodes,"Using a laterally-fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N = 3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B = 1.2 T. This means that, in the Kondo regime, an inverse effective magnetic field of B ~ 1.2 T can be applied to the QD in the parallel magnetic configuration of the ferromagnetic electrodes.",0711.2124v2 2007-11-27,Nonequilibrium interacting electrons in a ferromagnet,"Dynamics of the magnetization in ferromagnets is examined in the presence of transport electrons allowing the latter to interact. It is found that the existence of inhomogeneities such as domain wall (DW) structures, leads to changes that affect the dynamical structure of the equations of motion for the magnetization. Only in the limit of uniform magnetizations or sufficiently wide DW's, the equations of motion maintain the form they have in the noninteracting case. In this limit, results like the spin torques, the Gilbert parameter, and the DW velocities become renormalized. However the length scale that defines such a limit depends on the strength of the interaction. It is shown that if large ferromagnetic fluctuations exist in the metallic band then the range for which conformity with the noninteracting case holds extends to the limit of arbitrarily narrow DW's.",0711.4170v2 2007-12-14,Compensation of the Kondo effect in quantum dots coupled to ferromagnetic leads within equation of motion approach,"We propose a new approximation scheme within equation of motion approach (EOM) to spin polarized transport through a quantum dot coupled to ferromagnetic leads. It has some advantages over a widely used in the literature standard EOM technique, in particular when we are interested in spin polarized quantities. Namely, it gives the values of the dot spin polarization which are closer to the ones obtained within numerical renormalization group (NRG), than the standard EOM approach. While restoring the Kondo effect, the spin polarization vanishes and the transport becomes unpolarized, in agreement with NRG and a real time diagrammatic calculations. The standard EOM procedure gives nonzero values of the spin polarization, and the transport is still spin polarized. Both approximations give the same correct splitting of the Kondo peaks due to ferromagnetism in the electrodes.",0712.2414v1 2007-12-14,The Spin Reorientation Transition and Phase Diagram of Ultrathin Ferromagnetic Films,"We show results from Monte Carlo simulations of a two dimensional Heisenberg model for ultrathin films with perpendicular anisotropy. A complete phase diagram is obtained as a function of anisotropy and temperature, spanning a wide range of behavior. We discuss our results in relation with experimental findings in different ultrathin films. We observe and characterize a line of Spin Reorientation Transitions . This transition from out of plane stripe order to in plane ferromagnetic order presents a paramagnetic gap in between in a finite region in parameter space, as reported in experiments. For large anisotropies direct transitions from a low temperature stripe phase to a paramagnetic or tetragonal phase with dominant perpendicular magnetization is observed, also in agreement with experiments. We also show the phase diagram for a system without exchange, i.e. with pure dipolar and anisotropy interactions. It shows a similar behavior to the ferromagnetic case with antiferromagnetic instead of stripe phases at low temperatures. A Spin Reorientation Transition is also found in this case.",0712.2426v1 2008-01-08,Tunneling anisotropic magnetoresistance in multilayer-(Co/Pt)/AlOx/Pt structures,"We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.",0801.1192v1 2008-01-15,"New High-$T_c$ Half-Heusler Ferromagnets NiMnZ (Z = Si, P, Ge, As)","Based on the first principle calculation, we propose a new class of high-$T_c$ half-heusler ferromagnets NiMnZ (Z = Si, P, Ge, As). The structural and magnetic properties are investigated through the calculation of the electronic structure, phase stability, equilibrium lattice constant, magnetic exchange interaction $J_{ij}$ and Curie temperature $T_c$. It is found that all alloys show half-metallicity and ferromagnetism at temperatures much higher than room temperature in a wide range of lattice expansion (compression). At the equilibrium lattice constant, $T_c$ of 715K, 840K, 875K and 1050K are predicted by Monte Carlo simulation for NiMnP, NiMnAs, NiMnGe and NiMnSi, respectively. Following these results, these alloys are strongly expected to be promising candidates for spintronic applications.",0801.2225v1 2008-01-22,"Phase coherent transport in (Ga,Mn)As","Quantum interference effects and resulting quantum corrections of the conductivity have been intensively studied in disordered conductors over the last decades. The knowledge of phase coherence lengths and underlying dephasing mechanisms are crucial to understand quantum corrections to the resistivity in the different material systems. Due to the internal magnetic field and the associated breaking of time-reversal symmetry quantum interference effects in ferromagnetic materials have been scarcely explored. Below we describe the investigation of phase coherent transport phenomena in the newly discovered ferromagnetic semiconductor (Ga,Mn)As. We explore universal conductance fluctuations in mesoscopic (Ga,Mn)As wires and rings, the Aharonov-Bohm effect in nanoscale rings and weak localization in arrays of wires, made of the ferromagnetic semiconductor material. The experiments allow to probe the phase coherence length L_phi and the spin flip length L_SO as well as the temperature dependence of dephasing.",0801.3363v1 2008-01-29,Temperature Dependence of the Spin Resistivity in Ferromagnetic Thin Films,"The magnetic phase transition is experimentally known to give rise to an anomalous temperature-dependence of the electron resistivity in ferromagnetic crystals. Phenomenological theories based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. In this paper, we show by extensive Monte Carlo (MC) simulation the behavior of the resistivity of the spin current calculated as a function of temperature ($T$) from low-$T$ ordered phase to high-$T$ paramagnetic phase in a ferromagnetic film. We analyze in particular effects of film thickness, surface interactions and different kinds of impurities on the spin resistivity across the critical region. The origin of the resistivity peak near the phase transition is shown to stem from the existence of magnetic domains in the critical region. We also formulate in this paper a theory based on the Boltzmann's equation in the relaxation-time approximation. This equation can be solved using numerical data obtained by our simulations. We show that our theory is in a good agreement with our MC results. Comparison with experiments is discussed.",0801.4444v1 2008-01-31,Impurity-Ion pair induced high-temperature ferromagnetism in Co-doped ZnO,"Magnetic 3d-ions doped into wide-gap oxides show signatures of room temperature ferromagnetism, although their concentration is two orders of magnitude smaller than that in conventional magnets. The prototype of these exceptional materials is Co-doped ZnO, for which an explanation of the room temperature ferromagnetism is still elusive. Here we demonstrate that magnetism originates from Co2+ oxygen-vacancy pairs with a partially filled level close to the ZnO conduction band minimum. The magnetic interaction between these pairs is sufficiently long-ranged to cause percolation at moderate concentrations. However, magnetically correlated clusters large enough to show hysteresis at room temperature already form below the percolation threshold and explain the current experimental findings. Our work demonstrates that the magnetism in ZnO:Co is entirely governed by intrinsic defects and a phase diagram is presented. This suggests a recipe for tailoring the magnetic properties of spintronics materials by controlling their intrinsic defects.",0801.4945v1 2008-02-04,Adiabatic charge and spin pumping through quantum dots with ferromagnetic leads,"We study adiabatic pumping of electrons through quantum dots attached to ferromagnetic leads. Hereby we make use of a real-time diagrammatic technique in the adiabatic limit that takes into account strong Coulomb interaction in the dot. We analyze the degree of spin polarization of electrons pumped from a ferromagnet through the dot to a nonmagnetic lead (N-dot-F) as well as the dependence of the pumped charge on the relative leads' magnetization orientations for a spin-valve (F-dot-F) structure. For the former case, we find that, depending on the relative coupling strength to the leads, spin and charge can, on average, be pumped in opposite directions. For the latter case, we find an angular dependence of the pumped charge, that becomes more and more anharmonic for large spin polarization in the leads.",0802.0422v2 2008-02-10,Oxygen Vacancy Induced Ferromagnetism in V$_2$O$_{5-x}$,"{\it Ab initio} calculations within density functional theory with generalized gradient approximation have been performed to study the effects of oxygen vacancies on the electronic structure and magnetism in undoped V$_2$O$_{5-x}$ ($0 < x < 0.5$). It is found that the introduction of oxygen vacancies would induce ferromagnetism in V$_2$O$_{5-x}$ with the magnetization being proportional to the O vacancy concentration $x$. The calculated electronic structure reveals that the valence electrons released by the introduction of oxygen vacancies would occupy mainly the neighboring V $d_{xy}$-dominant band which then becomes spin-polarized due to intra-atomic exchange interaction, thereby giving rise to the half-metallic ferromagnetism.",0802.1313v1 2008-02-26,Electron-mediated ferromagnetism and small spin-orbit interaction in a molecular-beam-epitaxy grown n-type $GaAs/Al_{0.3}Ga_{0.7}As$ heterostructure with Mn $δ$-doping,"We report the first evidence of electron-mediated ferromagnetism in a molecular-beam-epitaxy (MBE) grown $GaAs/Al_{0.3}Ga_{0.7}As$ heterostructure with Mn $\delta$-doping. The interaction between the magnetic dopants (Mn) and the Two-Dimensional Electron Gas (2DEG) realizes magnetic ordering when the temperature is below the Curie temperature ($T_{C} \sim 1.7K$) and the 2DEG is brought in close proximity to the Mn layer by gating. The Anomalous Hall Effect (AHE) contribution to the total Hall resistance is shown to be about three to four orders of magnitude smaller than in the case of hole-mediated ferromagnetism indicating the presence of small spin-orbit interaction.",0802.3871v3 2008-02-27,Cationic Ordering and Microstructural Effects in the Ferromagnetic Perovskite La0.5Ba0.5CoO3: Impact upon Magnetotransport Properties,"The synthesis and structural study of the stoichiometric perovskite La0.5Ba0.5CoO3 have allowed three forms to be isolated. Besides the disordered La0.5Ba0.5CoO3 and the perfectly ordered layered LaBaCo2O6, a third form called nanoscale-ordered LaBaCo2O6, is obtained. As evidenced by transmission electron microscopy investigations, the latter consists of 112-type 90 degree oriented domains fitted into each other at a nanometer scale which induce large strains and consequently local atomic scale lattice distortions. These three ferromagnetic perovskites exhibit practically the same Tc (174-179 K), but differently from the other phases, the nanoscale-ordered LaBaCo2O6 is a hard ferromagnet, with Hc = 4.2 kOe, due to the strains which may pin domain walls, preventing the reversal of the spins in a magnetic field. The magnetotransport properties of these phases show that all of them exhibit a maximum intrinsic magnetoresistance, close to 6-7 % around Tc under 70 kOe but that the ordered phase exhibits a much higher tunnelling magnetoresistance effect at low temperature of about 15 % against 4 % due to the grain boundary effects.",0802.3984v1 2008-03-03,Introduction to Monte Carlo methods for an Ising Model of a Ferromagnet,"This discussion serves as an introduction to the use of Monte Carlo simulations as a useful way to evaluate the observables of a ferromagnet. Key background is given about the relevance and effectiveness of this stochastic approach and in particular the applicability of the Metropolis-Hastings algorithm. Importantly the potentially devastating effects of spontaneous magnetization are highlighted and a means to avert this is examined. An Ising model is introduced and used to investigate the properties of a two dimensional ferromagnet with respect to its magnetization and energy at varying temperatures. The observables are calculated and a phase transition at a critical temperature is also illustrated and evaluated. Lastly a finite size scaling analysis is undertaken to determine the critical exponents and the Curie temperature is calculated using a ratio of cumulants with differing lattice sizes. The results obtained from the simulation are compared to exact calculations to endorse the validity of this numerical process. A copy of the code used, written in C++, is enclosed and is freely available for use and modification under the General Public License.",0803.0217v1 2008-03-04,Inhomogeneous vortex-state-driven enhancement of superconductivity in nanoengineered ferromagnet-superconductor heterostructures,"Thin film heterostructures provide a powerful means to study the antagonism between superconductivity (SC) and ferromagnetism (FM). One interesting issue in FM-SC hybrids which defies the notion of antagonistic orders is the observation of magnetic field induced superconductivity (FIS). Here we show that in systems where the FM domains/islands produce spatial inhomogeneities of the SC order parameter, the FIS can derive significant contribution from different mobilities of the magnetic flux identified by two distinct critical states in the inhomogeneous superconductor. Our experiments on nanoengineered bilayers of ferromagnetic CoPt and superconducting NbN where CoPt/NbN islands are separated by a granular NbN, lend support to this alternative explanation of FIS in certain class of FM-SC hybrids.",0803.0454v1 2008-03-08,Spin structure factors and valence-bond-solid states of the trimerized Heisenberg chains in a magnetic field,"By means of the density matrix renormalization group (DMRG) method, the static spin structure factors and the magnetization plateaus of the trimerized Heisenberg ferromagnet-ferromagnet-antiferromagnet and antiferromagnet-antiferromagnet-ferromagnet spin chains in the presence of a magnetic field are elaborately studied. It is found that in the plateau states, the static structure factor with three peaks does not vary with the external magnetic field as well as the exchange couplings; the spin correlation function behaves as a perfect sequence and has a simple relation with the magnetization per site. An approximate wave function for the plateau states is proposed, and a picture based on the valence-bond-solid states is presented in order to understand the origin and the total number of the magnetization plateaus, which are shown to be in agreement with the DMRG results.",0803.1238v1 2008-03-26,Nanosized superparamagnetic precipitates in cobalt-doped ZnO,"The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc... These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, x-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution x-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.",0803.3774v2 2008-04-04,"Spin-charge coupling in a band ferromagnet: magnon-energy reduction, anomalous softening, and damping","The effects of correlation-induced coupling between spin and charge fluctuations on spin-wave excitations in a band ferromagnet are investigated by including self-energy and vertex corrections within a systematic inverse-degeneracy expansion scheme which explicitly preserves the Goldstone mode. Arising from the scattering of a magnon into intermediate spin-excitation states (including both magnon and Stoner excitations) accompanied with charge fluctuations in the majority spin band, this spin-charge coupling results not only in a substantial reduction of magnon energies but also in anomalous softening and significant magnon damping for zone-boundary modes lying within the Stoner gap. Our results are in good qualitative agreement with recent spin-wave excitation measurements in colossal magneto-resistive manganites and ferromagnetic ultrathin films of transition metals.",0804.0680v1 2008-04-09,"Curie point singularity in the temperature derivative of resistivity in (Ga,Mn)As","We observe a singularity in the temperature derivative $d\rho/dT$ of resistivity at the Curie point of high-quality (Ga,Mn)As ferromagnetic semiconductors with $T_c$'s ranging from approximately 80 to 185 K. The character of the anomaly is sharply distinct from the critical contribution to transport in conventional dense-moment magnetic semiconductors and is reminiscent of the $d\rho/dT$ singularity in transition metal ferromagnets. Within the critical region accessible in our experiments, the temperature dependence on the ferromagnetic side can be explained by dominant scattering from uncorrelated spin fluctuations. The singular behavior of $d\rho/dT$ on the paramagnetic side points to the important role of short-range correlated spin fluctuations.",0804.1578v2 2008-04-17,Andreev-Klein reflection in graphene ferromagnet-superconductor junctions,"We show that Andreev reflection in a junction between ferromagnetic (F) and superconducting (S) graphene regions is fundamentally different from the common FS junctions. For a weakly doped F graphene with an exchange field $h$ larger than its Fermi energy $E_{\rm F}$, Andreev reflection of massless Dirac fermions is associated with a Klein tunneling through an exchange field p-n barrier between two spin-split conduction and valence subbands. We find that this Andreev-Klein process results in an enhancement of the subgap conductance of a graphene FS junction by $h$ up to the point at which the conductance at low voltages $eV\ll \Delta$ is greater than its value for the corresponding non-ferromagnetic junction. We also demonstrate that the Andreev reflection can be of retro or specular types in both convergent and divergent ways with the reflection direction aligned, respectively, closer to and farther from the normal to the junction as compared to the incidence direction.",0804.2774v1 2008-04-21,CeFePO: A Heavy Fermion Metal with Ferromagnetic Correlations,"The ground state properties of CeFePO, a homologue of the new high temperature superconductors RFePnO(1-x)Fx, were studied by means of susceptibility, specific heat, resistivity, and NMR measurements on polycrystals. All the results demonstrate that this compound is a magnetically non-ordered heavy Fermion metal with a Kondo temperature TK~10K, a Sommerfeld coefficient gamma=700mJ/molK2 and a mass enhancement factor of the order of 200. The absence of a Fe-contribution to the effective moment at high temperatures indicates that the magnetism in CeFePO is completely dominated by the effect of Ce. Thus the strong electronic correlation effects originate from the Ce-4f electrons rather than from the Fe-3d electrons. An enhanced Sommerfeld-Wilson ratio R=5.5 as well as a Korringa product S0/T1TK2~0.065 well below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO appears to be on the non-magnetic side of a ferromagnetic instability.",0804.3250v2 2008-04-22,Effects of the crystal structure on the ferromagnetic correlations in ZnO with magnetic impurities,"We study the ferromagnetism in the compound (Zn,Mn)O within the Haldane-Anderson impurity model by using the quantum Monte Carlo technique and the tight-binding approximation for determining the host band-structure and the impurity-host hybridization. This computational approach allows us to determine how the host crystal structure influences the impurity bound state, which plays an important role in the development of the ferromagnetic (FM) correlations between the impurities. We find that the FM correlations are strongly influenced by the crystal structure. In particular, in p-type (Zn,Mn)O, we observe the development of FM correlations with an extended range at low temperatures for wurtzite and zincblende crystal structures. However, for the rocksalt structure no FM correlations are observed between the impurities. In addition, in n-type ZnO with magnetic impurities, the impurity bound state and FM correlations are not found.",0804.3436v1 2008-04-23,Coexistence of itinerant ferromagnetism and a non-unitary superconducting state with line nodes: possible application to UGe$_2$,"We construct a mean-field theory for itinerant ferromagnetism coexisting with a non-unitary superconducting state, where only the majority-spin band is gapped and contains line nodes, while the minority-spin band is gapless at the Fermi level. Our study is motivated by recent experimental results indicating that this may be the physical situation realized in the heavy-fermion compound UGe$_2$. We investigate the stability of the mean-field solution of the magnetic and superconducting order parameters. Also, we provide theoretical predictions for experimentally measurable properties of such a non-unitary superconductor: the specific heat capacity, the Knight shift, and the tunneling conductance spectra. Our study should be useful for direct comparison with experimental results and also for further predictions of the physics that may be expected in ferromagnetic superconductors.",0804.3803v1 2008-05-02,Domain-wall structure of a classical Heisenberg ferromagnet on a Mobius strip,"We study theoretically the structure of domain walls in ferromagnetic states on Mobius strips. A two-dimensional classical Heisenberg ferromagnet with single-site anisotropy is treated within a mean-field approximation by taking into account the boundary condition to realize the Mobius geometry. It is found that two types of domain walls can be formed, namely, parallel or perpendicular to the circumference, and that the relative stability of these domain walls is sensitive to the change in temperature and an applied magnetic field. The magnetization has a discontinuity as a function of temperature and the external field.",0805.0183v3 2008-05-10,Teleportation in the presence of common bath decoherence at the transmitting station,"We investigate the effect of common bath decoherence on the qubits of Alice in the usual teleportation protocol. The system bath interaction is studied under the central spin model, where the qubits are coupled to the bath spins through isotropic Heisenberg interaction. We have given a more generalized representation of the protocol in terms of density matrices and calculated the average fidelity of the teleported state for different Bell state measurements performed by Alice. The common bath interaction differentiates the outcome of various Bell state measurements made by Alice. There will be a high fidelity teleportation for a singlet measurement made by Alice when both the qubits of Alice interact either ferromagnetically or antiferromagnetically with bath. In contrast if one of the Alice's qubits interact ferromagnetically and the other anti-ferromagnetically then measurement of Bell states belonging to the triplet sector will give better fidelity. We have also evaluated the average fidelity when Alice prefers non-maximally entangled states as her basis for measurement.",0805.1456v2 2008-05-20,Proposed realization of itinerant ferromagnetism in optical lattices,"We propose to realize the itinerant ferromagnetism of two-component cold fermionic atoms in the $p$-orbital bands in optical lattices. The band flatness in the two-dimensional honeycomb lattice dramatically amplifies the interaction effect driving the ferromagnetic transition even with a relatively weak repulsive interaction. This scheme has the advantage that the stability of the system can be maintained without suffering decaying to the molecular state as one approaches the Feshbach resonance from the side with positive scattering length. Experimental signatures and detections are also discussed.",0805.3031v7 2008-06-01,"Itinerant Ferromagnetism in layered crystals LaCoOX (X = P, As)","The electronic and magnetic properties of cobalt-based layered oxypnictides, LaCoOX (X = P, As), are investigated. LaCoOP and LaCoOAs show metallic type conduction, and the Fermi edge is observed by hard x-ray photoelectron spectroscopy. Ferromagnetic transitions occur at 43 K for LaCoOP and 66 K for LaCoOAs. Above the transition temperatures, temperature dependence of the magnetic susceptibility follows the Curie-Weiss law. X-ray magnetic circular dichroism (XMCD) is observed at the Co L2,3-edge, but not at the other edges. The calculated electronic structure shows a spin polarized ground state. These results indicate that LaCoOX are itinerant ferromagnets and suggest that their magnetic properties are governed by spin fluctuation.",0806.0123v1 2008-06-02,Voltage Dependence of Spin Transfer Torque in Magnetic Tunnel Junctions,"Theoretical investigations of spin transfer torque in magnetic tunnel junctions using the tight-binding model in the framework of non-equilibrium Green functions formalism are presented. We show that the behavior of the spin transfer torque as a function of applied voltage can vary over a wide range depending on the band parameters of the ferromagnetic electrodes and the insulator that comprise the magnetic tunnel junction. The behavior of both the parallel and perpendicular components of the spin torque is addressed. This behavior is explained in terms of the spin and charge current dependence and on the interplay between evanescent states in the insulator and the Fermi surfaces of ferromagnetic electrodes comprising the junction. The origin of the perpendicular (field-like) component of spin transfer torque at zero bias, i.e. exchange coupling through the barrier between ferromagnetic electrodes is discussed.",0806.0396v1 2008-06-10,Optical spectra of the heavy fermion uniaxial ferromagnet UGe$_2$,"We report a detailed study of UGe$_{2}$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition $T_C = 53 K$. The characteristic changes are exhibited most strongly at an energy scale of around 12 meV (100 cm$^{-1}$). They recover their unrenormalized value above $T_C$ and for $\omega >$ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature $T_x \sim$ 30 K, observed in transport and thermodynamic experiments. In the ferromagnetic state we find signatures of a strong coupling to the longitudinal magnetic excitations that have been proposed to mediate unconventional superconductivity in this compound.",0806.1617v1 2008-06-16,Interplay of Local-Moment Ferromagnetism and Superconductivity in ErRh$_4$B$_4$ Single Crystals,"Tunnel-diode resonator technique was used to study crystals of ferromagnetic re-entrant superconductor, ErRh$_{4}$B$_{4}$. At the boundary between ferromagnetism (FM) and superconductivity (SC), dynamic magnetic susceptibility, $\chi(T,H)$, exhibits highly asymmetric behavior upon warming and cooling as well as enhanced diamagnetism on the SC side. SC phase nucleates upon warming in a cascade of discontinuous jumps in magnetic susceptibility $\chi(T,H)$, whereas FM phase develops gradually as reported in detail in \cite{prozorov2008}. Here we further investigate enhanced diamagnetism. We find that when a magnetic field is applied along the magnetic easy axes, a region of enhanced diamagnetic screening is smaller than in the perpendicular orientation. A discussion of possible causes of this effect is provided.",0806.2479v1 2008-06-20,Ferromagnetic spin fluctuation in LaFeAsO1-xFx,"The F doped LaFeAsO, a recently discovered superconductor with the high Tc of 26 K, has been studied by the resistivity, magnetic susceptibility, and heat capacity measurements in the F doping range from 0 to 0.14 (x in LaFeAsO1-xFx). In the low temperature region, a T3lnT term in the heat capacity and a T2 term in the magnetic susceptibility, which are derived from the spin fluctuation, are observed. The nearly ferromagnetic nature evidenced by a large Wilson ratio (6.5 for x = 0, and 11.2 for x = 0.025) suggests that the superconductivity in the LaFeAsO system is mediated by ferromagnetic spin fluctuation.",0806.3304v1 2008-06-20,Impurity bands and the character of the electronic states in ferromagnetic GaMnAs layers,"The interplay between disorder and spin polarization in a GaMnAs thin layer results into spin-polarized impurity hole bands. A figure of merit is defined to label the hole state as being extended or localized. The calculation leads to a phase diagram determining the metallic or non-metallic character of the sample. It is shown that samples with the highest figures of merit have a ratio between the extended hole density and the Mn concentration near 0.2, in agreement with the ratio of 0.1-0.25 known to occur among samples produced with the highest Curie temperatures. Both the non-metal-to-metal and the metal-to-non-metal transitions experimentally observed in the ferromagnetic regime are obtained, as the Mn concentration increases. An explanation is given for the occurrence of a maximal Curie temperature in ferromagnetic GaMnAs samples.",0806.3423v2 2008-06-25,Experimental Observation of the Inverse Proximity Effect in Superconductor/Ferromagnet Layered Structures,"We have studied the nuclear magnetic resonance (NMR) of 51V nuclei in the superconductor/ferromagnet thin film heterostructures Ni/V/Ni and Pd{1-x}Fe{x}/V/Pd{1-x}Fe{x} in the normaland superconducting state. Whereas the position and shape of the NMR line in the normal state for the trilayers is identical to that observed in a single V-layer, in the superconducting state the line shape definitely changes, developing a systematic distortion of the high-field wing of the resonance line. We consider this as the first experimental evidence for the penetration of ferromagnetism into the superconducting layer, a phenomenon which has been theoretically predicted recently and dubbed the inverse proximity effect.",0806.4104v1 2008-08-05,Evidence for the band broadening across the ferromagnetic transition in Cr$_{1/3}$NbSe$_2$,"The electronic structure of Cr$_{1/3}$NbSe$_2$ is studied via optical spectroscopy. We observe two low-energy interband transitions in the paramagnetic phase, which split into four peaks as the compound enters the ferromagnetic state. The band structure calculation indicates the four peaks are interband transitions to the spin up Cr e$_g$ states. We show that the peak splitting below the Curie temperature is \emph{not} due to the exchange splitting of spin up and down bands, but directly reflects a band broadening effect in Cr-derived states upon the spontaneous ferromagnetic ordering.",0808.0678v1 2008-08-07,On the phase diagrams of the ferromagnetic superconductors ZrZn2 and UGe2,"A general phenomenological theory is presented for the phase behavior of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory accounts in detail for the temperature-pressure phase diagram of ZrZn$_2$, while the main features of the diagram for UGe$_2$ are also described. Quantitative criteria are deduced for the U-type (type I) and Zr-type (type II) unconventional ferromagnetic superconductors with spin-triplet Cooper electron pairing. Some basic properties of quantum phase transitions are also elucidated.",0808.0939v5 2008-08-13,"Spin Filter, Spin Amplifier and Other Spintronic Applications in Graphene Nanodisks","Graphene nanodisk is a graphene derivative with a closed edge. The trigonal zigzag nanodisk with size $N$ has $N$-fold degenerated zero-energy states. A nanodisk can be interpletted as a quantum dot with an internal degree of freedom. The grand state of nanodisk has been argued to be a quasi-ferromagnet, which is a ferromagnetic-like states with a finite but very long life time. We investigate the spin-filter effects in the system made of nanodisks and leads based on the master equation. The finite-size effect on spin filter is intriguing due to a reaction from the polarization of incoming current to a quasi-ferromagnet. Analyzing the relaxation process with the use of the Landau-Lifshitz-Gilbert equation, we explore the response to four types of incoming currents, namely, unpolarized current, perfectly polarized current, partially polarized current and pulse polarized current. We propose some applications for spintronics, such as spin memory, spin amplifier, spin valve, spin-field-effect transistor and spin diode.",0808.1779v1 2008-08-16,All Oxide Ferromagnet/Semiconductor Epitaxial Heterostructures,"Oxide based ferromagnet/semiconductor heterostructures offer substantial advantages for spin electronics. We have grown (111) oriented Fe3O4 thin films and Fe3O4/ZnO heterostructures on ZnO(0001) and Al2O3(0001) substrates by pulsed laser deposition. High quality crystalline films with mosaic spread as small as 0.03 degree, sharp interfaces, and rms surface roughness of 0.3 nm were achieved. Magnetization measurements show clear ferromagnetic behavior of the magnetite layers with a saturation magnetization of 3.2 muB/f.u. at 300 K. Our results demonstrate that the Fe3O4/ZnO system is an intriguing and promising candidate for the realization of multi-functional heterostructures.",0808.2245v1 2008-08-25,Josephson tunnel junctions with strong ferromagnetic interlayer,"The dependence of the critical current density j_c on the ferromagnetic interlayer thickness d_F was determined for Nb/Al_2O_3/Cu/Ni/Nb Josephson tunnel junctions with ferromagnetic \Ni interlayer from very thin film thicknesses (\sim 1 nm) upwards and classified into F-layer thickness regimes showing a dead magnetic layer, exchange, exchange + anisotropy and total suppression of j_c. The Josephson coupling changes from 0 to pi as function of d_F, and -very close to the crossover thickness- as function of temperature. The strong suppression of the supercurrent in comparison to non-magnetic \Nb/Al_2O_3/Cu/Nb junctions indicated that the insertion of a F-layer leads to additional interface scattering. The transport inside the dead magnetic layer was in dirty limit. For the magnetically active regime fitting with both the clean and the dirty limit theory were carried out, indicating dirty limit condition, too. The results were discussed in the framework of literature",0808.3332v2 2008-08-26,Theoretical prediction and experimental study of a ferromagnetic shape memory alloy: Ga_2MnNi,"We predict the existence of a new ferromagnetic shape memory alloy Ga_2MnNi using density functional theory. The martensitic start temperature (T_M) is found to be approximately proportional to the stabilization energy of the martensitic phase (deltaE_tot) for different shape memory alloys. Experimental studies performed to verify the theoretical results show that Ga_2MnNi is ferromagnetic at room temperature and the T_M and T_C are 780K and 330K, respectively. Both from theory and experiment, the martensitic transition is found to be volume conserving that is indicative of shape memory behavior.",0808.3469v1 2008-09-06,Extended scaling for ferromagnetic Ising models with zero-temperature transitions,"We study the second-moment correlation length and the reduced susceptibility of two ferromagnetic Ising models with zero-temperature ordering. By introducing a scaling variable motivated by high-temperature series expansions, we are able to scale data for the one-dimensional Ising ferromagnet rigorously over the entire temperature range. Analogous scaling expressions are then applied to the two-dimensional fully frustrated Villain model where excellent finite-size scaling over the entire temperature range is achieved. Thus we broaden the applicability of the extended scaling method to Ising systems having a zero-temperature critical point.",0809.1161v2 2008-09-06,Local Structure and It's Effect on The Ferromagnetic Properties of La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films},"We have used high-resolution Extended X-ray Absorption Fine-Structure and diffraction techniques to measure the local structure of strained La$_{0.5}$Sr$_{0.5}$CoO$_3$ films under compression and tension. The lattice mismatch strain in these compounds affects both the bond lengths and the bond angles, though the larger effect on the bandwidth is due to the bond length changes. The popular double exchange model for ferromagnetism in these compounds provides a correct qualitative description of the changes in Curie temperature $T_C$, but quantitatively underestimates the changes. A microscopic model for ferromagnetism that provides a much stronger dependence on the structural distortions is needed.",0809.1178v2 2008-09-10,Multiferroic Properties of Nanocrystalline BaTiO3,"Some of the Multiferroics [1] form a rare class of materials that exhibit magnetoelectric coupling arising from the coexistence of ferromagnetism and ferroelectricity, with potential for many technological applications.[2,3] Over the last decade, an active research on multiferroics has resulted in the identification of a few routes that lead to multiferroicity in bulk materials.[4-6] While ferroelectricity in a classic ferroelectric such as BaTiO3 is expected to diminish with the reducing particle size,[7,8] ferromagnetism cannot occur in its bulk form.[9] Here, we use a combination of experiment and first-principles simulations to demonstrate that multiferroic nature emerges in intermediate size nanocrystalline BaTiO3, ferromagnetism arising from the oxygen vacancies at the surface and ferroelectricity from the core. A strong coupling between a surface polar phonon and spin is shown to result in a magnetocapacitance effect observed at room temperature, which can open up possibilities of new electro-magneto-mechanical devices at the nano-scale.",0809.1835v2 2008-09-12,Theory of electrical spin-detection at a ferromagnet/semiconductor interface,"We present a theoretical model that describes electrical spin-detection at a ferromagnet/semiconductor interface. We show that the sensitivity of the spin detector has strong bias dependence which, in the general case, is dramatically different from that of the tunneling current spin polarization. We show that this bias dependence originates from two distinct physical mechanisms: 1) the bias dependence of tunneling current spin polarization, which is of microscopic origin and depends on the specific properties of the interface, and 2) the macroscopic electron spin transport properties in the semiconductor. Numerical results show that the magnitude of the voltage signal can be tuned over a wide range from the second effect which suggests a universal method for enhancing electrical spin-detection sensitivity in ferromagnet/semiconductor tunnel contacts. Using first-principles calculations we examine the particular case of a Fe/GaAs Schottky tunnel barrier and find very good agreement with experiment. We also predict the bias dependence of the voltage signal for a Fe/MgO/GaAs tunnel structure spin detector.",0809.2278v1 2008-09-18,Spin fluctuations and ferromagnetic order in two-dimensional itinerant systems with Van Hove singularities,"The quasistatic approach is used to analyze the criterion of ferromagnetism for two-dimensional (2D) systems with the Fermi level near Van Hove (VH) singularities of the electron spectrum. It is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction between electrons and paramagnons, determined by the Hubbard on-site repulsion U, is sufficiently large. Due to incommensurate spin fluctuations near the ferromagnetic quantum phase transition, the critical interaction Uc remains finite at VH filling and exceeds considerably its value obtained from the Stoner criterion. A comparison with the functional renormalization group results and mean-field approximation which yields a phase separation is also performed.",0809.3191v1 2008-10-06,Superconducting/ferromagnetic diffusive bilayer with a spin-active interface: a numerical study,"We calculate the density of states (DOS) in a diffusive superconducting/ferromagnetic bilayer with a spin-active interface. We use a self-consistent numerical treatment to make a systematic study of the effects of the Spin-Dependence of Interfacial Phase Shifts (SDIPS) on the self-consistent superconducting gap and the DOS. Strikingly, we find that the SDIPS can induce a double gap structure (DGS) in the DOS of the ferromagnet, even when the superconducing layer is much thicker than the superconducting coherence lenght. We thus obtain DOS curves which have interesting similarities with those of Phys. Rev. Lett. 100, 237002 (2008).",0810.0904v3 2008-10-20,Optical Properties of III-Mn-V Ferromagnetic Semiconductors,"We review the first decade of extensive optical studies of ferromagnetic, III-Mn-V diluted magnetic semiconductors. Mn introduces holes and local moments to the III-V host, which can result in carrier mediated ferromagnetism in these disordered semiconductors. Spectroscopic experiments provide direct access to the strength and nature of the exchange between holes and local moments; the degree of itineracy of the carriers; and the evolution of the states at the Fermi energy with doping. Taken together, diversity of optical methods reveal that Mn is an unconventional dopant, in that the metal to insulator transition is governed by the strength of the hybridization between Mn and its p-nictogen neighbor. The interplay between the optical, electronic and magnetic properties of III-Mn-V magnetic semiconductors is of fundamental interest and may enable future spin-optoelectronic devices.",0810.3669v1 2008-10-22,Exchange bias effect in the phase separated Nd_{1-x}Sr_{x}CoO_3 at the spontaneous ferromagnetic/ferrimagnetic interface,"We report the new results of exchange bias effect in Nd_{1-x}Sr_{x}CoO_3 for x = 0.20 and 0.40, where the exchange bias phenomenon is involved with the ferrimagnetic (FI) state in a spontaneously phase separated system. The zero-field cooled magnetization exhibits the FI (T_{FI}) and ferromagnetic (T_C) transitions at ~ 23 and \sim 70 K, respectively for x = 0.20. The negative horizontal and positive vertical shifts of the magnetic hysteresis loops are observed when the system is cooled through T_{FI} in presence of a positive static magnetic field. Training effect is observed for x = 0.20, which could be interpreted by a spin configurational relaxation model. The unidirectional shifts of the hysteresis loops as a function of temperature exhibit the absence of exchange bias above T_{FI} for x = 0.20. The analysis of the cooling field dependence of exchange bias field and magnetization indicates that the ferromagnetic (FM) clusters consist of single magnetic domain with average size around \sim 20 and ~ 40 \AA ~ for x = 0.20 and 0.40, respectively. The sizes of the FM clusters are close to the percolation threshold for x = 0.20, which grow and coalesce to form the bigger size for x = 0.40 resulting in a weak exchange bias effect.",0810.4013v1 2008-10-30,Magnetoelectric Spin Wave Amplifier for Spin Wave Logic Circuits,"We propose and analyze a spin wave amplifier aimed to enhance the amplitude of the propagating spin wave via the magnetoelectric effect. The amplifier is a two-layer multiferroic structure, which comprises piezoelectric and ferromagnetic materials. By applying electric field to the piezoelectric layer, the stress is produced. In turn, the stress changes the direction of the easy axis in the ferromagnetic layer and the direction of the anisotropy field. The rotation frequency of the easy axis is the same as the frequency of the spin wave propagating through the ferromagnetic layer. As a result of this two-stage process, the amplitude of the spin wave can be amplified depending on the angle of the easy axis rotation. We present results of numerical simulations illustrating the operation of the proposed amplifier. According to numerical estimates, the amplitude of the spin wave signal can be increased by several orders of magnitude. The energy efficiency of the electric-to-magnetic power conversion is discussed. The proposed amplifier preserves the phase of the initial signal, which is important for application to logic circuits based on spin waves.",0810.5586v1 2008-10-31,Quenched-Vacancy Induced Spin-Glass Order,"The ferromagnetic phase of an Ising model in d=3, with any amount of quenched antiferromagnetic bond randomness, is shown to undergo a transition to a spin-glass phase under sufficient quenched bond dilution. This general result, demonstrated here with the numerically exact renormalization-group solution of a d=3 hierarchical lattice, is expected to hold true generally, for the cubic lattice and for quenched site dilution. Conversely, in the ferromagnetic-spinglass-antiferromagnetic phase diagram, the spin-glass phase expands under quenched dilution at the expense of the ferromagnetic and antiferromagnetic phases. In the ferro-spinglass phase transition induced by quenched dilution reentrance is seen, as previously found for the ferro-spinglass transition induced by increasing the antiferromagnetic bond concentration.",0811.0025v2 2008-11-03,Electronic transport driven spin-dynamics,"We propose a model to explore the dynamics of spin-systems coupled by exchange interaction to the conduction band electrons of a semiconductor material that forms the channel in a ferromagnet/semiconductor/ferromagnet spin-valve structure. We show that recent observation of the novel transient transport signature in a MnAs/GaAs/MnAs spin-valve structure with paramagnetic Mn impurities [D. Saha et al., Phys. Rev. Lett., 100, 196603 (2008)] can be quantitatively understood in terms of current driven dynamical polarization of Mn spins. Using our model of spin polarized transport through Schottky barriers at the two ferromagnet/semiconductor junctions in a spin-valve structure and a dynamical equation describing the paramagnetic impurities coupled to conduction band electrons we explain the scaling behaviour of observed transient features such as the magnitude and time-scale with temperature.",0811.0204v1 2008-11-05,Ferromagnetism and orbital order in the two-orbital Hubbard model,"We investigate spin and orbital states of the two-orbital Hubbard model on a square lattice by using a variational Monte Carlo method at quarter-filling, i.e., the electron number per site is one. As a variational wave function, we consider a Gutzwiller projected wave function of a mean-field type wave function for a staggered spin and/or orbital ordered state. Then, we evaluate expectation value of energy for the variational wave functions by using the Monte Carlo method and determine the ground state. In the strong Coulomb interaction region, the ground state is the perfect ferromagnetic state with antiferro-orbital (AF-orbital) order. By decreasing the interaction, we find that the disordered state becomes the ground state. Although we have also considered the paramagnetic state with AF-orbital order, i.e., purely orbital ordered state, and partial ferromagnetic states with and without AF-orbital order, they do not become the ground state.",0811.0733v1 2008-11-06,Exchange coupling between two ferromagnetic electrodes separated by a graphene nanoribbon,"In this study, based on the self-energy method and the total energy calculation, the indirect exchange coupling between two semi-infinite ferromagnetic strips (FM electrodes) separated by metallic graphene nanoribbons (GNRs) is investigated. In order to form a FM/GNR/FM junction, a graphitic region of finite length is coupled to the FM electrodes along graphitic zigzag or armchair interfaces of width $N$. The numerical results show that, the exchange coupling strength which can be obtained from the difference between the total energies of electrons in the ferromagnetic and antiferromagnetic couplings, has an oscillatory behavior, and depends on the Fermi energy and the length of the central region.",0811.0934v1 2008-11-07,Valence Bond States: Link models,"An isotropic anti-ferromagnetic quantum state on a square lattice is characterized by symmetry arguments only. By construction, this quantum state is the result of an underlying valence bond structure without breaking any symmetry in the lattice or spin spaces. A detailed analysis of the correlations of the quantum state is given (using a mapping to a 2D classical statistical model and methods in field theory like mapping to the non-linear sigma model or bosonization techniques) as well as the results of numerical treatments (regarding exact diagonalization and variational methods). Finally, the physical relevance of the model is motivated. A comparison of the model to known anti-ferromagnetic Mott-Hubbard insulators is given by means of the two-point equal-time correlation function obtained i) numerically from the suggested state and ii) experimentally from neutron scattering on cuprates in the anti-ferromagnetic insulator phase.",0811.1049v2 2008-11-14,Superconductivity induced by phosphorus doping and its coexistence with ferromagnetism in EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$,"We have studied EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu$^{2+}$ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.",0811.2390v3 2008-11-17,Dynamical properties of the one-dimensional spin-1/2 Bose-Hubbard model near Mott-insulator to ferromagnetic liquid transition,"We investigate the dynamics of the one-dimensional strongly repulsive spin-1/2 Bose-Hubbard model for filling $\nu\le1.$ While at $\nu=1$ the system is a Hubbard-Mott insulator exhibiting dynamical properties of the Heisenberg ferromagnet, at $\nu<1$ it is a ferromagnetic liquid with complex spin dynamics. We find that close to the insulator-liquid transition the system admits for a complete separation of spin and density degrees of freedom valid at {\it all} energy and momentum scales within the $t-J$ approximation. This allows us to derive the propagator of transverse spin waves and the shape of the magnon peak in the dynamic spin structure factor.",0811.2676v2 2008-11-18,The phase diagram of the extended anisotropic ferromagnetic-antiferromagnetic Heisenberg chain,"By using Density Matrix Renormalization Group (DMRG) technique we study the phase diagram of 1D extended anisotropic Heisenberg model with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. We analyze the static correlation functions for the spin operators both in- and out-of-plane and classify the zero-temperature phases by the range of their correlations. On clusters of $64,100,200,300$ sites with open boundary conditions we isolate the boundary effects and make finite-size scaling of our results. Apart from the ferromagnetic phase, we identify two gapless spin-fluid phases and two ones with massive excitations. Based on our phase diagram and on estimates for the coupling constants known from literature, we classify the ground states of several edge-sharing materials.",0811.2973v3 2008-11-20,Thermo-spin effects in a quantum dot connected to ferromagnetic leads,"We study a system composed of a quantum dot in contact with ferromagnetic leads, held at different temperatures. Spin analogues to the thermopower and thermoelectric figures of merit are defined and studied as a function of junction parameters. It is shown that in contrast to bulk ferromagnets, the spin thermopower coefficient in a junction can be as large as the Seebeck coefficient, resulting in a large spin figure of merit. In addition, it is demonstrated that the junction can be tuned to supply only spin current but no charge current. We also discuss experimental systems where our predictions can be verified.",0811.3265v2 2008-11-21,Thermodynamics of the frustrated ferromagnetic spin-1/2 Heisenberg chain,"We studied the thermodynamics of the one-dimensional J1-J2 spin-1/2 Heisenberg chain for ferromagnetic nearest-neighbor bonds J1 < 0 and frustrating antiferromagnetic next-nearest-neighbor bonds J1 > 0 using full diagonalization of finite rings and a second-order Green-function formalism. Thereby we focus on J2 < |J1|/4 where the ground state is still ferromagnetic, but the frustration influences the thermodynamic properties. We found that their critical indices are not changed by J2. The analysis of the low-temperature behavior of the susceptibility chi leads to the conclusion that this behavior changes from chi \propto T^{-2} at J2 < |J1|/4 to chi \propto T^{-3/2} at the quantum-critical point J2=|J1|/4. Another effect of the frustration is the appearance of an extra low-T maximum in the specific heat C_v(T) for J2 \gtrsim |J1|/8, indicating its strong influence on the low-energy spectrum.",0811.3549v1 2008-11-27,"Theory of Weak Localization in Ferromagnetic (Ga,Mn)As","We study quantum interference corrections to the conductivity in (Ga,Mn)As ferromagnetic semiconductors using a model with disordered valence band holes coupled to localized Mn moments through a p-d kinetic-exchange interaction. We find that at Mn concentrations above 1% quantum interference corrections lead to negative magnetoresistance, i.e. to weak localization (WL) rather than weak antilocalization (WAL). Our work highlights key qualitative differences between (Ga,Mn)As and previously studied toy model systems, and pinpoints the mechanism by which exchange splitting in the ferromagnetic state converts valence band WAL into WL. We comment on recent experimental studies and theoretical analyses of low-temperature magnetoresistance in (Ga,Mn)As which have been variously interpreted as implying both WL and WAL and as requiring an impurity-band interpretation of transport in metallic (Ga,Mn)As.",0811.4585v1 2008-12-06,Voltage-Controlled Surface Magnetization of Itinerant Ferromagnet Ni_(1-x)Cu_x,"We argue that surface magnetization of a metallic ferromagnet can be turned on and off isothermally by an applied voltage. For this, the material's electron subsystem must be close enough to the boundary between para- and ferromagnetic regions on the electron density scale. For the 3d series, the boundary is between Ni and Cu, which makes their alloy a primary candidate. Using Ginzburg-Landau functional, which we build from Ni_(1-x)Cu_x empirical properties, ab-initio parameters of Ni and Cu, and orbital-free LSDA, we show that the proposed effect is experimentally observable.",0812.1309v1 2008-12-08,Dilution Robustness for Mean Field Ferromagnets,"In this work we compare two different random dilution of a mean field ferromagnet: the first model is built on a Bernoulli-diluted network while the second lives on a Poisson-diluted network. While it is known that the two models have in the thermodynamic limit the same free energy we investigate on the structural constraints that the two models must fulfill. We rigorously derive for each model the set of identities for the multi-overlaps distribution using different methods for the two dilutions: constraints in the former model are obtained by studying the consequences of the self-averaging of the internal energy density, while in the latter are obtained by a stochastic-stability technique. Finally we prove that the identities emerging in the two models are the same, showing ""robustness"" of the ferromagnetic properties of diluted networks with respect to the details of dilution.",0812.1568v2 2008-12-11,What drives the insulating state in ultrathin films of SrRuO3?,"Using density functional calculations we have examined the evolution of the electronic structure of SrRuO3 films grown on SrTiO3 substrates as a function of film thickness. At the ultrathin limit of two monolayers (RuO2-terminated surface) the films are found to be at the brink of a spin-state transition which drives the system to an antiferromagnetic and insulating state. Increasing the film thickness to four monolayers, one finds the surprising result that two entirely different solutions coexist. An antiferromagnetic insulating solution coexists with a metallic solution corresponding to an antiferromagnetic surface and a ferromagnetic bulk. The electronic structure found at the ultrathin limit persists for thicker films and an unusual result is predicted. Thicker films are found to be metallic as expected for the bulk, but the magnetism does not directly evolve to the bulk ferromagnetic state. The surface remains antiferromagnetic while the bulk exhibits ferromagnetic ordering.",0812.2098v2 2008-12-29,Density matrix for the kink ground state of the ferromagnetic XXZ chain,"The exact expression for the density matrix of the kink ground state of the ferromagnetic XXZ chain is obtained. Utilizing this, we exactly calculate various correlation functions such as the longitudinal and transverse spin-spin correlation functions, and the ferromagnetic and antiferromagnetic string formation probabilities. The asymptotic behaviors of these correlation functions are also analyzed. As a consequence, we find that the spin-spin correlation functions decay exponentially for large distances, while the string formation probabilities exhibit Gaussian decay for large strings. We also evaluate the entanglement entropy, which shows interesting behaviors due to the lack of the translational invariance of the state.",0812.4947v1 2009-01-19,Hysteresis and noise in ferromagnetic materials with parallel domain walls,"We investigate dynamic hysteresis and Barkhausen noise in ferromagnetic materials with a huge number of parallel and rigid Bloch domain walls. Considering a disordered ferromagnetic system with strong in-plane uniaxial anisotropy and in-plane magnetization driven by an external magnetic field, we calculate the equations of motion for a set of coupled domain walls, considering the effects of the long-range dipolar interactions and disorder. We derive analytically an expression for the magnetic susceptivity, related to the effective demagnetizing factor, and show that it has a logarithmic dependence on the number of domains. Next, we simulate the equations of motion and study the effect of the external field frequency and the disorder on the hysteresis and noise properties. The dynamic hysteresis is very well explained by means of the loss separation theory.",0901.2918v1 2009-01-30,Theory of magnetism with temporal disorder applied to magnetically doped ZnO,"A dynamic model of the asymmetric Ising glass is presented: an Ising model with antiferromagnet bonds with probabilities q arranged at random in a ferromagnetic matrix. The dynamics is introduced by changing the arrangement of the antiferromagnetic bonds after n Monte Carlo steps but keeping the same value of q and spin configuration. In the region where there is a second order transition between the ferromagnetic and paramagnetic states the dynamic behaviour follows that expected for motional narrowing and reverts to the static behaviour only for large n. There is a different dynamic behaviour where there is a first order transition between the ferromagnetic and spin glass states where it shows no effects of motional narrowing. The implications of this are discussed. This model is devised to explain the properties of doped ZnO where the magnetisation is reduced when the exchange interactions change with time.",0901.4947v1 2009-02-27,Little-Parks Oscillations in Hybrid Ferromagnet-Superconductor Systems,"On the basis of of linearized Usadel equations we consider superconductivity nucleation in multiply connected mesoscopic superconductor/ferromagnet hybrids such as thin-walled superconducting cylinders placed in electrical contact with a ferromagnetic metal. We study the interplay between the oscillations of $T_c$ due to the Little--Parks effect and the oscillations due to the exchange field. We demonstrate that the exchange field provokes the switching between the superconducting states with different vorticities and this may result in the increase the critical temperature of the superconducting transition in the magnetic field. Moreover we analyse the influence of the S/F transparency on the realisation of the states with higher vorticities.",0902.4860v1 2009-03-05,"Escape Rate Measurements and Microwave Spectroscopy of 0, pi, and 0-pi ferromagnetic Josephson Tunnel Junctions","We present experimental studies of high quality underdamped 0, pi, and 0-pi ferromagnetic Josephson tunnel junctions of intermediate length L (lambda_J < L < 5 lambda_J, where lambda_J is the Josephson penetration depth). The junctions are fabricated as Nb/Al_2O_3/Cu_40Ni_60/Nb Superconductor-Insulator-Ferromagnet-Superconductor heterostructures. Using microwave spectroscopy, we have investigated the eigenfrequencies of 0, pi, and 0-pi Josephson junctions in the temperature range 1.9K...320mK. Harmonic, subharmonic and superharmonic pumping is observed in experiment, and the experimental data are compared with numerical simulations. Escape rate measurements without applied microwaves at temperatures T down to 20mK show that the width of the switching current histogram decreases with temperature and saturates below T=150mK. We analyze our data in the framework of the short junction model. The differences between experimental data and theoretical predictions are discussed.",0903.1046v1 2009-03-10,Spin-induced charge correlations in transport through interacting quantum dots with ferromagnetic leads,"We study the full counting statistics of electronic transport through a single-level quantum dot weakly coupled to two leads, with either one or both of them being ferromagnetic. The interplay of Coulomb interaction and finite spin polarization implies spin-correlation induced charge correlations that give rise to super-Poissonian transport behavior and positive cross correlations of the currents of the two spin species. In the case of two ferromagnetic leads, we analyze the nontrivial dependence of the cumulants on the angle between the polarization directions of the leads. We find diverging second and higher cumulants for spin polarizations approaching unity.",0903.1759v2 2009-03-16,Quantum phase transition and underscreened Kondo effect in electron transport through parallel double quantum dots,"We investigate electronic transport through parallel double quantum dot(DQD) system with strong on-site Coulomb interaction and capacitive interdot coupling. By applying numerical renormalization group(NRG) method, the ground state of the system and the transmission probability at zero temperature have been obtained. For a system of quantum dots with degenerate energy levels and small interdot tunnel coupling, the spin correlations between the DQDs is ferromagnetic, and the ground state of the system is a spin 1 triplet state. The linear conductance will reach the unitary limit ($2e^2/h$) due to the underscreened Kondo effect at low temperature. As the interdot tunnel coupling increases, there is a quantum phase transition from ferromagnetic to anti-ferromagnetic spin correlation in DQDs and the linear conductance is strongly suppressed.",0903.2685v2 2009-03-30,Room-temperature ferromagnetism in the mixtures of the TiO2 and Co3O4 powders,"We report here the observation of ferromagnetism (FM) at 300 K in mixtures of TiO2 and Co3O4 powders despite the antiferromagnetic and diamagnetic character of both oxides respectively. The ferromagnetic behavior is found in the early stages of reaction and only for TiO2 in anatase structure; no FM is found for identical samples prepared with rutile-TiO2. Optical spectroscopy and X-ray absorption spectra confirm a surface reduction of octahedral Co+3->Co+2 in the mixtures which is in the origin of the observed magnetism",0903.5214v1 2009-04-03,Stabilization and functional properties of La3NiAlMnO9 and La3CoAlMnO9 magnetoelectric triple perovskites,"Ferromagnetic La3NiAlMnO9 (LNAMO) and La3CoAlMnO9 (LCAMO) triple-perovskite thin films are stabilized in the 750-860 oC temperature range in 100 to 900 mTorr O2 pressure range using pulsed-laser deposition. The LCAMO and LNAMO films exhibit ferromagnetism up to 190 K and 130 K respectively. The structural, optical and magnetic properties of these films demonstrate that the B-site 3d-cations, Al, Mn and Co or Ni ions, are structurally short-range ordered. The strong spin-lattice-polarization coupling in LCAMO is evidenced by the temperature dependence of the dielectric constant and the softening of the phonon frequencies starting in the vicinity of the ferromagnetic-to-paramagnetic phase transition mimicking the behaviours of La2CoMnO6 double perovskite.",0904.0604v1 2009-05-06,Josephson tunnel junctions with ferromagnetic $\Fe_{0.75}\Co_{0.25}$ barriers,"Josephson tunnel junctions with the strong ferromagnetic alloy $\Fe_{0.75}\Co_{0.25}$ as the barrier material were studied. The junctions were prepared with high quality down to a thickness range of a few monolayers of Fe-Co. An oscillation length of $\xi_{F2}\approx 0.79\:{\rm {nm}}$ between 0 and $\pi$-Josephson phase coupling and a very short decay length $\xi_{F1}\approx 0.22\:{\rm {nm}}$ for the amplitude of the superconducting pair wave function in the Fe-Co layer were determined. The rapid damping of the pair wave function inside the Fe-Co layer is caused by the strong ferromagnetic exchange field and additional magnetic pair breaking scattering. Josephson junctions with Fe-Co barriers show a significantly increased tendency towards magnetic remanence and flux trapping for larger thicknesses $d_{F}$.",0905.0761v1 2009-05-13,Correlation between Organic Magnetoresistance (OMAR) and Ferromagnetic ordering,"We report observation of ferromagnetic (FM) ordering in organic semiconductors, namely regio-regular poly (3-hexyl thiophene) (RRP3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM), in the temperature range of 5-300 K in addition to magnetoresistance (OMAR) observed in the diodes made from the same materials. Particle induced x-ray emission spectroscopy confirms the presence of dilute magnetic impurities in the materials mainly as residues from the synthesis process. However, upon blending these two materials with FM signal, the FM ordering is suppressed by a huge paramagnetic (PM) signal indicating ground state charge transfer formation in the blend. Together with the magneto-transport studies, these results indicate that OMAR response is observed in a device only when the corresponding active materials are FM. In the diodes with P3HT:PCBM complex, that as a blend shows PM response, OMAR vanishes almost completely. We propose that ferromagnetism in the active material can have important correlation with the OMAR response in the diodes.",0905.2021v1 2009-05-16,Local-moment ferromagnetism and unusual magnetic domains in Fe$_{1/4}$TaS$_{2}$ crystals,"Single crystals of Fe$_{1/4}$TaS$_{2}$ have been studied by using magneto-optical (MO) imaging and radio-frequency (rf) magnetic susceptibility, $\chi$. Real time MO images reveal unusual, slow dynamics of dendritic domain formation, the details of which are strongly dependent upon magnetic and thermal history. Measurements of $\chi(T)$ show well-defined, local moment ferromagnetic transition at $T\approx 155$ K as well as thermal hysteresis for 50 K$ 6at%. The critical temperatures are not significantly increased by the P alloying.",0908.0063v2 2009-08-01,Magnetic solitons in frustrated ferromagnetic spin chain,"We study the classical anisotropic ferromagnetic spin chain with frustration. The behavior of soliton and kink solutions in the vicinity of the ground state phase transition from the ferromagnetic to the spiral phase is studied. The dependence of the soliton energy on small anisotropy parameter is established using scaling estimates and numerical minimization of the energy functional. Conditions of the existence of the solitons are determined. It is shown that solitons survive in the spiral phase though with some restrictions on their size. A comparison of the energies of the classical solitons and the bound magnon complexes in the quantum model shows the functional similarity between them. The influence of the finite-size effects on the soliton states is studied and it is shown that the localized solitons originate from the uniform state when the system size exceeds some critical value depending on the anisotropy.",0908.0068v1 2009-08-05,"Synthesis and characterization of ferromagnetic cobalt nanospheres, nanodiscs and nanocubes","We report the synthesis of cobalt nanoparticles with different shapes and sizes by rapid pyrolysis of cobalt carbonyl in the presence of various surfactants. The size and shape of the nanoparticles were influenced by reaction conditions, such as type of the surfactant, molar ratio of surfactant to precursor, reflux temperature and reaction time. The shapes that we have achieved include spherical, nearly spherical, disc and cube. The presence of linear amine yielded nanodiscs and they spontaneously self-assembled into long ribbons. The effect of shape anisotropy on magnetic nanoparticles has been investigated. Spherical nanoparticles of diameter 14.5 nm show strong ferromagnetic behavior at low temperature and superparamagnetism at room temperature. On the other hand the cubic nanoparticles of 45 nm sides showed negligible coercive field at T = 10 K and ferromagnetism that persisted above T = 300 K. The cobalt nanospheres were oxidized to grow cobalt oxide shell of varying thickness to study exchange bias effect. A pronounced exchange bias and a strong temperature dependant magnetization were observed in oxidized cobalt nanospheres.",0908.0665v1 2009-08-18,AC- and DC-driven noise and I-V characteristics of magnetic nanostructures,"We study a structure consisting of a ferromagnetic (F) layer coupled to two normal metal (N) leads. The system is driven out of equilibrium by the simultaneous application of external dc and ac voltages across the N/F/N structure. Using the Keldysh diagrammatic approach, and modeling the ferromagnet as a classical spin of size S >> 1, we derive the Langevin equation for the magnetization dynamics and calculate the noise correlator. We find that the noise has an explicit frequency dependence in addition to depending on the characteristics of the ac and dc drive. Further, we calculate the current-voltage characteristics of the structure to O(1/S^2) and find that the nonequilibrium dynamics of the ferromagnetic layer gives rise to corrections to the current that are both linear and nonlinear in voltage.",0908.2483v2 2009-08-24,Interplay between antiferromagnetic order and spin polarization in ferromagnetic metal/electron-doped cuprate superconductor junctions,"Recently we proposed a theory of point-contact spectroscopy and argued that the splitting of zero-bias conductance peak (ZBCP) in electron-doped cuprate superconductor point-contact spectroscopy is due to the coexistence of antiferromagnetic (AF) and d-wave superconducting orders [Phys. Rev. B {\bf 76}, 220504(R) (2007)]. Here we extend the theory to study the tunneling in the ferromagnetic metal/electron-doped cuprate superconductor (FM/EDSC) junctions. In addition to the AF order, the effects of spin polarization, Fermi-wave vector mismatch (FWM) between the FM and EDSC regions, and effective barrier are investigated. It is shown that there exits midgap surface state (MSS) contribution to the conductance to which Andreev reflections are largely modified due to the interplay between the exchange field of ferromagnetic metal and the AF order in EDSC. Low-energy anomalous conductance enhancement can occur which could further test the existence of AF order in EDSC. Finally, we propose a more accurate formula in determining the spin polarization value in combination with the point-contact conductance data.",0908.3366v1 2009-08-30,Spin conservation and Fermi liquid near a ferromagnetic quantum critical point,"We propose a new low-energy theory for itinerant fermions near a ferromagnetic quantum critical point. We show that the full low-energy model includes, in addition to conventional interaction via spin fluctuations, another type of interaction, whose presence is crucial for the theory to satisfy SU(2) spin conservation. We demonstrate the consistency between a loop-wise expansion and a Fermi liquid description for the full model. We further show that, prior to the ferromagnetic instability, the system develops a Pomeranchuk-type instability into a state with zero magnetization but with p-wave deformations of the Fermi surfaces of spin-up and -down electrons (a spin nematic).",0908.4433v2 2009-09-02,Tunneling through ferromagnetic barriers on the surface of a topological insulator,"We study the transmission through single and double ferromagnetic barriers on the surface of a topological insulator. By adjusting the gate voltage and magnetization oreintation, the ferromagnetic barrier can be tuned into various transmission regions, where the wavevector-dependent tunnelings are quite different. We find that the Klein tunneling can be manipulated or even be turned off. These special properties offer the possibility to control electron beams on the ""topological metal"". Various novel devices, such as electronic collimation, wavevector filter, magnetic and electric switchs, and wavevector-based spin valve, may be constructed based on our observed phenomena.",0909.0378v1 2009-09-07,$A$-site substitution effect on physical properties of Sr$_3$Fe$_{2-x}$Co$_x$O$_{7-δ}$,"We have investigated the $Ln^{3+}$-substitution ($Ln$ = lanthanoid) effect of a quasi two-dimensional ferromagnet Sr$_3$Fe$_{2-x}$Co$_x$O$_{7-\delta}$ ($x$$ = $0.5). %$Ln^{3+}$-substitution creates antiferromagnetic insulating clusters around $Ln^{3+}$. With increasing $Ln^{3+}$-concentration, the ferromagnetism is gradually suppressed and the resistivity is increasing, which are ascribed to an increase in antiferromagnetic (AFM) clusters created by $Ln^{3+}$-substitution. In Sr$_{2.7}$Gd$_{0.3}$Fe$_{1.5}$Co$_{0.5}$O$_{7-\delta}$, the magnetoresistance (MR) is enhanced by about 20 % compared with that of Sr$_3$Fe$_{1.5}$Co$_{0.5}$O$_{7-\delta}$. Coexistence of ferromagnetic (FM) and AFM phases is essential for the enhancement of the MR\@. Applied magnetic fields align the FM clusters in the same direction, resulting in a reduction in the resistivity. A metamagnetic transition observed in the $Ln^{3+}$-doped samples also contributes to the enhancement of the MR.",0909.1172v1 2009-09-11,Nagaoka ferromagnetism in large-spin systems -Fermion and Boson systems--,"We study magnetic properties of itinerant quantum magnetic particles described by a generalized Hubbard model with large spin ($S>1/2$) which may be realized in optical lattices of laser-cooled atom systems. In fermion systems (half-integer spins), an extended form of Nagaoka ferromagnetism may be realized. However, as novel aspects of the large spin cases, we found that the condition on the lattice connectivity is more stringent than in the case of $S=1/2$ particles, and that the system shows a peculiar degenerate structure of the ground statein which the ferromagnetic state is included. In contrast, it turns out that the ground state of itinerant bosonic systems (integer spins) has a degenerate structure similar to that of fermion system with $S>1/2$ regardless of the shape, connectivity or filling of the lattice, and that the state with the maximum total spin is always one of the ground states.",0909.2082v1 2009-09-21,"Experimental probing of the interplay between ferromagnetism and localisation in (Ga,Mn)As","The question whether the Anderson-Mott localisation enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga,Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed. Here, by direct magnetisation measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin-spin coupling is diminished by the gate electric field in metal/insulator/semiconductor structures of (Ga,Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models. We find that the transition from the ferromagnetic to the paramagnetic state proceeds via the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localisation leads to a phase separation into ferromagnetic and nonmagnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson-Mott quantum transition.",0909.3694v1 2009-09-23,Magnetization Densities as Replica Parameters: The Dilute Ferromagnet,"In this paper we compute exactly the ground state energy and entropy of the dilute ferromagnetic Ising model. The two thermodynamic quantities are also computed when a magnetic field with random locations is present. The result is reached in the replica approach frame by a class of replica order parameters introduced by Monasson. The strategy is first illustrated considering the SK model, for which we will show the complete equivalence with the standard replica approach. Then, we apply to the diluted ferromagnetic Ising model with a random located magnetic field, which is mapped into a Potts model. This formalism can be, in principle, applied to all random systems, and we believe that it could be of help in many other contexts.",0909.4215v3 2009-10-06,Magnetoelectric Coupling and Electric Control of Magnetization in Ferromagnet-Ferroelectric-Metal Superlattices,"Ferromagnet-ferroelectric-metal superlattices are proposed to realize the large room-temperature magnetoelectric effect. Spin dependent electron screening is the fundamental mechanism at the microscopic level. We also predict an electric control of magnetization in this structure. The naturally broken inversion symmetry in our tri-component structure introduces a magnetoelectric coupling energy of $P M^2$. Such a magnetoelectric coupling effect is general in ferromagnet-ferroelectric heterostructures, independent of particular chemical or physical bonding, and will play an important role in the field of multiferroics.",0910.1031v1 2009-10-09,Realization of random-field dipolar Ising ferromagnetism in a molecular magnet,"The longitudinal magnetic susceptibility of single crystals of the molecular magnet Mn$_{12}$-acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase due to dipolar interactions. With increasing magnetic field applied transverse to the easy axis, the transition temperature decreases considerably more rapidly than predicted by mean field theory to a T=0 quantum critical point. Our results are consistent with an effective Hamiltonian for a random-field Ising ferromagnet in a transverse field, where the randomness is induced by an external field applied to Mn$_{12}$-acetate crystals that are known to have an intrinsic distribution of locally tilted magnetic easy axes.",0910.1754v1 2009-10-10,"Effect of inversion asymmetry on the intrinsic anomalous Hall effect in ferromagnetic (Ga,Mn)As","The relativistic nature of the electron motion underlies the intrinsic part of the anomalous Hall effect, believed to dominate in ferromagnetic (Ga,Mn)As. In this paper, we concentrate on the crystal band structure as an important facet to the description of this phenomenon. Using different k.p and tight-binding computational schemes, we capture the strong effect of the bulk inversion asymmetry on the Berry curvature and the anomalous Hall conductivity. At the same time, we find it not to affect other important characteristics of (Ga,Mn)As, namely the Curie temperature and uniaxial anisotropy fields. Our results extend the established theories of the anomalous Hall effect in ferromagnetic semiconductors and shed new light on its puzzling nature.",0910.1907v4 2009-10-16,Ferromagnetic proximity effect in F-QDot-S device,"Ferromagnetic proximity effect is studied in InAs nanowire (NW) based quantum dots (QD) strongly coupled to a ferromagnetic (F) and a superconducting (S) lead. The influence of the F lead is detected through the splitting of the spin-1/2 Kondo resonance. We show that the F lead induces a local exchange field on the QD, which has varying amplitude and a sign depending on the charge states. The interplay of the F and S correlations generates an exchange field related supgap feature. This novel mini-gap allows now the visualization of the exchange field also in even charge states",0910.3237v2 2009-10-28,"Orbital fluctuations, spin-orbital coupling, and anomalous magnon softening in an orbitally degenerate ferromagnet","The correlated motion of electrons in the presence of strong orbital fluctuations and correlations is investigated with respect to magnetic couplings and excitations in an orbitally degenerate ferromagnet. Introduction of the orbital degree of freedom results in a class of diagrams representing spin-orbital coupling which become particularly important near the orbital ordering instability. Low-energy staggered orbital fluctuation modes, particularly with momentum near (\pi/2,\pi/2,0) (corresponding to CE-type orbital correlations), are shown to generically yield strong intrinsically non-Heisenberg (1-\cos q)^2 magnon self energy correction, resulting in no spin stiffness reduction, but strongly suppressed zone-boundary magnon energies in the Gamma-X direction. The zone-boundary magnon softening is found to be strongly enhanced with increasing hole doping and for narrow-band materials, which provides insight into the origin of zone-boundary anomalies observed in ferromagnetic manganites.",0910.5321v2 2009-10-28,Ferromagnetism of electron gas,"In current work, we investigate the density and temperature dependence of polarization parameter; using the relativistic formalism for the electron-electron interaction within the Fermi liquid model. we calculate the spin dependent scattering matrix elements in relativistic region, and then obtain the non-relativistic behavior to study the magnetic properties of an electron gas. By varying the polarization parameter, we minimized the free energy and then obtain the polarization of the system as a function of density and temperature. At zero temperature the exact results for polarization and magnetic susceptibility have obtained. It has been shown that for a given temperature (density) there is critical density (temperature) that the ferromagnetic phase can appears in electron gas. Our results show that at nonzero temperatures and in very low and very high densities the ferromagnetism phase can not be exist.",0910.5332v4 2009-11-02,Effect of helical edge states on the tunneling conductance in ferromagnet/noncentrosymmetric superconductors junctions,"Helical edge states exist in the mixed spin-singlet and spin-triplet phase of a noncentrosymmetric (NCS) superconductor. In this article we have considered a planar ferromagnetic metal/NCS superconductor tunnel junction and have studied the effect of these helical edge states which manifests itself through the charge and spin tunneling conductance across the junction. We have shown the behavior of conductances for the entire range of variation of $\gamma = \Delta_-/\Delta_+$ where $\Delta_\pm$ are the order parameters in the positive and negative helicity bands of the NCS superconductor.There exists a competition between the Rashba parameter $\alpha$ and the exchange energy $E_{ex}$ which is crucial for determining the variation of the conductance with the applied bias voltage across the junction. We have found a nonzero spin current across the junction which appears due to the exchange energy in the Ferromagnet and modulates with the bias voltage. It also changes its profile when the strength of the exchange energy is varied.",0911.0302v2 2009-11-04,Dissipative current in SIFS Josephson junctions,"We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We consider the case of a strong tunnel barrier such that the left S layer and the right FS bilayer are decoupled. We calculate quantitatively the density of states (DOS) in the FS bilayer for arbitrary length of the ferromagnetic layer, using a self-consistent numerical method. We compare these results with a known analytical DOS approximation, which is valid when the ferromagnetic layer is long enough. Finally we calculate quantitatively the current-voltage characteristics of a SIFS junction.",0911.0846v2 2009-11-17,Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. XI: Stabilizing neutron stars against a ferromagnetic collapse,"We construct a new Hartree-Fock-Bogoliubov (HFB) mass model, labeled HFB-18, with a generalized Skyrme force. The additional terms that we have introduced into the force are density-dependent generalizations of the usual $t_1$ and $t_2$ terms, and are chosen in such a way as to avoid the high-density ferromagnetic instability of neutron stars that is a general feature of conventional Skyrme forces, and in particular of the Skyrme forces underlying all the HFB mass models that we have developed in the past. The remaining parameters of the model are then fitted to essentially all the available mass data, an rms deviation $\sigma$ of 0.585 MeV being obtained. The new model thus gives almost as good a mass fit as our best-fit model HFB-17 ($\sigma$ = 0.581 MeV), and has the advantage of avoiding the ferromagnetic collapse of neutron stars.",0911.3346v1 2009-11-27,Domain growth within the backbone of the three-dimensional $\pm J$ Edwards-Anderson spin glass,"The goal of this work is to show that a ferromagnetic-like domain growth process takes place within the backbone of the three-dimensional $\pm J$ Edwards-Anderson (EA) spin glass model. To sustain this affirmation we study the heterogeneities displayed in the out-of-equilibrium dynamics of the model. We show that both correlation function and mean flipping time distribution present features that have a direct relation with spatial heterogeneities, and that they can be characterized by the backbone structure. In order to gain intuition we analyze the pure ferromagnetic Ising model, where we show the presence of dynamical heterogeneities in the mean flipping time distribution that are directly associated to ferromagnetic growing domains. We extend a method devised to detect domain walls in the Ising model to carry out a similar analysis in the three-dimensional EA spin glass model. This allows us to show that there exists a domain growth process within the backbone of this model.",0911.5264v1 2009-11-30,"Nonlocal Electronic Spin Detection, Spin Accumulation and the Spin Hall effect","In recent years, electrical spin injection and detection has grown into a lively area of research in the field of spintronics. Spin injection into a paramagnetic material is usually achieved by means of a ferromagnetic source, whereas the induced spin accumulation or associated spin currents are detected by means of a second ferromagnet or the reciprocal spin Hall effect, respectively. This article reviews the current status of this subject, describing both recent progress and well-established results. The emphasis is on experimental techniques and accomplishments that brought about important advances in spin phenomena and possible technological applications. These advances include, amongst others, the characterization of spin diffusion and precession in a variety of materials, such as metals, semiconductors and graphene, the determination of the spin polarization of tunneling electrons as a function of the bias voltage, and the implementation of magnetization reversal in nanoscale ferromagnetic particles with pure spin currents.",0911.5619v1 2010-01-06,Sustained ferromagnetism induced by H-vacancies in graphane,"The electronic and magnetic properties of graphane with H-vacancies are investigated with the help of quantum-chemistry methods. The hybridization of the edges is found to be absolutely crucial in defining the size of the bandgap, which is increased from 3.04 eV to 7.51 eV when the hybridization is changed from the sp^2 to the sp^3 type. The H-vacancy defects also influence the size of the gap depending on the number of defects and their distribution between the two sides of the graphane plane. Further, the H-vacancy defects induced on one side of the graphane plane and placed on the neighboring carbon atoms are found to be the source of ferromagnetism which is distinguished by the high stability of the state with a large spin number in comparison to that of the singlet state and is expected to persist even at room temperatures. However, the ferromagnetic ordering of the spins is obtained to be limited by the concentration of H-vacancy defects and ordering would be preserved if number of defects do not exceed eight.",1001.0919v2 2010-01-08,Thermodynamics of a two-dimensional frustrated spin-1/2 Heisenberg ferromagnet,"Using the spin-rotation-invariant Green's function method we calculate the thermodynamic quantities (correlation functions , uniform static spin susceptibility \chi, correlation length \xi, and specific heat C_V) of the two-dimensional spin-1/2 J1-J2 Heisenberg ferromagnet for J2 < J2^c \approx 0.44|J1|, where J2^c is the critical frustrating antiferromagnetic next-nearest neighbor coupling at which the ferromagnetic ground state gives way for a ground-state phase with zero magnetization. Examining the low-temperature behavior of \chi and \xi, in the limit T \to 0 both quantities diverge exponentially, i.e., \chi \propto \exp(b/T) and \xi \propto\exp(b/2T), respectively. We find a linear decrease of the coefficient b with increasing frustration according to b=-(\pi/2)(J1+2J2), i.e., the exponential divergence of \chi and \xi is present up to J2^c. Furthermore, we find an additional low-temperature maximum in the specific heat when approaching the critical point, J2 \to J2^c.",1001.1222v2 2010-01-10,Underscreened Kondo effect in quantum dots coupled to ferromagnetic leads,"We analyze the equilibrium transport properties of underscreened Kondo effect in the case of a two-level quantum dot coupled to ferromagnetic leads. Using the numerical renormalization group (NRG) method, we have determined the gate voltage dependence of the dot's spin and level-resolved spectral functions. We have shown that the polarization of the dot is very susceptible to spin imbalance in the leads and changes sign in the middle of the S=1 Coulomb valley. Furthermore, we have also found that by fine-tuning an external magnetic field one can compensate for the presence of ferromagnetic leads and restore the Kondo effect in the case of $S=1/2$ Coulomb valley. However, the underscreened Kondo effect cannot be fully recovered due to its extreme sensitivity with respect to the magnetic field.",1001.1506v2 2010-01-14,Kondo effect in a quantum dot coupled to ferromagnetic leads and side-coupled to a nonmagnetic reservoir,"Equilibrium transport properties of a single-level quantum dot tunnel-coupled to ferromagnetic leads and exchange-coupled to a side nonmagnetic reservoir are analyzed theoretically in the Kondo regime. The equilibrium spectral functions and conductance through the dot are calculated using the numerical renormalization group (NRG) method. It is shown that in the antiparallel magnetic configuration, the system undergoes a quantum phase transition with increasing exchange coupling $J$, where the conductance drops from its maximum value to zero. In the parallel configuration, on the other hand, the conductance is generally suppressed due to an effective spin splitting of the dot level caused by the presence of ferromagnetic leads, irrespective of the strength of exchange constant. However, for $J$ ranging from J=0 up to the corresponding critical value, the Kondo effect and quantum critical behavior can be restored by applying properly tuned compensating magnetic field.",1001.2475v1 2010-01-28,Stability of a Fully Magnetized Ferromagnetic state in Repulsively Interacting Ultracold Fermi Gases,"We construct a variational wave function to study whether a fully polarized Fermi sea is energetically stable against a single spin flip. Our variational wave function contains sufficient short-range correlation at least to the same level as Gutzwiller's projected wave function. For Hubbard lattice model and continuum model with pure repulsive interaction, we show a fully polarized Fermi sea is generally unstable even when the repulsive strength becomes infinite. While for a resonance model, ferromagnetic state is possible if the s-wave scattering length is positive and sufficiently large, and the system is prepared in scattering state orthogonal to molecular bound state. However, we can not rule out the possibility that more exotic correlation can destabilize the ferromagnetic state.",1001.5139v2 2010-02-03,Tailoring Interlayer Exchange Coupling of Ferromagnetic Films Across MgO with Fe Nanoclusters,"We investigate the interlayer exchange coupling in Fe/MgO/Fe and Fe/MgO/Co systems with magnetic Fe nanoclusters embedded in the MgO spacer. Samples are grown by molecular beam epitaxy (MBE) and utilize wedged MgO films to independently vary the film thickness and the position of the Fe nanoclusters. Depending on the position of the Fe nanoclusters, the bilinear coupling (J1) exhibits strong variations in magnitude and can even switch between antiferromagnetic and ferromagnetic. This effect is explained by the magnetic coupling between the ferromagnetic films and the magnetic nanoclusters. Interestingly, the coupling of Fe nanoclusters to a Co film is 160% stronger than their coupling to a Fe film (at MgO spacing of 0.56 nm). This is much greater than the coupling difference of 20% observed in the analogous thin film systems (i.e. Fe/MgO/Co vs. Fe/MgO/Fe), identifying an interesting nano-scaling effect related to the coupling between films and nanoclusters.",1002.0627v1 2010-02-07,Collective modes for an array of magnetic dots with perpendicular magnetization,"The dispersion relations of collective oscillations of the magnetic moment of magnetic dots arranged in square-planar arrays and having magnetic moments perpendicular to the array plane are calculated. The presence of the external magnetic field perpendicular to the plane of array, as well as the uniaxial anisotropy for single dot are taken into account. The ferromagnetic state with all the magnetic moments parallel, and chessboard antiferromagnetic state are considered. The dispersion relation yields information about the stability of different states of the array. There is a critical magnetic field below which the ferromagnetic state is unstable. The antiferromagnetic state is stable for small enough magnetic fields. The dispersion relation is non-analytic as the value of the wave vector approaches zero. Non-trivial Van Hove anomalies are also found for both ferromagnetic and antiferromagnetic states.",1002.1477v1 2010-02-16,"Orbital Ordering and Unfrustrated $(π,0)$ Magnetism from Degenerate Double Exchange in the Iron Pnictides","The magnetic excitations of the iron pnictides are explained within a degenerate double-exchange model. The local-moment spins are coupled by superexchanges $J_1$ and $J_2$ between nearest and next-nearest neighbors, respectively, and interact with the itinerant electrons of the degenerate $d_{xz}$ and $d_{yz}$ orbitals via a ferromagnetic Hund exchange. The latter stabilizes $(\pi,0)$ stripe antiferromagnetism due to emergent ferro-orbital order and the resulting kinetic energy gain by hopping preferably along the ferromagnetic spin direction. Taking the quantum nature of the spins into account, we calculate the magnetic excitation spectra in the presence of both, super- and double-exchange. A dramatic increase of the spin-wave energies at the competing N\'eel ordering wave vector is found, in agreement with recent neutron scattering data. The spectra are fitted to a spin-only model with a strong spatial anisotropy and additional longer ranged couplings along the ferromagnetic chains. Over a realistic parameter range, the effective couplings along the chains are negative corresponding to unfrustrated stripe antiferromagnetism.",1002.3165v3 2010-02-25,Ferromagnetic Resonance of Co/Gd and Co/Tb Multilayers,"The in-plane dynamics of ferrimagnetic Co/Gd multilayers are investigated by means of ferromagnetic resonance, magneto-optical Kerr effect and SQUID magnetometry. The power absorbed from these multilayers is strongly temperature dependent. For example, the resonant peak for a (Co 40 A /Gd 40 A)8 multilayer vanishes approximately 50 K below room temperature. We have further investigated Gd/Co/Gd and Tb/Co/Tb trilayers with different thicknesses of Gd (5-7 A), Tb (1-7 A) and Co (30-40 A). At room temperature, these Co-based trilayers show a shift of approximately 600 Oe at 24 GHz in the uniform ferromagnetic resonance field, compared to pure Co film, indicating the exchange coupling between the Co and Gd. The shift in the field for the resonance increases as the temperature is decreased. Furthermore the resonance linewidth increases as the temperature is decreased. The experimental results are in good agreement with our theoretical calculations.",1002.4889v1 2010-03-05,Finite size effects with variable range exchange coupling in thin-film Pd/Fe/Pd trilayers,"The magnetic properties of thin-film Pd/Fe/Pd trilayers in which an embedded ~1.5 A-thick ultrathin layer of Fe induces ferromagnetism in the surrounding Pd have been investigated. The thickness of the ferromagnetic trilayer is controlled by varying the thickness of the top Pd layer over a range from 8 A to 56 A. As the thickness of the top Pd layer decreases, or equivalently as the embedded Fe layer moves closer to the top surface, the saturated magnetization normalized to area and the Curie temperature decrease whereas the coercivity increases. These thickness-dependent observations for proximity-polarized thin-film Pd are qualitatively consistent with finite size effects that are well known for regular thin-film ferromagnets. The critical exponent $\beta$ of the order parameter (magnetization) is found to approach the mean field value of 0.5 as the thickness of the top Pd layer increases. The functional forms for the thickness dependences, which are strongly modified by the nonuniform exchange interaction in the polarized Pd, provide important new insights to understanding nanomagnetism in two-dimensions.",1003.1305v1 2010-03-08,Stability of 2pi domain walls in ferromagnetic nanorings,"The stability of 2pi domain walls in ferromagnetic nanorings is investigated via calculation of the minimum energy path that separates a 2pi domain wall from the vortex state of a ferromagnetic nanoring. Trapped domains are stable when they exist between certain types of transverse domain walls, i.e., walls in which the edge defects on the same side of the magnetic strip have equal sign and thus repel. Here the energy barriers between these configurations and vortex magnetization states are obtained using the string method. Due to the geometry of a ring, two types of 2pi walls must be distinguished that differ by their overall topological index and exchange energy. The minimum energy path corresponds to the expulsion of a vortex. The energy barrier for annihilation of a 2pi wall is compared to the activation energy for transitions between the two ring vortex states.",1003.1758v1 2010-03-09,Generation and Manipulation of Spin Current in Graphene Nanodisks: Robustness against Randomness and Lattice Defects,"Trigonal zigzag graphene nanodisk exhibits magnetism whose spin is proportional to the edge length of the nanodisk. Its spin can be designed from 1/2 to a huge value. The spins form a quasiferromagnet, which has intermediate properties between a single spin and a ferromagnet. In other words, the ferromagnet order has a relatively long life time, and yet the nanodisk spin can be rotated by external field or current. We consider a nanodisk connected with two leads. This system acts as a spin filter just as in a metal-ferromagnet-metal junction. In this way we can generate a spin current. Furthermore we can manipulate spin current by spin valve, spin switch and other spintronic devices made of graphene nanodisks. We also show that nanodisk spins are robust against the effect of randomness in site energy, transfer energy and lattice defects.",1003.1766v1 2010-03-09,Effect of induced shape anisotropy on magnetic properties of ferromagnetic cobalt nanocubes,"We report on the synthesis of ferromagnetic cobalt nanocubes of various sizes using thermal pyrolysis method and the effect of shape anisotropy on the static and dynamic magnetic properties were studied. Shape anisotropy of approximately 10 % was introduced in nanocubes by making nanodiscs using a linear chain amine surfactant during synthesis process. It has been observed that, ferromagnetism persisted above room temperature and a sharp drop in magnetic moment at low temperatures in zero-field cooled magnetization may be associated with the spin disorder due to the effective anisotropy present in the system. Dynamic magnetic properties were studied using RF transverse susceptibility measurements at different temperatures and the singularities due to anisotropy fields were probed at low temperatures. Symmetrically located broad peaks are observed in the frozen state at the effective anisotropy fields and the peak structure is strongly affected by shape anisotropy and temperature. Irrespective of size the shape anisotropy gave rise to higher coercive fields and larger transverse susceptibility ratio at all temperatures. The role of shape anisotropy and the size of the particles on the observed magnetic behaviour were discussed.",1003.1855v1 2010-03-10,Evidence for triplet superconductivity in Josephson junctions with ferromagnetic Cu$_{2}$MnAl-Heusler barriers,"We have studied Josephson junctions with barriers prepared from the Heusler compound Cu$_2$MnAl. In the as-prepared state the Cu$_2$MnAl layers are non ferromagnetic and the critical Josephson current density $j_{c}$ decreases exponentially with the thickness of the Heusler layers $d_{F}$. On annealing the junctions at 240\degree C the Heusler layers develop ferromagnetic order and we observe a dependence $j_{c}(d_{F}$) with $j_{c}$ strongly enhanced and weakly thickness dependent in the thickness range 7.0 nm < $d_{F}$ < 10.6 nm. We attribute this feature to a triplet component in the superconducting pairing function generated by the specific magnetization profile inside thin Cu$_2$MnAl layers.",1003.2082v1 2010-03-11,"Magnetic and Electrical Properties of Ordered 112-type Perovskite LnBaCoMnO5+δ(Ln = Nd, Eu)","Investigation of the oxygen-deficient 112-type ordered oxides of the type LnBaCoMnO5+\delta (Ln = Nd, Eu) evidences certain unusual magnetic behavior at low temperatures, compared to the LnBaCo2O5+\delta cobaltites. One observes that the substitution of manganese for cobalt suppresses the ferromagnetic state and induces strong antiferromagnetic interactions. Importantly, NdBaCoMnO5.9 depicts a clear paramagnetic to antiferromagnetic type transition around 220 K, whereas for EuBaCoMnO5.7 one observes an unusual magnetic behavior below 177 K which consists of ferromagnetic regions embedded in an antiferromagnetic matrix. The existence of two sorts of crystallographic sites for Co/Mn and their mixed valence states favor the ferromagnetic interaction whereas antiferromagnetism originates from the Co3+-O-Co3+ and Mn4+-O-Mn4+ interactions. Unlike the parent compounds, the present Mn-substituted phases do not exhibit prominent magnetoresistance effects in the temperature range 75-400K.",1003.2262v1 2010-03-12,Anomalous Hall Effect in Disordered Multiband Metals,"We present a microscopic theory of the anomalous Hall effect in metallic multi-band ferromagnets, which accounts for all scattering-independent contributions, i.e., both the intrinsic and the so-called side jump. For a model of Gaussian disorder, the anomalous Hall effect is expressed solely in terms of the electronic band structure of the host material. Our theory handles systematically the interband-scattering coherence effects. We demonstrate the method in the two-dimensional Rashba and three-dimensional ferromagnetic (III,Mn)V semiconductor models. Our formalism is directly amenable to ab initio treatments for a wide range of ferromagnetic metals.",1003.2626v2 2010-03-13,Ferromagnetic Clusters in the Brownmillerite Bilayered Compounds Ca2.5-xLaxSr0.5GaMn2O8: An Approach to Achieve Layered Spintronics Materials,"We report the effect of La-substitution on the magnetic and magnetotransport properties of Brownmillerite-like bilayered compounds Ca2.5-xLaxSr0.5GaMn2O8 (x = 0, 0.05, 0.075, and 0.1) by using dc-magnetization, resistivity and magnetoresistance techniques. The Rietveld analysis of the room temperature x-ray diffraction patterns confirms no observable change of average crystal structure with the La-substitution. Both magnetic and magnetotransport properties are found to be very sensitive to the La-substitution. Interestingly, the La-substituted compounds show ferromagnetic-like behavior (due to the occurrence of a double exchange mechanism) whereas, the parent compound is an antiferromagnet (TN 150 K). All compounds show an insulating behavior, in the measured temperature range of 100 - 300 K, with an overall decrease in the resistivity with the substitution. A higher value of magnetoresistance has been successfully achieved by the La-substitution. We have proposed an electronic phase separation model, considering the formation of ferromagnetic clusters in the antiferromagnetic matrix, to interpret the observed magnetization and magnetotransport results for the La-substituted samples. The present study demonstrates an approach to achieve new functional materials, based on naturally occurring layered system like Ca2.5-xLaxSr0.5GaMn2O8, for possible spintronics applications.",1003.2685v1 2010-03-26,Tunnel effect in ferromagnetic half-metal Co$_2$CrAl-superconductor heterostructures,"Ferromagnetic half-metal Co$_2$CrAl films and tunnel contacts Co$_2$CrAl - insulator (I) - Pb are fabricated and investigated. It is found that the normalized differential conductivity $\sigma ^{\rm FS} $ of such tunnel junctions with low resistance is larger than the normalized differential conductivity $\sigma ^{\rm NS} $ of known normal metal - I - superconductor type tunnel junctions. It is shown that the observed increase in $\sigma ^{\rm FS} $ is caused by the accumulation of spin polarized electrons in a superconductor and can be used for estimating the spin polarization degree $P$ in ferromagnets. This method shows that $P$ of L2$_1$-type ordered Co$_2$CrAl Heusler alloy films at $T = 4.2{\rm K}$ is close to 1.",1003.5093v1 2010-04-05,Hysteresis in the magneto-transport of Manganese-doped Germanium: evidence for carrier-mediated ferromagnetism,"We report the fabrication of Ge:Mn ferromagnetic semiconductors by Mn-ion implantation into Ge followed by pulsed laser annealing. Benefiting from the short time annealing, the hole concentration in Mn-implanted Ge has been increased by two orders of magnitude from 10$^{18}$ to over 10$^{20}$ cm$^{-3}$. Likely due to the high hole concentration, we observe that the longitudinal and Hall resistances exhibit the same hysteresis as the magnetization, which is usually considered as a sign of carrier-mediated ferromagnetism.",1004.0568v1 2010-04-07,Itinerant ferromagnetism of a repulsive atomic Fermi gas: a quantum Monte Carlo study,"We investigate the phase diagram of a two-component repulsive Fermi gas at T=0 by means of quantum Monte Carlo simulations. For a given value of the positive s-wave scattering length, both purely repulsive and purely attractive model potentials are considered in order to analyze the limits of the universal regime where the details of interatomic forces can be neglected. The equation of state of both balanced and unbalanced systems is calculated as a function of the interaction strength and the critical density for the onset of ferromagnetism is determined. The energy per particle of the strongly polarized gas is calculated and parametrized in terms of the physical properties of repulsive polarons, which are relevant for the stability of the fully magnetized ferromagnetic state. Finally, we analyze the phase diagram in the polarization/interaction plane under the assumption that only phases with homogeneous magnetization can be produced.",1004.1169v1 2010-04-26,Spin transfer in a ferromagnet-quantum dot and tunnel barrier coupled Aharonov-Bohm ring system with Rashba spin-orbit interactions,"The spin transfer effect in ferromagnet-quantum dot (insulator)-ferromagnet Aharonov-Bohm (AB) ring system with Rashba spin-orbit (SO) interactions is investigated by means of Keldysh nonequilibrium Green function method. It is found that both the magnitude and direction of the spin transfer torque (STT) acting on the right ferromagnet electrode can be effectively controlled by changing the magnetic flux threading the AB ring or the gate voltage on the quantum dot. The STT can be greatly augmented by matching a proper magnetic flux and an SO interaction at a cost of low electrical current. The STT, electrical current, and spin current are uncovered to oscillate with the magnetic flux. The present results are expected to be useful for information storage in nanospintronics.",1004.4507v1 2010-04-28,Is room temperature ferromagnetism possible in K-doped SnO2?,"Ab initio studies have theoretically predicted room temperature ferromagnetism in crystalline SnO2, ZrO2 and TiO2 doped with non magnetic element from the 1A column as K and Na. Our purpose is to address experimentally the possibility of magnetism in both Sn1-xKxO2 and Sn1-xCaxO2 compounds. The samples have been prepared using equilibrium methods of standard solid state route. Our study has shown that both Sn1-xCaxO2 and Sn1-xKxO2 structure is thermodynamically unstable and leads to a phase separation, as shown by X-ray diffraction and detailed micro-structural analyses with high resolution transmission electron microscopy (TEM). In particular, the crystalline SnO2 grains are surrounded by K-based amorphous phase. In contrast to Ca: SnO2 samples we have obtained a magnetic phase in K: SnO2 ones, but no long range ferromagnetic order. The K: SnO2 samples exhibit a moments of the order of 0.2 {\mu}B/K /ion, in contrast to ab-initio calculations which predict 3{\mu}B, where K atoms are on the Sn crystallographic site. The apparent contradictions between our experiments and first principle studies are discussed.",1004.5001v2 2010-04-30,Mean-field study of itinerant ferromagnetism in trapped ultracold Fermi gases: Beyond the local density approximation,"We theoretically investigate the itinerant ferromagnetic transition of a spherically trapped ultracold Fermi gas with spin imbalance under strongly repulsive interatomic interactions. Our study is based on a self-consistent solution of the Hartree-Fock mean-field equations beyond the widely used local density approximation. We demonstrate that, while the local density approximation holds in the paramagnetic phase, after the ferromagnetic transition it leads to a quantitative discrepancy in various thermodynamic quantities even with large atom numbers. We determine the position of the phase transition by monitoring the shape change of the free energy curve with increasing the polarization at various interaction strengths.",1004.5443v2 2010-06-03,Ferromagnetically coupled Shastry-Sutherland quantum spin singlets in (CuCl)LaNb$_2$O$_7$,"Using inelastic neutron scattering, x-ray, neutron diffraction, and the first-principle calculation techniques, we show that the crystal structure of the two-dimensional quantum spin system (CuCl)LaNb$_2$O$_7$ is orthorhombic with $Pbam$ symmetry in which CuCl$_4$O$_2$ octahedra are tilted from their high symmetry positions and the Cu$^{2+} (s = 1/2)$ ions form a distorted square lattice. The dominant magnetic interactions are the fourth nearest neighbor antiferromagnetic interactions with a Cu-Cl--Cl-Cu exchange path, which lead to the formation of spin singlets. The two strongest interactions between the singlets are ferromagnetic, which makes (CuCl)LaNb$_2$O$_7$ the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.",1006.0755v1 2010-06-07,Interface ferromagnetism and orbital reconstruction in BiFeO3- La0.7Sr0.3MnO3 heterostructures,"We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with La0.7Sr0.3MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L2,3-edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related with an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at oxygen K-edge.",1006.1194v1 2010-06-10,Probing the exchange field of a quantum-dot spin valve by a superconducting lead,"Electrons in a quantum-dot spin valve, consisting of a single-level quantum dot coupled to two ferromagnetic leads with magnetizations pointing in arbitrary directions, experience an exchange field that is induced on the dot by the interplay of Coulomb interaction and quantum fluctuations. We show that a third, superconducting lead with large superconducting gap attached to the dot probes this exchange field very sensitively. In particular, we find striking signatures of the exchange field in the symmetric component of the supercurrent with respect to the bias voltage applied between the ferromagnets already for small values of the ferromagnets' spin polarization.",1006.1976v2 2010-06-17,Nonlocal conductance via overlapping Andreev bound states in ferromagnet-superconductor heterostructures,"In a setup where two ferromagnetic electrodes are attached to a superconductor, Andreev bound states are induced at both ferromagnet/superconductor (FM/SC) interfaces. We study how these states propagate through the SC and interact with each other. As a result of this interaction, the energetic positions of the Andreev states are not anymore determined solely by the magnetic properties of a single interface, but also depend on the interface distance and the relative magnetization orientation of the FM contacts. We discuss how these bound states show up as distinct peaks in the nonlocal conductance signal, and lead to marked asymmetries with respect to the applied voltage. We relate our results to nonlocal crossed Andreev and elastic co-tunneling processes.",1006.3422v1 2010-06-18,Superfluid/ferromagnet/superfluid junction and $π$-phase in a superfluid Fermi gas,"We investigate the possibility of superfluid/ferromagnet/superfluid (SFS)-junction in a superfluid Fermi gas. To examine this possibility in a simple manner, we consider an attractive Hubbard model at $T=0$ within the mean-field theory. When a potential barrier is embedded in a superfluid Fermi gas with population imbalance ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of atoms with pseudospin $\sigma=\uparrow,\downarrow$), this barrier is shown to be {\it magnetized} in the sense that excess $\uparrow$-spin atoms are localized around it. The resulting superfluid Fermi gas is spatially divided into two by this {\it ferromagnet}, so that one obtains a junction similar to the superconductor/ferromagnet/superconductor-junction discussed in superconductivity. Indeed, we show that the so-called $\pi$-phase, which is a typical phenomenon in the SFS-junction, is realized, where the superfluid order parameter changes its sign across the junction. Our results would be useful for the study of magnetic effects on fermion superfluidity using an ultracold Fermi gas.",1006.3673v1 2010-06-26,Critical current diffraction pattern of SIFS Josephson junctions with step-like F-layer,"We present the latest generation of superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions with a step-like thickness of the ferromagnetic (F) layer. The F-layer thicknesses $d_1$ and $d_2$ in both halves were varied to obtain different combinations of positive and negative critical current densities $j_{c,1}$ and $j_{c,2}$. The measured dependences of the critical current on applied magnetic field can be well described by a model which takes into account different critical current densities (obtained from reference junctions) and different net magnetization of the multidomain ferromagnetic layer in both halves.",1006.5109v2 2010-07-07,"Three attractively interacting fermions in a harmonic trap: Exact solution, ferromagnetism, and high-temperature thermodynamics","Three fermions with strongly repulsive interactions in a spherical harmonic trap, constitute the simplest nontrivial system that can exhibit the onset of itinerant ferromagnetism. Here, we present exact solutions for three trapped, attractively interacting fermions near a Feshbach resonance. We analyze energy levels on the upper branch of the resonance where the atomic interaction is effectively repulsive. When the s-wave scattering length a is sufficiently positive, three fully polarized fermions are energetically stable against a single spin-flip, indicating the possibility of itinerant ferromagnetism, as inferred in the recent experiment. We also investigate the high-temperature thermodynamics of a strongly repulsive or attractive Fermi gas using a quantum virial expansion. The second and third virial coefficients are calculated. The resulting equations of state can be tested in future quantitative experimental measurements at high temperatures and can provide a useful benchmark for quantum Monte Carlo simulations.",1007.1041v1 2010-07-08,Observation of the Spin-Seebeck Effect in a Ferromagnetic Semiconductor,"The spin-Seebeck effect was recently discovered in a metallic ferromagnet and consists of a thermally generated spin distribution that is electrically measured utilizing the inverse spin Hall effect. Here this effect is reproduced experimentally in a ferromagnetic semiconductor, GaMnAs, which allows for flexible design of the magnetization directions, a larger spin polarization, and measurements across the magnetic phase transition. The spin-Seebeck effect in GaMnAs is observed even in the absence of longitudinal charge transport. The spatial distribution of spin-currents is maintained across electrical breaks highlighting the local nature of the effect, which is therefore ascribed to a thermally induced spin redistribution.",1007.1364v1 2010-07-10,Dynamical instability of a spin spiral in an interacting Fermi gas as a probe of the Stoner transition,"We propose an experiment to probe ferromagnetic phenomena in an ultracold Fermi gas, while alleviating the sensitivity to three-body loss and competing many-body instabilities. The system is initialized in a small pitch spin spiral, which becomes unstable in the presence of repulsive interactions. To linear order the exponentially growing collective modes exhibit critical slowing down close to the Stoner transition point. Also, to this order, the dynamics are identical on the paramagnetic and ferromagnetic sides of the transition. However, we show that scattering off the exponentially growing modes qualitatively alters the collective mode structure. The critical slowing down is eliminated and in its place a new unstable branch develops at large wave vectors. Furthermore, long-wavelength instabilities are quenched on the paramagnetic side of the transition. We study the experimental observation of the instabilities, specifically addressing the trapping geometry and how phase-contrast imaging will reveal the emerging domain structure. These probes of the dynamical phenomena could allow experiments to detect the transition point and distinguish between the paramagnetic and ferromagnetic regimes.",1007.1743v2 2010-07-12,The phases of deuterium at extreme densities,"We consider deuterium compressed to higher than atomic, but lower than nuclear densities. At such densities deuterium is a superconducting quantum liquid. Generically, two superconducting phases compete, a ""ferromagnetic"" and a ""nematic"" one. We provide a power counting argument suggesting that the dominant interactions in the deuteron liquid are perturbative (but screened) Coulomb interactions. At very high densities the ground state is determined by very small nuclear interaction effects that probably favor the ferromagnetic phase. At lower densities the symmetry of the theory is effectively enhanced to SU(3), and the quantum liquid enters a novel phase, neither ferromagnetic nor nematic. Our results can serve as a starting point for investigations of the phase dynamics of deuteron liquids, as well as exploration of the stability and dynamics of the rich variety of topological objects that may occur in phases of the deuteron quantum liquid, which range from Alice strings to spin skyrmions to Z_2 vortices.",1007.1972v2 2010-07-19,Spin waves in zigzag graphene nanoribbons and the stability of edge ferromagnetism,"We study the low energy spin excitations of zigzag graphene nanoribbons of varying width. We find their energy dispersion at small wave vector to be dominated by antiferromagnetic correlations between the ribbon's edges, in accrodance with previous calculations. We point out that spin wave lifetimes are very long due to the semi-conducting nature of the electrically neutral nanoribbons. However, application of very modest gate voltages cause a discontinuous transition to a regime of finite spin wave lifetime. By further increasing doping the ferromagnetic alignments along the edge become unstable against transverse spin fluctuations. This makes the experimental detection of ferromagnetism is this class of systems very delicate, and poses a difficult challenge to the possible uses of these nanoribbons as basis for spintronic devices.",1007.3285v2 2010-07-24,The itinerant ferromagnetic phase of the Hubbard model,"Using a newly developed quantum Monte Carlo technique, we provide strong evidence for the stability of a saturated ferromagnetic phase in the high-density regime of the two-dimensional infinite-U Hubbard model. By decreasing the electron density, a discontinuous transition to a paramagnetic phase is observed, accompanied by a divergence of the susceptibility on the paramagnetic side. This behavior, resulting from a high degeneracy among different spin sectors, is consistent with an infinite-order phase transition. The remarkable stability of itinerant ferromagnetism renews the hope to describe this phenomenon within a purely kinetic mechanism and will facilitate the validation of experimental quantum simulators with cold atoms loaded in optical lattices.",1007.4260v2 2010-07-27,Low-temperature properties of classical zigzag spin chain at the ferromagnet-helimagnet transition point,"Low-temperature thermodynamics of the classical frustrated ferromagnetic spin chain near the ferromagnet-helimagnet transition point is studied by means of mapping to the continuum limit. The calculation of the partition function and spin correlation function is reduced to quantum problem of a particle in potential well. It is shown that exactly at the transition point the correlation length behaves as $T^{-1/3}$ and the magnetic susceptibility diverges as $T^{-4/3}$ in the low-temperature limit. Corresponding numerical factors for the correlation length and the susceptibility is calculated. It is shown that the low-temperature susceptibility in the helical phase near the transition point has a maximum at some temperature. Such behavior as well as the location and the magnitude of the maximum as a function of deviation from the transition point are in agreement with that observed in several materials described by the quantum $s=1/2$ version of this model.",1007.4625v1 2010-07-30,Gapped Ferromagnetic Graphene Nanoribbons,"We theoretically design a graphene-based all-organic ferromagnetic semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic magnets. A large spin-split gap with 100% spin polarized density of states near the Fermi energy is obtained, which is of potential application in spin transistors. The interplays among electron, spin and lattice degrees of freedom are studied using the first-principles calculations combined with fundamental model analysis. All of the calculations consistently demonstrate that although no d electrons existing, the antiferromagnetic \pi-\pi exchange together with the strong spin-lattice interactions between organic magnets and ZGNRs make the ground state ferromagnetic. The fundamental physics makes it possible to optimally select the organic magnets towards practical applications.",1007.5416v2 2010-08-07,Disorder Induced Negative Magnetization in LaSrCoRuO6,"This paper reports effect of thermally induced disorder on the magnetic properties of LaSrCoRuO6 double perovskite. While the ordered sample is antiferromagnetic, the disordered sample exhibits negative values of magnetization measured in low applied fields. Isothermal magnetization on this sample shows hysteresis due to presence of ferromagnetic interactions. Based on neutron diffraction and X-ray Absorption Fine Structure (XAFS) studies, these results have been interpreted to be due disorder in site occupancy of Co and Ru leading to octahedral distortions and formation of Ru-O-Ru ferromagnetic linkages. Below 150K these ferromagnetic Ru spins polarize the Co spins in a direction opposite to that of the applied field resulting in observed negative magnetization.",1008.1314v1 2010-08-11,Nanoscale Dichotomy of Ti 3d Carriers Mediating the Ferromagnetism in Co:TiO2 Anatase Thin Films,"We study the surface and bulk electronic structure of the room-temperature ferromagnet Co:TiO2 anatase films using soft and hard x-ray photoemission spectroscopy with probe sensitivities of ~1 nm and ~10 nm, respectively. We obtain direct evidence of metallic Ti$^{3+}$ states in the bulk, which get suppressed to give a surface semiconductor, thus indicating a surface-bulk dichotomy. X-ray absorption and high-sensitivity resonant photoemission spectroscopy reveal Ti$^{3+}$ electrons at the Fermi level (E$_F$) and high-spin Co$^{2+}$ electrons occurring away from E$_F$. The results show the importance of the charge neutrality condition: Co$^{2+}$ + V$_{O}$$^{2-}$ + 2Ti$^{4+}$ $\leftrightarrow$ Co$^{2+}$ + 2Ti$^{3+}$ (V$_O$ is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.",1008.1908v1 2010-08-16,Thermal expansion of the superconducting ferromagnet UCoGe,"We report measurements of the coefficient of linear thermal expansion, $\alpha (T)$, of the superconducting ferromagnet UCoGe. The data taken on a single-crystalline sample along the orthorhombic crystal axes reveal a pronounced anisotropy, with the largest length changes along the $b$ axis. The large values of the step sizes $\Delta \alpha$ at the magnetic and superconducting phase transitions provide solid evidence for bulk magnetism and superconductivity. Specific-heat measurements corroborate bulk superconductivity. Thermal-expansion measurements in magnetic fields $B \parallel a,b$ show $\Delta \alpha$ at $T_C$ grows rapidly, which indicates the character of the ferromagnetic transition becomes first-order-like.",1008.2635v1 2010-08-19,Limited influence of diluted ferromagnetic dimers on Curie temperature in complex magnetic systems,"In this work we investigate Ising and classical Heisenberg models for two and three dimensional lattices in presence of diluted ferromagnetic dimers. For such models the Curie temperature as a function of ratio of intra-dimer exchange coupling constant I_A and other inter-site coupling constants I_B is calculated. In case dimer is treated exactly and environment within the mean-field approach it was found that even for I_A/I_B\to\infty T_C remains finite. Similar analysis is proposed for rhombohedral phase of intermetallic compound Gd2Fe17 where so-called Fe1-Fe1 ""dumbbell"" forms the diluted ferromagnetic dimer. It was shown that for such complex magnetic systems T_C is determined by all variety of exchange interactions and for the interval 0 < I_A/I_B < \infty T_C changes are not more than +/-10%.",1008.3219v1 2010-09-07,Itinerant ferromagnetism in a Fermi gas with contact interaction: Magnetic properties in a dilute Hubbard model,"Ground state properties of the repulsive Hubbard model on a cubic lattice are investigated by means of the auxiliary-field quantum Monte Carlo method. We focus on low-density systems with varying on-site interaction $U/t$, as a model relevant to recent experiments on itinerant ferromagnetism in a dilute Fermi gas with contact interaction. Twist-average boundary conditions are used to eliminate open-shell effects and large lattice sizes are studied to reduce finite-size effects. The sign problem is controlled by a generalized constrained path approximation. We find no ferromagnetic phase transition in this model. The ground-state correlations are consistent with those of a paramagnetic Fermi liquid.",1009.1409v1 2010-09-09,Antisite Domains in Double Perovskite Ferromagnets: Impact on Magnetotransport and Half-metallicity,"Several double perovskite materials of the form A_2BB'O_6 exhibit high ferromagnetic T_c, and significant low field magnetoresistance. They are also a candidate source of spin polarized electrons. The potential usefulness of these materials is, however, frustrated by mislocation of the B and B' ions, which do not organise themselves in the ideal alternating structure. The result is a strong dependence of physical properties on preparative conditions, reducing the magnetization and destroying the half-metallicity. We provide the first results on the impact of spatially correlated antisite disorder, as observed experimentally, on the ferromagnetic double perovskites. The antisite domains suppress magnetism and half-metallicity, as expected, but lead to a dramatic enhancement of the low field magnetoresistance.",1009.1702v1 2010-09-10,Improved half-metallic ferromagnetism of transition-metal pnictides and chalcogenides calculated with a modified Becke-Johnson exchange potential,"We use a density-functional-theory (DFT) approach with a modified Becke-Johnson exchange plus local density approximation (LDA) correlation potential (mBJLDA) [semi-local, orbital-independent, producing accurate semiconductor gaps. see F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)] to investigate the electronic structures of zincblende transition-metal (TM) pnictides and chalcogenides akin to semiconductors. Our results show that this potential does not yield visible changes in wide TM d-t_{2g} bands near the Fermi level, but makes the occupied minority-spin p-bands lower by 0.25~0.35 eV and the empty (or nearly empty) minority-spin e_g bands across the Fermi level higher by 0.33~0.73 eV. Consequently, mBJLDA, having no atom-dependent parameters, makes zincblende MnAs become a truly half-metallic (HM) ferromagnet with a HM gap (the key parameter) 0.318eV, being consistent with experiment. For zincblende MnSb, CrAs, CrSb, CrSe, or CrTe, the HM gap is enhanced by 19~56% compared to LDA and generalized gradient approximation results. The improved HM ferromagnetism can be understood in terms of the mBJLDA-enhanced spin exchange splitting.",1009.1920v1 2010-09-12,Universal valence-band picture of the ferromagnetic semiconductor GaMnAs,"The origin of ferromagnetism in the prototype ferromagnetic semiconductor GaMnAs is still controversial due to the insufficient understanding of its band structure and Fermi level position. Here, we show the universal valence-band (VB) picture of GaMnAs obtained by resonant tunneling spectroscopy for a variety of surface GaMnAs layers with the Mn concentrations from 6 to 15% and the Curie temperatures from 71 to 154 K. We find that the Fermi level exists in the bandgap, and that the VB structure of GaAs is almost perfectly maintained in all the GaMnAs samples, i.e. VB is not merged with the impurity band. Furthermore, the p-d exchange splitting of VB is found to be quite small (only several meV) even in GaMnAs with a high Curie temperature (154 K). These results indicate that the VB structure of GaMnAs is quite insensitive to the Mn doping.",1009.2235v1 2010-09-20,The cavity method for quantum disordered systems: from transverse random field ferromagnets to directed polymers in random media,"After reviewing the basics of the cavity method in classical systems, we show how its quantum version, with some appropriate approximation scheme, can be used to study a system of spins with random ferromagnetic interactions and a random transverse field. The quantum cavity equations describing the ferromagnetic-paramagnetic phase transition can be transformed into the well-known problem of a classical directed polymer in a random medium. The glass transition of this polymer problem translates ino the existence of a `Griffith phase' close to the quantum phase transition of the quantum spin problem, where the physics is dominated by rare events.",1009.3725v2 2010-09-22,Optimal control of magnetization dynamics in ferromagnetic heterostructures by spin--polarized currents,"We study the switching-process of the magnetization in a ferromagnetic-normal-metal multilayer system by a spin polarized electrical current via the spin transfer torque. We use a spin drift-diffusion equation (SDDE) and the Landau-Lifshitz-Gilbert equation (LLGE) to capture the coupled dynamics of the spin density and the magnetization dynamic of the heterostructure. Deriving a fully analytic solution of the stationary SDDE we obtain an accurate, robust, and fast self-consistent model for the spin-distribution and spin transfer torque inside general ferromagnetic/normal metal heterostructures. Using optimal control theory we explore the switching and back-switching process of the analyzer magnetization in a seven-layer system. Starting from a Gaussian, we identify a unified current pulse profile which accomplishes both processes within a specified switching time.",1009.4296v2 2010-09-25,Micro-Structured Ferromagnetic Tubes for Spin Wave Excitation,"Micron scale ferromagnetic tubes placed on the ends of ferromagnetic CoTaZr spin waveguides are explored in order to enhance the excitation of Backward Volume Magnetostatic Spin Waves. The tubes produce a closed magnetic circuit about the signal line of the coplanar waveguide and are, at the same time, magnetically contiguous with the spin waveguide. This results in a 10 fold increase in spin wave amplitude. However, the tube geometry distorts the magnetic field near the spin waveguide and relatively high biasing magnetic fields are required to establish well defined spin waves. Only the lowest (uniform) spin wave mode is excited.",1009.4986v1 2010-09-28,Non-Collinear Ferromagnetic Luttinger Liquids,"The presence of electron-electron interactions in one dimension profoundly changes the properties of a system. The separation of charge and spin degrees of freedom is just one example. We consider what happens when a system consisting of a ferromagnetic region of non-collinearity, i.e. a domain wall, is coupled to interacting electrons in one-dimension (more specifically a Luttinger liquid). The ferromagnetism breaks spin charge separation and the presence of the domain wall introduces a spin dependent scatterer into the problem. The absence of spin charge separation and the effects of the electron correlations results in very different behaviour for the excitations in the system and for spin-transfer-torque effects in this model.",1009.5509v1 2010-09-29,Acoustically driven ferromagnetic resonance,"Surface acoustic waves (SAW) in the GHz frequency range are exploited for the all-elastic excitation and detection of ferromagnetic resonance (FMR) in a ferromagnetic/ferroelectric (nickel/lithium niobate) hybrid device. We measure the SAW magneto-transmission at room temperature as a function of frequency, external magnetic field magnitude, and orientation. Our data are well described by a modified Landau-Lifshitz-Gilbert approach, in which a virtual, strain-induced tickle field drives the magnetization precession. This causes a distinct magnetic field orientation dependence of elastically driven FMR that we observe in both model and experiment.",1009.5798v2 2010-10-20,Strain-driven non-collinear magnetic ordering in orthorhombic epitaxial YMnO3 thin films,"We show that using epitaxial strain and chemical pressure in orthorhombic YMnO3 and Co-substituted (YMn0.95Co0.05O3) thin films, a ferromagnetic response can be gradually introduced and tuned. These results, together with the measured anisotropy of the magnetic response, indicate that the unexpected observation of ferromagnetism in orthorhombic o-RMnO3 (R= Y, Ho, Tb, etc) films originates from strain-driven breaking of the fully compensated magnetic ordering by pushing magnetic moments away from the antiferromagnetic [010] axis. We show that the resulting canting angle and the subsequent ferromagnetic response, gradually increase (up to ~ 1.2\degree) by compression of the unit cell. We will discuss the relevance of these findings, in connection to the magnetoelectric response of orthorhombic manganites.",1010.4118v2 2010-10-26,Partial annealing of a coupled mean-field spin-glass model with an embedded pattern,"A partially annealed mean-field spin-glass model with a locally embedded pattern is studied. The model consists of two dynamical variables, spins and interactions, that are in contact with thermal baths at temperatures T_S and T_J, respectively. Unlike the quenched system, characteristic correlations among the interactions are induced by the partial annealing. The model exhibits three phases, which are paramagnetic, ferromagnetic and spin-glass phases. In the ferromagnetic phase, the embedded pattern is stably realized. The phase diagram depends significantly on the ratio of two temperatures n=T_J/T_S. In particular, a reentrant transition from the embedded ferromagnetic to the spin-glass phases with T_S decreasing is found only below at a certain value of n. This indicates that above the critical value n_c the embedded pattern is supported by local field from a non-embedded region. Some equilibrium properties of the interactions in the partial annealing are also discussed in terms of frustration.",1010.5346v2 2010-10-27,Anomalous Josephson current through a ferromagnetic trilayer junction,"We studied the anomalous Josephson current appearing at zero phase difference in junctions coupled with a ferromagnetic trilayer which has noncoplanar magnetizations. A $\pi/2$ junction with an equilibrium phase difference $\pi/2$ is obtained under suitable conditions. The equilibrium phase difference and the amplitude of the supercurrent are all tunable by the structure parameters. In addition to calculating the anomalous current using the Bogoliubov-de Gennes equation, we also developed a clear physical picture explaining the anomalous Josephson effect in the structure. We show that the triplet proximity correlation and the phase shift in the anomalous current-phase relation all stem from the spin precession in the first and third ferromagnet layers.",1010.5554v2 2010-10-28,Density functional study of weak ferromagnetism in a thick BiCrO3 film,"Very thick films of BiCrO$_3$ have been grown on a SrTiO$_3$ substrate, maintaining a tetragonal lattice up to thicknesses of 14,000\AA. Assuming we can treat films of this thickness as bulk crystals, we first calculated the experimentally undetermined atomic positions within the unit cell with the measured lattice constant of the film, then relaxed the lattice constants along with the atomic positions. All the calculations result in an antiferroelectric crystal with the {\it Pnma} space group with noncollinear Bi displacements. We find G-type antiferromagnetism with a spin-orbit induced weak ferromagnetic component, however, the weak ferromagnetic component can cancel if the antiferromagnetic spins are oriented along a particular one of the three 2-fold rotation axes.",1010.6003v1 2010-11-25,Model for domain wall avalanches in ferromagnetic thin films,"The Barkhausen jumps or avalanches in magnetic domain-walls motion between succesive pinned configurations, due the competition among magnetic external driving force and substrum quenched disorder, appear in bulk materials and thin films. We introduce a model based in rules for the domain wall evolution of ferromagnetic media with exchange or short-range interactions, that include disorder and driving force effects. We simulate in 2-dimensions with Monte Carlo dynamics, calculate numerically distributions of sizes and durations of the jumps and find power-law critical behavior. The avalanche-size exponent is in excellent agreement with experimental results for thin films and is close to predictions of the other models, such as like random-field and random-bond disorder, or functional renormalization group. The model allows us to review current issues in the study of avalanches motion of the magnetic domain walls in thin films with ferromagnetic interactions and opens a new approach to describe these materials with dipolar or long-range interactions.",1011.5643v3 2010-11-26,Weak itinerant ferromagnetism in Heusler type Fe2VAl0.95,"We report measurements of the magnetic, transport and thermal properties of the Heusler type compound Fe2VAl0.95. We show that while stoichiometric Fe2VAl is a non-magnetic semi-metal a 5% substitution on the Al-site with the 3d elements Fe and V atoms leads to a ferromagnetic ground state with a Curie temperature TC = 33+-3 K and a small ordered moment ms = 0.12 mB/Fe in Fe2VAl0.95. The reduced value of the ratio ms/mp = 0.08, where mp = 1.4 mB/Fe is the effective Curie-Weiss moment, together with the analysis of the magnetization data M(H,T), show magnetism is of itinerant nature. The specific heat shows an unusual temperature variation at low temperatures with an enhanced Sommerfeld coefficient, g = 12 mJK-2mol-1. The resistivity, r(T), is metallic and follows a power law behavior r(T) = r0+AT^n with n = 1.5 below TC. With applying pressure, TC decreases with the rate of (1/TC)(dTC /dP) = -0.061 GPa-1. We conclude substitution on the Al-site with Fe and V atoms results in itinerant ferromagnetism with a low carrier density.",1011.5704v1 2010-12-02,Electronic structure and magnetism of transition metal doped Zn12O12 clusters: Role of defects,"We present a comprehensive study of the energetics and magnetic properties of ZnO clusters doped with 3d transition metals (TM) using ab initio density functional calculations in the framework of generalized gradient approximation + Hubbard U (GGA+U) method. Our results within GGA+U for all 3d dopants except Ti indicate that antiferromagnetic interaction dominates in a neutral, defect-free cluster. Formation energies are calculated to identify the stable defects in the ZnO cluster. We have analyzed in details the role of these defects to stabilize ferromagnetism when the cluster is doped with Mn, Fe, and Co. Our calculations reveal that in the presence of charged defects the transition metal atoms residing at the surface of the cluster may have an unusual oxidation state, that plays an important role to render the cluster ferromagnetic. Defect induced magnetism in ZnO clusters without any TM dopants is also analyzed. These results on ZnO clusters may have significant contributions in the nanoengineering of defects to achieve desired ferromagnetic properties for spintronic applications.",1012.0430v1 2010-12-03,Dissipationless mechanism of skyrmion Hall current in double-exchange ferromagnets,"We revisit a theory of skyrmion transport in ferromagnets. On a basis of an effective U(1) gauge theory for spin-chirality fluctuations in double-exchange ferromagnets, we derive an expression for the velocity of a skyrmion core driven by the dc electric field. We find that mutual feedback effects between conduction electrons and localized spins give rise to Chern-Simons terms, suggesting a dissipationless mechanism for the skyrmion Hall current. A conventional description of the current-induced skyrmion motion, appearing through the spin transfer torque and scattering events, is reproduced in a certain limit of our description, where the Chern-Simons terms are not fully incorporated. Our theory is applicable to not only metallic but also insulating systems, where the purely topological and dissipationless skyrmion Hall current can be induced in the presence of an energy gap.",1012.0631v3 2010-12-09,Superconductivity Reinforced by Magnetic Field and the Magnetic Instability in Uranium Ferromagnets,"We review our recent results on ferromagnetic superconductors, URhGe and UCoGe. High quality single crystals of both compounds were successfully grown. The specific heat shows a clear jump related to the superconducting transition in UCoGe. The finite values of C/T at 0K are discussed in terms of the self-induced vortex state and the value of the ordered moment. With increasing fields for H // b-axis in URhGe, the jump of thermal expansion increases and shifts to lower temperature. The re-entrant and S-shaped superconducting phases for URhGe and UCoGe respectively are explained by the unusual field dependence of the effective mass, which is induced by the ferromagnetic instability when the field is applied along the hard magnetization b-axis. The magnetic fluctuations are very sensitive to the field orientation. This is reflected in the Hc2 and the anisotropy of the effective mass.",1012.1987v1 2010-12-11,Magnetism in C or N-doped MgO and ZnO: density-functional study of impurity pairs,"It is shown that substitution of C or N for O recently proposed as a way to create ferromagnetism in otherwise nonmagnetic oxide insulators is curtailed by formation of impurity pairs, and the resultant C2 spin=1 dimers as well as the isoelectronic N2^{2+} interact antiferromagneticallly in p-type MgO. For C-doped ZnO, however, we demonstrate using the HSE hybrid functional that a resonance of the spin-polarized C2 pp\pi* states with the host conduction band results in a long-range ferromagnetic interaction. Magnetism of open-shell impurity molecules is proposed as a possible route to d0-ferromagnetism in oxide spintronic materials.",1012.2439v1 2010-12-14,Scaling behavior of the spin pumping effect in ferromagnet/platinum bilayers,"We systematically measured the DC voltage V_ISH induced by spin pumping together with the inverse spin Hall effect in ferromagnet/platinum bilayer films. In all our samples, comprising ferromagnetic 3d transition metals, Heusler compounds, ferrite spinel oxides, and magnetic semiconductors, V_ISH invariably has the same polarity. V_ISH furthermore scales with the magnetization precession cone angle with a universal prefactor, irrespective of the magnetic properties, the charge carrier transport mechanism or type. These findings quantitatively corroborate the present theoretical understanding of spin pumping in combination with the inverse spin Hall effect.",1012.3017v2 2011-01-09,Semiconducting (Half-Metallic) Ferromagnetism in Mn(Fe) Substituted Pt and Pd Nitrides,"Using first principles calculations as based on density functional theory, we propose a class of so far unexplored diluted ferromagnetic semiconductors and half-metals. Here, we study the electronic properties of recently synthesized $ 4d $ and $ 5d $ transition metal dinitrides. In particular, we address Mn- and Fe-substitution in PtN$_2$ and PdN$_2$. Structural relaxation shows that the resulting ordered compounds, Pt$_{0.75}$(Mn,Fe)$_{0.25}$N$_2$ and Pd$_{0.75}$(Mn,Fe)$_{0.25}$N$_2$, maintain the cubic crystal symmetry of the parent compounds. On substitution, all compounds exhibit long-range ferromagnetic order. While both Pt$_{0.75}$Mn$_{0.25}$N$_2$ and Pd$_{0.75}$Mn$_{0.25}$N$_2$ are semiconducting, Fe-substitution causes half-metallic behavior for both parent materials.",1101.1635v1 2011-01-21,Multiple spin state analysis of magnetic nano graphene,"Recent experiments indicate room-temperature ferromagnetism in graphite-like materials. This paper offers multiple spin state analysis applied to asymmetric graphene molecule to find out mechanism of ferromagnetic nature. First principle density functional theory is applied to calculate spin density, energy and atom position depending on each spin state. Molecules with dihydrogenated zigzag edges like C64H27, C56H24, C64H25, C56H22 and C64H23 show that in every molecule the highest spin state is the most stable one with over 3000 K energy difference with next spin state. This result suggests a stability of room temperature ferromagnetism in these molecules. In contrast, nitrogen substituted molecules like C59N5H22, C52N4H20, C61N3H22, C54N2H20 and C63N1H22 show opposite result that the lowest spin state is the most stable. Magnetic stability of graphene molecule can be explained by three key issues, that is, edge specified localized spin density, parallel spins exchange interaction inside of a molecule and atom position optimization depending on spin state. Those results will be applied to design a carbon-base ferro-magnet, an ultra high density 100 tera bit /inch2 class information storage and spintronic devices.",1101.4080v1 2011-01-27,Asymmetric graphene model applied to graphite-like carbon-based ferromagnetism,"Several experiments have recently found room-temperature ferromagnetism in graphite-like carbon based materials. This paper offers a model explaining such ferromagnetism by using an asymmetric nano-graphene. Our first typical model is C48H24 graphene molecule, which has three dihydrogenated (-CH2) zigzag edges. There are several multiple spin states competing for stable minimum energy in the same atomic topology. Both molecular orbital and density function theory methods indicate that the quartet state(S=3/2) is more stable than that of doublet (S=1/2), which means that larger saturation magnetization will be achieved. We also enhanced this molecule to an infinite length ribbon having many (-CH2) edges. Similar results were obtained where the highest spin state was more stable than lower spin state. In contrast, a nitrogen substituted (-NH) molecule C45N3H21 demonstrated opposite results. that is, the lowest spin state(S=1/2) is more stable than that of highest one(S=3/2), which arises from the slight change in atom position.",1101.5209v1 2011-01-27,Anomalous magnetic transport in ferromagnetic graphene junctions,"We investigate magnetotransport in a ferromagnetic/normal/ferromagnetic graphene junction where a gate electrode is attached to the normal segment. It is shown that the charge conductance can be maximal at an antiparallel configuration of the magnetizations. Moreover, we demonstrate that both the magnitude and the sign of the spin-transfer torque can be controlled by means of the gate voltage in the normal segment. In this way, the present system constitutes a spin-transfer torque transistor. These anomalous phenomena are attributed to the combined effect of the exchange field and the Dirac dispersion of graphene. Our prediction opens up the possibility of moving domain walls parallel or antiparallel to the current in a controllable fashion by means of a local gate voltage.",1101.5236v1 2011-01-31,Random-field p-spin glass model on regular random graphs,"We investigate in detail the phase diagrams of the p-body +/-J Ising model with and without random fields on random graphs with fixed connectivity. One of our most interesting findings is that a thermodynamic spin glass phase is present in the three-body purely ferromagnetic model in random fields, unlike for the canonical two-body interaction random-field Ising model. We also discuss the location of the phase boundary between the paramagnetic and spin glass phases that does not depend on the change of the ferromagnetic bias. This behavior is explained by a gauge transformation, which shows that gauge-invariant properties generically do not depend on the strength of the ferromagnetic bias for the +/-J Ising model on regular random graphs.",1101.5863v1 2011-02-01,One-dimentional magnonic crystal as a medium with magnetically tunable disorder on a periodical lattice,"We show that periodic magnetic nanostructures (magnonic crystals) represent an ideal system for studying excitations on disordered periodical lattices because of the possibility of controlled variation of the degree of disorder by varying the applied magnetic field. Ferromagnetic resonance (FMR) data collected inside minor hysteresis loops for a periodic array of Permalloy nanowires of alternating width and magnetic force microscopy images of the array taken after running each of these loops were used to establish convincing evidence that there is a strong correlation between the type of FMR response and the degree of disorder of the magnetic ground state. We found two types of dynamic responses: anti-ferromagnetic (AFM) and ferromagnetic (FM), which represent collective spin wave modes or collective magnonic states. Depending on the history of sample magnetization either AFM or FM state is either the fundamental FMR mode or represents a state of a magnetic defect on the artificial crystal. A fundamental state can be transformed into a defect one and vice versa by controlled magnetization of the sample.",1102.0069v1 2011-02-06,Microscopic model for the semiconductor-to-ferromagnetic-metal transition in FeSi$_{1-x}$Ge$_{x}$ Alloys,"The simplified bandstructure introduced by Mazurenko et al to model FeSi is used to analyze the singlet semiconductor to ferromagnetic metal transition in the isoelectronic isostructural alloys, FeSi$_{1-x}$Ge$_x$. The complex bandstructure of the alloy is replaced by an alternating chain of doubly and singly degenerate atoms to represent Fe and Si/Ge respectively. The former(latter) form narrow(broad) bands with a substantial hybridization between them. A substantial onsite repulsion including a Hund's rule coupling is introduced on the Fe sites. The mean field phase diagram contains a first order phase transition from the singlet semiconductor to a ferromagnetic metal with increasing temperature and interaction strength similar to the alloys. The analysis also reproduces the rapid rise of the spin susceptibility in the semiconductor with a crossover to a Curie-Weiss form at higher temperatures. Good agreement is found at zero temperature between the mean field and accurate DMRG calculations.",1102.1190v1 2011-02-12,Electrical control of ferromagnetism in Mn-doped semiconductor heterostructures,"The interplay of tunneling transport and carrier-mediated ferromagnetism in narrow semiconductor multi-quantum well structures containing layers of GaMnAs is investigated within a self-consistent Green's function approach, accounting for disorder in the Mn--doped regions and unwanted spin-flips at heterointerfaces on phenomenological ground. We find that the magnetization in GaMnAs layers can be controlled by an external electric bias. The underlying mechanism is identified as spin-selective hole tunneling in and out of the Mn-doped quantum wells, whereby the applied bias determines both hole population and spin polarization in these layers. In particular we predict that, near resonance, ferromagnetic order in the Mn doped quantum wells is destroyed. The interplay of both magnetic and transport properties combined with structural design potentially leads to several interrelated physical phenomena, such as dynamic spin filtering, electrical control of magnetization in individual magnetic layers, and, under specific bias conditions, to self-sustained current and magnetization oscillations (magneticmulti-stability). Relevance to recent experimental results is discussed.",1102.2507v2 2011-02-16,Magnetization dynamics and Majorana fermions in ferromagnetic Josephson junctions along the quantum spin Hall edge,"We investigate the interplay between ferromagnetic and superconducting order at the edge of a quantum spin Hall insulator. Using complementary analytical and self-consistent numerical approaches, we study a ferromagnetic Josephson junction and show how the direct coupling between magnetism and the superconducting U(1) phase gives rise to several unusual phenomena which distinguishes the present system from its non-topological equivalent. In particular, we demonstrate how the anomalous current-phase relation triggers supercurrent-induced magnetization dynamics and also study the spatial localization of the Majorana fermions appearing in the junction.",1102.3403v2 2011-02-18,Ground state properties of the one-dimensional electron liquid,"We present calculations of the energy, pair correlation function (PCF), static structure factor (SSF), and momentum density (MD) for the one-dimensional electron gas using the quantum Monte Carlo method. We are able to resolve peaks in the SSF at even-integer-multiples of the Fermi wave vector, which grow as the coupling is increased. Our MD results show an increase in the effective Fermi wave vector as the interaction strength is raised in the paramagnetic harmonic wire; this appears to be a result of the vanishing difference between the wave functions of the paramagnetic and ferromagnetic systems. We have extracted the Luttinger liquid exponent from our MDs by fitting to data around the Fermi wave vector, finding good agreement between the exponent of the ferromagnetic infinitely-thin wire and the ferromagnetic harmonic wire.",1102.3854v2 2011-02-24,Noncoplanar spin canting in lightly-doped ferromagnetic Kondo lattice model on a triangular lattice,"Effect of the coupling to mobile carriers on the 120$^\circ$ antiferromagnetic state is investigated in a ferromagnetic Kondo lattice model on a frustrated triangular lattice. Using a variational calculation for various spin orderings up to a four-site unit cell, we identify the ground-state phase diagram with focusing on the lightly-doped region. We find that an electron doping from the band bottom immediately destabilizes a 120$^\circ$ coplanar antiferromagnetic order and induces a noncoplanar three-sublattice ordering accompanied by an intervening phase separation. This noncoplanar phase has an umbrella-type spin configuration with a net magnetic moment and a finite spin scalar chirality. This spin-canting state emerges in competition between the antiferromagnetic superexchange interaction and the ferromagnetic double-exchange interaction under geometrical frustration. In contrast, a hole doping from the band top retains the 120$^\circ$-ordered state up to a finite doping concentration and does not lead to a noncolpanar ordering.",1102.4940v1 2011-02-26,Electron scattering from domain walls in ferromagnetic Luttinger liquids,"We study the properties of interacting electrons in a one-dimensional conduction band coupled to bulk non-collinear ferromagnetic order. The specific form of non-collinearity we consider is that of an extended domain wall. The presence of ferromagnetic order breaks spin-charge separation and the domain wall introduces a spin-dependent scatterer active over the length of the wall l. Both forward and backward scattering off the domain wall can be relevant perturbations of the Luttinger liquid and we discuss the possible low temperature phases. Our main finding is that backward scattering, while determining the ultimate low temperature physics, only becomes important at temperatures T/J < exp(-l/l_+) with J being the magnetic exchange and l_+ the backward scattering length scale. In physical realizations, l >> l_+ and the physics will be dominated by forward scattering which can lead to a charge conducting but spin insulating phase. In a perturbative regime at higher temperatures we furthermore calculate the spin and charge densities around the domain wall and quantitatively discuss the interaction induced changes.",1102.5446v2 2011-03-02,Anomalous Hall effect in NiPt thin films,"We study Hall effect in sputtered NixPt1-x thin films with different Ni concentrations. Temperature, magnetic field and angular dependencies are analyzed and the phase diagram of NiPt thin films is obtained. It is found that films with sub-critical Ni concentration exhibit cluster-glass behavior at low temperatures with a perpendicular magnetic anisotropy below the freezing temperature. Films with over-critical Ni concentration are ferromagnetic with parallel anisotropy. At the critical concentration the state of the film is strongly frustrated. Such films demonstrate canted magnetization with the easy axis rotating as a function of temperature. The magnetism appears via consecutive paramagnetic - cluster glass - ferromagnetic transitions, rather than a single second-order phase transition. But most remarkably, the extraordinary Hall effect changes sign at the critical concentration. We suggest that this is associated with a reconstruction of the electronic structure of the alloy at the normal metal - ferromagnet quantum phase transition.",1103.0367v2 2011-03-21,Coexistence of superconductivity and ferromagnetism in two dimensions,"Ferromagnetism is usually considered to be incompatible with conventional superconductivity, as it destroys the singlet correlations responsible for the pairing interaction. Superconductivity and ferromagnetism are known to coexist in only a few bulk rare-earth materials. Here we report evidence for their coexistence in a two-dimensional system: the interface between two bulk insulators, LaAlO$_3$ (LAO) and SrTiO$_3$ (STO), a system that has been studied intensively recently. Magnetoresistance, Hall and electric-field dependence measurements suggest that there are two distinct bands of charge carriers that contribute to the interface conductivity. The sensitivity of properties of the interface to an electric field make this a fascinating system for the study of the interplay between superconductivity and magnetism.",1103.4006v2 2011-03-23,"Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies","We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature whereas they are antiferromagnetic below 200 K approximately. Temperature dependence of the carrier mobility was determined using Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled modes. It shows that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electrons between spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn(1-x)CoxO films.",1103.4463v1 2011-03-27,Measurements of Nanoscale Domain Wall Flexing in a Ferromagnetic Thin Film,"We use the high spatial sensitivity of the anomalous Hall effect in the ferromagnetic semiconductor Ga1-xMnxAs, combined with the magneto-optical Kerr effect, to probe the nanoscale elastic flexing behavior of a single magnetic domain wall in a ferromagnetic thin film. Our technique allows position sensitive characterization of the pinning site density, which we estimate to be around 10^14 cm^{-3}. Analysis of single site depinning events and their temperature dependence yields estimates of pinning site forces (10 pN range) as well as the thermal deactivation energy. Finally, our data hints at a much higher intrinsic domain wall mobility for flexing than previously observed in optically-probed micron scale measurements.",1103.5240v1 2011-04-05,"Ferromagnetic Enhancement of CE-type Spin Ordering in (Pr,Ca)MnO$_3$","We present resonant soft X-ray scattering (RSXS) results from small band width manganites (Pr,Ca)MnO$_3$, which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance (CMR) effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic (FM-M) state and CE-type ordering.",1104.0757v1 2011-04-07,Finite-size effects on the magnetoelectric response of field-driven ferroelectric/ferromagnetic chains,"We study theoretically the coupled multiferroic dynamics of one-dimensional ferroelectric/ferromagnet chains driven by harmonic magnetic and electric fields as a function of the chain length. A linear magnetoelectric coupling is dominated by the spin-polarized screening charge at the interface. We performed Monte-Carlo simulations and calculations based on the coupled Landau-Lifshitz-Gilbert and Landau-Khalatnikov equations showing that the net magnetization and the total polarization of thin heterostructures, i.e. with up to ten ferroelectric and ferromagnetic sites counted from the interface, can be completely reversed by external electric and magnetic fields, respectively. However, for larger system solely a certain magnetoelectrical control can be achieved.",1104.1269v3 2011-04-08,Magnetization Dissipation in Ferromagnets from Scattering Theory,"The magnetization dynamics of ferromagnets are often formulated in terms of the Landau-Lifshitz-Gilbert (LLG) equation. The reactive part of this equation describes the response of the magnetization in terms of effective fields, whereas the dissipative part is parameterized by the Gilbert damping tensor. We formulate a scattering theory for the magnetization dynamics and map this description on the linearized LLG equation by attaching electric contacts to the ferromagnet. The reactive part can then be expressed in terms of the static scattering matrix. The dissipative contribution to the low-frequency magnetization dynamics can be described as an adiabatic energy pumping process to the electronic subsystem by the time-dependent magnetization. The Gilbert damping tensor depends on the time derivative of the scattering matrix as a function of the magnetization direction. By the fluctuation-dissipation theorem, the fluctuations of the effective fields can also be formulated in terms of the quasistatic scattering matrix. The theory is formulated for general magnetization textures and worked out for monodomain precessions and domain wall motions. We prove that the Gilbert damping from scattering theory is identical to the result obtained by the Kubo formalism.",1104.1625v1 2011-04-11,Synthesis and study of alpha-Fe1.4Ga0.6O3: An advanced Ferromagnetic Semiconductor,"We report the synthesis of alpha-Fe1.4Ga0.6O3 compound and present its structural phase stability and interesting magnetic, dielectric and photo-absorption properties. In our work Ga doped alpha-Fe2O3 samples are well stabilized in alpha phase (rhombohedral crystal structure with space group R3C). Properties of the present composition of Ga doped alpha-Fe2O3 system are remarkably advanced in comparison with recently most studied FeGaO3 composition. At room temperature the samples are typical soft ferromagnet, as well as direct band gap semiconductor. Dielectric study showed low dielectric loss in the samples with large enhancement of ac conductivity at higher frequencies. Optical absorption in the visible range has been enhanced by 4 to 5%. This composition has exhibited large scope of tailoring room temperature ferromagnetic moment and optical band gap by varying grain size and non-ambient (vacuum) heat treatment of the as prepared samples by mechanical alloying.",1104.1982v1 2011-04-12,Hybridization driven quantum critical behavior in weakly-itinerant ferromagnets,"We investigate the unusual magnetic properties of nearly-critical, weakly-itinerant ferromagnets with general formula UTX, where T=Rh,Co and X=Ge,Si. As a unique feature about these systems, we show that changes in the V_{df} hybridization control their proximity to a ferromagnetic instability, and determine the evolution of: the ground state magnetization, M_0, the Curie Temperature, T_C, the density of states at the Fermi level, N(E_F), the T^2 resistivity coefficient, A, and the specific heat coefficient, \gamma. The universal aspect of our findings comes from the dependence on only two parameters: the T_d bandwidth, W_d, and the distance between T_d and U_f band centers, C_{T_d}-C_{U_f}.",1104.2287v1 2011-04-15,Structural and Magnetic Phase Transitions in NdCoAsO under High Pressures,"We have investigated structural and magnetic phase transitions under high pressures in a quaternary rare earth transition metal arsenide oxide NdCoAsO compound that is isostructural to high temperature superconductor NdFeAsO. Four-probe electrical resistance measurements carried out in a designer diamond anvil cell show that the ferromagnetic Curie temperature and anti-ferromagnetic Neel temperature increase with an increase in pressure. High pressure x-ray diffraction studies using a synchrotron source show a structural phase transition from a tetragonal phase to a new crystallographic phase at a pressure of 23 GPa at 300 K. The NdCoAsO sample remained anti-ferromagnetic and non-superconducting to temperatures down to 10 K and to the highest pressure achieved in this experiment of 53 GPa. A P-T phase diagram for NdCoAsO is presented to a pressure of 53 GPa and low temperatures of 10 K.",1104.3172v1 2011-04-18,Enhanced photon-assisted spin transport in a quantum dot attached to ferromagnetic leads,"We investigate real-time dynamics of spin-polarized current in a quantum dot coupled to ferromagnetic leads in both parallel and antiparallel alignments. While an external bias voltage is taken constant in time, a gate terminal, capacitively coupled to the quantum dot, introduces a periodic modulation of the dot level. Using non equilibrium Green's function technique we find that spin polarized electrons can tunnel through the system via additional photon-assisted transmission channels. Owing to a Zeeman splitting of the dot level, it is possible to select a particular spin component to be photon-transfered from the left to the right terminal, with spin dependent current peaks arising at different gate frequencies. The ferromagnetic electrodes enhance or suppress the spin transport depending upon the leads magnetization alignment. The tunnel magnetoresistance also attains negative values due to a photon-assisted inversion of the spin-valve effect.",1104.3483v2 2011-05-05,Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons,"Ferromagnetism and superconductivity are in most cases adverse. However, recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3. We analyze the magnetic state within density functional theory and provide evidence that magnetism is not an intrinsic property of the two-dimensional electron liquid at the interface. We demonstrate that the robust ferromagnetic state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer. This allows for the notion that areas with increased density of oxygen vacancies produce ferromagnetic puddles and account for the previous observation of a superparamagnetic behavior in the superconducting state.",1105.1163v2 2011-05-13,Room temperature p-induced surface ferromagnetism,"We prove a spontaneous magnetization of the oxygen-terminated ZnO (0001) surface by utilizing a multi-code, SIESTA and KKR, first-principles approach, involving both LSDA+U and selfinteraction corrections (SIC) to treat electron correlation effects. Critical temperatures are estimated from Monte Carlo simulations, showing that at and above 300 K the surface is thermodynamically stable and ferromagnetic. The observed half-metallicity and long-range magnetic order originate from the presence of p-holes in the valence band of the oxide. The mechanism is universal in ionic oxides and points to a new route for the design of ferromagnetic low dimensional systems.",1105.2777v1 2011-05-27,Resonances in a dilute gas of magnons and metamagnetism of isotropic frustrated ferromagnetic spin chains,"We show that spin-S chains with SU(2)-symmetric, ferromagnetic nearest-neighbor and frustrating antiferromagnetic next-nearest-neighbor exchange interactions exhibit metamagnetic behavior under the influence of an external magnetic field for small S, in the form of a first-order transition to the fully polarized state. The corresponding magnetization jump increases gradually starting from an S-dependent critical value of exchange couplings and takes a maximum in the vicinity of a ferromagnetic Lifshitz point. The metamagnetism results from resonances in the dilute magnon gas caused by an interplay between quantum fluctuations and frustration.",1105.5626v2 2011-06-03,Origin of in-plane uniaxial magnetic anisotropy in CoFeB amorphous ferromagnetic thin-films,"Describing the origin of uniaxial magnetic anisotropy (UMA) is generally problematic in systems other than single crystals. We demonstrate an in-plane UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of the interface anisotropy in ferromagnetic films on GaAs(001), but strength which is independent of, rather than in inverse proportion to, the film thickness. We show that this volume UMA is consistent with a bond-orientational anisotropy, which propagates the interface-induced UMA through the thickness of the amorphous film. It is explained how, in general, this mechanism may describe the origin of in-plane UMAs in amorphous ferromagnetic films.",1106.0606v1 2011-06-12,Critical Behavior of AC Antiferromagnetic and Ferromagnetic Susceptibilities of a Spin-1/2 Metamagnetic Ising System,"In this study, the temperature variations of the equilibrium and the non-equilibrium antiferromagnetic and ferromagnetic susceptibilities of a metamagnetic system are examined near the critical point. The kinetic equations describing the time dependencies of the total and staggered magnetizations are derived by utilizing linear response theory. In order to obtain dynamic magnetic relaxation behavior of the system, the stationary solutions of the kinetic equations in existence of sinusoidal staggered and physical external magnetic fields are performed. In addition, the static and dynamical mean field critical exponents are calculated in order to formulate the critical behavior of antiferromagnetic and ferromagnetic magnetic response of a metamagnetic system. Finally, a comparison of the findings of this study with previous theoretical and experimental studies is represented and it is shown that a good agreement is found with our results.",1106.2306v1 2011-06-17,Field-controlled domain wall pinning-depinning effects in ferromagnetic nanowire-nanoparticles system,"We represent the results of micromagnetic modeling and experimental magnetic force microscopy investigations of domain wall (DW) pinning-depinning effects in the hybrid systems consisting of ferromagnetic nanowire (NW) and two ferromagnetic nanoparticles (NPs). It was demonstrated that the special NW-NPs configuration including the NW with DW nucleating bulb at one end and two-NP gate enables the realization of controlled DW pinning-depinning based on switching of magnetization in NPs subsystem. The algorithm of external driving field manipulation and independent NPs switching, which allow one to develop a new type of magnetic logic cells, is proposed.",1106.3420v2 2011-06-24,Ferromagnetic frustrated spin systems on the square lattice: a Schwinger boson study,"We study a ferromagnetic Heisenberg spin system on the square lattice, with nearest neighbors interaction J_1 frustrated by second J_2 and third J_3 neighbors antiferromagnetic interactions, using a mean field theory for the Schwinger boson representation of spins. For J_3=0 we find that the boundary between the ferromagnetic and the collinear classical phases shifts to smaller values of J_2 when quantum fluctuations are included. Along the line J_2/|J_1|= 1 the boundaries between the collinear and incommensurate regions are strongly shifted to larger values with respect to the classical case. We do not find clear evidence for spin gapped phases within the present approximation.",1106.5018v2 2011-06-27,Projective studies of spin nematics in a quantum frustrated ferromagnet,"We study the ground state properties of the spin-1/2 frustrated ferromagnetic J1-J2 Heisenberg model on the square lattice, employing projected BCS wavefunctions with spin-triplet pairings of the spinon fields as trial wavefunctions. Based on the variational Monte Carlo analysis, we argue that, in the competing coupling regime, a certain type of the projected BCS wavefunction, dubbed the projected Z2 planar state, achieves the best optimal energy among the other competing states such as the ferromagnetic state and collinear antiferromagnetic state. Like in quantum spin liquids, the projected Z2 planar state preserves the translational symmetry of the square lattice. However, it is also accompanied by a d-wave ordering of the quadrupole moments, breaking the spin rotational symmetry. The state thus describes a quantum spin analogue of the nematic liquid crystals. The calculated static correlation functions also reveal that the projected Z2 planar state has a strong collinear antiferromagnetic fluctuation.",1106.5333v2 2011-06-29,Hole mediated ferromagnetism in Cu-doped ZnO thin films,"We report the successful synthesis of ZnO:Cu thin films doped with holes, resulting in room temperature ferromagnetism. Hole doping is achieved by As-diffusion from the GaAs substrate into ZnO films, assisted by thermal annealing. The As-diffusion is probed with the help of x-ray absorption spectra collected at the As K-edge which show enhanced signature of diffusion in the annealed samples. Introduction of holes, due to the As doping, in ZnO films is further evidenced by the Cu L3,2-edge spectra. XMCD and magnetic measurements show that the ferromagnetic interaction between doped Cu ions is enhanced after hole doping.",1106.5831v1 2011-07-04,Continuous Generation of Spinmotive Force in a Patterned Ferromagnetic Film,"We study, both experimentally and theoretically, the generation of a dc spinmotive force. By exciting a ferromagnetic resonance of a comb-shaped ferromagnetic thin film, a continuous spinmotive force is generated. Experimental results are well reproduced by theoretical calculations, offering a quantitative and microscopic understanding of this spinmotive force.",1107.0549v4 2011-07-06,Counterexamples to Ferromagnetic Ordering of Energy Levels,"The Heisenberg ferromagnet has symmetry group ${\rm SU}(2)$. The property known as ferromagnetic ordering of energy levels (FOEL) states that the minimum energy eigenvalue among eigenvectors with total spin $s$ is monotone decreasing as a function of $s$. While this property holds for certain graphs such as open chains, in this note we demonstrate some counterexamples. We consider the spin 1/2 model on rings of length $2n$ for $n=2,3,...,8$, and show that the minimum energy among all spin singlets is less than or equal to the minimum energy among all spin triplets, which violates FOEL. This also shows some counterexamples to the ""Aldous ordering"" for the symmetric exclusion process. We also review some of the literature related to these examples.",1107.1266v2 2011-07-12,Spinmotive Force due to Intrinsic Energy of Ferromagnetic Nanowires,"We study, both analytically and numerically, a spinmotive force arising from inherent magnetic energy of a domain wall in a wedged ferromagnetic nanowire. In a spatially-nonuniform nanowire, domain walls are subjected to an effective magnetic field, resulting in spontaneous motion of the walls. The spinmotive force mechanism converts the ferromagnetic exchange and demagnetizing energy of the nanowire into the electrical energy of the conduction electrons through the domain wall motion. The calculations show that this spinmotive force can be several microvolts, which is easily detectable by experiments.",1107.2209v2 2011-07-12,Magnetic interactions in iron superconductors studied with a five-orbital model within the Hartree-Fock and Heisenberg approximations,"We have analyzed the magnetic interactions of a five orbital model for iron superconductors treated both within Hartree-Fock and Heisenberg approximations. We have found that the exchange constants depend non-trivially on the Fe-As-Fe angle and on the charge and orbital filling. Within the localized picture, columnar ordering is found for intermediate Hund's coupling J_H. At smaller J_H, an unusual orbital reorganization stabilizes checkerboard ordering. Ferromagnetism appears at large J_H. Ferromagnetic correlations are enhanced with electron doping while large hole doping stabilizes checkerboard antiferromagnetism, explaining the change in magnetic interactions upon substitution of Fe by Co or Mn. For intermediate and large values of U, Hartree-Fock shows similar results as strong coupling though with a double stripe phase instead of ferromagnetism. Itinerancy enhances the stability of the columnar ordering. Comparison of the two approaches reveals a metallic region of the phase diagram where strong coupling physics is determinant.",1107.2279v2 2011-07-14,The dimerized ferromagnetic Heisenberg chain,"Ferromagnetic, in contrast to antiferromagnetic, Heisenberg chains can undergo a Spin-Peierls dimerization only at finite temperatures. They show reentrant behavior as a function of temperature, which might play a role for systems with small effective elastic constants as, for example, monatomic chains on surfaces. We investigate the physical properties of the dimerized ferromagnetic Heisenberg chain using a modified spin-wave theory. We calculate the exponentially decaying spin and dimer correlation functions, analyze the temperature dependence of the corresponding coherence lengths, the susceptibility, as well as the static and dynamic spin structure factor. By comparing with numerical data obtained by the density-matrix renormalization group applied to transfer matrices, we find that the modified spin wave theory yields excellent results for all these quantities for a wide range of dimerizations and temperatures.",1107.2772v1 2011-07-15,On the Possibility of Ferromagnetism and Half-Metallicity in Local Moment Systems,"We use the Kondo lattice model to investigate the possibility of ferromagnetism and half-metallicity in local moment systems. Using the spectral density approach and making use of the fact that the spontaneous magnetization of local moment and the itinerant electron polarization are coupled, we derive an expression for the paramagnetic susceptibility. The magnetic ordering temperature is determined from the singularities of the susceptibility. The magnetic phase diagram is constructed in $T-n$(band filling) plane. It is found that ferromagnetism is possible only for small values of $n$. It is also found that the temperature drives the transition of the system from half-metal to metal.",1107.3061v1 2011-07-19,Enhancement of vortex pinning in superconductor/ferromagnet bilayers via angled demagnetization,"We use local and global magnetometry measurements to study the influence of magnetic domain width w on the domain-induced vortex pinning in superconducting/ferromagnetic bilayers, built of a Nb film and a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy, with an insulating layer to eliminate proximity effect. The quasi-periodic domain patterns with different and systematically adjustable width w, as acquired by a special demagnetization procedure, exert tunable vortex pinning on a superconducting layer. The largest enhancement of vortex pinning, by a factor of more than 10, occurs when w ~ 310 nm is close to the magnetic penetration depth.",1107.3743v1 2011-07-29,"Spin-wave excitations in the ferromagnetic-metallic and in the charge, orbital and spin ordered states in Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5","Inelastic neutron scattering experiments have been performed on single crystals of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5. Colossal magnetoresistance (CMR) in the manganites arises from the interplay between a ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating state. Therefore, it appears important to compare these phases concerning their underlying magnetic interaction parameters. Our investigations of the spin-wave disperion in the AFM ordered state of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half-doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. Furthermore, magnetic excitations in the ferromagnetic metallic state of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x=0.49 and x=0.50 exhibit nearly the same magnon dispersion which can be described with a Heisenberg model including nearest-neighbor interactions.",1107.5882v1 2011-08-10,Bias-induced destruction of ferromagnetism and disorder effects in GaMnAs heterostructures,"The magneto-electric properties of resonant tunneling double barrier structures using GaMnAs for the quantum well is investigated within a self-consistent Green's function approach and a tight-binding electronic structure model. The magnetic state of the well is determined self-consistently by the tunneling current which controls the hole spin density and, hence, the degree of exchange splitting of the subbands inside the well. Prompted by recent experiments we compare model systems of increasing defect concentration (substitutional disorder) regarding their I-V curve, magnetic state, and spin polarization. We predict that, near resonance, the ferromagnetic order which may be present at zero bias in the GaMnAs well tends to be destroyed. Resonance peaks are found to be more sensitive to disorder than ferromagnetic ordering and spin polarization of the steady-state current.",1108.2108v1 2011-08-14,Observation of non-conventional spin waves in composite fermion ferromagnets,"We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.",1108.2901v1 2011-08-22,Electron-phonon-scattering dynamics in ferromagnetic metals and its influence on ultrafast demagnetization processes,"We theoretically investigate spin-dependent carrier dynamics due to the electron-phonon interaction after ultrafast optical excitation in ferromagnetic metals. We calculate the electron-phonon matrix elements including the spin-orbit interaction in the electronic wave functions and the interaction potential. Using the matrix elements in Boltzmann scattering integrals, the momentum-resolved carrier distributions are obtained by solving their equation of motion numerically. We find that the optical excitation with realistic laser intensities alone leads to a negligible magnetization change, and that the demagnetization due to electron-phonon interaction is mostly due to hole scattering. Importantly, the calculated demagnetization quenching due to this Elliot-Yafet type depolarization mechanism is not large enough to explain the experimentally observed result. We argue that the ultrafast demagnetization of ferromagnets does not occur exclusively via an Elliott-Yafet type process, i.e., scattering in the presence of the spin-orbit interaction, but is influenced to a large degree by a dynamical change of the band structure, i.e., the exchange splitting.",1108.4454v1 2011-08-25,Spin Accumulation at Ferromagnet/Non-magnetic Material Interfaces,"Many proposed and realized spintronic devices involve spin injection and accumulation at an interface between a ferromagnet and a non-magnetic material. We examine the electric field, voltage profile, charge distribution, spin fluxes, and spin accumulation at such an interface. We include the effects of both screening and spin scattering. We also include both the spin-dependent chemical potentials {\mu}_{\uparrow,\downarrow} and the effective magnetic field H* that is zero in equilibrium. For a Co/Cu interface, we find that the spin accumulation in the copper is an order of magnitude larger when both chemical potential and effective magnetic field are included. We also show that screening contributes to the spin accumulation in the ferromagnet; this contribution can be significant.",1108.4969v2 2011-08-31,Area-dependence of spin-triplet supercurrent in ferromagnetic Josephson junctions,"In 2010, several experimental groups obtained compelling evidence for spin-triplet supercurrent in Josephson junctions containing strong ferromagnetic materials. Our own best results were obtained from large-area junctions containing a thick central Co/Ru/Co ""synthetic antiferromagnet"" and two thin outer layers made of Ni or PdNi alloy. Because the ferromagnetic layers in our samples are multi-domain, one would expect the sign of the local current-phase relation inside the junctions to vary randomly as a function of lateral position. Here we report measurements of the area dependence of the critical current in several samples, where we find some evidence for those random sign variations. When the samples are magnetized, however, the critical current becomes clearly proportional to the area, indicating that the current-phase relation has the same sign across the entire area of the junctions.",1108.6243v1 2011-09-05,"Carrier mediated interlayer exchange, ground state phase diagrams and transition temperatures of magnetic thin films","We investigate the influence of the carrier density and other parameters on the interlayer exchange in magnetic thin film systems. The system consists of ferromagnetic and non-magnetic layers where the carriers are allowed to move from layer to layer. For the ferromagnetic layers we use the Kondo-lattice model to describe interactions between itinerant electrons and local moments. The electrons' properties are calculated by a Green's function's equation of motion approach while the magnetization of the local moments is determined by a minimization of the free energy. As results we present magnetic phase diagrams and the interlayer exchange over a broad parameter range. Additionally we can calculate the transition temperatures for different alignments of the ferromagnetic layers' magnetizations.",1109.0830v1 2011-09-16,Elementary excitations and the phase transition in the bimodal Ising spin glass model,"We show how the nature of the the phase transition in the two-dimensional bimodal Ising spin glass model can be understood in terms of elementary excitations. Although the energy gap with the ground state is expected to be 4J in the ferromagnetic phase, a gap 2J is in fact found if the finite lattice is wound around a cylinder of odd circumference $L$. This 2J gap is really a finite size effect that should not occur in the thermodynamic limit of the ferromagnet. The spatial influence of the frustration must be limited and not wrap around the system if $L$ is large enough. In essence, the absence of 2J excitations defines the ferromagnetic phase without recourse to calculating magnetisation or investigating the system response to domain wall defects. This study directly investigates the response to temperature. We also estimate the defect concentration where the phase transition to the spin glass glass state occurs. The value $p_c = 0.1045(11)$ is in reasonable agreement with the literature.",1109.3548v1 2011-09-16,Mesoscopic Current-In-Plane Giant Magneto-Resistance,"We develop a three dimensional semiclassical theory which generalizes the Valet-Fert model in order to account for non-collinear systems with magnetic texture, including e.g. domain walls or magnetic vortices. The theory allows for spin transverse to the magnetization to penetrate inside the ferromagnet over a finite length and properly accounts for the Sharvin resistances. For ferromagnetic-normal-ferromagnetic multilayers where the current is injected in the plane of the layers (CIP), we predict the existence of a non zero mesoscopic CIP Giant Magneto-Resistance (GMR) at the diffusive level. This mesoscopic CIP-GMR, which adds to the usual ballistic contributions, has a non monotonic spatial variation and is reminiscent of conductance quantization in the layers. Furthermore, we study the spin transfer torque in spin valve nanopillars. We find that when the magnetization direction is non uniform inside the free layer, the spin torque changes very significantly and simple one-dimensional calculations cease to be reliable.",1109.3602v1 2011-09-16,Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators,"Spin-based electronics in topological insulators (TIs) is favored by the long spin coherence1,2 and consequently fault-tolerant information storage. Magnetically doped TIs are ferromagnetic up to 13 K,3 well below any practical operating condition. Here we demonstrate that the long range ferromagnetism at ambient temperature can be induced in Bi2-xMnxTe3 by the magnetic proximity effect through deposited Fe overlayer. This result opens a new path to interface-controlled ferromagnetism in TI-based spintronic devices.",1109.3609v2 2011-09-21,Anisotropic critical magnetic fluctuations in the ferromagnetic superconductor UCoGe,"We report neutron scattering measurements of critical magnetic excitations in the weakly ferromagnetic superconductor UCoGe. The strong non-Landau damping of the excitations we observe, although unusual has been found in another related ferromagnet, UGe2 at zero pressure. However, we also find there is a significant anisotropy of the magnetic correlation length in UCoGe that contrasts with an almost isotropic length for UGe2. The values of the magnetic correlation length and damping are found to be compatible with superconductivity on small Fermi surface pockets. The anisotropy may be important to explain why UCoGe is a superconductor at zero pressure while UGe2 is not.",1109.4541v1 2011-09-26,Electric-field control of magnetic domain wall motion and local magnetization reversal,"Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here, we report on an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in multiferroic CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer from ferroelastic domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. Optical polarization microscopy of both ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning persist in an applied electric field. This leads to an unprecedented electric controllability over the ferromagnetic microstructure, an accomplishment that produces giant magnetoelectric coupling effects and opens the way to multiferroic spintronic devices.",1109.5514v1 2011-10-12,Spin supercurrent in Josephson contacts with noncollinear ferromagnets,"We present a theoretical study of the Josephson coupling of two s-wave superconductors which are connected through a diffusive contact consisting of noncollinear ferromagnetic domains. First, we consider a contact with two domains with magnetization vectors misoriented by an angle $\theta$. Using the quantum circuit theory, we find that in addition to the charge supercurrent, a spin supercurrent, which is even in $\phi$ and odd in $\theta$, with a spin polarization normal to the magnetization vectors flows between the domains. Furthermore, with asymmetric insulating barriers at the interfaces of the junction, the system may experience an antiferromagnetic-ferromagnetic phase transition for $\phi=\pi$. Secondly, we discuss the spin supercurrent in an extended magnetic texture with multiple domainwalls. We find the position-dependent spin supercurrent. The magnitude of the spin supercurrent strongly depends on the phase difference between the superconductors and the number of domain walls. Our results demonstrate the possibility to couple the superconducting phase to the magnetization dynamics.",1110.2568v1 2011-10-25,"Magnetoresistance of individual ferromagnetic GaAs/(Ga,Mn)As core-shell nanowires","We investigate, angle dependent, the magnetoresistance (MR) of individual self-assembled ferromagnetic GaAs/(Ga,Mn)As core-shell nanowires at cryogenic temperatures. The shape of the MR traces and the observed strong anisotropies in transport can be ascribed to the interplay of the negative magnetoresistance effect and a strong uniaxial anisotropy with the magnetic easy direction pointing along the wire axis. The magnetoresistance can be well described by a quantitative analysis based on the concept of the effective magnetic field, usually used to describe ferromagnetic resonance phenomena. The nanowires we investigate exhibit a uniaxial anisotropy which is approximately 5 times larger than the strain induced anisotropy observed in lithographically prepared (Ga,Mn)As stripes.",1110.5507v1 2011-10-31,Thermal Transport Properties of Graphene-Based F/S/F Junctions,"We present an investigation of heat transport in gapless graphene-based Ferromagnetic /singlet Superconductor/Ferromagnetic (FG$\mid$SG$\mid$FG) junctions. We find that unlike uniform increase of thermal conductance vs temperature, the thermal conductance exhibits intensive oscillatory behavior vs width of the sandwiched s-wave superconducting region between the two ferromagnetic layers. This oscillatory form is occurred by interference of the massless Dirac fermions in graphene. Also we find that the thermal conductance vs exchange field $h$ displays a minimal value at $h/E_F\simeq 1$ within the low temperature regime where this finding demonstrates that propagating modes of the Dirac fermions in this value reach at their minimum numbers and verifies the previous results for electronic conductance. We find that for thin widths of superconducting region, the thermal conductance vs temperature shows linear increment i.e. $\Gamma\varpropto T$. At last we propose an experimental set-up to detect our predicted effects.",1110.6719v1 2011-12-09,Interfacial coupling across a modified interface studied with ferromagnetic resonance,"Using spin waves we directly probe the interface of an exchange biased Ni$_{80}$Fe$_{20}$/Ir$_{25}$Mn$_{75}$ film which has been modified by the presence of an Au dusting layer. Combining this experimental data with a discretised simulation model, parameters relating to interface exchange coupling and modification of interface magnetisation are determined. Exchange coupling is found to be relatively uniform as gold thickness is increased, and undergoes a sudden drop at 1.5$\textrm{\AA}$ of gold. Interface magnetisation decreases as a function of the gold dusting thickness. Antiparallel alignment of the ferromagnet and antiferromagnet supress the interface magnetisation compared to when they are in parallel alignment. These findings imply that the interface region has specific magnetisation states which depend on the ferromagnet orientation.",1112.2045v1 2011-12-15,"Selective spin wave excitation in ferromagnetic (Ga,Mn)As layers by picosecond strain pulses","We demonstrate selective excitation of a spin wave mode in a ferromagnetic (Ga,Mn)As film by picosecond strain pulses. For a certain range of magnetic fields applied in the layer plane only a single frequency is detected for the magnetization precession. We explain this selectivity of spin mode excitation by the necessity of spatial matching of magnon and phonon eigenfunctions, which represents a selection rule analogous to momentum conservation for magnon-phonon interaction in bulk ferromagnetic materials.",1112.3394v1 2011-12-26,Thin-Film Magnetization Dynamics on the Surface of a Topological Insulator,"We theoretically study the magnetization dynamics of a thin ferromagnetic film exchange-coupled with a surface of a strong three-dimensional topological insulator. We focus on the role of electronic zero modes imprinted by domain walls (DW's) or other topological textures in the magnetic film. Thermodynamically reciprocal hydrodynamic equations of motion are derived for the DW responding to electronic spin torques, on the one hand, and fictitious electromotive forces in the electronic chiral mode fomented by the DW, on the other. An experimental realization illustrating this physics is proposed based on a ferromagnetic strip, which cuts the topological insulator surface into two gapless regions. In the presence of a ferromagnetic DW, a chiral mode transverse to the magnetic strip acts as a dissipative interconnect, which is itself a dynamic object that controls (and, inversely, responds to) the magnetization dynamics.",1112.5884v2 2012-02-01,Long-range proximity effect for opposite-spin pairs in S/F heterostructures under non-equilibrium quasiparticle distribution,"By now it is known that in a singlet superconductor/ferromagnet (S/F) structure the superconducting correlations carried by opposite-spin pairs penetrate into the ferromagnet over a short distance of the order of magnetic coherence length. The long-range proximity effect (LRPE), taking place on the length scale of the normal metal coherence length, can only be maintained by equal-spin pairs, which can be generated by magnetic inhomogeneities in the system. In this work we have predicted a new type of LRPE, which can take place in S/F heterostructures under non-equilibrium conditions. The superconducting correlations in the F region are generated by opposite-spin Cooper pairs and equal-spin pairs are not involved. The possibility for an opposite-spin pair to penetrate into the ferromagnet over a large distance is provided by creation of the proper non-equilibrium quasiparticle distribution there. This leads to a sharp increase (up to a few orders of magnitude) of the critical Josephson current through a S/F/S junction at some values of the voltage controlling the nonequilibrium distribution in the F interlayer.",1202.0165v1 2012-02-04,Non-equilibrium spatial distribution of Rashba spin torque in ferromagnetic metal layer,"We study the spatial distribution of spin torque induced by a strong Rashba spin-orbit coupling (RSOC) in a ferromagnetic (FM) metal layer, using the Keldysh non-equilibrium Green's function method. In the presence of the s-d interaction between the non-equilibrium conduction electrons and the local magnetic moments, the RSOC effect induces a torque on the moments, which we term as the Rashba spin torque. A correlation between the Rashba spin torque and the spatial spin current is presented in this work, clearly mapping the spatial distribution of Rashba Spin torque in a nano-sized ferromagnetic device. When local magnetism is turned on, the out-of-plane (Sz) Spin Hall effect (SHE) is disrupted, but rather unexpectedly an in-plane (Sy) SHE is detected. We also study the effect of Rashba strength (\alpha_R) and splitting exchange (\Delta) on the non-equilibrium Rashba spin torque averaged over the device. Rashba spin torque allows an efficient transfer of spin momentum such that a typical switching field of 20 mT can be attained with a low current density of less than 10^6 A/cm^2.",1202.0890v2 2012-02-06,Critical Temperature Oscillations and Reentrant Superconductivity due to the FFLO like State in F/S/F Trilayers,"Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu 41 Ni 59 /Nb/Cu 41 Ni 59 trilayers investigated in the present work.",1202.1193v1 2012-02-17,Tuning the disorder of superglasses,"We study the interplay of superfluidity, glassy and magnetic orders of hardcore bosons with random frustrating interactions. In the classical limit, this model reduces to a $\pm J$ Edwards-Anderson Ising model with concentration $p$ of the ferromagnetic bonds, which hosts a glassy-ferromagnetic transition at a critical concentration $p_c^{\rm cl}\sim 0.77$ on a 3D cubic lattice. Our quantum Monte Carlo simulation results using the worm algorithm show that quantum fluctuations stabilize the coexistence of superfluidity and glassy order (""superglass""), and shift the (super)glassy-ferromagnetic transition to $p_c> p_c^{\rm cl}$. In contrast, antiferromagnetic order coexists with superfluidity to form a supersolid, and the transition to the glassy phase occurs at a higher $p$.",1202.3908v3 2012-02-22,Bulk Intergrowth of a Topological Insulator with a Room Temperature Ferromagnet,"We demonstrate that the layered room temperature ferromagnet Fe7Se8 and the topological insulator Bi2Se3 form crystallographically oriented bulk composite intergrowth crystals. The morphology of the intergrowth in real space and reciprocal space is described. Critically, the basal planes of Bi2Se3 and Fe7Se8 are parallel and hence the good cleavage inherent in the bulk phases is retained. The intergrowth is on the micron scale. Both phases in the intergrowth crystals display their intrinsic bulk properties: the ferromagnetism of the Fe7Se8 is anisotropic, with magnetization easy axis in the plane of the crystals, and ARPES characterization shows that the topological surface states remain present on the Bi2Se3. Analogous behavior is found for what has been called ""Fe-doped Bi2Se3.""",1202.4957v1 2012-02-23,Nodeless superconductivity in Ca3Ir4Sn13: evidence from quasiparticle heat transport,"We report resistivity $\rho$ and thermal conductivity $\kappa$ measurements on Ca$_3$Ir$_4$Sn$_{13}$ single crystals, in which superconductivity with $T_c \approx 7$ K was claimed to coexist with ferromagnetic spin-fluctuations. Among three crystals, only one crystal shows a small hump in resistivity near 20 K, which was previously attributed to the ferromagnetic spin-fluctuations. Other two crystals show the $\rho \sim T^2$ Fermi-liquid behavior at low temperature. For both single crystals with and without the resistivity anomaly, the residual linear term $\kappa_0/T$ is negligible in zero magnetic field. In low fields, $\kappa_0(H)/T$ shows a slow field dependence. These results demonstrate that the superconducting gap of Ca$_3$Ir$_4$Sn$_{13}$ is nodeless, thus rule out nodal gap caused by ferromagnetic spin-fluctuations.",1202.5164v2 2012-02-23,Spinons and helimagnons in the frustrated Heisenberg chain,"We investigate the dynamical spin structure factor S(q,w) for the Heisenberg chain with ferromagnetic nearest (J1<0) and antiferromagnetic next-nearest (J2>0) neighbor exchange using bosonization and a time-dependent density-matrix renormalization group algorithm. For |J1|<< J2 and low energies we analytically find and numerically confirm two spinon branches with different velocities and different spectral weights. Following the evolution of S(q,w) with decreasing J1/J2 we find that helimagnons develop at high energies just before entering the ferromagnetic phase. Furthermore, we show that a recent interpretation of neutron scattering data for LiCuVO4 in terms of two weakly coupled antiferromagnetic chains (|J_1|<< J2) is not viable. We demonstrate that the data are instead fully consistent with a dominant ferromagnetic coupling, J1/J2 ~ -2.",1202.5225v1 2012-02-26,Quantum Anomalous Hall Effect in Flat Band Ferromagnet,"We proposed a theory of quantum anomalous Hall effect in a flat-band ferromagnet on a two-dimensional (2D) decorated lattice with spin-orbit coupling. Free electrons on the lattice have dispersionless flat bands, and the ground state is highly degenerate when each lattice site is occupied averagely by one electron, i.e., the system is at half filling. The on-site Coulomb interaction can remove the degeneracy and give rise to the ferrimagnetism, which is the coexistence of the ferromagnetic and antiferromagnetic long-range orders. On the other hand the spin-orbit coupling makes the band structure topologically non-trivial, and produces the quantum spin Hall effect with a pair of helical edge states around the system boundary. Based on the rigorous results for the Hubbard model, we found that the Coulomb interaction can provide an effective staggered potential and turn the quantum spin Hall phase into a quantum anomalous Hall phase.",1202.5747v1 2012-02-27,"Crystalline anisotropic magnetoresistance with two-fold and eight-fold symmetry in (In,Fe)As ferromagnetic semiconductor","We have investigated the anisotropic magnetoresistance (AMR) of (In,Fe)As ferromagnetic semiconductor (FMS) layers grown on semi-insulating GaAs substrates. In a 10 nm-thick (In,Fe)As layer which is insulating at low temperature, we observed crystalline AMR with two-fold and eight-fold symmetries. In a metallic 100 nm-thick (In,Fe)As layer with higher electron concentration, only two-fold symmetric crystalline AMR was observed. Our results demonstrate the macroscopic ferromagnetism in (In,Fe)As with magnetic anisotropy that depends on the electron concentration. Non-crystalline AMR is also observed in the 100 nm-thick layer, but its magnitude is as small as 10^-5, suggesting that there is no s-d scattering near the Fermi level of (In,Fe)As. We propose the origin of the eight-fold symmetric crystalline anisotropy in (In,Fe)As.",1202.5874v1 2012-03-08,Ferromagnetism and non-local correlations in the Hubbard model,"We study the possibility and stability of band-ferromagnetism in the single-band Hubbard model for the simple cubic (SC) lattice. A non-local self-energy is derived within a modified perturbation theory. Results for the spectral density and quasiparticle density of states are shown with special attention to the effects of k-dependence. The importance of non-local correlations for the fulfillment of the Mermin-Wagner theorem is our main result. A phase digram showing regions of ferromagnetic order is calculated for the three dimensional lattice. Besides, we show results for the optical conductivity and prove that already the renormalized one-loop contribution to the conductivity cancels the Drude peak exactly in case of a local self-energy which is not anymore true for a non-local self-energy.",1203.1726v1 2012-03-08,"Controlling Curie temperature in (Ga,Ms)As through location of the Fermi level within the impurity band","The ferromagnetic semiconductor (Ga,Mn)As has emerged as the most studied material for prototype applications in semiconductor spintronics. Because ferromagnetism in (Ga,Mn)As is hole-mediated, the nature of the hole states has direct and crucial bearing on its Curie temperature TC. It is vigorously debated, however, whether holes in (Ga,Mn)As reside in the valence band or in an impurity band. In this paper we combine results of channeling experiments, which measure the concentrations both of Mn ions and of holes relevant to the ferromagnetic order, with magnetization, transport, and magneto-optical data to address this issue. Taken together, these measurements provide strong evidence that it is the location of the Fermi level within the impurity band that determines TC through determining the degree of hole localization. This finding differs drastically from the often accepted view that TC is controlled by valence band holes, thus opening new avenues for achieving higher values of TC.",1203.1852v1 2012-03-12,Ferromagnetic transition of a two-component Fermi gas of Hard Spheres,"We use microscopic many-body theory to analyze the problem of itinerant ferromagnetism in a repulsive atomic Fermi gas of Hard Spheres. Using simple arguments, we show that the available theoretical predictions for the onset of the ferromagnetic transition predict a transition point at a density ($k_F a \sim 1$) that is too large to be compatible with the universal low-density expansion of the energy. We present new variational calculations for the hard-sphere Fermi gas, in the framework of Fermi hypperneted chain theory, that shift the transition to higher densities ($k_F a \sim 1.8$). Backflow correlations, which are mainly active in the unpolarized system, are essential for this shift.",1203.2521v1 2012-03-10,Room temperature ferromagnetism and giant permittivity in chemical routed Co1.5Fe1.5O4 ferrite particles and their composite with NaNO3,"We report structural, magnetic and dielectric properties of Co1.5Fe1.5O4 nanoparticles and their composites with non-magnetic NaNO3. The samples were derived from metal nitrates solution at different pH values. The chemical routed sample was air heated at 200 0C and 500 0C. Heating of the material showed unusual decrease of crystallite size, but cubic spinel structure is seen in all samples. The samples of Co1.5Fe1.5O4 showed substantially large room temperature ferromagnetic moment, electrical conductivity, dielectric constant, and low dielectric loss. The samples are soft ferromagnet and electrically highly polarized. The interfaces of grains and grain boundaries are actively participating to determine the magnetic and dielectric properties of the ferrite grains. The effects of interfacial contribution are better realized using the ferrite and NaNO3 composite samples. We have examined different scopes of modifying the magnetic and dielectric parameters using same material in pure and composite form.",1203.2944v1 2012-03-14,Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets,"A theoretical study has been carried out to analyse the available results from the inelastic neutron scattering experiment performed on a quasi-two dimensional spin-1/2 ferromagnetic material $K_2CuF_4$. Our formalism is based on a conventional semi-classical like treatment involving a model of an ideal gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg ferromagnet on a square lattice. The results for dynamical structure functions for our model corresponding to spin-1/2, show occurrence of negative values in a large range of energy transfer even encompassing the experimental range, when convoluted with a realistic spectral window function. This result indicates failure of the conventional theoretical framework to be applicable to the experimental situation corresponding to low spin systems. A full quantum formalism seems essential for treating such systems.",1203.3069v2 2012-03-19,Injection and detection of spin in a semiconductor by tunneling via interface states,"Injection and detection of spin accumulation in a semiconductor having localized states at the interface is evaluated. Spin transport from a ferromagnetic contact by sequential, two-step tunneling via interface states is treated not in itself, but in parallel with direct tunneling. The spin accumulation induced in the semiconductor channel is not suppressed, as previously argued, but genuinely enhanced by the additional spin current via interface states. Spin detection with a ferromagnetic contact yields a weighted average of the spin accumulation in the channel and in the localized states. In the regime where the spin accumulation in the localized states is much larger than that in the channel, the detected spin signal is insensitive to the spin accumulation in the localized states and the ferromagnet probes the spin accumulation in the semiconductor channel.",1203.4034v1 2012-03-25,First Principles Calculation of Helical Spin Order in Iron Perovskite SrFeO3 and BaFeO3,"Motivated by recent discovery of ferromagnetism in cubic perovskite BaFeO3 under small magnetic field, we investigate spin order in BaFeO3 and isostructual SrFeO3 by the first principles calculation. The on-site Coulomb and exchange interactions are necessary for the helical spin order consistent with experiments. SrFeO3 exhibits stable G-type helical order, while A- and G-type helical orders in BaFeO3 are almost degenerate at short propagating vector with tiny energetic barrier with respect to ferromagnetic spin order, explaining ferromagnetism under small field. The results are consistent with model calculation where negative charge-transfer energy is explicitly taken into account.",1203.5470v1 2012-03-28,Theory of integer quantum Hall effect in insulating bilayer graphene,"A variational ground state for insulating bilayer graphene (BLG), subject to quantizing magnetic fields, is proposed. Due to the Zeeman coupling, the layer anti-ferromagnet (LAF) order parameter in fully gapped BLG gets projected onto the spin easy plane, and simultaneously a ferromagnet order, which can further be enhanced by exchange interaction, develops in the direction of the magnetic field. The activation gap for the $\nu=0$ Hall state then displays a crossover from quadratic to linear scaling with the magnetic field, as it gets stronger, and I obtain excellent agreement with a number of recent experiments with realistic strengths for the ferromagnetic interaction. A component of the LAF order, parallel to the external magnetic field, gives birth to additional incompressible Hall states at filling $\nu=\pm 2$, whereas the remote hopping in BLG yields $\nu=\pm 1$ Hall states. Evolution of the LAF order in tilted magnetic fields, scaling of the gap at $\nu=2$, the effect of external electric fields on various Hall plateaus, and different possible hierarchies of fractional quantum Hall states are highlighted.",1203.6340v3 2012-06-06,Fractional Spin Josephson Effect and Electrically Controlled Magnetization in Quantum Spin Hall Edges,"We explore a spin Josephson effect in a system of two ferromagnets coupled by a tunnel junction formed of 2D time-reversal invariant topological insulators. In analogy with the more commonly studied instance of the Josephson effect for charge in superconductors, we investigate properties of the phase-coherent {\it spin} current resulting from the misalignment of the in-plane magnetization angles of the two ferromagnets. We show that the topological insulating barrier offers the exciting prospect of hosting a {\it fractional} spin Josephson effect mediated by bound states at the ferromagnet-topological insulator interface. We provide multiple perspectives to understand the $4\pi$ periodic nature of this effect. We discuss several measurable consequences, such as, the generation of a transverse voltage signal which allows for purely electrical measurements, an inverse of this effect where an applied voltage gives rise to a transverse spin-current, and a fractional AC spin-Josephson effect.",1206.1295v2 2012-06-07,"Hybridization of electromagnetic, spin and acoustic waves in magnetic having conical spiral ferromagnetic order","The spectrum of hybrid electromagnetic-spin-acoustic waves for magnetic having conical spiral ferromagnetic structure defined by heterogeneous exchange and relativistic interactions have been received. The possibility of resonant interaction of spin, electromagnetic and acoustic waves has been shown. The electromagnetic waves reflectance from the half-infinity layer of magnetic having conical spiral ferromagnetic order has been calculated for different values of external magnetic field (angle of spiral). The acoustic Faradey effect has been considered.",1206.1421v2 2012-06-08,Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves,"We have experimentally determined the spin-dependent Seebeck coefficient of permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices. The devices were specifically designed to completely separate heat related effects from charge related effects. A pure heat current through the nanopillar spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and spin-down electrons. By using a three-dimensional finite-element model (3D-FEM) based on spin-dependent thermoelectric theory, whose input material parameters were measured in separate devices, we were able to accurately determine a spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5 microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where S_{F} is the Seebeck coefficient of the ferromagnet. The results are in agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve structures.",1206.1659v1 2012-06-13,"The nature of 4f electron magnetism in the diluted ferromagnetic Kondo lattice, CeIr2B2","We report on the physical properties of the series Ce_(1-x)La_xIr_2B_2 (x D 0-0.9), obtained by means of magnetization, heat capacity and electrical resistivity measurements as a function of temperature (down to 0.7 K for the latter two measurements). The Curie temperature of CeIr2B2 (?5 K) is lowered due to La substitution, as expected. However, no quantum critical point or 'non-Fermi liquid' behavior was observed even in the dilute limit of x D 0:9. Interestingly, ferromagnetic ordering persists even for Ce0:1La0:9Ir2B2, below 0.8 K. Among the Ce systems, CeIr2B2 is one of the compounds in which direct 4f-4f interaction does not appear to play any role in the magnetism, which is controlled by the indirect exchange interaction alone. In this compound, the Kondo effect persists in the ferromagnetic ordered state, as inferred from the entropy data.",1206.2737v1 2012-06-18,Competing ferromagnetic and nematic alignment in self-propelled polar particles,"We study a Vicsek-style model of self-propelled particles where ferromagnetic and nematic alignment compete in both the usual ""metric"" version and in the ""metric-free"" case where a particle interacts with its Voronoi neighbors. We show that the phase diagram of this out-of-equilibrium XY model is similar to that of its equilibrium counterpart: the properties of the fully-nematic model, studied before in [F. Ginelli, F. Peruani, M. Baer, and H. Chat\'e, Phys. Rev. Lett. 104, 184502 (2010)], are thus robust to the introduction of a modest bias of interactions towards ferromagnetic alignment. The direct transitions between polar and nematic ordered phases are shown to be discontinuous in the metric case, and continuous, belonging to the Ising universality class, in the metric-free version.",1206.3811v1 2012-06-27,Nano-pattern induced ferromagnetism in strongly correlated electrons,"Band ferromagnetism in strongly correlated electron systems is one of the most challenging issue in today's condensed-matter physics. In this theoretical work, we study the competition between kinetic term, Coulomb repulsion, and on-site correlated disorder for various lattice geometries. Unconventional and complex ferromagnetic phase diagrams are obtained: wide region of stability, cascade of transitions, re-entrance, high sensitivity to the carrier concentration and strongly inhomogeneous ground states for relatively weak on-site potential. The direct and systematic comparison with Exact Diagonalization shows that the Unrestricted Hartree-Fock method is unexpectedly accurate for such systems, which allows large size cluster calculations. A match of the order of 99.9% for weak and intermediate couplings is found, slightly reduced to about 95% in the large repulsion regime. Nano-patterned lattices appear to be particularly promising candidates that could, with the tremendous progress in growing and self-organized techniques, be synthesized in a near future.",1206.6218v2 2012-07-06,Ferromagnetic Quantum critical behavior in three-dimensional Hubbard model with transverse anisotropy,"One-band Hubbard model with transverse anisotropy is considered at density of electrons $n=0.4$. It is shown that when the anisotropy is appropriately chosen, the ground state is ferromagnetic with magnetic order perpendicular to the anisotropy. The increasing of the ratio $\frac tU$, where $t$ is the hopping parameter and $U$ is the Coulomb repulsion, decreases the Curie temperature, and the system arrives at the quantum critical point $(T_C=0)$. The result is obtained introducing Schwinger bosons and slave Fermions representation of the electron operators. Integrating out the spin-singlet Fermi fields an effective Heisenberg model with ferromagnetic exchange constant is obtained for vectors which identifies the local orientation of the spin of the itinerant electrons. The amplitude of the spin vectors is an effective spin of the itinerant electrons accounting for the fact that some sites, in the ground state, are doubly occupied or empty. Owing to the anisotropy, the magnon fluctuations drive the system to quantum criticality and when the effective spin is critically small these fluctuations suppress the magnetic order.",1207.1674v1 2012-07-09,Collective Edge Modes of a Quantum Hall Ferromagnet in Graphene,"We derive an effective field-theoretical model for the one-dimensional collective mode associated with a domain wall in a quantum Hall ferromagnetic state, as realized in confined graphene systems at zero filling. To this end, we consider the coupling of a quantum spin ladder forming near a kink in the Zeeman field to the spin fluctuations of a neighboring spin polarized two-dimensional environment. It is shown, in particular, that such coupling may induce anisotropy of the exchange coupling in the legs of the ladder. Furthermore, we demonstrate that the resulting ferromagnetic spin-1/2 ladder, subject to a kinked magnetic field, can be mapped to an antiferromagnetic spin chain at zero magnetic field.",1207.2069v1 2012-07-12,Griffiths singularity and magnetic phase diagram of La1-xCaxCoO3,"Magnetic properties of La$_{1-x}$Ca$_{x}$CoO$_{3}$ (0.10 $\leq x\leq$ 0.25) are systemically studied in this work. All the samples exhibits the ferromagnetic state at low temperatures. However, their inverse low-field magnetic susceptibilities shows a sharply downward deviation from high-temperature Curie-Weiss paramagnetic behavior well above the ferromagnetic transition temperature ($T_C$), which indicates the presence of a ferromagnetic clustered state above $T_C$. A detailed analysis on the susceptibilities reveals that the short-range state in these Ca-doped samples can be well described as the Griffiths phase. This characteristic is quite different from those of the clustered states above $T_C$ recently reported in Sr- and Ba-doped cobaltites, which are non-Griffith-like. It is proposed that this difference possibly arises from the unique dependence of magnetic interactions among Co$^{3+}$ ions on the size of the dopant in the doped cobaltites. Based on these results, the magnetic diagram of the Ca-doped cobaltites is established.",1207.2818v1 2012-07-12,"Exchange interactions and Tc in rhenium doped silicon: DFT, DFT+U and Monte Carlo calculations","Interactions between rhenium impurities in silicon are investigated by means of the density functional theory (DFT) and the DFT+U scheme. All couplings between impurities are ferromagnetic except the Re-Re dimers which in the DFT method are nonmagnetic, due to formation of the chemical bond supported by substantial relaxation of the geometry. The critical temperature is calculated by means of classical Monte Carlo (MC) simulations with the Heisenberg hamiltonian. The uniform ferromagnetic phase is obtained with the DFT exchange interactions at room temperature for the impurities concentration of 7%. With the DFT+U exchange interactions, the ferromagnetic clusters form above room temperature in MC samples containing only 3% Re.",1207.3059v1 2012-07-18,Temperature dependence of the electronic structure and Fermi-surface reconstruction of Eu(1-x)Gd(x)O through the ferromagnetic metal-insulator transition,"We present angle-resolved photoemission spectroscopy of Eu(1-x)Gd(x)O through the ferromagnetic metal-insulator transition. In the ferromagnetic phase, we observe Fermi surface pockets at the Brillouin zone boundary, consistent with density functional theory, which predicts a half metal. Upon warming into the paramagnetic state, our results reveal a strong momentum-dependent evolution of the electronic structure, where the metallic states at the zone boundary are replaced by pseudogapped states at the Brillouin zone center due to the absence of magnetic long-range order of the Eu 4f moments.",1207.4490v1 2012-08-12,Enhanced Andreev reflection in gapped graphene,"We theoretically demonstrate unusual features of superconducting proximity effect in gapped graphene which presents a pseudospin symmetry-broken ferromagnet with a net pseudomagnetization. We find that the presence of a band gap makes the Andreev conductance of graphene superconductor/pseudoferromagnet (S/PF) junction to behave similar to that of a graphene ferromagnet-superconductor junction. The energy gap $\Delta_N$ enhance the pseudospin inverted Andreev conductance of S/PF junction to reach a limiting maximum value for $\Delta_N\gg \mu$, which depending on the bias voltage can be larger than the value for the corresponding junction with no energy gap. We further demonstrate a damped-oscillatory behavior for the local density of states of the PF region of S/PF junction and a long-range crossed Andreev reflection process in PF/S/PF structure with antiparallel alignment of pseudomagnetizations of PFs, which confirm that, in this respect, the gapped normal graphene behaves like a ferromagnetic graphene.",1208.2461v1 2012-08-21,Dissipationless Spin Current between Two Coupled Ferromagnets,"We demonstrate the general principle which states that a dissipationless spin current flows between two coupled ferromagnets if their magnetic orders are misaligned. This principle applies regardless the two ferromagnets are metallic or insulating, and also generally applies to bulk magnetic insulators. On a phenomenological level, this principle is analogous to Josephson effect, and yields a dissipationless spin current that is independent from scattering. The microscopic mechanisms for the dissipationless spin current depend on the systems, which are elaborated in details. A uniform, static magnetic field is further proposed to be an efficient handle to create the misaligned configuration and stabilize the dissipationless spin current.",1208.4274v2 2012-08-26,Analysis of the Kondo effect in ferromagnetic atomic-sized contacts,"Atomic contacts made of ferromagnetic metals present zero-bias anomalies in the differential conductance due to the Kondo effect. These systems provide a unique opportunity to perform a statistical analysis of the Kondo parameters in nanostructures since a large number of contacts can be easily fabricated using break-junction techniques. The details of the atomic structure differ from one contact to another so a large number of different configurations can be statistically analyzed. Here we present such a statistical analysis of the Kondo effect in atomic contacts made from the ferromagnetic transition metals Ni, Co and Fe. Our analysis shows clear differences between materials that can be understood by fundamental theoretical considerations. This combination of experiments and theory allow us to extract information about the origin and nature of the Kondo effect in these systems and to explore the influence of geometry and valence in the Kondo screening of atomic-sized nanostructures.",1208.5257v1 2012-08-29,Spin-orbital coupling in a triplet superconductor-ferromagnet junction,"We study a novel type of coupling between spin and orbital degrees of freedom which appears at triplet superconductor-ferromagnet interfaces. Using a self-consistent spatially-dependent mean-field theory, we show that increasing the angle between the ferromagnetic moment and the triplet vector order parameter enhances or suppresses the p-wave gap close to the interface, according as the gap antinodes are parallel or perpendicular to the boundary, respectively. The associated change in condensation energy establishes an orbitally-dependent preferred orientation for the magnetization. When both gap components are present, as in a chiral superconductor, we observe a first-order transition between different moment orientations as a function of the exchange field strength.",1208.5871v2 2012-09-20,Transport in multi-terminal superconductor/ferromagnet junctions having spin-dependent interfaces,"We study electronic transport in junctions consisting of a superconductor electrode and two ferromagnet (F) leads in which crossed Andreev reflections (CAR) and elastic cotunnelings are accommodated. We model the system using an extended Blonder-Tinkham-Klapwijk treatment with a key modification that accounts for spin-dependent interfacial barriers (SDIB). We compute current-voltage relations as a function of parameters characterizing the SDIB, magnetization in the F leads, geometry of the junction, and temperature. Our results reveal a rich range of significantly altered physics due to a combination of interfering spin-dependent scattering processes and population imbalance in the ferromagnets, such as a significant enhancement in CAR current and a sign change in the relative difference between resistance of two cases having a antiparallel or parallel alignment of the magnetization in the F leads, respectively. Our model accounts for the surprising experimental findings of positive relative resistance by M. Colci et al. [Phys. Rev. B 85, 180512(R) (2012)] as well as previously measured negative relative resistance results, both within sufficiently large parameter regions.",1209.4478v2 2012-09-21,Dominant itinerant ferromagnetism in Eu0.5Sr0.5CoO3: evidences from both critical analysis and Rhodes-Wohlfarth's criterion,"The critical behaviors of ferromagnet Eu0.5Sr0.5CoO3 arround TC=140.5K have been comprehensively investigated by analyzing a series of isothermal magnetization M(H) curves. Both Modified Arrott plot and Kouvel-Fisher methods give nearly the same critical exponents, which scale nicely the M(H) curves into two different branches below and above TC. The exponents {\gamma}=1.044 and {\delta}=3.06 demonstrate the relevance of mean-field characters for this material. The conclusion of mean-field behavior proves a dominant itinerant ferromagnetism (FM) due to a long range exchange interaction in the system. Meanwhile, by using Rhodes-Wohlfarth's criterion [P. Rhodes and E. P. Wohlfarth, Proc. R. Soc. Lond. A 273, 247 (1963)], it is further confirmed that the itinerant FM dominates in the system.",1209.4790v2 2012-09-27,Theory of Strain-Controlled Magnetotransport and Stabilization of the Ferromagnetic Insulating Phase in Manganite Thin Films,"We show that applying strain on half-doped manganites makes it possible to tune the system to the proximity of a metal-insulator transition and thereby generate a colossal magnetoresistance (CMR) response. This phase competition not only allows control of CMR in ferromagnetic metallic manganites but can be used to generate CMR response in otherwise robust insulators at half-doping. Further, from our realistic microscopic model of strain and magnetotransport calculations within the Kubo formalism, we demonstrate a striking result of strain engineering that, under tensile strain, a ferromagnetic charge-ordered insulator, previously inaccessible to experiments, becomes stable.",1209.6174v2 2012-10-04,Ferromagnetic order in Yb(Rh$_{0.73}$Co$_{0.27}$)$_{2}$Si$_{2}$,"We present the discovery of ferromagnetism in single crystalline Yb(Rh$_{0.73}$Co$_{0.27}$)$_{2}$Si$_{2}$ below $T_{C} = 1.30\,$K with a spontaneous magnetic moment of $0.1\,\mu_{B}$/Yb along the crystallographic c-axis. This is shown by the huge c-susceptibility peak, the hysteresis in magnetization as well as by the field-dependent susceptibility and specific heat data. This discovery motivates the reinvestigation of the magnetic order in Yb(Rh$_{1-x}$Co$_{x}$)$_{2}$Si$_{2}$ with $x < 0.27$ and in pure YbRh$_{2}$Si$_{2}$ under small pressures with field along the $c$-direction to look for low-lying ferromagnetic order.",1210.1345v2 2012-10-12,Topological Magnon Insulator in Insulating Ferromagnet,"In the ferromagnetic insulator with Dzyaloshinskii-Moriya interaction, we theoretically predict and numerically verify a topological magnon insulator, where the charge-free magnon is topologically protected to transport along the edge while it is insulating in the bulk. Within the bulk band gaps, edge states form a connected loop as a $4\pi$- or $8\pi$-period M\""{o}bius strip in the wave vector space. As a consequence, the chiral energy current traveling along the corresponding edge is topologically protected from defects or disorders. Using the nonequilibrium Green's function method, we demonstrate that the energy current carried by magnons with energy in the bulk gap localizes at edges and prefers to travel along one edge in only one direction at nonequilibrium steady state. Our prediction about topological magnon insulator could be observed at a wide energy range in the thin film of the insulating ferromagnet, such as Lu$_2$V$_2$O$_7$.",1210.3487v2 2012-10-18,Measurement of Magnetic Exchange in Ferromagnet-Superconductor La2/3Ca1/3MnO3/YBa2Cu3O7 Bilayers,"The existence of coherent magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult to measure directly. Here we report on a study of ferromagnetic-superconductor bilayers of La2/3Ca1/3MnO3/YBa2Cu3O7 (LCMO/YBCO) with varying YBCO layer thicknesses. Using x-ray magnetic circular dichroism, we demonstrate that the ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperatures below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker than 12 unit cells, the Cu moment is suppressed for temperatures above the superconducting transition, suggesting this to be a direct measurement of magnetic coherence in the normal state of a superconducting oxide.",1210.5049v1 2012-10-18,Proximity effects in spin-triplet superconductor-ferromagnet heterostucture with spin-active interface,"We study the physical properties of a ballistic heterostructure made of a ferromagnet (FM) and a spin-triplet superconductor (TSC) with a layered structure stacking along the direction perpendicular to the planes where a chiral px+ipy pairing occurs and assuming spin dependent processes at the interface. We use a self-consistent Bogoliubov-de Gennes approach on a three-dimensional lattice to obtain the spatial profiles of the pairing amplitude and the magnetization. We find that, depending on the strength of the ferromagnetic exchange field, the ground state of the system can have two distinct configurations with a parallel or anti-parallel collinearity between the magnetic moments in the bulk and at the interface. We demonstrate that a magnetic state having non coplanar interface, bulk and Cooper pairs spins may be stabilized if the bulk magnetization is assumed to be fixed along a given direction. The study of the density of states reveals that the modification of the electronic spectrum in the FM plays an important role in the setting of the optimal magnetic configuration. Finally, we find the existence of induced spin-polarized pair correlations in the FM-TSC system.",1210.5160v2 2012-10-31,Scattering-Independent Anomalous Nernst Effect in Ferromagnets,"Using the full-potential linearized augmented plane-wave method within the density functional theory, we compute all contributions to the scattering independent part of the thermoelectric conductivity tensor, namely the intrinsic contribution and the side-jump contribution. For the ferromagnetic materials bcc Fe, hcp Co, fcc Ni and L1_0 ordered alloys FePd and FePt, our investigations of the energy and temperature dependence of the intrinsic and side-jump contributions show that they are both of equal importance. Overall, our calculations are able to correctly reproduce the order of magnitude and sign of the experimentally measured signal, suggesting that the scattering independent part plays an important role in the anomalous Nernst effect of ferromagnets.",1210.8283v2 2012-11-07,Correlation-mediated processes for electron-induced switching between Neel states of Fe anti-ferromagnetic chains,"The controlled switching between two quasi-stable Neel states in adsorbed anti-ferromagnetic Fe chains has recently been achieved by Loth et al [Science 335, 196 (2012)]. In order to rationalize their data, we evaluate the rate of tunneling electron-induced switching between the N\'eel states. Good agreement is found with the experiment permitting us to identify three switching mechanisms: (i) low-bias direct electron-induced transitions, (ii) intermediate-bias switching via spin-wave-like excitation, and (iii) high-bias transitions mediated by domain wall formation. Spin correlations in the anti-ferromagnetic chains are the switching driving force leading to a marked chain-size dependence.",1211.1538v1 2012-11-13,Singlet-triplet conversion and the long-range proximity effect in superconductor-ferromagnet structures with generic spin dependent fields,"The long-range proximity effect in superconductor/ferromagnet (S/F) hybrid nano-structures is observed if singlet Cooper pairs from the superconductor are converted into triplet pairs which can diffuse into the fer- romagnet over large distances. It is commonly believed that this happens only in the presence of magnetic inhomogeneities. We show that there are other sources of the long-range triplet component (LRTC) of the con- densate and establish general conditions for their occurrence. As a prototypical example we consider first a system where the exchange field and spin-orbit coupling can be treated as time and space components of an effective SU(2) potential. We derive a SU(2) covariant diffusive equation for the condensate and demonstrate that an effective SU(2) electric field is responsible for the long-range proximity effect. Finally, we extend our analysis to a generic ferromagnet and establish a universal condition for the LRTC. Our results open a new avenue in the search for such correlations in S/F structures and make a hitherto unknown connection between the LRTC and Yang-Mills electrostatics.",1211.3084v2 2012-12-04,Dynamics of Ising models near zero temperature : Real Space Renormalization Approach,"We consider the stochastic dynamics of Ising ferromagnets (either pure or random) near zero temperature. The master equation satisfying detailed balance can be mapped onto a quantum Hamiltonian which has an exact zero-energy ground state representing the thermal equilibrium. The largest relaxation time $t_{eq}$ governing the convergence towards this Boltzmann equilibrium in finite-size systems is determined by the lowest non-vanishing eigenvalue $E_1=1/t_{eq}$ of the quantum Hamiltonian $H$. We introduce and study a real-space renormalization procedure for the quantum Hamiltonian associated to the single-spin-flip dynamics of Ising ferromagnets near zero temperature. We solve explicitly the renormalization flow for two cases. (i) For the one-dimensional random ferromagnetic chain with free boundary conditions, the largest relaxation time $t_{eq}$ can be expressed in terms of the set of random couplings for various choices of the dynamical transition rates. The validity of these RG results in $d=1$ is checked by comparison with another approach. (ii) For the pure Ising model on a Cayley tree of branching ratio $K$, we compute the exponential growth of $t_{eq}(N)$ with the number $N$ of generations.",1212.0643v2 2012-12-12,Strong fluctuations near the frustration point in cubic lattice ferromagnets with localized moments,"Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient $D$ in the spin-wave spectrum $E_{\mathbf{k}}\sim D k^{2}$ vanishes. Within the Dyson-Maleev formalism it is found that at low temperatures thermal fluctuations stabilize ferromagnetism by increasing the value of $D$. For not too strong frustration this leads to an unusual ""concave"" shape of the temperature dependence of magnetization, which is in agreement with experimental data on the europium chalcogenides. The phase diagram is constructed by means of Monte Carlo simulation, and suppression of magnetization and Curie temperature is found in comparison with the results of the spin-wave theory. This effect is explained by the the presence of non-analytical corrections to the spin-wave spectrum which are represented in the lowest order by the term $\sim (T/S)^{2} k^{2}\log{k}$.",1212.2773v2 2012-12-13,Spin injection from a normal metal into a mesoscopic superconductor,"We report on nonlocal transport in superconductor hybrid structures, with ferromagnetic as well as normal-metal tunnel junctions attached to the superconductor. In the presence of a strong Zeeman splitting of the density of states, both charge and spin imbalance is injected into the superconductor. While previous experiments demonstrated spin injection from ferromagnetic electrodes, we show that spin imbalance is also created for normal-metal injector contacts. Using the combination of ferromagnetic and normal-metal detectors allows us to directly discriminate between charge and spin injection, and demonstrate a complete separation of charge and spin imbalance. The relaxation length of the spin imbalance is of the order of several $\mu$m and is found to increase with a magnetic field, but is independent of temperature. We further discuss possible relaxation mechanisms for the explanation of the spin relaxation length.",1212.3164v1 2012-12-19,"Theory of the spin Hall effect, and its inverse, in a ferromagnetic metal near the Curie temperature","We give a theory of the inverse spin Hall effect (ISHE) in ferromagnetic metals based on skew scattering via collective spin fluctuations. This extends Kondo's theory of the anomalous Hall effect (AHE) to include short-range spin-spin correlations. We find a relation between the ISHE and the four-spin correlations near the Curie temperature TC. Such four-spin correlations do not contribute to the AHE, which relates to the three-spin correlations. Thus our theory shows an essential difference between the AHE and ISHE, providing an essential complement to Kondo's classic theory of the AHE in metals. We note the relation to skew-scattering mechanisms based on impurity scattering. Our theory can be compared to recent experimental results byWei et al. [Nat. Commun. 3, 1058 (2012)] for the ISHE in ferromagnetic alloys.",1212.4561v1 2012-12-20,Global phase diagram of a doped Kitaev-Heisenberg model,"The global phase diagram of a doped Kitaev-Heisenberg model is studied using an SU(2) slave-boson mean-field method. Near the Kitaev limit, p-wave superconducting states which break the time-reversal symmetry are stabilized as reported by You {\it et al.} [Phys. Rev. B {\bf 86}, 085145 (2012)] irrespective of the sign of the Kitaev interaction. By further doping, a d-wave superconducting state appears when the Kitaev interaction is antiferromagnetic, while another p-wave superconducting state appears when the Kitaev interaction is ferromagnetic. This p-wave superconducting state does not break the time-reversal symmetry as reported by Hyart {\it et al.} [Phys. Rev. B {\bf 85}, 140510 (2012)], and such a superconducting state also appears when the antiferromagnetic Kitaev interaction and the ferromagnetic Heisenberg interaction compete. This work, thus, demonstrates the clear difference between the antiferromagnetic Kitaev model and the ferromagnetic Kitaev model when carriers are doped while these models are equivalent in the undoped limit, and how novel superconducting states emerge when the Kitaev interaction and the Heisenberg interaction compete.",1212.5218v2 2012-12-21,Domain-wall-controlled transverse spin injection,"We propose an effect whereby an electric current along the interface between a ferromagnetic and normal metal leads to injection of pure spin current into the normal metal, if the magnetization-direction in the ferromagnet varies along the direction of current. For the specific example of a ferromagnetic domain wall, we compute the inverse spin-Hall effect voltage this spin current gives rise to when injected into a Pt layer. Furthermore, we show that this pure spin current leads to modification of the parameters that govern spin transfer and current-driven domain-wall motion, which can be use to optimize the latter in layered magnetic systems. This effect in principle enables control over the location of spin-current injection in devices.",1212.5435v1 2012-12-23,Magnetoelectric coupling in a ferroelectric/ferromagnetic chain revealed by ferromagnetic resonance,"Understanding the multiferroic coupling is one of the key issues in the feld of multiferroics. As shown here theoretically, the ferromagnetic resonance (FMR) renders possible an access to the magnetoelectric coupling coefficient in composite multiferroics. This we evidence by a detailed analysis and numerical calculations of FMR in an unstrained chain of BaTiO3 in the tetragonal phase in contact with Fe, including the effect of depolarizing field. The spectra of the absorbed power in FMR are found to be sensitive to the orientation of the interface electric polarization and to an applied static electric field. Here we propose a method for measuring the magnetoelectric coupling coefficient by means of FMR.",1212.5794v1 2013-01-01,Using x-ray diffraction to identify precipitates in transition metal doped semiconductors,"In the past decade, room temperature ferromagnetism was often observed in transition metal doped semiconductors, which were claimed as diluted magnetic semiconductors (DMS). Nowadays intensive activities are devoted to clarify wether the observed ferromagnetism stems from carrier mediated magnetic impurities, ferromagnetic precipitates, or spinodal decomposition. In this paper, we have correlated the structural and magnetic properties of transition metal doped ZnO, TiO2, and Si, prepared by ion implantation. Crystalline precipitates, i.e., transition metal (Co, Ni) and Mn-silicide nanocrystals, are responsible for the magnetism. Additionally due to their orientation nature with respect to the host, these nanocrystals in some cases are not detectable by conventional x-ray diffraction (XRD). This nature results in the pitfall of using XRD to exclude magnetic precipitates in DMS materials.",1301.0100v1 2013-01-09,Nonlocal Thermoelectric Effects and Nonlocal Onsager Relations in a Three-Terminal Proximity-Coupled Superconductor-Ferromagnet Device,"We study thermal and charge transport in a three-terminal setup consisting of one superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin filtering and of spin- dependent scattering phase shifts at each of the two interfaces will lead to very large nonlocal thermo- electric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a nonlocal version of the Onsager relations for thermoelectric currents holds in a three-terminal quantum coherent ferromagnet-superconductor heterostructure including a spin-dependent crossed Andreev reflection and coherent electron transfer processes.",1301.1840v2 2013-01-10,"Magnetization precession induced by quasi-transverse picosecond strain pulses in (311) ferromagnetic (Ga,Mn)As","Quasi-longitudinal and quasi-transverse picosecond strain pulses injected into a ferromagnetic (311) (Ga,Mn)As film induce dynamical shear strain in the film, thereby modulating the magnetic anisotropy and inducing resonant precession of the magnetization at a frequency ~10 GHz. The modulation of the out-of-plane magnetization component by the quasitransverse strain reaches amplitudes as large as 10% of the equilibrium magnetization. Our theoretical analysis is in good agreement with the observed results, thus providing a strategy for ultrafast magnetization control in ferromagnetic films by strain pulses.",1301.2188v1 2013-01-14,Polarised Electromagnetic wave propagation through the ferromagnet: Phase boundary of dynamic phase transition,"The dynamical responses of ferromagnet to the propagating electromagnetic field wave passing through it are modelled and studied here by Monte Carlo simulation in two dimensional Ising ferromagnet. Here, the electromagnetic wave is linearly polarised in such a way that the direction of magnetic field is parallel to that of the magnetic momemts (spins). The coherent propagating mode of spin-clusters is observed. The time average magnetisation over the full cycle (time) of the field defines the order parameter of the dynamic transition. Depending on the value of the temperature and the amplitude of the propagating magnetic field wave, a dynamical phase transition is observed. The dynamic transition was detected by studying the temperature dependences of the dynamic order parameter, the variance of the dynamic order parameter, the derivative of the dynamic order parameter and the dynamic specific heat. The phase boundaries of the dynamic transitions were drawn for two different values of the wave lengths of the propagating magnetic field wave. The phase boundary was observed to shrink (inward) for lower speed of propagation of the EM wave. The divergence of the releavant length scale was observed at the transition point.",1301.3071v2 2013-01-14,Effect of Uniaxial Strain on Ferromagnetic Instability and Formation of Localized Magnetic States on Adatoms in Graphene,"We investigate the effect of an applied uniaxial strain on the ferromagnetic instability due to long- range Coulomb interaction between Dirac fermions in graphene. In case of undeformed graphene the ferromagnetic exchange instability occurs at sufficiently strong interaction within the Hartree- Fock approximation. In this work we show that using the same theoretical framework but with an additional applied uniaxial strain, the transition can occur for much weaker interaction, within the range in suspended graphene. We also study the consequence of strain on the formation of localized magnetic states on adatoms in graphene. We systematically analyze the interplay between the anisotropic (strain- induced) nature of the Dirac fermions in graphene, on- site Hubbard interaction at the impurity and the hybridization between the graphene and impurity electrons. The polarization of the electrons in the localized orbital is numerically calculated within the mean- field self- consistent scheme. We obtain complete phase diagram containing non- magnetic as well as magnetic regions and our results can find prospective application in the field of carbon- based spintronics.",1301.3169v1 2013-01-16,Epitaxially Strained BiMnO3 Films: High-Temperature Robust Multiferroic Materials with Novel Magnetoelectric Coupling,"Multiferroics with the coexistence of ferroelectric and ferromagnetic orders are ideal candidates for magnetoelectric applications. Unfortunately, only very few ferroelectric-ferromagnetic multiferroics (with low magnetic critical temperature) were discovered. Here we perform first principles calculations to investigate the effects of the epitaxial strain on the properties of BiMnO3 films grown along the pseudocubic [001] direction. Unlike the ground state with the centrosymmetric C2/c space group in bulk, we reveal that the tensile epitaxial strain stabilizes the ferromagnetic and ferroelectric Cc state with a large polarization (P > 80 {\mu}C/cm2) and high Curie temperature (Tc is predicted to be between 169 K and 395 K). More importantly, there is a novel intrinsic magnetoelectric coupling in the multiferroic Cc state with the easy magnetization axis controllable by the external electric field.",1301.3660v2 2013-01-28,Spin filter effect at room temperature in GaN/GaMnN ferromagnetic resonant tunneling diode,"We have investigated the spin current polarization without the external magnetic field in the resonant tunneling diode with the emitter and quantum well layers made from the ferromagnetic GaMnN. For this purpose we have applied the self-consistent Wigner-Poisson method and studied the spin-polarizing effect of the parallel and antiparallel alignment of the magnetization in the ferromagnetic layers. The results of our calculations show that the antiparallel magnetization is much more advantageous for the spin filter operation and leads to the full spin current polarization at low temperatures and 35 % spin polarization of the current at room temperature.",1301.6544v1 2013-02-06,Ferromagnetic Kondo effect in a triple quantum dot system,"We propose that a simple device of three laterally-coupled quantum dots, the central one contacted by metal leads, can realize the ferromagnetic Kondo model, which is characterized by interesting properties like a non-analytic inverted zero-bias anomaly and an extreme sensitivity to a magnetic field. Furthermore, by tuning the gate voltages of the lateral dots, this device may allow to study the transition from ferromagnetic to antiferromagnetic Kondo effect, a simple case of a Berezinskii-Kosterlitz-Thouless transition. We model the device by three coupled Anderson impurities that we study by numerical renormalization group. We calculate the single-particle spectral function of the central dot, which at zero frequency is proportional to the zero-bias conductance, across the transition, both in the absence and in the presence of a magnetic field.",1302.1499v2 2013-02-07,Ferromagnetic Exchange Anisotropy from Antiferromagnetic Superexchange in the Mixed 3d-5d Transition-Metal Compound Sr3CuIrO6,"We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr$_3$CuIrO$_6$. Utilizing Ir $L_3$ edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the $5d$ Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu$^{2+}$O$_4$ plaquettes and Ir$^{4+}$O$_6$ octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.",1302.1818v2 2013-02-09,Current-driven dynamics of chiral ferromagnetic domain walls,"In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically-rich spin textures such as spin-spirals and skyrmions via the Dzyaloshinskii-Moriya interaction (DMI). Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin texture enables extremely efficient current-driven motion. We show that spin torque from the spin Hall effect drives DWs in opposite directions in Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a N\'eel configuration with left-handed chirality. We directly confirm the DW chirality and rigidity by examining current-driven DW dynamics with magnetic fields applied perpendicular and parallel to the spin spiral. This work resolves the origin of controversial experimental results and highlights a new path towards interfacial design of spintronic devices.",1302.2257v1 2013-02-13,Ferromagnetism and the formation of interlayer As2-dimers in Ca(Fe1-xNix)2As2,"The compounds Ca(Fe1-xNix)2As2 with the tetragonal ThCr2Si2-type structure (space group I4/mmm) show a continuous transition of the interlayer As-As distances from a non-bonding state in CaFe2As2 (dAs-As = 313 pm) to single-bonded As2-dimers in CaNi2As2 (dAs-As = 260 pm). Magnetic measurements reveal weak ferromagnetism which develops near the composition Ca(Fe0.5Ni0.5)2As2, while the compounds with lower and higher nickel concentrations both are Pauli-paramagnetic. DFT band structure calculations reveal that the As2-dimer formation is a consequence of weaker metal-metal in MAs4-layers (M = Fe1-xNix) of Ni-richer compounds, and depends not on depopulation or shift of As-As antibonding states as suggested earlier. Our results also indicate that the ferromagnetism of Ca(Fe0.5Ni0.5)2As2 and related compounds like SrCo2(Ge0.5P0.5)2 is probably not induced by dimer breaking as recently suggested, but arises from the high density of states generated by the transition metal 3d bands near the Fermi level without contribution of the dimers.",1302.3046v1 2013-02-17,Long-Range Interaction of Spin-Qubits via Ferromagnets,"We propose a mechanism of coherent coupling between distant spin qubits interacting dipolarly with a ferromagnet. We derive an effective two-spin interaction Hamiltonian and estimate the coupling strength. We discuss the mechanisms of decoherence induced solely by the coupling to the ferromagnet and show that there is a regime where it is negligible. Finally, we present a sequence for the implementation of the entangling CNOT gate and estimate the corresponding operation time to be a few tens of nanoseconds. A particularly promising application of our proposal is to atomistic spin-qubits such as silicon-based qubits and NV-centers in diamond to which existing coupling schemes do not apply.",1302.4017v2 2013-03-02,Super-magnetoresistance effect in triplet spin valves,"We study a triplet spin valve obtained by intercalating a triplet superconductor spacer between two ferromagnetic regions with non-collinear magnetizations. We demonstrate that the magnetoresitance of the triplet spin valve depends on the relative orientations of the d-vector, characterizing the superconducting state, and the magnetization directions of the ferromagnetic layers. For devices characterized by a long superconductor, the Cooper pairs spintronics regime is reached allowing to observe the properties of a polarized current sustained by Cooper pairs only. In this regime a super-magnetoresistance effect emerges, and the chiral symmetry of the order parameter of the superconducting spacer is easily recognized. Our findings open new perspectives in designing devices based on the cooperative nature of ferromagnetic and triplet correlations in a spintronic framework.",1303.0375v1 2013-03-13,Ferromagnetic quantum critical point in UCo1-xFexGe,"We have carried out a comprehensive study of the UCo1-xFexGe series across the entire range of compositions 0 <= x <= 1, and report the results of x-ray diffraction, magnetization, specific heat, and electrical resistivity to uncover the T-x phase diagram. Substitution of Fe into UCoGe initially results in an increase in the Curie temperature and a rapid destruction of the superconductivity. Near x = 0.22, the ferromagnetic transition is suppressed to zero temperature at an apparent ferromagnetic itinerant electron quantum critical point, where the temperature dependence of the electrical resistivity and specific heat in this region reveal non-Fermi liquid behavior.",1303.3228v1 2013-03-18,Helical glasses near ferromagnetic quantum criticality,"We study the effects of quenched charge disorder on the phase reconstruction near itinerant ferromagnetic quantum critical points in three spatial dimensions. Combining a replica disorder average with a fermionic version of the quantum order-by-disorder mechanism, we show that weak disorder destabilizes the ferromagnetic state and enhances the susceptibility towards incommensurate, spiral magnetic ordering. The Goldstone modes of the spiral phase are governed by a 3d-XY model. The induced disorder in the pitch of the spiral generates a random anisotropy for the Goldstone modes, inducing vortex lines in the phase of the helical order and rendering the magnetic correlations short ranged with a strongly anisotropic correlation length.",1303.4300v2 2013-04-02,Continuous- and discrete-time Glauber dynamics. First- and second-order phase transitions in mean-field Potts models,"As is known, at the Gibbs-Boltzmann equilibrium, the mean-field $q$-state Potts model with a ferromagnetic coupling has only a first order phase transition when $q\geq 3$, while there is no phase transition for an antiferromagnetic coupling. The same equilibrium is asymptotically reached when one considers the continuous time evolution according to a Glauber dynamics. In this paper we show that, when we consider instead the Potts model evolving according to a discrete-time dynamics, the Gibbs-Boltzmann equilibrium is reached only when the coupling is ferromagnetic while, when the coupling is anti-ferromagnetic, a period-2 orbit equilibrium is reached and a stable second-order phase transition in the Ising mean-field universality class sets in for each component of the orbit. We discuss the implications of this scenario in real-world problems.",1304.0814v1 2013-04-19,Interfacial Coupling in Multiferroic-Ferromagnet Heterostructures,"We report local probe investigations of the magnetic interaction between BiFeO3 films and a ferromagnetic Co0.9Fe0.1 layer. Within the constraints of intralayer exchange coupling in the Co0.9Fe0.1, the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO3 ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO3 films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO3 film based on our X-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO3.",1304.5394v1 2013-04-25,Critical behavior and magnetic relaxation dynamics of Nd0.4Sr0.6MnO3 nanoparticles,"Detailed dc and ac magnetic properties of chemically synthesized Nd0.4Sr0.6MnO3 with different particle size (down to 27 nm) have been studied in details. We have found ferromagnetic state in the nanoparticles, whereas, the bulk Nd0.4Sr0.6MnO3 is known to be an A-type antiferromagnet. A Griffiths-like phase has also been identified in the nanoparticles. Further, critical behavior of the nanoparticles has been studied around the second order ferromagnetic-paramagnetic transition region (|(T-TC)/TC|{\pounds} 0.04) in terms of modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis. The estimated critical exponents (b,g,d) are quite different from those predicted according to three-dimensional mean-field, Heisenberg and Ising models. This signifies a quite unusual nature of the size-induced ferromagnetic state in Nd0.4Sr0.6MnO3. The nanoparticles are found to be interacting and do not behave like ideal superparamagnet. Interestingly, we find spin glass like slow relaxation of magnetization, aging and memory effect in the nanometric samples. These phenomena have been attributed to very broad distribution of relaxation time as well as to inter-particle interaction. Experimentally, we have found out that the dynamics of the nanoparticle systems can be best described by hierarchical model of spin glasses.",1304.6964v1 2013-04-26,Universality Aspects of Layering Transitions in Ferromagnetic Blume-Capel Thin Films,"Critical phenomena and universality behavior of ferromagnetic thin films described by a spin-1 Blume-Capel Hamiltonian has been examined for various thickness values ranging from 3 to 40 layers. Using effective field theory, we have found that crystal field interactions significantly affects the critical value of surface to bulk ratio of exchange interactions $R_{c}$ at which the critical temperature becomes independent of film thickness $L$. Moreover, we have extracted the shift exponent $\lambda$ from computed data. Based on the results, we have shown that in the presence of surface exchange enhancement, the system may exhibit a dimensional crossover. We have also found that presence of crystal field interactions does not affect the value of $\lambda$. Hence, a ferromagnetic spin-1/2 thin film is in the same universality class with its spin-1 counterpart.",1304.7250v1 2013-04-26,Landau-Lifshitz theory of the longitudinal spin Seebeck effect,"Thermal-bias-induced spin angular momentum transfer between a paramagnetic metal and ferromagnetic insulator is studied theoretically based on the stochastic Landau-Lifshitz-Gilbert (LLG) phenomenology. Magnons in the ferromagnet establish a nonequilibrium steady state by equilibrating with phonons via bulk Gilbert damping and electrons in the paramagnet via spin pumping, according to the fluctuation-dissipation theorem. Subthermal magnons and the associated spin currents are treated classically, while the appropriate quantum crossover is imposed on high-frequency magnetic fluctuations. We identify several length scales in the ferromagnet, which govern qualitative changes in the dependence of the thermally-induced spin current on the magnetic film thickness.",1304.7295v2 2013-04-30,Spinmotive force with static and uniform magnetization induced by a time-varying electric field,"A new spinmotive force is predicted in ferromagnets with spin-orbit coupling. By extending the theory of spinmotive force, we show that a time-varying electric field can induce a spinmotive force with static and uniform magnetization. This spinmotive has two advantages; it can be detected free from the inductive voltage owing to the absence of dynamical magnetization and it can be tuned by electric fields. To observe the effect, we propose two experimental setups: electric voltage measurement in a single ferromagnet and spin injection from a ferromagnet into an attached nonmagnetic conductor.",1304.7822v5 2013-05-01,Weyl fermions and the anomalous Hall effect in metallic ferromagnets,"We reconsider the problem of the anomalous Hall effect in ferromagnetic SrRuO$_3$, incorporating insights from the recently developed theory of Weyl semimetals. We demonstrate that SrRuO$_3$ possesses a large number of Weyl nodes, separated in momentum space, in its bandstructure. While the nodes normally do not coincide with the Fermi energy, unless the material is doped, we show that even the nodes inside the Fermi sea have a significant effect on the physical properties of the material. In particular, we show that the common belief that (non-quantized part of) the intrinsic anomalous Hall conductivity of a ferromagnetic metal is entirely a Fermi surface property, is incorrect: there generally exists a contribution to the anomalous Hall conductivity that arises from topological Fermi-arc surface states, associated with the Weyl nodes, which is of the same order of magnitude as the Fermi surface contribution.",1305.0183v2 2013-05-01,Charge and spin supercurrents in triplet superconductor--ferromagnet--singlet superconductor Josephson junctions,"We study the Josephson effect in a triplet superconductor--ferromagnet--singlet superconductor junction. We show that the interaction of tunneling Cooper pairs with the interface magnetization can permit a Josephson current at the lowest order of a tunneling Hamiltonian perturbation theory. Two conditions must be satisfied for this to occur: the magnetization of the ferromagnet has a component parallel to the ${\bf d}$-vector of the triplet superconductor, and the gaps of the superconductors have the same parity with respect to the interface momentum. The resulting charge current displays an unconventional dependence on the orientation of the magnetic moment and the phase difference. This is accompanied by a phase-dependent spin current in the triplet superconductor, while a phase-independent spin current is always present. The tunneling perturbation theory predictions are confirmed using a numerical Green's function method. An analytical treatment of a one-dimensional junction demonstrates that our conclusions are robust far away from the tunneling regime, and reveals signatures of the unconventional Josephson effect in the critical currents.",1305.0211v2 2013-05-10,Long-Range Interaction of Singlet-Triplet Qubits via Ferromagnets,"We propose a mechanism of a long-range coherent interaction between two singlet-triplet qubits dipolarly coupled to a dogbone-shaped ferromagnet. An effective qubit-qubit interaction Hamiltonian is derived and the coupling strength is estimated. Furthermore we derive the effective coupling between two spin-1/2 qubits that are coupled via dipolar interaction to the ferromagnet and that lie at arbitrary positions and deduce the optimal positioning. We consider hybrid systems consisting of spin-1/2 and ST qubits and derive the effective Hamiltonian for this case. We then show that operation times vary between 1MHz and 100MHz and give explicit estimates for GaAs, Silicon, and NV-center based spin qubits. Finally, we explicitly construct the required sequences to implement a CNOT gate. The resulting quantum computing architecture retains all the single qubit gates and measurement aspects of earlier approaches, but allows qubit spacing at distances of order 1$\,\mu$m for two-qubit gates, achievable with current semiconductor technology.",1305.2451v1 2013-05-15,Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets,"We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon spin-flip scattering is computed from first-principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Using the developed formalism we compute the phonon-induced spin lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated demagnetization rate is evaluated for laser-created thermalized and nonequilibrium electron distributions. Nonthermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the subpicosecond regime.",1305.3511v1 2013-05-23,Spin pumping and interlayer exchange coupling through palladium,"The magnetic behaviour of ultrathin ferromagnetic films deposited on substrates is strongly affected by the properties of the substrate. We investigate the spin pumping rate, interlayer exchange coupling and dynamic exchange coupling between ultrathin ferromagnetic films through palladium, a non-magnetic substrate that displays strong Stoner enhancement. We find that the interlayer exchange coupling, both in the static and dynamic versions, is qualitatively affected by the substrate's Stoner enhancement. For instance, the oscillatory behavior that is a hallmark property of the RKKY exchange coupling is strongly suppressed by Stoner enhancement. Although the spin pumping rate of ferromagnetic films atop palladium is only mildly changed by Stoner enhancement the change is large enough to be detected experimentally. The qualitative aspects of our results for palladium are expected to remain valid for any non-magnetic substrate where Coulomb repulsion is large.",1305.5459v1 2013-05-28,Ground-State Ferromagnetic Transition in Strongly Repulsive One-Dimensional Fermi Gases,"We prove that as a one-dimensional Fermi gas is brought across the resonance adiabatically from large repulsion to large attraction, the singlet ground state will give way to the maximum spin state, which is the lowest energy state among the states accessible to the system in this process. In the presence of tiny symmetry breaking fields that destroy spin conservation, the singlet ground state can evolve to the ferromagnetic state or a spin segregated state. We have demonstrated these effects by exact calculations on fermion cluster relevant to current experiments, and have worked out the quantum mechanical wavefunction that exhibits phase separation.",1305.6361v3 2013-05-29,Hydrodynamic theory of coupled current and magnetization dynamics in spin-textured antiferromagnets,"Antiferromagnets with vanishingly small (or zero) magnetization are interesting candidates for spintronics applications. In the present paper we propose two models for description of the current-induced phenomena in antiferromagnetic textures. We show that the magnetization that originates from rotation or oscillations of antiferromagnetic vector can, via $sd$-exchange coupling, polarize the current and give rise to adiabatic and nonadiabatic spin torques. Due to the Lorentz-type dynamics of antiferromagnetic moments (unlike the Galilenian-like dynamics in ferromagnets), the adiabatic spin torque affects the characteristic lengthscale of the moving texture. Nonadiabatic spin torque contributes to the energy pumping and can induce the stable motion of antiferromagnetic texture, but, in contrast to ferromagnets, has pure dynamic origin. We also consider the current-induced phenomena in artificial antiferromagnets where the current maps the staggered magnetization of the structure. In this case the effect of nonadiabatic spin torque is similar to that in ferromagnetic constituents of the structure. In particular, the current can remove degeneracy of the translational antiferromagnetic domains indistinguishable in the external magnetic field and thus can set into motion the 180$^\circ$ domain wall.",1305.6734v2 2013-06-03,Thermodynamics of Ferromagnetic Spin Chains in a Magnetic Field: Impact of the Spin-Wave Interaction,"The thermodynamic properties of ferromagnetic spin chains have been the subject of many publications. Still, the problem of how the spin-wave interaction manifest itself in these low-temperature series has been neglected. Using the method of effective Lagrangians, we explicitly evaluate the partition function of ferromagnetic spin chains at low temperatures and in the presence of a magnetic field up to three loops in the perturbative expansion where the spin-wave interaction sets in. We discuss in detail the renormalization and numerical evaluation of a particular three-loop graph and derive the low-temperature series for the free energy density, energy density, heat capacity, entropy density, as well as the magnetization and the susceptibility. In the low-temperature expansion for the free energy density, the spin-wave interaction starts manifesting itself at order $T^{5/2}$. In the pressure, the coefficient of the $T^{5/2}$-term is positive, indicating that the spin-wave interaction is repulsive. While it is straightforward to go up to three-loop order in the effective loop expansion, the analogous calculation on the basis of conventional condensed matter methods, such as spin-wave theory, appears to be beyond reach.",1306.0600v1 2013-06-05,Ferromagnetism and Ferroelectricity in epitaxial ultra-thin BiMnO3 films,"We studied the ferroelectric and ferromagnetic properties of compressive strained and unstrained BiMnO3 thin films grown by rf-magnetron sputtering. BiMnO3 samples exhibit a 2D cube-on-cube growth mode and a pseudo-cubic struc-ture up to a thickness of 15 nm and of 25 nm when deposited on (001) SrTiO3 and (110) DyScO3, respectively. Above these thicknesses we observe a switching to a 3D island growth and a simultaneous structural change to a monoclinic structure characterized by a (00l) orientation of the monoclinic unit cell. While ferromagnetism is observed below Tc = 100 K for all samples, signatures of room temperature ferroelectricity were found only in the pseudo-cubic ultra-thin films, indicating a correlation between electronic and structural orders.",1306.1192v1 2013-06-09,Emerging Weak Localization Effects on Topological Insulator-Insulating Ferromagnet (Bi_2Se_3-EuS) Interface,"Thin films of topological insulator Bi_2Se_3 were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature (T_C), resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above T_C. Such negative magnetoresistance was only observed for Bi_2Se_3 layers thinner than t~4nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.",1306.2038v2 2013-06-10,Ferromagnetism in neutron and charge neutral beta-equilibrated nuclear matter,"Ferromagnetism in infinite neutron matter as well as beta equilibrated, charge neutral, dense, and infinite nuclear matter is investigated using a model of interacting baryons and mesons. The standard minimal couplings between the magnetic field and the particle charges as well as the baryon dipole moments are included in the Lagrangian density. Minimizing the energy density with respect to the magnetic field yields a self-consistent expression for the ferromagnetic field. We calculate the phase boundary at a given density by increasing the strength of the baryon dipole moments till the energy density of magnetized matter is lower than that of unmagnetized matter. We find that, depending on the density, it is crossed when the baryon dipole moments are increased by a factor of 35. It is also sensitive to the details of the nuclear matter parameterizations and crossing it induces a magnetic field of $\sim 10^{17}$ gauss.",1306.2175v5 2013-06-10,Angular Preisach analysis of Hysteresis loops and FMR lineshapes of ferromagnetic nanowire arrays,"Preisach analysis is applied to the study of hysteresis loops measured for different angles between the applied magnetic field and the common axis of ferromagnetic Nickel nanowire arrays. When extended to Ferromagnetic Resonance (FMR) lineshapes, with same set of parameters extracted from the corresponding hysteresis loops, Preisach analysis shows that a different distribution of interactions or coercivities ought to be used in order to explain experimental results. Inspecting the behavior of hysteresis loops and FMR linewidth versus field angle, we infer that angular dependence might be exploited in angle sensing devices that could compete with Anisotropic (AMR) or Giant Magnetoresistive (GMR) based devices.",1306.2216v2 2013-06-12,Magnon self energy in the correlated ferromagnetic Kondo lattice model: spin-charge coupling effects on magnon excitations in manganites,"Magnon self energy due to spin-charge coupling is calculated for the correlated ferromagnetic Kondo lattice model using a diagrammatic expansion scheme. Systematically incorporating correlation effects in the form of self-energy and vertex corrections, the expansion scheme explicitly preserves the continuous spin rotation symmetry and hence the Goldstone mode. Due to a near cancellation of the correlation-induced quantum correction terms at intermediate coupling and optimal band filling relevant for ferromagnetic manganites, the renormalized magnon energies for the correlated FKLM are nearly independent of correlation term. Even at higher band fillings, despite exhibiting overall non-Heisenberg behavior, magnon dispersion in the \Gamma-X direction retains nearly Heisenberg form. Therefore, the experimentally observed doping dependent zone-boundary magnon softening must be ascribed to spin-orbital coupling effects.",1306.2769v1 2013-06-13,"Investigation of the quaternary Fe2-xCoxMnSi alloys by structural, magnetic, resistivity and spin polarization measurements","Effects of the Co substitution have been observed on the structural, magnetic and magneto-transport properties of Fe2-xCoxMnSi alloy. Curie temperature (TC) and saturation magnetization (MS) of these alloys increased linearly with the Co substitution. Competitive magnetic interaction between ferromagnetic (FM) and anti-ferromagnetic (AFM) phases exists in Fe2-xCoxMnSi for x less than 0.2, AFM phase is completely disappears for x greater than or equal to 0.2. The value of Rhodes-Wohlfarth ratio pc/ps is greater than one for these alloys which is the characteristics of iterant magnetism present in the system. M\""ossbauer spectroscopic measurements have been done to investigate the atomic disorder and local magnetic moment for some x values. Resistivity measurements also confirm the stability of ferromagnetism with the concentration of Co and also show a sign of half metallicity. Resistivity shows semiconducting behaviour for x = 0.4 which is interesting in view of spin gapless semiconductors.",1306.3086v3 2013-06-24,Magnon Energy Renormalization and Low-Temperature Thermodynamics of O(3) Heisenberg Ferromagnets,"We present the perturbation theory for lattice magnon fields of $D$-dimensional O(3) Heisenberg ferromagnet. The effective Hamiltonian for the lattice magnon fields is obtained starting from the effective Lagrangian, with two dominant contributions that describe magnon-magnon interactions identified as a usual gradient term for the unit vector field and a part originating in the Wess-Zumino-Witten term of effective Lagrangian. Feynman diagrams for lattice scalar fields with derivative couplings are introduced, on basis of which we investigate the influence of magnon-magnon interactions on magnon self-energy and ferromagnet free energy. We also comment appearance of spurious terms in the low-temperature series for the free energy by examining magnon-magnon interactions and internal symmetry of the effective Hamiltonian (Lagrangian).",1306.5584v1 2013-06-25,Entanglement entropy scaling of the XXZ chain,"We study the entanglement entropy scaling of the XXZ chain. While in the critical XY phase of the XXZ chain the entanglement entropy scales logarithmically with a coefficient that is determined by the associated conformal field theory, at the ferromagnetic point, however, the system is not conformally invariant yet the entanglement entropy still scales logarithmically albeit with a different coefficient. We investigate how such an nontrivial scaling at the ferromagnetic point influences the estimation of the central charge $c$ in the critical XY phase. In particular we use the entanglement scaling of the finite or infinite system, as well as the finite-size scaling of the ground state energy to estimate the value of $c$. In addition, the spin-wave velocity and the scaling dimension are also estimated. We show that in all methods the evaluations are influenced by the nearby ferromagnetic point and result in crossover behavior. Finally we discuss how to determine whether the central charge estimation is strongly influenced by the crossover behavior and how to properly evaluate the central charge.",1306.5828v3 2013-07-05,Spin-orbit coupled fermions in ladder-like optical lattices at half-filling,"We study the ground-state phase diagram of two-component fermions loaded in a ladder-like lattice at half filling in the presence of spin-orbit coupling. For repulsive fermions with unidirectional spin-orbit coupling along the legs we identify a N\'{e}el state which is separated from rung-singlet and ferromagnetic states by Ising phase transition lines. These lines cross for maximal spin-orbit coupling and a direct Gaussian phase transition between rung-singlet and ferro phases is realized. For the case of Rashba-like spin-orbit coupling, besides the rung singlet phases two distinct striped ferromagnetic phases are formed. In case of attractive fermions with spin-orbit coupling at half-filling for decoupled chains we identify a dimerized state that separates a singlet superconductor and a ferromagnetic states.",1307.1607v1 2013-07-06,Probing the Spin Pumping Mechanism: Exchange Coupling with Exponential Decay in Y3Fe5O12/barrier/Pt Heterostructures,"Ferromagnetic resonance driven spin pumping of pure spin currents from a ferromagnet into a nonmagnetic material promises new spin-functional devices with low energy consumption. The mechanism of spin pumping is under intense investigation and it is widely believed that exchange interaction between the ferromagnet and nonmagnetic material is responsible for this phenomenon. We observe a thousand-fold exponential decay of the spin pumping from 20-nm thick Y3Fe5O12 films to platinum across insulating barriers, from which the exponential decay lengths of 0.16 and 0.23 nm are extracted for oxide barriers with band gaps of 4.93 eV and 2.36 eV, respectively. This prototypical signature of quantum tunneling through a barrier underscores the importance of exchange coupling for spin pumping and reveals its dependence on the characteristics of the barrier material.",1307.1816v2 2013-07-25,Bell inequality and nonlocality in a two-dimensional mixed spin systems,"In this paper, we use Bell inequality and nonlocality to study the bipartite correlation in an exactly soluble two-dimensional mixed spin system. Bell inequality turns out to be a valuable detector for phase transitions in this model. It can detect not only the quantum phase transition, but also the thermal phase transitions, of the system. The property of bipartite correlation in the system is also analyzed. In the quantum anti-ferromagnetic phase, the Bell inequality is violated thus nonlocality is present. It is interesting that the nonlocality is enhanced by thermal fluctuation, and similar results have not been observed in anti-ferromagnetic phase. In the ferromagnetic phase, the quantum correlation turns out to be very novel, which cannot be captured by entanglement or nonlocality.",1307.6680v1 2013-10-01,Temperature and Field Dependence of Magnetic Domains in La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$,"Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic Magnetic Force Microscopy, domain structures for the layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature T$_C$, and appear either via the application of a c-axis magnetic field, or by decreasing the temperature further. At temperatures well below T$_C$, new domain walls, stable at zero field, can be formed by the application of a c-axis field. Magnetic structures are seen also at temperatures above T$_C$: these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.",1310.0258v5 2013-10-02,The effect of normal metal layers in ferromagnetic Josephson junctions,"Using the Usadel equation approach, we provide a compact formalism to calculate the critical current density of 21 different types of ferromagnetic (F) Josephson junctions containing insulating (I) and normal metal (N) layers in the weak link regions. In particular, we obtain that even a thin additional N layer may shift the 0-$\pi$ transitions to larger or smaller values of the thickness $d_F$ of the ferromagnet, depending on its conducting properties. For certain values of $d_F$, a 0-$\pi$ transition can even be achieved by changing only the N layer thickness. We use our model to fit experimental data of SIFS and SINFS tunnel junctions, where S is a superconducting electrode.",1310.0567v2 2013-10-07,Large in-plane deformation of RuO6 octahedron and ferromagnetism of bulk SrRuO3,"SrRuO3 is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below Tc, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO6 octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO6 octahedron, a weak band Jahn-Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO3 is related to the temperature-induced localization of Ru 4d bands.",1310.1686v2 2013-10-15,Thermodynamics of the ferromagnetic phase transition in nearly half metallic CoS2 at high pressures,"The volume change and heat capacity at the ferromagnetic phase transition in CoS2 were measured at high pressures using X-rays generated by the Argonne synchrotron light source and by ac-calorimetry, respectively. The transition entropy, calculated on the basis of these experimental data, drops along the transition line due to quantum degradation, as required by Nernst law. The volume change increases strongly along the transition line, which is explained by specifics of the compressibility difference of coexisting phases that results from nearly half metallic nature of the ferromagnetic phase of CoS2.",1310.3994v1 2013-10-18,Three `species' of Schrödinger cat states in an infinite-range spin model,"We explore a transverse-field Ising model that exhibits both spontaneous symmetry-breaking and eigenstate thermalization. Within its ferromagnetic phase, the exact eigenstates of the Hamiltonian of any large but finite-sized system are all Schr\""odinger cat states: superpositions of states with `up' and `down' spontaneous magnetization. This model exhibits two dynamical phase transitions {\it within} its ferromagnetic phase: In the lowest-temperature phase the magnetization can macroscopically oscillate between up and down. The relaxation of the magnetization is always overdamped in the remainder of the ferromagnetic phase, which is divided in to phases where the system thermally activates itself {\it over} the barrier between the up and down states, and where it quantum tunnels.",1310.4992v3 2013-10-24,Minority-spin $t_{2g}$ states and the degree of spin polarization in ferromagnetic metallic La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($x=0.38$),"Using angle-resolved photoemission spectroscopy (ARPES), we investigate the electronic band structure and Fermi surface of ferromagnetic La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($x=0.38$). Besides the expected two hole pockets and one electron pocket of majority-spin $e_g$ electrons, we show an extra electron pocket around the $\Gamma$ point. A comparison with first-principles spin-polarized band-structure calculations shows that the extra electron pocket arises from $t_{2g}$ electrons of minority-spin character, indicating this compound is not a complete half-metallic ferromagnet, with similar expectations for lightly-doped cubic manganites. However, our data suggest that a complete half-metallic state is likely to be reached as long as the bandwidth is mildly reduced. Moreover, the band-resolved capability of ARPES enables us to investigate the band structure effects on spin polarization for different experimental conditions.",1310.6476v1 2013-11-01,Superfluid Spin Transport through Easy-Plane Ferromagnetic Insulators,"Superfluid spin transport $-$ dissipationless transport of spin $-$ is theoretically studied in a ferromagnetic insulator with easy-plane anisotropy. We consider an open geometry where spin current is injected into the ferromagnet from one side by a metallic reservoir with a nonequilibrium spin accumulation, and ejected into another metallic reservoir located downstream. Spin transport through the device is studied using a combination of magnetoelectric circuit theory, Landau-Lifshitz-Gilbert phenomenology, and microscopic linear-response theory. We discuss how spin superfluidity can be probed using a magnetically-mediated electron-drag experiment.",1311.0288v4 2013-11-05,Quantum criticality and Lifshitz transition in the Ising system CeRu2Si2: Comparison with YbRh2Si2,"New thermoelectric power (TEP) measurements on prototype heavy-fermion compounds close to magnetic quantum criticality are presented. The highly sensitive technique of TEP is an unique tool to reveal Fermi surface instabilities, referred here as Lifshitz transitions. The first focus is on the Ising CeRu2Si2 series. Doping CeRu2Si2 with Rh produces a decoupling between the first order metamagnetic transition and the pseudo-metamagnetism observed in the pure compound. Comparison is made with the case of YbRh2Si2 which is often considered as the archetype of local quantum criticality by contrast to CeRu2Si2, taken as an example of spin-density wave criticality. Up to now for ferromagnetic materials showing ferromagnetic wings, no simple case appears where the Fermi surface is preserved between the ferromagnetic and paramagnetic phases. An open issue is the consequence of Lifshitz transitions on superconductivity in these multiband systems.",1311.1239v1 2013-11-11,The Vortex Signature of Discrete Ferromagnetic Dipoles at the LaAlO$_3$/SrTiO$_3$ Interface,"A hysteretic in-plane magnetoresistance develops below the superconducting transition of LaAlO$_3$/SrTiO$_3$ interfaces for $\left|H_{/\!/}\right|<$ 0.15 T, independently of the carrier density or oxygen annealing. We show that this hysteresis arises from vortex depinning within a thin superconducting layer, in which the vortices are created by discrete ferromagnetic dipoles located solely above the layer. We find no evidence for finite-momentum pairing or bulk magnetism and hence conclude that ferromagnetism is strictly confined to the interface, where it competes with superconductivity.",1311.2323v1 2013-11-11,Color Magnetism in Non-Abelian Vortex Matter,"We propose color magnetism as a generalization of the ordinary Heisenberg (anti-)ferro magnets on a triangular lattice. Vortex matter consisting of an Abrikosov lattice of non-Abelian vortices with color magnetic fluxes shows a color ferro or anti-ferro magnetism, depending on the interaction among the vortex sites. A prime example is a non-Abelian vortex lattice in rotating dense quark matter, showing a color ferromagnetism. We show that the low-energy effective theory for the vortex lattice system in the color ferromagnetic phase is described by a 3+1 dimensional $CP^{N-1}$ nonlinear sigma model with spatially anisotropic couplings. We identify gapless excitations independent from Tkachenko modes as color magnons, that is, Nambu-Goldstone modes propagating in the vortex lattice with an anisotropic linear dispersion relation $\omega_p^2 = c_{xy}^2(p_x^2+p_y^2) + c_z^2 p_z^2$. We calculate the transition temperature between the ordered and disordered phases, and apply it to dense quark matter. We also identify the order parameter spaces for color anti-ferromagnets.",1311.2399v2 2013-11-11,Ferromagnetic two-dimensional crystals: Single layers of K2CuF4,"The successful isolation of graphene ten years ago has evoked a rapidly growing scientific interest in the nature of two-dimensional (2D) crystals. A number of different 2D crystals has been produced since then, with properties ranging from superconductivity to insulating behavior. Here, we predict the possibility for realizing ferromagnetic 2D crystals by exfoliating atomically thin films of K2CuF4. From a first-principles theoretical analysis, we find that single layers of K2CuF4 form exactly 2D Kosterlitz-Thouless systems. The 2D crystal can form a free-standing membrane, and exhibits an experimentally accessible transition temperature and robust magnetic moments of 1 Bohr magneton per formula unit. 2D K2CuF4 unites ferromagnetic and insulating properties and is a demonstration of principles for nanoelectronics such as novel 2D-based heterostructures.",1311.2410v1 2013-11-21,Existence of Anisotropic Spin Fluctuations at Low Temperature in the Normal Phase of the Superconducting Ferromagnet UCoGe,"Thermal conductivity measurements have been performed on the superconducting ferromagnet UCoGe down to very low temperature and under magnetic field. In addition to the electronic quasiparticle thermal conductivity, additional contributions to the thermal transport are detected: they are sensitive to the amplitude and direction of the magnetic field, and at low temperature, they display a strong anisotropy with the heat current direction. We identify these contributions as arising from magnetic fluctuations. Detection of such fluctuations on the thermal transport in 3D weak ferromagnets is very rare if not unique, and pledges for a strongly itinerant character of the magnetism of UCoGe.",1311.5468v1 2013-12-03,Tensor networks for frustrated systems: emergence of order from simplex entanglement,"We consider a frustrated anti-ferromagnetic triangular lattice Hamiltonian and show that the properties of the manifold of its degenerated ground state are represented by a novel type of tensor networks. These tensor networks are not based on ancillary maximally entangled pairs, but rather on triangular W-like simplices. Anti-ferromagnetic triangular frustration is then related to ancillary W-states in contrast to ferromagnetic order which emerges from the contraction of GHZ-like triangular simplices. We further discuss the outwards entangling power of various simplices. This analysis suggests the emergence of distinct macroscopic types of order from the classification of entanglement residing on the simplices that define a tensor network.",1312.0952v1 2013-12-05,Strong impact of the eddy-current shielding on ferromagnetic resonance response of sub-skin-depth-thick conducting magnetic multilayers,"Exchange-coupled nonmagnetic (NM) and ferromagnetic (FM) conducting multilayers are crucial for microwave spintronic devices of the future. We demonstrate, experimentally and theoretically, that in broadband measurements of ferromagnetic resonance (FMR) 10-70 nm-thick permalloy (Py) layers are shielded from the dynamic magnetic field of the microstrip line by eddy currents circulating in the NM capping layers, which strongly diminishes the amplitude of magnetisation precession in the FM material. Our findings have direct implications for designing broadband FMR and measurements of spin current injection through interfaces realised by placing a conducting multilayer above a microwave microstrip line. We show that the eddy-current shielding is very strong at high microwave frequencies (30 GHz) even when the thickness of the NM capping layer is <5 nm, which is well below the microwave skin depth for Au, Cu, Ta, Pd, Pt and other NM metals technologically important for spintronics.",1312.1404v2 2013-12-20,Cation-ordered A$'_{1/2}$A$''_{1/2}$B$_2$X$_4$ magnetic spinels as magnetoelectrics,"We show that 1:1 ordering of A$'$ and A$''$ cations in A$'_{1/2}$A$''_{1/2}$B$_2$X$_4$ magnetic spinels results in appearance of magnetoelectric properties. Possible value of magnetically induced electric polarization is calculated using the recently proposed microscopic model, which takes into account spin-dependent electric dipole moments of magnetic ions located in noncentrosymmetric crystallographic positions. We build phenomenological models of magnetic phase transitions in cation-ordered spinels, which describe ferromagnetic and antiferromagnetic ordering patterns of B cation spins, and calculate the respective magnetoelectric responses. We find that magnetoelectric coefficients diverge at ferromagnetic or weak ferromagnetic phase transitions in ordered spinels.",1312.5925v2 2014-01-09,Edge proximity-induced magnetoresistance and spin polarization in ferromagnetic gated bilayer graphene nanoribbon,"Coherent spin-dependent transport through a junction containing of Normal/Ferromagnetic/Normal bilayer graphene nanoribbon with zigzag edges is investigated by using Landauer formalism. In a more realistic set-up, the exchange field is induced by two ferromagnetic insulator strips deposited on the ribbon edges while a perpendicular electric field is applied by the top gated electrodes. Our results show that, for antiparallel configuration, a band gap is opened giving rise a semiconducting behavior, while for parallel configuration, the band structure has no band gap. As a result, a giant magnetoresistance is achievable by changing the alignment of induced magnetization. Application of a perpendicular electric field on the parallel configuration, results in a spin field-effect transistor where a fully spin polarization occurs around the Dirac point. To be comparable our results with the one for monolayer graphene, we demonstrate that the reflection symmetry and so the parity conservation fails in bilayer graphene nanoribbons with the zigzag edges.",1401.2171v1 2014-01-17,Layer Anti-Ferromagnetism on Bilayer Honeycomb Lattice,"Bilayer honeycomb lattice, with inter-layer tunneling energy, has a parabolic dispersion relation, which causes the charge imbalance between two sublattices. Here, we investigate the metal-insulator and magnetic phase transitions on the strongly correlated bilayer honeycomb lattice by cellular dynamical mean-field theory combined with continuous time quantum Monte Carlo method. We find that different kinds of magnetic spontaneous symmetry breaking on dimer and non-dimer sites, cause a novel phase transition between normal anti-ferromagnet and layer anti-ferromagnet. We sketch the phase diagrams as the function of temperature, interaction and inter-layer hopping. Finally, we set up an experimental protocol for cold atoms in optical lattice to observe these phenomena in future experiments.",1401.4249v1 2014-01-18,"Magnetic Moment and Band Structure Analysis of Fe, Co, Ni-modified Graphene-nano- ribbon","Magnetic properties and band characteristics of graphene-nano-ribbon (GNR) modified by Fe, Co, and Ni were analyzed by the first principles DFT calculation. Typical unit cell is [C32H2Fe1], [C32H2Co1] and [C32H2Ni1] respectively. The most stable spin state was Sz=4/2 for Fe-modified GNR, whereas Sz=3/2 for Co-case and Sz=2/2 for Ni-case. Atomic magnetic moment of Fe, Co and Ni were 3.63, 2.49 and 1.26 {\mu}B, which were reduced values than that of atomic Hund-rule due to magnetic coupling with graphene network. There is a possibility for a ferromagnetic Fe, Co and Ni spin array through an interaction with carbon pi-conjugated spin system. By expanding a unit cell of Co-modified case as [C96H6Co3], ferromagnetic like spin state and ferrimagnetic like one were compared. It was concluded that ferromagnetic state was more stable magnetic state. Band calculations of Co-modified case show half-metal like structure with relatively large band gap (0.55eV) for up-spin, whereas small gap (0.05eV) for down-spin. This suggested a capability of spintronics application like a spin fiter.",1401.4504v1 2014-01-24,Zero-temperature spinglass-ferromagnetic transition : scaling analysis of the domain-wall energy,"For the Ising model with Gaussian random coupling of average $J_0$ and unit variance, the zero-temperature spinglass-ferromagnetic transition as a function of the control parameter $J_0$ can be studied via the size-$L$ dependent renormalized coupling defined as the domain-wall energy $J^R(L) \equiv E_{GS}^{(AF)}(L)-E_{GS}^{(F)}(L)$ (i.e. the difference between the ground state energies corresponding to AntiFerromagnetic and and Ferromagnetic boundary conditions in one direction). We study numerically the critical exponents of this zero-temperature transition within the Migdal-Kadanoff approximation as a function of the dimension $d=2,3,4,5,6$. We then compare with the mean-field spherical model. Our main conclusion is that in low dimensions, the critical stiffness exponent $\theta^c$ is clearly bigger than the spin-glass stiffness exponent $\theta^{SG}$, but that they turn out to coincide in high enough dimension and in the mean-field spherical model. We also discuss the finite-size scaling properties of the averaged value and of the width of the distribution of the renormalized couplings.",1401.6342v2 2014-01-24,Wavenumber-dependent Gilbert damping in metallic ferromagnets,"New terms to the dynamical equation of magnetization motion, associated with spin transport, have been reported over the past several years. Each newly identified term is thought to possess both a real and an imaginary effective field leading to fieldlike and dampinglike torques on magnetization. Here we show that three metallic ferromagnets possess an imaginary effective-field term which mirrors the well-known real effective-field term associated with exchange in spin waves. Using perpendicular standing spin wave resonance between 2-26 GHz, we evaluate the magnitude of the finite-wavenumber ($k$) dependent Gilbert damping $\alpha$ in three typical device ferromagnets, Ni$_{79}$Fe$_{21}$, Co, and Co$_{40}$Fe$_{40}$B$_{20}$, and demonstrate for the first time the presence of a $k^2$ term as $\Delta\alpha=\Delta\alpha_0+A_{k}\cdot k^2$ in all three metals. We interpret the new term as the continuum analog of spin pumping, predicted recently, and show that its magnitude, $A_{k}$=0.07-0.1 nm$^2$, is consistent with transverse spin relaxation lengths as measured by conventional (interlayer) spin pumping.",1401.6467v2 2014-01-29,Magnetic polaron structures in the one-dimensional double and super-exchange model,"An analytical and numerical study of the one-dimensional double and super-exchange model is presented. A phase separation between ferromagnetic and anti-ferromagnetic phases occurs at low super-exchange interaction energy. When the super-exchange interaction energy gets larger, the conduction electrons are self-trapped within separate small magnetic polarons. These magnetic polarons contain a single electron inside two or three sites depending on the conduction electron density and form a Wigner crystallization. A new phase separation is found between these small polarons and the anti-ferromagnetic phase. Our results could explain the spin-glass-like behavior observed in the nickelate one-dimensional compound Y(2-x)CaxBaNiO5.",1401.7656v1 2014-02-04,Exact Free Energies of Statistical Systems on Random Networks,"Statistical systems on random networks can be formulated in terms of partition functions expressed with integrals by regarding Feynman diagrams as random networks. We consider the cases of random networks with bounded but generic degrees of vertices, and show that the free energies can be exactly evaluated in the thermodynamic limit by the Laplace method, and that the exact expressions can in principle be obtained by solving polynomial equations for mean fields. As demonstrations, we apply our method to the ferromagnetic Ising models on random networks. The free energy of the ferromagnetic Ising model on random networks with trivalent vertices is shown to exactly reproduce that of the ferromagnetic Ising model on the Bethe lattice. We also consider the cases with heterogeneity with mixtures of orders of vertices, and derive the known formula of the Curie temperature.",1402.0740v3 2014-02-10,Monte Carlo simulation of stoquastic Hamiltonians,"Stoquastic Hamiltonians are characterized by the property that their off-diagonal matrix elements in the standard product basis are real and non-positive. Many interesting quantum models fall into this class including the Transverse field Ising Model (TIM), the Heisenberg model on bipartite graphs, and the bosonic Hubbard model. Here we consider the problem of estimating the ground state energy of a local stoquastic Hamiltonian $H$ with a promise that the ground state of $H$ has a non-negligible correlation with some `guiding' state that admits a concise classical description. A formalized version of this problem called Guided Stoquastic Hamiltonian is shown to be complete for the complexity class MA (a probabilistic analogue of NP). To prove this result we employ the Projection Monte Carlo algorithm with a variable number of walkers. Secondly, we show that the ground state and thermal equilibrium properties of the ferromagnetic TIM can be simulated in polynomial time on a classical probabilistic computer. This result is based on the approximation algorithm for the classical ferromagnetic Ising model due to Jerrrum and Sinclair (1993).",1402.2295v2 2014-02-12,Ferromagnetic barrier induced negative differential conductance on the surface of a topological insulator,"We theoretically investigate the effect of the negative differential conductance of a ferromagnetic barrier on the surface of a topological insulator. Due to the changes of the shape and position of the Fermi surfaces in the ferromagnetic barrier, the transport processes can be divided into three kinds: the total, partial and blockade transmission mechanisms. The bias voltage can give rise to the transition of the transport processes from partial to blockade transmission mechanisms, which results in a giant effect of negative differential conductance. With appropriate structural parameters, the current-voltage characteristics show that the minimum value of the current can reach to zero in a wide range of the bias voltage, and a large peak-to-valley current ratio can be obtained.",1402.2747v1 2014-02-14,Electronic relaxation rates in metallic ferromagnets,"We show that the single-particle and transport relaxation rates in ferromagnetic metals, which determine the thermal and electrical conductivity, respectively, at asymptotically low temperature do not obey a power law as previously thought, but rather show an exponential temperature dependence. The reason is the splitting of the conduction band that inevitably results from a nonzero magnetization. At higher temperatures there is a sizable temperature window where the transport rate shows a T^2 temperature dependence, in accord with prior results. This window is separated from the asymptotic regime by a temperature scale that is estimated to range from tens of mK to tens of K for typical ferromagnets. We motivate and derive a very general effective theory for metallic magnets that we then use to derive these results. Comparisons with existing experiments are discussed, and predictions for future experiments at low temperatures are made.",1402.3347v2 2014-02-24,Coplanar waveguide based ferromagnetic resonance in ultrathin film magnetic nanostructures: impact of conducting layers,"We report broadband ferromagnetic resonance (FMR) measurements based on a coplanar waveguide (CPW) of ultrathin magnetic film structures that comprise in-plane/out-of-plane decoupled layers deposited on nonmagnetic buffer layers of various thickness or other buffer structures with a diverse sheet resistance. We show that the excitation of the fundamental mode can be substantially (up to 10 times) enhanced in the structures deposited on buffer layers with a low sheet resistance in comparison to the structures deposited on thin or weakly conducting buffer layers. The results are analyzed in terms of shielding of the electromagnetic field of CPW by the conducting buffer layers. The effect of enhancement of FMR absorption can be attractive for applications in spintronic devices that utilize magnetization dynamics of ultrathin ferromagnetic layers.",1402.5844v2 2014-02-25,Hysteretic ac loss in a coated superconductor subjected to oscillating magnetic field: ferromagnetic effect and frequency dependence,"Numerical simulations of the hysteretic ac loss in a coated superconductor with a more realistic version of architecture were performed via finite-element technique in the presence of an oscillating magnetic field. The coated superconductor was electromagnetically modeled by resorting to the quasistatic approximation of a vector potential approach in conjunction with the nonlinear descriptions of the superconducting layer and ferromagnetic substrate therein by a power law model and the Langevin equation respectively. A diverse effect of the ferromagnetic substrate on the hysteretic ac loss, depending on the strength of the applied magnetic field, was displayed and its underlying cause was identified. The dependence of the hysteretic ac loss on the applied frequency is found to be related to a critical amplitude of the applied magnetic field, and the eddy-current loss dissipated in the metal coatings becomes prominent as the frequency augments merely at high applied magnetic fields.",1402.6071v1 2014-02-26,Raman spectra and Magnetization of all-ferromagnetic superlattices grown on (110) oriented SrTiO3,"Superlattices consist of two ferromagnets La0.7Sr0.3MnO3(LSMO)and SrRuO3(SRO) were grown in (110)-orientation on SrTiO3(STO) substrates. The x-ray diffraction and Raman spectra of these superlattices show the presence of in-plane compressive strain and orthorhombic structure of less than 4 u.c. thick LSMO spacer,respectively. Magnetic measurements reveal several features including reduced magnetization, enhanced coercivity, antiferromagnetic coupling, and switching from antiferromagnetic to ferromagnetic coupling with magnetic field orientations. These magnetic properties are explained by the observed orthorhombic structure of spacer LSMO in Raman scattering which occurs due to the modification in the stereochemistry of Mn at the interfaces of SRO and LSMO.",1402.6410v1 2014-02-26,Improved Domain Wall Dynamics and Magnonic Torques using Topological Insulators,"We investigate the magnetization dynamics that arise when a thin-film ferromagnet is deposited on a topological insulator (TI), focusing in particular on domain-wall motion via current and the possibility of a spin-wave torque acting on the magnetization. We show analytically that the coupling between the magnetic domain wall and the TI removes the degeneracy of the wall profile with respect to its chirality and topological charge. Moreover, we find that the threshold for Walker breakdown of domain wall motion is substantially increased and determined by the interaction with the TI, allowing for higher attainable wall velocities than in the conventional case where the hard axis anisotropy determines the Walker threshold. Finally, we show that the allowed modes of spin-wave excitations and the ensuing magnetization dynamics in the presence of a TI coupling enable a magnonic torque acting even on homogeneous magnetization textures. Our results indicate that the TI-ferromagnet interaction has a similar effect on the magnetization dynamics as an intrinsic Dzyaloshinskii-Moriya interaction in ferromagnets.",1402.6701v2 2014-02-27,On the longitudinal spin current induced by a temperature gradient in a ferromagnetic insulator,"Based on the solution of the stochastic Landau-Lifshitz-Gilbert equation discretized for a ferromagnetic chain subject to a uniform temperature gradient, we present a detailed numerical study of the spin dynamics with a focus particularly on finite-size effects. We calculate and analyze the net longitudinal spin current for various temperature gradients, chain lengths, and external static magnetic fields. In addition, we model an interface formed by a nonuniformly magnetized finite-size ferromagnetic insulator and a normal metal and inspect the effects of enhanced Gilbert damping on the formation of the space-dependent spin current within the chain. A particular aim of this study is the inspection of the spin Seebeck effect beyond the linear response regime. We find that within our model the microscopic mechanism of the spin Seebeck current is the magnon accumulation effect quantified in terms of the exchange spin torque. According to our results, this effect drives the spin Seebeck current even in the absence of a deviation between the magnon and phonon temperature profiles. Our theoretical findings are in line with the recently observed experimental results by M. Agrawal et al., Phys. Rev. Lett. 111, 107204 (2013).",1402.6899v1 2014-04-02,Impurity-assisted tunneling magnetoresistance under weak magnetic field,"Injection of spins into semiconductors is essential for the integration of the spin functionality into conventional electronics. Insulating layers are often inserted between ferromagnetic metals and semiconductors for obtaining an efficient spin injection, and it is therefore crucial to distinguish between signatures of electrical spin injection and impurity-driven effects in the tunnel barrier. Here we demonstrate an impurity-assisted tunneling magnetoresistance effect in nonmagnetic-insulator-nonmagnetic and ferromagnetic-insulator-nonmagnetic tunnel barriers. In both cases, the effect reflects on/off switching of the tunneling current through impurity channels by the external magnetic field. The reported effect, which is universal for any impurity-assisted tunneling process, finally clarifies the controversy of a widely used technique that employs the same ferromagnetic electrode to inject and detect spin accumulation.",1404.0633v2 2014-04-03,Anomalously Weak Cooper Pair-breaking by Exchange Energy in Ferromagnet/Superconductor Bilayers,"We report the superconducting transition temperature $T_c$ vs. thickness $d_F$ of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong $3d$ ferromagnet (Ni, Ni$_{0.81}$Fe$_{0.19}$ (Permalloy), Co$_{0.5}$Fe$_{0.5}$) and S = Nb, taken from superfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness $d_F$ where $T_c$ vs. $d_F$ begins to flatten out. Using this rearranged theory we conclude: 1) the effective exchange energy, $E_{ex}$, is about 15 times smaller than measured by ARPES and 5 times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, $\xi_{F}$, for Cooper pairs in F is larger than the electron mean free path, $\ell_F$; and 3) the $3d$-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.",1404.1032v1 2014-04-03,Itinerant magnetism in URhGe revealed by angle resolved photoelectron spectroscopy,"The electronic structure of the ferromagnetic superconductor URhGe in the paramagnetic phase has been studied by angle-resolved photoelectron spectroscopy using soft x rays (hn=595-700 eV). Dispersive bands with large contributions from U 5f states were observed in the ARPES spectra, and form Fermi surfaces. The band structure in the paramagnetic phase is partly explained by the band-structure calculation treating all U 5f electrons as being itinerant, suggesting that an itinerant description of U 5f states is a good starting point for this compound. On the other hand, there are qualitative disagreements especially in the band structure near the Fermi level (E_B < 0.5 eV). The experimentally observed bands are less dispersive than the calculation, and the shape of the Fermi surface is different from the calculation. The changes in spectral functions due to the ferromagnetic transition were observed in bands near the Fermi level, suggesting that the ferromagnetism in this compound has an itinerant origin.",1404.1324v1 2014-04-07,Effect of carbon content on magnetostructural properties of Mn$_3$GaC,"Effect of carbon content on magnetostructural transformation in antiperovskites of the type Mn$_3$GaC$_x$ ($x$ = 0.8, 1.0 and 1.05) has been investigated. It is found that, increase in carbon content changes the ground state from ferromagnetic metallic ($x$ = 0.8) to antiferromagnetic semiconducting ($x$ = 1.05) type. This has been attributed to localization of itinerant Mn $3d$ electrons due to increased Mn$3d$ - C$2p$ hybridization. Such a hybridization strengthens Mn-C-Mn antiferromagnetic interactions over Mn-Mn ferromagnetic interactions. Further, magnetic field can be used as a tool to modulate the relative strengths of these ferromagnetic and antiferromagnetic interactions thereby affecting the nature and strength of magnetocaloric properties.",1404.1751v1 2014-04-08,Dispersion-driven ferromagnetism in a flat-band Hubbard system,"We investigate a mechanism to establish ground-state ferromagnetism in flat-band Hubbard systems based on a kind of {\it order-from-disorder} effect driven by dispersion. As a paradigm we consider a frustrated diamond chain, where for ideal diamond geometry the lowest one-electron band is flat, but the ground state remains paramagnetic for arbitrary on-site repulsion $U$. We focus on half filling of the flat band. By using numerical and analytical arguments we present the ground-state phase diagram for a distorted diamond chain, i.e., the former flat band becomes dispersive. Driven by the interplay of dispersion and interaction the ground state is ferromagnetic if the interaction exceeds a critical value $U_c$.",1404.2230v1 2014-04-09,Proximity effect on spin-dependent conductance and thermopower of correlated quantum dots,"We study the electric and thermoelectric transport properties of correlated quantum dots coupled to two ferromagnetic leads and one superconducting electrode. Transport through such hybrid devices depends on the interplay of ferromagnetic-contact induced exchange field, superconducting proximity effect and correlations leading to the Kondo effect. We consider the limit of large superconducting gap. The system can be then modeled by an effective Hamiltonian with a particle-non-conserving term describing the creation and annihilation of Cooper pairs. By means of the full density-matrix numerical renormalization group method, we analyze the behavior of electrical and thermal conductances, as well as the Seebeck coefficient as a function of temperature, dot level position and the strength of the coupling to the superconductor. We show that the exchange field may be considerably affected by the superconducting proximity effect and is generally a function of Andreev bound state energies. Increasing the coupling to the superconductor may raise the Kondo temperature and partially restore the exchange-field-split Kondo resonance. The competition between ferromagnetic and superconducting proximity effects is reflected in the corresponding temperature and dot level dependence of both the linear conductance and the (spin) thermopower.",1404.2615v2 2014-04-18,Transport Properties of Dirac Ferromagnet,"We propose a model ferromagnet based on the Dirac Hamiltonian in three spatial dimensions, and study its transport properties which include anisotropic magnetoresistance (AMR) and anomalous Hall (AH) effect. This relativistic extension allows two kinds of ferromagnetic order parameters, denoted by $\bm{M}$ and $\bm{S}$, which are distinguished by the relative sign between the positive- and negative-energy states (at zero momentum) and become degenerate in the non-relativistic limit. Because of the relativistic coupling between the spin and the orbital motion, both $\bm{M}$ and $\bm{S}$ induce anisotropic deformations of the energy dispersion (and the Fermi surfaces) but in mutually opposite ways. The AMR is determined primarily by the anisotropy of the Fermi surface (group velocity), and secondarily by the anisotropy of the damping; the latter becomes important for ${\bm M}=\pm{\bm S}$, where the Fermi surfaces are isotropic. Even when the chemical potential lies in the gap, the AH conductivity is found to take a finite non-quantized value, $\sigma_{ij} = -(\alpha /3\pi^2 \hbar) \epsilon_{ijk} S_k $, where $\alpha$ is the (effective) fine structure constant. This offers an example of Hall insulator in three spatial dimensions.",1404.4741v2 2014-04-22,Spin-wave-induced correction to the conductivity of ferromagnets,"We calculate the correction to the conductivity of a disordered ferromagnetic metal due to spin-wave-mediated electron--electron interactions. This correction is the generalization of the Altshuler-Aronov correction to spin-wave-mediated interactions. We derive a general expression for the conductivity correction to lowest order in the spin-wave-mediated interaction and for the limit that the exchange splitting $\Delta$ is much smaller than the Fermi energy. For a ""clean"" ferromagnet with $\Delta\tau_{\rm el}/\hbar \gg 1$, with $\tau_{\rm el}$ the mean time for impurity scattering, we find a correction $\delta \sigma \propto -T^{5/2}$ at temperatures $T$ above the spin wave gap. In the opposite, ""dirty"" limit, $\Delta\tau_{\rm el}/\hbar \ll 1$, the correction is a non-monotonous function of temperature.",1404.5437v1 2014-07-03,A Poor Man's Derivation of Quantum Compass-Heisenberg Interaction: Superexchange Interaction in J-J Coupling Scheme,"The exchange interaction between 5d electrons in t$_{2g}$ orbitals is derived in the J-J coupling scheme which is the appropriate basis in the case of strong spin-orbit interaction. From simple calculations, it is found that ferromagnetic Ising interaction (quantum compass model) occurs due to a selection rule of hybridization between $\Gamma_7$ and $\Gamma_8$ orbitals, while antiferromagnetic Heisenberg interaction appears from hybridization between $\Gamma_7$ orbitals. It is also found that the ferromagnetic Ising interaction becomes small as the spin-orbit interaction increases. Thus, the sign of exchange interaction changes from ferromagnetic Ising to antiferromagnetic Heisenberg interaction as the spin-orbit interaction increases.",1407.0811v1 2014-07-04,Temperature Dependent Ferromagnetic Resonance via the Landau-Lifshitz-Bloch Equation: Application to FePt,"Using the Landau-Lifshitz-Bloch (LLB) equation for ferromagnetic materials, we derive analytic expressions for temperature dependent absorption spectra as probed by ferromagnetic resonance (FMR). By analysing the resulting expressions, we can predict the variation of the resonance frequency and damping with temperature and coupling to the thermal bath. We base our calculations on the technologically relevant L1$_0$ FePt, parameterised from atomistic spin dynamics simulations, with the Hamiltonian mapped from ab-initio parameters. By constructing a multi-macrospin model based on the LLB equation and exploiting GPU acceleration we extend the study to investigate the effects on the damping and resonance frequency in ${\mu}$m sized structures.",1407.1174v1 2014-07-22,Microscale Motion Control Through Ferromagnetic Films,"Actuation and control of motion in micro-mechanical systems are technological challenges, since they are accompanied by mechanical friction and wear, principal and well known sources of device lifetime reduction. In this theoretical work we propose a non-contact motion control technique based on the introduction of a tunable magnetic interaction. The latter is realized by coating two non-touching sliding bodies with ferromagnetic films. The resulting dynamics is determined by shape, size and ordering of magnetic domains arising in the films below the Curie temperature. We demonstrate that the domain behavior can be tailored by acting on handles like ferromagnetic coating preparation, external magnetic fields and the finite distance between the plates. In this way, motion control can be achieved without mechanical contact. Moreover, we discuss how such handles can disclose a variety of sliding regimes. Finally, we propose how to practically implement the proposed model sliding system.",1407.6058v1 2014-07-25,Magnetic Proximity Effect and Interlayer Exchange Coupling of Ferromagnetic/Topological Insulator/Ferromagnetic Trilayer,"Magnetic proximity effect between topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of coupling constant. Such FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of FMI/TI interface, but also provides insights into designing the magnetic storage devices with ultrafast response.",1407.6900v1 2014-07-28,Majorana fermions in ferromagnetic chains on the surface of bulk spin-orbit coupled $s$-wave superconductors,"Majorana fermion (MF) excitations in solid state system have non-Abelian statistics which is essential for topological quantum computation. Previous proposals to realize MF, however, generally requires fine-tuning of parameters. Here we explore a platform which avoids the fine-tuning problem, namely a ferromagnetic chain deposited on the surface of a spin-orbit coupled $s$-wave superconductor. We show that it generically supports zero-energy topological MF excitations near the two ends of the chain with minimal fine-tuning. Depending on the strength of the ferromagnetic moment in the chain, the number of MFs at each end, $n$, can be either one or two, and should be revealed by a robust zero-bias peak (ZBP) of height $2ne^2/h$ in scanning tunneling microscopy (STM) measurements which would show strong (weak) signals at the ends (middle) of the chain. The role of an approximate chiral symmetry which gives an integer topological invariant to the system is discussed.",1407.7519v2 2014-07-30,Room Temperature Spin Pumping in Topological Insulator Bi2Se3,"Three-dimensional (3D) topological insulators are known for their strong spin-orbit coupling and the existence of spin-textured topological surface states which could be potentially exploited for spintronics. Here, we investigate spin pumping from a metallic ferromagnet (CoFeB) into a 3D topological insulator (Bi2Se3) and demonstrate successful spin injection from CoFeB into Bi2Se3 and the direct detection of the electromotive force generated by the inverse spin Hal effect (ISHE) at room temperature. The spin pumping, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) linewidth. We find that the FMR linewidth more than quintuples, the spin mixing conductance can be as large as 3.4*10^20m^-2 and the spin Hall angle can be as large as 0.23 in the Bi2Se3 layer.",1407.7940v1 2014-08-07,Critical behavior and universality properties of uniaxial ferromagnetic thin films in the presence of random magnetic fields,"Critical phenomena in uniaxial ferromagnetic thin films in the presence of random magnetic fields have been studied within the framework of effective field theory. When the type of the random field distribution is bimodal, the system exhibits tricritical behavior. Furthermore, the critical value of surface to bulk ratio of exchange interactions at which the transition temperature becomes independent of film thickness is insensitive to the presence of disorder whether the distribution is bimodal or trimodal. Regarding the universality properties, neither $p$, nor $h$ variations in the system can affect the value of the shift exponent $\lambda$. In this regard, it can be concluded that pure ferromagnetic thin films are in the same universality class with those under the influence of random discrete magnetic fields.",1408.1513v1 2014-08-07,Ferromagnetic resonance of a magnetic dimer with dipolar coupling,"We develop a general formalism for analyzing the ferromagnetic resonance characteristics of a magnetic dimer consisting of two magnetic elements (in a horizontal or vertical configuration) coupled by dipolar interaction, taking account of their finite-size and aspect ratio. We study the effect on the resonance frequency and resonance field of the applied magnetic field (in amplitude and direction), the inter-element coupling, and the uniaxial anisotropy in various configurations. We obtain analytical expressions for the resonance frequency in various regimes of the interlayer coupling. We (numerically) investigate the behavior of the resonance field in the corresponding regimes. The critical value of the applied magnetic field at which the resonance frequency vanishes may be an increasing or a decreasing function of the dimer's coupling, depending on the anisotropy configuration. It is also a function of the nanomagnets aspect ratio in the case of in-plane anisotropy. This and several other results of this work, when compared with experiments using the standard ferromagnetic resonance with fixed frequency, or the network analyzer with varying frequency and applied magnetic field, provide a useful means for characterizing the effective anisotropy and coupling within systems of stacked or assembled nanomagnets.",1408.1594v1 2014-08-25,Spin-Scattering Rates in Metallic Thin Films Measured by Ferromagnetic Resonance Damping Enhanced by Spin-Pumping,"We determined the spin-transport properties of Pd and Pt thin films by measuring the increase in ferromagnetic resonance damping due to spin-pumping in ferromagnetic (FM)-nonferromagnetic metal (NM) multilayers with varying NM thicknesses. The increase in damping with NM thickness depends strongly on both the spin- and charge-transport properties of the NM, as modeled by diffusion equations that include both momentum- and spin-scattering parameters. We use the analytical solution to the spin-diffusion equations to obtain spin-diffusion lengths for Pt and Pd. By measuring the dependence of conductivity on NM thickness, we correlate the charge- and spin-transport parameters, and validate the applicability of various models for momentum-scattering and spin-scattering rates in these systems: constant, inverse-proportional (Dyakanov-Perel), and linear-proportional (Elliot-Yafet). We confirm previous reports that the spin-scattering time can be shorter than the momentum scattering time in Pt, and the Dyakanov-Perel-like model is the best fit to the data.",1408.5921v2 2014-08-29,Spin-dependent ballistic transport properties and electronic structures of pristine and edge-doped zigzag silicene nanoribbons: large magnetoresistance,"The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magnetic configuration of the two electrodes, are discussed. Switching on and off the external magnetic field, we may convert the metallic ferromagnetic ZSiNRs into insulating antiferromagnetic ZSiNRs. In the ferromagnetic state, even- or odd-width ZSiNRs exhibit a drastically different magnetoresistance. In an odd-width edge-doped ZSiNR a large magnetoresistance occurs compared to that in a pristine ZSiNR. The situation is reversed in even-width ZSiNRs. These phenomena result from the drastic change of the conductance in the antiparallel configuration.",1408.6899v1 2014-09-01,Proximity effect in superconductor/conical magnet heterostructures,"The presence of a spin-flip potential at the interface between a superconductor and a ferromagnetic metal allows for the generation of equal-spin spin-triplet Cooper pairs. These Cooper pairs are compatible with the exchange interaction within the ferromagnetic region and hence allow for the long-range proximity effect through a ferromagnet or half-metal. One suitable spin-flip potential is provided by incorporating the conical magnet Holmium (Ho) into the interface. The conical magnetic structure is characterised by an opening angle $\alpha$ with respect to the crystal $c$-axis and a turning (or pitch) angle $\beta$ measuring the rotation of magnetisation with respect to the adjacent layers. Here, we present results showing the influence of conical magnet interface layers with varying $\alpha$ and $\beta$ on the efficiency of the generation of equal-spin spin-triplet pairing. The results are obtained by self-consistent solutions of the microscopic Bogoliubov$-$de Gennes equations in the clean limit within a tight-binding model of the heterostructure. In particular, the dependence of unequal-spin and equal-spin spin-triplet pairing correlations on the conical magnetic angles $\alpha$ and $\beta$ are discussed in detail.",1409.0366v1 2014-09-01,Imaging and Control of Ferromagnetism in a Polar Antiferromagnet,"Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO3 (001) films on SrTiO3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.",1409.0520v1 2014-09-08,Strain induced edge magnetism at zigzag edge in graphene quantum dot,"We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot by using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction $U$ may lead to a edge ferromagnetic like behavior in the strained graphene quantum dot, and a possible room temperature transition is suggested. Around half filling, the ferromagnetic fluctuations at the zigzag edge is strengthened both markedly by the on-site Coulomb interaction and the strain, especially in low temperature region. The resultant strongly enhanced ferromagnetic like behavior may be important for the development of many applications.",1409.2341v2 2014-09-12,Reversal-mechanism of in-plane current induced perpendicular switching in a single ferromagnetic layer,"We propose a magnetization reversal model to explain the perpendicular switching of a single ferromagnetic layer induced by an in-plane current. Contrary to previously proposed reversal mechanisms that such magnetic switching is directly from the Rashba or spin Hall effects, we suggest that this type of switching arises from the current-induced chirality dependent domain wall motion. By measuring the field dependent switching behaviors, we show that such switching can also be achieved between any two multidomain states, and all of these switching behaviors can be well explained by this model. This model indicates that the spin Hall angle in such structures may be overestimated and also predicts similar switching behaviors in other ferromagnetic structures with chiral domain walls or skyrmions.",1409.3652v1 2014-09-22,Phase separation in a spin-orbit coupled Bose-Einstein condensate,"We study a spin-orbit (SO) coupled hyperfine spin-1 Bose-Einstein condensate (BEC) in a quasi-one-dimensional trap. For a SO-coupled BEC in a one-dimensional box, we show that in the absence of the Rabi term, any non-zero value of SO coupling will result in a phase separation among the components for a ferromagnetic BEC, like $^{87}$Rb. On the other hand, SO coupling favors miscibility in a polar BEC, like $^{23}$Na. In the presence of a harmonic trap, which favors miscibility, a ferromagnetic BEC phase separates, provided the SO-coupling strength and number of atoms are greater than some critical value. The Rabi term favors miscibility irrespective of the nature of the spin interaction: ferromagnetic or polar.",1409.6211v2 2014-09-23,Combining ferromagnetic resonator and digital image correlation to study the strain induced resonance tunability in magnetoelectric heterostructures,"This paper reports the development of a methodology combining microstrip ferromagnetic resonance (MS-FMR) and digital image correlation (DIC) in order to silmuteanously measure the voltage-induced strains and the magnetic resonance in artificial magnetoelectric heterostructures (magnetic films/piezoelectric substrate? or magnetic films/flexible substrate/piezoelectric actuator ?). The overall principle of the technique and the related analytical modelling are described. It is powerful to estimate the magnetostriction coefficient of ferromagnetic thin films and can be used to determine the effective magnetoelectric coefficient of the whole heterostructures in addition to the piezoelectric coefficient related to the in-plane voltage-induced strains. This methodology can be applied to system for which the strains are well transmitted at the different interfaces.",1409.6641v1 2014-10-02,Observation of Majorana Fermions in Ferromagnetic Atomic Chains on a Superconductor,"Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero energy end states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.",1410.0682v1 2014-10-08,Flat-band ferromagnetism in the multilayer Lieb optical lattice,"We theoretically study magnetic properties of two-component cold fermions in half-filled multilayer Lieb optical lattices, i.e., two, three, and several layers, using the dynamical mean-field theory. We clarify that the magnetic properties of this system become quite different depending on whether the number of layers is odd or even. In odd-number-th layers in an odd-number-layer system, finite magnetization emerges even with an infinitesimal interaction. This is a striking feature of the flatband ferromagnetic state in multilayer systems as a consequence of the Lieb theorem. In contrast, in even-number layers, magnetization develops from zero on a finite interaction. These different magnetic behaviours are triggered by the flat bands in the local density of states and become identical in the limit of the infinite-layer (i.e., three-dimensional) system. We also address how interlayer hopping affects the magnetization process. Further, we point out that layer magnetization, which is a population imbalance between up and down atoms on a layer, can be employed to detect the emergence of the flat-band ferromagnetic state without addressing sublattice magnetization.",1410.2166v1 2014-10-13,Anomalous temperature and field behaviors of magnetization in cubic lattice frustrated ferromagnets,"Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient $D$ in the spin-wave spectrum $E_{\mathbf{k}}\sim D k^{2}$ vanishes. Within the Dyson-Maleev formalism it is found that at low temperatures thermal fluctuations stabilize ferromagnetism by increasing the value of $D$. For not too strong frustration this leads to an unusual ""concave"" shape of the temperature dependence of magnetization, which is in agreement with experimental data on the europium chalcogenides. Anomalous temperature behavior of magnetization is confirmed by Monte Carlo simulation. Strong field dependence of magnetization (paraprocess) at finite temperature is found near the frustration point.",1410.3264v2 2014-10-14,Spin precession mapping at ferromagnetic resonance via nuclear resonant scattering,"We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance by nuclear resonant scattering of synchrotron radiation at the 14.4 keV resonance of $^{57}$Fe. The precession of the magnetization leads to an apparent reduction of the magnetic hyperfine field acting at the $^{57}$Fe nuclei. The spin dynamics is described in a stochastic relaxation model adapted to the ferromagnetic resonance theory by Smit and Beljers to model the decay of the excited nuclear state. From the fits of the measured data the shape of the precession cone of the spins is determined. Our results open a new perspective to determine magnetization dynamics in layered structures with very high depth resolution by employing ultrathin isotopic probe layers.",1410.3689v1 2014-10-15,Equivalence of a mixing condition and the LSI in spin systems with infinite range interaction,"We investigate unbounded continuous spin-systems with infinite-range interactions. We develop a new technique for deducing decay of correlations from a uniform Poincar\'e inequality based on a directional Poincar\'e inequality, which we derive through an averaging procedure. We show that this decay of correlations is equivalent to the Dobrushin-Shlosman mixing condition. With this, we also state and provide a partial answer to a conjecture regarding the relationship between the relaxation rates of non-ferromagnetic and ferromagnetic systems. Finally, we show that for a symmetric, ferromagnetic system with zero boundary conditions, a weaker decay of correlations can be bootstrapped.",1410.3924v3 2014-10-15,Non-linear spin transport in a rectifying ferromagnet/semiconductor Schottky contact,"The electrical creation and detection of spin accumulation in ferromagnet/semiconductor Schottky contacts that exhibit highly non-linear and rectifying electrical transport is evaluated. If the spin accumulation in the semiconductor is small, the expression for the spin voltage is identical to that of linear transport. However, if the spin accumulation is comparable to the characteristic energy scale that governs the degree of non-linearity, the spin detection sensitivity and the spin voltage are notably reduced. Moreover, the non-linearity enhances the back-flow of spins into the ferromagnet and its detrimental effect on the injected spin current, and the contact resistance required to avoid back-flow is larger than for linear transport. It is also shown that by virtue of the non-linearity, a non-magnetic metal contact can be used to electrically detect spin accumulation in a semiconductor.",1410.3994v2 2014-10-15,Transport gap and hysteretic behavior of the Ising quantum Hall ferromagnets in Landau levels $\left\vert N\right\vert >0$ of bilayer graphene,"The chiral two-dimensional electron gas in Landau levels $\left\vert N\right\vert >0$ of a Bernal-stacked graphene bilayer has a valley-pseudospin Ising quantum Hall ferromagnetic behavior at odd filling factors $\nu _{N}=1,3$ of these fourfold degenerate states. At zero interlayer electrical bias, the ground state at these fillings is spin polarized and electrons occupy one valley or the other while a finite electrical bias produces a series of valley pseudospin-flip transitions. In this work, we extend the Ising behavior to chirally-stacked multilayer graphene and discuss the hysteretic behavior of the Ising quantum Hall ferromagnets. We compute the transport gap due to different excitations: bulk electron-hole pairs, electron-hole pairs confined to the coherent region of a valley-pseudospin domain wall, and spin or valley-pseudospin skyrmion-antiskyrmion pairs. We determine which of these excitations has the lowest energy at a given value of the Zeeman coupling, bias, and magnetic field.",1410.4232v1 2014-10-24,Phenomenological theory of magneto-electric coupling in granular multiferroics,"We study coupling between the ferroelectric polarization and magnetization of granular ferromagnetic film using a phenomenological model of combined multiferroic system consisting of granular ferromagnetic film placed above the ferroelectric (FE) layer. The coupling is due to screening of Coulomb interaction in the granular film by the FE layer. Below the FE Curie temperature the magnetization has hysteresis as a function of electric field. Below the magnetic ordering temperature the polarization has hysteresis as a function of magnetic field. We study the magneto-electric coupling for weak and strong spatial dispersion of the FE layer. The effect of mutual influence decreases with increasing the spatial dispersion of the FE layer. For weak dispersion the strongest coupling occurs in the vicinity of the ferroelectric-paraelectric phase transition. For strong dispersion the situation is the opposite. We study the magneto-electric coupling as a function of distance between the FE layer and the granular film. For large distances the coupling decays exponentially due to the exponential decrease of electric field produced by the oscillating charges in the granular ferromagnetic film.",1410.6536v1 2014-10-24,Non-Fermi Liquid Behavior Close to a Quantum Critical Point in a Ferromagnetic State without Local Moments,"A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc_{3.1}In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid (NFL) behavior, manifested in the logarithmic divergence of the specific heat both in the ferro- and the paramagnetic states. Sc_{3.1}In displays critical scaling and NFL behavior in the ferromagnetic state, akin to what had been observed only in f-electron, local moment systems. With doping, critical scaling is observed close to the QCP, as the critical exponents, and delta, gamma and beta have weak composition dependence, with delta nearly twice, and beta almost half of their respective mean-field values. The unusually large paramagnetic moment mu_PM~1.3 mu_B/F.U. is nearly composition-independent. Evidence for strong spin fluctuations, accompanying the QCP at x_c = 0.035 +- 0.005, may be ascribed to the reduced dimensionality of Sc_{3.1}In, associated with the nearly one-dimensional Sc-In chains.",1410.6850v1 2014-10-30,Intrinsic synchronization of an array of spin-torque oscillators driven by the spin-Hall effect,"This paper micromagnetically studies the magnetization dynamics driven by the spin-Hall effect in a Platinum/Permalloy bi-layer. For a certain field and current range, the excitation of a uniform mode, characterized by a power with a spatial distribution in the whole ferromagnetic cross section, is observed. We suggest to use the ferromagnet of the bi-layer as basis for the realization of an array of spin-torque oscillators (STOs): the Permalloy ferromagnet will act as shared free layer, whereas the spacers and the polarizers are built on top of it. Following this strategy, the frequency of the uniform mode will be the same for the whole device, creating an intrinsic synchronization. The synchronization of an array of parallely connected STOs will allow to increase the output power, as necessary for technological applications.",1410.8342v1 2014-11-01,Ferromagnetic interactions and martensitic transformation in Fe doped Ni-Mn-In shape memory alloys,"The structure, magnetic and martensitic properties of Fe doped Ni-Mn-In magnetic shape memory alloys have been studied by differential scanning calorimetry, magnetization, resistivity, X-ray diffraction (XRD) and EXAFS. While Ni$_{2}$MnIn$_{1-x}$Fe$_{x}$ ($0 \le x \le 0.6$) alloys are ferromagnetic and non martensitic, the martensitic transformation temperature in Ni$_{2}$Mn$_{1.5} $In$_{1-y}$Fe$_{y}$ and Ni$_{2}$Mn$ _{1.6} $In$_{1-y}$Fe$_{y}$ increases for lower Fe concentrations ($y \le 0.05$) before decreasing sharply for higher Fe concentrations. XRD analysis reveals presence of cubic and tetragonal structural phases in Ni$_{2}$MnIn$_{1-x}$Fe$_{x}$ at room temperature with tetragonal phase content increasing with Fe doping. Even though the local structure around Mn and Ni in these Fe doped alloys is similar to martensitic Mn rich Ni-Mn-In alloys, presence of ferromagnetic interactions and structural disorder induced by Fe affect Mn-Ni-Mn antiferromagnetic interactions resulting in suppression of martensitic transformation in these Fe doped alloys.",1411.0058v1 2014-11-03,Strain dependence of antiferromagnetic interface coupling in La0.7Sr0.3MnO3/SrRuO3 superlattices,"We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg1/3Nb2/3O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu0HAF = 2.8 T is found to decrease by deltaHAF/delta epsilon ~ -520 mT %-1 under reversible biaxial strain mu0HAF at 80 K in a [La0.7Sr0.3MnO3(22)/SrRuO3(55)]15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO3 layers at the interface. Strain-dependent orbital occupation in a single-ion picture cannot explain the sign of the observed strain dependence, whereas the enhanced Mn order at the interface is qualitatively in line with it.",1411.0411v2 2014-11-04,Magnetoimpedance effect at the high frequency range for the thin film geometry: Numerical calculation and experiment,"The magnetoimpedance effect is a versatile tool to investigate ferromagnetic materials, revealing aspects on the fundamental physics associated to magnetization dynamics, broadband magnetic properties, important issues for current and emerging technological applications for magnetic sensors, as well as insights on ferromagnetic resonance effect at non-saturated magnetic states. Here, we perform a theoretical and experimental investigation of the magnetoimpedance effect for the thin film geometry in a wide frequency range. We calculate the longitudinal magnetoimpedance for single layered, multilayered or exchange biased systems from an approach that considers a magnetic permeability model for planar geometry and the appropriate magnetic free energy density for each structure. From numerical calculations and experimental results found in literature, we analyze the magnetoimpedance behavior, and discuss the main features and advantages of each structure. To test the robustness of the approach, we directly compare theoretical results with experimental magnetoimpedance measurements obtained in a wide range of frequencies for an exchange biased multilayered film. Thus, we provide experimental evidence to confirm the validity of the theoretical approach employed to describe the magnetoimpedance in ferromagnetic films, revealed by the good agreement between numerical calculations and experimental results.",1411.0971v1 2014-11-11,Striped Ferronematic ground states in a spin-orbit coupled $S=1$ Bose gas,"We theoretically establish the mean-field phase diagram of a homogeneous spin-$1$, spin-orbit coupled Bose gas as a function of the spin-dependent interaction parameter, the Raman coupling strength and the quadratic Zeeman shift. We find that the interplay between spin-orbit coupling and spin-dependent interactions leads to the occurrence of ferromagnetic or ferronematic phases which also break translational symmetry. For weak Raman coupling, increasing attractive spin-dependent interactions (as in $^{87}$Rb or $^7$Li) induces a transition from a uniform to a stripe XY ferromagnet (with no nematic order). For repulsive spin-dependent interactions however (as in $^{23}$Na), we find a transition from an $XY$ spin spiral phase ($= 0$ and uniform total density) with uniaxial nematic order, to a biaxial ferronematic, where the total density, spin vector and nematic director oscillate in real space. We investigate the stability of these phases against the quadratic Zeeman effect, which generally tends to favor uniform phases with either ferromagnetic or nematic order but not both. We discuss the relevance of our results to ongoing experiments on spin-orbit coupled, spinor Bose gases.",1411.2990v1 2014-11-12,Dependence of the Efficiency of Spin Hall Torque on the Transparency of Pt-Ferromagnetic Layer Interfaces,"We report that spin current transport across Pt-ferromagnet (FM) interfaces is strongly dependent on the type and the thickness of the FM layer and on post-deposition processing protocols. By employing both harmonic voltage measurements and spin-torque ferromagnetic resonance measurements, we find that the efficiency of the Pt spin Hall effect in exerting a damping-like spin torque on the FM ranges from < 0.05 to > 0.10 under different interfacial conditions. We also show that the temperature dependence of the spin torque efficiencies for both the damping-like torque and field-like torque is dependent upon the details of the Pt-FM interface. The ""internal"" spin Hall angle of the Pt thin films used in this study, after taking the interfacial spin transmission factor into account, is estimated to be ~ 0.20. This suggests that a careful engineering of Pt-FM interfaces can improve the spin-Hall-torque efficiency of Pt-based spintronic devices.",1411.3379v1 2014-11-13,Tunnel Magnetoresistance with Atomically Thin Two-Dimensional Hexagonal Boron Nitride Barriers,"The two-dimensional atomically thin insulator hexagonal boron nitride (h-BN) constitutes a new paradigm in tunnel based devices. A large band gap along with its atomically flat nature without dangling bonds or interface trap states makes it an ideal candidate for tunnel spin transport in spintronic devices. Here, we demonstrate the tunneling of spin-polarized electrons through large area monolayer h-BN prepared by chemical vapor deposition in magnetic tunnel junctions. In ferromagnet/h-BN/ferromagnet heterostructures fabricated over a chip scale, we show tunnel magneto resistance at room temperature. Measurements at different bias voltages and on multiple devices with different ferromagnetic electrodes establish the spin polarized tunneling using h-BN barriers. These results open the way for integration of 2D monolayer insulating barriers in active spintronic devices and circuits operating at ambient temperature, and for further exploration of their properties and prospects.",1411.3524v1 2014-11-14,Lifetimes of Magnons in Two-Dimensional Diluted Ferromagnetic Systems,"Spin dynamics in low dimensional magnetic systems has been of fundamental importance for a long time and has currently received an impetus owing to the emerging field of nanoelectronics. Knowledge of the spin wave lifetimes, in particular, can be favorable for future potential applications. We investigate the low-temperature spin wave excitations in two-dimensional disordered ferromagnetic systems, with a particular focus on the long wavelength magnon lifetimes. A semi-analytical Green's functions based approach is used to determine the dynamical spectral functions, for different magnetic impurity concentrations, from which the intrinsic linewidth is extracted. We obtain an unambiguous $q^4$ scaling of the magnon linewidth which is ascribed to the disorder induced damping of the spin waves, thereby settling a longstanding unresolved issue on the wave-vector dependence. Our findings are also in good agreement with previous theoretical studies on Heisenberg ferromagnets. Additionally, we demonstrate the futility of using the low moments associated with the spectral densities to evaluate the magnon dispersions and lifetimes.",1411.3876v2 2014-11-22,On the origin of CE-type orbital fluctuations in the ferromagnetic metallic La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$,"We investigate the orbital fluctuations in the ferromagnetic-metallic phase of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ by considering a two orbital model within a tight-binding description which reproduces the ARPES Fermi surface. We find strong antisymmetric transverse orbital fluctuations at wavevector ($ 0.5 \pi, 0.5 \pi$) resulting from the Fermi-surface nesting between the portions of bonding and antibonding bands instead of the widely believed nesting between the portions of bonding band despite their flat segments, which provide an insight into the origin of so called CE-type orbital fluctuations in the ferromagnetic-metallic phase. Subsequent renormalization of the phonons near wavevector ($0.5 \pi,0.5 \pi$) and the behavior of the phonon linewidth as a function of momentum are in agreement with the inelastic neutron scattering experiments.",1411.6088v1 2014-12-23,Temperature controlled motion of an antiferromagnet-ferromagnet interface within a dopant-graded FeRh epilayer,"Chemically ordered B2 FeRh exhibits a remarkable antiferromagnetic-ferromagnetic phase transition that is first order. It thus shows phase coexistence, usually by proceeding though nucleation at random defect sites followed by propagation of phase boundary domain walls. The transition occurs at a temperature that can be varied by doping other metals onto the Rh site. We have taken advantage of this to yield control over the transition process by preparing an epilayer with oppositely directed doping gradients of Pd and Ir throughout its height, yielding a gradual transition that occurs between 350~K and 500~K. As the sample is heated, a horizontal antiferromagnetic-ferromagnetic phase boundary domain wall moves gradually up through the layer, its position controlled by the temperature. This mobile magnetic domain wall affects the magnetisation and resistivity of the layer in a way that can be controlled, and hence exploited, for novel device applications.",1412.7346v1 2014-12-24,Direct experimental evidence of multiferroicity in a nanocrystalline Zener polaron ordered manganite,"We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in Pr0.67Ca0.33MnO3(PCMO) with the help of x ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the positive up negative down method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron(ZP) ordered P21nm and disordered Pbnm structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroyed while the long-range antiferromagnetic ordering at low temperature in bulk system is replaced by ferromagnetic correlations in nanoparticles. The conclusion that by reducing the crystallite size it might be possible to make ferromagnetism and ferroelectricity coexist near room temperature should be generally applicable to all ZP ordered manganites.",1412.7723v1 2014-12-26,Domain wall of a ferromagnet on a three-dimensional topological insulator,"Topological insulators (TIs) show rich phenomena and functions which can never be realized in ordinary insulators. Most of them come from the peculiar surface or edge states. Especially, the quantized anomalous Hall effect (QAHE) without an external magnetic field is realized in the two-dimensional ferromagnet on a three-dimensional TI which supports the dissipationless edge current. Here we demonstrate theoretically that the domain wall of this ferromagnet, which carries edge current, is charged and can be controlled by the external electric field. The chirality and relative stability of the Neel wall and Bloch wall depend on the position of the Fermi energy as well as the form of the coupling between the magnetic moments and orbital of the host TI. These findings will pave a path to utilize the magnets on TI for the spintronics applications.",1412.7910v2 2014-12-29,Electrically Tunable Magnetism in Magnetic Topological Insulators,"The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modification of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a topological transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. In particular, the field-controlled ferromagnetism in magnetic topological insulator can be used for voltage based writing of magnetic random access memories in magnetic tunnel junctions. The simultaneous electrical control of magnetic order and chiral edge transport in such devices may lead to electronic and spintronic applications for topological insulators.",1412.8237v5 2015-01-05,A novel platform for two-dimensional chiral topological superconductivity,"We show that the surface of an $s$-wave superconductor decorated with a two-dimensional lattice of magnetic impurities can exhibit chiral topological superconductivity. If impurities order ferromagnetically and the superconducting surface supports a sufficiently strong Rashba-type spin-orbit coupling, Shiba sub-gap states at impurity locations can hybridize into Bogoliubov bands with non-vanishing, sometimes large, Chern number $C$. This topological superconductor supports $C$ chiral Majorana edge modes. We construct phase diagrams for model two-dimensional superconductors, accessing the dilute and dense magnetic impurity limits analytically and the intermediate regime numerically. To address potential experimental systems, we identify stable configurations of ferromagnetic iron atoms on the Pb (111) surface and conclude that ferromagnetic adatoms on Pb surfaces can provide a versatile platform for two-dimensional topological superconductivity.",1501.00999v1 2015-01-09,"Collective magnetization dynamics in ferromagnetic (Ga,Mn)As mediated by photo-excited carriers","We present a study of photo-excited magnetization dynamics in ferromagnetic (Ga,Mn)As films observed by time-resolved magneto-optical measurements. The magnetization precession triggered by linearly polarized optical pulses in the absence of an external field shows a strong dependence on photon frequency when the photo-excitation energy approaches the band-edge of (Ga,Mn)As. This can be understood in terms of magnetic anisotropy modulation by both laser heating of the sample and by hole-induced non-thermal paths. Our findings provide a means for identifying the transition of laser-triggered magnetization dynamics from thermal to non-thermal mechanisms, a result that is of importance for ultrafast optical spin manipulation in ferromagnetic materials via non-thermal paths.",1501.02083v1 2015-01-12,Electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation,"The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized two-dimensional angular correlation of annihilation radiation (ACAR). The spectra were compared with theoretical results obtained within LDA+DMFT, a combination of the local density approximation (LDA) and the many-body dynamical mean-field theory (DMFT). The self-energy describing the electronic correlations in Ni is found to make important anisotropic contributions to the momentum distribution which are not present in LDA. Based on a detailed comparison of the theoretical and experimental results the strength of the local electronic interaction U in ferromagnetic Ni is determined as 2.0 +- 0.1 eV.",1501.02584v2 2015-01-13,Bound states of a ferromagnetic wire in a superconductor,"We consider the problem of bound states in strongly anisotropic ferromagnetic impurities in a superconductor, motivated by recent experiments that claim to observe Majorana modes at the ends of ferromagnetic wires on a superconducting substrate [S. Nadj-Perge et al., Science 346, 602 (2014)]. Generalizing the successful theory of bound states of spherically symmetric impurities, we consider a wire-like potential using both analytical and numerical approaches. We find that away from the ends of the wire the bound states form bands with pronounced van Hove singularities, giving rise to subgap peaks in the local density of states. For sufficiently strong magnetization of the wire, we show that this process generically produces a sharp peak at zero energy in the local density of states near the ends of the wire. This zero-energy peak has qualitative similarities to the claimed signature of a Majorana mode observed in the aforementioned experiment.",1501.03149v3 2015-01-17,Kondo dynamics in one-dimensional doped ferromagnetic insulators,"Some well-established examples of itinerant-electron ferromagnetism in one dimension occur in a Mott-insulating phase. We examine the consequences of doping a ferromagnetic insulator and cou- pling magnons to gapless charge fluctuations. Using a bosonization scheme for strongly interacting electrons, we derive an effective field theory for the magnon-holon interaction. When the magnon momentum matches the Fermi momentum of the holons, the backscattering of the magnon at low energies gives rise to a Kondo effect of a pseudospin defined from the chirality degree of freedom (right- or left-moving particles). The crossover between weak-coupling and strong-coupling fixed points of the effective mobile-impurity model is then investigated using a numerical renormalization group approach.",1501.04220v1 2015-01-20,"Structural, electronic, and magnetic properties of tris(8-hydroxyquinoline)iron(III) molecules and its magnetic coupling with ferromagnetic surface: first-principles study","Using first-principles calculations, we have systematically investigated the structural, electronic, and magnetic properties of facial (fac-) and meridional (mer-) tris(8-hydroxyquinoline)iron(III) (Feq3) molecules and their interaction with ferromagnetic substrate. Our calculation results show that for the isolated Feq3, mer-Feq3 is more stable than the fac-Feq3; and both Feq3 isomers have a high spin-state of 5 {\mu}B as the ground state when an on-site Hubbard-U term is included to treat the highly localized Fe 3d electrons; while the standard DFT calculations produce a low spin-state of 1 {\mu}B. These magnetic behaviors can be understood by the octahedral ligand field splitting theory. Furthermore, we found that fac-Feq3 has a stronger bonding to the Co surface than mer-Feq3 and an anti-ferromagnetic coupling was discovered between Fe and Co substrate, originating from the superexchange coupling between Fe and Co mediated by the interface oxygen and nitrogen atoms. These findings suggest that Feq3 molecular films may serve as a promising spin-filter material in spintronic devices.",1501.04926v1 2015-01-22,Coexistence and interconvertibility of ferromagnetic and antiferromagnetic phases in the single crystal of Mn$_3$ single-molecule magnet,"We report the coexistence of ferromagnetic and antiferromagnetic phases in the single crystal of Mn$_3$ single-molecule magnet. The coexistent state appears within a certain period of time in the progress of either oxidation or reduction during a reversible oxidation-reduction process, when the sample is exposed in the air (oxygen) or the methyl vapor. We noticed an apparent change in the molecular structure, which is also reversible in terms of that the methyl group is dropped or added to the molecules during the oxidation or reduction. The absence or presence of the methyl group in the molecules exert an essential impact upon the intermolecular exchange interaction, and the ferromagnetic phase comes from the heterogenous intermolecular bonds between pairs of molecules of which one molecule has a methyl group whereas the other has lost the methyl group. The reversible change in molecular structure suggests the magnetic structure of Mn$_3$ might be designed and modulated at molecular scale, which implies Mn$_3$ has a great application potential.",1501.05484v1 2015-01-27,Spontaneous PT symmetry breaking of a ferromagnetic superfluid in a gradient field,"We consider the interaction of a ferromagnetic spinor Bose-Einstein condensate with a magnetic field gradient. The magnetic field gradient realizes a spin-position coupling that explicitly breaks time-reversal symmetry T and space parity P, but preserves the combined PT symmetry. We observe using numerical simulations, a first-order phase transition spontaneously breaking this re-maining symmetry. The transition to a low-gradient phase, in which gradient effects are frozen out by the ferromagnetic interaction, suggests the possibility of high-coherence magnetic sensors unaffected by gradient dephasing.",1501.06830v3 2015-01-28,Spin Amplification by Controlled Symmetry Breaking for Spin-Based Logic,"Spin amplification is one of the most critical challenges for spintronics and spin-based logic in order to achieve spintronic circuits with fan-out. We propose a new concept for spin amplification that will allow a small spin current in a non-magnetic spin channel to control the magnetization of an attached ferromagnet. The key step is to bring the ferromagnet into an unstable symmetric state (USS), so that a small spin transfer torque from a small spin current can provide a magnetic bias to control the spontaneous symmetry breaking and select the final magnetization direction of the ferromagnet. Two proposed methods for achieving the USS configuration are voltage-controlled Curie temperature (VC-TC) and voltage-controlled magnetic anisotropy (VC-MA). We believe the development of new 2D magnetic materials with greater tunability of VC-TC and VC-MA will be needed for practical applications. A successful realization of spin amplification by controlled symmetry breaking will be important for the implementation of existing spin-logic proposals (e.g. ""All Spin Logic"") and could inspire alternative ideas for spintronic circuits and devices.",1501.07125v2 2015-02-02,Resonant magneto-tunneling between normal and ferromagnetic electrodes in relation to the three-terminal spin transport,"The recently suggested mechanism [Y. Song and H. Dery, Phys. Rev. Lett. 113, 047205 (2014)] of the three-terminal spin transport is based on the resonant tunneling of electrons between ferromagnetic and normal electrodes via an impurity. The sensitivity of current to a weak external magnetic field stems from a spin blockade, which, in turn, is enabled by strong on-site repulsion. We demonstrate that this sensitivity exists even in the absence of repulsion when a single-particle description applies. Within this description, we calculate exactly the resonant-tunneling current between the electrodes. The mechanism of magnetoresistance, completely different from the spin blocking, has its origin in the interference of virtual tunneling amplitudes. Spin imbalance in ferromagnetic electrode is responsible for this interference and the resulting coupling of the Zeeman levels. This coupling also affects the current in the correlated regime.",1502.00350v1 2015-02-06,Transport across a carbon nanotube quantum dot contacted with ferromagnetic leads: experiment and non-perturbative modeling,"We present measurements of tunneling magneto-resistance (TMR) in single-wall carbon nanotubes attached to ferromagnetic contacts in the Coulomb blockade regime. Strong variations of the TMR with gate voltage over a range of four conductance resonances, including a peculiar double-dip signature, are observed. The data is compared to calculations in the ""dressed second order"" (DSO) framework. In this non-perturbative theory, conductance peak positions and linewidths are affected by charge fluctuations incorporating the properties of the carbon nanotube quantum dot and the ferromagnetic leads. The theory is able to qualitatively reproduce the experimental data.",1502.02005v3 2015-02-16,Effect of non-uniform exchange field in ferromagnetic graphene,"We have presented here the consequences of the non-uniform exchange field on the spin transport issues in spin chiral configuration of ferromagnetic graphene. Taking resort to the spin orbit coupling (SOC) term and non-uniform exchange coupling term we are successful to express the expression of Hall conductivity in terms of the exchange field and SOC parameters through the Kubo formula approach. However, for a specific configuration of the exchange parameter we have evaluated the Berry curvature of the system. We also have paid attention to the study of SU(2) gauge theory of ferromagnetic graphene. The generation of anti damping spin orbit torque in spin chiral magnetic graphene is also briefly discussed.",1502.04453v1 2015-02-18,Strain-induced magnetic phase transition in SrCoO$_{3-δ}$ thin films,"It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO$_{3-\delta}$ ($\delta < 0.2$) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic to antiferromagnetic phase transition in SrCoO$_{3-\delta}$ films grown on DyScO$_3$ substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO$_3$ substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Ne\'el temperatures between $T_N \sim 135\,\pm\,10\,K$ and $\sim 325\,\pm\,10\,K$ depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO$_{3-\delta}$ thin films under large epitaxial strain.",1502.05192v1 2015-02-18,Ferromagnetism of Pd(001) substrate induced by Antiferromagnetic CoO,"Our first-principles study has revealed unexpected spin polarization of the Pd(001) substrate in contact with antiferromagnetic CoO overlayers. We give an evidence that the ferromagnetism of Pd is caused by the zigzag positions of Co atoms with respect to the Pd interface, resulted from the lattice-mismatch driven structural relaxation. Thanks to the itinerant nature of its 4d electrons, we see that the ferromagnetic properties of Pd are highly sensitive to the local environment and can be enhanced further by increasing the thickness of CoO overlayer film or/and by applying an additional uniaxial pressure along c-axis exerted externally on the bottom layers of the Pd substrate. Our finding provides new functionality for the interfacial moments of the CoO/Pd system, which can be accessed experimentally, e.g., by the magneto-optical Kerr effect (MOKE) or/and by element-resolved X-ray magnetic circular dichroism (XMCD) measurement.",1502.05208v1 2015-01-18,Theoretical Exploration on the Magnetic Properties of Ferromagnetic Metallic Glass: An Ising Model on Random Recursive Lattice,"The ferromagnetic Ising spins are modeled on a recursive lattice constructed from random-angled rhombus units with stochastic configurations, to study the magnetic properties of the bulk Fe-based metallic glass. The integration of spins on the structural glass model well represents the magnetic moments in the glassy metal. The model is exactly solved by the recursive calculation technique. The magnetization of the amorphous Ising spins, i.e. the glassy metallic magnet is investigated by our modeling and calculation on a theoretical base. The results show that the glassy metallic magnets has a lower Curie temperature, weaker magnetization, and higher entropy comparing to the regular ferromagnet in crystal form. These findings can be understood with the randomness of the amorphous system, and agrees well with others' experimental observations.",1502.06561v2 2015-02-27,Magnetoanisotropic Andreev Reflection in Ferromagnet/Superconductor Junctions,"Andreev reflection spectroscopy of ferromagnet/superconductor (FS) junctions is an important probe of spin polarization. We theoretically investigate spin-polarized transport in FS junctions in the presence of Rashba and Dresselhaus interfacial spin-orbit fields and show that Andreev reflection can be controlled by changing the magnetization orientation. We predict a giant in- and out-of-plane magnetoanisotropy of the junction conductance. If the ferromagnet is highly spin polarized---in the half-metal limit---the magnetoanisotropic Andreev reflection depends universally on the spin-orbit fields only. Our results show that Andreev reflection spectroscopy can be used for sensitive probing of interfacial spin-orbit fields in FS junction.",1502.08022v2 2015-03-02,Measuring the magnetic moment density in patterned ultrathin ferromagnets with submicron resolution,"We present a new approach to infer the surface density of magnetic moments $I_s$ in ultrathin ferromagnetic films with perpendicular anisotropy. It relies on quantitative stray field measurements with an atomic-size magnetometer based on the nitrogen-vacancy center in diamond. The method is applied to microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements of $I_s$ with a few percent uncertainty and a spatial resolution in the range of $(100$ nm)$^2$, an improvement by several orders of magnitude over existing methods. As an example of application, we measure the modifications of $I_s$ induced by local irradiation with He$^+$ ions in an ultrathin ferromagnetic wire. This method offers a new route to study variations of magnetic properties at the nanoscale.",1503.00705v1 2015-03-07,"Modulation of ferromagnetism in (In,Fe)As quantum wells via electrically controlled deformation of the electron wavefunctions","We demonstrate electrical control of ferromagnetism in field-effect transistors with a trilayer quantum well (QW) channel containing an ultrathin n-type ferromagnetic semiconductor (In,Fe)As layer. A gate voltage is applied to control the electron wavefunctions {\phi}i in the QW, such that the overlap of {\phi}i and the (In,Fe)As layer is modified. The Curie temperature is largely changed by 42%, whereas the change in sheet carrier concentration is 2 - 3 orders of magnitude smaller than that of previous gating experiments. This result provides a new approach for versatile, low power, and ultrafast manipulation of magnetization.",1503.02174v2 2015-03-12,Current induced torques in structures with ultra-thin IrMn antiferromagnet,"Relativistic current induced torques and devices utilizing antiferromagnets have been independently considered as two promising new directions in spintronics research. Here we report electrical measurements of the torques in structures comprising a $\sim1$~nm thick layer of an antiferromagnet IrMn. The reduced N\'eel temperature and the thickness comparable to the spin-diffusion length allow us to investigate the role of the antiferromagnetic order in the ultra-thin IrMn films in the observed torques. In a Ta/IrMn/CoFeB structure, IrMn in the high-temperature phase diminishes the torque in the CoFeB ferromagnet. At low temperatures, the antidamping torque in CoFeB flips sign as compared to the reference Ta/CoFeB structure, suggesting that IrMn in the antiferromagnetic phase governs the net torque acting on the ferromagnet. At low temperatures, current induced torque signatures are observed also in a Ta/IrMn structure comprising no ferromagnetic layer.",1503.03729v2 2015-03-23,Ferromagnetic Mn doped InSb studied at the atomic scale by cross-sectional STM,"We present an atomically resolved study of metal-organic vapor epitaxy grown Mn doped InSb that is ferromagnetic at room-temperature. Both topographic and spectroscopic measurements have been performed by cross-sectional scanning tunneling microscopy. The measurements show a perfect crystal structure without any precipitates and reveal that Mn acts as a shallow acceptor. The Mn concentration obtained from the cross-sectional STM data compares well with the intended doping concentration. No second phase material or (nano)clustering of the Mn was observed. While the pair correlation function of the Mn atoms showed that their local distribution is uncorrelated beyond the STM resolution for observing individual dopants, disorder in the Mn ion location is clearly noted. We discuss the implications of the observed disorder for a number of suggested explanations of the room-temperature ferromagnetism in Mn doped InSb grown by metal-organic vapor epitaxy.",1503.06825v1 2015-03-30,Van Hove singularity and ferromagnetic instability in phosphorene,"Using Wannier function-based interpolation techniques, we present compelling numerical evidence for the presence of a saddle-point van Hove singularity at the $\Gamma $ point near the phosphorene Fermi energy. We show that in proximity of the van Hove singularity the spin susceptibility presents the logarithmic temperature dependence typical of Liftshitz phase transitions. Furthermore, we demonstrate that the critical temperature for the ferromagnetic transition can be greatly increased (up to 0.05 K) if strain along the zigzag ridges is applied. Although the ferromagnetic state would be very difficult to experimentally reach, the logarithmic temperature behaviour of the spin susceptibility due to the van Hove singularity is found to persist at much higher temperatures (up to $\sim$97 K).",1503.08649v2 2015-04-02,Avoided ferromagnetic quantum critical point in CeRuPO,"CeRuPO is a rare example of a ferromagnetic (FM) Kondo-lattice system. External pressure suppresses the ordering temperature to zero at about $p_c\approx3$ GPa. Our ac-susceptibility and electrical-resistivity investigations evidence that the type of magnetic ordering changes from FM to antiferromagnetic (AFM) at about $p^*\approx0.87$ GPa. Studies in applied magnetic fields suggest that ferromagnetic and antiferromagnetic correlations compete for the ground state at $p>p^*$, but finally the AFM correlations win. The change in the magnetic ground-state properties is closely related to the pressure evolution of the crystalline-electric-field level (CEF) scheme and the magnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction. The N\'{e}el temperature disappears abruptly in a first-order-like fashion at $p_c$, hinting at the absence of a quantum critical point. This is consistent with the low-temperature transport properties exhibiting Landau-Fermi-liquid (LFL) behavior in the whole investigated pressure range up to 7.5 GPa.",1504.00612v1 2015-04-20,A-site driven ferroelectricity in strained ferromagnetic L2NiMnO6 thin films,"We report on theoretical and experimental investigation of A-site driven ferroelectricity in ferromagnetic La2NiMnO6 thin films grown on SrTiO3 substrates. Structural analysis and density functional theory calculations show that epitaxial strain stretches the rhombohedral La2NiMnO6 crystal lattice along the [111]cubic direction, triggering a displacement of the A-site La ions in the double perovskite lattice. The lattice distortion and the A-site displacements stabilize a ferroelectric polar state in ferromagnetic La2NiMnO6 crystals. The ferroelectric state only appears in the rhombohedral La2NiMnO6 phase, where MnO6 and NiO6 octahedral tilting is inhibited by the 3-fold crystal symmetry. Electron localization mapping showed that covalent bonding with oxygen and 6s orbital lone pair formation are negligible in this material.",1504.04905v1 2015-04-23,Spin-Reorientation and Weak Ferromagnetism in Antiferromagnetic TbMn_{0.5}Fe_{0.5}O_3,"Orthorhombic single crystals of TbMn0.5Fe0.5O3 are found to exhibit spin-reorientation, magnetization reversal and weak ferromagnetism. Strong anisotropy effects are evident in the temperature dependent magnetization measurements along the three crystallographic axes a, b and c. A broad magnetic transition is visible at T_N (Fe/Mn) = 286 K due to paramagnetic to AxGyCz ordering. A sharp transition is observed at T_SR (Fe/Mn) = 28 K, which is pronounced along c axis in the form of a sharp jump in magnetization where the spins reorient to GxAyFz configuration. The negative magnetization observed below TSR Fe/Mn along c axis is explained in terms of domain wall pinning. A component of weak ferromagnetism is observed in field-scans along c-axis but below 28 K. Field-induced steps-like transitions are observed in hysteresis measurement along b axis below 28 K. It is noted that no sign of Tb-order is discernible down to 2 K. TbMn0.5Fe0.5O3 could be highlighted as a potential candidate to evaluate its magneto-dielectric effects across the magnetic transitions.",1504.06060v1 2015-04-28,CaMn$_2$Al$_{10}$: itinerant Mn magnetism on the verge of ferromagnetic order,"We report the discovery of \CMA, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of $0.83\,\mathrm{\mu_B}$/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only $\approx 9$ \% of $R \mathrm{ln}\,2$. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility $\chi$ is anisotropic below 20 K, with a maximum ratio of $\chi_{[010]}/\chi_{[001]}\approx 3.5$. A strong power-law divergence $\chi(T)\sim T^{-1.2}$ below 20 K implies incipient ferromagnetic order, and an Arrott plot analysis of the magnetization suggests a vanishingly low Curie temperature $T_C\sim 0$. Our experiments indicate that \CMA~is a rare example of a Mn-based weak itinerant magnet that is poised on the verge of ferromagnetic order.",1504.07649v1 2015-04-29,Half-metallic ferromagnetism in the RbSe and CsTe compounds with CsCl structure: A first-principles study,"We investigate the electronic structures and magnetic properties of RbSe and CsTe compounds in CsCl, RS and ZB structures by using first-principles calculation. It is shown that these two compounds exhibit half-metallic ferromagnetism with an integer magnetic moment of 1.00 $\mu_B$ per formula in all the three structures. Total energy calculations indicate the CsCl phase is more stable than the other two phases. We investigate these two compounds with CsCl structure in detail. The ferromagnetism results mainly from the spin-polarization of p states of anion Se (Te) for the two compounds. The half-metallicities of RbSe and CsTe compounds can be preserved up to 2.5\% and 0.8\% compression of lattice constants with respect to their equilibrium lattices, respectively.",1504.07705v1 2015-05-12,Nonreciprocal spin wave propagation in chiral-lattice ferromagnets,"Spin current, i.e. the flow of spin angular momentum or magnetic moment, has recently attracted much attention as the promising alternative for charge current with better energy efficiency. Genuine spin current is generally carried by the spin wave (propagating spin precession) in insulating ferromagnets, and should hold the chiral symmetry when it propagates along the spin direction. Here, we experimentally demonstrate that such a spin wave spin current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice ferromagnet. This phenomenon originates from the interference of chirality between the SWSC and crystal-lattice, which is mediated by the relativistic spin-orbit interaction. The present finding enables the design of perfect spin current diode, and highlights the importance of the chiral aspect in SWSC.",1505.02868v1 2015-05-24,Coexistence of superconductivity and complex 4$f$ magnetism in Eu$_{0.5}$Ce$_{0.5}$BiS$_{2}$F,"EuBiS$_{2}$F is a self-doped superconductor due to the mixed valence of Eu. Here we report that, with the Ce substitution for Eu by 50 at.\%, the material exhibits ferromagnetic ordering at 8 K for the Ce-4$f$ moment, superconductivity at 2.2 K in the BiS$_2$ layers, and possibly antiferromagnetic ordering at 2.1 K for the Eu-4$f$ spins. The Eu valence is essentially divalent with the Ce incorporation. We tentatively interpret the coexistence of ferromagnetism and superconductivity by considering different Bi-6$p$ orbitals that are responsible for superconductivity itself and for mediating the ferromagnetic interaction, respectively. We argue that the antiferromagnetic ordering of the Eu-4$f$ spins is most likely due to a magnetic dipole-dipole interaction.",1505.06447v1 2015-05-26,Observation of pure inverse spin Hall effect in ferromagnetic metals by FM/AFM exchange bias structures,"We report that the spin current generated by spin Seebeck effect (SSE) in yttrium iron garnet (YIG) can be detected by a ferromagnetic metal (NiFe). By using the FM/AFM exchange bias structure (NiFe/IrMn), inverse spin Hall effect (ISHE) and planar Nernst effect (PNE) of NiFe can be unambiguously separated, allowing us to observe a pure ISHE signal. After eliminating the in plane temperature gradient in NiFe, we can even observe a pure ISHE signal without PNE from NiFe itself. It is worth noting that a large spin Hall angle (0.098) of NiFe is obtained, which is comparable with Pt. This work provides a kind of FM/AFM exchange bias structures to detect the spin current by charge signals, and highlights ISHE in ferromagnetic metals can be used in spintronic research and applications.",1505.06911v2 2015-05-27,Proximity-free enhancement of anomalous Nernst effects in metallic multilayers,"The anomalous Nernst effect (ANE) has been investigated in alternately-stacked multilayer films comprising paramagnetic and ferromagnetic metals. We found that the ANE is enhanced with increasing the number of the paramagnet/ferromagnet interfaces with keeping the total thickness of the films constant, and that the enhancement appears even in the absence of magnetic proximity effects; similar behavior was observed not only in Pt/Fe multilayers but also in Au/Fe and Cu/Fe multilayers free from proximity ferromagnetism. This universal enhancement of the ANE in the metallic multilayers suggests the presence of unconventional interface-induced thermoelectric conversion in the Fe films attached to the paramagnets.",1505.07183v1 2015-05-28,Determining Spin Polarization of Seebeck Coefficients via Anomalous Nernst Effect,"Recently, Seebeck coefficients of ferromagnetic conductors are found to be spin-dependent. However straightforward method of accurately determining its spin polarization is still to be developed. Here, we have derived a linear dependence of anomalous Nernst coefficient on anomalous Hall angle with scaling factor related to spin polarization of Seebeck coefficient, which has been experimentally verified in [Co/Pt]n superlattices. Based on the dependence, we have also evaluated spin polarization of Seebeck coefficient of some ferromagnetic conductors. Besides, we have also found a new mechanism to generate pure spin current from temperature gradient in ferromagnetic/nonmagnetic hybrid system, which could improve efficiency from thermal energy to spin current.",1505.07662v1 2015-05-28,On the dynamical stability of ferromagnetic Ru and Os in the bct structure: a first-principles study,"Recent theoretical studies have predicted magnetic states for Ru and Os in the body centered tetragonal structure (bct) with c/a < 1. In this study, we present first principles calculations of the phonon dispersion for ferromagnetic Ru- and Os-bct along the epitaxial and uniaxial Bain paths, to evaluate their dynamical stability. The phonon dispersions were computed using the density function perturbation theory, including the gradient corrections to the exchange-correlation functional within the plane-waves ultrasoft-pseudopotential approximation. The phonon dispersion for the local minimum in the Bain path with c/a < 1 as well as the uniaxial and epitaxial strained structures are analyzed. We find imaginary frequencies along different directions of the Brillouin zone, which indicates that both systems are dynamically unstable. Consequently, ferromagnetic Ru and Os in the body centered tetragonal structure with c/a < 1 are not truly metastable phases.",1505.07869v2 2015-06-01,Electrical detection of magnetization reversal without auxiliary magnets,"First-generation magnetic random access memories based on anisotropic magnetoresistance required magnetic fields for both writing and reading. Modern all-electrical read/write memories use instead non-relativistic spin-transport connecting the storing magnetic layer with a reference ferromagnet. Recent studies have focused on electrical manipulation of magnetic moments by relativistic spin-torques requiring no reference ferromagnet. Here we report the observation of a counterpart magnetoresistance effect in such a relativistic system which allows us to electrically detect the sign of the magnetization without an auxiliary magnetic field or ferromagnet. We observe the effect in a geometry in which the magnetization of a uniaxial (Ga,Mn)As epilayer is set either parallel or antiparallel to a current-induced non-equilibrium spin polarization of carriers. In our structure, this linear-in-current magnetoresistance reaches 0.2\% at current density of $10^6$ A cm$^{-2}$.",1506.00400v1 2015-06-04,Electronic structure and weak itinerant magnetism in metallic Y2Ni7,"We report a density functional study of the electronic structure and magnetism of Y2Ni7. The results show itinerant magnetism very similar to that in the weak itinerant ferromagnet Ni3Al. The electropositive Y atoms in Y2Ni7 serves to donate charge to the Ni host mostly in the form of $s$ electrons. The non-spin-polarized state shows a high density of states at the Fermi level, N(EF) due to flat bands. This leads to the ferromagnetic instability. However, there are also several much more dispersive bands crossing E(F), which should promote the conductivity. Spin fluctuation effects appear to be comparable to or weaker than Ni3Al, based on comparison with experimental data. Y2Ni7 provides a uniaxial analogue to cubic Ni3Al for studying weak itinerant ferromagnetism, suggesting detailed measurements of its low temperature physical properties and spin fluctuations, as well experiments under pressure.",1506.01553v2 2015-06-06,Spin filter and spin valve in ferromagnetic graphene,"We propose and demonstrate that a EuO-induced and top-gated graphene ferromagnetic junction can be simultaneously operated as a spin filter as well as a spin valve. We attribute such a remarkable result to a coexistence of a half-metal band and a common energy gap for opposite spins in ferromagnetic graphene. We show that, both the spin filter and the spin valve can be effectively controlled by a back gate voltage, and they survive for practical metal contacts and finite temperature. Specifically, larger single spin currents and on-state currents can be reached with contacts with work functions similar to graphene, and the spin filter can operate at higher temperature than the spin valve.",1506.02127v1 2015-06-11,A continuous family of equilibria in ferromagnetic media are ground states,"We show that a foliation of equilibria (a continuous family of equilibria whose graph covers all the configuration space) in ferromagnetic models are ground states. The result we prove is very general, and it applies to models with long range interactions and many body. As an application, we consider several models of networks of interacting particles including models of Frenkel-Kontorova type on $\mathbb{Z}^d$ and one-dimensional quasi-periodic media. The result above is an analogue of several results in the calculus variations (fields of extremals) and in PDE's. Since the models we consider are discrete and long range, new proofs need to be given. We also note that the main hypothesis of our result (the existence of foliations of equilibria) is the conclusion (using KAM theory) of several recent papers. Hence, we obtain that the KAM solutions recently established are minimizers when the interaction is ferromagnetic (and transitive).",1506.03582v1 2015-06-17,Continuous wave approach for simulating Ferromagnetic Resonance in nanosized elements,"We present a numerical approach to simulate the Ferromagnetic Resonance (FMR) of micron and nanosized magnetic elements by a micromagnetic finite difference method. In addition to a static magnetic field a linearly polarized oscillating magnetic field is utilized to excite and analyze the spin wave excitations observed by Ferromagnetic Resonance in the space- and time-domain. Our continuous wave approach (CW) provides an alternative to the common simulation method, which uses a pulsed excitation of the magnetic system. It directly models conventional FMR-experiments and permits the determination of the real and imaginary part of the complex dynamic susceptibility without the need of post-processing. Furthermore not only the resonance fields, but also linewidths, ellipticity, phase relations and relative intensities of the excited spin wave modes in a spectrum can be determined and compared to experimental data. The magnetic responses can be plotted as a function of spatial dimensions yielding a detailed visualization of the spin wave modes and their localization as a function of external magnetic field and frequency. This is illustrated for the case of a magnetic micron sized stripe.",1506.05292v1 2015-06-19,Inverse spin Hall effect in a complex ferromagnetic oxide heterostructure,"Complex oxide heterostructures are hot candidates for post CMOS multi-functional devices. Especially in spintronics applications ferromagnetic oxides may play a key role because they can exhibit extraordinary high spin polarization. Indeed, there are already plenty of examples in spintronics, notably in the area of spin pumping and inverse spin Hall effect (ISHE) \cite{Azevedo2011, Czeschka2011, Hahn2013, Obstbaum2014}. Although complex oxides have been used in these experiments as a source of spin currents, they have never been demonstrated to act as a spin sink that exhibits ISHE. Here we show that in a heterostructure consisting of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and SrRuO$_{3}$ (SRO) the low temperature ferromagnet SRO can act as a spin sink and exhibit a sizeable ISHE which persists even below its Curie temperature. This result opens up new possibilities for application of all oxide heterostructures in spintronics and may significantly extend the research on spin Hall effect and related phenomena.",1506.05921v1 2015-06-20,"Common effect of chemical and external pressures on the magnetic properties of $R$CoPO ($R$ = La, Pr, Nd, Sm). II","The direct correspondence between Co band ferromagnetism and structural parameters is investigated in the pnictide oxides $R$CoPO for different rare-earth ions ($R$ = La, Pr, Nd, Sm) by means of muon-spin spectroscopy and {\it ab-initio} calculations, complementing our results published previously [G. Prando {\it et al.}, {\it Phys. Rev. B} {\bf 87}, 064401 (2013)]. Both the transition temperature to the ferromagnetic phase $T_{_{\textrm{C}}}$ and the volume of the crystallographic unit cell $V$ are found to be conveniently tuned by the $R$ ionic radius and/or external pressure. A linear correlation between $T_{_{\textrm{C}}}$ and $V$ is reported and {\it ab-initio} calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, $R$ ions are shown to be influencing the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizeable magnetic contribution at much lower temperatures.",1506.06211v1 2015-06-26,Macroscopic quantum tunneling in spin filter ferromagnetic Josephson junctions,"The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F) is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunneling in Josephson junctions with GdN ferromagnetic insulator barriers. We prove a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions which demonstrate a clear signature of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits.",1506.08108v1 2015-07-06,Delta chain with anisotropic ferromagnetic and antiferromagnetic interactions,"We consider analytically and numerically an anisotropic spin-$\frac{1}{2}$ delta-chain (sawtooth chain) with nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic interactions. For certain values of the interactions a lowest one-particle band becomes flat and there is a class of localized-magnon eigenstates which form a ground state with a macroscopic degeneracy. In this case the model depends on a single parameter which can be chosen as the anisotropy of the exchange interactions. When this parameter changes from zero to infinity the model interpolates between the one-dimensional isotropic ferromagnet and the frustrated Ising model on the delta-chain. It is shown that the low-temperature thermodynamic properties in these limiting cases are governed by the specific structure of the excitation spectrum. In particular, the specific heat has one or infinite number of low-temperature maxima for the small or the large anisotropy parameter, correspondingly. Numerical calculations of finite chains demonstrate that this behavior is generic for definite values of the anisotropy parameter.",1507.01389v1 2015-07-13,Direct surface charging and alkali-metal doping for tuning the interlayer magnetic order in planar nanostructures,"The continuous reduction of magnetic units to ultra small length scales inspires efforts to look for a suitable means of controlling magnetic states. In this study we show two surface charge alteration techniques for tuning the interlayer exchange coupling (IEC) of ferromagnetic layers separated by paramagnetic spacers. Our study reveals that already a modest amount of extra charge can switch the mutual alignment of the magnetization from anti-ferromagnetic to ferromagnetic or vice verse. We also propose adsorption of alkali metals as an alternative way of varying the electronic and chemical properties of magnetic surfaces. Clear evidence is found that the interlayer magnetic order can be reversed by adsorbing alkali metals on the magnetic layer. Moreover, alkali metal overlayers strongly enhance the perpendicular magnetic anisotropy in FePt thin films. These findings combined with atomistic spin model calculations suggest that electronic or ionic way of surface charging can have a crucial role for magnetic hardening and spin state control.",1507.03365v1 2015-07-30,"First-order ferromagnetic transition in single-crystalline (Mn,Fe)2(P,Si)","(Mn,Fe)2(P,Si) single crystals have been successfully grown by flux method. Single crystal diffraction demonstrates that Mn0.83Fe1.17P0.72Si0.28 crystallizes in a hexagonal crystal structure (space group P-62m) at both 100 and 280 K, in the ferromagnetic and paramagnetic states, respectively. The magnetization measurements show that the crystals display a first-order ferromagnetic phase transition at their Curie temperature (TC). The preferred magnetization direction in is along the c axis. A weak magnetic anisotropy of K1 = 0.25x106 J/m3 and K2 = 0.19x106 J/m3 is found at 5 K. These values indicate a soft magnetic behaviour favourable for magnetic refrigeration. A series of discontinuous magnetization jumps is observed far below TC by increasing the field at a constant temperature. These magnetization jumps are irreversible, occur spontaneously at constant temperature and magnetic field, but can be restored by cycling across the first-order phase transition.",1507.08655v1 2015-07-31,Topological spin transport by Brownian diffusion of domain walls,"We propose thermally-populated domain walls (DWs) in an easy-plane ferromagnetic insulator as robust spin carriers between two metals. The chirality of a DW, which serves as a topological charge, couples to the metal spin accumulation via spin-transfer torque and results in the chirality-dependent thermal nucleation rates of DWs at the interface. After overpopulated DWs of a particular (net) chirality diffuse and leave the ferromagnet at the other interface, they reemit the spin current by spin pumping. The conservation of the topological charge supports an algebraic decay of spin transport as the length of the ferromagnet increases; this is analogous to the decaying behavior of superfluid spin transport but contrasts with the exponential decay of magnon spin transport. We envision that similar spin transport with algebraic decay may be implemented in materials with exotic spin phases by exploiting topological characteristics and the associated conserved quantities of their excitations.",1507.08767v2 2015-08-01,Defect induced d0 ferromagnetism in a ZnO grain boundary,"Several experimental studies have referred to the grain boundary (GB) defect as the origin of the ferromagnetism in ZnO. However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnO GB using the GGA+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. Unlike O vacancy (VO), Zn vacancy (VZn) is preferentially aggregated at the GB, relative to the bulk-like region, indicating the possibility of obtaining p-type conductivity in polycrystalline ZnO. Although no magnetization is obtained from point defect-free GB, VZn induces spin polarization as large as 0.68 {\mu}B/atom to the O sites at the GB. Ferromagnetic exchange energy > 150 eV is obtained by increasing the concentration of VZn and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d0 semiconductors.",1508.00100v1 2015-08-02,Magnetism and electronic structure of YTiO$_3$ thin films,"High-quality (001)-oriented (pseudo-cubic notation) ferromagnetic YTiO$_3$ thin films were epitaxially synthesized in a layer-by-layer way by pulsed laser deposition. Structural, magnetic and electronic properties were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, vibrating sample magnetometry, and element-resolved resonant soft X-ray absorption spectroscopy. To reveal ferromagnetism of the constituent titanium ions, X-ray magnetic circular dichroism spectroscopy was carried out using four detection modes probing complimentary spatial scale, which overcomes a challenge of probing ferromagnetic titanium with pure Ti3+(3d$^1$). Our work provides a pathway to distinguish between the roles of titanium and A-site magnetic rare-earth cations in determining the magnetism in rare-earth titanates thin films and heterostructures.",1508.00198v2 2015-08-04,Phase diagram of a frustrated asymmetric ferromagnetic spin ladder,"We perform a systematic investigation on the ground state of an asymmetric two-leg spin ladder (where exchange couplings of the legs are unequal) with ferromagnetic (FM) nearest-neighbor interaction and diagonal anti-ferromagnetic frustration using the Density Matrix Renormalization Group (DMRG) method. When the ladder is strongly asymmetric with moderate frustration, a magnetic canted state is observed between a FM state and a singlet dimerized state. The phase boundaries are dependent on the asymmetric strength $\alpha_{a}$. On the other hand, when the asymmetric strength is intermediate, a so-called spin-stripe state (spins align parallel on same legs, but antiparallel on rungs) is discovered, and the system experiences a first-order phase transition from the FM state to the spin-stripe state upon increasing frustration. We present numerical evidence to interpret the phase diagram in terms of frustration and the asymmetric strength.",1508.00723v2 2015-08-06,Interface-driven spin-torque ferromagnetic resonance by Rashba coupling at the interface between non-magnetic materials,"The Rashba-Edelstein effect stems from the interaction between the electron's spin and its momentum induced by spin-orbit interaction at an interface or a surface. It was shown that the inverse Rashba-Edelstein effect can be used to convert a spin- into a charge current. Here, we demonstrate that a Bi/Ag Rashba interface can even drive an adjacent ferromagnet to resonance. We employ a spin-torque ferromagnetic resonance excitation/detection scheme which was developed originally for a bulk spin-orbital effect, the spin Hall effect. In our experiment, the direct Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, Ni80Fe20) layer. Electrical detection of the magnetization dynamics is achieved by a rectification mechanism of the time dependent multilayer resistance arising from the anisotropic magnetoresistance.",1508.01410v1 2015-08-07,Ferromagnetic properties of SrRuO3 thin films deposited on the spin-triplet superconductor Sr2RuO4,"We report magnetic properties of epitaxial thin films of the itinerant ferromagnet SrRuO3 deposited on the cleaved ab surface of the spin-triplet superconductor Sr2RuO4. The films exhibit ferromagnetic transition near 160 K as in the bulk SrRuO3, although the films are under 1.7% compressive strain. The observed magnetization is even higher than that of the bulk SrRuO3. In addition, we newly found that the magnetization relaxation after field removal is strongly anisotropic: two relaxation processes are involved when magnetic domains are aligned along the ab-plane.",1508.01628v1 2015-06-03,Stable room-temperature ferromagnetic phase at the FeRh(100) surface,"Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at $\sim$400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1\%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. We thus reveal that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that fully support the experimental findings.",1508.01777v1 2015-08-09,Magnon-Magnon Interactions in O(3) Ferromagnets and Equations of Motion for Spin Operators,"The method of equations of motion for spin operators in the case of O(3) Heisenberg ferromagnet is systematically analyzed starting from the effective Lagrangian. It is shown that the random phase approximation and the Callen approximation can be understood in terms of perturbation theory for type B magnons. Also, the second order approximation of Kondo and Yamaji for one dimensional ferromagnet is reduced to the perturbation theory for type A magnons. An emphasis is put on the physical picture, i.e. on magnon-magnon interactions and symmetries of the Heisenberg model. Calculations demonstrate that all three approximations differ in manner in which the magnon-magnon interactions arising from the Wess-Zumino term are treated, from where specific features and limitations of each of them can be deduced.",1508.01974v1 2015-08-10,The role of dimensionality in the Kondo Ce$TX_{2}$ family: the case of CeCd$_{1-δ}$Sb$_{2}$,"Motivated by the presence of competing magnetic interactions in the heavy fermion family Ce$TX_2$ ($T$ = transition metal, $X$ = pnictogen), here we study the novel parent compound CeCd$_{1-\delta}$Sb$_{2}$ by combining magnetization, electrical resistivity, and heat-capacity measurements. Contrary to the antiferromagnetic (AFM) ground state observed in most members of this family, the magnetic properties of our CeCd$_{1-\delta}$Sb$_{2}$ single crystals revealed a ferromagnetic (FM) ordering at $T_{\rm c}$ = 3 K with an unusual soft behavior. By using a mean field model including anisotropic nearest-neighbors interactions and the tetragonal crystalline electric field (CEF) Hamiltonian, a systematic analysis of our macroscopic data was obtained. Our fits allowed us to extract a simple but very distinct CEF scheme, as compared to the AFM counterparts. As in the previously studied ferromagnet CeAgSb$_{2}$, a pure $|\pm 1/2 \rangle$ ground state is realized, hinting at a general trend within the ferromagnetic members. We propose a general scenario for the understanding of the magnetism in this family of compounds based on the subtle changes of dimensionality in the crystal structure.",1508.02116v1 2015-08-17,Universality of the mean-field for the Potts model,"We consider the Potts model with $q$ colors on a sequence of weighted graphs with adjacency matrices $A_n$, allowing for both positive and negative weights. Under a mild regularity condition the mean-field prediction for the log partition function of the Potts model on a sequence of matrices $A_n$ is asymptotically correct, whenever $\text{tr}(A_n^2)=o(n)$. In particular, our results are applicable for the Ising and the Potts models on any sequence of graphs with average degree going to $+\infty$. Using this, we establish the universality of the limiting log partition function of the ferromagnetic Potts model for a sequence of asymptotically regular graphs, and that of the Ising model for bi-regular bipartite graphs in both ferromagnetic and anti-ferromagnetic domain. We also derive a large deviation principle for the empirical measure of the colors for the Potts model on asymptotically regular graphs.",1508.03949v2 2015-08-24,Magneto-Electric Effect for Multiferroic Thin Film by Monte Carlo Simulation,"Magneto-electric effect in a multiferroic heterostructure film, i.e. a coupled ferromagnetic-ferroelectric thin film, has been investigated through the use of the Metropolis algorithm in Monte Carlo simulations. A classical Heisenberg model describes the energy stored in the ferromagnetic film, and we use a pseudo-spin model with a transverse Ising Hamiltonian to characterise the energy of electric dipoles in the ferroelectric film. The purpose of this article is to demonstrate the dynamic response of polarisation is driven by an external magnetic field, when there is a linear magneto-electric coupling at the interface between the ferromagnetic and ferroelectric components.",1508.05701v2 2015-08-27,Magnetic structure and phase stability of the van der Waals bonded ferromagnet Fe3-xGeTe2,"The magnetic structure and phase diagram of the layered ferromagnetic compound Fe$_3$GeTe$_2$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single crystal neutron diffraction finds ferromagnetic order with moments of 1.11(5)$\mu_B$/Fe aligned along the $c$-axis at 4K. These flux-grown crystals have a lower Curie temperature $T_{\textrm{c}}\approx$150K compared to crystals previously grown by vapor transport ($T_{\textrm{c}}$=220K). The difference is a reduced Fe content in the flux grown crystals, as illustrated by the behavior observed in a series of polycrystalline samples. As Fe-content decreases, so does the Curie temperature, magnetic anisotropy, and net magnetization. In addition, Hall effect and thermoelectric measurements on flux-grown crystals suggest multiple carrier types contribute to electrical transport in Fe$_{3-x}$GeTe$_2$ and structurally-similar Ni$_{3-x}$GeTe$_2$.",1508.06959v3 2015-09-04,Stoner Magnetism in an Inversion Layer,"Motivated by recent experimental work on magnetic properties of Si-MOSFETs, we report a calculation of magnetisation and susceptibility of electrons in an inversion layer, taking into account the co-ordinate dependence of electron wave function in the direction perpendicular to the plane. It is assumed that the inversion-layer carriers interact via a contact repulsive potential, which is treated at a mean-field level, resulting in a self-consistent change of profile of the wave functions. We find that the results differ significantly from those obtained in the pure 2DEG case (where no provision is made for a quantum motion in the transverse direction). Specifically, the critical value of interaction needed to attain the ferromagnetic (Stoner) instability is decreased and the Stoner criterion is therefore relaxed. This leads to an increased susceptibility and ultimately to a ferromagnetic transition deep in the high-density metallic regime. In the opposite limit of low carrier densities, a phenomenological treatment of the in-plane correlation effects suggests a ferromagnetic instability above the metal-insulator transition. Results are discussed in the context of the available experimental data.",1509.01320v1 2015-09-07,Spin-polarized edge currents and Majorana fermions in one- and two-dimensional topological superconductors,"We investigate the persistent currents, spin-polarized local density of states, and spectral functions of topological superconductors constructed by placing ferromagnetic impurities on top of an s-wave superconductor with Rashba spin-orbit interaction. We solve self-consistently for the superconducting order parameter and investigate both two-dimensional blocks and one-dimensional wires of ferromagnetic impurities, with the magnetic moments pointing both perpendicular and parallel to the surface. We find that the topologically protected edge states of ferromagnetic blocks give rise to spin-polarized edge currents, but that the total persistent current flows in opposite direction to what is expected from the dispersion relation of the edge states. We also show that the Majorana fermions at the end points of one-dimensional wires are spin-polarized, which can be directly related to the spin-polarization of the edge currents of two-dimensional blocks. Connections are also made to the physics of the Yu-Shiba-Rusinov states for zero-dimensional impurities.",1509.02080v1 2015-09-08,Magnetic and structural properties of ferromagnetic Fe5PB2 and Fe5SiB2 and effects of Co and Mn substitutions,"Crystallographic and magnetic properties of Fe5PB2, Fe4CoPB2, Fe4MnPB2, Fe5SiB2, Fe4CoSiB2, and Fe4MnSiB2 are reported. All adopt the tetragonal Cr5B3 structure-type and are ferromagnetic at room temperature with easy axis of magnetization along the c-axis. The spin reorientation in Fe5SiB2 is observed as an anomaly in the magnetization near 170 K, and is suppressed by substitution of Co or Mn for Fe. The silicides are found to generally have larger magnetic moments than the phosphides, but the data suggests smaller magnetic anisotropy in the silicides. Cobalt substitution reduces the Curie temperatures by more than 100 K and ordered magnetic moments by 16-20%, while manganese substitution has a much smaller effect. This suggests Mn moments align ferromagnetically with the Fe and that Co does not have an ordered moment in these structures. Anisotropic thermal expansion is observed in Fe5PB2 and Fe5SiB2, with negative thermal expansion seen along the c-axis of Fe5SiB2. First principles calculations of the magnetic properties of Fe5SiB2 and Fe4MnSiB2 are reported. The results, including the magnetic moment and anisotropy, and are in good agreement with experiment.",1509.02544v1 2015-09-14,Ground-state phases of rung-alternated spin-1/2 Heisenberg ladder,"The ground-state phase diagram of Heisenberg spin-1/2 system on a two-leg ladder with rung alternation is studied by combining analytical approaches with numerical simulations. For the case of ferromagnetic leg exchanges a unique ferrimagnetic ground state emerges, whereas for the case of antiferromagnetic leg exchanges several different ground states are stabilized depending on the ratio between exchanges along legs and rungs. For the more general case of a honeycomb-ladder model for the case of ferromagnetic leg exchanges besides usual rung-singlet and saturated ferromagnetic states we obtain a ferrimagnetic Luttinger liquid phase with both linear and quadratic low energy dispersions and ground state magnetization continuously changing with system parameters. For the case of antiferromagnetic exchanges along legs, different dimerized states including states with additional topological order are suggested to be realized.",1509.04062v1 2015-09-15,Dynamical current-induced ferromagnetic and antiferromagnetic resonances,"We demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect. Due to the spin-orbit interaction, a time-dependent spin density is generated by an oscillatory electric field applied parallel to the atomic planes of Fe/W(110) multilayers. For symmetric trilayers of Fe/W/Fe in which the Fe layers are ferromagnetically coupled, we demonstrate that only the collective out-of-phase precession mode is excited, while the uniform (in-phase) mode remains silent. When they are antiferromagnetically coupled, the oscillatory electric field sets the Fe magnetizations into elliptical precession motions with opposite angular velocities. The manipulation of different collective spin-wave dynamical modes through the engineering of the multilayers and their thicknesses may be used to develop ultrafast spintronics devices. Our work provides a general framework that probes the realistic responses of materials in the time or frequency domain.",1509.04599v2 2015-09-16,Fano resonance in a normal metal/ferromagnet-quantum dot-superconductor device,"We investigate theoretically the Andreev transport through a quantum dot strongly coupled with a normal metal/ferromagnet and a superconductor (N/F-QD-S), in which the interplay between the Kondo resonance and the Andreev bound states (ABSs) has not been clearly clarified yet. Here we show that the interference between the Kondo resonance and the ABSs modifies seriously the lineshape of the Kondo resonance, which manifests as a Fano resonance. The ferromagnetic lead with spin-polarization induces an effective field, which leads to splitting both of the Kondo resonance and the ABSs. The spin-polarization together with the magnetic field applied provides an alternative way to tune the lineshape of the Kondo resonances, which is dependent of the relative positions of the Kondo resonance and of the ABSs. These results indicate that the interplay between the Kondo resonance and the ABSs can significantly affect the Andreev transport, which could be tested by experiments.",1509.04801v1 2015-09-16,Microwave excitation of spin wave beams in thin ferromagnetic films,"We present an approach enabling generation of narrow spin wave beams in thin homogeneous ferromagnetic films. The main idea is to match the wave vector of the spin wave with that corresponding to the spectral maximum of the exciting microwave magnetic field only locally, in the region of space from which the beam should be launched. We show that this can be achieved with the aid of a properly designed coplanar waveguide transducer generating a nonuniform microwave magnetic field. The resulting two-dimensional spin wave beams obtained in micromagnetic simulations propagate over distances of several micrometers. The proposed approach requires neither inhomogeneity of the ferromagnetic film nor nonuniformity of the biasing magnetic field, and it can be generalized to yield multiple spin wave beams of different width at the same frequency. Other possible excitation scenarios and applications of spin wave beam magnonics are also discussed.",1509.05061v2 2015-09-18,FPTAS for Hardcore and Ising Models on Hypergraphs,"Hardcore and Ising models are two most important families of two state spin systems in statistic physics. Partition function of spin systems is the center concept in statistic physics which connects microscopic particles and their interactions with their macroscopic and statistical properties of materials such as energy, entropy, ferromagnetism, etc. If each local interaction of the system involves only two particles, the system can be described by a graph. In this case, fully polynomial-time approximation scheme (FPTAS) for computing the partition function of both hardcore and anti-ferromagnetic Ising model was designed up to the uniqueness condition of the system. These result are the best possible since approximately computing the partition function beyond this threshold is NP-hard. In this paper, we generalize these results to general physics systems, where each local interaction may involves multiple particles. Such systems are described by hypergraphs. For hardcore model, we also provide FPTAS up to the uniqueness condition, and for anti-ferromagnetic Ising model, we obtain FPTAS where a slightly stronger condition holds.",1509.05494v1 2015-09-26,Spin relaxation near a ferromagnetic transition,"We study spin relaxation in dilute magnetic semiconductors near a ferromagnetic transition, where spin fluctuations become strong. An enhancement in the scattering rate of itinerant carriers from the spin fluctuations of localized impurities leads to a change in the dominant spin relaxation mechanism from Dyakonov-Perel to spin flips in scattering. On the ferromagnetic side of the transition, we show that due to the presence of two magnetic components -- the itinerant carriers and the magnetic impurities -- with different gyromagnetic ratios, the relaxation rate of the total magnetization can be quite different from the relaxation rate of the spin. Following a disturbance of the equilibrium magnetization, the spin is initially redistributed between the two components to restore the equilibrium magnetization. It is only on a longer time scale, controlled by the spin-orbit interaction, that the total spin itself relaxes to its equilibrium state.",1509.07926v1 2015-09-29,Field-free magnetization reversal by spin-Hall effect and exchange bias,"Magnetic random-access memory (MRAM) driven by spin-transfer torque (STT) is a major contender for future memory applications. The energy dissipation involved in writing remains problematic, even with the advent of more efficient perpendicular magnetic anisotropy (PMA) devices. A promising alternative switching mechanism employs spin-orbit torques and the spin-Hall effect (SHE) in particular, but additional symmetry breaking is required to achieve deterministic switching in PMA devices. Currently used methods rely on in-plane magnetic fields or anisotropy gradients, which are not suitable for practical applications. Here, we interface the magnetic layer with an anti-ferromagnetic material. An in-plane exchange bias (EB) is created, and shown to enable field-free SHE-driven magnetization reversal of a perpendicularly magnetized Pt/Co/IrMn structure. Aside from the potential technological implications, our experiment provides additional insight into the local spin structure at the ferromagnetic/anti-ferromagnetic interface.",1509.08752v2 2015-10-07,Controllable intrinsic DC spin/valley Hall conductivity in ferromagnetic silicene: Exploring a fully spin/valley polarized transport,"We study intrinsic DC spin and valley Hall conductivity in doped ferromagnetic silicene in the presence of an electric filed applied perpendicular to silicene sheet. By calculating its energy spectrum and wavefunction and by making use of Kubo formalism, we obtain a general relation for the transverse Hall conductivity which can be used to obtain spin- and valley-Hall conductivity. Our results, in the zero limit of the exchange field, reduces to the previous results. Furthermore we discuss electrically tunable spin and valley polarized transport in ferromagnetic silicene and obtain the necessary conditions for observing a fully spin or valley polarized transport.",1510.02109v1 2015-10-13,Transportation of Static Magnetic Fields by a practically realizable Magnetic Hose,"A practically realizable magnetic hose, constructed by wrapping a ferromagnetic cylinder with alternate superconductor-ferromagnet heterostructure, was developed and its capability to transfer the static magnetic fields, e.g., generated by an Nd-Fe-B magnet, was examined in this letter. A diverse dependence of the transfer efficiency on the diameter of the inner cylinder was found in the magnetic hose demonstrators and the underlying cause was clarified by the finite-element simulations. Transfer efficiency of over 50% in terms of a moderate field has been achieved in the best demonstrator of this study, even with a thin sheet merely having moderate magnetism to embody the ferromagnet in the heterostructure. This work links the theoretically derived model with a physical reality and may also conceive fantastic solutions to form a magnetic circuit with minimum leakage or to create a magnetically shielded space, both of which are deemed promising in most electromagnetic devices.",1510.03559v1 2015-10-15,Disorder-induced stabilization of the quantum Hall ferromagnet,"We report on an absolute measurement of the electronic spin polarization of the $\nu=1$ integer quantum Hall state. The spin polarization is extracted in the vicinity of $\nu=1$ (including at exactly $\nu=1$) via resistive NMR experiments performed at different magnetic fields (electron densities), and Zeeman energy configurations. At the lowest magnetic fields, the polarization is found to be complete in a narrow region around $\nu=1$. Increasing the magnetic field (electron density) induces a significant depolarization of the system, which we attribute to a transition between the quantum Hall ferromagnet and the Skyrmion glass phase theoretically expected as the ratio between Coulomb interactions and disorder is increased. These observations account for the fragility of the polarization previously observed in high mobility 2D electron gas, and experimentally demonstrate the existence of an optimal amount of disorder to stabilize the ferromagnetic state.",1510.04448v1 2015-10-16,Adjacent Fe-Vacancy Interactions as the Origin of Room Temperature Ferromagnetism in (In$_{1-x}$Fe$_x$)$_2$O$_3$,"Dilute magnetic semiconductors (DMSs) show great promise for applications in spin-based electronics, but in most cases continue to elude explanations of their magnetic behavior. Here, we combine quantitative x-ray spectroscopy and Anderson impurity model calculations to study ferromagnetic Fe-substituted In$_2$O$_3$ films, and we identify a subset of Fe atoms adjacent to oxygen vacancies in the crystal lattice which are responsible for the observed room temperature ferromagnetism. Using resonant inelastic x-ray scattering, we map out the near gap electronic structure and provide further support for this conclusion. Serving as a concrete verification of recent theoretical results and indirect experimental evidence, these results solidify the role of impurity-vacancy coupling in oxide-based DMSs.",1510.05030v1 2015-10-19,Critical behavior of a triangular lattice Ising AF/FM bilayer,"We study a bilayer Ising spin system consisting of antiferromagnetic (AF) and ferromagnetic (FM) triangular planes, coupled by ferromagnetic exchange interaction, by standard Monte Carlo and parallel tempering methods. The AF/FM bilayer is found to display the critical behavior completely different from both the single FM and AF constituents as well as the FM/FM and AF/AF bilayers. Namely, by finite-size scaling (FSS) analysis we identify at the same temperature a standard Ising transition from the paramagnetic to FM state in the FM plane that induces a ferrimagnetic state with a finite net magnetic moment in the AF plane. At lower temperatures there is another phase transition, that takes place only in the AF plane, to different ferrimagnetic state with spins on two sublattices pointing parallel and on one sublattice antiparallel to the spins on the FM plane. FSS indicates that the corresponding critical exponents are close to the two-dimensional three-state ferromagnetic Potts model values.",1510.05383v1 2015-10-20,Evaluation of Spin Waves and Ferromagnetic Resonance Contribution to the Spin Pumping in Ta/CoFeB Structure,"The spin waves and ferromagnetic resonance (FMR) contribution to the spin pumping signal is studied in the Ta/CoFeB interface under different excitation bias fields. Ferromagnetic resonance is excited utilizing a coplanar waveguide and a microwave generator. Using a narrow waveguide of about 3 {\mu}m, magnetostatic surface spin waves with large wavevector (k) of about 0.81 {\mu}m^-1 are excited. A large k value results in dissociation of spin waves and FMR frequencies according to the surface spin wave dispersion relation. Spin waves and FMR contribution to the spin pumping are calculated based on the area under the Lorentzian curve fitting over experimental results. It is found that the FMR over spin waves contribution is about 1 at large bias fields in Ta/CoFeB structure. Based on our spin pumping results, we propose a method to characterize the spin wave decay constant which is found to be about 5.5 {\mu}m in the Ta/CoFeB structure at a bias field of 600 Oe.",1510.05745v1 2015-10-21,"Equilibrium, Metastability, and Hysteresis in a Model Spin-crossover Material with Nearest-neighbor Antiferromagnetic-like and Long-range Ferromagnetic-like Interactions","Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical endpoints and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.",1510.06357v2 2015-10-23,Goldstone mode stochastization in quantum Hall ferromagnet,"Experimental and theoretical studies of the coherent spin dynamics of two-dimensional GaAs/AlGaAs electron gas were performed. The system in the quantum Hall ferromagnet state exhibits a spin relaxation mechanism that is determined by many-particle Coulomb interactions. In addition to the spin exciton with changes in the spin quantum numbers of $\delta S\!=\!\delta S_z \!=\!-1$, the quantum Hall ferromagnet supports a Goldstone spin exciton that changes the spin quantum numbers to $\delta S\!=\!0$ and $\delta S_z\!=\!-1$, which corresponds to a coherent spin rotation of the entire electron system to a certain angle. The Goldstone spin exciton decays through a specific relaxation mechanism that is unlike any other collective spin state.",1510.06900v1 2015-10-23,Mapping the domain wall pinning landscape in ferromagnetic films,"The propagation of domain walls in a ferromagnetic film is largely determined by domain wall pinning at defects in the material. In this letter we map the effective potential landscape for domain wall pinning in Permalloy films by raster scanning a single ferromagnetic vortex and monitoring the hysteretic vortex displacement vs. applied magnetic field. The measurement is carried out using a differential magneto-optical microscopy technique which yields spatial sensitivity $\sim 10$ nm. We present a simple algorithm for extracting an effective pinning potential from the measurement of vortex displacement vs. applied field. The resulting maps of the pinning potential reveal distinct types of pinning sites, which we attribute to quasi-zero-, one-, and two-dimensional defects in the Permalloy film.",1510.07059v2 2015-10-23,"Fast, nanoscale addressability of nitrogen-vacancy spins via coupling to a dynamic ferromagnetic vortex","The core of a ferromagnetic vortex domain creates a strong, localized magnetic field which can be manipulated on nanosecond timescales, providing a platform for addressing and controlling individual nitrogen-vacancy center spins in diamond at room temperature, with nanometer-scale resolution. First, we show that the ferromagnetic vortex can be driven into proximity with a nitrogen-vacancy defect using small applied magnetic fields, inducing significant nitrogen-vacancy spin splitting. Second, we find that the magnetic field gradient produced by the vortex is sufficient to address spins separated by nanometer length scales. By applying a microwave-frequency magnetic field, we drive both the vortex and the nitrogen-vacancy spins, resulting in enhanced coherent rotation of the spin state. Finally we demonstrate that by driving the vortex on fast timescales, sequential addressing and coherent manipulation of spins is possible on $\sim100$ ns timescales.",1510.07073v2 2015-10-26,Spatial evolution of the ferromagnetic phase transition in an exchange graded film,"A combination of experiments and numerical modeling was used to study the spatial evolution of the ferromagnetic phase transition in a thin film engineered to have a smooth gradient in exchange strength. Mean-field simulations predict, and experiments confirm that a 100 nm Ni[x]Cu[1-x] alloy film with Ni concentration that varies by 9 % as a function of depth behaves predominantly as if comprised of a continuum of uncoupled ferromagnetic layers with continuously varying Curie temperatures. A mobile boundary separating ordered and disordered regions emerges as temperature is increased. We demonstrate continuous control of the boundary position with temperature, and reversible control of the magnetically ordered sample volume with magnetic field.",1510.07535v1 2015-10-26,A Panoply of Orders from a Quantum Lifshitz Field Theory,"We propose a universal non-linear sigma model field theory for one dimensional frustrated ferromagnets, which applies in the vicinity of a ""quantum Lifshitz point"", at which the ferromagnetic state develops a spin wave instability. We investigate the phase diagram resulting from perturbations of the exchange and of magnetic field away from the Lifshitz point, and uncover a rich structure with two distinct regimes of different properties, depending upon the value of a marginal, dimensionless, parameter of the theory. In the regime relevant for one dimensional systems with low spin, we find a metamagnetic transition line to a vector chiral phase. This line terminates in a critical endpoint from which emerges a cascade of multipolar phases. We show that the field theory has the property of ""asymptotic solubility"", so that a particular saddle point approximation becomes asymptotically exact near the Lifshitz point. Our results provide an analytic framework for prior numerical results on frustrated ferromagnets, and can be applied much more broadly.",1510.07640v1 2015-10-27,Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures,"We investigate spin transport through ferromagnetic graphene vertical heterostructures where a sandwiched tunneling layer is either a normal or ferroelectric insulator. We show that the spin-polarization of the tunneling current is electronically controlled via gate voltages. We also demonstrate that the tunneling current of Dirac fermions can be prohibited when the spin configuration of ferromagnetic graphene sheets is opposite. The giant electroresistance can thus be developed by using the proposed heterostructure in this study. The effects of temperature on the spin transport and the giant electroresistance ratio are also investigated. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures through an electric fields via gate and bias electrodes.",1510.07858v1 2015-10-28,Spin-$1$ $J_1-J_2-J_3$ ferromagnetic Heisenberg model with an easy-plane crystal field on the cubic lattice: A bosonic approach,"We examine the phase diagram of the spin-$1$ $J_1-J_2-J_3$ ferromagnetic Heisenberg model with an easy-plane crystal field on the cubic lattice, in which $J_1$ is the ferromagnetic exchange interaction between nearest neighbors, $J_2$ is the antiferromagnetic exchange interaction between next-nearest neighbors and $J_3$ is the antiferromagnetic exchange interaction between next-next-nearest neighbors. Using the bond-operator formalism, we investigate the phase transitions between the disordered paramagnetic phase and the ordered ones. We show that the nature of the quantum phase transitions change as the frustration parameters ($\frac{J_2}{J_1}$, $\frac{J_3}{J_1}$) are varied. The zero-temperature phase diagram exhibits second- and first-order transitions, depending on the energy gap behavior. Remarkably, we find a disordered nonmagnetic phase, even in the absence of a crystal field, which is suggested to be a quantum spin liquid candidate. We also depict the phase diagram at finite temperature for some values of crystal field and frustration parameters.",1510.08412v1 2015-10-20,Exact asymmetric Skyrmion in anisotropic ferromagnet and its helimagnetic application,"Topological Skyrmions as intricate spin textures were observed experimentally in helimagnets on 2d plane. Theoretical foundation of such solitonic states to appear in pure ferromagnetic model, as exact solutions expressed through any analytic function, was made long ago by Belavin and Polyakov (BP). We propose an innovative generalization of the BP solution for an anisotropic ferromagnet, based on a physically motivated geometric (in-)equality, which takes the exact Skyrmion to a new class of functions beyond analyticity. The possibility of stabilizing such metastable states in helimagnets is discussed with the construction of individual Skyrmion and Skyrmion crystal with asymmetry, likely to be detected in precision experiments.",1511.00712v2 2015-11-05,"Structural, Magnetic and Magneto-caloric studies of Ni50Mn30Sn20Shape Memory Alloy","We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc melting technique. Rietveld refinement confirms the austenite L21 structure in Fm-3m space group. Electrical resistivity has been found to clearly exhibiting two different phenomena viz. a magnetic transition from paramagnetic to ferromagnetic and a structural transition from austenite to martensitic phase. Thermo-magnetization measurements M(T) confirms ferromagnetic transition temperature TC at 222 K and martensitic transition starting at 127 K(MS). Magnetization measurement M(H) at 10 K confirms the ferromagnetic state. Frequency dependence of ac susceptibility \c{hi}' at low temperature suggests spin glass behavior in the system. The isothermal magnetic entropy change values have been found to be 1.14 J/Kg.K, 2.69 J/Kg.K and 3.9 J/Kg.K, with refrigeration capacities of 19.6 J/kg, 37.8 J/kg and 54.6 J/kg for the field change of 1, 2 and 3 Tesla respectively at 227 K.",1511.01632v1 2015-11-16,On the ferromagnetic ground state of SmN,"SmN is a ferromagnetic semiconductor with the unusual property of an orbital-dominant magnetic moment that is largely cancelled by an antiparallel spin contribution, resulting in a near-zero net moment. However, there is a basic gap in the understanding of the ferromagnetic ground state, with existing density functional theory calculations providing values of the $4f$ magnetic moments at odds with experimental data. To clarify the situation, we employ an effective $4f$ Hamiltonian incorporating spin-orbit coupling, exchange, the crystal field, and $J$-mixing to calculate the ground state $4f$ moments. Our results are in excellent agreement with experimental data, revealing moderate quenching of both spin and orbital moments to magnitudes of $\sim 2~\mu_B$ in bulk SmN, enhanced to an average of $\sim 3~\mu_B$ in SmN layers within a SmN/GdN superlattice. These calculations provide insight into recent studies of SmN showing that it is an unconventional superconductor at low temperatures and displays twisted magnetization phases in magnetic heterostructures.",1511.04820v1 2015-11-25,Ferromagnetic resonance phase imaging in spin Hall multilayers,"We experimentally image the magnetic precession phase of patterned spin Hall multilayer samples to study the rf driving field vector using time-resolved anomalous Nernst effect (TRANE) microscopy. Our ferromagnetic resonance (FMR) measurements quantify the phase and amplitude for both the magnetic precession and the electric current, which allows us to establish the total driving field orientation and the strength of spin Hall effect. In a channel of uniform width, we observe spatial variation of the FMR phase laterally across the channel. We interpret our findings in the context of electrical measurement using the spin-transfer torque ferromagnetic resonance technique and show that observed phase variation introduces a systematic correction into the spin Hall angle if spatial phase and amplitude variations are not taken into account.",1511.08126v1 2015-11-27,Resonance spin-charge phenomena and mechanism of magnetoresistance anisotropy in manganite/metal bilayer structures,"The dc voltage generated under ferromagnetic resonance has been studied in bilayer structures based on manganite thin epitaxial films La0.67Sr0.33MnO3 (LSMO) and non-magnetic metals (Au, Pt, and SrRuO3) in the temperature range up to the Curie point. The effect is shown to be caused by two different phenomena: (1) the resonance dc electromotive force related to anisotropic magnetoresistance (AMR) in the manganite film and (2) pure spin current (spin pumping) registered by means of the inverse spin Hall effect in normal metal. The two phenomena were separated using the angular dependence of the effect, the external magnetic field H0 being rotated in the film plane. It was found that the AMR mechanism in the manganite films differs substantially from that in traditional ferromagnetic metals being governed by the colossal magnetoresistance together with the in-plane magnetic anisotropy. The spin pumping effect registered in the bilayers was found to be much lower than that reported for common ferromagnets; possible reasons are discussed.",1511.08664v1 2015-11-28,Andreev reflection and subgap conductance in monolayer $MoS_2$ ferromagnet/$s$ and $d$-wave superconductor junction,"The accurate and proper form of electron-hole excitations and corresponding Dirac-like spinors of monolayer molybdenum disulfide superconductor are exactly obtained. Andreev reflection and resulting subgap conductance in a $MoS_2$-based ferromagnetic superconducting (F/S) junction is accurately investigated in terms of dynamical characteristics of system. Due to spin-splitting energy gap in the valence band and nondegenerate $K$ and $K'$ valleys, the ferromagnetic exchange energy $\sigma h$ can cause a distinct behavior of Andreev process between spin-up and spin-down charge carriers belonging to different valleys. The chemical potential is necessarily fixed by a determined range in order to occur the retro Andreev reflection. Given one-particle superconducting bispinors enable us to explicitly involve the anisotropic superconducting gap $\Delta_S$ under electron-hole conversion, i.e., taking place in $d$-wave pair coupling. The effect of such gap is exactly explained in terms of the dependence of the Andreev process on the electron incidence angle at the interface.",1511.08872v2 2015-11-30,Engineering quantum magnetism in one-dimensional trapped Fermi gases with p-wave interactions,"The highly controllable ultracold atoms in a one-dimensional (1D) trap provide a new platform for the ultimate simulation of quantum magnetism. In this regard, the Neel-antiferromagnetism and the itinerant ferromagnetism are of central importance and great interest. Here we show that these magnetic orders can be achieved in the strongly interacting spin-1/2 trapped Fermi gases with additional p-wave interactions. In this strong coupling limit, the 1D trapped Fermi gas exhibit an effective Heisenberg spin XXZ chain in the anisotropic p-wave scattering channels. For a particular p-wave attraction or repulsion within the same species of fermionic atoms, the system displays ferromagnetic domains with full spin segregation or the anti-ferromagnetic spin configuration in the ground state. Such engineered magnetisms are likely to be probed in a quasi-1D trapped Fermi gas of $^{40}$ K atoms with very close s-wave and p-wave Feshbach resonances.",1511.09377v4 2016-01-22,Long-range supercurrents induced by the interference effect of opposite-spin triplet state in clean superconductor-ferromagnet structures,"By now it is known that in an s-wave superconductor-ferromagnet-superconductor ($SFS$) structure the supercurrent induced by spin singlet pairs can only transmit a short distance of the order of magnetic coherence length. The long-range supercurrent, taking place on the length scale of the normal metal coherence length, will be maintained by equal-spin triplet pairs, which can be generated by magnetic inhomogeneities in the system. In this paper, we show an unusual long-range supercurrent, which can take place in clean $SF_1F_2S$ junction with non-parallel orientation of magnetic moments. The mechanism behind the enhancement of Josephson current is provided by the interference of the opposite-spin triplet states derived from $S/F_1$ and $F_2/S$ interfaces when both ferromagnetic layers have the same values of the length and exchange field. This finds can provide a natural explanation for recent experiment [Robinson et al., Phys. Rev. Lett. 104, 207001 (2010)].",1601.06045v1 2016-02-04,Magnetic anisotropy of the single-crystalline ferromagnetic insulator Cr2Ge2Te6,"Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, has attracted a great deal of interest recently owing to its potential for integration with Dirac materials to realize the quantum anomalous Hall effect (QAHE) and to develop novel spintronics devices. Here, we study the uniaxial magnetic anisotropy energy of single-crystalline CGT and determine that the magnetic easy axis is directed along the c-axis in its ferromagnetic phase. In addition, CGT is an insulator below the Curie temperature. These properties make CGT a potentially promising candidate substrate for integration with topological insulators for the realization of the high-temperature QAHE.",1602.01556v1 2016-02-05,A novel type of splayed ferromagnetic order observed in Yb2Ti2O7,"The pyrochlore insulator Yb2Ti2O7 has attracted the attention of experimentalists and theoreticians alike for about 15 years. Conflicting neutron diffraction data on the possible existence of magnetic Bragg reflections at low temperature have been published. Here we report the observation of magnetic Bragg reflections by neutron powder diffraction at 60 mK. The magnetic diffraction pattern is analyzed using representation theory. We find Yb2Ti2O7 to be a splayed ferromagnet as reported for Yb2Sn2O7, a sibling compound with also dominating ferromagnetic interactions as inferred from the positive Curie-Weiss temperature. However, the configuration of the magnetic moment components perpendicular to the easy axis is of the all-in--all-out type in Yb2Ti2O7 while it is two-in--two-out in Yb2Sn2O7. An overall experimental picture of the magnetic properties emerges.",1602.02025v2 2016-02-11,Exact diagonalization study of double quantum dots in parallel geometry in zero-bandwidth limit,"Exact eigenstates of the parallel coupled double quantum dots attached to the non-interacting leads taken in zero-bandwidth limit are analytically obtained in each particle and spin sector. The ground state of the half-filled system is identified from a four dimensional subspace of the twenty dimensional Hilbert space for different values of tunable parameters of the system viz. the energy levels of the quantum dots, the interdot tunneling matrix-element, the ondot and interdot Coulomb interactions and quantities like spin-spin correlation between the dots, occupancies of the dots are calculated. In the parameter space of the interdot tunneling matrix-element and ondot Coulomb interaction, the dots exhibit both ferromagnetic and antiferromagnetic correlation. There is a critical dependency of the interdot tunneling matrix-element on the ondot Coulomb interaction which leads to transition from the ferromagnetic correlation to the antiferromagnetic correlation as the interdot tunneling matrix-element is increased. The ferromagnetic and antiferromagnetic correlations also exist in the absence of interdot tunneling matrix-element through indirect exchange via the leads. The interdot Coulomb interaction is found to affect this dependency considerably.",1602.03900v1 2016-02-15,Ferromagnetic Cluster Glass Phase Embedded in a Paramagnetic and Metallic Host in Non-uniform Magnetic System CaRu_{1-x}Sc_{x}O_{3},"We have investigated both static and dynamic magnetic properties of polycrystalline CaRu$_{1-x}$Sc$_{x}$O$_{3}$ system in order to clarify the role of Sc ions as a disorder for magnetic ordering. We have observed typical features of a ferromagnetic cluster glass state below around 40 K: (i) a broad, frequency-dependent peak in the ac magnetic susceptibility, (ii) a slow relaxation of the magnetization, and (iii) a continuous increase in the dc magnetic susceptibility in field cooling process. The composition dependence of characteristic parameters for the cluster glass state suggests that chemical segregation can hardly explain the clustering mechanism. We propose a possible picture that the ferromagnetic clusters are distributed uniformly and form the glassy state embedded in the paramagnetic and metallic host of CaRuO$_{3}$.",1602.04647v1 2016-02-23,Ferromagnetism and superconductivity with possible $p+ip$ pairing symmetry in partially hydrogenated graphene,"By means of first-principles calculations, we predict two new types of partially hydrogenated graphene systems: C$_{6}$H$_{1}$ and C$_{6}$H$_{5}$, which are shown to be ferromagnetic (FM) semimetal and FM narrow-gap semiconductor, respectively. When properly doped, the Fermi surface of the two systems consists of an electron pocket or six hole patches in the first Brillouin zone with completely spin-polarized charge carries. If superconductivity exists in these systems, the stable pairing symmetries are shown to be $p+ip$ for electron doped case, and anisotropic $p+ip$ for hole doped case. The predicted systems may provide fascinating platforms for studying the novel properties of ferromagnetism and triplet-pairing superconductivity as well as two-dimensional spintronics.",1602.07329v3 2016-03-03,Direct penetration of spin-triplet superconductivity into a ferromagnet in Au/SrRuO3/Sr2RuO4 junctions,"Efforts have been ongoing to establish superconducting spintronics utilizing ferromagnet/superconductor heterostructures1. Previously reported devices are based on spin-singlet superconductors (SSCs), where the spin degree of freedom is lost. Spin-polarized supercurrent induction in ferromagnetic metals (FMs) is achieved even with SSCs, but only with the aid of interfacial complex magnetic structures, which severely affect information imprinted to the electron spin. Use of spin-triplet superconductors (TSCs) with active spins potentially overcomes this difficulty and further leads to novel functionalities. Here, we report spin-triplet superconductivity induction into a FM SrRuO3 from a leading TSC candidate Sr2RuO4, by fabricating microscopic devices using an epitaxial SrRuO3/Sr2RuO4 hybrid. The differential conductance, exhibiting Andreev-reflection features with multiple energy scales up to around half tesla, indicates the penetration of superconductivity over a considerable distance of 15 nm across the SrRuO3 layer without help of interfacial complex magnetism. This demonstrates the first FM/TSC device exhibiting the spin-triplet proximity effect.",1603.00971v2 2016-03-03,Observation of Quantum Griffiths Singularity and Ferromagnetism at Superconducting LaAlO3/SrTiO3(110) Interface,"Diverse phenomena emerge at the interface between band insulators LaAlO3 and SrTiO3, such as superconductivity and ferromagnetism, showing an opportunity for potential applications as well as bringing fundamental research interests. Particularly, the two-dimensional electron gas formed at LaAlO3/SrTiO3 interface offers an appealing platform for quantum phase transition from a superconductor to a weakly localized metal. Here we report the superconductor-metal transition in superconducting two-dimensional electron gas formed at LaAlO3/SrTiO3(110) interface driven by a perpendicular magnetic field. Interestingly, when approaching the quantum critical point, the dynamic critical exponent is not a constant but a diverging value, which is a direct evidence of quantum Griffiths singularity raised from quenched disorder at ultralow temperatures. Furthermore, the hysteretic property of magnetoresistance was firstly observed at LaAlO3/SrTiO3(110) interfaces, which suggests potential coexistence of superconductivity and ferromagnetism.",1603.01011v2 2016-03-07,Muon Spin Relaxation and Neutron Diffraction Studies of Cluster-Glass States in Sr1-xLaxRuO3,"To clarify the magnetic properties of cluster-glass states in Sr1-xLaxRuO3 (0.3 <= x <= 0.5), we report herein the results of muon spin relaxation (muSR) and neutron powder diffraction measurements. The muSR experiments showed that magnetic clusters start developing well above the peak temperature T^* in the ac susceptibility. The volume fraction of the magnetically ordered region increases continuously with decreasing temperature, showing no anomaly at T^*, and reaches nearly 100% at the lowest temperature. The temperature variation of the volume fraction is essentially independent of the La concentration in the x range presently investigated, although the dc magnetization is significantly suppressed with increasing x. Neutron powder diffraction experiments revealed that the ground state for x = 0.3 is a long-range ferromagnetic ordered state. These results indicate that, with decreasing temperature, cluster-glass states in Sr1-xLaxRuO3 gradually develop into long-range ferromagnetic ordered states with decreasing temperature, and that the magnetic ordering process differs strikingly from that expected for a conventional second-order ferromagnetic transition.",1603.01962v1 2016-03-11,Critical temperature of two-dimensional hydrogenated multilayer graphene-based diluted ferromagnet,"In the paper a theoretical study of critical (Curie) temperature of diluted ferromagnet based on multilayer graphene (or graphite) with hydrogen adatoms deposited over carbon atoms belonging to single sublattice is presented. The calculations are performed within Pair Approximation (PA) for diluted ferromagnetic system with long-range interactions. The method is able to take into account the spin-space anisotropy of coupling. The results obtained within Mean Field Approximation (MFA) are also presented for comparison. The assumed interaction between hydrogen adatom spins is inversely proportional to their mutual distance, with the additional exponential attenuation reflecting the presence of disorder in the system. The results obtained for a wide range of impurity concentrations and interaction decay length are discussed. The strongly non-linear behaviour of critical temperature as a function of dilution is predicted, at variance with MFA predictions. Moreover, MFA tends to overestimate heavily the critical temperature values compared to PA. An universal dependence of critical temperature on impurity concentration and interaction decay length is found for strong dilution regime.",1603.03810v3 2016-03-23,Thermoelectric power quantum oscillations in the ferromagnet UGe$_2$,"We present thermoelectric power and resistivity measurements in the ferromagnet UGe$_2$ as a function of temperature and magnetic field. At low temperature, huge quantum oscillations are observed in the thermoelectric power as a function of the magnetic field applied along the $a$ axis. The frequencies of the extreme orbits are determined and an analysis of the cyclotron masses is performed following different theoretical approaches for quantum oscillations detected in the thermoelectric power. They are compared to those obtained by Shubnikov-de Haas experiments on the same crystal and previous de Haas-van Alphen experiments. The agreement of the different probes confirms thermoelectric power as an excellent probe to extract simultaneously both microscopic and macroscopic information on the Fermi-surface properties. Band-structure calculations of UGe$_2$ in the ferromagnetic state are compared to the experiment.",1603.07121v1 2016-03-24,Voltage induced switching dynamics of a coupled spin pair in a molecular junction,"Molecular spintronics is made possible by the coupling between electronic configuration and magnetic po- larization of the molecules. For control and application of the individual molecular states it is necessary to both read and write their spin states. Conventionally, this is achieved by means of external magnetic fields or ferromagnetic contacts, which may change the intentional spin state and may present additional challenges when downsizing devices. Here, we predict that coupling magnetic molecules together opens up possibilities for all electrical control of both the molecular spin states as well as the current flow through the system. Tuning between the regimes of ferromagnetic and anti-ferromagnetic exchange interaction, the current can be, at least, an order of magnitude enhanced or reduced. The effect is susceptible to the tunnel coupling and molecular level alignment which can be used to achieve current rectification.",1603.07652v1 2016-03-24,Role of Berry phase theory for describing orbital magnetism: From magnetic heterostructures to topological orbital ferromagnets,"We address the importance of the modern theory of orbital magnetization for spintronics. Based on an all-electron first-principles approach, we demonstrate that the predictive power of the routinely employed ""atom-centered"" approximation is limited to materials like elemental bulk ferromagnets, while the application of the modern theory of orbital magnetization is crucial in chemically or structurally inhomogeneous systems such as magnetic thin films, and materials exhibiting non-trivial topology in reciprocal and real space,~e.g.,~Chern insulators or non-collinear systems. We find that the modern theory is particularly crucial for describing magnetism in a class of materials that we suggest here $-$ topological orbital ferromagnets.",1603.07683v2 2016-03-30,Coexistence of charge and ferromagnetic order in fcc Fe,"Phase coexistence phenomena have been intensively studied in strongly correlated materials where several ordered states simultaneously occur or compete. Material properties critically depend on external parameters and boundary conditions, where tiny changes result in qualitatively different ground states. However, up to date, phase coexistence phenomena have exclusively been reported for complex compounds composed of multiple elements. Here we show that charge- and magnetically ordered states coexist in double-layer Fe on Rh(001). Scanning tunneling microscopy and spectroscopy measurements reveal periodic charge order stripes below a temperature of 130 K. Close to liquid helium temperature, they are superimposed by ferromagnetic domains as observed by spin-polarized scanning tunneling microscopy. Temperature-dependent measurements reveal a pronounced cross-talk between charge and spin order at the ferromagnetic ordering temperature about 70 K, which is successfully modeled within an effective Landau theory including sixth-order terms. Our results show that subtle balance between structural modifications can lead to competing ordering phenomena.",1603.09100v1 2016-04-05,Micromagnetic simulation of exchange coupled ferri-/ferromagnetic composite in bit patterned media,"Ferri-/ferromagnetic exchange coupled composites are promising candidates for bit patterned media because of the ability to control the magnetic properties of the ferrimagnet by its composition. A micromagnetic model for the bilayer system is presented where we also incorporate the microstructural features of both layers. Micromagnetic finite element simulations are performed to investigate the magnetization reversal behaviour of such media. By adding the exchange coupled ferrimagnet to the ferromagnet, the switching field could be reduced by up to $40\,\%$ and also the switching field distribution is narrowed. To reach these significant improvements, an interface exchange coupling strength of $2\,\mathrm{mJ/m^2}$ is required.",1604.01180v1 2016-04-06,Temperature Dependence of Angular Momentum Transport Across Interfaces,"Angular momentum transport in magnetic multilayered structures plays a central role in spintronic physics and devices. The angular momentum currents or spin currents are carried by either quasi-particles such as electrons and magnons, or by macroscopic order parameters such as local magnetization of ferromagnets. Based on the generic interface exchange interaction, we develop a microscopic theory that describes interfacial spin conductance for various interfaces among non-magnetic metals, ferromagnetic and antiferromagnetic insulators. Spin conductance and its temperature dependence are obtained for different spin batteries including spin pumping, temperature gradient and spin Hall effect. As an application of our theory, we calculate the spin current in a trilayer made of a ferromagnetic insulator, an antiferromagnetic insulator and a non-magnetic heavy metal. The calculated results on the temperature dependence of spin conductance quantitatively agree with the existing experiments.",1604.01714v2 2016-04-07,Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions,"Thermoelectric effects result from the coupling of charge and heat transport, and can be used for thermometry, cooling and harvesting of thermal energy. The microscopic origin of thermoelectric effects is a broken electron-hole symmetry, which is usually quite small in metal structures, and vanishes at low temperatures. We report on a combined experimental and theoretical investigation of thermoelectric effects in superconductor/ferromagnet hybrid structures. We investigate the depencence of thermoelectric currents on the thermal excitation, as well as on the presence of a dc bias voltage across the junction. Large thermoelectric effects are observed in superconductor/ferromagnet and superconductor/normal-metal hybrid structures. The spin-independent signals observed under finite voltage bias are shown to be reciprocal to the physics of superconductor/normal-metal microrefrigerators. The spin-dependent thermoelectric signals in the linear regime are due to the coupling of spin and heat transport, and can be used to design more efficient refrigerators",1604.01940v1 2016-04-10,Interfacial Dzyaloshinskii-Moriya interaction in Pt/CoFeB films: effect of the heavy-metal thickness,"We report the observation of a Pt layer thickness dependence on the induced interfacial Dzyaloshinskii-Moriya interaction in ultra-thin Pt($d_{\text{Pt}}$)/CoFeB films. Taking advantage of the large spin-orbit coupling of the heavy metal, the interfacial Dzyaloshinskii-Moriya interaction is quantified by Brillouin light scattering measurements of the frequency non-reciprocity of spin-waves in the ferromagnet. The magnitude of the induced Dzyaloshinskii-Moriya coupling is found to saturate to a value $0.45$ mJ$/$m${}^2$ for Pt thicknesses larger than $\sim 2$ nm. The experimental results are explained by analytical calculations based on the 3-site indirect exchange mechanism that predicts a Dzyaloshinskii-Moriya interaction at the interface between a ferromagnetic thin layer and a heavy metal. Our findings open up a way to control and optimize chiral effects in ferromagnetic thin films through the thickness of the heavy metal layer.",1604.02626v2 2016-04-14,Quantum Nucleation of Skyrmions in Magnetic Films by Inhomogeneous Fields,"Recent experiments have reported on controlled nucleation of individual skyrmions in chiral magnets. Here we show that in magnetic ultra-thin films with interfacial Dzyaloshinskii-Moriya interaction, single skyrmions of different radii can be nucleated by creating a local distortion in the magnetic field. In our study, we have considered zero temperature quantum nucleation of a single skyrmion from a ferromagnetic phase. The physical scenario we model is one where a uniform field stabilizes the ferromagnet, and an opposing local magnetic field over a circular spot, generated by the tip of a local probe, drives the skyrmion nucleation. Using spin path integrals and a collective coordinate approximation, the tunneling rate from the ferromagnetic to the single skyrmion state is computed as a function of the tip's magnetic field and the circular spot radius. Suitable parameters for the observation of the quantum nucleation of single skyrmions are identified.",1604.04010v1 2016-04-20,Competition between Kondo effect and RKKY physics in graphene magnetism,"The cooperative behavior of quantum impurities on 2D materials, such as graphene and bilayer graphene, is characterized by a non-trivial competition between screening (Kondo effect), and Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetism. In addition, due to the small density of states at the Fermi level, impurities may not couple to the conduction electrons at all, behaving as free moments. Employing a recently developed {\em{exact}} numerical method to study multi-impurity lattice systems, we obtain non-perturbative results that dramatically depart from expectations based on the conventional RKKY theory. At half-filling and for weak coupling, impurities remain in the local moment regime when they are on opposite sublattices, up to a critical value of the interactions when they start coupling anti-ferromagnetically with correlations that decay very slowly with inter-impurity distance. At finite doping, away from half-filling, ferromagnetism is completely absent and the physics is dominated by a competition between anti-ferromagnetism and Kondo effect. In bilayer graphene, impurities on opposite layers behave as free moments, unless the interaction is of the order of the hopping or larger.",1604.06109v2 2016-04-26,Intrinsic spin orbit torque in a single domain nanomagnet,"We present theoretical studies of the intrinsic spin orbit torque (SOT) in a single domain ferromagnetic layer with Rashba spin-orbit coupling (SOC) using the non-equilibrium Green's function formalism for a model Hamiltonian. We find that, to the first order in SOC, the intrinsic SOT has only the field-like torque symmetry and can be interpreted as the longitudinal spin current induced by the charge current and Rashba field. We analyze the results in terms of the material related parameters of the electronic structure, such as band filling, band width, exchange splitting, as well as the Rashba SOC strength. On the basis of these numerical and analytical results, we discuss the magnitude and sign of SOT. Our results show that the different sign of SOT in identical ferromagnetic layers with different supporting layers, e.g. Co/Pt and Co/Ta, could be attributed to electrostatic doping of the ferromagnetic layer by the support.",1604.07885v2 2016-04-29,Current induced magnetization dynamics and magnetization switching in superconducting ferromagnetic hybrid (F$|$S$|$F) structures,"We investigate the current induced magnetization dynamics and magnetization switching in an unconventional p-wave superconductor sandwiched between two misaligned ferromagnetic layers by numerically solving Landau-Lifshitz-Gilbert equation modified with current induced Slonczewski's spin torque term. A modified form of Ginzburg-Landau free energy functional has been used for this purpose. We demonstrated the possibility of current induced magnetization switching in the spin-triplet ferromagnetic superconducting hybrid structures with strong easy axis anisotropy and the condition for magnetization reversal. The switching time for such arrangement is calculated and is found to be highly dependent on the magnetic configuration along with the biasing current. This study would be useful in designing practical superconducting-spintronic devices.",1604.08704v3 2016-04-29,Ferromagnetic Mott-Insulating State in Double Perovskites Gd2MgIrO6,"We have studied the electronic and magnetic properties of double perovskites Gd2MgIrO6 by first-principles density functional theory (DFT). Based on the DFT calculations, Gd2MgIrO6 is found to have a ferromagnetic (FM) ground state. The material undergo half-metallic ferromagnets to Mott-Hubbard insulator transition which happens due to strong correlation in Gd-4f and Ir-5d states. Our results shows that the 5d electrons of Ir hybridize strongly with O-2p states near the Fermi level giving rise to the insulating state of Gd2MgIrO6. Our study suggests that the enhanced magnetic moment is a result of itinerant exchange rather than the exchange interaction involving individual ions of Gd and Ir atoms. The total magnetic moment calculated in the present studies is 15 {\mu}B per formula unit for Gd2MgIrO6.",1604.08931v1 2016-06-06,Switching field distribution of exchange coupled ferri-/ferromagnetic composite bit patterned media,"We investigate the switching field distribution and the resulting bit error rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by micromagnetic simulations. Using islands with varying microstructure and anisotropic properties, the intrinsic switching field distribution is computed. The dipolar contribution to the switching field distribution is obtained separately by using a model of a triangular patterned island array resembling $1.4\,\mathrm{Tb/in}^2$ bit patterned media. Both contributions are computed for different thickness of the soft exchange coupled ferrimagnet and also for ferromagnetic single phase FePt islands. A bit patterned media with a bilayer structure of FeGd($5\,\mathrm{nm}$)/FePt($5\,\mathrm{nm}$) shows a bit error rate of $10^{-4}$ with a write field of $1.16\,\mathrm{T}$.",1606.01874v2 2016-07-08,Mott lobes of the $S=1$ Bose-Hubbard model with three-body interactions,"Using the density matrix renormalization group method, we studied the ground state of the one-dimensional $S=1$ Bose-Hubbard model with local three-body interactions, which can be a superfluid or a Mott insulator state. We drew the phase diagram of this model for both ferromagnetic and antiferromagnetic interaction. Regardless of the sign of the spin-dependent coupling, we obtained that the Mott lobes area decreases as the spin-dependent strength increases, which means that the even-odd asymmetry of the two-body antiferromagnetic chain is absent for local three-body interactions. For antiferromagnetic coupling, we found that the density drives first-order superfluid-Mott insulator transitions for even and odd lobes. Ferromagnetic Mott insulator and superfluid states were obtained with a ferromagnetic coupling, and a tendency to a ""long-range"" order was observed.",1607.02302v2 2016-07-08,Effect of demagnetization factor dependence on energy of ultra-thin ferromagnetic films with four layers,"Simple cubic and body centered cubic ferromagnetic lattices with four layers were studied using Heisenberg Hamiltonian. According to 3-D plots, the films with four layers can be easily oriented in certain directions under the influence of particular demagnetization factor and angles for both sc(001) and bcc(001) ferromagnetic lattice structures. A flat part can be seen in the middle of 3-D plots in addition to periodic variations. When the demagnetization factor is given by =6, sc(001) film with four layers can be easily oriented in 0.6 radians direction for the energy parameter values used in this simulation. Under the influence of demagnetization factor given by =5.2 , thin film of bcc(001) lattice with four layers can be easily oriented along 0.63 radians direction.",1607.02313v1 2016-07-12,Current-driven periodic domain wall creation in ferromagnetic nano-wires,"We predict the electrical generation and injection of domain walls into a ferromagnetic nano-wire without the need of an assisting magnetic field. Our analytical and numerical results show that above a critical current $j_{c}$ domain walls are injected into the nano-wire with a period $T \sim (j-j_{c})^{-1/2}$. Importantly, domain walls can be produced periodically even in a simple exchange ferromagnet with uniaxial anisotropy, without requiring any standard ""twisting"" interaction like Dzyaloshinskii-Moriya or dipole-dipole interactions. We show analytically that this process and the period exponents are universal and do not depend on the peculiarities of the microscopic Hamiltonian. Finally we give a specific proposal for an experimental realization.",1607.03336v1 2016-07-12,Effect of Quantum Tunneling on Spin Hall Magnetoresistance,"We present a formalism that simultaneously incorporates the effect of quantum tunneling and spin diffusion on spin Hall magnetoresistance observed in normal metal/ferromagnetic insulator bilayers (such as Pt/YIG) and normal metal/ferromagnetic metal bilayers (such as Pt/Co), in which the angle of magnetization influences the magnetoresistance of the normal metal. In the normal metal side the spin diffusion is known to affect the landscape of the spin accumulation caused by spin Hall effect and subsequently the magnetoresistance, while on the ferromagnet side the quantum tunneling effect is detrimental to the interface spin current which also affects the spin accumulation. The influence of generic material properties such as spin diffusion length, layer thickness, interface coupling, and insulating gap can be quantified in a unified manner, and experiments that reveal the quantum feature of the magnetoresistance are suggested.",1607.03409v1 2016-07-28,"Tunneling magnetoresistance in trilayer structures composed of group-IV ferromagnetic semiconductor Ge1-xFex, MgO, and Fe","Group-IV-based ferromagnetic semiconductor Ge1-xFex (GeFe) is one of the most promising materials for efficient spin injectors and detectors for Si and Ge. Recent first principles calculations (Sakamoto et al., Ref. 9) suggested that the Fermi level is located in two overlapping largely spin-polarized bands formed in the bandgap of GeFe; spin-down d(e) band and spin-up p-d(t2) band. Thus, it is important to clarify how these bands contribute to spin injection and detection. In this study, we show the first successful observation of the tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) containing a group-IV ferromagnetic semiconductor, that is, in MTJs composed of epitaxially grown Fe/MgO/Ge0.935Fe0.065. We find that the p-d(t2) band in GeFe is mainly responsible for the tunneling transport. Although the obtained TMR ratio is small (0.3%), the TMR ratio is expected to be enhanced by suppressing leak current through amorphous-like crystal domains observed in MgO.",1607.08298v1 2016-09-08,Evolution of an isolated monopole in a spin-1 Bose-Einstein condensate,"We simulate the decay dynamics of an isolated monopole defect in the nematic vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic phase transition of the system. Importantly, the decay of the monopole occurs in the absence of external magnetic fields and is driven principally by the dynamical instability due to the ferromagnetic spin-exchange interactions. An initial isolated monopole is observed to relax into a polar-core spin vortex, thus demonstrating the spontaneous transformation of a point defect of the polar order parameter manifold to a line defect of the ferromagnetic manifold. We also investigate the dynamics of an isolated monopole pierced by a quantum vortex line with winding number k. It is shown to decay into a coreless Anderson-Toulouse vortex if k = 1 and into a singular vortex with an empty core if k = 2. In both cases, the resulting vortex is also encircled by a polar-core vortex ring.",1609.02506v2 2016-09-09,Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces,"The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces.",1609.02905v1 2016-09-12,Estimation of spin relaxation lengths in spin valves of In and In2O3 nanostructures,"We report the electrical injection and detection of spin polarized current in lateral ferromagnet-nonmagnet-ferromagnet spin valve devices, ferromagnet being cobalt and nonmagnet being indium (In) or indium oxide (In2O3) nanostructures. The In nanostructures were grown by depositing pure In on lithographically pre-patterned structures. In2O3 nanostructures were obtained by oxidation of In nanostructures. Spin valve devices were fabricated by depositing micro magnets over the nanostructures with connecting nonmagnetic electrodes via two steps of e-beam lithography. Clear spin switching behavior was observed in the both types of spin valve devices measured at 10 K. From the measured spin signal, the spin relaxation length ({\lambda}N) of In and In2O3 nanostructures were estimated to be 449.6 nm and 788.6 nm respectively.",1609.03609v1 2016-09-15,Magnetoelectric Response in Multiferroic SrFe12O19 Ceramics,"We report here the realization of ferroelectricity, ferromagnetism and magnetocapacitance effect in SrFe12O19 ceramics at room temperature. The ceramics demonstrate a saturated polarization hysteresis loop, two I-V peaks and large anomaly of dielectric constant near Curie temperature. These evidences confirmed the ferroelectricity of SrFe12O19 ceramics after annealing in O2 atmosphere. The remnant polarization of the SrFe12O19 ceramic is 103 {\mu}C/cm2. The material also exhibits strong ferromagnetic characterization, the coercive field and remnant magnetic moment are 6192Oe and 35.8emu/g, respectively. Subsequent annealing SrFe12O19 ceramics in O2 not only reveals its innate ferroelectricity but also improves the ferromagnetic properties through transforming Fe2+ into Fe3+. By applying a magnetic field, the capacitance demonstrates remarkable change along with B field, the maximum relative change of dielectric constant is 1174%, which reflects a giant magnetocapacitance effect in SrFe12O19. These combined functional responses in SrFe12O19 ceramics opens substantial possibilities for applications in novel electric devices.",1609.04483v1 2016-09-20,Magnon nodal-line semimetals and drumhead surface states in anisotropic pyrochlore ferromagnets,"We introduce a new type of topological magnon matter: the magnonic pendant to electronic nodal-line semimetals. Magnon spectra of anisotropic pyrochlore ferromagnets feature twofold degeneracies of magnon bands along a closed loop in reciprocal space. These magnon nodal lines are topologically protected by the coexistence of inversion and time-reversal symmetry; they require the absence of spin-orbit interaction (no Dzyaloshinskii-Moriya interaction). We calculate the topological invariants of the nodal lines and show that details of the associated magnon drumhead surface states depend strongly on the termination of the surface. Magnon nodal-line semimetals complete the family of topological magnons in three-dimensional ferromagnetic materials.",1609.06131v1 2016-09-26,Ferromagnetic resonance study of composite Co/Ni - FeCoB free layers with perpendicular anisotropy,"We study the properties of composite free layers with perpendicular anisotropy. The free layers are made of a soft FeCoB layer ferromagnetically coupled by a variable spacer (Ta, W, Mo) to a very anisotropic [Co/Ni] multilayer embodied in a magnetic tunnel junction meant for spin torque memory applications. For this we use broadband ferromagnetic resonance to follow the field dependence of the acoustical and optical excitation of the composite free layer in both in-plane and out-of-plane applied fields. The modeling provides the interlayer exchange coupling, the anisotropies and the damping factors. The popular Ta spacer are outperformed by W and even more by Mo, which combines the strongest interlayer exchange coupling without sacrificing anisotropies, damping factors and transport properties.",1609.07863v1 2016-09-28,Ferromagnetism and conductivity in atomically thin SrRuO3,"Atomically thin ferromagnetic and conducting electron systems are highly desired for spintronics, because they can be controlled with both magnetic and electric fields. We present (SrRuO3)1-(SrTiO3)5 superlattices and single-unit-cell-thick SrRuO3 samples that are capped with SrTiO3. We achieve samples of exceptional quality. In these samples, the electron systems comprise only a single RuO2 plane. We observe conductivity down to 50 mK, a ferromagnetic state with a Curie temperature of 25 K, and signals of magnetism persisting up to approximately 100 K.",1609.08901v2 2016-10-02,Spin-transfer torque in ferromagnetic bilayers generated by anomalous Hall effect and anisotropic magnetoresistance,"We propose an experimental scheme to determine the spin-transfer torque efficiency excited by the spin-orbit interaction in ferromagnetic bilayers from the measurement of the longitudinal magnetoresistace. Solving a diffusive spin-transport theory with appropriate boundary conditions gives an analytical formula of the longitudinal charge current density. The longitudinal charge current has a term that is proportional to the square of the spin-transfer torque efficiency and that also depends on the ratio of the film thickness to the spin diffusion length of the ferromagnet. Extracting this contribution from measurements of the longitudinal resistivity as a function of the thickness can give the spin-transfer torque efficiency.",1610.00222v1 2016-10-03,Magnetic microscopy and simulation of strain-mediated control of magnetization in Ni/PMN-PT nanostructures,"Strain-mediated thin film multiferroics comprising piezoelectric/ferromagnetic heterostructures enable the electrical manipulation of magnetization with much greater efficiency than other methods; however, the investigation of nanostructures fabricated from these materials is limited. Here we characterize ferromagnetic Ni nanostructures grown on a ferroelectric PMN-PT substrate using scanning electron microscopy with polarization analysis (SEMPA) and micromagnetic simulations. The magnetization of the Ni nanostructures can be controlled with a combination of sample geometry and applied electric field, which strains the ferroelectric substrate and changes the magnetization via magnetoelastic coupling. We evaluate two types of simulations of ferromagnetic nanostructures on strained ferroelectric substrates: conventional micromagnetic simulations including a simple uniaxial strain, and coupled micromagnetic-elastodynamic simulations. Both simulations qualitatively capture the response of the magnetization changes produced by the applied strain, with the coupled solution providing more accurate representation.",1610.00746v1 2016-10-04,Resolving magnon number states in quantum magnonics,"Collective excitation modes in solid state systems play a central role in circuit quantum electrodynamics, cavity optomechanics, and quantum magnonics. In the latter, quanta of collective excitation modes in a ferromagnet, called magnons, interact with qubits to provide the nonlinearity necessary to access quantum phenomena in magnonics. A key ingredient for future quantum magnonics systems is the ability to probe magnon states. Here we observe individual magnons in a millimeter-sized ferromagnet coherently coupled to a superconducting qubit. Specifically, we resolve magnon number states in spectroscopic measurements of a transmon qubit with the hybrid system in the strong dispersive regime. This enables us to detect a change in the magnetic dipole of the ferromagnet equivalent to a single spin flipped among more than $10^{19}$ spins. The strong dispersive regime of quantum magnonics opens up the possibility of encoding superconducting qubits into non-classical magnon states, potentially providing a coherent interface between a superconducting quantum processor and optical photons.",1610.00839v1 2016-10-06,Giant Mesoscopic Fluctuations and Long Range Superconducting Correlations in Superconductor--Ferromagnet structures,"The fluctuating superconducting correlations emerging in dirty hybrid structures in conditions of the strong proximity effect are demonstrated to affect the validity range of the widely used formalism of Usadel equations at mesoscopic scales. In superconductor -- ferromagnet (SF) structures these giant mesoscopic fluctuations originating from the interference effects for the Cooper pair wave function in the presence of the exchange field can be responsible for an anomalously slow decay of superconducting correlations in a ferromagnet even when the non-collinear and spin-orbit effects are negligible. The resulting sample-to-sample fluctuations of the Josephson current in SFS junctions and local density of states in SF hybrid structures can provide an explanation of the long range proximity phenomena observed in mesoscopic samples with collinear magnetization.",1610.01775v1 2016-10-06,Quantum Anomalous Hall State in Ferromagnetic SrRuO$_3$ (111) Bilayers,"SrRuO$_3$ heterostructures grown in the (111) direction are a rare example of thin film ferromagnets. By means of density functional theory plus dynamical mean field theory we show that the half-metallic ferromagnetic state with an ordered magnetic moment of 2$\mu_{B}$/Ru survives the ultimate dimensional confinement down to a bilayer, even at elevated temperatures of 500$\,$K. In the minority channel, the spin-orbit coupling opens a gap at the linear band crossing corresponding to $\frac34$ filling of the $t_{2g}$ shell. We demonstrate that the respective state is Haldane's quantum anomalous Hall state with Chern number $C$=1, without an external magnetic field or magnetic impurities.",1610.01948v2 2016-10-13,Spin-wave spectra in periodically surface-modulated ferromagnetic thin films,"This article presents theoretical results for the dynamic response of periodically surface-modulated ferromagnetic thin films. For such system, the role of the periodic dipolar field induced by the modulation is addressed by using the plane-wave method. By controlling the geometry of the modulated volumes within the film, the frequency modes and spatial profiles of spin waves can be manipulated. The angular dependence of the frequency band-gaps unveils the influence of both dynamic and static magnetic charges, which reside in the edges of the etching periodic zones, and it is stablished that band-gap widths created by static magnetic charges are broader than the one created by dynamic magnetic charges. To corroborate the validity of the model, the theoretical results are compared with ferromagnetic resonance simulations, where a very good agreement is achieved between both methods. The theoretical model allows for a detailed understanding of the physics underlying these kind of systems, thereby providing an outlook to potential applications associated with magnonic crystals-based devices.",1610.04176v1 2016-10-14,Anisotropy of a Cubic Ferromagnet at Criticality,"Critical fluctuations change the effective anisotropy of cubic ferromagnet near the Curie point. If the crystal undergoes phase transition into orthorhombic phase and the initial anisotropy is not too strong, reduced anisotropy of nonlinear susceptibility acquires at $T_c$ the universal value $\delta_4^* = {{2v^*} \over {3(u^* + v^*)}}$ where $u^*$ and $v^*$ -- coordinates of the cubic fixed point on the flow diagram of renormalization group equations. In the paper, the critical value of the reduced anisotropy is estimated within the pseudo-$\epsilon$ expansion approach. The six-loop pseudo-$\epsilon$ expansions for $u^*$, $v^*$, and $\delta_4^*$ are derived for the arbitrary spin dimensionality $n$. For cubic crystals ($n = 3$) higher-order coefficients of the pseudo-$\epsilon$ expansions obtained turn out to be so small that use of simple Pad\'e approximants yields reliable numerical results. Pad\'e resummation of the pseudo-$\epsilon$ series for $u^*$, $v^*$, and $\delta_4^*$ leads to the estimate $\delta_4^* = 0.079 \pm 0.006$ indicating that detection of the anisotropic critical behavior of cubic ferromagnets in physical and computer experiments is certainly possible.",1610.04332v1 2016-10-18,Spin- and valley-dependent Goos-Hanchen effect in silicene and gapped graphene structures,"We investigate the Goos-H\""anchen shift for ballistic electrons (i) reflected from a step-like inhomogeneity of the potential energy and (or) effective mass, and (ii) transmitted through a ferromagnetic barrier region in monolayer silicene or gapped graphene. For the electrons reflected from a single interface we found that the Goos-H\""anchen shift is valley-polarized for gapped graphene structure, and valley- and spin-polarized for silicene due to the spin-valley coupling. Incontrast, for example, to gapless graphene the lateral beam shift in gapped structures occurs not only in the case of total, but also of partial, reflection, i.e. at the angles smaller than the critical angle of total reflection. We have also demonstrated that the valley- and spin-polarized displacement of the electron beam, transmitted through a ferromagnetic silicene barrier, resonantly depends on the barrier width. The resonant values of the displacement can be controlled by adjusting the electric potential, the external perpendicular electric field, and the exchange field induced by an insulating ferromagnetic substrate.",1610.05491v1 2017-01-31,Anomalous current in diffusive ferromagnetic Josephson junctions,"We demonstrate that in diffusive superconductor/ferromagnet/superconductor (S/F/S) junctions a finite, {\it anomalous}, Josephson current can flow even at zero phase difference between the S electrodes. The conditions for the observation of this effect are non-coplanar magnetization distribution and a broken magnetization inversion symmetry of the superconducting current. The latter symmetry is intrinsic for the widely used quasiclassical approximation and prevent previous works, based on this approximation, from obtaining the Josephson anomalous current. We show that this symmetry can be removed by introducing spin-dependent boundary conditions for the quasiclassical equations at the superconducting/ferromagnet interfaces in diffusive systems. Using this recipe we considered generic multilayer magnetic systems and determine the ideal experimental conditions in order to maximize the anomalous current.",1702.00056v2 2017-02-06,Magnetic phases of spin-1 lattice gases with random interactions,"A spin-1 atomic gas in an optical lattice, in the unit-filling Mott Insulator (MI) phase and in the presence of disordered spin-dependent interaction, is considered. In this regime, at zero temperature, the system is well described by a disordered rotationally-invariant spin-1 bilinear-biquadratic model. We study, via the density matrix renormalization group algorithm, a bounded disorder model such that the spin interactions can be locally either ferromagnetic or antiferromagnetic. Random interactions induce the appearance of a disordered ferromagnetic phase characterized by a non-vanishing value of spin-glass order parameter across the boundary between a ferromagnetic phase and a dimer phase exhibiting random singlet order. The study of the distribution of the block entanglement entropy reveals that in this region there is no random singlet order.",1702.01765v2 2017-02-07,Ferromagnetism in chiral multilayer 2D semimetals,"We calculate the temperature dependent long-range magnetic coupling in the presence of dilute concentrations of random magnetic impurities in chiral multilayer two-dimensional semimetals, i.e., undoped intrinsic multilayer graphene. Assuming a carrier-mediated indirect RKKY exchange interaction among the well-separated magnetic impurities with the itinerant carriers mediating the magnetic interaction between the impurities, we investigate the magnetic properties of intrinsic multilayer graphene using an effective chiral Hamiltonian model. We find that due to the enhanced density of states in the rhombohedral stacking sequence of graphene layers, the magnetic ordering of multilayer graphene is ferromagnetic in the continuum limit. The ferromagnetic transition temperature is calculated using a finite-temperature self-consistent field approximation and found to be within the experimentally accessible range for reasonable values of the impurity-carrier coupling.",1702.01859v1 2017-02-09,Monte Carlo simulation of quantum Potts model,"Using Monte Carlo simulations in the frame of stochastic series expansion (SSE), we study the three-state quantum Potts model. The cluster algorithm we used is a direct generalization of that for the quantum Ising model. The simulations include the one dimensional and two dimensional ferromagnetic three-state quantum Potts model and the two dimensional antiferromagnetic three-state quantum Potts model. Our results show that the phase transition of the one dimensional ferromagnetic quantum Potts model belongs to the same universality class of the two dimensional classical Potts model, the two dimensional ferromagnetic quantum Potts model undergoes a first order transition, which is also in analogy to its classical correspondence. The phase transition of the antiferromagnetic quantum Potts model is continuous, whose universality class belongs to the three-dimensional classical XY model, owing to an `emergent' O(2) symmetry at the critical point, although its ordered phase breaks the Z_6 symmetry.",1702.02675v1 2017-02-09,Long-range proximity effect in Nb-based heterostructures induced by a magnetically inhomogeneous permalloy layer,"Odd-frequency triplet Cooper pairs are believed to be the carriers of long-range superconducting correlations in ferromagnets. Such triplet pairs are generated by inhomogeneous magnetism at the interface between a superconductor (S) and a ferromagnet (F). So far, reproducible long-range effects were reported only in complex layered structures designed to provide the magnetic inhomogeneity. Here we show that spin triplet pair formation can be found in simple unstructured Nb/Permalloy (Py = Ni_0.8Fe_0.2)/Nb trilayers and Nb/Py bilayers, but only when the thickness of the ferromagnetic layer ranges between 140 and 250 nm. The effect is related to the emergence of an intrinsically inhomogeneous magnetic state, which is a precursor of the well-known stripe regime in Py that in our samples sets in at thickness larger than 300 nm.",1702.02765v1 2017-03-13,Artificial control of the bias-voltage dependence of tunnelling anisotropic magnetoresistance using quantization in a single-crystal ferromagnet,"A major issue in the development of spintronic memory devices is the reduction of the power consumption for the magnetization reversal. For this purpose, the artificial control of the magnetic anisotropy of ferromagnetic materials is of great importance. Here, we demonstrate the control of the carrier-energy dependence of the magnetic anisotropy of the density of states (DOS) using the quantum size effect in a single-crystal ferromagnetic material, GaMnAs. We show that the mainly two-fold symmetry of the magnetic anisotropy of DOS, which is attributed to the impurity band, is changed to a four-fold symmetry by enhancing the quantum size effect in the valence band of the GaMnAs quantum wells. By combination with the gate-electric field control technique, our concept of the usage of the quantum size effect for the control of the magnetism will pave the way for the ultra-low-power manipulation of magnetization in future spintronic devices.",1703.04294v1 2017-03-23,Magnon activation by hot electrons via non-quasiparticle states,"We consider the situation when a femtosecond laser pulse creates a hot electron state in half-metallic ferromagnet (e. g. ferromagnetic semiconductor) on a picosecond timescale but do not act directly on localized spin system. We show that the energy and magnetic moment transfer from hot itinerant electrons to localized spins is facilitated by the so-called non-quasiparticle states, which are the scattering states of a magnon and spin-majority electron. The magnon distribution is described by a quantum kinetic equation that we derive using the Keldysh diagram technique. In a typical ferromagnetic semiconductor such as EuO magnons remain essentially in non-equilibrium on a scale of the order of microsecond after the laser pulse.",1703.08106v1 2017-03-27,Dipolar ferromagnetism in three-dimensional superlattices of nanoparticles,"A series of atomistic finite temperature simulations on a model of an FCC lattice of maghemite nanoparticles using the stochastic Landau-Lifshitz-Gilbert (sLLG) equation are presented. The model exhibits a ferromagnetic transition that is in good agreement with theoretical expectations. The simulations also reveal an orientational disorder in the orientational order parameter for $T < 0.5 T_c$ due to pinning of the surface domain walls of the nanoparticles by surface vacancies. The extent of the competition between surface pinning and dipolar interactions provides support for the conjecture that recent measurements on systems of FCC superlattices of iron-oxide nanoparticles provide evidence for dipolar ferromagnetism is discussed.",1703.09290v1 2017-03-29,Current cross-correlations in double quantum dot based Cooper pair splitters with ferromagnetic leads,"We investigate the current cross-correlations in a double quantum dot based Cooper pair splitter coupled to one superconducting and two ferromagnetic electrodes. The analysis is performed by assuming a weak coupling between the double dot and ferromagnetic leads, while the coupling to the superconductor is arbitrary. Employing the perturbative real-time diagrammatic technique, we study the Andreev transport properties of the device, focusing on the Andreev current cross-correlations, for various parameters of the model, both in the linear and nonlinear response regimes. Depending on parameters and transport regime, we find both positive and negative current cross-correlations. Enhancement of the former type of cross-correlations indicates transport regimes, in which the device works with high Cooper pair splitting efficiency, contrary to the latter type of correlations, which imply negative influence on the splitting. The processes and mechanisms leading to both types of current cross-correlations are thoroughly examined and discussed, giving a detailed insight into the Andreev transport properties of the considered device.",1703.09957v1 2017-03-30,Study of spin pumping in Co thin film vis-a-vis seed and capping layer using ferromagnetic resonance spectroscopy,"We investigated the dependence of the seed [Ta/Pt, Ta/Au] and capping [Pt/Ta, Au/Ta] layers on spin pumping effect in the ferromagnetic 3 nm thick Co thin film using ferromagnetic resonance spectroscopy. The data is fitted with Kittel equation to evaluate damping constant and g-factor. A strong dependence of seed and capping layers on spin pumping has been discussed. The value of damping constant {alpha} is found to be relatively large i.e. 0.0326 for the Ta{3}/Pt{3}/Co{3}/Pt{3}/Ta{3} {nm} multi-layer structure, while it is 0.0104 for Ta{3}/Co{3}/Ta{3} {nm}. Increase in {alpha} is observed due to Pt layer that works as a good sink for spins due to high spin orbit coupling. In addition, we measured the effective spin conductance = 2.0e18 m-2 for the trilayer structure Pt{3}/Co{3}/Pt{3} {nm} as a result of the enhancement in {alpha} relative to its bulk value. We observed that the evaluated g-factor decreases as effective demagnetizing magnetic field increases in all the studied samples. The azimuthal dependence of magnetic resonance field and line width showed relatively high anisotropy in the trilayer Ta{3}/Co{3}/Ta{3} {nm} structure.",1703.10630v1 2017-07-31,Perfect spin filter by periodic drive of a ferromagnetic quantum barrier,"We consider the problem of particle tunneling through a periodically driven ferromagnetic quantum barrier connected to two leads. The barrier is modeled by an impurity site representing a ferromagnetic layer or quantum dot in a tight-binding Hamiltonian with a local magnetic field and an AC-driven potential, which is solved using the Floquet formalism. The repulsive interactions in the quantum barrier are also taken into account. Our results show that the time-periodic potential causes sharp resonances of perfect transmission and reflection, which can be tuned by the frequency, the driving strength, and the magnetic field. We demonstrate that a device based on this configuration could act as a highly-tunable spin valve for spintronic applications.",1708.00004v1 2017-08-05,Spin Injection and Detection via the Anomalous Spin Hall Effect in a Ferromagnetic Metal,"We report a novel spin injection and detection mechanism via the anomalous Hall effect in a ferromagnetic metal. The anomalous spin Hall effect (ASHE) refers to the transverse spin current generated within the ferromagnet. We utilize the ASHE and its reciprocal effect to electrically inject and detect magnons in a magnetic insulator in a non-local geometry. Our experiments reveal that permalloy can have a higher spin injection and detection efficiency to that of platinum, owing to the ASHE. We also demonstrate the tunability of the ASHE via the orientation of the permalloy magnetization, thus creating new possibilities for spintronic applications.",1708.01752v1 2017-08-06,Bias induced ferromagnetism and half-metallicity in graphene nano-ribbons,"Towards spin selective electronics made of three coordinated carbon atoms, here we computationally propose robust and reversibly bias driven evolution of pristine undoped graphene nano-ribbons(GNR) into ferromagnetic-semiconductor, metal or a half metal, irrespective of their edge configurations. The evolution is a result of a rare ferromagnetic(FM) order emerging among nearest neighbouring(n-n) sites, in positively biased regions in their in-homogeneous bias unit-cells, in attempt to cooperatively minimise on-site Coulomb repulsion and kinetic energy, while maximising localization of electrons at the positively biased sites. The phenomenon appears to be a general property of in-homogeneously biased Coulomb correlated bipartite systems. Consequences are particularly rich in zigzag edged graphene nano-ribbons(ZGNR) due to the contest of bias driven n-n FM order and the inter-edge antiferromagnetic order inherent to ZGNRs, leading to systematic closing of gap for one of the spins, amounting to bias controlled unmissable opening of window for FM-semiconducting and half-metallic transport.",1708.01860v1 2017-08-10,Spin Hall magnetoresistance and spin Nernst magnetothermopower: role of the inverse spin galvanic effect,"In ferromagnet/normal-metal bilayers, the sensitivity of the spin Hall magnetoresistance and the spin Nernst magnetothermopower to the boundary conditions at the interface is of central importance. In general, such boundary conditions can be substantially affected by current-induced spin polarizations. In order to quantify the role of the latter, we consider a Rashba two-dimensional electron gas with a ferromagnet attached to one side of the system. The geometry of such a system maximizes the effect of current-induced spin polarization on the boundary conditions, and the spin Hall magnetoresistance is shown to acquire a non-trivial and asymmetric dependence on the magnetization direction of the ferromagnet.",1708.03165v1 2017-08-21,Transport Coefficients of Dirac Ferromagnet: Effects of Vertex Corrections,"As a strongly spin-orbit coupled metallic model with ferromagnetism, we have considered an extended Stoner model to the relativistic regime, named Dirac ferromagnet in three dimensions. In the previous paper~[Phys. Rev. B 90, 214418 (2014)], we studied the transport properties giving rise to the anisotropic magnetoresistance~(AMR) and the anomalous Hall effect~(AHE) with the impurity potential being taken into account only as the self-energy. The effects of the vertex corrections~(VCs) to AMR and AHE are reported in this paper. AMR is found not to change quantitatively when the VCs is considered, although the transport lifetime is different from the one-electron lifetime and the charge current includes additional contributions from the correlation with spin currents. The side-jump and the skew-scattering contributions to AHE are also calculated. The skew-scattering contribution is dominant in the clean case as can be seen in the spin Hall effect in the non-magnetic Dirac electron system.",1708.06105v1 2017-08-21,"On the origin of magnetism in (Ga,Mn)As: from paramagnetic through superparamagnetic to ferromagnetic phase","The high-spectral-resolution spectroscopic studies of the energy gap evolution, supplemented with electronic, magnetic and structural characterization, show that the modification of the GaAs valence band caused by Mn incorporation occurs already for a very low Mn content, much lower than that required to support ferromagnetic spin - spin coupling in (Ga,Mn)As. Only for n-type (Ga,Mn)As with the Mn content below about 0.3% the Mn-related extended states are visible as a feature detached from the valence-band edge and partly occupied with electrons. The combined magnetic and low-temperature photoreflectance studies presented here indicate that the paramagnetic - ferromagnetic transformation in p-type (Ga,Mn)As takes place without imposing changes of the unitary character of the valence band with the Fermi level located therein. The whole process is rooted in the nanoscale fluctuations of the local (hole) density of states and the formation of a superparamagnetic-like state. The Fermi level in (Ga,Mn)As is coarsened by the carrier concentration of the itinerant valence band holes and further fine-tuned by the many-body interactions.",1708.06435v1 2017-08-26,Magneto-thermopower in the Weak Ferromagnetic Oxide CaRu0.8Sc0.2O3: An Experimental Test for the Kelvin Formula in a Magnetic Material,"We have measured the resistivity, the thermopower, and the specific heat of the weak ferromagnetic oxide CaRu0.8Sc0.2O3 in external magnetic fields up to 140 kOe below 80 K. We have observed that the thermopower Q is significantly suppressed by magnetic fields at around the ferromagnetic transition temperature of 30 K, and have further found that the magneto-thermopower {\Delta}Q(H, T) = Q(H, T) - Q(0, T) is roughly proportional to the magneto-entropy {\Delta}S(H, T) = S(H, T)-S(0, T).We discuss this relationship between the two quantities in terms of the Kelvin formula, and find that the observed {\Delta}Q is quantitatively consistent with the values expected from the Kelvin formula, a possible physical meaning of which is discussed.",1708.07992v1 2018-02-07,"Structure, magnetic and transport properties of epitaxial thin films of equiatomic CoFeMnGe quaternary Heusler alloy","Future spintronics requires the realization of thin film of half-metallic ferromagnets having high Curie temperature and 100\% spin polarization at the Fermi level for potential spintronics applications. In this paper, we report the epitaxial thin films growth of half-metallic CoFeMnGe Heusler alloy on MgO (001) substrate using pulsed laser deposition system, along with the study of structural, magnetic and transport properties. The magnetic property measurements of the thin film suggest a soft ferromagnetic state at room temperature with an in-plane magnetic anisotropy and a Curie temperature well above the room temperature. Anisotropic magnetoresistance (AMR) ratio and temperature dependent electrical resistivity measurements of the thin film indicate the compound to be half-metallic in nature and therefore suitable for the fabrications of spintronics devices.",1802.02413v1 2018-02-13,Inducing ferromagnetism and Kondo effect in platinum by paramagnetic ionic gating,"Electrically controllable magnetism, which requires the field-effect manipulation of both charge and spin degrees of freedom, has attracted growing interests since the emergence of spintronics. In this work, we report the reversible electrical switching of ferromagnetic (FM) states in platinum (Pt) thin films by introducing paramagnetic ionic liquid (PIL) as the gating media. The paramagnetic ionic gating controls the movement of ions with magnetic moments, which induces itinerant ferromagnetism on the surface of Pt films with large coercivity and perpendicular anisotropy mimicking the ideal two-dimensional Ising-type FM state. The electrical transport of the induced FM state shows Kondo effect at low temperature suggesting spatially separated coexistence of Kondo scattering beneath the FM interface. The tunable FM state indicates that paramagnetic ionic gating could serve as a versatile method to induce rich transport phenomena combining field effect and magnetism at PIL-gated interfaces.",1802.04448v2 2018-02-17,Potential and spin-exchange interaction between Anderson impurities in graphene,"The effective interaction between resonant magnetic Anderson impurities in graphene, mediated by conduction electrons, is studied as a function of the strength of the onsite energy level of the impurities and the amplitude of coupling to conduction electrons. The sign and character of the interaction depend on whether the impurities reside on the same or opposite sublattices. For the same (opposite) sublattice, the potential interaction is attractive (repulsive) in the weak coupling limit with $1/R^3$ dependence on the distance; the interaction reverses sign and becomes repulsive (attractive) in the strong coupling limit and displays $1/R$ behavior. The spin-exchange coupling is ferromagnetic (antiferromagnetic) at both large and small distances, but reverses sign and becomes anti-ferromagnetic (ferromagnetic) for intermediate distances. For opposite sublattices, the effective spin exchange coupling is resonantly enhanced at distances where the energy levels cross the Dirac points.",1802.06171v1 2018-02-19,Strong enhancement of the spin Hall effect by spin fluctuations near the Curie point of FexPt1-x alloys,"Robust spin Hall effects (SHE) have recently been observed in non-magnetic heavy metal systems with strong spin-orbit interactions. These SHE are either attributed to an intrinsic band-structure effect or to extrinsic spin-dependent scattering from impurities, namely side-jump or skew scattering. Here we report on an extraordinarily strong spin Hall effect, attributable to spin fluctuations, in ferromagnetic FexPt1-x alloys near their Curie point, tunable with x. This results in a damping-like spin-orbit torque being exerted on an adjacent ferromagnetic layer that is strongly temperature dependent in this transition region, with a peak value that indicates a lower bound 0.34 (+-) 0.02 for the peak spin Hall ratio within the FePt. We also observe a pronounced peak in the effective spin-mixing conductance of the FM/FePt interface, and determine the spin diffusion length in these FexPt1-x alloys. These results establish new opportunities for fundamental studies of spin dynamics and transport in ferromagnetic systems with strong spin fluctuations, and a new pathway for efficiently generating strong spin currents for applications.",1802.06911v1 2018-02-21,Magnonic Spin-Transfer Torque in Ferromagnet/Antiferromagnet/Ferromagnet Trilayer,"In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down magnons coexist and form an intrinsic degree of freedom resembling electrons. When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can be thermally generated in an AF. We explore thermal magnon transport in an insulating F/AF/F trilayer where propagating magnons inside the AF spacer can transfer angular momenta between the two Fs. We find that a sufficiently large temperature gradient can switch the downstream F via magnonic spin-transfer torque if it is initially antiparallel with the upstream F. A reciprocal switching is achievable by reversing the temperature gradient. Using typical material parameters, we estimate the threshold to be less than 1 K/nm at room temperature, which can be reduced by raising temperature and enhancing the interfacial exchange coupling.",1802.07709v2 2018-02-26,Devil's Staircases in SFS Josephson Junctions,"We study the effect of coupling between the superconducting current and magnetization in the superconductor/ferromagnet/superconductor Josephson junction under an applied circularly polarized magnetic field. Manifestation of ferromagnetic resonance in the frequency dependence of the amplitude of the magnetization and the average critical current density is demonstrated numerically. The IV-characteristics show subharmonic steps that form devil's staircases, following a continued fraction algorithm. The origin of the found steps is related to the effect of the magnetization dynamics on the phase difference in the Josephson junction. The dynamics of our system is described by a generalized RCSJ model coupled to the Landau-Lifshitz-Gilbert equation. In the suplement we justify analytically the appearance of the fractional steps in IV-characteristics of the superconductor/ferromagnet/superconductor Josephson junction.",1802.09212v2 2018-04-05,Two-Stage Proximity-Induced Gap-Opening in Topological Insulator - Insulating Ferromagnet (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ - EuS Bilayers,"To further investigate the interplay between ferromagnetism and topological insulators, thin films of the low-carrier topological insulator (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies between $T\approx30~\mathrm{K}$ and $T\approx60~\mathrm{K}$, well above the Curie temperature $T_C \approx 15~\mathrm{K}$ of EuS. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance with decreasing temperatures were observed concurrently with the magnetic anomalies. Positive-negative magnetoresistance crossovers were observed at the Curie temperature, which seem only to appear when the sheet resistance exceeds the Mott-Ioffe-Regel limit $h/e^2$. A two-stage gap-opening process due to magnetic proximity is proposed.",1804.02061v2 2018-04-06,Exact Results on Itinerant Ferromagnetism and the 15-puzzle Problem,"We apply a result from graph theory to prove exact results about itinerant ferromagnetism. Nagaoka's theorem of ferromagnetism is extended to all non-separable graphs except single polygons with more than four vertices by applying the solution to the generalized 15-puzzle problem, which studies whether the hole's motion can connect all possible tile configurations. This proves that the ground state of a $U\to\infty$ Hubbard model with one hole away from the half filling on a 2D honeycomb lattice or a 3D diamond lattice is fully spin-polarized. Furthermore, the condition of connectivity for $N$-component fermions is presented, and Nagaoka's theorem is also generalized to $SU(N)$-symmetric fermion systems on non-separable graphs.",1804.02347v1 2018-04-09,Genuine multipartite nonlocality in the one-dimensional ferromagnetic spin-1/2 chain,"Genuine multipartite entanglement has been found in some spin chain systems. However, genuine multipartite nonlocality, which is much rarer than genuine multipartite entanglement, has never been found in any spin chain system. Here we present genuine multipartite nonlocality in a spin chain system. After introducing the definition of genuine multipartite nonlocality and a multipartite Bell-type inequality, we construct a group of joint measurements for the inequality in a one-dimensional ferromagnetic $N$-qubit chain with nearest-neighbor XXZ interaction, and many violations to the inequality have been found. The violations do indicate that genuine multipartite nonlocality exists in this ferromagnetic spin-1/2 chain system. Last but not least, we also calculate genuine multipartite entanglement concurrence in the same spin chain to demonstrate the difference and relationship between genuine multipartite nonlocality and genuine multipartite entanglement.",1804.02901v1 2018-04-09,Van der Waals spin valves,"We propose spin valves where a 2D non-magnetic conductor is intercalated between two ferromagnetic insulating layers. In this setup, the relative orientation of the magnetizations of the insulating layers can have a strong impact on the in-plane conductivity of the 2D conductor. We first show this for a graphene bilayer, described with a tight-binding model, placed between two ferromagnetic insulators. In the anti-parallel configuration, a band gap opens at the Dirac point, whereas in the parallel configuration, the graphene bilayer remains conducting. We then compute the electronic structure of graphene bilayer placed between two monolayers of the ferromagnetic insulator CrI$_3$, using density functional theory. Consistent with the model, we find that a gap opens at the Dirac point only in the antiparallel configuration.",1804.03021v1 2018-04-17,Emergent Magnetism at the 3$d$-5$d$ Interface: SrMnO$_3/$SrIrO$_3$,"Recent experiments have found new magnetic behaviors, which are different from the parent bulk materials, at the interfaces between 3$d$ and 5$d$ oxides such as SrMnO$_3$ (SMO) and SrIrO$_3$ (SIO). The system is of considerable interest due to the strong spin-orbit coupling in the 5$d$ materials on one hand and the double exchange physics in SMO on the other, which belongs to the class of the colossal magnetoresistive (CMR) manganites. In order to gain insight into the physics of the system, we have performed density-functional studies on a selected interface structure, viz., the (SMO)$_1$(SIO)$_1$ superlattice, which has been experimentally grown and studied. Our density-functional results show that the interfacial magnetism is controlled by a net charge transfer at the interface from the SIO to the SMO side, turning both of them into ferromagnetic metal from the original antiferromagnetic insulating state in the bulk. The transferred electrons to the SMO side make it ferromagnetic through the Anderson-Hasegawa double exchange interaction, while the SIO part becomes ferromagnetic due to the doping of the half-filled Mott-Hubbard insulator as suggested by the Nagaoka Theorem. Our results are discussed in the context of the experiments for the same structure.",1804.06381v1 2018-04-17,Anomalous Hall effect in van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$,"We report anomalous Hall effect (AHE) in single crystals of quasi-two-dimensional Fe$_{3-x}$GeTe$_2$ ($x \approx 0.36$) ferromagnet grown by the flux method which induces defects on Fe site and bad metallic resistivity. Fe K-edge x-ray absorption spectroscopy was measured to provide information on local atomic environment in such crystals. The dc and ac magnetic susceptibility measurements indicate a second-stage transition below 119 K in addition to the paramagnetic to ferromagnetic transition at 153 K. A linear scaling behavior between the modified anomalous Hall resistivity $\rho_{xy}/\mu_0H_{eff}$ and longitudinal resistivity $\rho_{xx}^2M/\mu_0H_{eff}$ implies that the AHE in Fe$_{3-x}$GeTe$_2$ should be dominated by the intrinsic Karplus-Luttinger mechanism rather than the extrinsic skew-scattering and side-jump mechanisms. The observed deviation in the linear-M Hall conductivity $\sigma_{xy}^A$ below 30 K is in line with its transport characteristic at low temperatures, implying the scattering of conduction electrons due to magnetic disorder and the evolution of the Fermi surface induced by possible spin-reorientation transition.",1804.06392v1 2018-04-20,The nature of transport and ferromagnetic properties of the GaAs structures with the Mn δ-doped layer,"We reveal the nature of transport and ferromagnetic properties of the GaAs structure with the single Mn {\delta}-doped layer. To modify the properties of the structure the electrically active radiation defects are created by irradiation with helium ions. The investigations show that the transport properties of the structure are determined by two parallel conduction channels (the channel associated with hole transport in the valence band and the channel associated with electron transport in the Mn impurity band) and that the ferromagnetic properties are determined by the electrons localized at the allowed states within the Mn impurity band. The results also help to understand the features of structures with the Mn {\delta}-layer nearby the quantum well.",1804.07650v3 2018-04-20,Fluctuation-induced Néel and Bloch skyrmions at topological insulator surfaces,"Ferromagnets in contact with a topological insulator have become appealing candidates for spintronics due to the presence of Dirac surface states with spin-momentum locking. Because of this bilayer Bi$_2$Se$_3$-EuS structures, for instance, show a finite magnetization at the interface at temperatures well exceeding the Curie temperature of bulk EuS. Here we determine theoretically the effective magnetic interactions at a topological insulator-ferromagnet interface {\it above} the magnetic ordering temperature. We show that by integrating out the Dirac fermion fluctuations an effective Dzyaloshinskii-Moriya interaction and magnetic charging interaction emerge. As a result individual magnetic skyrmions and extended skyrmion lattices can form at interfaces of ferromagnets and topological insulators, the first indications of which have been very recently observed experimentally.",1804.07684v3 2018-04-29,Diluted magnetic Dirac-Weyl materials: Susceptibility and ferromagnetism in three-dimensional chiral gapless semimetals,"We theoretically investigate the temperature-dependent static susceptibility and long-range magnetic coupling of three-dimensional (3D) chiral gapless electron-hole systems (semimetals) with arbitrary band dispersion [i.e., $\varepsilon(k) \sim k^N$, where $k$ is the wave vector and $N$ is a positive integer]. We study the magnetic properties of these systems in the presence of dilute random magnetic impurities. Assuming carrier-mediated Ruderman-Kittel-Kasuya-Yosida indirect exchange interaction, we find that the magnetic ordering of intrinsic 3D chiral semimetals in the presence of dilute magnetic impurities is ferromagnetic for all values of $N$. Using finite-temperature self-consistent field approximation, we calculate the ferromagnetic transition temperature ($T_{\rm c}$). We find that $T_{\rm c}$ increases with increasing $N$ due to the enhanced density of states, and the calculated $T_{\rm c}$ is experimentally accessible assuming reasonable coupling between the magnetic impurities and itinerant carriers.",1804.10867v2 2018-11-04,Coexistence of exchange bias training effect and spin-orbit torque in IrMn-layer/ferromagnetic-ribbon heterostructures via magnetoimpedance effect,"We investigate a possible correlation between spin orbit torque (SOT) and exchange-bias (EB) in IrMn-layer/ferromagnetic-ribbon heterostructure by performing magnetoimpedance (MI) measurements. To uncover this correlation, we benefit from EB training effect probed by MI effect at room temperature. A damping-like SOT driven by ac current through the antiferromagnetic IrMn applies to the ferromagnetic ribbon layer, determined by MI magnetic field and frequency sweeps. Importantly, magnitude of the SOT is observed to remain intact against EB training and decrease of EB through alternative magnetic field sweep cycles. Our results pave the way to better elucidate the EB effect, EB training and the SOT, useful for future spintronic elements.",1811.01352v4 2018-11-14,Relation between spin Hall effect and anomalous Hall effect in 3$d$ ferromagnetic metals,"We study the mechanisms of the spin Hall effect (SHE) and anomalous Hall effect (AHE) in 3$d$ ferromagnetic metals (Fe, Co, permalloy (Ni$_{81}$Fe$_{19}$; Py), and Ni) by varying their resistivities and temperature. At low temperatures where the phonon scattering is negligible, the skew scattering coefficients of the SHE and AHE in Py are related to its spin polarization. However, this simple relation breaks down for Py at higher temperatures as well as for the other ferromagnetic metals at any temperature. We find that, in general, the relation between the SHE and AHE is more complex, with the temperature dependence of the SHE being much stronger than that of AHE.",1811.05589v2 2018-11-22,Magnetoelectric effects in superconductor/ferromagnet bilayers,"We demonstrate that the hybrid structures consisting of a superconducting layer with an adjacent spin-textured ferromagnet demonstrate the variety of equilibrium magnetoelectric effects originating from coupling between the conduction electron spin and superconducting current. By deriving and solving the generalized Usadel equation which takes into account the spin-filtering effect we find that a supercurrent generates spin polarization in the superconducting film which is non-coplanar with the local ferromagnetic moment. The inverse magnetoelectric effect in such structures is shown to result in the spontaneous phase difference across the magnetic topological defects such as a domain wall and helical spin texture. The possibilities to obtain dissipationless spin torques and detect domain wall motion through the superconducting phase difference are discussed.",1811.09304v2 2018-11-25,Temperature-Dependent Magnetization Reversal in Exchange Bias NiFe/IrMn/NiFe Structures,"We demonstrate the magnetization reversal features in NiFe/IrMn/NiFe thin-film structures with 40% and 75% relative content of Ni in Permalloy in the temperature range from 80 K to 300 K. At the descending branches of the hysteresis loops, the magnetization reversal sequence of the two ferromagnetic layers is found to depend on the type of NiFe alloy. In the samples with 75% relative content of Ni, the bottom ferromagnetic layer reverses prior to the top one. On the contrary, in the samples with 40% of Ni, the top ferromagnetic layer reverses prior to the bottom one. These tendencies of magnetization reversal are preserved in the entire range of temperatures. These distinctions can be explained by the morphological and structural differences of interfaces in the samples based on two types of Permalloy.",1811.09963v1 2019-04-30,Ferromagnetism and Spin-Valley liquid states in Moiré Correlated Insulators,"Motivated by the recent observation of evidences of ferromagnetism in correlated insulating states in systems with Moir\'{e} superlattices, we study a two-orbital quantum antiferromagnetic model on the triangular lattice, where the two orbitals physically correspond to the two valleys of the original graphene sheet. For simplicity this model has a SU(2)$^s$$\otimes$SU(2)$^v$ symmetry, where the two SU(2) symmetries correspond to the rotation within the spin and valley space respectively. Through analytical argument, Schwinger boson analysis and also DMRG simulation, we find that even though all the couplings in the Hamiltonian are antiferromagnetic, there is still a region in the phase diagram with fully polarized ferromagnetic order. We argue that a Zeeman field can drive a metal-insulator transition in our picture, as was observed experimentally. We also construct spin liquids and topological ordered phases at various limits of this model. Then after doping this model with extra charge carriers, the system most likely becomes spin-triplet/valley-singlet $d+id$ topological superconductor as was predicted previously.",1905.00033v2 2019-05-02,Negligible thermal contributions to the spin pumping signal in ferromagnetic metal-Platinum bilayers,"Spin pumping by ferromagnetic resonance is one of the most common technique to determine spin hall angles, Edelstein lengths or spin diffusion lengths of a large variety of materials. In recent years, rising concerns have appeared regarding the interpretation of these experiments, underlining that the signal could arise purely from thermoelectric effects, rather than from coherent spin pumping. Here, we propose a method to evaluate the presence or absence of thermal effects in spin pumping signals, by combining bolometry and spin pumping by ferromagnetic resonance measurements, and comparing their timescale. Using a cavity to perform the experiments on Pt\Permalloy and La0.7Sr0.3MnO3\Pt samples, we conclude on the absence of any measurable thermoelectric contribution such as the spin Seebeck and anomalous Nernst effects at resonance",1905.00771v2 2019-05-04,Theoretical Study on Four-fold Symmetric Anisotropic Magnetoresistance Effect in Cubic Single-crystal Ferromagnetic Model,"In this study, we present a theoretical interpretation of the experimental results that the anisotropic magnetoresistance (AMR) effect has a four-fold symmetric component, $c_4$, in cubic ferromagnetic metals. The theoretical model that we employ is based on the Anderson impurity model that includes a four-fold symmetric crystalline electric field, and we assume that the impurities have 3d electron orbitals and spin--orbit interaction (SOI). We describe the DC conductivity on the basis of the Kubo formula, and we investigate $c_4$ by analyzing the magnetization direction dependence of the resultant AMR ratio. Analytical and numerical calculations are performed; the analytical calculation reveals that $c_4$ arises from the fourth-order contribution of the SOI, and the numerical calculation provides the parameter dependencies of $c_{4}$ in our model. From the calculation results, we observe that the splitting of impurity 3d levels due to SOI is responsible for the existence of $c_{4}$ in cubic ferromagnetic metals.",1905.01424v2 2019-05-07,Atomic scale electronic structure of the ferromagnetic semiconductor Cr2Ge2Te6,"Cr2Ge2Te6 is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure, thus represents a promising material for novel electronic and spintronic devices. Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr2Ge2Te6. Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition, as well as a peculiar double-peak electronic state on the Cr-site defect. These features can be quantitatively explained by density functional theory calculations, which uncover a close relationship between the electronic structure and magnetic order. These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr2Ge2Te6.",1905.02444v1 2019-05-19,Magnetic Vortex induced by Nonmagnetic Impurity in Ferromagnets: Magnetic Multipole and Toroidal around the Vacancy,"We report our theoretical study on nucleation of a magnetic vortex around a nonmagnetic impurity doped into ferromagnets. The mechanism lies in the asymmetric Dzyaloshinskii-Moriya interaction arising from the breaking of spatial inversion symmetry by the impurity. By using the spin-wave analysis and Monte Carlo simulations, we show that the asymmetric interaction induces a magnetic vortex with nonzero vorticity $l=+1$. The vortex is stabilized even for less frustration in exchange interactions and in the absence of an external magnetic field. We also find that the magnetic vortex is characterized by magnetic multipoles according to its vorticity and helicity. We demonstrate a potential realization of such a magnetic vortex by considering a monolayer ferromagnet on a nonmagnetic substrate, which results in the magnetic monopole and toroidal dipole.",1905.07669v1 2019-05-19,Long-range triplet proximity effect in multiply connected ferromagnet-superconductor hybrids,"Applying the linearized Usadel equations, we consider the nucleation of superconductivity in multiply connected mesoscopic superconductor/ferromagnet (S/F) hybrids such as a thin superconducting ring on a ferromagnet with a uniform in-plane magnetization M and a spin-active S/F interface. We demonstrate that the exchange field in F provokes a switching between superconducting states with different vorticities which may increase the critical temperature ( Tc ) of the superconductor in a magnetic field. We study the interplay between oscillations in Tc due to the Little--Parks effect and oscillations in Tc induced by the exchange field. Furthermore, we analyse the influence of long-range spin-triplet correlations on the switching between different vorticities.",1905.07683v1 2019-05-22,Competitive 0 and π states in S/F multilayers: multimode approach,"We have investigated the critical temperature behavior in periodic superconductor/ ferromagnet (S/F) multilayers as a function of the ferromagnetic layer thickness $d_f$ and the interface transparency. The critical temperature $T_c(d_f)$ exhibits a damped oscillatory behavior in these systems due to an exchange field in the ferromagnetic material. In this work we have performed $T_c$ calculations using the self-consistent multimode approach, which is considered to be exact solving method. Using this approach we have derived the conditions of 0 or $\pi$ state realization in periodic S/F multilayers. Moreover, we have presented the comparison between the single-mode and multimode approaches and established the limits of applicability of the single-mode approximation, frequently used by experimentalists.",1905.09193v2 2019-05-22,Floquet Second-Order Topological Superconductor Driven via Ferromagnetic Resonance,"We consider a Floquet triple-layer setup composed of a two-dimensional electron gas with spin-orbit interactions, proximity coupled to an s-wave superconductor and to a ferromagnet driven at resonance. The ferromagnetic layer generates a time-oscillating Zeeman field which competes with the induced superconducting gap and leads to a topological phase transition. The resulting Floquet states support a second-order topological superconducting phase with a pair of localized zero-energy Floquet Majorana corner states. Moreover, the phase diagram comprises a Floquet helical topological superconductor, hosting a Kramers pair of Majorana edge modes protected by an effective time-reversal symmetry, as well as a gapless Floquet Weyl phase. The topological phases are stable against disorder and parameter variations and are within experimental reach.",1905.09241v1 2019-05-29,Skew Andreev reflection in ferromagnet/superconductor junctions,"Andreev reflection (AR) in ferromagnet/superconductor junctions is an indispensable spectroscopic tool for measuring spin polarization. We study theoretically how the presence of a thin semiconducting interface in such junctions, inducing Rashba and Dresselhaus spin-orbit coupling, modifies AR processes. The interface gives rise to an effective momentum- and spin-dependent scattering potential, making the probability of AR strongly asymmetric with respect to the sign of the incident electrons' transverse momenta. This skew AR creates spatial charge carrier imbalances and transverse Hall currents flow in the ferromagnet. We show that the effect is giant, as compared to the normal regime. We provide a quantitative analysis and a qualitative picture of this phenomenon, and finally show that skew AR also leads to a widely tunable transverse supercurrent response in the superconductor.",1905.12525v2 2020-07-03,Dynamic spin-triplet order induced by alternating electric fields in superconductor-ferromagnet-superconductor Josephson junctions,"Dynamic states offer extended possibilities to control the properties of quantum matter. Recent efforts are focused on studying the ordered states which appear exclusively under the time-dependent drives. Here we demonstrate a class of systems which feature dynamic spin-triplet superconducting order stimulated by the alternating electric field. The effect is based on the interplay of ferromagnetism, interfacial spin-orbital coupling (SOC) and the condensate motion driven by the field, which converts hidden static p-wave order, produced by the joint action of the ferromagnetism and the SOC, into dynamical s-wave equal-spin triplet correlations. We demonstrate that the critical current of Josephson junctions hosting these states is proportional to the electromagnetic power, supplied either by the external irradiation or by the ac current source. Based on these unusual properties we propose the scheme of a Josephson transistor which can be switched by the ac voltage and demonstrates an even-numbered sequence of Shapiro steps. Combining the photo-active Josephson junctions with recently discovered Josephson phase batteries we find photo-magnetic SQUID devices which can generate spontaneous magnetic fields while being exposed to irradiation.",2007.01805v2 2020-07-07,Magnetization control by angular momentum transfer from surface acoustic wave to ferromagnetic spin moments,"The angular momentum interconversion between electron spin and other type of angular momenta is useful to develop new spintronic functionalities. The conversions from the angular momentum of photon and mechanical rotation to ferromagnetic spin moment have been well studied. While the recent studies theoretically suggested circular vibration of atoms works as angular momentum of phonon, the direct experimental demonstration of conversion to spin moments remains to be performed. Here we demonstrate that the phonon angular momentum of surface acoustic wave can control the magnetization of a ferromagnetic Ni film by means of the phononic-to-electronic conversion of angular momentum in Ni/LiNbO$_3$ hybrid device. This result clearly shows the phonon angular momentum is useful to further fictionalize spintronic devices.",2007.03192v1 2010-05-04,Effect of disorder studied with ferromagnetic resonance for arrays of tangentially magnetized sub-micron Permalloy discs fabricated by nanosphere lithography,"Tangentially magnetized trigonal arrays of sub-micron Permalloy discs are characterized with ferromagnetic resonance to determine the possible contributions to frequency and linewidth from array disorder. Each array is fabricated by a water-surface self-assembly lithographic technique, and consists of a large trigonal array of 700 nm diameter magnetic discs. Each array is characterized by a different degree of ordering. Two modes are present in the ferromagnetic resonance spectra: a large amplitude, `fundamental' mode and a lower amplitude mode at higher field. Angular dependence of the resonance field in a very well ordered array is found to be negligible for both modes. The relationship between resonance frequency and applied magnetic field is found to be uncorrelated with array disorder. Linewidth is found to increase with increasing array disorder.",1005.0452v3 2010-05-07,Effect of ferromagnetic film thickness on magnetoresistance of thin-film superconductor-ferromagnet hybrids,"We study the influence of the thickness Df of the plain ferromagnetic (F) film on the electrical resistance of the flux-coupled hybrids, consisting of superconducting (S) Al film and multilayer [Co/Pt] F film with out-of-plain magnetization. The behavior of such hybrids at high and low temperatures is found to be different: the nucleation of superconductivity at high temperatures is governed mainly by the typical lateral dimensions of the magnetic domains, while low temperature properties are determined by topology of the magnetic template. We show that an increase in the Df value leads to a broadening of the field- and temperature intervals where non-monotonous dependence of the superconducting critical temperature Tc on the applied magnetic field H is observed (for demagnetized F films). Further increase in the Df value results in a global suppression of superconductivity. Thus, we determined an optimal thickness, when the non-monotonous dependence Tc(H) can be observed in rather broad T and H range, what can be interesting for further studies of the localized superconductivity in planar Al-based S/F hybrids and for development of the devices which can exploit the localized superconductivity.",1005.1246v1 2010-05-13,Anomalous spin distribution in the superconducting ferromagnet UCoGe studied by polarized neutron diffraction,"We report a polarized neutron diffraction study conducted to reveal the nature of the weak ferromagnetic moment in the superconducting ferromagnet UCoGe. We find that the ordered moment in the normal phase in low magnetic fields (B // c) is predominantly located at the U atom and has a magnitude of about 0.1 muB at 3 T, in agreement with bulk magnetization data. By increasing the magnetic field the U moment grows to about 0.3 muB in 12 T and most remarkably, induces a substantial moment (about 0.2 muB) on the Co atom directed antiparallel to the U moment. The anomalous polarizability of the Co 3d orbitals is unique among uranium intermetallics and might reflect the proximity to a magnetic quantum critical point of UCoGe in zero field.",1005.2390v1 2010-05-14,Spin State Transfer in Laterally Coupled Quantum Dot Chains with Disorders,"Quantum dot arrays are a promising media for transferring quantum information between two distant points without resorting to mobile qubits. Here we study two most common disorders namely, hyperfine interaction and exchange coupling fluctuations, in quantum dot arrays and their effects on quantum communication through these chains. Our results show that the hyperfine interaction is more destructive than the exchange coupling fluctuations. The average optimal time for communication is not affected by any disorder in the system and our simulations show that anti-ferromagnetic chains are much more resistive than the ferromagnetic ones against both kind of disorders. Even when time modulation of a coupling and optimal control is employed to improve the transmission, the anti-ferromagnetic chain performs much better. We have assumed the quasi-static approximation for hyperfine interaction and time dependent fluctuations in the exchange couplings. Particularly, for studying exchange coupling fluctuations we have considered the static disorder, white noise and $1/f$ noise.",1005.2571v2 2013-08-05,Controlling the Polarity of the Transient Ferromagnetic-Like State in Ferrimagnets,"After the application of an ultrashort laser pulse, the antiferromagnetic alignment in rare earth-transition metal alloys can temporarily become ferromagnetic with the rare-earth polarity. Proposed models merely describe this effect, without showing the route for its manipulation. Here we use extensive atomistic spin model simulations and micromagnetic theory for ferrimagnets at elevated temperatures to predict that the polarity of this transient ferromagnetic-like state can be controlled by initial temperature. We show that this arises because the magnetic response of each lattice has a different temperature dependence, at low temperatures the transition metal responds faster than the rare earth, while at high temperatures this role is interchanged. Our findings contribute to the physical understanding and control of this state and thus open new perspectives for its use in ultrafast magnetic devices.",1308.0993v2 2013-08-15,Spin heat accumulation induced by tunneling from a ferromagnet,"An electric current from a ferromagnet into a non-magnetic material can induce a spin-dependent electron temperature. Here it is shown that this spin heat accumulation, when created by tunneling from a ferromagnet, produces a non-negligible voltage signal that is comparable to that due to the coexisting electrical spin accumulation and can give a different Hanle spin precession signature. The effect is governed by the spin polarization of the Peltier coefficient of the tunnel contact, its Seebeck coefficient, and the spin heat resistance of the non-magnetic material, which is related to the electrical spin resistance by a spin-Wiedemann-Franz law. Moreover, spin heat injection is subject to a heat conductivity mismatch that is overcome if the tunnel interface has a sufficiently large resistance.",1308.3365v2 2013-08-21,"Reducing influence of antiferromagnetic interactions on ferromagnetic properties of p-(Cd,Mn)Te quantum wells","In order to explain the absence of hysteresis in ferromagnetic p-type (Cd,Mn)Te quantum wells (QWs), spin dynamics was previously investigated by Monte Carlo simulations combining the Metropolis algorithm with the determination of hole eigenfunctions at each Monte Carlo sweep. Short-range antiferromagnetic superexchange interactions between Mn spins - which compete with the hole-mediated long-range ferromagnetic coupling - were found to accelerate magnetization dynamics if the the layer containing Mn spins is wider than the vertical range of the hole wave function. Employing this approach it is shown here that appreciate magnitudes of remanence and coercivity can be obtained if Mn ions are introduced to the quantum well in a delta-like fashion.",1308.4525v1 2013-08-22,"New Diluted ferromagnetic semiconductor Li(Zn,Mn)P with decoupled charge and spin doping","We report the discovery of a new diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn2+ for Zn2+, respectively. Isostructural to (Ga,Mn)As, Li(Zn,Mn)P was found to be a p-type ferromagnetic semiconductor with excess Lithium providing charge doping. First principles calculations indicate that excess Li is favored to partially occupy the Zn site, leading to hole doping. Ferromagnetism is mediated in semiconducting samples of relative low mobile carriers with a small coercive force, indicating an easy spin flip.",1308.4929v1 2013-08-27,Experimental realization of magnetic energy concentration and transmission at a distance by metamaterials,"Controlling electromagnetic energy is essential for an efficient and sustainable society. A key requirement is concentrating magnetic energy in a desired volume of space in order to either extract the energy to produce work or store it. Metamaterials have opened new possibilities for controlling electromagnetic energy. Recently, a superconductor-ferromagnetic metamaterial that allows unprecedented concentration and amplification of magnetic energy, and also its transmission at distance through free space, has been devised theoretically. Here we design and build an actual version of the superconductor-ferromagnetic metamaterial and experimentally confirm these properties. We show that also a ferromagnetic metamaterial, without superconducting parts, can achieve concentration and transmission of energy with only a slight decrease in the performance. Transmission of magnetic energy at a distance by magnetic metamaterials may provide new ways of enhancing wireless power transmission, where efficiency depends critically on the magnetic coupling strength between source and receiver.",1308.5878v1 2014-03-04,Off-Resonant Manipulation of Spins in Diamond via Precessing Magnetization of a Proximal Ferromagnet,"We report the manipulation of nitrogen vacancy (NV) spins in diamond when nearby ferrimagnetic insulator, yttrium iron garnet, is driven into precession. The change in NV spin polarization, as measured by changes in photoluminescence, is comparable in magnitude to that from conventional optically detected magnetic resonance, but relies on a distinct mechanism as it occurs at a microwave frequency far removed from the magnetic resonance frequency of the NV spin. This observation presents a new approach to transferring ferromagnetic spin information into a paramagnet and then transducing the response into a robust optical signal. It also opens new avenues for studying ferromagnetism and spin transport at the nanoscale.",1403.0656v1 2014-03-04,All-optical control of ferromagnetic thin films and nanostructures,"The interplay of light and magnetism has been a topic of interest since the original observations of Faraday and Kerr where magnetic materials affect the light polarization. While these effects have historically been exploited to use light as a probe of magnetic materials there is increasing research on using polarized light to alter or manipulate magnetism. For instance deterministic magnetic switching without any applied magnetic fields using laser pulses of the circular polarized light has been observed for specific ferrimagnetic materials. Here we demonstrate, for the first time, optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed. These results challenge the current theoretical understanding and will have a major impact on data memory and storage industries via the integration of optical control of ferromagnetic bits.",1403.0784v1 2014-03-08,Ab initio study of CrNF: the first half-metallic ferromagnet nitride fluoride mimicking CrO2,"Based on the different covalent versus ionic chemical bonding and isoelectronic rutile CrO2, CrNF is proposed starting from ordered rutile derivative structures subjected to full geometry optimizations. The ground state structure defined from cohesive energies is of MgUO4-type, characterized by short covalent Cr-N and long ionic Cr-F distances. Like CrO2 it is a half-metallic ferromagnet with M = 2 Bohr Magnetons integer magnetization per formula unit with reduced band gap at minority spins. Major differences of magnetic response to pressure characterize CrNF as a soft ferromagnet versus hard magnetic CrO2. The chemical bonding properties point to prevailing covalent Cr-N versus ionic Cr-F bonding. Synthesis routes with two different protocols are proposed and analyzed.",1403.1961v1 2014-03-17,Ba(Zn1-2xMnxCox)2As2: A Bulk Form Diluted Magnetic Semiconductor with n-type Carriers,"We report the synthesis and characterization of bulk form diluted magnetic semiconductors Ba(Zn1-2xMnxCox)2As2 (0 <= x <= 0.15) with a crystal structure identical to that of 122-type Fe-based superconductors. Mn and Co co-doping into the parent compound BaZn2As2 results in a ferromagnetic ordering below TC ~ 80 K. Hall effect measurements indicate that the carrier are n-type with the density of ~10^17/cm3. The common crystal structure and excellent lattice matching between the p-type ferromagnetic (Ba1-yKy)(Zn1-xMnx)2As2, the n-type ferromagnetic Ba(Zn1-2xMnxCox)2As2, the antiferrmagnetic BaMn2As2 and the superconducting Ba(Fe1-xCox)2As2 systems make it possible to make various junctions between these systems through the As layer.",1403.4019v1 2014-03-17,Magnetic phase transition in coherently coupled Bose gases in optical lattices,"We describe the ground state of a gas of bosonic atoms with two coherently coupled internal levels in a deep optical lattice in a one dimensional geometry. In the single-band approximation this system is described by a Bose-Hubbard Hamiltonian. The system has a superfluid and a Mott insulating phase which can be either paramagnetic or ferromagnetic. We characterize the quantum phase transitions at unit filling by means of a density matrix renormalization group technique and compare it with a mean-field approach. The presence of the ferromagnetic Ising-like transition modifies the Mott lobes. In the Mott insulating region the system maps to the ferromagnetic spin-1/2 XXZ model in a transverse field and the numerical results compare very well with the analytical results obtained from the spin model. In the superfluid regime quantum fluctuations strongly modify the phase transition with respect to the well established mean-field three dimensional classical bifurcation.",1403.4185v2 2014-03-21,Interface boundary conditions for dynamic magnetization and spin wave dynamics in a ferromagnetic layer with the interface Dzyaloshinskii-Moriya interaction,"In this work we derive the interface exchange boundary conditions for the classical linear dynamics of magnetization in ferromagnetic layers with the interface Dzyaloshinskii-Moriya interaction (IDMI). We show that IDMI leads to pinning of dynamic magnetization at the interface. An unusual peculiarity of the IDMI-based pinning is that its scales as the spin-wave wave number. We incorporate these boundary conditions into an existing numerical model for the dynamics of the Damon-Eshbach spin wave in ferromagnetic films. IDMI affects the dispersion and the frequency non-reciprocity of the traveling Damon-Eshbach spin wave. For a broad range of film thicknesses L and wave numbers the results of the numerical simulations of the spin wave dispersion are in a good agreement with a simple analytical expression which shows that the contribution of IDMI to the dispersion scales as 1/L, similarly to the effect of other types of interfacial anisotropy. Suggestions to experimentalists how to detect the presence of IDMI in a spin wave experiment are given.",1403.5385v4 2014-03-21,Quantum Monte Carlo study of one-dimensional transition metal organometallic cluster systems and their suitability as spin filters,"We present calculations of electronic and magnetic structures of vanadium-benzene multidecker clusters V$_{n}$Bz$_{n+1}$ ($n$ = 1 - 3) using advanced quantum Monte Carlo methods. These and related systems have been identified as prospective spin filters in spintronic applications, assuming that their ground states are half-metallic ferromagnets. Although we find that magnetic properties of these multideckers are consistent with ferromagnetic coupling, their electronic structures do not appear to be half-metallic as previously assumed. In fact, they are ferromagnetic insulators with large and broadly similar $\uparrow$-/$\downarrow$-spin gaps. This makes the potential of these and related materials as spin filtering devices very limited, unless they are further modified or functionalized.",1403.5411v1 2014-03-25,Critical temperature of site-diluted spin-1/2 systems with long-range ferromagnetic interactions,"In the paper the Pair Approximation (PA) method for studies of the site-diluted spin-1/2 systems of arbitrary dimensionality with the long-range ferromagnetic interactions is adopted. The method allows to take into account arbitrary anisotropy of the interactions in the spin space, so it is not limited to purely Ising couplings. Within this approach, the Gibbs free energy is obtained, which allows to derive all the further interesting thermodynamic properties. In particular, we obtain an equation for the critical temperature of the second-order phase transitions for the model in question. In the study we focus our attention on the systems with ferromagnetic interactions decaying with the distance according to the power law $J(r)\propto r^{-n}$. We discuss the dependence of the critical temperature on the concentration of magnetic component and the index $n$ for selected one-, two- and three-dimensional lattices.We confirm the absence of the critical concentration for a diluted magnet with infinite interaction range. In the regime of the low concentrations of magnetic component, we find a non-linear increase of the critical temperature with the concentration in the form of $T_{c}\propto p^{n/d}$, depending on the system dimensionality $d$ and the index $n$, whereas $n > d$.",1403.6283v1 2014-03-26,Parity violation in ferromagnet-superconductor heterostructures with strong spin-orbit coupling,"We study spectroscopic properties of ferromagnetic-superconductor heterostructures with strong spin-orbit coupling of the Rashba type and in the presence of exchange fields. The superconducting layer (film) experiences both an intrinsic spin-orbit field and an exchange field due to the proximity to ferromagnetic layers (films). We analyse the temperature dependence of the order parameter for superconductivity at various values of exchange field and spin-orbit coupling, and describe momentum-dependent properties that exhibit parity violation. Furthermore, we show that parity violation can be probed in tunneling experiments of the single-particle density of states and in photoemission experiments of the momentum distribution.",1403.6858v1 2016-11-04,Quantum critical scaling in the disordered itinerant ferromagnet UCo$_{1-x}$Fe$_x$Ge,"Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first-order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition. For disordered metals BKV theory predicts that the second order nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive. Our low-temperature magnetization study identifies the ferromagnetic QPT in the disordered metal UCo$_{1-x}$Fe$_x$Ge as the first clear example that exhibits the associated critical exponents predicted by BKV theory.",1611.01539v2 2016-07-17,Thick ferromagnetic films and their anisotropies as described by second order perturbed Heisenberg Hamiltonian,"Second and fourth order anisotropy dependence of energy of thick simple cubic ferromagnetic films with 10000 layers is explained using Heisenberg Hamiltonian with second order perturbation in this manuscript. The second and fourth order anisotropy constants were assumed to be constants through out the film. When the fourth order anisotropy is given by fourth order anisotropy of 6, the sc(001) ferromagnetic thick films with 10000 layers can be easily oriented in 0.6 radians direction for the energy parameters given this report. Under the influence of the second order anisotropy given by second order anisotropy of 6.3, the easy direction of sc(001) film with 10000 layers is given by 0.66 radians. Although the energy varies periodically in all cases, the maximum energy considerably decreases with fourth order anisotropy constant. According to 3-D plots, energy under influence of second order anisotropy is larger than energy under influence of fourth order anisotropy.",1611.02228v1 2016-11-09,Possible Weyl fermions in the magnetic Kondo system CeSb,"Materials where the electronic bands have unusual topologies allow for the realization of novel physics and have a wide range of potential applications. When two electronic bands with linear dispersions intersect at a point, the excitations could be described as Weyl fermions which are massless particles with a particular chirality. Here we report evidence for the presence of Weyl fermions in the ferromagnetic state of the low-carrier density, strongly correlated Kondo lattice system CeSb, from electronic structure calculations and angle-dependent magnetoresistance measurements. When the applied magnetic field is parallel to the electric current, a pronounced negative magnetoresistance is observed within the ferromagnetic state, which is destroyed upon slightly rotating the field away. These results give evidence for CeSb belonging to a new class of Kondo lattice materials with Weyl fermions in the ferromagnetic state.",1611.02927v2 2016-11-10,Laser-induced ultrafast demagnetization time and spin moment in ferromagnets: First-principles calculation,"When a laser pulse excites a ferromagnet, its spin undergoes a dramatic change. The initial demagnetization process is very fast. Experimentally, it is found that the demagnetization time is related to the spin moment in the sample. In this study, we employ the first-principles method to directly simulate such a process. We use the fixed spin moment method to change the spin moment in ferromagnetic nickel, and then we employ the Liouville equation to couple the laser pulse to the system. We find that in general the dependence of demagnetization time on the spin moment is nonlinear: It decreases with the spin moment up to a point, after which an increase with the spin moment is observed, followed by a second decrease. To understand this, we employ an extended Heisenberg model, which includes both the exchange interaction and spin-orbit coupling. The model directly links the demagnetization rate to the spin moment itself and demonstrates analytically that the spin relaxes more slowly with a small spin moment. A future experimental test of our predictions is needed.",1611.03154v1 2016-11-21,Coherence and stiffness of spin waves in diluted ferromagnets,"We present results of a numerical analysis of magnon spectra in supercells simulating two-dimensional and bulk random diluted ferromagnets with long-ranged pair exchange interactions. We show that low-energy spectral regions for these strongly disordered systems contain a coherent component leading to interference phenomena manifested by a pronounced sensitivity of the lowest excitation energies to the adopted boundary conditions. The dependence of configuration averages of these excitation energies on the supercell size can be used for an efficient determination of the spin-wave stiffness D. The developed formalism is applied to the ferromagnetic Mn-doped GaAs semiconductor with optional incorporation of phosphorus; the obtained concentration trends of D are found in reasonable agreement with recent experiments. Moreover, a relation of the spin stiffness to the Curie temperature Tc has been studied for Mn-doped GaAs and GaN semiconductors. It is found that the ratio Tc/D exhibits qualitatively the same dependence on Mn concentration in both systems.",1611.06691v1 2016-11-21,Effect of surface hybridization on RKKY coupling in ferromagnet/topological insulator/ferromagnet trilayer system,"We theoretically investigate the RKKY exchange coupling between two ferromagnets (FM) separated by a thin topological insulator film (TI). We find an unusual dependence of the RKKY exchange coupling on the TI thickness ($t_{TI}$). First, when $t_{TI}$ decreases, the coupling amplitude increases at first and reaches its maximum value at some critical thickness, below which the amplitude turns to diminish. This trend is attributed to the hybridization between surfaces of the TI film, which opens a gap below critical thickness and thus turns the surfaces into insulating state from semi-metal state. In insulating phase, diamagnetism induced by the gap-opening compensates paramagnetism of Dirac state, resulting in a diminishing magnetic susceptibility and RKKY coupling. For typical parameters, the critical thickness in Bi2Se3 thin film is estimated to be in the range of 3-5 nm.",1611.06710v2 2017-01-23,Unidirectional spin Hall magnetoresistance in topological insulator/ferromagnetic layer heterostructures,"The large spin orbit coupling in topological insulators results in helical spin-textured Dirac surface states that are attractive for topological spintronics. These states generate an efficient spin-orbit torque on proximal magnetic moments at room temperature. However, memory or logic spin devices based upon such switching require a non-optimal three terminal geometry, with two terminals for the writing current and one for reading the state of the device. An alternative two terminal device geometry is now possible by exploiting the recent discovery of a unidirectional spin Hall magnetoresistance in heavy metal/ferromagnet bilayers and (at low temperature) in magnetically doped topological insulator heterostructures. We report the observation of unidirectional spin Hall magnetoresistance in a technologically relevant device geometry that combines a topological insulator with a conventional ferromagnetic metal. Our devices show a figure-of-merit (magnetoresistance per current density per total resistance) that is comparable to the highest reported values in all-metal Ta/Co bilayers.",1701.06505v1 2017-01-24,Pumping of magnons in a Dzyaloshinskii-Moriya ferromagnet,"We formulate a microscopic linear response theory of nonequilibrium magnonic torques and magnon pumping applicable to multiple-magnonic-band uniform ferromagnets with Dzyaloshinskii-Moriya interactions. From the linear response theory, we identify the extrinsic and intrinsic contributions where the latter is expressed via the Berry curvature of magnonic bands. We observe that in the presence of a time-dependent magnetization Dzyaloshinskii-Moriya interactions can act as fictitious electric fields acting on magnons. We study various current responses to this fictitious field and analyze the role of Berry curvature. After identifying the magnon-mediated contribution to the equilibrium Dzyaloshinskii-Moriya interaction, we also establish the Onsager reciprocity between the magnon-mediated torques and heat pumping. We apply our theory to the magnonic heat pumping and torque responses in honeycomb and kagome lattice ferromagnets.",1701.06990v3 2017-01-28,Novel Majorana mode and magnetoresistance in ferromagnetic superconducting topological insulator,"Among the potential applications of topological insulators, we investigate theoretically the effect of coexistence of proximity-induced ferromagnetism and superconductivity on the surface states of 3-dimensional topological insulator, where the superconducting electron-hole excitations can be significantly affected by the magnetization of ferromagnetic order. We find that, Majorana mode energy, as a verified feature of TI F/S structure, along the interface sensitively depends on the magnitude of magnetization $m_{zfs}$ in FS region, while its slope in perpendicular incidence presents steep and no change. Since the superconducting gap is renormalized by a factor $\eta(m_{zfs})$, hence Andreev reflection is more or less suppressed, and, in particular, resulting subgap tunneling conductance is more sensitive to the magnitude of magnetizations in FS and F regions. Furthermore, an interesting scenario happens at the antiparallel configuration of magnetizations $m_{zf}$ and $m_{zfs}$ resulting in magnetoresistance in N/F/FS junction, which can be controlled and decreased by tuning the magnetization magnitude in FS region.",1701.08275v1 2017-04-03,Itinerant ferromagnetism of two-dimensional repulsive fermions with Rabi coupling,"We study a two-dimensional fermionic cloud of repulsive alkali-metal atoms characterized by two hyperfine states which are Rabi coupled. Within a variational Hartree-Fock scheme, we calculate analytically the ground-state energy of the system. Then we determine the conditions under which there is a quantum phase transition with spontaneous symmetry breaking from a spin-balanced configuration to a spin-polarized one, an effect known as itinerant ferromagnetism. Interestingly, we find that the transition appears when the interaction energy per particle exceedes both the kinetic energy per particle and the Rabi coupling energy. The itinerant ferromagnetism of the polarized phase is analyzed, obtaining the population imbalance as a function of interaction strength, Rabi coupling, and number density. Finally, the inclusion of a external harmonic confinement is investigated by adopting the local density approximation. We predict that a single atomic cloud can display population imbalance near the center of the trap and a fully balanced configuration at the periphery.",1704.00483v1 2017-04-11,Role of polar compensation in interfacial ferromagnetism of LaNiO$_3$/CaMnO$_3$ superlattices,"Polar compensation can play an important role in the determination of interfacial electronic and magnetic properties in oxide heterostructures. Using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, bulk magnetometry, and transport measurements, we find that interfacial charge redistribution via polar compensation is essential for explaining the evolution of interfacial ferromagnetism in LaNiO$_3$/CaMnO$_3$ superlattices as a function of LaNiO$_3$ layer thickness. In insulating superlattices (4 unit cells or less of LaNiO$_3$), magnetism is dominated by Ni-Mn superexchange, while itinerant electron-based Mn-Mn double-exchange plays a role in thicker metallic superlattices. X-ray magnetic circular dichroism and resonant x-ray scattering show that Ni-Mn superexchange contributes to the magnetization even in metallic superlattices. This Ni-Mn superexchange interaction can be explained in terms of polar compensation at the LaNiO$_3$-CaMnO$_3$ interface. These results highlight the different mechanisms responsible for interfacial ferromagnetism and the importance of understanding compensation due to polar mismatch at oxide-based interfaces when engineering magnetic properties.",1704.03163v1 2017-04-12,Cycloidally modulated magnetic order stabilized by thermal fluctuations in the Néel-type skyrmion host GaV$_4$S$_8$,"We report small-angle neutron scattering studies of the lacunar spinel GaV$_4$S$_8$, which reveal the long-wavelength magnetic states to be cycloidally modulated. This provides direct support for the formation of N\'eel-type skyrmions recently claimed to exist in this compound. In striking contrast with all other bulk skyrmion host materials, upon cooling the modulated magnetic states transform into a ferromagnetic state. These results indicate all of the modulated states in GaV$_4$S$_8$, including the skyrmion state, gain their stability from thermal fluctuations, while at lower temperature the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. In the vicinity of the transition between the ferromagnetic and modulated states, both a phase coexistence and a soliton-like state are also evidenced by our study.",1704.03621v1 2017-04-22,Spin Density wave instability in a ferromagnet,"Ferromagnetic (FM) and incommensurate spin-density wave (ISDW) states are an unusual set of competing magnetic orders that are seldom observed in the same material without application of a polarizing magnetic field. We report, for the first time, the discovery of an ISDW state that is derived from a FM ground state through a Fermi surface (FS) instability in Fe$_3$Ga$_4$. This was achieved by combining neutron scattering experiments with first principles simulations. Neutron diffraction demonstrates that Fe$_3$Ga$_4$ is in an ISDW state at intermediate temperatures and that there is a conspicuous re-emergence of ferromagnetism above 360 K. First principles calculations show that the ISDW ordering wavevector is in excellent agreement with a prominent nesting condition in the spin-majority FS demonstrating the discovery of a novel instability for FM metals; ISDW formation due to Fermi surface nesting in a spin-polarized Fermi surface.",1704.06727v2 2017-04-25,Unified description of dynamics of a repulsive two-component Fermi gas,"We study a binary spin-mixture of a zero-temperature repulsively interacting $^6$Li atoms using both the atomic-orbital and the density functional approaches. The gas is initially prepared in a configuration of two magnetic domains and we determine the frequency of the spin-dipole oscillations which are emerging after the repulsive barrier, initially separating the domains, is removed. We find, in agreement with recent experiment (G. Valtolina et al., arXiv:1605.07850 (2016)), the occurrence of a ferromagnetic instability in an atomic gas while the interaction strength between different spin states is increased, after which the system becomes ferromagnetic. The ferromagnetic instability is preceded by the softening of the spin-dipole mode.",1704.07707v2 2017-04-27,Edge-controlled half-metallic ferromagnetism and direct gap in ZrS2 nanoribbons,"The electronic and magnetic properties of ZrS2 nanoribbons (NRs) are investigated based on the first-principles calculations. It is found that the ZrS2 NRs with armchair edges are all indirect-band-gap semiconductors without magnetism and the band-gap exhibits odd-even oscillation behavior with the increase of the ribbon width. For the NRs with zigzag edges, those with both edges S-terminated are nonmagnetic direct-band-gap semiconductors and the gap decreases monotonically as a function of the ribbon width. However, the NRs with one edge S-terminated and the other edge Zr-terminated are ferromagnetic half-metal, while those with both edges Zr-terminated tend to be ferromagnetic half-metal when the width $N\geq9$. The magnetism of both systems mainly originates from the unsaturated edge Zr atoms. Depending on the different edge configurations and ribbon widths, the ZrS2 NRs exhibit versatile electronic and magnetic properties, making them promising candidates for the applications of electronics and spintronics.",1704.08459v1 2018-05-07,Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets,"The concepts of Weyl fermions and topological semimetals emerging in three-dimensional momentum space are extensively explored owing to the vast variety of exotic properties that they give rise to. On the other hand, very little is known about semimetallic states emerging in two-dimensional magnetic materials, which present the foundation for both present and future information technology. Here, we demonstrate that including the magnetization direction into the topological analysis allows for a natural classification of topological semimetallic states that manifest in two-dimensional ferromagnets as a result of the interplay between spin-orbit and exchange interactions. We explore the emergence and stability of such mixed topological semimetals in realistic materials, and point out the perspectives of mixed topological states for current-induced orbital magnetism and current-induced domain wall motion. Our findings pave the way to understanding, engineering and utilizing topological semimetallic states in two-dimensional spin-orbit ferromagnets.",1805.02549v3 2018-05-08,Effects of Lifshitz transitions in ferromagnetic superconductors: the case of URhGe,"In ferromagnetic superconductors, like URhGe, superconductivity co-exists with magnetism near zero field, but then re-appears again in a finite field range, where the system also displays mass enhancement in the normal state. We present theoretical understanding of this non-monotonic behavior. We explore the multi-band nature of URhGe and associate re-entrant superconductivity and mass enhancement with the finite field Lifshitz transition in one of the bands. We found good agreement between our theory and a number of experimental results for URhGe, such as weakly first order reentrant transition, the dependence of superconducting $T_c$ on a magnetic field, and the field dependence of the effective mass, the specific heat and the resistivity in the normal state. Our theory can be applied to other ferromagnetic multi-band superconductors.",1805.02949v1 2018-05-22,Spin-orbit torque in completely compensated synthetic antiferromagnet,"Synthetic antiferromagnets (SAF) have been proposed to replace ferromagnets in magnetic memory devices to reduce the stray field, increase the storage density and improve the thermal stability. Here we investigate the spin-orbit torque in a perpendicularly magnetized Pt/[Co/Pd]/Ru/[Co/Pd] SAF structure, which exhibits completely compensated magnetization and an exchange coupling field up to 2100 Oe. The magnetizations of two Co/Pd layers can be switched between two antiparallel states simultaneously by spin-orbit torque. The magnetization switching can be read out due to much stronger spin-orbit coupling at bottom Pt/[Co/Pd] interface compared to its upper counterpart without Pt. Both experimental and theoretical analyses unravel that the torque efficiency of antiferromagnetic coupled stacks is significantly higher than the ferromagnetic counterpart, making the critical switching current of SAF comparable to the conventional single ferromagnet. Besides adding an important dimension to spin-orbit torque, the efficient switching of completely compensated SAF might advance magnetic memory devices with high density, high speed and low power consumption.",1805.08486v2 2018-05-23,Special temperatures in frustrated ferromagnets,"The description and detection of unconventional magnetic states such as spin liquids is a recurring topic in condensed matter physics. While much of the efforts have traditionally been directed at geometrically frustrated antiferromagnets, recent studies reveal that systems featuring competing antiferromagnetic and ferromagnetic interactions are also promising candidate materials. We find that this competition leads to the notion of special temperatures, analogous to those of gases, at which the competing interactions balance, and the system is quasi-ideal. Although induced by weak perturbing interactions, these special temperatures are surprisingly high and constitute an accessible experimental diagnostic of eventual order or spin liquid properties. The well characterised Hamiltonian and extended low-temperature susceptibility measurement of the canonical frustrated ferromagnet Dy$_2$Ti$_2$O$_7$ enables us to formulate both a phenomenological and microscopic theory of special temperatures for magnets. Other members of this new class of magnets include kapellasite Cu$_3$Zn(OH)$_6$Cl$_2$ and the spinel GeCo$_2$O$_4$.",1805.09332v1 2018-05-31,Second-harmonic current-phase relation in Josephson junctions with ferromagnetic barriers,"We report the observation of a current-phase relation dominated by the second Josephson harmonic in superconductor-ferromagnet-superconductor junctions. The exotic current-phase relation is realized in the vicinity of a temperature-controlled 0-to-$\pi$ junction transition, at which the first Josephson harmonic vanishes. Direct current-phase relation measurements, as well as Josephson interferometry, non-vanishing supercurrent and half-integer Shapiro steps at the 0-$\pi$ transition self-consistently point to an intrinsic second harmonic term, making it possible to rule out common alternative origins of half-periodic behavior. While surprising for diffusive multimode junctions, the large second harmonic is in agreement with theory predictions for thin ferromagnetic interlayers.",1805.12546v2 2008-07-11,"Structural Disorder, Octahedral Coordination, and 2-Dimensional Ferromagnetism in Anhydrous Alums","The crystal structures of the triangular lattice, layered anhydrous alums KCr(SO4)2, RbCr(SO4)2 and KAl(SO4)2 are characterized by X-ray and neutron powder diffraction at temperatures between 1.4 and 773 K. The compounds all crystallize in the space group P-3, with octahedral coordination of the trivalent cations. In all cases, small amounts of disorder in the stacking of the triangular layers of corner sharing MO6 octahedra and SO4 tetrahedra is seen, with the MO6-SO4 network rotated in opposite directions between layers. The electron diffraction study of KCr(SO4)2 supports this model, which on average can be taken to imply trigonal prismatic coordination for the M3+ ions; as was previously reported for the prototype anhydrous alum KAl(SO4)2. The temperature dependent magnetic susceptibilities for ACr(SO4)2 (A = K,Rb,Cs) indicate the presence of predominantly ferromagnetic interactions. Low temperature powder neutron diffraction reveals that the magnetic ordering is ferromagnetic in-plane, with antiferromagnetic ordering between planes below 3 K.",0807.1895v1 2008-07-12,Micromagnetic simulations of small arrays of submicron ferromagnetic particles,"We report the results of a set of simulations of small arrays of submicron ferromagnetic particles. The actions of dipolar and exchange interactions were qualitatively investigated by analysing the ferromagnetic resonance spectra at 9.37 GHz resulting from the magnetization response of con- nected and unconnected particles in the array as a function of the applied dc magnetic field. We find that the magnetization precession movement (at resonance) observed in individual particles in the array presents a distinctive behaviour (an amplitude mismatch) in comparison to isolated, one-particle reference simulations, a result that we attribute to the action of interparticle dipolar couplings. Exchange interactions appear to have an important role in modifying the spectra of connected particles, even through a small contact surface.",0807.1978v1 2008-07-15,Physical realization and possible identification of topological excitations in quantum Heisenberg anti-ferromagnet on a two dimensional lattice,"Physical spin configurations corresponding to topological excitations, expected to be present in the XY limit of a quantum spin 1/2 Heisenberg anti-ferromagnet, are probed on a two dimensional square lattice . Quantum vortices (anti-vortices) are constructed in terms of coherent staggered spin field components, as limiting case of meronic (anti-meronic) configurations . The crucial role of the associated Wess-Zumino-like (WZ-like) term is highlighted in our procedure . The time evolution equation of coherent spin fields used in this analysis is obtained by applying variational principle on the quantum Euclidean action corresponding to the Heisenberg anti-ferromagnet on lattice . It is shown that the WZ-like term can distinguish between vortices and anti-vortices only in a charge sector with odd topological charges. Our formalism is distinctly different from the conventional approach for the construction of quantum vortices (anti-vortices) .",0807.2359v2 2008-07-23,Metal-insulator transition by isovalent anion substitution in Ga1-xMnxAs: Implications to ferromagnetism,"We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4% of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4 % As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature TC from 113 K to 60 K (100 K to 65 K) upon the introduction of 3.1 % P (1% N) into the As sublattice.",0807.3722v1 2008-07-28,Critical Scaling of the Magnetization and Magnetostriction in the Weak Itinerant Ferromagnet UIr,"The weak itinerant ferromagnet UIr is studied by magnetization and magnetostriction measurements. Critical behavior, which surprisingly extends up to several Tesla, is observed at the Curie temperature $T_C\simeq45$ K and is analyzed using Arrott and Maxwell relations. Critical exponents are found that do not match with any of the well-known universality classes. The low-temperature magnetization $M_s\simeq0.5$ $\mu_B \cong const.$ below 3 T rises towards higher fields and converges asymptotically around 50 T with the magnetization at $T_C$. From the magnetostriction and magnetization data, we extract the uniaxial pressure dependences of $T_C$, using a new method presented here, and of $M_s$. These results should serve as a basis for understanding spin fluctuations in anisotropic itinerant ferromagnets.",0807.4411v1 2009-12-01,Observation of spin-triplet superconductivity in Co-based Josephson Junctions,"We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly-ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.",0912.0205v1 2009-12-06,The Correlation Functions of the XXZ Heisenberg Chain for Zero or Infinite Anisotropy and Random Walks of Vicious Walkers,"The XXZ Heisenberg chain is considered for two specific limits of the anisotropy parameter: $\Dl\to 0$ and $\Dl\to -\infty$. The corresponding wave functions are expressed by means of the symmetric Schur functions. Certain expectation values and thermal correlation functions of the ferromagnetic string operators are calculated over the base of N-particle Bethe states. The thermal correlator of the ferromagnetic string is expressed through the generating function of the lattice paths of random walks of vicious walkers. A relationship between the expectation values obtained and the generating functions of strict plane partitions in a box is discussed. Asymptotic estimate of the thermal correlator of the ferromagnetic string is obtained in the limit of zero temperature. It is shown that its amplitude is related to the number of plane partitions.",0912.1138v2 2009-12-08,Evidence of superconductivity on the border of quasi-2D ferromagnetism in Ca2RuO4 at high pressure,"The layered perovskite Ca2RuO4 is a spin-one Mott insulator at ambient pressure and exhibits metallic ferromagnetism at least up to ~ 80 kbar with a maximum Curie temperature of 28 K. Above ~ 90 kbar and up to 140 kbar, the highest pressure reached, the resistivity and ac susceptibility show pronounced downturns below ~ 0.4 K in applied magnetic fields of up to ~10 mT. This indicates that our specimens of Ca2RuO4 are weakly superconducting on the border of a quasi-2D ferromagnetic state.",0912.1513v1 2009-12-17,Surface magnetism in ZnO/Co3O4 mixtures,"We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides respectively. Here we present a detailed study on the electronic structure of this material in order to account for this unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction of Co+3 to Co2+ at the particle surface takes place as evidenced by XAS measurements and optical spectrocopy. This reduction allows to xplain the observed ferromagnetic signal within the well established theories of magnetism.",0912.3458v1 2011-11-09,A dynamical phase transition in ferromagnetic granular materials,"We study, using simulations the dynamical properties of complex ferromagnetic granular materials. The system of grains is modeled by a disordered two-dimensional lattice in which the grains are embedded, while the magnitude and direction of the easy axis are random. Using the monte-carlo method we track the dynamics of the magnetic moments of the grains. We observe a transition of the system from a macroscopic blocked (ferromagnetic) phase at low temperature in which the grain's magnetic moment do not flip to the other direction to an unblocked (superparamagnetic) phase at high temperature in which the magnetic moment is free to rotate. Our results suggest that this transition exhibits the characteristics of a second order phase transition such as the appearance of a giant cluster of unblocked grains which is fractal at the critical temperature, a peak in the size of the second largest cluster at the same temperature and a power law distribution of cluster sizes near the criticality.",1111.2130v1 2011-11-14,Callen-like method for the classical Heisenberg ferromagnet,"A study of the d-dimensional classical Heisenberg ferromagnetic model in the presence of a magnetic field is performed within the two-time Green function's framework in classical statistical physics. We extend the well known quantum Callen method to derive analytically a new formula for magnetization. Although this formula is valid for any dimensionality, we focus on one- and three- dimensional models and compare the predictions with those arising from a different expression suggested many years ago in the context of the classical spectral density method. Both frameworks give results in good agreement with the exact numerical transfer-matrix data for the one-dimensional case and with the exact high-temperature-series results for the three-dimensional one. In particular, for the ferromagnetic chain, the zero-field susceptibility results are found to be consistent with the exact analytical ones obtained by M.E. Fisher. However, the formula derived in the present paper provides more accurate predictions in a wide range of temperatures of experimental and numerical interest.",1111.3177v1 2011-11-14,Nambu-Goldstone dynamics and generalized coherent-state functional integrals,"The present paper gives a new method of attack on the Nambu-Goldstone dynamics in spontaneously broken theories. Since the target space of the Nambu-Goldstone fields is a group coset space, their effective quantum dynamics can be naturally phrased in terms of generalized coherent-state functional integrals. As an explicit example of this line of reasoning we construct a low-energy effective Lagrangian for the Heisenberg ferromagnet in broken phase. The leading field configuration in the WKB approximation leads to the Landau-Lifshitz equation for quantum ferromagnet. The corresponding linearized equations allow to identify the Nambu-Goldstone boson with ferromagnetic magnon.",1111.3228v2 2011-11-16,Adiabatic quantum pumping through surface states in 3D topological insulators,"We investigate adiabatic quantum pumping of Dirac fermions on the surface of a strong 3D topological insulator. Two different geometries are studied in detail, a normal metal -- ferromagnetic -- normal metal (NFN) junction and a ferromagnetic -- normal metal -- ferromagnetic (FNF) junction. Using a scattering matrix approach, we first calculate the tunneling conductance and then the adiabatically pumped current using different pumping mechanisms for both types of junctions. We explain the oscillatory behavior of the conductance by studying the condition for resonant transmission in the junctions and find that each time a new resonant mode appears in the transport window, the pumped current diverges. We also predict an experimentally distinguishable difference between the pumped current and the rectified current.",1111.3927v1 2011-11-21,Electron-phonon scattering dynamics in ferromagnets on ultrafast timescales: Influence of the phonon temperature,"The magnetization response of bulk ferromagnets after excitation by an ultrashort optical pulse is calculated using a dynamical model of the Elliott-Yafet type that includes the effects of the spin-orbit interaction in the ab-initio ferromagnetic band structure, the electron-phonon interaction at the level of Boltzmann scattering integrals, and dynamical changes in the temperature of the phonon bath. Using realistic parameters for the ultrashort optical pulse, the computed maximum magnetization quenching achievable with electron-phonon scattering in a fixed band structure is much smaller than the quenching observed in experiments. Heating of the phonon bath is found to not appreciably change the magnetization dynamics on ultrashort timescales.",1111.4783v1 2011-11-21,Ab initio theory of galvanomagnetic phenomena in ferromagnetic metals and disordered alloys,"We present an ab initio theory of transport quantities of metallic ferromagnets developed in the framework of the fully relativistic tight-binding linear muffin-tin orbital method. The approach is based on the Kubo-Streda formula for the conductivity tensor, on the coherent potential approximation for random alloys, and on the concept of interatomic electron transport. The developed formalism is applied to pure 3d transition metals (Fe, Co, Ni) and to random Ni-based ferromagnetic alloys (Ni-Fe, Ni-Co, Ni-Mn). High values of the anisotropic magnetoresistance (AMR), found for Ni-rich alloys, are explained by a negligible disorder in the majority spin channel while a change of the sign of the anomalous Hall effect (AHE) on alloying is interpreted as a band-filling effect without a direct relation to the high AMR. The influence of disorder on the AHE in concentrated alloys is investigated as well.",1111.4793v2 2011-11-23,Anomalous polarization-dependent transport in nanoscale double-barrier superconductor/ferromagnet/superconductor junctions,"We study the transport properties of nanoscale superconducting (S) devices in which two superconducting electrodes are bridged by two parallel ferromagnetic (F) wires, forming an SFFS junction with a separation between the two wires less than the superconducting coherence length. This allows crossed Andreev reflection to take place. We find that the resistance as a function of temperature exhibits behavior reminiscent of the re-entrant effect and, at low temperatures and excitation energies below the superconducting gap, the resistance corresponding to antiparallel alignment of the magnetization of the ferromagnetic wires is higher than that of parallel alignment, in contrast to the behavior expected from crossed Andreev reflection. We present a model based on spin-dependent interface scattering that explains this surprising result and demonstrates the sensitivity of the junction transport properties to interfacial parameters.",1111.5415v3 2011-11-28,Investigation of weak itinerant ferromagnetism and critical behavior of Y$_2$Ni$_7$,"The weak itinerant ferromagnetic behavior of Y$_2$Ni$_7$ is investigated through magnetic, transport and calorimetric measurements. The low value of saturation moment, large Rhodes-Wohlfarth ratio, large value of the linear term in heat capacity and Fermi liquid like resistivity behavior with enhanced electron-electron scattering contribution establish firmly the weak itinerant ferromagetic nature of the sample. The critical exponents associated with the paramagnetic to ferromagnetic transition are also investigated from magnetization isotherms using modified Arrott plot, Kouvel-Fisher plot and critical isotherm technique. The more accurate Kouvel-Fisher plot provides the critical exponents to be $\beta$ = 0.306, $\gamma$ = 1.401 and $\delta$ = 5.578. These values are markedly different from the mean field values and correspond closely to the two dimensional Ising spin system with long range spin spin interaction.",1111.6343v1 2012-01-16,^31P NMR investigations on the ferromagnetic quantum critical system YbNi_4P_2,"We studied the new heavy-fermion system YbNi$_{4}$P$_{2}$, which presents strong ferromagnetic correlations, using the local $^{31}$P NMR probe over a wide field (0.2-8.6 \textsf{T}) and temperature (1.8-200 K) range. $^{31}$P NMR Knight shift provides the static spin susceptibility which tracks the bulk susceptibility whereas the spin-lattice relaxation rate $^{31}(1/T_{1}$) provide information about the fluctuations of the Yb 4f moment. The Korringa law is valid over a wide range in temperature and field. The Korringa product $^{31}(1/T_{1}TK^{2}$S$_{0}$) $\ll$ 1 gives evidence for the presence of strong ferromagnetic correlations. Over two decades in temperature a $^{31}(1/T_{1}T) \sim T^{-3/4}$ behaviour was found.",1201.3260v1 2012-01-18,Quantum order-by-disorder driven phase reconstruction in the vicinity of ferromagnetic quantum critical points,"The formation of new phases close to itinerant electron quantum critical points has been observed experimentally in many compounds. We present a unified analytical model that explains the emergence of new types of order around itinerant ferromagnetic quantum critical points. The central idea of our analysis is that certain Fermi-surface deformations associated with the onset of the competing order enhance the phase-space available for low-energy quantum fluctuations and so self-consistently lower the free energy. We demonstrate that this quantum order-by-disorder mechanism leads to instabilities towards the formation of spiral and d-wave spin nematic phases close to itinerant ferromagnetic quantum critical points in three spatial dimensions.",1201.3916v2 2012-01-19,Spin and valley quantum Hall ferromagnetism in graphene,"In a graphene Landau level (LL), strong Coulomb interactions and the fourfold spin/valley degeneracy lead to an approximate SU(4) isospin symmetry. At partial filling, exchange interactions can spontaneously break this symmetry, manifesting as additional integer quantum Hall plateaus outside the normal sequence. Here we report the observation of a large number of these quantum Hall isospin ferromagnetic (QHIFM) states, which we classify according to their real spin structure using temperature-dependent tilted field magnetotransport. The large measured activation gaps confirm the Coulomb origin of the broken symmetry states, but the order is strongly dependent on LL index. In the high energy LLs, the Zeeman effect is the dominant aligning field, leading to real spin ferromagnets with Skyrmionic excitations at half filling, whereas in the `relativistic' zero energy LL, lattice scale anisotropies drive the system to a spin unpolarized state, likely a charge- or spin-density wave.",1201.4167v1 2012-04-02,On the influence of nanometer-thin antiferromagnetic surface layer on ferromagnetic CrO$_2$,"We present magnetic stray field measurements performed on a single micro-crystal of the half metallic ferromagnet CrO$_2$, covered by a naturally grown 2\,-\,5\,nm surface layer of antiferromagnetic (AFM) Cr$_2$O$_3$. The temperature variation of the stray field of the micro-crystal measured by micro-Hall magnetometry shows an anomalous increase below $\sim$\,60\,K. We find clear evidence that this behavior is due to the influence of the AFM surface layer, which could not be isolated in the corresponding bulk magnetization data measured using SQUID magnetometry. The distribution of pinning potentials, analyzed from Barkhausen jumps, exhibits a similar temperature dependence. Overall, the results indicate that the surface layer plays a role in defining the potential landscape seen by the domain configuration in the ferromagnetic grain.",1204.0531v1 2012-04-04,Theory of ferromagnetic unconventional superconductors with spin-triplet electron pairing,"A general phenomenological theory is presented for the phase behavior of ferromagnetic superconductors with spin-triplet electron Cooper pairing. The theory describes in details the temperature-pressure phase diagrams of real inter-metallic compounds exhibiting the remarkable phenomenon of coexistence of spontaneous magnetic moment of the itinerant electrons and spin-triplet superconductivity. The quantum phase transitions which may occur in these systems are also described. The theory allows for a classification of these itinerant ferromagnetic superconductors in two types: type I and type II. The classification is based on quantitative criteria.The comparison of theory and experiment is performed and outstanding problems are discussed.",1204.1007v2 2012-04-04,Carrier-controlled ferromagnetism in SrTiO3,"Magnetotransport and superconducting properties are investigated for uniformly La-doped SrTiO3 films and GdTiO3/SrTiO3 heterostructures, respectively. GdTiO3/SrTiO3 interfaces exhibit a high-density two-dimensional electron gas on the SrTiO3-side of the interface, while for the SrTiO3 films carriers are provided by the dopant atoms. Both types of samples exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. For the uniformly doped SrTiO3 films, the Curie temperature is found to increase with doping and to coexist with superconductivity for carrier concentrations on the high-density side of the superconducting dome. The Curie temperature of the GdTiO3/SrTiO3 heterostructures scales with the thickness of the SrTiO3 quantum well. The results are used to construct a stability diagram for the ferromagnetic and superconducting phases of SrTiO3.",1204.1081v2 2012-04-13,"Comment on: ""Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite"", by M. Sepioni, R.R. Nair, I.-Ling Tsai, A.K. Geim and I.V. Grigorieva, EPL 97 (2012) 47001","This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity contamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.",1204.2992v1 2012-04-22,Spin Hall effect versus Rashba torque: a Diffusive Approach,"Current-driven magnetization dynamics of single ferromagnets in heavy metal/ferromagnet bilayers has been recently realized. In this work, spin torque induced by so-called Rashba spin-orbit coupling and spin Hall effect are considered within a diffusive model. The dependence of the resulting torque as a function of the thicknesses of the ferromagnet and heavy metal is analyzed. We show that (i) both torques are on the form ${\bf T}=T_{||}{\bf m}\times{\bf y}+T_\bot{\bf m}\times({\bf y}\times{\bf m})$, (ii) the ratio $T_{||}/T_\bot$ strongly depends on the thickness of the layers and (iii) the thickness dependence of the spin torque provides an indication of the origin of the (Rashba- or spin Hall effect-induced) spin torque .",1204.4869v1 2012-04-22,Anomalous ferromagnetism and non-Fermi-liquid behavior in the Kondo lattice CeRuSi2,"The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi2 are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and \mu SR measurements. Data on specific heat, resistivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, the non-Fermi-liquid behavior is observed at low temperatures, which is unusual for a ferromagnetic Kondo system. A comparison with other magnetic Kondo lattices is performed.",1204.4903v3 2012-04-24,Nonlocal feedback in ferromagnetic resonance,"Ferromagnetic resonance in thin films is analyzed under the influence of spatiotemporal feedback effects. The equation of motion for the magnetization dynamics is nonlocal in both space and time and includes isotropic, anisotropic and dipolar energy contributions as well as the conserved Gilbert- and the non-conserved Bloch-damping. We derive an analytical expression for the peak-to-peak linewidth. It consists of four separate parts originated by Gilbert damping, Bloch-damping, a mixed Gilbert-Bloch component and a contribution arising from retardation. In an intermediate frequency regime the results are comparable with the commonly used Landau-Lifshitz-Gilbert theory combined with two-magnon processes. Retardation effects together with Gilbert damping lead to a linewidth the frequency dependence of which becomes strongly nonlinear. The relevance and the applicability of our approach to ferromagnetic resonance experiments is discussed.",1204.5342v1 2012-04-25,Interface Metallic States between a Topological Insulator and a Ferromagnetic Insulator,"We study electronic structures at an interface between a topological insulator and a ferromagnetic insulator by using three-dimensional two-band model. In usual ferromagnetic insulators, the exchange potential is much larger than the bulk gap size in the topological insulators and electronic structures are asymmetric with respect to the fermi level. In such situation, we show that unusual metallic states appear under the magnetic moment pointing the perpendicular direction to the junction plane, which cannot be described by the two-dimensional effective model around the Dirac point. When the magnetic moment is in the parallel direction to the plane, the number of Dirac cones becomes even integers. The conclusions obtained in analytical calculations are confirmed by numerical simulations on tight-binding lattice.",1204.5562v1 2012-04-27,Magnetically-driven electronic phase separation in the semimetallic ferromagnet EuB$_6$,"From measurements of fluctuation spectroscopy and weak nonlinear transport on the semimetallic ferromagnet EuB$_6$ we find direct evidence for magnetically-driven electronic phase separation consistent with the picture of percolation of magnetic polarons (MP), which form highly conducting magnetically-ordered clusters in a paramagnetic and 'poorly conducting' background. These different parts of the conducting network are probed separately by the noise spectroscopy/nonlinear transport and the conventional linear resistivity. We suggest a comprehensive and 'universal' scenario for the MP percolation, which occurs at a critical magnetization either induced by ferromagnetic order at zero field or externally applied magnetic fields in the paramagentic region.",1204.6222v1 2013-09-12,Cosmic Neutrino Background as a Ferromagnet,"If cosmic background neutrinos interact very weakly with each other, through spin-spin interactions, then they may have experienced a phase transition, leading to a ferromagnetic ordering. The small magnetic field resulting from ferromagnetic ordering -- if present before galaxy formation -- could act as a primordial seed of the magnetic fields observed in several galaxies. Our findings suggest that the magnetization could occur in the right epoch, if the exchange boson of neutrino-neutrino interaction is a massless boson beyond the Standard Model, with a coupling constant of $2.2\times 10^{-13} \left(\frac{m_\nu}{10^{-4}\,\rm{eV}}\right)^21/2, there is an noncollinear canted state. For the quantum case, we found that, although the classical Neel state is absent, the canted state exists if the frustration parameter a exceeds a critical point ac1. The precise critical point ac1 can be determined by using the coupled cluster method and the numerical exact diagonalization method separately. The results of the coupled cluster method and the exact diagonalization method both disclose that the type of phase transition occurring at ac1 changes from a classical second-order transition to a quantum first-order transition due to quantum fluctuation. Although there is another critical point ac2 in a finite system at which the ground state evolves from the canted state to the collinear Neel plus ferromagnetic state, that state is absent because ac2 tends to infinity in the thermodynamic limit.",1406.4317v1 2014-06-17,Paramagnetic to ferromagnetic phase transition in lightly Fe-doped Cr$_2$B,"Cr$_2$B displays temperature independent paramagnetism. We induce ferromagnetism by replacing less than $3\,\%$ of the Cr atoms by Fe. By the lowest Fe doping level made, Curie-Weiss behavior is observed; $\Theta_{CW}$ changes from $-20\,$K for $0.5\,\%$ Fe-doped Cr$_2$B to positive values of about 50 K by $5\,\%$ Fe doping. The ferromagnetic T$_C$ is 8 K for $2.5\,\%$ Fe doping and increases linearly to 46 K by $5\,\%$ doping; we infer that a quantum phase transition occurs near the $2.0\,\%$ Fe level. Magnetic fluctuations at the intermediate doping levels are reflected in the linear resistance and an anomalous heat capacity at low temperatures. Imaging and chemical analysis down to the atomic scale show that the Fe dopant is randomly distributed.",1406.4527v1 2014-06-25,Magnetic structure of GdBiPt: A candidate antiferromagnetic topological insulator,"A topological insulator is a state of matter which does not break any symmetry and is characterized by topological invariants, the integer expectation values of non-local operators. Antiferromagnetism on the other hand is a broken symmetry state in which the translation symmetry is reduced and time reversal symmetry is broken. Can these two phenomena coexist in the same material? A proposal by Mong {\it et al.}\cite{Mong2010} asserts that the answer is yes. Moreover, it is theoretically possible that the onset of antiferromagnetism enables the non-trivial topology since it may create spin-orbit coupling effects which are absent in the non-magnetic phase. The current work examines a real system, half-Heusler GdBiPt, as a candidate for topological anti ferromagnetism. We find that the magnetic moments of the gadolinium atoms form ferromagnetic sheets which are stacked antiferromagnetically along the body diagonal. This magnetic structure may induce spin orbit coupling on band electrons as they hop perpendicular to the ferromagnetic sheets.",1406.6663v1 2015-12-04,Magnetic proximity effect at interface between a cuprate superconductor and an oxide spin valve,"Heterostructures consisting of a cuprate superconductor YBa2Cu3O7x and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve have been studied by SQUID magnetometry, ferromagnetic resonances and neutron reflectometry. It was shown that due to the influence of magnetic proximity effect a magnetic moment is induced in the superconducting part of heterostructure and at the same time the magnetic moment is suppressed in the ferromagnetic spin valve. The experimental value of magnetization induced in the superconductor has the same order of magnitude with the calculations based on the induced magnetic moment of Cu atoms due to orbital reconstruction at the superconductor-ferromagnetic interface. It corresponds also to the model that takes into account the change in the density of states at a distance of order of the coherence length in the superconductor. The experimentally obtained characteristic length of penetration of the magnetic moment into superconductor exceeds the coherence length for cuprate superconductor. This fact points on the dominance of the mechanism of the induced magnetic moment of Cu atoms due to orbital reconstruction.",1512.01336v1 2015-12-04,Suppression of the ferromagnetic order in the Heusler alloy Ni50Mn35In15 by hydrostatic pressure,"We report the effect of hydrostatic pressure on the magnetic and structural properties of the shape-memory Heusler alloy Ni50Mn35In15. Magnetization and x-ray diffraction experiments were performed at hydrostatic pressures up to 5 GPa using diamond anvil cells. Pressure stabilizes the martensitic phase, shifting the martensitic transition to higher temperatures and suppresses the ferromagnetic austenitic phase. Above ~3 GPa, where the martensitic-transition temperature approaches the Curie temperature in the austenite, the magnetization shows no indication of ferromagnetic ordering anymore. We further find an extremely large temperatureregion with a mixture of martensite and austenite phases, which directly relates to the magnetic properties.",1512.01407v1 2015-12-17,Spatially resolved detection of complex ferromagnetic dynamics using optically detected NV spins,"We demonstrate optical detection of a broad spectrum of ferromagnetic excitations using nitrogen-vacancy (NV) centers in an ensemble of nanodiamonds. Our recently developed approach exploits a straightforward CW detection scheme using readily available diamond detectors, making it easily implementable. The NV center is a local detector, giving the technique spatial resolution, which here is defined by our laser spot, but in principle can be extended far into the nanoscale. Among the excitations we observe are propagating dipolar and dipolar-exchange spinwaves, as well as dynamics associated with the multi-domain state of the ferromagnet at low fields. These results offer an approach, distinct from commonly used ODMR techniques, for spatially resolved spectroscopic study of magnetization dynamics at the nanoscale.",1512.05418v2 2015-12-19,Large thermoelectric power and figure of merit in a ferromagnetic-quantum dot-superconducting device,"We investigate the thermoelectric properties of a quantum dot coupled to ferromagnetic and superconducting electrodes. The combination of spin polarized tunneling at the ferromagnetic-quantum dot interface and the application of an external magnetic field that Zeeman splits the dot energy level leads to large values of the thermopower (Seebeck coefficient). Importantly, the thermopower can be tuned with an external gate voltage connected to the dot. We compute the figure of merit that measures the efficiency of thermoelectric conversion and find that it attains high values. We discuss the different contributions from Andreev reflection processes and quasiparticle tunneling into and out of the superconducting contact. Furthermore, we obtain dramatic variations of both the magnetothermopower and the spin Seebeck effect, which suggest that in our device spin currents can be controlled with temperature gradients only.",1512.06242v2 2016-05-11,Avoided ferromagnetic quantum critical point: Antiferromagnetic ground state in substituted CeFePO,"We have investigated single crystals of two substitution series Ce(Ru$_{1-x}$Fe$_{x}$)PO and CeFe(As$_{1-y}$P$_{y}$)O in the vicinity to the quantum critical material CeFePO by means of magnetic-susceptibility and specific-heat measurements. We observe an antiferromagnetic ground state in the vicinity of the quantum critical point, with pronounced metamagnetic transitions for $H\parallel c$, which is the magnetically hard direction. Our results verify that a ferromagnetic quantum critical point is avoided in substituted CeFePO, because we clearly demonstrate that the ferromagnetic ground state changes into an antiferromagnetic one, when approaching the quantum critical point.",1605.03515v1 2016-05-12,Classical limit of Rabi nutations in spins of ferromagnets,"Rabi oscillations describe the interaction of a two-level system with a rotating electromagnetic field. As such, they serve as the principle method for manipulating quantum bits. By using a combination of femtosecond laser pulses and microwave excitations, we have observed the classical form of Rabi nutations in a ferromagnetic system whose equations of motion mirror the case of a precessing quantum two-level system. Key to our experiments is the selection of a subset of spins that is in resonance with the microwave excitation and whose coherence time is thereby extended. Taking advantage of Gilbert damping, the relaxation times are further increased such that mode-locking takes place. The observation of such Rabi nutations is the first step towards potential applications based on phase-coherent spin manipulation in ferromagnets.",1605.03996v1 2016-05-13,Ferromagnetism in orthorhombic RAgAl3 (R = Ce and Pr) compounds,"We present a detailed study on magnetic, thermodynamic and transport properties of polycrystalline RAgAl3(R = Ce and Pr) compounds. Both compounds crystallize in orthorhombic structure, which is distorted from the tetragonal BaAl4 structure with the space group Cmcm. Heat capacity measurement indicates the bulk magnetic ordering of the compounds. CeAgAl3 and PrAgAl3 order ferromagnetically at TC = 3.8 K and 5.8 K, respectively as it was confirmed from magnetic measurements. CeAgAl3 exhibits heavy Fermion behaviour. The Schottky behaviour in heat capacity data was observed in both compounds. The crystalline electric field (CEF) analysis of the magnetic parts of heat capacity of CeAgAl3 and PrAgAl3 yielded to a CEF level scheme with three doublets and nine singlets and with an overall splitting of 51 K and 180 K, respectively. Fit yielded a magnetic doublet state for CeAgAl3, whereas for PrAgAl3 a pseudo-doublet ground-state with an energy difference of 15 K has been obtained. The resistivity measurements display a low temperature drop at the magnetic ordering temperature of the compounds. Negative magnetoresistance (MR) due to the ferromagnetic ordering has been observed for both Ce and Pr compounds.",1605.04261v3 2016-05-14,Observation of orbital two-channel Kondo effect in a ferromagnetic L10-MnGa film,"The experimental existence and stability of the quantum criticality point of the two-channel Kondo (2CK) effect displaying exotic non-Fermi liquid physics has been buried in persistent confusion despite the intensive theoretical and experimental efforts in past three decades. Here we report an experimental realization of the two-level system scattering-induced orbital 2CK effect in a ferromagnetic L10-MnGa film, which is signified by a magnetic field-independent resistivity upturn that has a logarithmic and square-root temperature dependence beyond and below the Kondo temperature of ~14.5 K, respectively. Our result not only evidences the robust existence of orbital 2CK effect even in the presence of strong magnetic fields and long-range ferromagnetic ordering but also extends the scope of 2CK host materials from nonmagnetic nanoscale point contacts to diffusive conductors of disordered alloys.",1605.04366v4 2016-04-26,"Conductivity of weakly disordered metals close to a ""ferromagnetic"" quantum critical point","We calculate analytically the conductivity of weakly disordered metals close to a ""ferromagnetic"" quantum critical point in the low temperature regime. Ferromagnetic in the sense that the effective carrier potential $V(q,\omega)$, due to critical fluctuations, is peaked at zero momentum $q=0$. Vertex corrections, due to both critical fluctuations and impurity scattering, are explicitly considered. We find that only the vertex corrections due to impurity scattering, combined with the self-energy, generate appreciable effects as a function of the temperature $T$ and the control parameter $a$, which measures the proximity to the critical point. Our results are consistent with resistivity experiments in several materials displaying typical Fermi liquid behavior, but with a diverging prefactor of the $T^2$ term for small $a$.",1605.06129v3 2016-05-25,Chirality-induced magnon transport in AA-stacked bilayer honeycomb chiral magnets,"In this Letter, we study the magnetic transport in AA-stacked bilayer honeycomb chiral magnets coupled either ferromagnetically or antiferromagnetically. For both couplings, we observe chirality-induced gaps, chiral protected edge states, magnon Hall and magnon spin Nernst effects of magnetic spin excitations. For ferromagnetically coupled layers, thermal Hall and spin Nernst conductivities do not change sign as function of magnetic field or temperature similar to single-layer honeycomb ferromagnetic insulator. In contrast, for antiferromagnetically coupled layers, we observe a sign change in the thermal Hall and spin Nernst conductivities as the magnetic field is reversed. We discuss possible experimental accessible honeycomb bilayer quantum materials in which these effects can be observed.",1605.07971v5 2016-05-26,Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet,"When a topological insulator (TI) is in contact with a ferromagnet, both time reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI-FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces.",1605.08137v2 2016-08-02,Spin dynamics and magnetic interactions of Mn dopants in the topological insulator Bi$_2$Te$_3$,"The magnetic and electronic properties of the magnetically doped topological insulator Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ were studied using electron spin resonance (ESR) and measurements of static magnetization and electrical transport. The investigated high quality single crystals of Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ show a ferromagnetic phase transition for $x\geq 0.04$ at $T_{C}\approx 12$ K. The Hall measurements reveal a p-type finite charge-carrier density. Measurements of the temperature dependence of the ESR signal of Mn dopants for different orientations of the external magnetic field give evidence that the localized Mn moments interact with the mobile charge carriers leading to a Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling between the Mn spins of order 2-3 meV. Furthermore, ESR reveals a low-dimensional character of magnetic correlations that persist far above the ferromagnetic ordering temperature.",1608.00827v2 2016-08-02,Topological Phases of Spin-1/2 Ferromagnetic--Antiferromagnetic Alternating Heisenberg Chains with Alternating Next-Nearest-Neighbour Interaction,"A series of symmetry-protected topological(SPT) and trivial spin-gap phases in the spin-1/2 ferromagnetic--antiferromagnetic alternating Heisenberg chain with alternating next-nearest-neighbour interaction are investigated using two kinds of entanglement spectra defined by different divisions of the whole chain. In the case one of the next-nearest-neighbor interaction vanishes, the model reduces to the $\Delta$-chain in which a series of spin-gap phases are found in J. Phys. Soc. Jpn. 77, 044707 (2008). From the degeneracy of the entanglement spectra, these phases are identified as the SPT and trivial phases. It is found that the ground state phases boundaries are insensitive to the strength of the alternation in next-nearest neighbor interaction. These results are consistent with the analysis based on the nonlinear $\sigma$ model and exact solution on the ferromagnetic-nonmagnetic phase boundary.",1608.00837v2 2016-08-06,Zero-temperature study of a tetrameric spin-$1/2$ chain in a transverse magnetic field,"We consider an alternating Heisenberg spin-$1/2$ antiferromagnetic-ferromagnetic ($AF-F$) chain with the space modulated dominant antiferromagnetic exchange and anisotropic ferromagnetic coupling (tetrameric spin-$1/2$ chain). The zero-temperature effect of a symmetry breaking transverse magnetic field on the model is studied numerically. It is found that the anisotropy effect on the ferromagnetic coupling induces two new gapped phases. We identified their orderings as a kind of the stripe-antiferromagnetic phase. As a result, the magnetic phase diagram of the tetrameric chain shows five gapped quantum phases and the system is characterized by four critical fields which mark quantum phase transitions in the ground state of the system with the changing transverse magnetic field. We have also exploited the well known bipartite entanglement (name as concurrence) and global entanglement tools to verify the occurrence of quantum phase transitions and the corresponding critical points.",1608.02145v5 2016-08-10,Exchange bias in a mixed metal oxide based magnetocaloric compound YFe0.5Cr0.5O3,"We report a detailed investigation of magnetization, magnetocaloric effect and exchange bias studies on a mixed metal oxide YFe0.5Cr0.5O3 belonging to perovskite family. Our results reveal that the compound is in canted magnetic state (CMS) where ferromagnetic correlations are present in an antiferromagnetic state. Magnetic entropy change of this compound follows a power law (deltaS_M ~ H^m) dependence of magnetic field. In this compound, inverse magnetocaloric effect (IMCE) is observed below 260 K while conventional magnetocaloric effect (CMCE) above it. The exponent m is found to be independent of temperature and field only in the IMCE region. Investigation of temperature and magnetic field dependence studies of exchange bias, reveal a competition between effective Zeeman energy of the ferromagnetic regions and anisotropic exchange energy at the interface between ferromagnetic and antiferromagnetic regions. Variation of exchange bias due to temperature and field cycling is also investigated.",1608.03223v1 2016-08-12,Intertwined nematic orders in a frustrated ferromagnet,"We investigate the quantum phases of the frustrated spin-$\frac{1}{2}$ $J_1$-$J_2$-$J_3$ Heisenberg model on the square lattice with ferromagnetic $J_1$ and antiferromagnetic $J_2$ and $J_3$ interactions. Using the pseudo-fermion functional renormalization group technique, we find an intermediate paramagnetic phase located between classically ordered ferromagnetic, stripy antiferromagnetic, and incommensurate spiral phases. We observe that quantum fluctuations lead to significant shifts of the spiral pitch angles compared to the classical limit. By computing the response of the system with respect to various spin rotation and lattice symmetry-breaking perturbations, we identify a complex interplay between different nematic spin states in the paramagnetic phase. While retaining time-reversal invariance, these phases either break spin-rotation symmetry, lattice-rotation symmetry, or a combination of both. We therefore propose the $J_1$-$J_2$-$J_3$ Heisenberg model on the square lattice as a paradigmatic example where different intimately connected types of nematic orders emerge in the same model.",1608.03751v2 2016-08-18,Manipulation of the electroluminescence of organic light-emitting diodes via fringe fields from patterned magnetic domains,"We predict very large changes in the room-temperature electroluminescence of thermally-activated delayed fluorescence organic light emitting diodes near patterned ferromagnetic films. These effects exceed the changes in a uniform magnetic field by as much as a factor of two. We describe optimal ferromagnetic film patterns for enhancing the electroluminescence. A full theory of the spin-mixing processes in exciplex recombination, and how they are affected by hyperfine fields, spin-orbit effects, and ferromagnetic fringe field effects is introduced, and is used to describe the effect of magnetic domain structures on the luminescence in various regimes. This provides a method of enhancing light emission rates from exciplexes and also a means of efficiently coupling information encoded in magnetic domains to organic light emitting diode emission.",1608.05242v1 2016-08-19,Progressive magnetic softening of ferromagnetic layers in multilayer ferromagnet-nonmagnet systems and the role of granularity,"We report a study of the structural and magnetic behavior of the topmost magnetic layer in a ferromagnet-nonmagnet (Co-Au) multilayer system. Glancing angle X-ray diffraction measurements performed on a series of multilayers showed a gradually increasing degree of amorphization of the topmost magnetic layer with increasing number of bilayers. Concurrently, the magnetic hardness and magneto-crystalline anisotropy of the top Co layer were found to decrease, as observed by magneto-optical Kerr effect measurements. This magnetic softening has been discussed in the light of Herzer's random anisotropy model. Micromagnetic simulations of the multilayer system also corroborated these observations.",1608.05568v5 2016-08-24,Magnetic Phases in Periodically Rippled Graphene,"We study the effects that ripples induce on the electrical and magnetic properties of graphene. The variation of the interatomic distance created by the ripples translates in a modulation of the hopping parameter between carbon atoms. A tight binding Hamiltonian including a Hubbard interaction term is solved self consistently for ripples with different amplitudes and periods. We find that, for values of the Hubbard interaction $U$ above a critical value $U_C$, the system displays a superposition of local ferromagnetic and antiferromagnetic ordered states. Nonetheless the global ferromagnetic order parameter is zero. The $U_C$ depends only on the product of the period and hopping amplitude modulation. When the Hubbard interaction is close to the critical value of the antiferromagnetic transition in pristine graphene, the antiferromagnetic order parameter becomes much larger than the ferromagnetic one, being the ground state similar to that of flat graphene.",1608.06803v1 2016-12-05,The 2D percolation transition at finite temperature: The phase boundary for in-plane ferromagnetism in $\approx$2 ML Fe/W(110) films,"A two dimensional (2D) percolation transition in Fe/W(110) ultrathin magnetic films occurs when islands in the second atomic layer percolate and resolve a frustrated magnetic state to produce long-range in-plane ferromagnetic order. Novel measurements of the magnetic susceptibility $\chi(\theta)$ as the films are deposited at a constant temperature, allow the long-range percolation transition to be observed as a sharp peak consistent with a critical phase transition. The measurements are used to trace the paramagnetic-to-ferromagnetic phase boundary between the $T=0$ percolation magnetic transition and the thermal Curie magnetic transition of the undiluted film. A quantitative comparison to critical scaling theory is made by fitting the functional form of the phase boundary. The fitted parameters are then used in theoretical expressions for $\chi(T)$ in the critical region of the paramagnetic state to provide an excellent, independent representation of the experimental measurements.",1612.01460v1 2016-12-07,Intrinsic Ferromagnetism in the Diluted Magnetic Semiconductor Co:TiO$_2$,"Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO$_2$ is an intrinsic diluted magnetic semiconductor.",1612.02235v1 2016-12-08,Synthesis and pressure and field dependent magnetic properties of the Kagome-bilayer spin liquid Ca$_{10}$Cr$_7$O$_{28}$,"We report synthesis of polycrystalline samples of the recently discovered spin liquid material Ca$_{10}$Cr$_7$O$_{28}$ and present measurements of the ambient and high pressure magnetic susceptibility $\chi$ versus temperature $T$, magnetization $M$ versus magnetic field $H$ at various $T$, and heat capacity $C$ versus $T$ at various $H$. The ambient pressure magnetic measurements indicate the presence of both ferromagnetic and antiferromagnetic exchange interactions with dominant ferromagnetic interactions and with the largest magnetic energy scale $\sim 10$~K\@. The $\chi(T)$ measurements under externally applied pressure of up to $P \approx 1$~GPa indicate the robust nature of the spin-liquid state despite relative increase in the ferromagnetic exchanges. $C(T)$ shows a broad anomaly at $T\approx 2.5$~K which moves to higher temperatures in a magnetic field. The evolution of the low temperature $C(T,H)$ and the magnetic entropy is consistent with frustrated magnetism in Ca$_{10}$Cr$_7$O$_{28}$.",1612.02692v2 2016-12-14,Transport in Ferromagnet/Superconductor spin valves,"We consider charge transport properties in realistic, fabricable, Ferromagnet/Superconductor spin valves having a layered structure $F_1/N/F_2/S$, where $F_1$ and $F_2$ denote the ferromagnets, $S$ the superconductor, and $N$ the normal metal spacer usually inserted in actual devices. Our calculation is fully self-consistent, as required to ensure that conservation laws are satisfied. We include the effects of scattering at all the interfaces. We obtain results for the device conductance $G$, as a function of bias voltage, for all values of the angle $\phi$ between the magnetizations of the $F_1$ and $F_2$ layers and a range of realistic values for the material and geometrical parameters in the sample. We discuss, in the context of our results for $G$, the relative influence of all parameters on the spin valve properties. We study also the spin current and the corresponding spin transfer torque in $F_1/F_2/S$ structures.",1612.04696v1 2016-12-16,Polynomial-time classical simulation of quantum ferromagnets,"We consider a family of quantum spin systems which includes as special cases the ferromagnetic XY model and ferromagnetic Ising model on any graph, with or without a transverse magnetic field. We prove that the partition function of any model in this family can be efficiently approximated to a given relative error E using a classical randomized algorithm with runtime polynomial in 1/E, system size, and inverse temperature. As a consequence we obtain a polynomial time algorithm which approximates the free energy or ground energy to a given additive error. We first show how to approximate the partition function by the perfect matching sum of a finite graph with positive edge weights. Although the perfect matching sum is not known to be efficiently approximable in general, the graphs obtained by our method have a special structure which facilitates efficient approximation via a randomized algorithm due to Jerrum and Sinclair.",1612.05602v1 2016-12-20,Ferromagnetic resonance and interlayer exchange coupling in magnetic multilayers with compositional gradients,"Ferromagnetic resonance (FMR) in magnetic multilayers of type F1/f/F2, where two strongly ferromagnetic layers F1 and F2 are separated by a weakly magnetic spacer f with a compositional gradient along its thickness, is investigated. The method allows to detect the weak signal from the spacer in additional to the more pronounced and readily measured signal from the outer strongly-magnetic layers, and thereby study the properties of the spacer as well as the interlayer exchange interaction it mediates. Variable temperature FMR measurements, especially near the relevant Curie points, reveal a rich set of properties of the exchange interactions in the system. The obtained results are useful for designing and optimizing nanostructures with thermally-controlled magnetic properties.",1612.06785v1 2016-12-29,Chiral Majorana Fermion Modes on the Surface of Superconducting Topological Insulators,"The surface of superconducting topological insulators (STIs) has been recognized as an effective $p\pm ip$ superconductivity platform for realizing elusive Majorana fermions. Chiral Majorana modes (CMMs), which are different from Majorana bound states localized at points, can be achieved readily in experiments by depositing a ferromagnetic overlayer on top of the STI surface. Here we simulate this heterostructure by employing a realistic tight-binding model and show that the CMM appears on the edge of the ferromagnetic islands only after the superconducting gap is inverted by the exchange coupling between the ferromagnet and the STI. In addition, multiple CMMs can be generated by tuning the chemical potential of the topological insulator. These results can be applied to both proximity-effect induced superconductivity in topological insulators and intrinsic STI compounds such as PbTaSe$_2$, BiPd and their chemical analogues, providing a route to engineering CMMs in those materials.",1612.09276v1 2017-06-07,Coupled multiferroic domain switching in the canted conical spin spiral system Mn$_{2}$GeO$_{4}$,"Despite remarkable progress in developing multifunctional materials, spin-driven ferroelectrics featuring both spontaneous magnetization and electric polarization are still rare. Among such ferromagnetic ferroelectrics are conical spin spiral magnets with a simultaneous reversal of magnetization and electric polarization that is still little understood. Such materials can feature various multiferroic domains that complicates their study. Here we study the multiferroic domains in ferromagnetic ferroelectric Mn$_{2}$GeO$_{4}$ using neutron diffraction, and show that it features a double-Q conical magnetic structure that, apart from trivial 180 degree commensurate magnetic domains, can be described by ferromagnetic and ferroelectric domains only. We show unconventional magnetoelectric couplings such as the magnetic-field-driven reversal of ferroelectric polarization with no change of spin-helicity, and present a phenomenological theory that successfully explains the magnetoelectric coupling. Our measurements establish Mn$_{2}$GeO$_{4}$ as a conceptually simple multiferroic in which the magnetic-field-driven flop of conical spin spirals leads to the simultaneous reversal of magnetization and electric polarization.",1706.02363v1 2017-06-21,Uniform magnetization dynamics of a submicron ferromagnetic coin driven by the spin-orbit coupled spin torque,"A simple model of magnetization dynamics in a ferromagnet/doped semiconductor hybrid structure with Rashba spin-orbit interaction (SOI) driven by an applied pulse of the electric field is proposed. The electric current excited by the applied field is spin-polarized due to the SOI and therefore it induces the magnetization rotation in the ferromagnetic layer via s-d exchange coupling. Magnetization dynamics dependence on the electric pulse shape and magnitude is analyzed for realistic values of parameters. We show that it is similar to the dynamics of a damped nonlinear oscillator with the time-dependent frequency proportional to the square root of the applied electric field. The magnetization switching properties of an elliptic magnetic element are examined as a function of the applied field magnitude and direction.",1706.06909v1 2017-06-22,Critical behavior of quasi-two-dimensional semiconducting ferromagnet CrGeTe$_3$,"The critical properties of the single-crystalline semiconducting ferromagnet CrGeTe$_3$ were investigated by bulk dc magnetization around the paramagnetic to ferromagnetic phase transition. Critical exponents $\beta = 0.200\pm0.003$ with critical temperature $T_c = 62.65\pm0.07$ K and $\gamma = 1.28\pm0.03$ with $T_c = 62.75\pm0.06$ K are obtained by the Kouvel-Fisher method whereas $\delta = 7.96\pm0.01$ is obtained by the critical isotherm analysis at $T_c = 62.7$ K. These critical exponents obey the Widom scaling relation $\delta = 1+\gamma/\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M(H)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m = f_\pm(h)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The determined exponents match well with those calculated from the results of renormalization group approach for a two-dimensional Ising system coupled with long-range interaction between spins decaying as $J(r)\approx r^{-(d+\sigma)}$ with $\sigma=1.52$.",1706.07324v1 2017-06-22,Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO$_3$,"The combination of ferromagnets with topological superconductors or insulators allows for new phases of matter that support excitations such as chiral edge modes and Majorana fermions. EuS, a wide-band-gap ferromagnetic insulator with a Curie temperature around 16 K, and SrTiO$_3$ (STO), an important substrate for engineering heterostructures, may support these phases. We present scanning superconducting quantum interference device (SQUID) measurements of EuS grown epitaxially on STO that reveal micron-scale variations in ferromagnetism and paramagnetism. These variations are oriented along the STO crystal axes and only change their configuration upon thermal cycling above the STO cubic-to-tetragonal structural transition temperature at 105 K, indicating that the observed magnetic features are due to coupling between EuS and the STO tetragonal structure. We speculate that the STO tetragonal distortions may strain the EuS, altering the magnetic anisotropy on a micron-scale. This result demonstrates that local variation in the induced magnetic order from EuS grown on STO needs to be considered when engineering new phases of matter that require spatially homogeneous exchange.",1706.07374v1 2017-06-27,Analytical and numerical $K_u - B$ phase diagrams for cobalt nanostructures: stability region for a Bloch skyrmion,"In this letter we calculate the energies corresponding to the different magnetic phases present in a ferromagnetic cylinder by means of analytical calculations. From the comparison of these energies, it is possible to construct magnetic phase diagrams as a function of the uniaxial anisotropy of the sample and the external magnetic field applied. As proof of concept, we analyzed the magnetic phase diagrams for a cobalt dot of 240 nm in diameter and 70 nm in length, with an easy axis parallel to the dot axis, and with a magnetic field applied towards or perpendicular to this axis. From these diagrams we have obtained the stability regions for a Bloch skyrmion (Sk), a vortex core (VC) and a ferromagnetic (F) configuration, which can point in any $\psi$ direction. Our results provide a pathway to engineer the formation and controllability of a skyrmion in a ferromagnetic dot to different anisotropy constants and magnetic fields.",1706.08876v2 2017-06-27,"Intrinsic 2D ferromagnetism, quantum anomalous Hall conductivity, and fully-spin-polarized edge states of FeBr3 monolayer","It is of great interest to explore intrinsic two-dimensional ferromagnetism and seek better two-dimensional quantum anomalous Hall insulator materials. Here, we show that the FeBr$_3$ monolayer is an intrinsic two-dimensional ferromagnetic material whose Curie temperature is 140 K thanks to its strong spin exchange interaction and giant uniaxial magnetic anisotropy. Our phonon spectra and mechanical analysis indicate that the FeBr$_3$ monolayer is dynamically and mechanically stable. Our electronic structure calculation shows that there is one Dirac cone at K point in the Brillouin zone and the spin-orbit coupling opens a semiconductor gap of 33.5 meV. Further tight-binding analysis reveals that the Chern number is equivalent to 1 and there is a quantum anomalous Hall conductivity $\sigma _{xy} = e^2/h$, and the chiral edge states are fully spin-polarized when an edge is created. Furthermore, it is shown that the main results are not affected by electron correlation effects and biaxial strain. Therefore, this FeBr$_3$ monolayer as 2D material would be useful for spintronic applications.",1706.08943v2 2017-07-05,Thermodynamics of the pyrochlore Heisenberg ferromagnet with arbitrary spin $S$,"We use the rotation-invariant Green's function method (RGM) and the high-temperature expansion (HTE) to study the thermodynamic properties of the spin-$S$ Heisenberg ferromagnet on the pyrochlore lattice. We examine the excitation spectra as well as various thermodynamic quantities, such as the order parameter (magnetization), the uniform static susceptibility, the correlation length, the spin-spin correlations, and the specific heat, as well as the static and dynamic structure factors. We discuss the influence of the spin quantum number $S$ on the temperature dependence of these quantities. We compare our results for the pyrochlore ferromagnet with the corresponding ones for the simple-cubic lattice both having the same coordination number $z=6$. We find a significant suppression of magnetic ordering for the pyrochlore lattice due to its geometry with corner-sharing tetrahedra.",1707.01529v1 2017-07-19,Stability of skyrmion textures and the role of thermal fluctuations in cubic helimagnets: a new intermediate phase at low temperature,"The stability of the four known stationary points of the cubic helimagnet energy functional: the ferromagnetic state, the conical helix, the conical helicoid, and the skyrmion lattice, is studied by solving the corresponding spectral problem. The only stable points are the ferromagnetic state at high magnetic field and the conical helix at low field, and there is no metastable state. Thermal fluctuations around the stationary point, included to quadratic order in the saddle point expansion, destabilize the conical helix in a region where the ferromagnetic state is unstable. Thus, a new intermediate phase appears which, in a region of the phase diagram, is a skyrmion lattice stabilized by thermal fluctuations. The skyrmion lattice lost the stability by lowering temperature, and a new intermediate phase of unknown type, presumably with three dimensional modulations, appears in the lower temperature region.",1707.06016v2 2017-07-31,Spin wave beam propagation in ferromagnetic thin film with graded refractive index: mirage effect and prospective applications,"Using analysis of iso-frequency contours of the spin-wave dispersion relation, supported by micromagnetic simulations, we study the propagation of spin-wave (SW) beams in thin ferromagnetic films through the areas of the inhomogeneous refractive index. We compare the transmission and reflection of SWs in areas with gradual and step variation of the SW refractive index. In particular, we show the mirage effect for SWs with narrowing SW beam width, and an application of the gradual modulation of the SWs refractive index as a diverging lens. Furthermore, we study the propagation of SWs in ferromagnetic stripe with modulated refractive index. We demonstrate that the system can be considered as the graded-index waveguide, which preserves the width of the SW beam for a long distance-the property essential for prospective applications of magnonics.",1707.09768v4 2017-10-04,Ferromagnetic order in dipolar systems with anisotropy: application to magnetic nanoparticle supracrystals,"Single domain magnetic nanoparticles (MNP) interacting through dipolar interactions (DDI) in addition to the magnetocrystalline energy may present a low temperature ferromagnetic (SFM) or spin glass (SSG) phase according to the underlying structure and the degree of order of the assembly. We study, from Monte Carlo simulations in the framework of the effective one-spin or macrospin models, the case of a monodisperse assembly of single domain MNP fixed on the sites of a perfect lattice with fcc symmetry and randomly distributed easy axes. We limit ourselves to the case of a low anisotropy, namely the onset of the disappearance of the dipolar long-range ferromagnetic (FM) phase obtained in the absence of anisotropy due to the disorder introduced by the latter.",1710.01532v2 2017-10-06,Work probability distribution for a ferromagnet with long-ranged and short-ranged correlations,"Work fluctuations and work probability distributions are fundamentally different in systems with short- ranged versus long-ranged correlations. Specifically, in systems with long-ranged correlations the work distribution is extraordinarily broad compared to systems with shortranged correlations. This difference profoundly affects the possible applicability of fluctuation theorems like the Jarzynski fluctuation theorem. The Heisenberg ferromagnet , well below its Curie temperature, is a system with long-ranged correlations in very low magnetic fields due to the presence of Goldstone modes. As the magnetic field is increased the correlations gradually become short-ranged. Hence, such a ferromagnet is an ideal system for elucidating the changes of the work probability distribution as one goes from a domain with long-ranged correlations to a domain with short-ranged correlations by tuning the magnetic field. A quantitative analysis of this crossover behaviour of the work probability distribution and the associated fluctuations is presented.",1710.02361v1 2017-10-15,Itinerant ferromagnetism of the Pd-terminated polar surface of PdCoO$_2$,"We study the electronic structure of the Pd-terminated surface of the non-magnetic delafossite oxide metal PdCoO$_2$. Combining angle-resolved photoemission spectroscopy and density-functional theory, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability towards itinerant surface ferromagnetism. Our results reveal how this leads to a rich multi-band surface electronic structure, and provide spectroscopic evidence for an intriguing sample-dependent coupling of the surface electrons to a bosonic mode which we attribute to electron-magnon interactions. Moreover, we find similar surface state dispersions in PdCrO$_2$, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.",1710.05392v1 2017-10-18,Magnetic Dynamics of Phase Separation Domains in GdMn2O5 and Gd0.8Ce0.2Mn2O5 Multiferroics,"Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn2O5 and Gd0.8Ce0.2Mn2O5 have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn2O5 and Gd0.8Ce0.2Mn2O5 are compared with the properties of the previously investigated RMn2O5 (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn2O5 compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn2O5 is built.",1710.06695v1 2017-10-23,"Electrically tunable exchange splitting in bilayer graphene on monolayer Cr$_2$X$_2$Te$_6$ with X=Ge, Si, and Sn","We investigate the electronic band structure and the proximity exchange effect in bilayer graphene on a family of ferromagnetic multilayers Cr$_2$X$_2$Te$_6$, X=Ge, Si, and Sn, with first principles methods. In each case the intrinsic electric field of the heterostructure induces an orbital gap on the order of 10 meV in the graphene bilayer. The proximity exchange is strongly band dependent. For example, in the case of Cr$_2$Ge$_2$Te$_6$, the low-energy valence band of bilayer graphene has exchange splitting of 8 meV, while the low energy conduction band's splitting is 30 times less (0.3 meV). This striking discrepancy stems from the layer-dependent hybridization with the ferromagnetic substrate. Remarkably, applying a vertical electric field of a few V/nm reverses the exchange, allowing us to effectively turn ON and OFF proximity magnetism in bilayer graphene. Such a field-effect should be generic for van der Waals bilayers on ferromagnetic insulators, opening new possibilities for spin-based devices.",1710.08117v2 2017-10-25,Josephson Effect in Singlet Superconductor-Ferromagnet-Triplet Superconductor Junction,"We study the current-phase relation of a ballistic SIFIT junction, consisting of a spin-singlet superconductor (S), a weak ferromagnetic metal (F), a spin-triplet superconductor (T), and insulating ferromagnetic interfaces (I). We use the generalized quasiclassical formalism developed by A. Millis et al. to compute the current density and the free energy of the junction for arbitrary orientation of the magnetizations of the junction barrier. We investigate in detail the effect of the distribution of magnetization on the various harmonics of the current-phase relation and the transition of the ground state of the junction. The $\phi$-state junction can be realized for a noncollinear orientation of the barrier magnetizations in the plane perpendicular to the d-vector of the triplet superconductor.",1710.09082v1 2017-10-27,Single shot all optical switching of intrinsic micron size magnetic domains of a Pt/Co/Pt ferromagnetic stack,"We demonstrate that the magnetization reversal in a ferromagnetic Pt/Co/Pt stack can be induced by a single femtosecond laser pulse. We find that the size of the switched spot is comparable to the size of the intrinsic magnetic domains. It requires an absorbed energy density of $\sim$4 mJ.cm$^{-2}$, beyond which the excited spot fragments into a multidomain structure. The switching process occurs back and forth with subsequent laser pulses and it is helicity-independent. Furthermore, the dynamics of the magnetization reversal occurs in a timescale less than one microsecond. These results suggest that all optical switching in ferromagnetic films requires to match the laser spot with the specific domain sizes.",1710.10341v1 2017-10-28,High frequency dynamics modulated by collective magnetization reversal in artificial spin ice,"Spin-torque ferromagnetic resonance (ST-FMR) arises in heavy metal/ferromagnet heterostructures when an alternating charge current is passed through the bilayer stack. The methodology to detect the resonance is based on the anisotropic magnetoresistance, which is the change in the electrical resistance due to different orientations of the magnetization. In connected networks of ferromagnetic nanowires, known as artificial spin ice, the magnetoresistance is rather complex owing to the underlying collective behavior of the geometrically frustrated magnetic domain structure. Here, we demonstrate ST-FMR investigations in a square artificial spin-ice system and correlate our observations to magnetotransport measurements. The experimental findings are described using a simulation approach that highlights the importance of the correlated dynamics response of the magnetic system. Our results open the possibility of designing reconfigurable microwave oscillators and magnetoresistive devices based on connected networks of nanomagnets.",1710.10534v1 2017-12-05,Majorana bound states in hybrid 2D Josephson junctions with ferromagnetic insulators,"We consider a Josephson junction consisting of superconductor/ferromagnetic insulator (S/FI) bilayers as electrodes which proximizes a nearby 2D electron gas. By starting from a generic Josephson hybrid planar setup we present an exhaustive analysis of the the interplay between the superconducting and magnetic proximity effects and the conditions under which the structure undergoes transitions to a non-trivial topological phase. We address the 2D bound state problem using a general transfer matrix approach that reduces the problem to an effective 1D Hamiltonian. This allows for straightforward study of topological properties in different symmetry classes. As an example we consider a narrow channel coupled with multiple ferromagnetic superconducting fingers, and discuss how the Majorana bound states can be spatially controlled by tuning the superconducting phases. Following our approach we also show the energy spectrum, the free energy and finally the multiterminal Josephson current of the setup.",1712.01684v2 2017-12-05,Third order perturbed modified Heisenberg Hamiltonian of fcc structured ferromagnetic films with seventy spin layers,"Magnetic properties of fcc structured ferromagnetic films with the number of spin layers up to seventy was described using third order perturbed Heisenberg Hamiltonian. The variation of magnetic easy direction, magnetic energies in easy and hard directions, magnetic anisotropy energy and the angle between easy and hard directions was investigated by varying the number of spin layers. Spin exchange interaction, magnetic dipole interaction, second and fourth order magnetic anisotropies, in and out of plane applied magnetic fields, demagnetization factor and stress induced anisotropy were considered in the model. Because magnetic dipole interaction and demagnetization factor represent microscopic and macroscopic properties of the sample, respectively, both these terms were incorporated in our theoretical model. Although our model is a semi-classical model, some discrete variations of angle of easy axis were observed. Our theoretical data qualitatively agree with experimental data of Fe and Ni ferromagnetic films.",1712.02636v1 2017-12-08,Spin Hall and spin swapping torques in diffusive ferromagnets,"A complete set of the generalized drift-diffusion equations for a coupled charge and spin dynamics in ferromagnets in the presence of extrinsic spin-orbit coupling is derived from the quantum kinetic approach, covering major transport phenomena, such as the spin and anomalous Hall effects, spin swapping, spin precession and relaxation processes. We argue that the spin swapping effect in ferromagnets is enhanced due to spin polarization, while the overall spin texture induced by the interplay of spin-orbital and spin precessional effects displays a complex spatial dependence that can be exploited to generate torques and nucleate/propagate domain walls in centrosymmetric geometries without use of external polarizers, as opposed to the conventional understanding of spin-orbit mediated torques.",1712.03009v1 2017-12-14,Quantum simulation of ferromagnetic Heisenberg model,"Large quantum simulators, with sufficiently many qubits to be impossible to simulate classically, become hard to experimentally validate. We propose two tests of a quantum simulator with Heisenberg interaction in a linear chain of spins. In the first, we propagate half of a singlet state through a chain of spin with a ferromagnetic interaction and subsequently recover the state with an antiferromagnetic interaction. The antiferromagnetic interaction is intrinsic to the system while the ferromagnetic one can be simulated by a sequence of time-dependent controls of the antiferromagnetic interaction and Suzuki-Trotter approximations. In the second test, we use the same technique to transfer a spin singlet state from one end of a spin chain to the other. We show that the tests are robust against parametric errors in operation of the simulator and may be applicable even without error correction.",1712.05282v1 2017-12-20,Planar Hall torque,"Spin-orbit torques in bilayers of ferromagnetic and nonmagnetic materials hold promise for energy efficient switching of magnetization in nonvolatile magnetic memories. Previously studied spin Hall and Rashba torques originate from spin-orbit interactions within the nonmagnetic material and at the bilayer interface, respectively. Here we report a spin-orbit torque that arises from planar Hall current in the ferromagnetic material of the bilayer and acts as either positive or negative magnetic damping. This planar Hall torque exhibits unusual biaxial symmetry in the plane defined by the applied electric field and the bilayer normal. The magnitude of the planar Hall torque is similar to that of the giant spin Hall torque and is large enough to excite auto-oscillations of the ferromagnetic layer magnetization.",1712.07335v1 2017-12-21,Spin-triplet paired phases inside ferromagnet induced by Hund's rule coupling and electronic correlations: Application to $\mathrm{UGe}_2$,"We discuss a mechanism of real-space spin-triplet pairing, alternative to that due to quantum paramagnon excitations, and demonstrate its applicability to $\mathrm{UGe_2}$. Both the Hund's rule ferromagnetic exchange and inter-electronic correlations contribute to the same extent to the equal-spin pairing, particularly in the regime in which the weak-coupling solution does not provide any. The theoretical results, obtained within the orbitally-degenerate Anderson lattice model, match excellently the observed phase diagram for $\mathrm{UGe_2}$ with the coexistent ferromagnetic (FM1) and superconducting ($A_1$-type) phase. Additionally, weak $A_2$- and $A$-type paired phases appear in very narrow regions near the metamaganetic (FM2 $\rightarrow$ FM1) and FM1 $\rightarrow$ paramagnetic first-order phase-transition borders, respectively. The values of magnetic moments in the FM2 and FM1 states are also reproduced correctly in a semiquantitative manner. The Hund's metal regime is also singled out as appearing near FM1-FM2 boundary.",1712.08028v2 2018-01-01,Optical Control of Exchange Interaction and Kondo Temperature in cold Atom Gas,"The relevance of magnetic impurity problems in cold atom systems depends crucially on the nature of exchange interaction between itinerant fermionic atoms and a localized impurity atom. In particular, Kondo physics occurs only if the exchange interaction is anti-ferromagnetic, and strong enough to yield high enough Kondo temperature ($T_K/T_F \ge 0.1$). Focusing, as an example, on the experimentally accessible system of ultra-cold $^{173}$Yb atoms, it is shown that the sign and strength of an exchange interaction between an itinerant Yb($^{1}$S$_{0}$) atom and a trapped Yb($^{3}$P$_{0}$) atom can be optically controlled. Explicitly, as the light intensity increases (from zero), the exchange interaction changes from ferromagnetic to anti-ferromagnetic. When the light intensity is just below a singlet Feshbach resonance, the singlet scattering length $a_S$ is large and negative, and the Kondo temperature increases sharply.",1801.00482v3 2018-01-23,Magnetic and Structural Properties of A-Site Ordered Chromium Spinel Sulfides: Alternating Antiferromagnetic and Ferromagnetic Interactions in the Breathing Pyrochlore Lattice,"We report a comprehensive study on the magnetic and structural properties of the spinel sulfides LiInCr4S8, LiGaCr4S8, and CuInCr4S8, where Li+/Cu+ and Ga3+/In3+ ions form a zinc-blende-type order. On the basis of synchrotron X-ray diffraction and magnetization data obtained using polycrystalline samples, these three sulfides are suggested to be breathing pyrochlore magnets with alternating antiferromagnetic and ferromagnetic interactions on the small and large tetrahedra, respectively. The measured magnetization processes of the three sulfides up to 72 T are significantly different. The magnetization curves of LiInCr4S8 and CuInCr4S8 have large hysteresis loops with different shapes, while there is no hysteresis in that of LiGaCr4S8. Geometrical frustration of the small tetrahedron is likely to give rise to a wide variety of ground states, indicating the rich physics in these antiferromagnetic-ferromagnetic breathing pyrochlore magnets.",1801.07354v1 2018-01-24,Critical behaviors of half-metallic ferromagnet Co3Sn2S2,"We have investigated the critical behavior of a shandite-type half-metal ferromagnet Co3Sn2S2. It exhibits a second-order paramagnetic-ferromagnetic phase transition with TC = 174 K. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical components beta, gamma, and delta determined using the modified Arrott plot, the Kouvel-Fisher method as well as the critical isotherm analysis are match reasonably well and follow the scaling equation, confirming that the exponents are unambiguous and intrinsic to the material. The determined exponents of Co3Sn2S2 deviates from theoretical estimated short-range universal models. Instead, Co3Sn2S2 exhibits long-range order in the nature of magnetic interaction with the spin decay as J(r) ~ 1/r^[-(d + sigma)] with sigma = 1.28.",1801.08048v1 2018-01-25,Electrical generation and detection of spin waves in a quantum Hall ferromagnet,"Spin waves are collective excitations of magnetic systems. An attractive setting for studying long-lived spin-wave physics is the quantum Hall (QH) ferromagnet, which forms spontaneously in clean two-dimensional electron systems at low temperature and in a perpendicular magnetic field. We used out-of-equilibrium occupation of QH edge channels in graphene to excite and detect spin waves in magnetically ordered QH states. Our experiments provide direct evidence for long distance spin wave propagation through different ferromagnetic phases in the N=0 Landau level, as well as across the insulating canted antiferromagnetic phase. Our results will enable experimental investigation of the fundamental magnetic properties of these exotic two-dimensional electron systems.",1801.08534v2 2018-09-05,Microscopic understanding of magnetic interactions in bilayer CrI$_3$,"We performed the detailed microscopic analysis of the inter-layer magnetic couplings for bilayer CrI$_3$. As the first step toward understanding the recent experimental observations and utilizing them for device applications, we estimated magnetic force response as well as total energy. Various van der Waals functionals equivocally point to the ferromagnetic ground state for the low-temperature structured bilayer CrI$_3$ which is further confirmed independently by magnetic force response calculations. The calculated orbital-dependent magnetic forces clearly show that $e_g$-$t_{2g}$ interaction is the key to stabilize this ferromagnetic order. By suppressing this ferromagnetic interaction and enhancing antiferromagnetic orbital channels of $e_g$-$e_g$ and $t_{2g}$-$t_{2g}$, one can realize the desirable antiferromagnetic order. We showed that high-temperature monoclinic stacking can be the case. Our results provide unique information and insight to understand the magnetism of multi-layer CrI$_3$ paving the way to utilize it for applications.",1809.01388v3 2018-09-09,Spin transfer torques and spin-dependent transport in a metallic F/AF/N tunneling junction,"We study spin-dependent electron transport through a ferromagnetic-antiferromagnetic-normal metal tunneling junction subject to a voltage or temperature bias, in the absence of spin-orbit coupling. We derive microscopic formulas for various types of spin torque acting on the antiferromagnet as well as for charge and spin currents flowing through the junction. The obtained results are applicable in the limit of slow magnetization dynamics. We identify a parameter regime in which an unconventional damping-like torque can become comparable in magnitude to the equivalent of the conventional Slonczewski's torque generalized to antiferromagnets. Moreover, we show that the antiferromagnetic sublattice structure opens up a channel of electron transport which does not have a ferromagnetic analogue and that this mechanism leads to a pronounced field-like torque. Both charge conductance and spin current transmission through the junction depend on the relative orientation of the ferromagnetic and the antiferromagnetic vectors (order parameters). The obtained formulas for charge and spin currents allow us to identify the microscopic mechanisms responsible for this angular dependence and to assess the efficiency of an antiferromagnetic metal acting as a spin current polarizer.",1809.02950v1 2018-09-22,Global Memory from Local Hysteresis in an Amorphous Solid,"A disordered material that cannot relax to equilibrium, such as an amorphous or glassy solid, responds to deformation in a way that depends on its past. In experiments we train a 2D athermal amorphous solid with oscillatory shear, and show that a suitable readout protocol reveals the shearing amplitude. When shearing alternates between two amplitudes, signatures of both values are retained only if the smaller one is applied last. We show that these behaviors arise because individual clusters of rearrangements are hysteretic and dissipative, and because different clusters respond differently to shear. These roles for hysteresis and disorder are reminiscent of the return-point memory seen in ferromagnets and many other systems. Accordingly, we show how a simple model of a ferromagnet can reproduce key results of our experiments and of previous simulations. Unlike ferromagnets, amorphous solids' disorder is unquenched; they require ""training"" to develop this behavior.",1809.08505v3 2018-09-25,Axion Insulator State in a Ferromagnet/Topological Insulator/Antiferromagnet Heterostructure,"We propose to use ferromagnetic insulator MnBi2Se4/Bi2Se3/antiferromagnetic insulator Mn2Bi2Se5 heterostructures for the realization of the axion insulator state. Importantly, the axion insulator state in such heterostructures only depends on the magnetization of the ferromagnetic insulator and hence can be observed in a wide range of external magnetic field. Using density functional calculations and model Hamiltonian simulations, we find that the top and bottom surfaces have opposite half-quantum Hall conductance, with a sizable global spin gap of 5.1 meV opened for the topological surface states of Bi2Se3. Our work provides a new strategy for the search of axion insulators by using van der Waals antiferromagnetic insulators along with three-dimensional topological insulators.",1809.09265v2 2018-09-26,The spin-$\frac{1}{2}$ Heisenberg ferromagnet on the pyrochlore lattice: A Green's function study,"We consider the pyrochlore-lattice quantum Heisenberg ferromagnet and discuss the properties of this spin model at arbitrary temperatures. To this end, we use the Green's function technique within the random-phase (or Tyablikov) approximation as well as the linear spin-wave theory and quantum Monte Carlo simulations. We compare our results to the ones obtained recently by other methods to corroborate our findings. Finally, we contrast our results with the ones for the simple-cubic-lattice case: both lattices are identical at the mean-field level. We demonstrate that thermal fluctuations are more efficient in the pyrochlore case (finite-temperature frustration effects). Our results may be of use for interpreting experimental data for ferromagnetic pyrochlore materials.",1809.09951v1 2018-09-27,Magnetization induced shape transformations in flexible ferromagnetic rings,"Flexible ferromagnetic rings are spin-chain magnets, in which the magnetic and mechanical subsystems are coupled. The coupling is achieved through the tangentially oriented anisotropy axis. The possibility to operate the mechanics of the nanomagnets by controlling their magnetization is an important issue for the nanorobotics applications. A minimal model for the deformable curved anisotropic Heisenberg ferromagnetic wire is proposed. An equilibrium phase diagram is constructed for the closed loop geometry: (i) A vortex state with vanishing total magnetic moment is typical for relatively large systems; in this case the wire has the form of a regular circle. (ii) A topologically trivial onion state with the planar magnetization distribution is realized in small enough systems; magnetic loop is elliptically deformed. By varying geometrical and elastic parameters a phase transition between the vortex and onion states takes place. The detailed analytical description of the phase diagram is well confirmed by numerical simulations.",1809.10622v1 2018-10-01,Energy-momentum tensor of a ferromagnet,"The energy-momentum tensor of a ferromagnet derived according to the standard prescription of Noether's theorem has a major flaw: the term originating from the spin Berry phase is gauge-dependent. As a consequence, some physical quantities computed from the tensor show unphysical behavior. For example, the presence of a spin-polarized current does not affect the energy of the domain wall in the commonly accepted gauge, which implies-incorrectly-the absence of the adiabatic spin torque. In other gauges, the spin torque shows unphysical glitches occurring when the plane of magnetization crosses the Dirac string associated with a magnetic monopole in spin space. We derive a gauge-invariant energy-momentum tensor that is free from these artifacts but requires the addition of an extra spatial dimension, with the ferromagnet living on its boundary. It can be obtained most directly from the Wess-Zumino action for spins, which relies on the same extra dimension.",1810.01006v2 2018-10-02,Switchable Weyl nodes in topological Kagome ferromagnet Fe3Sn2,"The control of topological quantum materials is the prerequisite for novel devices exploiting these materials. Here we propose that the room temperature ferromagnet Fe3Sn2, whose fundamental building blocks are Kagome bilayers of iron, hosts Weyl nodes at the Fermi level which can move in momentum space depending on the direction of the magnetization, itself readily controlled either by modest external fields or temperature. The proposal is derived from density functional calculations, including a mean field treatment of Hubbard repulsion U, which have been validated by comparison with angle-resolved photoemission data. Ferromagnetism with magnetization along certain directions is shown to lift the Weyl degeneracies, while at the same time inducing texture in the quasiparticle spin polarizations mapped in reciprocal space. In particular, the polarization is attenuated and then rotated from parallel to perpendicular to antiparallel to the magnetization as Weyl points derived from crossing of majority and minority spin bands are traversed.",1810.01514v1 2018-10-03,Location of zeros for the partition function of the Ising model on bounded degree graphs,"The seminal Lee-Yang theorem states that for any graph the zeros of the partition function of the ferromagnetic Ising model lie on the unit circle in $\mathbb C$. In fact the union of the zeros of all graphs is dense on the unit circle. In this paper we study the location of the zeros for the class of graphs of bounded maximum degree $d\geq 3$, both in the ferromagnetic and the anti-ferromagnetic case. We determine the location exactly as a function of the inverse temperature and the degree $d$. An important step in our approach is to translate to the setting of complex dynamics and analyze a dynamical system that is naturally associated to the partition function.",1810.01699v3 2018-10-15,Ferromagnetic and Antiferromagnetic Coupling of Spin Molecular Interfaces with High Thermal Stability,"We report an advanced organic spin-interface architecture with magnetic remanence at room temperature, constituted by metal phthalocyanine molecules magnetically coupled with Co layer(s), mediated by graphene. Fe- and Cu-phthalocyanines assembled on graphene/Co have identical structural configurations, but FePc couples antiferromagnetically with Co up to room temperature, while CuPc couples ferromagnetically with weaker coupling and thermal stability, as deduced by element-selective X-ray magnetic circular dichroic signals. The robust antiferromagnetic coupling is stabilized by a superexchange interaction, driven by the out-of-plane molecular orbitals responsible of the magnetic ground state and electronically decoupled from the underlying metal via the graphene layer, as confirmed by ab initio theoretical predictions. These archetypal spin interfaces can be prototypes to demonstrate how antiferromagnetic and/or ferromagnetic coupling can be optimized by selecting the molecular orbital symmetry.",1810.06317v1 2018-10-17,Role of Tin and Carbon in the magnetic interactions in Mn$_3$SnC,"In this paper we attempt to understand the role of tin and carbon in magnetic interactions in Mn$_3$SnC. Mn$_3$SnC exhibits a time dependent magnetic configuration and a complex magnetic ground state with both ferromagnetic and antiferromagnetic orders. Such a magnetic state is attributed to presence of distorted Mn$_6$C octahedra with long and short Mn--Mn bonds. Our studies show that C deficiency increases the tensile strain on the Mn$_6$C octahedra which elongates Mn--Mn bonds and strengthens ferromagnetic interactions while Sn deficiency tends to ease out the strain resulting in shorter as well as longer Mn--Mn bond distances in comparison with stoichiometric Mn$_3$SnC. Such a variation strengthens both, ferromagnetic and antiferromagnetic interactions. Thus the structural strain caused by both Sn and C is responsible for complex magnetic ground state of Mn$_3$SnC.",1810.07482v1 2018-10-18,A novel chiral spin texture: Antiferromagnetic Skyrmionium,"Exotic spin textures viz. chiral domain wall, vortices, skyrmion, skyrmionium, etc. have recently emerged as active field of research because of their potential applications in high density data storage technology and logic gate computing. Magnetic skyrmionium is a skyrmion like soliton, which carries zero topological quantum number. Skyrmioniums are superior to conventional skyrmions in ferromagnets due to their negligible skyrmion hall effect and higher velocity. The physical properties of both skyrmion and skyrmionium have been investigated rigorously in ferromagnetic systems. Recent observations hint that such chiral spin structures in antiferromagnetic (AFM) systems are more promising in comparison to the ferromagnetic ones because of their robustness towards external perturbation, absence of Skyrmion hall effect, etc. However skyrmionium in AFM materials are not reported in literature so far. In this work, we demonstrate that skyrmionium can be created and stabilized in AFM materials by application of spin polarized current in an experimentally feasible geometry. We have further studied the dynamics of AFM skyrmionium by applying spin polarized current.",1810.08262v2 2018-10-24,Photoinduced topological spin texture in a metallic ferromagnet,"Photoinduced nonequilibrium spin structure is examined in the double-exchange model, in which itinerant electrons couple with localized spins through the ferromagnetic Hund coupling. In particular, we focus on the transient spin structure from the initial ferromagnetic metallic state to the steady antiferromagnetic ordered state reported in [Phys. Rev. Lett. 119, 207202 (2017)]. By solving the Schr\""odinger equation combined with the Landau-Lifshitz-Gilbert equation, we find finite winding number and chirality, which implies emergence of topological chiral spin textures. These observations are reproduced by a calculation where spin dynamics after sudden quench of the chemical potential are examined in larger clusters. A possible mechanism of the topological spin texture in the transient dynamics is discussed.",1810.10244v1 2018-10-26,Trajectory Generation for Millimeter Scale Ferromagnetic Swimmers: Theory and Experiments,"Microrobots have the potential to impact many areas such as microsurgery, micromanipulation and minimally invasive sensing. Due to their small size, microrobots swim in a regime that is governed by low Reynolds number hydrodynamics. In this paper, we consider small scale artificial swimmers that are fabricated using ferromagnetic filaments and locomote in response to time varying external magnetic fields. We motivate the design of previously proposed control laws using tools from geometric mechanics and also demonstrate how new control laws can be synthesized to generate net translation in such swimmers. We further describe how to modify these control inputs to make the swimmers track rich trajectories in the workspace by investigating stability properties of their limit cycles in the orientation angles phase space. Following a systematic design optimization, we develop a principled approach to encode internal magnetization distributions in millimeter scale ferromagnetic filaments. We verify and demonstrate this procedure experimentally and finally show translation, trajectory tracking and turning in place locomotion in these optimal swimmers using a Helmholtz coils setup.",1810.11191v1 2018-10-29,Hydrodynamic sound modes and Galilean symmetry breaking in a magnon fluid,"The non-interacting magnon gas description in ferromagnets breaks down at finite magnon density where momentum-conserving collisions between magnons become important. Observation of the collision-dominated regime, however, has been hampered by the lack of probes to access the energy and lengthscales characteristic of this regime. Here we identify a key signature of the collision-dominated hydrodynamic regime---a magnon sound mode---which governs dynamics at low frequencies and can be detected with recently-introduced spin qubit magnetometers. The magnon sound mode is an excitation of the longitudinal spin component with frequencies below the spin wave continuum in gapped ferromagnets. We also show that, in the presence of exchange interactions with SU(2) symmetry, the ferromagnet hosts an usual hydrodynamic regime that lacks Galilean symmetry at all energy and lengthscales. The hydrodynamic sound mode, if detected, can lead to a new platform to explore hydrodynamic behavior in quantum materials.",1810.12333v2 2019-06-05,Large Anomalous Hall Effect in Topological Insulators with Proximitized Ferromagnetic Insulators,"We report a proximity-driven large anomalous Hall effect in all-telluride heterostructures consisting of ferromagnetic insulator Cr2Ge2Te6 and topological insulator (Bi,Sb)2Te3. Despite small magnetization in the (Bi,Sb)2Te3 layer, the anomalous Hall conductivity reaches a large value of 0.2e2/h in accord with a ferromagnetic response of the Cr2Ge2Te6. The results show that the exchange coupling between the surface state of the topological insulator and the proximitized Cr2Ge2Te6 layer is effective and strong enough to open the sizable exchange gap in the surface state.",1906.02159v1 2019-06-07,Angle resolved relaxation of spin currents by antiferromagnets in spin valves,"We observe and analyze tunable relaxation of a pure spin current by an antiferromagnet in spin-valves. This is achieved by carefully controlling the angle between a resonantly excited ferromagnetic layer pumping the spin current and the N\'eel vector of the antiferromagnetic layer. The effect is observed as an angle-dependent spin-pumping contribution to the ferromagnetic resonance linewidth. An interplay between spin-mixing conductance and, often disregarded, longitudinal spin conductance is found to underlie our observations, which is in agreement with a recent prediction for related ferromagnetic spin valves.",1906.03124v3 2019-06-05,Uncovering A Two-Dimensional Semiconductor with Intrinsic Ferromagnetism at Room Temperature,"Two-dimensional materials have been gaining great attention as they displayed a broad series of electronic properties that ranging from superconductivity to topology. Among them, those which possess magnetism are most desirable, enabling us to manipulate charge and spin simultaneously. Here, based on first-principles calculation, we demonstrate monolayer chromium iodide arsenide (CrIAs), an undiscovered stable two-dimensional material, is an intrinsic ferromagnetic semiconductor with out-of-plane spin magnetization. The indirect bandgaps are predicted to be 0.32 eV for majority spin and 3.31 eV for minority spin, large enough to preserve semiconducting features at room temperature. Its Curie temperature, estimated by Heisenberg model with magnetic anisotropic energy using Monte Carlo method, is as high as 655 K that well above the room temperature, owing to strong direct exchange interaction between chromium d and iodine p orbitals. This work offers the affirmative answer of whether there exists two-dimensional ferromagnetic semiconductor at room temperature. And the practical realization of quantum spintronic devices, which have been suppressed because of lacking suitable room temperature magnetic materials, would embrace a great opportunity.",1906.05444v1 2019-06-17,Local Photothermal Control of Phase Transitions for On-demand Room-temperature Rewritable Magnetic Patterning,"The ability to make controlled patterns of magnetic structures within a nonmagnetic background is essential for several types of existing and proposed technologies. Such patterns provide the foundation of magnetic memory and logic devices, allow the creation of artificial spin-ice lattices and enable the study of magnon propagation. Here, we report a novel approach for magnetic patterning that allows repeated creation and erasure of arbitrary shapes of thin-film ferromagnetic structures. This strategy is enabled by epitaxial Fe$_{0.52}$Rh$_{0.48}$ thin films designed so that both ferromagnetic and antiferromagnetic phases are bistable at room temperature. Starting with the film in a uniform antiferromagnetic state, we demonstrate the ability to write arbitrary patterns of the ferromagnetic phase by local heating with a focused laser. If desired, the results can then be erased by cooling with a thermoelectric cooler and the material repeatedly re-patterned.",1906.07239v1 2019-06-19,Correlation between the Dzyaloshinskii-Moriya interaction and the orbital angular momentum at an oxide / ferromagnet interface,"We report on the Dzyaloshinskii-Moriya (DMI) interaction at the interface between a ferromagnet and an oxide. We demonstrate experimentally that oxides can give rise to DMI. By comparison of systems comprised of Pt/Co90Fe10/Oxide and Cu/Co90Fe10/Oxide, we also show how oxidation of one interface can enhance and add to the total DMI of that generated by the Pt interface. This is due to the fact that the DMI on both interfaces promotes left-handed chirality. Finally, by use of ferromagnetic resonance spectroscopy, we show that the DMI and the spectroscopic splitting factor, which is a measure of the orbital momentum, are correlated. This indicates the importance of hybridization and charge transfer at the oxide interface for the DMI.",1906.08395v1 2019-06-25,Itinerant Ferromagnetism in p-doped Monolayers of MoS2,"Density functional theory is used to explore the possibility of inducing impurity band ferromagnetism in monolayers of semiconducting MoS2 by introducing holes into the narrow Mo 4d band that forms the top of the valence band. A large out of plane anisotropy is found for unpaired spins bound to the substitutional acceptor impurities V, Nb and Ta that couple ferromagnetically for all but the shortest separations. Using the separation dependent exchange interactions as input to Monte Carlo calculations, we estimate ordering temperatures as a function of the impurity concentration. For about 9% of V impurities, Curie temperatures in excess of 160 K are predicted. The singlet formation at short separations that limits the ordering temperature is explained and we suggest how it can be circumvented.",1906.10433v1 2019-07-04,Topological Magnon-Phonon Hybrid Excitations in Two-Dimensional Ferromagnets with Tunable Chern Numbers,"We theoretically investigate magnon-phonon hybrid excitations in two-dimensional ferromagnets. The bulk bands of hybrid excitations, which are referred to as magnon-polarons, are analytically shown to be topologically nontrivial, possessing finite Chern numbers. We also show that the Chern numbers of magnon-polaron bands and the number of band-crossing lines can be manipulated by an external magnetic field. For experiments, we propose to use the thermal Hall conductivity as a probe of the finite Berry curvatures of magnon-polarons. Our results show that a simple ferromagnet on a square lattice supports topologically nontrivial magnon-polarons, generalizing topological excitations in conventional magnetic systems.",1907.02224v1 2019-07-07,Rotating magnetocaloric effect in the ferromagnetic Weyl semi-metal Co$_{3}$Sn$_{2}$S$_{2}$,"The rotating magnetocaloric effect (RMCE) is a recent interest in magnetic refrigeration technique in which the cooling effect is attained by rotating the anisotropic magnetocaloric material from one orientation to the other in a fixed magnetic field. In this work, we report the anisotropic magnetocaloric properties of single crystals of the ferromagnetic Weyl semimetal Co$_{3}$Sn$_{2}$S$_{2}$ for magnetic field $H\parallel c$ axis and $H\parallel ab$ plane. We observed a significant (factor of $2$) difference between the magnetocaloric effect measured in both orientations. The rotating magnetocaloric effect has been extracted by taking the difference of the magnetic entropy change ($\Delta S_{M}$) for fields applied in the two crystallographic orientations. In a scaling analysis of $\Delta S_{M}$, the rescaled $\Delta S_{M}(T,H)$ vs reduced temperature $\theta$ curves collapse onto a single universal curve, indicating that the transition from paramagnetic to ferromagnetic phase at 174~K is a second order transition. Furthermore, using the power law dependence of $\Delta S_{M}$ and relative cooling power RCP, the critical exponents $\beta$ and $\gamma$ are calculated, which are consistent with the recent critical behavior study on this compound \cite{Yan2018}.",1907.03317v1 2019-07-11,Hole compensation effect in III-Mn-V dilute ferromagnetic semiconductors,"A systematic study of hole compensation effect on magnetic properties, which is controlled by defect compensation through ion irradiation, in (Ga,Mn)As, (In,Mn)As and (Ga,Mn)P is presented in this work. In all materials, both Curie temperature and magnetization decrease upon increasing the hole compensation, confirming the description of hole mediated ferromagnetism according to the p-d Zener model. The material dependence of Curie temperature and magnetization versus hole compensation reveals that the manipulation of magnetic properties in III-Mn-V dilute ferromagnetic semiconductors by ion irradiation is strongly influenced by the energy level location of the produced defect relative to the band edges in semiconductors.",1907.05160v1 2019-07-19,A cryogenic memory element based on an anomalous Josephson junction,"We propose a non-volatile memory element based on a lateral ferromagnetic Josephson junction with spin-orbit coupling and out-of-plane magnetization. The interplay between the latter and the intrinsic exchange field of the ferromagnet leads to a magnetoelectric effect that couples the charge current through the junction and its magnetization, such that by applying a current pulse the direction of the magnetic moment in F can be switched. The two memory states are encoded in the direction of the out-of-plane magnetization. With the aim to determine the optimal working temperature for the memory element, we explore the noise-induced effects on the averaged stationary magnetization by taking into account thermal fluctuations affecting both the Josephson phase and the magnetic moment dynamics. We investigate the switching process as a function of intrinsic parameters of the ferromagnet, such as the Gilbert damping and strength of the spin-orbit coupling, and proposed a non-destructive readout scheme based on a dc-SQUID. Additionally, we analyze a way to protect the memory state from external perturbations by voltage gating in systems with a both linear-in-momentum Rashba and Dresselhaus spin-orbit coupling.",1907.08454v2 2019-07-19,Optical control of synchronous phases in a programmable polariton cell,"We demonstrate deterministic control of the nearest and next-nearest neighbor coupling in the unit cell of a square lattice of microcavity exciton-polariton condensates. We tune the coupling in a continuous and reversible manner by optically imprinting potential barriers of variable height, in the form of spatially localized incoherent exciton reservoirs that modify the particle flow between condensates. By controlling the couplings in a $2\times2$ polariton cluster, we realize ferromagnetic, anti-ferromagnetic and paired ferromagnetic phases. Our approach paves the way towards simulating complex condensed matter phases through precise control of the individual couplings in networks of optical nonlinear oscillators.",1907.08580v1 2019-07-24,Semi-Quantized Spin Pumping and Spin-Orbit Torques in Topological Dirac Semimetals,"We study the time-development processes of spin and charge transport phenomena in a topological Dirac semimetal attached to a ferromagnetic insulator with a precessing magnetization. Compared to conventional normal metals, topological Dirac semimetals manifest a large inverse spin Hall effect when a spin current is pumped from the attached ferromagnetic insulator. It is shown that the induced charge current is semi-quantized, i.e., it depends only on the distance between the two Dirac points in momentum space and hardly depends on the disorder strength when the system remains in the topological Dirac semimetal phase. As an inverse effect, we show that the electric field applied to the topological Dirac semimetal exerts a spin torque on the local magnetization in the ferromagnetic insulator via the exchange interaction and the semi-quantized spin Hall effect. Our study demonstrates that the topological Dirac semimetal offers a less-dissipative platform for spin-charge conversion and spin switching.",1907.10459v2 2019-07-29,Co-existence of Novel Ferromagnetic/Anti-ferromagnetic Phases in Two-dimensional Ti3C2 MXene,"This study reports first synthesis of MXene-derived co-existing phases. New family of two-dimensional materials such as Ti3C2 namely MXene, having transition metal forming hexagonal structure with carbon atoms have attracted tremendous interest now a days. We have reported structural, optical and magnetic properties of undoped and La-doped Ti3C2Tx MXene synthesized using co-precipitation method. The c-lattice parameters (c-LP) calculated for La-MXene is c=18.3{\AA} which is slightly different from the parent un-doped MXene (c=19.2{\AA}), calculated from X-ray diffraction data. The doping of La+3 ions shrinks Ti3C2Tx layers perpendicular to the planes but expands slightly the in-plane lattice parameters. The band gap for MXene is calculated to be 1.06 eV which is increased to 1.44 eV after the doping of La+3 ion that shows its good semiconducting nature. The experimental results for magnetic properties of both the samples have been presented and discussed, indicating the presence of ferromagnetic-antiferromagnetic phases co-existing. The results presented here are unique and first report on magnetic properties of two-dimensional carbides for magnetic data storage applications.",1907.12588v1 2019-09-04,High-field moment polarization in the itinerant ferromagnet URhSi,"We report a high-magnetic-field study of the itinerant ferromagnet URhSi. Magnetization and electrical resistivity were measured under magnetic fields $\mu_0H$ up to 58~T applied along the directions $\mathbf{a}$, $\mathbf{b}$, and $\mathbf{c}$ of the orthorhombic structure and temperatures $T$ ranging from 1.5 to 50 K. For $\mathbf{H}\parallel\mathbf{b}$, pseudo-metamagnetism at $\mu_0H_m\simeq30-40$~T is associated with a broad step in the magnetization and a maximum in the resistivity. The properties of URhSi are discussed and compared with those of the isostructural superconducting ferromagnets URhGe and UCoGe and of the superconducting paramagnet UTe$_2$.",1909.01810v2 2019-09-10,Crystal Structures and Phase Transitions of the van-der-Waals Ferromagnet VI3,"The results of a single-crystal X-ray-diffraction study of the evolution of crystal structures of VI3 with temperature with emphasis on phase transitions are presented. Some related specific-heat and magnetization data are included. The existence of the room-temperature trigonal crystal structure R-3 (148) has been confirmed. Upon cooling, VI3 undergoes a structural phase transition to a monoclinic phase at Ts ~ 79 K. Ts is reduced in magnetic fields applied along the trigonal c-axis. When VI3 becomes ferromagnetic at TFM1 ~ 50 K, magnetostriction-induced changes of the monoclinic-structure parameters are observed. Upon further cooling, the monoclinic structure transforms into a triclinic variant at 32 K which is most likely occurring in conjunction with the previously reported transformation of the ferromagnetic structure. The observed phenomena are preliminarily attributed to strong magnetoelastic interactions.",1909.04459v2 2019-09-13,Anomalous ferromagnetism and magneto-optical Kerr effect in semiconducting double perovskite Ba2NiOsO6 and its (111) (Ba2NiOsO6)/(BaTiO3)10 superlattice,"We carry out a first-principles investigation on magnetism, electronic structure, magneto-optical effects and topological property of newly grown cubic double perovskite Ba2NiOsO6 and its (111) (Ba2NiOsO6)/(BaTiO3)10 superlattice, based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Couloumb interactions. Interestingly, we find that both structures are rare ferromagnetic (FM) semiconductors with estimated Curie temperatures of ~150 and 70 K, respectively. The calculated near-neighbor exchange coupling parameters reveal that the ferromagnetism is driven by exotic FM coupling between Ni and Os atoms, which is due to the FM superexchange interaction caused by the abnormally strong hybridization between the half-filled Nieg and unoccupied Os eg orbitals. The strong spin-orbit coupling (SOC) on the Os atom is found to not only open the semiconducting gap but also produce a large antiparallel orbital magnetic moment on the Os atom, thus reducing the total magnetization from 4.0 uB/f.u.,",1909.05996v1 2019-09-16,Enhanced spin triplet superconductivity due to Kondo destabilization,"In a Kondo lattice system, suppression of effective Kondo coupling leads to the breakdown of the heavy-electron metal and a change in the electronic structure. Spin triplet superconductivity in the Kondo lattice UTe2 appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of spin-triplet superconductivity near the pressure-driven suppression of the Kondo coherence, implying that superconductivity is strengthened by the affiliated growth of both spin and charge fluctuations. The coherent Kondo state discontinuously transitions to ferromagnetic order at higher pressures. Application of magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe2. In addition to spin fluctuations associated with ferromagnetism, our results show that a Kondo-driven Fermi surface instability may be playing a role in stabilizing spin triplet superconductivity.",1909.06932v1 2019-09-18,Magnetic domain dynamics in an insulating quantum ferromagnet,"The statistics and form of avalanches in a driven system reveal the nature of the underlying energy landscape and dynamics. In conventional metallic ferromagnets, eddy-current back action can dominate the dynamics. Here, we study Barkhausen noise in Li(Ho,Y)F4, an insulating Ising ferromagnet that cannot sustain eddy currents. For large avalanches at temperatures approaching the Curie point, we find a symmetric response free of drag effects. In the low temperature limit, drag effects contribute to the dynamics, which we link to enhanced pinning from local random fields that are enabled by the microscopic dipole-coupled Hamiltonian (the Ising model in transverse field).",1909.08591v1 2019-09-25,Stability and Absence of a Tower of States in Ferrimagnets,"Antiferromagnets and ferromagnets are archetypes of the two distinct (type-A and type-B) ways of spontaneously breaking a continuous symmetry. Although type-B Nambu--Goldstone modes arise in various systems, the ferromagnet was considered pathological due to the stability and symmetry-breaking nature of its exact ground state. However, here we show that symmetry-breaking in ferrimagnets closely resembles the ferromagnet. In particular, there is an extensive ground state degeneracy, there is no Anderson tower of states, and the maximally polarized ground state is thermodynamically stable. Our results are derived analytically for the Lieb--Mattis ferrimagnet and numerically for the Heisenberg ferrimagnet. We argue that these properties are generic for type-B symmetry-broken systems, where the order parameter operator is a symmetry generator.",1909.11381v1 2019-09-26,Superconductivity driven helical magnetic structure in EuRbFe$_4$As$_4$ ferromagnetic superconductor,"Recently the evidence of the helical magnetic structure modulated along $c$-axis with the period of four lattice parameters was obtained in easy $ab$ plane ferromagnetic superconductor EuRbFe$_4$As$_4$ [K. Iida et al., Phys. Rev. B 100, 014506 (2019)]. We argue that such structure may appear due to the presence of superconductivity. In spite of the very small value of the exchange field acting on the superconducting electrons in EuRbFe$_4$As$_4$, the exchange mechanism of interaction between superconductivity and ferromagnetism could dominate over the electromagnetic one and this circumstance could favor the emergence of the short-period magnetic structure (with the period less than the superconducting coherence length). Such a situation differs from one in the similar compound P-doped EuFe$_2$As$_2$, where the electromagnetic mechanism dominates and results in the magnetic structure with significantly larger period (of the order of London penetration depth). We also analyze the effect of the external magnetic field on the onset temperature of the modulated magnetic structure.",1909.12082v2 2019-09-27,Optical second harmonic generation in a ferromagnetic liquid crystal,"A comparative experimental investigation of the dependence of second harmonic generation (SHG) on applied external voltage between the standard nematic liquid crystalline material and an analogue ferromagnetic nematic liquid crystalline material was performed by using a fundamental optical beam at 800 nm wavelength. For a ferromagnetic material, the dependence of SHG on an applied magnetic field was also examined. Three different polarization combinations of the fundamental and the second harmonic radiation were analysed. The SHG signal observed in the former material is attributed to a combination of electric field-induced SHG (EFISHG) and flexoelectric deformation-induced SHG, while SHG signal observed in the latter material is attributed solely to flexoelectric deformation-induced SHG. The obtained dependences of the SHG signal on the associated optical retardation show that in the most favourable polarization combination the two contributions generate about the same effective nonlinear optical susceptibility.",1909.12724v1 2019-10-21,Generation of exchange magnons in thin ferromagnetic films by ultrashort acoustic pulses,"We investigate generation of exchange magnons by ultrashort, picosecond acoustic pulses propagating through ferromagnetic thin films. Using the Landau-Lifshitz-Gilbert equations we derive the dispersion relation for exchange magnons for an external magnetic field tilted with respect to the film normal. Decomposing the solution in a series of standing spin wave modes, we derive a system of ordinary differential equations and driven harmonic oscillator equations describing the dynamics of individual magnon mode. The external magnetoelastic driving force is given by the time-dependent spatial Fourier components of acoustic strain pulses inside the layer. Dependencies of the magnon excitation efficiencies on the duration of the acoustic pulses and the external magnetic field highlight the role of acoustic bandwidth and phonon-magnon phase matching. Our simulations for ferromagnetic nickel evidence the possibility of ultrafast magneto-acoustic excitation of exchange magnons within the bandwidth of acoustic pulses in thin samples under conditions readily obtained in femtosecond pump-probe experiments.",1910.09481v1 2019-10-28,Tuning interfacial spins in antiferromagnetic / ferromagnetic / heavy metal heterostructures via spin-orbit torque,"Antiferromagnets are outstanding candidates for the next generation of spintronic applications, with great potential for downscaling and decreasing power consumption. Recently, the manipulation of bulk properties of antiferromagnets has been realized by several different approaches. However, the interfacial spin order of antiferromagnets is an important integral part of spintronic devices, thus the successful control of interfacial antiferromagnetic spins is urgently desired. Here, we report the high controllability of interfacial spins in antiferromagnetic / ferromagnetic / heavy metal heterostructure devices using spin-orbit torque (SOT) assisted by perpendicular or longitudinal magnetic fields. Switching of the interfacial spins from one to another direction through multiple intermediate states is demonstrated. The field-free SOT-induced switching of antiferromagnetic interfacial spins is also observed, which we attribute to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. Our work provides a precise way to modulate the interfacial spins at an antiferromagnet / ferromagnet interface via SOT, which will greatly promote innovative designs for next generation spintronic devices.",1910.12445v1 2019-10-29,Ferromagnetic van der Waals compound MnSb$_{1.8}$Bi$_{0.2}$Te$_4$,"The intersection of topology and magnetism represents a new playground to discover novel quantum phenomena and device concepts. In this work, we show that a van der Waals compound MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ exhibits a ferromagnetic ground state with a Curie temperature of 26 K, in contrast to the antiferromagnetic order previously found for other members of the Mn(Sb, Bi)$_2$Te$_4$ family. We employ magneto-transport, bulk magnetization and neutron scattering studies to illustrate the magnetic and electrical properties of MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ and report on the observation of an unusual anomalous Hall effect. Our results are an important step in the synthesis and understanding of ferromagnetic topological insulators.",1910.13057v1 2019-10-30,Large anomalous Nernst effect in a van der Waals ferromagnet Fe$_3$GeTe$_2$,"Anomalous Nernst effect, a result of charge current driven by temperature gradient, provides a probe of the topological nature of materials due to its sensitivity to the Berry curvature near the Fermi level. Fe3GeTe2, one important member of the recently discovered two-dimensional van der Waals magnetic materials, offers a unique platform for anomalous Nernst effect because of its metallic and topological nature. Here, we report the observation of large anomalous Nernst effect in Fe3GeTe2. The anomalous Hall angle and anomalous Nernst angle are about 0.07 and 0.09 respectively, far larger than those in common ferromagnets. By utilizing the Mott relation, these large angles indicate a large Berry curvature near the Fermi level, consistent with the recent proposal for Fe3GeTe2 as a topological nodal line semimetal candidate. Our work provides evidence of Fe3GeTe2 as a topological ferromagnet, and demonstrates the feasibility of using two-dimensional magnetic materials and their band topology for spin caloritronics applications.",1910.13617v1 2020-01-08,Odd triplet superconductivity induced by the moving condensate,"It has been commonly accepted that magnetic field suppresses superconductivity by inducing the ordered motion of Cooper pairs. We demonstrate that magnetic field can instead provide a generation of superconducting correlations by inducing the motion of superconducting condensate. This effect arises in superconductor/ferromagnet heterostructures in the presence of Rashba spin-orbital coupling. We predict the odd-frequency spin-triplet superconducting correlations called the Berezinskii order to be switched on at large distances from the superconductor/ferromagnet interface by the application of a magnetic field. This is shown to result in the unusual behaviour of Josephson effect and local density of states in superconductor/ferromagnet structures.",2001.02507v2 2020-01-08,Magnetic correlations in the disordered ferromagnetic alloy Ni-V revealed with small angle neutron scattering,"We present small angle neutron scattering (SANS) data collected on polycrystalline Ni$_{1-x}$V$_x$ samples with $x\geq0.10$ with confirmed random atomic distribution. We aim to determine the relevant length scales of magnetic correlations in ferromagnetic samples with low critical temperatures $T_c$ that show signs of magnetic inhomogeneities in magnetization and $\mu$SR data. The SANS study reveals signatures of long-range order and coexistence of short-range magnetic correlations in this randomly disordered ferromagnetic alloy. We show the advantages of a polarization analysis in identifying the main magnetic contributions from the dominating nuclear scattering.",2001.02706v1 2020-01-20,The topological magnon bands in the Flux state in Sashtry-Sutherland lattice,"We investigate low energy magnon excitations above the non-collinear flux state and non-coplanar canted flux state in a Heisenberg anti-ferromagnet with Dzyaloshinskii-Moriya interaction~(DMI) on a Sashtry-Sutherland lattice.While previous studies have shown the presence of topological magnetic excitation in the dimer and ferromagnetic phases on the Shastry-Sutherland lattice, our results establish the non-trivial topology of magnons in the anti-ferromagnetic flux and canted flux states. Our results uncover the existence of a multitude of topological phase transitions in the magnon sector -- evidenced by the changing Chern numbers of the single magnon bands -- as the Hamiltonian parameters are varied, even when the ground state remains unchanged. The thermal Hall conductivity is calculated and its derivative is shown to exhibit a logarithmic divergence at the phase transitions, independent of the type of band touching involved. This may provide a useful means to identify the energy at which the transition occurs. Finally, we propose the way to realize the studied model in a practical material.",2001.07008v1 2020-01-21,Contrasting Ferromagnetism in Pyrite FeS$_2$ Induced by Chemical Doping versus Electrostatic Gating,"Recent advances in electrostatic gating provide a novel way to modify the carrier concentration in materials via electrostatic means instead of chemical doping, thus minimizing the impurity scattering. Here, we use first-principles Density Functional Theory combined with a tight-binding approach to compare and contrast the effects of electrostatic gating and Co chemical doping on the ferromagnetic transition of FeS$_2$, a transition metal disulfide with the pyrite structure. Using tight-binding parameters obtained from maximally-localized Wannier functions, we calculate the magnetic susceptibility across a wide doping range. We find that electrostatic gating requires a higher electron concentration than the equivalent in Co doping to induce ferromagnetism via a Stoner-like mechanism. We attribute this behavior to the formation of a narrow Co band near the bottom of the conduction band under chemical doping, which is absent in the electrostatic gating case. Our results reveal that the effects of electrostatic gating go beyond a simple rigid band shift, and highlight the importance of the changes in the crystal structure promoted by gating.",2001.07319v1 2020-01-23,Relaxation of ferromagnetic domains in a disordered lattice in 2D,"We investigate the relaxation process of ferromagnetic domains in 2D subjected to the influence of both, static disorder of variable strength and weak interactions. The domains are represented by a two species bosonic mixture of $^{87}$Rb ultracold atoms, such that initially each specie lies on left and right halves of a square lattice. The dynamics of the double domain is followed by describing the two-component superfluid, at mean field level, through the time dependent Gross-Pitaevskii coupled equations, considering values of the intra and inter-species interaction, reachable in current experimental setups, that guaranty miscibility of the components. A robust analysis for several values inter-species interaction leads us to conclude that the presence of structural disorder leads to slowdown the relaxation process of the initial ferromagnetic order. As shown by our numerical experiments, magnetization is maintained up to 60 percent of its initial value for the largest disorder amplitude.",2001.08337v1 2020-01-27,Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves,"Voltage induced magnetization dynamics of magnetic thin films is a valuable tool to study anisotropic fields, exchange couplings, magnetization damping and spin pumping mechanism. A particularly well established technique is the ferromagnetic resonance (FMR) generated by the coupling of microwave photons and magnetization eigenmodes in the GHz range. Here we review the basic concepts of the so-called acoustic ferromagnetic resonance technique (a-FMR) induced by the coupling of surface acoustic waves (SAW) and magnetization of thin films. Interestingly, additional to the benefits of the microwave excited FMR technique, the coupling between SAW and magnetization also offers fertile ground to study magnon-phonon and spin rotation couplings. We describe the in-plane magnetic field angle dependence of the a-FMR by measuring the absorption / transmission of SAW and the attenuation of SAW in the presence of rotational motion of the lattice, and show the consequent generation of spin current by acoustic spin pumping.",2001.09581v1 2020-02-06,Spin Reversal of a Quantum Hall Ferromagnet at a Landau Level Crossing,"When Landau levels (LLs) become degenerate near the Fermi energy in the quantum Hall regime, interaction effects can drastically modify the electronic ground state. We study the quantum Hall ferromagnet formed in a two-dimensional hole gas around the LL filling factor $\nu=1$ in the vicinity of a LL crossing in the heave-hole valence band. Cavity spectroscopy in the strong-coupling regime allows us to optically extract the two-dimensional hole gas' spin polarization. By analyzing this polarization as a function of hole density and magnetic field, we observe a spin flip of the ferromagnet. Furthermore, the depolarization away from $\nu=1$ accelerates close to the LL crossing. This is indicative of an increase in the size of Skyrmion excitations as the effective Zeeman energy vanishes at the LL crossing.",2002.02240v2 2020-02-06,Domain structure dynamics in the ferromagnetic Kagome-lattice Weyl semimetal Co$_3$Sn$_2$S$_2$,"Co$_3$Sn$_2$S$_2$, a Weyl semimetal that consists of layers of Kagome lattices, transitions from a high-temperature paramagnetic phase to a low-temperature ferromagnetic phase below 177 K. The phase transition occurs through an intermediate non-trivial magnetic phase, the so-called ""A""-phase just below the Curie temperature. The ""A""-phase was earlier linked with a competing anti-ferromagnetic phase, a spin-glass phase and certain indirect measurements indicated the possibility of magnetic Skyrmions in this phase. We have imaged the magnetic domain structure in a single crystal of Co$_3$Sn$_2$S$_2$ at different temperatures, magnetic fields and field-angles by magnetic force microscopy. At low temperatures, we observed stripe domains indicating presence of uniaxial anisotropy. Above 130 K, the domain walls become mobile and they tend to align relatively easily when the magnetic field is increased along the $c$-axis than in the $a-b$ plane. Our detailed study of field-dependent domain dynamics reveal that the anomalous nature of the phase transition just below $T_c$ is dominantly governed by domain wall motion.",2002.02494v1 2020-02-07,Spin-dependent transport through a Weyl semimetal surface,"We experimentally compare two types of interface structures with magnetic and non-magnetic Weyl semimetals. They are the junctions between a gold normal layer and magnetic Weyl semimetal Ti$_2$MnAl, and a ferromagnetic nickel layer and non-magnetic Weyl semimetal WTe$_2$, respectively. Due to the ferromagnetic side of the junction, we investigate spin-polarized transport through the Weyl semimetal surface. For both structures, we demonstrate similar current-voltage characteristics, with hysteresis at low currents and sharp peaks in differential resistance at high ones. Despite this behavior resembles the known current-induced magnetization dynamics in ferromagnetic structures, evolution of the resistance peaks with magnetic field is unusual. We connect the observed effects with current-induced spin dynamics in Weyl topological surface states.",2002.02673v2 2020-02-10,Spontaneous ferromagnetism and finite surface energy gap in the topological insulator Bi$_2$Se$_3$ from surface Bi$_\rm{Se}$ antisite defects,"We perform ab-initio calculations on Bi$_\mathrm{{Se}}$ antisite defects in the surface of Bi$_2$Se$_3$, finding strong low-energy defect resonances with a spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptional large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper lying defects, such hybridization is largely absent, the magnetic moments becomes more localized, and no energy gap is present.",2002.03962v2 2020-02-18,Dynamic percolation of ferromagnetic regions in phase separated manganites using non-uniform electric fields,"Thin films of the manganite (La$_{1-y}$Pr$_y$)$_{1-x}$Ca$_x$MnO$_3$ exhibit dynamic phase coexistence with micrometer scale, fluid-like ferromagnetic metallic (FMM) regions interspersed in a charge-order insulating (COI) background. It has been previously reported that a uniform electric field realigns the fluid-like FMM regions due to a phenomenon similar to dielectrophoresis. Here we report that non-uniform electric fields have a stronger effect on the FMM regions as expected from the dielectrophoresis model. The dynamic percolation of the FMM regions is observed over a wider range of temperatures compared to the results in a uniform field. Additionally, in a non-uniform electric field, the time required for dynamic percolation along the magnetic hard axis ($t_{\mathrm{B}}$) decreased with increasing applied voltage ($V_{\mathrm{A}}$) as a power law, $V_{\mathrm{A}}^{-\delta}$ with $\delta \approx 5$ while $\delta < 2$ for a uniform electric field. Our results in a non-uniform electric field provide strong evidence in favor of the dielectrophoresis model and a unique method for manipulating micrometer-sized ferromagnetic regions using electric fields.",2002.07783v1 2020-04-01,Importance of site occupancy and absence of strain glassy phase in Ni$_{2-x}$Fe$_{x}$Mn$_{1.5}$In$_{0.5}$,"Martensitic transition temperature steadily decreases in Ni$_{2-x}$Fe$_{x}$Mn$_{1.5}$In$_{0.5}$ and is completely suppressed at $x$ = 0.2. Despite suppression of martensitic transition, Ni$_{1.8}$Fe$_{0.2}$Mn$_{1.5}$In$_{0.5}$ does not display the expected strain glassy phase. Instead, a ground state with dominant ferromagnetic interactions is observed. A study of structural and magnetic properties of $x$ = 0.2 reveal that the alloy consists of a major Fe rich cubic phase and a minor Fe deficient monoclinic phase favoring a ferromagnetic ground state. This is exactly opposite of that observed in Ni$_2$Mn$_{1-y}$Fe$_{y}$In$_{0.5}$ wherein a strain glassy phase is observed for $y$ = 0.1. The change in site symmetry of Fe when doped for Ni in contrast to Mn in the Heusler composition seems to support the growth of the ferromagnetic phase.",2004.00256v1 2020-04-07,Very large thermal rectification in ferromagnetic insulator-based superconducting tunnel junctions,"We investigate electronic thermal rectification in ferromagnetic insulator-based superconducting tunnel junctions. Ferromagnetic insulators coupled to superconductors are known to induce sizable spin splitting in the superconducting density of states, and also lead to efficient spin filtering if operated as tunnel barriers. The combination of spin splitting and spin filtering is shown to yield a substantial self-amplification of the electronic heat diode effect due to breaking of the electron-hole symmetry in the system which is added to the thermal asymmetry of the junction. Large spin splitting and large spin polarization can potentially lead to thermal rectification efficiency exceeding 5 .10^4 for realistic parameters in a suitable temperature range, thereby outperforming up to a factor of 250 the heat diode effect achievable with conventional superconducting tunnel junctions. These results could be relevant for improved mastering of the heat currents in innovative phase-coherent caloritronic nanodevices, and for enhanced thermal management of quantum circuits at the nanoscale.",2004.03620v1 2020-04-08,Charge-to-spin conversion efficiency in ferromagnetic nanowires by spin torque ferromagnetic resonance: Reconciling lineshape and linewidth analysis methods,"Spin orbit torques are of great interest for switching the magnetization direction in nanostructures, moving skyrmions and exciting spin waves. The standard method of determining their efficiency is by spin torque ferromagnetic resonance (ST-FMR), a technique that involves analyzing the resonance linewidth or lineshape. On microstuctures these two analysis methods are quite consistent. Here we present ST-FMR results on permalloy (Ni$_{80}$Fe$_{20}$) nanowires -- with widths varying from $150$ to 800 nm -- that show that the standard model used to analyze the resonance linewidth and lineshape give different results; the efficiency appears greatly enhanced in nanowires when the lineshape method is used. A ST-FMR model that properly accounts for the sample shape is presented and shows much better consistency between the two methods. Micromagnetic simulations are used to verify the model. These results and the more accurate nanowire model presented are of importance for characterizing and optimizing charge-to-spin conversion efficiencies in nanostructures.",2004.03784v1 2020-04-29,Terahertz Emission From an Exchange-Coupled Synthetic Antiferromagnet,"We report on terahertz emission from FeMnPt/Ru/FeMnPt and Pt/CoFeB/Ru/CoFeB/Pt synthetic antiferromagnet (SAF) structures upon irradiation by a femtosecond laser; the former is via the anomalous Hall effect, whereas the latter is through the inverse spin Hall effect. The antiparallel alignment of the two ferromagnetic layers leads to a terahertz emission peak amplitude that is almost double that for a corresponding single-layer or bilayer emitter with the same equivalent thickness. In addition, we demonstrate by both simulation and experiment that terahertz emission provides a powerful tool to probe the magnetization reversal processes of individual ferromagnetic layers in a SAF structure, as the terahertz signal is proportional to the vector difference of the magnetizations of the two ferromagnetic layers.",2004.14128v1 2020-12-10,Antichiral Ferromagnetism,"Here by combining a symmetry-based analysis with numerical computations we predict a new kind of magnetic ordering - antichiral ferromagnetism. The relationship between chiral and antichiral magnetic order is conceptually similar to the relationship between ferromagnetic and antiferromagnetic order. Without loss of generality, we focus our investigation on crystals with full tetrahedral symmetry where chiral interaction terms - Lifshitz invariants - are forbidden by symmetry. However, we demonstrate that leading chirality-related term leads to nontrivial smooth magnetic textures in the form of helix-like segments of alternating opposite chiralities. The unconventional order manifests itself beyond the ground state by stabilizing excitations such as domains and skyrmions in an antichiral form.",2012.05835v2 2020-12-18,Domain wall skew scattering in ferromagnetic Weyl metals,"We study transport in the presence of magnetic domain walls (DWs) in a lattice model of ferromagnetic type-I Weyl metals. We compute the diagonal and Hall conductivities in the presence of a DW, using both Kubo and Landauer formalisms, and uncover the effect of DW scattering. When the Fermi level lies near Weyl points, we find a strong skew scattering at the DW which leads to a significant additional Hall effect. We estimate the average Hall resistivity for multi-domain configurations and identify the limit where the DW scattering contribution becomes significant. We show that a continuum model obtained by linearizing the lattice dispersion around the Weyl points does not correctly capture this DW physics. Going beyond the linearized theory, and incorporating leading curvature terms, leads to a semi-quantitative agreement with our lattice model results. Our results are relevant for experiments on the Hall resistivity of spin-orbit coupled ferromagnets, which can have Weyl points near the Fermi energy.",2012.10457v2 2020-12-20,Tuning the Magnetic and Electronic Properties of Strontium Titanate by Carbon Doping,"The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation (GGA) and the GGA+U approach. Our results show that the structural stability, electronic properties and magnetic properties of C-doped SrTiO3 strongly depend on the distance between carbon dopants. In both GGA and GGA+U calculations, the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors, which can be ascribed to the formation of a C-C dimer pair accompanied by stronger C-C and weaker C-Ti hybridizations as the C-C distance becomes smaller. As the C-C distance increases, C-doped SrTiO3 changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations, while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method. Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO3, which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.",2012.10963v1 2020-12-23,Effect of Fullerene on domain size and relaxation in a perpendicularly magnetized Pt/Co/C60/Pt system,"Buckminsterfullerene (C60) can exhibit ferromagnetism at the interface (called as a spinterface) when it is placed next to a ferromagnet (FM). Formation of such spinterface happens due to orbital hybridization and spin polarized charge transfer at the interface. The spinterface can influence the domain size and dynamics of the organic/ferromagnetic heterostructure. Here, we have performed magnetic domain imaging and studied the relaxation dynamics in Pt/Co/C60/Pt system with perpendicular anisotropy. We have compared the results with its parent Pt/Co/Pt system. It is observed that presence of C60 in the Pt/Co/Pt system increases the anisotropy and a decrease in the bubble domain size. Further the switching time of Pt/Co/C60/Pt system is almost two times faster than Pt/Co/Pt system. We have also performed the spin polarized density functional theory (DFT) calculations to understand the underneath mechanism. DFT results show formation of a spin polarized spinterface which leads to an enhancement in anisotropy.",2012.12777v3 2020-12-31,Detection of magnetic gap in the topological surface states of MnBi2Te4,"Recently, intrinsic antiferromagnetic topological insulator MnBi2Te4 has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect, axion insulator state, and other quantum phases. An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi2Te4. However, the experimental observations of the surface gap remain controversial. Here, we report the observation of the surface gap via the point contact tunneling spectroscopy. In agreement with theoretical calculations, the gap size is around 50 meV, which vanishes as the sample becomes paramagnetic with increasing temperature. The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field, substantiating the surface ferromagnetism. Furthermore, the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy. Combining these results, the magnetism-induced gap in topological surface states of MnBi2Te4 is revealed.",2012.15591v1 2021-01-01,Quantum Quench dynamics in XY spin chain with ferromagnetic and antiferromagnetic interactions,"In this manuscript we investigate the one-dimensional anisotropic XY model with ferromagnetic and antiferromagnetic interactions, which gives more interesting phase diagrams and dynamic critical behaviors. By using quantum renormalization-group method, we find that there are three phases in the system: antiferromagnetic Ising phase ordered in ""x direction"", spin-fluid phase and ferromagnetic Ising phase ordered in ""y direction"". In order to study the dynamical critical behaviors of the system, two quantum quenching methods are used. In both cases, the concurrence, a measure of entanglement, oscillates periodically over time. We show that the periods are the same and can be used as a new order parameter for quantum phase transitions. For further discussion, we derive the scaling exponent, {\theta}, and correlation length exponent, {\nu}, from the scaling behavior of the evolution period.",2101.00226v2 2021-01-06,Ferromagnetic order of ultra-thin La0.7Ba0.3MnO3 sandwiched between SrRuO3 layers,"We demonstrate the stability of ferromagnetic order of one unit cell thick optimally doped manganite (La0.7Ba0.3MnO3, LBMO) epitaxially grown between two layers of SrRuO3 (SRO) by using x-ray magnetic circular dichroism. At low temperature LBMO shows an inverted hysteresis loop due to the strong antiferromagnetic coupling to SRO. Moreover, above SRO TC the manganite still exhibits magnetic remanence. Density Functional Theory calculations show that coherent interfaces of LBMO with SRO hinder electronic confinement and the strong magnetic coupling enables the increase of the LBMO TC. From the structural point of view, interfacing with SRO enables LBMO to have octahedral rotations similar to bulk. All these factors jointly contribute for stable ferromagnetism up to 130 K for a one unit cell LBMO film.",2101.01940v1 2021-01-10,Inverse Proximity Effects at Spin-Triplet Superconductor-Ferromagnet Interface,"We investigate inverse proximity effects in a spin-triplet superconductor (TSC) interfaced with a ferromagnet (FM), assuming different types of magnetic profiles and chiral or helical pairings. The region of the coexistence of spin-triplet superconductivity and magnetism is significantly influenced by the orientation and spatial extension of the magnetization with respect to the spin configuration of the Cooper pairs, resulting into clearcut anisotropy signatures. A characteristic mark of the inverse proximity effect arises in the induced spin-polarization at the TSC interface. This is unexpectedly stronger when the magnetic proximity is weaker, thus unveiling immediate detection signatures for spin-triplet pairs. We show that an anomalous magnetic proximity can occur at the interface between the itinerant ferromagnet, SrRuO$_3$, and the unconventional superconductor Sr$_2$RuO$_4$. Such scenario indicates the potential to design characteristic inverse proximity effects in experimentally available SrRuO$_3$-Sr$_2$RuO$_4$ heterostructures and to assess the occurrence of spin-triplet pairs in the highly debated superconducting phase of Sr$_2$RuO$_4$.",2101.03611v1 2021-01-13,Hinge Spin Polarization in Magnetic Topological Insulators Revealed by Resistance Switch,"We report on the possibility to detect hinge spin polarization in magnetic topological insulators by resistance measurements. By implementing a three-dimensional model of magnetic topological insulators into a multi-terminal device with ferromagnetic contacts near the top surface, local spin features of the chiral edge modes are unveiled. We find local spin polarization at the hinges that inverts sign between top and bottom surfaces. At the opposite edge, the topological state with inverted spin polarization propagates in the reverse direction. Large resistance switch between forward and backward propagating states is obtained, driven by the matching between the spin polarized hinges and the ferromagnetic contacts. This feature is general to the ferromagnetic, antiferromagnetic and canted-antiferromagnetic phases, and enables the design of spin-sensitive devices, with the possibility of reversing the hinge spin polarization of the currents.",2101.05293v2 2021-01-17,Momentum-Dependent Local Ansatz Approach to the Metallic Ferromagnetism,"The first principles momentum dependent local ansatz wavefunction method (MLA) has been extended to the ferromagnetic state by introducing spin-dependent variational parameters. The theory is applied to the ferromagnetic Fe, Co, and Ni. It is shown that the MLA yields the magnetizations being comparable to the results obtained by the GGA (generalized gradient approximation) in the density functional theory. The projected momentum distribution functions as well as the mass enhancement factors are also calculated on the same footing, and are compared with those in the paramagnetic state. It is shown that the calculated mass enhancement factor of Fe is strongly suppressed by the spin polarization due to exchange splitting of the e${}_{\rm g}$ flat bands, while those of Co and Ni remain unchanged by the polarization. These results are shown to be consistent with the experimental results obtained from the low-temperature specific heats.",2101.06593v1 2021-01-19,Robust ferromagnetism in insulating La$_2$NiMnO$_6$ thin films,"The field of oxide spintronics can strongly benefit from the establishment of robust ferromagnetic insulators with near room-temperature Curie temperature. Here we investigate the structural, electronic, and magnetic properties of atomically-precise epitaxially-strained thin films of the double perovskite La$_2$NiMnO$_6$ (LNMO) grown by off-axis radio-frequency magnetron sputtering. We find that the films retain both a strong insulating behavior and a bulk-like Curie temperature in the order of 280 K, nearly independently from epitaxial strain conditions. These results suggest a prospective implementation of LNMO films in multi-layer device architectures where a high-temperature ferromagnetic insulating state is a prerequisite.",2101.07530v2 2000-06-07,Theory of Magnetic Anisotropy in III_{1-x}Mn_{x}V Ferromagnets,"We present a theory of magnetic anisotropy in ${\rm III}_{1-x}{\rm Mn}_{x}{\rm V}$ diluted magnetic semiconductors with carrier-induced ferromagnetism. The theory is based on four and six band envelope functions models for the valence band holes and a mean-field treatment of their exchange interactions with ${\rm Mn}^{++}$ ions. We find that easy-axis reorientations can occur as a function of temperature, carrier density $p$, and strain. The magnetic anisotropy in strain-free samples is predicted to have a $p^{5/3}$ hole-density dependence at small $p$, a $p^{-1}$ dependence at large $p$, and remarkably large values at intermediate densities. An explicit expression, valid at small $p$, is given for the uniaxial contribution to the magnetic anisotropy due to unrelaxed epitaxial growth lattice-matching strains. Results of our numerical simulations are in agreement with magnetic anisotropy measurements on samples with both compressive and tensile strains. We predict that decreasing the hole density in current samples will lower the ferromagnetic transition temperature, but will increase the magnetic anisotropy energy and the coercivity.",0006093v1 2000-06-19,Multi-band Gutzwiller wave functions for itinerant ferromagnetism,"Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The approximate evaluation of these variational ground states becomes exact in the limit of large coordination number. The result allows the identification of quasi-particle band structures for correlated electron systems. As a first application, we summarize a study of itinerant ferromagnetism in a two-band model, thereby elucidating the co-operation of the Coulomb repulsion and the Hund's-rule exchange. Then, we present results of calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis of 4s, 4p and 3d valence electrons. Good agreement with the experimental ground-state properties of nickel is obtained. In particular, the quasi-particle energy bands agree much better with the photo-emission and Fermi surface data than the band structure obtained from spin-density functional theory. Finally, we present results for the variational spinwave dispersion for our two-band model.",0006283v1 2000-06-28,Theory of interlayer tunneling in bi-layer quantum Hall ferromagnets,"Spielman et al. have recently observed a large zero-bias peak in the tunnel conductance of a bi-layer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size and destroy the predicted Josephson effect. We predict that the peak would be split and shifted by an in-plane magnetic field in a way that maps the dispersion relation of the ferromagnet's Goldstone mode. We also predict resonant structures in the DC I-V characteristic under bias by an {\em ac} electric field.",0006457v1 2012-05-02,Orbital polaron in double exchange ferromagnets,"We investigate the spectral properties of the two-orbital Hubbard model, including the double hopping term, by means of the dynamical mean field method. This Hamiltonian describes materials in which ferromagnetism is realized by the double exchange mechanism, like for instance manganites, nickelates or diluted magnetic semiconductors. The spectral function of the unoccupied states is characterized by a specific equidistant three peak structure. We emphasize the importance of the double hopping term on the spectral properties. We show the existence of a ferromagnetic phase due to electron doping near n=1 by the double exchange mechanism. A quasi-particle excitation at the Fermi energy is found that we attribute to what we will call an orbital polaron. We derive an effective spin-pseudospin Hamiltonian for the two-orbital double exchange model at n=1 filling to explain the existence and dynamics of this quasi-particle.",1205.0511v3 2012-05-07,Ferromagnetic behavior of ultrathin manganese nanosheets,"Ferromagnetic behaviour has been observed experimentally for the first time in nanostructured Manganese. Ultrathin ($\sim$ 0.6 nm) Manganese nanosheets have been synthesized inside the two dimensional channels of sol-gel derived Na-4 mica. The magnetic properties of the confined system are measured within 2K-300K temperature range. The confined structure is found to show a ferromagnetic behaviour with a nonzero coercivity value. The coercivity value remains positive throughout the entire temperature range of measurement. The experimental variation of susceptibility as a function of temperature has been satisfactorily explained on the basis of a two dimensional system with a Heisenberg Hamiltonian involving direct exchange interaction.",1205.1313v1 2012-05-08,Novel disordering mechanism in ferromagnetic systems with competing interactions,"Ferromagnetic Ising systems with competing interactions are considered in the presence of a random field. We find that in three space dimensions the ferromagnetic phase is disordered by a random field which is considerably smaller than the typical interaction strength between the spins. This is the result of a novel disordering mechanism triggered by an underlying spin-glass phase. Calculations for the specific case of the long-range dipolar LiHo_xY_{1-x}F_4 compound suggest that the above mechanism is responsible for the peculiar dependence of the critical temperature on the strength of the random field and the broadening of the susceptibility peaks as temperature is decreased, as found in recent experiments by Silevitch et al. [Nature (London) 448, 567 (2007)]. Our results thus emphasize the need to go beyond the standard Imry-Ma argument when studying general random-field systems.",1205.1572v2 2012-05-15,Ferromagnetic Josephson switching device with high characteristic voltage,"We develop a fast Magnetic Josephson Junction (MJJ) - a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient Single Flux Quantum (SFQ) circuits. We present experimental results for Superconductor-Insulator-Ferromagnet-Superconductor (SIFS) MJJs with high characteristic voltage IcRn of >700 uV proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ IcRn product is only ~30% lower than that of conventional junction co-produced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.",1205.3372v1 2012-05-18,Controlling exchange coupling strength in NixCu100-x thin films,"Thickness (dF) and concentration (x) dependence of the Curie temperature of NixCu100-x(dF) ferromagnetic alloy layers (x =0.55,0.65, dF =[3nm{\div}12nm]) being in contact with a vanadium layer was studied. The Curie temperature of the ferromagnetic layers depends on the thickness when it is comparable with the interface layer between the F and the vanadium layers, which is attributed to the proximity coupling of the interface region with the rest of the F layer. The present study provides valuable information for fabrication of samples with controlled exchange coupling strength for studies of superconductor/ferromagnet (S/F) proximity effects.",1205.4149v1 2017-05-02,Magnetic Field-Free Giant Magnetoresistance in a Proximity- and Gate-Induced Graphene Spin Valve,"Due to its two dimensional nature, ferromagnetism and charge doping can be induced by proximity and electric field effects in graphene. Taking advantage of these features, we propose an electrically engineered spin valve by combining two magnetic insulators (using EuO, EuS, or YIG) and three coating gates. Two top gates are used to cancel the heavy electron doping's in these magnets and one back gate is used to utilize the normal or half-metallic ferromagnetisms. We demonstrate that, when the second top gate is tuned to utilize the insulating or spin insulating states, huge giant magnetoresistance (GMR) at high temperature (several times of $10^5\%$ at 68K and 100K) can be achieved for EuO and YIG. These results imply a distinguished GMR that is magnetism tunable, vertical configured (ferromagnetism versus insulating), and magnetic field-free. Our work may offer a viable path to a tantalizing magnetic field-free spintronics.",1705.00773v1 2017-05-02,Spin-dependent thermoelectric phenomena in a quantum dot attached to ferromagnetic and superconducting electrodes,"We investigate transport and thermoelectric properties of hybrid systems based on a single-level quantum dot and one superconducting lead. The other lead is generally normal-metallic ferromagnet. In the latter case single-particle transport is spin-polarized. Our main interest is in the interplay of Andreev tunneling of Cooper pairs and single-particle tunneling. The latter is responsible for relatively large thermopower and figure of merit due to a diverging density of single-particle states at the superconducting gap edges. System with ferromagnetic and superconducting leads can also reveal spin thermoelectric phenomena. Finite superconducting gap is considered within the BCS theory, and the thermoelectric coefficients are calculated by means of nonequilibrium Green's function technique within Hartree-Fock like approximation with respect to the intradot Coulomb interaction.",1705.01007v1 2017-05-13,"Partial Ferrimagnetism in S=1/2 Heisenberg Ladders with a Ferromagnetic Leg, an Antiferromagnetic Leg, and Antiferromagnetic Rungs","Ground-state and finite-temperature properties of $S=1/2$ Heisenberg ladders with a ferromagnetic leg, an antiferromagnetic leg, and antiferromagnetic rungs are studied. It is shown that a partial ferrimagnetic phase extends over a wide parameter range in the ground state. The numerical results are supported by an analytical calculation based on a mapping onto the nonlinear $\sigma$ model and a perturbation calculation from the strong-rung limit. It is shown that the partial ferrimagnetic state is a spontaneously magnetized Tomonaga--Luttinger liquid with incommensurate magnetic correlation, which is confirmed by a DMRG calculation. The finite-temperature magnetic susceptibility is calculated using the thermal pure quantum state method. It is suggested that the susceptibility diverges as $T^{-2}$ in the ferrimagnetic phases as in the case of ferromagnetic Heisenberg chains.",1705.04836v2 2017-05-19,The phonon softening due to melting of the ferromagnetic order in elemental iron,"We study the fundamental question of the lattice dynamics of a metallic ferromagnet in the regime where the static long range magnetic order is replaced by the fluctuating local moments embedded in a metallic host. We use the \textit{ab initio} Density Functional Theory(DFT)+embedded Dynamical Mean-Field Theory(eDMFT) functional approach to address the dynamic stability of iron polymorphs and the phonon softening with increased temperature. We show that the non-harmonic and inhomogeneous phonon softening measured in iron is a result of the melting of the long range ferromagnetic order, and is unrelated to the first order structural transition from the BCC to the FCC phase, as is usually assumed. We predict that the BCC structure is dynamically stable at all temperatures at normal pressure, and is only thermodynamically unstable between the BCC-$\alpha$ and the BCC-$\delta$ phase of iron.",1705.06877v2 2017-05-19,Controlling supercurrents and their spatial distribution in ferromagnets,"Spin-triplet Cooper pairs induced in ferromagnets form the centrepiece of the emerging field of superconducting spintronics [1,2]. Usually the focus of research is on the spin polarization of the triplets, potentially enabling low-dissipation magnetization switching and domain wall motion. However, the fundamental mechanism for generating triplet pairs [3,4] also permits control over a parameter which has not been addressed before, namely the spatial distribution of the supercurrent. Here we demonstrate this control by tailoring distinct supercurrent pathways in the ferromagnetic weak link of a Josephson junction. Combining micromagnetic simulations with three-dimensional critical current calculations, based on the Usadel description of mesoscopic superconductivity [5], we designed a disk-shaped structure with a magnetic vortex, which induces two distinct supercurrent channels across the junction. The design was successfully tested with superconducting quantum interferometry (SQI). Moreover, we show how the position of the pathways can be controlled by moving the vortex with a magnetic field. This novel approach allows adaptable supercurrent paths to be dynamically reconfigured to switch between different functionalities in the same device.",1705.07020v1 2017-05-19,Ferromagnetic Dirac-Nodal Semimetal Phase of Stretched Chromium Dioxide,"We show by first-principles calculations that the Dirac nodal-line semimetal phase can co-exist with the ferromagnetic order at room temperature in chromium dioxide, a widely used material in magnetic tape applications, under small tensile hydrostatic strains. An ideally flat Dirac nodal ring close to the Fermi energy is placed in the reflection-invariant boundary of the Brillouin zone perpendicular to the magnetic order, and is topologically protected by the unitary mirror symmetry of the magnetic group $D_{4h}(C_{4h})$, which quantizes the corresponding Berry phase into integer multiples of $\pi$. The symmetry-dependent topological stability is demonstrated through showing that only the topologically protected nodal ring can persistently exist under small anisotropic stains preservingthe symmetry $D_{4h}(C_{4h})$, while the other seeming band touching points are generically gapped. Our work provides a practical platform for the investigation of novel physics and potential applications of the Dirac nodal-line and drumhead fermions, in particular those related to ferromagnetic properties",1705.07076v1 2017-05-24,Effect of magnetic nanoparticles on the nematic-smectic-A phase transition,"Recent experiments on mixed liquid crystals have highlighted the hugely significant role of ferromagnetic nanoparticle impurities in defining the nematic-smectic-A phase transition point. Structured around a Flory-Huggins free energy of isotropic mixing and Landau-de Gennes free energy, this article presents a phenomenological mean-field model that quantifies the role of such impurities in analyzing thermodynamic phases, in a mixture of thermotropic smectic liquid crystal and ferromagnetic nanoparticles. First we discuss the impact of ferromagnetic nanoparticles on the isotropic-ferronematic and ferronematic-ferrosmectic phase transitions and their transition temperatures. This is followed by analysis of various topologies in the phase diagrams. Our model results indicate that there exists a critical concentration of nanoparticle impurities for which the second order N-SmA transition becomes first order at a tricritical point. Calculations based on this model show remarkable agreement with experiment.",1705.08702v1 2017-05-25,"High-temperature intrinsic ferromagnetism in the (In,Fe)Sb semiconductor","The (In,Fe)Sb layers with the Fe content up to 13 at. % have been grown on (001) GaAs substrates using the pulsed laser deposition. The TEM investigations show that the (In,Fe)Sb layers are epitaxial and free of the inclusions of a second phase. The observation of the hysteretic magnetoresistance curves at temperatures up to 300 K reveals that the Curie point is above room temperature. The resonant character of magnetic circular dichroism confirms the intrinsic ferromagnetism in the (In,Fe)Sb layers. We suggest that the ferromagnetism of the (In,Fe)Sb matrix is not carrier-mediated and apparently is determined by the mechanism of superexchange interaction between Fe atoms (This work was presented at the XXI Symposium Nanophysics and Nanoelectronics, Nizhny Novgorod, March, 13-16, 2017 (book of proceedings v.1, p. 195), http://nanosymp.ru/UserFiles/Symp/2017_v1.pdf).",1705.09318v3 2017-05-30,"Spin-transport, spin-torque and memory in antiferromagnetic devices: Part of a collection of reviews on antiferromagnetic spintronics","Ferromagnets are key materials for sensing and memory applications. In contrast, antiferromagnets that represent the more common form of magnetically ordered materials, have so far found less practical application beyond their use for establishing reference magnetic orientations via exchange bias. This might change in the future due to the recent progress in materials research and discoveries of antiferromagnetic spintronic phenomena suitable for device applications. Experimental demonstrations of the electrical switching and electrical detection of the N\'eel order open a route towards memory devices based on antiferromagnets. Apart from the radiation and magnetic-field hardness, memory cells fabricated in antiferromagnets are inherently multilevel which could be used for neuromorphic computing. Switching speeds attainable in antiferromagnets far exceed those of the ferromagnetic and semiconductor memory technologies. Here we review the recent progress in electronic spin-transport and spin-torque phenomena in antiferromagnets that are dominantly of the relativistic quantum mechanics origin. We discuss their utility in pure antiferromagnetic or hybrid ferromagnetic/antiferromagnetic memory devices",1705.10675v1 2017-09-07,Room temperature magnetization switching in topological insulator-ferromagnet heterostructures by spin-orbit torques,"Topological insulators (TIs) with spin momentum locked topological surface states (TSS) are expected to exhibit a giant spin-orbit torque (SOT) in the TI/ferromagnet systems. To date, the TI SOT driven magnetization switching is solely reported in a Cr doped TI at 1.9 K. Here, we directly show giant SOT driven magnetization switching in a Bi2Se3/NiFe heterostructure at room temperature captured using a magneto-optic Kerr effect microscope. We identify a large charge to spin conversion efficiency of ~1-1.75 in the thin TI films, where the TSS is dominant. In addition, we find the current density required for the magnetization switching is extremely low, ~6x10^5 A cm-2, which is one to two orders of magnitude smaller than that with heavy metals. Our demonstration of room temperature magnetization switching of a conventional 3d ferromagnet using Bi2Se3 may lead to potential innovations in TI based spintronic applications.",1709.02159v1 2017-09-08,Low temperature enhancement of ferromagnetic Kitaev correlations in α-RuCl3,"Kitaev-type interactions between neighbouring magnetic moments emerge in the honeycomb material ${\alpha}$-RuCl3. It is debated however whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a d4 triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperature the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for ${\alpha}$-RuCl3 with ferromagnetic Kitaev coupling.",1709.02712v1 2017-09-11,Controlling the superconducting transition by spin-orbit coupling,"Whereas there exists considerable evidence for the conversion of singlet Cooper pairs into triplet Cooper pairs in the presence of inhomogeneous magnetic fields, recent theoretical proposals have suggested an alternative way to exert control over triplet generation: intrinsic spin-orbit coupling in a homogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple Nb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit coupled ferromagnet owing to structural inversion asymmetry. Unconventional modulation of the superconducting critical temperature as a function of in-plane and out-of- plane applied magnetic fields suggests the presence of triplets that can be controlled by the magnetic orientation of a single homogeneous ferromagnet. Our studies demonstrate for the first time an active role of spin-orbit coupling in controlling the triplets -- an important step towards the realization of novel superconducting spintronic devices.",1709.03504v2 2017-09-14,Proximity Effect in Superconducting-Ferromagnetic Granular Structures,"We examined the proximity effect in granular films made of Pb, a superconductor, and Ni, a ferromagnet, with various compositions. Slow decay of the critical temperature as a function of the relative volume concentration of Ni per sample was demonstrated by our measurements, followed by a saturation of T$_c$. Using an approximate theoretical description of our granular system in terms of a layered one, we show that our data can only be reasonably fitted by a multilayer model. This indicates the importance of the interplay between different ferromagnetic grains; when non-collinearly magnetized they should lead to triplet Cooper pairing.",1709.04760v2 2017-09-28,Tailoring the exchange bias effect by in-plane magnetic anisotropy,"We report an unusual, non-monotonous dependence of the exchange bias on the thickness of the ferromagnetic layer in a ferromagnet/antiferromagnet bilayer system. We show that in epitaxial CoO/Fe(110) bilayers, the evolution of the Fe magnetic anisotropy, which drives the thickness-induced in-plane spin-reorientation process, controls the interfacial CoO spin directions in the 0-90 degrees range and, consequently, drastically modifies the magnitude of the hysteresis loop shift and its dependence on the thickness of the Fe layer. Our results present a new recipe for tailoring the exchange bias and antiferromagnetic spin structure by utilizing the spin-reorientation process that occurs in a ferromagnetic layer adjacent to an antiferromagnetic layer.",1709.09925v1 2017-11-02,Fulde-Ferrell state in ferromagnetic chiral superconductor with magnetic domain wall,"Motivated by the recent theoretical and experimental progress in the heavy fermion system UCoGe, we study ferromagnetic chiral superconductors in the presence of magnetic domains. Within mean field approximations, it is shown that chiral superconducting domains are naturally induced by the ferromagnetic domains. The domain wall current flows in the opposite direction to the naively expected one as in $^3$He-A phase due to contributions from ""unpaired electrons"". Consequently, the domain wall current flows in the same direction with that of surface currents when the magnetic domain wall lies parallel to the sample surface, and therefore they contribute to the net current along the whole sample. We find that, due to the non-cancellation between the domain wall current and surface current, a Fulde-Ferrell-like superconducting state can be stabilized in an anisotropic sample for all the temperatures below the superconducting transition temperature.",1711.00635v2 2017-11-08,Pressure Tuning of Collapse of Helimagnetic Structure in Au$_2$Mn,"We identify the phase boundary between spiral spin and ferromagnetic phases in Au$_2$Mn at a critical pressure of 16.4 kbar, as determined by neutron diffraction, magnetization and magnetoresistance measurements. The temperature-dependent critical field at a given pressure is accompanied by a peak in magnetoresistance and a step in magnetization. The critical field decreases with increasing temperature and pressure. The critical pressure separating the spiral phase and ferromagnetism coincides with the disappearance of the magnetroresistance peak, where the critical field goes to zero. The notable absence of an anomalous Hall effect in the the ferromagnetic phase is attributable to the high conductivity of this material.",1711.03203v1 2017-11-16,Anisotropic magnetic properties of the ferromagnetic semiconductor CrSbSe$_3$,"Single crystals of CrSbSe$_3$, a structurally pseudo-one-dimensional ferromagnetic semiconductor, were grown using a high-temperature solution growth technique and were characterized by x-ray diffraction, anisotropic, temperature- and field-dependent magnetization, temperature-dependent resistivity and optical absorption measurements. A band gap of 0.7 eV was determined from both resistivity and optical measurements. At high temperatures, CrSbSe$_3$ is paramagnetic and isotropic with a Curie-Weiss temperature of $\sim$145 K and an effective moment of $\sim$4.1 $\mu_B$/Cr. A ferromagnetic transition occurs at $T_c$ = 71 K. The $a$-axis, perpendicular to the chains in the structure, is the magnetic easy axis, while the chain axis direction, along $b$, is the hard axis. Magnetic isotherms measured around $T_c$ do not follow the behavior predicted by simple mean field critical exponents for a second order phase transition. A tentative set of critical exponents is estimated based on a modified Arrott plot analysis, giving $\beta\sim$0.25, $\gamma\sim$1.38 and $\delta\sim$6.6.",1711.06342v1 2017-11-17,Thickness dependence of unidirectional spin-Hall magnetoresistance in metallic bilayers,"A nonlinear magnetoresistance - called unidirectional spin-Hall magnetoresistance - is recently experimentally discovered in metallic bilayers consisting of a heavy metal and a ferromagnetic metal. To study the fundamental mechanism of the USMR, both ferromagnetic and heavy metallic layer thickness dependence of the USMR are presented in a Pt/Co/AlOx trilayer at room temperature. To avoid ambiguities, second harmonic Hall measurements are used for separating spin-Hall and thermal contributions to the non-linear magnetoresistance. The experimental results are fitted by using a drift-diffusion theory, with parameters extracted from an analysis of longitudinal resistivity of the Co layer within the framework of the Fuchs-Sondheimer model. A good agreement with the theory is found, demonstrating that the USMR is governed by both the spin-Hall effect in the heavy metallic layer and the metallic diffusion process in the ferromagnetic layer.",1711.06488v1 2017-11-21,Coupling magneto-elastic Lagrangians to spin transfer torque sources,"The consequences of coupling magnetic and elastic degrees of freedom, where spins and deformations are carried by point-like objects subject to local interactions, are studied, theoretically and by detailed numerical simulations. From the constrained Lagrangians we derive consistent equations of motion for the coupled dynamical variables. In order to probe the dynamics of such a system, we consider external perturbations, such as spin transfer torques for the magnetic part, and homogeneous stresses for the elastic part, associated to their corresponding damping. This approach is applied to the study of ultrafast switching processes in anti-ferromagnetic systems, which have recently attracted attention as candidates for anti-ferromagnetic spintronic devices. Our strategy is then checked in simple, but instructive, situations. We carried out numerical experiments to study, in particular, how the magnetostrictive coupling and external stresses affect the nature of the switching processes in a prototype anti-ferromagnetic material.",1711.08062v2 2017-11-26,On orbital angular momentum conservation in Brillouin light scattering within a ferromagnetic sphere,"Magnetostatic modes supported by a ferromagnetic sphere have been known as the Walker modes, each of which possesses an orbital angular momentum as well as a spin angular momentum along a static magnetic field. The Walker modes with non-zero orbital angular momenta exhibit topologically non-trivial spin textures, which we call \textit{magnetic quasi-vortices}. Photons in optical whispering gallery modes supported by a dielectric sphere possess orbital and spin angular momenta forming \textit{optical vortices}. Within a ferromagnetic, as well as dielectric, sphere, two forms of vortices interact in the process of Brillouin light scattering. We argue that in the scattering there is a selection rule that dictates the exchange of orbital angular momenta between the vortices. The selection rule is shown to be responsible for the experimentally observed nonreciprocal Brillouin light scattering.",1711.09321v1 2017-11-29,Spin-orbit torque in 3D topological insulator-ferromagnet heterostructure: crossover between bulk and surface transport,"Current-driven spin-orbit torques are investigated in a heterostructure composed of a ferromagnet deposited on top of a three dimensional topological insulator using the linear response formalism. We develop a tight-binding model of the heterostructure adopting a minimal interfacial hybridization scheme that promotes induced magnetic exchange on the topological surface states, as well as induced Rashba-like spin-orbit coupling in the ferromagnet. Therefore, our model accounts for spin Hall effect from bulk states together with inverse spin galvanic and magnetoelectric effects at the interface on equal footing. By varying the transport energy across the band structure, we uncover a crossover from surface-dominated to bulk-dominated transport regimes. We show that the spin density profile and the nature of the spin-orbit torques differ substantially in both regimes. Our results, which compare favorably with experimental observations, demonstrate that the large damping torque reported recently is more likely attributed to interfacial magnetoelectric effect, while spin Hall torque remains small even in the bulk-dominated regime.",1711.11016v2 2018-03-04,Ultrafast optically induced ferromagnetic/anti-ferromagnetic phase transition in GdTiO$_3$ from first principles,"Epitaxial strain and chemical substitution have been the workhorses of functional materials design. These static techniques have shown immense success in controlling properties in complex oxides through the tuning of subtle structural distortions. Recently, an approach based on the excitation of an infrared active phonon with intense mid-infrared light has created an opportunity for dynamical control of structure through special nonlinear coupling to Raman phonons. We use first-principles techniques to show that this approach can dynamically induce a magnetic phase transition from the ferromagnetic ground state to a hidden antiferromagnetic phase in the rare earth titanate GdTiO$_3$ for realistic experimental parameters. We show that a combination of a Jahn-Teller distortion, Gd displacement, and infrared phonon motion dominate this phase transition with little effect from the octahedral rotations, contrary to conventional wisdom.",1803.01323v1 2018-03-04,Three-dimensional magnetic critical behavior in CrI$_3$,"CrI$_3$ is a promising candidate for the van der Waals bonded ferromagnetic devices since its ferromagnetism can be maintained upon exfoliating of bulk crystals down to single layer. In this work we studied critical properties of bulk CrI$_3$ single crystals around the paramagnetic to ferromagnetic phase transition. Critical exponents $\beta$ = 0.260(4) with a critical temperature $T_c$ = 60.05(13) K and $\gamma$ = 1.136(6) with $T_c$ = 60.43(4) K are obtained by the Kouvel-Fisher method, whereas $\delta$ = 5.32(2) is obtained by a critical isotherm analysis at $T_c$ = 60 K. The critical exponents determined in bulk CrI$_3$ single crystals suggest a three-dimensional long-range magnetic coupling with the exchange distance decaying as $J(r)\approx r^{-4.69}$.",1803.01326v1 2018-03-05,Off-resonant all-optical switching dynamics in a ferromagnetic model system,"We present a theoretical study of the the effects of off-resonant polarized optical fields on a ferromagnetic model system. We determine the light-induced dynamics of itinerant carriers in a system that includes magnetism at the mean-field level and spin-orbit coupling. We investigate an all-optical switching process for ferromagnets, which is close to the one proposed by Qaiumzadeh et al. [Phys. Rev. B 88, 064416] for the inverse Faraday effect. By computing the optically driven coherent dynamics together with incoherent scattering mechanisms we go beyond a perturbation expansion in powers of the optical field. We find an important contribution of a dynamic Stark effect coupling of the Raman type between the magnetic bands, which leads to a polarization-dependent effect on the magnetization that may support or oppose switching, but also contributes to demagnetization via an increase in electronic energy.",1803.01924v1 2018-03-06,Supercurrent in ferromagnetic Josephson junctions with heavy metal interlayers,"The lengthscale over which supercurrent from conventional BCS, $s$-wave, superconductors ($S$) can penetrate an adjacent ferromagnetic ($F$) layer depends on the ability to convert singlet Cooper pairs into triplet Cooper pairs. Spin aligned triplet Cooper pairs are not dephased by the ferromagnetic exchange interaction, and can thus penetrate an $F$ layer over much longer distances than singlet Cooper pairs. These triplet Cooper pairs carry a dissipationless spin current and are the fundamental building block for the fledgling field of superspintronics. Singlet-triplet conversion by inhomogeneous magnetism is well established. Here, we describe an attempt to use spin orbit coupling as a new mechanism to mediate singlet-triplet conversion in $S-F-S$ Josephson junctions. We report that the addition of thin Pt spin-orbit coupling layers in our Josephson junctions significantly increases supercurrent transmission, however the decay length of the supercurrent is not found to increase. We attribute the increased supercurrent transmission to Pt acting as a buffer layer to improve the growth of the Co $F$ layer.",1803.01965v2 2018-03-09,Spin transport across antiferromagnets induced by the spin Seebeck effect,"For prospective spintronics devices based on the propagation of pure spin currents, antiferromagnets are an interesting class of materials that potentially entail a number of advantages as compared to ferromagnets. Here, we present a detailed theoretical study of magnonic spin current transport in ferromagnetic-antiferromagnetic multilayers by using atomistic spin dynamics simulations. The relevant length scales of magnonic spin transport in antiferromagnets are determined. We demonstrate the transfer of angular momentum from a ferromagnet into an antiferromagnet due to the excitation of only one magnon branch in the antiferromagnet. As an experimental system, we ascertain the transport across an antiferromagnet in YIG$|$Ir$_{20}$Mn$_{80}|$Pt heterostructures. We determine the spin transport signals for spin currents generated in the YIG by the spin Seebeck effect and compare to measurements of the spin Hall magnetoresistance in the heterostructure stack. By means of temperature-dependent and thickness-dependent measurements, we deduce conclusions on the spin transport mechanism across IrMn and furthermore correlate it to its paramagnetic-antiferromagnetic phase transition.",1803.03416v1 2018-03-12,Critical behavior of quasi-two-dimensional weak itinerant ferromagnet trigonal chromium telluride Cr$_{0.62}$Te,"The critical properties of flux-grown single-crystalline quasi-two-dimensional weak itinerant ferromagnet Cr$_{0.62}$Te were investigated by bulk dc magnetization around the paramagnetic (PM) to ferromagnetic (FM) phase transition. Critical exponents $\beta = 0.315(7)$ with a critical temperature $T_c = 230.6(3)$ K and $\gamma = 1.81(2)$ with $T_c = 229.1(1)$ K are obtained by the Kouvel-Fisher method whereas $\delta = 6.35(4)$ is obtained by a critical isotherm analysis at $T_c = 230$ K. With these obtained exponents, the magnetization-field-temperature curves collapse into two independent curves following a single scaling equation $M|\frac{T-T_c}{T_c}|^{-\beta} = f_\pm(H|\frac{T-T_c}{T_c}|^{-\beta\delta})$ around $T_c$, suggesting the reliability of the obtained exponents. Additionally, the determined exponents of Cr$_{0.62}$Te exhibit an Ising-like behavior with a change from short-range order to long-range order in the nature of magnetic interaction and with an extension from 2D to 3D on cooling through $T_c$.",1803.04482v1 2018-03-16,Motion of vortices in ferromagnetic spin-1 BEC,"The paper investigates dynamics of nonsingular vortices in a ferromagnetic spin-1 BEC, where spin and mass superfluidity coexist in the presence of uniaxial anisotropy (linear and quadratic Zeeman effect). The analysis is based on hydrodynamics following from the Gross-Pitaevskii theory. Cores of nonsingular vortices are skyrmions with charge, which is tuned by uniaxial anisotropy and can have any fractal value between 0 and 1. There are circulations of mass and spin currents around these vortices. The results are compared with the equation of vortex motion derived earlier in the Landau-Lifshitz-Gilbert theory for magnetic vortices in easy-plane ferromagnetic insulators. In the both cases the transverse gyrotropic force (analog of the Magnus force in superfluid and classical hydrodynamics) is proportional to the charge of skyrmions in vortex cores.",1803.06939v1 2018-03-24,Topological order generated by random field in a 2D exchange model,"We study a 2D exchange model with a weak static random field on lattices containing over one hundred million spins. Ferromagnetic correlations persist on the Imry-Ma scale inversely proportional to the random-field strength and decay exponentially at greater distances. We find that the average energy of the correlated area is close to the ground-state energy of a skyrmion, while the topological charge of the area is close to $\pm 1$. Correlation function of the topological charge density exhibits oscillations with a period determined by the ferromagnetic correlation length, while its Fourier transform exhibits a maximum. These findings suggest that static randomness transforms a 2D ferromagnetic state into a skyrmion-antiskyrmion glass.",1803.09117v1 2018-03-28,"Charge-density-wave phase, mottness and ferromagnetism in monolayer $1T$-NbSe$_2$","The recently investigated $1T$-polymorph of monolayer NbSe$_2$ revealed an insulating behaviour suggesting a star-of-David phase with $\sqrt{13}\,\times\sqrt{13}$ periodicity associated with a Mott insulator, reminiscent of $1T$-TaS$_2$. In this work, we examine this novel two-dimensional material from first principles. We find an instability towards the formation of an incommensurate charge-density-wave (CDW) and establish the star-of-David phase as the most stable commensurate CDW. The mottness in the star-of-David phase is confirmed and studied at various levels of theory: the spin-polarized generalized gradient approximation (GGA) and its extension involving the on-site Coulomb repulsion (GGA+$U$), as well as the dynamical mean-field theory (DMFT). Finally, we estimate Heisenberg exchange couplings in this material and find a weak nearest-neighbour ferromagnetic coupling, at odds with most Mott insulators. We point out the close resemblance between this star-of-David phase and flat-band ferromagnetism models.",1803.10727v1 2018-06-08,Effective low-energy description of the two impurity Anderson model: RKKY interaction and quantum criticality,"We show that the RKKY interaction in the two-impurity Anderson model comprise two contributions: a ferromagnetic part stemming from the symmetrized hybridization functions and an anti-ferromagnetic part. We demonstrate that this anti-ferromagnetic contribution can also be generated by an effective local tunneling term between the two impurities. This tunneling can be analytically calculated for particle-hole symmetric impurities. Replacing the full hybridization functions by the symmetric part and this tunneling term leads to the identical low-temperature fixed point spectrum in the numerical renormalization group. Compensating this tunneling term is used to restore the Varma-Jones quantum critical point between a strong coupling phase and a local singlet phase even in the absence of particle-hole symmetry in the hybridization functions. We analytically investigate the spatial frequencies of the effective tunneling term based on the combination of the band dispersion and the shape of the Fermi surface. Numerical renormalization group calculations provide a comparison of the distance dependent tunneling term and the local spin-spin correlation function. Derivations between the spatial dependency of the full spin-spin correlation function and the textbook RKKY interaction are reported.",1806.03130v1 2018-06-15,On the coexistence of dipolar frustration and criticality in ferromagnets,"In real magnets the tendency towards ferromagnetism, promoted by exchange coupling, is usually frustrated by dipolar interaction. As a result, the uniformly ordered phase is replaced by modulated (multi-domain) phases, characterized by different order parameters rather than the global magnetization. The transitions occurring within those modulated phases and towards the disordered phase are generally not of second-order type. Nevertheless, strong experimental evidence indicates that a standard critical behavior is recovered when comparatively small fields are applied that stabilize the uniform phase. The resulting power laws are observed with respect to a putative critical point that falls in the portion of the phase diagram occupied by modulated phases, in line with an avoided-criticality scenario. Here we propose a generalization of the scaling hypothesis for ferromagnets, which explains this observation assuming that the dipolar interaction acts as a relevant field, in the sense of renormalization group.",1806.06046v1 2018-06-18,Current-driven domain wall motion along ferromagnetic strips with periodically-modulated perpendicular anisotropy,"The dynamics of magnetic domain walls along ferromagnetic strips with spatially modulated perpendicular magnetic anisotropy is theoretically studied by means of micromagnetic simulations. Ferromagnetic layers with a periodic sawtooth profile of the anisotropy depict a well-defined set of energy minima where the walls are pinned in the absence of external stimuli, and favor the unidirectional propagation of domain walls. The performance of the current-driven domain wall motion along these ratchet-like systems is compared to the field-driven case. Our study indicates that the current-driven domain wall motion exhibits significant improvements with respect to the field-driven case in terms of bit shifting speed and storage density, and therefore, it is suggested for the development of novel devices. The feasibility of these current-driven ratchet devices is studied by means of realistic micromagnetic simulations and supported by a one-dimensional model updated to take into account the periodic sawthooth anisotropy profile. Finally, the current-driven domain wall motion is also evaluated in systems with a triangular modulation of the anisotropy designed to promote the bidirectional shifting of series of walls, a functionality that cannot be achieved by magnetic fields.",1806.06585v1 2018-06-25,Ferromagnetism above 1000 K in highly cation-ordered double-perovskite insulator Sr3OsO6,"Magnetic insulators have been intensively studied for over 100 years, and they, in particular ferrites, are considered to be the cradle of magnetic exchange interactions in solids. Their wide range of applications include microwave devices and permanent magnets . They are also suitable for spintronic devices owing to their high resistivity, low magnetic damping, and spin-dependent tunneling probabilities. The Curie temperature is the crucial factor determining the temperature range in which any ferri/ferromagnetic system remains stable. However, the record Curie temperature has stood for over eight decades in insulators and oxides (943 K for spinel ferrite LiFe5O8). Here we show that a highly B-site ordered double-perovskite, Sr2(SrOs)O6 (Sr3OsO6), surpasses this long standing Curie temperature record by more than 100 K. We revealed this B-site ordering by atomic-resolution scanning transmission electron microscopy. The density functional theory (DFT) calculations suggest that the large spin-orbit coupling (SOC) of Os6+ 5d2 orbitals drives the system toward a Jeff = 3/2 ferromagnetic (FM) insulating state. Moreover, the Sr3OsO6 is the first epitaxially grown osmate, which means it is highly compatible with device fabrication processes and thus promising for spintronic applications.",1806.09308v1 2018-07-01,Interplay between interlayer exchange and stacking in CrI$_3$ bilayers,"We address the interplay between stacking and interlayer exchange for ferromagnetically ordered CrI$_3$, both for bilayers and bulk. Whereas bulk CrI$_3$ is ferromagnetic, both magneto-optical and transport experiments show that interlayer exchange for CrI$_3$ bilayers is antiferromagnetic. Bulk CrI$_3$ is known to assume two crystal structures, rhombohedral and monoclinic, that differ mostly in the stacking between monolayers. Below 210-220 Kelvin, bulk CrI$_3$ orders in a rhombohedral phase. Our density functional theory calculations show a very strong dependence of interlayer exchange and stacking. Specifically, the ground states of both bulk and free-standing CrI$_3$ bilayers are ferromagnetic for the rhombohedral phase. In contrast, the energy difference between both configurations is more than one order of magnitude smaller for the monoclinic phase, and eventually becomes antiferromagnetic when either positive strain or on-site Hubbard interactions ($U \geq 3$) are considered. We also explore the interplay between interlayer hybrydization and stacking, using a Wannier basis, and between interlayer hybrydization and relative magnetic alignment for CrI$_3$ bilayers, that helps to account for the very large tunnel magnetoresistance obvserved in recent experiments.",1807.00357v2 2018-07-05,"Rogue breather modes: Topological sectors, and the `belt-trick', in a one-dimensional ferromagnetic spin chain","We present explicit solutions for breather soliton modes of excitation in the one-dimensional Heisenberg ferromagnetic spin chain. We identify a characteristic geometrical feature of these breather modes wherein a helicoidal configuration of spins is continuously transformed to one which differs from the initial helicoid by a total twist of `2'. This is a curious manoeuvre popularly known as the `belt trick', an illustration of the simple connectedness of the $\rm{SU}(2)$ group manifold, and its rotation period $4\pi$. We show that this effectively splits the configuration space of the ferromagnetic chain in one-dimension into two topological sectors, distinguished by their total twist -- either `0', or `1'. Further, the energy lower bound of the two sectors is separated by a finite gap varying inversely with the size of the lattice.",1807.01867v2 2018-07-06,Controlled ordering of topological charges in an exciton-polariton chain,"We demonstrate, experimentally and theoretically, controlled loading of an exciton-polariton vortex chain into a 1D array of trapping potentials. Switching between two types of vortex chains, with topological charges of the same or alternating sign, is realised by means of appropriate shaping of an incoherent pump beam that drives the system to the regime of bosonic condensation. In analogy to spin chains, these vortex sequences realise either a ""ferromagnetic"" or an ""anti-ferromagnetic"" order, whereby the role of spin is played by the orbital angular momentum. The ""ferromagnetic"" ordering of vortices is associated with the formation of a persistent chiral current. Our results pave the way for controlled creation of nontrivial distributions of orbital angular momentum and topological order in a periodic exciton-polariton system.",1807.02343v1 2018-07-11,Unique interplay between superconducting and ferromagnetic orders in EuRbFe$_4$As$_4$,"Transport, magnetic and optical investigations on EuRbFe$_4$As$_4$ single crystals evidence that the ferromagnetic ordering of the Eu$^{2+}$ magnetic moments at $T_N=15$ K, below the superconducting transition ($T_c=36$ K), affects superconductivity in a weak but intriguing way. Upon cooling below $T_N$, the zero resistance state is preserved and the superconductivity is affected by the in-plane ferromagnetism mainly at domain boundaries; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below $T_c$, associated with the opening of a complete superconducting gap at $2\Delta = 10$ meV. A gap smaller than the weak coupling limit suggests the strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap.",1807.04135v1 2018-07-16,Control of Ultracold Atoms with a Chiral Ferromagnetic Film,"We show that the magnetic field produced by a chiral ferromagnetic film can be applied to control ultracold atoms. The film will act as a magnetic mirror or a reflection grating for ultracold atoms when it is in the helical phase or the skyrmion crystal phase respectively. By applying a bias magnetic field and a time-dependent magnetic field, one-dimensional or two-dimensional magnetic lattices including honeycomb, Kagome, triangular types can be created to trap the ultracold atoms. We have also discussed the trapping height, potential barrier, trapping frequency, and Majorana loss rate for each lattice. Our results suggest that the chiral ferromagnetic film can be a platform to develop artificial quantum systems with ultracold atoms based on modern spintronics technologies.",1807.05647v2 2018-07-17,Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy,"In this work, we report the structural, magnetic and electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of temperature dependence of Seebeck coefficient indicate a semi-metallic ground state with a pseudo-gap that is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2*10-3) in Ru2NbAl, it is the largest among all the reported non-doped full Heusler alloys.",1807.06608v1 2018-07-23,Cross-correlations in a quantum dot Cooper pair splitter with ferromagnetic leads,"We investigate Andreev transport through a quantum dot attached to two external ferromagnetic leads and one superconducting electrode. The transport properties of the system are studied by means of the real-time diagrammatic technique in the sequential tunneling regime. To distinguish various contributions to Andreev current we calculate the current cross-correlations, i.e. correlations between currents flowing through two junctions with normal leads. We analyze dependence of current cross-correlations on various parameters of the considered model, both in linear and nonlinear transport regimes. The processes and mechanisms leading to enhancement, suppression or sign change of current cross-correlations are examined and discussed. Interestingly, our results show that for specific transport regimes splitted Cooper pair results in two uncorrelated electrons. However, utilizing ferromagnetic leads instead of non-magnetic electrodes can result in positive current cross-correlations.",1807.08850v1 2018-07-24,Impact of magnetic moment and anisotropy of Co$_\textrm{1-x}$Fe$_\textrm{x}$ thin films on the magnetic proximity effect of Pt,"We present a systematic study of the magnetic proximity effect in Pt, depending on the magnetic moment and anisotropy of adjacent metallic ferromagnets. Element-selective x-ray resonant magnetic reflectivity measurements at the Pt absorption edge (11565$\,$eV) are carried out to investigate the spin polarization of Pt in Pt/Co$_\textrm{1-x}$Fe$_\textrm{x}$ bilayers. We observe the largest magnetic moment of (0.72$\,\pm\,$0.03)$\, \mu_\textrm{B}$ per spin polarized Pt atom in Pt/Co$_\textrm{33}$Fe$_\textrm{67}$, following the Slater-Pauling curve of magnetic moments in Co-Fe alloys. In general, a clear linear dependence is observed between the Pt moment and the moment of the adjacent ferromagnet. Further, we study the magnetic anisotropy of the magnetized Pt which clearly adopts the magnetic anisotropy of the ferromagnet below. This is depicted for Pt on Fe(001) and on Co$_\textrm{50}$Fe$_\textrm{50}$(001), which have a 45$^{\circ}$ relative rotation of the fourfold magnetocrystalline anisotropy.",1807.09032v2 2018-07-24,Giant Electrostatic Modification of Magnetism via Electrolyte-Gate-Induced Cluster Percolation in La$_{1-x}$Sr$_x$CoO$_{3-δ}$,"Electrical control of magnetism is a long-standing goal in physics and technology, recently developed electrolyte gating techniques providing a promising route to realization. Validating a recent theoretical prediction, here we demonstrate large enhancement of electrostatic modulation of ferromagnetic order in ion-gel-gated ultrathin La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$ by thickness-tuning to the brink of a magnetic percolation transition. Application of only 3-4 V then drives a transition from a short-range-ordered insulator to a robust long-range ferromagnetic metal, realizing giant electrostatic Curie temperature modulation over a 150 K window. In operando polarized neutron reflectometry confirms gate-controlled ferromagnetism, also demonstrating unusually deep penetration of induced magnetization, in further agreement with theory.",1807.09364v1 2018-07-25,Left-handed polarized spin waves in ferromagnets induced by spin-transfer torque,"Polarization is a fundamental property of waves that refers to the orientation of the oscillations. It has been widely used to encode information in photonics and phononics. However, the polarization of spin waves is rarely used yet in magnonics. The reason for this is that only the right-handed polarized spin waves can be accommodated in ferromagnets. Here, we report that stable left-handed polarized spin waves can be introduced into ferromagnets if a spin-polarized electrical current is presented. The right-handed and left-handed polarized spin waves coexist when the current density is larger than a critical value while the system keeps stable. The results are confirmed by micromagnetic simulations. This work provides new playgrounds to study spin waves and points to new findings for future experimental studies.",1807.09456v2 2018-07-29,Readable Racetrack Memory via Ferromagnetically Coupled Chiral Domain Walls,"Current-induced motion of domain walls (CIMDW) with interfacial Dzyaloshinskii-Moriya interaction (DMI) in heavy metal (HM)/ferromagnetic (FM) metal multilayers have attracted attention owing to their potential application in novel magnetic memories. In recent years, the CIMDW at ultrahigh speed has been observed in a synthetic antiferromagnetic (SAF) multilayer. However, due to the zero net magnetization, the reading of information from the SAF multilayer is still challenging. In this work, we propose a readable racetrack memory consisting of a synthetic ferromagnetic multilayer composed of two FM layers with an interlayer FM coupling. One FM layer had an isotropic DMI, while the other had an anisotropic DMI. This difference of DMIs resulted in the opposite tilting directions of the DW planes in the two layers. This tilting was inhibited by a strong interlayer FM coupling, resulting in an increase in the DW velocity and the reduction of the minimum allowed spacing between two adjacent DWs. In addition, the FM coupling enhanced the stray field, and the stored information could be read conveniently using a conventional reading head. Therefore, our proposal paves a way for the fabrication of a racetrack memory with high reading speed, large storage density, and good readability.",1807.11063v1 2018-08-01,Giant tunable nonreciprocity of light in Weyl semimetals,"The propagation of light in Weyl semimetal films is analyzed. The magnetic family of these materials is known by anomalous Hall effect, which, being enhanced by the large Berry curvature, allows one to create strong gyrotropic and nonreciprocity effects without external magnetic field. The existence of nonreciprocal waveguide electromagnetic modes in ferromagnetic Weyl semimetal films in the Voigt configuration is predicted. Thanks to the strong dielectric response caused by the gapless Weyl spectrum and the large Berry curvature, ferromagnetic Weyl semimetals combine the best waveguide properties of magnetic dielectrics or semiconductors with strong anomalous Hall effect in ferromagnets. The magnitude of the nonreciprocity depends both on the internal Weyl semimetal properties, the separation of Weyl nodes, and the external factor, the optical contrast between the media surrounding the film. By tuning the Fermi level in Weyl semimetals, one can vary the operation frequencies of the waveguide modes in THz and mid-IR ranges. Our findings pave the way to the design of compact, tunable, and effective nonreciprocal optical elements.",1808.00342v3 2018-08-03,Ferromagnetic resonance in thin films - cross-validation analysis of numerical solutions of Smit-Beljers equation. Application to GaMnAs,"(Dated: August 3, 2018) The new method of numerical analysis of experimental ferromagnetic resonance (FMR) spectra in thin films is developed and applied to (Ga,Mn)As thin films. Specifically, it starts with the finding of numerical solutions of Smit-Beljers (SB) equation and continues with their subsequent statistical analysis within the cross-validation (CV) approach taken from machine learning techniques. As a result of this treatment, we are able to reinterpret the available FMR experimental results in diluted ferromagnetic semiconductor (Ga,Mn)As thin films with the resulting determination of magnetocrystalline anisotropy constants. The outcome of CV analysis points out that it is necessary to take into account terms describing the bulk cubic anisotropy up to the fourth order to reproduce FMR experimental results for (Ga,Mn)As correctly. This finding contradicts the wide-spread conviction in the literature that only first order cubic anisotropy term is important in this material. We also provide numerical values of these higher order cubic anisotropy constants for (Ga,Mn)As thin films resulting from SB-CV approach.",1808.01347v1 2018-08-07,Temperature controlled FFLO instability in superconductor-ferromagnet hybrids,"We show that a wide class of layered superconductor-ferromagnet (S/F) hybrids demonstrate the emergence of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase well below the superconducting transition temperature. Decreasing the temperature one can switch the system from uniform to the FFLO state which is accompanied by the damping of the diamagnetic Meissner response down to zero and also by the sign change in the curvature of the current-velocity dependence. Our estimates show that an additional layer of the normal metal (N) covering the ferromagnet substantially soften the conditions required for the predicted FFLO instability and for existing S/F/N systems the temperature of the transition into the FFLO phase can reach several kelvins.",1808.02440v1 2018-08-10,Ferromagnetic Josephson junctions for cryogenic memory,"Josephson junctions containing ferromagnetic materials have attracted intense interest both because of their unusual physical properties and because they have potential application for cryogenic memory. There are two ways to store information in such a junction: either in the amplitude of the critical current or in the ground-state phase difference across the junction; the latter is the topic of this paper. We have recently demonstrated two different ways to achieve phase control in such junctions: the first uses junctions containing two magnetic layers in a ""pseudo spin valve"" configuration, while the second uses junctions containing three magnetic layers with non-collinear magnetizations. The demonstration devices, however, have not yet been optimized for use in a large-scale cryogenic memory array. In this paper we outline some of the issues that must be considered to perform such an optimization, and we provide a speculative ""phase-diagram"" for the nickel-permalloy spin-valve system showing which combinations of ferromagnetic layer thicknesses should produce useful devices.",1808.03639v1 2018-08-11,Role of Ce 4f hybridization in the origin of magnetism in nanoceria,"Nanoscale CeO2 (nanoceria) is a prototypical system that presents d0 ferromagnetism. Using a combination of x-ray absorption spectroscopy, x-ray magnetic circular dichroism and modelling, we show that nanostructure, defects and disorder, and non-stoichiometry create magnetically polarized Ce 4f and O 2p hybridized states captured by the vacancy orbitals (Vorb) that are vital to ferromagnetism. Further, we demonstrate that foreign ions (Fe and Co) enhance the moment at Ce 4f sites while the number of Vorb is unchanged, pointing clearly to the mechanism of orbital hybridization being key missing ingredient to understanding the unexpected ferromagnetism in many nanoscale dilute magnetic oxides and semiconductors.",1808.03758v3 2018-08-17,Director switching dynamics of ferromagnetic nematic liquid crystals,"Successful realization of ferromagnetic nematic liquid crystals has opened up the possibility to experimentally study a completely new set of fundamental physical phenomena. In this contribution we present a detailed investigation of some aspects of the static response and the complex dynamics of ferromagnetic liquid crystals under the application of an external magnetic field. Experimental results are then compared with a macroscopic model. Dynamics of the director were measured by optical methods and analyzed in terms of a theoretical macroscopic model. A dissipative cross-coupling coefficient describing the dynamic coupling between the two system order parameters, the magnetization and the nematic director, is needed to explain the results. In this contribution we examine the dependency of this coefficient on material parameters and the saturation magnetization and the liquid crystal host. Despite the complexity of the system, the theoretical description allows for a proper interpretation of the results and is connected to several microscopic aspects of the colloidal suspension.",1808.05843v1 2018-08-27,Magnetostriction in Antiferromagnets,"We consider magnetostriction in magnetic materials. Starting from a microscopic lattice model of the magneto-elastic coupling, we derive the continuum magnetostriction Hamiltonian in cubic ferromagnets and antiferromagnets. In ferromagnets, our results agree with established results. In antiferromagnets, we find a new central contribution which reflects the spin ordering. The additional term is essential in certain antiferromagnets where magnetic moments order in antiparallel ferromagnetic planes characterized by a normal axis. In such antiferromagnets, we predict that the magnetostrictive strain depends on the direction of the normal axis. Our findings are relevant for materials such as NiO and CoO, where there is a new spin-structure dependent strain, but not in RbMnF$_3$. We propose an experimental procedure to verify the importance of our findings in NiO that can be readily generalized to other materials. We also generalize the theory to include long-wavelength magnetoelastic coupling. We expect that the spin-structure dependent term critically affects the magnon-phonon coupling.",1808.09013v2 2018-08-28,An Intrinsic Spin Orbit Torque Nano-Oscillator,"Spin torque and spin Hall effect nanooscillators generate high intensity spin wave auto oscillations on the nanoscale enabling novel microwave applications in spintronics, magnonics, and neuromorphic computing. For their operation, these devices require externally generated spin currents either from an additional ferromagnetic layer or a material with a high spin Hall angle. Here we demonstrate highly coherent field and current tunable microwave signals from nanoconstrictions in single 15 and 20 nm thick permalloy layers. Using a combination of spin torque ferromagnetic resonance measurements, scanning microBrillouin light scattering microscopy, and micromagnetic simulations, we identify the autooscillations as emanating from a localized edge mode of the nanoconstriction driven by spin orbit torques. Our results pave the way for greatly simplified designs of auto oscillating nanomagnetic systems only requiring a single ferromagnetic layer.",1808.09330v1 2018-08-28,Negative-pressure-induced helimagnetism in ferromagnetic cubic perovskites Sr$_{1-x}$Ba$_{x}$CoO$_{3}$,"Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries, as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite SrCoO$_3$ with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content $x$ in Sr$_{1-x}$Ba$_x$CoO$_3$ continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near $x$=0.4, where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron diffraction measurements. The emergence of helimagnetism is semi-quantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong $d$-$p$ hybridisation can exhibit a variety of novel magnetic phases originating from competing exchange interactions.",1808.09423v1 2018-08-30,Ground-state phase diagram of the $S = 1$ one-dimensional Kondo lattice model with a uniaxial anisotropy under transverse fields,"We study the effects of transverse magnetic fields on the $S=1$ one-dimensional Kondo lattice model with a uniaxial anisotropy using the density matrix renormalization group. The model can be regarded as a simplified one for analyzing the Ising ferromagnetic superconductor URhGe. We find various phases such as ferromagnetic and antiferromagnetic phases, Kondo plateau (KP) phases, Tomonaga-Luttinger liquids, and fully polarized phases. In the KP phase, a pseudoplateau emerges in the magnetization due to strongly bound pairs between the local and the conduction electron spins. This is why we call this phase the Kondo plateau. At the critical field between the ferromagnetic and KP phases, we find metamagnetic behavior in the magnetization curve. We discuss various correlation functions and the Friedel oscillations in detail, and the experimental data under transverse magnetic fields in URhGe are discussed on the basis of the present results.",1808.10142v3 2018-12-04,Klein tunnelling and Hartman effect in graphene junctions with proximity exchange field,"Tunnelling of electrons in graphene-based junctions is studied theoretically. Graphene is assumed to be deposited either directly on a ferromagnetic insulator or on a few atomic layers of boron nitride which separate graphene from a metallic ferromagnetic substrate. Such junctions can be formed by appropriate external gating of the corresponding system. To describe low-energy electronic states near the Dirac points, certain effective Hamiltonians available in the relevant literature are used. These Hamiltonians include staggered potential and exchange interaction due to ferromagnetic substrates. Tunnelling in the systems under consideration is then spin-dependent. The main focus is on Klein tunnelling and also on the group delay and the associated Hartman effect. The impact of a gap induced in the spectrum at the Dirac points on tunnelling is analysed in detail.",1812.01421v1 2018-12-06,Multistep Bloch-line-mediated Walker breakdown in ferromagnetic strips,"A well-known feature of magnetic field driven dynamics of domain walls in ferromagnets is the existence of a threshold driving force at which the internal magnetization of the domain wall starts to precess -- a phenomenon known as the Walker breakdown -- resulting in an abrupt drop of the domain wall propagation velocity. Here, we report on micromagnetic simulations of magnetic field driven domain wall dynamics in thin ferromagnetic strips with perpendicular magnetic anisotropy which demonstrate that in wide enough strips Walker breakdown is a multistep process: It consists of several distinct velocity drops separated by short linear parts of the velocity vs field curve. These features originate from the repeated nucleation, propagation and annihilation of an increasing number of Bloch lines within the domain wall as the driving field magnitude is increased. This mechanism arises due to magnetostatic effects breaking the symmetry between the two ends of the domain wall.",1812.02545v2 2018-12-10,Optical second harmonic generation from interfaces between heavy and ferromagnetic metals,"Surfaces and interfaces of magnetic nanostructures can reveal rather interesting and unusual properties that differ substantially from those of bulky materials. Here we apply the surface-sensitive method of optical second harmonic generation (SHG) for the studies of magnetization induced effects that appear in the nonlinear reflection from interfaces between ferromagnetic (Co) and heavy metals (Pt, Ta, W, Au, Ag, Cu). We demonstrate the appearance of magnetization induced variation in the p-polarized SHG intensity in the geometry of the longitudinal magneto-optical Kerr effect that is forbidden for homogeneous magnetic structures. This confirms the existence of chiral magnetic states at heavy metal/ferromagnet interfaces that appear due to the surface-induced Dzyaloshinskii-Moriya interaction. The related nonlinear chiroptical effect in the SHG intensity is proportional to the dc flexo-electric polarization that is shown to exist for chiral magnetic states at the considered interfaces.",1812.03922v2 2018-12-13,Demonstration of defect-defect ferromagnetic coupling in Gd doped GaN epitaxial films: A polarization selective magneto-photoluminescence study,"Magnetic field dependent polarization selective photoluminescence(PL) study has been carried out at 1.5~K on Gd-doped GaN epitaxial layers grown on c-SiC substrates by molecular beam epitaxy technique. It has been found that the incorporation of Gd in GaN leads to the generation of three types of donor like defects that result in neutral donor bound excitonic features in low temperature PL. The study reveals that the rate of spin-flip scattering for all the three excitonic features becomes almost $B$-independent suggesting that these signals must be stemming from defects which are ferromagnetically coupled with each other. This is further confirmed by the study carried out on a GaN sample co-doped with Si and Gd, where defects are found to be ferromagnetically coupled, while Si-donors do not show any involvement in coupling.",1812.05350v1 2018-12-17,The Doping effect of Chalcogen on the Two-Dimensional Ferromagnetic Material Chromium Tribromide,"Recently the discovery of magnetic order in two-dimensional monolayer chromium trihalides opens the new research field in two-dimensional materials. We use first-principles calculations to systematically examine the doping effect of chalcogen on CrBr3. In the case of S-doping, four stable configurations, Cr2Br5S, Cr2Br4S2-A, Cr2Br4S2-B and Cr2Br3S3-A, are predicted to be ferromagnetic semiconductors. It is found that the new bands appearing in the original bandgap are made up of S-p and Cr-d-egorbits, lead to the obvious reduce of bandgap and the enhanced optical absorption in the visible range. Due to the decrease of valence electron after chalcogen doping, the magnetic moment also decreases with the increase of S atoms, and the character of ferromagnetic semiconductor is always hold in a wide range of strain. The results shown that monolayer CrBr3with chalcogen doping supply a effectual way to control the magnetism and extend the optoelectronic applications.",1812.06567v1 2018-12-18,Twists in Ferromagnetic Monolayers With Trigonal Prismatic Symmetry,"Two-dimensional materials such as graphene or hexagonal boron nitride are indispensable in industry. The recently discovered 2D ferromagnetic materials also promise to be vital for applications. In this work, we develop a phenomenological description of non-centrosymmetric 2D ferromagnets with trigonal prismatic crystal structure. We chose to study this special symmetry group since these materials do break inversion symmetry and therefore, in principle, allow for chiral spin structures such as magnetic helices and skyrmions. However, unlike all non-centrosymmetric magnets known so far, we show that the symmetry of magnetic trigonal prismatic monolayers neither allow for an internal relativistic Dzyaloshinskii-Moriya interaction (DMI) nor a reactive spin-orbit torque. We demonstrate that the DMI only becomes important at the boundaries, where it modifies the boundary conditions of the magnetization and leads to a helical equilibrium state with a helical wavevector that is inherently linked to the internal spin orientation. Furthermore, we find that the helical wavevector can be electrically manipulated via dissipative spin-torque mechanisms. Our results reveal that 2D magnets offer a large potential for unexplored magnetic effects.",1812.07361v1 2018-12-19,Origins of thermal spin depolarization in half-metallic ferromagnet CrO$_2$,"Using high-resolution spin-resolved photoemission spectroscopy, we observed a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore we observed a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristics of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.",1812.07792v1 2018-12-27,Ferromagnetism in nitrogen doped graphene,"Inducing a robust long-range magnetic order in diamagnetic graphene remains a challenge. While nitrogen-doped graphene is reported to be a promising candidate, the corresponding exchange mechanism endures unclear and is essential to tune further and manipulate magnetism. Within the first-principles calculations, we systematically investigate the local moment formation and the concurrent interaction between various defect complexes. The importance of adatom diffusion on the differential defect abundance is discussed. The individual nitrogen complexes that contribute toward itinerant and a local magnetic moment are identified. The magnetic interaction between the complexes is found to depend on the concentration, complex type, sublattice, distance, and orientation. We propose that the direct exchange mechanism between the delocalized magnetic moment originating from the itinerant $\pi$-electron at the prevalent graphitic complexes to be responsible for the observed ferromagnetism. We show that B co-doping further improves ferromagnetism. Present results will assist in the microscopic understanding of the current experimental results and motivate experiments to produce robust magnetism following the proposed synthesis strategy.",1812.10643v1 2019-01-03,Effect of hydrostatic pressure on ferromagnetism in two-dimensional CrI$_3$,"We have investigated the magnetic properties of highly anisotropic layered ferromagnetic semiconductor CrI$_3$ in presence of hydrostatic pressure ($P$). At ambient pressure, magnetization exhibits a clear anomaly below 212 K along with a thermal hysteresis over a wide temperature range (212-180 K), where a first-order structural transition is observed. CrI$_3$ undergoes a second-order ferromagnetic-paramagnetic phase transition with Curie temperature $T_C$=60.4 K. With application of pressure, the transition becomes sharper and $T_C$ is found to increase from 60.4 to 64.9 K as $P$ increases from 0 to 1.0 GPa. $T_C$ increases with $P$ in a sublinear fashion. The thermal hysteresis in magnetization and the increase of $T_C$ with pressure suggest that the spin and lattice degrees of freedom are coupled. The observed increase in $T_C$ has been explained on the basis of change in inter-layer coupling and Cr-I-Cr bond angle with pressure.",1901.00706v2 2019-01-04,How to accurately determine a saturation magnetization of the sample in a ferromagnetic resonance experiment?,"The phenomenon of ferromagnetic resonance (FMR) is still being exploited for determining the magnetocrystalline anisotropy constants of magnetic materials. We show that one can also determine accurately the saturation magnetization of the sample using results of FMR experiments after taking into account the relationship between resonance frequency and curvature of the spatial distribution of free energy at resonance. Specifically, three examples are given of calculating saturation magnetization from FMR data: we use historical Bickford's measurements from 1950 for bulk magnetite, Liu's measurements from 2007 for a 500 mn thin film of a weak ferromagnet (Ga, Mn)As, and Wang's measurements from 2014 for an ultrathin film of YIG. In all three cases, the magnetization values we have determined are consistent with the results of measurements.",1901.01207v1 2019-01-08,Current-driven Dynamics of Magnetic Skyrmion in Chiral Ferromagnetic Film with Spatially Modulated Dzyaloshinskii-Moriya Interaction,"The dynamics of magnetic skyrmion driven by spin-polarized current is theoretically studied in the chiral ferromagnetic film with spatially modulated Dzyaloshinskii-Moriya interaction. Three cases including linear, sinusoidal, and periodic rectangular modulations have been considered, where the increase, decrease, and the periodic modification of the size and velocity of the skyrmion have been observed in the microscopic simulations. These phenomena are well explained by the Thiele equation, where an effective force on the skyrmion is induced by the inhomogeneous Dzyaloshinskii-Moriya interaction. The results here suggest that the dynamics of skyrmion can be manipulated by artificially tuning the Dzyaloshinskii-Moriya interaction in chiral ferromagnetic film with material engineering methods, which will be useful to design skyrmion-based spintronics devices.",1901.02120v1 2019-01-11,Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene,"When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interactions. Here we present evidence that near three-quarters ($3/4$) filling of the conduction miniband these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. We observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as $10.4\ \mathrm{k\Omega}$ and signs of chiral edge states in a narrow density range around an apparent insulating state at $3/4$. Surprisingly, the magnetization of the sample can be reversed by applying a small DC current. Although the AH resistance is not quantized and dissipation is significant, we suggest that the system is an incipient Chern insulator.",1901.03520v1 2019-01-18,Magnon-Fluxon interaction in a ferromagnet/superconductor heterostructure,"Ferromagnetism and superconductivity are most fundamental phenomena in condensed matter physics. Entailing opposite spin orders, they share an important conceptual similarity: Disturbances in magnetic ordering in magnetic materials can propagate in the form of spin waves (magnons) while magnetic fields penetrate superconductors as a lattice of magnetic flux quanta (fluxons). Despite a rich choice of wave and quantum phenomena predicted, magnon-fluxon coupling has not been observed experimentally so far. Here, we clearly evidence the interaction of spin waves with a flux lattice in ferromagnet/superconductor Py/Nb bilayers. We demonstrate that, in this system, the magnon frequency spectrum exhibits a Bloch-like band structure which can be tuned by the biasing magnetic field. Furthermore, we observe Doppler shifts in the frequency spectra of spin waves scattered on a flux lattice moving under the action of a transport current in the superconductor.",1901.06156v1 2019-01-21,Ferromagnetic Kondo lattice behavior in Ce11Pd4In9,"We report on the low-temperature physical properties of a novel compound Ce$_{11}$Pd$_4$In$_9$ that crystallizes with the orthorhombic Nd$_{11}$Pd$_4$In$_9$-type crystal structure (space group $Cmmm$). The compound exhibits ferromagnetic ordering at $T_{\rm {C}}$ = 18.6 K and an order-order transition at $T_{\rm {t}}$ $\sim$~1.6 K, as inferred from the low-temperature magnetic susceptibility, heat capacity and electrical resistivity data. In the paramagnetic region, the electrical transport in Ce$_{11}$Pd$_4$In$_9$ is dominated by Kondo effect. Below $T_{\rm {C}}$, a distinct contribution due to ferromagnetic spin waves dominates the electrical resistivity data, while at the lowest temperatures, the electrical transport and thermodynamic properties are governed by strong electron-electron correlations. The features observed conjointly hint at strongly correlated ground state in Ce$_{11}$Pd$_4$In$_9$ .",1901.06888v1 2019-01-26,"Robust p-orbital half-metallicity and high Curie-temperature in the hole-doped anisotropic TcS2 (X=S, Se) nanosheets","Here, we study the magnetism of the distorted 1T-TcX2 (X=S,Se) based on first-principles calculation. The magnetism originates from the hole doping due to the density of states near the valence band edge having van Hove singularity feature. The calculated results show that the TcS2 monolayer can develop an interesting ferromagnetic (FM) half-metallic phase with tunable spin-polarization orientation. The FM half-metallicity and magnetic moments of the hole-doped TcS2 monolayer are primarily derived from the p orbital of S atoms, then, a FM ground phase with a high Curie temperature (Tc) (larger than 800 K) is obtained due to the strong Sp-Sp direct exchange interaction. The magnetic order is robust against thermal excitations at finite temperatures because of magnetic anisotropic energy. In the TcS2 bilayer, the electrons near Fermi level are redistributed when introducing the interlayer interaction, which suppresses the ferromagnetism induced by hole doping. The ferromagnetism can be recovered when the interlayer interaction is weakened.",1901.09147v1 2019-01-30,Modelling long-range interactions in multiscale simulations of ferromagnetic materials,"Atomistic-continuum multiscale modelling is becoming an increasingly popular tool for simulating the behaviour of materials due to its computational efficiency and reliable accuracy. In the case of ferromagnetic materials, the atomistic approach handles the dynamics of spin magnetic moments of individual atoms, while the continuum approximations operate with volume-averaged quantities, such as magnetisation. One of the challenges for multiscale models in relation to physics of ferromagnets is the existence of the long-range dipole-dipole interactions between spins. The aim of the present paper is to demonstrate a way of including these interactions into existing atomistic-continuum coupling methods based on the partitioned-domain and the upscaling strategies. This is achieved by modelling the demagnetising field exclusively at the continuum level and coupling it to both scales. Such an approach relies on the atomistic expression for the magnetisation field converging to the continuum expression when the interatomic spacing approaches zero, which is demonstrated in this paper.",1901.11401v1 2019-02-07,Unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling,"We report strong unidirectional anisotropy in bulk polycrystalline B20 FeGe measured by ferromagnetic resonance spectroscopy. Bulk and micron-sized samples were produced and analytically characterized. FeGe is a B20 compound with inherent Dzyaloshinskii-Moriya interaction. Lorenz microscopy confirms a skyrmion lattice at $190 \; \text{K}$ in a magnetic field of 150 mT. Ferromagnetic resonance was measured at $276 \; \text{K} \pm 1 \; \text{K}$, near the Curie temperature. Two resonance modes were observed, both exhibit a unidirectional anisotropy of $K=1153 \; \text{J/m}^3 \pm 10 \; \text{J/m}^3$ in the primary, and $K=28 \; \text{J/m}^3 \pm 2 \; \text{J/m}^3$ in the secondary mode, previously unknown in bulk ferromagnets. Additionally, about 25 standing spin wave modes are observed inside a micron-sized FeGe wedge, measured at room temperature ($\sim \; 293$ K). These modes also exhibit unidirectional anisotropy.",1902.02665v2 2019-02-19,The random field XY model on sparse random graphs shows replica symmetry breaking and marginally stable ferromagnetism,"The ferromagnetic XY model on sparse random graphs in a randomly oriented field is analyzed via the belief propagation algorithm. At variance with the fully connected case and with the random field Ising model on the same topology, we find strong evidences of a tiny region with Replica Symmetry Breaking (RSB) in the limit of very low temperatures. This RSB phase is robust against different choices of the external field direction, while it rapidly vanishes when increasing the graph mean degree, the temperature or the directional bias in the external field. The crucial ingredients to have such a RSB phase seem to be the continuous nature of vector spins, mostly preserved by the O(2)-invariant random field, and the strong spatial heterogeneity, due to graph sparsity. We also uncover that the ferromagnetic phase can be marginally stable despite the presence of the random field. Finally, we study the proper correlation functions approaching the critical points to identify the ones that become more critical.",1902.07132v3 2019-02-22,Coexistent spin-triplet superconducting and ferromagnetic phases induced by the Hund's rule coupling and electronic correlations II: Effect of applied magnetic field,"Recently proposed local-correlation-driven pairing mechanism, describing ferromagnetic phases (FM1 and FM2) coexisting with spin-triplet superconductivity (SC) within a single orbitally degenerate Anderson lattice model, is extended to the situation with applied Zeeman field. The model provides and rationalizes in a semiquantitative manner the principal features of the phase diagram observed for $\mathrm{UGe_2}$ in the field absence [cf. Phys. Rev. B $\mathbf{97}$, 224519 (2018)]. As spin-dependent effects play a crucial role for both the ferromagnetic and SC states, the role of the Zeeman field is to single out different stable spin-triplet SC phases. This analysis should thus be helpful in testing the proposed real-space pairing mechanism, which may be regarded as complementary to spin-fluctuation theory suitable for $\mathrm{^3He}$. Specifically, we demonstrate that the presence of the two distinct phases, FM1 and FM2, and associated field-driven metamagnetic transition between them, induce respective metasuperconducting phase transformation. At the end, we discuss briefly how the spin fluctuations might be incorporated as a next step into the considered here renormalized quasiparticle picture.",1902.08444v2 2019-03-01,Probing Magnetism in Insulating Cr2Ge2Te6 by Induced Anomalous Hall Effect in Pt,"Two-dimensional ferromagnet Cr2Ge2Te6 (CGT) is so resistive below its Curie temperature that probing its magnetism by electrical transport becomes extremely difficult. By forming heterostructures with Pt, however, we observe clear anomalous Hall effect (AHE) in 5 nm thick Pt deposited on thin (< 50 nm) exfoliated flakes of CGT. The AHE hysteresis loops persist to ~ 60 K, which matches well to the Curie temperature of CGT obtained from the bulk magnetization measurements. The slanted AHE loops with a narrow opening indicate magnetic domain formation, which is confirmed by low-temperature magnetic force microscopy (MFM) imaging. These results clearly demonstrate that CGT imprints its magnetization in the AHE signal of the Pt layer. Density functional theory calculations of CGT/Pt heterostructures suggest that the induced ferromagnetism in Pt may be primarily responsible for the observed AHE. Our results establish a powerful way of investigating magnetism in 2D insulating ferromagnets which can potentially work for monolayer devices.",1903.00569v1 2019-03-07,Proximity effect in a ferromagnetic semiconductor with spin-orbit interactions,"We study theoretically the proximity effect in a ferromagnetic semiconductor with Rashba spin-orbit interaction. The exchange potential generates opposite-spin-triplet Cooper pairs which are transformed into equal-spin-triplet pairs by the spin-orbit interaction. In the limit of strong spin-orbit interaction, symmetry of the dominant Cooper pair depends on the degree of disorder in a ferromagnet. In the clean limit, spin-singlet $s$-wave Cooper pairs are the most dominant because the spin-momentum locking stabilizes a Cooper pair consisting of a time-reversal partner. In the dirty limit, on the other hand, equal-spin-triplet $s$-wave pairs are dominant because random impurity potentials release the locking. We also discuss the effects of the spin-orbit interaction on the Josephson current.",1903.02746v1 2019-03-08,Magnon quantum anomalies in Weyl ferromagnets,"When subjected to parallel electric field $\mathbf E$ and magnetic field $\mathbf B$, Weyl semimetals exhibit the exotic transport property known as the chiral anomaly due to the pumping of electrons between Weyl cones of opposite chiralities. When one or both electromagnetic (EM) fields are replaced by strain-induced chiral pseudo-electromagnetic (pseudo-EM) fields, other types of quantum anomalies occur. In the present paper, we will show that such quantum anomalies can be reproduced in a completely different system -- a Weyl ferromagnet whose magnonic structure remarkably encodes Weyl physics. By analytical and numerical calculations, we will show that the magnon bands can be Landau-quantized by either an inhomogeneous electric field $\mathbf E$ or a chiral pseudo-electric field $\mathbf e$ induced by a torsional strain, and magnons can be pumped along bands by either an inhomogeneous magnetic field $\mathbf B$ or a chiral pseudo-magnetic field $\mathbf b$ due to a dynamic uniaxial strain. We list the magnon quantum anomalies and the associated anomalous spin and heat currents in the Weyl ferromagnet, and show they are distinct from their Weyl semimetal counterparts.",1903.03664v1 2019-03-10,Holographic paramagnetic-ferromagnetic phase transition with Power-Maxwell electrodynamics,"We explore the effects of Power-Maxwell nonlinear electrodynamics on the properties of holographic s-wave paramagnetic-ferromagnetic phase transition in the background of Schwarzchild Anti-de Sitter (AdS) black hole. For this purpose, we introduce a massive 2-form coupled to the Power-Maxwell field. We perform the numerical shooting method in the probe limit by assuming the Power-Maxwell and the 2-form fields do not back react on the background geometry. We observe that increasing the strength of the power parameter causes the formation of magnetic moment in the black hole background harder and critical temperature lower. In the absence of external magnetic field and at the low temperatures, the spontaneous magnetization and the ferromagnetic phase transition happen. In this case, the critical exponent for magnetic moment is always 1/2 which is in agreement with the result from the mean field theory. In the presence of external magnetic field, the magnetic susceptibility satisfies the Cure-Weiss law.",1903.05451v2 2019-03-19,Pump-probe nuclear spin relaxation study of the quantum Hall ferromagnet at filling factor nu = 2,"The nuclear spin-lattice relaxation time T1 of the nu = 2 quantum Hall ferromagnet (QHF) formed in a gate-controlled InSb two-dimensional electron gas has been characterized using a pump-probe technique. In contrast to a long T1 of quantum Hall states around nu = 1 that possesses a Korringa-type temperature dependence, the temperature-independent short T1 of the nu = 2 QHF suggests the presence of low energy collective spin excitations in a domain wall. Furthermore, T1 of this ferromagnetic state is also found to be filling- and current-independent. The interpretation of these results as compared to the T1 properties of other QHFs is discussed in terms of the domain wall skyrmion, which will lead to a better understanding of the QHF.",1903.07778v2 2019-03-19,Physical origin of giant excitonic and magneto-optical responses in two-dimensional ferromagnetic insulators,"The recent discovery of magnetism in atomically thin layers of van der Waals crystals has created great opportunities for exploring light-matter interactions and magneto-optical phenomena in the two-dimensional limit. Optical and magneto-optical experiments have provided insights into these topics, revealing strong magnetic circular dichroism and giant Kerr signals in atomically thin ferromagnetic insulators. However, the nature of the giant magneto-optical responses and their microscopic mechanism remain unclear. Here, by performing first-principles GW and Bethe-Salpeter equation calculations, we show that excitonic effects dominate the optical and magneto-optical responses in the prototypical two-dimensional ferromagnetic insulator, CrI$_3$. We simulate the Kerr and Faraday effects in realistic experimental setups, and based on which we predict the sensitive frequency- and substrate-dependence of magneto-optical responses. These findings provide physical understanding of the phenomena as well as potential design principles for engineering magneto-optical and optoelectronic devices using two-dimensional magnets.",1903.07787v3 2019-03-25,Quench dynamics of spin in quantum dots coupled to spin-polarized leads,"We investigate the quench dynamics of a quantum dot strongly coupled to spin-polarized ferromagnetic leads. The real-time evolution is calculated by means of the time-dependent density-matrix numerical renormalization group method implemented within the matrix product states framework. We examine the system's response to a quench in the spin-dependent coupling strength to ferromagnetic leads as well as in the position of the dot's orbital level. The spin dynamics is analyzed by calculating the time-dependent expectation values of the quantum dot's magnetization and occupation. Based on these, we determine the time-dependence of a ferromagnetic-contact-induced exchange field and predict its nonmonotonic build-up. In particular, two time scales are identified, describing the development of the exchange field and the dot's magnetization sign change. Finally, we study the effects of finite temperature on the dynamical behavior of the system.",1903.10381v1 2019-03-27,Switching of magnetism via modifying phase shift of quantum-well states by tailoring the interface electronic structure,"We demonstrate control of the magnetism of Pd(100) ultrathin films, which show d-electron quantum-well induced ferromagnetism, via modulation of the interface electronic state using density functional calculation. From an analysis based on the phase model, forming the Au/Pd(100) interface induces hybridization of the wave function of d-electron quantum-well states, and modulates the term of the scattering phase shift as a function of the reciprocal lattice point. In contrast, forming the Al interface, which has only s-electrons at the Fermi energy, cannot modify the scattering phase shift. Our finding indicates the possibility of modifying the phase shift by tailoring the interface electronic states using hybridization of the wave function, and this efficiently changes the density of states near the Fermi energy of Pd films, and the switching between paramagnetism and ferromagnetism occurs based on the condition for ferromagnetism (Stoner criterion).",1903.11404v3 2019-03-28,Topological phases of higher Chern numbers in Kitaev-Heisenberg ferromagnet with further-neighbor interactions,"Emergence of multiple topological phases with a series of Chern numbers, $\pm 1$, $\mp 1$, $\pm 2$, $\mp 2$, $\pm 3$ and $\mp 4$, are observed in a ferromagnetic Kitaev-Heisenberg-spin-anisotropic model on honeycomb lattice with further neighbor interactions in the presence of an external magnetic field. Magnon Chern insulating dispersions of this two-band model are studied by using linear spin-wave theory formulated on the exact ferromagnetic ground state. Magnon edge states are obtained for the respective topological phases along with density of states. A topological phase diagram of this model is presented. Behavior of thermal Hall conductivity for those phases is studied. Sharp jumps of thermal hall conductivity is noted near the vicinity of phase transition points. Topological phases of the Kitaev spin-liquid compounds, $\alpha$-RuCl$_3$, X$_2$IrO$_3$, X = (Na, Li) and CrY$_3$, Y = (Cl, Br, I) are characterized based on this theoretical findings.",1903.11882v2 2019-03-31,Odd-parity linear magnetoresistance and the planar Hall effect,"The phenomena of odd-parity magnetoresistance and the planar Hall effect are deeply entwined with ferromagnetism. The intrinsic magnetization of the ordered state permits these unusual and rarely observed manifestations of Onsager's theorem when time reversal symmetry is broken at zero applied field. Here we study two classes of ferromagnetic materials, rare-earth magnets with high intrinsic coercivity and antiferromagnetic pyrochlores with strongly-pinned ferromagnetic layers at domain walls, which both exhibit odd-parity magnetoresistive behavior. The peculiar angular variation of the response with respect to the relative alignments of the magnetization, magnetic field, and current reveal the two underlying microscopic mechanisms: spin-polarization-dependent scattering of a Zeeman-shifted Fermi surface and magnetoresistance driven by the anomalous velocity physics usually associated with the anomalous Hall effect.",1904.00330v1 2019-04-11,Coexistence of high electrical conductivity and weak ferromagnetism in Cr doped Y$_2$Ir$_2$O$_7$ pyrochlore iridate,"We report the structural, magnetic and electrical transport properties of Y$_2$Ir$_{2-x}$Cr$_x$O$_7$ pyrochlore iridates. The chemical doping leads to order of magnitude enhancement of electrical conductivity. The introduction of Cr3+ at Ir4+ site tends to distort the Ir-O6 octahedra and suppresses antiferromagnetic correlation. The X-ray photoemission spectroscopy measurements suggest the coexistence of Ir4+ and Ir5+ valence states in the Y$_2$Ir$_{2-x}$Cr$_x$O$_7$ compounds. The concentration of Ir5+ is enhanced with Cr doping, leading to weak ferromagnetism and enhanced electrical conductivity. A cluster-glass like transition is also observed at low temperature with Cr doping, possibly due to competing ferromagnetic and antiferromagnetic interaction.",1904.05541v1 2019-04-12,Antiferromagnetic Stacking of Ferromagnetic Layers and Doping Controlled Phase Competition in Ca$_{1-x}$Sr$_{x}$Co$_{2-y}$As$_{2}$,"In search of a quantum phase transition between the two-dimensional ($2$D) ferromagnetism of CaCo$_{2-y}$As$_{2}$ and stripe-type antiferromagnetism in SrCo$_{2}$As$_{2}$, we rather find evidence for $1$D magnetic frustration between magnetic square Co layers. We present neutron diffraction data for Ca$_{1-x}$Sr$_{x}$Co$_{2-y}$As$_{2}$ that reveal a sequence of $x$-dependent magnetic transitions which involve different stacking of $2$D ferromagnetically-aligned layers with different magnetic anisotropy. We explain the $x$-dependent changes to the magnetic order by utilizing classical analytical calculations of a $1$D Heisenberg model where single-ion magnetic anisotropy and frustration of antiferromagnetic nearest- and next-nearest-layer exchange are all composition dependent.",1904.06444v2 2019-04-15,A Rigorous Demonstration of Superconductivity in a Repulsive Hubbard Model,"We have rigorously shown that a strong Hubbard repulsion can cause superconductivity. The model, which has a particular set of local symmetries, manifests the phase diagram of many unconventional superconductors; anti-ferromagnetism paramagnetism, superconductivity, and even ferromagnetism are all observed. The key technique in the analysis is an exact non-linear fermion transformation, which preserves the correlated motion of electrons while removing the strong interactions. Using resolvent formalism, it is exactly shown that two holes at the Mott point bind to form a localised Cooper pair. As interactions are now weak, we then use BCS mean field theory to calculate the energy, excess pairing, and superconducting gap. These results are compared to exact diagonalisation of finite sized systems and show good agreement. At the Mott point the system is an anti-ferromagnet, and a superconducting phase quickly appears upon doping, and then vanishes.",1904.07138v1 2019-04-27,Phase diagram of a magnetic topological nodal semimetal: Stable nodal line in an easy-plane ferromagnet,"We study a topological phase diagram of a ferromagnetic topological nodal semimetal. We consider a lattice model for three-dimensional topological insulators with ferromagnetic ordering. The exchange coupling between the magnetization and the electron spin leads to the nodal band structure. The topology of the nodal band structure depends on the direction of the magnetization and both the Weyl points and the nodal line emerge. We find that the nodal line structure is stable under an easy-plane magnetization in appropriate model parameters. In this case, the nodal line phase emerges as a phase boundary between two topologically distinct Weyl semimetal phases.",1904.12122v1 2019-04-30,Violation of single-length scaling dynamics via spin vortices in an isolated spin-1 Bose gas,"We consider the phase ordering dynamics of an isolated quasi-two-dimensional spin-1 Bose gas quenched into an easy-plane ferromagnetic phase. Preparing the initial system in an unmagnetized anti-ferromagnetic state the subsequent ordering involves both polar core and Mermin-Ho spin vortices, with the ratio between the different vortices controllable by the quench parameter. Ferromagnetic domain growth occurs as these vortices annihilate. The distinct dynamics of the two types of vortices means that the domain growth law is determined by two macroscopic length scales, violating the standard dynamic scaling hypothesis. Nevertheless we find that universality of the ordering process manifests in the decay laws for the spin vortices.",1904.13222v1 2019-08-01,Chiral Magnetic Josephson junction: a base for low-noise superconducting qubits?,"Superconducting materials with non-centrosymmetric lattices lacking the space inversion symmetry are known to exhibit a variety of interesting parity-breaking phenomena, including the anomalous Josephson effect. Here we consider a Josephson junction consisting of two non-centrosymmetric superconductors (NCSs) connected by a uniaxial ferromagnet, and demonstrate that it exhibits a direct analog of the Chiral Magnetic Effect observed in Dirac and Weyl semimetals. We propose to use this ""Chiral Magnetic Josephson junction"" (CMJ junction) as an element of a qubit with a Hamiltonian tunable by the ferromagnet's magnetization. The CMJ junction allows to avoid the use of an offset magnetic flux in inductively shunted qubits, thus enabling a simpler and more robust architecture. The resulting""`chiral magnetic qubit"" is protected from the noise caused by fluctuations in magnetization when the easy axis of the uniaxial ferromagnet is directed across the junction.",1908.00392v1 2019-08-01,Universal prethermal dynamics in Heisenberg ferromagnets,"We study the universal far from equilibrium dynamics of magnons in Heisenberg ferromagnets. We show that such systems exhibit universal scaling in momentum and time of the quasiparticle distribution function, with the universal exponents distinct from those recently observed in Bose-Einstein condensates. This new universality class originates from the SU(2) symmetry of the Hamiltonian, which leads to a strong momentum-dependent magnon-magnon scattering amplitude. We compute the universal exponents using the Boltzmann kinetic equation and incoherent initial conditions that can be realized with microwave pumping of magnons. We compare our numerical results with analytic estimates of the scaling exponents and demonstrate the robustness of the scaling to variations in the initial conditions. Our predictions can be tested in quench experiments of spin systems in optical lattices and pump-probe experiments in ferromagnetic insulators such as yttrium iron garnet.",1908.00554v2 2019-08-02,An accelerating approach of designing ferromagnetic materials via machine learning modeling of magnetic ground state and Curie temperature,"Magnetic materials have a plethora of applications ranging from informatics to energy harvesting and conversion. However, such functionalities are limited by the magnetic ordering temperature. In this work, we performed machine learning on the magnetic ground state and the Curie temperature (TC), using generic chemical and crystal structural descriptors. Based on a database of 2805 known intermetallic compounds, a random forest model is trained to classify ferromagnetic and antiferromagnetic compounds and to do regression on the TC for the ferromagnets. The resulting accuracy is about 86% for classification and 92% for regression (with a mean absolute error of 58K). Composition based features are sufficient for both classification and regression, whereas structural descriptors improve the performance. Finally, we predict the magnetic ordering and TC for all the intermetallic magnetic materials in the Materials Project. Our work paves the way to accelerate the development of magnetic materials for technological applications.",1908.00926v2 2019-08-06,Densities of states in Fe-doped III-V semiconductors: a first-principles study,"The electronic structures of Fe-doped III-V semiconductors were studied by first-principles supercell calculation. It was found that their electronic structures are basically the same as those of Mn-doped ones except that the extra electron of Fe compared to Mn occupies either majority-spin $p$-$d$ hybridized antibonding states ($t_{a,\uparrow}$) or minority-spin $e$ states ($e_{\downarrow}$) and that the center of gravity of the $d$ partial density of states is higher for Fe than for Mn. The present calculations suggest that ferromagnetism appears when the $e_{\downarrow}$ states start to be occupied. The band splitting due to $s$-$d$ hybridization was found to be significantly smaller than the one due to $p$-$d$ hybridization. This indicates that the $s,p$-$d$ exchange interaction is not responsible for the high-temperature ferromagnetism of the Fe-doped ferromagnetic semiconductors even in $n$-type compounds.",1908.02311v1 2019-08-07,Deformation of flexible ferromagnetic filaments under a rotating magnetic field,"Research on magnetic particles dispersed in a fluid medium, actuated by a rotating magnetic field, is becoming increasingly active for both lab-on-chip and bio-sensing applications. In this study, we experimentally investigate the behaviour of ferromagnetic filaments in a rotating field. Filaments are synthesized by linking micron-sized ferromagnetic particles with DNA strands. The experiments were conducted under different magnetic field strengths, frequencies and filament sizes, and deformation of the filaments was registered via microscope and camera. The results obtained showed that the body deformation is larger for longer filaments and higher frequencies and lower for larger magnetic field. The angle between the filament tangent at the centre and the magnetic field direction increases linearly with frequency at low-frequency regime. A further increase in the frequency will result in filament movement out of plane when the angle approaches 90 degrees. The experimental results were used to estimate magnetic moment and the bending elasticity of the filament.",1908.02604v2 2019-08-08,Study of the ferromagnetic quantum phase transition in Ce$_{3-x}$Mg$_x$Co$_{9}$,"The Ce$_{3-x}$Mg$_x$Co$_{9}$ system evolves from a Pauli paramagnetic ground state for $x = 0$ to a ferromagnetic ground state for $x\approx0.80$ in single phase, polycrystalline samples [Phys. Rev. Applied 9, 024023 (2018)]. In order to better understand this behavior, single crystalline samples of Ce$_{3-x}$Mg$_x$Co$_{9}$ for \textit{x} = 0.01, 0.16, 0.24, 0.35, 0.43 and 0.50 were grown using the flux growth technique, and electrical transport and magnetic properties were studied. The \textit{T}$_C$-\textit{x} phase diagram we infer shows that the system has a quantum phase transition near \textit{x} = 0.35, transforming to a ferromagnetic ground state.",1908.03126v1 2019-08-11,Spin Hall Magnetoresistance in Metallic Bilayers with In-plane Magnetized Ferromagnets,"We revisit the theory and experiment on spin Hall magnetoresistance (SMR) in bilayers consisting of a heavy metal (H) coupled to in-plane magnetized ferromagnetic metal (F), and determine contributions to the magnetoresistance due to SMR and anisotropic magnetoresistance (AMR) in four different bilayer systems: W/$\text{Co}_{20}\text{Fe}_{60}\text{B}_{20}$, W/Co, $\text{Co}_{20}\text{Fe}_{60}\text{B}_{20}$/Pt, and Co/Pt. To do this, the AMR is explicitly included in the diffusion transport equations in the ferromagnet. The results allow precise determination of different contributions to the magnetoresistance, which can play an important role in optimizing prospective magnetic stray field sensors. They also may be useful in the determination of spin transport properties of metallic magnetic heterostructures in other experiments based on magnetoresistance measurements.",1908.03906v1 2019-08-12,Superconductor-ferromagnet tunnel junction thermoelectric bolometer and calorimeter with a SQUID readout,"Superconductor-ferromagnet thermoelectric detector (SFTED) is a novel ultrasensitive radiation detector based on the giant thermoelectric effect in superconductor-ferromagnet tunnel junctions. This type of detector can be operated without the need of additional bias lines, and is predicted to provide a performance rivaling transition-edge sensors and kinetic inductance detectors. Here we report our numerical studies on the SFTED noise equivalent power, energy resolution and time constant, and the feasibility of a SQUID readout in both bolometric and calorimetric regimes, with the goal to provide practical design parameters for the detector fabrication and the readout circuitry implementation.",1908.04097v2 2019-08-21,"Two-dimensional Ferromagnetic van der Waals CrX3 (X=Cl, Br, I) Monolayers with Enhanced Anisotropy and Curie Temperature","Among the recently widely studied van der Waals layered magnets CrX3 (X=Cl, Br, I), CrCl3 monolayer (ML) is particularly puzzling as it is solely shown by experiments to have an in-plane magnetic easy axis and, furthermore, all of previous first-principles calculation results contradict this. Through systematical first-principles calculations,we unveil that its in-plane shape anisotropy that dominates over its weak perpendicular magnetocrystalline anisotropy is responsible for the in-plane magnetic easy axis of CrCl3 ML. To tune the in-plane ferromagnetism of CrCl3 ML into the desirable perpendicular one, we propose substituting Cr with isovalent tungsten (W). We find that CrWCl6 has a strong perpendicular magnetic anisotropy and a high Curie temperature up to 76 K. Our work not only gives insight into understanding the two-dimensional ferromagnetism of van der Waals MLs but also sheds new light on engineering their performances for nanodevices.",1908.07710v2 2019-08-23,Magnetic ordering of the martensite phase in Ni-Co-Mn-Sn-based ferromagnetic shape memory alloys,"The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of dc magnetization, ac susceptibility and small angle neutron scattering (SANS) studies provide a clear evidence for AF order in martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition of Ni44Co6Mn40Sn10 point to presence of SG order in martensitic phase and absence of SPM behavior, contrary to earlier report. SANS results do show presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys.",1908.08860v2 2019-11-07,Quantum Oscillations of Gilbert Damping in Ferromagnetic/Graphene Bilayer Systems,"We study the spin dynamics of a ferromagnetic insulator on which graphene is placed. We show that the Gilbert damping is enhanced by the proximity exchange coupling at the interface. The modulation of the Gilbert damping constant is proportional to the product of the spin-up and spin-down densities of states of graphene. Consequently, the Gilbert damping constant in a strong magnetic field oscillates as a function of the external magnetic field that originates from the Landau level structure of graphene. We find that a measurement of the oscillation period enables the strength of the exchange coupling constant to be determined. The results demonstrate in theory that the ferromagnetic resonance measurements may be used to detect the spin resolved electronic structure of the adjacent materials, which is critically important for future spin device evaluations.",1911.02775v2 2019-11-21,Strain-induced phase transition in CrI$_{3}$ bilayers,"A monolayer of CrI$_3$ is a two-dimensional crystal that in its equilibrium configuration is a ferromagnetic semiconductor, however, two coupled layers can be ferromagnetic or antiferromagnetic depending on the stacking. We study the magnetic phase diagram upon the strain of the antiferromagnetically coupled bilayer with C2/m symmetry. We found that strain may be an efficient tool to tune the magnetic phase of the structure. A tensile strain stabilizes the antiferromagnetic phase, while a compressive strain turns the system ferromagnetic. We understood that behavior by looking at the relative displacement between layers due to the strain. We also study the evolution of the magnetic anisotropy, the magnetic exchange coupling between Cr atoms, and how the Curie temperature is affected by the strain.",1911.09734v4 2019-11-22,Superconducting spin-valve effect in heterostructures with ferromagnetic Heusler alloy layers,"We report a comparative analysis and theoretical description of the superconducting properties of two spin-valve-valve structures containing the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al$_{y}$ as one of two ferromagnetic (F1 or F2) layers of the F1/F2/S structure, where S stands for the superconducting Pb layer. In our experiments we used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a half-metal on the place of the F1 layer. In the first case, we obtained a large ordinary superconducting spin-valve effect $\Delta T_c$ assisted by the triplet superconducting spin-valve effect $\Delta T_c^{trip}$. In the second case, we observed a giant magnitude of $\Delta T_c^{trip}$ reaching 0.5 K. An underlying theory based on the solution of the Usadel equations using Kupriyanov-Lukichev boundary conditions with arbitrary material parameters for all layers and arbitrary boundary parameters for all interfaces is presented in Appendix. We find a good agreement between our experimental data and theoretical results.",1911.09984v1 2019-11-22,Proximity effect in [Nb(1.5nm)/Fe(x)]$_{10}$/Nb(50nm) superconducting/ferromagnet heterostructures,"We have investigated the structural, magnetic and superconducting properties of [Nb(1.5nm)/Fe(x)]$_{10}$ superlattices deposited on a thick Nb(50nm) layer. Our investigation showed that the Nb(50nm) layer grows epitaxially at 800$^\circ$C on Al$_2$O$_3$(1$\bar{1}$02) substrate. Samples grown at this condition posses a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with $x<$ 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has properties of a weak ferromagnet. Proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by co-existing superconducting and normal-state domains. By increasing the thickness of the Fe layer to $x$ = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the periodic Fe/Nb structure.",1911.10168v2 2019-11-26,Large magnetoresistance and spin-dependent output voltage in a lateral MnGa/GaAs/MnGa spin-valve device,"We investigated the spin-dependent transport properties of a lateral spin-valve device with a 600 nm-long GaAs channel and ferromagnetic MnGa electrodes with perpendicular magnetization. Its current-voltage characteristics show nonlinear behavior below 50 K, indicating that tunnel transport through the MnGa/GaAs Schottky barrier is dominant at low temperatures. We observed clear magnetoresistance (MR) ratio up to 12% at 4 K when applying a magnetic field perpendicular to the film plane. Furthermore, a large spin-dependent output voltage of 33 mV is obtained. These values are the highest in lateral ferromagnetic metal / semiconductor / ferromagnetic metal spin-valve devices reported so far.",1911.11295v1 2019-11-27,Phase diagram of a three-dimensional dipolar Ising model with textured Ising axes,"We study from tempered Monte Carlo simulations the magnetic phase diagram of a textured dipolar Ising model on a face centered cubic lattice. The Ising coupling of the model follow the dipole-dipole interaction. The Ising axes are distributed with a uniaxial symmetry along the $\hat{z}$ direction with a gaussian probability density of the polar angles. This distribution provides a quenched disorder realization of the dipolar Ising model making a continuous link between the parallel axes dipoles and the random axes dipole models. As expected the phase diagram presents three distinctive phases: paramagnetic, ferromagnetic and spin-glass. A quasi long range ferromagnetic and a reentrant spin-glass phases are obtained in the vicinity of the ferromagnetic spin-glass line. This model provides a way to predict the magnetic phases of magnetic nanoparticles supracrystals in terms of the texturation of the easy axes distribution in the strong anisotropy limit.",1911.12121v1 2019-11-28,First and Second Order Topological Phases on Ferromagnetic Breathing Kagome Lattice,"In this work, topological properties of a ferromagnetic Heisenberg model on a breathing kagome lattice are investigated extensively in the presence of Dzyaloshinskii-Moriya interaction. While the kagome ferromagnet hosts only a single first order topological phase, the breathing kagome system exhibits multiple first and second order topological phases along with their coexistence. Magnon dispersion relation is obtained by using linear spin wave theory. Flat band and Dirac cones are obtained in the absence of Dzyaloshinskii-Moriya interaction. A topological phase diagram is presented where several first and second order phases as well as their overlap are identified. Values of thermal Hall conductivity for all the first order phases are obtained. Distinct first order phases are characterized by different sets of Chern numbers in association with the necessary chiral edge states in accordance to the first order bulk-boundary-correspondence rule. Second order phase is characterized by polarization along with the emergence of corner states. Violation of the second order bulk-corner-correspondence rule has been noted in some regions.",1911.12620v2 2019-12-11,Pressure-induced superconductivity and structural transition in ferromagnetic Cr2Si2Te6,"The discovery of intrinsic magnetism in atomically thin two-dimensional transition-metal trichalcogenides has attracted intense research interest due to the exotic properties of magnetism and potential applications in devices. Pressure has proven to be an effective tool to manipulate the crystal and electronic structures of the materials. Here, we report investigations on ferromagnetic van der Waals Cr2Si2Te6 via high-pressure synchrotron x-ray diffraction, electrical resistance, Hall resistance, and magnetoresistance measurements. Under compression, Cr2Si2Te6 simultaneously undergoes a structural transition, emergence of superconductivity at 3 K, sign change of the magnetoresistance, and dramatic change of the Hall coefficient at ~8 GPa. The superconductivity persists up to the highest measured pressure of 47.1 GPa with a maximum Tc = 4.5 K at ~30 GPa. The discovery of superconductivity in the two-dimensional van der Waals ferromagnetic Cr-based Cr2Si2Te6 provides new perspectives to explore superconductivity and the interplay between superconductivity and magnetism.",1912.05166v1 2019-12-19,Magnetic Topological Kagome Systems,"The recently discovered material Co$_3$Sn$_2$S$_2$ shows an impressive behavior of the quantum anomalous Hall (QAH) conductivity driven by the interplay between ferromagnetism in the $z$ direction and antiferromagnetism in the $xy$ plane. Motivated by these facts, first we build and study a spin-1/2 model to describe the magnetism of Co-atoms on the Kagome planes. Then, we include conduction electrons which are coupled to the spins-1/2 through a strong Hund's coupling. The spin-orbit coupling results in topological low-energy bands. For 2/3 on-site occupancy, we find a topological transition from a QAH ferromagnetic insulating phase with Chern number one to a quantum spin Hall (QSH) antiferromagnetic phase. The QAH phase is metallic when slightly changing the on-site occupancy. To account for temperature effects, we include fluctuations in the direction of the Hund's coupling. We show how the Hall conductivity can now smoothly evolve when spins develop a $120^o$ antiferromagnetism in the $xy$ plane and can synchronize with the ferromagnetic fraction.",1912.09214v2 2019-12-19,Tuning Ginzburg-Landau theory to quantitatively study thin ferromagnetic materials,"Along with experiments, numerical simulations are key to gaining insight into the underlying mechanisms governing domain wall motion in thin ferromagnetic systems. However, a direct comparison between numerical simulation of model systems and experimental results still represents a great challenge. Here, we present a tuned Ginzburg-Landau model to quantitatively study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy. This model incorporates material and experimental parameters and the micromagnetic prescription for thermal fluctuations, allowing us to perform material-specific simulations and at the same time recover universal features. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal perpendicular magnetic anisotropy Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we present a statistical analysis of the domain wall width parameter, showing that our model can provide detailed nano-scale information while retaining the complex behavior of a statistical disordered model.",1912.09392v2 2019-12-20,Exchange bias training effect in IrMn-layer/ferromagnetic-ribbon heterostructures probed with magnetoimpedance,"The exchange-bias training effect in IrMn-layer/ferromagnetic-ribbon heterostructure is studied by performing magnetoimpedance (MI) measurements. Asymmetric, hysteretic and single peak behavior of the MI response and a shift in the MI peak to zero fields accompanied by 52% increase in MI has been detected as the signature of the exchange bias (EB) and training effect (TE), respectively. Also, during the consecutive filed sweep of MI response, both EB field and the degree of asymmetry of MI decrease that is another reason for existence of TE in our sample. The analysis of the magneto-optical Kerr effect (MOKE) and MI behavior establish that Hoffmann s model (Phys. Rev. Lett. 93, 097203 (2004)) is a good description of our experimental data, due to the existence of a strong single cycle TE in our IrMn-layer/ferromagnetic-ribbon system.",1912.10032v1 2019-12-23,Applied electric field on zigzag graphene nanoribbons: reduction of spin stiffness and appearance of spiral spin density waves,"We investigated the reduction of the spin stiffness and the appearance of the spiral spin density waves when the electric field is applied on the zigzag graphene nanoribbons for the ferromagnetic and antiferromagnetic edge states. For that purpose, we exploited the generalized Bloch theorem combined with a constraint method to keep the direction of the magnetic moment of the carbon atom at the edges. We found that the ground state of ferromagnetic configuration is unstable and leads to the spiral ground state while the ground state of the antiferromagnetic configuration is robust. We also showed that the spin stiffness in the ferromagnetic and antiferromagnetic configurations reduces as the electric field increases. Thus, we justified that not only the spin stiffness but also the ground state of the zigzag graphene nanoribbons can be controlled by the electric field.",1912.10670v1 2019-12-31,Ferromagnetic kinetic exchange interaction in magnetic insulators,"The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms/groups. Here we show that explicit consideration of magnetic and (leading) bridging orbitals, together with the electron transfer between the former, reveals a strong ferromagnetic kinetic exchange contribution. First principle calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges. In particular, it is responsible for a very large ferromagnetic coupling ($-10$ meV) between the iron ions in a Fe$^{3+}$-Co$^{3+}$-Fe$^{3+}$ complex.",1912.13274v3 2020-03-02,Tailored flux pinning in superconductor/ferromagnet multilayers with engineered magnetic domain morphology from stripes to skyrmions,"Superconductor/Ferromagnet (S/F) hybrid systems show interesting magneto-transport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of applications. Here we show that engineering the F magnetization reversal allows tuning the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This is done by using magnetic multilayers in which Co thin films are combined with different heavy metals (Ru, Ir, Pt). By choosing the materials, thicknesses, and stacking order of the layers, we can design the characteristic domain size and morphology, from out-of-plane magnetized stripe domains to much smaller magnetic skyrmions. These changes strongly affect the magneto-transport properties. The underlying mechanisms are identified by comparing the experimental results to a magnetic pinning model.",2003.01065v1 2020-03-08,Influence of the velocity barrier on the massive Dirac electron transport in a monolayer MoS$_{2}$ quantum structure,"Using the transfer matrix method, spin- and valley-dependent electron transport properties modulated by the velocity barrier were studied in the normal/ferromagnetic/normal monolayer MoS$_{2}$ quantum structure. Based on Snell's Law in optics, we define the velocity barrier as $\xi=v_{2}/v_{1}$ by changing the Fermi velocity of the intermediate ferromagnetic region to obtain a deflection condition during the electron transport process in the structure. The results show that both the magnitude and the direction of spin- and valley-dependent electron polarization can be regulated by the velocity barrier. $-100\%$ polarization of spin- and valley-dependent electron can be achieved for $\xi>1$, while $100\%$ polarization can be obtained for $\xi<1$. Furthermore, it is determined that perfect spin and valley transport always occur at a large incident angle. In addition, the spin- and valley-dependent electron transport considerably depends on the length $k_{F}L$ and the gate voltage $U(x)$ of the intermediate ferromagnetic region. These findings provide an effective method for designing novel spin and valley electronic devices.",2003.03718v1 2020-03-08,3D motion of flexible ferromagnetic filaments under rotating magnetic field,"Ferromagnetic filaments in a rotating magnetic field are studied both numerically and experimentally. The filaments are made from micron-sized ferromagnetic particles linked with DNA strands. It is found that at low frequencies of the rotating field a filament rotates synchronously with the field and beyond a critical frequency it undergoes a transition to a three dimensional regime. In this regime the tips of the filament rotate synchronously with the field on circular trajectories in the plane parallel to the plane of the rotating field. The characteristics of this motion found numerically match the experimental data and allow us to obtain the physical properties of such filaments. We also discuss the differences in behaviour between magnetic rods and filaments and the applicability of filaments in mixing.",2003.03737v1 2020-03-24,Magnetic Kondo regimes in a frustrated half-filled trimer,"We analyze theoretically the phase diagram of a triangular triple quantum dot with strong onsite repulsion coupled to ferromagnetic leads. This model includes the competition of magnetic ordering of local or itinerant magnetic moments, geometric frustration and Kondo screening. We identify all the phases resulting from this competition. We find that three Kondo phases -- the conventional one, the two-stage underscreened one, and the one resulting from the ferromagnetic Kondo effect -- can be realized at zero temperature, and all are very susceptible to the proximity of ferromagnetic leads. In particular, we find that the quantum dots are spin-polarized in each of these phases. Further, we discuss the fate of the phases at non-zero temperatures, where a plethora of competing energy scales gives rise to complex landscape of crossovers. Each Kondo regime splits into a pair of phases, one not magnetized and one comprising magnetically polarized quantum dots. We discuss our results in the context of heavy-fermion physics in frustrated Kondo lattices.",2003.11140v2 2020-03-26,Magnetoelastic waves in thin films,"This paper discusses the physics of magnetoelasticity and magnetoelastic waves as well as their mathematical description. Magnetoelastic waves occur as a result of strong coupling between spin waves and elastic waves in magnetostrictive ferromagnetic media. In a first part, the basic behavior of spin waves is reviewed in both bulk ferromagnets as well as in thin films. Next, elastic waves are discussed with a focus on thin films. Finally, the interactions between the elastic and magnetic domains are described and it is shown how this results in the formation of magnetoelastic waves. Based on the description of bulk magnetoelastic waves, a theory for magnetoelastic waves in thin films is developed and their dispersion relations are derived. It is shown that the behavior strongly depends on the geometry of the system, especially on the polarization of spin and elastic waves and the direction of the magnetization of the magnetostrictive ferromagnet.",2003.12099v1 2020-03-29,Hypergiant spin polarons photogenerated in ferromagnetic europium chalcogenides,"We find that in the ferromagnetic semiconductor EuS, near its Curie temperature, a single band-edge photon generates a spin polaron (SP), whose magnetic moment approaches 20,000 Bohr magnetons. This is much larger than the supergiant photoinduced SPs in antiferromagnetic europium chalcogenides, reported previously. The larger SP in ferromagnetic EuS, and still larger expected for EuO, is explained by a larger Bohr radius of the photoexcited electron state, which encircles and polarizes a greater number of lattice spins. However, because the wave function of the photoexcited electron spreads over a greater volume, the photoexcited electron exchange interaction with individual lattice spins weakens, which makes the SP more easily quenched thermally.",2003.12956v1 2020-03-31,Nonlinear localized excitations in a topological ferromagnetic honeycomb lattice,"We theoretically investigate nonlinear localized excitations in a Heisenberg honeycomb ferromagnet with a second neighbour Dzialozinskii-Moriya interaction, which has been proved to possess the topological band structure. Using the time-dependent variational principle, we obtain the equation of motion for the system in the Glauber coherent-state representation. By means of the semidiscrete multiple-scale method, different types of nonlinear localized excitations are gotten.Our results show that both two-dimensional discrete breathers and bulk gap solitons may occur in the Heisenberg honeycomb ferromagnet.",2003.14268v3 2020-05-01,Pt and CoB trilayer Josephson $π$ junctions with perpendicular magnetic anisotropy,"We report on the electrical transport properties of Nb based Josephson junctions with Pt/Co$_{68}$B$_{32}$/Pt ferromagnetic barriers. The barriers exhibit perpendicular magnetic anisotropy, which has the main advantage for potential applications over magnetisation in-plane systems of not affecting the Fraunhofer response of the junction. In addition, we report that there is no magnetic dead layer at the Pt/Co$_{68}$B$_{32}$ interfaces, allowing us to study barriers with ultra-thin Co$_{68}$B$_{32}$. In the junctions, we observe that the magnitude of the critical current oscillates with increasing thickness of the Co$_{68}$B$_{32}$ strong ferromagnetic alloy layer. The oscillations are attributed to the ground state phase difference across the junctions being modified from zero to $\pi$. The multiple oscillations in the thickness range $0.2~\leqslant~d_\text{CoB}~\leqslant~1.4$~nm suggests that we have access to the first zero-$\pi$ and $\pi$-zero phase transitions. Our results fuel the development of low-temperature memory devices based on ferromagnetic Josephson junctions.",2005.00384v2 2020-05-04,Quantum Magnetism in Wannier-Obstructed Mott Insulators,"We develop a strong coupling approach towards quantum magnetism in Mott insulators for Wannier obstructed bands. Despite the lack of Wannier orbitals, electrons can still singly occupy a set of exponentially-localized but nonorthogonal orbitals to minimize the repulsive interaction energy. We develop a systematic method to establish an effective spin model from the electron Hamiltonian using a diagrammatic approach. The nonorthogonality of the Mott basis gives rise to multiple new channels of spin-exchange (or permutation) interactions beyond Hartree-Fock and superexchange terms. We apply this approach to a Kagome lattice model of interacting electrons in Wannier obstructed bands (including both Chern bands and fragile topological bands). Due to the orbital nonorthogonality, as parameterized by the nearest neighbor orbital overlap $g$, this model exhibits stable ferromagnetism up to a finite bandwidth $W\sim U g$, where $U$ is the interaction strength. This provides an explanation for the experimentally observed robust ferromagnetism in Wannier obstructed bands. The effective spin model constructed through our approach also opens up the possibility for frustrated quantum magnetism around the ferromagnet-antiferromagnet crossover in Wannier obstructed bands.",2005.01439v1 2020-05-06,Understanding the Curious Magnetic State of Sr$_{3}$OsO$_{6}$,"Motivated by the recent report on high T$_c$ ferromagnetic insulating state of single transition metal containing double perovskite compound, Sr$_3$OsO$_6$ (Wakabayashi et. al., Nature Commun {\bf 10} 535, 2019), we study this curious behavior by employing first-principles calculations in conjunction with exact diagonalization of full $t_{2g}$ multiplet problem of two Os sites. Our analysis highlights the fact that stabilization of Sr$_3$OsO$_6$ in the cubic phase in epitaxially grown thin film is the key to both ferromagnetic correlation and high temperature scale associated to it. This also provides a natural explanation for the sister compound, Ca$_3$OsO$_6$ to exhibit low T$_N$ antiferromagnetism in its monoclinic structure. Further the insulating property is found to be driven by opening of Mott gap in the half filled spin-orbit coupled $j=3/2$ manifold of $d^{2}$ Os. We point out that Sr$_2$CaOsO$_6$ which naturally forms in the cubic phase would be worthwhile to explore as a future candidate to exhibit high T$_c$ ferromagnetic insulating state in bulk form.",2005.02582v2 2020-05-14,Hall effect in ferromagnetic nanomagnets: magnetic field dependence as an evidence of inverse spin Hall effect contribution,"We measure magnetic field dependence of the Hall angle in a metallic ferromagnetic nanomagnet with stable local magnetic moments where the adopted mechanisms of Hall effect predict linear plus a constant dependence on the external field originating from the ordinary and anomalous Hall effects, respectively. We suggest that the experimentally observed deviations from this dependence is caused by the inverse spin Hall effect (ISHE) and develop a phenomenological theory, which predicts a unique nonlinear dependence of the ISHE contribution on the external magnetic field. Perfect agreement between theory and experiment supports the considerable role of the ISHE in the Hall transport in ferromagnetic metals.",2005.06675v1 2020-05-16,Independence of the inverse spin Hall effect with the magnetic phase in thin NiCu films,"Large spin Hall angles have been observed in 3d ferromagnets, but their origin, and especially their link with the ferromagnetic order, remain unclear. Here, we investigate the inverse spin Hall effect of Ni60Cu40 and Ni50Cu50 across their Curie temperature using spin pumping experiments. We evidence that the inverse spin Hall effect in these samples is comparable to that of platinum, and that it is insensitive to the magnetic order. These results points towards a Heisenberg localized model of the transition, and suggest that the large spin Hall effects in 3d ferromagnets can be independent of the magnetic phase.",2005.08066v1 2020-05-19,Anomalous Nernst effect in ferromagnetic Mn$_5$Ge$_3$C$_x$ thin films on insulating sapphire,"Investigating the thermoelectric properties of ferromagnets is important for the development of future microelectronic devices for efficient energy conversion purposes. Ferromagnetic Mn$_5$Ge$_3$C$_x$ thin films with a Curie temperature up to $T_{\rm C}$ = 450 K well above room temperature are potential candidates for spintronic applications by integration into CMOS heterostructures. In this work, the thermoelectric power, in particular, the anomalous Nernst effect (ANE) has been investigated experimentally for magnetron sputtered thin films on sapphire (11-20) substrates. The ANE gradually increases with increasing carbon content x up to a maximum value obtained for x=0.8 in line with earlier investigations of the magnetization and anomalous Hall effect. The ANE is strongly enhanced by a factor three compared to the parent Mn$_5$Ge$_3$ compound. However, for x=0.8 we observe a clear deviation of the calculated ANE from the measured values.",2005.09320v1 2020-05-21,Reentrant superconductivity in YBa$_2$Cu$_3$O$_{7-d}$ microstructured particles,"A reentrant superconducting effect was observed in YBCO particles as a consequence of their size and microstructure. It rises from the coexistence between bulk superconductivity and surface ferromagnetism, where the enhanced magneto-electric coupling induced by broken spatial-inversion symmetry in layers near structural defects plays a very important role on surface ferromagnetism. The study assumes grain boundaries inside the particles act as additional surface regions that enhance the ferromagnetic properties. Magnetic and superconducting properties as a function of particle size were studied. Particularly, the 385 nm particles which were the ones that exhibited the reentrant superconducting effect. The Ginzburg-Landau parameter and the critical temperatures are $k \sim 1.29$, T$_{c1}$ = 92 $\pm$ 5 K, and T$_{c2}$ = 10 $\pm$ 3 K.",2005.10894v1 2020-05-21,Surface lattice resonance based magneto-plasmonic switch in NiFe patterned nano-structure,"In this work, a 2D magneto-plasmonic grating structure combining materials with ferromagnetic and plasmonic properties is demonstrated. NiFe composite ferromagnetic material, as an active medium with tunable physical properties, and Au metal, as a plasmonic excitation layer, were the materials of choice. Here, we have experimentally investigated the active control of the plasmonic characteristics in Au/NiFe bilayer by the action of an external magnetic field, as well as the switching effect of the system. The active plasmonic control, can be achieved by the magnetization switching of the ferromagnetic material, opening a new path in the development of active plasmonic devices. To our best knowledge, this is the first demonstration of such a magneto-optical plasmonic switch based on the coupling of plasmons with magneto-optical active materials, in which the response time was estimated to be in the range of microseconds.",2005.10913v1 2020-05-26,First-order ferromagnetic transitions of lanthanide local moments in divalent compounds: An itinerant electron positive feedback mechanism and Fermi surface topological change,"Around discontinuous (first-order) magnetic phase transitions the strong caloric response of materials to the application of small fields is widely studied for the development of solid-state refrigeration. Typically strong magnetostructural coupling drives such transitions and the attendant substantial hysteresis dramatically reduces the cooling performance. In this context we describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant non-hysteretic magnetocaloric effect recently discovered in a Eu$_2$In compound. There is positive feedback between the magnetism of itinerant valence electrons and the ferromagnetic ordering of local $f$-electron moments, which appears as a topological change to the Fermi surface. The origin of this electronic mechanism stems directly from Eu's divalency, which explains the absence of a similar discontinuous transition in Gd$_2$In.",2005.12659v1 2020-06-11,Observation of a strongly ferromagnetic spinor Bose-Einstein condensate,"We report the observation of strongly ferromagnetic $F=1$ spinor Bose-Einstein condensates of $^7$Li atoms. The condensates are generated in an optical dipole trap without using magnetic Feshbach resonances, so that the condensates have internal spin degrees of freedom. Studying the non-equilibrium spin dynamics, we have measured the ferromagnetic spin interaction energy and determined the $s$-wave scattering length difference among total spin $f$ channels to be $a_{f=2}-a_{f=0} =-18(3)$ Bohr radius. This strong collision-channel dependence leads to a large variation in the condensate size with different spin composition. We were able to excite a radial monopole mode after a spin-flip transition between the $|m_F=0\rangle$ and $|m_F=1\rangle$ spin states. From the experiments, we estimated the scattering length ratio $a_{f=2}/a_{f=0}=0.27(6)$, and determined $a_{f=2}$ = 7(2) and $a_{f=0}$ = 25(5) Bohr radii, respectively. The results indicate the spin-dependent interaction energy of our system is as large as 46$\%$ of the condensate chemical potential.",2006.06228v1 2020-06-22,Absence of Spin Hall Magnetoresistance in Pt/(CoNi)n multilayers,"We systematically studied the magnetoresistance effect in a Pt/(CoNi)n multilayer system with perpendicular magnetic anisotropy and the fcc (111) texture. The angular dependence of magnetoresistance, including high-order cosine terms, was observed in a plane perpendicular to the electrical current; this was attributed to the geometrical-size effects caused by crystal symmetry, the ordered arrangement of grains, and the anisotropic interface magnetoresistance effect caused by the breaking of the symmetry at interfaces. Based on the accuracy of our experimental results, the magnitude of spin Hall magnetoresistance (SMR) in Pt/(CoNi)n was expected to be below $1\times10^{-4}$. However, on evaluating the spin Hall angle of $\geq$ 0.07 for Pt using spin-torque ferromagnetic resonance measurements, the theoretical magnitude of SMR in our samples was estimated to exceed $7\times10^{-4}$. This absence of SMR in the experimental results can be explained by the Elliott-Yafet spin relaxation of itinerant electrons in the ferromagnetic metal, which indicates that the boundary conditions of the spin current in the heavy metal/ferromagnetic insulator may not be applicable to all-metallic heterostructures.",2006.12029v1 2020-06-22,Spin-orbit torque induced magnetisation dynamics and switching in CoFeB/Ta/CoFeB system with mixed magnetic anisotropy,"Spin-orbit torque (SOT) induced magnetisation switching in CoFeB/Ta/CoFeB trilayer with two CoFeB layers exhibiting in-plane magnetic anisotropy (IPMA) and perpendicular magnetic anisotropy (PMA) is investigated. Interlayer exchange coupling (IEC), measured using ferromagnetic resonance technique is modified by varying thickness of Ta spacer. The evolution of the IEC leads to different orientation of the magnetic anisotropy axes of two CoFeB layers: for thicker Ta layer where magnetisation prefers antiferromagnetic ordering and for thinner Ta layer where ferromagnetic coupling exists. Magnetisation state of the CoFeB layer exhibiting PMA is controlled by the spin-polarized current originating from SOT in $\mu m$ sized Hall bars. The evolution of the critical SOT current density with Ta thickness is presented, showing an increase with decreasing $t_\mathrm{Ta}$, which coincides with the coercive field dependence. In a narrow range of $t_\mathrm{Ta}$ corresponding to the ferromagnetic IEC, the field-free SOT-induced switching is achieved.",2006.12068v1 2020-06-23,Canted antiferromagnetism in high purity $\mathrm{NaFeF_3}$ prepared by a novel wet-chemical synthesis method,"We report a novel synthesis method for, and structural and magnetic characterization of the fluoroperovskite $\mathrm{NaFeF_3}$. We have developed a wet-chemical method that allows preparation of large volumes of air-sensitive fluoroperovskites with high purity. $\mathrm{NaFeF_3}$ has a N\'eel temperature ($T_N$) of 90 K and a Weiss constant ($\theta$) of -124 K, corresponding to dominant antiferromagnetic interactions. Below $T_N$, a slight difference is observed between zero-field and field cooled samples, indicating spin-canting and weak ferromagnetism. AC magnetometry confirms that weak ferromagnetism is inherent to $\mathrm{NaFeF_3}$ and not due to impurities. From powder neutron diffraction data, we describe the magnetic structure precisely as a weakly canted G-type (magnetic space group $Pn'ma'$). A ferromagnetic component is allowed in $Pn'ma'$, however, this component may be absent in zero magnetic fields and is too small to be confirmed on the basis of powder neutron diffraction data.",2006.12891v2 2020-06-27,A Comprehensible Review: Magnonic Magnetoelectric Coupling in Ferroelectric/ Ferromagnetic Composites,"Composite materials consisting of coupled magnetic and ferroelectric layers hold the promise for new emergent properties such as controlling magnetism with electric fields. Obviously, the interfacial coupling mechanism plays a crucial role and its understanding is the key for exploiting this material class for technological applications. This short review is focused on the magnonic-based magnetoelectric coupling that forms at the interface of a metallic ferromagnet with a ferroelectric insulator. After analyzing the physics behind this coupling, the implication for the magnetic, transport, and optical properties of these composite materials is discussed. Furthermore, examples for the functionality of such interfaces are illustrated by the electric field controlled transport through ferroelectric/ferromagnetic tunnel junctions, the electrically and magnetically controlled optical properties, and the generation of electromagnon solitons for the use as reliable information carriers.",2006.15290v2 2020-06-30,Ferromagnetic contamination of Ultra-Low-Field-NMR sample containers. Quantification of the problem and possible solutions,"The presence of a weak remanence in Ultra-Low-Field (ULF) NMR sample containers is investigated on the basis of proton precession. The high-sensitivity magnetometer used for the NMR detection, enables simultaneously the measurement of the static field produced in the sample proximity by ferromagnetic contaminants. The presence of the latter is studied by high resolution chemical analyses of the surface, based on X-ray fluorescence spectroscopy and secondary ions mass spectroscopy. Methodologies to reduce the contamination are explored and characterized. This study is of relevance in any ULF-NMR experiment, as in the ULF regime spurious ferromagnetism becomes easily a dominant cause of artefacts.",2006.16568v1 2020-08-07,Controllable supercurrent in mesoscopic superconductor-normal metal-ferromagnet crosslike Josephson structures,"A nonmonotonic dependence of the critical Josephson supercurrent on the injection current through a normal metal/ferromagnet weak link from a single domain ferromagnetic strip has been observed experimentally in nanofabricated planar crosslike S-N/F-S Josephson structures. This behavior is explained by 0-pi and pi-0 transitions, which can be caused by the suppression and Zeeman splitting of the induced superconductivity due to interaction between N and F layers, and the injection of spin-polarized current into the weak link. A model considering both effects has been developed. It shows the qualitative agreement between the experimental results and the theoretical model in terms of spectral supercurrent-carrying density of states of S-N/F-S structure and the spin-dependent double-step nonequilibrium quasiparticle distribution.",2008.03217v1 2020-08-14,Cooperation and competition between magnetism and chemisorption,"Chemisorption on ferromagnetic and non-magnetic surfaces is discussed within the Newns-Anderson-Grimley model along with the Stoner model of ferromagnetism. In the case of ferromagnetic surfaces, the adsorption energy is formulated in terms of the change in surface magnetic moments. Using such a formulation, we address the issue of how an adsorbate's binding strength depends on the magnetic moments of the surface and how the adsorption process reduces/enhances the magnetic moments of the surface. Our results indicates a possible adsorption energy scaling relationship in terms surface magnetic moments. In the case of non-magnetic surfaces, we formulate a modified stoner criterion and discuss the condition for the appearance of magnetism due to chemisorption on an otherwise non-magnetic surface.",2008.06316v4 2020-08-15,Spin-crossover induced ferromagnetism and layer stacking-order change in pressurized 2D antiferromagnet MnPS$_3$,"High-pressure properties of MnPS$_3$ are investigated by using the hybrid functional, we report a spin-crossover pressure of 35 GPa consisting with experimental observation (30 GPa), less than half of existing report (63 GPa) using the Hubbard U correction. Interestingly, a spin-crossover induced antiferromagnetism-to-ferromagnetism transition combined with stacking-order change from monoclinic to rhombohedral are founded, and the ferromagnetism origins from the partially occupied $t_{2g}$ orbitals. Different from previous understanding, the Mott metal-insulator transition of MnPS$_3$ does not occur simultaneously with the spin-crossover but in pressurized low-spin phase.",2008.06663v1 2020-08-18,Modulation of Dirac electrons in epitaxial Bi2Se3 ultrathin films on van-der-Waals ferromagnet Cr2Si2Te6,"We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi2Se3 was epitaxially grown on a van-der-Waals ferromagnet Cr2Si2Te6 (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in 6 quintuple-layer (QL) Bi2Se3 on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in 2QL counterpart. Intriguingly, this gap is much larger than those for Bi2Se3 films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small characteristic length and energy scale of the magnetic proximity effect. The present results suggest a crucial role of interfacial coupling for modulating Dirac electrons in topological-insulator hybrids.",2008.07927v1 2020-08-18,Effects of defects in the XY chain with frustrated boundary conditions,"It has been recently proven that new types of bulk, local order can ensue due to frustrated boundary condition, that is, periodic boundary conditions with an odd number of lattice sites and anti-ferromagnetic interactions. For the quantum XY chain in zero external fields, the usual antiferromagnetic order has been found to be replaced either by a mesoscopic ferromagnet or by an incommensurate AFM order. In this work we examine the resilience of these new types of orders against a defect that breaks the translational symmetry of the model. We find that, while a ferromagnetic defect restores the traditional, staggered order, an AFM one stabilizes the incommensurate order. The robustness of the frustrated order to certain kinds of defects paves the way for its experimental observability.",2008.08102v2 2020-08-18,Dark soliton-like magnetic domain walls in a two-dimensional ferromagnetic superfluid,"We report a stable magnetic domain wall in a uniform ferromagnetic spin-1 condensate, characterized by the magnetization having a dark soliton profile with nonvanishing superfluid density. We find exact stationary solutions for a particular ratio of interaction parameters with and without magnetic fields, and develop an accurate analytic solution applicable to the whole ferromagnetic phase. In the absence of magnetic fields, this domain wall relates various distinct solitary excitations in binary condensates through $\textrm{SO}(3)$ spin rotations, which otherwise are unconnected. Remarkably, studying the dynamics of a quasi-two-dimensional (quasi-2D) system we show that standing wave excitations of the domain wall oscillate without decay, being stable against the snake instability. The domain wall is dynamically unstable to modes that cause the magnetization to grow perpendicularly while leaving the domain wall unchanged. Real time dynamics in the presence of white noise reveals that this ""spin twist"" instability does not destroy the topological structure of the magnetic domain wall.",2008.08175v2 2020-08-26,Magnon bands in twisted bilayer honeycomb quantum magnets,"We study the magnon bands of twisted bilayer honeycomb quantum magnets using linear spin wave theory. Although the interlayer coupling can be ferromagnetic or antiferromagnetic, we keep the intralayer one ferromagnetic to avoid possible frustration. For the interlayer ferromagnetic case, we find the magnon bands have similar features with the corresponding electronic energy spectrums. Although the linear dispersions near the Dirac points are preserved in the magnon bands of twisted bilayer magnets, their slopes are graduately reduced with the decrease of the twist angles. On the other hand, the interlayer antiferromagnetic couplings generate quite different magnon spectra. The two single-layered magnon spectra are usually undecoupled due to the opposite orientations of the spins in the two layers. We also develop a low-energy continuous theory for very small twist angles, which has been verified to fit well with the exact tight-binding calculations. Our results may be experimentally observed due to the rapid progress in two-dimensional magnetic materials.",2008.11640v1 2020-08-26,Field Induced Modulated State in the Ferromagnet PrPtAl,"The theory of quantum order-by-disorder (QOBD) explains the formation of modulated magnetic states at the boundary between ferromagnetism and paramagnetism in zero field. PrPtAl has been argued to provide an archetype for this. Here, we report the phase diagram in magnetic field, applied along both the easy $a$-axis and hard $b$-axis. For field aligned to the $b$-axis, we find that the magnetic transition temperatures are suppressed and at low temperature there is a single modulated fan state, separating an easy $a$-axis ferromagnetic state from a field polarised state. This fan state is well explained with the QOBD theory in the presence of anisotropy and field. Experimental evidence supporting the QOBD explanation is provided by the large increase in the $T^2$ coefficient of the resistivity and direct detection of enhanced magnetic fluctuations with inelastic neutron scattering, across the field range spanned by the fan state. This shows that the QOBD mechanism can explain field induced modulated states that persist to very low temperature.",2008.11644v2 2020-08-27,Lattice strain accommodation and absence of pre-transition phases in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$,"The stoichiometric Ni$_{50}$Mn$_{25}$In$_{25}$ Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ alloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value of $x$ = 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state.",2008.11982v1 2020-08-28,High-Curie-temperature ferromagnetism in bilayer CrI3 on bulk semiconducting substrates,"Two-dimensional (2D) ferromagnetic (FM) semiconductors with high Curie temperature have long been pursued for electronic and spintronic applications. Here we provide a general strategy to achieve robust FM state in bilayer CrI3 of the monoclinic stacking, which intrinsically has interlayer antiferromagnetic (AFM) order and weak in-plane FM coupling. We showed that the proximity effect from bulk semiconducting substrates induces electronic doping and significantly increases the FM nearest-neighbor exchange for bilayer CrI3, leading to the AFM-to-FM transition for the interlayer spin configuration as well as enhanced intralayer FM coupling. By first-principles calculations and Monte Carlo simulations, bulk and 2D semiconductors providing different interaction strengths from strong covalent bonding to weak van der Waals (vdW) interaction with CrI3 are compared to thoroughly address the substrate effect on magnetic behavior and Curie temperature of bilayer CrI3. These theoretical results offer a facile route for direct synthesis of 2D ferromagnets on proper semiconducting substrates to achieve high Curie temperature for device implementation.",2008.12519v1 2020-09-02,Effect of the Optical Pumping and Magnetic Field on the States of Phase Separation Domains in Eu_{0.8}Cr_{0.2}Mn_2O_5,"The effect of optical pumping and applied magnetic field on the characteristics of ferromagnetic layers in one-dimensional superlattices is studied. At low enough temperatures, these layers correspond to phase separation domains in RMn_2O_5 and R_{0.8}Ce_{0.2}Mn_2O_5 multiferroics. The formation of such domains occurs owing to the charge ordering of Mn^{3+} and Mn^{4+} ions and to the finite probability for e_g electrons to tunnel between these pairs of ions. The volume occupied by such superlattices is rather small, and they can be treated as isolated ferromagnetic semiconductor heterostructures, spontaneously formed in the host crystal. The sequences of ferromagnetic resonances related to the superlattice layers in Eu_{0.8}Ce_{0.2}Mn_2O_5 are studied. The characteristics of these resonances give information on the properties of such layers. For the first time, it is demonstrated that the optical pumping gives rise to a new metastable state of superlattices, which can be recovered by the magnetic field cycling to the state existing before the optical pumping. It is found that the superlattices recovered by the magnetic field exist up to temperatures higher than those in as-grown crystals.",2009.01057v1 2020-09-08,Ferromagnetism in $d$-dimensional SU($n$) Hubbard models with nearly flat bands,"We present rigorous results for the SU($n$) Fermi-Hubbard models with finite-range hopping in $d$ ($\ge 2$) dimensions. The models are defined on a class of decorated lattices. We first study the models with flat bands at the bottom of the single-particle spectrum and prove that the ground states exhibit SU($n$) ferromagnetism when the number of particles is equal to the number of unit cells. We then perturb the models by adding particular hopping terms and make the bottom bands dispersive. Under the same filling condition, it is proved that the ground states remain SU($n$) ferromagnetic when the bottom bands are sufficiently flat and the Coulomb repulsion is sufficiently large.",2009.03580v3 2020-09-28,Tunable SSH model in ferromagnetic systems,"It is well known that the topology of Su-Schrieffer-Heeger(SSH) model, which belongs to AIII symmetry class, is protected by chiral symmetry. In this article, instead of chiral symmetry, we constrain the bulk Hamiltonian by a magnetic point group symmetry, which can be generated by a unitary symmetry and an anti-unitary symmetry. Under these symmetries, a four-band model can be block diagonalized into two 2-band models and each 2-band model is analogous to an SSH model. As the two 2-band models are individual, we call the four-band model double independent SSH (DISSH) model. Interestingly, since the symmetry requirements of DISSH model can be fulfilled in ferromagnetic systems, the discovery in this manuscript extends SSH model into ferromagnetic systems. Furthermore, we presented an example of DISSH model with a set of reasonable parameters to show that it is possible to manipulate the topological phase of DISSH model by tuning the magnetic moment in experiment.",2009.13034v2 2020-10-01,Prediction of two-dimensional ferromagnetic ferroelectric VOF$_2$ monolayer,"Nowadays, designing and searching for materials with multiple functional characteristics are the key to achieving high-performance electronic devices. Among many candidates, two-dimensional multiferroic materials have great potential to be applied in highly-integrated magnetoelectric devices, such as high-density non-volatile memories. Here, we predict a two-dimensional material VOF2 monolayer to possess intrinsic ferroelectric and ferromagnetic properties. The VOF$_2$ monolayer own the largest in-plane ferroelectric polarization ($332$ pC/m) in the family of VO$X_2$ ($X$: halogen) oxyhalides. Different from other VO$X_2$ monolayers whose magnetic ground states are antiferromagnetic or noncollinear spiral textures, the VOF$_2$ monolayer owns a robust ferromagnetic ground state, which is rare but highly desirable. Our theoretical prediction provides a good candidate and starting point for the further pursuit of more two-dimensional multiferroic materials with high-performance magnetoelectricity.",2010.00234v1 2020-10-05,Exact Solution to Sodium-Iridate-BCS-Hubbard Model along the Symmetric Line: Non-trivial topology in the ferromagnetic order,"We study the sodium-iridates model on the honeycomb lattice with both BCS pairing potential and Hubbard interaction term. It is shown that this model can be exactly solved with appropriate choices of amplitude of pairing gaps, where the interacting terms are transformed to external field terms. The band structures of these exact solutions on both torus and cylinder geometry are discussed in great details. It is found that the ground state assumes an anti-ferromagnetic configuration, which breaks the time reversal symmetry spontaneously and renders the superconductor topologically trivial. On the other hand, the nontrivial topology is preserved with ferromagnetic configuration and can be characterized by the isospin Chern number.",2010.01754v1 2020-10-06,"Inverse Spin Hall effect in ferromagnetic nanomagnet. Dependencies on magnetic field, current and current polarity","The measured Hall angle in a ferromagnetic nanomagnet shows a substantial non-linear dependence on an external magnetic field, which cannot be explained by adopted mechanisms of the Ordinary and Anomalous (AHE) Hall effects implying a linear plus constant dependence on the external magnetic field. We suggest that there is an additional non-linear contribution from the Inverse Spin Hall effect (ISHE). The significant contribution of ISHE in a ferromagnet is supported by perfect agreement of experiment with a phenomenological theory of ISHE. We observed different dependencies of AHE and ISHE on current suggesting their different thermal dependencies. We also observe dependence of the Hall angle of the current polarity which is due to the Spin Hall effect.",2010.02409v1 2020-10-06,Electronic correlations in the van der Waals ferromagnet Fe$_3$GeTe$_2$ revealed by its charge dynamics,"The layered van der Waals ferromagnetic Fe$_3$GeTe$_2$ harbours an unconventional interplay between topology and magnetism, leading to a large anomalous Hall conductivity at low temperatures. Here, we investigate the temperature dependence of its charge dynamics and reveal that upon entering the ferromagnetic state at $T_C \sim 200$ K and further lowering the temperature there is the onset of a gradual spectral weight reshuffling from the mid-infrared range towards far- as well as near-infrared frequencies. This two-fold spectral weight transfer indicates the important role of the Hund's coupling as primary source for electronic correlations and signals an incoherent-coherent crossover at low temperatures. Our findings also convey the electronic environment, based on nodal-line topological states, favouring the large anomalous Hall conductivity.",2010.02662v1 2020-10-13,Effects of spin-orbit torque on the ferromagnetic and exchange spin wave modes in ferrimagnetic CoGd alloy,"We use micro-focus Brillouin light scattering spectroscopy to study the effects of spin-orbit torque on thermal spin waves in almost angular-momentum compensated ferrimagnetic CoGd alloy films. The spin-orbit torque is produced by the electric current flowing in the Pt layer adjacent to CoGd. Both the ferromagnetic and the exchange modes are detected in our measurements. The intensity and the linewidth of the ferromagnetic mode are modified by the spin-orbit torque. In contrast, the properties of the exchange mode are unaffected by the spin-orbit torque. We also find that the frequencies and the linewidths of both modes are significantly modified by Joule heating, due to the strong temperature dependence of the magnetic properties of CoGd in the vicinity of angular momentum compensation point. Our results provide insight into the mechanisms that can enable the implementation of sub-THz magnetic nano-oscillators based on ferrimagnetic materials, as well as related effects in antiferromagnets.",2010.06615v1 2020-10-18,Light-Induced Static Magnetization: Nonlinear Edelstein Effect,"We theoretically and computationally demonstrate that static magnetization can be generated under light illumination via nonlinear Edelstein effect (NLEE). NLEE is applicable to semiconductors under both linearly and circularly polarized light, and there are no constraints from either spatial inversion or time-reversal symmetry. Remarkably, magnetization can be induced under linearly polarized light in nonmagnetic materials. With ab initio calculations, we reveal several prominent features of NLEE. We find that the orbital contributions can be significantly greater than the spin contributions. And magnetization with various orderings, including anti-ferromagnetic, ferromagnetic, etc., are all realizable with NLEE, which may facilitate many applications, such as unveiling hidden physical effects, creating a spatially varying magnetization, or manipulating the magnetization of anti-ferromagnetic materials. The relationship between NLEE and other magneto-optic effects, including the inverse Faraday effect and inverse Cotton-Mouton effect, is also discussed.",2010.09143v2 2020-10-25,Criticality-enhanced quantum sensing in ferromagnetic Bose-Einstein condensates: role of readout measurement and detection noise,"We theoretically investigate estimation of the control parameter in a ferromagnetic Bose-Einstein condensate near second order quantum phase transitions. We quantify sensitivity by quantum and classical Fisher information and using the error-propagation formula. For these different metrics, we find the same, beyond-standard-quantum-limit (SQL) scaling with atom number near critical points, and SQL scaling away from critical points. We find that both depletion of the $m_f=0$ Zeeman sub-level and transverse magnetization provide signals of sufficient quality to saturate the sensitivity scaling. To explore the effect of experimental imperfections, we study the scaling around criticality at nonzero temperature and with nonzero detection noise. Our results suggest the feasibility of sub-SQL sensing in ferromagnetic condensates with current experimental capabilities.",2010.13133v1 2020-10-30,Characterization of microscopic ferromagnetic defects in thin films using magnetic microscope based on Nitrogen-Vacancy centres,"In this work we present results acquired by applying magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal for characterization of magnetic thin films defects. We used the constructed wide-field magnetic microscope for measurements of two kinds of magnetic defects in thin films. One family of defects under study was a result of non-optimal thin film growth conditions. The magnetic field maps of several regions of the thin films created under very similar conditions to previously published research revealed microscopic impurity islands of ferromagnetic defects, that potentially could disturb the magnetic properties of the surface. The second part of the measurements was dedicated to defects created post deposition - mechanical defects introduced in ferromagnetic thin films. In both cases, the measurements identify the magnetic field amplitude and distribution of the magnetic defects. In addition, the magnetic field maps were correlated with the corresponding optical images. As this method has great potential for quality control of different stages of magnetic thin film manufacturing process and it can rival other widely used measurement techniques, we also propose solutions for the optimization of the device in the perspective of high throughput.",2010.16197v1 2020-11-03,Symmetry breaking induced magnon-magnon coupling in synthetic antiferromagnets,"We propose a general theory of microwave absorption spectroscopy for symmetry-breaking synthetic antiferromagnets (SAFs). Generally, inhomogeneity or different thickness of the two ferromagnetic sublayers of a SAF results in the intrinsic symmetry breaking, while out-of-plane components of dc magnetic fields lead to the extrinsic one. The broken symmetry of SAFs excludes the original symmetry-protected crossing between pure in-phase and out-of-phase resonance modes with opposite parity. Alternatively, new frequency branches become hybridization of original bare modes in terms of symmetry-breaking-induced magnon-magnon coupling, which results in an indirect gap in ferromagnetic resonance frequencies. Also, the dependence of gap width on the degree of symmetry breaking for several typical cases are presented and compared with existing experiments. Our theory provides a simple but physical understanding on the rich structure of ferromagnetic resonance spectra for asymmetric SAFs.",2011.01583v1 2020-11-05,Avoided ferromagnetic quantum critical point in pressurized La$_5$Co$_2$Ge$_3$,"We present the pressure-temperature phase diagram La$_5$Co$_2$Ge$_3$ up to $\sim$ 5\,GPa, which was constructed from magnetization, resistivity and specific heat measurements. At ambient pressure, La$_5$Co$_2$Ge$_3$ is an itinerant ferromagnet with a Curie temperature $T_\textrm C\sim$ 4\,K. Upon increasing pressure up to $\sim$ 1.7\,GPa, $T_\textrm C$ is suppressed down to $\sim$ 3\,K. Upon further increasing pressure, our results suggest that La$_5$Co$_2$Ge$_3$ enters a different low-temperature ground state. The corresponding transition temperature, $T^*$, has a nonmonotonic pressure dependence up to $\sim$ 5\,GPa. Our results demonstrate that the ferromagnetic quantum critical point in La$_5$Co$_2$Ge$_3$ is avoided by the appearance of a different, likely magnetically ordered state that has an antiferromagnetic component.",2011.03086v1 2020-11-11,Characterization of room-temperature in-plane magnetization in thin flakes of CrTe$_2$ with a single spin magnetometer,"We demonstrate room-temperature ferromagnetism with in-plane magnetic anisotropy in thin flakes of the CrTe$_2$ van der Waals ferromagnet. Using quantitative magnetic imaging with a single spin magnetometer based on a nitrogen-vacancy defect in diamond, we infer a room-temperature in-plane magnetization in the range of $M\sim 27$ kA/m for flakes with thicknesses down to $20$ nm. In addition, our measurements indicate that the orientation of the magnetization is not determined solely by shape anisotropy in micron-sized CrTe$_2$ flakes, which suggest the existence of a non-negligible magnetocrystalline anisotropy. These results make CrTe$_2$ a unique system in the growing family of van der Waals ferromagnets, as it is the only material platform known to date which offers an intrinsic in-plane magnetization and a Curie temperature above $300$ K in thin flakes.",2011.05722v2 2020-11-12,Picoscale Magnetoelasticity Governs Heterogeneous Magnetic Domains in a Noncentrosymmetric Ferromagnetic Weyl Semimetal,"Magnetic Weyl semimetals are predicted to host emergent electromagnetic fields at heterogeneous strained phases or at the magnetic domain walls. Tunability and control of the topological and magnetic properties is crucial for revealing these phenomena, which are not well understood or fully realized yet. Here, we use a scanning SQUID microscope to image spontaneous magnetization and magnetic susceptibility of CeAlSi, a noncentrosymmetric ferromagnetic Weyl semimetal candidate. We observe large metastable domains alongside stable ferromagnetic domains. The metastable domains most likely embody a type of frustrated or glassy magnetic phase, with excitations that may be of an emergent and exotic nature. We find evidence that the heterogeneity of the two types of domains arises from magnetoelastic or magnetostriction effects. We show how these domains form, how they interact, and how they can be manipulated or stabilized with estimated lattice strains on picometer levels. CeAlSi is a frontier material for straintronics in correlated topological systems.",2011.06168v1 2020-11-20,Hund's coupling-assisted ferromagnetic percolation transition in a multiorbital flat band,"By connecting Hund's physics with flat band physics, we establish an exact result for studying ferromagnetism in a multiorbital system. We consider a two-layer model consisting of a $p_x$, $p_y$-orbital honeycomb lattice layer and an $f$-orbital triangular lattice layer with sites aligned with the centers of the honeycomb plaquettes. The system features a flat band that admits a percolation representation for an appropriate chemical potential difference between the two layers. In this representation, the ground state space is spanned by maximum-spin clusters of localized single-particle states, and averaging over the ground states yields a correlated percolation problem with weights due to the spin degeneracy of the clusters. A paramagnetic-ferromagnetic transition occurs as the band approaches half filling and the ground states become dominated by states with a large maximum-spin cluster, as shown by Monte Carlo simulation.",2011.10554v4 2020-11-24,Evidence of interfacial asymmetric spin scattering at ferromagnet-Pt interfaces,"We measure the spin-charge interconversion by the spin Hall effect in ferromagnetic/Pt nanodevices. The extracted effective spin Hall angles (SHAs) of Pt evolve drastically with the ferromagnetic (FM) materials (CoFe, Co, and NiFe), when assuming transparent interfaces and a bulk origin of the spin injection/detection by the FM elements. By carefully measuring the interface resistance, we show that it is quite large and cannot be neglected. We then evidence that the spin injection/detection at the FM/Pt interfaces are dominated by the spin polarization of the interfaces. We show that interfacial asymmetric spin scattering becomes the driving mechanism of the spin injection in our samples.",2011.12207v2 2020-11-27,d0 Ferromagnetism in Mg-doped Rutile TiO2 Nanoparticles,"In a quest of enriching the area of d0 magnetism in oxide materials, we have undertaken to study Mg-doped TiO2 compounds. The Ti1-xMgxO2 (x=0, 0.02, 0.04 and 0.06) nanoparticles were prepared by solid-state reaction route. The X-ray diffractions (XRD) patterns of these samples indicate single phase of tetragonal rutile-structure of TiO2. The refinement of the XRD patterns reveals no change in the crystallographic lattice parameters in comparison to pure TiO2 upon Mg doping and it indicates that Mg2+ ions do not enter core grains and form core/shell structure. SEM observations reveal the uniform morphology with nanometric grains in the range of 150-200 nm. The measurement of magnetic properties of these compounds indicates that pure TiO2 and Ti0.98Mg0.02 compounds exhibit paramagnetic behavior and Ti0.96Mg0.04 compound exhibits ferromagnetic (FM) phase superimposed with the dominating paramagnetic phase. However, Ti0.94Mg0.06 compound exhibits ferromagnetic to paramagnetic transition with FM transition temperature of 180.2 K. The measurements of zero field and field cooled magnetization data indicate low temperature magnetic irreversibility for x=0.06 sample and it was attributed to the competing AFM (core) and the FM (shell) interactions. The measurement of hysteresis curves at various temperatures indicates domain wall pinning and an exchange-bias behavior.",2011.13718v1 2021-02-05,Penetration depth of Cooper pairs in the IrMn antiferromagnet,"Suppression of superconductivity due to the proximity effect between a superconductor and a ferromagnet can be partially alleviated when a Cooper pair simultaneously samples different directions of the short-range exchange field. The superconductor's critical temperature, TC, is therefore expected to partially recover when the ferromagnet is in a multi-domain state, as opposed to a single-domain state. Here, we discuss series of experiments performed with ferromagnet(Pt/Co)/spacer(IrMn and Pt)/superconductor(NbN) heterostructures. By tuning the various parameters in play, e.g., superconducting coherence length-to-thicknesses ratio, and domain sizes, we obtained up to 10 percent recovery of the superconducting critical temperature {\Delta}TC/TC. This large-scale recovery made novel investigations possible. In particular, from the spacer thickness-dependence of {\Delta}TC/TC, it was possible to deduce the characteristic length for Cooper pair penetration in an IrMn antiferromagnet. This information is crucial for electronic transport, and up to now has been difficult to access experimentally for antiferromagnets.",2102.03425v1 2021-02-07,Spinterface Induced Modification in Magnetic Properties in Co40Fe40B20/Fullerene Bilayers,"Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel properties due to the formation of the spinterface at the interface. Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the interface when it is placed next to a ferromagnet (FM) such as Fe or Co. Formation of spinterface occurs due to the orbital hybridization and spin polarized charge transfer at the interface. In this work, we have demonstrated that one can enhance the magnetic anisotropy of the low Gilbert damping alloy CoFeB by introducing a C60 layer. We have shown that anisotropy increases by increasing the thickness of C60 which might be a result of the formation of spinterface. However, the magnetic domain structure remains same in the bilayer samples as compared to the reference CoFeB film.",2102.03914v4 2021-02-11,Phase transitions in superconductor/ferromagnet bilayer driven by spontaneous supercurrents,"We investigate superconducting phase transition in superconductor(S)/ferromagnet(F) bilayer with Rasba spin-orbit interaction at S/F interface. This spin-orbit coupling produces spontaneous supercurrents flowing inside the atomic-thickness region near the interface, which are compensated by the screening Meissner currents [S. Mironov and A. Buzdin, Phys. Rev. Lett \textbf{118}, 077001 (2017)]. In the case of thin superconducting film the emergence of the spontaneous surface currents causes the increase of the superconducting critical temperature and we calculate the actual value of the critical temperature shift. We also show that in the case of type-I superconducting film this phase transition can be of the first order. In the external magnetic field the critical temperature depends on the relative orientation of the external magnetic field and the exchange field in the ferromagnet. Also we predict the in-plane anisotropy of the critical current which may open an alternative way for the experimental observation of the spontaneous supercurrents generated by the SOC.",2102.05885v1 2021-02-12,"Carrier-induced Phase Transition in Metal Dichlorides XCl$_{2}$ (X: Fe, Co, and Ni)","We investigated the ground state of monolayer 1T-XCl$_{2}$ (X: Fe, Co, and Ni) using the generalized Bloch theorem, which can generate ferromagnetic, spiral, and antiferromagnetic states. Each state was represented by a unique spiral vector that arranges the magnetic moment of magnetic atom in the primitive unit cell. We found the ferromagnetic ground state for the FeCl$_{2}$ and NiCl$_{2}$ while the spiral ground state appears for the CoCl$_{2}$. We also showed that the ground state depends sensitively on the lattice constant. When the hole-electron doping was taken into account, we found the phase transition, which involves the ferromagnetic, spiral, and antiferromagnetic states, for all the systems. Since the spin-spin interaction in the monolayer metal dichlorides is influenced by the competition between the direct exchange and the superexchange, we justify that the carrier concentration determines which interaction should dominate.",2102.06478v1 2021-02-14,Destabilization of ferromagnetism by frustration and realization of a nonmagnetic Mott transition in the quarter-filled two-orbital Hubbard model,"The two-orbital Hubbard model on a square lattice at quarter filling (electron number per site $n=1$) is investigated by the variational Monte Carlo method. For the variational wave function, we include short-range doublon-holon binding factors. We find that the energy of this wave function is lower than that of the density-density Jastrow wave function partially including long-range correlations used in a previous study. We introduce frustration to the model by the next-nearest-neighbor hopping $t'$ in addition to the nearest-neighbor hopping $t$. For $t'=0$, a ferromagnetic state with staggered orbital order occurs by increasing the Coulomb interaction $U$ before the Mott transition takes place. By increasing $t'$, the region of this ferromagnetic phase shrinks, and the Mott transition without magnetic order occurs.",2102.07262v1 2021-02-15,Tunable moire spinons in magnetically encapsulated twisted van der Waals quantum spin-liquids,"Quantum spin-liquid van der Waals magnets such as TaS$_2$, TaSe$_2$, and RuCl$_3$ provide a natural platform to explore new exotic phenomena associated with spinon physics, whose properties can be controlled by exchange proximity with ferromagnetic insulators such as CrBr$_3$. Here we put forward a twisted van der Waals heterostructure based on a quantum spin-liquid bilayer encapsulated between ferromagnetic insulators. We demonstrate the emergence of spinon flat bands and topological spinon states in such heterostructure, where the emergence of a topological gap is driven by the twist. We further show that the spinon bandstructure can be controlled via exchange proximity effect to the ferromagnetic leads. We finally show how by combining small magnetic fields with tunneling spectroscopy, magnetically encapsulated heterostructures provide a way of characterizing the nature of the quantum spin-liquid state. Our results put forward twisted quantum spin-liquid bilayers as potential platforms for exotic moire spinon phenomena, demonstrating the versatility of magnetic van der Waals heterostructures.",2102.07484v1 2021-02-16,Holographic paramagnetic-ferromagnetic phase transition of Power-Maxwell-Gauss-Bonnet black holes,"Based on the shooting method, we numerically investigate the properties of holographic paramagnetism-ferromagnetism phase transition in the presence of higher order Gauss-Bonnet (\emph{GB}) correction terms on the gravity side. On the matter field side, however, we consider the effects of the Power-Maxwell (\emph{PM}) nonlinear electrodynamics on the phase transition of this system. For this purpose, we introduce a massive $2-$form coupled to \emph{PM} field, and neglect the effects of $2-$form fields and gauge field on the background geometry. We observe that increasing the strength of both the power parameter $q$ and \emph{GB} coupling constant $\alpha$ decrease the critical temperature of the holographic model, and lead to the harder formation of magnetic moment in the black hole background. Interestingly, we find out that at low temperatures, the spontaneous magnetization and ferromagnetic phase transition happen in the absence of external magnetic field. In this case, the critical exponent for magnetic moment has the mean field value, $1/2$, regardless of the values of $q$ and $\alpha$. In the presence of external magnetic field, however, the magnetic susceptibility satisfies the Curie-Weiss law.",2102.08053v2 2021-02-16,Metamagnetic phase transition in ferromagnetic superconductor URhGe,"Ferromagnetic superconductor URhGe has orthorhombic structure and possesses spontaneous magnetisation along the c-axis. Magnetic field directed along the $b$-axis suppresses ferromagnetism in $c$-direction and leads to a metamagnetic transition into polarised paramagnetic state in the $b$-direction. The theory of these phenomena based on the specific magnetic anisotropy of this material in $(b,c)$ plane is given. Line of the first order metamagnetic transition ends at a critical point. The Van der Waals - type description of behaviour of physical properties near this point is developed. The triplet superconducting state destroyed by orbital effect is recreated in vicinity of the transition. It is shown that the reentrance of superconductivity is caused by the sharp increase of magnetic susceptibility in $b$ direction near the metamagnetic transition. The specific behaviour of the upper critical field in direction of spontaneous magnetisation in UCoGe and in UGe$_2$ related to the field dependence of magnetic susceptibility is discussed.",2102.08091v2 2021-02-16,Spin-glass behavior in Shastry-Sutherland lattice of Tm$_\textbf{2}$Cu$_\textbf{2}$In,"The spin-glass behavior in the ferromagnetic phase of $\mathrm{Tm_2Cu_2In}$ was investigated by dc-, ac-magnetization, and non-equilibrium dynamics characterizations. Arc-melted polycrystalline compound of $\mathrm{Tm_2Cu_2In}$ crystallizes in $\mathrm{Mo_2FeB_2}$-type tetragonal structure (space group $P_4/mbm$). The temperature variation dc-magnetization exhibits a ferromagnetic behavior with Curie temperature $T_C$ = 32.5 K. The zero-field cooled (ZFC) and field cooled (FC) magnetic curves show a thermomagnetic irreversible behavior below $T_C$. The field dependence of irreversible temperature follows Almeida-Thouless line. The frequency and ac-driven field-dependent anomalies in ac-susceptibility results indicate the existence of spin-glass state in $\mathrm{Tm_2Cu_2In}$. The time dependence of magnetization further supports the spin-glass behavior observed in $\mathrm{Tm_2Cu_2In}$. The frequency dependence of the freezing temperature in the real part of ac-susceptibility has been analyzed on the basis power-law divergence and Vogel-Fulcher law. The obtained results revealed that $\mathrm{Tm_2Cu_2In}$ belongs to a ferromagnetic canonical spin-glass class of compounds.",2102.08230v2 2021-02-23,Strain glass versus antisite disorder induced ferromagnetic state in Fe doped Ni-Mn-In Heusler martensites,"Fe doping in Ni$_2$Mn$_{1.5}$In$_{0.5}$ results in suppression of the martensitic phase via two contrasting routes. In Ni$_2$Mn$_{1.5-x}$Fe$_{x}$In$_{0.5}$, the martensitic phase is converted to a strain glassy phase, while in Ni$_{2-y}$Fe$_y$Mn$_{1.5}$In$_{0.5}$, a cubic ferromagnetic phase results at the expense of the martensite. Careful studies of magnetic and structural properties reveal the presence of the impurity $\gamma -$(Fe,Ni) phase as the reason for the emergence of non-ergodic strain glassy phase when Fe is sought to be doped at Y/Z (Mn) sites of X$_2$YZ Heusler alloy. Whereas attempts to dope Fe in the X (Ni) sublattice result in an A2 type antisite disorder that promotes a ferromagnetic ground state.",2102.11611v1 2021-02-26,Nematic versus ferromagnetic shells: new insights in curvature-induced effects,"Within the framework of continuum theory, we draw a parallel between ferromagnetic materials and nematic liquid crystals confined on curved surfaces, which are both characterized by local interaction and anchoring potentials. We show that the extrinsic curvature of the shell combined with the out-of-plane component of the director field gives rise to chirality effects. This interplay produces an effective energy term reminiscent of the chiral term in cholesteric liquid crystals, with the curvature tensor acting as a sort of anisotropic helicity. We discuss also how the different nature of the order parameter, a vector in ferromagnets and a tensor in nematics, yields different textures on surfaces with the same topology as the sphere. In particular, we show that the extrinsic curvature governs the ground state configuration on a nematic spherical shell, favouring two antipodal disclinations of charge +1 on small particles and four $+1/2$ disclinations of charge located at the vertices of a square inscribed in a great circle on larger particles.",2102.13497v2 2021-02-27,Covalent 2D Cr$_2$Te$_3$ ferromagnet,"To broaden the scope of van der Waals 2D magnets, we report the synthesis and magnetism of covalent 2D magnetic Cr$_2$Te$_3$ with a thickness down to one-unit-cell. The 2D Cr$_2$Te$_3$ crystals exhibit robust ferromagnetism with a Curie temperature of 180 K, a large perpendicular anisotropy of 7*105 J m-3, and a high coercivity of ~ 4.6 kG at 20 K. First-principles calculations further show a transition from canted to collinear ferromagnetism, a transition from perpendicular to in-plane anisotropy, and emergent half-metallic behavior in atomically-thin Cr$_2$Te$_3$, suggesting its potential application for injecting carriers with high spin polarization into spintronic devices.",2103.00339v1 2021-03-01,Electron-induced nuclear magnetic ordering in n-type semiconductors,"Nuclear magnetism in n-doped semiconductors with positive hyperfine constant is revisited. Two kinds of nuclear magnetic ordering can be induced by resident electrons in a deeply cooled nuclear spin system. At positive nuclear spin temperature below a critical value, randomly oriented nuclear spin polarons similar to that predicted by I. Merkulov [I. Merkulov, Physics of the Solid State 40, 930 (1998)] should emerge. These polarons are oriented randomly and within each polaron nuclear and electron spins are aligned antiferromagnetically. At negative nuclear spin temperature below a critical value we predict another type of magnetic ordering - dynamically induced nuclear ferromagnet. This is a long-range ferromagnetically ordered state involving both electrons and nuclei. It can form if electron spin relaxation is dominated by the hyperfine coupling, rather than by the spin-orbit interaction. Application of the theory to the n-doped GaAs suggests that the ferromagnetic order may be reached at experimentally achievable nuclear spin temperature $\Theta_N \approx 0.5$ $\mu$K and lattice temperature $T_L \approx 5$ K.",2103.01120v1 2021-03-07,Robust noncoplanar magnetism in band filling-tuned (Nd$_{1-x}$Ca$_x$)$_2$Mo$_2$O$_7$,"In the metallic pyrochlore Nd$_2$Mo$_2$O$_7$, the conducting Molybdenum sublattice adopts canted, yet nearly collinear ferromagnetic order with nonzero scalar spin chirality. The chemical potential may be controlled by replacing Nd$^{3+}$ with Ca$^{2+}$, while introducing only minimal additional disorder to the conducting states. Here, we demonstrate the stability of the canted ferromagnetic state, including the tilting angle of Molybdenum spins, in (Nd$_{1-x}$Ca$_{x}$)$_2$Mo$_2$O$_7$ (NCMO) with $x\le 0.15$ using magnetic susceptibility measurements. Mo-Mo and Mo-Nd magnetic couplings both change sign above $x=0.22$, where the canted ferromagnetic state gives way to a spin-glass metallic region. Contributions to the Curie-Weiss law from two magnetic sublattices are separated systematically.",2103.04295v1 2021-03-14,Electron-beam Floating-zone Refined UCoGe,"The interplay between unconventional superconductivity and quantum critical ferromagnetism in the U-Ge compounds represents an open problem in strongly correlated electron systems. Sample quality can have a strong influence on both of these ordered states in the compound UCoGe, as is true for most unconventional superconductors. We report results of a new approach at UCoGe crystal growth using a floating-zone method with potential for improvements of sample quality and size as compared with traditional means such as Czochralski growth. Single crystals of the ferromagnetic superconductor UCoGe were produced using an ultra-high vacuum electron-beam floating-zone refining technique. Annealed single crystals show well-defined signatures of bulk ferromagnetism and superconductivity at $T_c \sim$2.6 K and $T_s \sim$0.55 K, respectively, in the resistivity and heat capacity. Scanning electron microscopy of samples with different surface treatments shows evidence of an off-stoichiometric uranium rich phase of UCoGe collected in cracks and voids that might be limiting sample quality.",2103.08000v1 2021-03-16,Large Enhancement of Ferro-Magnetism under Collective Strong Coupling of YBCO Nanoparticles,"Light-matter strong coupling in the vacuum limit has been shown to enhance material properties over the past decade. Oxide nanoparticles are known to exhibit weak ferromagnetism due to vacancies in the lattice. Here we report the 700-fold enhancement of the ferromagnetism of YBa$_2$Cu$_3$O$_{7-x}$ nanoparticles under cooperative strong coupling at room temperature. The magnetic moment reaches 0.90 $\mu_{\rm B}$/mol, and with such a high value, it competes with YBa$_2$Cu$_3$O$_{7-x}$ superconductivity at low temperature. This strong ferromagnetism at room temperature suggest that strong coupling is a new tool for the development of next generations of magnetic and spintronic nanodevices.",2103.09331v2 2021-03-24,PDE/statistical mechanics duality: relation between Guerra's interpolated $p$-spin ferromagnets and the Burgers hierarchy,"We examine the duality relating the equilibrium dynamics of the mean-field $p$-spin ferromagnets at finite size in the Guerra's interpolation scheme and the Burgers hierarchy. In particular, we prove that - for fixed $p$ - the expectation value of the order parameter on the first side w.r.t. the generalized partition function satisfies the $p-1$-th element in the aforementioned class of nonlinear equations. In the light of this duality, we interpret the phase transitions in the thermodynamic limit of the statistical mechanics model with the development of shock waves in the PDE side. We also obtain the solutions for the $p$-spin ferromagnets at fixed $N$, allowing us to easily generate specific solutions of the corresponding equation in the Burgers hierarchy. Finally, we obtain an effective description of the finite $N$ equilibrium dynamics of the $p=2$ model with some standard tools in PDE side.",2103.13116v1 2021-04-08,"Comment on ""Unconventional enhancement of ferromagnetic interactions in Cd-doped GdFe2Zn20 single crystals studied by ESR and 57Fe Mossbauer spectroscopies""","In the recent publication, Phys. Rev. B 102, 144420 (2020), Cabrera-Baez et al. present a study of the effects of Cd-substitution for Zn in the ferromagnetic compound GdFe2Zn20. As part of this paper, they claim that for GdFe2Zn18.6Cd1.4 the effective moment of Gd is reduced by 25% and the saturated moment of Gd is reduced by over 40%. We regrew representative members of the GdFe2Zn(20-x)Cdx series and did not find any such reductions. In addition, we measured several crystals from the growth batch that was used by Cabrera-Baez et al. and did not see such reductions. Although there is a modest increase in TC with Cd substitution, there is no significant change in the Gd effective moment or the saturated moment associated with the low temperature ferromagnetic state.",2104.03770v1 2021-04-12,Controllable Spin Current in van der Waals Ferromagnet Fe3GeTe2,"The control of spin current is pivotal for spintronic applications, especially for spin-orbit torque devices. Spin Hall effect (SHE) is a prevalent method to generate spin current. However, it is difficult to manipulate its spin polarization in nonmagnet. Recently, the discovery of spin current in ferromagnet offers opportunity to realize the manipulation. In the present work, the spin current in van der Waals ferromagnet Fe3GeTe2 (FGT) with varying magnetization is theoretically investigated. It has been observed that the spin current in FGT presents the nonlinear behavior with respect to magnetization. The in-plane and out-of-plane spin polarization emerges simultaneously, and the bilayer FGT can even exhibit arbitrary spin polarization thanks to the reduced symmetry. More intriguingly, the correlation between anomalous Hall effect (AHE) and spin anomalous Hall effect (SAHE) has been interpreted from the aspect of Berry curvature. This work illustrates that the interplay of symmetry and magnetism can effectively control the magnitude and spin polarization of the spin current, providing a practical method to realize exotic spin-orbit torques.",2104.05308v2 2021-04-15,Defect Engineered Room-Temperature Ferromagnetism in Quasi-Two-Dimensional Nitrided CoTa2O6,"Thermal ammonolysis of quasi-two-dimensional (quasi-2D) CoTa2O6 yields the O2-/N3- and anionic vacancy ordered Co2+Ta5+2O6-xN2x/3$\Box$x/3 (x $\leq$ 0.15) that exhibits a transition from antiferromagnetism to defect engineered above room-temperature ferromagnetism as evidenced by diffraction, spectroscopic and magnetic characterizations. First-principles calculations reveal the origin of ferromagnetism is a particular CoON configuration with N located at Wyckoff position 8j, which breaks mirror symmetry about ab plane. A pressure-induced electronic phase transition is also predicted at around 24.5 GPa, accompanied by insulator-to-metal transition and magnetic moment vanishing.",2104.07804v1 2021-04-28,Polarisation-dependent single-pulse ultrafast optical switching of an elementary ferromagnet,"The ultimate control of magnetic states of matter at femtosecond (or even faster) timescales defines one of the most pursued paradigm shifts for future information technology. In this context, ultrafast laser pulses developed into extremely valuable stimuli for the all-optical magnetisation reversal in ferrimagnetic and ferromagnetic alloys and multilayers, while this remains elusive in elementary ferromagnets. Here we demonstrate that a single laser pulse with sub-picosecond duration can lead to the reversal of the magnetisation of bulk nickel, in tandem with the expected demagnetisation. As revealed by realistic time-dependent electronic structure simulations, the central mechanism is ultrafast light-induced torques acting on the magnetisation, which are only effective if the laser pulse is circularly polarised on a plane that contains the initial orientation of the magnetisation. We map the laser pulse parameter space enabling the magnetisation switching and unveil rich intra-atomic orbital-dependent magnetisation dynamics featuring transient inter-orbital non-collinear states. Our findings open further perspectives for the efficient implementation of optically-based spintronic devices.",2104.13850v1 2021-04-29,Surpassing the Energy Resolution Limit with ferromagnetic torque sensors,"We discuss the fundamental noise limitations of a ferromagnetic torque sensor based on a levitated magnet in the tipping regime. We evaluate the optimal magnetic field resolution taking into account the thermomechanical noise and the mechanical detection noise at the standard quantum limit (SQL). We find that the Energy Resolution Limit (ERL), pointed out in recent literature as a relevant benchmark for most classes of magnetometers, can be surpassed by many orders of magnitude. Moreover, similarly to the case of a ferromagnetic gyroscope, it is also possible to surpass the standard quantum limit for magnetometry with independent spins, arising from spin-projection noise. Our finding indicates that magnetomechanical systems optimized for magnetometry can achieve a magnetic field resolution per unit volume several orders of magnitude better than any conventional magnetometer. We discuss possible implications, focusing on fundamental physics problems such as the search for exotic interactions beyond the standard model.",2104.14425v2 2021-04-30,Finite-temperature critical behavior of long-range quantum Ising models,"We study the phase diagram and critical properties of quantum Ising chains with long-range ferromagnetic interactions decaying in a power-law fashion with exponent $\alpha$, in regimes of direct interest for current trapped ion experiments. Using large-scale path integral Monte Carlo simulations, we investigate both the ground-state and the nonzero-temperature regimes. We identify the phase boundary of the ferromagnetic phase and obtain accurate estimates for the ferromagnetic-paramagnetic transition temperatures. We further determine the critical exponents of the respective transitions. Our results are in agreement with existing predictions for interaction exponents $\alpha > 1$ up to small deviations in some critical exponents. We also address the elusive regime $\alpha < 1$, where we find that the universality class of both the ground-state and nonzero-temperature transition is consistent with the mean-field limit at $\alpha = 0$. Our work not only contributes to the understanding of the equilibrium properties of long-range interacting quantum Ising models, but can also be important for addressing fundamental dynamical aspects, such as issues concerning the open question of thermalization in such models.",2104.15070v2 2021-05-04,Magnetic properties and magnetocaloric effect of $\mathrm{Tb_2Rh_3Ge}$,"We report the structural, magnetic properties, and magnetocaloric effect of a new polycrystalline compound of $\mathrm{Tb_2Rh_3Ge}$. This present compound crystallizes with $\mathrm{Mg_2Ni_3Si}$-type of rhombohedral Laves phases (space group $R\overline{3}m$, hR18). The magnetic properties and magnetocaloric effect of the $\mathrm{Tb_2Rh_3Ge}$ is explored through dc-magnetization measurements. Temperature dependence of magnetization revealed that the compound exhibits ferromagnetic behavior with $T_C$$=$56 K. The field dependence of magnetization indicates that $\mathrm{Tb_2Rh_3Ge}$ is a soft ferromagnet. The obtained isothermal magnetic entropy changes ($\Delta S_m$), and refrigeration capacity (relative power cooling) for a change of magnetic field 0-9 T are 12.74 J/kg-K, and 497(680) J/kg respectively. The Arrott plots and universal curve of normalized $\Delta S_m$ indicate that this compound undergoes second order ferromagnetic phase transition",2105.01364v2 2021-05-12,Magnon spin current induced by triplet Cooper pair supercurrents,"At the interface between a ferromagnetic insulator and a superconductor there is a coupling between the spins of the two materials. We show that when a supercurrent carried by triplet Cooper pairs flows through the superconductor, the coupling induces a magnon spin current in the adjacent ferromagnetic insulator. The effect is dominated by Cooper pairs polarized in the same direction as the ferromagnetic insulator, so that charge and spin supercurrents produce similar results. Our findings demonstrate a way of converting Cooper pair supercurrents to magnon spin currents.",2105.05861v2 2021-05-13,The essential role of magnetic frustration in the phase diagrams of doped cobaltites,"Doped perovskite cobaltites (e.g., La$_{1-x}$Sr$_x$CoO$_3$) have been extensively studied for their spin-state physics, electronic inhomogeneity, and insulator-metal transitions. Ferromagnetically-interacting spin-state polarons emerge at low $x$ in the phase diagram of these compounds, eventually yielding long-range ferromagnetism. The onset of long-range ferromagnetism ($x \approx 0.18$) is substantially delayed relative to polaron percolation ($x \approx 0.05$), however, generating a troubling inconsistency. Here, Monte-Carlo simulations of a disordered classical spin model are used to establish that previously ignored magnetic frustration is responsible for this effect, enabling faithful reproduction of the magnetic phase diagram.",2105.06402v1 2021-05-14,An energy harvesting technology controlled by ferromagnetic resonance,"We have successfully demonstrated electrical charging using the electromotive force (EMF) generated in a ferromagnetic metal (FM) film under ferromagnetic resonance (FMR). In the case of Ni80Fe20 films, electrical charge due to the EMF generated under FMR can be accumulated in a capacitor; however, the amount of charge is saturated well below the charging limit of the capacitor. Meanwhile in the case of Co50Fe50, electrical charge generated under FMR can be accumulated in a capacitor and the amount of charge increases linearly with the FMR duration time. The difference between the Ni80Fe20 and Co50Fe50 films is due to the respective magnetic field ranges for the FMR excitation. When the FM films were in equivalent thermal states during FMR experiments, Co50Fe50 films could maintain FMR in a detuned condition, while Ni80Fe20 films were outside the FMR excitation range. The EMF generation phenomenon in an FM film under FMR can be used an energy harvesting technology by appropriately controlling the thermal conditions of the FM film.",2105.06606v1 2021-05-16,Voltage-Controlled Reconfigurable Magnonic Crystal at the Submicron Scale,"Multiferroics offer an elegant means to implement voltage-control and on the fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic materials. These properties are particularly interesting for the field of magnonics, where spin waves are used to perform advanced logical or analogue functions. Recently, the emergence of nano-magnonics {\color{black} is expected to} eventually lead to the large-scale integration of magnonic devices. However, a compact voltage-controlled, on demand reconfigurable magnonic system has yet to be shown. Here, we introduce the combination of multiferroics with ferromagnets in a fully epitaxial heterostructure to achieve such voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic $\mathrm{BiFeO_3}$ with a periodicity of $500\,\mathrm{nm}$ yields a modulation of the effective magnetic field in the micron-scale, ferromagnetic $\mathrm{La_{2/3}Sr_{1/3}MnO_3}$ magnonic waveguide. We evidence the magneto-electrical coupling by characterizing the spin wave propagation spectrum in this artificial, voltage induced, magnonic crystal and demonstrate the occurrence of a robust magnonic bandgap with $>20 \,\mathrm{dB}$ rejection.",2105.07418v1 2021-05-19,Origin of insulating ferromagnetism in iron oxychalcogenide Ce$_2$O$_2$FeSe$_2$,"An insulating ferromagnetic (FM) phase exists in the quasi-one-dimensional iron chalcogenide Ce$_2$O$_2$FeSe$_2$ but its origin is unknown. To understand the FM mechanism, here a systematic investigation of this material is provided, analyzing the competition between ferromagnetic and antiferromagnetic tendencies and the interplay of hoppings, Coulomb interactions, Hund's coupling, and crystal-field splittings. Our intuitive analysis based on second-order perturbation theory shows that large entanglements between doubly-occupied and half-filled orbitals play a key role in stabilizing the FM order in Ce$_2$O$_2$FeSe$_2$. In addition, via many-body computational techniques applied to a multi-orbital Hubbard model, the phase diagram confirms the proposed FM mechanism, in agreement with experiments.",2105.09239v2 2021-05-23,Multifunctional operation of the double-layer ferromagnetic structure coupled by a rectangular nanoresonator,"The use of spin waves as a signal carrier requires developing the functional elements allowing for multiplexing and demultiplexing information coded at different wavelengths. For this purpose, we propose a system of thin ferromagnetic layers dynamically coupled by a rectangular ferromagnetic resonator. We show that a single and double, clockwise and counter-clockwise, circulating modes of the resonator offer a wide possibility of control of propagating waves. Particularly, at frequency related to the double-clockwise circulating spin-wave mode of the resonator, the spin wave excited in one layer is transferred to the second one where it propagates in the backward direction. Interestingly, the wave excited in the second layer propagates in the forward direction only in that layer. This demonstrates add-drop filtering, as well as circulator functionality. Thus, the proposed system can become an important part of future magnonic technology for signal routing.",2105.10875v1 2021-05-25,Logarithmic Criticality in Transverse Thermoelectric Conductivity of the Ferromagnetic Topological Semimetal CoMnSb,"We report the results of our experimental studies on the magnetic, transport and thermoelectric properties of the ferromagnetic metal CoMnSb. Sizable anomalous Hall conductivity $\sigma_{yx}$ and transverse thermoelectric conductivity $\alpha_{yx}$ are found experimentally and comparable in size to the values estimated from density-functional theory. Our experiment further reveals that CoMnSb exhibits $-T\ln T$ critical behavior in $\alpha_{yx}(T)$, deviating from Fermi liquid behavior $\alpha_{yx}\sim T$ over a decade of temperature between 10 K to 400 K, similar to ferromagnetic Weyl and nodal-line semimetals. Our theoretical calculation for CoMnSb also predicts the $-T\ln T$ behavior when the Fermi energy locates near the Weyl nodes in momentum space.",2105.12098v1 2021-05-31,Two-dimensional Ferroelectric Ferromagnetic Half Semiconductor in VOF monolayer,"Two-dimensional (2D) multiferroics have been casted great attention owing to their promising prospects for miniaturized electronic and memory devices.Here, we proposed a highly stable 2D multiferroic, VOF monolayer, which is an intrinsic ferromagnetic half semiconductor with large spin polarization ~2 $\mu_{B}/V$ atom and a significant uniaxial magnetic anisotropy along a-axis (410 $\mu eV/V$ atom). Meanwhile, it shows excellent ferroelectricity with a large spontaneous polarization 32.7 $\mu C/cm^{2}$ and a moderate energy barrier (~43 meV/atom) between two ferroelectric states, which can be ascribed to the Jahn-Teller distortion.Moreover, VOF monolayer harbors an ultra-large negative Poisson's ratio in the in-plane direction (~-0.34). The Curie temperature evaluated from the Monte Carlo simulations based on the Ising model is about 215 K, which can be enhanced room temperature under -4% compressive biaxial strain.The combination of ferromagnetism and ferroelectricity in the VOF monolayer could provide a promising platform for future study of multiferroic effects and next-generation multifunctional nanoelectronic device applications.",2105.14765v1 2021-06-02,"Growth and characterization of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb","We study the growth and properties of quaternary-alloy ferromagnetic semiconductor (FMS) (In0.94-x,Gax,Fe0.06)Sb (x = 5% - 30%, Fe concentration is fixed at 6%) grown by low temperature molecular beam epitaxy (LT-MBE).Reflection high-energy electron diffraction (RHEED) patterns, scanning transmission electron microscopy (STEM) lattice images, and X-ray diffraction (XRD) spectra indicate that the (In0.94-x,Gax,Fe0.06)Sb layers have a zinc-blende crystal structure without any other second phase. The lattice constant of the (In0.94-x,Gax,Fe0.06)Sb films changes linearly with the Ga concentration x, indicating that Ga atoms substitute In atoms in the zinc-blend structure. We found that the carrier type of can be systematically controlled by varying x, being n-type when x \le 10% and p-type when x \ge 20%. Characterizations using magnetic circular dichroism (MCD) spectroscopy indicate that the (In0.94-x,Gax,Fe0.06)Sb layers have intrinsic ferromagnetism with relatively high Curie temperatures (TC = 40 - 120 K). The ability to widely control the fundamental material properties (lattice constant, bandgap, carrier type, magnetic property) of (In0.94-x,Gax,Fe0.06)Sb demonstrated in this work is essential for spintronic device applications.",2106.00938v1 2021-06-02,Enhanced spin-orbit coupling and orbital moment in ferromagnets by electron correlations,"In atomic physics, the Hund rule says that the largest spin and orbital state is realized due to the interplay of the spin-orbit coupling (SOC) and the Coulomb interactions. Here, we show that in ferromagnetic solids the effective SOC and the orbital magnetic moment can be dramatically enhanced by a factor of $1/[1-(2U^\prime-U-J_H)\rho_0]$, where $U$ and $U^\prime$ are the on-site Coulomb interaction within the same oribtals and between different orbitals, respectively, $J_H$ is the Hund coupling, and $\rho_0$ is the average density of states. This factor is obtained by using the two-orbital as well as five-orbital Hubbard models with SOC. We also find that the spin polarization is more favorable than the orbital polarization, being consistent with experimental observations. This present work provides a fundamental basis for understanding the enhancements of SOC and orbital moment by Coulomb interactions in ferromagnets, which would have wide applications in spintronics.",2106.01046v1 2021-06-15,Thickness Dependent Magnetic Transition in Few Layer 1T Phase CrTe2,"Room temperature two-dimensional (2D) ferromagnetism is highly desired in practical spintronics applications. Recently, 1T phase CrTe2 (1T-CrTe2) nanosheets with five and thicker layers have been successfully synthesized, which all exhibit the properties of ferromagnetic (FM) metals with Curie temperatures around 305 K. However, whether the ferromagnetism therein can be maintained when continuously reducing the nanosheet's thickness to monolayer limit remains unknown. Here, through first-principles calculations, we explore the evolution of magnetic properties of 1 to 6 layers CrTe2 nanosheets and several interesting points are found: First, unexpectedly, monolayer CrTe2 prefers a zigzag antiferromagnetic (AFM) state with its energy much lower than that of FM state. Second, in 2 to 4 layers CrTe2, both the intralayer and interlayer magnetic coupling are AFM. Last, when the number of layers is equal to or greater than five, the intralayer and interlayer magnetic coupling become FM. Theoretical analysis reveals that the in-plane lattice contraction of few layer CrTe2 compared to bulk is the main factor producing intralayer AFM-FM magnetic transition. At the same time, as long as the intralayer coupling gets FM, the interlayer coupling will concomitantly switch from AFM to FM. Such highly thickness dependent magnetism provides a new perspective to control the magnetic properties of 2D materials.",2106.07960v1 2021-06-15,First-principles calculations on the spin anomalous Hall effect of ferromagnetic alloys,"The spin anomalous Hall effect (SAHE) in ferromagnetic metals, which can generate spin-orbit torque to rotate the magnetization of another ferromagnetic layer through a non-magnetic spacer in magnetic junctions, has attracted much attention. We theoretically investigated the spin anomalous Hall conductivity (SAHC) of the L1$_0$-type alloys $X$Pt($X$=Fe,Co,Ni) on the basis of first-principles density functional theory and linear response theory. We found that the SAHC of FePt is much smaller than the anomalous Hall conductivity (AHC), leading to very small polarization for the anomalous Hall effect $\zeta$=SAHC/AHC of around 0.1. On the other hand, the SAHC increases with increasing number of valence electrons($N_{\rm v}$), and CoPt and NiPt show relatively large values of $|\zeta|$, greater than 1. The negative contribution of the spin-down-down component of AHC is the origin of the large SAHC and $\zeta$ in CoPt and NiPt, which is due to the anti-bonding states of Pt around the Fermi level in the minority-spin states.",2106.07993v2 2021-06-21,Resonant Measurement of Non-Reorientable Spin-Orbit Torque from a Ferromagnetic Source Layer Accounting for Dynamic Spin Pumping,"Using a multilayer structure containing (cobalt detector layer)/(copper spacer)/(Permalloy source layer), we show experimentally how the non-reorientable spin-orbit torque generated by the Permalloy source layer (the component of spin-orbit torque that does not change when the Permalloy magnetization is rotated) can be measured using spin-torque ferromagnetic resonance (ST-FMR) with lineshape analysis. We find that dynamic spin pumping between the magnetic layers exerts torques on the magnetic layers as large or larger than the spin-orbit torques, so that if dynamic spin pumping is neglected the result would be a large overestimate of the spin-orbit torque. Nevertheless, the two effects can be separated by performing ST-FMR as a function of frequency. We measure a non-reorientable spin torque ratio $\xi_{\text{Py}} = 0.04 \pm 0.01$ for spin current flow from Permalloy through an 8 nm Cu spacer to the Co, and a strength of dynamic spin pumping that is consistent with previous measurements by conventional ferromagnetic resonance.",2106.11127v1 2021-06-23,Spin-glass state induced by Mn-doping into a moderate gap layered semiconductor SnSe$_2$,"Various types of magnetism can appear in emerging quantum materials such as van der Waals layered ones. Here, we report the successful doping of manganese atoms into a post-transition metal dichalcogenide semiconductor: SnSe$_2$. We synthesized a single crystal Sn$_{1-x}$Mn$_x$Se$_{2}$ with $\textit{x}$ = 0.04 by the chemical vapor transport (CVT) method and characterized it by x-ray diffraction (XRD) and energy-dispersive x-ray spectroscopy (EDS). The magnetic properties indicated a competition between coexisting ferromagnetic and antiferromagnetic interactions, from the temperature dependence of the magnetization, together with magnetic hysteresis loops. This means that magnetic clusters having ferromagnetic interaction within a cluster form and the short-range antiferromagnetic interaction works between the clusters; a spin-glass state appears below ~ 60 K. Furthermore, we confirmed by $\textit{ab initio}$ calculations that the ferromagnetic interaction comes from the 3$\textit{d}$ electrons of the manganese dopant. Our results offer a new material platform to understand and utilize the magnetism in the van der Waals layered materials.",2106.12334v1 2021-06-24,Large-scale epitaxy of two-dimensional van der Waals room-temperature ferromagnet Fe5GeTe2,"In recent years, two-dimensional van der Waals materials have emerged as an important platform for the observation of long-range ferromagnetic order in atomically thin layers. Although heterostructures of such materials can be conceived to harness and couple a wide range of magneto-optical and magneto-electrical properties, technologically relevant applications require Curie temperatures at or above room-temperature and the ability to grow films over large areas. Here we demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe5GeTe2 on an insulating substrate using molecular beam epitaxy. Magnetic measurements show the persistence of soft ferromagnetism up to room temperature, with a Curie temperature of 293 K, and a weak out-of-plane magnetocrystalline anisotropy. Surface, chemical, and structural characterizations confirm the layer-by-layer growth, 5:1:2 Fe:Ge:Te stoichiometric elementary composition, and single crystalline character of the films.",2106.12808v1 2021-06-24,Structure and magnetism in Fe-doped FeVSb and epitaxial Fe/FeVSb nanocomposite films,"The combination of ferromagnetism and semiconducting behavior offers an avenue for realizing novel spintronics and spin-enhanced thermoelectrics. Here we demonstrate the synthesis of doped and nanocomposite half Heusler Fe$_{1+x}$VSb films by molecular beam epitaxy. For dilute excess Fe ($x < 0.1$), we observe a decrease in the Hall electron concentration and no secondary phases in X-ray diffraction, consistent with Fe doping into FeVSb. Magnetotransport measurements suggest weak ferromagnetism that onsets at a temperature of $T_{c} \approx$ 5K. For higher Fe content ($x > 0.1$), ferromagnetic Fe nanostructures precipitate from the semiconducting FeVSb matrix. The Fe/FeVSb interfaces are epitaxial, as observed by transmission electron microscopy and X-ray diffraction. Magnetotransport measurements suggest proximity-induced magnetism in the FeVSb, from the Fe/FeVSb interfaces, at an onset temperature of $T_{c} \approx$ 20K.",2106.13165v1 2021-07-05,Ferromagnetic half-metallicity in YBaCo2O6 and spin-states driven metal-insulator transition,"Cobaltates have rich spin-states and diverse properties. Using spin-state pictures and firstprinciples calculations, here we study the electronic structure and magnetism of the mixed-valent double perovskite YBaCo2O6. We find that YBaCo2O6 is in the formal intermediate-spin (IS) Co3+/low-spin (LS) Co4+ ground state. The hopping of eg electron from IS-Co3+ to LS-Co4+ via double exchange gives rise to a ferromagnetic half-metallicity, which well accounts for the recent experiments. The reduction of both magnetization and Curie temperature by oxygen vacancies is discussed, aided with Monte Carlo simulations. We also explore several other possible spin-states and their interesting electronic/magnetic properties. Moreover, we predict that a volume expansion more than 3% would tune YBaCo2O6 into the high-spin (HS) Co3+/LS Co4+ ferromagnetic state and simultaneously drive a metal-insulator transition. Therefore, spin-states are a useful parameter for tuning the material properties of cobaltates.",2107.02038v1 2021-07-07,Strain-induced phase diagram of the $S = \frac32$ Kitaev material $\rm{CrSiTe_3}$,"The interplay among anisotropic magnetic terms, such as the bond-dependent Kitaev interactions and single-ion anisotropy, plays a key role in stabilizing the finite-temperature ferromagnetism in the two-dimensional compound $\rm{CrSiTe_3}$. While the Heisenberg interaction is predominant in this material, a recent work shows that it is rather sensitive to the compressive strain, leading to a variety of phases, possibly including a sought-after Kitaev quantum spin liquid [C. Xu, \textit{et. al.}, Phys. Rev. Lett. \textbf{124}, 087205 (2020)]. To further understand these states, we establish the quantum phase diagram of a related bond-directional spin-$3/2$ model by the density-matrix renormalization group method. As the Heisenberg coupling varies from ferromagnetic to antiferromagnetic, three magnetically ordered phases, i.e., a ferromagnetic phase, a $120^\circ$ phase and an antiferromagnetic phase, appear consecutively. All the phases are separated by first-order phase transitions, as revealed by the kinks in the ground-state energy and the jumps in the magnetic order parameters. However, no positive evidence of the quantum spin liquid state is found and possible reasons are discussed briefly.",2107.02971v1 2021-07-16,Defect engineering of magnetic ground state in EuTiO$_3$ epitaxial thin films,"Atomistic defect engineering through the pulsed laser epitaxy of perovskite transition metal oxides offers facile control of their emergent opto-electromagnetic and energy properties. Among the various perovskite oxides, EuTiO3 exhibits a strong coupling between the lattice, electronic, and magnetic degrees of freedom. This coupling is highly susceptible to atomistic defects. In this study, we investigated the magnetic phase of EuTiO$_3$ epitaxial thin films via systematic defect engineering. A magnetic phase transition from an antiferromagnet to a ferromagnet was observed when the unit cell volume of EuTiO3 expanded due to the introduction of Eu-O vacancies. Optical spectroscopy and density functional theory calculations show that the change in the electronic structure as the ferromagnetic phase emerges can be attributed to the weakened Eu-Ti-Eu super-exchange interaction and the developed ferromagnetic Eu-O-Eu interaction. Facile defect engineering in EuTiO$_3$ thin films facilitates understanding and tailoring of their magnetic ground state.",2107.07711v1 2021-07-17,Magnetic anisotropy and critical behavior of the quaternary van der Waals ferromagnetic material $\bf CrGe_δSi_{1-δ}Te_3$,"Recently, two-dimensional ferromagnetism in the family of Chromium compounds $\rm CrXTe_3 (X=Si, Ge)$ has attracted a broad research interest. Despite the structural similarity in $\rm CrTe_6$ octahedra, the size effect of inserted Ge or Si dimer contributes to significant differences in magnetism. Here, we report a new quaternary van der Waals ferromagnetic material $\rm CrGe_{\delta}Si_{1-\delta}Te_3$ synthesized by flux method. Ge substitution in Si site results in the lattice expansion, further increasing the Curie temperature and reducing the magnetic anisotropy. The critical behavior of $\rm Cr_{0.96}Ge_{0.17}Si_{0.82}Te_3$ has been studied by specific heat as well as magnetization measurements. And the extracted critical exponents are self-consistent and well-obeying the scaling laws, which are closer to the 2D Ising model with interaction decaying as $J(r)\approx r^{-3.44}$.",2107.08222v1 2021-07-21,Dynamics of two ferromagnetic insulators coupled by superconducting spin current,"A conventional superconductor sandwiched between two ferromagnets can maintain coherent equilibrium spin current. This spin supercurrent results from the rotation of odd-frequency spin correlations induced in the superconductor by the magnetic proximity effect. In the absence of intrinsic magnetization, the superconductor cannot maintain multiple rotations of the triplet component but instead provides a Josephson type weak link for the spin supercurrent. We determine the analogue of the current-phase relation in various circumstances and show how it can be accessed in experiments on dynamic magnetization. In particular, concentrating on the magnetic hysteresis and the ferromagnetic resonance response, we show how the spin supercurrent affects the nonequilibrium dynamics of magnetization which depends on a competition between spin supercurrent mediated static exchange contribution and a dynamic spin pumping contribution. Depending on the outcome of this competition, a mode crossing in the system can either be an avoided crossing or mode locking.",2107.09959v2 2021-07-23,Carrier-induced ferromagnetism in 2D magnetically-doped semiconductor structures,"We show theoretically that the magnetic ions, randomly distributed in a two-dimensional (2D) semiconductor system, can generate a ferromagnetic long-range order via the RKKY interaction. The main physical reason is the discrete (rather than continuous) symmetry of the 2D Ising model of the spin-spin interaction mediated by the spin-orbit coupling of 2D free carriers, which precludes the validity of the Mermin-Wagner theorem. Further, the analysis clearly illustrates the crucial role of the molecular field fluctuations as opposed to the mean field. The developed theoretical model describes the desired magnetization and phase-transition temperature $T_c$ in terms of a single parameter; namely, the chemical potential $\mu$. Our results highlight a path way to reach the highest possible $T_c$ in a given material as well as an opportunity to control the magnetic properties externally (e.g., via a gate bias). Numerical estimations show that magnetic impurities such as Mn$^{2+}$ with spins $S=5/2$ can realize ferromagnetism with $T_c$ close to room temperature.",2107.11377v1 2021-07-28,Ce-site dilution in the ferromagnetic Kondo lattice CeRh$_6$Ge$_4$,"The heavy fermion ferromagnet CeRh$_6$Ge$_4$ is the first example of a clean stoichiometric system where the ferromagnetic transition can be continuously suppressed by hydrostatic pressure to a quantum critical point. In order to reveal the outcome when the magnetic lattice of CeRh$_6$Ge$_4$ is diluted with non-magnetic atoms, this study reports comprehensive measurements of the physical properties of both single crystal and polycrystalline samples of La$_x$Ce$_{1-x}$Rh$_6$Ge$_4$. With increasing $x$, the Curie temperature decreases, and no transition is observed for $x$ $>$ 0.25, while the system evolves from exhibiting coherent Kondo lattice behaviors at low $x$, to the Kondo impurity scenario at large $x$. Moreover, non-Fermi liquid behavior (NFL) is observed over a wide doping range, which agrees well with the disordered Kondo model for 0.52 $\leq$ $x$ $\leq$ 0.66, while strange metal behavior is revealed in the vicinity of $x_c$ = 0.26.",2107.13131v1 2021-08-03,Atomic scale control of spin current transmission at interfaces,"Spin transmission at ferromagnet/heavy metal interfaces is of vital importance for many spintronic devices. Usually the spin current transmission is limited by the spin mixing conductance and loss mechanisms such as spin memory loss. In order to understand these effects, we study the interface transmission when an insulating interlayer is inserted between the ferromagnet and the heavy metal. For this we measure the inverse spin Hall voltage generated from optically injected spin current pulses as well as the magnitude of the spin pumping using ferromagnetic resonance. From our results we conclude that significant spin memory loss only occurs for 5d metals with less than half filled d-shell.",2108.01770v1 2021-08-04,Spin-scattering asymmetry at half-metallic ferromagnet/ferromagnet interface,"We study spin-scattering asymmetry at the interface of two ferromagnets (FMs) based on a half-metallic Co$_{2}$Fe$_{0.4}$Mn$_{0.6}$Si (CFMS)/CoFe interface. First-principles ballistic transport calculations based on Landauer formula for (001)-CoFe/CFMS/CoFe indicate strong spin-dependent conductance at the CFMS/CoFe interface, suggesting a large interface spin-scattering asymmetry coefficient ($\gamma$). Fully epitaxial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) pseudo-spin-valve (PSV) devices involving CoFe/CFMS/Ag/CFMS/CoFe structures exhibit an enhancement in MR output owing to the formation of the CFMS/CoFe interface at room temperature (RT). This is well reproduced qualitatively by a simulation based on a generalized two-current series-resistor model with considering the presence of $\gamma$ at the CFMS/CoFe interface, half-metallicity of CFMS, and combinations of terminated atoms at the interfaces in the CPP-GMR PSV structure. We show direct evidence for a large $\gamma$ at a half-metallic FM/FM interface and its impact on CPP-GMR effect even at RT.",2108.02045v3 2021-08-05,Triaxial magnetic anisotropy in the two-dimensional ferromagnetic semiconductor CrSBr,"Two-dimensional (2D) ferromagnets have recently drawn extensive attention, and here we study the electronic structure and magnetic properties of the bulk and monolayer of CrSBr, using first-principles calculations and Monte Carlo simulations. Our results show that bulk CrSBr is a magnetic semiconductor and has the easy magnetization b-axis, hard c-axis, and intermediate a-axis. Thus, the experimental triaxial magnetic anisotropy (MA) is well reproduced here, and it is identified to be the joint effects of spin-orbit coupling (SOC) and magnetic dipole-dipole interaction. We find that bulk CrSBr has a strong ferromagnetic (FM) intralayer coupling but a marginal interlayer one. We also study CrSBr monolayer in detail and find that the intralayer FM exchange persists and the shape anisotropy has a more pronounced contribution to the MA. Using the parameters of the FM exchange and the triaxial MA, our Monte Carlo simulations show that CrSBr monolayer has Curie temperature Tc = 175 K. Moreover, we find that a uniaxial tensile (compressive) strain along the a (b) axis would further increase the Tc.",2108.02418v2 2021-08-09,Realization of 0 - $π$ states in SFIS Josephson junctions. The role of spin-orbit interaction and lattice impurities,"Josephson devices with ferromagnetic barriers have been widely studied. Much less is known when the ferromagnetic layer is insulating. In this manuscript we investigate the transport properties of superconductor-ferromagnetic insulator-superconductor (SFIS) junctions with particular attention to the temperature behavior of the critical current, that may be used as a fingerprint of the junction. We investigate the specific role of impurities as well as of possible spin mixing mechanisms, due to the spin orbit coupling. Transition between the 0 and the $\pi$ phases can be properly tuned, thus achieving stable $\pi$ junctions over the whole temperature range, that may be possibly employed in superconducting quantum circuits.",2108.04292v2 2021-08-14,Voltage-controlled magnetism enabled by resistive switching,"The discovery of new mechanisms of controlling magnetic properties by electric fields or currents furthers the fundamental understanding of magnetism and has important implications for practical use. Here, we present a novel approach of utilizing resistive switching to control magnetic anisotropy. We study a ferromagnetic oxide that exhibits an electrically triggered metal-to-insulator phase transition producing a volatile resistive switching. This switching occurs in a characteristic spatial pattern: the formation of a transverse insulating barrier inside a metallic matrix resulting in an unusual ferromagnetic/paramagnetic/ferromagnetic configuration. We found that the formation of this voltage-driven paramagnetic insulating barrier is accompanied by the emergence of a strong uniaxial magnetic anisotropy that overpowers the intrinsic material anisotropy. Our results demonstrate that resistive switching is an effective tool for manipulating magnetic properties. Because resistive switching can be induced in a very broad range of materials, our findings could enable a new class of voltage-controlled magnetism systems.",2108.06445v1 2021-08-20,Probing anisotropy in epitaxial Fe/Pt bilayers by spin-orbit torque ferromagnetic resonance,"We report the generation and detection of spin-orbit torque ferromagnetic resonance (STFMR) in micropatterned epitaxial Fe/Pt bilayers grown by molecular beam epitaxy. The magnetic field dependent measurements at an in-plane magnetic field angle of 45 degrees with respect to the microwave-current direction reveal the presence of two distinct voltage peaks indicative of a strong magnetic anisotropy. We show that STFMR can be employed to probe the underlying magnetic properties including the anisotropies in the Fe layer. We compare our STFMR results with broadband ferromagnetic resonance spectroscopy of the unpatterned bilayer thin films. The experimental STFMR measurements are interpreted using an analytical formalism and further confirmed using micromagnetic modeling, which shed light on the field-dependent magnetization alignment in the microstructures responsible for the STFMR rectification. Our results demonstrate a simple and efficient method for determining magnetic anisotropies in microstructures by means of rf spectroscopy.",2108.09132v1 2021-09-10,Controlling magnetism through Ising superconductivity in magnetic van der Waals heterostructures,"Van der Waals heterostructures have risen as a tunable platform to combine different electronic orders, due to the flexibility in stacking different materials with competing symmetry broken states. Among them, van der Waals ferromagnets such as CrI3 and superconductors as NbSe2 provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, NbSe2 is well known for hosting strong spin-orbit coupling effects that influence the properties of the superconducting state. Here we put forward a ferromagnet/NbSe2/ferromagnet heterostructure where the interplay between Ising superconductivity in NbSe2 and magnetism controls the magnetic alignment of the heterostructure. In particular, we show that the interplay between spin-orbit coupling and superconductivity allows controlling magnetic states in van der Waals materials. Our results show how hybrid van der Waals ferromagnet/superconductor heterostructure can be used as a tunable materials platform for superconducting spin-orbitronics.",2109.04788v2 2021-09-10,"Non-reciprocal magnetoresistance, directional inhomogeneity and mixed symmetry Hall devices","Phenomenology similar to the nonreciprocal charge transport violating Onsagers reciprocity relations can develop in directionally inhomogeneous conducting films with nonuniform Hall coefficient along the current trajectory. The effect is demonstrated in ferromagnetic CoPd films and analyzed in comparison with the unidirectional magnetoresistance phenomena. We suggest to use an engineered inhomogeneity for spintronics applications and present the concept of mixed symmetry Hall devices in which transverse to current Hall signal is measured in a longitudinal contacts arrangement. Magnetization reversal and memory detection is demonstrated in the three terminal and the partitioned normal metal-ferromagnet (NM - FM) device designs. Multi-bit memory is realized in the partitioned FM-NM-FM structure. The relative amplitude of the antisymmetric signal in the engineered ferromagnetic devices is few percent which is 10 to 1000 times higher than in their unidirectional magnetoresistance analogues.",2109.04820v1 2021-09-18,Strain Tunable Intrinsic Ferromagnetic in 2D Square CrBr$_2$,"Two-dimensional (2D) intrinsic magnetic materials with high Curie temperature (Tc) coexisting with 100% spin-polarization are highly desirable for realizing promising spintronic devices. In the present work, the intrinsic magnetism of monolayer square CrBr2 is predicted by using first-principles calculations. The monolayer CrBr2 is an intrinsic ferromagnetic (FM) half-metal with the half-metallic gap of 1.58 eV. Monte Carlo simulations based on the Heisenberg model estimates Tc as 212 K. Furthermore, the large compressive strain makes CrBr2 undergo ferromagnetic-antiferromagnetic phase transition, when the biaxial tensile strain larger than 9.3% leads to the emergence of semiconducting electronic structures. Our results show that the intrinsic half-metal with a high Tc and controllable magnetic properties endow monolayer square CrBr2 a potential material for spintronic applications.",2109.08808v1 2021-09-20,Giant anomalous Nernst signal in the antiferromagnet YbMnBi2,"Searching for a high anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show high ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. Herein, we demonstrate that YbMnBi2, a canted antiferromagnet, has a large ANE conductivity of ~10 Am-1K-1 that surpasses the common high values (i.e. 3-5 Am-1K-1) observed so far in ferromagnets. The canted spin structure of Mn guarantees a nonzero Berry curvature but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin-orbit coupling enables a high ANE and low thermal conductivity, whereas its highly dispersive px/y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization makes YbMnBi2 an excellent candidate for transverse thermoelectrics.",2109.09382v1 2021-09-27,Half-integer Shapiro Steps in Strong Ferromagnetic Josephson Junctions,"We report the experimental observation of half-integer Shapiro steps in the strong ferromagnetic Josephson junction (Nb-NiFe-Nb) by investigating the current-phase relation under radiofrequency microwave excitation. The half-integer Shapiro steps are robust in a wide temperature range from T = 4 to 7 K. The half-integer Shapiro steps could be attributed to co-existence of 0- and pi-states in the strong ferromagnetic NiFe Josephson junctions with the spatial variation of the2 NiFe thickness. This scenario is also supported by the high-resolution transmission electron microscopy characterization of the Nb/NiFe/Nb junction.",2109.13181v1 2021-09-29,Magnetic interlayer coupling between ferromagnetic SrRuO$_3$ layers through a SrIrO$_3$ spacer,"A key element to tailor the properties of magnetic multilayers is the coupling between the individual magnetic layers. In case of skyrmion hosting multilayers, coupling of skyrmions across the magnetic layers is highly desirable. Here the magnetic interlayer coupling was studied in epitaxial all-oxide heterostructures of ferromagnetic perovskite SrRuO$_3$ layers separated by spacers of the strong spin-orbit coupling oxide SrIrO$_3$. This combination of oxide layers is being discussed as a potential candidate system to host N\'{e}el skyrmions. First order reversal curve (FORC) measurements were performed in order to distinguish between magnetic switching processes of the individual layers and to disentangle the signal of soft magnetic impurities from the samples$'$ signal. Additionally, FORC investigations enabled to determine whether the coupling between the magnetic layers is ferromagnetic or antiferromagnetic. The observed interlayer coupling strength was weak for all the heterostructures, with SrIrO$_3$ spacers between 2 monolayers and 12 monolayers thick.",2109.14292v2 2021-09-29,Non-chiral spin frustration versus highly degenerate ferromagnetic state with local chiral degrees of freedom of an exactly solvable spin-electron planar model of inter-connected trigonal bipyramids,"The frustration phenomenon in an exactly solvable spin-electron planar model constituted by identical bipyramidal plaquettes is discussed within the Toulouse's and dos Santos and Lyra's frustration concepts. It is shown that the ground state of the model contains the unfrustrated spontaneously ordered quantum ferromagnetic phase with local chiral degrees of freedom in the electron sub-lattice and the disordered quantum one, where both the Ising and electron sub-lattices are frustrated. The frustration of the latter sub-lattice persists at finite temperatures, but only in the disordered region. It finally vanishes at a certain frustration temperature. The reentrant behaviour of the frustration in the electron sub-lattice with three consecutive frustration temperatures due to a competition with the unfrustrated ferromagnetic spin arrangement near the ground-state phase transition can also be observed.",2109.14366v1 2021-10-19,Superconductivity-driven ferromagnetism and spin manipulation using vortices in the magnetic superconductor EuRbFe4As4,"Magnetic superconductors are specific materials exhibiting two antagonistic phenomena, superconductivity and magnetism, whose mutual interaction induces various emergent phenomena, such as the reentrant superconducting transition associated with the suppression of superconductivity around the magnetic transition temperature (Tm), highlighting the impact of magnetism on superconductivity. In this study, we report the experimental observation of the ferromagnetic order induced by superconducting vortices in the high-critical-temperature (high-Tc) magnetic superconductor EuRbFe4As4. Although the ground state of the Eu2+ moments in EuRbFe4As4 is helimagnetism below Tm, neutron diffraction and magnetization experiments show a ferromagnetic hysteresis of the Eu2+ spin alignment. We demonstrate that the direction of the Eu2+ moments is dominated by the distribution of pinned vortices based on the critical state model. Moreover, we demonstrate the manipulation of spin texture by controlling the direction of superconducting vortices, which can help realize spin manipulation devices using magnetic superconductors.",2110.09675v1 2021-10-21,Massless Dirac magnons in the two dimensional van der Waals honeycomb magnet CrCl3,"Two dimensional van der Waals ferromagnets with honeycomb structures are expected to host the bosonic version of Dirac particles in their magnon excitation spectra. Using inelastic neutron scattering, we study spin wave excitations in polycrystalline CrCl$_3$, which exhibits ferromagnetic honeycomb layers with antiferromagnetic stackings along the $c$-axis. For comparison, polycrystal samples of CrI$_3$ with different grain sizes are also studied. We find that the powder-averaged spin wave spectrum of CrCl$_3$ at $T$ = 2 K can be adequately explained by the two dimensional spin Hamiltonian including in-plane Heisenberg exchanges only. The observed excitation does not exhibit noticeable broadening in energy, which is in remarkable contrast to the substantial broadening observed in CrI$_3$. Based on these results, we conclude that the ferromagnetic phase of CrCl$_3$ hosts massless Dirac magnons and is thus not topological.",2110.10894v1 2021-10-21,Three-dimensional ferromagnetism and magnetotransport in van der Waals Mn-intercalated tantalum disufide,"Van der Waals (vdW) ferromagnets are an important class of materials for spintronics applications. The recent discovery of atomically vdW magnets CrI$_3$ and Cr$_2$Ge$_2$Te$_6$ has triggered a renaissance in the area of two-dimensional (2D) magnetism. Herein we systematically studied 2H-Mn$_{0.28}$TaS$_2$ single crystal, a 2D vdW ferromagnet with $T_c$ $\sim$ 82.3 K and a large in-plane magnetic anisotropy. Mn $K$-edge x-ray absorption spectroscopy was measured to provide information on its electronic state and local atomic environment. The detailed magnetic isotherms measured in the vicinity of $T_c$ indicates that the spin coupling inside 2H-Mn$_{0.28}$TaS$_2$ is of a three-dimensional (3D) Heisenberg-type coupled with the attractive long-range interaction between spins that decay as $J(r)\approx r^{-4.85}$. Both resistivity $\rho(T)$ and thermopower $S(T)$ exhibit anomalies near $T_c$, confirming that the hole-type transport carriers strongly interact with local moments. An unusual angle-dependent magnetoresistance is further observed, suggesting a possible field-induced novel magnetic structure.",2110.10908v1 2021-10-26,Topological phase transitions in strongly correlated systems: application to Co$_3$Sn$_2$S$_2$,"The topological transition in the strongly correlated half-metallic ferromagnetic compound Co$_3$Sn$_2$S$_2$ from Weyl semimetal (including chiral massless fermions) to a non-magnetic state is treated. This transition goes with a change in topological invariant, and is accompanied by a non-topological transition from saturated ferromagnetic to paramagnetic state, the minority Fermi surface being transformed from ghost (hidden) to real. A corresponding description is given in terms of slave fermion representation for the effective narrow-band Hubbard model. The system Co$_3$Sn$_2$S$_2$ provides a bright example of coexistence of non-trivial topology and strong low-dimensional ferromagnetism. A comparison is performed with other compounds where frustrations result in formation of a correlated paramagnetic state.",2110.13709v1 2021-10-31,Analytical results for the unusual Grüneisen ratio in the quantum Ising model with Dzyaloshinskii-Moriya interaction,"The Gr\""uneisen ratio (GR) has emerged as a superb tool for the diagnosis of quantum phase transitions, which diverges algebraically upon approaching critical points of continuous phase transitions. However, this paradigm has been challenged recently by observations of a finite GR for self-dual criticality and divergent GR at symmetry-enhanced first-order transitions. To unveil the fascinating GR further, we exemplify the idea by studying an exactly solvable quantum Ising model with Dzyaloshinskii-Moriya interaction, which harbors a ferromagnetic phase, a paramagnetic phase, and a chiral Luttinger liquid. Although the self-dual criticality of the ferromagnetic--paramagnetic transition is undermined by the Dzyaloshinskii-Moriya interaction, we find that the GR at the transition is still finite albeit with an increasing value, signifying a proximate self-dual relation. By contrast, the GR at the transition between the gapped ferromagnetic phase and the gapless Luttinger liquid diverges and changes its sign when crossing the first-order transition. This implies that the GR could also probe the first-order transition between the gapped and gapless phases.",2111.00536v2 2021-11-09,Magnetotransport in ferromagnetic Fe$_2$Ge semimetallic thin films,"Thin films of the ferromagnet Fe$_2$Ge were grown via molecular beam epitaxy, and their electrical and magneto-transport properties measured for the first time. X-ray diffraction and vibrating sample magnetometry measurements confirmed the crystalline ferromagnetic Fe$_2$Ge phase. The observed high temperature maximum in the longitudinal resistivity, as well as the observed suppression of electron-magnon scattering at low temperatures, point to the presence of strong spin polarization in this material. Measurements of the Hall resistivity, $\rho_{xy}$, show contributions from both the ordinary Hall effect and anomalous Hall effect, $\rho_{xy}^{AH}$, from which we determined the charge carrier concentration and mobility. Measurements also show a small negative magnetoresistance in both the longitudinal and transverse geometries. Fe$_2$Ge holds promise as a useful spintronic material, especially for its semiconductor compatibility.",2111.05417v1 2021-11-11,Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet VI3,"Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with \k{appa}_xy ~ 1x10^(-2) W K^(-1) m^(-1) in an insulating van der Waals ferromagnet VI3. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI3 exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI3 as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics/magnonics applications.",2111.06039v1 2021-11-15,Nonstanding spin waves in a single rectangular permalloy microstrip under uniform magnetic excitation,"Ferromagnetic resonance modes in a single rectangular Ni$_{80}$Fe$_{20}$ microstrip were directly imaged using time-resolved scanning transmission x-ray microscopy combined with a phase-locked ferromagnetic resonance excitation scheme and the findings were corroborated by micromagnetic simulations. Although under uniform excitation in a single confined microstructure typically standing spin waves are expected, all imaged spin waves showed a nonstanding character both, at and off resonance, the latter being additionally detected with microantenna-based ferromagnetic resonance. The effect of the edge quality on the spin waves was observed in micromagnetic simulations.",2111.07773v3 2021-11-16,Optical sensing of magnons via the magnetoelastic displacement,"We show how to measure a steady-state magnon population in a magnetostatic mode of a ferromagnet or ferrimagnet, such as yttrium iron garnet. We adopt an optomechanical approach and utilize the magnetoelasticity of the ferromagnet. The magnetostrictive force dispersively couples magnons to the deformation displacement of the ferromagnet, which is proportional to the magnon population. By further coupling the mechanical displacement to an optical cavity that is resonantly driven by a weak laser, the magnetostrictively induced displacement can be sensed by measuring the phase quadrature of the optical field. The phase shows an excellent linear dependence on the magnon population for a not very large population, and can thus be used as a `magnometer' to measure the magnon population. We further study the effect of thermal noises, and find a high signal-to-noise ratio even at room temperature. At cryogenic temperatures, the resolution of magnon excitation numbers is essentially limited by the vacuum fluctuations of the phase, which can be significantly improved by using a squeezed light.",2111.08376v2 2021-11-16,Ultrathin ferrimagnetic GdFeCo films with very low damping,"Ferromagnetic materials dominate as the magnetically active element in spintronic devices, but come with drawbacks such as large stray fields, and low operational frequencies. Compensated ferrimagnets provide an alternative as they combine the ultrafast magnetization dynamics of antiferromagnets with a ferromagnet-like spin-orbit-torque (SOT) behavior. However to use ferrimagnets in spintronic devices their advantageous properties must be retained also in ultrathin films (t < 10 nm). In this study, ferrimagnetic Gdx(Fe87.5Co12.5)1-x thin films in the thickness range t = 2-20 nm were grown on high resistance Si(100) substrates and studied using broadband ferromagnetic resonance measurements at room temperature. By tuning their stoichiometry, a nearly compensated behavior is observed in 2 nm Gdx(Fe87.5Co12.5)1-x ultrathin films for the first time, with an effective magnetization of Meff = 0.02 T and a low effective Gilbert damping constant of {\alpha} = 0.0078, comparable to the lowest values reported so far in 30 nm films. These results show great promise for the development of ultrafast and energy efficient ferrimagnetic spintronic devices.",2111.08768v1 2021-11-19,Asymptotic States of Ising Ferromagnets with Long-range Interactions,"It is known that, after a quench to zero temperature ($T=0$), two-dimensional ($d=2$) Ising ferromagnets with short-range interactions do not always relax to the ordered state. They can also fall in infinitely long-lived striped metastable states with a finite probability. In this paper, we study how the abundance of striped states is affected by long-range interactions. We investigate the relaxation of $d=2$ Ising ferromagnets with power-law interactions by means of Monte Carlo simulations at both $T=0$ and $T \ne 0$. For $T=0$ and the finite system size, the striped metastable states are suppressed by long-range interactions. In the thermodynamic limit, their occurrence probabilities are consistent with the short-range case. For $T \ne 0$, the final state is always ordered. Further, the equilibration occurs at earlier times with an increase in the strength of the interactions.",2111.10118v2 2021-12-10,Spin anomalous-Hall unidirectional magnetoresistance,"We predict a spin anomalous-Hall unidirectional magnetoresistance (AH-UMR) in conducting bilayers composed of a ferromagnetic layer and a nonmagnetic layer, which does $\textit{not}$ rely on the spin Hall effect in the normal metal layer$-$in stark contrast to the well-studied unidirectional spin-Hall magnetoresistance$-$but, instead, arises from the spin anomalous Hall effect in the ferromagnetic layer. Physically, it is the charge-spin conversion induced by the spin anomalous Hall effect that conspires with the structural inversion asymmetry to generate a net nonequilibrium spin density in the ferromagnetic layer, which, in turn, modulates the resistance of the bilayer when the direction of the applied current or the magnetization is reversed. The dependences of the spin AH-UMR effect on materials and geometric parameters are analyzed and compared with other nonlinear magnetoresistances. In particular, we show that, in magnetic bilayers where spin anomalous Hall and spin Hall effects are comparable, the overall UMR may undergo a sign change when the thickness of either layer is varied, suggesting a scheme to quantify the spin Hall or spin anomalous Hall angle via a nonlinear transport measurement.",2112.05703v2 2021-12-13,XY model with competing higher-order interactions,"We study effects of competing pairwise higher-order interactions (HOI) with alternating signs and exponentially decreasing intensity on critical behavior of the XY model. It is found that critical properties of such a generalized model can be very different from the standard XY model and can strongly depend on whether the number of HOI terms is odd or even. Inclusion of any odd number of HOI terms results in two consecutive phase transitions to distinct ferromagnetic quasi-long-range order phases. Even number of HOI terms leads to two phase transitions only if the decay of the HOI intensities is relatively slow. Then the high-temperature transition to the ferromagnetic phase is followed by another transition to a peculiar competition-induced canted ferromagnetic phase. In the limit of an infinite number of HOI terms only one phase transition is confirmed, and under the conditions of fierce competition between the even and odd terms the transition temperature can be suppressed practically to zero.",2112.06791v1 2021-12-23,Nutation spin waves in ferromagnets,"Magnetization dynamics and spin waves in ferromagnets are investigated using the inertial Landau-Lifshitz-Gilbert equation. Taking inertial magnetization dynamics into account, dispersion relations describing the propagation of nutation spin waves in an arbitrary direction relative to the applied magnetic field are derived via Maxwell's equations. It is found that the inertia of magnetization causes the hybridization of electromagnetic waves and nutation spin waves in ferromagnets, hybrid nutation spin waves emerge, and the redshift of frequencies of precession spin waves is initiated, which transforms to precession-nutation spin waves. These effects depend sharply on the direction of wave propagation relative to the applied magnetic field. Moreover, the waves propagating parallel to the applied field are circularly polarized, while the waves propagating perpendicular to that field are elliptically polarized. The characteristics of these spin nutation waves are also analyzed.",2112.12503v3 2021-12-22,Modification of the magnetic and electronic properties of the graphene-Ni(111) interface via halogens intercalation,"Electronic decoupling of graphene from metallic and semiconducting substrates via intercalation of different species is one of the widely used approaches in studies of graphene. In the present work the modification of the electronic and magnetic properties of graphene on ferromagnetic Ni(111) layer via intercalation of halogen atoms (X = F, Cl, Br) is studied using the state-of-the-art density-functional theory approach. It is found that in all gr/X/Ni(111) intercalation systems a graphene layer is fully electronically decoupled from the ferromagnetic substrate; however, different kind (electron or hole) and level of doping can be achieved. Despite the extremely small magnetic moment of C-atoms in graphene observed after halogens intercalation, the sizeable spin-splitting up to $35$ meV for the linearly dispersing graphene $\pi$ bands is found. The obtained theoretical data bring new ideas on the formation of the graphene-ferromagnet interfaces where spin polarized free-standing graphene layer can be formed with the possible application of these systems in electronics and spintronics.",2112.12528v1 2021-12-29,Electric and magnetic fields tuned spin-polarized topological phases in two-dimensional ferromagnetic MnBi$_4$Te$_7$,"Applying electric or magnetic fields is widely used to not only create and manipulate topological states but also facilitate their observations in experiments. In this work, we show by first-principles calculations and topological analysis that the time-reversal (TR) symmetry-broken quantum spin Hall (QSH) state emerges in a two-dimensional ferromagnetic MnBi$_4$Te$_7$ monolayer. This TR-symmetry broken QSH phase possesses a highly tunable nontrivial band gap under an external electric field (or tuning interlayer distance). Furthermore, based on the Wannier-function-based tight-binding approach, we reveal that a topological phase transition from the TR-symmetry broken QSH phase to the quantum anomalous Hall (QAH) phase occurs with the increase of magnetic fields. Besides, we also find that a reverse electric fields can facilitate the realization of QAH phase. Our work not only uncovers the ferromagnetic topological properties the MnBi$_4$Te$_7$ monolayer tuned by electric and magnetic fields, but also can stimulate further applications to spintronics and topological devices.",2112.14352v1 2022-01-04,Magnetization reversal of a ferromagnetic Pt/Co/Pt film by helicity dependent absorption of visible to near-infrared laser pulses,"The practical difficulty in distinguishing the impact of magnetic circular dichroism and the inverse Faraday effect fuels intense debates over which mechanism predominantly drives the process of helicity dependent all-optical switching of magnetization in ferromagnets. Here, we quantitatively measure the efficiency of the switching process in a Pt/Co/Pt multilayered stack using visible- to near-infrared optical pulses. We find that the switching efficiency increases by a factor of 8.6 upon increasing the pumping wavelength from 0.5 $ \mu $m to 1.1 $ \mu $m, becoming 100 % efficient at even longer wavelengths up to 2.0 $ \mu $m. Our experimental results can be successfully explained by the phenomenon of magnetic circular dichroism, making a significant step towards resolving the long-standing controversy over the origin of the all-optical process of magnetization reversal in ferromagnets.",2201.01141v1 2021-12-10,Enhanced Planar Antenna Efficiency Through Magnetic Thin-Films,"This work proposes to use magnetic material as the substrate of planar antennas to overcome the platform effect caused by the conducting ground plane. The upper bound of the radiation efficiency of an electric-current-driven low-profile antenna is theoretically derived, which is inversely proportional to the Gilbert damping factor of the magnetic material. Meanwhile, the improvement of radiation due to the use of magnetic material is demonstrated by a three-dimensional (3D) multiphysics and multiscale time-domain model. The simulation results match the theoretical derivation, showing 25% radiation efficiency from a planar antenna backed by a FeGaB thin film with 2.56 um thickness. Furthermore, for conductive ferromagnetic materials, it is shown that the eddy current loss can be well suppressed by laminating the thin film into multiple layers. The radiation efficiency of the modeled antenna with a conductive ferromagnetic substrate is improved from 2.2% to 11.8% by dividing the substrate into 10 layers, with a ferromagnetic material fill factor of 93%.",2201.04932v1 2022-01-19,Supersolid-like solitons in spin-orbit coupled spin-2 condensate,"We study supersolid-like crystalline structures emerging in the stationary states of a quasi-two-dimensional spin-orbit (SO)-coupled spin-2 condensate in the ferromagnetic, cyclic, and antiferro-magnetic phases by solving a mean-field model.Interplay of different strengths of SO coupling and interatomic interactions gives rise to a variety of non-trivial density patterns in the emergent solutions. For small SO-coupling strengths $\gamma$ ($\gamma \approx 0.5$), the ground state is an axisymmetric multi-ring soliton for polar, cyclic and weakly-ferromagnetic interactions, whereas for stronger-ferromagnetic interactions a circularly-asymmetric soliton emerges as the ground state.Depending on the values of interaction parameters, with an increase in SO-coupling strength, a stripe phase may also emerge as the ground state for polar and cyclic interactions. For intermediate values of SO-coupling strength ($\gamma \approx 1$), in addition to these solitons, one could have a quasi-degenerate triangular-lattice soliton in all magnetic phases. On further increasing the SO-coupling strength ($\gamma \gtrapprox 4$), a square-lattice and a superstripe soliton emerge as quasi-degenerate states. The emergence of all these solitons can be inferred from a study of solutions of the single-particle Hamiltonian.",2201.07500v1 2022-01-21,Eigenmodes of twisted spin-waves in a thick ferromagnetic nanodisk,"Magnetic vortex is topologically nontrivial and commonly found in ferromagnetic nanodisks. So far, three classes spin-wave eigenmodes, i.e., gyrotropic, azimuthal and radial modes, have been identified in ferromagnetic nanodisks. Here, using micromagnetic simulation and analytical calculation, we reveal twisted spin-wave modes in a thick permalloy (Ni0.8Fe0.2) nanodisk. The twisted spin-waves carry topological charges, which sign depends on the core polarity of the magnetic vortex in the nanodisk. By applying rotating magnetic fields at one end of the sample, we observe continuous generation of twisted spin-waves that have characteristic spiral phase front and carry topological charge l = 1, -1, 2 and -2. The dispersion relation of twisted spin-waves is derived analytically and the result is in good agreement with micromagnetic numerical calculations.",2201.08621v1 2022-01-28,Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe3-xGeTe2,"Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal that the competing magnetic fluctuations are driven by an orbital selective Mott transition, where only the plane-perpendicular a1g orbital of the Fe(3d) manifold remains itinerant. Our results highlight the multi-orbital character in Fe3-xGeTe2 that supports a rare coexistence of local and itinerant physics within this material.",2201.12375v2 2022-01-29,Gibbs measures of the Ising model with mixed spin-1 and spin-1/2 on a Cayley tree,"In the present paper, the Ising model with mixed spin-(1,1/2) is considered on the second order Cayley tree. A construction of splitting Gibbs measures corresponding the model is given which allows to establish the existence of the phase transition (non-uniqueness of Gibbs measures). We point out that, in the phase transition region, the considered model has three translation-invariant Gibbs measures in the ferromagnetic and anti-ferromagnetic regimes, while the classical Ising model does not possesses such Gibbs measures in the anti-ferromagnetic regime. It turns out that the considered model, like the Ising model, exhibits a disordered Gibbs measure. Therefore, non-extremity and extremity of such disordered Gibbs measures is investigated by means of tree-indexed Markov chains.",2201.12615v1 2022-02-02,Microscopic Origin Of Room Temperature Ferromagnetism in a Double Perovskite Sr$_2$FeReO$_6$: a first principle and model Hamiltonian study,"The puzzling observation of room temperature ferromagnetism in double perovskites (A$_2$BB$'$O$_6$), despite having the magnetic lattice of B-ions diluted by non-magnetic B$'$-ions, have been examined for Sr$_2$FeReO$_6$. {\it Ab-initio} spin spiral electronic structure calculations along various high symmetry directions in reciprocal space are used to determine the exchange interactions entering an extended Heisenberg model, which is then solved classically using Monte Carlo simulations to determine the ferromagnetic transition temperature T$_c$. We find that one must consider onsite Coulomb interactions at the nonmagnetic Re sites ($U$) in order to obtain a T$_c$ close to the experimental value. Analysis of the $ab$-$initio$ electronic structure as well as an appropriate model Hamiltonian trace the origin of enhancement in T$_c$ with $U$ to the enhanced exchange splitting that is introduced at these sites. This in turn destabilizes the antiferromagnetic exchange channels, thereby enhancing the T$_c$. The role of occupancy at the non-magnetic sites is examined by contrasting with the case of Sr$_2$FeMoO$_6$.",2202.00994v1 2022-02-02,Defect Controlled Ferromagnetic Ordering in Au Implanted TiSe$_2$ Nanocrystals,"Layered transition metal dichalcogenides (TMDs) are attracting increasing attention because they exhibit unconventional magnetic properties due to crystal imperfections in their usually non-magnetic 2D structure. This work aims to investigate the magnetic response of self-engineered Se deficient TiSe$_2$ thin films, synthesized using chemical vapour deposition. We demonstrate tunability of the ferromagnetic order with the introduction of Au atoms using low energy Au ion implantation, which works as a controlling knob to vary the stoichiometry of Se in TiSe$_{2-x}$. The corresponding isothermal field-magnetization curves fit well with a modified Brillouin J function with J value of 1.5 for Ti$^{3+}$, and 4 for Au$^{3+}$, accounting for the diamagnetism that arises from Au implantation. We propose a qualitative model for the experimentally observed magnetization as a function of ion fluence, corroborated with high-resolution transmission electron microscopy. Depending on the Au nanoparticle size in the implanted samples, magnetization saturates faster at a much lower applied magnetic field than the pristine sample. Our findings hold potential to expand the range of 2D ferromagnetic materials for spintronics and magnetic sensing applications.",2202.01152v1 2022-02-03,Magnetism of Al$_x$Fe$_{2-x}$GeO$_5$ with Andalusite Structure,"The magnetism of Al$_x$Fe$_{2-x}$GeO$_5$ from $x$ = 0.09 to $x$ = 0.91 with andalusite structure was examined. The magnetic properties of Al$_x$Fe$_{2-x}$GeO$_5$ at low temperatures were found to be weak ferromagnetic-like state for $x$ $<$ 0.3 and spin-glass state for $x$ $>$ 0.3. The small spontaneous magnetization that appears in the weak ferromagnetic-like phase would be due to the presence of Dzyaloshinsky-Moriya interaction or to the difference in the magnitude of the magnetic moment of Fe$^{3+}$ in the octahedral and trigonal bipyramidal sites. The appearance of the spin-glass phase indicates that the dilution of Fe ions by Al ions in Al$_x$Fe$_{2-x}$GeO$_5$ causes the competition between ferromagnetic and antiferromagnetic interactions. The $x$ dependence of the site occupancy of Fe ions suggests that Fe$_2$GeO$_5$ with andalusite structure cannot be synthesized.",2202.01491v1 2022-02-06,Theoretical Study on Anisotropic Magnetoresistance Effects of Arbitrary Directions of Current and Magnetization for Ferromagnets: Application to Transverse Anisotropic Magnetoresistance Effect,"We develop a theory of the anisotropic magnetoresistance (AMR) effects of arbitrary directions of current and magnetization for ferromagnets. Here, we use the electron scattering theory with the $s$--$s$ and $s$--$d$ scattering processes, where $s$ is the conduction electron state and $d$ is the localized d states. The resistivity due to electron scattering is expressed by the probability density of the d states of the current direction. The d states are numerically obtained by applying the exact diagonalization method to the Hamiltonian of the d states with the exchange field, crystal field, and spin--orbit interaction. Using the theory, we investigate the transverse AMR (TAMR) effect for strong ferromagnets with a crystal field of cubic or tetragonal symmetry. The cubic systems exhibit the fourfold symmetric TAMR effect, whereas the tetragonal systems show the twofold and fourfold symmetric TAMR effect. On the basis of the above results, we also comment on the experimental results of the TAMR effect for Fe$_4$N.",2202.02810v1 2022-02-18,Large anomalous Hall effect induced by weak ferromagnetism in the noncentrosymmetric antiferromagnet $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$,"We study the mechanism of the exceptionally large anomalous Hall effect (AHE) in the noncentrosymmetric antiferromagnet $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ by angle-resolved photoemission spectroscopy (ARPES) and magnetotransport measurements. From ARPES measurements of $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ and its family compounds ($\mathrm{Fe}\mathrm{Nb}_3\mathrm{S}_6$ and $\mathrm{Ni}\mathrm{Nb}_3\mathrm{S}_6$), we find a band dispersion unique to the Co intercalation existing near the Fermi level. We further demonstrate that a slight deficiency of sulfur in $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ eliminates the ferromagnetism and the AHE simultaneously while hardly changing the band structure, indicating that the weak ferromagnetism is responsible for the emergence of the large AHE. Based on our results, we propose Weyl points near the Fermi level to cause the large AHE.",2202.09280v1 2022-02-22,Certain exact many-body results for Hubbard model ground states testable in small quantum dot arrays,"We present several interesting phenomena related to flatband ferromagnetism in the Hubbard model. The first is a mathematical theorem stating certain conditions under which a flatband ferromagnetic must necessarily be degenerate with a nonferromagnetic state. This theorem is generally applicable and geometry-independent, but holds only for a small number of holes in an otherwise filled band. The second phenomenon is a peculiar example where the intuition fails that particles prefer to doubly occupy low-energy states before filling higher-energy states. Lastly, we show a pattern of ferromagnetism which appears in small pentagonal and hexagonal plaquettes at filling factors of roughly 3/10 and 1/4. These examples require only a small number of lattice sites, and may be observable in quantum dot arrays currently available as laboratory spin qubit arrays.",2202.11115v4 2022-02-23,Probing the magnetic band gap of the ferromagnetic topological insulator MnSb$_2$Te$_4$,"Mn-rich MnSb$_2$Te$_4$ is a ferromagnetic topological insulator with yet the highest Curie temperature T_C = 45-50 K. It exhibits a magnetic gap at the Dirac point of the topological surface state that disappears above T_C. By scanning tunneling spectroscopy, we probe this gap at different magnetic fields and temperatures. We firstly reveal that the gap size shrinks, when an in-plane magnetic field of up to B = 3 T is applied, but does not close completely as the magnetization is only partially rotated in-plane. This corroborates the magnetic origin of the gap and the complex magnetic structure. In addition, we demonstrate significant spatiotemporal fluctuations of the gap size at temperatures as low as T_C/2, above which the remanent magnetization indeed decays. This temperature is close to the antiferromagnetic transition temperature observed for bulk-type single crystals of MnSb$_2$Te$_4$, highlighting the important role of competing magnetic orders in the formation of the favorable ferromagnetic topological insulator. Our study, thus, provides crucial insights into the complex magnetic gap opening of topological insulators that is decisive for quantum anomalous Hall devices.",2202.11540v1 2022-03-07,Ultrafast optical observation of spin-pumping induced dynamic exchange coupling in ferromagnetic semiconductor/metal bilayer,"Spin angular momentum transfer in magnetic bilayers offers the possibility of ultrafast and low-loss operation for next-generation spintronic devices. We report the field- and temperature- dependent measurements on the magnetization precessions in Co$_2$FeAl/(Ga,Mn)As by time-resolved magneto-optical Kerr effect (TRMOKE). Analysis of the effective Gilbert damping and phase shift indicates a clear signature of an enhanced dynamic exchange coupling between the two ferromagnetic (FM) layers due to the reinforced spin pumping at resonance. The temperature dependence of the dynamic exchange-coupling reveals a primary contribution from the ferromagnetism in (Ga,Mn)As.",2203.03225v2 2022-03-08,Topological phase transition in magnon bands in a honeycomb ferromagnet driven by sublattice symmetry breaking,"Ferromagnetic honeycomb systems are known to exhibit a magnonic topological phase under the existence of the next-nearest neighbor Dzyaloshinskii-Moriya interaction (DMI). Motivated by the recent progress in the sublattice-specific control of magnetic anisotropy, we study the topological phase of magnon bands of honeycomb ferromagnetic monolayer and bilayer with the sublattice symmetry breaking due to the different anisotropy energy in the presence of the DMI. We show that there is a topological phase transition between the topological magnon insulator and the topologically trivial magnon phase driven by the change of the relative size of the DMI and the anisotropy differences between the sublattices. The magnon thermal Hall conductivity is proposed as an experimental probe of the magnon topology.",2203.03845v1 2022-03-08,Enhancement of thermal spin pumping by orbital angular momentum of rare earth iron garnet,"In a bilayer of ferromagnetic and non-magnetic metal, spin pumping can be generated by a thermal gradient. The spin current generation depends on the spin mixing conductance of the interface and the magnetic properties of the ferromagnetic layer. Due to its low intrinsic damping, rare earth iron garnet is often used for the ferromagnetic layer in the spin Seebeck experiment. However, it is actually a ferrimagnetic with antiferromagnetically coupled magnetic lattices and the contribution of rare earth magnetic lattice of rare earth iron garnet on thermal spin pumping is not well understand. Here we focus on the effect of magnetic properties of lanthanide and show that the orbital angular momentum of rare earth iron garnet enhances thermal spin current generation of lanthanide substituted yttrium iron garnet.",2203.03915v2 2022-03-09,"Interplay between Charge-Density-Wave, Superconductivity, and Ferromagnetism in CuIr2-xCrxTe4 Chalcogenides","We report the crystal structure, charge-density-wave (CDW), superconductivity (SC), and ferromagnetism (FM) in CuIr2-xCrxTe4 chalcogenides. Powder x-ray diffraction (PXRD) results reveal that CuIr2-xCrxTe4 series are distinguished between two structural types and three different regions: (i) layered trigonal structure region, (ii) mixed phase regions, (iii) spinel structure region. Besides, Cr substitution for Ir site results in rich physical properties including the collapse of CDW, the formation of dome-shaped like SC, and the emergence of magnetism. Cr doping slightly elevates the superconducting critical temperature (Tsc) to its highest Tsc = 2.9 K around x = 0.06. As x increases from 0.3 to 0.4, the ferromagnetic Curie temperature (Tc) increases from 175 to 260 K. However, the Tc remains unchanged in the spinel range from 1.9 to 2. This finding provides a comprehensive material platform for investigating the interplay between CDW, SC, and FM multipartite quantum states.",2203.04599v1 2022-03-17,Mechanism-based Tuning of Room-temperature Ferromagnetism in Mn-doped \b{eta}-Ga2O3 by Annealing Atmospher,"Mn-doped \b{eta}-Ga2O3 (GMO) films with room-temperature ferromagnetism (RTFM) are synthesized by polymer-assisted deposition and the effects of annealing atmosphere (air or pure O2 gas) on their structures and physical properties are investigated. The characterizations show that the concentrations of vacancy defects and Mn dopants in various valence states and lattice constants of the samples are all modulated by the annealing atmosphere. Notably, the samples annealed in air (GMO-air) exhibit a saturation magnetization as strong as 170% times that of the samples annealed in pure O2 gas (GMO-O2), which can be quantitatively explained by oxygen vacancy (VO) controlled ferromagnetism due to bound magnetic polarons established between delocalized hydrogenic electrons of VOs and local magnetic moments of Mn2+, Mn3+, and Mn4+ ions in the samples. Our results provide insights into mechanism-based tuning of RTFM in Ga2O3 and may be useful for design, fabrication, and application of related spintronic materials.",2203.09331v4 2022-03-22,Weak ferromagnetism linked to the high-temperature spiral phase of YBaCuFeO5,"The layered perovskite YBaCuFeO5 is a rare example of cycloidal spiral magnet whose ordering temperature Tspiral can be tuned far beyond room temperature by adjusting the degree of Cu2+/Fe3+ chemical disorder in the structure. This unusual property qualifies this material as one of the most promising spin-driven multiferroic candidates. However, very little is known about the response of the spiral to magnetic fields, crucial for magnetoelectric cross-control applications. Using bulk magnetization and neutron powder diffraction measurements under magnetic fields up to 9 T we report here the first temperature-magnetic field phase diagram of this material. Besides revealing a strong stability of the spiral state, our data uncover the presence of weak ferromagnetism coexisting with the spiral modulation. Since ferromagnets can be easily manipulated with magnetic fields, this observation opens new perspectives for the control of the spiral orientation, directly linked to the polarization direction, as well as for a possible future use of this material in technological applications.",2203.11767v1 2022-03-23,Unconventional Surface State Pairs in a High-Symmetry Lattice with Anti-ferromagnetic Band-folding,"Many complex magnetic structures in a high-symmetry lattice can arise from a superposition of well-defined magnetic wave vectors. These ""multi-q"" structures have garnered much attention because of interesting real-space spin textures such as skyrmions. However, the role multi-q structures play in the topology of electronic bands in momentum space has remained rather elusive. Here we show that the type-I anti-ferromagnetic 1q, 2q and 3q structures in an face-centered cubic sublattice with band inversion, such as NdBi, can induce unconventional surface state pairs inside the band-folding hybridization bulk gap. Our density functional theory calculations match well with the recent experimental observation of unconventional surface states with hole Fermi arc-like features and electron pockets below the Neel temperature. We further show that these multi-q structures have Dirac and Weyl nodes. Our work reveals the special role that band-folding from anti-ferromagnetism and multi-q structures can play in developing new types of surface states.",2203.12541v2 2022-03-26,Ferromagnetic Impurity Induced Majorana Zero Mode in Iron-Based Superconductor,"Recent experiments reported the puzzling zero energy modes associated with ferromagnetic impurities in some iron-based superconductors with topological band structures. Here, we show that the sufficiently strong exchange coupling between a ferromagnetic impurity and substrate can trigger a quantum phase transition, beyond which, the phase of the topological surface superconducting order parameter around the impurity acquires a sign-change. In such a case, we prove that a Kramers degenerate pair of Majorana modes can be induced at the boundary separating the two sign-change regimes and trapped around the impurity in the topological surface superconducting state. Furthermore, we show that our theory can explain the controversial observations and confusing features of the zero energy modes from recent experiments in some iron-based superconductors.",2203.14017v3 2022-03-30,Large surface acoustic wave nonreciprocity in synthetic antiferromagnets,"We have studied the transmission of surface acoustic waves (SAWs) in ferromagnetic/non-magnetic/ferromagnetic tryilayers. The SAW scattering matrix is studied for devices with various non-magnetic spacer thickness, which defines the strength of the interlayer exchange coupling. We find the SAW transmission amplitude depends on their propagation direction when the two ferromagnetic layers are coupled antiferromagnetically. The degree of such SAW nonreciprocity increases with increasing exchange coupling strength and reaches 37 dB/mm for a device with the thinnest spacer layer. These results show the potential of interlayer exchange coupled synthetic antiferromagnets for viable acoustic nonreciprocal transmission devices, such as circulators and isolators.",2203.16192v1 2022-03-30,Ferromagnetism and doublon localization in a Wannier-Hubbard chain,"We derive a ""Wannier-Hubbard"" model consisting of an array of overlapping atomic orbitals interacting via a local Coulomb interaction. Transforming to an orthogonal Wannier basis set, the resulting Hamiltonian displays long range hopping and interactions, with new terms such as correlated hopping and ferromagnetic direct exchange, among others. We numerically study the one-dimensional version of the model at half-filling using the density matrix renormalization group (DMRG) method, unveiling a rich phase diagram as a function of the interaction $U$ and the overlap $s$ with metallic, and ferromagnetic phases, separated by a ferrimagnetic region. Our results indicate a path toward understanding new emergent phases under pressure and beyond standard model Hamiltonians.",2203.16483v2 2022-04-01,Effect of interfacial spin mixing conductance on gyromagnetic ratio of Gd substituted Y$_{3}$Fe$_{5}$O$_{12}$,"Due to its low intrinsic damping, Y$_3$Fe$_5$O$_{12}$ and its substituted variations are often used for ferromagnetic layer at spin pumping experiment. Spin pumping is an interfacial spin current generation in the interface of ferromagnet and non-magnetic metal, governed by spin mixing conductance parameter $G^{\uparrow\downarrow}$. $G^{\uparrow\downarrow}$ has been shown to enhance the damping of the ferromagnetic layer. The theory suggested that the effect of $G^{\uparrow\downarrow}$ on gyromagnetic ratio only come from its negligible imaginary part. In this article, we show that the different damping of ferrimagnetic lattices induced by $G^{\uparrow\downarrow}$ can affect the gyromagnetic ratio of Gd-substituted Y$_3$Fe$_5$O$_{12}$.",2204.00310v1 2022-04-05,Schottky's forgotten step to the Ising model,"A longstanding problem in natural science and later in physics was the understanding of the existence of ferromagnetism and its disappearance under heating to high temperatures. Although a qualitative description was possible by the Curie-Weiss theory it was obvious that a microscopic model was necessary to explain the tendency of the elementary magnetons to prefer parallel ordering at low temperatures. Such a model was proposed in 1922 by W. Schottky within the old Bohr-Sommerfeld quantum mechanics and claimed to explain the high values of the Curie temperatures of certain ferromagnets. Based on this idea Ising formulated a new model for ferromagnetism in solids. Simultaneously the old quantum mechanics was replaced by new concepts of Heisenberg and Schr\""odinger and the discovery of spin. Thus Schottky's idea was outperformed and finally replaced in 1928 by Heisenberg exchange interaction. This led to a reformulation of Ising's model by Pauli at the Solvay conference in 1930. Nevertheless one might consider Schottky's idea as a forerunner of this development explaining and asserting that the main point is the Coulomb energy leading to the essential interaction of neighboring elementary magnets.",2204.02036v2 2022-04-06,Muon spin rotation and relaxation study on Nb$_{1-y}$Fe$_{2+y}$,"We present a detailed study of the magnetic properties of weakly ferromagnetic/quantum critical Nb$_{1-y}$Fe$_{2+y}$ using muon spin rotation and relaxation ($\mu$SR). By means of an angular dependent study of the muon spin rotation signal in applied magnetic fields on a single crystal in the paramagnetic state we establish the muon stopping site in the crystallographic lattice of NbFe$_2$. With this knowledge we develop models to describe the muon spin rotation and relaxation signals in the weakly ferromagnetic, spin density wave and quantum critical phases of Nb$_{1-y}$Fe$_{2+y}$ and fit the corresponding experimental data. In particular, we quantify the $\mu$SR response for quantum critical behavior in Nb$_{1.0117}$Fe$_{1.9883}$ and extract the influence of residual weak structural disorder. From our analysis, Nb$_{1-y}$Fe$_{2+y}$ emerges to be uniquely suited to study quantum criticality close to weak itinerant ferromagnetic order.",2204.02852v3 2022-04-14,Huge out-of-plane piezoelectric response in ferromagnetic monolayer NiClI,"The combination of piezoelectricity and ferromagnetic (FM) order in a two-dimensional (2D) material, namely 2D piezoelectric ferromagnetism (PFM), may open up unprecedented opportunities for novel device applications. Here, we predict an in-plane FM semiconductor Janus monolayer NiClI with considerably large magnetic anisotropy energy (MAE) of 1.439 meV, exhibiting dynamic, mechanical and thermal stabilities. The NiClI monolayer possesses larger in-plane piezoelectricity ($d_{11}$$=$5.21 pm/V) comparable to that of $\mathrm{MoS_2}$. Furthermore, NiClI has huge out-of-plane piezoelectricity ($d_{31}$$=$1.89 pm/V), which is highly desirable for ultrathin piezoelectric device application. It is proved that huge out-of-plane piezoelectricity is robust against electronic correlation, which confirms reliability of huge $d_{31}$. Finally, being analogous to NiClI, PFM with large out-of-plane piezoelectricity can also be achieved in the Janus monolayers of NiClBr and NiBrI, with the predicted $d_{31}$ of 0.73 pm/V and 1.15 pm/V, respectively. The predicted huge out-of-plane piezoelectric response makes Janus monolayer NiClI a good platform for multifunctional semiconductor spintronic applications, which is also compatible with the bottom/top gate technologies of conventional semiconductor nanoelectronic devices.",2204.06715v1 2022-04-14,Tuning the coexistence regime of incomplete and tubular skyrmions in ferro/ferri/ferromagnetic trilayers,"The development of skyrmionic devices requires a suitable tuning of material parameters in order to stabilize skyrmions and control their density. It has been demonstrated recently that different skyrmion types can be simultaneously stabilized at room temperature in heterostructures involving ferromagnets, ferrimagnets and heavy metals, offering a new platform of coding binary information in the type of skyrmion instead of the presence/absence of skyrmions. Here, we tune the energy landscape of the two skyrmion types in such heterostructures by engineering the geometrical and material parameters of the individual layers. We find that a fine adjustment of the ferromagnetic layer thickness and thus its magnetic anisotropy, allows the trilayer system to support either one of the skyrmion types or the coexistence of both and with varying densities.",2204.06808v2 2022-04-15,Dynamical mean-field theory study of a ferromagnetic CrI3 monolayer,"We have employed one of the well-known many-body techniques, density functional theory plus dynamical mean-field theory (DFT + DMFT), to investigate the electronic structure of ferromagnetic monolayer CrI3 as a function of temperature and hole-doping concentration. The computed magnetic susceptibility follows the Curie's law, indicating that the ferromagnetism of monolayer CrI3 originates from localized magnetic moments of Cr atoms rather than Stoner-type itinerant ones. The DFT + DMFT calculations show a different coherent temperature for each spin component, demonstrating apparent strong spin-dependent electronic correlation effects in monolayer CrI3. Furthermore, we have explored the doping-dependent electronic structure of monolayer CrI3 and found that its electronic and magnetic properties are easily tunable by the hole-doping.",2204.07307v1 2022-04-18,Influence of the presence of multiple resonances on material parameter determination using broadband ferromagnetic resonance spectroscopy,"The influence of the presence of multiple resonances in ferromagnetic resonance spectra on extracted material parameters is investigated using numerical simulations. Our results show that the systematic error of assuming an incorrect number of resonances for a material can lead to the extraction of material parameters that significantly deviate from any of the true material parameters. When noise is present in experimental ferromagnetic resonance spectra increasing the frequency range of the broadband characterization can significantly reduce the error-margins when the data is analyzed assuming the correct number of resonances present in the material. For the cases investigated in this study it was found that analyzing the data using a single resonance results in extracted gyromagnetic ratios and effective magnetization parameters that are consistent with the weighted average of the true material parameters. We further provide a cautionary example regarding the extraction of the inhomogeneous linewidth broadening and damping parameters of materials that contain an unknown number of resonances.",2204.08500v1 2022-04-23,Theory of inertial spin dynamics in anisotropic ferromagnets,"Recent experimental observation of inertial spin dynamics calls upon holistic reevaluation of the theoretical framework of magnetic resonance in ferromagnets. Here, we derive the secular equation of an inertial spin system in analogy to the ubiquitous Smit-Beljers formalism. We find that the frequency of precessional ferromagnetic resonances is decreased as compared to non-inertial case. We also find that the frequency of nutational resonances is generally increased due to the presence of magnetic anisotropy and applied magnetic field. We obtain exact solutions of the secular equation and approximations that employ the terminology of non-inertial theory and thus allow for convenient estimates of the inertial effects.",2204.11057v2 2022-05-02,"Enhanced Seebeck coefficient through the magnetic fluctuations in Sr$_2$Ru$_{1-x}M_x$O$_4$ ($M = $ Co, Mn)","The layered perovskite Sr$_2$RuO$_4$ is a most intensively studied superconductor, but its pairing mechanism, which is often coupled intimately with magnetic fluctuations in correlated materials, is still an open question. Here we present a systematic evolution of the Seebeck coefficient in Co- and Mn-substituted Sr$_2$RuO$_4$ single crystals, in which ferromagnetic and antiferromagnetic glassy states respectively emerge in proximity to the superconducting phase of the parent compound. We find that the Seebeck coefficient $S$ divided by temperature $T$, $S/T$, shows a maximum near characteristic temperatures seen in the irreversible magnetization $M_{\rm ir}$ in both of the Co- and Mn-substituted crystals, demonstrating both of the ferromagnetic and antiferromagnetic fluctuations to enhance the Seebeck coefficient. Interestingly, $S/T$ increases with lowering temperature in the parent compound, reminiscent of non-Fermi-liquid behavior, indicating an essential role of coexisting ferromagnetic and antiferromagnetic fluctuations for the itinerant electrons in Sr$_2$RuO$_4$.",2205.00599v2 2022-05-02,Complex network growth model: Possible isomorphism between nonextensive statistical mechanics and random geometry,"In the realm of Boltzmann-Gibbs statistical mechanics there are three well known isomorphic connections with random geometry, namely (i) the Kasteleyn-Fortuin theorem which connects the $\lambda \to 1$ limit of the $\lambda$-state Potts ferromagnet with bond percolation, (ii) the isomorphism which connects the $\lambda \to 0$ limit of the $\lambda$-state Potts ferromagnet with random resistor networks, and (iii) the de Gennes isomorphism which connects the $n \to 0$ limit of the $n$-vector ferromagnet with self-avoiding random walk in linear polymers. We provide here strong numerical evidence that a similar isomorphism appears to emerge connecting the energy $q$-exponential distribution $\propto e_q^{-\beta_q \varepsilon}$ (with $q=4/3$ and $\beta_q \omega_0 =10/3$) optimizing, under simple constraints, the nonadditive entropy $S_q$ with a specific geographic growth random model based on preferential attachment through exponentially-distributed weighted links, $\omega_0$ being the characteristic weight.",2205.00998v1 2022-04-26,In situ transport characterization of magnetic states in Nb/Co superconductor/ferromagnet heterostructures,"Employment of the non-trivial proximity effect in superconductor/ferromagnet (S/F) heterostructures for the creation of novel superconducting devices requires accurate control of magnetic states in complex thin-film multilayers. In this work, we study experimentally in-plane transport properties of microstructured Nb/Co multilayers. We apply various transport characterization techniques, including magnetoresistance, Hall effect, and the first-order-reversal-curves (FORC) analysis. We demonstrate how FORC can be used for detailed in situ characterization of magnetic states. It reveals that upon reduction of the external field, the magnetization in ferromagnetic layers first rotates in a coherent scissor-like manner, then switches abruptly into the antiparallel state and after that splits into the polydomain state, which gradually turns into the opposite parallel state.",2205.01534v1 2022-05-06,A Monte Carlo study on the temperature dependence of hysteresis loops in Ising Spin-1 Square Bilayers,"A Metropolis Monte Carlo simulation is used in this paper to investigate the temperature dependency of the hysteresis loops of a spin-1 bilayer with square monolayers. In this system, the atoms interact ferromagnetically in-plane, with either ferromagnetic or antiferromagnetic interplane interactions. The effects of four distinct combinations of the Hamiltonian parameters on the hysteresis behaviours are discussed in detail. The geometry of the hysteresis loops changes depending on how the exchange couplings are combined. With ferromagnetic interlayer coupling, only the central hysteresis loop opens while for the antiferromagnetic case, the hysteresis loop becomes a double loop for the specific combination of coupling strengths. Additionally, in all these cases, the area of the hysteresis loops grows with the gradual lowering of the temperature.",2205.03349v2 2022-06-01,"A hybrid ferromagnetic transmon qubit: circuit design, feasibility and detection protocols for magnetic fluctuations","We propose to exploit currently available tunnel ferromagnetic Josephson junctions to realize a hybrid superconducting qubit. We show that the characteristic hysteretic behavior of the ferromagnetic barrier provides an alternative and intrinsically digital tuning of the qubit frequency by means of magnetic field pulses. To illustrate functionalities and limitation of the device, we discuss the coupling to a read-out resonator and the effect of magnetic fluctuations. The possibility to use the qubit as a noise detector and its relevance to investigate the subtle interplay of magnetism and superconductivity is envisaged.",2206.00715v1 2022-06-14,Mesoscopic superconducting memory based on bistable magnetic textures,"With the ever-increasing energy need to process big data, the realization of low-power computing technologies, such as superconducting logic and memories, has become a pressing issue. Developing fast and non-volatile superconducting memory elements, however, remains a challenge. Superconductor-ferromagnet hybrid devices offer a promising solution, as they combine ultra-fast manipulation of spins with dissipationless readout. Here, we present a new type of non-volatile Josephson junction memory that utilizes the bistable magnetic texture of a single mesoscopic ferromagnet. We use micromagnetic simulations to design an ellipse-shaped planar junction structured from a Nb/Co bilayer. The ellipse can be prepared as uniformly magnetized or as a pair of vortices at zero applied field. The two states yield considerably different critical currents, enabling reliable electrical readout of the element. We describe the mechanism used to control the critical current by applying numerical calculations to quantify the local stray field from the ferromagnet, which shifts the superconducting interference pattern. By combining micromagnetic modeling with bistable spin-textured junctions, our approach presents a novel route towards realizing superconducting memory applications.",2206.06816v1 2022-06-15,Polarization dependent photoemission as a probe of the magnetic ground state in the layered ferromagnet VI3,"Layered ferromagnets are thrilling materials from both a fundamental and technological point of view. VI3 is an interesting example, with a complex magnetism that differentiates it from the first reported Cr based layered ferromagnets. Here, we show in an indirect way through Angle Resolved Photoemission Spectroscopy (ARPES) experiments, the importance of spin-orbit coupling setting the electronic properties of this material. Our light polarized photoemission measurements point to a ground state with a half-filled e'_+- doublet, where a gap opening is triggered by spin-orbit coupling enhanced by electronic correlations.",2206.07233v1 2022-06-16,Defect-induced edge ferromagnetism and fractional spin excitations of the SU(4) $π$-flux Hubbard model on honeycomb lattice,"Recently, a SU(4) $\pi$-flux Hubbard model on the honeycomb lattice has been proposed to study the spin-orbit excitations of $\alpha$-ZrCl$_3$ [Phys.~Rev.~Lett. 121.097201~(2017)]. Based on this model with a zigzag edge, we show the edge defects can induce edge flat bands that result in a SU(4) edge ferromagnetism. We develop an effective one-dimensional interaction Hamiltonian to study the corresponding SU(4) spin excitations. Remarkably, SU(4) spin excitations of the edge ferromagnet appear as a continuum covering the entire energy region rather than usual magnons. Through further entanglement entropy analysis, we suggest that the continuum consists of fractionalized spin excitations from the disappeared magnons, except for that from the particle-hole Stoner excitations. Moreover, in ribbon systems with finite widths, the disappeared magnons can be restored in the gap formed by the finite-size effect and the optical branch of the restored magnons are found to be topological nontrivial.",2206.08036v1 2022-06-17,Magnetism and Superconductivity in Hydrogenated Graphite Foils,"Unique to certain unconventional superconductors is the coexistence of magnetism and superconductivity. We have previously found ferromagnetism and superconductivity in hydrogenated graphitic materials. Herein we present similar as well as completely new findings this time applicable to hydrogenated graphite foils. As the strength of the magnetic field is increased, the temperature-dependent magnetization shows several important transitions. From a Neel paramagnetic-antiferromagnetic transition, to a ferromagnetic superconductor state, to an orbital paramagnetic glass high-temperature superconductor with critical temperature for the dominant phase at Tc = 50 to 60 K. The ferromagnetic state is observed up to room temperature. Thus, the magnetism of hydrogenated low-density carbon graphite foils plays an important role in establishing electronic correlations of which some are superconducting in nature.",2206.08562v1 2022-06-23,Anomalous ferromagnetic behavior in the orthorhombic Li$_3$Co$_2$SbO$_6$,"Monoclinic Li$_3$Co$_2$SbO$_6$ has been proposed as a Kitaev spin liquid candidate and investigated intensively, whereas the properties of its polymorph, the orthorhombic phase, is less known. Here we report the magnetic properties of the orthorhombic Li$_3$Co$_2$SbO$_6$ as revealed by dc and ac magnetic susceptibility, muon spin relaxation ($\mu$SR) and neutron diffraction measurements. Successive magnetic transitions at (115, 89 and 71) K were observed in the low field dc susceptibility measurements. The transitions below $T_N$ (= 115 K), are suppressed in higher applied fields. However, zero field, ac susceptibility measurements reveals distinct frequency independent transitions at about (114, 107, 97, 79 and 71) K. A long range magnetic ordered state was confirmed by specific heat, $\mu$SR and neutron diffraction measurements, all indicating a single transition at about 115 K. The discrepancy between different measurements is attributed to possible stacking faults and/or local disorders of the ferromagnetic zig-zag chains, resulting in ferromagnetic boundaries within the overall antiferromagnetic matrix.",2206.11466v1 2022-06-23,Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe$_3$,"Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and break-down of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons.",2206.11962v1 2022-07-01,Ferromagnetic diagonal stripe states in the two-dimensional Hubbard model with $U\lesssim\infty$,"We have performed a variational Monte Carlo simulation to study the ground state of a two-dimensional Hubbard model on a square lattice in the strong coupling region. The energy gain of possible inhomogeneous electron states are computed as a function of $U$ when the hole density $\epsilon=1/8$ and next nearest-neighbor hopping $t'/t=-0.30$. The bond-centered ferromagnetic diagonal stripe state is stabilized in the strong coupling region ($U/t\geq$16), which is due to the gain of both kinetic energy and on-site Coulomb interaction energy due to the holon moving over the ferromagnetic domain and the gain of kinetic-exchange-interaction energy at the antiferromagnetic domain wall.",2207.00250v1 2022-07-05,"Observation of enhanced spin-spin correlations at triple point in 2D ferromagnetic Cr2X2Te6 (X=Si, Ge)","The domain dynamics and spin-spin correlation of 2D ferromagnets Cr2X2Te6 (X=Si, Ge) are investigated by a composite magnetoelectric method. The magnetic field-temperature phase diagrams for both in-plane and out-of-plane magnetic fields disclose a triple point around TC and 1 kOe, where ferromagnetic, paramagnetic, and spin-polarized phases coexist. The magnetoelectric signal shows peak features at the phase boundaries and reaches the maximum at the triple point, suggesting significant enhancement of spin-spin correlations at this point. A comparison between two systems reveals that the spin-spin correlations in Cr2Si2Te6 are stronger than that in Cr2Ge2Te6.",2207.01846v1 2022-07-10,Out-of-plane high-temperature ferromagnetic monolayer CrSCl with large vertical piezoelectric response,"For two-dimensional (2D) material, piezoelectric ferromagnetism (PFM) with large out-of-plane piezoresponse is highly desirable for multifunctional ultrathin piezoelectric device application. Here, we predict that Janus monolayer CrSCl is an out-of-plane ferromagnetic (FM) semiconductor with large vertical piezoelectric response and high Curie temperature. The predicted out-of-plane piezoelectric strain coefficient $d_{31}$ is -1.58 pm/V, which is higher than ones of most 2D materials (compare absolute values of $d_{31}$). The large out-of-plane piezoelectricity is robust against electronic correlation and biaxial strain, confirming reliability of large $d_{31}$. Calculated results show that tensile strain is conducive to high Curie temperature, large magnetic anisotropy energy (MAE) and large $d_{31}$. Finally, by comparing $d_{31}$ of CrYX (Y=S; X=Cl, Br I) and CrYX (Y=O; X=F, Cl, Br), we conclude that the size of $d_{31}$ is positively related to electronegativity difference of X and Y atoms. Such findings can provide valuable guidelines for designing 2D piezoelectric materials with large vertical piezoelectric response.",2207.04425v1 2022-07-18,Nutational switching in ferromagnets and antiferromagnets,"It was demonstrated recently that on ultrashort time scales magnetization dynamics does not only exhibit precession but also nutation. Here, we investigate how nutation can contribute to spin switching leading towards ultrafast data writing. We use analytic theory and atomistic spin simulations to discuss the behavior of ferromagnets and antiferromagnets in high-frequency magnetic fields. In ferromagnets, linearly polarized fields align the magnetization perpendicular to the external field, enabling $90^{\circ}$ switching. For circularly polarized fields in the $xy$ plane, the magnetization tilts to the $z$ direction. During this tilting, it rotates around the $z$ axis, allowing $180^{\circ}$ switching. In antiferromagnets, external fields with frequencies higher than the nutation frequency align the order parameter parallel to the field direction, while for lower frequencies it is oriented perpendicular to the field. The switching frequency increases with the magnetic field strength, and it deviates from the Larmor frequency, making it possible to outpace precessional switching in high magnetic fields.",2207.08566v1 2022-07-18,Magnetoelectric effects in Josephson junctions,"The review is devoted to the fundamental aspects and characteristic features of the magnetoelectric effects, reported in the literature on Josephson junctions (JJs). The main focus of the review is on the manifestations of the direct and inverse magnetoelectric effects in various types of Josephson systems. They provide a coupling of the magnetization in superconductor/ferromagnet/superconductor JJs to the Josephson current. The direct magnetoelectric effect is a driving force of spin torques acting on the ferromagnet inside the JJ. Therefore it is of key importance for the electrical control of the magnetization. The inverse magnetoelectric effect accounts for the back action of the magnetization dynamics on the Josephson subsystem, in particular, making the JJ to be in the resistive state in the presence of the magnetization dynamics of any origin. The perspectives of the coupling of the magnetization in JJs with ferromagnetic interlayers to the Josephson current via the magnetoelectric effects are discussed.",2207.08876v1 2022-07-21,Evidence for electronic signature of magnetic transition in topological magnet HoSbTe,"Topological insulators with intrinsic magnetic order are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. To study these systems and their physics, people have proposed a variety of magnetic topological insulator systems, including HoSbTe, an antiferromagnetic weak topological insulator candidate. In this work, we use scanning tunneling microscopy to probe the electronic structure of HoSbTe with antiferromagnetic and ferromagnetic orders that are tuned by applying an external magnetic field. Although around the Fermi energy, we find minor differences between the quasi-particle interferences under the ferromagnetic and antiferromagnetic orders, deep inside the valance region, a new quasi-particle interference signal emerges with ferromagnetism. This observation is consistent with our first-principles calculations indicating the magnetism-driven transition of the electronic states in this spin-orbit coupled topological magnet.",2207.10291v1 2022-07-22,Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer,"In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature (Tc) in thick samples. It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of Tc, which is distinct from both the monolayer and the bulk. Our observation potentially offers a 2D localized spin system on honeycomb lattice to explore magnetic frustration.",2207.10945v1 2022-07-23,Enhanced Curie temperature and skyrmion stability in room temperature ferromagnetic semiconductor CrISe monolayer,"We report CrISe monolayer as a room temperature ferromagnetic semiconductor with the Curie temperature ($T_C$), magnetic anisotropy energy (MAE) and band gap being 322 K, 113 $\mu$eV and 0.67 eV, respectively. The $T_C$ and MAE can be further enhanced up to 385 K and 313 $\mu$eV by tensile strain. More interestingly, the magnetic easy axis can be switched between off-plane and in-plane by compressive strain. Particularly, due to the broken inversion symmetry and strong spin-orbit coupling of Se atoms, a large Dzyaloshinskii-Moriya interaction (DMI) of 2.40 meV is obtained. More importantly, by micromagnetic simulations, stable skyrmions with sub-10 nm radius are stabilized by the large DMI above room temperature in a wide range of strain from $-2\%$ to $6\%$. Our work demonstrates CrISe as a promising candidate for next-generation skyrmion-based information storage devices and provides guidance for the research of DMI and skyrmions in room temperature ferromagnetic semiconductors.",2207.11418v1 2022-07-26,Anderson-Higgs mass of magnons in superconductor/ferromagnet/superconductor systems,"Anderson-Higgs mechanism of mass generation is a generic concept in high-energy and condensed matter physics. It shows up through the Meissner effect providing the expulsion of static and low-frequency magnetic fields from superconductors. However, it does not affect propagating electromagnetic waves with a spectrum gap determined by the plasma frequency, which is too large to be sensitive to the superconducting transition. Here we demonstrate the spectroscopic manifestation of the Anderson-Higgs mass, showing that it determines the spectrum gap of magnons in superconductor/ferromagnet/superconductor multilayers. Moreover, we show that this effect has been observed in recent experiments as a spontaneous ferromagnetic resonance frequency shift in such systems. Our theory explains many unusual experimental features and suggests effective controls over the magnon spectrum with tunable spectral gap and group-velocity reversal. These findings pave the way to a wide range of advanced functionalities for possible applications in magnonics.",2207.13201v2 2022-07-27,Selectively controlled ferromagnets by electric fields in van der Waals ferromagnetic heterojunctions,"Charge transfer plays a key role at the interfaces of heterostructures, which can affect electronic structures and ultimately the physical properties of the materials. However, charge transfer is difficult to manipulate externally once the interface formed. Here, we report electrically tunable charge transfer in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 all-magnetic van der Waals heterostructures, which can be exploited to selectively modify the magnetic properties of the top or bottom Fe3GeTe2 electrodes. The directional charge transfer from metallic Fe3GeTe2 to semiconducting Cr2Ge2Te6 remarkably modifies magnetic anisotropy energy of Fe3GeTe2, leading to the dramatically suppressed coercivity. The electrically selective control of ferromagnets demonstrated in this study could stimulate the development of spintronic devices based on van der Waals magnets.",2207.13338v1 2022-07-25,Integrable generalized Heisenberg ferromagnet equations with self-consistent potentials and related Yajima-Oikawa type equations,"We consider some nonlinear models describing interactions of long and short (LS) waves. Such LS models have been derived and proposed with various motivations, which mainly come from fluid and plasma physics. In this paper, we study some of integrable LS models, namely, the Yajima-Oikawa equation, the Newell equation, the Ma equation, the Geng-Li equation and etc. In particular, the gauge equivalent counterparts of these integrable LS models (equations) are found. In fact, these gauge equivalents of the LS equations are integrable generalized Heisenberg ferromagnet equations (HFE) with self-consistent potentials (HFESCP). The associated Lax representations of these HFESCP are given. We also presented several spin-phonon equations which describe nonlinear interactions of spin and lattice subsystems in ferromagnetic materials.",2207.13520v1 2022-07-28,Magnetic properties of CrSnS3: A new Van der Waals ferromagnet,"We report an experimental discovery of CrSnS3, a new Van der Waals ferromagnetic (FM) with Curie temperature TC ~119K. The Curie temperature is in qualitative agreement of Chittari et. al. DFT prediction (TC =112.3K) for an Ising model with the addition of Coulomb potential. The FM ordering temperature in CrSnS3 is the highest among Cr-based Van der Waals materials: CrI3, CrSiTe3 and CrGeTe3. The dc-susceptibility revealed a sharp increase at Tc (~119K) indicative of a first order transition; then raises to a wide maximum at AFM Neel temperature TN~90K.The ac-susceptibility reveals the presence of two sharp peaks; one close to Tc (119K) and the other at TN~90K. Ac-susceptibility measurements in an applied dc-magnetic field indicates that CrSnS3 undergoes two successive phase transitions: a sharp increase in the susceptibility indicates a first order PM to FM at 119K, followed by more gradual decrease at the onset of AFM transition at TN~90K. The magnetization isotherms below 30K indicates that the materials orders in an AFM state; while the magnetic isotherms at T >30K rises sharply to its saturated values confirming the ferromagnetic state.",2207.14129v1 2022-08-02,Proximity-Enhanced Magnetocaloric Effect in Ferromagnetic Trilayers,"The demagnetization and associated magnetocaloric effect in strong-weak-strong ferromagnetic trilayers, upon a reorientation of the strong ferromagnets from parallel to antiparallel magnetization, is simulated using atomistic spin dynamics. The simulations yield non-trivial spin distributions in the antiparallel state, which in turn allows entropy to be calculated directly. Empirical functional forms are obtained for the magnetization distribution in the spacer, differing significantly from some of the commonly used models. Finally, we find that the magnetocaloric effect in the system can be significantly improved by allowing the local exchange to vary through the spacer, which in practice can be implemented by spatially tailoring the spacer's magnetic dilution.",2208.01379v3 2022-08-10,Efficient spin-to-charge interconversion in Weyl semimetal TaP at room temperature,"In this paper we present spin-to-charge current conversion properties in the Weyl semimetal TaP by means of the inverse Rashba-Edelstein effect (IREE) with the integration of this quantum material with the ferromagnetic metal Permalloy $(Py=Ni_{81}Fe_{19})$. The spin currents are generated in the Py layer by the spin pumping effect (SPE) from microwave-driven ferromagnetic resonance and are detected by a dc voltage along the TaP crystal, at room temperature. We observe a field-symmetric voltage signal without the contamination of asymmetrical lines due to spin rectification effects observed in studies using metallic ferromagnets. The observed voltage is attributed to spin-to-charge current conversion based on the IREE, made possible by the spin-orbit coupling induced intrinsically by the bulk band structure of Weyl semimetals. The measured IREE coefficient ${\lambda}_{IREE}=(0.30 \pm{0.01})$ nm is two orders of magnitude larger than in graphene and is comparable to or larger than the values reported for some metallic interfaces and for several topological insulators.",2208.05151v1 2022-08-10,Spin-carrier coupling induced ferromagnetism and giant resistivity peak in EuCd$_2$P$_2$,"EuCd$_2$P$_2$ is notable for its unconventional transport: upon cooling the metallic resistivity changes slope and begins to increase, ultimately 100-fold, before returning to its metallic value. Surprisingly, this giant peak occurs at 18K, well above the N\'{e}el temperature ($T_N$) of 11.5K. Using a suite of sensitive probes of magnetism, including resonant x-ray scattering and magneto-optical polarimetry, we have discovered that ferromagnetic order onsets above $T_N$ in the temperature range of the resistivity peak. The observation of inverted hysteresis in this regime shows that ferromagnetism is promoted by coupling of localized spins and itinerant carriers. The resulting carrier localization is confirmed by optical conductivity measurements.",2208.05499v1 2022-08-11,Ferromagnetism in the SU(N) Kondo lattice model -- SU(N) double exchange and supersymmetry,"We study the ground-state properties of the SU(N)-generalization of the Kondo-lattice model in one dimension when the Kondo coupling J_K (both ferromagnetic and antiferromagnetic) is sufficiently strong. Both cases can be realized using alkaline-earth-like cold gases in optical lattices. Specifically, we first carry out the strong-coupling expansion and identify two insulating phases (one of which is the SU(N)-analogue of the well-known gapped Kondo singlet phase). We then rigorously establish that the ground state in the low-density (for J_K<0) or the high-density (for J_K>0) region is ferromagnetic. The results are accounted for by generalizing the double-exchange mechanism to SU(N) ""spins"". Possible realizations of Bose-Fermi supersymmetry SU(N|1) in the (generalized) SU(N) Kondo-lattice model are discussed as well.",2208.05899v2 2022-08-22,Absence of spontaneous time-reversal symmetry breaking and ferromagnetism in superconducting NiBi3 single crystal,"Recent experiments have pointed to a chiral p-wave-like superconductivity in epitaxial Bi/Ni bilayers that are spontaneously time-reversal symmetry breaking (TRSB), making it a promising platform for exploring physics useful for topologically protected quantum computing. Quite intriguingly, evidence has emerged that in non-epitaxial Bi/Ni bilayers, superconductivity arises due to the formation of NiBi3, which has been reported to host coexisting ferromagnetic and superconducting orders at the surface. We perform high resolution surface magneto-optic Kerr effect (SMOKE) measurements using a Sagnac interferometer on single crystal NiBi3 and find no sign of any spontaneous Kerr signal except for contributions from trapped vortices. This strongly indicates the absence of TRSB in NiBi3, whether due to TRSB in the superconducting state or any coexisting ferromagnetism, and we conclude that the superconductivity found in non-epitaxial Bi/Ni is distinctively different from that in epitaxial Bi/Ni.",2208.10645v2 2022-09-01,Magneto-optical Kerr effect and magnetoelasticity in weak ferromagnetic RuF$_4$ monolayer,"Considerable research interest has been attracted to noncollinear magnetic structures for their intriguing physics and promising applications. In this work, based on relativistic density functional theory, we reveal the interesting magnetic order and relevant properties in monolayer RuF$_4$, which can be exfoliated from its bulk phase. Although the spins on Ru ions are almost antiferromagnetically aligned between nearest-neighbors, weak ferromagnetism is generated because of the antisymmetric Dzyaloshinskii-Moriya interaction as well as the single-ion anisotropy. A prominent magneto-optical Kerr effect can be observed for this antiferromagnet, similar to those of regular strong ferromagnets. In addition, a uniaxial strain can induce a ferroelastic switching together with the in-plane rotation of spin direction, giving rise to a strong intrinsic magnetoelasticity. Our work not only suggests an alternative direction for two-dimensional magnetic materials, but also provides hints to future devices based on antiferromagnetic magnetoelastic or magneto-optical materials.",2209.00178v1 2022-09-29,"Magnetic properties, electrical resistivity, and hardness of high-entropy alloys FeCoNiPd and FeCoNiPt","We report the magnetic properties, electrical resistivity, and Vickers microhardness of as-cast and annealed high-entropy alloys (HEAs) FeCoNiPd and FeCoNiPt with the face-centered cubic structure. The heat treatment at 800 $^{\circ}$C does not largely affect the physical properties in each HEA. The values of the Curie temperature and the saturation moment at 50 K are 955 K and 1.458 $\mu_\mathrm{B}$/f.u. for the annealed FeCoNiPd, and 851 K and 1.456 $\mu_\mathrm{B}$/f.u. for the annealed FeCoNiPt, respectively. Each HEA is a soft ferromagnet and shows metallic resistivity. The electronic structure calculations of both HEAs support the ferromagnetic ground states. The comparisons between experimental and theoretical values are made for the Curie temperature, the saturation moment, and the residual resistivity. The Vickers microhardness of annealed FeCoNiPd and FeCoNiPt are both 188 HV. The hardness vs. valence electron count (VEC) per atom plot of these HEAs does not largely deviate from an expected universal relation forming a broad peak at VEC$\sim$6.8. This study would give some hints on designing a soft ferromagnetic HEA with high hardness.",2209.14506v1 2022-10-05,Detection of long-range orbital-Hall torques,"We report and quantify a large orbital-Hall torque generated by Nb and Ru, which we identify from a strong dependence of torques on the ferromagnets. This is manifested as a sign reversal and strong enhancement in the damping-like torques measured in Nb (or Ru)/Ni bilayers as compared to Nb (or Ru)/FeCoB bilayers. The long-range nature of orbital transport in the ferromagnet is revealed by the thickness dependences of Ni in Nb (or Ru)/Ni bilayers which are markedly different from the regular spin absorption in the ferromagnet that takes place within a few angstroms and thus it uniquely distinguishes the orbital Hall torque from the spin Hall torque.",2210.02283v2 2022-10-03,Ferromagnetic Heisenberg model with the Dzyaloshinskii-Moriya interaction,"The spin-1/2 Heisenberg model is formulated in terms of a mean-field approximation (MFA) by using the matrix forms of spin operators $\hat{S}_x,\hat{S}_y$ and $\hat{S}_z$ in three-dimensions. The considered Hamiltonian consists of bilinear exchange interaction parameters $(J_x,J_y,J_z)$, Dzyaloshinskii-Moriya interactions $(\Delta_x,\Delta_y,\Delta_z)$ and external magnetic field components $(H_x,H_y,H_z)$. The magnetization and its components are obtained in the MFA with the general anisotropic case with $J_x\neq J_y \neq J_z$ for various values of coordination numbers $q$. Then, the thermal variations of magnetizations are investigated in detail to obtain the phase diagrams of the model for the isotropic case with $J_x=J_y=J_z>0$. It is found that the model exhibits ferromagnetic, paramagnetic, random phase regions and an extra ferromagnetic phase at which the components of magnetizations present branching.",2210.03542v1 2022-10-08,Compact Model of a Topological Transistor,"The precession of a ferromagnet leads to the injection of spin current and heat into an adjacent non-magnetic material. Besides, spin-orbit entanglement causes an additional charge current injection. Such a device has been recently proposed where a quantum-spin hall insulator (QSHI) in proximity to a ferromagnetic insulator (FI) and superconductor (SC) leads to the pumping of charge, spin, and heat. Here we build a circuit-compatible Verilog-A-based compact model for the QSHI-FI-SC device capable of generating two topologically robust modes enabling the device operation. Our model also captures the dependence on the ferromagnetic precision, drain voltage, and temperature with an excellent (> 99%) accuracy.",2210.03874v1 2022-10-10,Orbital order and ferromagnetism in LaMnO3 doped with Ga,"We study from first principles the magnetic, electronic, orbital and structural properties of the LaMnO3 doped with gallium replacing the Mn-site. The gallium doping reduces the Jahn-Teller effect, and consequently the bandgap. Surprisingly, the system does not go towards a metallic phase because of the Mn-bandwidth reduction. The Ga-doping tends to reduce the orbital order typical of bulk antiferromagnetic LaMnO3 and consequently weakens the antiferromagnetic phase. The Ga-doping favors the G-type orbital order and layered-ordered ferromagnetic perovskite at x=0.50, both effects contribute to the formation of the insulating ferromagnetic phase in LaMn1-xGaxO3.",2210.04516v1 2022-10-11,New Gd-based magnetic compound GdPt$_2$B with a chiral crystal structure,"Herein, we report the discovery of a novel Gd-based magnetic compound GdPt$_2$B with a chiral crystal structure. X-ray diffraction and chemical composition analyses reveal a CePt$_2$B-type crystal structure (space group: $P6_422$) for GdPt$_2$B. Moreover, we successfully grew single crystals of GdPt$_2$B using the Czochralski method. Magnetization measurements and the Curie$-$Weiss analysis demonstrate that the ferromagnetic interaction is dominant in GdPt$_2$B. A clear transition is observed in the temperature dependence of electrical resistivity, magnetic susceptibility, and specific heat at $T_{\rm O}$ = 87 K. The magnetic phase diagram of GdPt$_2$B, which consists of a field-polarized ferromagnetic region and a magnetically ordered region, resembles those of known chiral helimagnets. Furthermore, magnetic susceptibility measurements reveal a possible spin reorientation within the magnetically ordered phase in magnetic fields perpendicular to the screw axis. The results demonstrate that GdPt$_2$B is a suitable platform for investigating the competing effects of ferromagnetic and antisymmetric exchange interactions in rare-earth-based chiral compounds.",2210.05099v1 2022-10-19,Competing ferromagnetic superconducting states in europium-based iron pnictides,"In europium-based iron pnictides superconducting Fe-planes can be influenced by a Zeeman field originated from the neighboring Eu-planes. The field tends to induce spin-density waves with a ferromagnetic average which coexists with the superconducting order by forming complementary patterns of the superconducting and magnetic order parameters in a Fulde-Ferrell-Larkin-Ovchinnikov phase and a two-dimensional textured-superconducting phase. The hard gap around the Fermi energy disappears in these fragile inhomogeneous superconducting states, which features, instead, V-shaped spin-resolved local density of states. The inhomogeneous states are also competing with either a homogeneous superconducting or a homogeneous ferromagnetic state, manifesting the intertwining influences of the magnetic orders in Fe and Eu planes, the spin-density wave band structure, and the superconducting pairing order.",2210.10312v1 2022-10-24,Optimal anti-ferromagnets for light dark matter detection,"We propose anti-ferromagnets as optimal targets to hunt for sub-MeV dark matter with spin-dependent interactions. These materials allow for multi-magnon emission even for very small momentum transfers, and are therefore sensitive to dark matter particles as light as the keV. We use an effective theory to compute the event rates in a simple way. Among the materials studied here, we identify nickel oxide (a well-assessed anti-ferromagnet) as an ideal candidate target. Indeed, the propagation speed of its gapless magnons is very close to the typical dark matter velocity, allowing the absorption of all its kinetic energy, even through the emission of just a single magnon.",2210.13516v2 2022-10-26,Nonreciprocal collective magnetostatic wave modes in geometrically asymmetric bilayer structure with nonmagnetic spacer,"Nonreciprocity, i.e. inequivalence in amplitudes and frequencies of spin waves propagating in opposite directions, is a key property underlying functionality in prospective magnonic devices. Here we demonstrate experimentally and theoretically a simple approach to induce frequency nonreciprocity in a magnetostatically coupled ferromagnetic bilayer structure with a nonmagnetic spacer by its geometrical asymmetry. Using Brillouin light scattering, we show the formation of two collective spin wave modes in Fe$_{81}$Ga$_{19}$/Cu/Fe$_{81}$Ga$_{19}$ structure with different thicknesses of ferromagnetic layers. Experimental reconstruction and theoretical modeling of the dispersions of acoustic and optical collective spin wave modes reveal that both possess nonreciprocity reaching several percent at the wavenumber of $22~\cdot~10^4$ rad cm$^{-1}$. The analysis demonstrates that the shift of the amplitudes of counter-propagating coupled modes towards either of the layers is responsible for the nonreciprocity because of the pronounced dependence of spin wave frequency on the layers thickness. The proposed approach enables the design of multilayered ferromagnetic structures with a given spin wave dispersion for magnonic logic gates.",2210.14882v2 2022-10-27,Topological magnons in one-dimensional ferromagnetic Su-Schrieffer-Heeger model with anisotropic interaction,"Topological magnons in a one-dimensional (1D) ferromagnetic (FM) Su-Schrieffer-Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the inter-cellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI), also can induce the emergence of the non-trivial phase with two degenerate in-gap edge states separately localized at the two ends of the 1D chain, while the intracellular interactions instead unfavors the topological phase. The interplay among them has synergistic effects on the topological phase transition, very different from that in the two-dimensional (2D) ferromagnet. These results demonstrate that the 1D magnons possess rich topological phase diagrams distinctly different from the electronic version of the SSH model and even the 2D magnons. Due to the lower dimensional structural characteristics of this 1D topological magnonic system, the magnonic crystals can be constructed from bottom to top, which has important potential applications in the design of novel magnonic devices.",2210.15530v1 2022-10-30,Signature of topological band crossing in ferromagnetic Cr1/3NbSe2 epitaxial thin film,"In intercalated transition metal dichalcogenides (I-TMDC), transition metal intercalation introduces magnetic phases which in some cases induce topological band crossing. However, evidence of the topological properties remains elusive in such materials. Here we employ angle-resolved photoemission spectroscopy to reveal the band structure of epitaxially grown ferromagnetic Cr1/3NbSe2. Experimental evidence of the Weyl crossing shows Cr1/3NbSe2 to be a topological ferromagnet. This work highlights I-TMDC as platform towards the interplay of magnetic and topological physics in low-dimensional systems.",2210.16748v1 2022-11-03,Disentangling the magneto-optic Kerr effect of manganite epitaxial heterostructures,"Magneto-optic Kerr effect can probe the process of magnetization reversal in ferromagnetic thin films and thus be used as an alternative to magnetometry. Kerr effect is wavelength-dependent and the Kerr rotation can reverse sign, vanishing at particular wavelengths. We investigate epitaxial heterostructures of ferromagnetic manganite, La$_{0.7}$Sr$_{0.3}$Mn$_{0.9}$Ru$_{0.1}$O$_3$, by polar Kerr effect and magnetometry. The manganite layers are separated by or interfaced with a layer of nickelate, NdNiO$_3$. Kerr rotation hysteresis loops of trilayers, with two manganite layers of different thickness separated by a nickelate layer, have intriguing humplike features, when measured with light of 400 nm wavelength. By investigating additional reference samples we disentangle the contributions of the individual layers to the loops: we show that the humps originate from the opposite sense of the Kerr rotation of the two different ferromagnetic layers, combined with the additive behavior of the Kerr signal.",2211.01750v1 2022-11-04,Two-dimensional Rare-earth Halide Based Single Phase Triferroic,"Two-dimensional multiferroic materials are highly sought after due to their huge potential for applications in nanoelectronic and spintronic devices. Here, we predict, based on first-principle calculations, a single phase {\it triferroic} where three ferroic orders; ferromagnetism, ferroelectricity and ferroelasticity, coexist simultaneously in hole doped GdCl$_2$ monolayer (a ferromagnetic semiconductor). This is achieved by substituting 1/3rd of the Gd$^{2+}$ ions with Eu$^{2+}$ in the hexagonal structure of GdCl$_2$ monolayer. The resulting metallic state undergoes a bond-centered charge ordering driving a distortion in the hexagonal structure making it semiconducting again and {\it ferroelastic}. Further, the lattice distortion accompanied by a breaking of the lattice centrosymmetry renders a non-centrosymmetric charge distribution which makes the monolayer {\it ferroelectric}, at the same time. The two ferroic orders, ferroelectricity and ferroelasticity, present in Eu doped GdCl$_2$ monolayer are found to be strongly coupled making it a promising candidate for device applications. The doped monolayer remains a ferromagnetic semiconductor with large 4f magnetic moment just like the parent monolayer and possesses an even higher (out-of-plane) magnetic anisotropy energy (MAE) than its pristine counterpart as desired for two dimensional magnets to have high transition temperature.",2211.02398v1 2022-11-16,Superconductor/Ferromagnet Heterostructures: A Platform for Superconducting Spintronics and Quantum Computation,"The interplay between superconductivity and ferromagnetism in the superconductor/ferromagnet (SC/FM) heterostructures generates many interesting physical phenomena, including spin-triplet superconductivity, superconducting order parameter oscillation, and topological superconductivity. The unique physical properties make the SC/FM heterostructures as promising platforms for future superconducting spintronics and quantum computation applications. In this article, we review important research progress of SC/FM heterostructures from superconducting spintronics to quantum computation, and it is organized as follows. Firstly, we discuss the progress of spin current carriers in SC/FM heterostructures including Bogoliubov quasiparticles, superconducting vortex, and spin-triplet Cooper pairs which might be used for long-range spin transport. Then, we will describe the {\pi} Josephson junctions and its application for constructing {\pi} qubits. Finally, we will briefly review experimental signatures of Majorana states in the SC/FM heterostructures and the theoretically proposed manipulation, which could be useful to realize fault-tolerant topological quantum computing.",2211.08625v1 2022-11-16,Strongly coupled magnon-plasmon polaritons in graphene- 2D ferromagnet heterostructures,"Magnons and plasmons are two very different types of collective modes, acting on the spin and charge degrees of freedom, respectively. At first sight, the formation of hybrid plasmon-magnon polaritons in heterostructures of plasmonic and magnetic systems would face two challenges, the small mutual interaction, via Zeeman coupling of the electromagnetic field of the plasmon with the spins, and the energy mismatch, as in most systems plasmons have energies in the eV range, orders of magnitude larger than magnons. Here we show that graphene plasmons form polaritons with the magnons of two-dimensional ferrromagnetic insulators, placed up to to half a micron apart, with Rabi couplings in the range of 100 GHz (dramatically larger than cavity QED magnonics). This strong coupling is facilitated both by the small energy of graphene plasmons and the cooperative super-radiant nature of the plasmon-magnon coupling afforded by phase matching. We show that the Rabi coupling can be modulated both electrically and mechanically and we propose a attenuated total internal reflection experiment to implement ferromagnetic resonance experiments on 2D ferromagnets driven by plasmon excitation.",2211.08949v1 2022-11-19,Anomalous sign inversion of spin-orbit torque in ferromagnetic/nonmagnetic bilayer systems due to self-induced spin-orbit torque,"Self-induced spin-orbit torques (SI-SOTs) in ferromagnetic (FM) layers have been overlooked when estimating the spin Hall angle (SHA) of adjacent nonmagnetic (NM) layers. In this work, we observe anomalous sign inversion of the total SOT in the spin-torque ferromagnetic resonance due to the enhanced SI-SOT, and successfully rationalize the sign inversion through a theoretical calculation considering the SHE in both the NM and FM layers. The findings show that using an FM layer whose SHA sign is the same as that of the NM achieves efficient SOT-magnetization switching with the assistance of the SI-SOT. The contribution of the SI-SOT becomes salient for a weakly conductive NM layer, and conventional analyses that do not consider the SI-SOT can overestimate the SHA of the NM layer by a factor of more than 150.",2211.10692v1 2022-11-25,Single-walled Ising nanotube with opposite sign of interactions using Wang-Landau algorithm,"The effect of opposite sign of interactions in a single-walled Ising nanotube is investigated using the Wang-Landau algorithm. The thermodynamic observables are calculated from the estimated density of states (DOS) with and without the presence of an external magnetic field. Irrespective of the applied magnetic field, a symmetric trend of DOS is observed for opposite sign of interactions which is in contrast to the asymmetric trend for same sign of interactions. Further, two types of anti-ferromagnetic (AFM) orderings, namely A-type and C-type anti-ferromagnetic order, are observed for opposite sign of interactions. These AFM spin orientations are switched to ferromagnetic (FM) phase by increasing the applied magnetic field ($B$). However, the spin ordering changes from the ordered AFM/FM phase to a disordered paramagnetic phase by increasing the temperature. Phase diagram shows that these three phases coexist around $B=2.0$. This study indicates that, by properly tuning the magnetic properties, the single-walled nanotube can be used for fabrication of new types of magnetic storage nano materials.",2211.13880v1 2022-11-27,High-temperature ferromagnetism and strong $π$-conjugation feature in two-dimensional manganese tetranitride,"Two-dimensional (2D) magnetic materials have attracted tremendous research interest because of the promising application in the next-generation microelectronic devices. Here, by the first-principles calculations, we propose a two-dimensional ferromagnetic material with high Curie temperature, manganese tetranitride MnN$_4$ monolayer, which is a square-planar lattice made up of only one layer of atoms. The structure is demonstrated to be stable by the phonon spectra and the molecular dynamic simulations, and the stability is ascribed to the $\pi$-d conjugation between $\pi$ orbital of N=N bond and Mn $d$ orbital. More interestingly, the MnN$_4$ monolayer displays robust 2D ferromagnetism, which originates from the strong exchange couplings between Mn atoms due to the $\pi$-d conjugation. The high critical temperature of 247 K is determined by solving the Heisenberg model with the Monte Carlo method.",2211.14772v1 2022-11-30,Ferromagnetic Crossover within the Ferromagnetic Order of U$_{7}$Te$_{12}$,"We investigate the physical properties of a single crystal of uranium telluride U$_{7}$Te$_{12}$. We have confirmed that U$_{7}$Te$_{12}$ crystallizes in the hexagonal structure with three nonequivalent crystallographic uranium sites. The paramagnetic moments are estimated to be approximately 1 $\mu_{\rm B}$ per the uranium site, assuming a uniform moment on all the sites. A ferromagnetic phase transition occurs at $T_{\rm C}=48$ K, where the in-plane magnetization increases sharply, whereas the out-of-plane component does not increase significantly. With decreasing temperature further below $T_{\rm C}$ under field-cooling conditions, the out-of-plane component increases rapidly around $T^{\star}=26$ K. In contrast, the in-plane component hardly changes at $T^{\star}$. Specific heat measurement indicates no $\lambda$-type anomaly around $T^{\star}$, so this is a cross-over suggesting a reorientation of the ordering moments or successive magnetic ordering on the part of the multiple uranium sites.",2211.16760v1 2022-12-01,"Tuning Structural, Transport and Magnetic Properties of Epitaxial SrRuO3 through Ba-Substitution","The perovskite ruthenates (ARuO3, A = Ca, Ba, or Sr) exhibit unique properties owing to a subtle interplay of crystal structure and electronic-spin degrees of freedom. Here, we demonstrate an intriguing continuous tuning of crystal symmetry from orthorhombic to tetragonal (no octahedral rotations) phases in epitaxial SrRuO3 achieved via Ba-substitution (Sr1-xBaxRuO3 with 0 < x < 0.7). An initial Ba-substitution to SrRuO3 not only changes the ferromagnetic properties, but also tunes the perpendicular magnetic anisotropy via flattening the Ru-O-Ru bond angle (to 180{\deg}), resulting in the maximum Curie temperature and an extinction of RuO6 rotational distortions at x = 0.20. For x > 0.2, the suppression of RuO6 octahedral rotational distortion dominantly enhances the ferromagnetism in the system, though competing with the impact of the RuO6 tetragonal distortion. Further increasing x > 0.2 gradually enhances the tetragonal-type distortion, resulting in the tuning of Ru-4d orbital occupancy and suppression of ferromagnetism. Our results demonstrate that isovalent substitution of the A-site cations significantly and controllably impacts both electronic and magnetic properties of perovskite oxides.",2212.00267v2 2022-12-05,Prediction of topological phases in metastable ferromagnetic MPX$_3$ monolayers,"Density functional theory calculations are carried out to study the electronic and topological properties of $M$P$X_3$ ($M$ = Mn, Fe, Co, Ni, and $X$ = S, Se) monolayers in the ferromagnetic (FM) metastable magnetic state. We find that FM MnPSe$_3$ monolayers host topological semimetal signatures that are gapped out when spin-orbit coupling (SOC) is included. These findings are supported by explicit calculations of the Berry curvature and the Chern number. The choice of the Hubbard-$U$ parameter to describe the $d$-electrons is thoroughly discussed, as well as the influence of using a hybrid-functional approach. The presence of band inversions and the associated topological features are found to be formalism-dependent. Nevertheless, routes to achieve the topological phase via the application of external biaxial strain are demonstrated. Within the hybrid-functional picture, topological band structures are recovered under a pressure of 15% (17 GPa). The present work provides a potential avenue for uncovering new topological phases in metastable ferromagnetic phases.",2212.02431v2 2022-12-05,Spin wavepackets in the Kagome ferromagnet Fe$_3$Sn$_2$: propagation and precursors,"The propagation of spin waves in magnetically ordered systems has emerged as a potential means to shuttle quantum information over large distances. Conventionally, the arrival time of a spin wavepacket at a distance, $d$, is assumed to be determined by its group velocity, $v_g$. He we report time-resolved optical measurements of wavepacket propagation in the Kagome ferromagnet Fe$_3$Sn$_2$ that demonstrate the arrival of spin information at times significantly less than $d/v_g$. We show that this spin wave ""precursor"" originates from the interaction of light with the unusual spectrum of magnetostatic modes in Fe$_3$Sn$_2$. Related effects may have far-reaching consequences toward realizing long-range, ultrafast spin wave transport in both ferromagnetic and antiferromagnetic systems.",2212.02498v1 2022-12-11,Surface-induced ferromagnetism and anomalous Hall transport at Zr2S(001),"Two-dimensional layered electrides possessing anionic excess electrons in the interstitial spaces between cationic layers have attracted much attention due to their promising opportunities in both fundamental research and technological applications. Using first-principles calculations, we predict that the layered bulk electride Zr2S is nonmagnetic with massive Dirac nodal-line states arising from Zr-4d cationic and interlayer anionic electrons. However, the Zr2S(001) surface increases the density of states at the Fermi level caused by the surface potential, thereby inducing a ferromagnetic order at the outermost Zr layer via the Stoner instability. Consequently, the time-reversal symmetry breaking at the surface not only generates highly spin-polarized topological surface states with intricate helical spin textures, but also hosts an intrinsic anomalous Hall effect originating from the Berry curvature generated by spin-orbit coupling. Our findings offer a playground to investigate the emergence of ferromagnetism and anomalous Hall transport at the surface of nonmagnetic topological electrides.",2212.05472v1 2022-12-14,Effective interfacial Dzyaloshinskii-Moriya interaction and skyrmion stabilization in ferromagnet/paramagnet and ferromagnet/superconductor hybrid systems,"It is shown that a term in the form of Dzyaloshinskii-Moriya interaction (DMI) contributes to the free energy of a ferromagnetic (FM) film on a paramagnetic (PM) (an FM above the critical temperature, Tc) or superconducting (SC) substrate occurring in the London limit. This contribution results from magnetostatic interaction between the film and substrate under which the substrate affects FM magnetization back via its magnetic field produced by magnetization inhomogeneity in the film. Strikingly, in the FM/PM system this effective DMI stabilizes chiral magnetic textures, e.g., magnetic skyrmions (MSk's) of the Neel-type, which is in contrast to that in the FM/SC one. A strong temperature sensitivity of the effective DMI allows for tuning the coupling between the FM film and PM or SC substrate and thus controlling the MSk radius in FM/PM.",2212.07315v2 2022-12-19,NaRuO$_2$: Kitaev-Heisenberg exchange in triangular-lattice setting,"Kitaev exchange, a new paradigm in quantum magnetism research, occurs for 90$^{\circ}$ metal-ligand-metal links, $t_{2g}^5$ transition ions, and sizable spin-orbit coupling. It is being studied in honeycomb compounds but also on triangular lattices. While for the former it is known by now that the Kitaev intersite couplings are ferromagnetic, for the latter the situation is unclear. Here we pin down the exchange mechanisms and determine the effective coupling constants in the $t_{2g}^5$ triangular-lattice material NaRuO$_2$, recently found to host a quantum spin liquid ground state. We show that, compared to honeycomb compounds, the characteristic triangular-lattice cation surroundings dramatically affect exchange paths and effective coupling parameters, changing the Kitaev interactions to antiferromagnetic. The quantum chemical analysis and subsequent effective spin model computations provide perspective onto the nature of the experimentally observed quantum spin liquid -- it seemingly implies finite longer-range exchange, and the atypical proximity to ferromagnetic order is related to sizable ferromagnetic Heisenberg nearest-neighbor couplings.",2212.09365v2 2022-12-23,Thickness Dependent Itinerant Ferromagnetism in Ultrathin Ba-Doped SrRuO3 films,"The electronic and magnetic properties in ABO3 perovskite oxides are extremely sensitive to lattice structure but also the dimensionality, such as the thickness in thin film form. Here, we report the thickness-dependent electro-magnetic properties of ultrathin epitaxially stabilized Sr1-xBaxRuO3 (x = 0.08, 0.2) thin films on SrTiO3 (001) substrate. The results reveal that the Ba-doping (0.08 < x < 0.20) reduces RuO6 orthorhombic distortions existing in SrRuO3 and induces a tetragonal distortion as evidenced by out-of-plane lattice expansion. A metal-to-insulator transition, accompanied by a ferromagnetic to non-magnetic transition occurs with reducing film thickness from 10 to 3 unit-cell (u.c.) for both x = 0.08 and 0.2, regardless of the doping level. The results suggest that the effects of compositional vacancies and surface/interface contributions introduced via dimensional confinement are more dominant than A-site chemical disorder or structural distortion for the loss of metallicity and ferromagnetism in ultra-thin epitaxial films.",2212.12120v1 2022-12-27,"Datasets on materials research of hard ferromagnet in TM-Fe-Si (TM=Ti, Zr, Hf, V, Nb, and Ta) ternary systems","The datasets presented in this article are related to materials research on hard ferromagnet in TM-Fe-Si (TM=Ti, Zr, Hf, V, Nb, and Ta) ternary systems. The motivation for data collection is based on the research paper entitled ""Novel hard magnetic phase with Zr$_{11.5}$Fe$_{53}$Si$_{35.5}$ composition"". The datasets are composed of scanning electron microscope images, X-ray diffraction (XRD) patterns, and magnetization data for TM$_{7}$Fe$_{52}$Si$_{41}$ annealed at 1050 $^{\circ}$C. The chemical compositions of constituent phases were determined by an energy dispersive X-ray spectrometer (EDS). The phase analysis was performed using XRD and EDS results. The Curie temperature of each sample was obtained using magnetization data, and the coercive field was determined for hard ferromagnet samples Zr$_{7}$Fe$_{52}$Si$_{41}$ and Hf$_{7}$Fe$_{52}$Si$_{41}$. The datasets would be useful for developing an Fe-based rare-earth-free permanent magnet, which is one of the central issues of materials science.",2212.13595v1 2023-01-06,Interfacial magnetic anisotropy controlled spin pumping in Co60Fe20B20/Pt stack,"Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of CoFeB thin film tilts the magnetic easy axis from in-plane to out-of-plane due to surface anisotropy. The Ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 0.11 to 0.24 MA/m2, with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices.",2301.02370v1 2023-01-09,dc Josephson Effect in Altermagnets,"The ability of magnetic materials to modify superconducting systems is an active research area for possible applications in thermoelectricity, quantum sensing, and spintronics. We consider the fundamental properties of the Josephson effect in a third class of magnetic materials beyond ferromagnets and antiferromagnets: altermagnets. We show that despite having no net magnetization, altermagnets induce $0$-$\pi$ oscillations. The decay length and oscillation period of the Josephson coupling are qualitatively different from ferromagnetic junctions and depend on the crystallographic orientation of the altermagnet. The Josephson effect in altermagnets thus serves a dual purpose: it acts as a signature that distinguishes altermagnetism from conventional (anti)ferromagnetism and offers a way to tune the supercurrent via flow direction anisotropy.",2301.03603v2 2023-01-21,Hard Ferromagnets as a New Perspective on Materials for Thermomagnetic Power Generation Cycles,"We consider the ways in which magnetically hard materials can be used as the working materials in thermomagnetic power generation (TMG) cycles in order to expand the area in the magnetisation vs. applied field ($M-H$) plane available for energy conversion. There are 3 parts to this Perspective. First, experiments on commercially available hard ferrites reveal that, while these materials are not yet good TMG candidates, hard ferromagnets with higher thermal conductivity and a greater change of magnetization with temperature could outperform existing TMG materials. Second, computational results indicate that biasing a soft magnet with a hard ferromagnet is essentially equivalent to shifting the $M-H$ loop by an amount proportional to the field of the biasing magnet. Work outputs under biased conditions show a substantial improvement over unbiased cycles, but experimental verification is needed. Third, we discuss the rationale for exploring artificial spin reorientation materials as novel TMG working materials.",2301.08854v1 2023-01-22,Entanglement gap in 1D long-range quantum spherical models,"We investigate the finite-size scaling of the entanglement gap in the one dimensional long-range quantum spherical model (QSM). We focus on the weak long-range QSM, for which the thermodynamic limit is well-defined. This model exhibits a continuous phase transition, separating a paramagnetic from a ferromagnet phase. The universality class of the transition depends on the long-range exponent $\alpha$. We show that in the thermodynamic limit the entanglement gap is finite in the paramagnetic phase, and it vanishes in the ferromagnetic phase. In the ferromagnetic phase the entanglement gap is understood in terms of standard magnetic correlation functions. The entanglement gap decays as $\delta\xi\simeq C_\alpha L^{-(1/2-\alpha/4)}$, where the constant $C_\alpha$ depends on the low-energy properties of the model. This reflects that the lower part of the dispersion is affected by the long range physics. Finally, multiplicative logarithmic corrections are absent in the scaling of the entanglement gap, in contrast with the higher-dimensional case.",2301.09143v2 2023-02-04,"Room Temperature Ferroelectricity, Ferromagnetism, and Anomalous Hall Effect in Half-metallic Monolayer CrTe","Two-dimensional materials hosting ferroelectricity and ferromagnetism are crucial for low-power and high-speed information processing technologies. However, intrinsic 2D multiferroics in the monolayer limit are rare. Here, we demonstrate that monolayer CrTe, obtained by cleaving the [002] surface, is dynamically stable multiferroic at temperatures beyond room temperature. We show that it orders ferromagnetically with significant in-plane magnetocrystalline anisotropy, and it is a half-metal featuring a large half-metal gap. Remarkably, the broken inversion symmetry and buckled geometry of monolayer CrTe make it a ferroelectric with a large spontaneous out-of-plane polarization and significant magnetoelectric coupling. In addition, we demonstrate polarization or electric field-induced tunability of the anomalous Hall effect, accompanied by substantial bandstructure modulation. Our findings establish monolayer CrTe as a room-temperature multiferroic with great potential for applications in spintronics and ferroelectric devices.",2302.02145v2 2023-02-08,Nonsinusoidal current-phase relations in semiconductor-superconductor-ferromagnetic insulator devices,"Coherent tunneling processes of multiple Cooper pairs across a Josephson junction give rise to higher harmonics in the current phase relation. In this work, we propose and study Josephson junctions based on semiconductor-superconductor-ferromagnetic insulator heterostructures to engineer nonsinusoidal current-phase relations. The gate-tunability of charge carriers density in the semiconductor, together with the adjustable magnetization of the ferromagnetic insulator, provides control over the content of the supercurrent harmonics. At finite exchange field, hybrid junctions can undergo a 0\,--\,$\pi$ phase transition, resulting in the supercurrent reversal. Close to the transition, single-pair tunneling is suppressed and the current-phase relation is dominated by the second-harmonic, indicating transport primarily by pairs of Cooper pairs. Finally, we demonstrate that non-collinear magnetization or spin-orbit coupling in the leads and the junction can lead to a gate-tunable Josephson diode effect with efficiencies of up to $\sim30\%$.",2302.04267v2 2023-02-17,Effect of electron- and hole-doping on properties of kagome-lattice ferromagnet Fe3Sn2,"We report a theoretical investigation of effects of Mn and Co substitution in the transition metal sites of the kagome-lattice ferromagnet, Fe3Sn2. Herein, hole- and electron-doping effects of Fe3Sn2 have been studied by density-functional theory calculations on the parent phase and on the substituted structural models of Fe3-xMxSn2 (M = Mn, Co; x = 0.5, 1.0). All optimized structures favor the ferromagnetic ground state. Analysis of the electronic density of states (DOS) and band structure plots reveals that the hole (electron) doping leads to a progressive decrease (increase) in the magnetic moment per Fe atom and per unit cell overall. The high DOS is retained nearby the Fermi level in the case of both Mn and Co substitutions. The electron doping with Co results in the loss of nodal band degeneracies, while in the case of hole doping with Mn emergent nodal band degeneracies and flatbands initially are suppressed in Fe2.5Mn0.5Sn2 but re-emerge in Fe2MnSn2. These results provide key insights into potential modifications of intriguing coupling between electronic and spin degrees of freedom observed in Fe3Sn2.",2302.08944v1 2023-02-22,Anomalous Nernst effect induced terahertz emission in a single ferromagnetic film,"By developing a bidirectional-pump terahertz (THz) emission spectroscopy, we reveal an anomalous Nernst effect (ANE) induced THz emission in a single ferromagnetic film. Based on the distinctive symmetry of the THz signals, ANE is unequivocally distinguished from the previously attributed ultrafast demagnetization and anomalous Hall effect mechanisms. A quantitative method is established to separate the different contributions, demonstrating a significant ANE contribution that even overwhelms other competing mechanisms. Our work not only clarifies the origin of the ferromagnetic-based THz emission, but also offers a fertile platform for investigating the ultrafast magnetism and THz spintronics.",2302.11134v2 2023-02-27,Robust Spin Polarization of Yu-Shiba-Rusinov States in Superconductor/Ferromagnetic Insulator Heterostructures,"Yu-Shiba-Rusinov (YSR) states arise as sub-gap excitations of a magnetic impurity in a superconducting host. Taking into account the quantum nature of the impurity spin in a single-site approximation, we study the spectral properties of the YSR excitations of a system of magnetic impurity in a spin-split superconductor, i.e. a superconductor in proximity to a ferromagnetic insulator at zero external magnetic fields. The YSR excitations of this system exhibit a robust spin-polarization that is protected from fluctuations and environmental noise by the exchange field of the ferromagnetic insulator, which can be as large as a few Tesla. We compare the results of this quantum approach to the classical approach, which conventionally predicts fully polarized YSR excitations even in the absence of exchange and external magnetic field. Turning on a small magnetic field, we show the latter splits the YSR excitations in the regime where the impurity is strongly coupled to the superconductor, whilst the classical approach predicts no such splitting. The studied system can potentially be realized in a tunnel junction connected to a quantum dot in proximity to a spin-split superconductor.",2302.13864v1 2023-02-28,Uncertainties in experiments on strongly-coupled vacuum field modification of superconductivity and ferromagnetism in solids,"We discuss recent experiments in which fine particles of the organic superconductor Rb$_3$C$_{60}$ or the cuprate superconductor YBa$_2$Cu$_3$O$_{6+x}$ are held in a polystyrene film that is spin-coated on to a silicon substrate with or without an intervening gold, or another inert metallic layer. From SQUID magnetisation data for Rb$_3$C$_{60}$ there appears to be a striking and completely unexpected increase in the superconducting transition temperature from $30$ to $45$~K, which is ascribed to coupling between the electrons in the superconductor and vacuum fluctuations in the electromagnetic field just above the metallic film. We argue that this could be a non-intrinsic effect associated with the presence of solid oxygen in the Pyrex sample tube. We suggest that the ferromagnetic SQUID signal observed for YBa$_2$Cu$_3$O$_{6+x}$ particles in polystyrene could be attributed to ferromagnetic particles or magnetic clusters of unknown origin.",2303.00090v1 2023-03-03,Unusual coercivity and zero field stabilization of fully saturated magnetization in single crystals of LaCrGe$_3$,"LaCrGe$_3$ is an itinerant, metallic ferromagnet with a Curie temperature ($T_C$) of $\sim$ 86 K. Whereas LaCrGe$_3$ has been studied extensively as a function of pressure as an example of avoided ferromagnetic quantum criticality, questions about its ambient pressure ordered state remain; specifically, whether there is a change in the nature of the ferromagnetically ordered state below $T_C$ $\sim$ 86 K. We present anisotropic $M$($H$) isotherms, coupled with anisotropic AC susceptibility data, and demonstrate that LaCrGe$_3$ has a remarkable, low temperature coercivity associated with exceptionally sharp, complete magnetization reversals to and from fully polarized states. This coercivity is temperature dependent, it drops to zero in the 40 - 55 K region and reappears in the 70 - 85 K regions. At low temperatures LaCrGe$_3$ has magnetization loops and behavior that has previously associated with micromagnetic/nanocrystalline materials, not bulk, macroscopic samples.",2303.02062v1 2023-03-06,Strain Engineering of Photo-induced Topological Phases in 2D Ferromagnets,"We argue that strain engineering is a powerful tool which may facilitate the experimental realization and control of topological phases in laser-driven 2D ferromagnetic systems. To this extent, we show that by applying a circularly polarized laser field to a 2D honeycomb ferromagnet which is uniaxially strained in either the zig-zag or armchair direction, it is possible to generate a synthetic Dzyaloshinskii-Moriya interaction (DMI) tunable by the intensity of the applied electric field, as well as by the magnitude of applied strain. Such deformations enable transitions to phases with opposite sign of Chern number, or to trivial phases. These are basic results that could pave the way for the development of a new field of Strain Engineered Topological Spintronics (SETS).",2303.03305v3 2023-03-07,Electrically tunable Gilbert damping in van der Waals heterostructures of two-dimensional ferromagnetic metals and ferroelectrics,"Tuning the Gilbert damping of ferromagnetic (FM) metals via a nonvolatile way is of importance to exploit and design next-generation novel spintronic devices. Through systematical first-principles calculations, we study the magnetic properties of the van der Waals heterostructure of two-dimensional FM metal CrTe2 and ferroelectric (FE) In2Te3 monolayers. The ferromagnetism of CrTe2 is maintained in CrTe2/In2Te3 and its magnetic easy axis can be switched from in-plane to out-of-plane by reversing the FE polarization of In2Te3. Excitingly, we find that the Gilbert damping of CrTe2 is tunable when the FE polarization of In2Te3 is reversed from upward to downward. By analyzing the k-dependent contributions to the Gilbert damping, we unravel that such tunability results from the changed intersections between the bands of CrTe2 and Fermi level on the reversal of the FE polarizations of In2Te3 in CrTe2/In2Te3. Our work provides an appealing way to electrically tailor Gilbert dampings of two-dimensional FM metals by contacting them with ferroelectrics.",2303.03852v1 2023-03-07,Universal approach to p-wave triplet superconductivity in the Hubbard models,"Spin-triplet superconductivity is actively pursued in condensed matter physics due to the potential applications in topological quantum computations. The related pairing mechanism involving the interaction remains an important research topic. Here we propose a universal approach to obtain p-wave triplet superconductivity in the Hubbard models by simply changing the sign of the hopping amplitudes of the spin-down electrons, and apply it to three prototype two-dimensional lattices (honeycomb, square, and triangular). The parent Hamiltonian at half filling has long-range magnetic order, which is ferromagnetic in all three directions for the frustrated triangular lattices, and ferromagnetic (antiferromagnetic) in the xy plane (z direction) for the bipartite honeycomb and square lattices. The magnetic transitions occur at some critical interactions on honeycomb and triangular lattices, which are estimated by finite-size scalings. When the systems are doped, we find the triplet p-wave pairing is a dominating superconducting instability. We demonstrate its emergence is closely related to the strong ferromagnetic spin fluctuations induced by the doping. Our results provide an understanding of the microscopical triplet-pairing mechanism, and will be helpful in the search for spin-triplet superconducting materials.",2303.04004v1 2023-03-08,Andreev spin qubit: A nonadiabatic geometric gate,"We study a hybrid structure of a ferromagnetic-insulator and a superconductor connected by a weak link, which accommodates Andreev bound states whose spin degeneracy is lifted due to the exchange interaction with the ferromagnet. The resultant spin-resolved energy levels realize a two-state quantum system, provided that a single electron is trapped in the bound state, i.e., an Andreev spin qubit. The qubit state can be manipulated by controlling the magnetization dynamics of the ferromagnet, which mediates the coupling between external fields and the qubit. In particular, our hybrid structure provides a simple platform to manipulate and control the spin qubit using spintronic techniques. By employing a modified Hahn spin echo protocol for the magnetization dynamics, we show that our Andreev spin qubit can realize a nonadiabatic geometric gate.",2303.04344v1 2023-03-09,Antiferromagnetic resonances in superconductor-ferromagnet multilayers,"In this work, we study magnetization dynamics in superconductor-ferromagnet (S-F) thin-film multilayer. Theoretical considerations supported by the broad-band ferromagnetic resonance spectroscopy reveal development of acoustic and optic resonance modes in S-F multilayers at significantly higher frequencies in comparison to the Kittel mode of individual F-layers. These modes are formed due to antiferromagnetic-like interaction between F-layers via shared circulating superconducting currents in S-layers. The gap between resonance modes is determined by the thickness and superconducting penetration depth in S-layers. Overall, rich spectrum of S-F multilayers and its tunability opens wide prospects for application of these multialyers in magnonics as well as in various superconducting hybrid systems.",2303.05175v2 2023-03-10,Ultrafast magnetization reversal in ferromagnetic spin-valves: an s-d model perspective,"We present an extension to simple s-d models, aiming at simulating ultrafast magnetization dynamics and spin transport in metallic heterostructures. In particular, we consider an alternative spin dissipation channel due to a finite exchange splitting of the s band. From this theory, we show three different mechanisms governing the dynamics of spin accumulation. On top of the already widely discussed ""-dM/dt"" electron-magnon mechanism, we study the role of a dynamic change of exchange splitting (of conduction electrons) as well as the rotation of spins reflected at an interface with a ferromagnet. Finally, we use the presented theory to explain the recent observation of subpicosecond reversal of a ferromagnet in rare-earth free spin-valves. Our conclusion agrees with the one of reference [1] favoring magnetization reversal due to the rotation of the spin polarization of a reflected spin current.",2303.05907v1 2023-03-22,Electronic Correlation Effects on Stabilizing a Perfect Kagome Lattice and Ferromagnetic Fluctuation in LaRu$_3$Si$_2$,"A perfect Kagome lattice features flat bands that usually lead to strong electronic correlation effects, but how electronic correlation, in turn, stabilizes a perfect Kagome lattice has rarely been explored. Here, we study such effect in a superconducting ($T_c \sim 7.8$ K) Kagome metal LaRu$_3$Si$_2$ with a distorted Kagome plane consisting of pure Ru ions, using density functional theory plus $U$ and plus dynamical mean-field theory. We find that increasing electronic correlation can stabilize a perfect Kagome lattice and induce substantial ferromagnetic fluctuations in LaRu$_3$Si$_2$. By comparing the calculated magnetic susceptibilities to experimental data, LaRu$_3$Si$_2$ is found to be on the verge of becoming a perfect Kagome lattice. It thus shows moderate but non-negligible electronic correlations and ferromagnetic fluctuations, which are crucial to understanding the experimentally observed non-Fermi-liquid behavior and the pretty high superconducting $T_c$ of LaRu$_3$Si$_2$.",2303.12273v1 2023-04-13,Magnetism in twisted triangular bilayer graphene quantum dots,"Using a tight-binding model along with the mean-field Hubbard method, we investigate the effect of twisting angle on the magnetic properties of twisted bilayer graphene (tBLG) quantum dots (QDs) with triangular shape and zigzag edges. We consider such QDs in two configurations: when their initial untwisted structure is a perfect AA- or AB-stacked BLG, referred to as AA- or AB-like dots. We find that AA-like dots exhibit an antiferromagnetic spin polarization for small twist angles, which transits to a ferromagnetic spin polarization beyond a critical twisting angle $\theta_c$. Our analysis shows that $\theta_c$ decreases as the dot size increases, obeying a criterion, according to which once the maximum energy difference between electron and hole edge states (in the single-particle picture) is less than $(U / \gamma_0)\, t_0$, the spin-polarized energy levels are aligned ferromagnetically [$U$ is the Hubbard parameter and $\gamma_0$ ($t_0$) the graphene intralayer (interlayer) hopping]. Unlike AA-like dots, AB-like dots exhibit finite magnetization for any twist angle. Furthermore, in the ferromagnetic polarization state, the ground net spin for both dot configurations agrees with prediction from Lieb's theorem.",2304.06228v1 2023-04-14,Hall effect of ferro/antiferromagnetic wallpaper fermions,"Nonsymmorphic crystals can host characteristic double surface Dirac cones with fourfold degeneracy on the Dirac points, called wallpaper fermion, protected by wallpaper group symmetry. We clarify the charge and spin Hall effect of wallpaper fermions in the presence of the (anti)ferromagnetism.Based on a four-sublattice model, we construct the effective Hamiltonian of wallpaper fermions coupled with the ferromagnetic or antiferromagnetic moment.Both ferromagnetic and antiferromagnetic moments induce an energy gap for the wallpaper fermions, leading to quantized (spin) Hall conductivity. The ferromagnetic wallpaper fermion induces the Hall conductivity quantized into $e^2/h$, which is twice that for a single Dirac fermion on the surface of topological insulators. On the other hand, the spin Hall conductivity decays and reaches to be a finite value as the antiferromagnetic coupling increases. We also show that the results above are valid for a general model of wallpaper fermions from symmetry consideration.",2304.06982v3 2023-05-03,Zigzag edge ferromagnetism of triangular-graphene-quantum-dot-like system,"Here, the magnetic susceptibility of a triangular-graphene-quantum-dot-like system was examined by using the determinant quantum Monte Carlo method. We focused on three zigzag edge quantum dots or rings, namely, the triangular graphene quantum ring, bilayer triangular graphene quantum dot, and bilayer triangular graphene quantum ring. The triangular-graphene-quantum-dot-like system exhibited robust edge ferromagnetic behavior, which was independent of size, monolayer or bilayer, or dot or ring shape, according to the numerical results. At half filling, the edge magnetic susceptibility is increased by on-site interactions, especially in the low-temperature region. Spintronics systems may benefit from use of this system due to its robust edge ferromagnetic behavior.",2305.03145v2 2023-05-06,Giant magnetostriction in La2CoMnO6 synthesized by microwave irradiation,"Polycrystalline insulating ferromagnetic double perovskite La2CoMnO6 possessing monoclinic structure and a high ferromagnetic Curie temperature (TC = 222 K) was rapidly synthesized ( 30 min) by irradiating stoichiometric mixture of oxides with the microwave. The sample exhibits negative magnetostriction, i.e., contraction of length along the magnetic field direction in the ferromagnetic state. At 10 K, the parallel magnetostriction does not show saturation up to a magnetic field of 50 kOe where it reaches 610 ppm which is one of the highest values of magnetostriction found so far among perovskite oxides with 3d ions. The magnitude of magnetostriction decreases monotonically as the temperature increases and becomes negligible above TC. The giant magnetostriction in this double perovskite is suggested to originate from large spin-orbit coupling associated with Co2+ (d7) cation. The obtained magnetostriction value is comparable to 630 ppm in an identical composition obtained through solid-state reaction over several days in a conventional furnace which indicates the advantages of microwave-assisted synthesis in saving reaction time and electric power without deteriorating physical properties.",2305.03886v1 2023-05-23,Current-driven motion of magnetic topological defects in ferromagnetic superconductors,"Recent years have seen a number of instances where magnetism and superconductivity intrinsically coexist. Our focus is on the case where spin-triplet superconductivity arises out of ferromagnetism, and we make a hydrodynamic analysis of the effect of a charge supercurrent on magnetic topological defects like domain walls and merons. We find that the emergent electromagnetic field that arises out of the superconducting order parameter provides a description for not only the physical quantities such as the local energy flux density and the interaction between current and defects but also the energy dissipation through magnetic dynamics of the Gilbert damping, which becomes more prominent compared to the normal state as superconductivity attenuates the energy dissipation through the charge sector. In particular, we reveal that the current-induced dynamics of domain walls and merons in the presence of the Gilbert damping give rise to the nonsingular $4\pi$ and $2\pi$ phase slips, respectively, revealing the intertwined dynamics of spin and charge degrees of freedom in ferromagnetic superconductors.",2305.13564v1 2023-05-30,Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions,"Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.",2305.18791v2 2023-05-31,Boundedness of Susceptibility in Spin Glass Transition of Transverse Field Mixed $p$-spin Glass Models,"Spontaneous symmetry breaking phenomena in the transverse field mixed $p$-spin glass model in finite dimensions are studied with Nishimori's gauge theory. Useful identities in the gauge theory enable us to study $\mathbb Z_2$-symmetry breaking. It is proven that ferromagnetic long-range order and spontaneous magnetization at an arbitrary temperature are bounded by those on the Nishimori line in the corresponding classical model. These bounds imply that neither ferromagnetic long-range order nor spontaneous magnetization exists in spin glass phase. It is proven also that the ferromagnetic susceptibility has an upper bound in paramagnetic and spin glass phases.",2305.19630v2 2023-06-11,Strong ferromagnetic fluctuations in a doped checkerboard lattice,"Using the determinant quantum Monte Carlo method, we study the magnetic susceptibility in the parameter space of the on-site interaction $U$, temperature $T$, electron filling $\avg{n}$, and the frustration control parameter $t^{\prime}$ within the Hubbard model on a two-dimensional checkerboard lattice. It is shown that the system exhibits stable and strong ferromagnetic fluctuations about the electron filling $\avg{n}\ge1.2$ for different $t^{\prime}$, and the ferromagnetic susceptibility is strongly enhanced by the increasing interaction and decreasing tempeture. We also discuss the sign problem to clarify which parameter region is accessible and reliable. Our findings not only demonstrate important implications for modulating magnetism in the checkerboard lattice, but will also provide a theoretical platform for a flat-band model that demonstrates a variety of physical properties.",2306.06697v1 2023-06-13,On Weyl Nodes in Ferromagnetic Weyl Semimetal,"The ferromagnetic Weyl semimetals, such as Co3Sn2S2, feature pairs of Weyl points characterized by the opposite chiralities.We model this type of semimetals by the inversion symmetry protected and the time reversal symmetry broken Bloch Hamiltonian. It involves terms representing the tunnelling effect, exchange field corresponding to the ferromagnetic order, chirality index of Weyl points with related energy parameters, and the angle formed by the spin magnetic moments and the axis perpendicular to the system-plane. While for the in-plane spin moment order the Weyl nodes are absent at some points of the first Brillouin zone , the bands of opposite chirality non-linearly cross each other with band inversion at Weyl points for the spin moment order along the perpendicular axis. The absence of linearity implies that the system is unable to host massless Weyl fermions. We also show that, in the absence of the exchange field, the incidence of the circularly polarized radiation leads to the emergence of a novel state with broken time reversal symmetry.",2306.07882v3 2023-06-28,Exchange interaction between two quantum dots coupled through a superconducting island,"We present a theoretical study of a system consisting of a superconducting island and two quantum dots, a possible platform for building qubits and Cooper pair splitters, in the regime where each dot hosts a single electron and, hence, carries a magnetic moment. We focus on the case where the dots are coupled to overlapping superconductor states and we study whether the spins are ferromagnetically or antiferromagnetically aligned. We show that if the total number of particles is even, the spins align antiferromagnetically in the flatband limit, i.e., when the bandwidth of the superconductor is negligibly small, but ferromagnetically if the bandwidth is finite and above some value. If the total number of particles is odd, the alignment is ferromagnetic independently from the bandwidth. This implies that the results of the flatband limit are applicable only within restricted parameter regime for realistic superconducting qubit systems and that some care is required in applying simplified models to devices such as Cooper pair splitters.",2306.16211v1 2023-06-30,Achiral dipoles on a ferromagnet can affect its magnetization direction,"We demonstrate the possibility of a coupling between the magnetization direction of a ferromagnet and the tilting angle of adsorbed achiral molecules. To illustrate the mechanism of the coupling, we analyze a minimal Stoner model that includes Rashba spin-orbit coupling due to the electric field on the surface of the ferromagnet. The proposed mechanism allows us to study magnetic anisotropy of the system with an extended Stoner-Wohlfarth model, and argue that adsorbed achiral molecules can change magnetocrystalline anisotropy of the substrate. Our research's aim is to motivate further experimental studies of the current-free chirality induced spin selectivity effect involving both enantiomers.",2306.17592v2 2023-07-02,Ferromagnetic filament shapes in a rotating field reveal their magnetoelastic properties,"Flexible ferromagnetic filaments can be used to control the flow on the micro-scale with external magnetic field. To accurately model them, it is crucial to know their parameters such as their magnetization and bending modulus, the latter of which is hard to determine precisely. We present a method how the ferromagnetic filament's shape in a rotating field can be used to determine the magnetoelastic number $Cm$ - the ratio of magnetic to elastic forces. Then once the magnetization of the filament is known, it is possible to determine its bending modulus. The main idea of the method is that $Cm$ is the only parameter that determines whether the filament is straight or whether its tips are bent towards the magnetic field direction. Comparing with numerical solutions, we show that the method results in an error of $15...20\%$ for the determined $Cm$, what is more precise than estimations from other methods. This method will allow to improve the comparability between theoretical filament models and experimental measurements.",2307.00685v1 2023-07-05,Theoretical determination of Ising-type transition by using the Self-Consistent Harmonic Approximation,"Over the years, the Self-Consistent Harmonic Approximation (SCHA) has been successfully utilized to determine the transition temperature of many different magnetic models, particularly the Berezinskii-Thouless-Kosterlitz transition in two-dimensional ferromagnets. More recently, the SCHA has found application in describing ferromagnetic samples in spintronic experiments. In such a case, the SCHA has proven to be an efficient formalism for representing the coherent state in the ferromagnetic resonance state. One of the main advantages of using the SCHA is the quadratic Hamiltonian, which incorporates thermal spin fluctuations through renormalization parameters, keeping the description simple while providing excellent agreement with experimental data. In this article, we investigate the SCHA application in easy-axis magnetic models, a subject that has not been adequately explored to date. We obtain both semiclassical and quantum approaches of the SCHA for a general anisotropic magnetic model and employ them to determine various quantities such as the transition temperature, spin-wave energy spectrum, magnetization, and critical exponents. To verify the accuracy of the method, we compare the SCHA results with experimental and Monte Carlo simulation data for many distinct well-known magnetic materials.",2307.02596v2 2023-07-06,Tuning the Magnetism in Ultrathin CrxTey Films by Lattice Dimensionality,"Two-dimensional (2D) magnetic transition metal compounds with atomic thickness exhibit intriguing physics in fundamental research and great potential for device applications. Understanding the correlations between their macrosopic magnetic properties and the dimensionality of microscopic magnetic exchange interactions are valuable for the designing and applications of 2D magnetic crystals. Here, using spin-polarized scanning tunneling microscopy, magnetization and magneto-transport measurements, we identify the zigzag-antiferromagnetism in monolayer CrTe2, incipient ferromagnetism in bilayer CrTe2, and robust ferromagnetism in bilayer Cr3Te4 films. Our density functional theory calculations unravel that the magnetic ordering in ultrathin CrTe2 is sensitive to the lattice parameters, while robust ferromagnetism with large perpendicular magnetic anisotropy in Cr3Te4 is stabilized through its anisotropic 3D magnetic exchange interactions.",2307.02857v1 2023-07-10,Increasing Flips per Second and Speed of p-Computers by Using Dilute Magnetic Semiconductors to Implement Binary Stochastic Neurons,"Probabilistic computing with binary stochastic neurons (BSN) implemented with low- or zero-energy barrier nanoscale ferromagnets (LBMs) possessing in-plane magnetic anisotropy has emerged as an efficient paradigm for solving computationally hard problems. The fluctuating magnetization of an LBM at room temperature encodes a p-bit which is the building block of a BSN. Its only drawback is that the dynamics of common (transition metal) ferromagnets are relatively slow and hence the number of uncorrelated p-bits that can be generated per second - the so-called ""flips per second"" (fps) - is insufficient, leading to slow computational speed in autonomous co-processing with p-computers. Here, we show that a simple way to increase fps is to replace commonly used ferromagnets (e.g. Co, Fe, Ni), which have large saturation magnetization Ms, with a dilute magnetic semiconductor like GaMnAs with much smaller saturation magnetization. The smaller Ms reduces the energy barrier within the LBM and increases the fps significantly. It also offers other benefits such as increased packing density for increased parallelization and reduced device to device variation. This provides a way to realize the hardware acceleration and energy efficiency promise of p-computers.",2307.04663v1 2023-07-20,"Double exchange, itinerant ferromagnetism and topological Hall effect in moiré heterobilayer","Motivated by the recent experiments and the wide tunability on the MoTe$_2$/WSe$_2$ moir\'{e} heterobilayer, we consider a physical model to explore the underlying physics for the interplay between the itinerant carriers and the local magnetic moments. In the regime where the MoTe$_2$ is tuned to a triangular lattice Mott insulator and the WSe$_2$ layer is doped with the itinerant holes, we invoke the itinerant ferromagnetism from the double exchange mechanism for the itinerant holes on the WSe$_2$ layer and the local moments on the MoTe$_2$ layer. Together with the antiferromagnetic exchange on the MoTe$_2$ layer, the itinerant ferromagnetism generates the scalar spin chirality distribution in the system. We further point out the presence of spin-assisted hopping in addition to the Kondo coupling between the local spin and the itinerant holes, and demonstrate the topological Hall effect for the itinerant electrons in the presence of the non-collinear spin configurations. This work may improve our understanding of the correlated moir\'{e} systems and inspire further experimental efforts.",2307.10613v1 2023-07-23,Unconventional spin polarization at Argon ion milled SrTiO3 Interfaces,"Interfacial two-dimensional electron gas (2DEG) formed at the perovskite-type oxide, such as SrTiO3, has attracted significant attention due to its properties of ferromagnetism, superconductivity, and its potential application in oxide-based low-power consumption electronics. Recent studies have investigated spin-to-charge conversion at the STO interface with different materials, which could affect the efficiency of this 2DEG interface. In this report, we presented an Ar^+ ion milling method to create a 2DEG at STO directly by inducing oxygen vacancies. To quantify the spin-to-charge conversion of this interface, we measured the angular-dependent spin-torque ferromagnetic resonance (ST-FMR) spectra, revealing an unconventional spin polarization at the interface of Argon ion-milled STO and NiFe. Furthermore, a micromagnetic simulation for angular-dependent spin-torque ferromagnetic resonance (ST-FMR) has been performed, confirming the large unconventional spin polarization at the interface.",2307.12390v1 2023-07-25,Spin waves in bilayers of transition-metal dichalcogenides,"Van der Waals magnetic materials are currently of great interest as materials for applications in future ultrathin nanoelectronics and nanospintronics. Due to weak coupling between individual monolayers, these materials can be easily obtained in the monolayer and bilayer forms. The latter are of specific interest as they may be considered as natural two-dimensional spin valves. In this paper, we study theoretically spin waves in bilayers of transition metal dichalcogenides. The considerations are carried within the general spin wave theory based on effective spin Hamiltonian and Hollstein-Primakoff-Bogolubov transformation. The spin Hamiltonian includes intra-layer as well as inter-layer nearest-neighbour exchange interactions, easy-plane anisotropy, and additionally a weak in-plane easy-axis anisotropy. The bilayer systems consist of two ferromagnetic (in-plane magnetization) monolayers that are coupled either ferromagnetically or antiferromagnetically. In the latter case, we analyse the spin wave spectra in all magnetic phases, i.e. in the antiferromagnetic, spin-flop, and ferromagnetic ones.",2307.13414v2 2023-07-30,Oscillatory superconducting transition temperature in superconductor/antiferromagnet heterostructures,"One of the most famous proximity effects at ferromagnet/superconductor (F/S) interfaces is partial conversion of singlet superconductivity to triplet pairing correlations. Due to the presence of macroscopic exchange field in the ferromagnet the Cooper pairs penetrating into the ferromagnet from the superconductor acquire a finite momentum there. The finite-momentum pairing manifests itself, in particular, as a nonmonotonic dependence of the critical temperature of the bilayer on the thickness of the F layer. Here we predict that despite the absence of the macroscopic exchange field the critical temperature of the antiferromagnet/superconductor (AF/S) bilayers also exhibit nonmonotonic (oscillating) dependence on the AF layer thickness. It is a manifestation of the proximity-induced Neel-type triplet correlations, which acquire finite total pair momentum and oscillate in the AF layer due to the Umklapp electron scattering processes at the AF/S interface. Our prediction can provide a possible explanation for a number of recently published experimental observations of the critical temperature of AF/S bilayers.",2307.16320v2 2023-08-08,Magnetic Phases of Bilayer Quantum-Dot Hubbard Model Plaquettes,"It has been demonstrated that small plaquettes of quantum dot spin qubits are capable of simulating condensed matter phenomena which arise from the Hubbard model, such as the collective Coulomb blockade and Nagaoka ferromagnetism. Motivated by recent materials developments, we investigate a bilayer arrangement of quantum dots with four dots in each layer which exhibits a complex ground state behavior. We find using a generalized Hubbard model with long-range Coulomb interactions, several distinct magnetic phases occur as the Coulomb interaction strength is varied, with possible ground states that are ferromagnetic, antiferromagnetic, or having both one antiferromagnetic and one ferromagnetic layer. We map out the full phase diagram of the system as it depends on the inter- and intra-layer Coulomb interaction strengths, and find that for a single layer, a similar but simpler effect occurs. We also predict interesting contrasts among electron, hole, and electron-hole bilayer systems arising from complex correlation physics. Observing the predicted magnetic configuration in already-existing few-dot semiconductor bilayer structures could prove to be an important assessment of current experimental quantum dot devices, particularly in the context of spin-qubit-based analog quantum simulations.",2308.04504v1 2023-08-17,Efficient Quantum Transduction Using Anti-Ferromagnetic Topological Insulators,"Transduction of quantum information between distinct quantum systems is an essential step in various applications, including quantum networks and quantum computing. However, mediating photons of vastly different frequencies and designing high-performance transducers are challenging, due to multifaceted and sometimes conflicting requirements. In this work, we first discuss some general principles for quantum transducer design, and then propose solid-state anti-ferromagnetic topological insulators to serve as highly effective transducers. First, topological insulators exhibit band-inversion, which can greatly enhance their optical responses. This property, coupled with robust spin-orbit coupling and high spin density, results in strong nonlinear interaction in magnetic topological insulators, thereby substantially improving transduction efficiency. Second, the anti-ferromagnetic order can minimize the detrimental influence on other neighboring quantum systems due to magnetic interactions. Using MnBi2Te4 as an example, we showcase that single-photon quantum transduction efficiency exceeding 80% can be achieved with modest experimental requirements, while the transduction bandwidth can reach the GHz range. The strong nonlinear photonic interactions in magnetic topological insulators can find diverse applications, including the generation of entanglement between photons of disparate frequencies and quantum squeezing.",2308.09048v2 2023-08-17,Dielectric Screening and Electric Field Control of Ferromagnetism at the CaMnO$_3$/CaRuO$_3$ Interface,"Control of magnetism by an applied electric field is a desirable technique for the functionalization of magnetic materials. Motivated by recent experiments, we study the electric field control of the interfacial magnetism of CaRuO$_3$/CaMnO$_3$ (CRO/CMO) (001), a prototype interface between a non-magnetic metal and an antiferromagnetic insulator. Even without the electric field, the interfacial CMO layer acquires a ferromagnetic moment due to a spin-canted state, caused by the Anderson-Hasegawa double exchange (DEX) between the Mn moments and the leaked electrons from the CRO side. An electric field would alter the carrier density at the interface, leading to the possibility of controlling the magnetism, since DEX is sensitive to the carrier density. We study this effect quantitatively usingdensity-functional calculations in the slab geometry. We find a text-book like dielectric screening of the electric field, which introduces polarization charges at the interfaces and the surfaces. The extra charge at the interface enhances the ferromagnetism via the DEX interaction, while away from the interface the original AFM state of the Mn layers remains unchanged. The effect could have potential application in spintronics devices.",2308.09155v1 2023-08-22,Effect of Electron-Phonon Scattering on the Anomalous Hall Conductivity of Fe$_3$Sn: A Kagome Ferromagnetic Metal,"We report on magnetic and magnetotransport studies of a Kagome ferromagnetic metal, Fe$_3$Sn. Our studies reveal a large anomalous Hall conductivity ($\sigma_{zx}$) in this system, mainly contributed by temperature independent intrinsic Hall conductivity ($\sigma^{int}_{zx}$=485$\pm$60 S/cm) and temperature dependent extrinsic Hall conductivity ($\sigma^{ext}_{zx}$) due to skew-scattering. Although $\sigma^{ext}_{zx}$ value is large and almost equivalent to the intrinsic Hall conductivity at low temperatures, it drastically decreases with increasing temperature, following the relation $\sigma^{ext}_{zx}=\frac{\sigma_{zx0}^{ext}}{(aT+1)^2}$, under the influence of electron-phonon scattering. The presence of electron-phonon scattering in this system is also confirmed by the linear dependence of longitudinal electrical resistivity at higher temperatures [$\rho(T)\propto T$]. We further find that Fe$_3$Sn is a soft ferromagnet with an easy-axis of magnetization lying in the $\it{ab}$ plane of the crystal with magnetocrystalline anisotropy energy density as large as 1.02 $\times$ 10$^6$",2308.11177v1 2023-08-23,Spin pumping from a ferromagnetic insulator into an altermagnet,"A class of antiferromagnets with spin-polarized electron bands, yet zero net magnetization, called altermagnets is attracting increasing attention due to their potential use in spintronics. Here, we study spin injection into an altermagnet via spin pumping from a ferromagnetic insulator. We find that the spin pumping behaves qualitatively different depending on how the altermagnet is crystallographically oriented relative the interface to the ferromagnetic insulator. The altermagnetic state can enhance or suppress spin pumping, which we explain in terms of spin-split altermagnetic band structure and the spin-flip probability for the incident modes. Including the effect of interfacial Rashba spin-orbit coupling, we find that the spin-pumping effect is in general magnified, but that it can display a non-monotonic behavior as a function of the spin-orbit coupling strength. We show that there exists an optimal value of the spin-orbit coupling strength which causes an order of magnitude increase in the pumped spin current, even for the crystallographic orientation of the altermagnet which suppresses the spin pumping.",2308.12335v1 2023-08-24,Highly efficient room-temperature nonvolatile magnetic switching by current in Fe3GaTe2 thin flakes,"Effectively tuning magnetic state by using current is essential for novel spintronic devices. Magnetic van der Waals (vdW) materials have shown superior properties for the applications of magnetic information storage based on the efficient spin torque effect. However, for most of known vdW ferromagnets, the ferromagnetic transition temperatures lower than room temperature strongly impede their applications and the room-temperature vdW spintronic device with low energy consumption is still a long-sought goal. Here, we realize the highly efficient room-temperature nonvolatile magnetic switching by current in a single-material device based on vdW ferromagnet Fe3GaTe2. Moreover, the switching current density and power dissipation are about 300 and 60000 times smaller than conventional spin-orbit-torque devices of magnet/heavymetal heterostructures. These findings make an important progress on the applications of magnetic vdW materials in the fields of spintronics and magnetic information storage.",2308.12710v1 2023-08-29,Efficient thermo-spin conversion in van der Waals ferromagnet FeGaTe,"Recent discovery of 2D van der Waals (vdW) magnetic materials has spurred progress in developing advanced spintronic devices. A central challenge lies in enhancing the spin-conversion efficiency for building spin-logic or spin-memory devices. We systematically investigated the anomalous Hall effect and anomalous Nernst effect in above-room-temperature van der Waals ferromagnet FeGaTe with perpendicular anisotropy, uncovering significant spin-conversion effects. The anomalous Hall effect demonstrated an efficient electric spin-charge conversion, with a notable spin Hall angle of 6 $\%$ - 10.38 $\%$. The temperature-dependent behavior of the anomalous Nernst voltage primarily results from the thermo-spin conversion. Uniquely, we have experimentally achieved thermo-spin polarization values of over 690 $\%$ at room temperature and extremely large of 4690 $\%$ at about 93 K. This study illuminates the potential of vdW ferromagnets in advancing efficient spin conversion devices.",2308.14977v1 2023-09-05,Probing defect induced room temperature ferromagnetism in CVD grown MoO3 flakes: A correlation with electronic structure and first principle-based calculations,"In this paper, we report the growth of pure {\alpha}-MoO3 micro-flakes by CVD technique and their structural, electronic, optical, and magnetic properties. Samples are annealed at various temperatures in an H2 atmosphere to induce ferromagnetism. All the samples exhibit ferromagnetism at room temperature, and 250oC annealed sample shows the highest magnetic moment of 0.087 emu/g. It is evident from PL data that pristine as well as annealed samples contain different types of defects like oxygen vacancies, surface defects, interstitial oxygen, etc. It is deduced from the analysis of Mo3d and O1s core-level XPS spectra that oxygen vacancies increase up to an annealing temperature of 250oC that correlates with the magnetic moment. Significant changes in the total density of states and also in the magnetic moment for two and three oxygen vacancies are noticed through first-principle-based calculations. It is concluded that the magnetic moment is produced by oxygen vacancies or vacancy clusters, which is consistent with our experimental findings.",2309.02277v1 2023-09-10,Deterministic and non-volatile switching of all-van der Waals spin-orbit torque system above room temperature without external magnetic fields,"Two-dimensional van der Waals (vdW) magnetic materials hold promise for the development of high-density, energy-efficient spintronic devices for memory and computation. Recent breakthroughs in material discoveries and spin-orbit torque (SOT) control of vdW ferromagnets have opened a path for integration of vdW magnets in commercial spintronic devices. However, a solution for field-free electric control of perpendicular magnetic anisotropy (PMA) vdW magnets at room temperatures, essential for building compact and thermally stable spintronic devices, is still missing. Here, we report the first demonstration of field-free deterministic and non-volatile switching of a PMA vdW ferromagnet, Fe$_3$GaTe$_2$ above room temperature (up to 320 K). We use the unconventional out-of-plane anti-damping torque from an adjacent WTe$_2$ layer to enable such switching with a low current density of $2.23 \times 10^6$ A/cm$^2$. This study exemplifies the efficacy of low-symmetry vdW materials for spin-orbit torque control of vdW ferromagnets and provides an all-vdW solution for the next generation of scalable and energy-efficient spintronic devices.",2309.04930v1 2023-09-11,Itinerant ferromagnetism in transition metal dichalcogenides moiré superlattices,"Moir\'e materials are artificial crystals formed at van der Waals heterojunctions that have emerged as a highly tunable platform to realize much of the rich quantum physics of electrons in atomic scale solids, also providing opportunities to discover new quantum phases of matter. Here we use finite-size exact diagonalization methods to explore the physics of single-band itinerant electron ferromagnetism in semiconductor moir\'e materials. We predict where ferromagnetism is likely to occur in triangular-lattice moir\'e systems, and where it is likely to yield the highest Curie temperatures.",2309.05556v3 2023-09-22,Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor,"Electric control of magnetization dynamics in two-dimensional (2D) magnetic materials is an essential step for the development of novel spintronic nanodevices. Electrostatic gating has been shown to greatly affect the static magnetic properties of some van der Waals magnets, but the control over their magnetization dynamics is still largely unexplored. Here we show that the optically-induced magnetization dynamics in the van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$ can be effectively controlled by electrostatic gates, with a one order of magnitude change in the precession amplitude and over 10% change in the internal effective field. In contrast to the purely thermally-induced mechanisms previously reported for 2D magnets, we find that coherent opto-magnetic phenomena play a major role in the excitation of magnetization dynamics in Cr$_2$Ge$_2$Te$_6$. Our work sets the first steps towards electric control over the magnetization dynamics in 2D ferromagnetic semiconductors, demonstrating their potential for applications in ultrafast opto-magnonic devices.",2309.12776v1 2023-10-02,Spin-orbit enabled unconventional Stoner magnetism,"The Stoner instability remains a cornerstone for understanding metallic ferromagnets. This instability captures the interplay of Coulomb repulsion, Pauli exclusion, and two-fold fermionic spin degeneracy. In materials with spin-orbit coupling, this fermionic spin is generalized to a two-fold degenerate pseudospin which is typically believed to have symmetry properties as spin. Here we identify a distinct symmetry of this pseudospin that forbids it to couple to a Zeeman field. This `spinless' property is required to exist in five non-symmorphic space groups and has non-trivial implications for superconductivity and magnetism. With Coulomb repulsion, Fermi surfaces composed primarily of this spinless pseudospin feature give rise to Stoner instabilities into magnetic states that are qualitatively different than ferromagnets. These spinless-pseudospin ferromagnets break time-reversal symmetry, have a vanishing magnetization, are non-collinear, and exhibit altermagnetic-like energy band spin-splittings. In superconductors, for all pairing symmetries and field orientations, this spinless pseudospin extinguishes paramagnetic limiting. We discuss applications to superconducting UCoGe and magnetic NiS$_{2-x}$Se$_x$.",2310.00838v2 2023-10-06,"Kagome KMn$_3$Sb$_5$ metal: Magnetism, lattice dynamics, and anomalous Hall conductivity","Kagome metals are reported to exhibit remarkable properties, including superconductivity, charge density wave order, and a large anomalous Hall conductivity, which facilitate the implementation of spintronic devices. In this work, we study a novel kagome metal based on Mn magnetic sites in a KMn$_3$Sb$_5$ stoichiometry. By means of first-principles density functional theory calculations, we demonstrate that the studied compound is dynamically stable, locking the ferromagnetic order as the ground state configuration, thus preventing the charge-density-wave state as reported in its vanadium-based counterpart KV$_3$Sb$_5$. Our calculations predict that KMn$_3$Sb$_5$ exhibits an out-of-plane (001) ferromagnetic response as the ground state, allowing for the emergence of topologically protected Weyl nodes near the Fermi level and nonzero anomalous Hall conductivity ($\sigma_{ij}$) in this centrosymmetric system. We obtain a tangible $\sigma_{xy} = 314$ S$\cdot$cm$^{-1}$ component, which is comparable to that of other kagome metals. Finally, we explore the effect of the on-site Coulomb repulsion ($+U$) on the structural and electronic properties and find that, although the lattice parameters and $\sigma_{xy}$ moderately vary with increasing $+U$, KMn$_3$Sb$_5$ stands as an ideal stable ferromagnetic kagome metal with a large anomalous Hall conductivity response.",2310.04339v1 2023-10-10,Skyrmions and magnetic bubbles in spin-orbit coupled metallic magnets,"Motivated by the observation of Skyrmion-like magnetic textures in 2D itinerant ferromagnets Fe$_n$GeTe$_2$ ($n \geq3$), we develop a microscopic model combining itinerant magnetism and spin-orbit coupling on a triangular lattice. The ground state of the model in the absence of magnetic field consists of filamentary magnetic domain walls revealing a striking similarity with our magnetic force microscopy experiments on Fe$_3$GeTe$_2$. In the presence of magnetic field, these filaments were found to break into large size magnetic bubbles in our experiments. We identify uniaxial magnetic anisotropy as an important parameter in the model that interpolates between magnetic Skyrmions and ferromagnetic bubbles. Consequently, our work uncovers new topological magnetic textures that merge properties of Skyrmions and ferromagnetic bubbles.",2310.06521v1 2023-10-05,Manipulating Vortices with Domain Walls in Superconductor-Ferromagnet Heterostructures,"Vortices are point-like topological defects in superconductors whose motion dictates superconducting properties and controls device performance. In superconductor-ferromagnet heterostructures, vortices interact with topological defects in the ferromagnet such as line-like domain walls. While in previous heterostructure generations, vortex-domain wall interactions were mediated by stray fields; in new heterostructure families, more important become exchange fields and spin-orbit coupling. However, spin-orbit coupling's role in vortex-domain wall interactions remains unexplored. Here we uncover, via numerical simulations and Ginzburg-Landau theory, that Rashba spin-orbit coupling induces magnetoelectric interactions between vortices and domain walls that crucially depend on the wall's winding direction$-$its helicity. The wall's helicity controls whether vortices are pushed or dragged by N\'eel walls, and their gliding direction along Bloch walls. Our work capitalizes on interactions between topological defects from different order parameters and of different dimensionality to engineer enhanced functionality.",2310.06866v1 2023-09-09,Investigation into the nature behind the interesting half levitation behavior of claimed superconductor LK-99,"A recent article published by Lee et.al. claimed to have successfully achieved superconductivity at room temperature (RT) has become a topical issue. Besides the research paper, Lee and his team provided a demonstration video of LK-99 half levitating (HL) on a magnet. Such interesting HL appearance has drawn tremendous sensation both in academia and the network. However, the true identity of LK-99 still remains unclear, i.e., whether the HL behavior can necessarily indicate the diamagnetism behavior of the sample. Here, we fabricated our own LK-99 samples following the procedures reported by Lee et al. We found quite a few sample pieces showing the typical HL that is similar to those reported. Meanwhile, oxidation during the sample preparation was found to deleterious to acquiring HL in the sample, while furnace cooling or water quenching in the last step revealed little effect. However, our careful observations indicated that those HL pieces are more likely simple ferromagnetic. Then we conducted a comprehensive study on the behavior patterns of typical diamagnetism and ferromagnetic substances interacting with a Nd2Fe14B magnet, and provided instructions to distinguish the characteristics between ferromagnetic and diamagnetic to prevent misunderstanding of LK-99 like levitation behavior.",2310.08594v1 2023-10-12,Electrical noise spectroscopy of magnons in a quantum Hall ferromagnet,"Collective spin-wave excitations-magnons-in a quantum Hall ferromagnet are promising quasi-particles for next-generation spintronics devices, including platforms for information transfer. Detection of these charge-neutral excitations relies on the conversion of magnons into electrical signals in the form of excess electrons and holes, but if these signals are equal the magnon detection remains elusive. In this work, we overcome this shortcoming by measuring the electrical noise generated by magnons. We use the symmetry-broken quantum Hall ferromagnet of the zeroth Landau level in graphene to launch magnons. Absorption of these magnons creates excess noise above the Zeeman energy and remains finite even when the average electrical signal is zero. Moreover, we formulate a theoretical model in which the noise is generated by equilibration (partial or full, depending on the bias voltage) between edge channels and propagating magnons. Our model, which agrees with experimental observations, also allows us to pinpoint the regime of ballistic magnon transport in our device.",2310.08703v1 2023-10-27,Superconducting Spintronic Heat Engine,"Heat engines are key devices that convert thermal energy into usable energy. Strong thermoelectricity, at the basis of electrical heat engines, is present in superconducting spin tunnel barriers at cryogenic temperatures where conventional semiconducting or metallic technologies cease to work. Here we realize a superconducting spintronic heat engine consisting of a ferromagnetic insulator/superconductor/insulator/ferromagnet tunnel junction (EuS/Al/AlO$_x$/Co). The efficiency of the engine is quantified for bath temperatures ranging from 25 mK up to 800 mK, and at different load resistances. Moreover, we show that the sign of the generated thermoelectric voltage can be inverted according to the parallel or anti-parallel orientation of the two ferromagnetic layers, EuS and Co. This realizes a thermoelectric spin valve controlling the sign and strength of the Seebeck coefficient, thereby implementing a thermoelectric memory cell. We propose a theoretical model that allows describing the experimental data and predicts the engine efficiency for different device parameters.",2310.18132v1 2023-10-29,Quantum Phase Transitions in a Generalized Dicke Model,"We investigate a generalized Dicke model by introducing two interacting spin ensembles coupled with a single-mode bosonic field. Apart from the normal to superradiant phase transition induced by the strong spin-boson coupling, interactions between the two spin ensembles enrich the phase diagram by introducing ferromagnetic, antiferromagnetic and paramagnetic phases. The mean-field approach reveals a phase diagram comprising three phases: paramagnetic-normal phase, ferromagnetic-superradiant phase, and antiferromagnetic-normal phase. Ferromagnetic spin-spin interaction can significantly reduce the required spin-boson coupling strength to observe the superradiant phase, where the macroscopic excitation of the bosonic field occurs. Conversely, antiferromagnetic spin-spin interaction can strongly suppress the superradiant phase. To examine higher-order quantum effects beyond the mean-field contribution, we utilize the Holstein-Primakoff transformation, which converts the generalized Dicke model into three coupled harmonic oscillators in the thermodynamic limit. Near the critical point, we observe the close of the energy gap between the ground and the first excited states, the divergence of entanglement entropy and quantum fluctuation in certain quadrature. These observations further confirm the quantum phase transition and offer additional insights into critical behaviors.",2310.18978v1 2023-10-31,Microwave Spin-Pumping from an Antiferromagnet FeBO3,"Recently, canted antiferromagnets offer great potential for fundamental research and applications due to their unique properties. The presence of the Dzyaloshinskii-Moriya interaction leads to the existence of a weak ferromagnetic moment at room temperature. We study both theoretically and experimentally microwave spin pumping by the quasi-ferromagnetic mode from a canted easy plane antiferromagnet with weak ferromagnetism FeBO3. The conversion of a microwave signal into the constant voltage is realized using the inverse spin Hall effect in an iron borate/heavy metal heterostructure. We use an additional bias magnetic field to selectively tune the resonance frequency of such a microwave detector over a wide range up to 43.5 GHz with potential sensitivity near 2.5 microV/W. We confirm the pure spin current nature by changing polarity of the detected via inverse spin Hall effect voltage by switching the direction of the bias magnetic field. We believe that our results will be useful for the development of highly tunable, portable and sensitive microwave antiferromagnet-based functional devices.",2310.20422v1 2023-11-01,Giant anomalous Hall effect in epitaxial Mn$_{3.2}$Ge films with a cubic kagome structure,"We report on the first example of epitaxial Mn$_{3 + \delta}$Ge thin films with a cubic $L1_2$ structure. The films are found to exhibit frustrated ferromagnetism with an average magnetization corresponding to 0.98$~\pm~$0.06$~\mu_B$/Mn, far larger than the parasitic ferromagnetism in hexagonal Mn$_3$Ge and the partially compensated ferrimagnetism in tetragonal Mn$_3$Ge. The Hall conductivity is the largest reported for the kagome magnets with a low temperature value of $\sigma_{xy} = 1587~$S/cm. Density functional calculations predict that a chiral antiferromagnetic structure is lower in energy than a ferromagnetic configuration in an ordered stoichiometric crystal. However, chemical disorder driven by the excess Mn in our films explains why a frustrated 120$^\circ$ spin structure is not observed. Comparisons between the magnetization and the Hall resistivity indicate that a non-coplanar spin structure contributes the Hall signal. Anisotropic magnetoresistance and planar Hall effect with hysteresis up to 14 T provides further insights into this material.",2311.00683v1 2023-11-03,Exchange energy of the ferromagnetic electronic ground-state in a monolayer semiconductor,"Mobile electrons in the semiconductor monolayer-MoS$_2$ form a ferromagnetic state at low temperature. The Fermi sea consists of two circles, one at the $K$-point, the other at the $\tilde{K}$-point, both with the same spin. Here, we present an optical experiment on gated MoS$_2$ at low electron-density in which excitons are injected with known spin and valley quantum numbers. The resulting trions are identified using a model which accounts for the injection process, the formation of antisymmetrized trion states, electron-hole scattering from one valley to the other, and recombination. The results are consistent with a complete spin polarization. From the splittings between different trion states, we measure the exchange energy, $\Sigma$, the energy required to flip a single spin within the ferromagnetic state, as well as the intervalley Coulomb exchange energy, $J$. We determine $\Sigma=11.2\,$meV and $J=5\,$meV at $n=1.5 \times 10^{12}\,$cm$^{-2}$, and find that $J$ depends strongly on the electron density, $n$.",2311.02164v1 2023-11-08,Pairing symmetries of multiple superconducting phases in UTe2: Competition between ferromagnetic and antiferromagnetic fluctuations,"The putative spin-triplet superconductor UTe2 exhibits multiple superconducting phases under applied pressure [D. Braithwaite et al., Commun. Phys. 2, 147 (2019)]. The clarification of pairing mechanisms and symmetries of gap functions are essentially important for understanding the multiple-phase diagram. Since the coexistence of ferromagnetic and antiferromagnetic spin fluctuations with Ising-like anisotropy is suggested from measurements of magnetic susceptibilities and neutron scattering measurements, it is expected that the interplay between these spin fluctuations plays a crucial role in the emergence of the multiple superconducting phases. Motivated by these observations, we examine the spin-fluctuation-mediated pairing mechanism, analyzing the linearized Eliashberg equations for an effective model of f-electron bands. It is found that the Ising-like ferromagnetic fluctuations stabilize spin-triplet pairings in either the Au or B3u states, whereas Ising-like antiferromagnetic fluctuations stabilize spin-triplet pairings in the B1u state. These results provide a plausible scenario elucidating the multiple superconducting phases under pressure.",2311.04629v3 2023-11-11,Spin transfer exchange torque in ferromagnet/ferromagnet structures made of half metals with large exchange gaps,"Spin torques in magnetic multilayers are produced by spin polarization $P$ of ferromagnetic (F) layers, and increase with growing $P$. The latter, however, cannot exceed the $P=1$ value found in half metals. We study the $P=1$ case to find what other parameters still influence spin torques in this extreme limit. It is found that the ratio of exchange gap to Fermi energy strongly affects the properties of the torque. For large values of the gap the magnitude of exchange spin torque exhibits a sharp peak at very small misalignment angles between magnetizations. This behavior is found to be linked to a transition between Ohmic and tunneling transport regimes through the F/F boundary.",2311.06626v1 2023-11-12,Scaling Theory of Magnetic Order and Microwave Absorption in Amorphous and Granular Ferromagnets,"Magnetic order and microwave absorption in amorphous ferromagnets and materials sintered from nanoscale ferromagnetic grains are investigated analytically and numerically within the random-anisotropy model. We show that a scaling argument specific to static randomness allows one to make conclusions about the behavior of a large system with a weak disorder by studying a smaller system with a strong disorder. The breakdown of the scaling on increasing the strength of the magnetic anisotropy and/or the size of the grain separates two distinct regimes in magnetic ordering and frequency dependence of the absorbed microwave power. Analytical results are confirmed by numerical experiments on spin lattices containing up to ten million spins. Our findings should help design materials with desired magnetic and microwave properties. The method can be extended to other systems with quenched randomness.",2311.06843v1 2023-11-20,Spectroscopy of elementary excitations from quench dynamics in a dipolar XY Rydberg simulator,"We use a Rydberg quantum simulator to demonstrate a new form of spectroscopy, called quench spectroscopy, which probes the low-energy excitations of a many-body system. We illustrate the method on a two-dimensional simulation of the spin-1/2 dipolar XY model. Through microscopic measurements of the spatial spin correlation dynamics following a quench, we extract the dispersion relation of the elementary excitations for both ferro- and anti-ferromagnetic couplings. We observe qualitatively different behaviors between the two cases that result from the long-range nature of the interactions, and the frustration inherent in the antiferromagnet. In particular, the ferromagnet exhibits elementary excitations behaving as linear spin waves. In the anti-ferromagnet, spin waves appear to decay, suggesting the presence of strong nonlinearities. Our demonstration highlights the importance of power-law interactions on the excitation spectrum of a many-body system.",2311.11726v1 2023-11-23,Superconductivity controlled bulk magnetism,"Ferromagnetism's ability to influence superconducting order is well known and well established, but the converse phenomena remains relatively less explored. Theoretical work on the subject includes Anderson and Suhl prediction of a crypto-ferromagnetic state, and De Gennes proposal of two ferromagnetic insulators exchange coupled through a superconductor. In this study, we present compelling evidence of coexistence of both phenomena in a superconducting spin valve system. We demonstrate that superconducting exchange coupling enables reliable bistable states, and the coexistence of SEC and CFM leads to a wide range of reproducible zero field micro-magnetic states in the SSV, which are a function of the strength of the superconducting state. These micromagnetic states can in turn influence the superconducting state, leading to multiple reproducible and non-volatile resistance states; thus paving the way for a novel direction in cryogenic in-memory computing.",2311.14170v2 2023-11-18,Thermal Spin Orbit Torque with Dresselhaus Spin Orbit Coupling,"Based on the spinor Boltzmann equation, we obtain a temperature dependent thermal spin-orbit torque in terms of the local equilibrium distribution function in a two dimensional ferromagnet with Dresselhaus spin-orbit coupling. We also derive the continuity equation of spin accumulation and spin current, the spin diffusion equation in Dresselhaus ferromagnet, which contains the thermal spin orbit torque under local equilibrium assumption. This temperature dependent thermal spin-orbit torque originates from the temperature gradient applied to the system. it is also sensitive to temperature due to the local equilibrium distribution function therein. In the spin diffusion equation, we can single out the usual spin-orbit torque as well as the spin transfer torque, which is conceded to our previous results. Finally, we illustrate them by an example of spin-polarized transport through a ferromagnet with Dresselhaus spin-orbit coupling driven by temperature gradient, those torques including thermal spin-orbit torque are demonstrated numerically.",2311.14719v1 2023-11-29,Non-reciprocal electron transport in finite-size superconductor/ferromagnet bilayers with strong spin-orbit coupling,"We show that spin-orbit coupling at the interface between a superconducting film of the finite lateral size and the underlying ferromagnetic insulator with in-plane exchange field gives rise to a series of non-reciprocal effects provided the superconducting pairing is enhanced near the boundaries of the superconductor due to, e.g., variation of the film thickness or of the interlayer electron transparency. Specifically, the critical temperature and the critical depairing current are shown to depend on the relative orientation between the exchange field in the ferromagnet and the superconducting film boundaries. The discovered anisotropy of the superconducting properties is promising for the design of diode-type elements in superconducting spintronics.",2311.17534v2 2023-12-11,"Impersonating a Superconductor: High-Pressure BaCoO$_3$, an Insulating Ferromagnet","We report the high-pressure synthesis (6 GPa, 1200 $^{\circ}$C) and ambient pressure characterization of hexagonal HP-BaCoO$_3$. The material (with the 2H crystal structure) has a short intrachain Co-Co distance of about 2.07 $\text{\r{A}}$. Our magnetization investigation revealed robust diamagnetic behavior below approximately 130 K when exposed to weak applied magnetic fields (10 Oe) and a distinct half-levitation phenomenon below that temperature, such as is often observed for superconductors. Its field-dependent magnetization profile, however, unveils the characteristics of ferromagnetism, marked by a substantial magnetic retentivity of 0.22(1) ${\mu}_B$/Co at a temperature of 2 K. Electrical resistivity measurements indicate that HP-BaCoO$_3$ is a ferromagnetic insulator, not a superconductor.",2312.14955v1 2024-01-01,Magnon Damping Minimum and Logarithmic Scaling in a Kondo-Heisenberg Model,"Recently, an anomalous temperature evolution of spin wave excitations has been observed in a van der Waals metallic ferromagnet Fe$_3$GeTe$_2$ (FGT) [S. Bao, et al., Phys. Rev. X 12, 011022 (2022)], whose theoretical understanding yet remains elusive. Here we study the spin dynamics of a ferromagnetic Kondo-Heisenberg lattice model at finite temperature, and propose a mechanism of magnon damping that explains the intriguing experimental results. In particular, we find the magnon damping rate $\gamma(T)$ firstly decreases as temperature lowers, due to the reduced magnon-magnon scatterings. It then reaches a minimum at $T_{\rm d}^*$, and rises up again following a logarithmic scaling $\gamma(T) \sim \ln{(T_0/T)}$ (with $T_0$ a constant) for $T < T_{\rm d}^*$, which can be attributed to electron-magnon scatterings of spin-flip type. Moreover, we obtain the phase diagram containing the ferromagnetic and Kondo insulator phases by varying the Kondo coupling, which may be relevant for experiments on pressured FGT. The presence of a magnon damping minimum and logarithmic scaling at low temperature indicates the emergence of the Kondo effect reflected in the collective excitations of local moments in a Kondo lattice system.",2401.00758v1 2024-01-06,Cavity magnonics with domain walls in insulating ferromagnetic wires,"Magnetic domain walls (DWs) are topological defects that exhibit robust low-energy modes that can be harnessed for classical and neuromorphic computing. However, the quantum nature of these modes has been elusive thus far. Using the language of cavity optomechanics, we show how to exploit a geometric Berry-phase interaction between the localized DWs and the extended magnons in short ferromagnetic insulating wires to efficiently cool the DW to its quantum ground state or to prepare nonclassical states exhibiting a negative Wigner function that can be extracted from the power spectrum of the emitted magnons. Moreover, we demonstrate that magnons can mediate long-range entangling interactions between qubits stored in distant DWs, which could facilitate the implementation of a universal set of quantum gates. Our proposal relies only on the intrinsic degrees of freedom of the ferromagnet, and can be naturally extended to explore the quantum dynamics of DWs in ferrimagnets and antiferromagnets, as well as quantum vortices or skyrmions confined in insulating magnetic nanodisks.",2401.03164v1 2024-01-08,Optically Helicity-Dependent Orbital and Spin Dynamics in Two-Dimensional Ferromagnets,"Disentangling orbital (OAM) and spin (SAM) angular momenta in the ultrafast spin dynamics of two-dimensional (2D) ferromagnets on subfemtoseconds is a challenge in the field of ultrafast magnetism. Herein, we employed non-collinear spin version of real-time time-dependent density functional theory to investigate the orbital and spin dynamics of 2D ferromagnets Fe3GeTe2 (FGT) induced by circularly polarized light. Our results show the demagnetization of Fe sublattice in FGT is accompanied by helicity-dependent precession of OAM and SAM excited by circularly polarized lasers. We further identify that precession of OAM and SAM in FGT is faster than the demagnetization within a few femtoseconds. Remarkably, circularly polarized lasers can significantly induce a periodically transverse response of OAM and SAM on very ultrafast timescales of ~250 attoseconds. Our finding suggests a powerful new route for attosecond regimes of the angular momentum manipulation to coherently control helicity-dependent orbital and spin dynamics in 2D limits.",2401.04038v1 2024-01-10,Self-generated spin-orbit torque driven by anomalous Hall current,"Spin-orbit torques enable energy-efficient manipulation of magnetization by electric current and hold promise for applications ranging from nonvolatile memory to neuromorphic computing. Here we report the discovery of a giant spin-orbit torque induced by anomalous Hall current in ferromagnetic conductors. This anomalous Hall torque is self-generated as it acts on magnetization of the ferromagnet that engenders the torque. The magnitude of the anomalous Hall torque is sufficiently large to fully negate magnetic damping of the ferromagnet, which allows us to implement a microwave spin torque nano-oscillator driven by this torque. The peculiar angular symmetry of the anomalous Hall torque favors its use over the conventional spin Hall torque in coupled nano-oscillator arrays. The universal character of the anomalous Hall torque makes it an integral part of the description of coupled spin transport and magnetization dynamics in magnetic nanostructures.",2401.05006v1 2024-01-27,Interplay of altermagnetism and weak ferromagnetism in two-dimensional RuF$_4$,"Gaining growing attention in spintronics is a class of magnets displaying zero net magnetization and spin-split electronic bands called altermagnets. Here, by combining density functional theory and symmetry analysis, we show that RuF$_4$ monolayer is a two-dimensional $d$-wave altermagnet. Spin-orbit coupling leads to pronounced spin splitting of the electronic bands at the $\Gamma$ point by $\sim 100$ meV and turns the RuF$_4$ into a weak ferromagnet due to non trivial spin-momentum locking that cants the Ru magnetic moments. The net magnetic moment scales linearly with the spin-orbit coupling strength. Using group theory we derive an effective spin Hamiltonian capturing the spin-splitting and spin-momentum locking of the electronic bands. Disentanglement of the altermagnetic and spin-orbit coupling induced spin splitting uncovers to which extent the altermagnetic properties are affected by the spin-orbit coupling. Our results move the spotlight to the non trivial spin-momentum locking and weak ferromagnetism in the two-dimensional altermagnets relevant for novel venues in this emerging field of material science research.",2401.15424v1 2024-02-01,A mechanism for electrostatically generated magnetoresistance in chiral systems without spin-dependent transport,"Significant attention has been drawn to electronic transport in chiral materials coupled to ferromagnets in the chirality induced spin selectivity (CISS) effect. A large magnetoresistance (MR) is usually observed which is widely interpreted to originate from spin (dependent) transport. However, there are severe discrepancies between the experimental results and theoretical interpretations, most notably the apparent failure of the Onsager reciprocity relation in the linear response regime. We provide an alternative explanation for the mechanism of the two terminal MR in chiral systems coupled to a ferromagnet. For this we point out that it was observed that the electrostatic contact potential of chiral materials on a ferromagnet depends on the magnetization direction and chirality. In our explanation this causes the transport barrier to be modified by the magnetization direction, already in equilibrium, in the absence of a bias current. This strongly alters the charge transport through/over the barrier, not requiring spin transport. This provides a mechanism that allows the linear response resistance to be sensitive to the magnetization direction and also explains the failure of the Onsager reciprocity relations. We propose experimental configurations to confirm our alternative mechanism for MR.",2402.00472v1 2024-02-01,Absence of Induced Ferromagnetism in Epitaxial Uranium Dioxide Thin Films,"Recently, Sharma et al. [Adv. Sci. 9, 2203473 (2022)] claimed that thin films (around 20 nm) of UO2 deposited on perovskite substrates exhibit strongly enhanced paramagnetism (called induced ferromagnetism by the authors). Moments of up to 3 Bohr magneton/U atom were claimed in magnetic fields of 6 T. We have reproduced such films and, after characterisation, have examined them with X-ray circular magnetic dichroism (XMCD) at the uranium M edges, a technique that is element specific. We do not confirm the published results. We find a small increase, as compared to the bulk, in the magnetic susceptibility of UO2 in such films, but the magnetisation versus field curves, measured by XMCD, are linear with field and there is no indication of any ferromagnetism. The absence of any anomaly around 30 K (the antiferromagnetic ordering temperature of bulk UO2) in the XMCD signal suggests the films do not order magnetically.",2402.00546v1 2024-02-06,Evidence of temperature-dependent interplay between spin and orbital moment in van der Waals ferromagnet VI3,"Van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and the fabrication of functional heterostructures. Among them, the trihalide VI3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI3 by means of temperature-dependent x-ray absorption spectroscopy and x-ray magnetic circular and linear dichroism. We detect localized electronic states and reduced magnetic dimensionality, due to electronic correlations. We furthermore provide experimental evidence of (a) an unquenched orbital magnetic moment (up to 0.66(7)) in the ferromagnetic state, and (b) an instability of the orbital moment in proximity of the spin reorientation transition. Our results support a coherent picture where electronic correlations give rise to a strong magnetic anisotropy and a large orbital moment, and establish VI3 as a prime candidate for the study of orbital quantum effects.",2402.04063v1 2024-02-07,Macroscopic Magnetic Dynamics,"Ferromagnetic metals and spin-polarized $^{3}$He are spin 1/2 systems with the same macroscopic symmetry, and thus should have macroscopic magnetic dynamics with the same structure. Using Onsager's irreversible thermodynamics, we develop a theory for these systems that contains two relaxation times (one for the magnetization $\vec{M}$ and the other for the spin current $\vec{J}_{i}$), a magnetic compressibility, and a mean-field parameter. Currently spintronics data on metallic ferromagnets are analyzed using a complex decay length from a theory employing a diffusion constant, a lifetime, and a mean-field parameter. The present theory leads to a complex decay length with the same structure. On neglecting decay of $\vec{M}$, the present theory applies to liquids and gases. For macroscopic equations the particle statistics is not relevant, so the theory also applies to bosons. The theory predicts a longitudinal spin wave whose velocity we estimate for liquid $^{3}$He and for paramagnetic metals; but such a wave should also occur for ferromagnets and for gases.",2402.04639v1 2024-02-08,Valley-dependent Multiple Quantum States and Topological Transitions in Germanene-based Ferromagnetic van der Waals Heterostructures,"Topological and valleytronic materials are promising for spintronic and quantum applications due to their unique properties. Using first principles calculations, we demonstrate that germanene (Ge)-based ferromagnetic heterostructures can exhibit multiple quantum states such as quantum anomalous Hall effect (QAHE) with Chern numbers of C=-1 or C=-2, quantum valley Hall effect (QVHE) with a valley Chern number of C$v$=2, valley-polarized quantum anomalous Hall effect (VP-QAHE) with two Chern numbers of C=-1 and C$v$=-1 as well as time-reversal symmetry broken quantum spin Hall effect (T-broken QSHE) with a spin Chern number of C$s$~1. Furthermore, we find that the transitions between different quantum states can occur by changing the magnetic orientation of ferromagnetic layers through applying a magnetic field. Our discovery provides new routes and novel material platforms with a unique combination of diverse properties that make it well suitable for applications in electronics, spintronics and valley electronics.",2402.05613v1 2024-02-21,Itinerant magnetism and magnetic polarons in the triangular lattice Hubbard model,"We use density matrix renormalization group to investigate the phase diagram of the Fermi Hubbard model on a triangular lattice with densities above half-filling, $1 \leq n < 2$. We discuss the important role of kinetic magnetism and magnetic polarons. For strong interactions and low doublon dopings, attractive interaction between polarons results in phase separation between the fully polarized state at finite doping and the commensurate spin-density wave state at half-filling. For intermediate interaction strength and small doping, competition between antiferromagnetic superexchange and kinetic magnetism gives rise to the incommensurate spin density wave (I-SDW) phase. Fully polarized ferromagnetic (FPF) phase for weak interactions is limited to dopings close to the van Hove singularity in the density of states. With increasing interactions the FPF phase expands to lower dopings. For strong interactions it reaches the low doping regime and is better understood as arising from proliferation of Nagaoka-type ferromagnetic polarons. Other phases that we find include a partially polarized phase, another type of I-SDW at high densities, and M\""uller-Hartmann ferromagnetism close to the band insulating regime.",2402.14074v1 2024-02-27,Semiclassical approach to spin dynamics of a ferromagnetic S=1 chain,"Motivated by recent experimental progress in the quasi-one-dimensional quantum magnet NiNb$_2$O$_6$, we study the spin dynamics of an S=1 ferromagnetic Heisenberg chain with single-ion anisotropy by using a semiclassical molecular dynamics approach. This system undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state under a transverse magnetic field, and the magnetic responses reflecting this transition is well described by our semiclassical method. We show that at low-temperature the transverse component of the dynamical structure factor depicts clearly the magnon dispersion, and the longitudinal component exhibits two continua associated with single- and two-magnon excitations, respectively. These spin excitation spectra show interesting temperature dependence as effects of magnon interactions.Our findings shed light on experimental detection of spin excitations in a large class of quasi-one-dimensional magnets.",2402.17416v1 2024-03-01,Minority magnons and mode branching in monolayer Fe$_3$GeTe$_2$,"In this letter, we predict the existence of minority magnons in a monolayer of Fe$_3$GeTe$_2$ using first principles calculations. Minority magnons constitute a new type of collective magnetic excitations which increase the magnetic moment rather than lower it, contrary to ordinary (majority) magnons. The presence of such quasi-particles is made possible by the nontrivial ferromagnetic band structure of Fe$_3$GeTe$_2$ originating from its nonequivalent Fe sublattices. The result is a strong peak in the dynamic spin-raising susceptibility $\chi^{-+}(\omega)$ in the long wavelength limit, which is the hallmark of minority magnon physics. We calculate the susceptibility using time-dependent density functional theory and perform a detailed mode analysis, which allows us to separate and investigate the individual magnon modes as well as the Stoner excitations that constitute the many-body spectrum. For minority as well as majority magnons, the analysis reveals a plethora of magnetic excitations, which in addition to the standard main magnon branches include both satellite, valley and spin-inversion magnons, originating from the itinerancy of the ferromagnetic order. The physics underlying this analysis is in no way restricted to Fe$_3$GeTe$_2$, and minority magnons are expected to be observable in many complex ferromagnetic materials.",2403.00525v1 2024-03-02,Zero-field magnetic skyrmions in exchange-biased ferromagnetic-antiferromagnetic bilayers,"We report on the stabilization of ferromagnetic skyrmions in zero external magnetic fields, in exchange-biased systems composed of ferromagnetic-antiferromagnetic (FM-AFM) bilayers. By performing atomistic spin dynamics simulations, we study cases of compensated, uncompensated, and partly uncompensated FM-AFM interfaces, and investigate the impact of important parameters such as temperature, inter-plane exchange interaction, Dzyaloshinskii-Moria interaction, and magnetic anisotropy on the skyrmions appearance and stability. The model with an uncompensated FM-AFM interface leads to the stabilization of individual skyrmions and skyrmion lattices in the FM layer, caused by the effective field from the AFM instead of an external magnetic field. Similarly, in the case of a fully compensated FM-AFM interface, we show that FM skyrmions can be stabilized. We also demonstrate that accounting for interface roughness leads to stabilization of skyrmions both in compensated and uncompensated interfaces. Moreover, in bilayers with a rough interface, skyrmions in the FM layer are observed for a wide range of exchange interaction values through the FM-AFM interface, and the chirality of the skyrmions depends critically on the exchange interaction.",2403.01175v1 2024-03-06,Competing magnetic correlations and uniaxial anisotropy in (Fe$_{1-x}$Mn$_{x}$)$_2$AlB$_2$ single crystals,"We have succeeded for the first time in synthesizing single crystals of nanolaminated borides (Fe$_{1-x}$Mn$_{x}$)$_2$AlB$_2$ in the entire Fe-Mn composition range using the Al self-flux method, and have established $T$-$x$, $H$-$T$ and three-dimensional $H$-$T$-$x$ magnetic phase diagrams from the results of magnetization measurements. The ferromagnetic correlation of Fe$_2$AlB$_2$ is weakened with the Mn substitution, whereas the antiferromagnetic correlation of Mn$_2$AlB$_2$ is enhanced with the Fe up to $x=0.65$. The spin direction in the magnetic ordered states changes from the $a$ to the $b$ axis with increasing Mn concentration and temperature. At $x$ = 0.31-0.46, there are three magnetic phases; ferromagnetic, antiferromganetic, and intermediate phases in between. At $x$ = 0.65 and 0.74, a spin-flop-like metamagnetic transition was observed at a finite field parallel to the spin direction. These observations indicate that in (Fe$_{1-x}$Mn$_{x}$)$_2$AlB$_2$ the ferromagnetic and antiferromagnetic correlations coexist and the uniaxial magnetic anisotropy competes between the $a$ and $b$ axes.",2403.03415v1 2024-03-08,Imprinting of Antiferromagnetic Vortex States in NiO-Fe Nanostructures,"Magnetic vortices are topological spin structures frequently found in ferromagnets, yet novel to antiferromagnets. By combining experiment and theory, we demonstrate that in a nanostructured antiferromagnetic-ferromagnetic NiO(111)-Fe(110) bilayer, a magnetic vortex is naturally stabilized by magnetostatic interactions in the ferromagnet and is imprinted onto the adjacent antiferromagnet via interface exchange coupling. We use micromagnetic simulations to construct a corresponding phase diagram of the stability of the imprinted antiferromagnetic vortex state. Our in depth analysis reveals that the interplay between interface exchange coupling and the antiferromagnet magnetic anisotropy plays a crucial role in locally reorienting the N\'eel vector out-of-plane in the prototypical in-plane antiferromagnet NiO and thereby stabilizing the vortices in the antiferromagnet.",2403.05151v1 2024-03-14,Exact matrix product state representations for a type of scale-invariant states,"Exact matrix product state representations for a type of scale-invariant states are presented, which describe highly degenerate ground states arising from spontaneous symmetry breaking with type-B Goldstone modes in one-dimensional quantum many-body systems. As a possible application, such a representation offers a convenient but powerful means for evaluating the norms of highly degenerate ground states. This in turn allows us to perform a universal finite system-size scaling analysis of the entanglement entropy. Moreover, this approach vividly explains why the entanglement entropy does not depend on what types of the boundary conditions are adopted, either periodic boundary conditions or open boundary conditions. Illustrative examples include the ${\rm SU}(2)$ spin-$s$ Heisenberg ferromagnetic model, the ${\rm SU}(2s+1)$ ferromagnetic model, and the staggered ${\rm SU}(3)$ spin-1 ferromagnetic biquadratic model.",2403.09458v1 2024-03-15,Half-metallic transport and spin-polarized tunneling through the van der Waals ferromagnet Fe${_4}$GeTe$_{2}$,"The recent emergence of van der Waals (vdW) ferromagnets has opened new opportunities for designing spintronic devices. We theoretically investigate the coherent spin-dependent transport properties of the vdW ferromagnet Fe$_4$GeTe$_2$, by using density functional theory combined with the non-equilibrium Green's functions method. We find that the conductance in the direction perpendicular to the layers is half-metallic, namely it is entirely spin-polarized, as a result of the material's electronic structure. This characteristic persists from bulk to single layer, even under significant bias voltages, and it is little affected by spin-orbit coupling and electron correlation. Motivated by this observation, we then investigate the tunnel magnetoresistance (TMR) effect in an magnetic tunnel junction, which comprises two Fe$_4$GeTe$_2$ layers separated by the vdW gap acting as insulating barrier. We predict a TMR ratio of almost 500\%, which can be further boosted by increasing the number of Fe$_4$GeTe$_2$ layers in the junction.",2403.10195v1 2024-03-18,Uncovering the lowest thickness limit for room-temperature ferromagnetism of Cr$_{1.6}$Te$_{2}$,"Half-metallic ferromagnetic transition metal dichalcogenides have emerged as important building blocks for scalable magnonics and memory applications. Downscaling such systems to the ultra-thin limit is critical to integrate them into technology. Here, we achieved layer-by-layer control over the transition metal dichalcogenide Cr$_{1.6}$Te$_{2}$ by using pulsed laser deposition, and we uncovered the minimum critical thickness above which room temperature magnetic order is maintained. The electronic and magnetic structure is explored experimentally and theoretically and it is shown that the films exhibit strong in-plane magnetic anisotropy as a consequence of large spin-orbit effects. Our study establishes Cr$_{1.6}$Te$_{2}$ as a platform material, viable for ferromagnetic nanoscale devices and magnetic-memory architectures.",2403.11977v1 2024-03-25,Detection of spin pumping free of rectification and thermal artefacts in molecular-based ferromagnetic insulator V[TCNE]x~2,"The molecular-based ferrimagnetic insulator V(TCNE)x has gained recent interest for efficient spin-wave excitation due to its low Gilbert damping ratio a=4E-5, and narrow ferromagnetic resonance linewidth f=1Oe. Here we report a clean spin pumping signal detected on V(TCNE)x/metal bilayer structures, free from spin rectification or thermal artifacts. On-chip coupling of microwave power is achieved via a coplanar waveguide to measure the in-plane angle-dependence of the inverse spin-Hall effect under ferromagnetic resonance conditions with respect to a constant external magnetic field. A signature of pure spin current from V(TCNE)x is observed in both platinum and permalloy metal layers, demonstrating the utility of V(TCNE)x for magnon spintronics studies in molecule/solid-state heterostructures.",2403.16429v2 2024-03-29,Electric field enhancement of the superconducting spin-valve effect via strain-transfer across a ferromagnetic/ferroelectric interface,"In a ferromagnet/superconductor/ferromagnet (F/S/F) superconducting spin-valve (SSV), a change of the magnetization alignment of the two F layers modulates the critical temperature (Tc) of the S layer. The Tc-switching (the SSV effect) is based on the interplay between superconductivity and magnetism. Fast and large resistive switching associated with the Tc-switching is suitable for nonvolatile cryogenic memory applications. However, external magnetic field-based operation of SSVs is hindering their miniaturization, and therefore, electric field control of the SSV effect is desired. Here, we report epitaxial growth of a La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3 SSV on a piezo-electric [Pb(Mg0.33Nb0.67)O3]0.7-[PbTiO3]0.3 (001) substrate and demonstrate electric field control of the SSV effect. Electric field-induced strain-transfer from the piezo-electric substrate increases the magnetization and Tc of the SSV, and leads to an enhancement of the magnitude of Tc-switching. The results are promising for the development of magnetic-field-free superconducting spintronic devices, in which the S/F interaction is not only sensitive to the magnetization alignment but also to an applied electric field.",2403.20210v1 2002-01-18,Electron Paramagnetic Resonance Study of Phase Segregation in Nd$_{0.5}$Sr$_{0.5}$MnO$_3$,"We present results of an electron paramagnetic resonance (EPR) study of Nd$_{1-x}$Sr$_x$MnO$_3$ with x = 0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T$_c$) and the antiferromagnetic, charge ordering transition (T$_N$ = T$_{co}$) at 150 K. The results are compared with those on Nd$_{0.45}$Sr$_{0.55}$MnO$_3$ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T$_N$ = 230 K and on La$_{0.77}$Ca$_{0.23}$MnO$_3$ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x = 0.5, the EPR signals below T$_c$ consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x = 0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La$_{0.77}$Ca$_{0.23}$MnO$_3$. The comparison of behaviour of A-type AFM signal observed in both Nd$_{0.5}$Sr$_{0.5}$MnO$_3$and Nd$_{0.45}$Sr$_{0.55}$MnO$_3with the two FM phases of La$_{0.77}$Ca$_{0.23}$MnO$_3$, vis-a-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd$_{0.5}$Sr$_{0.5}$MnO$_3$undergoes phase separation into A-type AFM and FM phases.",0201336v2 2008-09-30,Dynamics of diluted magnetic semiconductors from atomistic spin dynamics simulations: Mn doped GaAs as a case study,"The dynamical behavior of the magnetism of diluted magnetic semiconductors (DMS) has been investigated by means of atomistic spin dynamics simulations. The conclusions drawn from the study are argued to be general for DMS systems in the low concentration limit, although all simulations are done for 5% Mn-doped GaAs with various concentrations of As antisite defects. The magnetization curve, $M(T)$, and the Curie temperature $T_C$ have been calculated, and are found to be in good correspondence to results from Monte Carlo simulations and experiments. Furthermore, equilibrium and non-equilibrium behavior of the magnetic pair correlation function have been extracted. The dynamics of DMS systems reveals a substantial short ranged magnetic order even at temperatures at or above the ordering temperature, with a non-vanishing pair correlation function extending up to several atomic shells. For the high As antisite concentrations the simulations show a short ranged anti-ferromagnetic coupling, and a weakened long ranged ferromagnetic coupling. For sufficiently large concentrations we do not observe any long ranged ferromagnetic correlation. A typical dynamical response shows that starting from a random orientation of moments, the spin-correlation develops very fast ($\sim$ 1ps) extending up to 15 atomic shells. Above $\sim$ 10 ps in the simulations, the pair correlation is observed to extend over some 40 atomic shells. The autocorrelation function has been calculated and compared with ferromagnets like bcc Fe and spin-glass materials. We find no evidence in our simulations for a spin-glass behaviour, for any concentration of As antisites. Instead the magnetic response is better described as slow dynamics, at least when compared to that of a regular ferromagnet like bcc Fe.",0809.5187v1 2009-02-17,Scaling behavior of domain walls at the T=0 ferromagnet to spin-glass transition,"We study domain-wall excitations in two-dimensional random-bond Ising spin systems on a square lattice with side length L, subject to two different continuous disorder distributions. In both cases an adjustable parameter allows to tune the disorder so as to yield a transition from a spin-glass ordered ground state to a ferromagnetic groundstate. We formulate an auxiliary graph-theoretical problem in which domain walls are given by undirected shortest paths with possibly negative distances. Due to the details of the mapping, standard shortest-path algorithms cannot be applied. To solve such shortest-path problems we have to apply minimum-weight perfect-matching algorithms. We first locate the critical values of the disorder parameters, where the ferromagnet to spin-glass transition occurs for the two types of the disorder. For certain values of the disorder parameters close to the respective critical point, we investigate the system size dependence of the width of the the average domain-wall energy (~L^\theta) and the average domain-wall length (~L^df). Performing a finite-size scaling analysis for systems with a side length up to L=512, we find that both exponents remain constant in the spin-glass phase, i.e. \theta ~- 0.28 and df~1.275. This is consistent with conformal field theory, where it seems to be possible to relate the exponents from the analysis of Stochastic Loewner evolutions (SLEs) via df-1=3/[4(3+\theta)]. Finally, we characterize the transition in terms of ferromagnetic clusters of spins that form, as one proceeds from spin-glass ordered to ferromagnetic ground states.",0902.2938v2 2009-03-31,Repulsive Fermi gas in a harmonic trap: Ferromagnetism and spin textures,"We study ferromagnetism in a repulsively interacting two-component Fermi gas in a harmonic trap. Within a local density approximation, the two components phase-separate beyond a critical interaction strength, with one species having a higher density at the trap center. We discuss several easily observable experimental signatures of this transition. The mean field release energy, its separate kinetic and interaction contributions, as well as the potential energy, all depend on the interaction strength and contain a sharp signature of this transition. In addition, the conversion rate of atoms to molecules, arising from three-body collisions, peaks at an interaction strength just beyond the ferromagnetic transition point. We then go beyond the local density approximation, and derive an energy functional which includes a term that depends on the local magnetization gradient and acts as a `surface tension'. Using this energy functional, we numerically study the energetics of some candidate spin textures which may be stabilized in a harmonic trapping potential at zero net magnetization. We find that a hedgehog state has a lower energy than an `in-out' domain wall state in an isotropic trap. Upon inclusion of trap anisotropy we find that the hedgehog magnetization profile gets distorted due to the surface tension term, this distortion being more apparent for small atom numbers. We estimate that the magnetic dipole interaction does not play a significant role in this system. We consider possible implications for experiments on trapped Li-6 and K-40 gases.",0903.5343v5 2012-06-14,Topological superconductivity and Majorana fermions in hybrid structures involving cuprate high-T_c superconductors,"The possibility of inducing topological superconductivity with cuprate high-temperature superconductors (HTSC) is studied for various heterostructures. We first consider a ballistic planar junction between a HTSC and a metallic ferromagnet. We assume that inversion symmetry breaking at the tunnel barrier gives rise to Rashba spin-orbit coupling in the barrier and allows equal-spin triplet superconductivity to exist in the ferromagnet. Bogoliubov-de Gennes equations are obtained by explicitly modeling the barrier, and taking account of the transport anisotropy in the HTSC. By making use of the self-consistent boundary conditions and solutions for the barrier and HTSC regions, an effective equation of motion for the ferromagnet is obtained where Andreev scattering at the barrier is incorporated as a boundary condition for the ferromagnetic region. For a ferromagnet layer deposited on a (100) facet of the HTSC, triplet p-wave superconductivity is induced. For the layer deposited on a (110) facet, the induced gap does not have the p-wave orbital character, but has an even orbital symmetry and an odd dependence on energy. For the layer on the (001) facet, an exotic f-wave superconductivity is induced. We also consider the induced triplet gap in a one-dimensional half-metallic nanowire deposited on a (001) facet of a HTSC. We find that for a wire axis along the a-axis, a robust triplet p-wave gap is induced. For a wire oriented 45 degrees away from the a-axis the induced triplet p-wave gap vanishes. For the appropriately oriented wire, the induced p-wave gap should give rise to Majorana fermions at the ends of the half-metallic wire. Based on our result, topological superconductivity in a semi-conductor nanowire may also be possible given that it is oriented along the a-axis of the HTSC.",1206.3226v2 2012-08-03,"Spin dynamics, short-range order and superparamagnetism in superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10-δ","We report structural, detailed DC and linear/non-linear AC, isothermal and thermoremanent magnetization study of the rutheno-cuprate superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10-{\delta} (GdRu-1222). Structural analysis, by employing Rietveld refinement of X-ray diffraction pattern, reveals that GdRu-1222 crystallizes in tetragonal phase with I4/mmm space group. GdRu-1222 is a reported superconducting ferromagnet with Ru spins magnetic ordering at around 110 K and superconductivity below 40 K in Cu-O2 planes. Detailed linear/non-linear first and higher order harmonic of AC susceptibility studies unveiled the complex magnetism of GdRu-1222. A frequency dependent cusp is observed in AC susceptibility ({\chi}ac) vs. T measurements. The change in cusp position with applied frequency followed the well known Vogel-Fulcher law, which is a feature to describe a spin-glass (SG) system with possibility of embedded homogeneous/non-homogeneous magnetically interacting/non-interacting ferromagnetic clusters. Such an interpretation is also supported by thermoremanent magnetization (TRM) study at T = 60 K. Detailed interpretation of AC magnetization results revealed the formation of magnetic (ferromagnetic) homogenous/non-homogenous clusters of different sizes embedded in spin-glass (SG) matrix. The magnetization vs. applied field loops do not saturate, even at high applied fields (50 kOe), resulting in the short-range magnetic ordering in the system, which causes the formation of clusters that freeze at low temperatures. Temperature variation of first- and third-order susceptibility harmonics show good agreement with Wohlfarth's model (WM), leading to the superparamagnetism (SPM) state. Detailed magnetization (DC and AC both) results and their analysis helped in explaining the temperature dependent magnetism of the GdRu-1222 system.",1208.0760v2 2014-10-13,Topological Superconductivity induced by Ferromagnetic Metal Chains,"Recent experiments have provided evidence that one-dimensional (1D) topological superconductivity can be realized experimentally by placing transition metal atoms that form a ferromagnetic chain on a superconducting substrate. We address some properties of this type of systems by using a Slater-Koster tight-binding model. We predict that topological superconductivity is nearly universal when ferromagnetic transition metal chains form straight lines on superconducting substrates and that it is possible for more complex chain structures. The proximity induced superconducting gap is $\sim \Delta E_{so} / J$ where $\Delta$ is the $s$-wave pair-potential on the chain, $E_{so}$ is the spin-orbit splitting energy induced in the normal chain state bands by hybridization with the superconducting substrate, and $J$ is the exchange-splitting of the ferromagnetic chain $d$-bands. Because of the topological character of the 1D superconducting state, Majorana end modes appear within the gaps of finite length chains. We find, in agreement with experiment, that when the chain and substrate orbitals are strongly hybridized, Majorana end modes are substantially reduced in amplitude when separated from the chain end by less than the coherence length defined by the $p$-wave superconducting gap. We conclude that Pb is a particularly favorable substrate material for ferromagnetic chain topological superconductivity because it provides both strong $s-$wave pairing and strong Rashba spin-orbit coupling, but that there is an opportunity to optimize properties by varying the atomic composition and structure of the chain. Finally, we note that in the absence of disorder a new chain magnetic symmetry, one that is also present in the crystalline topological insulators, can stabilize multiple Majorana modes at the end of a single chain.",1410.3453v3 2014-12-08,Imaging Dirac-Mass Disorder from Magnetic Dopant-Atoms in the Ferromagnetic Topological Insulator Cr$_x$(Bi$_{0.1}$Sb$_{0.9}$)$_{2-x}$Te$_3$,"To achieve and utilize the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TI),it is necessary to open a ""Dirac-mass gap"" in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely used approach. But it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr$_{0.08}$(Bi$_{0.1}$Sb$_{0.9}$)$_{1.92}$Te$_3$. Simultaneous visualization of the Dirac-mass gap $\Delta(r)$ reveals its intense disorder, which we demonstrate directly is related to fluctuations in $n(r)$, the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of $\Delta(r)$ consistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship $\Delta(r)\propto n(r)$ is confirmed throughout, and exhibits an electron-dopant interaction energy $J^*$=145$meV\cdot nm^2$. These observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal-symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.",1412.2718v1 2015-01-27,Unconventional Strong Spin-Fluctuation Effects around the Critical Pressure of the Itinerant Ising-Type Ferromagnet URhAl,"Resistivity measurements were performed for the itinerant Ising-type ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5 GPa, using single crystals. We found that the critical pressure of the Curie temperature exists at around $P_c$ ~ 5.2 GPa. Near $P_c$, the $A$-coefficient of the $AT^{2}$ Fermi-liquid resistivity term below $T^*$ is largely enhanced with a maximum around 5.2-5.5 GPa. Above $P_c$, the exponent of the resistivity $\rho(T)$ deviates from 2. At $P_c$, it is close to $n = 5/3$, which is expected by the theory of three-dimensional ferromagnetic spin fluctuations for a 2nd-order quantum-critical point (QCP). However, $T_C(P)$ disappears as a 1st-order phase transition, and the critical behavior of resistivity in URhAl cannot be explained by the theory of a 2nd-order QCP. The 1st-order nature of the phase transition is weak, and the critical behavior is still dominated by the spin fluctuation at low temperature. With increasing pressure, the non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies point out a ferromagnetic wing structure with a tri-critical point (TCP) at ~ 4.8-4.9 GPa in URhAl. One open possibility is that the switch from the ferromagnetic to the paramagnetic states does not occur simply but an intermediate state arises below the TCP as suggested theoretically recently. Quite generally, if a drastic Fermi-surface change occurs through $P_c$, the nature of the interaction itself may change and lead to the observed unconventional behavior.",1501.06701v2 2015-05-27,Thermodynamics of the frustrated $J_1$-$J_2$ Heisenberg ferromagnet on the body-centered cubic lattice with arbitrary spin,"We use the spin-rotation-invariant Green's function method as well as the high-temperature expansion to discuss the thermodynamic properties of the frustrated spin-$S$ $J_{1}$-$J_{2}$ Heisenberg magnet on the body-centered cubic lattice. We consider ferromagnetic nearest-neighbor bonds $J_1 < 0$ and antiferromagnetic next-nearest-neighbor bonds $J_2 \ge 0$ and arbitrary spin $S$. We find that the transition point $J_2^c$ between the ferromagnetic ground state and the antiferromagnetic one is nearly independent of the spin $S$, i.e., it is very close to the classical transition point $J_2^{c,{\rm clas}}= \frac{2}{3}|J_1|$. At finite temperatures we focus on the parameter regime $J_2\alpha$ and it becomes impossible to distinguish the bound state and the scattering state. These positive-energy bound states become occupied and therefore the upper branch reaches an energy maximum at $k_{\rm F}a=\alpha$. In the zero range limit, there exists a narrow window ($0.861, and all S=1/2,1,3/2,.... For the isotropic model (Delta =1) we show that all ground states are translation invariant. For the proof of these statements we propose a strategy for demonstrating completeness of the list of the pure infinite-volume ground states of a quantum many-body system, of which the present results for the XXX and XXZ chains can be seen as an example. The result for Delta>1 can also be proved by an easy extension to general $S$ of the method used in [T. Matsui, Lett. Math. Phys. 37 (1996) 397] for the spin-1/2 ferromagnetic XXZ chain with $\Delta>1$. However, our proof is different and does not rely on the existence of a spectral gap. In particular, it also works to prove absence of non-translationally invariant ground states for the isotropic chains (Delta=1), which have a gapless excitation spectrum. Our results show that, while any small amount of the anisotropy is enough to stabilize the domain walls against the quantum fluctuations, no boundary condition exists that would stabilize a domain wall in the isotropic model (Delta=1).",9709208v2 1997-12-19,Multi-band Gutzwiller wave functions for general on-site interactions,"We introduce Gutzwiller wave functions for multi-band models with general on-site Coulomb interactions. As these wave functions employ correlators for the exact atomic eigenstates they are exact both in the non-interacting and in the atomic limit. We evaluate them in infinite lattice dimensions for all interaction strengths without any restrictions on the structure of the Hamiltonian or the symmetry of the ground state. The results for the ground-state energy allow us to derive an effective one-electron Hamiltonian for Landau quasi-particles, applicable for finite temperatures and frequencies within the Fermi-liquid regime. As applications for a two-band model we study the Brinkman-Rice metal-to-insulator transition at half band-filling, and the transition to itinerant ferromagnetism for two specific fillings, at and close to a peak in the density of states of the non-interacting system. Our new results significantly differ from those for earlier Gutzwiller wave functions where only density-type interactions were included. When the correct spin symmetries for the two-electron states are taken into account, the importance of the Hund's-rule exchange interaction is even more pronounced and leads to paramagnetic metallic ground states with large local magnetic moments. Ferromagnetism requires fairly large interaction strengths, and the resulting ferromagnetic state is a strongly correlated metal.",9712240v1 1998-03-20,"Dynamics of a ferromagnetic domain wall: avalanches, depinning transition and the Barkhausen effect","We study the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium. The avalanche-like motion of the domain walls between pinned configurations produces a noise known as the Barkhausen effect. We discuss experimental results on soft ferromagnetic materials, with reference to the domain structure and the sample geometry, and report Barkhausen noise measurements on Fe$_{21}$Co$_{64}$B$_{15}$ amorphous alloy. We construct an equation of motion for a flexible domain wall, which displays a depinning transition as the field is increased. The long-range dipolar interactions are shown to set the upper critical dimension to $d_c=3$, which implies that mean-field exponents (with possible logarithmic correction) are expected to describe the Barkhausen effect. We introduce a mean-field infinite-range model and show that it is equivalent to a previously introduced single-degree-of-freedom model, known to reproduce several experimental results. We numerically simulate the equation in $d=3$, confirming the theoretical predictions. We compute the avalanche distributions as a function of the field driving rate and the intensity of the demagnetizing field. The scaling exponents change linearly with the driving rate, while the cutoff of the distribution is determined by the demagnetizing field, in remarkable agreement with experiments.",9803253v1 1998-03-27,Skyrmions in quantum Hall ferromagnets as spin-waves bound to unbalanced magnetic flux quanta,"A microscopic description of (baby)skyrmions in quantum Hall ferromagnets is derived from a scattering theory of collective (neutral) spin modes by a bare quasiparticle. We start by mapping the low lying spectrum of spin waves in the uniform ferromagnet onto that of free moving spin excitons, and then we study their scattering by the defect of charge. In the presence of this disturbance, the local spin stiffness varies in space, and we translate it into an inhomogeneus metric in the Hilbert space supporting the excitons. An attractive potencial is then required to preserve the symmetry under global spin rotations, and it traps the excitons around the charged defect. The quasiparticle now carries a spin texture. Textures containing more than one exciton are described within a mean-field theory, the interaction among the excitons being taken into account through a new renormalization of the metric. The number of excitons actually bound depends on the Zeeman coupling, that plays the same role as a chemical potencial. For small Zeeman energies, the defect binds many excitons which condensate. As the bound excitons have a unit of angular momentum, provided by the quantum of magnetic flux left unbalanced by the defect of charge, the resulting texture turns out to be a topological excitation of charge 1. Its energy is that given by the non-linear sigma model for the ground state in this topological sector, i.e. the texture is a skyrmion.",9803333v1 1998-05-19,Free vortex and vortex-pair lifetimes in classical two-dimensional easy-plane magnets,"We report numerical simulation results for free-vortex lifetimes in the critical region of classical two-dimensional easy-plane ferro- and antiferromagnets having three-component order parameters. The fluctuations in the vortex number density in a spin dynamics simulation were used to estimate the lifetimes. The observed lifetimes are of the same order of magnitude but smaller than the characteristic time-scale above which a phenomenological ideal vortex-gas theory that has been used to account for the central peak in the dynamic structure factor $S^{\alpha\alpha}({\bf q},\omega)$ is expected to be valid. % For strong anisotropy, where the vortices are in-plane, the free vortex lifetimes for ferromagnets and antiferromagnets are the same, while for weak anisotropy, where the vortices have nonzero out-of-easy-plane components, the lifetimes in antiferromagnets are smaller than in ferromagnets. The dependence of the free-vortex and total vortex densities on the size dependent correlation length in the critical region is examined. We also determined the lifetimes of vortex-antivortex pairs for $T = T_{{\scriptsize KT}}$ and well below $T_{{\scriptsize KT}}$. The observed time-scales are very short, and the observed pair densities are extremely small. These results suggest that pair creation and annihilation are unlikely to play any role in the central peak in $S^{xx}({\bf q},\omega)$ observed in computer simulations for the ferromagnetic model for $T \leq T_{{\scriptsize KT}}$.",9805218v1 1998-11-25,Transport and magnetic anomalies due to A-site ionic size mismatch in La$_{0.5}$Ca$_{0.5-x}$Ba_{x}MnO$_3$,"We present results of electrical resistivity, magnetoresistance and ac and dc magnetic susceptibility on polycrystalline samples of the type La(0.5)Ca(0.5-x)Ba(x)MnO(3) synthesized under identical heat treatment conditions. The substitution of larger Ba ions for Ca results in a non- monotonic variation of the curie temperature as the system evolves from a charge ordered insulating state for x=0 to a ferromagnetic metallic state for x=0.5. An intermediate compositino, x=0.1, interestingly exhibits ferromagnetic. insulating behaviour with thermal hysteresis in ac chi around the curie tem- perature (120K). The x=0.2 and 0.3 compounds exhibit semiconducting like behavior as the temperature is lowered below 300K, with a broad peak in rho around 80-100K: These compositions exhibit a weak increase in rho as the temperature lowered below 30K, indicative of electron localization effects. These compositions also undergo ferromagnetic transitions below about 200 and 235K respectively, though these are non-hysteretic; above all, for these compositions, MR is large and conveniently measurable over the entire tempera- ture range of measurement below Tc. This experimental finding may be of interest from the application point of view. We infer that the A-site ionic-size mismatch plays a crucial role in the deciding these properties.",9811351v2 1998-12-31,Effect of disorder on quantum phase transitions in anisotropic XY spin chains in a transverse field,"We present some exact results for the effect of disorder on the critical properties of an anisotropic XY spin chain in a transverse field. The continuum limit of the corresponding fermion model is taken and in various cases results in a Dirac equation with a random mass. Exact analytic techniques can then be used to evaluate the density of states and the localization length. In the presence of disorder the ferromagnetic-paramagnetic or Ising transition of the model is in the same universality class as the random transverse field Ising model solved by Fisher using a real space renormalization group decimation technique (RSRGDT). If there is only randomness in the anisotropy of the magnetic exchange then the anisotropy transition (from a ferromagnet in the $x$ direction to a ferromagnet in the $y$ direction) is also in this universality class. However, if there is randomness in the isotropic part of the exchange or in the transverse field then in a non-zero transverse field the anisotropy transition is destroyed by the disorder. We show that in the Griffiths' phase near the Ising transition that the ground state energy has an essential singularity. The results obtained for the dynamical critical exponent, the typical correlation length, and the temperature dependence of the specific heat near the Ising transition agree with the results of the RSRGDT and numerical work.",9812430v1 1999-02-12,Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing,"Ru based perovskites demonstrate an amazing richness in their magnetic properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and unconventional superconductivity. Tendency to ferromagnetism, stemming from the unusually large involvement of O in magnetism in ruthenates, leads to ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave superconductivity. A related compound Ca2RuO4 was measured to be antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations in Sr2RuO4 as well. Here we report first principles calculations that demonstrate that in both compounds the ferro- and antiferromagnetic fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a close competition between p-wave and d-wave superconducting symmetries in Sr2RuO4. The antiferromagnetism in this system appears to be mostly related with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly, for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one everywhere except in close vicinity of the antiferromagnetic instability. The most stable state within the d-wave channel has vanishing order parameter at one out of three Fermi surfaces in Sr2RuO4, while in the p channel its amplitude is comparable at all three of them.",9902193v1 1999-06-18,First-Order Phase Transition in a Quantum Hall Ferromagnet,"The single-particle energy spectrum of a two-dimensional electron gas in a perpendicular magnetic field consists of equally-spaced spin-split Landau levels, whose degeneracy is proportional to the magnetic field strength. At integer and particular fractional ratios between the number of electrons and the degeneracy of a Landau level (filling factors n) quantum Hall effects occur, characterised by a vanishingly small longitudinal resistance and quantised Hall voltage. The quantum Hall regime offers unique possibilities for the study of cooperative phenomena in many-particle systems under well-controlled conditions. Among the fields that benefit from quantum-Hall studies is magnetism, which remains poorly understood in conventional material. Both isotropic and anisotropic ferromagnetic ground states have been predicted and few of them have been experimentally studied in quantum Hall samples with different geometries and filling factors. Here we present evidence of first-order phase transitions in n = 2 and 4 quantum Hall states confined to a wide gallium arsenide quantum well. The observed hysteretic behaviour and anomalous temperature dependence in the longitudinal resistivity indicate the occurrence of a transition between the two distinct ground states of an Ising quantum-Hall ferromagnet. Detailed many-body calculations allowed the identification of the microscopic origin of the anisotropy field.",9906294v4 1999-12-27,Melting of Charge/Orbital Ordered States in Nd$_{1/2}$Sr$_{1/2}$MnO$_3$: Temperature and Magnetic Field Dependent Optical Studies,"We investigated the temperature ($T=$ 15 $\sim $ 290 K) and the magnetic field ($H=$ 0 $\sim $ 17 T) dependent optical conductivity spectra of a charge/orbital ordered manganite, Nd$_{1/2}$Sr$_{1/2}$MnO$_3$. With variation of $T$ and $H$, large spectral weight changes were observed up to 4.0 eV. These spectral weight changes could be explained using the polaron picture. Interestingly, our results suggested that some local ordered state might remain above the charge ordering temperature, and that the charge/orbital melted state at a high magnetic field (i.e. at $H=$ 17 T and $% T=$ 4.2 K) should be a three dimensional ferromagnetic metal. We also investigated the first order phase transition from the charge/orbital ordered state to ferromagnetic metallic state using the $T$- and $H$% -dependent dielectric constants $\epsilon_1$. In the charge/orbital ordered insulating state, $\epsilon_1$ was positive and $d\epsilon_1/d\omega \approx 0$. With increasing $T$ and $H$, $\epsilon_1$ was increased up to the insulator-metal phase boundaries. And then, $\epsilon_1$ abruptly changed into negative and $d\epsilon_1/d\omega >0$, which was consistent with typical responses of a metal. Through the analysis of $% \epsilon_1$ using an effective medium approximation, we found that the melting of charge/orbital ordered states should occur through the percolation of ferromagnetic metal domains.",9912451v1 2000-05-22,Tricritical Points in the Sherrington-Kirkpatrick Model in the Presence of Discrete Random Fields,"The infinite-range-interaction Ising spin glass is considered in the presence of an external random magnetic field following a trimodal (three-peak) distribution. The model is studied through the replica method and phase diagrams are obtained within the replica-symmetry approximation. It is shown that the border of the ferromagnetic phase may present first-order phase transitions, as well as tricritical points at finite temperatures. Analogous to what happens for the Ising ferromagnet under a trimodal random field, it is verified that the first-order phase transitions are directly related to the dilution in the fields (represented by $p_{0}$). The ferromagnetic boundary at zero temperature also exhibits an interesting behavior: for $0p_{0}^{*}$ the critical frontier is completely continuous; however, for $p_{0}=p_{0}^{*}$, a fourth-order critical point appears. The stability analysis of the replica-symmetric solution is performed and the regions of validity of such a solution are identified; in particular, the Almeida-Thouless line in the plane field versus temperature is shown to depend on the weight $p_{0}$.",0005366v1 2000-07-27,Short range ferromagnetism and spin glass state in $\mathrm{Y_{0.7}Ca_{0.3}MnO_{3}}$,"Dynamic magnetic properties of $\mathrm{Y_{0.7}Ca_{0.3}MnO_{3}}$ are reported. The system appears to attain local ferromagnetic order at $T_{\mathrm{SRF}} \approx 70$ K. Below this temperature the low field magnetization becomes history dependent, i.e. the zero field cooled (ZFC) and field cooled (FC) magnetization deviate from each other and closely logarithmic relaxation appears at our experimental time scales (0.3-$10^{4}$ sec). The zero field cooled magnetization has a maximum at $T_{\mathrm{f}}\approx 30$ K, whereas the field cooled magnetization continues to increase, although less sharply, also below this temperature. Surprisingly, the dynamics of the system shows non-equilibrium spin glass (SG) features not only below the maximum in the ZFC magnetization, but also in the temperature region between this maximum and $T_{\mathrm{SRF}}$. The aging and temperature cycling experiments show only quantitative differences in the dynamic behavior above and below the maximum in the ZFC-magnetization; similarly, memory effects are observed in both temperature regions. We attribute the high temperature behavior to the existence of clusters of short range ferromagnetic order below $T_{\mathrm{SRF}}$; the configuration evolves into a conventional spin glass state at temperatures below $T_{\mathrm{f}}$.",0007439v2 2001-01-26,Approach to the metal-insulator transition in La(1-x)CaxMnO3 (00.5 while there no magnetic behavior or local moments reported for x<0.5. We suggest that the phase diagram is characterized by a crossover from effective single-band character with U >> W for x>0.5 into a three-band regime for x<0.5, where U --> U_eff <= U/\sqrt(3) ~ W and correlation effects are substantially reduced.",0403018v2 2004-03-28,Fully polarized states and decoherence,"The aim of this review is to show how ``ferromagnetic'' states, that is, states having a fully polarization, can produce intrinsic decoherence by unitary evolution. This effect can give an understanding of recent experiments on mesoscopic devices as quantum point contacts showing the 0.7 conductance anomaly and the wide number of data about saturation of dephasing time observed at very low temperatures, as a fully polarized two dimensional electron gas. But similar effects can be seen in different area of physics as for example the Dicke model describing the interaction of two-level systems with a radiation mode. In this case one can show that decoherence is intrinsic and remove a Schr\""odinger cat state leaving a single coherent state, collapsing the wave function in the thermodynamic limit. So, saturation of dephasing time at low temperatures in mesoscopic devices can be understood by a fully polarized two dimensional electron gas that, by an exchange model, can be reduced to a generalized form of the Dicke Hamiltonian and where the quasiparticles are spin excitations interacting with magnons. In this way, one can see that several experiments on nanowires and quantum dots can be satisfactorily explained. The existence of intrinsic decoherence in the thermodynamic limit could have deep implications in fundamental problems like quantum measurement and irreversibility. Recent experiments with cavities with a large number of photons and with nuclear magnetic resonance in organic molecular crystals give a first strong support to this view.",0403678v1 2004-05-26,Microwave Magnetoelectric Effects in Single Crystal Bilayers of Yttrium Iron Garnet and Lead Magnesium Niobate-Lead Titanate,"The first observation of microwave magnetoelectric (ME) interactions through ferromagnetic resonance (FMR) in bilayers of single crystal ferromagnetic-piezoelectric oxides and a theoretical model for the effect are presented. An electric field E produces a mechanical deformation in the piezoelectric phase, resulting in a shift dHE in the resonance field for the ferromagnet. The strength of ME coupling is obtained from data on dHE vs E. Studies were performed at 9.3 GHz on bilayers of (111) yttrium iron garnet (YIG) films and (001) lead magnesium niobate-lead titanate (PMN-PT). The samples were positioned outside a TE102-reflection type cavity. Resonance profiles were obtained for E = 0-8 kV/cm for both in-plane and out-of-plane magnetic fields H. Important results are as follows. (i) The ME coupling in the bilayers is an order of magnitude stronger than in polycrystalline composites and is in the range 1-5.4 Oe cm/kOe, depending on the YIG film thickness. (ii) The coupling strength is dependent on the magnetic field orientation and is higher for out-of-plane H than for in-plane H. (iii) Estimated ME constant and its dependence on volume ratio for the two phases are in good agreement with the data.",0405622v1 2004-06-13,Green's-function theory of the Heisenberg ferromagnet in a magnetic field,"We present a second-order Green's-function theory of the one- and two-dimensional S=1/2 ferromagnet in a magnetic field based on a decoupling of three-spin operator products, where vertex parameters are introduced and determined by exact relations. The transverse and longitudinal spin correlation functions and thermodynamic properties (magnetization, isothermal magnetic susceptibility, specific heat) are calculated self-consistently at arbitrary temperatures and fields. In addition, exact diagonalizations on finite lattices and, in the one-dimensional case, exact calculations by the Bethe-ansatz method for the quantum transfer matrix are performed. A good agreement of the Green's-function theory with the exact data, with recent quantum Monte Carlo results, and with the spin polarization of a $\nu=1$ quantum Hall ferromagnet is obtained. The field dependences of the position and height of the maximum in the temperature dependence of the susceptibility are found to fit well to power laws, which are critically analyzed in relation to the recently discussed behavior in Landau's theory. As revealed by the spin correlation functions and the specific heat at low fields, our theory provides an improved description of magnetic short-range order as compared with the random phase approximation. In one dimension and at very low fields, two maxima in the temperature dependence of the specific heat are found. The Bethe-ansatz data for the field dependences of the position and height of the low-temperature maximum are described by power laws. At higher fields in one and two dimensions, the temperature of the specific heat maximum linearly increases with the field.",0406302v1 2004-06-21,Aging dynamics in reentrant ferromagnet: Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound,"Aging dynamics of a reentrant ferromagnet Cu$_{0.2}$Co$_{0.8}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound has been studied using AC and DC magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures $T_{c}$ ($= 9.7$ K) and $T_{RSG}$ ($= 3.5$ K). The relaxation rate $S(t)$ exhibits a characteristic peak at $t_{cr}$ close to a wait time $t_{w}$ below $T_{c}$, indicating that the aging phenomena occur in both the reentrant spin glass (RSG) phase below $T_{RSG}$ and the ferromagnetic (FM) phase between $T_{RSG}$ and $T_{c}$. The relaxation rate $S(t)$ ($=\text{d}\chi_{ZFC}(t)/\text{d}\ln t$) in the FM phase exhibits two peaks around $t_{w}$ and a time much shorter than $t_{w}$ under the positive $T$-shift aging, indicating a partial rejuvenation of domains. The aging state in the FM phase is fragile against a weak magnetic-field perturbation. The time ($t$) dependence of $\chi_{ZFC}(t)$ around $t \approx t_{cr}$ is well approximated by a stretched exponential relaxation: $\chi_{ZFC}(t) \approx \exp[-(t/\tau)^{1-n}]$. The exponent $n$ depends on $t_{w}$, $T$, and $H$. The relaxation time $\tau$ ($\approx t_{cr}$) exhibits a local maximum around 5 K, reflecting a chaotic nature of the FM phase. It drastically increases with decreasing temperature below $T_{RSG}$.",0406473v1 2004-07-07,Odd triplet pairing effects induced by interface spin-flip scatterings: critical temperature of superconductor/ferromagnet bilayers,"The superconducting critical temperature $T_C$ of a superconductor/ferromagnet (S/F) bilayer with spin-flip scatterings at the interface is calculated as a function of the ferromagnet thickness $d_F$ in the dirty limit employing the Usadel equation. The appropriate boundary conditions from the spin-flip scatterings at the S/F interface are derived for the Usadel equation which includes the spin triplet pairing components as well as the spin singlet one. The spin-flip processes induce the spin triplet pairing components with s-wave in momentum and odd symmetry in frequency from the s-wave singlet order parameter $\Delta$ of the superconductor region. The induced triplet components alter the singlet order parameter in the superconductor through boundary conditions at the interface and, consequently, change the $T_C$ of an S/F bilayer system. The calculated $T_C(d_F)$, like the case of no spin-flips, shows non-monotonic behavior which typically decreases as $d_F$ is increased from 0 and shows a shallow minimum and then saturates slowly as $d_F$ is further increased. It is well established that as the interface resistance (parameterized in terms of $\gamma_b$) is increased, the $T_C$ is increased for a given $d_F$ and the non-monotonic feature in $T_C(d_F)$ is strongly suppressed. As the spin flip scattering (parameterized in terms of $\gamma_m$) is increased, on the other hand, the $T_C$ is also increased for a given $d_F$, but the non-monotonic feature in $T_C(d_F)$ is less suppressed or even enhanced, through the formation of the spin triplet components.",0407149v1 2004-07-16,"A Theoretical Study on Spin-Dependent Transport of ""Ferromagnet/Carbon Nanotube Encapsulating Magnetic Atoms/Ferromagnet"" Junctions with 4-Valued Conductances","As a novel function of ferromagnet (FM)/spacer/FM junctions, we theoretically investigate multiple-valued (or multi-level) cell property, which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs; (up,up), (up,down), (down,up), (down,down). In order to sense all the states, 4-valued conductances corresponding to the respective states are necessary. We previously proposed that 4-valued conductances are obtained in FM1/spin-polarized spacer (SPS)/FM2 junctions, where FM1 and FM2 have different spin polarizations, and the spacer depends on spin [J. Phys.: Condens. Matter 15, 8797 (2003)]. In this paper, an ideal SPS is considered as a single-wall armchair carbon nanotube encapsulating magnetic atoms, where the nanotube shows on-resonance or off-resonance at the Fermi level according to its length. The magnitude of the obtained 4-valued conductances has an opposite order between the on-resonant nanotube and the off-resonant one, and this property can be understood by considering electronic states of the nanotube. Also, the magnetoresistance ratio between (up,up) and (down,down) can be larger than the conventional one between parallel and anti-parallel configurations.",0407439v1 2004-08-06,"Low temperature synthesis, magnetic and magnetotransport properties of (La1-xLux)0.67Ca0.33MnO3 (0 < x < 0.12) system","We have been able to synthesize Lu+3 substituted La0.67Ca0.33MnO3 (LCMO) by an auto-combustion method. Synthesis of this compound is not successful by conventional ceramic or other chemical methods. Magnetic and electrical transport properties of the Lu substituted LCMO [(La1-xLux)0.67Ca0.33MnO3 (0 < x < 0.12)] system have been investigated and compared with those of the Y+3, Pr+3, Dy+3 and Tb+3 substituted LCMO systems. All the compounds show a ferromagnetic metal to paramagnetic insulator transition at TC. The tolerance factor reduces from 0.917 for x = 0 to 0.909 for x = 0.12 and for this range all are ferromagnetic metals indicating the dominance of the coupling between spins due to double exchange over the antiferromagnetic superexchange interaction. The transition temperatures and magnetization decrease as the Lu concentration increases. This is satisfactorily accounted for on the basis of transition from ferromagnetic at x = 0 to canted spin order for x > 0. All the samples show higher magnitude of MR compared to that in pure LCMO at 80 kOe field in the temperature range of 5 to 320K. A fairly high value of low field magnetoresistance (LFMR) of about 30% is obtained in all the samples at a field less than 5 kOe.",0408129v1 2004-08-18,Irreversible spin-transfer and magnetization reversal under spin-injection,"In the context of spin electronics, the two spin-channel model assumes that the spin carriers are composed of two distinct populations: the conduction electrons of spin up, and the conduction electrons of spin down. In order to distinguish the paramagnetic and ferromagnetic contributions in spin injection, we describe the current injection with four channels : the two spin populations of the conduction bands ($s$ or paramagnetic) and the two spin populations of the more correlated electrons ($d$ or ferromagnetic). The redistribution of the conduction electrons at the interface is described by relaxation mechanisms between the channels. Providing that the $d$ majority-spin band is frozen, $s-d$ relaxation essentially concerns the minority-spin channels. Accordingly, even in the abscence of spin-flip scattering (i.e. without standard spin-accumulation or giant magnetoresistance), the $s-d$ relaxation leads to a $d$ spin accumulation effect. The coupled diffusion equations for the two relaxation processes ($s-d$ and spin-flip) are derived. The link with the ferromagnetic order parameter $\vec{M}$ is performed by assuming that only the $d$ channel contributes to the Landau-Lifshitz-Gilbert equation. The effect of magnetization reversal induced by spin injection is explained by these relaxations under the assumption that the spins of the conduction electrons act as environmental degrees of freedom on the magnetization.",0408410v1 2004-09-21,Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNi$_{2}$B$_{2}$C: A penetration depth study,"The coexistence of weak ferromagnetism and superconductivity in ErNi$_{2}$B$% _{2}$C suggests the possibility of a spontaneous vortex phase (SVP) in which vortices appear in the absence of an external field. We report evidence for the long-sought SVP from the in-plane magnetic penetration depth $\Delta \lambda (T)$ of high-quality single crystals of ErNi$_{2}$B$_{2}$C. In addition to expected features at the N\'{e}el temperature $T_{N}$ = 6.0 K and weak ferromagnetic onset at $T_{WFM}=2.3 $K, $\Delta \lambda (T)$ rises to a maximum at $T_{m}=0.45$ K before dropping sharply down to $\sim $0.1 K. We assign the 0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing $\Delta \lambda (T)$ as a consequence of increasing vortex density, and its subsequent decrease below $T_{m}$ as defect pinning suppresses vortex hopping.",0409521v2 2005-01-10,Orbital polarons versus itinerant e_g electrons in doped manganites,"We study an effective one-dimensional (1D) orbital t-J model derived for strongly correlated e_g electrons in doped manganites. The ferromagnetic spin order at half filling is supported by orbital superexchange prop. to J which stabilizes orbital order with alternating x^2-y^2 and 3z^2-r^2 orbitals. In a doped system it competes with the kinetic energy prop. to t. When a single hole is doped to a half-filled chain, its motion is hindered and a localized orbital polaron is formed. An increasing doping generates either separated polarons or phase separation into hole-rich and hole-poor regions, and eventually polarizes the orbitals and gives a it metallic phase with occupied 3z^2-r^2 orbitals. This crossover, investigated by exact diagonalization at zero temperature, is demonstrated both by the behavior of correlation functions and by spectral properties, showing that the orbital chain with Ising superexchange is more classical and thus radically different from the 1D spin t-J model. At finite temperature we derive and investigate an effective 1D orbital model using a combination of exact diagonalization with classical Monte-Carlo for spin correlations. A competition between the antiferromagnetic and ferromagnetic spin order was established at half filling, and localized polarons were found for antiferromagnetic interactions at low hole doping. Finally, we clarify that the Jahn-Teller alternating potential stabilizes the orbital order with staggered orbitals, inducing the ferromagnetic spin order and enhancing the localized features in the excitation spectra. Implications of these findings for colossal magnetoresistance manganites are discussed.",0501190v1 2005-03-10,Crystal and magnetic structure of La$_{1-x}$Ca$_{x}$MnO$_{3}$ compound $(0.11\leq x\leq 0.175,"We studied the crystal and magnetic structure of the La$_{1-x}$Ca$_{x}$MnO$_{3}$ compound for $(0.11\leq x\leq 0.175)$ using stoichiometric samples. For $x<0.13$ the system's ground state is insulating canted antiferromagnetic. For $0.13\le x \le 0.175$ e below the Jahn Teller transition temperature ($T_{\rm JT}$) the crystal structure undergoes a monoclinic distortion. The crystal structure can be described with $P 2_1/c$ space group which permits two Mn sites. The unit cell strain parameter $s=2(a-c)/(a+c)$ increases for $Ta>b/\sqrt{2}$) structure.",0503253v2 2005-03-28,The role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO2,"We have investigated the role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO2 using LSDA+U approach which takes into account strong on-cite Coulomb correlations for electronic structure calculations. Several model systems such as supercells of rutile TiO2 with Co{+2} ion in high-spin state substituted into titanium cite and with oxygen vacancies were considered. We found that exchange interaction of Co magnetic ions is ferromagnetic, but very weak due to the large average impurity-impurity distance. However, its strength becomes three times stronger when there is a magnetic vacancy nearby. The magnetic moment values are 3\mu_B for Co{2+} and 1\mu_B for vacancy with the opposite directions of them. Our investigation showed that exchange interaction energy of Co and vacancy moments varies from 330meV to 40meV depending on the distance between them in the supercell. We suppose that strong interaction between Co and vacancy moments should be taken into account for the explanation of high Curie temperature value in Co-doped TiO2.",0503625v2 2005-08-10,"Low-temperature magnetization of (Ga,Mn)As semiconductors","We report on a comprehensive study of the ferromagnetic moment per Mn atom in (Ga,Mn)As ferromagnetic semiconductors. Theoretical discussion is based on microscopic calculations and on an effective model of Mn local moments antiferromagnetically coupled to valence band hole spins. The validity of the effective model over the range of doping studied is assessed by comparing with microscopic tight-binding/coherent-potential approximation calculations. Using the virtual crystal k.p model for hole states, we evaluate the zero-temperature mean-field contributions to the magnetization from the hole kinetic and exchange energies, and magnetization suppression due to quantum uctuations of Mn moment orientations around their mean-field ground state values. Experimental low-temperature ferromagnetic moments per Mn are obtained by superconducting quantum interference device and x-ray magnetic circular dichroism measurements in a series of (Ga,Mn)As semiconductors with nominal Mn doping ranging from ~2% to 8%. Hall measurements in as-grown and annealed samples are used to estimate the number of uncompensated substitutional Mn moments. Based on our comparison between experiment and theory we conclude that all these Mn moments in high quality (Ga,Mn)As materials have nearly parallel ground state alignment.",0508255v1 2005-09-07,Magnetization Reversal and Nanoscopic Magnetic Phase Separation in Doped La1-xSrxCoO3,"The doped perovskite cobaltite La1-xSrxCoO3 (LSCO) has been advanced as a model system for studying intrinsic magnetic phase separation. We have employed a first-order reversal curve (FORC) method to probe the amount of irreversible switching in bulk polycrystalline LSCO as a function of Sr doping, field cooling procedure, and temperature. The value of the FORC distribution, rho, is used as a measure of the extent of irreversible switching. For x < 0.18, the small values of rho and its ridge-like distribution along local coercivity (Hc) and zero bias (Hb), are characteristic of non-interacting single domain particles. This is consistent with the formation of an array of isolated nanoscopic ferromagnetic clusters, as observed in previous work. For x >= 0.18, the much larger values of rho, the tilting of its distribution towards negative bias field, and the emergence of regions with negative rho, are consistent with increased long-range ferromagnetic ordering. The FORC distributions display little dependence on the cooling procedure. With increasing temperature, the fraction of irreversible switching determined from the FORC distribution follows closely the ferromagnetic phase fraction measured by La nuclear magnetic resonance. Our results furthermore demonstrate that the FORC method is a valuable first-pass characterization tool for magnetic phase separation.",0509190v1 2005-09-08,2D Ferromagnetism in the High-Tc Analogue Cs_2AgF_4,"Although the precise mechanism of high-Tc superconductivity in the layered cuprates remains unknown, it is generally thought that strong 2D Heisenberg antiferromagnetism combined with disruptive hole doping is an essential aspect of the phenomenon. Intensive studies of other layered 3d transition metal systems have greatly extended our understanding of strongly correlated electron states, but to date have failed to show strong 2D antiferromagnetism or high-Tc superconductivity. For this reason the largely unexplored 4d^9 Ag^II fluorides, which are structurally and perhaps magnetically similar to the 3d^9 Cu^II cuprates, merit close study. Here we present a comprehensive study of magnetism in the layered Ag^II fluoride Cs_2AgF_4, using magnetic susceptometry, neutron diffraction and inelastic neutron scattering techniques. We find that this material is well described as a 2D Heisenberg ferromagnet, in sharp contrast to the high-Tc cuprates. The exchange constant J is the largest known for any material of this type. We suggest that orbital ordering may be the origin of the ferromagnetism we observe in this material.",0509194v1 2005-10-21,Fermi Velocity Spectrum and Incipient Magnetism in TiBe2,"We address the origin of the incipient magnetism in TiBe$_2$ through precise first principles calculations, which overestimate the ferromagnetic tendency and therefore require correction to account for spin fluctuations. TiBe$_2$ has sharp fine structure in its electronic density of states, with a van Hove singularity only 3 meV above the Fermi level. Similarly to the isovalent weak ferromagnet ZrZn$_2$, it is flat bands along the K-W-U lines of hexagonal face of the fcc Brillouin zone make the system prone to magnetism, and more so if electrons are added. We find that the Moriya $B$ coefficient (multiplying $\frac{\omega}{q}$ in the fluctuation susceptibility $\Delta \chi(q,\omega)$) is divergent when the velocity vanishes at a point on the Fermi surface, which is very close (3 meV) to occurring in TiBe$_2$. In exploring how the FM instability (the $q$=0 Stoner enhancement is $S\approx 60$) might be suppressed by fluctuations in TiBe$_2$, we calculate that the Moriya A coefficient (of $q^2$) is negative, so $q$=0 is not the primary instability. Explicit calculation of $\chi_o(q)$ shows that its maximum occurs at the X point $(1,0,0)\frac{2\pi}{a}$; TiBe$_2$ is thus an incipient {\it anti}ferromagnet rather than ferromagnet as has been supposed. We further show that simple temperature smearing of the peak accounts for most of the temperature dependence of the susceptibility, which previously had been attributed to local moments (via a Curie-Weiss fit), and that energy dependence of the density of states also strongly affects the magnetic field variation of $\chi$.",0510598v1 2005-10-28,Electron magnetic resonance studies of nanomanganite Nd0.67Sr0.33MnO3,"Nd_0.67Sr_0.33MnO_3 nanoparticles with the grain size of about 30 nm are prepared by sol-gel method.These nanopowders are annealed at four different temperatures viz. 800 oC, 900 oC, 1000 oC and 1100 oC to study the effect of particle size on magnetic, transport and electron magnetic resonance (EMR) spectral parameters. The samples are characterized by XRD, SEM, EDAX and TEM. The ac susceptibility experiments show that as the particle size increases the ferromagnetic to paramagnetic transition temperature (Tc) decreases. The metal-insulator transition temperature also changes with the particle size as revealed by resistivity measurements. EMR spectra of the nanopowders are recorded from room temperature down to 4K using an X-band EPR spectrometer. the spectra could be fitted using two broad-Gaussian lineshapes below Tc and suggested the ferromagnetic nature of the samples. Above Tc a single Lorenzian fits the signals as expetced for paramagnetic samples. The EMR spectral parameters are found to be different from the bulk (polycrystalline)sample data. The spectral parameters show variation with the particle size. The presence of the two signals in the ferromagnetic phase is attributed to core and shell regions in the nanoparticles. We could estimate the shell thickness from the EMR intensity data as 0.7 - 1 nm which agrees with other measurements.",0510780v1 2005-12-09,"Superconducting Junctions with Ferromagnetic, Antiferromagnetic or Charge-Density-Wave Interlayers","Spectra and spin structures of Andreev interface states and the Josephson current are investigated theoretically in junctions between clean superconductors (SC) with ordered interlayers. The Josephson current through the ferromagnet-insulator-ferromagnet interlayer can exhibit a nonmonotonic dependence on the misorientation angle. The characteristic behavior takes place if the pi state is the equilibrium state of the junction in the particular case of parallel magnetizations. We find a novel channel of quasiparticle reflection (Q reflection) from the simplest two-sublattice antiferromagnet (AF) on a bipartite lattice. As a combined effect of Andreev and Q reflections, Andreev states arise at the AF/SC interface. When the Q reflection dominates the specular one, Andreev bound states have almost zero energy on AF/ s-wave SC interfaces, whereas they lie near the edge of the continuous spectrum for AF/d-wave SC boundaries. For an s-wave SC/AF/s-wave SC junction, the bound states are found to split and carry the supercurrent. Our analytical results are based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets. Similar effects can take place on interfaces of superconductors with charge density wave materials (CDW), including the possible d-density wave state (DDW) of the cuprates.",0512204v1 2005-12-15,Ferromagnetic Resonance Linewidths in Ultrathin Structures: Theoretical Studies of Spin Pumping,"We present theoretical studies of the spin pumping contribution to the ferromagnetic resonance linewidth for various ultrathin film ferromagnetic structures. We consider the isolated film on a substrate, with Fe on Au(100) and Fe on W(110) as examples. We explore as well the linewidth from this mechanism for the optical and acoustical collective modes of FM/Cu$_{\rm N}$/FM/Cu(100) structures. The calculations employ a realistic electronic structure, with self consistent ground states generated from the empirical tight binding method, with nine bands for each material in the structure. The spin excitations are generated through use of the random phase approximation applied to the system, including the semi infinite substrate on which the structure is grown. We calculate the frequency response of the system directly by examining the spectral density associated with collective modes whose wave vector parallel to the surface is zero. Linewidths with origin in leakage of spin angular momentum from the adsorbed structure to the semi infinite substrate may be extracted from these results. We discuss a number of issues, including the relationship between the interfilm coupling calculated adiabatically for trilayers, and that extracted from the (dynamical) spin wave spectrum. We obtain excellent agreement with experimental data, within the framework of calculations with no adjustable parameters.",0512333v1 2006-01-19,Oscillations of Induced Magnetization in Superconductor-Ferromagnet Heterostructures,"We study a change in the spin magnetization of a superconductor-ferromagnet (SF) heterostructure, when temperature is lowered below the superconducting transition temperature. It is assumed that the SF interface is smooth on the atomic scale and the mean free path is not too short. Solving the Eilenberger equation we show that the spin magnetic moment induced in the superconductor is an oscillating sign-changing function of the product $hd$ of the exchange field $h$ and the thickness $d$ of the ferromagnet. Therefore the total spin magnetic moment of the system in the superconducting state can be not only smaller (screening) but also greater (anti-screening) than that in the normal state, in contrast with the case of highly disordered (diffusive) systems, where only screening is possible. This surprising effect is due to peculiar periodic properties of localized Andreev states in the system. It is most pronounced in systems with ideal ballistic transport (no bulk disorder in the samples, smooth ideally transparent interface), however these ideal conditions are not crucial for the very existence of the effect. We show that oscillations exist (although suppressed) even for arbitrary low interface transparency and in the presence of bulk disorder, provided that $h \tau \gg 1$ ($\tau$ -- mean free path). At low interface transparency we solve the problem for arbitrary strength of disorder and obtain oscillating magnetization in ballistic regime ($h \tau \gg 1$) and nonoscillating magnetization in diffusive one ($h \tau \ll 1$) as limiting cases of one formula.",0601443v1 2006-01-30,Correlation in the transition metal based Heusler compounds Co$_2$MnSi and Co$_2$FeSi,"Half-metallic ferromagnets like the full Heusler compounds with formula X$_2$YZ are supposed to show an integer value of the spin magnetic moment. Calculations reveal in certain cases of X = Co based compounds non-integer values, in contrast to experiments. In order to explain deviations of the magnetic moment calculated for such compounds, the dependency of the electronic structure on the lattice parameter was studied theoretically. In local density approximation (LDA), the minimum total energy of Co$_2$FeSi is found for the experimental lattice parameter, but the calculated magnetic moment is about 12% too low. Half-metallic ferromagnetism and a magnetic moment equal to the experimental value of $6\mu_B$ are found, however, only after increasing the lattice parameter by more than 6%. To overcome this discrepancy, the LDA$+U$ scheme was used to respect on-site electron correlation in the calculations. Those calculations revealed for Co$_2$FeSi that an effective Coulomb-exchange interaction $U_{eff}=U-J$ in the range of about 2eV to 5eV leads to half-metallic ferromagnetism and the measured, integer magnetic moment at the measured lattice parameter. Finally, it is shown in the case of Co$_2$MnSi that correlation may also serve to destroy the half-metallic behavior if it becomes too strong (for Co$_2$MnSi above 2eV and for Co$_2$FeSi above 5eV). These findings indicate that on-site correlation may play an important role in the description of Heusler compounds with localized moments.",0601671v1 2006-02-24,Quantum Criticality in doped CePd_1-xRh_x Ferromagnet,"CePd_1-xRh_x alloys exhibit a continuous evolution from ferromagnetism (T_C= 6.5 K) at x = 0 to a mixed valence (MV) state at x = 1. We have performed a detailed investigation on the suppression of the ferromagnetic (F) phase in this alloy using dc-(\chi_dc) and ac-susceptibility (\chi_ac), specific heat (C_m), resistivity (\rho) and thermal expansion (\beta) techniques. Our results show a continuous decrease of T_C (x) with negative curvature down to T_C = 3K at x*= 0.65, where a positive curvature takes over. Beyond x*, a cusp in cac is traced down to T_C* = 25 mK at x = 0.87, locating the critical concentration between x = 0.87 and 0.90. The quantum criticality of this region is recognized by the -log(T/T_0) dependence of C_m/T, which transforms into a T^-q (~0.5) one at x = 0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegard's law (at x_V~0.75) and increasing hybridization effects on high temperature \chi_dc and \rho. Coincidentally, a Fermi liquid contribution to the specific heat arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C_m/T flattening is observed for x > x_cr rather the mentioned power law divergence, which coincides with a change of sign of \beta. The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.",0602588v1 2006-02-27,Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves,"We demonstrate composite media with ferromagnetic wires that exhibit a frequency region at the microwave regime with scattering spectra strongly dependent on an external magnetic field or stress. These tunable composite materials have recently been proposed theoretically; however, no direct experimental verification has been reported. We used composite materials with predominantly oriented CoFeCrSiB glass-coated amorphous wires having large magnetoimpedance at GHz frequencies. The free space measurements of reflection and transmission coefficients were conducted in the frequency range 1-8 GHz in the presence of an external static magnetic field or stress applied to the whole sample. In general, the transmission spectra show greater changes in the range of 10dB for a relatively small magnetic field of few Oe or stress of 0.1 MPa. The obtained results are quantitatively consistent with the analytical expressions predicted by the effective medium arguments. The incident electromagnetic wave induces an electrical dipole moment in each wire, the aggregate of which forms the effective dipole response of the whole composite structure in the radiative near or far field region. The field and stress dependences of the effective response arise from a field or tensile stress sensitivity of the ac surface impedance of a ferromagnetic wire. In the vicinity of the antenna resonance the variations in the magneto-impedance of the wire inclusions result in large changes of the total effective response. A number of applications of proposed materials is discussed including the field tunable microwave surfaces and the self-sensing media for the remote non-destructive evaluation of structural materials.",0602644v2 2006-04-26,Graphene integer quantum Hall effect in the ferromagnetic and paramagnetic regimes,"Starting from the graphene lattice tight-binding Hamiltonian with an on-site U and long-range Coulomb repulsion, we derive an interacting continuum Dirac theory governing the low-energy behavior of graphene in an applied magnetic field. Initially, we consider a clean graphene system within this effective theory and explore integer quantum Hall ferromagnetism stabilized by exchange from the long-range Coulomb repulsion. We study in detail the ground state and excitations at nu = 0 and nu = \pm 1, taking into account small symmetry-breaking terms that arise from the lattice-scale interactions, and also explore the ground states selected at nu = \pm 3, \pm 4, and \pm 5. We argue that the ferromagnetic regime may not yet be realized in current experimental samples, which at the above filling factors perhaps remain paramagnetic due to strong disorder. In an attempt to access the latter regime where the role of exchange is strongly suppressed by disorder, we apply Hartree theory to study the effects of interactions. Here, we find that Zeeman splitting together with symmetry-breaking interactions can in principle produce integer quantum Hall states in a paramagnetic system at nu = 0, \pm 1 and \pm 4, but not at nu = \pm 3 or \pm 5, consistent with recent experiments in high magnetic fields. We make predictions for the activation energies in these quantum Hall states which will be useful for determining their true origin.",0604601v1 2006-05-15,Magnetic reconfiguration of MnAs/GaAs(001) observed by Magnetic Force Microscopy and Resonant Soft X-ray Scattering,"We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft X-ray scattering and magnetic force microscopy. The results of the diffuse satellite X-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane mono-domain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9 times the film thickness d. This is in good agreement with the expected theoretical value of L = 2.6d.",0605356v1 2006-05-19,Phase diagram of the anisotropic multichannel Kondo Hamiltonian revisited,"The phase diagram of the multichannel Kondo Hamiltonian with an XXZ spin-exchange anisotropy is revisited, revealing a far richer fixed-point structure than previously appreciated. For a spin-1/2 impurity and k > 2 conduction-electron channels, a second ferromagnetic-like domain is found deep in the antiferromagnetic regime. The new domain extends above a (typically large) critical longitudinal coupling J_z^{\ast} > 0, and is separated from the antiferromagnetic domain by a second Kosterliz-Thouless line. A similar line of stable ferromagnetic-like fixed points with a residual isospin-1/2 local moment is shown to exist for large J_z >> |J_{\perp}| > 0 and arbitrary k and s obeying |k - 2s| > 1. Here J_z is the longitudinal spin-exchange coupling, J_{\perp} is the transverse coupling, and s is the impurity spin. Near the free-impurity fixed-point, spin-exchange anisotropy is a relevant perturbation for s > 1/2 and arbitrary k. Depending on the sign of J_z^2 - J_{\perp}^2 and the parity of 2s, the system flows either to a conventional Fermi liquid with no residual degeneracy, or to a k-channel, spin-1/2 Kondo effect, or to a line of ferromagnetic-like fixed points with a residual isospin-1/2 local moment. These results are obtained through a combination of perturbative renormalization-group techniques, Abelian bosonization, a strong-coupling expansion in 1/J_z, and explicit numerical renormalization-group calculations.",0605474v1 2006-06-05,Phenomenological theory of current driven exchange switching in ferromagnetic nanojunctions,"Phenomenological approach is developed in the theory of spin-valve type ferromagnetic junctions to describe exchange switching by current flowing perpendicular to interfaces. Forward and backward current switching effects are described and they may be principally different in nature. Mobile electron spins are considered as being free in all the contacting ferromagnetic layers. Joint action of the following two current effects is investigated: the nonequilibrium longitudinal spin-injection effective field and the transverse spin-transfer surface torque. Dispersion relation for fluctuations is derived and solved for a junction model having spatially localized spin transfer torque: depth of the torque penetration into the free layer is assumed much smaller than the total free layer thickness. Some critical value of the well known Gilbert damping constant is established for the first time. Spin transfer torque dominates in the instability threshold determination for small enough damping constants, while the spin-injection effective field dominates for high damping. Fine interplay between spin transfer torque and spin injection is necessary to provide a hysteretic behavior of the resistance versus current dependence. The state diagram building up shows the possibility of non-stationary (time dependent) nonlinear states arising due to instability development. Calculations lead to the instability rise time values of the order of 0.1 ns. Spin wave resonance frequency spectrum softening occurs under the current growing to the instability threshold. Magnetization fluctuations above the threshold rise oscillating with time for low damping, but rise aperiodically and much more rapid for high damping.",0606102v2 2006-06-16,Multiferroic tunnel junctions,"Multiferroics are singular materials that can display simultaneously electric and magnetic orders. Some of them can be ferroelectric and ferromagnetic and, for example, provide the unique opportunity of encoding information independently in electric polarization and magnetization to obtain four different logic states. However, schemes allowing a simple electrical readout of these different states have not been demonstrated so far. In this article, we show that this can be achieved if a multiferroic material is used as the tunnel barrier in a magnetic tunnel junction. We demonstrate that thin films of ferromagnetic-ferroelectric La0.1Bi0.9MnO3 (LBMO) retain both ferroic properties down to a thickness of only 2 nm. We have used such films as spin-filtering tunnel barriers the magnetization and electric polarization of which can be switched independently. In that case, the tunnel current across the structure is controlled by both the magnetic and ferroelectric configuration of the barrier, which gives rise to four distinct resistance states. This can be explained by the combination of spin filtering by the ferromagnetic LBMO barrier and the partial charge screening of electrical charges at the barrier/electrode interfaces due to ferroelectricity. We anticipate our results to be a starting point for more studies on the interplay between ferroelectricity and spin-dependent tunneling, and for the use of nanometric multiferroic elements in prototype devices. On a wider perspective, they may open the way towards novel reconfigurable logic spintronics architectures and to electrically controlled readout in quantum computing schemes using the spin-filter effect.",0606444v1 2006-07-25,Spin dynamics in the diluted ferromagnetic Kondo lattice model,"The interplay of disorder and competing interactions is investigated in the carrier-induced ferromagnetic state of the Kondo lattice model within a numerical finite-size study in which disorder is treated exactly. Competition between impurity spin couplings, stability of the ferromagnetic state, and magnetic transition temperature are quantitatively investigated in terms of magnon properties for different models including dilution, disorder, and weakly-coupled spins. A strong optimization is obtained for T_c at hole doping p << x, highlighting the importance of compensation in diluted magnetic semiconductors. The estimated T_c is in good agreement with experimental results for Ga_{1-x}Mn_x As for corresponding impurity concentration, hole bandwidth, and compensation. Finite-temperature spin dynamics is quantitatively studied within a locally self-consistent magnon renormalization scheme, which yields a substantial enhancement in T_c due to spin clustering, and highlights the nearly-paramagnetic spin dynamics of weakly-coupled spins. The large enhancement in density of low-energy magnetic excitations due to disorder and competing interactions results in a strong thermal decay of magnetization, which fits well with the Bloch form M_0(1-BT^{3/2}) at low temperature, with B of same order of magnitude as obtained in recent squid magnetization measurements on Ga_{1-x}Mn_x As samples.",0607633v1 2006-08-04,Theoretical study of the conductance of ferromagnetic atomic-sized contacts,"Recently, different experiments on the transport through atomic-sized contacts made of ferromagnetic materials have produced contradictory results. In particular, several groups have reported the observation of half-integer conductance quantization, which requires having full spin polarization and perfectly conducting channels. Motivated by these surprising results, we have studied theoretically the conductance of ideal atomic contact geometries of the ferromagnetic 3d materials Fe, Co, and Ni using a realistic tight-binding model. Our analysis shows that in the absence of magnetic domains, the d bands of these transition metals play a key role in the electrical conduction. In the contact regime this fact has the following important consequences for the three materials: (i) there are partially open conduction channels and therefore conductance quantization is not expected, (ii) the conductance of the last plateau is typically above G_0=2e^2/h, (iii) both spin species contribute to the transport and thus there is in general no full current polarization, and (iv) both the value of the conductance and the current polarization are very sensitive to the contact geometry and to disorder. In the tunneling regime we find that a strong current polarization can be achieved.",0608132v1 2006-09-21,Model of the Interplay of Band J-T Effect with Magnetic Order Mediated by Exchange Interaction,"A model calculation is presented with the aim to study the interplay between magnetic and structural transitions. The model consists of an orbitally doubly degenerate conduction band and a periodic array of local moments. The band electrons interact with the local spins via the s-f interaction. The interaction of the band electrons with phonons is introduced by including band Jahn-Teller (J-T) interaction. The model Hamiltonian, including the above terms, is solved for the single particle Greens function. In doing this an ansatz for selfenergy of electrons, which was developed earlier has been utilized. The quasiparticle density of states (QDOS) and hence the orbital populations are calculated treating the ferromagnetism of local moments in the mean field approximation. The critical value of electron-phonon interaction ($G$) for the appearance of the band J-T distortion is higher in the ferromagnetic state. The strain appears at a critical temperature ($T_s$) when $G$ is greater than the critical value. The onset of ferromagnetism at $T_C$ ($ 6) the long time relaxation appeared at 10 K and the magnetization reversed its sign and became strongly negative in wide temperature range, even under an applied magnetic field of 15 kOe. The observed field and temperature dependences of the magnetization in this state are reversed in comparison with the ordinary ferromagnetic ones. Above TC, the observed diamagnetic susceptibility of the reversed magnetization state at T = 120 K is ~ - 0.9 x 10-4 emu g-1 Oe-1. Only after some storage time at room temperature, the abnormal magnetic state is erasable. It is suggested that the negative magnetization observed results from a specific coupling of the nano/micro-size ferromagnetic regions with a surrounding diamagnetic matrix formed, in a puzzled way, by the repeating training (quick cooling) cycles.",0609626v1 2006-09-29,Influence of strain on magnetization and magnetoelectric effect in La0.7A0.3MnO3 / PMN-PT(001) (A = Sr; Ca),"We investigate the influence of a well-defined reversible biaxial strain <=0.12 % on the magnetization (M) of epitaxial ferromagnetic manganite films. M has been recorded depending on temperature, strain and magnetic field in 20 - 50 nm thick films. This is accomplished by reversibly compressing the isotropic in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT (001) substrates by application of an electric field E <= 12 kV cm-1. The magnitude of the total variable in-plane strain has been derived. Strain-induced shifts of the ferromagnetic Curie temperature (Tc) of up to 19 K were found in La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 films and are quantitatively analysed for LSMO within a cubic model. The observed large magnetoelectric coupling coefficient alpha=mu0 dM/dE <= 6 10-8 s m-1 at ambient temperature results from the strain-induced M change in the magnetic-film-ferroelectric-substrate system. It corresponds to an enhancement of mu0 DeltaM <= 19 mT upon biaxial compression of 0.1 %. The extraordinary large alpha originates from the combination of three crucial properties: (i) the strong strain dependence of M in the ferromagnetic manganites, (ii) large piezo-strain of the PMN-PT substrates and (iii) effective elastic coupling at the film-substrate interface.",0609760v1 2006-10-10,Critical properties in single crystals of Pr1-xPbxMnO3,"The critical properties at the ferromagnetic - paramagnetic transition have been analysed from data of static magnetization measurements on single crystals of Pr1-xPbxMnO3, for x = 0.23 and x = 0.30. In Pr1-xPbxMnO3 the ferromagnetic ordering and the metal-insulator transition do not coincide in parts of the phase diagram. The crystal with x = 0.23 is a ferromagnetic insulator with Curie temperature Tc = 173 K, while the crystal with x =0.30 has Tc = 198 K and remains metallic up to a metal-insulator transition temperature Tmi = 235 K. The dc magnetization measurements were carried out in the field range from 0 to 5 T for an interval in the critical temperature range Tc+_10 K corresponding to a reduced temperature interval 0.003 < epsilon < 0.6. The exponents beta for spontaneous magnetization, gamma for the initial susceptibility above Tc and delta for the critical magnetization isotherm at Tc were obtained by static scaling analysis from modified Arrott plots and by the Kouvel Fisher method for the insulating crystal with composition x = 0.23. The data are well described by critical exponents similar to those expected for the Heisenberg universality class relevant for conventional isotropic magnets Systematic deviations from scaling in the data for the metallic crystal with composition x = 0.30 are demonstrated from effective critical exponents near the assumed ordering transition. The unconventional magnetic ordering in this system indicates the presence of frustrated magnetic couplings that suppresses magnetic ordering and lowers the transition temperature.",0610252v1 2006-10-24,31P-NMR and muSR Studies of Filled Skutterudite Compound SmFe4P12: Evidence for Heavy Fermion Behavior with Ferromagnetic Ground State,"The 31P-NMR (nuclear magnetic resonance) and muSR (muon spin relaxation) measurements on the filled skutterudite system SmFe4P12 have been carried out. The temperature T dependence of the 31P-NMR spectra indicates the existence of the crystalline electric field effect splitting of the Sm3+$ (J = 5/2) multiplet into a ground state and an excited state of about 70 K. The spin-lattice relaxation rate 1/T1 shows the typical behavior of the Kondo system, i.e., 1/T1 is nearly T independent above 30 K, and varies in proportion to T (the Korringa behavior, 1/T1 \propto T) between 7.5 K and 30 K. The T dependence deviated from the Korringa behavior below 7 K, which is independent of T in the applied magnetic field of 1 kOe, and suppressed strongly in higher fields. The behavior is explained as 1/T1is determined by ferromagnetic fluctuations of the uncovered Sm3+ magnetic moments by conduction electrons. The muSR measurements in zero field show the appearance of a static internal field associated with the ferromagnetic order below 1.6 K.",0610648v1 2006-11-17,Electronic structure and magnetism in doped semiconducting half-Heusler compounds,"We have studied in details the electronic structure and magnetism in M (Mn and Cr) doped semiconducting half-Heusler compounds FeVSb, CoTiSb and NiTiSn (XM$_{x}$Y$_{1-x}$Z) in a wide concentration range using local-spin density functional method in the framework of tight-binding linearized muffin tin orbital method(TB-LMTO) and supercell approach. Our calculations indicate that some of these compounds are not only ferromagnetic but also half-metallic and may be useful for spintronics applications. The electronic structure of the doped systems is analyzed with the aid of a simple model where we have considered the interaction between the dopant transition metal (M) and the valence band X-Z hybrid. We have shown that the strong X-d - M-d interaction places the M-d states close to the Fermi level with the M-t$_{2g}$ states lying higher in energy in comparison to the M-e$_{g}$ states. Depending on the number of available d-electrons, ferromagnetism is realized provided the d-manifold is partially occupied. The tendencies toward ferromagnetic(FM) or antiferromagnetic(AFM) behavior are discussed within Anderson-Hasegawa models of super-exchange and double-exchange. In our calculations for Mn doped NiTiSn, the strong preference for FM over AFM ordering suggests a possible high Curie temperature for these systems.",0611478v1 2006-11-27,Subtle competition between ferromagnetic and antiferromagnetic order in a Mn(II) - free radical ferrimagnetic chain,"The macroscopic magnetic characterization of the Mn(II) - nitronyl nitroxide free radical chain (Mn(hfac)2(R)-3MLNN) evidenced its transition from a 1-dimensional behavior of ferrimagnetic chains to a 3-dimensional ferromagnetic long range order below 3 K. Neutron diffraction experiments, performed on a single crystal around the transition temperature, led to a different conclusion : the magnetic Bragg reflections detected below 3 K correspond to a canted antiferromagnet where the magnetic moments are mainly oriented along the chain axis. Surprisingly in the context of other compounds in this family of magnets, the interchain coupling is antiferromagnetic. This state is shown to be very fragile since a ferromagnetic interchain arrangement is recovered in a weak magnetic field. This peculiar behavior might be explained by the competition between dipolar interaction, shown to be responsible for the antiferromagnetic long range order below 3 K, and exchange interaction, the balance between these interactions being driven by the strong intrachain spin correlations. More generally, this study underlines the need, in this kind of molecular compounds, to go beyond macroscopic magnetization measurements.",0611684v2 2007-02-02,Anomalous physical properties of underdoped weak-ferromagnetic superconductor RuSr$_2$EuCu$_{2}$O$_{8}$,"Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferromagnetism, T$_{Curie}$ = 131 K) and superconducting (T$_{c}$ = 56 K) RuSr$_{2}$GdCu$_{2}$O$_{8}$, the underdoped RuSr$_{2}$EuCu$_{2}$O$_{8}$ (T$_{Curie}$ = 133 K, T$_{c}$ = 36 K) also exhibited a spontaneous vortex state (SVS) between 16 K and 36 K. The low field ($\pm$20 G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of average lower critical field B$_{c1}^{ave}$(T) = B$_{c1}^{ave}$(0)[1 - (T/T$_{SVS}$)$^{2}$], with B$_{c1}^{ave}$(0) = 7 G and T$_{SVS}$ = 16 K. The vortex melting transition (T$_{melting}$ = 21 K) below T$_{c}$ obtained from the broad resistivity drop and the onset of diamagnetic signal indicates a vortex liquid region due to the coexistence and interplay between superconductivity and WFM order. No visible jump in specific heat was observed near T$_{c}$ for Eu- and Gd-compound. This is not surprising, since the electronic specific heat is easily overshadowed by the large phonon and weak-ferromagnetic contributions. Furthermore, a broad resistivity transition due to low vortex melting temperature would also lead to a correspondingly reduced height of any specific heat jump. Finally, with the baseline from the nonmagnetic Eu-compound, specific heat data analysis confirms the magnetic entropy associated with antiferromagnetic ordering of Gd$^{3+}$ (J = S = 7/2) at 2.5 K to be close to $\it{N_{A}k}$ ln8 as expected.",0702040v1 2007-03-15,Functional Keldysh Theory of Spin Torques,"We present a microscopic treatment of current-induced torques and thermal fluctuations in itinerant ferromagnets based on a functional formulation of the Keldysh formalism. We find that the nonequilibrium magnetization dynamics is governed by a stochastic Landau-Lifschitz-Gilbert equation with spin transfer torques. We calculate the Gilbert damping parameter $\alpha$ and the non-adiabatic spin transfer torque parameter $\beta$ for a model ferromagnet. We find that $\beta \neq \alpha$, in agreement with the results obtained using imaginary-time methods of Kohno, Tatara and Shibata [J. Phys. Soc. Japan 75, 113706 (2006)]. We comment on the relationship between $s-d$ and isotropic-Stoner toy models of ferromagnetism and more realistic density-functional-theory models, and on the implications of these relationships for predictions of the $\beta/\alpha$ ratio which plays a central role in domain wall motion. Only for a single-parabolic-band isotropic-Stoner model with an exchange splitting that is small compared to the Fermi energy does $\beta/\alpha$ approach one. In addition, our microscopic formalism incorporates naturally the fluctuations needed in a nonzero-temperature description of the magnetization. We find that to first order in the applied electric field, the usual form of thermal fluctuations via a phenomenological stochastic magnetic field holds.",0703414v2 2007-03-21,Hartree-Fock variational bounds for ground state energy of chargeless fermions with finite magnetic moment in presence of a hard core potential:A stable ferromagnetic state,"We use different types of determinantal Hartree-Fock (HF) wave functions to calculate variational bounds for the ground state energy of spin-half fermions in volume V_0, with mass m, electric charge zero, and magnetic moment mu, which are interacting through long range magnetic dipole-dipole interaction. We find that at high densities when the average inter particle distance r_0 becomes small compared to the magnetic length r_m, a ferromagnetic state with spheroidal occupation function, involving quadrupolar deformation, gives a lower energy compared to the variational energy for the uniform paramagnetic state. This HF variational bound to the ground state energy turns out to have a lower energy than our earlier calculation in which instead of a determinantal wavefunction we had used a positive semi-definite single particle density matrix operator whose eigenvalues, having quadrupolar deformation, were allowed to take any value from 0 to 1. This system is of course still unstable towards infinite density collapse, but we show here explicitly that a suitable short range repulsive (hard core) interaction of strength U_0 and range a can stop this collapse.The existence of a stable high density ferromagnetic state with spheroidal occupation function is possible as long as the ratio of hard-core and magnetic dipole coupling constants is not very small compared to 1.",0703547v1 2007-03-25,Magnetic field induced spin-flop transition in Na$_x$CoO$_2$ (0.5$<$x$<$0.55),"The isothermal magnetoresistance (MR) with magnetic field (H) parallel to and perpendicular to ab plane is systematically studied on the single crystal Na$_{0.52}$CoO$_2$ with charge ordering at $\sim 50$ K and an in-plane ferromagnetism below 25 K. The isothermal MR behavior with H $\parallel$ ab plane and H $\perp$ ab plane is quite different. When H $\parallel$ ab plane, the MR is always negative and the in-plane ferromagnetic behavior is enhanced. While the MR with H $\perp$ ab plane changes from negative to positive with decreasing temperature or increasing H, and the in-plane ferromagnetic behavior is suppressed. A striking feature is that the MR with H $\perp$ ab plane shows a hysteresis behavior below 25 K, which is absent for the case of H $\parallel$ ab plane. These results provide strong evidence for a spin-flop transition of small moments of Co$^{3.5-\delta}$ sites induced by H $\perp$ ab plane, leading to a metamagnetic transition for small moments of Co$^{3.5-\delta}$ sites. These complex magnetism suggests an unconventional superconductivity in Na$_x$CoO$_2$ system because the Na$_x$CoO$_2$ around x=0.5 is considered to be the parent compound of superconductivity.",0703644v1 2005-05-16,"Semiclassical circular strings in AdS_5 and ""long"" gauge field strength operators","We consider circular strings rotating with equal spins S_1=S_2=S in two orthogonal planes in AdS_5 and suggest that they may be dual to ""long"" gauge theory operators built out of self-dual components of gauge field strength. As was found in hep-th/0404187, the one-loop anomalous dimensions of the such gauge-theory operators are described by an anti-ferromagnetic XXX_1 spin chain and scale linearly with length L>>1. We find that in the case of rigid rotating string both the classical energy E_0 and the 1-loop string correction E_1 depend linearly on the spin S (within the stability region of the solution). This supports the relation between the rigid rotating string and the gauge-theory operator corresponding to the maximal-spin (ferromagnetic) state of the XXX_1 spin chain. The energy of more general rotating and pulsating strings also happens to scale linearly with both the spin and the oscillation number. Such solutions should be dual to other lower-spin states of the spin chain, with the anti-ferromagnetic ground state presumably corresponding to the string pulsating in two planes with no rotation.",0505130v3 2005-12-09,"Statistical mechanical systems on complete graphs, infinite exchangeability, finite extensions and a discrete finite moment problem","We show that a large collection of statistical mechanical systems with quadratically represented Hamiltonians on the complete graph can be extended to infinite exchangeable processes. This extends a known result for the ferromagnetic Curie--Weiss Ising model and includes as well all ferromagnetic Curie--Weiss Potts and Curie--Weiss Heisenberg models. By de Finetti's theorem, this is equivalent to showing that these probability measures can be expressed as averages of product measures. We provide examples showing that ``ferromagnetism'' is not however in itself sufficient and also study in some detail the Curie--Weiss Ising model with an additional 3-body interaction. Finally, we study the question of how much the antiferromagnetic Curie--Weiss Ising model can be extended. In this direction, we obtain sharp asymptotic results via a solution to a new moment problem. We also obtain a ``formula'' for the extension which is valid in many cases.",0512191v4 1996-08-24,Band Structure and Transport Properties of CrO_2,"Local Spin Density Approximation (LSDA) is used to calculate the energy bands of both the ferromagnetic and paramagnetic phases of metallic CrO_2. The Fermi level lies in a peak in the paramagnetic density of states, and the ferromagnetic phase is more stable. As first predicted by Schwarz, the magnetic moment is 2 \mu_B per Cr atom, with the Fermi level for minority spins lying in an insulating gap between oxygen p and chromium d states (""half-metallic"" behavior.) The A_1g Raman frequency is predicted to be 587 cm^{-1}. Drude plasma frequencies are of order 2eV, as seen experimentally by Chase. The measured resistivity is used to find the electron mean-free path l, which is only a few angstroms at 600K, but nevertheless, resistivity continues to rise as temperature increases. This puts CrO_2 into the category of ""bad metals"" in common with the high T_c superconductors, the high T metallic phase of VO_2, and the ferromagnet SrRuO_3. In common with both SrRuO_3 and Sr_2RuO_4, the measured specific heat \gamma is higher than band theory by a renormalization factor close to 4.",9608006v1 2007-06-27,Kondo quantum dot coupled to ferromagnetic leads: Numerical renormalization group study,"We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson's numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in the bands (governed by the band edges) and (iii) an arbitrary shape of the leads density of states. For a generic lead density of states the quantum dot favors being occupied by a particular spin-species due to exchange interaction with ferromagnetic leads leading to a suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry restoring the Kondo effect. We study both the gate-voltage dependence (for a fixed band structure in the leads) and the spin polarization dependence (for fixed gate voltage) of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantum dot in presence of leads with an energy dependent density of states is not only possible by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.",0706.3997v1 2007-07-17,"A study of the ferromagnetic transition of $SrRuO_3$ in nanometer thick bilayers with $YBa_2Cu_3O_y$, $La_{1.88}Sr_{0.12}CuO_{4-y}$, Au and Cr: Signature of injected carriers in the pseudogap regime","The hypothesis regarding the existence of uncorrelated pre-formed pairs in the pseudogap regime of superconducting $YBa_2Cu_3O_y$ is tested experimentally using bilayers of $YBa_2Cu_3O_y$ and the itinerant ferromagnet $SrRuO_3$. In our study, we monitor the influence of $YBa_2Cu_3O_y$ on $T_p$, the ferromagnetic ordering temperature of $SrRuO_3$. Here, $T_p$ is the temperature of maximum dM/dT or dR/dT where M and R are the magnetization and resistance of $SrRuO_3$, respectively. We compare the results with similar measurements carried out on bilayers of $La_{1.88}Sr_{0.12}CuO_{4-y}$, $Au$ and $Cr$ with $SrRuO_3$. We find that in bilayers made of underdoped 10 nm $YBa_2Cu_3O_y$/5 nm $SrRuO_3$, the $T_p$ values are shifted to lower temperatures by up to 6-8 K as compared to $T_p\approx 140$ K of the 5 nm thick reference $SrRuO_3$ film. In contrast, in the other type of bilayers, which are not in the pseudogap regime near $T_p$, only a smaller shift of up to $\pm$2 K is observed. These differences are discussed in terms of a proximity effect, where carriers from the $YBa_2Cu_3O_y$ layer are injected into the $SrRuO_3$ layer and vice versa. We suggest that correlated electrons in the pseudogap regime of $YBa_2Cu_3O_y$ are responsible for the observed large $T_p$ shifts.",0707.2497v1 2007-08-11,Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study,"A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented for the concentration range that spans the ferromagnetic-insulator (FMI) to ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22. All samples are ferromagnetic although the saturation magnetization for the 16% Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI samples have similar correlations between changes in the local Mn-O distortions and the magnetization as observed previously for the colossal magnetoresistance (CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully magnetized. The data show that there are at least two distinct types of distortions. The initial distortions removed as the insulating sample becomes magnetized are small and provides direct evidence that roughly 50% of the Mn sites have a small distortion/site and are magnetized first. The large remaining Mn-O distortions at low T are attributed to a small fraction of Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or unmagnetized. Thus the insulating samples are very similar to the behavior of the CMR samples up to the point at which the M/I transition occurs for the CMR materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of the sample is magnetic, implies that there must be preferred magnetized Mn sites and that such sites do not percolate at these concentrations.",0708.1546v1 2007-08-13,Optical response of ferromagnetic YTiO_3 studied by spectral ellipsometry,"We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric YTiO_3 single crystal with ferromagnetic Curie temperature T_C = 30 K. The optical response exhibits a weak but noticeable anisotropy. Using a classical dispersion analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV. Although the optical conductivity spectra are only weakly temperature dependent below 300 K, we are able to distinguish high- and low-temperature regimes with a distinct crossover point around 100 K. The low-temperature regime in the optical response coincides with the temperature range in which significant deviations from Curie-Weiss mean field behavior are observed in the magnetization. Using an analysis based on a simple superexchange model, the spectral weight rearrangement can be attributed to intersite d_i^1d_j^1 \longrightarrow d_i^2d_j^0 optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around 2.9 eV can be associated with the high-spin-state (^3T_1) optical transition. This indicates that other mechanisms, such as weakly dipole-allowed p-d transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at 2 eV. The recorded optical spectral weight gain of 2.9 eV optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbit-lattice phenomena near ferromagnetic ordering temperature in YTiO_3.",0708.1702v1 2007-09-17,Electrical expression of spin accumulation in ferromagnet/semiconductor structures,"We treat the spin injection and extraction via a ferromagnetic metal/semiconductor Schottky barrier as a quantum scattering problem. This enables the theory to explain a number of phenomena involving spin-dependent current through the Schottky barrier, especially the counter-intuitive spin polarization direction in the semiconductor due to current extraction seen in recent experiments. A possible explanation of this phenomenon involves taking into account the spin-dependent inelastic scattering via the bound states in the interface region. The quantum-mechanical treatment of spin transport through the interface is coupled with the semiclassical description of transport in the adjoining media, in which we take into account the in-plane spin diffusion along the interface in the planar geometry used in experiments. The theory forms the basis of the calculation of spin-dependent current flow in multi-terminal systems, consisting of a semiconductor channel with many ferromagnetic contacts attached, in which the spin accumulation created by spin injection/extraction can be efficiently sensed by electrical means. A three-terminal system can be used as a magnetic memory cell with the bit of information encoded in the magnetization of one of the contacts. Using five terminals we construct a reprogrammable logic gate, in which the logic inputs and the functionality are encoded in magnetizations of the four terminals, while the current out of the fifth one gives a result of the operation.",0709.2707v1 2007-10-16,Optical phase diagram of perovskite-type colossal magnetoresistance manganites with near-half doping,"We present a systematic optical study for a bandwidth-controlled series of nearly half doped colossal magnetoresistive manganites RE$_{0.55}$AE$_{0.45}$MnO$_3$ (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region $T=10-800$ K. The ground state of the compounds ranges from the charge and orbital ordered insulator through the spin glass to the ferromagnetic metal. The enhanced phase fluctuations, namely the short-range charge and orbital correlations dominate the paramagnetic region of the phase diagram above all the ground-state phases. This paramagnetic region is characterized by a full-gap to pseudo-gap crossover towards elevated temperatures where a broad low-energy electronic structure appears in the conductivity spectra over a large variation of the bandwidth. This pseudo-gap state with local correlations is robust against thermal fluctuations at least up to T=800 K. For small bandwidth the onset of the long-range charge order is accompanied by an instantaneous increase of the gap. The emergence of the ferromagnetic state is manifested in the optical spectra as a first-order insulator to metal transition for compounds with moderate bandwidth while it becomes a second-order transition on the larger bandwidth side. Unusually large scattering rate of the metallic carriers is observed in the ferromagnetic state which is attributed to orbital correlation with probably rod-like ($3z^2-r^2$-like) character.",0710.3015v1 2007-12-02,Role of the conduction electrons in mediating exchange interactions in Heusler alloys,"Because of large spatial separation of the Mn atoms in Heusler alloys the Mn 3d states belonging to different atoms do not overlap considerably. Therefore an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi- and full-Heusler alloys we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The Anderson's s-d model is used to perform a qualitative analysis of the obtained results. The conditions leading to diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied Mn 3d states lie well above the Fermi level, an RKKY-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied Mn 3d states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculational results are in good correlation with the conclusions made on the basis of the Anderson s-d model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.",0712.0158v1 2007-12-13,Andreev reflections at large ferromagnet/high-T_C superconductor area junctions with rough interface,"Sub-gap conductance at a large area junction with a rough interface of a ferromagnet and a high-T$_{C}$ superconductor is superimposed by multiple peaks which is not expected from an ideal point contact Andreev reflection process. We demonstrate this phenomenon by measuring resistance as a function of bias voltage of a Co/Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ junction with contact area 50 x 70 $\mu$ $m^{2}$ at various temperatures. In order to analyze such Andreev reflection data, the interface is assumed to have random potentials which can create local electric fields. The Blonder-Tinkham-Klapwijk theory is modified with the inclusion of a broadening parameter due to finite life time effects of quasi particles. An additional voltage drop due to local electric fields at the rough interface has been included in terms of an extra energy shift which may be related to the asymmetry of normalized resistance data. Spin polarization has been introduced for the ferromagnet. The presented model explains the multi-peak nature and asymmetry of Andreev reflection data experimentally observed at large area junctions. Extension of the model also interprets the experimentally observed anomalous enhancement of resistance peaks in the sub-gap region which may result from crossing the critical current limit across the junction.",0712.2131v2 2008-01-17,The effect of the three-spin interaction and the next-nearest neighbor interaction on the quenching dynamics of a transverse Ising model,"We study the zero temperature quenching dynamics of various extensions of the transverse Ising model (TIM) when the transverse field is linearly quenched from $-\infty$ to $+\infty$ (or zero) at a finite and uniform rate. The rate of quenching is dictated by a characteristic scale given by $\tau$. The density of kinks produced in these extended models while crossing the quantum critical points during the quenching process is calculated using a many body generalization of the Landau-Zener transition theory. The density of kinks in the final state is found to decay as $\tau^{-1/2}$. In the first model considered here, the transverse Ising Hamiltonian includes an additional ferromagnetic three spin interaction term of strength $J_3$. For $J_3<0.5$, the kink density is found to increase monotonically with $J_3$ whereas it decreases with $J_3$ for $J_3>0.5$. The point $J_3=0.5$ and the transverse field $h=-0.5$is multicritical where the density shows a slower decay given by $\tau^{-1/6}$. We also study the effect of ferromagnetic or antiferromagnetic next nearest neighbor (NNN) interactions on the dynamics of TIM under the same quenching scheme. In a mean field approximation, the transverse Ising Hamiltonians with NNN interactions are identical to the three spin Hamiltonian. The NNN interactions non-trivially modifies the dynamical behavior, for example an antiferromagnetic NNN interactions results to a larger number of kinks in the final state in comparison to the case when the NNN interaction is ferromagnetic.",0801.2621v1 2008-02-13,Magnetoresistance behavior of a ferromagnetic shape memory alloy: Ni_1.75Mn_1.25Ga,"A negative-positive-negative switching behavior of magnetoresistance (MR) with temperature is observed in a ferromagnetic shape memory alloy Ni_1.75Mn_1.25Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with anti-ferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favoured ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease of electron scattering at twin and domain boundaries; and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first order nature of the martensitic phase transition.",0802.1801v2 2008-02-14,"Enhanced annealing, high Curie temperature and low-voltage gating in (Ga,Mn)As: A surface oxide control study","(Ga,Mn)As and related diluted magnetic semiconductors play a major role in spintronics research because of their potential to combine ferromagnetism and semiconducting properties in one physical system. Ferromagnetism requires ~1-10% of substitutional Mn_Ga. Unintentional defects formed during growth at these high dopings significantly suppress the Curie temperature. We present experiments in which by etching the (Ga,Mn)As surface oxide we achieve a dramatic reduction of annealing times necessary to optimize the ferromagnetic film after growth, and report Curie temperature of 180 K at approximately 8% of Mn_Ga. Our study elucidates the mechanism controlling the removal of the most detrimental, interstitial Mn defect. The limits and utility of electrical gating of the highly-doped (Ga,Mn)As semiconductor are not yet established; so far electric-field effects have been demonstrated on magnetization with tens of Volts applied on a top-gate, field effect transistor structure. In the second part of the paper we present a back-gate, n-GaAs/AlAs/GaMnAs transistor operating at a few Volts. Inspired by the etching study of (Ga,Mn)As films we apply the oxide-etching/re-oxidation procedure to reduce the thickness (arial density of carriers) of the (Ga,Mn)As and observe a large enhancement of the gating efficiency. We report gatable spintronic characteristics on a series of anisotropic magnetoresistance measurements.",0802.2080v1 2008-03-19,Successive phase transitions to antiferromagnetic and weak-ferromagnetic long-range orders in quasi-one-dimensional antiferromagnet Cu$_3$Mo$_2$O$_9$,"Investigation of the magnetism of Cu$_3$Mo$_2$O$_9$ single crystal, which has antiferromagnetic (AF) linear chains interacting with AF dimers, reveals an AF second-order phase transition at $T_{\rm N} = 7.9$ K. Although weak ferromagnetic-like behavior appears at lower temperatures in low magnetic fields, complete remanent magnetization cannot be detected down to 0.5 K. However, a jump is observed in the magnetization below weak ferromagnetic (WF) phase transition at $T_{\rm c} \simeq 2.5$ K when a tiny magnetic field along the a axis is reversed, suggesting that the coercive force is very weak. A component of magnetic moment parallel to the chain forms AF long-range order (LRO) below $T_{\rm N}$, while a perpendicular component is disordered above $T_{\rm c}$ at zero magnetic field and forms WF-LRO below $T_{\rm c}$. Moreover, the WF-LRO is also realized with applying magnetic fields even between $T_{\rm c}$ and $T_{\rm N}$. These results are explainable by both magnetic frustration among symmetric exchange interactions and competition between symmetric and asymmetric Dzyaloshinskii-Moriya exchange interactions.",0803.2787v1 2008-04-08,Piezo-control of magnetic anisotropy in GaMnAs: Reversible manipulation of magnetization orientation and irreversible magnetization switching,"We have investigated the magnetic properties of a piezoelectric actuator/ferromagnetic semiconductor hybrid structure. Using a GaMnAs epilayer as the ferromagnetic semiconductor and applying the piezo-stress along its [110] direction, we quantify the magnetic anisotropy as a function of the voltage V_p applied to the piezoelectric actuator using anisotropic magnetoresistance techniques. We find that the easy axis of the strain-induced uniaxial magnetic anisotropy contribution can be inverted from the [110] to the [1-10] direction via the application of appropriate voltages V_p. At T=5K the magnetoelastic term is a minor contribution to the magnetic anisotropy. Nevertheless, we show that the switching fields of rho(H) loops are shifted as a function of V_p at this temperature. At 50K - where the magnetoelastic term dominates the magnetic anisotropy - we are able to tune the magnetization orientation by about 70 degree solely by means of the electrical voltage V_p applied. Furthermore, we derive the magnetostrictive constant lambda_111 as a function of temperature and find values consistent with earlier results. We argue that the piezo-voltage control of magnetization orientation is directly transferable to other ferromagnetic/piezoelectric hybrid structures, paving the way to innovative multifunctional device concepts. As an example, we demonstrate piezo-voltage induced irreversible magnetization switching at T=40K, which constitutes the basic principle of a nonvolatile memory element.",0804.1336v1 2008-05-08,Anomalous Hall Effect in Magnetite: Universal Scaling Relation Between Hall and Longitudinal Conductivity in Low-Conductivity Ferromagnets,"The anomalous Hall effect (AHE) has been studied systematically in the low-conductivity ferromagnetic oxide Fe$_{3-x}$Zn$_x$O$_4$ with $x = 0$, 0.1, and 0.5. We used (001), (110), and (111) oriented epitaxial Fe$_{3-x}$Zn$_x$O$_4$ films grown on MgO and sapphire substrates in different oxygen partial pressure to analyze the dependence of the AHE on crystallographic orientation, Zn content, strain state, and oxygen deficiency. Despite substantial differences in the magnetic properties and magnitudes of the anomalous Hall conductivity $\sigma_{xy}^{\rm AHE}$ and the longitudinal conductivity $\sigma_{xx}$ over several orders of magnitude, a universal scaling relation $\sigma_{xy}^{\rm AHE} \propto \sigma_{xx}^{\alpha}$ with $\alpha = 1.69 \pm 0.08$ was found for all investigated samples. Our results are in agreement with recent theoretical and experimental findings for ferromagnetic metals in the dirty limit, where transport is by metallic conduction. We find the same scaling relation for magnetite, where hopping transport prevails. The fact that this relation is independent of crystallographic orientation, Zn content, strain state, and oxygen deficiency suggests that it is universal and particularly does not depend on the nature of the transport mechanism.",0805.1120v1 2008-05-15,Thickness dependent magnetic anisotropy of ultrathin LCMO epitaxial thin films,"The magnetic properties of La0.7Ca0.3MnO3 (LCMO) manganite thin films were studied with magnetometry and ferromagnetic resonance as a function of film thickness. They maintain the colossal magnetoresistance behavior with a pronounced metal-insulator transition around 150-200 K, except for the very thinnest films studied (3 nm). Nevertheless, LCMO films as thin as 3 nm remain ferromagnetic, without a decrease in saturation magnetization, indicating an absence of dead-layers, although below approx. 6 nm the films remain insulating at low temperature. Magnetization hysteresis loops reveal that the magnetic easy axes lie in the plane of the film for thicknesses in the range of 4-15 nm. Ferromagnetic resonance studies confirm that the easy axes are in-plane, and find a biaxial symmetry in-plane with two, perpendicular easy axes. The directions of the easy axes with respect to the crystallographic directions of the cubic SrTiO3 substrate differ by 45 degrees in 4 nm and 15 nm thick LCMO films.",0805.2336v1 2008-06-10,Magnetic order in double-layer manganites (La(1-z)Pr(z))1.2Sr1.8Mn2O7: intrinsic properties and role of the intergrowths,"We report on an investigation of the double-layer manganite series (La(1-z)Pr(z))1.2Sr1.8Mn2O7 (0 <= z <= 1), carried out on single crystals by means of both macroscopic magnetometry and local probes of magnetism (muSR, 55Mn NMR). Muons and NMR demonstrate an antiferromagnetically ordered ground state at non-ferromagnetic compositions (z >= 0.6), while more moderate Pr substitutions (0.2 <= z <= 0.4) induce a spin reorientation transition within the ferromagnetic phase. A large magnetic susceptibility is detected at {Tc,TN} < T < 250K at all compositions. From 55Mn NMR spectroscopy, such a response is unambiguously assigned to the intergrowth of a ferromagnetic pseudocubic phase (La(1-z)Pr(z))(1-x)Sr(x)MnO3, with an overall volume fraction estimated as 0.5-0.7% from magnetometry. Evidence is provided for the coupling of the magnetic moments of these inclusions with the magnetic moments of the surrounding (La(1-z)Pr(z))1.2Sr1.8Mn2O7 phase, as in the case of finely dispersed impurities. We argue that the ubiquitous intergrowth phase may play a role in the marked first-order character of the magnetic transition and the metamagnetic properties above Tc reported for double-layer manganites.",0806.1599v1 2008-06-28,Spin effects in single electron tunneling,"An important consequence of the discovery of giant magnetoresistance in metallic magnetic multilayers is a broad interest in spin dependent effects in electronic transport through magnetic nanostructures. An example of such systems are tunnel junctions -- single-barrier planar junctions or more complex ones. In this review we present and discuss recent theoretical results on electron and spin transport through ferromagnetic mesoscopic junctions including two or more barriers. Such systems are also called ferromagnetic single-electron transistors. We start from the situation when the central part of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle. Transport characteristics reveal then single-electron charging effects, including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations. Single-electron ferromagnetic transistors based on semiconductor quantum dots and large molecules (especially carbon nanotubes) are also considered. The main emphasis is placed on the spin effects due to spin-dependent tunnelling through the barriers, which gives rise to spin accumulation and tunnel magnetoresistance. Spin effects also occur in the current-voltage characteristics, (differential) conductance, shot noise, and others. Transport characteristics in the two limiting situations of weak and strong coupling are of particular interest. In the former case we distinguish between the sequential tunnelling and cotunneling regimes. In the strong coupling regime we concentrate on the Kondo phenomenon, which in the case of transport through quantum dots or molecules leads to an enhanced conductance and to a pronounced zero-bias Kondo peak in the differential conductance.",0806.4719v1 2008-08-25,Josephson current in diffusive multilayer superconductor/ferromagnet/superconductor junctions,"We calculate the Josephson current in a diffusive superconductor/ferromagnet/superconductor junction, where the ferromagnetic region contains multiple layers (or domains). In particular, we study a configuration where there are two layers with an arbitrary relative in-plane magnetization orientation, and also include non-ideal interfaces and arbitrary spin-flip scattering. We study the 0-$\pi$ oscillations of the critical current for varying junction width $d$, and find that the $\pi$ state vanishes entirely when the magnetic misorientation angle of the two layers exceeds a critical angle $\phi_c$. While $\phi_c \to \pi/2$ in the limit of high temperatures, we find that $\phi_c$ becomes smaller than $\pi/2$ at low temperatures compared to $T_c$. 0-$\pi$ oscillations are also found when varying the temperature or the misorientation angle for fixed values of $d$, and we present phase diagrams that show qualitatively the conditions for the appearance of such oscillations. We also point out how one may obtain significant enhancement of the critical current in such a system by switching the magnetization for selected values of the junction width $d$, and comment on the necessary conditions for establishing a long range triplet Josephson effect.",0808.3359v1 2008-10-10,Junctions of Spin-Incoherent Luttinger Liquids with Ferromagnets and Superconductors,"We discuss the properties of a strongly interacting spin-charge separated one dimensional system coupled to ferromagnets and/or superconductors. Our results are valid for arbitrary temperatures with respect to the spin energy, but require temperature be small compared to the charge energy. We focus mainly on the spin-incoherent regime where temperature is large compared to the spin energy, but small compared to the charge energy. In the case of a ferromagnet we study spin pumping and the renormalized dynamics of a precessing magnetic order parameter. We find the interaction-dependent temperature dependence of the spin pumping can be qualitatively different in the spin-incoherent regime from the Luttinger liquid regime, allowing an identification of the former. Likewise, the temperature dependence of the renormlized magnetization dynamics can be used to identify spin-incoherent physics. For the case of a spin-incoherent Luttinger liquid coupled to two superconductors, we compute the ac and dc Josephson current for a wire geometry in the limit of tunnel coupled superconductors. Both the ac and dc response contain ""smoking gun"" signatures that can be used to identify spin-incoherent behavior. Experimental requirements for the observation of these effects are laid out.",0810.1788v1 2008-10-14,Field induced magnetic order in the frustrated magnet Gadolinium Gallium Garnet,"Gd3Ga5O12, (GGG), has an extraordinary magnetic phase diagram, where no long range order is found down to 25 mK despite \Theta_CW \approx 2 K. However, long range order is induced by an applied field of around 1 T. Motivated by recent theoretical developments and the experimental results for a closely related hyperkagome system, we have performed neutron diffraction measurements on a single crystal sample of GGG in an applied magnetic field. The measurements reveal that the H-T phase diagram of GGG is much more complicated than previously assumed. The application of an external field at low T results in an intensity change for most of the magnetic peaks which can be divided into three distinct sets: ferromagnetic, commensurate antiferromagnetic, and incommensurate antiferromagnetic. The ferromagnetic peaks (e.g. (112), (440) and (220)) have intensities that increase with the field and saturate at high field. The antiferromagnetic reflections have intensities that grow in low fields, reach a maximum at an intermediate field (apart from the (002) peak which shows two local maxima) and then decrease and disappear above 2 T. These AFM peaks appear, disappear and reach maxima in different fields. We conclude that the competition between magnetic interactions and alternative ground states prevents GGG from ordering in zero field. It is, however, on the verge of ordering and an applied magnetic field can be used to crystallise ordered components. The range of ferromagnetic and antiferromagnetic propagation vectors found reflects the complex frustration in GGG.",0810.2410v1 2008-10-23,Phase separation of electrons strongly coupled with phonons in cuprates and manganites,"Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron-electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron-phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott-Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localised by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Froehlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.",0810.4277v1 2008-11-04,"Perspectives in spintronics: magnetic resonant tunneling, spin-orbit coupling, and GaMnAs","Spintronics has attracted wide attention by promising novel functionalities derived from both the electron charge and spin. While branching into new areas and creating new themes over the past years, the principal goals remain the spin and magnetic control of the electrical properties, essentially the I-V characteristics, and vice versa. There are great challenges ahead to meet these goals. One challenge is to find niche applications for ferromagnetic semiconductors, such as GaMnAs. Another is to develop further the science of hybrid ferromagnetic metal/semiconductor heterostructures, as alternatives to all-semiconductor room temperature spintronics. Here we present our representative recent efiorts to address such challenges. We show how to make a digital magnetoresistor by combining two magnetic resonant diodes, or how introducing ferromagnetic semiconductors as active regions in resonant tunneling diodes leads to novel efiects of digital magnetoresistance and of magnetoelectric current oscillations. We also discuss the phenomenon of tunneling anisotropic magnetoresistance in Fe/GaAs junctions by introducing the concept of the spin-orbit coupling field, as an analog of such fields in all-semiconductor junctions. Finally, we look at fundamental electronic and optical properties of GaMnAs by employing reasonable tight-binding models to study disorder efiects.",0811.0500v1 2008-11-21,Anisotropic magnetic and superconducting properties of aligned weak-ferromagnetic superconductor RuSr$_2$RCu$_2$O$_8$ (R = rare earths),"The powder alignment method is used to investigate the anisotropic physical properties of the weak-ferromagnetic superconductor system RuSr2RCu2O8 (R = Pr, Nd, Sm, Eu, Gd, Gd0.5Dy0.5). The RuSr2GdCu2O8 cuprate is a weak-ferromagnetic superconductor with a magnetic ordering of Ru moments at TN(Ru) = 131 K, a superconducting transition in the CuO2 layers at Tc = 56 K, and a low temperature Gd antiferromagnetic ordering at TN(Gd) = 2.5 K. Due to weak magnetic anisotropy of this tetragonal system, highly c-axis aligned microcrystalline powder (diameter ~ 1-10 $\mu$m) in epoxy can be obtained only for R = Eu and Gd through the field-rotation powder alignment method where c-axis is perpendicular to the aligned magnetic field Ba = 0.9 T and parallel to the rotation axis. For smaller rare earth compound R = Gd0.5Dy0.5, powder alignment can be achieved using the simple field powder alignment method where c-axis is partially aligned along the aligned magnetic field. The anisotropic temperature dependence of magnetic susceptibility for the c-axis aligned powders exhibit weak anisotropy with $\chi_{c} > \chi_{ab}$ at room temperature due to anisotropic rare earth, Eu and Gd, contribution and crossover to $\chi_{c} < \chi_{ab}$ below 190 K where strong Ru anisotropic short-range exchange interaction overtakes the rare earth contribution. Anisotropic diamagnetic superconducting intragrain shielding signal of aligned microcrystalline RuSr2GdCu2O8 powder-in-epoxy below vortex lattice melting temperature at 39 K in 1-G field is much weaker than the intergrain polycrystalline bulk sample signal due to the small grain size (d ~ 1-10 $\mu$m), long penetration depth ($\lambda_{ab}$ ~ 0.55 $\mu$m, $\lambda_{c}$ ~ 0.66 $\mu$m) and the two-dimensional (2D) character of CuO2 layers.",0811.3457v1 2008-12-08,Dynamics of a quantum phase transition with decoherence: the quantum Ising chain in a static spin environment,"We consider a linear quench from the paramagnetic to ferromagnetic phase in the quantum Ising chain interacting with a static spin environment. Both decoherence from the environment and non-adiabaticity of the evolution near a critical point excite the system from the final ferromagnetic ground state. For weak decoherence and relatively fast quenches the excitation energy, proportional to the number of kinks in the final state, decays like an inverse square root of a quench time, but slow transitions or strong decoherence make it decay in a much slower logarithmic way. We also find that fidelity between the final ferromagnetic ground state and a final state after a quench decays exponentially with a size of a chain, with a decay rate proportional to average density of excited kinks, and a proportionality factor evolving from 1.3 for weak decoherence and fast quenches to approximately 1 for slow transitions or strong decoherence. Simultaneously, correlations between kinks randomly distributed along the chain evolve from a near-crystalline anti-bunching to a Poissonian distribution of kinks in a number of isolated Anderson localization centers randomly scattered along the chain.",0812.1455v1 2009-01-11,Nonextensive quantum method for itinerant-electron ferromagnetism: Fractorization approach,"Magnetic and thermodynamical properties of itinerant-electron (metallic) ferromagnets described by the Hubbard model have been discussed with the use of the generalized Fermi-Dirac (GFD) distribution for nonextensive quantum systems. We have derived the GFD distribution within the superstatistics, which is equivalent to that obtained by the maximum-entropy method to the Tsallis entropy with the factorization approimation. By using the Hartree-Fock approximation to the electron-electron interaction in the Hubbard model, we have calculated magnetic moment, energy, specific heat and Curie-Weiss-type spin susceptibility, as functions of the temperature and entropic index $q$ expressing the degree of the nonextensivity: $q=1.0$ corresponds to the Boltzmann-Gibbs statistics. It has been shown that with increasing the nonextensivity of $| q-1 |$, the temperature dependence of magnetic moment becomes more significant and the low-temperature electronic specific heat is much increased. This is attributed to enlarged Stoner excitations in the GFD distribution, which is elucidated by an analysis with the use of the generalized Sommerfeld expansion. We discuss the difference and similarity between the effects of the nonextensivity on metallic and insulating ferromagnets.",0901.1469v4 2009-01-21,Instability of the collinear phase in two-dimensional ferromagnet in strong in-plane magnetic field,"It is well-known that in thin ferromagnetic film with a net magnetization perpendicular to the film the collinear arrangement of spins is unstable in an in-plane field $H$ smaller than its saturation value $H_c$. Existence of a stripe phase was proposed with elongated domains of alternating direction of magnetization component perpendicular to the film. We consider in the present paper the strong-field regime $H 0, but does not lead to superconductivity. For various x values, temperature dependence of the resistivity, specific heat, magnetic susceptibility, Hall coefficient, and single crystal x-ray diffraction data are presented. The materials show signatures of approaching a ferromagnetic state with x, including a metamagnetic transition for x as little as 0.36, an enhanced magnetic susceptibility, and a large Sommerfeld coefficient. Such results reflect renormalization due to spin fluctuations and they are supported by density functional calculations at x = 1. Calculations show a strong interplay between magnetic ordering and chemical ordering of Fe and Cr, with a ferromagnetic ground state. This ferromagnetic ground state is explained in terms of the electronic structure. The resulting phase diagram is suggestive that superconductivity does not derive simply from the suppression of the structural/magnetic transitions.",0903.5546v1 2009-04-14,Room temperature ferromagnetic-like behavior in Mn-implanted and post-annealed InAs layers deposited by Molecular Beam Epitaxy,"We report on the magnetic and structural properties of Ar and Mn implanted InAs epitaxial films grown on GaAs (100) by Molecular Beam Epitaxy (MBE) and the effect of Rapid Thermal Annealing (RTA) for 30 seconds at 750C. Channeling Particle Induced X- ray Emission (PIXE) experiments reveal that after Mn implantation almost all Mn atoms are subsbtitutional in the In-site of the InAs lattice, like in a diluted magnetic semiconductor (DMS). All of these samples show diamagnetic behavior. But, after RTA treatment the Mn-InAs films exhibit room-temperature magnetism. According to PIXE measurements the Mn atoms are no longer substitutional. When the same set of experiments were performed with As as implantation ion all of the layers present diamagnetism without exception. This indicates that the appearance of room-temperature ferromagnetic-like behavior in the Mn-InAs-RTA layer is not related to lattice disorder produce during implantation, but to a Mn reaction produced after a short thermal treatment. X-ray diffraction patterns (XRD) and Rutherford Back Scattering (RBS) measurements evidence the segregation of an oxygen deficient-MnO2 phase (nominally MnO1.94) in the Mn-InAs-RTA epitaxial layers which might be on the origin of room temperature ferromagnetic-like response observed.",0904.2132v1 2009-04-14,Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach,"We study spin and charge currents pumped by precessing magnetization of a single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator; N-normal-metal) multilayers of nanoscale thickness attached to two normal metal electrodes with no applied bias voltage between them. Both simple one-dimensional model, consisting of a single precessing spin and a potential barrier as the ""sample,"" and realistic three-dimensional devices are investigated. In the rotating reference frame, where the magnetization appears to be static, these junctions are mapped onto a four-terminal dc circuit whose effectively half-metallic ferromagnetic electrodes are biased by the frequency $\hbar \omega/e$ of microwave radiation driving magnetization precession at the ferromagnetic resonance (FMR) conditions. We show that pumped spin current in F|I|F junctions, diminished behind the tunnel barrier and increased in the opposite direction, is filtered into charge current by the second $F$ layer to generate dc pumping voltage of the order of $\sim 1$ $\mu$V (at FMR frequency $\sim 10$ GHz) in an open circuit. In F|I|N devices, several orders of magnitude smaller charge current and the corresponding dc voltage appear concomitantly with the pumped spin current due to barrier induced asymmetry in the transmission coefficients connecting the four electrodes in the rotating frame picture of pumping.",0904.2198v1 2009-05-03,Exchange bias in Co/CoO core-shell nanowires: Role of the antiferromagnetic superparamagnetic fluctuations,"The magnetic properties of Co (=15 nm, =130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ K. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity S which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism.",0905.0268v4 2009-05-07,Evolutionary dynamics on rugged fitness landscapes: exact dynamics and information theoretical aspects,"The parallel mutation-selection evolutionary dynamics, in which mutation and replication are independent events, is solved exactly in the case that the Malthusian fitnesses associated to the genomes are described by the Random Energy Model (REM) and by a ferromagnetic version of the REM. The solution method uses the mapping of the evolutionary dynamics into a quantum Ising chain in a transverse field and the Suzuki-Trotter formalism to calculate the transition probabilities between configurations at different times. We find that in the case of the REM landscape the dynamics can exhibit three distinct regimes: pure diffusion or stasis for short times, depending on the fitness of the initial configuration, and a spin-glass regime for large times. The dynamic transition between these dynamical regimes is marked by discontinuities in the mean-fitness as well as in the overlap with the initial reference sequence. The relaxation to equilibrium is described by an inverse time decay. In the ferromagnetic REM, we find in addition to these three regimes, a ferromagnetic regime where the overlap and the mean-fitness are frozen. In this case, the system relaxes to equilibrium in a finite time. The relevance of our results to information processing aspects of evolution is discussed.",0905.1097v2 2009-05-13,"Spin Fluctuation Theory for Quantum Tricritical Point Arising in Proximity to First-Order Phase Transitions: Applications to Heavy-Fermion Systems, YbRh2Si2, CeRu2Si2, and beta-YbAlB4","We propose a phenomenological spin fluctuation theory for antiferromagnetic quantum tricritical point (QTCP), where the first-order phase transition changes into the continuous one at zero temperature. Under magnetic fields, ferromagnetic quantum critical fluctuations develop around the antiferromagnetic QTCP in addition to antiferromagnetic ones, which is in sharp contrast with the conventional antiferromagnetic quantum critical point. For itinerant electron systems,} we show that the temperature dependence of critical magnetic fluctuations around the QTCP are given as chiQ \propto T^{-3/2} (chi0\propto T^{-3/4}) at the antiferromagnetic ordering (ferromagnetic) wave number q=Q (q=0). The convex temperature dependence of chi0^{-1} is the characteristic feature of the QTCP, which is never seen in the conventional spin fluctuation theory. We propose that the general theory of quantum tricriticality that has nothing to do with the specific Kondo physics itself, solves puzzles of quantum criticalities widely observed in heavy-fermion systems such as YbRh2Si2, CeRu2Si2, and beta-YbAlB4. For YbRh2Si2, our theory successfully reproduces quantitative behaviors of the experimental ferromagnetic susceptibility and the magnetization curve by choosing the phenomenological parameters properly. The quantum tricriticality is also consistent with singularities of other physical properties such as specific heat, nuclear magnetic relaxation time 1/T_1T, and Hall coefficient. For CeRu2Si2 and beta-YbAlB4, we point out that the quantum tricriticality is a possible origin of the anomalous diverging enhancement of the uniform susceptibility observed in these materials.",0905.2046v1 2009-07-07,Tilted-Cone-induced easy-plane pseudo-spin ferromagnet and Kosterlitz-Thouless transition in massless Dirac fermions,"The possible quantum Hall ferromagnet at a filling factor $\nu =0$ is investigated for the zero-energy (N=0) Landau level of the two dimensional massless Dirac fermions in $\alpha$-(BEDT-TTF)$_2$I$_3$ under pressure with tilted cones and a twofold valley degeneracy resulting from time-reversal symmetry. In the case of the Dirac cones without tilting, the long-range Coulomb interaction in the N=0 Landau level exhibits the SU(2) valley-pseudo-spin symmetry even to the order $O(a/l_{\rm H})$, in contrast to $N \ne 0$ Landau levels, where $a$ and $l_{\rm H}$ represent the lattice constant and the magnetic length, respectively. Such a characteristic comes from a fact that zero-energy states in a particular valley are restricted to only one of the spinor components, whereas the other spinor component is necessarily zero. In the case of the tilted Dirac cones as found in $\alpha$-(BEDT-TTF)$_2$I$_3$, one obtains a non-zero value of the second component and then the ackscattering processes between valleys becomes non-zero. It is shown that this fact can lead to easy-plane pseudospin ferromagnetism (XY-type). In this case, the phase fluctuations of the order parameters can be described by the XY model leading to Kosterlitz-Thouless transition at lower temperature. In view of these theoretical results, experimental findings in resistivity of $\alpha$-(BEDT-TTF)$_2$I$_3$ are discussed.",0907.1160v1 2009-07-21,Ferromagnetic transition metal implanted ZnO: a diluted magnetic semiconductor?,"Recently theoretical works predict that some semiconductors (e.g. ZnO) doped with magnetic ions are diluted magnetic semiconductors (DMS). In DMS magnetic ions substitute cation sites of the host semiconductor and are coupled by free carriers resulting in ferromagnetism. One of the main obstacles in creating DMS materials is the formation of secondary phases because of the solid-solubility limit of magnetic ions in semiconductor host. In our study transition metal ions were implanted into ZnO single crystals with the peak concentrations of 0.5-10 at.%. We established a correlation between structural and magnetic properties. By synchrotron radiation X-ray diffraction (XRD) secondary phases (Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as the source for ferromagnetism. Due to their different crystallographic orientation with respect to the host crystal these nanocrystals in some cases are very difficult to be detected by a simple Bragg-Brentano scan. This results in the pitfall of using XRD to exclude secondary phase formation in DMS materials. For comparison, the solubility of Co diluted in ZnO films ranges between 10 and 40 at.% using different growth conditions pulsed laser deposition. Such diluted, Co-doped ZnO films show paramagnetic behaviour. However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d exchange interaction as compared to Co-implanted ZnO single crystals.",0907.3536v1 2009-09-04,Magnetic structure of an imbalanced Fermi gas in an optical lattice,"We analyze the repulsive fermionic Hubbard model on square and cubic lattices with spin imbalance and in the presence of a parabolic confinement. We analyze the magnetic structure as a function of the repulsive interaction strength and polarization. In the first part of the paper we perform unrestricted Hartree-Fock calculations for the 2D case and find that above a critical interaction strength $U_c$ the system turns ferromagnetic at the edge of the trap, in agreement with the ferromagnetic Stoner instability of a homogeneous system away from half-filling. For $U 0.18$, whereas for a lower Eu-doping level, there are no spin-state transitions, but a crossover between the ferromagnetic and paramagnetic states occurs. The effect of oxygen isotope substitution along with Eu doping on the magnetic/spin state is discussed. The oxygen-isotope substitution ($^{16}$O by $^{18}$O) is found to shift both the magnetic and spin-state phase boundaries to lower Eu concentrations. The isotope effect on the spin-state transition temperature ($y > 0.18$) is rather strong, but it is much weaker for the transition to a ferromagnetic state ($y < 0.18$). The ferromagnetic ordering in the low-Eu doped samples is shown to be promoted by the Co$^{4+}$ ions, which favor the formation of the intermediate-spin state of neighboring Co$^{3+}$ ions.",0909.2978v2 2009-10-26,Andreev tunneling through a double quantum-dot system coupled to a ferromagnet and a superconductor: effects of mean field electronic correlations,"We study the transport properties of a hybrid nanostructure composed of a ferromagnet, two quantum dots, and a superconductor connected in series. By using the non-equilibrium Green's function approach, we have calculated the electric current, the differential conductance and the transmittance for energies within the superconductor gap. In this regime, the mechanism of charge transmission is the Andreev reflection, which allows for a control of the current through the ferromagnet polarization. We have also included interdot and intradot interactions, and have analyzed their influence through a mean field approximation. In the presence of interactions, Coulomb blockade tend to localized the electrons at the double-dot system, leading to an asymmetric pattern for the density of states at the dots, and thus reducing the transmission probability through the device. In particular, for non-zero polarization, the intradot interaction splits the spin degeneracy, reducing the maximum value of the current due to different spin-up and spin-down densities of states. Negative differential conductance (NDC) appears for some regions of the voltage bias, as a result of the interplay of the Andreev scattering with electronic correlations. By applying a gate voltage at the dots, one can tune the effect, changing the voltage region where this novel phenomenon appears. This mechanism to control the current may be of importance in technological applications.",0910.4919v1 2010-01-05,Indication of antiferromagnetic interaction between paramagnetic Co ions in the diluted magnetic semiconductor Zn$_{1-x}$Co$_{x}$O,"The magnetic properties of Zn$_{1-x}$Co$_x$O ($x=0.07$ and 0.10) thin films, which were homo-epitaxially grown on a ZnO(0001) substrates with varying relatively high oxygen pressure, have been investigated using x-ray magnetic circular dichroism (XMCD) at Co $2p$ core-level absorption edge. The line shapes of the absorption spectra are the same in all the films and indicate that the Co$^{2+}$ ions substitute for the Zn sites. The magnetic-field and temperature dependences of the XMCD intensity are consistent with the magnetization measurements, indicating that except for Co there are no additional sources for the magnetic moment, and demonstrate the coexistence of paramagnetic and ferromagnetic components in the homo-epitaxial Zn$_{1-x}$Co$_{x}$O thin films, in contrast to the ferromagnetism in the hetero-epitaxial Zn$_{1-x}$Co$_{x}$O films studied previously. The analysis of the XMCD intensities using the Curie-Weiss law reveals the presence of antiferromagnetic interaction between the paramagnetic Co ions. Missing XMCD intensities and magnetization signals indicate that most of Co ions are non-magnetic probably because they are strongly coupled antiferromagnetically with each other. Annealing in a high vacuum reduces both the paramagnetic and ferromagnetic signals. We attribute the reductions to thermal diffusion and aggregation of Co ions with antiferromagnetic nanoclusters in Zn$_{1-x}$Co$_{x}$O.",1001.0712v1 2010-03-23,Unconventional Neel and dimer orders in a spin-1/2 frustrated ferromagnetic chain with easy-plane anisotropy,"We study the ground-state phase diagram of a one-dimensional spin-1/2 easy-plane XXZ model with a ferromagnetic nearest-neighbor (NN) coupling $J_1$ and a competing next-nearest-neighbor (NNN) antiferromagnetic coupling $J_2$ in the parameter range $0infty respectively, and which gives rise to equal magnetic and overlap correlation functions when T=S. While the high-temperature phase is always paramagnetic, at low temperatures there is a ferromagnetic phase for weak disorder (small S) and a glassy phase at large disorder (large S). These three phases are separated by transition lines with different magnetic and glassy critical behaviors. The disorder induced by the random shifts turns out to be irrelevant at the paramagnetic-ferromagnetic transition line, where the critical behavior belongs to the 3D XY universality class of pure systems; disorder gives only rise to very slowly decaying scaling corrections. The glassy critical behavior along the finite-temperature paramagnetic-glassy transition line belongs to the gauge-glass universality class, with a quite large critical exponent nu=3.2(4). These transition lines meet at a multicritical point M, located at T=S=0.7840(2). The low-temperature ferromagnetic and glassy phases are separated by a third transition line, from M down to the T=0 axis, which is slightly reentrant.",1012.2432v1 2011-01-01,Signature effects of spin clustering and distribution of spin couplings on magnetization behaviour in Ni-Fe-Mo and Ni-Fe-W alloys,"The spontaneous magnetization as a function of temperature is investigated for a number of disordered Ni-Fe-Mo and Ni-Fe-W alloys using superconducting quantum interference device magnetometry, with a focus on the low-T behavior as well as the critical exponents associated with the magnetic phase transition. While the low-T magnetization is found to be well described by Bloch's T^{3/2} law, an extraordinary enhancement of the spin-wave parameter B and the reduced coefficient B_{3/2}=BT_C ^{3/2} are observed with increasing Fe dilution as compared to conventional 3d ferromagnets, whereas the critical amplitudes are found to decrease systematically. Recent locally self-consistent calculations of finite-temperature spin dynamics in a generic diluted magnet provide an understanding in terms of two distinct energy scales associated with weakly coupled bulk spins in the FM matrix and strongly coupled cluster spins. In view of similar behaviour observed in diluted magnetic semiconductors and other ferromagnetic alloys, it is proposed that these distinctive features corresponding to the three important temperature regimes provide macroscopic indicators of signature effects of spin clustering on magnetization behaviour in disordered ferromagnets.",1101.0343v2 2011-02-01,Dzyaloshinskii-Moriya interaction and spin re-orientation transition in the frustrated kagome lattice antiferromagnet,"Magnetization, specific heat, and neutron scattering measurements were performed to study a magnetic transition in jarosite, a spin-5/2 kagome lattice antiferromagnet. When a magnetic field is applied perpendicular to the kagome plane, magnetizations in the ordered state show a sudden increase at a critical field H_c, indicative of the transition from antiferromagnetic to ferromagnetic states. This sudden increase arises as the spins on alternate kagome planes rotate 180 degrees to ferromagnetically align the canted moments along the field direction. The canted moment on a single kagome plane is a result of the Dzyaloshinskii-Moriya interaction. For H < H_c, the weak ferromagnetic interlayer coupling forces the spins to align in such an arrangement that the canted components on any two adjacent layers are equal and opposite, yielding a zero net magnetic moment. For H > H_c, the Zeeman energy overcomes the interlayer coupling causing the spins on the alternate layers to rotate, aligning the canted moments along the field direction. Neutron scattering measurements provide the first direct evidence of this 180-degree spin rotation at the transition.",1102.0185v1 2011-03-08,Collective Phenomena in the LiHo_xY_{1-x}F4 Quantum Ising Magnet: Recent Progress and Open Questions,"In LiHo_xY_{1-x}F4, the magnetic Holmium Ho^{3+} ions behave as effective Ising spins that can point parallel or antiparallel to the crystalline c-axis. The predominant inter-Holmium interaction is dipolar, while the Y^{3+} ions are non-magnetic. The application of a magnetic field Bx transverse to the c-axis Ising direction leads to quantum spin-flip fluctuations, making this material a rare physical realization of the celebrated transverse field Ising model. The problems of classical and transverse-field-induced quantum phase transitions in LiHo_xY_{1-x}F_4 in the dipolar ferromagnetic (x=1), diluted ferromagnetic (0.25 <~ x < 1) and highly diluted x<~0.25 dipolar spin glass regimes have attracted much experimental and theoretical interest over the past twenty-five years. Two questions have received particular attention: (i) is there an {\it antiglass} (quantum disordered) phase at low Ho^{3+} concentration and (ii) what is the mechanism responsible for the fast Bx-induced destruction of the ferromagnetic (0.25 <~ x < 1) and spin glass (x <~ 0.25) phases? This paper reviews some of the recent theoretical and experimental progress in our understanding of the collective phenomena at play in LiHo_xY_{1-x}F4, in both zero and nonzero Bx.",1103.1537v1 2011-04-11,Writing electronic ferromagnetic states in a high-temperature paramagnetic nuclear spin system,"In this paper we use the Nuclear Magnetic Resonance (NMR) to write eletronic states of a ferromagnetic system into a high-temperature paramagnetic nuclear spins. Through the control of phase and duration of radiofrequency pulses we set the NMR density matrix populations, and apply the technique of quantum state tomography to experimentally obtain the matrix elements of the system, from which we calculate the temperature dependence of magnetization for different magnetic fields. The effects of the variation of temperature and magnetic field over the populations can be mapped in the angles of spins rotations, carried out by the RF pulses. The experimental results are compared to the Brillouin functions of ferromagnetic ordered systems in the mean field approximation for two cases: the mean field is given by (i) $B=B_0+\lambda M$ and (ii) $B=B_0+\lambda M + \lambda^\prime M^3$, where $B_0$ is the external magnetic field, and $\lambda, \lambda^\prime$ are mean field parameters. The first case exhibits second order transition, whereas the second case has first order transition with temperature hysteresis. The NMR simulations are in good agreement with the magnetic predictions.",1104.1953v2 2011-04-25,Magnetic Catalysis and Quantum Hall Ferromagnetism in Weakly Coupled Graphene,"We study the realization in a model of graphene of the phenomenon whereby the tendency of gauge-field mediated interactions to break chiral symmetry spontaneously is greatly enhanced in an external magnetic field. We prove that, in the weak coupling limit, and where the electron-electron interaction satisfies certain mild conditions, the ground state of charge neutral graphene in an external magnetic field is a quantum Hall ferromagnet which spontaneously breaks the emergent U(4) symmetry to U(2)XU(2). We argue that, due to a residual CP symmetry, the quantum Hall ferromagnet order parameter is given exactly by the leading order in perturbation theory. On the other hand, the chiral condensate which is the order parameter for chiral symmetry breaking generically obtains contributions at all orders. We compute the leading correction to the chiral condensate. We argue that the ensuing fermion spectrum resembles that of massive fermions with a vanishing U(4)-valued chemical potential. We discuss the realization of parity and charge conjugation symmetries and argue that, in the context of our model, the charge neutral quantum Hall state in graphene is a bulk insulator, with vanishing longitudinal conductivity due to a charge gap and Hall conductivity vanishing due to a residual discrete particle-hole symmetry.",1104.4714v1 2011-05-01,Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices,"We demonstrate that delta-doping can be used to create a dimensionally confined region of metallic ferromagnetism in an antiferromagnetic (AF) manganite host, without introducing any explicit disorder due to dopants or frustration of spins. Delta-doped carriers are inserted into a manganite superlattice (SL) by a digital-synthesis technique. Theoretical consideration of these additional carriers show that they cause a local enhancement of ferromagnetic (F) double-exchange with respect to AF superexchange, resulting in local canting of the AF spins. This leads to a highly modulated magnetization, as measured by polarized neutron reflectometry. The spatial modulation of the canting is related to the spreading of charge from the doped layer, and establishes a fundamental length scale for charge transfer, transformation of orbital occupancy and magnetic order in these manganites. Furthermore, we confirm the existence of the canted, AF state as was predicted by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained elusive.",1105.0223v2 2011-05-09,Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids,"We studied experimentally the effect of a stripe-like domain structure in a ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a superconducting Pb microbridge. The system was designed in such a way that the bridge is oriented perpendicular to the domain walls. It is demonstrated that depending on the ratio between the amplitude of the nonuniform magnetic field B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the superconducting material, the regions of the reverse-domain superconductivity in the H-T plane can be isolated or can overlap (H is the external magnetic field, T is temperature). The latter case corresponds to the condition B_0/H_{c2}<1 and results in the formation of superconductivity above the magnetic domains of both polarities. We discovered the regime of edge-assisted reverse-domain superconductivity, corresponding to localized superconductivity near the edges of the bridge above the compensated magnetic domains. Direct verification of the formation of inhomogeneous superconducting states and external-field-controlled switching between normal state and inhomogeneous superconductivity were obtained by low-temperature scanning laser microscopy.",1105.1596v2 2011-06-05,"Checkerboards, stripes and corner energies in spin models with competing interactions","We study the zero temperature phase diagram of Ising spin systems in two dimensions in the presence of competing interactions, long range antiferromagnetic and nearest neighbor ferromagnetic of strength J. We first introduce the notion of a ""corner energy"" which shows, when the antiferromagnetic interaction decays faster than the fourth power of the distance, that a striped state is favored with respect to a checkerboard state when J is close to J_c, the transition to the ferromagnetic state, i.e., when the length scales of the uniformly magnetized domains become large. Next, we perform detailed analytic computations on the energies of the striped and checkerboard states in the cases of antiferromagnetic interactions with exponential decay and with power law decay r^{-p}, p>2, that depend on the Manhattan distance instead of the Euclidean distance. We prove that the striped phase is always favored compared to the checkerboard phase when the scale of the ground state structure is very large. This happens for J\lesssim J_c if p>3, and for J sufficiently large if 2 -1^+ for both open and periodic boundary conditions. At \Delta=-1 (ferromagnetic point) the model is equivalent to the Heisenberg ferromagnet and its degenerate ground state manifold is the SU(2) multiplet with maximal total spin. Any state in this so-called ""symmetric sector"" is an equal weight superposition of all possible spin configurations. In the gapless phase at \Delta>-1 this property is progressively lost as one moves away from the \Delta=-1 point. We investigate how the ES obtained from the states in this manifold reflects this change, using exact diagonalization and Bethe ansatz calculations. We find that in the limit \Delta ->-1^+ most of the ES levels show divergent behavior. Moreover, while at \Delta=-1 the ES contains no information about the boundaries, for \Delta>-1 it depends dramatically on the choice of boundary conditions. For both open and periodic boundary conditions the ES exhibits an elegant multiplicity structure for which we conjecture a combinatorial formula. We also study the entanglement eigenfunctions, i.e. the eigenfunctions of the reduced density matrix. We find that the eigenfunctions corresponding to the non diverging levels mimic the behavior of the state wavefunction, whereas the others show intriguing polynomial structures. Finally we analyze the distribution of the ES levels as the system is detuned away from \Delta=-1.",1206.0131v2 2012-06-10,Magnetic glass in Shape Memory Alloy : Ni45Co5Mn38Sn12,"The first order martensitic transition in the ferromagnetic shape memory alloy Ni45Co5Mn38Sn12 is also a magnetic transition and has a large field induced effect. While cooling in the presence of field this first order magnetic martensite transition is kinetically arrested. Depending on the cooling field, a fraction of the arrested ferromagnetic austenite phase persists down to the lowest temperature as a magnetic glassy state, similar to the one observed in various intermetallic alloys and in half doped manganites. A detailed investigation of this first order ferromagnetic austenite (FM-A) to low magnetization martensite (LM-M) state transition as a function of temperature and field has been carried out by magnetization measurements. Extensive cooling and heating in unequal field (CHUF) measurements and a novel field cooled protocol for isothermal MH measurements (FC-MH) are utilized to investigate the glass like arrested states and show a reverse martensite transition. Finally, we determine a field -temperature (HT) phase diagram of Ni45Co5Mn38Sn12 from various magnetization measurements which brings out the regions where thermodynamic and metastable states co-exist in the HT space clearly depicting this system as a 'Magnetic Glass'.",1206.2024v2 2012-06-10,Observation of the Triplet Spin-Valve Effect in a Superconductor-Ferromagnet Heterostructure,"The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu41Ni59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation.",1206.2045v1 2012-06-20,Ga$^{3+}$-substitution effects in the weak ferromagnetic oxide LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$,"Magnetization and x-ray diffraction have been measured on polycrystalline samples of LaCo$_{0.8-y}$Rh$_{0.2}$Ga$_{y}$O$_{3}$ for $0 \leq y \leq 0.15$ in order to understand the spin state of Co$^{3+}$ through the Ga$^{3+}$ substitution effect. The ferromagnetic order in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ below 15 K is dramatically suppressed by the Ga$^{3+}$ substitution, where the ferromagnetic volume fraction is linearly decreased. The normal state susceptibility also systematically decreases with the Ga content, from which we find that one Ga$^{3+}$ ion reduces 4.6 $\mu_{\rm B}$ per formula unit. We have evaluated how the concentration of the high-spin state Co$^{3+}$ changes with temperature by using an extended Curie-Weiss law, and have found that the substituted Rh$^{3+}$ ion stabilizes the high-spin state Co$^{3+}$ ion down to low temperatures. We find that Ga$^{3+}$ preferentially replaces the high-spin state Co$^{3+}$, which makes a remarkable contrast to our previous finding that Rh$^{3+}$ preferentially replaces the low-spin state Co$^{3+}$. These results strongly suggest that the magnetically excited state of LaCoO$_{3}$ at room temperature is a mixed state of high-spin state Co$^{3+}$ and low-spin state Co$^{3+}$.",1206.4472v1 2012-06-29,A computational mechanics approach to estimate entropy and (approximate) complexity for the dynamics of the 2D Ising Ferromagnet,"We present a numerical analysis of the entropy rate and statistical complexity related to the spin flip dynamics of the 2D Ising Ferromagnet at different temperatures T. We follow an information theoretic approach and test three different entropy estimation algorithms to asses entropy rate and statistical complexity of binary sequences. The latter are obtained by monitoring the orientation of a single spin on a square lattice of side-length L=256 at a given temperature parameter over time. The different entropy estimation procedures are based on the M-block Shannon entropy (a well established method that yields results for benchmarking purposes), non-sequential recursive pair substitution (providing an elaborate and an approximate estimator) and a convenient data compression algorithm contained in the zlib-library (providing an approximate estimator only). We propose an approximate measure of statistical complexity that emphasizes on correlations within the sequence and which is easy to implement, even by means of black-box data compression algorithms. Regarding the 2D Ising Ferromagnet simulated using Metropolis dynamics and for binary sequences of finite length, the proposed approximate complexity measure is peaked close to the critical temperature. For the approximate estimators, a finite-size scaling analysis reveals that the peak approaches the critical temperature as the sequence length increases. Results obtained using different spin-flip dynamics are briefly discussed. The suggested complexity measure can be extended to non-binary sequences in a straightforward manner.",1206.7032v1 2012-07-04,Ground-state phase diagram of a spin-1/2 frustrated ferromagnetic XXZ chain: Haldane dimer phase and gapped/gapless chiral phases,"The ground-state phase diagram of a spin-1/2 XXZ chain with competing ferromagnetic nearest-neighbor (J_1<0) and antiferromagnetic second-neighbor (J_2>0) exchange couplings is studied by means of the infinite time evolving block decimation algorithm and effective field theories. For the SU(2)-symmetric (Heisenberg) case, we show that the nonmagnetic phase in the range -40$), $0\leq \lambda\leq 1$ is an anisotropic constant and $D$ is the single-ion anisotropic constant. It is well-known that the single-ion anisotropy $D$ creates a competition between an ordered state (favored by the exchange interaction) and a disordered state, even at zero temperature. For small values of $D$, the system has a spontaneous magnetization $m_z\neq 0$, while in the large-D phase $m_z=0$ because a state with $\langle S^z\rangle\neq 0$ is energetically unfavorable. Therefore, a phase transition due to quantum fluctuations occurs in some critical value $D_c$. For systems with short-range interaction $D_c\approx 6J$, depending of $\lambda$ constant, but in our model we have found larger values of $D$ due to the higher cost to flip a spin. Since low-dimensional magnetic systems with long range interaction can be ordered at finite temperature, we also have analyzed the thermal phase transitions (similar to the BKT transition). The model has been studied by using a Schwinger boson formalism as well as the Self-consistent Harmonic Approximation (SCHA) and both methods provide according results.",1404.1939v1 2014-04-15,Hierarchy of Hofstadter states and replica quantum Hall ferromagnetism in graphene superlattices,"Self-similarity and fractals have fascinated researchers across various disciplines. In graphene placed on boron nitride and subjected to a magnetic field, self-similarity appears in the form of numerous replicas of the original Dirac spectrum, and their quantization gives rise to a fractal pattern of Landau levels, referred to as the Hofstadter butterfly. Here we employ capacitance spectroscopy to probe directly the density of states (DoS) and energy gaps in this spectrum. Without a magnetic field, replica spectra are seen as pronounced DoS minima surrounded by van Hove singularities. The Hofstadter butterfly shows up as recurring Landau fan diagrams in high fields. Electron-electron interactions add another twist to the self-similar behaviour. We observe suppression of quantum Hall ferromagnetism, a reverse Stoner transition at commensurable fluxes and additional ferromagnetism within replica spectra. The strength and variety of the interaction effects indicate a large playground to study many-body physics in fractal Dirac systems.",1404.3856v2 2014-04-16,Manipulation of edge magnetism in hexagonal graphene nanoflake,"We explore possible ways to manipulate the intrinsic edge magnetism in hexagonal graphene nanoflake with zigzag edges, using density functional theory supplemented with on-site Coulomb interaction. The effect of carrier doping, chemical modification at the edge, and finite temperature on the edge magnetism has been studied. The magnetic phase diagram with varied carrier doping, and on-site Coulomb interaction is found to be complex. In addition to the intrinsic antiferromag- netic solution, as predicted for charge neutral hexagonal nanoflake, fully polarized ferromagnetic, and mixed phase solutions are obtained depending on the doped carrier concentration, and on-site Coulomb interaction. The complexity arises due to the competing nature of local Coulomb in- teraction and carrier doping, favoring antiferromagnetic and ferromagnetic coupling, respectively. Chemical modification of the edge atoms by hydrogen leads to partial quenching of local moments, giving rise to a richer phase diagram consisting of antiferromagnetic, ferromagnetic, mixed, and nonmagnetic phases. We further report the influence of temperature on the long-range magnetic ordering at the edge using ab initio molecular dynamics. In agreement with the recent experimental observations, we find that temperature can also alter the magnetic state of neutral nanoflake, which is otherwise antiferromagnetic at zero temperature. These findings will have important implications in controlling magnetism in graphene based low dimensional structures for technological purpose, and in understanding varied experimental reports.",1404.4143v2 2014-07-01,Statistical properties of Barkhausen noise in amorphous ferromagnetic films,"We investigate the statistical properties of the Barkhausen noise in amorphous ferromagnetic films with thicknesses in the range between $100$ and $1000$ nm. From Barkhausen noise time series measured with the traditional inductive technique, we perform a wide statistical analysis and establish the scaling exponents $\tau$, $\alpha$, $1/\sigma \nu z$, and $\vartheta$. We also focus on the average shape of the avalanches, which gives further indications on the domain wall dynamics. Based on experimental results, we group the amorphous films in a single universality class, characterized by scaling exponents $\tau\sim 1.27$, $\alpha \sim 1.5$, $1/\sigma \nu z \sim \vartheta \sim 1.77$, values similar to that obtained for several bulk amorphous magnetic materials. Besides, we verify that the avalanche shape depends on the universality class. By considering the theoretical models for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium found in literature, we interpret the results and identify an experimental evidence that these amorphous films, within this thickness range, present a typical three-dimensional magnetic behavior with predominant short-range elastic interactions governing the domain wall dynamics. Moreover, we provide experimental support for the validity of a general scaling form for the average avalanche shape for non-mean-field systems.",1407.0396v1 2014-07-03,"Code optimization, frozen glassy phase and improved decoding algorithms for low-density parity-check codes","The statistical physics properties of low-density parity-check codes for the binary symmetric channel are investigated as a spin glass problem with multi-spin interactions and quenched random fields by the cavity method. By evaluating the entropy function at the Nishimori temperature, we find that irregular constructions with heterogeneous degree distribution of check (bit) nodes have higher decoding thresholds compared to regular counterparts with homogeneous degree distribution. We also show that the instability of the mean-field calculation takes place only after the entropy crisis, suggesting the presence of a frozen glassy phase at low temperatures. When no prior knowledge of channel noise is assumed (searching for the ground state), we find that a reinforced strategy on normal belief propagation will boost the decoding threshold to a higher value than the normal belief propagation. This value is close to the dynamical transition where all local search heuristics fail to identify the true message (codeword or the ferromagnetic state). After the dynamical transition, the number of metastable states with larger energy density (than the ferromagnetic state) becomes exponentially numerous. When the noise level of the transmission channel approaches the static transition point, there starts to exist exponentially numerous codewords sharing the identical ferromagnetic energy.",1407.0779v2 2014-07-21,Direct epitaxial integration of the ferromagnetic semiconductor EuO with silicon for spintronic applications,"Materials in which charge and spin degrees of freedom interact strongly offer applications known as spintronics. Following a remarkable success of metallic spintronics based on the giant-magnetoresistive effect, tremendous efforts have been invested into the less developed semiconductor spintronics, in particular, with the aim to produce three-terminal spintronic devices, e.g. spin transistors. One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers from a ferromagnetic semiconductor into a nonmagnetic semiconductor, preferably one of those currently used for industrial applications such as Si - a workhorse of modern electronics. Ferromagnetic semiconductor EuO is long believed to be the best candidate for integration of magnetic semiconductor with Si. Although EuO proved to offer optimal conditions for effective spin injection into silicon and in spite of considerable efforts, the direct epitaxial stabilization of stoichiometric EuO thin films on Si without any buffer layer has not been demonstrated to date. Here we report a new technique for control of EuO/Si interface on submonolayer level which may have general implications for the growth of functional oxides on Si. Using this technique we solve a long-standing problem of direct epitaxial growth on silicon of thin EuO films which exhibit structural and magnetic properties of EuO bulk material. This result opens up new possibilities in developing all-semiconductor spintronic devices.",1407.5431v1 2014-07-24,Emergent Spin Hall phase at a Lifshitz transition from competing orders,"The effects of competing orders, such as superconductivity and ferromagnetism, on a Fermi liquid are well established. A comprehensive understanding of such a competition in a metal whose Fermi surface has a non-trivial topology is yet to be achieved. Here, we address this question in a prototypical system: the 2D Rashba semimetal. We show that dominant superconductivity interplays with Rashba spin orbit interactions (SOI) in forming a novel intrinsic anomalous Hall effect (AHE) with gapless edge states of Bogoliubov-de Gennes (BdG) quasiparticles. As in the case of itinerant ferromagnets, the intrinsic AHE arises from Berry curvature effects in the band structure. This phenomenon is robust even as sub-dominant ferromagnetism dramatically changes the nature of pairing symmetry. An emergent spin Hall phase involving a change in Fermi-surface topology is found to accompany this Lifshitz quantum phase transition. We demonstrate the coexistence of the original and novel AHE in the presence of weak disorder. We offer a comparison of our results with experiments on the two dimensional electron gas at oxide hetero-interfaces as well as make some testable predictions.",1407.6539v2 2014-07-29,Noise Model of Relaxation Oscillators Due to Feedback Regeneration Based on Physical Phase Change,"A new approach to investigate noise spikes due to regeneration in a relaxation oscillator is proposed. Noise spikes have not been satisfactorily accounted for in traditional phase noise models. This paper attempts to explain noise spikes/jump phenomenon by viewing it as phase change in the thermodynamic system(for example, from gas to liquid or magnetization of ferromagnet). Both are due to regeneration (positive feedback in oscillator as well as alignment of spin due to positive feedback in ferromagnet). The mathematical tool used is the partition function in thermodynamics, and the results mapped between thermodynamic system and relaxation oscillator. Theory is developed and formula derived to predict the magnitude of the jump, as a function of design parameter such as regeneration parameter or loop gain. Formulas show that noise increases sharply as regeneration parameter/loop gain approaches one, in much the same way when temperature approaches critical temperature in phase change. Simulations on circuits (Eldo) using CMOS as well as Monte Carlo simulations (Metropolis) on ferromagnet (Ising model) were performed and both show jump behaviour consistent with formula. Measurements on relaxation oscillators fabricated in 0.13um CMOS technology verify such behaviour, where the sharp increase in noise when regeneration parameter/loop gain is close to one, matches closely with the theoretical formula. Using the formula the designer can quantify the variation of noise spikes dependency on design parameters such as gm (device transconductance), R, I0, via their influence on regeneration parameter/loop gain.",1407.7877v2 2014-08-01,Lattice Gauge Theory for Condensed Matter Physics: Ferromagnetic Superconductivity as its Example,"Recent theoretical studies of various strongly-correlated systems in condensed matter physics reveal that the lattice gauge theory(LGT) developed in high-energy physics is quite a useful tool to understand physics of these systems. Knowledges of LGT are to become a necessary item even for condensed matter physicists. In the first part of this paper, we present a concise review of LGT for the reader who wants to understand its basics for the first time. For illustration, we choose the abelian Higgs model, a typical and quite useful LGT, which is the lattice verison of the Ginzburg-Landau model interacting with a U(1) gauge field (vector potential). In the second part, we present an account of the recent progress in the study of ferromagnetic superconductivity (SC) as an example of application of LGT to topics in condensed matter physics, . As the ferromagnetism (FM) and SC are competing orders with each other, large fluctuations are expected to take place and therefore nonperturbative methods are required for theoretical investigation. After we introduce a LGT describing the FMSC, we study its phase diagram and topological excitations (vortices of Cooper pairs) by Monte-Carlo simulations.",1408.0089v2 2014-08-26,"Proximity induced ferromagnetism, superconductivity, and finite-size effects on the surface and edge states of topological insulator nanostructures","Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3}$ are well known 3D-topological insulators. Films made of these materials exhibit metal-like surface states with a Dirac dispersion and possess high mobility. The high mobility metal-like surface states can serve as channel material for TI-based field effect transistors. While such a transistor offers superior terminal characteristics, they suffer from an inherent zero band gap problem. The absence of a band gap for the surface states prevents an easy turn-off mechanism. In this work, techniques that can be employed to easily open a band gap for the TI surface states is introduced. Two approaches are described: 1) Coating the surface states with a ferromagnet which has a controllable magnetization axis. The magnetization strength of the ferromagnet is incorporated as an exchange interaction term in the Hamiltonian. 2) An \textit{s}-wave superconductor, because of the proximity effect, when coupled to a 3D-TI opens a band gap on the surface. This TI-superconductor heterostructure is modeled using the Bogoliubov-de Gennes Hamiltonian. A comparison demonstrating the finite size effects on surface states of a 3D-TI and edge states of a CdTe/HgTe/CdTe-based 2D-TI is also presented. 3D-TI nanostructures can be reduced to dimensions as low as 10.0 $ \mathrm{nm} $ in contrast to 2D-TI structures which require a thickness of at least 100.0 $ \mathrm{nm} $. All calculations are performed using the continuum four-band k.p Hamiltonian.",1408.6274v2 2014-09-25,Spin glass like ground state and observation of exchange bias in Mn_{0.8}Fe_{0.2}NiGe alloy,"The ground-state magnetic properties of hexagonal equiatomic alloy of nominal composition Mn_{0.8}Fe_{0.2}NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes first order martensitic transition below 140 K with simultaneous development of long range ferromagnetic ordering from the high temperature paramagnetic phase. The undoped compound MnNiGe has an antiferromagnetic ground state and it shows martensitic like structural instability well above room temperature. Fe doping at the Mn site not only brings down the martensitic transition temperature, it also induces ferromagnetism in the sample. Our study brings out two important aspects regarding the sample, namley (i) the observation of exchange bias at low temperature, and (ii) spin glass like ground state which prevails below the martensitic and magnetic transition points. In addition to the observed usual relaxation behavior the spin glass state is confirmed by zero field cooled memory experiment, thereby indicating cooperative freezing of spin and/or spin clusters rather than uncorrelated dynamics of superparamagnetic like spin clusters. We believe that doping disorder can give rise to some islands of antiferromagnetic clusters in the otherwise ferromagnetic background which can produce interfacial frustration and exchange pinning responsible for spin glass and exchange bias effect. A comparison is made with doped rare-earth manganites where similar phase separation can lead to glassy ground state.",1409.7221v1 2014-10-09,Electrical Characteristics of Superconducting-Ferromagnetic Transistors,"We report experimental results on characteristics of SFIFS junctions and multi-terminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS junctions serve as injectors in the SFIFSIS devices which have transistor-like properties; for this reason we call them Superconducting-Ferromagnetic Transistors (SFTs). We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic (CVC) becomes linear. Furthermore, we investigated the DC and AC characteristics of SFTs of two types: ordinary devices with a single acceptor (SIS) junction, and devices with a double acceptor. In the first case, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For devices of the second type, we studied voltage amplification properties when the operating point was chosen in the sub-gap region of the acceptor CVC. By applying an AC signal (in the kHz range) while biasing the injector (SFIFS) junction with a constant DC current, we observed a voltage gain above 25 on the double acceptor. In the reverse transmission experiment, we applied DC current and an AC modulation to the acceptor junction and, within the accuracy of the experiment, observed no response on the injector junction, which implies an excellent input-output isolation in our SFIFSIS devices. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.",1410.2348v1 2014-10-12,"Spin-orbit coupling, strong correlation, and insulator-metal transitions: the J$_{\rm eff}$ =3\2 ferromagnetic Mott insulator Ba$_{2}$NaOsO$_{6}$","The double perovskite Ba$_{2}$NaOsO$_{6}$ (BNOO), an exotic example of a very high oxidation state (heptavalent) osmium $d^1$ compound and also uncommon by being a ferromagnetic Mott insulator without Jahn-Teller (JT) distortion, is modeled using the density functional theory (DFT) hybrid functional based exact exchange for correlated electrons (oeeHyb) method and including spin-orbit coupling (SOC). The experimentally observed narrow gap ferromagnetic insulating ground state is obtained, with easy axis along [110] in accord with experiment, providing support that this approach provides a realistic method for studying this system. The predicted spin density for [110] spin orientation is nearly cubic (unlike for other directions), providing an explanation for the absence of JT distortion. An orbital moment of -0.4$\mu_B$ strongly compensates the +0.5$\mu_B$ spin moment on Os, leaving a strongly compensated moment more in line with experiment. Remarkably, the net moment lies primarily on the oxygen ions. An insulator-metal transition by rotating the magnetization direction with an external field under moderate pressure is predicted as one consequence of strong SOC, and metallization under moderate pressure is predicted. Comparison is made with the isostructural, isovalent insulator Ba$_2$LiOsO$_6$ which however orders antiferromagnetically.",1410.3167v1 2014-10-28,Critical exponents and scaling invariance in the absence of a critical point,"The paramagnetic-to-ferromagnetic phase transition is believed to proceed through a critical point, at which power laws and scaling invariance, associated with the existence of one diverging characteristic length scale -- the so called correlation length -- appear. We indeed observe power laws and scaling behavior over extraordinarily many decades of the suitable scaling variables at the paramagnetic-to-ferromagnetic phase transition in ultrathin Fe films. However, we find that, when the putative critical point is approached, the singular behavior of thermodynamic quantities transforms into an analytic one: the critical point does not exist, it is replaced by a more complex phase involving domains of opposite magnetization, below as well as $above$ the putative critical temperature. All essential experimental results are reproduced by Monte-Carlo simulations in which, alongside the familiar exchange coupling, the competing dipole-dipole interaction is taken into account. Our results imply that a scaling behavior of macroscopic thermodynamic quantities is not necessarily a signature for an underlying second-order phase transition and that the paramagnetic-to-ferromagnetic phase transition proceeds, very likely, in the presence of at least two long spatial scales: the correlation length and the size of magnetic domains.",1410.7686v2 2014-11-15,Spin-transfer-torque through antiferromagnetic IrMn,"Spin-transfer-torque, a transfer of angular momentum between the electron spin and the local magnetic moments, is a promising and key mechanism to control ferromagnetic materials in modern spintronic devices . However, much less attention has been paid to the same effect in antiferromagnets. For the sake of investigating how the spin current interacts with the magnetic moments in antiferromagnets, we perform spin-torque ferromagnetic resonance measurements on Co20Fe60B20 4nm/Ir25Mn75 tIrMn nm/Pt 4 nm multilayers under a spin Hall effect of Pt. The effective magnetic damping in Co20Fe60B20 is modified by the spin current injected from the Pt layer via the IrMn layer. The results indicate that the spin current interacts with IrMn magnetic moments and exerts the anti-damping torque on the magnetic moments of Co20Fe60B20 through the IrMn. It is also found that the reduction of the exchange bias in the IrMn/Pt interface degrades the anti-damping torque exerted on the Co20Fe60B20 layer, suggesting the transmission of the spin torque becomes less efficient as the interface exchange coupling degrades. Our work infers that the magnetic moments in IrMn can be manipulated by spin torque similarly to the one in a ferromagnetic layer.",1411.4100v4 2014-12-01,Quantifying magnetic anisotropy dispersion: Theoretical and experimental study of the magnetic properties of anisotropic FeCuNbSiB ferromagnetic films,"The Stoner-Wohlfarth model is a traditional and efficient tool to calculate magnetization curves and it can provides further insights on the fundamental physics associated to the magnetic properties and magnetization dynamics. Here, we perform a theoretical and experimental investigation of the quasi-static magnetic properties of anisotropic systems. We consider a theoretical approach which corresponds to a modified version of the Stoner-Wohlfarth model to describe anisotropic systems and a distribution function to express the magnetic anistropy dispersion. We propose a procedure to calculate the magnetic properties for the anisotropic case of the SW model from experimental results of the quadrature of magnetization curves, thus quantifying the magnetic anisotropy dispersion. To test the robustness of the approach, we apply the theoretical model to describe the quasi-static magnetic properties of amorphous FeCuNbSiB ferromagnetic films. We perform calculations and directly compare theoretical results with longitudinal and transverse magnetization curves measured for the films. Thus, our results provide experimental evidence to confirm the validity of the theoretical approach to describe the magnetic properties of anisotropic amorphous ferromagnetic films, revealed by the excellent agreement between numerical calculation and experimental results.",1412.0563v1 2014-12-01,Experimental and Numerical Understanding of Localized Spin Wave Mode Behavior in Broadly Tunable Spatially Complex Magnetic Configurations,"Spin wave modes confined in a ferromagnetic film by the spatially inhomogeneous magnetic field generated by a scanned micromagnetic tip of a ferromagnetic resonance force microscope (FMRFM) enable microscopic imaging of the internal fields and spin dynamics in nanoscale magnetic devices. Here we report a detailed study of spin wave modes in a thin ferromagnetic film localized by magnetic field configurations frequently encountered in FMRFM experiments, including geometries in which the probe magnetic moment is both parallel and antiparallel to the applied uniform magnetic field. We demonstrate that characteristics of the localized modes, such as resonance field and confinement radius, can be broadly tuned by controlling the orientation of the applied field relative to the film plane. Micromagnetic simulations accurately reproduce our FMRFM spectra allowing quantitative understanding of the localized modes. Our results reveal a general method of generating tightly confined spin wave modes in various geometries with excellent spatial resolution that significantly facilitates the broad application of FMRFM. This paves the way to imaging of magnetic properties and spin wave dynamics in a variety of contexts for uncovering new physics of nanoscale spin excitations.",1412.0726v1 2014-12-07,Stoner ferromagnetism of a strongly interacting Fermi gas in the quasirepulsive regime,"Recent advances in rapidly quenched ultracold atomic Fermi gases near a Feshbach resonance have brought about a number of interesting problems, in the context of observing the long-sought Stoner ferromagnetic phase transition. The possibility of experimentally obtaining a ""quasirepulsive"" regime in the upper branch of the energy spectrum due to the rapid quench is currently being debated, and the Stoner transition has mainly been investigated theoretically by using perturbation theory or at high polarization, due to the limited theoretical approaches in the strongly repulsive regime. In this work, we present a nonperturbative theoretical approach to the quasirepulsive upper branch of a Fermi gas near a broad Feshbach resonance, and we determine the finite-temperature phase diagram for the Stoner instability. Our results agree well with the known quantum Monte-Carlo simulations at zero temperature, and we recover the known virial expansion prediction at high temperature for arbitrary interaction strengths. At resonance, we find that the Stoner transition temperature becomes of the order of the Fermi temperature, around which the molecule formation rate becomes vanishingly small. This suggests a feasible way to observe Stoner ferromagnetism in the nondegenerate temperature regime.",1412.2412v3 2014-12-22,Modeling Ferro- and Antiferromagnetic Interactions in Metal-Organic Coordination Networks,"Magnetization curves of two rectangular metal-organic coordination networks formed by the organic ligand TCNQ (7,7,8,8-tetracyanoquinodimethane) and two different (Mn and Ni) 3d transition metal atoms [M(3d)] show marked differences that are explained using first principles density functional theory and model calculations. We find that the existence of a weakly dispersive hybrid band with M(3d) and TCNQ character crossing the Fermi level is determinant for the appearance of ferromagnetic coupling between metal centers, as it is the case of the metallic system Ni-TCNQ but not of the insulating system Mn-TCNQ. The spin magnetic moment localized at the Ni atoms induces a significant spin polarization in the organic molecule; the corresponding spin density being delocalized along the whole system. The exchange interaction between localized spins at Ni centers and the itinerant spin density is ferromagnetic. Based on two different model Hamiltonians, we estimate the strength of exchange couplings between magnetic atoms for both Ni- and Mn-TCNQ networks that results in weak ferromagnetic and very weak antiferromagnetic correlations for Ni- and Mn-TCNQ networks, respectively.",1412.6898v1 2015-01-28,Spin-orbit coupled repulsive Fermi atoms in a one-dimensional optical lattice,"Motivated by recent experimental development, we investigate spin-orbit coupled repulsive Fermi atoms in a one-dimensional optical lattice. Using the density-matrix renormalization group method, we calculate momentum distribution function, gap, and spin-correlation function to reveal rich ground-state properties. We find that spin-orbit coupling (SOC) can generate unconventional momentum distribution, which depends crucially on the filling. We call the corresponding phase with zero gap the SOC-induced metallic phase. We also show that SOC can drive the system from the antiferromagnetic to ferromagnetic Mott insulators with spin rotating. As a result, a second-order quantum phase transition between the spin-rotating ferromagnetic Mott insulator and the SOC-induced metallic phase is predicted at the strong SOC. Here the spin rotating means that the spin orientations of the nearest-neighbor sites are not parallel or antiparallel, i.e., they have an intersection angle $\theta \in (0,\pi )$. Finally, we show that the momentum $k_{\mathrm{peak}}$, at which peak of the spin-structure factor appears, can also be affected dramatically by SOC. The analytical expression of this momentum with respect to the SOC strength is also derived. It suggests that the predicted spin-rotating ferromagnetic ($k_{\mathrm{peak}% }<\pi /2$) and antiferromagnetic ($\pi /2 Tc and T < Tc show three main results- (a) the two step magnetic reversal above Tc converts into a four step reversal below Tc in HSs with d-YBCO >= 30 nm, (b) the magnetic field corresponding to the additional two switching steps and their magnitude show characteristic evolution with T and d-YBCO and (c) the HS with d-YBCO = 20 nm shows radically different behaviour, where the two step magnetic reversal above Tc continues to persist below Tc and converts into a single step reversal at T << Tc. The first two results indicate magnetostatic coupling between the magnetic domains and the vortices across the two F/S interfaces resulting in reversal dynamics different from that deep within the LCMO layers. Whereas, the result c reveals indirect exchange coupling between LCMO layers through the superconducting YBCO layer, which is a clear experimental evidence of coexistence of ferromagnetism and superconductivity in nm scale F/S/F HSs expected theoretically by C.A.R. Sa de Melo (Physica C 387, 17-25 (2003)).",1609.04519v1 2016-09-23,Quantum Anomalous Hall and Half-metallic Phases in Ferromagnetic (111) Bilayers of 4d and 5d Transition Metal Perovskites,"Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices (ABO3)2/(AB'O3)10 of 4d and 5d transition metal perovskite (B = Ru, Rh, Ag, Re, Os, Ir, Au; AB'O3 = LaAlO3, SrTiO3), we demonstrate that due to quantum confinement, bilayers (LaBO3)2 (B = Ru, Re, Os) and (SrBO3)2 (B = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to room temperature. In particular, bilayer (LaOsO3)2 is an exotic spin-polarized quantum anomalous Hall insulator, while the other ferromagnetic bilayers are metallic with large Hall conductances comparable to the conductance quantum. Furthermore, bilayers (LaRuO3)2 and (SrRhO3)2 are half-metallic, while bilayer (SrIrO3)2 exhibits peculiar colossal magnetic anisotropy. Our findings thus show that 4d and 5d metal perovskite (111) bilayers are a class of quasi-two-dimensional materials for exploring exotic quantum phases and also for advanced applications such as low-power nanoelectronics and oxide spintronics.",1609.07383v1 2016-09-26,Quantum anomalous Hall effect in ferromagnetic transition metal halides,"The quantum anomalous Hall (QAH) effect is a novel topological spintronic phenomenon arising from inherent magnetization and spin-orbit coupling. Various theoretical and experimental efforts have been devoted in search of robust intrinsic QAH insulators. However, up to now, it has only been observed in Cr or V doped (Bi,Sb)2Te3 film in experiments with very low working temperature. Based on the successful synthesis of transition metal halides, we use first-principles calculations to predict that RuI3 monolayer is an intrinsic ferromagnetic QAH insulator with a topologically nontrivial global band gap of 11 meV. This topologically nontrivial band gap at the Fermi level is due to its crystal symmetry, thus the QAH effect is robust. Its Curie temperature, estimated to be ~360 K using Monte-Carlo simulation, is above room temperature and higher than most of two-dimensional ferromagnetic thin films. We also discuss the manipulation of its exchange energy and nontrivial band gap by applying in-plane strain. Our work adds a new experimentally feasible member to the QAH insulator family, which is expected to have broad applications in nanoelectronics and spintronics.",1609.08115v2 2016-10-01,Magnetic MoS2 Interface Monolayer on CdS Nanowire by Cation Exchange,"MoS2 atomic layers have recently attracted much interest because of their two-dimensional structure as well as tunable optical, electrical, and mechanical properties for next generation electronic and electro-optical devices. Here we have achieved facile fabrication of MoS2 thin films on CdS nanowires by cation exchange in solution at room temperature and importantly observed their extraordinary magnetic properties. We establish the atomic structure of the MoS2/CdS heterostructure by taking atomic images of the MoS2/CdS interface as well as performing first-principles density functional geometry optimizations and STEM-ADF image simulations. Furthermore, our first principles density functional calculations for the MoS2/CdS heterostructure reveal that the magnetism in the MoS2/CdS heterostructure stems from the ferromagnetic MoS2 monolayer next to the MoS2/CdS interface. The ferromagnetism is attributed to the partial occupation of the Mo dx2-y2/dxy conduction band in the interfacial MoS2 monolayer caused by the mixed covalent-ionic bonding among the MoS2 and CdS monolayers near the MoS2/CdS interface. The present findings of the ferromagnetic MoS2 monolayer with large spin polarization at the MoS2-semiconductor interface suggest a new route for fabrication of the transition metal dichalcogenide-based magnetic semiconductor multilayers for applications in spintronic devices.",1610.00096v1 2016-10-03,Spin-orbital liquids and insulator-metal transitions on the pyrochlore lattice,"The two orbital Hubbard model, with the electrons additionally coupled to a complex magnetic background, arises in the pyrochlore molybdates. The background involves local moments Hund's coupled to the electrons, driving double exchange ferromagnetism, and antiferromagnetic (AF) tendency arising from competing superexchange. The key scales include the Hubbard repulsion and the superexchange, both of which can be tuned in these materials. They control the phase transition from a ferromagnetic metal to a spin glass metal and then to a spin glass (Mott) insulator. We provide a comprehensive description of the ground state of this model using an unrestricted Hartree-Fock scheme implemented via a simulated annealing procedure and establish the metal-insulator transition line for varying Hubbard interaction and superexchange. The electrons see an effective disorder, due to orbital frustration, already in the ferromagnetic phase. The disorder is further enhanced by antiferromagnetic coupling and the resulting magnetic disorder. As a result, increasing AF coupling shifts the metal-insulator transition to lower Hubbard interaction and gives it an additional ""Anderson"" character. We provide detailed results on the magnetic and orbital correlations, the density of states, and the optical conductivity.",1610.00695v3 2016-10-07,Skyrmions and multi-sublattice helical states in a frustrated chiral magnet,"We investigate the existence and stability of skyrmions in a frustrated chiral ferromagnet by considering the competition between ferromagnetic (FM) nearest-neighbour (NN) interaction ($J_1$) and antiferromagnetic (AFM) next-nearest-neighbour (NNN) interaction ($J_2$). Contrary to the general wisdom that long-range ferromagnetic order is not energy preferable under frustration, the skyrmion lattice not only exists but is even stable for a large field range when $J_2 \leq J_1$ compared with frustration-free systems. We defend that the enlargement of stability window of skyrmions is a consequence of the reduced effective exchange interaction caused by the frustration. A multi-sublattice helical state is found below the skyrmion phase, which results from the competition between AFM coupling that favors a two-sublattice N\'{e}el state and the chiral interaction that prefers a helix. As a byproduct, the hysteresis loop of the frustrated chiral system shrinks as the magnetization goes to zero and then opens up again, known as wasp-waist hysteresis loop. The critical field that separates the narrow and wide part of the wasp-waist loop depends exponentially on the strength of NNN coupling. By measuring the critical field, it is possible to determine the strength of NNN coupling.",1610.02172v2 2016-10-17,Triplet Cooper pairs induced in diffusive s-wave superconductors interfaced with strongly spin-polarized magnetic insulators or half-metallic ferromagnets,"Interfacing superconductors with strongly spin-polarized magnetic materials opens the possibility to discover new spintronic devices in which spin-triplet Cooper pairs play a key role. Motivated by the recent derivation of spin-polarized quasiclassical boundary conditions capable of describing such a scenario in the diffusive limit, we consider the emergent physics in hybrid structures comprised of a conventional s-wave superconductor (e.g. Nb, Al) and either strongly spin-polarized ferromagnetic insulators (e.g. EuO, GdN) or halfmetallic ferromagnets (e.g. CrO2, LCMO). In contrast to most previous works, we focus on how the superconductor itself is influenced by the proximity effect, and how the generated triplet Cooper pairs manifest themselves in the self-consistently computed density of states (DOS) and the superconducting critical temperature Tc. We provide a comprehensive treatment of how the superconductor and its properties are affected by the triplet pairs, demonstrating that our theory can reproduce the recent observation of an unusually large zero-energy peak in a superconductor interfaced with a half-metal, which even exceeds the normal-state DOS. We also discuss the recent observation of a large superconducting spin-valve effect with a Tc change ~1K in superconductor/half-metal structures, in which case our results indicate that the experiment cannot be explained fully by a long-ranged triplet proximity effect.",1610.05300v2 2016-10-26,Atomistic origin of exchange anisotropy in $γ$-IrMn$_3$-CoFe bilayers,"The exchange interaction determines the ferromagnetic (FM) or antiferromagnetic (AFM) ordering of atomic spins. When ferromagnets and antiferromagnets are coupled together, they often exhibit the exchange bias effect, a unidirectional interface exchange field causing a shift of the magnetic hysteresis loop. The effective magnitude of this interface exchange field is at most a few percent of the bulk exchange, arising from pinned interfacial spins in the antiferromagnet. The pinned spins are known to comprise a small fraction of the total number of interface spins, yet their exact nature and physical origin has so far been elusive. Here we show that in the technologically important $\gamma - IrMn_3/CoFe$ structure the pinned interface spins are in fact delocalised over the whole interface layer. The pinned spins arise from the small imbalance of the number of spins in each magnetic sublattice in the antiferromagnet due to the natural atomic disorder. These pinned spins are strongly coupled to the bulk antiferromagnet explaining their remarkable stability. Moreover, we find that the ferromagnet strongly distorts the interface spin structure of the antiferromagnet, causing a large reversible interface magnetisation that does not contribute to exchange bias. The unexpected delocalised nature of the pinned interface spins explains both their small number and their stability, uncovering the mysterious microscopic origin of the exchange bias effect.",1610.08236v3 2016-10-27,Vortex-antivortex proliferation from an obstacle in thin film ferromagnets,"Magnetization dynamics in thin film ferromagnets can be studied using a dispersive hydrodynamic formulation. The equations describing the magnetodynamics map to a compressible fluid with broken Galilean invariance parametrized by the longitudinal spin density and a magnetic analog of the fluid velocity that define spin-density waves. A direct consequence of these equations is the determination of a magnetic Mach number. Micromagnetic simulations reveal nucleation of nonlinear structures from an impenetrable object realized by an applied magnetic field spot or a defect. In this work, micromagnetic simulations demonstrate vortex-antivortex pair nucleation from an obstacle. Their interaction establishes either ordered or irregular vortex-antivortex complexes. Furthermore, when the magnetic Mach number exceeds unity (supersonic flow), a Mach cone and periodic wavefronts are observed, which can be well-described by solutions of the steady, linearized equations. These results are reminiscent of theoretical and experimental observations in Bose-Einstein condensates, and further supports the analogy between the magnetodynamics of a thin film ferromagnet and compressible fluids. The nucleation of nonlinear structures and vortex-antivortex complexes using this approach enables the study of their interactions and effects on the stability of spin-density waves.",1610.08940v3 2017-02-03,Site dilution in SrRuO$_3$: Effects on structural and magnetic properties,"We have investigated the effect of site dilution with substitution of nonmagnetic element in SrRu$_{1-x}$Ti$_x$O$_3$ ($x$ $\leq$ 0.7). The nature of ferromagnetic state in SrRuO$_3$ is believed to be of itinerant type with transition temperature $T_c$ $\sim$ 162 K. Crystallographically, SrRuO$_3$ has a distorted orthorhombic structure. Substitution of Ti$^{+4}$ (3$d^0$) for Ru$^{+4}$ (4$d^4$), however, does not introduce significant structural modification due to their matching ionic radii. This substitution, on the other hand, is expected to tune the electronic correlation effect and the $d$ electron density in system. With Ti substitution, we find that magnetic moment and Curie temperature decreases but $T_c$ remains unchanged which has been attributed to opposite tuning of electron correlation effect and density of states within framework of itinerant ferromagnetism. The estimated critical exponent ($\beta$) related to magnetization implies a mean-field type of magnetic nature in SrRuO$_3$. The value of $\beta$ further increases with $x$ which is understood from the dilution effect of magnetic lattice. The system evolves to exhibit Griffiths phase like behavior above $T_c$ which is usually realized in diluted ferromagnet following local moment model of magnetism. Our detail analysis of magnetization data indicates that magnetic state in SrRuO$_3$ has contribution from both itinerant and local moment model of magnetism.",1702.01013v1 2017-02-22,Effect of Sr doping on the magnetic exchange interactions in manganites,"Strontium doping transforms manganites of type La(1-x)Sr(x)MnO(3) from an insulating antiferromagnet (x=0) to a metallic ferromagnet (x>0.16) due to the induced charge carriers (holes). Neutron scattering experiments were employed to investigate the effect of Sr doping on a tailor-made compound of composition La(0.7)Sr(0.3)Mn(0.1)Ti(0.3)Ga(0.6)O(3). By the simultaneous doping with Sr2+ and Ti4+ ions the compound remains in the insulating state, so that the magnetic interactions for large Sr doping can be studied in the absence of charge carriers. At TC=215 K there is a first-order reconstructive phase transition from the trigonal R-3c structure to the orthorhombic Pnma structure via an intermediate virtual configuration described by the common monoclinic subgroup P21/c. The magnetic excitations associated with Mn3+ dimers give evidence for two different nearest-neighbor ferromagnetic exchange interactions, in contrast to the undoped compound LaMnyA(1-y)O(3) where both ferromagnetic and antiferromagnetic interactions are present. The doping induced changes of the exchange coupling originates from different Mn-O-Mn bond angles determined by neutron diffraction. The large fourth-nearest-neighbor interaction found for metallic manganites is absent in the insulating state. We argue that the Ruderman-Kittel-Kasuya-Yosida interaction reasonably accounts for all the exchange couplings derived from the spin-wave dispersion in metallic manganites.",1702.06802v1 2017-03-06,Absence of magnetic long range order in Y$_{2}$CrSbO$_{7}$: bond-disorder induced magnetic frustration in a ferromagnetic pyrochlore,"The consequences of nonmagnetic-ion dilution for the pyrochlore family Y$_{2}$($M_{1-x}N_{x}$)$_{2}$O$_{7}$ ($M$ = magnetic ion, $N$ = nonmagnetic ion) have been investigated. As a first step, we experimentally examine the magnetic properties of Y$_{2}$CrSbO$_{7}$ ($x$ = 0.5), in which the magnetic sites (Cr$^{3+}$) are percolative. Although the effective Cr-Cr spin exchange is ferromagnetic, as evidenced by a positive Curie-Weiss temperature, $\Theta_\mathrm{{CW}}$ = 20.1(6) K, our high-resolution neutron powder diffraction measurements detect no sign of magnetic long range order down to 2 K. In order to understand our observations, we performed numerical simulations to study the bond-disorder introduced by the ionic size mismatch between $M$ and $N$. Based on these simulations, bond-disorder ($x_{b}$ $\simeq$ 0.23) percolates well ahead of site-disorder ($x_{s}$ $\simeq$ 0.61). This model successfully reproduces the critical region (0.2 < $x$ < 0.25) for the N\'eel to spin glass phase transition in Zn(Cr$_{1-x}$Ga$_{x}$)$_{2}$O$_{4}$, where the Cr/Ga-sublattice forms the same corner-sharing tetrahedral network as the $M/N$-sublattice in Y$_{2}$($M_{1-x}N_{x}$)$_{2}$O$_{7}$, and the rapid drop in magnetically ordered moment in the N\'eel phase [Lee $et$ $al$, Phys. Rev. B 77, 014405 (2008)]. Our study stresses the nonnegligible role of bond-disorder on magnetic frustration, even in ferromagnets.",1703.02097v1 2017-03-07,Ferromagnetic transition in a simple variant of the Ising model on multiplex networks,"Multiplex networks consist of a fixed set of nodes connected by several sets of edges which are generated separately and correspond to different networks (""layers""). Here, a simple variant of the Ising model on multiplex networks with two layers is considered, with spins located in the nodes and edges corresponding to ferromagnetic interactions between them. Critical temperatures for the ferromagnetic transition are evaluated for the layers in the form of random Erd\""os-R\'enyi graphs or heterogeneous scale-free networks using the mean-field approximation and the replica method, from the replica symmetric solution. Both methods require the use of different ""partial"" magnetizations, associated with different layers of the multiplex network, and yield qualitatively similar results. If the layers are strongly heterogeneous the critical temperature differs noticeably from that for the Ising model on a network being a superposition of the two layers, evaluated in the mean-field approximation neglecting the effect of the underlying multiplex structure on the correlations between the degrees of nodes. The critical temperature evaluated from the replica symmetric solution depends sensitively on the correlations between the degrees of nodes in different layers and shows satisfactory quantitative agreement with that obtained from Monte Carlo simulations. The critical behavior of the magnetization for the model with strongly heterogeneous layers can depend on the distributions of the degrees of nodes and is then determined by the properties of the most heterogeneous layer.",1703.02431v2 2017-03-09,Spin-glass transition in the Ising model on multiplex networks,"Multiplex networks consist of a fixed set of nodes connected by several sets of edges which are generated separately and correspond to different networks (""layers""). Here, the Ising model on multiplex networks with two layers is considered, with spins located in the nodes and edges corresponding to ferromagnetic or antiferromagnetic interactions between them. Critical temperatures for the spin glass and ferromagnetic transitions are evaluated for the layers in the form of random Erd\""os-R\'enyi graphs or heterogeneous scale-free networks using the replica method, from the replica symmetric solution. For the Ising model on multiplex networks with scale-free layers it is shown that the critical temperature is finite if the distributions of the degrees of nodes within both layers have a finite second moment, and that depending on the model parameters the transition can be to the ferromagnetic or spin glass phase. It is also shown that the correlation between the degrees of nodes within different layers significantly influences the critical temperatures for both transitions and thus the phase diagram. The scaling behavior for the spin glass order parameter is determined by a semi-analytic procedure and it is shown that for the Ising model on multiplex networks with scale-free layers the scaling exponent can depend on the distributions of the degrees of nodes within layers. The analytic results are partly confirmed by Monte Carlo simulations using the parallel tempering algorithm.",1703.03270v1 2017-03-11,Spin-wave dynamics in Permalloy/Cobalt magnonic crystals in the presence of a non-magnetic spacer,"In this paper, we theoretically study the influence of a non-magnetic spacer between ferromagnetic dots and ferromagnetic matrix on the frequency dispersion of the spin wave excitations in two-dimensional bi-component magnonic crystals. By means of the dynamical matrix method we investigate structures inhomogeneous across the thickness represented by square arrays of Cobalt or Permalloy dots in a Permalloy matrix. We show that the introduction of a non-magnetic spacer significantly modifies the total internal magnetic field especially at the edges of the grooves and dots. This permits the manipulation of the magnonic band structure of spin waves localized either at the edges of the dots or in matrix material at the edges of grooves. According to the micromagnetic simulations two types of end modes were found. The corresponding frequencies are significantly influenced by the end modes localization region. We also show that, with the use of a single ferromagnetic material, it is possible to design a magnonic crystal preserving properties of bi-component magnonic crystals and magnonic antidot lattices. Finally, the influence of the non-magnetic spacers on the technologically relevant parameters like group velocity and magnonic band width are discussed.",1703.03978v1 2017-03-28,Co-appearance of superconductivity and ferromagnetism in a Ca$_2$RuO$_4$ nanofilm crystal,"By tuning the physical and chemical pressures of layered perovskite materials we can realize the quantum states of both superconductors and insulators. By reducing the thickness of a layered crystal to a nanometer level, a nanofilm crystal can provide novel quantum states that have not previously been found in bulk crystals. Here we report the realization of high-temperature superconductivity in Ca$_2$RuO$_4$ nanofilm single crystals. Ca$_2$RuO$_4$ thin film with the highest transition temperature $T_c$ (midpoint) of 64~K exhibits zero resistance in electric transport measurements. The superconducting critical current exhibited a logarithmic dependence on temperature and was enhanced by an external magnetic field. Magnetic measurements revealed a ferromagnetic transition at 180~K and diamagnetic magnetization due to superconductivity. Our results suggest the co-appearance of superconductivity and ferromagnetism in Ca$_2$RuO$_4$ nanofilm crystals. We also found that the induced bias current and the tuned film thickness caused a superconductor-insulator transition. The fabrication of micro-nanocrystals made of layered material enables us to discuss rich superconducting phenomena in ruthenates.",1703.09459v2 2017-08-31,Search for Origin of Room Temperature Ferromagnetism Properties in Ni doped ZnO Nanostructure,"The origin of room temperature (RT) ferromagnetism (FM) in Ni doped ZnO samples are systematically investigated through physical, optical, and magnetic properties of nanostructure, prepared by simple low-temperature wet chemical method. Reitveld refinement of X-ray diffraction pattern displays an increase in lattice parameters with strain relaxation and contraction in Zn/O occupancy ratio by means of Ni-doping. Similarly scanning electron microscope demonstrates modification in the morphology from nanorods to nanoflakes with Ni doping, suggests incorporation of Ni ions in ZnO. More interestingly, XANES (X-ray absorption near edge spectroscopy) measurements confirm that Ni is being incorporated in ZnO as Ni2+. EXAFS (Extended X-ray Absorption Fine Structure) analysis reveals that structural disorders near the Zn sites in the ZnO samples upsurges with increasing Ni concentration. Raman spectroscopy exhibits additional defect driven vibrational mode at 275 cm-1, appeared with Ni-doped sample only and the shift with broadening in 580 cm-1 peak, which manifests the presence of the oxygen vacancy (VO) related defects. Moreover, in photoluminescence (PL) spectra we observed peak appears at 524 nm, indicates the presence of singly ionized VO+, which may activate bound magnetic polarons (BMPs) in dilute magnetic semiconductors (DMSs). Magnetization measurements indicate weak ferromagnetism at RT, which rises with increasing Ni consolidation. It is therefore proposed that effect of the Ni-ions as well as the inherent exchange interactions rising from VO+ assist to produce BMPs, which are accountable for the RT-FM in Zn1-xNixO (00$, our data suggest that the percolation transition is universal, irrespective of whether the ground state exhibits ferromagnetic or spin-glass order, and is in the universality class of standard percolation. This shows that correlations in the bond occupancy of the Fortuin-Kasteleyn clusters are irrelevant, except for $p=0$ where the clusters are tied to Ising correlations so the percolation transition is in the Ising universality class.",1905.04220v1 2019-05-27,Unidirectional planar Hall voltages induced by surface acoustic waves in ferromagnetic thin films,"The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on LiNbO$_3$ substrates are patterned into Hall-bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.",1905.11224v1 2019-05-31,Magnetic Anisotropy and Low Field Magnetic Phase Diagram of Quasi Two-Dimensional Ferromagnet Cr$_2$Ge$_2$Te$_6$,"In this work we present a comprehensive investigation on magnetic and thermodynamic properties of the two-dimensional layered honeycomb system Cr$_2$Ge$_2$Te$_6$. Using magnetization and specific heat measurements under magnetic field applied along two crystallographic directions we obtain the magnetic phase diagram for both directions. Cr$_2$Ge$_2$Te$_6$ is a ferromagnet with a Curie temperature $T_C=65$ K and exhibits an easy magnetization axis perpendicular to the structural layers in the $ab$-plane. Under magnetic fields applied parallel to the hard plane $ab$ below the magnetic saturation, a downturn with an onset temperature $T\textrm{*}$ is observed in the temperature dependent magnetization curve. $T\textrm{*}$ shows a monotonous shift towards lower temperatures with increasing field. The nature of this anisotropic and specific behavior for fields in the hard plane is discussed as an interplay among field, temperature and effective magnetic anisotropy. Similarities to structurally related compounds such as CrX$_3$ (X = Br, I) hint towards a universality of this behavior in ferromagnetic quasi two-dimensional honeycomb materials.",1905.13609v2 2020-07-06,Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques,"Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayer structures. In the course of a measurement, other resonant processes such as spin pumping (SP) and heating can cause spin current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects (NE). In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOTs. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in Pt (6 nm)/CoFeB samples the contribution from the artifacts is small compared to the SOT rectification signal for CoFeB layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of CoFeB thickness that we suggest may be due to a competition between a resonant heating effect and the SP/ISHE contribution.",2007.02850v1 2020-07-21,Interplay of itinerant magnetism and reentrant spin-glass behavior in Fe$_{x}$Cr$_{1-x}$,"When suppressing the itinerant antiferromagnetism in chromium by doping with the isostructual itinerant ferromagnet iron, a dome of spin-glass behavior emerges around a putative quantum critical point at an iron concentration $x \approx 0.15$. Here, we report a comprehensive investigation of polycrystalline samples of Fe$_{x}$Cr$_{1-x}$ in the range $0.05 \leq x \leq 0.30$ using x-ray powder diffraction, magnetization, ac susceptibility, and neutron depolarization measurements, complemented by specific heat and electrical resistivity data for $x = 0.15$. Besides antiferromagnetic ($x < 0.15$) and ferromagnetic regimes ($0.15 \leq x$), we identify a dome of reentrant spin-glass behavior at low temperatures for $0.10 \leq x \leq 0.25$ that is preceded by a precursor phenomenon. Neutron depolarization indicates an increase of the size of ferromagnetic clusters with increasing $x$ and the Mydosh parameter $\phi$, inferred from the ac susceptibility, implies a crossover from cluster-glass to superparamagnetic behavior. Taken together, these findings consistently identify Fe$_{x}$Cr$_{1-x}$ as an itinerant-electron system that permits to study the evolution of spin-glass behavior of gradually varying character in unchanged crystalline environment.",2007.10644v1 2020-07-30,Coexistence of distinct skyrmion phases observed in hybrid ferromagnetic/ferrimagnetic multilayers,"Materials hosting magnetic skyrmions at room temperature could enable new computing architectures as well as compact and energetically efficient magnetic storage such as racetrack memories. In a racetrack device, information is coded by the presence/absence of magnetic skyrmions forming a chain that is moved through the device. The skyrmion Hall effect that would eventually lead to an annihilation of the skyrmions at the edges of the racetrack can be suppressed for example by anti-ferromagnetically-coupled skyrmions. However, avoiding modifications of the inter-skyrmion distances in the racetrack remains challenging. As a solution to this issue, a chain of bits could also be encoded by two different solitons such as a skyrmion and a chiral bobber. The major limitation of this approach is that it has solely been realized in B20-type single crystalline material systems that support skyrmions only at low temperatures, thus hindering the efficacy for future applications. Here we demonstrate that a hybrid ferro/ferri/ferromagnetic multilayer system can host two distinct skyrmion phases at room temperature. By matching quantitative magnetic force microscopy data with micromagnetic simulations, we reveal that the two phases represent tubular skyrmions and partial skyrmions (similar to skyrmion bobbers). Furthermore, the tubular skyrmion can be converted into a partial skyrmion. Such multilayer systems may thus serve as a platform for designing skyrmion memory applications using distinct types of skyrmions and potentially for storing information using the vertical dimension in a thin film device.",2007.15427v1 2010-05-29,"Structural, Magnetic and Electron Transport Properties of Ordered-Disordered Perovskite Cobaltites","Rare earth perovskite cobaltites are increasingly recognized as materials of importance due to rich physics and chemistry in their ordered-disordered structure for the same composition. Apart from colossal magnetoresistance effect, like manganites, the different forms of cobaltites exhibit interesting phenomena including spin, charge and orbital ordering, electronic phase separation, insulator-metal transition, large thermoelectric power at low temperature. Moreover, the cobaltites which display colossal magnetoresistance effect could be used as read heads in magnetic data storage and also in other applications depending upon their particular properties. The A-site ordereddisordered cobaltites exhibit ferromagnetism and metal-insulator transitions as well as other properties depending on the composition, size of A-site cations and various external factors such as pressure, temperature, magnetic field etc. Ordered cobaltites, having a 112-type layered structure, are also reported to have an effectively stronger electron coupling due to layered A-site cationic ordering. Most importantly for the present article we focus on La-Ba-Co-O based ordered-disordered perovskite phases, which exhibit interesting magnetic and electron transport properties with ferromagnetic transition, TC ~ 177K, and it being the first member of lanthanide series. Zener double exchange mechanism considered to be crucial for understanding basic physics of the ferromagneticmetallic phase, yet does not explain clearly the insulating-type phase. In terms of electron transport the ferromagnetic-metallic or insulating/semiconducting states have been discussed in the present article with different types of hopping model.",1005.5426v1 2013-08-21,Intense low-energy ferromagnetic fluctuations in the antiferromagnetic heavy-fermion metal CeB6,"Heavy-fermion metals exhibit a plethora of low-temperature ordering phenomena, among them the so-called hidden-order phases that in contrast to conventional magnetic order are invisible to standard neutron diffraction. One of the oldest and structurally simplest hidden-order compounds, CeB6, became famous for an elusive phase that was attributed to the antiferroquadrupolar ordering of cerium-4f moments. In its ground state, CeB6 also develops a more usual antiferromagnetic (AFM) order. Hence, its essential low-temperature physics was always considered to be solely governed by AFM interactions between the dipolar and multipolar Ce moments. Here we overturn this established perspective by uncovering an intense ferromagnetic (FM) low-energy collective mode that dominates the magnetic excitation spectrum of CeB6. Our inelastic neutron-scattering data reveal that the intensity of this FM excitation by far exceeds that of conventional spin-wave magnons emanating from the AFM wave vectors, thus placing CeB6 much closer to a FM instability than could be anticipated. This propensity of CeB6 to ferromagnetism may account for much of its unexplained behavior, such as the existence of a pronounced electron spin resonance, and should lead to a substantial revision of existing theories that have so far largely neglected the role of FM interactions.",1308.4491v1 2014-03-18,The XYZ chain with Dzyaloshinsky-Moriya interactions: from spin-orbit-coupled lattice bosons to interacting Kitaev chains,"Using the density-matrix renormalization-group algorithm (DMRG) and a finite-size scaling analysis, we study the properties of the one-dimensional completely-anisotropic spin-1/2 XYZ model with Dzyaloshinsky-Moriya (DM) interactions. The model shows a rich phase diagram: depending on the value of the coupling constants, the system can display different kinds of ferromagnetic order and Luttinger-liquid behavior. Transitions from ferromagnetic to Luttinger-liquid phases are first order. We thoroughly discuss the transition between different ferromagnetic phases, which, in the absence of DM interactions, belongs to the XX universality class. We provide evidence that the DM exchange term turns out to split this critical line into two separated Ising-like transitions and that in between a disordered phase may appear. Our study sheds light on the general problem of strongly-interacting spin-orbit-coupled bosonic gases trapped in an optical lattice and can be used to characterize the topological properties of superconducting nanowires in the presence of an imposed supercurrent and of interactions.",1403.4568v3 2016-11-21,Purely Antiferromagnetic Magnetoelectric Random Access Memory,"Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50 fold reduction of the writing threshold compared to ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes of these novel systems, we construct a comprehensive model of the magnetoelectric selection mechanism in thin films of magnetoelectric antiferromagnets. We identify that growth induced effects lead to emergent ferrimagnetism, which is detrimental to the robustness of the storage. After pinpointing lattice misfit as the likely origin, we provide routes to enhance or mitigate this emergent ferrimagnetism as desired. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in purely antiferromagnetic spintronics devices.",1611.07027v1 2017-04-28,Electric Field Effect in Multilayer Cr2Ge2Te6: a Ferromagnetic Two-Dimensional Material,"The emergence of two-dimensional (2D) materials has attracted a great deal of attention due to their fascinating physical properties and potential applications for future nanoelectronic devices. Since the first isolation of graphene, a Dirac material, a large family of new functional 2D materials have been discovered and characterized, including insulating 2D boron nitride, semiconducting 2D transition metal dichalcogenides and black phosphorus, and superconducting 2D bismuth strontium calcium copper oxide, molybdenum disulphide and niobium selenide, etc. Here, we report the identification of ferromagnetic thin flakes of Cr2Ge2Te6 (CGT) with thickness down to a few nanometers, which provides a very important piece to the van der Waals structures consisting of various 2D materials. We further demonstrate the giant modulation of the channel resistance of 2D CGT devices via electric field effect. Our results illustrate the gate voltage tunability of 2D CGT and the potential of CGT, a ferromagnetic 2D material, as a new functional quantum material for applications in future nanoelectronics and spintronics.",1704.08862v1 2018-05-07,Electrically controlled long-distance spin transport through an antiferromagnetic insulator,"Spintronics uses spins, the intrinsic angular momentum of electrons, as an alternative for the electron charge. Its long-term goal is in the development of beyond-Moore low dissipation technology devices. Recent progress demonstrated the long-distance transport of spin signals across ferromagnetic insulators. Antiferromagnetically ordered materials are however the most common class of magnetic materials with several crucial advantages over ferromagnetic systems. In contrast to the latter, antiferromagnets exhibit no net magnetic moment, which renders them stable and impervious to external fields. In addition, they can be operated at THz frequencies. While fundamentally their properties bode well for spin transport, previous indirect observations indicate that spin transmission through antiferromagnets is limited to short distances of a few nanometers. Here we demonstrate the long-distance, over tens of micrometers, propagation of spin currents through hematite (\alpha-Fe2O3), the most common antiferromagnetic iron oxide, exploiting the spin Hall effect for spin injection. We control the spin current flow by the interfacial spin-bias and by tuning the antiferromagnetic resonance frequency with an external magnetic field. This simple antiferromagnetic insulator is shown to convey spin information parallel to the compensated moment (N\'eel order) over distances exceeding tens of micrometers. This newly-discovered mechanism transports spin as efficiently as the net magnetic moments in the best-suited complex ferromagnets. Our results pave the way to ultra-fast, low-power antiferromagnet-insulator-based spin-logic devices that operate at room temperature and in the absence of magnetic fields.",1805.02451v1 2018-05-25,Lattice relaxations in disordered Fe-based materials in the paramagnetic state from first principles,"In this work we propose a method for the structural relaxation of magnetic materials in the paramagnetic regime, in an adiabatic fast-magnetism approximation within the disordered local moment (DLM) picture in the framework of density functional theory (DFT). The method is straight forward to implement using any $ab$ $initio$ code that allows for structural relaxations. We illustrate the importance of considering the disordered magnetic state during lattice relaxations by calculating formation energies and geometries for an Fe vacancy and C insterstitial atom in bcc Fe as well as bcc Fe$_{1-x}$Cr$_x$ random alloys in the paramagnetic state. In the vacancy case, the nearest neighbors to the vacancy relax towards the vacancy of 0.16 {\AA} (-5% of the ideal bcc nearest neighbor distance), which is twice as large as the relaxation in the ferromagnetic case. The vacancy formation energy calculated in the DLM state on these positions is 1.60 eV, which corresponds to a reduction of about 0.1 eV compared to the formation energy calculated using DLM but on ferromagnetic-relaxed positions. The carbon interstitial formation energy is found to be 0.41 eV when the DLM relaxed positions are used, as compared to 0.59 eV when the FM-relaxed positions are employed. For bcc Fe$_{0.5}$Cr$_{0.5}$ alloys, the mixing enthalpy is reduced by 5 meV/atom, or about 10%, when the DLM state relaxation is considered, as compared to positions relaxed in the ferromagnetic state.",1805.10110v1 2018-05-31,Investigation into the role of the orbital moment in a series of isostructural weak ferromagnets,"The orbital contribution to the magnetic moment of the transition metal ion in the isostructural weak ferromagnets ACO$_3$ (A=Mn,Co,Ni) and FeBO$_3$ was investigated by a combination of first-principles calculations, non-resonant x-ray magnetic scattering and x-ray magnetic circular dichroism. A non-trivial evolution of the orbital moment as a function of the $3d$ orbitals filling is revealed, with an anomalously large value found in the Co member of the family. Here, the coupling between magnetic and lattice degrees of freedom produced by the spin-orbit interaction results in a large single-ion anisotropy and a peculiar magnetic-moment-induced electron cloud distortion, evidenced by the appearance of a subtle scattering amplitude at space group-forbidden reflections and significant magnetostrictive effects. Our results, which complement a previous investigation on the sign of the Dzyaloshinskii$-$Moriya interaction across the series, highlight the importance of spin-orbit coupling in the physics of weak ferromagnets and prove the ability of modern first-principles calculations to predict the properties of materials where the Dzyaloshinskii$-$Moriya interaction is a fundamental ingredient of the magnetic Hamiltonian.",1805.12423v2 2011-11-23,Smeared spin-flop transition in random antiferromagnetic Ising chain,"At T = 0 and a sufficiently large field, the nearest-neighbor antiferromagnetic Ising chain undergoes a first-order spin-flop transition into the ferromagnetic phase. We consider its smearing under the random-bond disorder such that all independent random bonds are antiferromagnetic (AF). It is shown that it can be described exactly for arbitrary distribution of AF bonds P(J). Moreover, the site magnetizations of finite chains can be found analytically in this model. We consider continuous P(J) which is zero above some -J_1 and behaves near it as (-J_1 - J)^\lambda, \lambda > -1. In this case ferromagnetic phase emerges continuously in a field H > H_c = 2J_1. At 0 > \lambda > -1 it has usual second-order anomalies near H_c with critical indices obeying the scaling relation and depending on \lambda . At \lambda > 0 the higher-order transitions appear (third, fourth etc.) marked by the divergence of corresponding nonlinear susceptibilities. In the chains with even number of spins the intermediate ""bow-tie"" phase with linearly modulated AF order exists between AF and ferromagnetic ones at J_1 < H < H_c. Its origin can be traced to the infinite correlation length of the degenerate AF phase from which it emerges. This implies the existence of similar inhomogeneous phases with size- and form-dependent order in a number of other systems with infinite correlation length. The possibility to observe the signs of ""bow-tie"" phase in low-T neutron diffraction experiments is discussed.",1111.5394v2 2012-01-04,Ferroelectricity at the Néel Temperature of Chromium in Rare-earth Orthochromites: Interplay of Rare-earth and Cr Magnetism,"We report ferroelectricity with fairly large spontaneous polarization (P ~ 0.2 - 0.8 \muC/cm2) below the N\'eel temperature of chromium (TNCr) in weakly ferromagnetic rare-earth orthochromites, RCrO3 (R=rare-earth) only when the rare-earth ion is magnetic. Intriguingly, the ferroelectricity in ErCrO3 (TC = 133 K) disappears at a spin-reorientation (Morin) transition (TSR ~ 22 K) below which the weak ferromagnetism associated with the Cr-sublattice also disappears, demonstrating the crucial role of weak ferromagnetism in inducing ferroelectricity. Further, the ferroelectric polarization (P) could be reversed by reversing the spontaneous magnetization (M) by changing the polarity of magnetic field, indicating a strong magnetoelectric effect. We suggest that the ferroelectricity occurs in RCrO3, due to the combined effect of poling field that breaks the symmetry and the exchange field on R-ion from Cr-sublattice stabilizes the polar state. We propose that a similar mechanism could work in the isostructural rare-earth orthoferrites, RFeO3 as well.",1201.0826v6 2012-01-06,Experimental observation of the optical spin transfer torque,"The spin transfer torque is a phenomenon in which angular momentum of a spin polarized electrical current entering a ferromagnet is transferred to the magnetization. The effect has opened a new research field of electrically driven magnetization dynamics in magnetic nanostructures and plays an important role in the development of a new generation of memory devices and tunable oscillators. Optical excitations of magnetic systems by laser pulses have been a separate research field whose aim is to explore magnetization dynamics at short time scales and enable ultrafast spintronic devices. We report the experimental observation of the optical spin transfer torque, predicted theoretically several years ago building the bridge between these two fields of spintronics research. In a pump-and-probe optical experiment we measure coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by circularly polarized laser pulses. During the pump pulse, the spin angular momentum of photo-carriers generated by the absorbed light is transferred to the collective magnetization of the ferromagnet. We interpret the observed optical spin transfer torque and the magnetization precession it triggers on a quantitative microscopic level. Bringing the spin transfer physics into optics introduces a fundamentally distinct mechanism from the previously reported thermal and non-thermal laser excitations of magnets. Bringing optics into the field of spin transfer torques decreases by several orders of magnitude the timescales at which these phenomena are explored and utilized.",1201.1436v1 2012-01-14,Effect of lattice geometry on magnon Hall effect in ferromagnetic insulators,"We have investigated the thermal Hall effect of magnons for various ferromagnetic insulators. For pyrochlore ferromagnetic insulators Lu$_2$V$_2$O$_7$, Ho$_2$V$_2$O$_7$, and In$_2$Mn$_2$O$_7$, finite thermal Hall conductivities have been observed below the Curie temperature $T_C$ . From the temperature and magnetic field dependences, it is concluded that magnons are responsible for the thermal Hall effect. The Hall effect of magnons can be well explained by the theory based on the Berry curvature in momentum space induced by the Dzyaloshinskii-Moriya (DM) interaction. The analysis has been extended to the transition metal (TM) oxides with perovskite structure. The thermal Hall signal was absent or far smaller in La$_2$NiMnO$_6$ and YTiO$_3$, which have the distorted perovskite structure with four TM ions in the unit cell. On the other hand, a finite thermal Hall response is discernible below $T_C$ in another ferromagentic perovskite oxide BiMnO$_3$, which shows orbital ordering with a larger unit cell. The presence or absence of the thermal Hall effect in insulating pyrochlore and perovskite systems reflect the geometric and topological aspect of DM-induced magnon Hall effect.",1201.3002v2 2012-04-21,"Ab-initio study of different structures of CaC: Magnetism, Bonding, and Lattice Dynamics","On the basis of ab-initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA*), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA*, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33-CaC is the energetically preferred system, while the more ionic structures prefer a ferromagnetic ground state with high Fermi level spin polarization. The observed ferromagnetism in the more ionic systems is attributed to the sharp partially filled $p$ states of carbon atom in the system. In the framework of density functional perturbation theory, the phonon spectra of these systems are computed and the observed dynamical instabilities of the NA* and B2 structures are explained in terms of the covalent bonds between carbon atoms. The calculated Helmholtz and Enthalpy free energies indicate the highest stability of the B33 structure in a wide range of temperatures and pressures. Among the ferromagnetic structures, RS-CaC and ZB-CaC are reported, respectively, to be the most and the least metastable systems in various thermodynamics conditions. Several mechanical properties of the dynamically stable structures of CaC are determined from their phonon spectra.",1204.4781v2 2013-09-04,Temperature Evolution of Itinerant Ferromagnetism in SrRuO3 Probed by Optical Spectroscopy,"The temperature ($T$) dependence of the optical conductivity spectra $\sigma(\omega)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$ range from 5 to 450 K. We observed significant $T$ dependence of the spectral weights of the charge transfer and interband $d$-$d$ transitions across the ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. $T$ scans of $\sigma(\Omega, T)$ at fixed frequencies $\Omega$ reveal a clear $T^2$ dependence below $T_c$, demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, $\sigma(\Omega, T)$ continues to evolve at temperatures above $T_c$, indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO$_3$ is an intriguing oxide system with itinerant ferromagnetism.",1309.0925v1 2013-09-09,Ferromagnetic cluster-glass state in itinerant electron system Sr1-xLaxRuO3,"The magnetic and electronic properties of Sr1-xLaxRuO3 were studied by means of dc-magnetization, ac-susceptibility, specific heat, and electrical resistivity measurements. The dc-magnetization and ac-susceptibility measurements have revealed that the transition temperature and the ordered moment of the ferromagnetic order are strongly suppressed as La is substituted for Sr. The ac-susceptibility exhibits a peak at T* due to the occurrence of spontaneous spin polarization. Furthermore, we observed that T* shows clear frequency variations for x>= 0.3. The magnitude of the frequency shifts of T* is comparable to that of cluster-glass systems, and the frequency dependence is well described in terms of the Vogel-Fulcher law. On the other hand, it is found that the linear specific heat coefficient gamma enhances with the suppression of the ferromagnetic order. The relatively large gamma values reflect the presence of the Ru 4d state at Fermi level, and hence, the magnetism of this system is considered to be tightly coupled with the itinerant characteristics of the Ru 4d electrons. The present experimental results and analyses suggest that the intrinsic coexistence of the spatially inhomogeneous magnetic state and the itinerant nature of the Ru 4d electrons is realized in this system, and such a feature may be commonly involved in La- and Ca-doped SrRuO3.",1309.2037v2 2013-09-10,Effect of Disorder in the Frustrated Ising FCC Antiferromagnet: Phase Diagram and Stretched Exponential Relaxation,"We study the phase transition in a face-centered-cubic antiferromagnet with Ising spins as a function of the concentration $p$ of ferromagnetic bonds randomly introduced into the system. Such a model describes the spin-glass phase at strong bond disorder. Using the standard Monte Carlo simulation and the powerful Wang-Landau flat-histogram method, we carry out in this work intensive simulations over the whole range of $p$. We show that the first-order transition disappears with a tiny amount of ferromagnetic bonds, namely $p\sim 0.01$, in agreement with theories and simulations on other 3D models. The antiferromagnetic long-range order is also destroyed with a very small $p$ ($\simeq 5%$). With increasing $p$, the system changes into a spin glass and then to a ferromagnetic phase when $p>0.65$. The phase diagram in the space ($T_c,p$) shows an asymmetry, unlike the case of the $\pm J$ Ising spin glass on the simple cubic lattice. We calculate the relaxation time around the spin-glass transition temperature and we show that the spin autocorrelation follows a stretched exponential relaxation law where the factor $b$ is equal to $\simeq 1/3$ at the transition as suggested by the percolation-based theory. This value is in agreement with experiments performed on various spin glasses and with Monte Carlo simulations on different SG models.",1309.2594v2 2013-09-16,Anomalous Hall effect arising from noncollinear antiferromagnetism,"In most conductors current flow perpendicular to electric field direction (Hall current) can be explained in terms of the Lorentz forces present when charged particles flow in an external magnetic field. However, as established in the very early work of Edwin Hall, ferromagnetic conductors such as Fe, Co, and Ni have an anomalous Hall conductivity contribution that cannot be attributed to Lorentz forces and therefore survives in the absence of a magnetic field. Although the anomalous Hall effect is experimentally strong, it has stood alone among metallic transport effects for much of the last century because it lacked a usefully predictive, generally accepted theory. Progress over the past decade has explained why. It is now clear that the anomalous Hall effect in ferromagnets has contributions from both extrinsic scattering mechanisms similar to those that determine most transport coefficients, and from an intrinsic mechanism that is independent of scattering. The anomalous Hall effect is also observed in paramagnets, which have nonzero magnetization induced by an external magnetic field. Although no explicit relationship has been established, the anomalous Hall effect in a particular material is usually assumed to be proportional to its magnetization. In this work we point out that it is possible to have an anomalous Hall effect in a noncollinear antiferromagnet with zero net magnetization provided that certain common symmetries are absent, and predict that Mn3Ir, a technologically important antiferromagnetic material with noncollinear order that survives to very high temperatures, has a surprisingly large anomalous Hall effect comparable in size to those of the elemental transition metal ferromagnets.",1309.4041v1 2013-09-20,Internal modes of a skyrmion in the ferromagnetic background in chiral magnets,"A spin texture called skyrmion has been recently observed in certain chiral magnets without inversion symmetry. The observed skyrmions are extended objects with typical linear sizes of 10 nm to 100 nm that contain $10^3$ to $10^5$ spins and can be deformed in response to external perturbations. Weak deformations are characterized by the internal modes which are localized around the skyrmion center. Knowledge of internal modes is crucial to assess the stability and rigidity of these topological textures. Here we compute the internal modes of a skyrmion in the ferromagnetic background state by numerical diagonalization of the dynamical matrix. We find several internal modes below the magnon continuum, such as the mode corresponding to the translational motion, and different kinds of breathing modes. The number of internal modes is larger for lower magnetic fields. Indeed, several modes become gapless in the low field region indicating that the single skyrmion solution becomes unstable, although a skyrmion lattice is thermodynamically stable. On the other hand, at high fields only three internal modes exist and the skyrmion is stable even at higher fields when the ferromagnetic state is thermodynamically stable. We also show that the presence of out-of-plane easy-axis anisotropy stabilizes the single skyrmion solution. Finally, we discuss the effects of damping and possible experimental observations of these internal modes.",1309.5168v2 2013-09-23,Zinc Oxide - From Dilute Magnetic Doping to Spin Transport,"During the past years there has been renewed interest in the wide-bandgap II-VI semiconductor ZnO, triggered by promising prospects for spintronic applications. First, ferromagnetism was predicted for dilute magnetic doping. In comprehensive investigation of ZnO:Co thin films based on the combined measurement of macroscopic and microscopic properties, we find no evidence for carrier-mediated itinerant ferromagnetism. Phase-pure, crystallographically excellent ZnO:Co is uniformly paramagnetic. Superparamagnetism arises when phase separation or defect formation occurs, due to nanometer-sized metallic precipitates. Other compounds like ZnO:(Li,Ni) and ZnO:Cu do not exhibit indication of ferromagnetism. Second, its small spin-orbit coupling and correspondingly large spin coherence length makes ZnO suitable for transporting or manipulating spins in spintronic devices. From optical pump/optical probe experiments, we find a spin dephasing time of the order of 15 ns at low temperatures which we attribute to electrons bound to Al donors. In all-electrical magnetotransport measurements, we successfully create and detect a spin-polarized ensemble of electrons and transport this spin information across several nanometers. We derive a spin lifetime of 2.6 ns for these itinerant spins at low temperatures, corresponding well to results from an electrical pump/optical probe experiment.",1309.5857v1 2013-09-25,Analytical estimates of the locations of phase transition points in the ground state for the bimodal Ising spin glass model in two dimensions,"We analytically estimate the locations of phase transition points in the ground state for the $\pm J$ random bond Ising model with asymmetric bond distributions on the square lattice. We propose and study the percolation transitions for two types of bond shared by two non-frustrated plaquettes. The present method indirectly treats the sizes of clusters of correlated spins for the ferromagnetic and spin glass orders. We find two transition points. The first transition point is the phase transition point for the ferromagnetic order, and the location is obtained as $p_c^{(1)} \approx 0.895 \, 399 \, 54$ as the solution of $[p^2 + 3 (1-p)^2 ]^2 \, p^3 - \frac{1}{2} = 0$. The second transition point is the phase transition point for the spin glass order, and the location is obtained as $p_c^{(2)} = \frac{1}{4} [2 + \sqrt{2 (\sqrt{5} - 1)}] \approx 0.893 \, 075 \, 69$. Here, $p$ is the ferromagnetic bond concentration, and $1 - p$ is the antiferromagnetic bond concentration. The obtained locations are reasonably close to the previously estimated locations. This study suggests the presence of the intermediate phase between $p_c^{(1)}$ and $p_c^{(2)}$; however, since the present method produces remarkable values but has no mathematical proof for accuracy yet, no conclusions are drawn in this article about the presence of the intermediate phase.",1309.6554v3 2013-09-26,Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons,"We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the form $\Omega/\Gamma (q)$ with $\Gamma (q) \propto q$ (the Landau damping). However, the experiments on uranium metallic compounds UGe$_2$ and UCoGe showed that $\Gamma (q)$ tends to a constant value at vanishing $q$. A non-zero $\Gamma (0)$ is impossible in systems with one type of carriers (either localized or itinerant) because it would violate the spin conservation. It has been conjectured recently that a non-zero $\Gamma (q)$ in UGe$_2$ and UCoGe may be due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancellation between the contributions to $\Gamma (0)$ from self-energy and vertex correction insertions into the spin polarization bubble and discuss the special role of the Aslamazov-Larkin processes. We show that $\Gamma (0)$ increases with $T$ both in the paramagnetic and ferromagnetic regions, but in-between it has a peak at $T_c$. We compare our theory with the available experimental data.",1309.7065v3 2014-05-19,"Comparison of micromagnetic parameters of ferromagnetic semiconductors (Ga,Mn)(As,P) and (Ga,Mn)As","We report on the determination of micromagnetic parameters of epilayers of the ferromagnetic semiconductor (Ga,Mn)As, which has easy axis in the sample plane, and (Ga,Mn)(As,P) which has easy axis perpendicular to the sample plane. We use an optical analog of ferromagnetic resonance where the laser-pulse-induced precession of magnetization is measured directly in the time domain. By the analysis of a single set of pump-and-probe magneto-optical data we determined the magnetic anisotropy fields, the spin stiffness and the Gilbert damping constant in these two materials. We show that incorporation of 10% of phosphorus in (Ga,Mn)As with 6% of manganese leads not only to the expected sign change of the perpendicular to plane anisotropy field but also to an increase of the Gilbert damping and to a reduction of the spin stiffness. The observed changes in the micromagnetic parameters upon incorporating P in (Ga,Mn)As are consistent with the reduced hole density, conductivity, and Curie temperature of the (Ga,Mn)(As,P) material. We report that the magnetization precession damping is stronger for the n = 1 spin wave resonance mode than for the n = 0 uniform magnetization precession mode.",1405.4677v1 2014-06-20,Cluster Altered Magnetic and Transport Properties in Eu Co-Doped Ge(1-x)Mn(x)Te,"Magnetic and transport properties of Ge(1-x-y)Mn(x)Eu(y)Te crystals with chemical compositions 0.041 < x < 0.092 and 0.010 < y < 0.043 are studied. Ferromagnetic order is observed at 150 < T < 160 K. Aggregation of magnetic ions into clusters is found to be the source of almost constant, composition independent Curie temperatures in our samples. Magnetotransport studies show the presence of both negative (at T < 25 K) and linear positive (for 25 320 K), using ferromagnetic resonance at room temperature. We show that the total MA energy (E) along the [001] direction changes its sign from positive (in-plane) to negative (perpendicular) with increasing d above an effective critical value \mathrm{d}_\mathrm{C}^\mathrm{*}\ ~ 42 nm. We reveal that (Ga,Fe)Sb has two-fold symmetry in the film plane. Meanwhile, in the plane perpendicular to the film including the in-plane [110] axis, the two-fold symmetry with the easy magnetization axis along [110] changes to four-fold symmetry with easy magnetization axis along <001> with increasing d. This peculiar behavior is different from that of (Ga,Mn)As, in which only the in-plane MA depends on the film thickness and has four-fold symmetry due to its dominant cubic anisotropy along the <100> axes. This work provides an important guide for controlling the easy magnetization axis of high-Tc FMS (Ga,Fe)Sb for room-temperature device applications.",1907.01188v1 2019-07-10,Quantum phase diagram of a frustrated spin-1/2 system on a Trellis Ladder,"We study an isotropic Heisenberg spin-1/2 model on a trellis ladder which is composed of two $J_1-J_2$ zigzag ladders interacting through anti-ferromagnetic rung couplings $J_3$. The $J_1$ and $J_2$ are ferromagnetic zigzag spin interaction between two legs and anti-ferromagnetic interaction along each leg of a zigzag ladder. A quantum phase diagram of this model is constructed using the density matrix renormalization group (DMRG) method and linearized spin wave analysis. In small $J_2$ limit a short range stripe collinear phase is found in the presence of $J_3$, whereas, in the large $J_2/J_3$ limit non-collinear quasi-long range phase is found. The system shows a short range non-collinear state in large $J_3$ limit. The short range order phase is the dominant feature of this phase diagram. We also show that the results obtained by DMRG and linearized spin wave analysis show similar phase boundary between stripe collinear and non-collinear short range phases, and the collinear phase region shrinks with increasing $J_3$. We apply this model to understand the magnetic properties of CaV$_2$O$_5$ and also fit the experimental data of susceptibility and magnetization. The variation of magnetic specific heat capacity as function of external magnetic field is also predicted. We note that $J_3$ is a dominant interaction in this system, whereas $J_1$ and $J_2$ are approximately half of $J_3$.",1907.04709v2 2019-07-17,Quasi 2-D magnetism in the Kagome layer compound FeSn,"Single crystals of the single Kagome layer compound FeSn are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, heat capacity, resistivity, Hall, Seebeck, thermal expansion, thermal conductivity measurements and density functional theory (DFT). FeSn is a planar antiferromagnet below TN = 365 K and exhibits ferromagnetic magnetic order within each Kagome layer. The in-plane magnetic susceptibility is sensitive to synthesis conditions. Resistivity, Hall and Seebeck results indicate multiple bands near the Fermi energy. The resistivity of FeSn is about 3 times lower for current along the stacking direction than in the plane, suggesting that transport and the bulk electronic structure of FeSn is not quasi 2D. FeSn is an excellent metal with Rho(300K)/Rho(2K) values about 100 in both directions. While the ordered state is antiferromagnetic, high temperature susceptibility measurements indicate a ferromagnetic Curie-Weiss temperature of 173 K, reflecting the strong in-plane ferromagnetic interactions. DFT calculations show a 3D electronic structure with the Dirac nodal lines along the K-H directions in the magnetic Brillouin zone about 0.3 eV below the Fermi energy, with the Dirac dispersions at the K points gapped by spin-orbit coupling except at the H point. The magnetism, however, is highly 2D with Jin-plane/Jout-of-plane = 10. The predicted spin-wave spectrum is presented.",1907.07719v1 2019-07-26,Ferromagnetism in narrow bands of moiré superlattices,"Many graphene moir\'e superlattices host narrow bands with non-zero valley Chern numbers. We provide analytical and numerical evidence for a robust spin and/or valley polarized insulator at total integer band filling in nearly flat bands of several different moir\'e materials. In the limit of a perfectly flat band, we present analytical arguments in favor of the ferromagnetic state substantiated by numerical calculations. Further, we numerically evaluate its stability for a finite bandwidth. We provide exact diagonalization results for models appropriate for ABC trilayer graphene aligned with hBN, twisted double bilayer graphene, and twisted bilayer graphene aligned with hBN. We also provide DMRG results for a honeycomb lattice with a quasi-flat band and non-zero Chern number, which extend our results to larger system sizes. We find a maximally spin and valley polarized insulator at all integer fillings when the band is sufficiently flat. We also show that interactions may induce effective dispersive terms strong enough to destabilize this state. These results still hold in the case of zero valley Chern number (for example, trivial side of TLG/hBN). We give an intuitive picture based on extended Wannier orbitals, and emphasize the role of the quantum geometry of the band, whose microscopic details may enhance or weaken ferromagnetism in moir\'e materials.",1907.11723v1 2019-07-29,Flat Band and Hole-induced Ferromagnetism in a Novel Carbon Monolayer,"In recent experiments, superconductivity and correlated insulating states were observed in twisted bilayer graphene (TBG) with small magic angles, which highlights the importance of the flat bands near Fermi energy. However, the moir\'e pattern of TBG consists of more than ten thousand carbon atoms that is not easy to handle with conventional methods. By density functional theory calculations, we obtain a flat band at E$_F$ in a novel carbon monolayer coined as cyclicgraphdiyne with the unit cell of eighteen atoms. By doping holes into cyclicgraphdiyne to make the flat band partially occupied, we find that cyclicgraphdiyne with 1/8, 1/4, 3/8 and 1/2 hole doping concentration shows ferromagnetism (half-metal) while the case without doping is nonmagnetic, indicating a hole-induced nonmagnetic-ferromagnetic transition. The calculated conductivity of cyclicgraphdiyne with 1/8, 1/4 and 3/8 hole doping concentration is much higher than that without doping or with 1/2 hole doping. These results make cyclicgraphdiyne really attractive. By studying several carbon monolayers, we find that a perfect flat band may occur in the lattices with both separated or corner-connected triangular motifs with only including nearest-neighboring hopping of electrons, and the dispersion of flat band can be tuned by next-nearest-neighboring hopping. Our results shed insightful light on the formation of flat band in TBG. The present study also poses an alternative way to manipulate magnetism through doping flat band in carbon materials.",1907.12362v2 2019-09-04,"Lattice dynamics, phonon chirality and spin-phonon coupling in 2D itinerant ferromagnet Fe3GeTe2","Fe3GeTe2 has emerged as one of the most fascinating van der Waals crystals due to its two-dimensional (2D) itinerant ferromagnetism, topological nodal lines and Kondo lattice behavior. However, lattice dynamics, chirality of phonons and spin-phonon coupling in this material, which set the foundation for these exotic phenomena, have remained unexplored. Here we report the first experimental investigation of the phonons and mutual interactions between spin and lattice degrees of freedom in few-layer Fe3GeTe2. Our results elucidate three prominent Raman modes at room temperature: two A1g({\Gamma}) and one E2g({\Gamma}) phonons. The doubly degenerate E2g({\Gamma}) mode reverses the helicity of incident photon, indicating the pseudo-angular momentum and chirality. Through analysis of temperature-dependent phonon energies and lifetimes, which strongly diverge from the anharmonic model below Curie temperature, we determine the spin-phonon coupling in Fe3GeTe2. Such interaction between lattice oscillations and spin significantly enhances the Raman susceptibility, allowing us to observe two additional Raman modes at the cryogenic temperature range. In addition, we reveal laser radiation induced degradation of Fe3GeTe2 in ambient conditions and the corresponding Raman fingerprint. Our results provide the first experimental analysis of phonons in this novel 2D itinerant ferromagnet and their applicability for further fundamental studies and application development.",1909.01598v1 2019-09-10,"Coexistence of nontrivial topological properties and strong ferromagnetic fluctuations in $A_2$Cr$_3$As$_3$ ($A$=Na, K, Rb and Cs)","Superconductivity in crystals without inversion symmetry has received extensive attention due to its unconventional pairing and possible nontrivial topological properties. Using first-principles calculations, we systemically study the electronic structure of noncentrosymmetric superconductors $A_2$Cr$_3$As$_3$ ($A$=Na, K, Rb and Cs). Topologically protected triply degenerate points connected by one-dimensional arcs appear along the $C_{3}$ axis, coexisting with strong ferromagnetic (FM) fluctuations in the non-superconducting state. Within random phase approximation, our calculations show that strong enhancements of spin fluctuations are present in K$_2$Cr$_3$As$_3$ and Rb$_2$Cr$_3$As$_3$, and are substantially reduced in Na$_2$Cr$_3$As$_3$ and Cs$_2$Cr$_3$As$_3$. Symmetry analysis of spin-orbit coupling $g_{k}$ suggests that the arc surface states might remain stable in the superconducting state, giving rise to possible nontrivial topological properties.",1909.04346v2 2019-09-19,Quenching of an antiferromagnet into high resistivity states using electrical or ultrashort optical pulses,"Ultra-fast dynamics, insensitivity to external magnetic fields, or absence of magnetic stray fields are examples of properties that make antiferromagnets of potential use in the development of spintronic devices. Similar to their ferromagnetic counterparts, antiferromagnets can store information in the orientations of the collective magnetic order vector. However, also in analogy to ferromagnets, the readout magnetoresistivity signals in simple antiferromagnetic films have been weak and the extension of the electrical reorientation mechanism to optics has not been achieved. Here we report reversible and reproducible quenching of an antiferromagnetic CuMnAs film by either electrical or ultrashort optical pulses into nano-fragmented domain states. The resulting resistivity changes approach 20\% at room temperature, which is comparable to the giant magnetoresistance ratios in ferromagnetic multilayers. We also obtain a signal readout by optical reflectivity. The analog time-dependent switching and relaxation characteristics of our devices can mimic functionality of spiking neural network components.",1909.09071v3 2019-09-20,Field-free spin-orbit torque switching through domain wall motion,"Deterministic current-induced spin-orbit torque (SOT) switching of magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic heterostructure with the ferromagnetic layer being perpendicularly-magnetized typically requires an externally-applied in-plane field to break the switching symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure, deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer can be realized without the need of in-plane bias field. Kerr imaging study further unveils that the observed switching is mainly dominated by domain nucleation and domain wall motion, which might limit the potentiality of using this type of multilayer stack design for nanoscale SOT-MRAM application. Comparison of the experimental switching behavior with micromagnetic simulations reveals that the deterministic switching in our devices cannot be explained by the stray field contribution of the in-plane magnetized layer, and the roughness-caused N\'eel coupling effect might play a more important role in achieving the observed field-free deterministic switching.",1909.09604v1 2019-09-30,Colossal magnetoresistance in the insulating ferromagnetic double perovskites Tl$_2$NiMnO$_6$: A neutron diffraction study,"In the family of double perovskites, colossal magnetoresistance (CMR) has been so far observed only in half-metallic ferrimagnets such as the known case Sr$_2$FeMoO$_6$ where it has been assigned to the tunneling MR at grain boundaries due to the half-metallic nature. Here we report a new material-Tl$_2$NiMnO$_6$, a relatively ordered double perovskite stablized by the high pressure and high temperature synthesis-showing CMR in the vicinity of its Curie temperature. We explain the origin of such effect with neutron diffraction experiment and electronic structure calculations that reveal the material is a ferromagnetic insulator. Hence the ordered Tl$_2$NiMnO$_6$ (~70% of Ni$^{2+}$/Mn$^{4+}$ cation ordering) represents the first realization of a ferromagnetic insulating double perovskite, showing CMR. The study of the relationship between structure and magnetic properties allows us to clarify the nature of spin glass behaviour in the disordered Tl$_2$NiMnO$_6$ (~31% of cation ordering), which is related to the clustering of antisite defects and associated with the short-range spin correlations. Our results highlight the key role of the cation ordering in establishing the long range magnetic ground state and lay out new avenues to exploit advanced magnetic materials in double perovskites.",1910.00042v1 2019-10-03,"Structural, dielectric and magnetic properties of multiferroic (1-x) La0.5Ca0.5MnO3-(x) BaTi0.8Sn0.2O3 laminated composites","High performance lead-free multiferroic composites are desired to replace the lead-based ceramics in multifunctional devices applications. Laminated compounds were prepared from ferroelectric and ferromagnetic materials. In this work, we present laminated ceramics compound by considering the ferromagnetic La0.5Ca0.5MnO3 (LCMO) and the ferroelectric BaTi0.8Sn0.2O3 (BTSO) in two different proportions. Compounds (1-x) LCMO-(x) BTSO with x=1 and 0 (pure materials) were synthesized by the sol gel method and x=0.7 and 0.5 (laminated) compounds were elaborated by welding appropriate mass ratios of each pure material by using the silver paste technique. Structural, dielectric, ferroelectric, microstructure and magnetic characterization were conducted on these samples. X-ray scattering results showed pure perovskite phases confirming the successful formation of both LCMO and BTSO. SEM images evidenced the laminated structure and good quality of the interfaces. The laminated composite materials have demonstrated a multiferroic behavior characterized by the ferroelectric and the ferromagnetic hysteresis loops. Furthermore, the enhancement of the dielectric constant in the laminated composite samples is mainly attributed to the Maxwell-Wagner polarization.",1910.01538v1 2019-10-06,Ground-State Phase Diagram of an Anisotropic S=1 Ferromagnetic-Antiferromagnetic Bond-Alternating Chain,"By using mainly numerical methods, we investigate the ground-state phase diagram (GSPD) of an $S=1$ ferromagnetic-antiferromagnetic bond-alternating chain with the $XXZ$ and the on-site anisotropies. This system can be mapped onto an anisotropic spin-2 chain when the ferromagnetic interaction is much stronger than the antiferromagnetic interaction. Since there are many quantum parameters in this system, we numerically obtained the GSPD on the plane of the magnitude of the antiferromagnetic coupling versus its $XXZ$ anisotropy, by use of the exact diagonalization, the level spectroscopy as well as the phenomenological renormalization group. The obtained GSPD consists of six phases. They are the $XY$1, the large-$D$ (LD), the intermediate-$D$ (ID), the Haldane (H), the spin-1 singlet dimer (SD), and the N\'eel phases. Among them, the LD, the H, and the SD phases are the trivial phases, while the ID phase is the symmetry-protected topological phase. The former three are smoothly connected without any quantum phase transitions. It is also emphasized that the ID phase appears in a wider region compared with the case of the GSPD of the anisotropic spin-2 chain with the $XXZ$ and the on-site anisotropies. We also compare the obtained GSPD with the result of the perturbation theory.",1910.02456v1 2019-10-11,Chiral Magnonic Edge States in Ferromagnetic Skyrmion Crystals Controlled by Magnetic Fields,"Achieving control over magnon spin currents in insulating magnets - where dissipation due to Joule heating is highly suppressed - is an active area of research that could lead to energy-efficient spintronics applications. However, magnon spin currents supported by conventional systems with uniform magnetic order have proven hard to control. An alternative approach that relies on topologically protected magnonic edge states of spatially periodic magnetic textures has recently emerged. A prime example of such textures is the ferromagnetic skyrmion crystal which hosts chiral edge states providing a platform for magnon spin currents. Here, we show, for the first time, an external magnetic field can drive a topological phase transition in the spin wave spectrum of a ferromagnetic skyrmion crystal. The topological phase transition is signaled by the closing of a low-energy bulk magnon gap at a critical field. In the topological phase, below the critical field, two topologically protected chiral magnonic edge states lie within this gap, but they unravel in the trivial phase, above the critical field. Remarkably, the topological phase transition involves an inversion of two magnon bands that at the $\Gamma$ point correspond to the breathing and anticlockwise modes of the skyrmions in the crystal. Our findings suggest that an external magnetic field could be used as a knob to switch on and off magnon spin currents carried by topologically protected chiral magnonic edge states.",1910.05214v1 2019-10-14,Enhanced ferromagnetism in cylindrically confined MnAs nanocrystals embedded in wurtzite GaAs nanowire shells,"Nearly 30% increase of the ferromagnetic phase transition temperature has been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix. Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing a hexagonal to orthorhombic structural phase transition, which in the bulk MnAs is combined with the magnetic one. This effect results in a remarkable shift of the magneto-structural phase transition temperature from 313 K in the bulk MnAs to above 400 K in the tensely strained MnAs nanocrystals. This finding is corroborated by the state of the art transmission electron microscopy, sensitive magnetometry and the first-principles calculations. The effect relies in defining a nanotube geometry of molecular beam epitaxy grown core-multishell wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires where the MnAs nanocrystals are formed during the thermal-treatment-induced phase separation of wurtzite (Ga,Mn)As into the GaAs:MnAs granular system. Such a unique combination of two types of hexagonal lattices provides possibility of attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise), leading to the substantial ferromagnetic phase transition temperature increase in this compound.",1910.06229v1 2019-10-16,Modeling magnetic evolution and exchange hardening in disordered magnets: The example of Mn$_{1-x}$Fe$_x$Ru$_2$Sn Heusler alloys,"We demonstrate how exchange hardening can arise in a chemically-disordered solid solution from a first-principles statistical mechanics approach. A general mixed-basis chemical and magnetic cluster expansion has been developed, and applied to the Mn$_{1-x}$Fe$_x$Ru$_2$Sn Heusler alloy system; single-phase solid solutions between antiferromagnetic \ch{MnRu2Sn} and ferromagnetic \ch{FeRu2Sn} with disorder on the Mn/Fe sublattice that exhibit unexpected exchange hardening. Monte Carlo simulations applied to the cluster expansion are able to reproduce the experimentally measured magnetic transition temperatures and the bulk magnetization as a function of composition. The magnetic ordering around a site is shown to be dependent not only on bulk composition, but also on the identity of the site and the local composition around that site. The simulations predict that local antiferromagnetic orderings form inside a bulk ferromagnetic region at intermediate compositions that drives the exchange hardening. Furthermore, the antiferromagnetic regions disorder at a lower temperature than the ferromagnetic regions, providing an atomistic explanation for the experimentally-observed decrease in exchange hardening with increasing temperature. These effects occur on a length scale too small to be resolved with previously-used characterization techniques.",1910.07543v1 2019-10-21,High-frequency magnon excitation due to femtosecond spin-transfer torques,"Femtosecond laser pulses can induce ultrafast demagnetization as well as generate bursts of hot electron spin currents. In trilayer spin valves consisting of two metallic ferromagnetic layers separated by a nonmagnetic one, hot electron spin currents excited by an ultrashort laser pulse propagate from the first ferromagnetic layer through the spacer reaching the second magnetic layer. When the magnetizations of the two magnetic layers are noncollinear, this spin current exerts a torque on magnetic moments in the second ferromagnet. Since this torque is acting only within the sub-ps timescale, it excites coherent high-frequency magnons as recently demonstrated in experiments. Here, we calculate the temporal shape of the hot electron spin currents using the superdiffusive transport model and simulate the response of the magnetic system to the resulting ultrashort spin-transfer torque pulse by means of atomistic spin-dynamics simulations. Our results confirm that the acting spin-current pulse is short enough to excite magnons with frequencies beyond 1 THz, a frequency range out of reach for current induced spin-transfer torques. We demonstrate the formation of thickness dependent standing spin waves during the first picoseconds after laser excitation. In addition, we vary the penetration depth of the spin-transfer torque to reveal its influence on the excited magnons. Our simulations clearly show a suppression effect of magnons with short wavelengths already for penetration depths in the range of 1 nm confirming experimental findings reporting penetration depths below $2\, {\rm nm}$.",1910.09412v1 2019-10-23,Multi-messenger nano-probes of hidden magnetism in a strained manganite,"The ground state properties of correlated electron systems can be extraordinarily sensitive to external stimuli, such as temperature, strain, and electromagnetic fields, offering abundant platforms for functional materials. We present a metastable and reversible photoinduced ferromagnetic transition in strained films of the doped manganite La(2/3)Ca(1/3)MnO3. Using the novel multi-messenger combination of atomic force microscopy, cryogenic scanning near-field optical microscopy, magnetic force microscopy, and ultrafast laser excitation, we demonstrate both ""writing"" and ""erasing"" of a metastable ferromagnetic metal phase with nanometer-resolved finesse. By tracking both optical conductivity and magnetism at the nano-scale, we reveal how spontaneous strain underlies the thermal stability, persistence, and reversal of this photoinduced metal. Our first-principles electronic structure calculations reveal how an epitaxially engineered Jahn-Teller distortion can stabilize nearly degenerate antiferromagnetic insulator and ferromagnetic metal phases. We propose a Ginzburg-Landau description to rationalize the co-active interplay of strain, lattice distortion, and magnetism we resolve in strained LCMO, thus guiding future functional engineering of epitaxial oxides like manganites into the regime of phase-programmable materials.",1910.10361v1 2019-10-24,DFT study of itinerant ferromagnetism in $p$-doped monolayers of MoS$_2$,"We use density functional theory to explore the possibility of making the semiconducting transition-metal dichalcogenide MoS$_2$ ferromagnetic by introducing holes into the narrow Mo $d$ band that forms the top of the valence band. In the single impurity limit, the repulsive Coulomb potential of an acceptor atom and intervalley scattering lead to a twofold orbitally degenerate effective-mass like $e'$ state being formed from Mo $d_{x^2-y^2}$ and $d_{xy}$ states, bound to the K and K$'$ valence band maxima. It also leads to a singly degenerate $a'_1$ state with Mo $d_{3z^2-r^2}$ character bound to the slightly lower lying valence band maximum at $\Gamma$. Within the accuracy of our calculations, these $e'$ and $a'_1$ states are degenerate for MoS$_2$ and accommodate the hole that polarizes fully in the local spin density approximation in the impurity limit. With spin-orbit coupling included, we find a single ion magnetic anisotropy of $\sim 5\,$meV favouring out-of-plane orientation of the magnetic moment. Pairs of such hole states introduced by V, Nb or Ta doping are found to couple ferromagnetically unless the dopant atoms are too close in which case the magnetic moments are quenched by the formation of spin singlets. Combining these exchange interactions with Monte Carlo calculations allows us to estimate ordering temperatures as a function of the dopant concentration $x$. For $x \sim 9\%$, Curie temperatures as high as 100K for Nb and Ta and in excess of 160K for V doping are predicted. Factors limiting the ordering temperature are identified and suggestions made to circumvent these limitations.",1910.10992v1 2020-01-14,Anomalous Josephson Hall effect charge and transverse spin currents in superconductor/ferromagnetic insulator/superconductor junctions,"Interfacial spin-orbit coupling in Josephson junctions offers an intriguing way to combine anomalous Hall and Josephson physics in a single device. We study theoretically how the superposition of both effects impacts superconductor/ferromagnetic insulator/superconductor junctions' transport properties. Transverse momentum-dependent skew tunneling of Cooper pairs through the spin-active ferromagnetic insulator interface creates sizable transverse Hall supercurrents, to which we refer as anomalous Josephson Hall effect currents. We generalize the Furusaki-Tsukada formula, which got initially established to quantify usual (tunneling) Josephson current flows, to evaluate the transverse current components and demonstrate that their amplitudes are widely adjustable by means of the spin-orbit coupling strengths or the superconducting phase difference across the junction. As a clear spectroscopic fingerprint of Josephson junctions, well-localized subgap bound states form around the interface. By analyzing the spectral properties of these states, we unravel an unambiguous correlation between spin-orbit coupling-induced asymmetries in their energies and the transverse current response, founding the currents' microscopic origin. Moreover, skew tunneling simultaneously acts like a transverse spin filter for spin-triplet Cooper pairs and complements the discussed charge current phenomena by their spin current counterparts. The junctions' universal spin-charge current cross ratios provide valuable possibilities to experimentally detect and characterize interfacial spin-orbit coupling.",2001.04691v2 2020-01-15,"Nonsaturating magnetoresistance, anomalous Hall effect, and magnetic quantum oscillations in ferromagnetic semimetal PrAlSi","We report a comprehensive investigation of the structural, magnetic, transport and thermodynamic properties of a single crystal PrAlSi, in comparison to its nonmagnetic analogue LaAlSi. PrAlSi exhibits a ferromagnetic transition at $T_C$ = 17.8 K which, however, is followed by two weak phase transitions at lower temperatures. Based on the combined dc and ac magnetic susceptibility measurements, we propose the two reentrant magnetic phases below $T_C$ to be spin glasses or ferromagnetic cluster glasses. When the magnetic glassy states are suppressed by small field, several remarkable features appear. These include a linear, nonsaturating magnetoresistance as a function of field that is reminiscent of a topological or charge-compensated semimetal, and a large anomalous Hall conductivity amounting to $\sim$2000 $\Omega ^{-1}$cm$^{-1}$. Specific-heat measurements indicate a non-Kramers doublet ground state and a relatively low crystal electric field splitting of the Pr$^{3+}$ multiplets of less than 100 K. Shubnikov-de Hass oscillations are absent in LaAlSi, whereas they are clearly observed below about 25 K in PrAlSi, with an unusual temperature dependence of the dominating oscillation frequency $F$. It increases from $F$ = 18 T at 25 K to $F$ = 33 T at 2 K, hinting at an emerging Fermi pocket upon cooling into the ordered phase. These results suggest that PrAlSi is a new system where a small Fermi pocket of likely relativistic fermions is strongly coupled to magnetism. Whether hybridization between $f$ and conduction band is also involved remains an intriguing open problem.",2001.05398v1 2020-01-16,Electrodynamics of highly spin-polarized tunnel Josephson junctions,"The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor/ferromagnet/superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-pi transition in tunnel-ferromagnetic spin-filter JJs could enhance the capabilities of SFS JJs also as active elements. Here, we provide a self-consistent electrodynamic characterization of NbN/GdN/NbN spin-filter JJs as a function of the barrier thickness, disentangling the high-frequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the IV characteristics at 4.2K and at 300mK by using the Tunnel Junction Microscopic model allows us to determine the subgap resistance Rsg, the quality factor Q and the junction capacitance C. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel ferromagnetic JJs as active elements in classical and quantum circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.",2001.06046v1 2020-01-19,Controlling Dzyaloshinskii-Moriya interactions in the skyrmion host candidates FePd$_{1-x}$Pt$_x$Mo$_3$N,"Ferromagnets crystallizing in structures described by chiral cubic space groups, including compounds with the B20 or $\beta$-Mn structures, are known to host long-period chiral spin textures such as skyrmion lattices. These spin textures are stabilized by a competition between ferromagnetic exchange and antisymmetric Dyzaloshinskii-Moriya (DM) exchange, which is enhanced by the spin-orbit coupling associated with high-atomic-number elements. For real-world application, it is desirable to find materials that can host compact skyrmion lattices at readily accessible temperatures. Here, we report on the crystal chemistry and magnetic phase diagrams of a family of compounds with the filled $\beta$-Mn structure, FePd$_{1-x}$Pt$_x$Mo$_3$N with $T_C$ ranging from 175 K to 240 K. DC and AC magnetization measurements reveal magnetic phase diagrams consistent with the formation of a skyrmion pocket just below $T_C$. The magnitudes of ferromagnetic and DM exchanges are determined from the phase diagrams, demonstrating that the introduction of increasing amounts of Pt can be used to increase spin-orbit coupling in order to control the expected skyrmion lattice parameter between 140 nm and 65 nm while simultaneously increasing $T_C$.",2001.06783v1 2020-01-23,Magnetism at iridate/manganite interface: influence of strong spin-orbit interaction,"The complex investigation of dc transport and magnetic properties of the epitaxial manganite/iridate heterostructure was carried out by mean of X-ray (XRD), dc resistance measurements, ferromagnetic resonance (FMR) and polarized neutron reflectivity (PNR). Epitaxial growth of the heterostructure proceeded according to the cube-to-cube mechanism with the small lattice turn. The dc measurement indicates the presence of a conduction channel at the iridate/manganite interface due to the charge leakage from iridate that makes it hole doped, while the manganite side becomes electron doped. This is confirmed by the first principles calculations based on density functional theory [Sayantika Bhowal, and Sashi Satpathy AIP Conference Proceedings 2005, 020007 (2018)] that show the charge transfer at the interface from the half-filled spin-orbit entangled Jeff = 1/2 state of the iridate to the empty e states of manganite. The neutron scattering data show the turn of magnetization vector of the heterostructure (mainly manganite) on 26 degree closer to the external field with reducing temperature down to 10K. Additional ferromagnetic state appearing at T<100K indicate on emergence of ferromagnetism in the thin (10 nm) paramagnetic SIO film close to the interface. We have measured the dc voltage aroused on the SIO film caused by spin pumping and the anisotropic magnetoresistance in the heterostructure.",2001.08403v1 2020-01-27,Resonant thermal energy transfer to magnons in a ferromagnetic nanolayer,"Energy harvesting is a modern concept which makes dissipated heat useful by transferring thermal energy to other excitations. Most of the existing principles for energy harvesting are realized in systems which are heated continuously, for example generating DC voltage in thermoelectric devices. Here we present the concept of high-frequency energy harvesting where the dissipated heat in a sample excites resonant magnons in a 5-nm thick ferromagnetic metal layer. The sample is excited by femtosecond laser pulses with a repetition rate of 10 GHz which results in temperature modulation at the same frequency with amplitude ~0.1 K. The alternating temperature excites magnons in the ferromagnetic nanolayer which are detected by measuring the net magnetization precession. When the magnon frequency is brought onto resonance with the optical excitation, a 12-fold increase of the amplitude of precession indicates efficient resonant heat transfer from the lattice to coherent magnons. The demonstrated principle may be used for energy harvesting in various nanodevices operating at GHz and sub-THz frequency ranges.",2001.09732v2 2020-01-28,Tailoring Magnetic Anisotropy in Cr$_2$Ge$_2$Te$_6$ by Electrostatic Gating,"Electrical control of magnetism of a ferromagnetic semiconductor offers exciting prospects for future spintronic devices for processing and storing information. Here, we report observation of electrically modulated magnetic phase transition and magnetic anisotropy in thin crystal of Cr$_2$Ge$_2$Te$_6$ (CGT), a layered ferromagnetic semiconductor. We show that heavily electron-doped ($\sim$ $10^{14}$ cm$^{-2}$) CGT in an electric double-layer transistor device is found to exhibit hysteresis in magnetoresistance (MR), a clear signature of ferromagnetism, at temperatures up to above 200 K, which is significantly higher than the known Curie temperature of 61 K for an undoped material. Additionally, angle-dependent MR measurements reveal that the magnetic easy axis of this new ground state lies within the layer plane in stark contrast to the case of undoped CGT, whose easy axis points in the out-of-plane direction. We propose that significant doping promotes double-exchange mechanism mediated by free carriers, prevailing over the superexchange mechanism in the insulating state. Our findings highlight that electrostatic gating of this class of materials allows not only charge flow switching but also magnetic phase switching, evidencing their potential for spintronics applications.",2001.10217v1 2020-02-28,Anomalous Anisotropy of the Lower Critical Field and Meissner Effect in UTe2,"We report on low temperature susceptibility and magnetization measurements made on single crystals of the recently discovered heavy-fermion superconductor UTe$_2$ and compare the results with the two ambient pressure ferromagnetic superconductors URhGe and UCoGe. Hysteresis curves in the superconducting phase show a familiar diamond shape superimposed on a large paramagnetic background. The Meissner state was measured by zero field cooling in small fields of a few Oe as well as ac susceptibility measurements in small fields and resulted in 100\% shielding, with a sharp transition. However the field cooling Meissner-Ochsenfeld effect (expulsion of flux) was negligible in fields greater than just a few Oe, but becomes nearly 30\% of the perfect diamagnetic signal when the field was reduced to 0.01~Oe. The critical current due to flux pinning was studied by ac susceptibility techniques. Over the range in fields and temperature of this study, no signature of a ferromagnetic transition could be discerned. The lower critical field $H_{\rm c1}$ has been measured along the three crystalographic axes, and surprisingly, the anisotropy of $H_{\rm c1}$ contradicts that of the upper critical field. We discuss this discrepancy and show that it may provide additional support for a magnetic field-dependent pairing mediated by ferromagnetic fluctuations in UTe$_2$.",2002.12724v2 2020-04-01,Field stability of Majorana spin liquids in antiferromagnetic Kitaev models,"Magnetic fields can give rise to a plethora of phenomena in Kitaev spin systems, such as the formation of non-trivial spin liquids in two and three spatial dimensions. For the original honeycomb Kitaev model, it has recently been observed that the sign of the bond-directional exchange is of crucial relevance for the field-induced physics, with antiferromagnetic couplings giving rise to an intermediate spin liquid regime between the low-field gapped Kitaev spin liquid and the high-field polarized state, which is not present in the ferromagnetically coupled model. Here, by employing a Majorana mean-field approach for a magnetic field pointing along the [001] direction, we present a systematic study of field-induced spin liquid phases for a variety of two and three-dimensional lattice geometries. We find that antiferromagnetic couplings generically lead to (i) spin liquid phases that are considerably more stable in field than those for ferromagnetic couplings, and (ii) an intermediate spin liquid phase which arises from a change in the topology of the Majorana band structure. Close inspection of the mean-field parameters reveal that the intermediate phase occurs due to a field-driven sign change in an 'effective' $z$-bond energy parameter. Our results clearly demonstrate the richness of the Majorana physics of the antiferromagnetic Kitaev models, in comparison to their ferromagnetic counterparts.",2004.00640v1 2020-04-02,Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide,"Magnetic Weyl fermions, which occur in magnets, have novel transport phenomena related to pairs of Weyl nodes, and they are, of both, scientific and technological interest, with the potential for use in high-performance electronics, spintronics and quantum computing. Although magnetic Weyl fermions have been predicted to exist in various oxides, evidence for their existence in oxide materials remains elusive. SrRuO3, a 4d ferromagnetic metal often used as an epitaxial conducting layer in oxide heterostructures, provides a promising opportunity to seek for the existence of magnetic Weyl fermions. Advanced oxide thin film preparation techniques, driven by machine learning technologies, may allow access to such topological matter. Here we show direct quantum transport evidence of magnetic Weyl fermions in an epitaxial ferromagnetic oxide SrRuO3: unsaturated linear positive magnetoresistance (MR), chiral-anomaly-induced negative MR, Pi Berry phase accumulated along cyclotron orbits, light cyclotron masses and high quantum mobility of about 10000 cm2/Vs. We employed machine-learning-assisted molecular beam epitaxy (MBE) to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic quantum transport properties. We also clarified the disorder dependence of the transport of the magnetic Weyl fermions, and provided a brand-new diagram for the Weyl transport, which gives a clear guideline for accessing the topologically nontrivial transport phenomena. Our results establish SrRuO3 as a magnetic Weyl semimetal and topological oxide electronics as a new research field.",2004.00810v2 2020-04-07,Structural Transition in Oxidized Ca$_2$N Electrenes: CaO/CaN 2D heterostructures,"Based on first-principles calculations we show that the oxidation of ultrathin films of Ca$_2$N electrides, electrenes, drives a hexagonal$\rightarrow$tetragonal structural transition. The ground state configuration of the oxidized monolayer (ML) and bilayer (BL) systems can be viewed as CaO/CaN and CaO/(CaN)$_2$/CaO two dimensional (2D) heterostructures. In both systems, we found nearly free electron (NFE) states lying near the vacuum level, and the spatial projection reveals that they are localized above the oxidized CaO surface. Focusing on the magnetic properties, we find that the nitrogen atoms of the oxidized Ca$_2$N becomes spin-polarized ($\sim$1 $\mu_{\rm B}$/N-atom); where (i) the ferromagnetic and the anti-ferromagnetic phases are nearly degenerated in the ML system, CaO/CaN, while (ii) there is an energetic preference for the ferromagnetic phase in CaO/(CaN)$_2$/CaO. We show that such a FM preference can be strengthened upon mechanical compression. Further electronic structure calculations reveal that the FM CaO/Ca$_2$N/CaO presents half-metallicity, where the metallic channels project (predominantly) on the N-$2p_{x,y}$ orbitals. In addition to the total energy results, molecular dynamic and phonon spectra calculations have been done in order to verify its thermal and structural stabilities. Those findings suggest that CaO/Ca$_2$N/CaO is a quite interesting, and structurally stable, 2D FM heterostructure characterized half-metallic bands sandwiched by NFE states lying on the oxidized surfaces.",2004.03528v2 2020-04-17,Reversible and magnetically unassisted voltage-driven switching of magnetization in FeRh/PMN-PT,"Reversible control of magnetization by electric fields without assistance from a subsidiary magnetic field or electric current could help reduce the power consumption in spintronic devices. When increasing temperature above room temperature, FeRh displays an uncommon antiferromagnetic to ferromagnetic phase transition linked to a unit cell volume expansion. Thus, using the strain exerted by an adjacent piezoelectric layer, the relative amount of antiferromagnetic and ferromagnetic regions can be tuned by an electric field applied to the piezoelectric material. Indeed, large variations in the saturation magnetization have been observed when straining FeRh films grown on suitable piezoelectric substrates. In view of its applications, the variations in the remanent magnetization rather than those of the saturation magnetization are the most relevant. Here, we show that in the absence of any bias external magnetic field, permanent and reversible magnetization changes as high as 34% can be induced by an electric field, which remain after this has been zeroed. Bulk and local magnetoelectric characterization reveals that the fundamental reason for the large magnetoelectric response observed at remanence is the expansion (rather than the nucleation) of ferromagnetic nanoregions.",2004.08087v1 2020-04-17,Many-body effects on second-order phase transitions in spinor Bose-Einstein condensates and breathing dynamics,"We unravel the correlation effects of the second-order quantum phase transitions emerging on the ground state of a harmonically trapped spin-1 Bose gas, upon varying the involved Zeeman terms, as well as its breathing dynamics triggered by quenching the trapping frequency. It is found that the boundaries of the associated magnetic phases are altered in the presence of interparticle correlations for both ferromagnetic and anti-ferromagnetic spin-spin interactions, an effect which becomes more prominent in the few-body scenario. Most importantly, we unveil a correlation-induced shrinking of the anti-ferromagnetic and broken-axisymmetry phases implying that ground states with bosons polarized in a single spin-component are favored. Turning to the dynamical response of the spinor gas it is shown that its breathing frequency is independent of the system parameters while correlations lead to the formation of filamentary patterns in the one-body density of the participating components. The number of filaments is larger for increasing spin-independent interaction strengths or for smaller particle numbers. Each filament maintains its coherence and exhibits an anti-correlated behavior while distinct filaments show significant losses of coherence and are two-body correlated. Interestingly, we demonstrate that for an initial broken-axisymmetry phase an enhanced spin-flip dynamics takes place which can be tuned either via the linear Zeeman term or the quench amplitude.",2004.09303v2 2020-04-21,Role of interfacial oxidation in generation of spin-orbit torques,"We report that current-induced spin-orbit torques (SOTs) in heavy-metal/ferromagnetic-metal bilayers are strongly altered by the oxidation of the ferromagnetic layer near the interface. We measured damping-like (DL) and field-like (FL) SOTs for Pt/Co and Pt/Ni$_{81}$Fe$_{19}$ (Pt/Py) films using spin-torque ferromagnetic resonance. In the Pt/Co film, we found that the oxidation of the Co layer near the interface enhances both DL and FL SOTs in spite of the insulating nature of the CoO$_x$ layer. The enhancement of the SOTs disappears by inserting a thin Ti layer at the Pt/CoO$_x$ interface, indicating that the dominant source of the SOTs in the Pt/CoO$_x$/Co film is the spin-orbit coupling at the Pt/CoO$_x$ interface. In contrast to the Pt/CoO$_x$/Co film, the SOTs in the Pt/PyO$_x$/Py film are dominated by the bulk spin-orbit coupling. Our result shows that the interfacial oxidation of the Pt/Py film suppresses the DL-SOT and reverses the sign of the FL-SOT. The change of the SOTs can be attributed to the change of the real and imaginary parts of the spin mixing conductance induced by the insertion of the insulating PyO$_x$ layer. These results show that the interfacial oxidation provides an effective way to manipulate the strength and sign of the SOTs.",2004.09837v1 2020-12-10,Absence of a Dirac gap in ferromagnetic Cr$_x$(Bi$_{0.1}$Sb$_{0.9}$)$_{2-x}$Te$_3$,"Magnetism breaks the time reversal symmetry expected to open a Dirac gap in 3D topological insulators that consequently leads to quantum anomalous Hall effect. The most common approach of inducing ferromagnetic state is by doping magnetic 3$d$ elements into bulk of 3D topological insulators. In Cr$_{0.15}$(Bi$_{0.1}$Sb$_{0.9}$)$_{1.85}$Te$_3$, the material where the quantum anomalous Hall effect was initially discovered at temperatures much lower than the ferromagnetic transition, $T_C$, the scanning tunneling microscopy studies have reported a large Dirac gap $\sim20-100$ meV. The discrepancy between the low temperature of quantum anomalous Hall effect ($\ll T_C$) and large spectroscopic Dirac gaps ($\gg T_C$) found in magnetic topological insulators remains puzzling. Here, we used angle-resolved photoemission spectroscopy to study the surface electronic structure of pristine and potassium doped surface of Cr$_{0.15}$(Bi$_{0.1}$Sb$_{0.9}$)$_{1.85}$Te$_3$. Upon potassium deposition, the $p$-type surface state of pristine sample was turned into an $n$-type, allowing spectroscopic observation of Dirac point. We find a gapless surface state, with no evidence of a large Dirac gap reported in tunneling studies.",2012.05884v1 2020-12-16,Exact Ground States and Domain Walls in One Dimensional Chiral Magnets,"We determine exactly the phase structure of a chiral magnet in one spatial dimension with the Dzyaloshinskii-Moriya (DM) interaction and a potential that is a function of the third component of the magnetization vector, $n_3$, with a Zeeman (linear with the coefficient $B$) term and an anisotropy (quadratic with the coefficient $A$) term, constrained so that $2A\leq \vert B\vert$. For large values of potential parameters $A$ and $B$, the system is in one of the ferromagnetic phases, whereas it is in the spiral phase for small values. In the spiral phase we find a continuum of spiral solutions, which are one-dimensionally modulated solutions with various periods. The ground state is determined as the spiral solution with the lowest average energy density. As the phase boundary approaches, the period of the lowest energy spiral solution diverges, and the spiral solutions become domain wall solutions with zero energy at the boundary. The energy of the domain wall solutions is positive in the homogeneous phase region, but is negative in the spiral phase region, signaling the instability of the homogeneous (ferromagnetic) state. The order of the phase transition between spiral and homogeneous phases and between polarized ($n_3=\pm 1$) and canted ($n_3\not=\pm 1$) ferromagnetic phases is found to be second order.",2012.08800v2 2020-12-17,Unconventional critical behaviour in weak ferromagnets Fe2-xMnxCrAl (00) as well as ferromagnetic (J1<0) nearest-neighbor interactions together with frustrating antiferromagnetic next-nearest-neighbor interaction J2>0. The strength of inter-plaquette interaction lambda varies between lambda=1 (that corresponds to the uniform J1-J2 model) and lambda=0 (that corresponds to isolated frustrated 4-spin plaquettes). While on the classical level (s \to \infty) both versions of models (i.e., with ferro- and antiferromagnetic J1) exhibit the same ground-state behavior, the ground-state phase diagram differs basically for the quantum case s=1/2. For the antiferromagnetic case (J1 > 0) Neel antiferromagnetic long-range order at small J2/J1 and lambda \gtrsim 0.47 as well as collinear striped antiferromagnetic long-range order at large J2/J1 and lambda \gtrsim 0.30 appear which correspond to their classical counterparts. Both semi-classical magnetic phases are separated by a nonmagnetic quantum paramagnetic phase. The parameter region, where this nonmagnetic phase exists, increases with decreasing of lambda. For the ferromagnetic case (J1 < 0) we have the trivial ferromagnetic ground state at small J2/|J1|. By increasing of J2 this classical phase gives way for a semi-classical plaquette phase, where the plaquette block spins of length s=2 are antiferromagnetically long-range ordered. Further increasing of J2 then yields collinear striped antiferromagnetic long-range order for lambda \gtrsim 0.38, but a nonmagnetic quantum paramagnetic phase lambda \lesssim 0.38.",1205.0681v1 2012-05-18,Interplay between dipole and quadrupole modes of field influence in liquid-crystalline suspensions of ferromagnetic particles,"In the framework of continuum theory we study orientational transitions induced by electric and magnetic fields in ferronematics, i.e., in liquid-crystalline suspensions of ferromagnetic particles. We have shown that in a certain electric field range the magnetic field can induce a sequence of re-entrant orientational transitions in ferronematic layer: nonuniform phase --- uniform phase --- nonuniform phase. This phenomenon is caused by the interplay between the dipole (ferromagnetic) and quadrupole (dielectric and diamagnetic) mechanisms of the field influence on a ferronematic structure. We have found that these re-entrant Freedericksz transitions exhibit tricritical behavior, i.e., they can be of the first or the second order. The character of the transitions depends on a degree of redistribution of magnetic admixture in the sample exposed to uniform magnetic field (magnetic segregation). We demonstrate how electric and magnetic fields can change the order of orientational transitions in ferronematics. We show that electric Freedericksz transitions in ferronematics subjected to magnetic field have no re-entrant nature. Tricritical segregation parameters for the transitions induced by electric or magnetic fields are obtained analytically. We demonstrate the re-entrant behavior of ferronematic by numerical simulations of the magnetization and optical phase lag.",1205.4179v1 2012-05-25,Anisotropic ferromagnetism in carbon doped zinc oxide from first-principles studies,"A density functional theory study of substitutional carbon impurities in ZnO has been performed, using both the generalized gradient approximation (GGA) and a hybrid functional (HSE06) as exchange-correlation functional. It is found that the non-spinpolarized C$_\mathrm{Zn}$ impurity is under almost all conditions thermodynamically more stable than the C$_\mathrm{O}$ impurity which has a magnetic moment of $2\mu_{\mathrm{B}}$, with the exception of very O-poor and C-rich conditions. This explains the experimental difficulties in sample preparation in order to realize $d^{0}$-ferromagnetism in C-doped ZnO. From GGA calculations with large 96-atom supercells, we conclude that two C$_\mathrm{O}$-C$_\mathrm{O}$ impurities in ZnO interact ferromagnetically, but the interaction is found to be short-ranged and anisotropic, much stronger within the hexagonal $ab$-plane of wurtzite ZnO than along the c-axis. This layered ferromagnetism is attributed to the anisotropy of the dispersion of carbon impurity bands near the Fermi level for C$_{\mathrm{O}}$ impurities in ZnO. From the calculated results, we derive that a C$_{\mathrm{O}}$ concentration between 2% and 6% should be optimal to achieve $d^{0}$-ferromagnetism in C-doped ZnO.",1205.5830v1 2017-09-01,Wing structure in the phase diagram of the Ising Ferromagnet URhGe close to its tricritical point investigated by angle-resolved magnetization measurements,"High-precision angle-resolved dc magnetization and magnetic torque studies were performed on a single-crystalline sample of URhGe, an orthorhombic Ising ferromagnet with the $c$ axis being the magnetization easy axis, in order to investigate the phase diagram around the ferromagnetic (FM) reorientation transition in a magnetic field near the $b$ axis. We have clearly detected first-order transition in both the magnetization and the magnetic torque at low temperatures, and determined detailed profiles of the wing structure of the three-dimensional $T$-$H_{b}$-$H_{c}$ phase diagram, where $H_{c}$ and $H_{b}$ denotes the field components along the $c$ and the $b$ axes, respectively. The quantum wing critical points are located at $\mu_0H_c\sim\pm$1.1 T and $\mu_0H_b\sim$13.5 T. Two second-order transition lines at the boundaries of the wing planes rapidly tend to approach with each other with increasing temperature up to $\sim 3$ K. Just at the zero conjugate field ($H_c=0$), however, a signature of the first-order transition can still be seen in the field derivative of the magnetization at $\sim 4$ K, indicating that the tricritical point exists in a rather high temperature region above 4 K. This feature of the wing plane structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at the triciritical point.",1709.00135v1 2017-09-06,Magnetic ground states and magnetodielectric effect in $R$Cr(BO$_3$)$_2$ ($R$ = Y and Ho),"The layered perovskites $R$Cr(BO$_3$)$_2$ ($R$ = Y and Ho) with magnetic triangular lattices were studied by performing ac/dc susceptibility, specific heat, elastic and inelastic neutron scattering, and dielectric constant measurements. The results show (i) both samples' Cr$^{3+}$ spins order in a canted antiferromagnetic structure with $T_N$ around 8-9 K, while the Ho$^{3+}$ ions do not order down to $T$ = 1.5 K in HoCr(BO$_3$)$_2$; (ii) when a critical magnetic field H$_{C}$ around 2-3 T is applied below $T_{N}$, the Cr$^{3+}$ spins in the Y-compound and both the Cr$^{3+}$ and Ho$^{3+}$ spins in the Ho-compound order in a ferromagnetic state; (iii) both samples exhibit dielectric constant anomalies around the transition temperature and critical field, but the Ho-compound displays a much stronger magnetodielectric response. We speculate that this is due to the magnetostriction which depends on both of the Cr$^{3+}$ and the Ho$^{3+}$ ions' ordering in the Ho-compound. Moreover, by using linear spin wave theory to simulate the inelastic neutron scattering data, we estimated the Y-compound's intralayer and interlayer exchange strengths as ferromagnetic J$_{1}$ = -0.12 meV and antiferromagnetic J$_{2}$ = 0.014 meV, respectively. The competition between different kinds of superexchange interactions results in the ferromagnetic intralayer interaction.",1709.03493v1 2017-09-27,Carrier and strain tunable intrinsic magnetism in two-dimensional MAX$_3$ transition metal chalcogenides,"We present a density functional theory study of the carrier-density and strain dependence of magnetic order in two-dimensional (2D) MAX$_3$ (M= V, Cr, Mn, Fe, Co, Ni; A= Si, Ge, Sn, and X= S, Se, Te) transition metal trichalcogenides. Our {\em ab initio} calculations show that this class of compounds includes wide and narrow gap semiconductors and metals and half-metals, and that most of these compounds are magnetic. Although antiferromagnetic order is most common, ferromagnetism is predicted in MSiSe$_3$ for M= Mn, Ni, in MSiTe$_3$ for M= V, Ni, in MnGeSe$_3$, in MGeTe$_3$ for M=Cr, Mn, Ni, in FeSnS$_3$, and in MSnTe$_3$ for M= V, Mn, Fe. Among these compounds CrGeTe$_3$ and VSnTe$_3$ are ferromagnetic semiconductors. Our calculations suggest that the competition between antiferromagnetic and ferromagnetic order can be substantially altered by strain engineering, and in the semiconductor case also by gating. The associated critical temperatures can be substantially enhanced by means of carrier doping and strains.",1709.09386v1 2017-11-20,Two-Dimensional Hyperferroelectric Metals: a Different Route to Ferromagnetic-Ferroelectric Multiferroics,"Recently, two-dimensional (2D) multiferroics have attracted numerous attention due to their fascinating properties and promising applications. Although the ferroelectric (FE)-ferroelastic and ferromagnetic (FM)-ferroelastic multiferroics have been observed/predicted in 2D systems, 2D ferromagnetic-ferroelectric (FM-FE) multiferroics remain to be discovered since FM insulators are very rare. Here, we proposed for the first time the concept of 2D hyperferroelectric metals, with which the insulating prerequisite for the FM-FE multiferroic is no longer required in 2D systems. We validate the concept of 2D hyperferroelectric metals and 2D metallic FM-FE multiferroics by performing first-principle calculations on 2D CrN and CrB2 systems. The 2D buckled monolayer CrN is found to be a hyperferroelectic metal with the FM ground state, i.e., a 2D FM-FE multiferroic. With the global optimization approach, we find the 2D CrB2 system has an antiferromagnetic (AFM)/planar ground state and a FM/FE metastable state, suggesting that it can be used to realize electric field control of magnetism. Our analysis demonstrates that the spin-phonon coupling and metal-metal interaction are two new mechanisms for stabilizing the out-of-plane electric polarization in 2D systems. Our work not only extends the concept of FE to metallic systems, but also paves a new way to search the long-sought high temperature FM-FE multiferroics.",1711.07342v2 2018-03-02,Antiferroelectric and Magnetodielectric Coupling Response of La0.2Sr0.7Fe12O19 Ceramics,"Multiferroics is a class of functional materials that simultaneously exhibit ferroelectricity and ferromagnetism in a single structure. We report here the integration of anti-ferroelectricity and ferromagnetism in a new M-type hexaferrite compound, in which 0.3 Sr ions was substituted by 0.2 La3+ ions in SrFe12O19 so as to keep the charge balance. This doped compound is expressed as a molecular formula of La0.2Sr0.7Fe12O19, which was confirmed by XRD to have the same structure as SrFe12O19 with a lattice contraction of 0.59%. Surprisingly, the doping effect turns La0.2Sr0.7Fe12O19 from ferroelectric to antiferroelectric phase, which displays double hysteresis loops with a maximum polarization of 154 uC/cm2 and a remnant one of 38uC/cm2. The dielectric constant is also greatly improved from 1462 to 1451778 at 10.2Hz by this substitution. This compound simultaneously demonstrates strong ferromagnetism, the remnant magnetic moment and coercive field are 52 emu/g and 5876 Oe, respectively. In addition, La0.2Sr0.7Fe12O19 also exhibits strong magnetodielectric (MD) response, in which a applying magnetic field B lifts the whole e1-f spectra up and right shifts e2-f spectra to higher frequency side. Both real and imaginary parts of the dielectric constant varies with frequency and B field and obey Debye relaxation model. The maximum MD increment in e1 reaches as high as 540830 upon a B field of 926mT. The capacitance of the La0.2Sr0.7Fe12O19 ceramics could be much more enhanced by applying a magnetic field B, which could induce an additional polarization P(H) upon the conventional P(E) by the cycloid conical spin in the intermediate phases. These results suggest that La0.2Sr0.7Fe12O19 exhibits a strong interplay between magnetic ordering and ferroelectricity, which makes it a good magnetoelectric coupling candidate.",1803.00731v1 2018-03-06,Magnon-assisted tunnelling in van der Waals heterostructures based on CrBr3,"The growing family of two-dimensional (2D) materials that are now available can be used to assemble van der Waals heterostructures with a wide range of properties. Of particular interest are tunnelling heterostructures, which have been used to study the electronic states both in the tunnelling barrier and in the emitter and collector contacts. Recently, 2D ferromagnets have been studied theoretically and experimentally. Here we investigate electron tunnelling through a thin (2-6 layers) ferromagnetic CrBr3 barrier. For devices with non-magnetic barriers, conservation of momentum can be relaxed by phonon-assisted tunnelling or by tunnelling through localised states. In the case of our ferromagnetic barrier the dominant tunnelling mechanisms are the emission of magnons at low temperatures or scattering of electrons on localised magnetic excitations above the Curie temperature. Tunnelling with magnon emission offers the possibility of injecting spin into the collector electrode.",1803.02120v2 2018-03-07,Magnetoresistance in Hybrid Pt/CoFe2O4 Bilayers Controlled by Competing Spin Accumulation and Interfacial Chemical Reconstruction,"Pure spin currents hold promises for an energy-friendlier spintronics. They can be generated by a flow of charge along a non-magnetic metal having a large spin-orbit coupling. It produces a spin accumulation at its surfaces, controllable by the magnetization of an adjacent ferromagnetic layer. Paramagnetic metals typically used are close to a ferromagnetic instability and thus magnetic proximity effects can contribute to the observed angular-dependent magnetoresistance (ADMR). As interface phenomena govern the spin conductance across the metal/ferromagnetic-insulator heterostructures, unraveling these distinct contributions is pivotal to full understanding of spin current conductance. We report here x-ray absorption and magnetic circular dichroism (XMCD) at Pt-M and (Co,Fe)-L absorption edges and atomically-resolved energy loss electron spectroscopy (EELS) data of Pt/CoFe2O4 bilayers where CoFe2O4 layers have been capped by Pt grown at different temperatures. It turns out that the ADMR differs dramatically, being either dominated by spin Hall magnetoresistance (SMR) associated to spin Hall effect or anisotropic magnetoresistance (AMR). The XMCD and EELS data indicate that the Pt layer grown at room temperature does not display any magnetic moment, whereas when grown at higher temperature it is magnetic due to interfacial Pt-(Co,Fe) alloying. These results allow disentangling spin accumulation from interfacial chemical reconstructions and for tailoring the angular dependent magnetoresistance.",1803.02611v1 2018-03-13,Magneto-optic dynamics in a ferromagnetic nematic liquid crystal,"We investigate dynamic magneto-optic effects in a ferromagnetic nematic liquid crystal experimentally and theoretically. Experimentally we measure the magnetization and the phase difference of the transmitted light when an external magnetic field is applied. As a model we study the coupled dynamics of the magnetization, M, and the director field, n, associated with the liquid crystalline orientational order. We demonstrate that the experimentally studied macroscopic dynamic behavior reveals the importance of a dynamic cross-coupling between M and n. The experimental data are used to extract the value of the dissipative cross-coupling coefficient. We also make concrete predictions about how reversible cross-coupling terms between the magnetization and the director could be detected experimentally by measurements of the transmitted light intensity as well as by analyzing the azimuthal angle of the magnetization and the director out of the plane spanned by the anchoring axis and the external magnetic field. We derive the eigenmodes of the coupled system and study their relaxation rates. We show that in the usual experimental set-up used for measuring the relaxation rates of the splay-bend or twist-bend eigenmodes of a nematic liquid crystal one expects for a ferromagnetic nematic liquid crystal a mixture of at least two eigenmodes.",1803.04898v1 2018-03-14,Critical behavior of the van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$,"The critical properties of the single-crystalline van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$ were investigated by bulk dc magnetization around the paramagnetic (PM) to ferromagnetic (FM) phase transition. The Fe$_{3-x}$GeTe$_2$ single crystals grown by self-flux method with Fe deficiency $x \approx 0.36$ exhibit bulk FM ordering below $T_c = 152$ K. The M\""{o}ssbauer spectroscopy was used to provide information on defects and local atomic environment in such crystals. Critical exponents $\beta = 0.372(4)$ with a critical temperature $T_c = 151.25(5)$ K and $\gamma = 1.265(15)$ with $T_c = 151.17(12)$ K are obtained by the Kouvel-Fisher method whereas $\delta = 4.50(1)$ is obtained by a critical isotherm analysis at $T_c = 151$ K. These critical exponents obey the Widom scaling relation $\delta = 1+\gamma/\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M(H)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m = f_\pm(h)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg ($d = 3, n = 3$) spins coupled with the attractive long-range interactions between spins that decay as $J(r)\approx r^{-(3+\sigma)}$ with $\sigma=1.89$.",1803.05905v1 2018-03-26,Domain-wall-assisted giant magnetoimpedance of thin-wall ferromagnetic nanotubes,"We study the effciency of the magnetoimpedance (MI) of thin-wall circumferentially ordered nanotubes in sub-GHz and GHz frequency regimes using micromagnetic simulations. We consider empty ferromagnetic tubes as well as tubes filled with non-magnetic conductors of circular cross-section (nanowire coverings) focusing on the low-field regime of MI (below the characteristic field of the low-frequency ferromagnetic resonance). In this field area, the effcient mechanism of MI is related to oscillations of the positions of (perpendicular to the tube axis) domain walls (DWs). Two mechanisms of driving the DW motion by the ac current are taken into account; the driving via the Oersted field and via the spintransfer torque. The simulations are performed for Co nanotubes of the diameter of 300nm. Achievable low-field MI exceeds 100%, while the field region of a high sensitivity of that DW-based giant MI is of the width of tens of kA/m. The later is widely adjustable with changing the density of the driving ac current, its frequency, and the nanotube length. Of particular interest is the resonant motion of DW due to the interaction with the nanotube ends the conditions of whom are discussed.",1803.09512v2 2018-03-28,Collapse of ferromagnetism with Ti doping in Sm$_{0.55}$Sr$_{0.45}$MnO$_3$: A combined experimental and theoretical study,"We have investigated the effect of Ti doping on the transport properties coupled with the magnetic ones in Sm$_{0.55}$Sr$_{0.45}$Mn$_{1-\eta}$Ti$_{\eta}$O$_3$ ($0 \leq \eta \leq 0.04$). The parent compound, Sm$_{0.55}$Sr$_{0.45}$MnO$_3$, exhibits a first-order paramagnetic-insulator to ferromagnetic-metal transition just below $T_{\rm c}$ = 128 K. With substitution of Ti at Mn sites ($B$-site), $T_{\rm c}$ decreases approximately linearly at the rate of 22 K$\%^{-1}$ while the width of thermal hysteresis in magnetization and resistivity increases almost in an exponential fashion. The most spectacular effect has been observed for the composition $\eta$=0.03, where a magnetic field of only 1 T yields a huge magnetoresistance, $1.2 \times 10^7$ $\%$ at $T_c\approx$ 63 K. With increasing magnetic field, the transition shifts towards higher temperature, and the first-order nature of the transition gets weakened and eventually becomes crossover above a critical field ($H_{cr}$) which increases with Ti doping. For Ti doping above 0.03, the system remains insulting without any ferromagnetic ordering down to 2 K. The Monte-Carlo calculations based on a two-band double exchange model show that the decrease of $T_{\rm c}$ with Ti doping is associated with the increase of the lattice distortions around the doped Ti ions.",1803.10588v1 2018-06-07,Electrically induced and detected Néel vector reversal in a collinear antiferromagnet,"Electrical detection of the 180 deg spin reversal, which is the basis of the operation of ferromagnetic memories, is among the outstanding challenges in the research of antiferromagnetic spintronics. Analogous effects to the ferromagnetic giant or tunneling magnetoresistance have not yet been realized in antiferromagnetic multilayers. Anomalous Hall effect (AHE), which has been recently employed for spin reversal detection in non-collinear antiferromagnets, is limited to materials that crystalize in ferromagnetic symmetry groups. Here we demonstrate electrical detection of the 180 deg N\'eel vector reversal in CuMnAs which comprises two collinear spin sublattices and belongs to an antiferromagnetic symmetry group with no net magnetic moment. We detect the spin reversal by measuring a second-order magnetotransport coefficient whose presence is allowed in systems with broken space inversion symmetry. The phenomenology of the non-linear transport effect we observe in CuMnAs is consistent with a microscopic scenario combining anisotropic magneto-resistance (AMR) with a transient tilt of the N\'eel vector due to a current-induced, staggered spin-orbit field. We use the same staggered spin-orbit field, but of a higher amplitude, for the electrical switching between reversed antiferromagnetic states which are stable and show no sign of decay over 25 hour probing times.",1806.02795v1 2018-06-25,Stacking tunable interlayer magnetism in bilayer CrI3,"Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasi-bonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g. magnetism. Inspired by several recent experiments showing interlayer anti-ferromagnetically coupled CrI3 bilayers, we carried out first-principles calculations for CrI3 bilayers. We found that the anti-ferromagnetic coupling results from a new stacking order with the C2/m space group symmetry, rather than the graphene-like one with R3 as previously believed. Moreover, we demonstrated that the intra- and inter-layer couplings in CrI3 bilayer are governed by two different mechanisms, namely ferromagnetic super-exchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong- and weak-interaction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a new paradigm for tuning interlayer magnetic properties with the freedom of stacking order in two dimensional layered materials.",1806.09274v2 2018-07-12,Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal,"In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$_2$MnGa, reaching $S_{yx}\sim -6$ $\mu$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $\alpha_{yx}$ shows a crossover between $T$-linear and $-T \log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $\alpha_{yx}$. The $T$ dependence of $\alpha_{yx}$ in experiments and numerical calculations can be understood in terms of a quantum critical scaling function predicted by the low energy effective theory over more than a decade of temperatures. Moreover, the observation of chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.",1807.04761v1 2018-07-20,Another view on Gilbert damping in two-dimensional ferromagnets,"A keen interest towards technological implications of spin-orbit driven magnetization dynamics requests a proper theoretical description, especially in the context of a microscopic framework, to be developed. Indeed, magnetization dynamics is so far approached within Landau-Lifshitz-Gilbert equation which characterizes torques on magnetization on purely phenomenological grounds. Particularly, spin-orbit coupling does not respect spin conservation, leading thus to angular momentum transfer to lattice and damping as a result. This mechanism is accounted by the Gilbert damping torque which describes relaxation of the magnetization to equilibrium. In this study we work out a microscopic Kubo-St\v{r}eda formula for the components of the Gilbert damping tensor and apply the elaborated formalism to a two-dimensional Rashba ferromagnet in the weak disorder limit. We show that an exact analytical expression corresponding to the Gilbert damping parameter manifests linear dependence on the scattering rate and retains the constant value up to room temperature when no vibrational degrees of freedom are present in the system. We argue that the methodology developed in this paper can be safely applied to bilayers made of non- and ferromagnetic metals, e.g., CoPt.",1807.07897v2 2018-07-24,Spin-split conductance and subgap peak in ferromagnet/superconductor spin valve heterostructures,"We consider the separate spin channel contributions to the charge conductance in superconducting/ferromagnetic spin valve $F_1/N/F_2/S$ structures. We find that the up- and down-spin conductance contributions may have a very different behavior in the subgap bias region (i.e. there is a spin-split conductance). This leads to a subgap %EM2 added spin-split conductance peak in the total conductance. This peak behavior, which can be prominent also in $N/F/S$ systems, is strongly dependent, in a periodic way, on the thickness of the intermediate ferromagnetic layer. We study this phenomenon using a numerical self consistent method, with additional insights gained from an approximate analytic calculation for an infinite $N/F/S$ structure. We study also the angular dependence on the relative magnetization angle between $F_1$ and $F_2$ of both the spin-split and the total conductance. We do so for realistic material parameters and layer thicknesses relevant to experimental studies on these devices. We also find that the spin-split conductance is highly dependent on the interfacial scattering in these devices, and we carefully include these effects for realistic systems. A strong valve-effect is found for the angularly dependent subgap peak conductance that is largely independent on the scattering and may prove useful in actual realizations of a superconducting spin valve device.",1807.09206v1 2018-08-01,Raman fingerprint of two terahertz spin wave branches in a two-dimensional honeycomb Ising ferromagnet,"Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. So far, a comprehensive understanding of the magnetic excitations in such 2D magnets remains missing. Here we report polarized micro-Raman spectroscopy studies on a 2D honeycomb ferromagnet CrI3. We show the definitive evidence of two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and 3.75 THz, respectively, that are three orders of magnitude higher than those of conventional ferromagnets. By tracking the thickness dependence of both spin waves, we reveal that both are surface spin waves with lifetimes an order of magnitude longer than their temporal periods. Our results of two branches of high-frequency, long-lived surface spin waves in 2D CrI3 demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.",1808.00168v3 2018-08-02,Generation of pure superconducting spin current in magnetic heterostructures via non-locally induced magnetism due to Landau Fermi-liquid effects,"We propose a mechanism for the generation of pure superconducting spin-current carried by equal-spin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (N$_{\rm so}$) with intrinsic spin-orbit coupling. We show that in the presence of Landau Fermi-liquid interactions the superconducting proximity effect can induce non-locally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Landau Fermi-liquid exchange field leads to the onset of a spin supercurrent associated with the generation of long-range spin-triplet superconducting correlations in the trilayer. We demonstrate that the magnitude of the spin supercurrent as well as the induced magnetic order in the N$_{\rm so}$ layer depends critically on the superconducting proximity effect between the S layer and the F and N$_{\rm so}$ layers and the magnitude of the relevant Landau Fermi-liquid interaction parameter. We investigate the effect of spin flip processes on this mechanism. Our results demonstrate the crucial role of Landau Fermi-liquid interaction in combination with spin-orbit coupling for the creation of spin supercurrent in superconducting spintronics, and give a possible explanation of a recent experiment utilizing spin-pumping via ferromagnetic resonance [Jeon $\it{ et}$ $\it{al.}$, Nat. Mat. ${\bf 17}$, 499 (2018)].",1808.01045v1 2018-08-03,Collapse of Kondo state and ferromagnetic quantum phase transition in YbFe$_2$Zn$_{20}$,"We present the electrical resistivity data under application of pressures up to $\sim$ 26 GPa and down to 50 mK temperatures on YbFe$_2$Zn$_{20}$. We find a pressure induced magnetic phase transition with an onset at $p_c$=18.2$\pm$0.8 GPa. At ambient pressure, YbFe$_2$Zn$_{20}$ manifests a heavy fermion, nonmagnetic ground state and the Fermi liquid behavior at low temperatures. As pressure is increased, the power law exponent in resistivity, $n$, deviates significantly from Fermi liquid behavior and tends to saturate with $n$ = 1 near $p_c$. A pronounced resistivity maximum, $T_\text{max}$, which scales with Kondo temperature is observed. $T_\text{max}$ decreases with increasing pressure and flattened out near $p_c$ indicating the suppression of Kondo exchange interaction. For $p>p_c$, $T_\text{max}$ shows a sudden upward shift, most likely becoming associated with crystal electric field scattering. Application of magnetic field for $p>p_c$ broadens the transition and shifts it toward the higher temperature, which is a typical behavior of the ferromagnetic transition. The magnetic transition appears to abruptly develop above $p_c$, suggesting probable first-order (with changing pressure) nature of the transition; once stabilized, the ordering temperature does not depend on pressure up to $\sim$ 26 GPa. Taken as a whole, these data suggest that YbFe$_2$Zn$_{20}$ has a quantum phase transition at $p_c$ = 18.2 GPa associated with the avoided quantum criticality in metallic ferromagnets.",1808.01367v1 2018-08-13,Ferromagnetism in graphene traced to an antisymmetric orbital combination of involved electronic states,"We reveal that the origin of ferromagnetism caused by $sp$ electrons in graphene with vacancies can be traced to electrons partially filling $sp^{2*}$-antibonding and $p_z^*$-nonbonding states, which are induced by the vacancies and appear near the Fermi level. Because the spatial wavefunctions of the both states are composed of atomic orbitals in an antisymmetric configuration, their spin wavefunctions should be symmetric according to the electron exchange antisymmetric principle, leading to electrons partially filling these states in spin polarization. Since this $p_z^*$ state originates not from interactions between the atoms but from the unpaired $p_z$ orbitals due to the removal of $p_z$ orbitals on the minority sublattice, the $p_z^*$ state is constrained, distributed on the atoms of the majority sublattice, and decays gradually from the vacancy as $\sim$ $1/r$. According to these characteristics, we concluded that the $p_z^*$ state plays a critical role in magnetic ordering in graphene with vacancies. If the vacancy concentration in graphene is large enough to cause the decay-length regions to overlap, constraining the $p_z^*$ orbital components as little as possible on the minority sublattice atoms in the overlap regions results in the vacancy-induced $p_z^*$ states being coherent. The coherent process in the overlap region leads to the wavefunctions in all the involved regions antisymmetrized, consequently causing ferromagnetism according to the electron exchange antisymmetric principle. This unusual mechanism concerned with the origin of $sp$-electron magnetism and magnetic ordering has never before been reported and is distinctly different from conventional mechanisms.",1808.04055v1 2018-08-15,Unconventional Planar Hall Effect in Exchange-Coupled Topological Insulator-Ferromagnetic Insulator Heterostructures,"The Dirac electrons occupying the surface states (SSs) of topological insulators (TIs) have been predicted to exhibit many exciting magneto-transport phenomena. Here we report on the first experimental observation of an unconventional planar Hall effect (PHE) and an electrically gate-tunable hysteretic planar magnetoresistance (PMR) in EuS/TI heterostructures, in which EuS is a ferromagnetic insulator (FMI) with an in-plane magnetization. In such exchange-coupled FMI/TI heterostructures, we find a significant (suppressed) PHE when the in-plane magnetic field is parallel (perpendicular) to the electric current. This behavior differs from previous observations of the PHE in ferromagnets and semiconductors. Furthermore, as the thickness of the 3D TI films is reduced into the 2D limit, in which the Dirac SSs develop a hybridization gap, we find a suppression of the PHE around the charge neutral point indicating the vital role of Dirac SSs in this phenomenon. To explain our findings, we outline a symmetry argument that excludes linear-Hall mechanisms and suggest two possible non-linear Hall mechanisms that can account for all the essential qualitative features in our observations.",1808.05268v1 2018-08-21,"Zero point fluctuations for magnetic spirals and Skyrmions, and the fate of the Casimir energy in the continuum limit","We study the role of zero-point quantum fluctuations in a range of magnetic states which on the classical level are close to spin-aligned ferromagnets. These include Skyrmion textures characterized by non-zero topological charge, and topologically-trivial spirals arising from the competition of the Heisenberg and Dzyaloshinskii-Moriya interactions. For the former, we extend our previous results on quantum exactness of classical Bogomolny-Prasad-Sommerfield (BPS) ground-state degeneracies to the general case of K\""ahler manifolds, with a specific example of Grassmann manifolds $\mathrm{Gr(M,N)}$. These are relevant to quantum Hall ferromagnets with $\mathrm{N}$ internal states and integer filling factor $\mathrm{M}$. A promising candidate for their experimental implementation is monolayer graphene with $\mathrm{N=4}$ corresponding to spin and valley degrees of freedom at the charge neutrality point with $\mathrm{M=2}$ filled Landau levels. We find that the vanishing of the zero-point fluctuations in taking the continuum limit occurs differently in the case of BPS states compared to the case of more general smooth textures, with the latter exhibiting more pronounced lattice effects. This motivates us to consider the vanishing of zero-point fluctuations in such near-ferromagnets more generally. We present a family of lattice spin models for which the vanishing of zero-point fluctuations is evident, and show that some spirals can be thought of as having nonzero but weak zero-point fluctuations on account of their closeness to this family. Between them, these instances provide concrete illustrations of how the Casimir energy, dependent on the full UV-structure of the theory, evolves as the continuum limit is taken.",1808.06783v1 2018-08-22,Role of magnons and the size effect in heat transport through an insulating ferromagnet/insulator interface,"While recent experiments on the spin Seebeck effect have revealed the decisive role of the magnon contribution to the heat current $Q$ in hybrid systems containing thin ferromagnetic layers, the available acoustic mismatch theory does not account for their magnetic properties. Here, we analyze theoretically the heat transfer through an insulating ferromagnet (F) sandwiched between two insulators (I). Depending on the relation between the F thickness, $d$, and the mean free path of phonons generated by magnons, $l_{ls}$, we reveal two qualitatively different regimes in the nonlinear heat transport through the F/I interfaces. Namely, in thick F layers the regime of conventional ""Joule"" heating with $Q \propto T_s^4$ is realized, in which the detailed structure of the F/I interfaces is inessential. Here $T_s$ is the magnon temperature. By contrast, in thin F layers with $d\ll l_{ls}$, most of phonons emitted by magnons can leave F without being absorbed in its interior, giving rise to the \emph{magnon overheating} regime with $Q \propto T_s^m$ and $m\gtrsim7$. Conditions for the examination of both regimes and the determination of $T_s$ from experiments are discussed. The reported results are relevant for the theoretical analysis of the spin Seebeck effect and the development of magnon-based spin caloritronic devices.",1808.07294v1 2018-12-01,"Strain and onsite-correlation tunable quantum anomalous Hall phases in ferromagnetic (111) LaXO$_3$ bilayers (X$=$Pd, Pt)","Quantum anomalous Hall (QAH) phases in magnetic topological insulators are characterized by the scattering-free chiral edge currents protected by their nontrivial bulk band topology. To fully explore these intriguing phenomena and application of topological insulators, high temperature material realization of QAH phases is crucial. In this paper, based on extensive first-principles density functional theory calculations, we predict that perovskite bilayers (LaXO$_3$)$_{2}$ (X = Pd, Pt) imbedded in the (111) (LaXO$_3$)$_{2}$/(LaAlO$_3$)$_{10}$ superlattices are high Curie temperature ferromagnets that host both QAH and Dirac semimetal phases, depending on the biaxial strain and onsite electron correlation. In particular, both the direction (the sign of Chern number) and spin-polarization of the chiral edge currents are tunable by either onsite electron correlation or biaxial in-plane strain. Furthermore, the nontrivial band gap can be enhanced up to 92 meV in the LaPdO$_3$ bilayer by the compressive in-plane strain, and can go up to as large as 242 meV when the Pd atoms are replaced by the heavier Pt atoms. Finally, the microscopic mechanisms of the ferromagnetic coupling and other interesting properties of the bilayers are uncovered by analyzing their underlying electronic band structures.",1812.01944v1 2018-12-12,First-principles prediction of half-Heusler half-metals above room temperature,"Half-metallicity (HM) offers great potential for engineering spintronic applications, yet only few magnetic materials present metallicity in just one spin channel. In addition, most HM systems become magnetically disordered at temperatures well below ambient conditions, which further hinders the development of spin-based electronic devices. Here, we use first-principles methods based on density functional theory (DFT) to investigate the electronic, magnetic, structural, mixing, and vibrational properties of $90$ $XYZ$ half-Heusler (HH) alloys ($X =$ Li, Na, K, Rb, Cs; $Y =$ V,Nb, Ta; $Z =$ Si, Ge, Sn, S, Se, Te). We disclose a total of $28$ new HH compounds that are ferromagnetic, vibrationally stable, and HM, with semiconductor band gaps in the range of $1$-$4$ eV and HM band gaps of $0.2$-$0.8$ eV. By performing Monte Carlo simulations of a spin Heisenberg model fitted to DFT energies, we estimate the Curie temperature, $T_{\rm C}$, of each HM compound. We find that $17$ HH HM remain magnetically ordered at and above room temperature, namely, $300 \le T_{\rm C} \le 450$ K, with total magnetic moments of $2$ and $4$ $\mu_{\rm B}$. A further materials sieve based on zero-temperature mixing energies let us to conclude $5$ overall promising ferromagnetic HH HM at and above room temperature: NaVSi, RbVTe, CsVS, CsVSe, and RbNbTe. We also predict $2$ ferromagnetic materials that are semiconductor and magnetically ordered at ambient conditions: LiVSi and LiVGe.",1812.04813v1 2018-12-18,Low-temperature marginal ferromagnetism explains anomalous scale-free correlations in natural flocks,"We introduce a new ferromagnetic model capable of reproducing one of the most intriguing properties of collective behaviour in starling flocks, namely the fact that strong collective order of the system coexists with scale-free correlations of the modulus of the microscopic degrees of freedom, that is the birds' speeds. The key idea of the new theory is that the single-particle potential needed to bound the modulus of the microscopic degrees of freedom around a finite value, is marginal, that is has zero curvature. We study the model by using mean-field approximation and Monte Carlo simulations in three dimensions, complemented by finite-size scaling analysis. While at the standard critical temperature, $T_c$, the properties of the marginal model are exactly the same as a normal ferromagnet with continuous symmetry-breaking, our results show that a novel zero-temperature critical point emerges, so that in its deeply ordered phase the marginal model develops divergent susceptibility and correlation length of the modulus of the microscopic degrees of freedom, in complete analogy with experimental data on natural flocks of starlings.",1812.07522v2 2019-01-08,Magnetoelectric Coupling by Giant Piezoelectric Tensor Design,"Strain-coupled magnetoelectric (ME) phenomena in piezoelectric / ferromagnetic thin-film bilayers are a promising paradigm for sensors and information storage devices, where strain is utilized to manipulate the magnetization of the ferromagnetic film. In-plane magnetization rotation with an electric field across the film thickness has been challenging due to the virtual elimination of in-plane piezoelectric strain by substrate clamping, and to the requirement of anisotropic in-plane strain in two-terminal devices. We have overcome both of these limitations by fabricating lithographically patterned devices with a piezoelectric membrane on a soft substrate platform, in which in-plane strain is freely generated, and a patterned edge constraint that transforms the nominally isotropic piezoelectric strain into the required uniaxial strain. We fabricated 500 nm thick, (001) oriented [Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$]$_{0.7}$-[PbTiO$_3$]$_{0.3}$ (PMN-PT) unclamped piezoelectric membranes with ferromagnetic Ni overlayers. Guided by analytical and numerical continuum elastic calculations, we designed and fabricated two-terminal devices exhibiting Ni magnetization rotation in response to an electric field across the PMN-PT. Similar membrane heterostructures could be used to apply designed strain patterns to many other materials systems to control properties such as superconductivity, band topology, conductivity, and optical response.",1901.02456v1 2019-01-21,Flat-band Ferromagnetism of the SU$(N)$ Hubbard Model on the Tasaki Lattice,"We investigate the para-ferro transition of the repulsive SU($N$) Hubbard model on one- and two-dimensional Tasaki lattices. Under certain restrictions for constructing localized many-particle ground states of flat-band ferromagnetism, the quantum strongly correlated electrons model is mapped to a classical statistical geometric site-percolation problem, where the nontrivial weights of different configurations must be considered. We prove rigorously the para-ferro transition for the SU($N$) Hubbard model on one-dimensional Tasaki lattice by transfer-matrix method, and numerically verify it for the model with symmetries up to SU($10$). In two dimensions, we numerically investigate the phase transition of SU($3$), SU($4$) and SU($10$) Hubbard models by Metropolis Monte Carlo simulation. And we find the critical density exceeds that of standard percolation, and it increases with spin degrees of freedom, implying that the effective repulsive interaction becomes stronger for larger $N$. We further rigorously prove the flat-band ferromagnetism of the SU$(N)$ Hubbard model, when the number of particles $N_e$ equals to the degeneracy $N_d$ of the lowest band in the single-particle energy spectrum.",1901.07004v2 2019-01-24,A Single Pair of Weyl Fermions in Half-metallic EuCd2As2 Semimetal,"An ideal Weyl semimetal with a single pair of Weyl points (WPs) may be generated by splitting a single Dirac point (DP) through the breaking of time-reversal symmetry by magnetic order. However, most known Dirac semimetals possess a pair of DPs along an axis that is protected by crystalline symmetry. Here, we demonstrate that a single pair of WPs may also be generated from a pair of DPs. Using first-principles band structure calculations, we show that inducing ferromagnetism in the AFM Dirac semimetal EuCd2As2 generates a single pair of WPs due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that this ideal Weyl semimetal is obtained in EuCd2As2 because the DPs are very close to the zone center and the ferromagnetic exchange splitting is large enough to push one pair of WPs to merge and annihilate at Gamma while the other pair survives. Furthermore, we predict that alloying with Ba at the Eu site can stabilize the ferromagnetic configuration and generate a single pair of Weyl points without application of a magnetic field.",1901.08234v2 2019-01-22,Ferromagnetic-like behavior of Bi0.9La0.1FeO3-KBr nanocomposites,"We studied magnetostatic response of the Bi0.9La0.1FeO3-KBr composites (BLFO-KBr) consisting of nanosized (about 100 nm) ferrite Bi0.9La0.1FeO3 (BLFO) conjugated with fine grinded ionic conducting KBr. When the fraction of KBr is rather small (less than 15 wt percent) the magnetic response of the composite is very weak and similar to that observed for the BLFO (pure KBr matrix without Bi1-xLaxFeO3 has no magnetic response as anticipated). However, when the fraction of KBr increases above 15percent, the magnetic response of the composite changes substantially and the field dependence of magnetization reveals ferromagnetic-like hysteresis loop with a remanent magnetization about 0.14 emu/g and coercive field about 1.8 Tesla (at room temperature). Nothing similar to the ferromagnetic-like hysteresis loop can be observed in BLFO ceramics, which magnetization quasi linearly increases with magnetic field. Different physical mechanisms were considered to explain the unusual experimental results for BLFO-KBr nanocomposites, but only those among them, which are highly sensitive to the interaction of antiferromagnetic Bi0.9La0.1FeO3 with ionic conductor KBr, can be relevant. An appropriate mechanism turned out to be ferro-magneto-ionic coupling.",1901.08913v1 2019-01-26,Hard-Core and Soft-Core Widom-Rowlinson models on Cayley trees,"We consider both Hard-Core and Soft-Core Widom-Rowlinson models with spin values $-1,0,1$ on a Cayley tree of order $k\geq 2$ and we are interested in the Gibbs measures of the models. The models depend on 3 parameters: the order $k$ of the tree, $\theta$ describing the strength of the (ferromagnetic or antiferromagnetic) interaction, and $\lambda$ describing the intensity for particles. The Hard-Core Widom-Rowlinson model corresponds to the case $\theta=0$. For the binary tree $k=2$, and for $k=3$ we prove that the ferromagnetic model has either one or three splitting Gibbs measures (tree-automorphism invariant Gibbs measures (TISGM) which are tree-indexed Markov chains). We also give the exact form of the corresponding critical curves in parameter space. For higher values of $k$ we give an explicit sufficient bound ensuring non-uniqueness which we conjecture to be the exact curve. Moreover, for the antiferromagnetic model we explicitly give two critical curves and prove that on these curves there are exactly two TISGMs; between these curves there are exactly three TISGMs; otherwise there exists a unique TISGM. Also some periodic and non-periodic SGMs are constructed in the ferromagnetic model.",1901.09258v1 2019-01-31,Triplet superconductivity in ferromagnets due to magnon exchange,"We consider the superconducting pairing induced by spin waves exchange in a ferromagnet with both conduction and localized electrons, the latter being described as spins. We use the microscopic Eliashberg theory to describe the pairing of conducting electrons and the RPA approach to treat the localized spins assuming an exchange coupling between the conducting electrons and spins. In the framework of non relativistic Hamiltonian twe found that he spin wave exchange results in equal spin electron pairing described by the two components of the order parameter, $\Delta^{\uparrow}$ (both spins up) and $\Delta^{\downarrow}$ (both spins down). Due to the conservation of total spin projection on the axis of the spontaneous ferromagnetic moment, the spin wave exchange at low temperatures includes an emission of magnons and an absorption of thermal magnons by the conduction electrons. The absorption and emission processes depend differently on the temperature, with the absorption being progressively suppressed as the temperature drops. As a result, the superconducting pairing exists only if the electron-spin wave exchange parameter $g$ exceeds some critical value $g_c$. At $g>g_c$ pairing vanishes if the temperature drops below the lowest point $T_{cl}$ or increases above the upper critical point $T_{ch} \approx T_m$ (the Curie temperature) where the spin waves cease to exist. This behavior inherent to the spin carrying glue is in an obvious disagreement with the results of conventional BCS approach which assumes that the effective electron-electron attraction is simply proportional to the static magnetic susceptibility.",1901.11248v1 2019-02-09,"Robustness of ferromagnetism in (In,Fe)Sb diluted magnetic semiconductor to variation of charge carrier concentration and Fermi level position","The influence of He+ ion irradiation on the transport and magnetic properties of epitaxial layers of a diluted magnetic semiconductor (DMS) (In,Fe)Sb, a two-phase (In,Fe)Sb composite and a nominally undoped InSb semiconductor has been investigated. In all layers, a conductivity type conversion from the initial n-type to the ptype has been found. The ion fluence at which the conversion occurs depends on the Fe concentration in the InSb matrix. Magnetotransport properties of the two-phase (In,Fe)Sb layer are strongly affected by ferromagnetic Fe inclusions. An influence of the number of electrically active radiation defects on the magnetic properties of the single-phase In0.75Fe0.25Sb DMS has been found. At the same time, the results show that the magnetic properties of the In0.75Fe0.25Sb DMS are quite resistant to significant changes of the charge carrier concentration and the Fermi level position. The results confirm a weak interrelation between the ferromagnetism and the charge carrier concentration in (In,Fe)Sb.",1902.03465v2 2019-02-10,Evidence of Pure Spin-Current Generated by Spin Pumping in Interface Localized States in Hybrid Metal-Silicon-Metal Vertical Structures,"Due to the difficulty to grow high quality semiconductors on ferromagnetic metals, the study of spin diffusion transport in Si was only limited to lateral geometry devices. In this work, by using ultra-high vacuum wafer-bonding technique, we have successfully fabricated metal semiconductor metal CoFeB/MgO/Si/Pt vertical structures. We hereby demonstrate pure spin-current injection and transport in the perpendicular current flow geometry over a distance larger than 2\mu m in n-type Si at room temperature. In those experiments, a pure propagating spin-current is generated via ferromagnetic resonance spin-pumping and converted into a measurable voltage by using the inverse spin-Hall effect occurring in the top Pt layer. A systematic study by varying both Si and MgO thicknesses reveals the important role played by the localized states at the MgO/Si interface for the spin-current generation. Proximity effects involving indirect exchange interactions between the ferromagnet and the MgO/Si interface states appears to be a prerequisite to establish the necessary out-of-equilibrium spin-population in Si under the spin-pumping action.",1902.03652v1 2019-02-12,Magnetoelectric effect in band insulator-ferromagnet heterostructures,"We theoretically study magnetoelectric effects in a heterostructure of a generic band insulator and a ferromagnet. In contrast to the kinetic magnetoelectric effect in metals, referred to as the Edelstein effect or the inverse spin galvanic effect, our mechanism relies on virtual interband transitions between the valence and conduction bands and therefore immune to disorder or impurity scattering. By calculating electric field-induced magnetization by the linear response theory, we reveal that the magnetoelectric effect shows up without specific parameter choices. The magnetoelectric effect qualitatively varies by changing the direction of the magnetic moment in the ferromagnet: the response is diagonal for the out-of-plane moment, whereas it is off-diagonal for the inplane moment. We also find out that in optical frequencies, the magnetoelectric signal can be drastically enhanced via interband resonant excitations. Finally, we estimate the magnitude of the magnetoelectric effect for a hybrid halide perovskite semiconductor as an example of the band insulator and compare it with other magnetoelectric materials. We underscore that our mechanism is quite general and widely expectable, only requiring the Rashba spin-orbit coupling and exchange coupling. Our result could potentially offer a promising method of Joule heating-free electric manipulation of magnetic moments in spintronic devices.",1902.04204v1 2019-02-18,Stacking order-dependent sign-change of microwave phase due to eddy currents in nanometer-scale NiFe/Cu heterostructures,"In the field of spintronics, ferromagnetic/non-magnetic metallic multilayers are core building blocks for emerging technologies. Resonance experiments using stripline transducers are commonly used to characterize and engineer these stacks for applications. Up to now in these experiments, the influence of eddy currents on the excitation of the dynamics of ferromagnetic magnetization below the skin-depth limit was most often neglected. Here, using a coplanar stripline transducer, we experimentally investigated the broadband ferromagnetic resonance response of NiFe/Cu bilayers a few nanometers thick in the sub-skin-depth regime. Asymmetry in the absorption spectrum gradually built up as the excitation frequency and Cu-layer thickness increased. Most significantly, the sign of the asymmetry depended on the stacking order. Experimental data were consistent with a quantitative analysis considering eddy currents generated in the Cu layers and the subsequent phaseshift of the feedback magnetic field generated by the eddy currents. These results extend our understanding of the impact of eddy currents below the microwave magnetic skin-depth and explain the lineshape asymmetry and phase lags reported in stripline experiments.",1902.06501v3 2019-02-20,Spin-charge conversion in NiMnSb Heusler alloy films,"Half-metallic Heusler alloys are attracting considerable attention because of their unique half-metallic band structures which exhibit high spin polarization and yield huge magnetoresistance ratios. Besides serving as ferromagnetic electrodes, Heusler alloys also have the potential to host spin-charge conversion which has been recently demonstrated in other ferromagnetic metals. Here, we report on the spin-charge conversion effect in the prototypical Heusler alloy NiMnSb. Spin currents were injected from Y3Fe5O12 into NiMnSb films by spin pumping, and then the spin currents were converted to charge currents via spin-orbit interactions. Interestingly, an unusual charge signal was observed with a sign change at low temperature, which can be manipulated by film thickness and ordering structure. It is found that the spin-charge conversion has two contributions. First, the interfacial contribution causes a negative voltage signal, which is almost constant versus temperature. The second contribution is temperature dependent because it is dominated by minority states due to thermally excited magnons in the bulk part of the film. This work provides a pathway for the manipulation of spin-charge conversion in ferromagnetic metals by interface-bulk engineering for spintronic devices.",1902.07406v1 2019-02-26,Two dimensional ordering and collective magnetic excitations in the dilute ferromagnetic topological insulator (Bi$_{0.95}$Mn$_{0.05}$)$_{2}$Te$_{3}$,"Employing elastic and inelastic neutron scattering (INS) techniques, we report on detailed microscopic properties of the ferromagnetism in he magnetic topological insulator (Bi$_{0.95}$Mn$_{0.05}$)$_{2}$Te$_{3}$. Neutron diffraction of polycrystalline samples show the ferromagnetic (FM) ordering is long-range within the basal plane, and mainly 2D in character with short-range correlations between layers below $T_{\mathrm{C}} \approx 13$ K. Despite the random distribution of the dliute Mn atoms, we find that the 2D-like magnetic peaks are commensurate with the chemical structure, and the absence of (00L) magnetic peaks denote that the Mn$^{2+}$ magnetic moments are normal to the basal planes. Surprisingly, we observed collective magnetic excitations, in this dilute magnetic system. Despite the dilute nature, the excitations are typical of quasi-2D FM systems, albeit are severely broadened at short wavelengths, likely due to the random spatial distribution of Mn atoms in the Bi planes. Detailed analysis of the INS provide energy scales of the exchange couplings and the single ion anisotropy.",1902.10174v2 2019-03-01,Highly efficient spin-orbit torque and switching of layered ferromagnet Fe3GeTe2,"Among van der Waals (vdW) layered ferromagnets, Fe3GeTe2 (FGT) is an excellent candidate material to form FGT/heavy metal heterostructures for studying the effect of spin-orbit torques (SOT). Its metallicity, strong perpendicular magnetic anisotropy built in the single atomic layers, relatively high Curie temperature (Tc about 225 K) and electrostatic gate tunability offer a tantalizing possibility of achieving the ultimate high SOT limit in monolayer all-vdW nanodevices. The spin current generated in Pt exerts a damping-like SOT on FGT magnetization. At about 2.5x1011 A/m2 current density,SOT causes the FGT magnetization to switch, which is detected by the anomalous Hall effect of FGT. To quantify the SOT effect, we measure the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane. Our analysis shows that the SOT efficiency is comparable with that of the best heterostructures containing three-dimensional (3D) ferromagnetic metals and much larger than that of heterostructures containing 3D ferrimagnetic insulators. Such large efficiency is attributed to the atomically flat FGT/Pt interface, which demonstrates the great potential of exploiting vdW heterostructures for highly efficient spintronic nanodevices.",1903.00571v1 2019-03-05,Observation of superconducting gap spectra of long-range proximity effect in Au/SrTiO$_3$/SrRuO$_3$/Sr$_2$RuO$_4$ tunnel junctions,"We observe an unconventional superconducting minigap induced into a ferromagnet SrRuO$_3$ from a spin-triplet superconductor Sr$_2$RuO$_4$ using a Au/SrTiO$_3$/SrRuO$_3$/Sr$_2$RuO$_4$ tunnel junction. Voltage bias differential conductance of the tunnel junctions exhibits V-shaped gap features around zero bias, corresponding to a decrease in the density-of-states with an opening of a superconducting minigap in SrRuO$_3$. Observation of a minigap at a surface of a 15~nm thick SrRuO$_3$ layers confirms the spin-triplet nature of induced superconductivity. The shape and temperature dependence of the gap features in the differential conductance indicate that the even-frequency $p$-wave correlations dominate, over odd-frequency $s$-wave correlations. Theoretical calculations support this $p$-wave scenario. Our work provides the density-of-states proof for $p$-wave Cooper pair penetration in a ferromagnet and significantly put forward our understanding of the $p$-wave spin-triplet proximity effect between spin-triplet superconductors and ferromagnets.",1903.02093v1 2019-03-08,Spin-transfer torques for domain walls in antiferromagnetically coupled ferrimagnets,"Antiferromagnetic materials are outstanding candidates for next generation spintronic applications, because their ultrafast spin dynamics makes it possible to realize several orders of magnitude higher-speed devices than conventional ferromagnetic materials1. Though spin-transfer torque (STT) is a key for electrical control of spins as successfully demonstrated in ferromagnetic spintronics, experimental understanding of STT in antiferromagnets has been still lacking despite a number of pertinent theoretical studies2-5. Here, we report experimental results on the effects of STT on domain-wall (DW) motion in antiferromagnetically-coupled ferrimagnets. We find that non-adiabatic STT acts like a staggered magnetic field and thus can drive DWs effectively. Moreover, the non-adiabaticity parameter {\beta} of STT is found to be significantly larger than the Gilbert damping parameter {\alpha}, challenging our conventional understanding of the non-adiabatic STT based on ferromagnets as well as leading to fast current-induced antiferromagnetic DW motion. Our study will lead to further vigorous exploration of STT for antiferromagnetic spin textures for fundamental physics on spin-charge interaction as wells for efficient electrical control of antiferromagnetic devices.",1903.03251v1 2019-03-13,Nutation spectroscopy of a nanomagnet driven into deeply nonlinear ferromagnetic resonance,"Strongly out-of-equilibrium regimes in magnetic nanostructures exhibit novel properties, linked to the nonlinear nature of magnetization dynamics, which are of great fundamental and practical interest. Here, we demonstrate that field-driven ferromagnetic resonance can occur with substantial spatial coherency at unprecedented large angle of magnetization precessions, which is normally prevented by the onset of spin-wave instabilities and magnetization turbulent dynamics. Our results show that this limitation can be overcome in nanomagnets, where the geometric confinement drastically reduces the density of spin-wave modes. The obtained deeply nonlinear ferromagnetic resonance regime is probed by a new spectroscopic technique based on the application of a second excitation field. This enables to resonantly drive slow coherent magnetization nutations around the large angle periodic trajectory. Our experimental findings are well accounted for by an analytical model derived for systems with uniaxial symmetry. They also provide new means for controlling highly nonlinear magnetization dynamics in nanostructures, which open interesting applicative opportunities in the context of magnetic nanotechnologies.",1903.05411v1 2019-03-12,Critical behavior and magnetocaloric effect in VI$_3$,"Layered van der Waals ferromagnets are promising candidates for designing new spintronic devices. Here we investigated the critical properties and magnetocaloric effect connected with ferromagnetic transition in layered van der Waals VI$_3$ single crystals. The critical exponents $\beta = 0.244(5)$ with a critical temperature $T_c = 50.10(2)$ K and $\gamma = 1.028(12)$ with $T_c = 49.97(5)$ K are obtained from the modified Arrott plot, whereas $\delta = 5.24(2)$ is obtained from a critical isotherm analysis at $T_c = 50$ K. The magnetic entropy change $-\Delta S_M(T,H)$ features a maximum at $T_c$, i.e., $-\Delta S_M^{max} \sim$ 2.64 (2.27) J kg$^{-1}$ K$^{-1}$ with out-of-plane (in-plane) field change of 5 T. This is consistent with $-\Delta S_M^{max}$ $\sim$ 2.80 J kg$^{-1}$ K$^{-1}$ deduced from heat capacity and the corresponding adiabatic temperature change $\Delta T_{ad}$ $\sim$ 0.96 K with out-of-plane field change of 5 T. The critical analysis suggests that the ferromagnetic phase transition in VI$_3$ is situated close to a three- to two-dimensional critical point. The rescaled $\Delta S_M(T,H)$ curves collapse onto a universal curve, confirming a second-order type of the magnetic transition and reliability of the obtained critical exponents.",1903.05477v3 2019-03-15,Topological Hall effect in bulk ferromagnet Cr$_2$Te$_3$ embedded with black-phosphorus-like bismuth nanosheets,"We implement the molecular beam epitaxy method to embed the black-phosphorus-like bismuth nanosheets into the bulk ferromagnet Cr$_2$Te$_3$. As a typical surfactant, bismuth lowers the surface tensions and mediates the layer-by-layer growth of Cr$_2$Te$_3$. Meanwhile, the bismuth atoms precipitate into black-phosphorus-like nanosheets with the lateral size of several tens of nanometers. In Cr$_2$Te$_3$ embedded with Bi-nanosheets, we observe simultaneously a large topological Hall effect together with the magnetic susceptibility plateau and magnetoresistivity anomaly. As a control experiment, none of these signals is observed in the pristine Cr$_2$Te$_3$ samples. Therefore, the Bi-nanosheets serve as seeds of topological Hall effect induced by non-coplanar magnetic textures planted into Cr$_2$Te$_3$. Our experiments demonstrate a new method to generates a large topological Hall effect by planting strong spin-orbit couplings into the traditional ferromagnet, which may have potential applications in spintronics.",1903.06486v1 2019-03-26,Complex exchange mechanism driven ferromagnetism in half-metallic Heusler Co$_{2}$TiGe: Evidence from critical behavior,"We have investigated the critical phenomenon associated with the magnetic phase transition in the half-metallic full-Heusler Co$_2$TiGe. The compound undergoes a continuous ferromagnetic to paramagnetic phase transition at the Curie temperature $T_{C}$=371.5 K. The analysis of magnetization isotherms in the vicinity of $T_{c}$, following modified Arrott plot method, Kouvel-Fisher technique, and critical isotherm plot, yields the asymptotic critical exponents $\beta$=0.495, $\gamma$=1.324, and $\delta$=3.67. The self-consistency and reliability of the obtained exponents are further verified by the Widom scaling relation and scaling equation of states. The mean-field-like value of the critical exponent $\beta$ suggests long-range nature of the exchange interactions, whereas the values of the critical exponents $\gamma$ and $\delta$, imply sizeable critical spin fluctuations. The half-metallic itinerant character of Co$_{2}$TiGe in the presence of magnetic inhomogeneity may result in such a strong deviation from the three-dimensional Heisenberg values ($\beta$=0.369, $\gamma$=1.38 and $\delta$=4.8) of the critical exponents towards the mean field values ($\beta$=0.5, $\gamma$=1 and $\delta$=3). The results suggest complex nature of exchange couplings that stabilize the long-range ferromagnetic ordering in the system and are consistent with the earlier theoretical studies on the exchange mechanism in Co$_2$TiGe.",1903.10987v1 2019-03-27,Predicting magnetization of ferromagnetic binary Fe alloys from chemical short range order,"Among the ferromagnetic binary alloys, body centered cubic (bcc) Fe-Co is the one showing the highest magnetization. It is known experimentally that ordered Fe-Co structures show a larger magnetization than the random solid solutions with the same Co content. In this work, based on density functional theory (DFT) studies, we aim at a quantitative prediction of this feature, and point out the role of the orbital magnetic moments. Then, we introduce a DFT-based analytical model correlating local magnetic moments and chemical compositions for Co concentrations ranging from 0 to 70 at.%. It is also extended to predict the global magnetization of both ordered and disordered structures at given concentration and chemical short range orders. The latter model is particularly useful for interpreting experimental data. Based on these models, we note that the local magnetic moment of a Fe atom is mainly dictated by the Co concentration in its first two neighbor shells. The detailed local arrangement of the Co atoms has a minor effect. These simple models can fully reproduce the difference in magnetization between the ordered and disordered Fe-Co alloys between 30% and 70% Co, in good agreement with experimental data. Finally, we show that a similar model can be established for another bcc binary Fe alloy, the Fe-Ni, also presenting ferromagnetic interactions between atoms.",1903.11468v1 2019-04-02,Efficient quantum and simulated annealing of Potts models using a half-hot constraint,"The Potts model is a generalization of the Ising model with $Q>2$ components. In the fully connected ferromagnetic Potts model, a first-order phase transition is induced by varying thermal fluctuations. Therefore, the computational time required to obtain the ground states by simulated annealing exponentially increases with the system size. This study analytically confirms that the transverse magnetic-field quantum annealing induces a first-order phase transition. This result implies that quantum annealing does not exponentially accelerate the ground-state search of the ferromagnetic Potts model. To avoid the first-order phase transition, we propose an iterative optimization method using a half-hot constraint that is applicable to both quantum and simulated annealing. In the limit of $Q \to \infty$, a saddle point equation under the half-hot constraint is identical to the equation describing the behavior of the fully connected ferromagnetic Ising model, thus confirming a second-order phase transition. Furthermore, we verify the same relation between the fully connected Potts glass model and the Sherrington--Kirkpatrick model under assumptions of static approximation and replica symmetric solution. The proposed method is expected to obtain low-energy states of the Potts models with high efficiency using Ising-type computers such as the D-Wave quantum annealer and the Fujitsu Digital Annealer.",1904.01522v2 2019-04-03,Haldane and Dimer phases in a frustrated spin chain: an exact groundstate and associated topological phase transition,"A Heisenberg spin-$s$ chain with alternating ferromagnetic ($-J_1^F<0$) and antiferromagnetic ($J_1^A>0$) nearest-neighbor (NN) interactions, exhibits the Dimer and spin-$2s$ Haldane phases in the limits $J_1^F/J_1^A \rightarrow 0$ and $J_1^F/J_1^A \rightarrow \infty$ respectively. These two phases are understood to be topologically equivalent. Induction of the frustration through the next nearest-neighbor ferromagnetic interaction ($-J_2^F<0$) produces a very rich quantum phase diagram. With frustration, the whole phase diagram is divided into a ferromagnetic (FM) and a nonmagnetic (NM) phase. For $s=1/2$, the full NM phase is seen to be of Haldane-Dimer type, but for $s>1/2$, a spiral phase comes between the FM and the Haldane-Dimer phases. The study of a suitably defined string-order parameter and spin-gap at the phase boundary indicates that the Haldane-Dimer and spiral phases have different topological characters. We also find that, along the $J_2^F=\frac 12 J_1^F$ line in the NM phase, an NN dimer state is the {\it exact} groundstate, provided $J_1^A>J_C=\kappa J_1^F$ where $\kappa \le s + h$ for applied magnetic field $h$. Without magnetic field, the position of $J_C$ is on the FM-NM phase boundary when $s=1/2$, but for $s>1/2$, the location of $J_C$ is on the phase separation line between the Haldane-Dimer and spiral phases.",1904.02102v3 2019-04-25,High Spin-Wave Propagation Length Consistent with Low Damping in a Metallic Ferromagnet,"We report ultra-low intrinsic magnetic damping in Co$_{\text{25}}$Fe$_{\text{75}}$ heterostructures, reaching the low $10^{-4}$ regime at room temperature. By using a broadband ferromagnetic resonance technique, we extracted the dynamic magnetic properties of several Co$_{\text{25}}$Fe$_{\text{75}}$-based heterostructures with varying ferromagnetic layer thickness. By estimating the eddy current contribution to damping, measuring radiative damping and spin pumping effects, we found the intrinsic damping of a 26\,nm thick sample to be $$\alpha_{\mathrm{0}} \lesssim 3.18\times10^{-4}$. Furthermore, using Brillouin light scattering microscopy we measured spin-wave propagation lengths of up to $(21\pm1)\,\mathrm{\mu m}$ in a 26 nm thick Co$_{\text{25}}$Fe$_{\text{75}}$ heterostructure at room temperature, which is in excellent agreement with the measured damping.",1904.11321v3 2019-04-30,Tunable ferromagnetic resonance in coupled trilayers with crossed in-plane and perpendicular magnetic anisotropies,"A novel approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni$_{80}$Fe$_{20}$ (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCo$_\text{x}$ film with perpendicular magnetic anisotropy (PMA) through a non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect (TMOKE), alternating gradient magnetometry (AGM) as well as vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both Co concentration and Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic field is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than $\pm$70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.",1904.13275v1 2019-08-01,Pursuing High-Temperature Quantum Anomalous Hall Effect in MnBi$_2$Te$_4$/Sb$_2$Te$_3$ Heterostructures,"Quantum anomalous Hall effect (QAHE) has been experimentally realized in magnetically-doped topological insulators or intrinsic magnetic topological insulator MnBi$_2$Te$_4$ by applying an external magnetic field. However, either the low observation temperature or the unexpected external magnetic field (tuning all MnBi$_2$Te$_4$ layers to be ferromagnetic) still hinders further application of QAHE. Here, we theoretically demonstrate that proper stacking of MnBi$_2$Te$_4$ and Sb$_2$Te$_3$ layers is able to produce intrinsically ferromagnetic van der Waals heterostructures to realize the high-temperature QAHE. We find that interlayer ferromagnetic transition can happen at $T_{\rm C}=42~\rm K$ when a five-quintuple-layer Sb$_2$Te$_3$ topological insulator is inserted into two septuple-layer MnBi$_2$Te$_4$ with interlayer antiferromagnetic coupling. Band structure and topological property calculations show that MnBi$_2$Te$_4$/Sb$_2$Te$_3$/MnBi$_2$Te$_4$ heterostructure exhibits a topologically nontrivial band gap around 26 meV, that hosts a QAHE with a Chern number of $\mathcal{C}=1$. In addition, our proposed materials system should be considered as an ideal platform to explore high-temperature QAHE due to the fact of natural charge-compensation, originating from the intrinsic n-type defects in MnBi$_2$Te$_4$ and p-type defects in Sb$_2$Te$_3$.",1908.00498v2 2019-08-20,Quantum anomalous Hall effect driven by magnetic proximity coupling in all-telluride based heterostructure,"The quantum anomalous Hall effect (QAHE) is an exotic quantum phenomenon originating from dissipation-less chiral channels at the sample edge. While the QAHE has been observed in magnetically doped topological insulators (TIs), exploiting magnetic proximity effect on the TI surface from adjacent ferromagnet layers may provide an alternative approach to the QAHE by opening an exchange gap with less disorder than that in the doped system. Nevertheless, the engineering of a favorable heterointerface that realizes the QAHE based on the magnetic proximity effect remains to be achieved. Here, we report on the observation of the QAHE in a proximity coupled system of non-magnetic TI and ferromagnetic insulator (FMI). We have designed sandwich heterostructures of (Zn,Cr)Te/(Bi,Sb)2Te3/(Zn,Cr)Te that fulfills two prerequisites for the emergence of the QAHE; the formation of a sizable exchange gap at the TI surface state and the tuning of the Fermi energy into the exchange gap. The efficient proximity coupling in the all-telluride based heterostructure as demonstrated here will enable a realistic design of versatile tailor-made topological materials coupled with ferromagnetism, ferroelectricity, superconductivity, and so on.",1908.07163v1 2019-08-20,Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by 75As NMR-NQR measurements,"Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe$_4$As$_{12}$ [Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs$_4$As$_{12}$ with a superconducting transition temperature of $T_{\rm c}$ $\sim$ 4.8 K. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, is nearly independent of temperature above $\sim$ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe$_4$As$_{12}$, we found that the values of $|K|$ and $1/T_1T$ in SrOs$_4$As$_{12}$ are smaller than those in SrFe$_4$As$_{12}$, indicating no obvious ferromagnetic spin correlations in SrOs$_4$As$_{12}$. From the temperature dependence of 1/$T_1$ in the superconducting state, an $s$-wave superconductivity is realized.",1908.07923v1 2019-08-28,The Frustration of being Odd: How Boundary Conditions can destroy Local Order,"A central tenant in the classification of phases is that boundary conditions cannot affect the bulk properties of a system. In this work, we show striking, yet puzzling, evidence of a clear violation of this assumption. We use the prototypical example of an XYZ chain with no external field in a ring geometry with an odd number of sites and both ferromagnetic and antiferromagnetic interactions. In such a setting, even at finite sizes, we are able to calculate directly the spontaneous magnetizations that are traditionally used as order parameters to characterize the system's phases. When ferromagnetic interactions dominate, we recover magnetizations that in the thermodynamic limit lose any knowledge about the boundary conditions and are in complete agreement with standard expectations. On the contrary, when the system is governed by antiferromagnetic interactions, the magnetizations decay algebraically to zero with the system size and are not staggered, despite the AFM coupling. We term this behavior {\it ferromagnetic mesoscopic magnetization}. Hence, in the antiferromagnetic regime, our results show an unexpected dependence of a local, one--spin expectation values on the boundary conditions, which is in contrast with predictions from the general theory.",1908.10876v3 2019-11-01,Role of spin mixing conductance in determining thermal spin pumping near the ferromagnetic phase transition in EuO_{1-x} and La2NiMnO6,"We present a comprehensive study of the temperature (T) dependence of the longitudinal spin Seebeck effect (LSSE) in Pt/EuO_{1-x} and Pt/La2NiMnO6 (LNMO) hybrid structures across their Curie temperatures (Tc). Both systems host ferromagnetic interaction below Tc, hence present optimal conditions for testing magnon spin current based theories against ferrimagnetic YIG. Notably, we observe an anomalous Nernst effect (ANE) generated voltage in bare EuO_{1-x}, however, we find LSSE predominates the thermal signals in the bilayers with Pt. The T-dependence of the LSSE in small T-range near Tc could be fitted to a power law of the form (Tc-T)^P. The derived critical exponent, P, was verified for different methods of LSSE representation and sample crystallinity. The results are explained based on the magnon-driven thermal spin pumping mechanism that relate the T-dependence of LSSE to the spin mixing conductance (Gmix) at the heavy metal/ferromagnet (HM/FM) interface, which in turn is known to vary in accordance with the square of the spontaneous magnetization (Ms). Additionally, the T-dependence of the real part of Gmix derived from spin Hall magnetoresistance measurements at different temperatures for the Pt/LNMO structure, further establish the interdependence.",1911.00211v1 2019-11-13,Nonlocal Exchange Interactions in Strongly Correlated Electron Systems,"We study the influence of ferromagnetic nonlocal exchange on correlated electrons in terms of a $SU(2)$-Hubbard-Heisenberg model and address the interplay of on-site interaction induced local moment formation and the competition of ferromagnetic direct and antiferromagnetic kinetic exchange interactions. In order to simulate thermodynamic properties of the system in a way that largely accounts for the on-site interaction driven correlations in the system, we advance the correlated variational scheme introduced in [M. Sch\""uler et al., Phys. Rev. Lett. 111, 036601 (2013)] to account for explicitily symmetry broken electronic phases by introducing an auxiliary magnetic field. After benchmarking the method against exact solutions of a finite system, we study the $SU(2)$-Hubbard-Heisenberg model on a square lattice. We obtain the $U$-$J$ finite temperature phase diagram of a $SU(2)$-Hubbard-Heisenberg model within the correlated variational approach and compare to static mean field theory. While the generalized variational principle and static mean field theory yield transitions from dominant ferromagnetic to antiferromagnetic correlations in similar regions of the phase diagram, we find that the nature of the associated phase tranistions differs between the two approaches. The fluctuations accounted for in the generalized variational approach render the transitions continuous, while static mean field theory predicts discontinuous transitions between ferro- and antiferromagnetically ordered states.",1911.05420v1 2019-11-14,"Minimal model for the magnetic phase diagram of CeTi$_{1-x}$Sc$_{x}$Ge, GdFe$_{1-x}$Co$_{x}$Si, and related materials","We present a theoretical analysis of the magnetic phase diagram of CeTi$_{1-x}$Sc$_{x}$Ge and GdFe$_{1-x}$Co$_{x}$Si as a function of the temperature and the Sc and Co concentration $x$, respectively. CeScGe and GdCoSi, as many other RTX (R=rare earth, T=transition metal, X=p-block element) compounds, present a tetragonal crystal structure where bilayers of R are separated by layers of T and X. While GdFeSi and CeTi$_{0.75}$Sc$_{0.25}$Ge are ferromagnetic, CeScGe and GdCoSi order antiferromagnetically with the R 4f magnetic moments on the same bilayer aligned ferromagnetically and magnetic moments in nearest neighbouring bilayers aligned antiferromagnetically. The antiferromagnetic transition temperature $T_N$ decreases with decreasing concentration $x$ in both compounds and for low enough values of $x$ the compounds show a ferromagnetic behavior. Based on these observations we construct a simplified model Hamiltonian that we solve numerically for the specific heat and the magnetization. We find a good qualitative agreement between the model and the experimental data. Our results show that the main magnetic effect of the Sc $\to$ Ti and Co $\to$ Fe substitution in these compounds is consistent with a change in the sign of the exchange coupling between magnetic moments in neighbouring bilayers. We expect a similar phenomenology for other magnetic RTX compounds with the same type of crystal structure.",1911.06235v1 2019-11-17,Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism as Operando Probes of Complex Oxide Electrolyte Gate Transistors,"Electrolyte-based transistors utilizing ionic liquids/gels have been highly successful in the study of charge-density-controlled phenomena, particularly in oxides. Experimental probes beyond transport have played a significant role, despite challenges to their application in electric double-layer transistors. Here, we demonstrate application of synchrotron soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) as operando probes of the charge state and magnetism in ion-gel-gated ferromagnetic perovskite films. Electrochemical response via oxygen vacancies at positive gate bias in LaAlO$_3$(001)/La$_{0.5}$Sr$_{0.5}$CoO$_{3-{\delta}}$ is used as a test case. XAS/XMCD measurements of 4-25 unit-cell-thick films first probe the evolution of hole doping (from the O K-edge pre-peak) and ferromagnetism (at the Co L-edges), to establish a baseline. Operando soft XAS/XMCD of electrolyte-gated films is then demonstrated, using optimized spin-coated gels with thickness $\sim$1 ${\mu}$m, and specific composition. Application of gate voltages up to +4 V is shown to dramatically suppress the O $K$-edge XAS pre-peak intensity and Co $L$-edge XMCD, thus enabling the Co valence and ferromagnetism to be tracked upon gate-induced reduction. Soft XAS and XMCD, with appropriate electrolyte design, are thus established as viable for the operando characterization of electrolyte-gated oxides.",1911.07139v1 2019-11-18,Thermally Activated Processes for Ferromagnet Intercalation in Graphene-Heavy Metal Interfaces,"The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by intercalation through graphene of different metals. Using photoelectron spectroscopy (XPS) and Scanning Tunneling Microscopy (STM), we achieve a nanoscale control of thermal activated intercalation of homogeneous ferromagnetic (FM) layer underneath epitaxial Gr grown onto (111)-oriented heavy metal (HM) buffers deposited in turn onto insulating oxide surfaces. XPS and STM demonstrate that Co atoms evaporated on top of Gr arrange in 3D clusters, and, upon thermal annealing, penetrate through and diffuse below Gr in a 2D fashion. The complete intercalation of the metal occurs at specific temperatures depending on the type of metallic buffer. The activation energy and the optimum temperature for the intercalation processes are determined. We describe a reliable method to fabricate and characterize in-situ high quality Gr-FM/HM heterostructures enabling the realization of novel spin-orbitronic devices that exploits the extraordinary properties of Gr.",1911.07549v2 2019-11-23,Evidence the ferromagnetic order on CoSb layer of LaCoSb$_2$,"The emergence of unconventional superconductivity is generally considered to be related to spin fluctuations. Unveiling the intriguing behaviors of spin fluctuations in parent compounds with layered transition-metal ions may shed light on the search for exotic unconventional superconductors. Here, based on the framework of the first-principles calculations, we theoretically propose that LaCoSb$_2$ is a weak antiferromagnetic layered metal with an in-plane ferromagnetic moment of 0.88 $\mu_B$ at the Co sites, as a candidate parent compound of the cobalt-based superconductors. Importantly, this theoretical finding is experimentally supported by our magnetization measurements on polycrystalline samples of LaCo$_{0.78}$Sb$_2$. Following the symmetry analysis, we suggest a possible $p$-wave superconductivity hosted in doped LaCoSb$_2$ emerging at the verge of ferromagnetic spin fluctuations, which implies potential applications in topological quantum computing in future.",1911.10347v5 2019-11-25,Large non-reciprocal propagation of surface acoustic waves in epitaxial ferromagnetic/semiconductor hybrid structures,"Non-reciprocal propagation of sound, that is, the different transmission of acoustic waves traveling along opposite directions, is a challenging requirement for the realization of devices like acoustic isolators and circulators. Here, we demonstrate the efficient non-reciprocal transmission of surface acoustic waves (SAWs) propagating along opposite directions of a GaAs substrate coated with an epitaxial Fe$_3$Si film. The non-reciprocity arises from the acoustic attenuation induced by the magneto-elastic (ME) interaction between the SAW strain field and spin waves in the ferromagnetic film, which depends on the SAW propagation direction and can be controlled via the amplitude and orientation of an external magnetic field. The acoustic transmission non-reciprocity, defined as the difference between the transmitted acoustic power for forward and backward propagation under ME resonance, reaches values of up to 20%, which are, to our knowledge, the largest non-reciprocity reported for SAWs traveling along a semiconducting piezoelectric substrate covered by a ferromagnetic film. The experimental results are well accounted for by a model for ME interaction, which also shows that non-reciprocity can be further enhanced by optimization of the sample design. These results make Fe$_3$Si/GaAs a promising platform for the realization of efficient non-reciprocal SAW devices.",1911.11072v3 2019-11-25,Magnetic response of FeRh to static and dynamic disorder,"Changes of the magnetic and crystal structure on the microscopic scale in 40 nm FeRh thin films have been applied to investigate the phenomena of a disorder induced ferromagnetism at room temperature initiated through light ion-irradiation with fluences up to 0.125 Ne$^+$/nm$^{-2}$. Magnetometry shows an increase of magnetic ordering at low temperatures and a decrease of the transition temperature combined with a broadening of the hysteresis with rising ion fluence. $^{57}$Fe M\""ossbauer spectroscopy reveals the occurrence of an additional magnetic contributions with an hyperfine splitting of 27.2 T - identical to that of ferromagnetic B2-FeRh. The appearance of an anti-site Fe-contribution can be assumed to be lower than 0.6 Fe-at%, indicating that no change of the chemical composition is evident. The investigation of the local structure shows an increase of the static mean square relative displacement determined by X-ray absorption fine structure spectroscopy, while an increase of the defect-concentration has been determined by positron annihilation spectroscopy. From the changes of the microscopic magnetic structure a similarity between the temperature induced and the structural disorder induced ferromagnetic phase can be observed. These findings emphasize the relationship between magnetic ordering and the microscopic defect structure in FeRh.",1911.11256v1 2019-12-05,Coexistence of surface and bulk ferromagnetism mimics skyrmion Hall effect in a topological insulator,"Here we report the investigation of the anomalous Hall effect in the magnetically doped topological insulator (V,Bi,Sb)2Te3. We find it contains two contributions of opposite sign. Both components are found to depend differently on carrier density, leading to a sign inversion of the total anomalous Hall effect as a function of applied gate voltage. The two contributions are found to have different magnetization reversal fields, which in combination with a temperature dependent study points towards the coexistence of two ferromagnetic orders in the system. Moreover, we find that the sign of total anomalous Hall response of the system depends on the thickness and magnetic doping density of the magnetic layer. The thickness dependence suggests that the two ferromagnetic components originate from the surface and bulk of the magnetic topological insulator film. We believe that our observations provide insight on the magnetic behavior, and thus will contribute to an eventual understanding of the origin of magnetism in this material class. In addition, our data bears a striking resemblance to anomalous Hall signals often associated with skyrmion contributions. Our analysis provides a straightforward explanation for both the magnetic field dependence of the Hall signal and the observed change in sign without needing to invoke skyrmions, and thus suggest that caution is needed when making claims of effects from skyrmion phases.",1912.02766v1 2019-12-18,Gravitational realization of magnons in a ferromagnetic spin chain,"A gravitational model of magnons in thermal equilibrium with a ferromagnetic spin chain is developed in a phenomenological bottom-up approach. A large Schwarzschild-AdS black hole background is used as the thermal reservoir and the magnon dynamics is obtained by scalar fields and branes in the bulk. The key feature of this model is that the coupling of the spin chain is related with the radial position in which the brane is located. We further study a ferromagnetic spin chain with a competing interaction and find that the couplings are related by the difference of positions of the branes. We show how to obtain the model from a weak limit of a dynamical gravitational system. This allows us to embed the model into a holographic system. The couplings can be related to entanglement entropy at finite temperature of the CFT since the turning point of minimal surfaces coincides with the position of the branes. The difference of entropy is used to define a notion of distance between the chain couplings.",1912.08761v2 2020-03-11,A two-dimensional electron gas at the (001) surface of ferromagnetic EuTiO$_{3}$(001),"Studies on oxide quasi-two dimensional electron gas (q2DEG) have been a playground for the discovery of novel and sometimes unexpected phenomena, like the reported magnetism at the surface and at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ non-magnetic materials. However, magnetism in this system is weak and there are evidences of a not intrinsic origin. Here, by using in-situ high-resolution angle resolved photoemission we demonstrate that ferromagnetic EuTiO$_{3}$, the magnetic counterpart of SrTiO$_{3}$ in the bulk, hosts a q2DEG at its (001) surface. This is confirmed by density functional theory calculations with Hubbard U terms in the presence of oxygen divacancies in various configurations, all of them leading to a spin-polarized q2DEG related to the ferromagnetic order of Eu-4f magnetic moments. The results suggest EuTiO$_{3}$(001) as a new material platform for oxide q2DEGs, characterized by broken inversion and time reversal symmetries.",2003.05494v2 2020-03-14,Unusual Intralayer Ferromagnetism Between S = 5/2 ions in MnBi$_2$Te$_4$: Role of Empty Bi $p$ States,"The layered magnetic topological insulator MnBi$_2$Te$_4$ is a promising platform to realize the quantum anomalous Hall effect because its layers possess intrinsic ferromagnetism. However, it is not well understood why the high-spin $d^5$ magnetic ions Mn$^{2+}$ forming the Mn-Te-Mn spin exchange paths prefer ferromagnetic (FM) coupling, contrary to the prediction of the Goodenough-Kanamori rule that a TM-L-TM spin exchange, where TM and L are a transition-metal magnetic cation and a main group ligand, respectively, is antiferromagnetic (AFM) even when the bond angle of the exchange path is 90$^{\circ}$. Using density functional theory (DFT) calculations, we show that the presence of Bi$^{3+}$ ions is essential for the FM coupling in MnBi$_2$Te$_4$. Then, using a tight-binding model Hamiltonian, we find that high-spin $d^5$ ions (S = 5/2) in TM-L-TM spin exchange paths prefer FM coupling if the empty p-orbitals of a nonmagnetic cation M (e.g., Bi$^{3+}$ ion) hybridize strongly with those of the bridging ligand L, but AFM coupling otherwise.",2003.06585v1 2020-03-17,"Anomalous critical point behavior in dilute magnetic semiconductor (Ca,Na)(Zn,Mn)2Sb2","In this paper we report successful synthesis and magnetic properties of (Ca,Na)(Zn,Mn)2Sb2 as a new ferromagnetic dilute magnetic semiconductor (DMS). In this DMS material the concentration of magnetic moments can be controlled independently from the concentration of electric charge carriers that are required for mediating magnetic interactions between these moments. This feature allows us to separately investigate the effect of carriers and of spins on the ferromagnetic properties of this new DMS alloy, and particularly of the critical ferromagnetic behavior. We use modified Arrott plot technique to establish critical exponents b, g, and d of this alloy. We find that at low Mn concentrations (< 10 at.%), it is governed by short-range 3D-Ising behavior, with experimental values of b, g, and d very close to theoretical 3D-Ising values of 0.325, 1.24, and 4.815. However, as the Mn concentration increases, this DMS material exhibits a mixed-phase behavior, with g retaining its 3D-Ising characteristics, but b crossing over to longer-range mean-field behavior.",2003.07617v3 2020-03-29,Robust ferromagnetism in highly strained SrCoO3 thin films,"Epitaxial strain provides important pathways to control the magnetic and electronic states in transition metal oxides. However, the large strain is usually accompanied by a strong reduction of the oxygen vacancy formation energy, which hinders the direct manipulation of their intrinsic properties. Here using a post-deposition ozone annealing method, we obtained a series of oxygen stoichiometric SrCoO3 thin films with the tensile strain up to 3.0%. We observed a robust ferromagnetic ground state in all strained thin films, while interestingly the tensile strain triggers a distinct metal to insulator transition along with the increase of the tensile strain. The persistent ferromagnetic state across the electrical transition therefore suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which then calls for further investigation of the intrinsic mechanism of this magnetic compound beyond the double-exchange mechanism.",2003.12982v2 2020-05-05,Dimensional crossover in spin Hall oscillators,"Auto-oscillations of magnetization driven by direct spin current have been previously observed in multiple quasi-zero-dimensional (0D) ferromagnetic systems such as nanomagnets and nanocontacts. Recently, it was shown that pure spin Hall current can excite coherent auto-oscillatory dynamics in quasi-one-dimensional (1D) ferromagnetic nanowires but not in quasi-two-dimensional (2D) ferromagnetic films. Here we study the 1D to 2D dimensional crossover of current-driven magnetization dynamics in wire-based Pt/$\mathrm{Ni}_{80}\mathrm{Fe}_{20}$ bilayer spin Hall oscillators via varying the wire width. We find that increasing the wire width results in an increase of the number of excited auto-oscillatory modes accompanied by a decrease of the amplitude and coherence of each mode. We also observe a crossover from a hard to a soft onset of the auto-oscillations with increasing the wire width. The amplitude of auto-oscillations rapidly decreases with increasing temperature suggesting that interactions of the phase-coherent auto-oscillatory modes with incoherent thermal magnons plays an important role in suppression of the auto-oscillatory dynamics. Our measurements set the upper limit on the dimensions of an individual spin Hall oscillator and elucidate the mechanisms leading to suppression of coherent auto-oscillations with increasing oscillator size.",2005.01925v2 2020-05-08,$s$-$d$ model for local and nonlocal spin dynamics in laser-excited magnetic heterostructures,"We discuss a joint microscopic theory for the laser-induced magnetization dynamics and spin transport in magnetic heterostructures based on the $s$-$d$ interaction. Angular momentum transfer is mediated by scattering of itinerant $s$ electrons with the localized ($d$ electron) spins. We use the corresponding rate equations and focus on a spin one-half $d$ electron system, leading to a simplified analytical expression for the dynamics of the local magnetization that is coupled to an equation for the non-equilibrium spin accumulation of the $s$ electrons. We show that this description converges to the microscopic three-temperature model in the limit of a strong $s$-$d$ coupling. The equation for the spin accumulation is used to introduce diffusive spin transport. The presented numerical solutions show that during the laser-induced demagnetization in a ferromagnetic metal a short-lived spin accumulation is created that counteracts the demagnetization process. Moreover, the spin accumulation leads to the generation of a spin current at the interface of a ferromagnetic and non-magnetic metal. Depending on the specific magnetic system, both local spin dissipation and interfacial spin transport are able to enhance the demagnetization rate by providing relaxation channels for the spin accumulation that is build up during demagnetization in the ferromagnetic material.",2005.03905v2 2020-05-13,Spin-polarized triplet supercurrent in Josephson junctions with perpendicular ferromagnetic layers,"Josephson junctions containing three ferromagnetic layers with non-collinear magnetizations between adjacent layers carry spin-triplet supercurrent under certain conditions. The signature of the spin-triplet supercurrent is a relatively slow decay of the maximum supercurrent as a function of the thickness of the middle ferromagnetic layer. In this work we focus on junctions where the middle magnetic layer is a [Co/Pd]$_N$ multilayer with perpendicular magnetic anisotropy (PMA), while the outer two layers have in-plane anisotropy. We compare junctions where the middle PMA layer is or is not configured as a synthetic antiferromagnet (PMA-SAF). We find that the supercurrent decays much more rapidly with increasing the number $N$ of [Co/Pd] bilayers in the PMA-SAF junctions compared to the PMA junctions. Similar behavior is observed in junctions containing [Co/Ni]$_N$ PMA multilayers. We model that behavior by assuming that each Co/Pd or Co/Ni interface acts as a partial spin filter, so that the spin-triplet supercurrent in the PMA junctions becomes more strongly spin-polarized as $N$ increases while the supercurrent in the PMA-SAF junctions is suppressed with increasing $N$. We also address a question raised in a previous work regarding how much spin-singlet supercurrent is transmitted through our nominally spin-triplet junctions. We do that by comparing spin-triplet junctions with similar junctions where the order of the magnetic layers has been shuffled. The results of this work are expected to be helpful in designing spin-triplet Josephson junctions for use in cryogenic memory.",2005.06657v1 2020-05-25,Dynamics of ferromagnetic bimerons driven by spin currents and magnetic fields,"Magnetic bimeron composed of two merons is a topological counterpart of magnetic skyrmion in in-plane magnets, which can be used as the nonvolatile information carrier in spintronic devices. Here we analytically and numerically study the dynamics of ferromagnetic bimerons driven by spin currents and magnetic fields. Numerical simulations demonstrate that two bimerons with opposite signs of topological numbers can be created simultaneously in a ferromagnetic thin film via current-induced spin torques. The current-induced spin torques can also drive the bimeron and its speed is analytically derived, which agrees with the numerical results. Since the bimerons with opposite topological numbers can coexist and have opposite drift directions, two-lane racetracks can be built in order to accurately encode the data bits. In addition, the dynamics of bimerons induced by magnetic field gradients and alternating magnetic fields are investigated. It is found that the bimeron driven by alternating magnetic fields can propagate along a certain direction. Moreover, combining a suitable magnetic field gradient, the Magnus-force-induced transverse motion can be completely suppressed, which implies that there is no skyrmion Hall effect. Our results are useful for understanding of the bimeron dynamics and may provide guidelines for building future bimeron-based spintronic devices.",2005.11924v2 2020-05-25,Magnetic order and transport in a spin-fermion model on a superlattice,"We consider a spin-fermion model consisting of free electrons coupled to classical spins, where the latter are embedded in a quasi one-dimensional superlattice structure consisting of spin blocks separated by spinless buffers. Using a spiral ansatz for the spins, we study the effect of the electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction on the $T=0$ ground state of the system. We find that the RKKY interaction can lead to ferromagnetic, antiferromagnetic, or intermediate spiral phases for different system parameters. When the width is much larger than the length of the individual blocks, the spiral phases are suppressed, and the ground state oscillates between ferromagnetic and antiferromagnetic order as the size of the buffer regions is varied. This is accompanied by a corresponding oscillation in the Drude weight reflecting an increased conductivity in the ferromagnetic state compared to the antiferromagnetic one. These results are reminiscent of classic giant magnetoresistance phenomena observed in a similar geometry of thin, sandwiched magnetic and non-magnetic layers. Our analysis provides a robust framework for understanding the role of the RKKY interaction on the ground state order and corresponding transport properties of such systems, extending beyond the conventional perturbative regime.",2005.12381v2 2020-06-03,Two-dimensional Weyl nodal line semimetal in high Curie temperature d0 ferromagnet K2N monolayer,"Nodal line semimetals in two-dimensional (2-D) materials have attracted intense attention currently. From fundamental physics and spintronic applications points of view, high Curie temperature ferromagnetic (FM) ones with nodal lines robust against spin-orbit coupling (SOC) are significantly in desirable. Here, we propose that FM K2N monolayer is such Weyl nodal line semimetal. We show that K2N monolayer is dynamically stable, and has a FM ground magnetic state with the out-of-plane [001] magnetization. It shows two nodal lines in the low-energy band structures. Both nodal lines are robust against SOC, under the protection of mirror symmetry. We construct an effective Hamiltonian, which can well characterize the nodal lines in the system. Remarkably, the nodal line semimetal proposed here is distinct from the previously studied ones in that K2N monolayer is 2-D d0-type ferromagnet with the magnetism arising from the partially filled N-p orbitals, which can bring special advantages in spintronic applications. Besides, the Curie temperature in K2N monolayer is estimated to be 942K, being significantly higher than previous FM nodal lines materials. We also find that, specific tensile strains can transform the nodal line from type-I to a type-II one, making its nodal line characteristics even more interesting.",2006.02018v1 2020-06-09,Ferromagnetism out of charge fluctuation of strongly correlated electrons in $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br,"We perform magnetic susceptibility and magnetic torque measurements on the organic $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br, which is recently suggested to host an exotic quantum dipole-liquid in its low-temperature insulating phase. Below the metal-insulator transition temperature, the magnetic susceptibility follows a Curie-Weiss law with a positive Curie-Weiss temperature, and a particular $M\propto \sqrt{H}$ curve is observed. The emergent ferromagnetically interacting spins amount to about 1/6 of the full spin moment of localized charges. Taking account of the possible inhomogeneous quasi-charge-order that forms a dipole-liquid, we construct a model of antiferromagnetically interacting spin chains in two adjacent charge-ordered domains, which are coupled via fluctuating charges on a Mott-dimer at the boundary. We find that the charge fluctuations can draw a weak ferromagnetic moment out of the spin singlet domains.",2006.05221v2 2020-06-18,Tunable anomalous Hall transport in bulk and two-dimensional 1$T$-CrTe$_{2}$: A first-principles study,"Layered materials with robust magnetic ordering have been attracting significant research interest. In recent experiments, a new layered material 1$T$-CrTe$_{2}$ has been synthesized and exhibits ferromagnetism above the room temperature. Here, based on first-principles calculations, we investigate the electronic, magnetic, and transport properties of 1$T$-CrTe$_{2}$, both in the bulk and in the two-dimensional (2D) limit. We show that 1$T$-CrTe$_{2}$ can be stable in the monolayer form, and has a low exfoliation energy. The monolayer structure is an intrinsic ferromagnetic metal, which maintains a high Curie temperature above the room temperature. Particularly, we reveal interesting features in the anomalous Hall transport. We show that in the ground state, both bulk and monolayer 1$T$-CrTe$_{2}$ possess vanishing anomalous Hall effect, because the magnetization preserves one vertical mirror symmetry. The anomalous Hall conductivity can be made sizable by tuning the magnetization direction or by uniaxial strains that break the mirror symmetry. The room-temperature 2D ferromagnetism and the tunable anomalous Hall effect make the material a promising platform for nanoscale device applications.",2006.10795v2 2020-06-21,Chemical Bonding Governs Complex Magnetism in MnPt5P,"Subtle changes in chemical bonds may result in dramatic revolutions in magnetic properties in solid state materials. MnPt5P, a new derivative of the rare-earth-free ferromagnetic MnPt5As, was discovered and is presented in this work. MnPt5P was synthesized and its crystal structure and chemical composition were characterized by X-ray diffraction as well as energy-dispersive X-ray spectroscopy. Accordingly, MnPt5P crystallizes in the layered tetragonal structure with the space group P4/mmm (No. 123), in which the face-shared Mn@Pt12 polyhedral layers are separated by P layers. In contrast to the ferromagnetism observed in MnPt5As, the magnetic properties measurements on MnPt5P show antiferromagnetic ordering occurs at ~188 K with a strong magnetic anisotropy in and out of the ab-plane. Moreover, a spin-flop transition appears when a high magnetic field is applied. An A-type antiferromagnetic structure was obtained from the analysis of powder neutron diffraction (PND) patterns collected at 150 K and 9 K. Calculated electronic structures imply that hybridization of Mn-3d and Pt-5d orbitals are critical for both the structural stability and observed magnetic properties. Semi-empirical molecular orbitals calculations on both MnPt5P and MnPt5As indicate that the lack of 4p character on the P atoms at the highest occupied molecular orbital (HOMO) in MnPt5P may cause the different magnetic behavior in MnPt5P compared to MnPt5As. The discovery of MnPt5P, along with our previously reported MnPt5As, parametrizes the end points of a tunable system to study the chemical bonding which tunes the magnetic ordering from ferromagnetism to antiferromagnetism with strong spin-orbit coupling (SOC) effect.",2006.11903v1 2020-06-25,Perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction at an oxide/ferromagnetic metal interface,"We report on the study of both perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) at an oxide/ferromagnetic metal (FM) interface, i.e. BaTiO3 (BTO)/CoFeB. Thanks to the functional properties of the BTO film and the capability to precisely control its growth, we are able to distinguish the dominant role of the oxide termination (TiO2 vs BaO), from the moderate effect of ferroelectric polarization in the BTO film, on the PMA and DMI at the oxide/FM interface. We find that the interfacial magnetic anisotropy energy of the BaO-BTO/CoFeB structure is two times larger than that of the TiO2-BTO/CoFeB, while the DMI of the TiO2-BTO/CoFeB interface is larger. We explain the observed phenomena by first-principles calculations, which ascribe them to the different electronic states around the Fermi level at the oxide/ferromagnetic metal interfaces and the different spin-flip processes. This study paves the way for further investigation of the PMA and DMI at various oxide/FM structures and thus their applications in the promising field of energy-efficient devices.",2006.14268v1 2020-06-26,Ab initio determination of magnetic ground state of pyrochlore Y$_2$Mn$_2$O$_7$,"There are two discrepant experimental results on the magnetic ground state of Y$_{2}$Mn$_{2}$O$_{7}$, one study proposes a spin glass state, while another introduces the material as a ferromagnet. In this study, we attempt to resolve this issue by employing density functional theory and Monte Carlo simulations. We derive different spin models by varying the Hubbard $U$ parameter in ab initio GGA+$U$ calculations. For the most range of Hubbard $U$, We obtain that the leading terms in the spin Hamiltonian are bi-quadratic and the nearest neighbor Heisenberg exchange interactions. By comparing Monte Carlo simulations of these models with the experiments, we find a ferromagnetic ground state for Y$_{2}$Mn$_{2}$O$_{7}$ as the most compatible with experiments. We also consider Y$_{2}$Mo$_{2}$O$_{7}$ as a prototype of the defect-free pyrochlore system with spin-glass behavior and compare it with Y$_{2}$Mn$_{2}$O$_{7}$. The orbital degrees of freedom are considered as a leading factor in converting a defect-free pyrochlore such as Y$_{2}$Mn$_{2}$O$_{7}$ to a spin glass system. By changing the $d$ orbital occupations of Mo atoms, our GGA+$U$ calculations for Y$_{2}$Mo$_{2}$O$_{7}$ indicate many nearly degenerate states with different $d$ orbital orientations which reveals $d$ orbital degrees of freedom in this material. While for Y$_{2}$Mn$_{2}$O$_{7}$, we find a single ground state with a fixed orbital orientation. Consequently, all of our ab initio approaches confirm Y$_{2}$Mn$_{2}$O$_{7}$ as a ferromagnetic system.",2006.15002v1 2020-08-13,Half-metallic ferromagnetism in layered CdOHCl induced by hole doping,"Next-generation spintronic devices will benefit from low-dimensionality, ferromagnetism, and half-metallicity, possibly controlled by electric fields. We find these technologically-appealing features to be combined with an exotic microscopic origin of magnetism in doped CdOHCl, a van der Waals material from which 2D layers may be exfoliated. By means of first principles simulations, we predict homogeneous hole-doping to give rise to $p$-band magnetism in both the bulk and monolayer phases and interpret our findings in terms of Stoner instability: as the Fermi level is tuned via hole-doping through singularities in the 2D-like density of states, ferromagnetism develops with large saturation magnetization of 1 $\mu_B$ per hole, leading to a half-metallic behaviour for layer carrier densities of the order of 10$^{14}$ cm$^{-2}$. Furthermore, we put forward electrostatic doping as an additional handle to induce magnetism in monolayers and bilayers of CdOHCl. Upon application of critical electric fields perpendicular to atomically-thin-films (as low as 0.2 V/$A{\deg}$ and 0.5 V/$A{\deg}$ in the bilayer and monolayer case, respectively), we envisage the emergence of a magnetic half-metallic state. The different behaviour of monolayer vs bilayer systems, as well as an observed asymmetric response to positive and negative electric fields in bilayers, are interpreted in terms of intrinsic polarity of CdOHCl atomic stacks, a distinctive feature of the material. In perspective, given the experimentally accessible magnitude of critical fields in bilayer of CdOHCl, one can envisage $p$ band magnetism to be exploited in miniaturized spintronic devices.",2008.05766v1 2020-08-29,Observation of nonreciprocal magneto-optical scattering in nonencapsulated few-layered CrI3,"Magneto-optical effect refers to a rotation of polarization plane, which has been widely studied in traditional ferromagnetic metal and insulator films and scarcely in two-dimensional layered materials. Here we uncover a new nonreciprocal magneto-inelastic light scattering effect in ferromagnetic few-layer CrI3. We observed a rotation of the polarization plane of inelastic light scattering between -20o and +60o that are tunable by an out-of-plane magnetic field from -2.5 to 2.5 T. It is experimentally observed that the degree of polarization can be magnetically manipulated between -20% and 85%. This work raises a new magneto-optical phenomenon and could create opportunities of applying 2D ferromagnetic materials in Raman lasing, topological photonics, and magneto-optical modulator for information transport and storage.",2008.12896v2 2020-08-29,Unveiling the polarity of the spin-to-charge current conversion in $Bi_2Se_3$,"We report an investigation of the spin- to charge-current conversion in sputter-deposited films of topological insulator $Bi_2Se_{3}$ onto single crystalline layers of YIG $(Y_{3}Fe_{5}O_{12})$ and polycrystalline films of Permalloy $(Py = Ni_{81}Fe_{19})$. Pure spin current was injected into the $Bi_{2}Se_{3}$ layer by means of the spin pumping process in which the spin precession is obtained by exciting the ferromagnetic resonance of the ferromagnetic film. The spin-current to charge-current conversion, occurring at the $Bi_{2}Se_{3}/$ferromagnet interface, was attribute to the inverse Rashba-Edelstein effect (IREE). By analyzing the data as a function of the $Bi_{2}Se_{3}$ thickness we calculated the IREE length used to characterize the efficiency of the conversion process and found that 1.2 pm $\leq|{\lambda}_{IREE}|\leq$ 2.2 pm. These results support the fact that the surface states of $Bi_{2}Se_{3}$ have a dominant role in the spin-charge conversion process, and the mechanism based on the spin diffusion process plays a secondary role. We also discovered that the spin- to charge-current mechanism in $Bi_{2}Se_{3}$ has the same polarity as the one in Ta, which is the opposite to the one in Pt. The combination of the magnetic properties of YIG and Py, with strong spin-orbit coupling and dissipationless surface states topologically protected of $Bi_{2}Se_{3}$ might lead to spintronic devices with fast and efficient spin-charge conversion.",2008.12900v1 2020-08-30,Microwave and spin transfer torque driven coherent control in ferromagnets,"Coherent control is a method used to manipulate the state of matter using oscillatory electromagnetic radiation which relies on the non-adiabatic interaction. It is commonly applied in quantum processing applications. This technique is interesting in the context of ferromagnetic materials because of the ability to combine it with spintronics for the purpose of fundamental spin transport research, low-power information processing, and potentially future quantum bit (Qubit) applications. In this work we address the theoretical grounds of coherent manipulation in practical ferromagnetic systems. We study electromagnetic radiation driven interaction that is enhanced in the presence of spin polarized currents and map the conditions that allow coherent manipulation for which Rabi oscillations take place. The role of the magnetic anisotropy field is shown to act as an additional oscillatory driving field. We discuss the Gilbert losses in the context of effective coherence decay rates and show that it is possible to control these rates by application of a static spin current. The case of coherent manipulation using oscillatory spin currents that is free of radiation is discussed as well. Our work paves the way towards spin current amplification as well as radiation-free coherent control schemes that may potentially lead to novel Qubits that are robust and scalable.",2008.13139v3 2020-09-01,Spin coherence on the ferromagnetic spherical surface,"Spintronics on flat surfaces has been studied over the years, and the scenario is relatively well-known; however, there is a lack of information when we consider non-flat surfaces. In this paper, we are concerned about the spin dynamics of the ferromagnetic model on the spherical surface. We use the Schwinger bosonic formalism for describing the thermodynamics of spin operators in terms of spinon operators. Opposite to the flat two-dimensional model, which is disordered at finite temperature, the curvature of the spherical surface provides non-zero critical temperature for Schwinger boson condensation, which characterizes order at finite temperature even in the absence of external magnetic fields. The thermodynamics is then analyzed in the low-temperature regime. In addition, we consider the presence of both static and oscillating magnetic fields, the necessary condition for inducing the ferromagnetic resonance, and we show systematically that the studied model is well-described by $SU(2)$ coherent states, which provides the correct dynamics of the magnetization. The archived results can be applied for describing a diversity of experiments such as spin superfluidity, angular momentum injection by spin pumping and spin-transfer torque in non-conventional junctions, magnon dissipation, and magnetoelectronics on the spherical surface.",2009.00636v2 2020-08-30,From Ising model to Kitaev Chain -- An introduction to topological phase transitions,"In this general article, we map the one-dimensional transverse field quantum Ising model of ferromagnetism to Kitaev's one-dimensional p-wave superconductor, which has its application in fault-tolerant topological quantum computing. Mapping Pauli's spin operators of transverse Ising chain to spinless fermionic creation and annihilation operators by Inverse Jordan-Wigner transformation leads to a Hamiltonian form closely related to Kitaev Chain, which exhibits topological phase transition where phases are characterized by different topological invariant that changes discontinuously at the transition point. Kitaev Chain supports two Majorana zero modes (MZMs) in the non-trivial topological phase, while none is in the trivial phase. The doubly degenerate ground state of the transverse Ising in ferromagnetic phase corresponds to non-local free fermion degree made from MZMs. The quasi-particle excitations of Ising chain, viz., domain wall formation in the ferromagnetic phase and spin-flip in paramagnetic phase maps to Bogoliubon excitations. The mapping suggests that a non-local order parameter can be defined for Kitaev Chain to work with the usual paradigm of Landau's theory.",2009.01078v6 2020-09-08,Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures,"In recent years, there has been a growing interest in spin-orbit torques (SOTs) for manipulating the magnetization in nonvolatile magnetic memory devices. SOTs rely on the spin-orbit coupling of a nonmagnetic material coupled to a ferromagnetic layer to convert an applied charge current into a torque on the magnetization of the ferromagnet (FM). Transition metal dichalcogenides (TMDs) are promising candidates for generating these torques with both high charge-to-spin conversion ratios, and symmetries and directions which are efficient for magnetization manipulation. Moreover, TMDs offer a wide range of attractive properties, such as large spin-orbit coupling, high crystalline quality and diverse crystalline symmetries. Although numerous studies were published on SOTs using TMD/FM heterostructures, we lack clear understanding of the observed SOT symmetries, directions, and strengths. In order to shine some light on the differences and similarities among the works in literature, in this mini-review we compare the results for various TMD/FM devices, highlighting the experimental techniques used to fabricate the devices and to quantify the SOTs, discussing their potential effect on the interface quality and resulting SOTs. This enables us to both identify the impact of particular fabrication steps on the observed SOT symmetries and directions, and give suggestions for their underlying microscopic mechanisms. Furthermore, we highlight recent progress of the theoretical work on SOTs using TMD heterostructures and propose future research directions.",2009.03710v2 2020-09-05,Transition to Coarse-Grained Order in Coupled Logistic Maps: Effect of Delay and Asymmetry,"We study one-dimensional coupled logistic maps with delayed linear or nonlinear nearest-neighbor coupling. Taking the nonzero fixed point of the map x* as reference, we coarse-grain the system by identifying values above x* with the spin-up state and values below x* with the spin-down state. We define persistent sites at time T as the sites which did not change their spin state even once for all even times till time T. A clear transition from asymptotic zero persistence to non-zero persistence is seen in the parameter space. The transition is accompanied by the emergence of antiferromagnetic, or ferromagnetic order in space. We observe antiferromagnetic order for nonlinear coupling and even delay, or linear coupling and odd delay. We observe ferromagnetic order for linear coupling and even delay, or nonlinear coupling and odd delay. For symmetric coupling, we observe a power-law decay of persistence. The persistence exponent is close to 0.375 for the transition to antiferromagnetic order and close to 0.285 for ferromagnetic order. The number of domain walls decays with an exponent close to 0.5 in all cases as expected. The persistence decays as a stretched exponential and not a power-law at the critical point, in the presence of asymmetry.",2009.04919v1 2020-09-25,Alternation of Magnetic Anisotropy Accompanied by Metal-Insulator Transition in Strained Ultrathin Manganite Heterostructures,"Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). We successfully detect the clear change of the magnetic behavior of the Mn ions concomitant with the thickness-dependent metal-insulator transition (MIT). Our results suggest that double-exchange interaction induces the ferromagnetism in the metallic LSMO film under tensile strain caused by the SrTiO$_3$ substrate, while superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via Mn 3$d_{3z^2-r^2}$ orbitals under compressive strain and the double-exchange interaction via the 3$d_{x^2-y^2}$ orbitals.",2009.12327v2 2020-09-28,Electric field induced topological phase transition and large enhancements of spin-orbit coupling and Curie temperature in two-dimensional ferromagnetic semiconductors,"Tuning topological and magnetic properties of materials by applying an electric field is widely used in spintronics. In this work, we find a topological phase transition from topologically trivial to nontrivial states at an external electric field of about 0.1 V/A in MnBi$_2$Te$_4$ monolayer that is a topologically trivial ferromagnetic semiconductor. It is shown that when electric field increases from 0 to 0.15 V/A, the magnetic anisotropy energy (MAE) increases from about 0.1 to 6.3 meV, and the Curie temperature Tc increases from 13 to about 61 K. The increased MAE mainly comes from the enhanced spin-orbit coupling due to the applied electric field. The enhanced Tc can be understood from the enhanced $p$-$d$ hybridization and decreased energy difference between $p$ orbitals of Te atoms and $d$ orbitals of Mn atoms. Moreover, we propose two novel Janus materials MnBi$_2$Se$_2$Te$_2$ and MnBi$_2$S$_2$Te$_2$ monolayers with different internal electric polarizations, which can realize quantum anomalous Hall effect (QAHE) with Chern numbers $C$=1 and $C$=2, respectively. Our study not only exposes the electric field induced exotic properties of MnBi2Te4 monolayer, but also proposes novel materials to realize QAHE in ferromagnetic Janus semiconductors with electric polarization.",2009.13328v2 2020-10-17,Ferromagnetic Gyroscopes for Tests of Fundamental Physics,"A ferromagnetic gyroscope (FG) is a ferromagnet whose angular momentum is dominated by electron spin polarization and that will precess under the action of an external torque, such as that due to a magnetic field. Here we model and analyze FG dynamics and sensitivity, focusing on practical schemes for experimental realization. In the case of a freely floating FG, we model the transition from dynamics dominated by libration in relatively high externally applied magnetic fields, to those dominated by precession at relatively low applied fields. Measurement of the libration frequency enables in situ measurement of the magnetic field and a technique to reduce the field below the threshold for which precession dominates the FG dynamics. We note that evidence of gyroscopic behavior is present even at magnetic fields much larger than the threshold field below which precession dominates. We also model the dynamics of an FG levitated above a type-I superconductor via the Meissner effect, and find that for FGs with dimensions larger than about 100 nm the observed precession frequency is reduced compared to that of a freely floating FG. This is akin to negative feedback that arises from the distortion of the field from the FG by the superconductor. Finally we assess the sensitivity of an FG levitated above a type-I superconductor to exotic spin-dependent interactions under practical experimental conditions, demonstrating the potential of FGs for tests of fundamental physics.",2010.08731v1 2020-10-19,Inhomogeneous ferromagnetism mimics signatures of the topological Hall effect in SrRuO$_3$ films,"Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the ``topological Hall effect'' (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal SrRuO$_3$ the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a non-random distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the non-collinear magnetization on the atomic scale.",2010.09349v1 2021-02-03,Unveiling excitonic properties of magnons in a quantum Hall ferromagnet,"Magnons enable transferring a magnetic moment or spin over macroscopic distance. In quantum Hall ferromagnet, it has been predicted in the early 90s that spin and charges are entangled, meaning that any change of the spin texture modifies the charge distribution. As a direct consequence of this entanglement, magnons carry an electric dipole moment. Here we report the first evidence of the existence of this electric dipole moment in a graphene quantum Hall ferromagnet using a Mach-Zehnder interferometer as a quantum sensor. By propagating towards the interferometer through an insulating bulk, the magnon electric dipole moment modifies the Aharonov-Bohm flux through the interferometer, changing both its phase and its visibility. In particular, we relate the phase shift to the sign of this electric dipole moment, and the exponential loss of visibility to the flux of emitted magnons. Finally, we probe the emission energy threshold of the magnons close to filling factor v=1. Approaching v=0, we observe that the emission energy threshold diminishes towards zero, which might be linked to the existence of gapless mode in the canted-antiferromagnetic (CAF) phase at v=0. The detection and manipulation of magnons based on their electric dipole open the field for a new type of coherent magnon quantum circuits that will be electrostatically controlled.",2102.02068v1 2021-02-11,Large topological Hall effect near room temperature in noncollinear ferromagnet LaMn2Ge2 single crystal,"Non-trivial spin structures in itinerant magnets can give rise to topological Hall effect (THE) due to the interacting local magnetic moments and conductive electrons. While, in series of materials, THE has mostly been observed at low temperatures far below room temperature (RT) limiting its potential applications. Here, we report the anisotropic anomalous Hall effect (AHE) near RT in LaMn2Ge2, a noncollinear ferromagnetic (FM) with Curie temperature TC=325 K. Large topological Hall resistivity of ~1.0 10-6 ohmcm in broad temperature range (190 K